© Copyright by Shuren Qu 2019
All Rights Reserved
TRIBOLOGY OF PTFE/PEEK COMPOSITE AT ELEVATED TEMPERATURE
A Dissertation
Presented to
the Faculty of the Interdisciplinary Program in Materials Engineering
University of Houston
In Partial Fulfillment
of the Requirements for the Degree
Doctor of Philosophy
in Materials Engineering
by
Shuren Qu
August 2019
TRIBOLOGY OF PTFE/PEEK COMPOSITE AT ELEVATED TEMPERATURE
Shuren Qu
Approved:
Chair of the Committee
Su Su Wang,
Hugh Roy and Lillie Cranz Cullen Professor,
Department of Mechanical Engineering
Committee Members:
Alamgir Karim,
Dow Chair and Welch Foundation Professor,
Department of Chemical & Biomolecular
Engineering
Megan Robertson,
Associate Professor,
Department of Chemical & Biomolecular
Engineering
Akira Miyase,
Research Professor,
Department of Mechanical Engineering
King Him Lo,
Research Professor,
National Wind Energy Center
Suresh K. Khator, Associate Dean,
Cullen College of Engineering
Alamgir Karim,
Dow Chair and Welch Foundation Professor
and Director of the Interdisciplinary Program
in Materials Engineering
Acknowledgements
I am sincerely thankful for all the remarkable people who had helped me,
personally and professionally, through my years as a Ph.D. student at the University of
Houston.
I would like to thank my advisor, Professor Su Su Wang for his support and
guidance throughout my research. He is not only a knowledgeable professor, but also a
mentor and a friend. I have been continuously improving myself professionally as a
student through his invaluable feedbacks and leadership. I also thank him for always
motivating and encouraging me throughout my Ph.D. career.
I would also like to express my gratitude towards Professor Alamgir Karim,
Professor Megan Robertson, Professor Akira Miyase and Professor King Him Lo who
served as members of my thesis committee for their effort in evaluating this work.
I thank Dr. Akira Miyase and Dr. King Him Lo for training me as a new
researcher at Composite Engineering and Applications Center and their significant
contributions to this research. I am fortunate to work with them in the past 5 years. Their
integrity, dedication and vision in research inspired me tremendously. I will certainly
bring these good characters to my future endeavors. I thank Dr. Boris Makarenko from
University of Houston Department of Chemistry for his kind help and fruitful discussion
on the surface elemental analysis with X-ray photoelectron spectroscopy.
I am also grateful to my group members, Jonathan Penaranda and Ethan Pedneau,
for their endless help on my research and making graduate school as enjoyable as
v
possible. Thank Jonathan and Ethan for being my friends. I cherish our friendships and it
is a true pleasure working with them.
I would like to dedicate this work to my parents, Zhibo Qu and Jinge Tang, my
grandmother, Jinhuan Gu, and my wife Ya Zhuo. I could not have finished my Ph.D.
study without their love and support. Praise the Lord that I have such a wonderful family
that always encourage me and give me hope. All the laughter and the tears we had
together now become worthy. Thank God for carrying me so far and I will never stop my
steps seeking truth.
Financial assistance from the Composites Engineering and Applications Center
for this research is also gratefully acknowledged.
vi
TRIBOLOGY OF PTFE/PEEK COMPOSITE AT ELEVATED TEMPERATURE
An Abstract
of a
Dissertation
Presented to
the Faculty of the Interdisciplinary Program in Materials Engineering
University of Houston
In Partial Fulfillment
of the Requirements for the Degree
Doctor of Philosophy
in Materials Engineering
by
Shuren Qu
August 2019
vii
Abstract
Engineering thermoplastics are used extensively in reciprocating and rotating
machinery. For example, Polyetheretherketone (PEEK) and its composites are commonly
used as sealing materials in compressors and pumps. Neat PEEK polymer is known for
its outstanding chemical resistance, thermal stability, and high-temperature mechanical
strength. Despite these attractive properties, a critical concern of the PEEK polymer in
tribological applications is its high friction, which could lead to serious problems, such as
high local flash temperature and severe wear. A common practice to reduce the high
friction is to incorporate solid-state lubricants, such as polytetrafluoroethylene (PTFE),
into the polymer. In view of the wide use of PTFE/PEEK composite for tribological
applications, predictive models and theories are needed to better understand friction and
wear mechanisms of the composite and accelerate development and applications of the
composite for tribological operations.
In the experimental phase of this study, friction and wear of PTFE/PEEK
composites with various PTFE contents were determined at different temperatures.
Elevated temperature mechanical properties of neat PEEK, PTFE and their composites,
such as elastic modulus and compressive yield stress, were also determined. Quantitative
methods for characterization of the transfer films were developed. The area coverage
ratio and composition of composite transfer films were investigated with an in-house
developed computer software and XPS, respectively. Important experimental results were
obtained and used for subsequent developments of friction and wear models and theories.
In the theoretical phase of the study, a power-law relationship between
PTFE/PEEK composite friction and wear is first established based on experimental
viii
results. Friction and wear models are established for developing theories on PTFE/PEEK
composite sliding friction and wear at low temperature. Solid film lubrication is
introduced along with the rule of the mixtures to derive the new friction and wear
theories for the polymer composite. For elevated temperature friction and wear of the
PTFE/PEEK composite, mechanism-based friction and wear models are proposed for the
development of the new tribological theories to predict the friction and wear of
PTFE/PEEK composite. Detailed mechanisms and mechanics of low and elevated
temperature friction and wear of the PTFE/PEEK composites are discussed.
ix
Table of Contents
Acknowledgements ........................................................................................................... v
Abstract
................................................................................................................... viii
Table of Contents .............................................................................................................. x
List of Figures ................................................................................................................. xiii
List of Tables ................................................................................................................ xviii
Chapter 1
Introduction ................................................................................................ 1
Chapter 2
Literature Review ...................................................................................... 8
2.1 Polymer Composites with Solid-State Lubricants .................................................... 8
2.2 Friction and Wear of PTFE/PEEK Composite........................................................ 13
2.3 Effect of Transfer Films on Tribological Properties of PTFE/PEEK Composite ... 15
2.3.1 PTFE ................................................................................................................. 15
2.3.2 PEEK ................................................................................................................ 17
2.3.3 PTFE/PEEK Composite ................................................................................... 18
2.4 Experimental Methods for Polymer Tribological Study ......................................... 19
2.4.1 Block-on-Ring Tribometer ............................................................................... 19
2.4.2 Pin-on-Disk Tribometer .................................................................................... 20
2.4.3 Linear-Reciprocating Tribometer ..................................................................... 21
2.4.4 Thrust-Washer Tribometer ............................................................................... 21
2.5 Friction and Wear Theories of Polymer Composite................................................ 21
2.5.1 Friction............................................................................................................ 21
2.5.2 Transfer Films................................................................................................... 23
2.5.3 Wear.................................................................................................................. 24
2.5.4 Flash Temperature ............................................................................................ 26
Chapter 3
Objectives and Scope of Research ........................................................... 27
Chapter 4
Materials System and Experimental Program ....................................... 29
4.1 Material System....................................................................................................... 29
4.1.1 Constituent Materials ........................................................................................ 29
4.1.2 PTFE/PEEK Composite Microstructure ........................................................... 29
x
4.1.3 Polymer and Composite Morphology............................................................... 30
4.2 Thermal and Mechanical Properties ........................................................................ 34
4.2.1 Thermal Properties ........................................................................................... 34
4.2.2 High-temperature Mechanical Properties ......................................................... 35
4.3 Experimental Facilities ............................................................................................ 45
4.3.1 High-Temperature Pin-on-Disk Tribometer ..................................................... 45
4.3.2 High Temperature Stage and Temperature Controller ..................................... 48
4.4 Experimental Program............................................................................................. 50
4.4.1 Sample Preparation ........................................................................................... 50
4.4.2 Friction and Wear Test Matrix ......................................................................... 51
4.4.3 Experimental Procedure ................................................................................... 54
4.4.4 Data acquisition and analysis ........................................................................... 54
Chapter 5
Relationship between Friction and Wear of PTFE/PEEK Composite
.................................................................................................................... 58
5.1 Experimental Results of Friction............................................................................. 58
5.2 Experimental Results of Wear................................................................................. 60
5.3 Relationship between Composite Friction and Wear .............................................. 62
5.4 Validation with Literature Data............................................................................... 64
Chapter 6
Transfer Films ........................................................................................... 69
6.1 Nature and Issues of Transfer films in PTFE, PEEK and Their Composite ........... 69
6.2 Experiment Methods for Transfer Film Evaluation ................................................ 70
6.2.1 Transfer Film Coverage .................................................................................... 70
6.2.2 Elemental and Compositional Analysis of Transfer Films ............................... 73
6.3 Experimental Observations ..................................................................................... 73
6.4 Characterization of Transfer Films on Counterface ................................................ 75
Chapter 7
Development of Friction and Wear Theories for PTFE/PEEK
Composite .................................................................................................. 79
7.1 Friction and Mechanical Properties of PTFE/PEEK Composite ............................ 79
7.2 Solid Film Lubrication and Associated Models ...................................................... 82
7.3 Friction Involving Transfer Films ........................................................................... 84
7.4 Wear with Transfer Films ....................................................................................... 88
xi
Chapter 8
Elevated Temperature Friction and Wear of PTFE/PEEK Composite
.................................................................................................................... 91
8.1 Elevated Temperature Friction and Wear Experimental Results ............................ 91
8.1.1 Friction.............................................................................................................. 91
8.1.2 Wear.................................................................................................................. 93
8.2 Characteristics of Friction and Wear at Elevated Temperature .............................. 95
8.3 Relationship between Friction and Wear at Elevated Temperature ...................... 100
8.4 Elevated Temperature Friction Theory ................................................................. 103
8.4.1 Validation of Friction Theory with Room Temperature Experiments ........... 109
8.4.2 Validation of Friction Theory with Elevated Temperature Experiments ....... 112
8.5 Elevated Temperature Wear Theory ..................................................................... 113
Chapter 9
Mechanisms of Friction and Wear of PTFE/PEEK Composite at
Elevated Temperature ........................................................................... 117
9.1 Friction .................................................................................................................. 117
9.1.1 Neat PEEK ...................................................................................................... 117
9.1.2 Neat PTFE ...................................................................................................... 119
9.1.3 PTFE/PEEK composite .................................................................................. 120
9.2 Wear ...................................................................................................................... 122
9.2.1 Neat PEEK ...................................................................................................... 122
9.2.2 Neat PTFE ...................................................................................................... 123
9.2.3 PTFE/PEEK Composite ................................................................................. 123
Chapter 10 Conclusions .............................................................................................. 125
References
.................................................................................................................. 128
Appendix A Numerical Simulation of Randomly Distributed Spherical Particle
Filled Composite ..................................................................................... 142
Appendix B Friction Coefficients by the Rule of Mixtures ...................................... 147
xii
List of Figures
Figure 1.1
Chemical structures of PTFE and PEEK polymers. ……………
4
Figure 2.1
Specific wear rate and friction coefficient of selected neat
polymers (blue dots) and polymer composites (orange dots)
sliding against a metal conterface……………………………….
9
Lamellar lattice structures of (a) graphite (b) molybdenum
disulfide and (c) polymer chain of polytetrafluoroethylene……
11
Figure 2.2
Figure 2.3
Schematic model of transfer film development of FTFE [69]….. 16
Figure 2.4
SEM pictures of steel counterface after (a) 1 cycle; (b) 10
cycles; (c) 100 cycles; (d) 10,000 cycles; (e) 70,000 cycles; and
(f) 141,000 cycles sliding of PEEK……………………………..
17
Figure 2.5
Common tribological test systems: (a) block-on-ring tribometer,
(b) pin-on-disk tribometer, (c) linear reciprocating tribometer
and (d) thrust washer tribometer………………………………... 20
Figure 2.6
Solid film lubrication at a polymer-steel contact junction……… 24
Figure 4.1
SEM micrographs of (a) neat PEEK, (b) C05, (c) C10, (d) C15,
and (e) C20 composites, and (f) neat PTFE…………………….. 30
Figure 4.2
XRD patterns of neat PEEK, neat PTFE and PTFE/PEEK
composite………………………………………………………..
31
Crystalline and amorphous peaks of the X-ray diffraction
pattern of C10…………………………………………………...
32
Figure 4.3
Figure 4.4
Crystallinity of neat PEEK, neat PTFE, and individual PEEK
and PTFE phases in the PTFE/PEEK composite……………….. 34
Figure 4.5
DSC traces of neat PEEK, neat PTFE and PTFE/PEEK
composites………………………………………………………
35
Hardness of neat PEEK, neat PTFE and PTFE/PEEK
composites at room temperature………………………………...
36
Figure 4.6
Figure 4.7
Storage modulus of neat PEEK, neat PTFE and PTFE/PEEK
composites as a function of temperature.……………………….. 37
Figure 4.8
Loss modulus of neat PEEK, neat PTFE and PTFE/PEEK
composites as a function of temperature ...……………………... 38
xiii
Figure 4.9
Loss tangent of neat PEEK, neat PTFE and PTFE/PEEK
composites as a function of temperature………………………... 38
Figure 4.10
Stress-strain curves of elevated temperature compression tests
of (a) PEEK, (b) C05, (c) C10, (d) C15, (e) C20 and (f) PTFE… 43
Figure 4.11
Elastic moduli of neat and composite samples as a function of
temperature ………………………..............................................
44
Figure 4.12
Compressive yield stresses of neat and composite samples as a
function of temperature…………………………………………. 44
Figure 4.13
Schematic of pin-on-disk tribometer……………………………
Figure 4.14
Drive train of the pin-on-disk tribometer……………………….. 46
Figure. 4.15
Assembly of load arm, load cell, and LVDT……………………
Figure 4.16
Temperature control stage for high-temperature pin-on-disk
tribometer……………………………………………………….. 49
Figure 4.17
Multi-channel temperature controller and recorder……………..
50
Figure 4.18
A steel counterface. (Dotted lines indicate the locations of
roughness measurements)……………………………………….
51
Structure of data acquisition and control system of the pin-ondisk tribometer…………………………………………………..
55
Figure 4.19
45
47
Figure 4.20
User-interface of the control software of the pin-on-disk
tribometer……………………………………………………….. 56
Figure 5.1
Friction coefficients of neat PEEK, neat PTFE and PTFE/PEEK
composites as a function of time………………………………... 58
Figure 5.2
Coefficients of friction of neat PEEK and PTFE/PEEK
composite as a function of PTFE volume fraction……………...
59
Figure 5.3
Wear (height) loss as a function of sliding time for PTFE/PEEK
composite, neat PEEK, and neat PTFE…………………………. 60
Figure 5.4
Specific wear rates of neat PEEK and PTFE/PEEK composites
as a function of PTFE volume fraction………………………….
xiv
61
Figure 5.5
Relationship between specific wear rate and friction coefficient
of the PTFE/PEEK composite from current experiments………. 63
Figure 5.6
Experimental results from [28] compared with power-law
predictions………………………………………………………. 65
Figure 5.7
Experimental results from [19] compared with power-law
predictions………………………………………………………. 66
Figure 5.8
Experimental results from [63] compared with power-law
predictions………………………………………………………. 67
Figure 5.9
Specific wear rates and friction coefficients of PTFE/PEEK
composite obtained from experiments with different test
methods…………………………………………………………. 68
Figure 6.1
Figure 6.1 Micrographs of transfer film on the counterface (C10
sliding on 1018 carbon steel counterface)……………………… 71
Figure 6.2
A method for analyzing images of counterface micrographs to
determine the transfer film covered area………………………..
72
Figure 6.3
Optical (non-polarized) images of steel counterface after wear
tests of (a) Neat PEEK polymer, (b) PTFE/PEEK composite
(C20) and (c) Neat PTFE polymer……………………………… 74
Figure 6.4
Optical (polarized light) images of steel counterface after wear
tests of (a) Neat PEEK polymer, (b) PTFE/PEEK composite
(C20) and (c) Neat PTFE polymer…………………………........ 74
Figure 6.5
Transfer films area coverage ratio for composites with different
PTFE volume fractions………………………………………….
76
Figure 6.6
XPS scan spectra of virgin and tested steel counterface sliding
over the PTFE/PEEK composite………………………………... 77
Figure 6.7
XPS C 1s spectra of virgin and tested steel counterface………... 77
Figure 7.1
PTFE/PEEK composite friction coefficients and mechanical
properties………………………………………………………..
80
Predicted μc (from Eq. 7.1) and test results on friction
coefficients of PTFE/PEEK composite…………………………
81
Comparison of PTFE/PEEK composite friction coefficient
predictions with experimental results…………………………...
87
Figure 7.2
Figure 7.3
xv
Figure 7.4
Figure 8.1
Figure 8.2
Figure 8.3
Figure 8.4
Figure 8.5
Comparison of specific wear rate solutions with experimental
results (β=4)……………………………………………………..
90
Friction coefficients during sliding wear of neat PEEK, neat
PTFE, C10 and C15 composites at 60 and 200 °C……………...
92
Wear of PEEK, PTFE, C10 and C15 composite at 60°C and
200°C……………………………………………………………
94
(a) Steel counterface (b) sample worn surface and SEM images
of (c) steel counterface and (d) worn surface of neat PEEK after
friction and wear sliding test at 60°C…………………………...
96
(a) Steel counterface, (b) sample worn surface and SEM images
of (c) steel counterface and (d) worn surface of neat PEEK after
firciton and wear sliding test at 200°C………….………………
96
(a) steel Steel counterface, (b) sample worn surface and SEM
images of (c) steel counterface and (d) worn surface of C15
composite after friction and wear sliding tests at 60°C….……..
97
Figure 8.6
(a) Steel counterface, (b) sample worn surface and SEM images
of (c) steel counterface and (d) worn surface of C15 composite
after friction and wear sliding tests at 200°C…………………… 98
Figure 8.7
Steel counterface, (a) and (d); PTFE sample worn surface, (b)
and (e), and micrographs of counterface, (c) and (f). (a), (b) and
(c) from tests at 60°C, and (d), (e) and (f), from 200°C………...
99
Power law relationship between friction and wear at various
temperatures, (a) below Tg of PEEK matrix (152 °C), (b) above
Tg of PEEK matrix……………………………………………...
102
Figure 8.8
Figure 8.9
Values of β in the power-law relationship and the storage
modulus of neat PEEK………………………………………….. 104
Figure 8.10
PTFE crystallite structure (following the illustrations in [5])…... 105
Figure 8.11
Schematic PTFE sliding mechanisms (following the
illustrations in [7] and [5])………………………………………
106
Approximate relation of
and Vf of PTFE in PTFE/PEEK
composite………………………………………………………..
108
Figure 8.12
xvi
Figure 8.13
Comparison between friction theory (Eq. 8.4) and current
experimental results……………………………………………..
110
Figure 8.14
Comparison between friction theory (Eq. 8.4) and experimental
results from [8]………………………………………………….. 110
Figure 8.15
Comparison between friction theory (Eq. 8.4) and experimental
results from [3]………………………………………………….. 111
Figure 8.16
Comparison between friction theory (Eq. 8.4) and experimental
results from [7]………………………………………………….. 111
Figure 8.17
Theoretical and experimental friction coefficients of neat
PEEK, neat PTFE and PTFE/PEEK composites at elevated
temperature……………………………………………………...
113
Specific wear rates of PTFE/PEEK composites (C10, C15, and
C20) and theoretical predictions………………………………...
115
Figure 8.18
Figure 9.1
SEM image of the trailing edge of PEEK slid at 200 °C……….. 118
Figure 9.2
SEM image (back-scattering mode) of transfer films of C15 slid
on steel counterface at 200°C…………………………………...
121
Figure A.1
Flow chart of the particle reinforced composite model…………
144
Figure A.2
Illustration of slicing of the cross section……………………….
145
Figure A.3
A cross section of 10% by volume particle filled composite…...
145
Figure A.4
Distribution of the area to volume fraction ratio of a 10%
composite for different particle mean radii……………………... 146
xvii
List of Tables
Table 4.1
Mechanical properties of PEEK, PTFE and PTFE/PEEK
composite materials at room temperature…...…………………..
39
Experimental matrix for room temperature friction and wear
tests……………………………………………………………...
52
Table 4.3
Elevated Temperature Tribological Test Matrix………………..
53
Table 5.1
Test conditions of PTFE/PEEK friction and wear experiments
from the literature……………………………………………….
65
Table 4.2
Table 7.1
Mechanical properties and friction coefficients of neat PEEK,
PTFE and PTFE/PEEK Composites at room temperature…...…. 79
Table 8.1
Results of friction and wear experiments of neat PEEK, neat
PTFE and the PTFE/PEEK composites in sliding contact at
different temperatures…………………………………………...
92
Experimental and theoretical predictions of friction and wear of
neat PEEK and PTFE/PEEK composites, and the exponent β in
power law relation………………………………………………
101
Parameters used for Eq. 8.4 to determine composite friction
coefficient……………………………………………………….
109
Parameters used for Eq. 8.5 to determine composite wear rate…
114
Table 8.2
Table 8.3
Table 8.4
xviii
Chapter 1
Introduction
Tribology is a discipline that studies surface interaction of contacting materials in
relative motion, which includes friction, wear and lubrication. The first appearance of the
term “tribology” is in the report by Jost [1] in 1966. The prefix “tribo”, which means
rubbing, is derived from the Greek language. Together with the suffix “ology”, the literal
translation of “tribology” would be “the science of rubbing”. From International Space
Station to common household appliances, tribology plays an important role in almost all
mechanical devices and structures with contacting surfaces in relative motion. Loss due
to friction and wear lowers the efficiency of machinery; causes unexpected damage and
repair and shortens their service life. According to the Jost Report [1], 1% of the total
gross domestic product (GDP) of Britain was lost due to friction and wear of machinery
in 1966; not counting the financial losses associated with machine down-time which
could be even more costly than machine maintenance and parts replacement. A recent
report [2] published by the United States Department of Energy on tribology
opportunities for enhancing America’s energy efficiency identified 2.1% of the GDP of
energy loss related to friction and wear could be saved annually. In 2017, Holmberg and
Erdemir [3] concluded that about 23% of the world’s total energy loss originated from
tribological contacts. Among them, 20% was used to overcome friction and 3% was used
to remanufacture worn parts and spare equipment due to wear and wear-related failures.
Thus, tribological considerations are of critical concerns to design and material selection
for moving contacting surfaces in mechanical systems and structural components. This
includes multiple factors such as temperature of contacting surfaces, their relative sliding
speed, contacting loads and surface geometry. Without careful tribological considerations
1
in design and material selection, extensive maintenance may be required during service
and unexpected catastrophic failure may occur.
Applications of tribological principles have had a long history, though the use of
the scientific term “tribology” and its study as an engineering discipline are still relatively
young. In 1880 B.C., ancient Egyptian poured liquid (likely water) into the path of a
moving sledge to reduce friction while transporting a heavy statue [4]. At the end of the
15th century, the beginning of European maritime exploration, Leonardo Da Vinci (14521519) firstly introduced the concept of fiction coefficient (𝜇). Observing sliding of a
wooden block on a flat surface, he concluded that the friction coefficient is the ratio of
tangential (frictional) force to normal load on the wooden block and is independent of the
magnitude of the normal load [5]. In 1699, two important rules of friction were proposed
by Guillaume Amontons (1663-1705), who investigated the sliding of two flat surfaces
against each other [6]. Similar to Da Vinci’s discovery, the first rule stated that frictional
force is directly proportional to the load applied perpendicular (normal) to the sliding
direction. The seconded rule, according to Amontons, is that friction coefficient is
independent of the apparent contact area of the two sliding bodies. Later in 1785, French
physicist Charles-Augustin de Coulomb (1736-1806) added the third rule of friction,
stating that frictional force is independent of the sliding speed once motion started [7];
thus a clear distinction between kinetic and static friction is drawn. Comparing to friction,
the study of wear has a shorter history. After the industrial revolution, wear issues in
mass production machinery and railroads were recognized but solutions to wear problems
were mostly empirical. With rapid industrial growth in the 20th century, the study of both
friction and wear in metallic materials has rapidly accelerated and achieved significant
2
advancements, evidenced by the development of contact mechanics theories [8], various
wear mechanisms [9, 10], semi-empirical friction [11] and wear models [12] and
lubrication theories [13] for metallic materials.
In the past few decades, thermoset and thermoplastic polymers are increasingly
used for various engineering applications, due to their outstanding strength/density ratio,
chemical resistance, and machinability. The friction and wear behavior of engineering
polymers and polymer composites has also been actively investigated [14–20]. Most
engineering polymers has high friction and wear which limited their application as
tribological-worthy materials. To overcome the disadvantages of the polymers,
particulate- and short-fiber reinforced polymer composites, with superior tribological
performance than neat polymers, were introduced [21]. Adding short fibers into polymer
matrix mainly improved the mechanical strength of a polymer composite. Other added
inorganic and organic particles, like graphite and PTFE, lubricate the composite when
sliding against another surface. The particles were considered as solid-state lubricants and
substantially reduced the friction coefficient of the polymer. Reduction in friction was
believed to be associated with transfer film formation on the harder surface of the
counterface [22–25]. Recently reported results on modeling the friction and wear
behavior of polymer composites suggested that friction reduction will ultimately lead to
higher wear resistance of polymer composites [26].
Among many tribological polymers and composites, PTFE/PEEK composite is of
particular interest due to its excellent tribological performance and outstanding
mechanical strength and chemical resistivity [19]. Both PEEK and PTFE are semi-
3
crystalline thermoplastics. Chemical structures of neat PEEK and PTFE are shown in
Figure 1.1.
Figure 1.1. Chemical structures of PTFE and PEEK polymers.
The PEEK polymer serves as the composite matrix due to its excellent combined
mechanical properties and thermal stability. The PTFE polymer possesses outstanding
lubrication properties and acts as a solid-state lubricant inside the PEEK matrix [19].
Even though the tribological performance of PTFE/PEEK polymer composite has been
studied and reported by many investigators [19, 27–29], the ability to predict, either
analytically or empirically, the friction and wear rate of PTFE/PEEK composite is still
lacking. In view of the wide use of PTFE/PEEK composite for tribological applications,
predictive models (analytical or engineering models) are needed to help understand
4
friction and wear mechanisms of the composite for accelerated development and
applications of the composite for tribological operations. The lack of knowledge on the
composite wear and frictional behavior introduces serious barriers in material
development, manufacturing, design and consequently, prediction of their reliability in
long-term applications.
Issues and difficulties in determining the PTFE/PEEK composite tribological
behavior and understanding its complicated mechanisms mainly result from inherently
inhomogeneous microstructures of the composite, interactions among the particles, the
surrounding matrix and the interface between them. Transfer film formation, chemical
reactions during sliding, and lack of quantitative evaluation of the transfer films introduce
additional complexities. Also, many parameters, including sliding speed, pressure,
temperature, PTFE volume fraction, and transfer film morphology, have an influence on
the tribological behavior of the composite and need to be considered.
The objective of this study is to conduct a comprehensive investigation on the
tribological behavior of PTFE/PEEK composite at both room and elevated temperatures.
A combined theoretical modeling and experimental investigation is used to resolve
several critical issues that have not been successfully addressed regarding friction and
wear of PTFE/PEEK composite: (1) the relationship between friction and wear at room
and elevated temperatures; (2) the role of transfer films on the friction coefficient of
PTFE/PEEK composite; (3) the tribological behavior of PTFE/PEEK at elevated
temperatures; and (4) quantitative predictive models for friction and wear at elevated
temperatures.
5
A comprehensive experimental program is developed first and later supports the
analytical modeling of friction and wear. Experiment efforts include: (1) pin-on-disk
tribological testing of PTFE/PEEK composite at various temperatures with a special inhouse design and fabricated heating and temperature control device; and (2) new
methodology for quantitative measurements of the areal coverage of transfer film.
Theoretical efforts include: (1) development of the power law of friction and wear of
PTFE/PEEK composite; and (2) derivation of a semi-empirical friction and wear model
of the composite at room and elevated temperatures. Numerical simulation is also
conducted to establish the relationship between the cross-sectional area fraction and
volume fraction of randomly distributed particulate composite. Such relationship is
essential to the development of friction and wear model for the PTFE/PEEK composite.
In the next chapter, a review of literature is conducted on the previous work
relevant to the current study, including self-lubricating polymer composites, friction and
wear of PTFE/PEEK composites, experimental methods and state-of-the-art theoretical
advances in polymer composite tribology. In Chapter 3, it provides a description of the
objective and detailed scope of this research. Chapter 4 includes a detailed experimental
program and materials information employed in this study. The results and discussions
are presented Chapters 5, 6, 7, 8, and 9, addressing the aforementioned critical
tribological issues of the PTFE/PEEK composite. Specifically, correlations between
friction and wear of PTFE/PEEK composite are presented in Chapter 5. Also included in
this chapter is the development of an empirical predictive wear model for the
PTFE/PEEK composite and validation of the wear model with test results obtained in this
study and from those available in the literature. In Chapter 6, details of the effect of
6
transfer films on the friction and wear behavior of polymer composite are given. An
experimental method for transfer film characterization method is also described. Chapter
7 addresses the development of composite friction models that take into considerations of
mechanical properties of neat polymers and the composites, and the presence of transfer
films on the counterface. Also included in this chapter is verification of the models with
test results. Chapter 8 presents the tribological study of PTFE/PEEK composite at
elevated temperatures. Models developed in Chapters 5 and 6 are included in this chapter
as references and compared with experimental data. Chapter 9 discussed microscopic
friction and wear mechanisms of the PTFE/PEEK composites at elevated temperatures.
Conclusions based on the experimental results acquired and the predictive models
developed in the study are discussed in Chapter 10.
7
Chapter 2
Literature Review
2.1 Polymer Composites with Solid-State Lubricants
Engineering semi-crystalline polymers have been used in many applications for
their high strength-weight ratio, good corrosion resistivity, thermal stability and
machinability [30]. For rotating and reciprocating machinery such as pumps and drilling
equipment, polymers are employed commonly as sealing materials [31]. Besides the
mechanical and chemical properties such as stiffness, strength, thermal and chemical
stabilities, tribological properties, i.e. friction coefficient (𝜇) and wear rate (ẇ), are also
critical to polymers used in such applications [13]. Literature reported friction
coefficients and wear rates of common polymers are collected and plotted in Figure 2.1
as blue circles. In Figure 2.1, friction and wear data of PPS, PES and PSU (0.2 m/s, 50N
in air) are cited from [33]. Data of POM, PETP, PEI and PA-66 (0.25 m/s, 50N in air) are
cited from [34]. Data of UHWMPE (50Hz, 1.1 MPa, 20°C) are cited from [35]. Data of
PFA (0.0508 m/s, 6.3 MPa in air) cited from [36]. Data of PS (0.431 m/s, 50N, 25°C) are
cited from [37]. Data of PET (0.025m/s, 250N, room temp.) are cited from [38]. Data of
PEEKK, PEN, PI and PA-66 (1 m/s, 1 MPa, 20°C) are cited from [39]. Data of Epoxy
(0.4 m/s, 3 MPa) are cited from [40]. Data of Al2O3/PTFE (0.05 m/s, 260 N, room
temp.) are cited from [41]. Data of Gr/PI, SCF/PI, Gr/SCF/PI, and SiO2/Gr/SCF/PI
(0.431 m/s, 200N, room temp.) are cited from [42]. Data of PTFE/PPS and
Al2O3/PTFE/PPS (2m/s, 200N, 24°C) are cited from [43]. Data of TiO2/PEEK, PBI and
PPP (0.1m/s, 1MPa, room temp.) are cited from [44]. A goal of material design is to get
close to the lower left corner of the figure (shaded by the color of blue) where combined
low wear and friction are desired. As shown in Figure 2.1, most polymers are poor
8
tribological materials. They suffer from either high wear or high friction. For example,
PTFE has outstanding fiction (μ < 0.2) but high wear (ẇ >10-4 mm3/Nm). PBI, quite the
opposite, suffers from the high friction (μ > 0.7) but enjoys a low wear rate (ẇ <10-6
mm3/Nm). Neither PTFE nor PBI is favorable for tribological applications. It is a
common problem of polymers that they normally do not possess a combination of low
friction and wear rate. Therefore, solid state lubricants are introduced to improve
tribological performance [32].
Figure 2.1. Specific wear rate and friction coefficient of selected neat polymers (blue
dots) and polymer composites (orange dots) sliding against a metal conterface.
9
Solid-state lubricant, normally exhibiting low friction during sliding, can be either
organic or inorganic substance. It has several advantages, such as light weight, easy
maintenance, low cost and contamination comparing to traditional liquid-phase lubricant
in high pressure and temperatures environment [45, 46]. In addition, when using solidstate lubricant, equipment required for circulating liquid lubricant like pumps, filters and
reservoirs are no longer necessary [46, 47].
Common inorganic solid-state lubricants for polymeric composites include
graphite, molybdenum disulfide (MoS2) and others [48]. The structures of them are
basically lattice structures with two-dimensional sheets stacking on each other and
bonded by week van der Waals force as illustrated in Figures 2.2 (a) and 2.2 (b). Each
layer can easily slide with respect to other layers with little resistance to the motion
resulting in low friction. Typical organic solid-state lubricants are polytetrafluoroethylene
(PTFE) and high-density polyethylene (HDPE) [49]. Molecular structures of all
polymeric solid-state lubricants share a common feature – only small single atoms (e.g.,
hydrogen or fluorine) are attached to carbon backbones directly without any side chains,
as shown in Figure 2.2 (c) (for PTFE). Small side atoms give minimum steric hindrance
and allow polymer chains pack tightly, which lead to low surface energy and may slip
easily over other chains. This feature yields low shear strength and large plastic flow
during sliding and eventually achieves low friction. Unlike inorganic lubricants, yielding
and large plastic flow result in material transfer from the bulk polymer to the mating
surface, which leave thin-layer of transfer films [50]. The transfer films left on the
counterface are proved to have significant impact on reduction of polymer fiction, wear,
or both [51].
10
As discussed previously, polymers suffer from high wear or friction for most
engineering applications. So, organic and inorganic solid-state lubricants are filled into
neat polymer to improve their tribological performance. Tribologically modified
polymers, such as PTFE/PEEK, Gr/PI, and Al2O3/PTFE composites, exhibit excellent
friction and wear properties as shown in Figure 2.1 (orange dots). Compared to friction
and wear of neat polymers (blue dots), both wear and friction for the polymer composites
are reduced. Although, some reduction in mechanical properties due to the incorporation
of soft solid-state lubricants are observed, they are outweighed by substantial
improvements on tribological performance.
Figure 2.2. Lamellar lattice structures of (a) graphite (b) molybdenum disulfide and (c)
polymer chain of polytetrafluoroethylene.
11
Instead of introducing solid-state lubricants into hard polymers, another method is
to incorporate hard particles into soft lubricous polymers like PTFE. The friction
coefficient of neat PTFE is low, but its wear is extremely high. The poor wear resistance
of PTFE is attributed to large sheet-like polymer continuously transferred to the
counterface, which then gets expelled from the sliding interface [51, 52]. The wear
resistance of PTFE composites reinforced with hard particles or fibers, such as alumina,
glass fibers, bronze or titanium oxide, has been improved by two orders of magnitude
[51, 53–57]. Although wear reduction mechanisms of hard-particle reinforced composites
are still unclear, several hypotheses are proposed in the literature. The first is that
particles in the composites arrest propagating cracks. Blanchet and Kennedy [51] claimed
that severe wear of neat PTFE was a consequence of subsurface delamination. The role of
the hard particles in PTFE is to interrupt subsurface deformation and to prevent crack
propagation, which otherwise produce large wear debris. The second hypothesis states
when fibers and particle fillers of suitable size are introduced into soft polymers, wear is
reduced because of the load-supporting action of particles. Tanaka and Kawakami [56]
explained the load-supporting action of fibers and hard particles theoretically by
modifying the analysis for effects of discontinues fibers in strengthening the composite.
The third hypothesis on nano-sized particle filled polymers claims that nanoscale
particles help improve transfer film uniformity and tenacity. Experimental results of
nanoscale ZnO [58] and alumina filled PTFE [41] have been reported supporting this
hypothesis. A more recent study compared friction and wear of micro- and nano- sized
alumina filled PTFE [54]. It concluded that friction was less affected by the size of the
alumina, but the wear resistance was significantly improved by nano-sized alumina
12
because it helped form thin uniform transfer films. Contrary to the third hypothesis,
Kandanur et al. [59] reported not all nanoscale particles were effective on reducing wear.
They compared various micro- and nano-scale particles reinforced PTFE and discovered
that the wear reduction introduced by micro-particles was diminished when the size of
particle reduced to a few tens of nanometers. To date, functionalities of hard particles and
wear reduction mechanisms are still in debate. Further investigation is required for a
better understanding of how particles reduce the wear of PTFE and other similar
polymers.
2.2 Friction and Wear of PTFE/PEEK Composite
Among many tribological polymer composites, PTFE/PEEK is commonly used in
rotating and reciprocating machinery and serving as a matrix for further compounding
with carbon fibers, graphite flakes and other fillers. In the literature, several papers
address tribological behavior of PTFE/PEEK composite. Briscoe [60] reported a study on
wear and friction of PTFE/PEEK polymer composite in pin-on-disk tests. PTFE fillers in
the composite reduced friction coefficient of the composite but increased the wear rate
slightly. Although this is the earliest attempt to measure friction and wear of PTFE/PEEK
composite, the wear rate obtained in this study is contradictory to later studies. Lu and
Friedrich [28] performed sliding tests on PTFE/PEEK composite against 100 Cr6 steel in
a pin-on-disk tester and reported that both friction and wear of the composite decreased
with increasing PTFE content. Hufenbach et al. [29] test PTFE/PEEK sliding against a
smooth steel surface and determined the optimum fraction of PTFE for tribological
applications is 7.5% (by weight). In addition to sliding PTFE/PEEK composite on a
smooth surface, Bijwe and co-workers [61] study the tribological behavior of
13
PTFE/PEEK composite sliding against abrasive surfaces and find that specific wear rate
of the composite increases with increasing PTFE weight fractions. They compare
abrasive wear test results with mechanical properties of PTFE/PEEK composite and
conclude that hardness and tensile strength are dominating wear-controlling material
properties in the case of abrasive wear. Burris and Sawyer [19] study filled PEEK
particles in a PTFE matrix and show that the PEEK particles decrease the wear rate and
friction of the composite. The synergetic effect on friction and wear of PTFE/PEEK
composite is attributed to networking and nanoscale penetration of PTFE through PEEK
particles. In another study [27], they also fabricate a compositionally graded PEEK/PTFE
composite with one end is PTFE-rich and test it using different methods (linear
reciprocating, rotating pin-on-disk, and thrust washer). They discover the compositionally
graded composite reduces both friction and wear comparing those to the bulk component
without sacrificing mechanical properties. Lal et al. [62] investigate the tribological
performance of PTFE/PEEK composite in harsh environments. The inclusion of PTFE in
PEEK improves lubrication and wear resistance. The geometrical effect of PTFE fillers
on tribological properties is studied by Vail et al. [63]. Vertically oriented PTFE fibers
reduce wear by an order of magnitude comparing to PTFE powder filled one. Qu et al.
[64] study the PTFE/PEEK composite with different PTFE contents and observe a
synergistic effect of PTFE (up to 20% by volume) in PEEK in reducing friction and wear
simultaneously. The reduction of wear and friction may result from transfer film
formation on the steel counterface during sliding. Further, Qu et al. [65] also study
elevated temperature friction and wear of PTFE/PEEK composite at 200°C. Tribological
performance are compared below and above the PEEK glass transition temperature. At
14
200°C, plastic flow of the composite develops during sliding which leads to thick transfer
films. The wear mechanism is therefore switched from adhesive wear to a plastic-flow
dominated wear.
2.3 Effect of Transfer Films on Tribological Properties of PTFE/PEEK
Composite
Though many fundamental investigations of polymer and polymer composite
tribology have been done with the aim at improving their mechanical and tribological
properties, transfer films in polymer friction are critical and worthy of great attention.
Polymer transfer films on mating counterface during sliding have been observed by many
researchers [24, 52, 66, 67].
2.3.1 PTFE
PTFE transfer films on a glass substrate and its friction mechanisms have been
studied by Makinson and Tabor [68]. They observe that at the beginning of sliding
contact, the friction is high (µ>0.1). However, the friction coefficient of PTFE drops to
below 0.07 after the initial period of time and find that within the initial period of sliding,
large lumps and slabs of PTFE adhere to the glass substrate. After sliding for a while, thin
transfer films of 0.1 – 0.4 µm thickness are observed on the counterface instead of lumpy
debris. Pooley and Tabor [49] further study the transfer of PTFE to glass substrate and
reveal more details of drawing of transfer films. The authors claim that drawing of
transfer films of PTFE is an intra-crystalline process, and the massive fracture of drawing
to be inter-crystalline. The transfer film formation process of PTFE is summarized by
Biswas and shown in Figure 2.3 [69]. In the beginning of sliding, lumpy transfer is
occurred due to high adhesion between the counterface and the polymer. Thin films are
15
then formed due to drawing of the lumps and slabs. With the presence of thin transfer
film, interaction between PTFE and the counterface surface becomes the interaction
between the bulk PTFE and transferred films. The latter interaction keeps the thin
oriented films to be drawn continuously out from the PTFE by a shear. The plastic
deformation and drawing of PTFE require lower shear forces and energy than those
associated with bulk failure, which results in a low steady-state friction of PTFE.
Figure 2.3. Schematic model of transfer film development of FTFE [69].
16
2.3.2 PEEK
Unlike PTFE, thin and continuous transfer films are not observed for neat PEEK.
Voort and Bahadur [70] study the sliding behavior of neat PEEK under test conditions of
19.6 N normal load, 1.0 m/s sliding speed and 0.11 μm center-line surface roughness.
Figure 2.4 shows steel counterfaces (Ra = 0.11 μm) during different sliding traverses of
neat PEEK. Up to 100 cycles, only small amount of PEEK (appears dark in color in the
image) deposits in the valleys of steel asperities.
Figure 2.4. SEM pictures of steel counterface after (a) 1 cycle; (b) 10 cycles; (c) 100
cycles; (d) 10,000 cycles; (e) 70,000 cycles; and (f) 141,000 cycles sliding of
PEEK.
17
After 10,000 cycles, patches and clumps of transferred PEEK start to show. With more
cycles, the same process occurs repeatedly, covering more groves over a larger area.
However, most of the counterface is still not covered with PEEK patches of any
significant thickness after 141,000 cycles. The lack of a continuous transfer film of PEEK
indicates that adhesion between the polymer transfer film and the steel surface is weak.
2.3.3 PTFE/PEEK Composite
Transfer films of PTFE/PEEK composite are discussed in two cases, depending
on the amount of PTFE in the composite. Adding a low volume fraction of PTFE into
PEEK matrix and introducing PEEK particles into PTFE matrix result in different
microstructures. The continuous phase of these two composites are PEEK and PTFE,
respectively. Burris and Sawyer [19] investigate both cases and discover for the case of
PTFE filled PEEK, the PTFE is drawn out of the matrix to form transfer films on the
counterface lubricating the contact surface. Below a specific amount of weight fraction,
the spacing between the PTFE particules become large such that transfer films cannot
completely cover the steel counterface to lubricate the PEEK. In the case of introducing
PEEK particles into PTFE matrix, with an increasing amount of PEEK, the PTFE friction
coefficient is reduced due to the shearing of low strength running films over an
increasingly stiff material with less real area in contact. Qu et al. [26] also point out that
for a PTFE/PEEK composite with less than 20% PTFE by volume, the area covered by
transfer films increase with PTFE content. The increase in transfer film area is believed
to result in a decrease in friction coefficient.
18
2.4 Experimental Methods for Polymer Tribological Study
The friction force and volumetric wear loss of a material during sliding contact
may be measured by a tribometer. The experimental apparatus normally holds a test
specimen mating with a stationary substrate or vice versa. The fiction force between the
sliding pair is measured by a load cell and the materials loss is determined by a linear
variable differential transducer (LVDT). Commercially available tribometers generally
allow control of experimental parameters, such as normal load, sliding velocity, specimen
geometry, temperature and humidity. Bayer [71] and Benzing et al. [72] review various
friction and wear testers that have been employed for tribological evaluation.
Experimental systems commonly used for polymer tribological tests are shown in Figure
2.5. Although these test systems are most commonly used, actual tribometers used by
different researchers may be custom-built machines. Instead of preforming tribological
tests with a tribometer, some tribological experiments may be performed directly on the
full-size machines under real application conditions. The following subsections discuss
the features of each selected tribo-test system.
2.4.1 Block-on-Ring Tribometer
In the block-on-ring apparatus shown in Figure 2.5 (a), the sample block is held
stationary and the ring rotates. A load is applied perpendicular to the test specimen along
axis of rotating ring. The sample block may be cylindrical or cubical. One end contacting
the ring has an arc of the same radius of the ring. This method is standardized by ASTM
G137 – 97 [73].
19
Figure 2.5. Common tribological test systems: (a) block-on-ring tribometer, (b) pin-ondisk tribometer, (c) linear reciprocating tribometer and (d) thrust washer
tribometer.
2.4.2 Pin-on-Disk Tribometer
The pin-on-disk tribometer, as shown in Figure 2.5(b), may be the most popular
test system for polymer tribological experiments. In this apparatus, either the pin is held
stationary with a rotating disk or vice versa. The sample pin can be a ball or a cylinder
with a flat end. The disk rotates at a constant speed during sliding contact with a normal
load applied on top of the specimen pin. The procedure for using the test method is
standardized by ASTM G99 – 17 [74].
20
2.4.3 Linear-Reciprocating Tribometer
In a linear reciprocating test apparatus, as shown in Figure 2.5(c), the specimen
pin is stationary and the flat substrate moves in reciprocating motion. The geometry of
the test specimen can be a ball, cylinder, cube or even parallelepiped. The substrate can
also oscillate with a small amplitude for fretting wear experiments. This test method is
standardized by ASTM G133 – 05 [75].
2.4.4 Thrust-Washer Tribometer
In the thrust-washer tester, as shown in Figure 2.5(d), the stationary flat
counterface is a washer/disk. The sample thrust rotates or oscillates on the washer
surface. The load is applied parallel to the axis of rotation. The test method is
standardized by ASTM D3702 – 94 [76].
2.5 Friction and Wear Theories of Polymer Composite
2.5.1 Friction
When a particulate-reinforced composite slides against a steel counterface, the total
normal load 𝐿 is taken by the particles 𝐿𝑝 and by the matrix 𝐿𝑚 along the contacting
surface,
𝐿 = 𝐿𝑝 + 𝐿𝑚 .
(2.1)
On any cross section of the composite, particles are randomly distributed across the surface.
The relationship between the area fraction (𝐴𝑓 ) and the volume fraction (𝑉𝑓 ) of the particle
in the composite is approximately equal,
𝐴𝑓 ≅ 𝑉𝑓 .
21
(2.2)
Detailed proof and numerical simulation of the relation between cross-sectional areal and
volumetric fraction of a randomly distributed particulate-reinforced composite is in
Appendix A. Assume that the contact shear stress (𝜏) caused by friction across the crosssection area is uniformly distributed, i.e.,
𝜏≅
𝐹 𝐹𝑝
𝐹𝑚
≅
≅
,
𝐴 𝐴𝑝 𝐴𝑚
(2.3)
where 𝐹 is the friction force and 𝐴 is the contact area. Subscripts, 𝑝 and 𝑚 , indicate
particles and the matrix of the composite, respectively. Note that 𝐴𝑝
𝑉𝑓 𝐴 and 𝐴𝑚
(1 − 𝑉𝑓 )𝐴. Since the friction coefficient is defined as, 𝜇𝑐 = 𝐹/𝐿, Eq. 2.1 may be written
as
𝐹𝑝 𝐹𝑚
𝐹
=
+
,
𝜇𝑐 𝜇𝑝 𝜇𝑚
(2.4)
where 𝜇𝑝 = 𝐹𝑝 /𝐿𝑝 and 𝜇𝑚 = 𝐹𝑚 /𝐿𝑚 . Substituting Eq. 2.3 into Eq. 2.4,
𝑉𝑓 1 − 𝑉𝑓
1
=
+
.
𝜇𝑐 𝜇𝑝
𝜇𝑚
(2.5)
Eq. 2.5 suggests an inverse rule-of-mixtures type friction equation for the composite.
Similarly, if uniform pressure along the contact surface is assumed,
,a
linear rule-of-mixtures type friction equation is obtained,
𝜇𝑐 = 𝑉𝑓 𝜇𝑝 + (1 − 𝑉𝑓 )𝜇𝑚 .
22
(2.6)
2.5.2 Transfer Films
The solid film-lubrication theory by Rabinowicz [13] introduces the effect of
transfer films on sliding friction. In a non-lubricated sliding contact i.e., dry sliding of
polymer without a lubricating film as shown in Figure 2.6, the friction coefficient 𝜇𝑚 may
be determined by the ratio of shear yield stress and hardness of the softer material. In the
case of a polymer-metal contact sliding, the polymer is the soft material, thus
𝜇𝑚 =
𝜏𝑚
,
𝐻𝑚
(2.7)
where 𝜏𝑚 is the shear yield stress and 𝐻𝑚 is the hardness of the polymer. When a soft
lubricating film is developed between the polymer and the steel counterface, as illustrated
in Fig. 2.6 (b), sliding motion takes place either in the film or along the film-solid interface.
In either case, the friction coefficient of the lubricating film can be shown to be
𝜇𝑙 =
𝜏𝑙
,
𝐻𝑚
(2.8)
where 𝜇𝑙 and 𝜏𝑙 are, respectively, the friction coefficient and shear stress of the lubricant
film. Taking the ratio of 𝜇𝑙 to 𝜇𝑚 , we have
𝜇𝑙
𝜏𝑙
𝜏𝑙
=
𝑜𝑟 𝜇𝑙 =
⋅𝜇 .
𝜇 𝑚 𝜏𝑚
𝜏𝑚 𝑚
23
(2.9)
Figure 2.6. Solid film lubrication at a polymer-steel contact junction.
If the lubricating film is assumed to have the same tribological and yield properties of the
composite, then the composite friction coefficient may be expressed in terms of friction
coefficient of the neat polymer matrix. Thus Eq. 2.9 may be rewritten as
𝜇𝑐 =
𝜏𝑐
⋅𝜇 .
𝜏𝑚 𝑚
(2.10)
Eq. 2.10 gives a simple relation between friction coefficient and shear yield stresses of the
composite and the matrix polymer.
2.5.3 Wear
Polymer-matrix composites for tribological applications normally contain short
reinforcing fibers and particles and solid-state lubricants. To better understand the
relation between wear and the composition of a polymer composite, phenomenological
wear theories have been developed [77, 78]. Assuming fillers are distributed uniformly
and non-interactive in the matrix, the rule-of-mixtures may be applied to approximate the
wear rate of the composite. Since a unit volume of a polymer composite is represented by
the sum of volume fraction of the fillers
and the matrix
24
, the total normal
load
is the sum of the loads on the fillers
and the matrix
. The normal load
on the matrix and on the fillers can be approximated as
𝐿𝑝 = 𝑉𝑓 ⋅ 𝐿 and 𝐿𝑚 = (1 − 𝑉𝑓 ) ⋅ 𝐿
(2.11)
if contact pressure is assumed to be uniform and the (cross-sectional) area fraction is
assumed equal to the composite volume fraction. Therefore, the specific wear rate of a
composite may be obtained in terms of the volume fractions of individual phase
materials. Khruschov [77] introduces a linear rule-of-mixtures wear model for composite
as
𝑤̇𝑐 = 𝑉𝑓 𝑤𝑝̇ + (1 − 𝑉𝑓 )𝑤𝑚
̇ ,
where
,
, and
(2.12)
are specific wear rates of the composite, the reinforcing particle
and the matrix, respectively. The model successfully predicted the wear of a brass/lead
composite [77]. However, it failed to predict other composite, such as tungsten
carbide/cobalt (a hard, brittle and porous composite). Based on Khruschov’s wear
equation, the inverse rule of mixture model of abrasive wear is proposed by Simm and
Freti [78],
𝑉𝑓 1 − 𝑉𝑓
1
=
+
.
𝑤𝑐̇
𝑤𝑝̇
𝑤𝑚
̇
(2.13)
The two linear rule-of-mixtures and inverse rule-of-mixtures wear models, Eqs 2.11 and
2.12, are derived for abrasive wear of metal composites. Transfer films are not
considered.
25
2.5.4 Flash Temperature
When two surfaces slide against each other, frictional heating raises the
temperature of the interface above that of the environment. This temperature rise is
termed the flash temperature raise. The maximum temperature rise 𝛩, at the counterface
during sliding contact may be estimated by
Θ=
2 𝑞𝑎 1
,
√𝜋 𝐾 √𝑃𝑒
(2.14)
where q is heat generated, assuming all the heat comes from frictional work; a is the
characteristic contact length, and K, thermal conductivity. Pe is the Peclet number,
defined as
𝑃𝑒 =
𝑎𝑣
,
2𝜅
(2.15)
with κ being thermal diffusivity. The heat source, q in general is the result of friction
during sliding and is dependent on the applied normal pressure P, friction coefficient µ,
and the sliding velocity v. Therefore, the value of q may be determined by
𝑞=𝜇𝑃𝑣
(2.16)
Thus, in the subsequent evaluation and modeling of the friction coefficients, temperature
effects need to be considered. Friction, wear and other relevant material properties should
be evaluated at the temperature associated with the flash temperature rise. To avoid issues
related with flash temperature, temperature at the interface between test sample and
counterface is controlled for all high temperature tests.
26
Chapter 3
Objectives and Scope of Research
Friction and wear behavior of polymer composites are complicated system
responses. Despite the widely use of PTFE/PEEK composites in tribological applications,
friction and wear behavior and their associated mechanisms and predictive theories are
not fully studied. Fundamental knowledge of tribological properties of PTFE/PEEK
composite is essential for future material development and applications. For elevated
temperature applications, thermal properties are critical. The objective of this study is to
understand the nature of the sliding friction and wear behavior of PTFE/PEEK composite
at both room and elevated temperatures. The scope of the research includes the following
items:
1. Develop a tribological testing system capable to evaluate friction and wear
properties of the PEEK polymer and the PEEK-based polymer composite at
elevated temperature.
2. Evaluate fundamental mechanical and thermal properties of neat PEEK, neat
PTFE and PTFE/PEEK composite.
3. Room temperature tribological experiments on neat PEEK, neat PTFE and
PTFE/PEEK composite with various PTFE volume fractions.
4. Elevated temperature tribological experiments on neat PEEK, neat PTFE and
PTFE/PEEK composite at various temperatures.
5. Develop a quantitative method for transfer film evaluation.
6. Establish proper relationships between friction and wear of PEEK polymer
and PTFE/PEEK composite.
27
7. Develop friction and wear theories for PTFE/PEEK composite at room
temperature.
8. Develop friction and wear theory for PTFE/PEEK composite at elevated
temperature.
28
Chapter 4
Materials System and Experimental Program
4.1 Material System
4.1.1 Constituent Materials
The neat PEEK, neat PTFE and PTFE/PEEK composites investigated in this study
were manufactured in the same manner as samples in previous works [26, 64, 65]. All
samples were prepared by compression molding followed by sintering. Molded
composite samples contained 5%, 10%, 15%, and 20% (by volume) PTFE in the PEEK
matrix. The PTFE/PEEK composite were denoted, according to their PTFE volume
fractions, as C05, C10, C15, C20, respectively. The neat PEEK and PTFE polymers
studied were also made by compression molding from their powders. The PEEK powders
were Victrex 450PF and the PTFE powders were Dakin M-15X. The actual powder size
was reported in a previous study [79] to be tens of microns.
4.1.2 PTFE/PEEK Composite Microstructure
Polished PTFE/PEEK samples were coated with conductive Au for scanning
electron microscope (SEM) examinations. Typical images of polished surfaces of the
composites were obtained by SEM secondary electron beam scanning and shown in
Figures 4.1 (a)-(f). The PTFE particulates appeared in light color with irregular shapes
and a major axis up to 100 µm. Phase segregation was observed and revealed the
immiscible nature of the PTFE and PEEK polymers.
29
Figure 4.1. SEM micrographs of (a) neat PEEK, (b) C05, (c) C10, (d) C15, and (e) C20
composites, and (f) neat PTFE.
4.1.3 Polymer and Composite Morphology
The degrees of crystallinity of neat PEEK, neat PTFE and the composites were
determined by X-ray diffraction (XRD). The X-ray diffraction was performed with a
Rigaku SmartLAB X-ray diffractometer, using a Cu target producing Kα X-rays (λ=1.54
Å) scanning from 10 – 30° 2θ at a rate of 5 degrees per minute. To perform the
morphological study, specimens were cut to a plaque (20 mm × 20 mm × 5mm) from the
bulk material and polished by 400, 600, and 1200 girt sand papers. XRD patterns of each
sample, as shown in Figure 4.2, resulted from crystalline peaks of the crystalline plane
superimposed on broad amorphous halos. By drawing a linear based line from 2θ 10° to
30°, XRD patterns were fitted by a Pseudo-Voigt function which was a convolution of
30
Gaussian and Lorentzian functions. The Gaussian 𝐺(2𝜃) and Lorentzian 𝐿(2𝜃) functions
were expressed by
(2𝜃 − 2𝜃0 )2
𝐺(2𝜃) = 𝐼𝑚𝑎𝑥 exp [−𝜋
]
𝛽2
(4.1)
1
1
2Г
𝐿(2𝜃) =
,
𝜋 1 2
2
(2 Г) + (2𝜃 − 2𝜃0 )
(4.2)
and
where
is the maximum intensity; 2𝜃0 , the position of the peak maximum, and 𝛽 is
the integral breadth related to the full width at half maximum (FWHM), Г, by
Г
. The C10 composite crystalline peaks and amorphous halos fitted and
separated by this method is shown in Figure 4.3.
T=23 °C
Figure 4.2. XRD patterns of neat PEEK, neat PTFE and PTFE/PEEK composites.
31
350000
T=23 °C
Experimental Results
Pseudo Voigt Fit
300000
PEEK amorphous
PEEK Crystalline
250000
Intensity (ct.)
PTFE Amorphous
PTFE Crystalline
200000
150000
100000
50000
0
10
15
20
2θ
25
30
Figure 4.3. Crystalline and amorphous peaks of the X-ray diffraction pattern of C10.
The degrees of crystallinity of neat PEEK, neat PTFE and the individual PEEK
and PTFE phases in the composites were determined by integrating crystalline and
amorphous peaks from X-ray diffraction traces using Ruland’s method [80] with
correction factors. The degrees of crystallinity, Xc was obtained by taking the ratio of
crystalline peaks areas to the total area under the curve
2𝜃
𝑋𝑐 =
𝜁 ∫2𝜃 2 𝐼𝑐 (2𝜃) 𝑑2𝜃
1
2𝜃
𝜁 ∫2𝜃 2 𝐼𝑐 (2𝜃) 𝑑2𝜃
1
2𝜃
+ ∫2𝜃 2 𝐼𝑎 (2𝜃) 𝑑2𝜃
1
32
⋅ 100% ,
(4.3)
where I is intensity of X-ray diffraction. Subscripts, c and a, indicate the crystalline and
the amorphous phases. The ζ is the Ryland’s correction factor which corrects the effects
of polarization, diffraction angle, and temperature. The value of ζ used in the calculation
was 1.0 for PEEK [81] and 1.8 for PTFE [82]. Degrees of crystallinity of PEEK and
PTFE constituent in the composite are shown in Figure 4.4. Dotted lines indicate the
degrees of crystallinity of neat PEEK (red) and neat PTFE (blue). The crystallinity of
PEEK remained almost the same in the composite as that in the neat resin. The
crystallinity of PTFE phase in the composite was about 2% higher than that in the neat
PTFE. The 2% difference in crystallinity was not significant enough to cause any
mechanical and thermal property differences. The degrees of crystallinity is mainly
affected by the cooling rate after sintering [83]. This is associated with the processing
conditions. Slow cooling in the processing normally leads to higher degrees of
crystallinity. On the other hand, quenching or rapid cooling result in lower degrees of
crystallinity for polymeric materials. Since all samples with different compositions were
processed in the same manner and the degrees of crystallinity of each individual
constituent phase was almost the constant, the effect of crystallinity was not considered in
the subsequent mechanical and tribological studies. Several factors may affect the crystal
size of a polymer, such as number of nucleation sites and presence of nucleating agents
[83].
33
T=23 °C
Figure 4.4. Crystallinity of neat PEEK, neat PTFE, and individual PEEK and PTFE
phases in the PTFE/PEEK composite.
4.2 Thermal and Mechanical Properties
4.2.1 Thermal Properties
Thermal properties, such as glass transitions and melting points, of neat PEEK, neat
PTFE, PTFE/PEEK composites were obtained from differential scanning calorimetry
(DSC). In Figure 4.5, DSC traces of neat PEEK and PTFE resins and composite samples
are shown. At 152 °C, a glass transition was observed for neat PEEK and the PEEK
matrix in the composite. The PEEK matrix and the PTFE particulates in the composite
exhibited same melting points as their neat resins at 340 and 332 °C, respectively. The
distinct melting peaks for the PEEK and PTFE phases in their composite confirmed phase
segregation in the composite.
34
EXO
0
-0.2
Heat Flow (a.u.)
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-1.6
PEEK
C05
C10
C15
C20
PTFE
PEEK
Tg=152°C
100
150
PTFE
melting
332°C
-1.8
50
200
T(°C)
250
300
PEEK
melting
340°C
350
Figure 4.5. DSC traces of neat PEEK, neat PTFE and PTFE/PEEK composites.
4.2.2 High-temperature Mechanical Properties
4.2.2 (a) Hardness
Plastic yielding and failure of neat PEEK, PTFE and PTFE/PEEK composites
were reported in [79]. Hardness was measured with a Rockwell hardness tester (Wilson
Rockwell 2000). The procedure of hardness measurements followed the ASTM D785
Standard on Hardness of Plastics and Electrical Insulating Materials [84]. Rockwell R
and M scales were applied to PTFE and other samples, respectively. A conversion from
Rockwell hardness to Brinell hardness was performed by the following equation,
𝐵𝐻𝑁 =
𝐹𝑎
,
0.004𝜋𝑟 ⋅ (130 − 𝑅𝐻)
35
(4.4)
where BHN is the Brinell hardness number; RH, Rockwell hardness; r, the indenter
radius, and 𝐹𝑎 , the applied force. The Rockwell hardness is determined from the depth of
penetration of a ball indenter under load. Brinell hardness is also determined using a ball
indenter. Unlike Rockwell hardness, Brinell hardness is determined using the area of
indentation made by the ball indenter and is expressed in megapascals (MPa). Though
both Rockwell and Brinell hardness are determined using ball indenters, the quantities
used to characterize material hardness are different but related to each other as shown in
Eq. 4.4. Selected room temperature mechanical properties of PEEK, PTFE and
PTFE/PEEK composites are tabulated in Table 4.1. The measured Brinell hardness of
neat PEEK, PTFE, and PTFE/PEEK composites are shown in Figure 4.6.
1000
T=23 °C
900
Brinell
Hardness
800
700
H (MPa)
600
500
400
300
200
PTFE
54.9 MPa
100
0
0
0.05
0.1
0.15
Vf (PTFE)
0.2
0.25
Figure 4.6. Hardness of neat PEEK, neat PTFE and PTFE/PEEK composites at room
temperature.
36
4.2.2 (b) High-Temperature Dynamic Mechanical Properties
Dynamic mechanical analysis (DMA) of the aforementioned materials were
performed with a TA RSA-III Thermal analyzer. Sample dimensions were 24 mm in
length, 12 mm in width, and 1.5 mm in thickness. Test conditions followed ASTM
D7028-07 Standard [85], which specified 1 Hz frequency and 5 ºC/min heating rate.
Three-point-bending fixtures were used for the DMA tests with a fixed strain of 0.2%.
Specimen were heated from room temperature to 280°C. Storage modulus, 𝐸′, loss
modulus, 𝐸", and loss tangent, tan 𝛿, were plotted as a function of temperature and
shown in Figures 4.7, 4.8 and 4.9, respectively.
5×109
4×109
PEEK
C05
3×109
E' (Pa)
C10
C15
2×109
C20
PTFE
109
0
0
50
100
150
200
250
T (°C)
Figure 4.7. Storage modulus of neat PEEK, neat PTFE and PTFE/PEEK composites as a
function of temperature.
37
4×108
PEEK
C05
C10
3×108
C15
E" (Pa)
C20
PTFE
2×108
1×108
0
0
50
100
150
200
250
T (°C)
Figure 4.8. Loss modulus of neat PEEK, neat PTFE and PTFE/PEEK composites as a
function of temperature.
0.2
0.18
0.16
0.14
Tan δ
0.12
0.1
0.08
PEEK
C05
C10
C15
C20
PTFE
0.06
0.04
0.02
0
0
50
100
150
200
250
T (°C)
Figure 4.9. Loss tangent of neat PEEK, neat PTFE and PTFE/PEEK composites as a
function of temperature.
38
39
112
91
61
49
43
12
PEEK
C05
C10
C15
C20
PTFE
Tension
20
80
85
98
128
143
Compression
9
49
50
54
59
76
Shear
Yield Stress (MPa) [79]
--
48
50
65
93
112
Tension
--
89
102
115
128
143
Compression
--
61
62
66
66
80
Shear
Failure Strength (MPa) [79]
HRR
HRM
Scale
18.1
75.3
86
92.9
98.2
105.4
Mean
0.9
0.9
0.4
0.5
0.3
0.1
Dev.
St.
Rockwell Hardness
54.9
404.3
502.4
596.1
695.1
899.2
(MPa)
Mean
2.7
5
2.2
3.2
1.8
0.9
Dev.
St.
Brinell Hardness
Table 4.1. Mechanical properties of PEEK, PTFE and PTFE/PEEK composite materials at room temperature.
4.2.2 (c) High-Temperature Stiffness and Yield Stress
Room temperature mechanical properties (yield stress and failure strength) of neat
PEEK, neat PTFE and PTFE/PEEK composite listed in Table 4.1 are cited from [79].
Tubular sample geometry and strain gauge were used in the cited study. Elevated
temperature compression tests on neat PEEK, neat PTFE and PTFE/PEEK composites
were performed with an Instron ElectroPuls E10000 Linear-Torsion electrical tester.
Dimensions of the test block were 6.35 × 6.35 × 12.7 mm (¼ × ¼ × ½ in.). The strain rate
was 0.1%/s. Stress-strain curves of neat PEEK, C05, C10, C15, C20 and neat PTFE were
shown in Figure 4.10.
Elastic modulus and compressive yield stress of each specimen were obtained for
all temperatures up to 200 °C. Elastic moduli and compressive yield stresses of neat
PEEK, neat PTFE and the composites are plotted in Figures 4.11 and 4.12 as a function
of temperature. A 2% offset yield stress was used in this study to illustrate the rapid drop
in yield stress with increasing temperature. Modulus of all samples decreased with
increasing temperature. At the glass transition temperature of PEEK (152 °C), a sudden
drop was observed for neat PEEK and PEEK-based composites. Compressive yield stress
showed a monotonic decrease with increasing temperature for all neat and composite
samples.
40
-160
(a)
PEEK
-140
25 °C
60 °C
-120
100 °C
125 °C
150 °C
-80
175 °C
-60
200 °C
-40
225 °C
-20
0
0
-0.1
-0.2
-0.3
ε
-160
(b)
C05
-140
25 °C
60 °C
-120
100 °C
125 °C
-100
σ (MPa)
σ (MPa)
-100
150 °C
175 °C
-80
200 °C
-60
-40
-20
0
0
-0.1
ε
41
-0.2
-0.3
-160
(c)
C10
-140
25 °C
-120
60 °C
100 °C
σ (MPa)
-100
125 °C
150 °C
-80
175 °C
-60
-40
200 °C
-20
0
0
-0.1
-0.2
ε
-0.3
-160
(d)
C15
-140
25 °C
-120
60 °C
-100
125 °C
100 °C
σ (MPa)
150 °C
-80
175 °C
-60
200 °C
-40
-20
0
0
-0.05
-0.1
-0.15
ε
42
-0.2
-0.25
-0.3
-160
(e)
C20
-140
-120
25 °C
σ (MPa)
-100
60 °C
-80
100 °C
125 °C
-60
175 °C
150 °C
-40
200 °C
-20
0
0
-0.05
-0.1
-0.15
ε
-0.2
-0.25
-0.3
-50
(f)
PTFE
-45
-40
25 °C
-35
σ (MPa)
-30
-25
60 °C
-20
100 °C
125 °C
150 °C
-15
-10
175 °C 200 °C
-5
0
0
-0.1
ε
-0.2
-0.3
Figure 4.10. Stress-strain curves of elevated temperature compression tests of (a) PEEK,
(b) C05, (c) C10, (d) C15, (e) C20 and (f) PTFE.
43
4500
PEEK
4000
C05
C10
EC (MPa)
3500
C15
3000
C20
2500
PTFE
2000
1500
1000
500
0
0
50
100
150
T (°C)
200
250
Figure 4.11. Elastic moduli of neat and composite samples as a function of temperature.
140
PEEK
C05
120
C10
C15
100
σyc (MPa)
C20
80
PTFE
60
40
20
0
0
50
100
T (°C)
150
200
250
Figure 4.12. Compressive yield stresses of neat and composite samples as a function of
temperature.
44
4.3 Experimental Facilities
4.3.1 High-Temperature Pin-on-Disk Tribometer
To conduct research on elevated temperature polymer composite tribology, and
overcome limitations of the existing commercially available tribometers, a new hightemperature tribotester with precise temperature control was designed and built.
Figure 4.13. Schematic of pin-on-disk tribometer.
The tribotester was constructed with the frame of a Lewis Research LRI-a tribometer
(Figure 4.13). The spindle, specimen fixtures, servo motor controller and data acquisition
devices were totally replaced. With completely re-programed computer control software,
45
the newly developed tester was capable to operate at high velocity and load for a
continuous long experiment.
The system was driven by a servo motor with a build-in encoder which monitored
and controlled rotating speed. The motor provided a speed of maximum 3500 rpm. The
rotational motion was transmitted to the spindle by pulleys and a timing belt with a drive
ratio of 1:0.44. A schematic of the driving train is shown in Figure 4.14.
Figure 4.14. Drive train of the pin-on-disk tribometer.
Friction force was measured by a load cell with a maximum capacity of 111.2N
(25 lbf). The tangential (friction) force of the polymer specimen during sliding was
46
transmitted through an arm to the load cell as shown in Figure 4.15. The transmission
ratio was 10/0.475.
The wear loss of the sample was measured by an LVDT mounted at the end of the
load beam. The LVDT converted a measured displacement into analog signals with a
sensitivity of 0.394 VDC/mm. The volumetric wear loss was calculated by the specimen
height loss since the cross-sectional area remained constant.
Figure. 4.15. Assembly of load arm, load cell, and LVDT.
47
4.3.2 High Temperature Stage and Temperature Controller
Several common methods for evaluating tribological properties of polymers and
polymer composite were described previously in Section 2.4. Friction and wear rates
measured by these methods show dependence on flash temperature rise at the contact
surface introduced by frictional heating [86]. Tests with high load and/or speed resulted
in large plastic flow or even melting of specimens due to excessive heat built up at the
sliding surface [87]. In this study, a precision-test method has been developed to conduct
friction and wear experiments with a controlled sliding contact surface temperature. By
controlling contact-surface temperature, failure modes of the test material were ensured
due to wear at a specific temperature.
To accurately control of the sliding contact surface temperature, a heating stage
and a control device for the pin-on-disk tribotester were designed and constructed. In
Figure 4.16, schematics are shown for the temperature control assembly for a hightemperature tribology test. The fixtures consisted of a stainless-steel base for mounting
the counterface disk. The base was attached to a copper cooling fin. The copper fins had
different sizes to control heat removal. For temperature below approximate 80 °C, the
temperature control was achieved by controlled removal of heat generated at the interface
of the test sample and the counter surface. Air at room temperature (approximately 23°C)
supplied by a cooling fan was passed through the cooper fin to remove heat generated at
the sliding surface.
48
Figure 4.16. Temperature control stage for high-temperature pin-on-disk tribometer.
In elevated temperature experiments, especially at the temperature above Tg, an
augmented controlled heating is required and a heating stage was introduced (Figure
4.16). Three compartments are designed in the heating stage for cartridge heaters of
choice.
The sliding contact surface temperature was controlled by a temperature
controller as shown in Figure 4.17. A PID (proportional–integral–derivative) algorithm
was implemented to prevent temperature overshooting. In addition to the precise control
of the temperature, the thermal system can record temperature profiles up to 4
thermocouple measurements. It equipped with a touch screen and a graphical user
interface for easy use.
49
Figure 4.17. Multi-channel temperature controller and recorder.
4.4 Experimental Program
4.4.1 Sample Preparation
Neat PEEK, neat PTFE and PTFE/PEEK composite samples were compressionmolded and followed by sintering. Specimens were cut from the bulk materials. Sample
pins were machined to a 6.35 × 6.35 × 9 mm cuboid. The steel counterface was made of
AISI 1018 carbon steel. Prior to each test, the sliding surface was grinded and polished
by hand, using a 320-grit silicon carbide sand paper on a rotating lapping machine with a
stream of tap water to remove any pre-existing machining marks. After all marks were
removed, the sample surface was polished by 400 grit and a600 grit sand papers. During
the surface polishing, the sample was turned at various angles to remove randomly
oriented polishing lines and their effect on the sliding friction and wear tests. After
50
polishing, the surface was rinsed and wiped with alcohol. The center line roughness of
the counterface was measured with a Mitutoyo SJ-210 stylus profilometer at 5 locations
as illustrated in Figure 4.18. The center-line roughness of each disk was determined as
the average of the 5 values measured at the indicated locations. This counterface
preparation procedure resulted in a roughness (Ra) of 0.1 µm ±0.02 µm. The polished
counterface provided a uniform background for subsequent studying of transfer films.
Figure 4.18. A steel counterface. (Dotted lines indicate the locations of roughness
measurements).
4.4.2 Friction and Wear Test Matrix
A comprehensive test program on friction and wear of neat PEEK, neat PTFE and
PTFE/PEEK composite was developed. Three samples were tested for each composition.
A detailed test matrix is shown in Table 4.2.
51
Table 4.2. Experimental matrix for room temperature friction and wear tests.
Test
No.
Number of
Test Samples
1
3
2
3
3
3
4
3
5
3
6
3
Material
Counterface
Material
Normal
Pressure
(MPa)
Sliding
Speed (m/s)
PEEK
5%
PTFE/PEEK
10%
PTFE/PEEK
15%
PTFE/PEEK
20%
PTFE/PEEK
PTFE
1018 Steel
0.5
1
(800 RPM)
Normal pressure and sliding speed were chosen based on consideration of real conditions
of material application (e.g., a compressor). Note that within a certain range of PV value
(the product of pressure and sliding velocity), friction and wear do not change
significantly. However, if the PV value is too high, different tribological behavior may be
expected due to seizure and material melt down.
Neat PEEK, neat PTFE and several PTFE/PEEK composites were selected for
elevated temperature tribological experiments. Detailed experimental matrix is shown
below in Table 4.3. The test temperatures selected in the study ranged from room
temperature up to 200 °C. The experiments were expected to reveal differences in
tribological behavior of neat resin and composite samples below and above the PEEK
glass transition temperature (152 °C).
52
Table 4.3. Elevated Temperature Tribological Test Matrix.
Test
Temperature
No.
(°C)
1
60
2
100
3
125
4
150
5
175
6
200
Sample
Material
Counterface
Material
Normal
Pressure (MPa)
Sliding
Speed (m/s)
PEEK
C10
C15
C20
PTFE
PEEK
C10
C15
C20
PTFE
PEEK
C10
C15
PTFE
PEEK
C10
C15
C20
PTFE
PEEK
C10
C15
PTFE
PEEK
C10
C15
C20
PTFE
1018 Steel
0.5
1
53
4.4.3 Experimental Procedure
Test sample pins prepared previously were mounted on a sample holder in the
pin-on-disk tribometer, with the sliding surface pre-conditioned at 0.5 MPa normal
pressure under 0.1 m/s sliding against a 600-grit sand paper for 1 minute. Machining
marks were removed from the sample surface by this procedure. The sample surface and
steel counterface were then wiped with alcohol. Compressed air was used to evaporate
alcohol residuals and remove any other solid contaminations from the sliding interface.
Friction and wear test were conducted at a specified contact surface temperature
in an ambient laboratory condition with ~50% relative humidity. Tests were continued
for 48 hours without any interruption. The height loss of the sample, the friction force
and the conterface temperature were measured with a LVDT, a load cell, and a
thermocouple, respectively.
4.4.4 Data acquisition and analysis
Friction, wear and temperature data were collected periodically by a personal
DAQ 55 data acquisition device during the test. Data were transmitted and stored at a rate
of 0.05 Hz, which yielded a 200-second logging frequency. To control and synchronize
the motor speed and the temperature measurements with the data collection, a computerbased control software was developed. Figure 4.19 illustrated the test system and its
control mechanisms.
54
Figure 4.19. Structure of data acquisition and control system of the pin-on-disk
tribometer.
The data acquisition and analysis were developed with a Microsoft Windows
operating system to accommodate all aforementioned functionalities with a graphical user
interface. It has a simple interaction logic and a dedicated window to print the testing
status. A screen shot of the interface of the program is shown in Figure 4.20. Collected
data during the test were written into a file for subsequent analysis.
55
Figure 4.20. User-interface of the control software of the pin-on-disk tribometer.
The raw data obtained from a test included measured frictional force, height loss
and temperature. Normal load and sliding speed were controlled and hold constant
throughout the entire experiment. For each measurement, the friction coefficient was
calculated by
μ(𝑡) =
𝐹𝑓 (𝑡)
,
𝐹𝑁 (𝑡)
where 𝐹𝑓 (𝑡) and 𝐹𝑁 (𝑡) were the frictional force and the normal load at time, 𝑡,
respectively in the experiment.
56
(4.5)
The height of the specimen, ℎ, was measured and recorded periodically. The
height loss rate was expressed as
ℎ̇ =
𝑑ℎ
.
𝑑𝑡
(4.6)
The specific wear rate, 𝑤̇ , was defined as the ratio of the wear volume loss, 𝑉, and the
product of the normal load and the sliding distance, 𝑑, as
𝑤̇ =
𝑉
𝐹𝑁 ⋅ 𝑑
(4.7)
and commonly expressed in units of 𝑚𝑚3 /𝑁 ⋅ 𝑚 .
Note that various methods are available to measure wear loss volume for
determining the specific wear rate. The measurement could be direct or indirect. An
indirect way is to weigh the mass of the sliding specimen and convert the mass loss to
volume by dividing the density. This method usually interrupts the sliding test and a
uniform density of the specimen is assumed. In the present study, a direct measurement
was employed to avoid experiment interruptions and density uncertainties. Sliding
samples were machined to a cuboid with constant cross-sectional area. Therefore, the
wear volume loss simply became the product of the height loss and cross section area.
Here, the specific wear was determined by
𝑤̇ =
ℎ̇
𝑃𝑣
where 𝑃 is the normal pressure and 𝑣 is the sliding velocity.
57
(4.8)
Chapter 5
Relationship between Friction and Wear of
PTFE/PEEK Composite
5.1 Experimental Results of Friction
The coefficients of friction of neat polymers and PTFE/PEEK polymer
composites with different PTFE fractions were obtained from the experiments and are
shown in Figure 5.1. All experiments lasted 48 hours except that on PTFE due to its high
wear rate. (The test duration for PTFE was 10 hours.) The first 24 hours (4 hours for
PTFE) of a friction test was considered as a running-in period. During the subsequent 24
hours of tests, friction coefficients were found to fluctuate around a constant value.
Friction in this period was considered in a steady state and its associated friction
coefficient was determined.
0.6
Ambient Temperature = 23 °C
0.5
PEEK
0.4
µ
C05
0.3
C10
C15
0.2
C20
PTFE
0.1
0
0
10
20
t (hour)
30
40
Figure 5.1. Friction coefficients of neat PEEK, neat PTFE and PTFE/PEEK composites
as a function of time.
58
Friction coefficients of neat PEEK and its composites determined in room
temperature experiments are given in Figure 5.2 as a function of PTFE volume fraction.
The friction coefficient decreased from 0.41 to 0.22 with an increasing PTFE volume
fraction from 0% to 20%. The addition of PTFE in the PEEK matrix significantly
reduced friction of the composite when it slides against a steel counterface. For a
PTFE/PEEK composite with less than 15% PTFE (by volume), its friction coefficient
decreased monotonically with PTFE volume fraction.
0.5
Ambient Temperature = 23 °C
0.4
µ
0.3
0.2
0.1
0
0
5
10
15
20
Vf (% PTFE)
Figure 5.2. Coefficients of friction of neat PEEK and PTFE/PEEK composite as a
function of PTFE volume fraction.
59
25
5.2 Experimental Results of Wear
The wear losses of neat PEEK, neat PTFE, and PTFE/PEEK composites were
determined as a function of sliding time (Figure 5.3). The neat PTFE wear loss was 1.3
mm (in height) in 7 hours of sliding. Wear resistance of neat PEEK was 1.3 mm in height
loss in a much longer time (48 hours). All tests for the composite wear lasted 48 hours.
The specific wear rates were determined over the second 24-hour period. In the (first 24
hours) running-in period, the wear sample experienced initial thermal expansion due to
frictional heating and unstable wear due to incomplete transfer-film development.
1.3
PTFE
PEEK
Ambient Temperature = 23 °C
1.1
C05
h (mm)
0.9
0.7
0.5
C10
0.3
C15
0.1
C20
-0.1
0
10
20
30
40
t (hour)
Figure 5.3. Wear (height) loss as a function of sliding time for PTFE/PEEK composite,
neat PEEK, and neat PTFE.
60
The specific wear rate was determined with the procedure described in Section
4.4.3. The (height) wear rate was obtained by taking linear regression of the wear height
loss of specimen after the running-in period. Specific wear rates of neat PEEK and
PTFE/PEEK composites were shown in Figure 5.4. The specific wear rate of neat PEEK
was 1.54×10-5 mm3/Nm. The PTFE/PEEK composites exhibited a monotonic decrease in
wear rate with increasing PTFE volume fraction. For the composite with 20% PTFE, the
specific wear rate of the composite was 1.20×10-6 mm3/Nm, which was less than 10% of
that of neat PEEK.
Ambient Temperature = 23 °C
ẇ (mm3/Nm)
10-5
10-6
10-7
0
5
10
15
20
25
Vf (% PTFE)
Figure 5.4. Specific wear rates of neat PEEK and PTFE/PEEK composites as a function
of PTFE volume fraction.
61
5.3 Relationship between Composite Friction and Wear
A power-law relationship between friction coefficient (µ) and wear coefficient
(K) for metals and non-metals is discussed in [13] with an exponent (n) of 4 for metals
and 2 for non-metals,
𝐾~𝜇 𝑛
(5.1)
The wear coefficient in [13] is defined as
𝐾=
𝐻ℎ̇
,
𝑃𝑣
(5.2)
where H is the material hardness. Comparing with equation 4.8, the difference between
wear coefficient and specific wear rate is the material hardness in the numerator. Since
determination of hardness of polymeric material depends heavily on test conditions and
size of test blocks, the specific wear rate is therefore used in this study to evaluate the
wear characteristics of polymeric material.
Experimental results show that both the friction coefficient and the specific wear
rate PTFE/PEEK composites decreased with an increasing amount of PTFE in the
composite. The specific wear rate of the PTFE/PEEK composite was found to relate to
the friction coefficient by a power law relationship shown in Figure 5.5.
62
10-4
Ambient Temperature = 23 °C
PEEK
C05
ẇ (mm3/Nm)
10-5
ẇ = C μβ
β=4
C10
C15
C20
10-6
10-7
0.1
1
μ
Figure 5.5. Relationship between specific wear rate and friction coefficient of the
PTFE/PEEK composite from current experiments.
The power-law relationship between specific wear rate and friction coefficient
may be expressed as
𝛽
ẇ(𝑉𝑓 ) = 𝐶 𝜇(𝑉𝑓 ) ,
where
is the exponent, C is a constant and ẇ and
(5.3)
are the specific wear rate and the
friction coefficient, respectively. Note that Eq. 5.3 is similar to the power-law
relationship discussed in [13] but the hardness of the material is not involved. In this
study, the PTFE volume fraction was in a range from 0 to 20%. In the case of neat PEEK,
i.e.,
, we have
63
𝛽
ẇ𝑃𝐾 ~ 𝜇𝑃𝐾 .
Similarly for the PTFE/PEEK composites,
(5.4)
,
𝛽
ẇ𝑐 ~ 𝜇𝑐 .
(5.5)
Subscripts PK and c denote properties of neat PEEK and the composite, respectively.
Taking the ratio of the specific wear rates of neat PEEK and PTFE/PEEK composite, we
have
𝜇𝑐 𝛽
ẇc = (
) ⋅ ẇ𝑃𝐾 .
𝜇𝑃𝐾
(5.6)
The equation established a clear relation between friction and wear of neat PEEK and the
PTFE/PEEK composite. Consequently, the specific wear rate of the composite can be
determined from friction coefficients of the composite and the matrix wear rate.
5.4 Validation with Literature Data
For PTFE/PEEK composites, a large amount of experimental data of friction and
wear are available in the literature. Note that some [14, 88] only reported the specific
wear rate without giving the friction information. The lack of the friction coefficient
information provides difficulty in assessing the wear model. However, three sets of
experimental results were available [19, 28, 63] to assess the validity of the power-law
relationship. While all of the experiments were carried out by sliding PTFE/PEEK
composites on smooth steel counterfaces, the sliding speed, normal load, and apparatus
used were different. Details of the experimental conditions were summarized in Table
5.1, along with test conditions conducted in this study, for comparison.
64
Table 5.1. Test conditions of PTFE/PEEK friction and wear experiments from the
literature.
Year
Pressure
(MPa)
Velocity
(m/s)
Counterface
Mat'l
Roughness
(µm)
Test
Configuration
Lu[28]
1995
1
1
100 Cr 6
0.2-0.3(Ra)
Pin-on-Disc
Burris[19]
2006
6.25
0.051
304 stainless
steel
0.161 (Rq)
Reciprocating
Vail[63]
2011
6.25
0.051
304 stainless
steel
0.15 (Ra)
Reciprocating
This Study
2018
0.5
1
1018 Carbon
Steel
0.1±0.02(Ra)
Pin-on-Disc
The specific wear rates of PTFE/PEEK composites obtained from [28] as shown
in Figure 5.6 and compared with the predictions from Eq. 5.6.
Ambient Temperature = 23 °C
Eq. (5.6), β=4
ẇ (mm3/Nm)
Lu et al. (1995) [28]
0.0
0.1
0.2
0.3
0.4
0.5
Vf (PTFE)
Figure 5.6. Experimental results from [28] compared with power-law predictions.
65
In the figure, red circles denote the experimental results and blue squares and lines
indicated the solutions determined from Eq. 5.6 with an exponent,
. The power-law
relationship correctly predicted the decreasing specific wear rate of the composite with
increasing amount of PTFE in the composite.
Burris and Sawyer examined friction and wear of PTFE/PEEK composite tested
on a reciprocal tribometer. The results are also used for validating the power-law
relationship. The experimental results and the predictions from Eq. 5.6 were compared in
Figure 5.7.
Ambient Temperature = 23 °C
Eq. (5.6), β=3
ẇ (mm3/Nm)
Burris et al (2006) [19]
10-8
0
0.1
0.2
Vf (PTFE)
0.3
0.4
Figure 5.7. Experimental results from [19] compared with power-law predictions.
Experimental data obtained by Lu et al. [28] and Burris et al. [19] were compared
with the power-law predictions. Good agreement was observed however the power-law
66
model underpredicted the wear rate of the PTFE/PEEK composite with15% PTFE. For
the composite with PTFE greater than 30%, specific wear rates of the composites
dropped below 10-7 mm3/Nm and the model predictions were also close to the
experimental data.
Vail et al. [63] performed friction and wear experiments on fibers filled
PTFE/PEEK composites with various amounts of PTFE particles. Their test results on
wear of PTFE reinforced PEEK were also taken to compare with Eq. 5.6 and shown in
Figure 5.8.
Ambient Temperature = 23 °C
Eq. (5.6), β=3
ẇ (mm3/Nm)
Vail et al. (2011) [63]
0
0.05
0.1
0.15
0.2
0.25
Vf (PTFE)
Figure 5.8. Experimental results from [63] compared with power-law predictions.
The experimental results showed good agreement with the power-law prediction except
the case of the composite with 20% PTFE. The wear rate of 20% PTFE/PEEK composite
67
was over 10-6 mm3/Nm while the predictions remained almost the same with that of 15%
PTFE/PEEK composite.
With all friction and wear results from the relevant literature [19, 28, 63] and from
the present research (Figure 5.9), the exponent β of the power law of friction and wear
was found to be related to the test methods used. For the pin-on-disk wear test, the β
appeared to have a value of 4, whereas the reciprocal friction and wear test gave a β of 3.
10-4
Ambient Temperature = 23 °C
ẇ (mm3/Nm)
10-5
β=3
β=4
Test Results
10-6
Vail 2011 [63]
Lu 1997 [28]
Burris 2006 [19]
10-7
0.1
μ
Figure 5.9. Specific wear rates and friction coefficients of PTFE/PEEK composite
obtained from experiments with different test methods.
68
1
Chapter 6
Transfer Films
6.1 Nature and Issues of Transfer films in PTFE, PEEK and Their
Composite
In tribological applications, thermoplastic polymer with low shear strength and
surface energy tends to adhere and transfer the material to the high strength and surface
energy metal. The layer of polymer on the metal counterface is known as transfer film
[89]. With the layer of transfer film developed between the soft polymer and hard
metallic counterface, the polymer is prevented from the direct contact with the
counterface as illustrated in Figure 2.6. During sliding contact between the PTFE/PEEK
polymer composite and the transfer film, the low shear strength of the film may greatly
reduce the friction coefficient [90]. The transfer films may also have profound influence
on wear of the polymer composite. Transfer films characterized as ‘patchy,’ and ‘nonuniform’ are often related to poor wear resistance, whereas thin and uniform transfer
films are associated with the polymer composites having low friction and wear [91–96].
In sliding contact, neat PTFE often forms continuous transfer films with large
flake-like debris [49, 51, 56]. Its transfer film is continuously removed and replaced to
form large debris during sliding. The layered lattice structure of the PTFE and the low
(inter-laminar) shear strength of the PTFE transfer film generally leads to a low friction
coefficient, and the rapid removal of PTFE transfer film during sliding results in high
wear loss.
Neat PEEK tends to form discontinuous transfer films on a metal counterface as
discussed in Section 2.3. Though some of small particles of the PEEK may get caught
within the valleys of counterface asperities, the total coverage of the PEEK transfer film
69
is not adequate to serve as a protective film to reduce its high friction and wear. Voort et
al. [70] observe only small patches of PEEK films on the conterface after sliding. Kalin et
al. [97] report that the transfer film coverage of neat PEEK in sliding is less than 27%.
Both of them attributed high friction of neat PEEK due to the lack of thin-layer
continuous transfer films.
The PTFE/PEEK composite, unlike neat PEEK, develops tenacious transfer films
on the metal counterface [19, 28, 66]. PEEK polymer filled with PTFE particles has been
observed to produce finer wear debris [64]. Accumulation of fine debris on the metal
counterface as a thin-layer transfer film results in improvements of friction and wear
resistance [19]. Although qualitative description of transfer films of PTFE/PEEK
composite suggests that the transfer film deposited on the counterface prevents the direct
contact between the composite and the metal surface and reduces both friction and wear,
a quantitative evaluation of the transfer film may lead to better understanding of the
effect of transfer films on the PTFE/PEEK friction and wear.
6.2 Experiment Methods for Transfer Film Evaluation
6.2.1 Transfer Film Coverage
A method for evaluating the transfer film geometry and characterizing the transfer
film coverage ratio has been developed in this study. To obtain accurate characterization
of its area coverage in sliding contact, the counterface surface was examined under an
optical microscope with a non-polarized reflective light source at 25X magnification. A
panorama image of the transfer film on the entire counterface surface was obtained by
taking multiple micrographs around different locations of the counterface to capture the
sectional transfer film images (Figure 6.1, for example).
70
Figure 6.1. Micrographs of transfer film on the counterface (C10 sliding on 1018 carbon
steel counterface).
The micrographs taken at individual locations were then patched togheter, with the aid of
an in-house built computer software, to obtain a high-definition image to determine the
total area on the steel counterface that was covered by the transfer film. The highdefinition image was then converted to a gray scale image in which contrast between the
transfer film and the bare metal surface was shown by different scales of gray. The
intensity of each pixel on the gray-scale image was represented by a digital number
(ranging from 0 (black) to 255 (white)). It was possible to establish a threshold intensity
(pixel level) to identify the area on the counterface that was covered by the transfer film.
71
Figure 6.2. A method for analyzing images of counterface micrographs to determine the
transfer film covered area.
A statistical method was used to minimize the error that may be introduced in
selecting the threshold intensity level. For example, a small area on the counterface that
was totally covered by transfer film was first selected and a histogram of the intensities of
the pixels within the area was constructed as shown in Figure 6.2. The mean intensity (μ)
and the standard deviation (σ) were then determined. The threshold intensity value that
was used to distinguish the area on the counterface that was covered by transfer film from
the (uncovered) bare metal surface was then taken as μ+2σ. The fraction of the
counterface surface that was covered by the transfer film (i.e., the transfer film areal
coverage ratio) was determined with the established threshold intensity. The process was
72
then repeated by choosing a different location on the counterface that was totally covered
with the transfer film. Several different locations on the counterface were chosen for
analysis and the corresponding fractions of the transfer films coverage were determined.
The average value of all the fractions so determined was taken as the average transfer
film areal coverage ratio.
6.2.2 Elemental and Compositional Analysis of Transfer Films
In addition, elemental and compositional analysis of the transfer film were
conducted by X-ray photoelectron spectroscopy (XPS). For the XPS analysis, samples
were cleansed by acetone before being placed in a sample chamber. An achromatic Al Kα
X-ray source (1486.6 eV) was operated at 350W. The area of measurement, the collection
solid angle cone and the take-off angle were set at 800 μm2, 5ºand 45º, respectively. The
passing energy of the hemispherical energy analyzer was set at 11.75 eV, which gave a
resolution of better than 0.51 eV. The pressure in the vacuum chamber was 5×10-9 torr.
Contaminated C 1S (284.9 eV) was employed to calibrate the surface charge effect on
binding energy.
6.3 Experimental Observations
A recent study of microstructure and function of transfer films formed in sliding
friction and wear of PTFE/PEEK composite is reported in [98]. The results of the study
indicate that transfer films from PTFE/PEEK composite have a gradient structure with
PTFE particles located on the topmost surface of the film and the PEEK polymer lying
mainly inside the transfer film. Apparently, the PEEK polymer migrates preferentially
first before the developing of the PTFE particles, to the counterface surface. Also,
73
friction reduction is mainly from the PTFE phase lying on the top surface of the thin
PEEK film.
In the present study, optical micrograph images (Figures 6.3 and 6.4) of wear
tracks on a steel counterface obtained from in-house friction tests of neat polymers
(PTFE and PEEK) and PTFE/PEEK composite with 20% volume fraction of PTFE. The
optical micrograph images shown in Figure 6.3 were obtained with a non-polarized
reflective light source. The transfer films formed on the steel counterface from testing
the composite (Figure 6.3(b)) and neat PTFE sample (Figure 6.3(c)) can be easily
identified in the micrographs.
Figure 6.3. Optical (non-polarized) images of steel counterface after wear tests of (a)
Neat PEEK polymer, (b) PTFE/PEEK composite (C20) and (c) Neat PTFE
polymer.
Figure 6.4. Optical (polarized light) images of steel counterface after wear tests of (a)
Neat PEEK polymer, (b) PTFE/PEEK composite (C20) and (c) Neat PTFE
polymer.
74
The counterface optical images shown in Figure 6.4 were obtained with a
polarized reflective light source. PTFE transfer films appeared shiny under the
microscope with the polarized light shown in Figure 6.4 (c). Comparing the micrograph
image of the counterface (Figure 6.4(b)) from the testing of PTFE/PEEK composite
sample with the counterface micrograph of neat PTFE shown in Figure 6.4(c), the same
shiny layer appeared on top of the composite transfer films. One may infer that the shiny
films on top of the transfer film in Figure 6.4(b) was also PTFE. Similar observations of
the PTFE layer were also reported in [98]. Since the PTFE on the top surface of the
transfer film behave as a solid-state lubricant, the apparent friction coefficient of transfer
films was expected to be similar to that of the neat PTFE polymer.
6.4 Characterization of Transfer Films on Counterface
During the sliding contact of PTFE/PEEK composite transfer films were observed
to form on the steel counterface surface. The area on the counterface surface covered
with the transfer film varied with the volume fraction of PTFE in the composite, from
scattered patches at a low PTFE content to almost full coverage at high PTFE content.
The transfer film areal coverage ratio (ACR, α) was determined by the method described
in Section 6.2 and the results were shown as a function of the PTFE volume fraction (Vf )
in Figure 6.5. The results were also curve-fitted with an error function,
𝜉𝑉𝑓
𝛼(𝑉𝑓 ) =
2
√𝜋
2
∫ 𝑒 −𝑡 𝑑𝑡 ,
(6.1)
0
where is a correlation factor. (The value of
75
equals to 7 is used for the curve fitting.)
1.2
Ambient Temperature = 23 °C
1
α
0.8
0.6
0.4
Eq.6.1, 𝜉=7
Measured α
0.2
0
0
0.05
0.1
0.15
Vf
0.2
0.25
0.3
Figure 6.5. Transfer films area coverage ratio for composites with different PTFE volume
fractions.
Transfer film area coverage ratio (α) was found to increase with the PTFE volume
fraction in the PTFE/PEEK composites tested in the study. For the composite with a 20%
volume of PTFE, almost the entire counterface was covered by the transfer film.
76
Figure 6.6. XPS scan spectra of virgin and tested steel counterface sliding over the
PTFE/PEEK composite.
Figure 6.7. XPS C 1s spectra of virgin and tested steel counterface.
77
The results of a compositional analysis of transfer films formed by a PTFE/PEEK
composite (analyzed by XPS) are shown in Figures 6.6 and 6.7. Figure 6.6 provided the
survey scan of C 1s spectra on a clean counterface before testing and the counterface
after 48-hour sliding wear test. On the counterface, after the wear test, a peak of 689 eV
was found from the survey scan spectrum. The peak was attributed to the CF2 species. In
contrast, no fluorine element peak was found on the virgin counterface. Therefore, one
may conclude that during the sliding, PTFE was transferred to the steel counterface. This
further confirmed the presence of PTFE in the transfer film together with the previously
shown polarized optical micrographs. The survey spectra exhibited decays in intensity of
iron and oxygen elements on the tested counterface, suggesting that the thickness of the
PTFE film was less than 10 nm, since the detection depth of the XPS was approximately
7 to 10 nm. The C 1s spectra of the virgin surface and tested counterface were shown in
Figure 6.7. The C 1s spectrum of the virgin surface exhibited three peaks at 284.8 eV,
285.2eV and 288.2eV. They were attributed to contaminated carbon (C-C), C-O, and
C=O, respectively. On the tested counterface, a C-F peak at 292eV was shown. The C-F
bond observed on the wear-tested surface must come from the PTFE since no other
fluorine source was present in the sliding pair. The XPS elemental study clearly
demonstrated the existence of the PTFE in transfer films on the top layer of the transfer
film.
78
Chapter 7
Development of Friction and Wear Theories
for PTFE/PEEK Composite
7.1 Friction and Mechanical Properties of PTFE/PEEK Composite
Mechanical properties (shear and compression yield stresses and elastic modulus)
of the neat PEEK and PTFE polymers and PTFE/PEEK composites with different PTFE
volume fractions were obtained experimentally in [64, 65, 79]. In Table 7.1 mechanical
properties and friction coefficients of the PTFE/PEEK composite are shown.
Table 7.1. Mechanical properties and friction coefficients of neat PEEK, PTFE and
PTFE/PEEK Composites at room temperature.
Vf
(PTFE)
μ
PEEK
0
C05
Yield Stress (MPa)
Compression
Shear
Modulus
(GPa)
0.407
143
76
3.87
0.05
0.373
128
59
3.56
C10
0.1
0.300
98
54
3.23
C15
0.15
0.231
85
50
2.97
C20
0.2
0.220
80
49
2.68
PTFE
1
0.208
20
9
0.57
Material
In Figure 7.1, friction coefficients of the PTFE/PEEK composites and the neat PEEK are
related to their shear, compressive yield stress (cited from [79]) and elastic modulus (E’
obtained from DMA tests). The results indicate that friction coefficient of PTFE/PEEK
composite μc (with up to 20% volume fraction of PTFE) may be related to the friction
coefficient of neat PEEK (μPK) and its constituent mechanical properties by
𝜇𝑐 =
𝑃𝑐
𝜇 .
𝑃𝑃𝐾 𝑃𝐾
79
(7.1)
The PPK and Pc in Eq. (7.1) represent, respectively, mechanical properties (shear yield
stresses, compression yield stresses or elastic moduli) of the neat PEEK and the
PTFE/PEEK composite.
160
7
Ambient Temperature = 23 °C
140
6
120
5
4
80
3
E (GPa)
σy (MPa)
100
60
Neat PEEK
40
2
Compression
1
Shear
20
Modulus
0
0.15
0.2
0.25
0.3
μ
0.35
0.4
0
0.45
Figure 7.1. PTFE/PEEK composite friction coefficients and mechanical properties.
Note that subscripts c and PK are used hereafter to denote the quantities associated with
PTFE/PEEK composite and neat PEEK polymer.
80
0.45
Ambient Temperature = 23 °C
Test Results
0.4
E
σyc
μ
0.35
τy
0.3
0.25
0.2
0.15
0
0.05
0.1
Vf
0.15
0.2
Figure 7.2. Predicted μc (from Eq. 7.1) and test results on friction coefficients of
PTFE/PEEK composite.
In Figure 7.2, results of friction experiments on the PTFE/PEEK composite (with up to
20% volume fraction of PTFE) are compared with the predictions from Eq. 7.1 using
compression yield stress, shear yield stress and the elastic modulus of the composite
(shown in Table 7.1). Good correlations are observed between the test results and the
predictions using compressive and shear yield stresses. The predictions with the
composite and neat PEEK compressive yield stresses (Eq. 7.1) appear to correlate the
best with the test results.
The composite friction equation, Eq. 7.1, is established based on the relationships
between friction test results and mechanical properties of the PTFE/PEEK composite
81
(with up to 20% volume of PTFE). For the composites with more than 20% volume of
PTFE, applicability of Eq. 7.1 for friction coefficient prediction has not been
investigated, due to lack of experimental data. A limitation of using Eq. 7.1 to predict
friction coefficient of the PTFE/PEEK composite requires, as a priori, shear yield stresses
(or compression yield stresses or moduli) of both the composite and the neat PEEK
polymer. Also, note that only friction coefficient of the neat PEEK is involved in Eq. 7.1
for predicting the composite friction coefficient. The effect of PTFE on composite
friction is implicit through mechanical properties of the composite and friction coefficient
of PTFE does not appear explicitly in Eq. 7.1.
7.2 Solid Film Lubrication and Associated Models
When a soft lubricating film is introduced as an interphase between a steel counterface
and a sliding PEEK polymer, the apparent friction coefficient between the neat PEEK and
the counterface is governed by the plastic flow of the lubricating film as
𝜇𝑙 =
𝜏𝑙
⋅𝜇 ,
𝜏𝑃𝐾 𝑃𝐾
(7.2)
where μl and τl are friction coefficient and shear yield stress of the lubricating film,
respectively, and τPK is shear yield stress of neat PEEK polymer. Two special cases are
considered below.
If the lubricating film has the same friction and mechanical properties as those of the
PTFE/PEEK composite, then μl or the composite friction coefficient (μc) may be
expressed in terms of the neat PEEK polymer friction coefficient as
𝜇 𝑙 = 𝜇𝑐 =
𝜏𝑐
⋅𝜇 ,
𝜏𝑃𝐾 𝑃𝐾
82
(7.3)
where τc is shear yield stress of the composite. If the sliding PEEK polymer is replaced
with the PTFE/PEEK composite, then τPK and μPK in Eq. 7.3 would take on the values of
τc and μc and it simply becomes an identity
𝜇𝑐 =
𝜏𝑐
⋅𝜇 .
𝜏𝑐 𝑐
(7.4)
Hence, friction coefficient behavior of a PTFE/PEEK composite may not be directly
determined with the conventional solid film lubrication theory [13]. Further, according to
the experimental relationship obtained in Section 7.1, the composite shear yield stress (τc)
and neat PEEK shear yield stress (τPK) are used as terms Pc and PPK in Eq. 7.1.
Substituting τc and τPK in Eq.7.1, it becomes identical to Eq. 7.3 for the PTFE/PEEK
composites with up to 20% volume of PTFE. This implies that if shear yielding is the
controlling mechanism of friction, then friction behavior of PTFE/PEEK composite (with
up to 20% volume of PTFE) would be similar to that of neat PEEK polymer sliding on
thin films of PTFE/PEEK composite located on the top surface of the steel counterface.
The friction coefficient of the composite, μc, was in fact originated from the shear of the
transfer film. Similar to the friction coefficient of lubricating film, μl, from the solid-film
lubrication theory, transfer-film friction coefficient, μTF, was defined associated with the
shear of the composite transfer film and approximated as
𝜇 𝑇𝐹 =
𝜏𝑐
⋅𝜇 .
𝜏𝑃𝐾 𝑃𝐾
83
(7.5)
7.3 Friction Involving Transfer Films
To account for the effect of transfer films on friction of PTFE/PEEK composite in
sliding contact, the contact area (test pin cross-section area) between the composite
sample and the steel counterface is assumed to consist of two distinct areas – one with
and the other without the transfer film coverage. Friction of the composite in the contact
area with transfer films is different from that of the area lacking the transfer film
coverage. Further, the contact area without the transfer film coverage is divided into two
parts: the area of PTFE covering the counterface and the other of neat PEEK on the steel
counterface. Modifications are made to the linear rule mixtures (LROM) and inverse rule
of mixtures (IROM) to incorporate the effect of transfer films on the overall friction of
the PTFE/PEEK composite. Details of the modifications are given in Appendix B. Two
expressions are obtained for predicting the friction coefficient (μc) of the PTFE/PEEK
composite. With the modified LROM, one has
𝜇𝑐 = 𝛼𝜇 𝑇𝐹 + (1 − 𝛼)𝑉𝑓 𝜇𝑃𝑇 + (1 − 𝛼)(1 − 𝑉𝑓 )𝜇𝑃𝐾 ,
(7.6)
where the first term on the right-hand side of Eq. 7.6 is the contribution of transfer films
to the friction coefficient of the composite. Similarly with the modified IROM, one
obtains
𝜇𝑐 =
1
(1 − 𝛼)𝑉𝑓 (1 − 𝛼)(1 − 𝑉𝑓 )
𝛼
+
+
𝜇 𝑇𝐹
𝜇𝑃𝑇
𝜇𝑃𝐾
84
.
(7.7)
In Eqs. 7.6 and 7.7, α, Vf, µTF and µPK are the transfer film areal coverage ratio (ACR),
the volume fraction of PTFE, the transfer film friction coefficient and friction coefficient
of neat PTFE polymer, respectively. Note that the areal coverage ratio (ACR) is a
function of Vf as shown in Fig. 6.5. In the following, two different values are assumed for
the apparent friction coefficient (µTF) of transfer films.
First, the transfer films are assumed to behave as a soft lubricant with a friction
coefficient given by Eq. 7.5, i.e.,
𝜇 𝑇𝐹 =
𝜏𝑐
⋅𝜇 .
𝜏𝑃𝐾 𝑃𝐾
(7.5)
The PTFE/PEEK composite coefficient of friction, µc, from Eqs. 7.6 and 7.7, are then obtained as
𝜇𝑐 = 𝛼
𝜏𝑐
𝜇 + (1 − 𝛼)𝑉𝑓 𝜇𝑃𝑇 + (1 − 𝛼)(1 − 𝑉𝑓 )𝜇𝑃𝐾
𝜏𝑃𝐾 𝑃𝐾
(7.8)
or
𝜇𝑐 =
1
(1 − 𝛼)𝑉𝑓 (1 − 𝛼)(1 − 𝑉𝑓 )
𝛼
+
+
𝜏𝑐
𝜇𝑃𝑇
𝜇𝑃𝐾
𝜇
𝑃𝐾
𝜏
,
(7.9)
𝑃𝐾
where μPT is the friction coefficient of neat PTFE.
Second, the PTFE material on top of the transfer films, as shown in Figure 6.4,
are assumed to behave as a solid-state lubricant and the friction coefficient of transfer
films is the same as that of the neat PTFE, i.e.,
85
𝜇 𝑇𝐹 = 𝜇𝑃𝑇 .
(7.10)
Incorporating Eq. 7.10 into Eqs. 7.6 and 7.7, the friction coefficient of the PTFE/PEEK
composite may be expressed as
𝜇𝑐 = [𝛼 + (1 − 𝛼)𝑉𝑓 ]𝜇𝑃𝑇 + (1 − 𝛼)(1 − 𝑉𝑓 )𝜇𝑃𝐾
(7.11)
or
𝜇𝑐 =
1
.
𝛼 + (1 − 𝛼)𝑉𝑓 (1 − 𝛼)(1 − 𝑉𝑓 )
+
𝜇𝑃𝑇
𝜇𝑃𝐾
(7.12)
The PTFE/PEEK composite friction coefficient determined by Eqs. 7.8 and 7.9 are
compared with the experimental results obtained in the study in Figure 7.3. The friction
coefficient predictions by Eqns. 7.11 and 7.12 are also shown in the figure. For
PTFE/PEEK composite with a low PTFE content (≤ 10% by volume), the composite
friction solutions by Eqs. 7.8 and 7.9 are closer to the test results than those predicted by
Eqs. 7.11 and 7.12. However, for the PTFE/PEEK composite with a high PTFE volume
fraction (more than 10%), the solution obtained by Eqs. 7.11 and 7.12 are closer to the
test results. Overall, the theoretical solution based on the linear rule-of-mixtures with the
transfer film having a friction coefficient of neat PTFE yields better results closer to the
experimental data.
86
0.45
Ambient Temperature = 23 °C
0.4
μ
0.35
0.3
0.25
Eq. 7.8
Eq. 7.9
Eq. 7.11
Eq. 7.12
Experimental results
0.2
0.15
0
0.05
0.1
Vf (PTFE)
0.15
0.2
Figure 7.3. Comparison of PTFE/PEEK composite friction coefficient predictions with
experimental results.
Equations 7.8 and 7.9 are obtained, based on modification of the LROM and
IROM for the PTFE/PEEK composite. The transfer films are considered to have friction
behavior similar to a solid (composite) lubricating film. Since the transfer-film ACR (α)
is required, in addition to the input of composite and PEEK matrix shear yield strengths,
Eqs. 7.8 and 7.9 are more involved than Eq. 7.1. As the PTFE content in the composite
increased beyond 20% (by volume), the transfer-film ACR (α) rapidly approched unity
(Figure 6.5), and Eqs. 7.8 and 7.9 revert to the one of the three cases represented by Eq.
7.1 (i.e., the case based on the composite and the neat PEEK shear yield stresses).
87
The friction solutions, Eqs. 7.11 and 7.12 are also derived based on modifications of the
LROM and IROM and may be convenient (compared with Eqs. 7.8 and 7.9) to use since
no composite mechanical properties are required. However, the transfer-film ACR (α) is
still needed for the composite friction coefficient determination. From microscopic
observations, surface compositional analysis and the literature [98], the transfer-film
friction coefficient is chosen to be the friction coefficient of neat PTFE. For the
composite lubricated with more than 10% PTFE, the PTFE-film lubrication-based model
yields the best theoretical results when compared with experimental data. For the PTFE
volume fraction below 10%, the model underestimated the friction coefficient of the
composite due to the insufficient amount of PTFE retained on top of the transfer film to
form a PTFE lubricating layer. Thus, the solid film lubrication theories with a composite
transfer film lead to analytical results closer to the experimental data.
7.4 Wear with Transfer Films
Wear of the PTFE/PEEK composite is reduced due to the synergetic effect of the
transfer film lubrication [26]. The large plastic flow and low surface energy of the
transfer film appear to reduce the friction during dry sliding. Wear, a form of progressive
material failure, is related to cohesive energy of the polymer material. With less frictional
force, the probability of material damage is lowered, leading to less wear. For a
PTFE/PEEK composite (with up to 20% of PTFE by volume), this behavior was shown
by a power-law relationship (between the friction and wear) as discussed in Section 5.3.
Since friction of PTFE/PEEK composite is successfully modeled by the modified LROM
and IROM with considerations of transfer films, wear of the PTFE/PEEK composite
88
material may also be modeled with a combination of transfer film lubrication friction
theory and the power law relationship.
Considering the LROM and IROM with composite transfer films (Eqs. 7.8 and
7.9), specific wear rate, ẇ, of PTFE/PEEK composite (with up to 20% PTFE by volume)
may be determined as
ẇ𝑐 = {(1 − 𝛼) [𝑉𝑓 (
𝜇𝑃𝑇
𝜏𝑐 𝛽
− 1) + 1] + 𝛼
} ẇ𝑃𝐾
μ𝑃𝐾
𝜏𝑃𝐾
(7.13)
and
𝛽
1
ẇ𝑐 = { 𝜏
} ẇ𝑃𝐾 .
𝜇
𝛼 𝜏𝑃𝐾 + (1 − 𝛼) [𝑉𝑓 ( 𝜇𝑃𝐾 − 1) + 1]
𝑐
(7.14)
𝑃𝑇
Similarly, if the specific wear rate of PTFE/PEEK composite is modeled with the PTFE
lubrication theory (Eqs. 7.11 and 7.12), are obtained
ẇ𝑐 = {[𝛼 + (1 − 𝛼)𝑉𝑓 ]
𝛽
𝜇𝑃𝑇
+ (1 − 𝛼)(1 − 𝑉𝑓 )} ẇ𝑃𝐾
𝜇𝑃𝐾
(7.15)
and
𝛽
1
ẇ𝑐 = {
} ẇ𝑃𝐾 .
𝜇𝑃𝐾
[𝛼 + (1 − 𝛼)𝑉𝑓 ]
+ (1 − 𝛼)(1 − 𝑉𝑓 )
𝜇𝑃𝑇
(7.16)
The PTFE/PEEK composite specific wear rate obtained by Eqs. 7.13 and 7.14 are shown
with the experimental data in Figure 7.4. The specific wear rate given by Eqns. 7.15 and
7.16 are also given in the figure. For the composite with a 5% PTFE volume content, the
solutions for Eqs. 7.13 and 7.14 are found closer to the experimental results than the
89
solutions determined by Eqs. 7.15 and 7.16. However, for the composite with more than
10% PTFE (by volume), the predictions obtained by Eqs. 7.15 and 7.16 exhibit better
agreements with experimental data.
10-4
Eq. 7.13
Eq. 7.14
Eq. 7.15
Eq. 7.16
Experimental results
Ambient Temperature = 23 °C
ẇ (mm3/Nm)
10-5
10-6
10-7
0
0.05
0.1
Vf
0.15
0.2
Figure 7.4. Comparison of specific wear rate solutions with experimental results (β=4).
The wear of PTFE/PEEK composite depends highly on its friction behavior. With the
presence of transfer films, adhesion between the composite test sample and the
counterface was reduced, especially with a layer of PTFE deposited on the top of the
transfer film. The reduction in adhesion resulted in low friction and wear at the same
time. This synergistic effect for PTFE lubricated PEEK composite is successfully
modeled by the modified LROM and IROM including the effect of lubricating transfer
films.
90
Chapter 8
Elevated Temperature Friction and Wear of
PTFE/PEEK Composite
8.1 Elevated Temperature Friction and Wear Experimental Results
Tribological tests of neat PEEK, neat PTFE and PTFE/PEEK composites (C10,
C15, and C20) were carried out at elevated temperature up to 200 °C. Test duration for
the neat PEEK and composite samples was 48 hours while for neat PTFE it was 8 hours
due to high wear rate. The test sliding speed and applied pressure in all tests were 1m/s
and 0.5 MPa, respectively. The experimental matrix of elevated temperature friction and
wear tests is shown in Table 4.3. During each test, contact surface temperature was
controlled to be a constant. Friction force and wear loss were measured and recorded
every 2 minutes.
8.1.1 Friction
The results of sliding-contact friction experiments of neat PEEK, neat PTFE, C10
and C15 composite at 60 °C and 200 °C were shown in Figure 8.1. At 200°C, the neat
PEEK reached the highest friction coefficient, 0.6. The friction coefficients of C10, C15,
C20 and neat PTFE were found to be 0.3, 0.24, 0.21 and 0.14, respectively. At 60 °C, the
friction coefficient of neat PEEK dropped from 0.6 to 0.4 while the C15 and neat PTFE
exhibited friction coefficients of around 0.27. The friction coefficient of C10 was about
0.3 at 60 °C, higher than these of neat PTFE and C15. The elevated temperature friction
and wear experimental results were summarized in Table 8.1.
91
0.7
PEEK @
200 °C
0.6
0.5
μ
PEEK @
60 °C
0.4
C10 @
200 °C
0.3
PTFE @
60 °C
C10 @
60 °C
C15 @
60 °C
0.2
C15 @
200 °C
PTFE @
200 °C
0.1
0
0
10
20
t (hour)
30
40
50
Figure 8.1. Friction coefficients during sliding wear of neat PEEK, neat PTFE, C10 and
C15 composites at 60 and 200 °C.
Table 8.1 Results of friction and wear experiments of neat PEEK, neat PTFE and the
PTFE/PEEK composites in sliding contact at different temperatures.
PEEK
C10
C15
C20
PTFE
T
(°C)
μ
ẇ
(mm3/Nm)
μ
ẇ
(mm3/Nm)
μ
ẇ
(mm3/Nm)
μ
ẇ
(mm3/Nm)
μ
ẇ
(mm3/Nm)
60
0.42
1.03×10-5
0.30
3.08×10-6
0.27
1.95×10-6
0.23
1.06×10-6
0.20
6.65×10-5
100
0.42
1.38×10-5
0.31
3.18×10-6
0.27
2.45×10-6
0.24
1.21×10-6
0.20
2.04×10-5
125
0.42
1.15×10-5
0.26
1.36×10-6
0.22
8.65×10-7
-
-
0.14
1.80×10-5
150
0.43
1.24×10-5
0.26
1.68×10-6
0.22
7.36×10-7
0.20
5.09×10-7
0.14
1.88×10-5
175
0.63
1.55×10-6
0.29
6.05×10-7
0.23
4.68×10-7
-
-
0.15
2.01×10-5
200
0.63
1.40×10-6
0.30
5.16×10-7
0.24
4.75×10-7
0.21
3.79×10-7
0.14
2.23×10-5
92
As shown in the Table, friction coefficient of neat PEEK increased significantly above at
the PEEK glass transition temperature while fiction coefficients of C10 and C15 dropped
between the PTFE α phase transition (116 °C) and PEEK glass transition (152 °C). A
monotonic decreasing trend of neat PTFE friction with increasing temperature was
observed. Similar results were reported in [99]. The effect of temperature on PTFE
friction may be related to its viscoelastic behavior, which depended on temperature and
strain rate. The temperature-dependent friction behavior of PTFE may be described with
a modified Arrhenius equation in [51].
8.1.2 Wear
Height losses during sliding contact of neat PEEK, neat PTFE, C10 and C15
composites at different temperatures were obtained and shown in Figure 8.2. the negative
height loss was observed at the beginning of the sliding test due to controlled (prescribed)
and frictional heating. The sliding contact between the steel counterface and polymer pins
resulted in smooth contact surfaces after the initial wear of the polymer pin and followed
by a steady-state wear. Similar to the room temperature experiment, the test period prior
to the steady-state wear was the “running-in” phase. From Figure 8.2, neat PEEK and
PTFE/PEEK composites, wear rates reached steady state after 24 hours into the tests.
Hence wear in the first 24-hour running-in time was not considered in the subsequent
analysis of the specific wear rate of the test material.
93
2.5
PTFE @
60 °C
2
PTFE @
200 °C
h (mm)
1.5
PEEK @
60 °C
1
C10 @
60 °C
0.5
C10 @
200 °C
C15 @
60 °C
0
C15 @
200 °C
24h
-0.5
0
10
20
t (hour)
30
PEEK @
200 °C
40
50
Figure 8.2. Wear of PEEK, PTFE, C10 and C15 composite at 60°C and 200°C.
In Table 8.1, the specific wear rate of neat PEEK was found around 10-5 mm3/Nm below
its glass transition temperature, Tg (152 °C). At elevated temperature above Tg, the wear
rate of neat PEEK was reduced by one order of magnitude to 1.55×10-6 mm3/Nm. The
specific wear rate of the neat PTFE decreased from 6.65×10-5 mm3/Nm to 2×10-5
mm3/Nm during the temperature increase from 60°C to 100 °C. Above 100 °C, the
specific wear rate remained almost constant. Wear rates of all composites decreased with
increasing temperature. For example, the C15 composite had a specific wear rate of
2×10-6 mm3/Nm at 60 °C and 5×10-7 mm3/Nm at 200 °C.
94
8.2 Characteristics of Friction and Wear at Elevated Temperature
At the end of a sliding test of the neat PEEK at 60 °C, only a limited amount of
PEEK residual was observed on the steel counterface. As shown in Figure 8.3 (c), only a
small amount of PEEK accumulated along the polishing marks on the counterface and no
patchy transfer films were observed. The sample worn surface was smooth with slight
scratching bands observed along the sliding direction. The scratching marks may result
from abrasion by contaminated hard particles or PEEK wear debris. Small (scale-like)
debris were produced during the sliding test as shown in Figures 8.3 (b) and (d). No
polymer drawing nor significant plastic flow was observed at the leading and trailing
edges of the PEEK sample pin after each test.
At elevated temperature (200°C), neat PEEK exhibited different behavior during
the sliding experiment. As shown in Figures 8.4 (a) and (c), neat PEEK formed
continuous transfer films on the steel counterface. Above its glass transition temperature,
PEEK became viscoplastic and had much lower yield stress with large elongation at
fracture (as discussed in Section 4.2). Observed transfer films and the worn pin surface
indicated PEEK had large plastic flow at 200 °C during the sliding experiment. In
addition, the polymer extrusion and drawing at the trailing edge of the pin further
confirmed the plastic flow of neat PEEK when sliding at elevated temperature. The
PEEK friction had a combined effect of adhesion at the contact interface and drawing of
polymer chains.
95
Figure 8.3. (a) Steel counterface (b) sample worn surface and SEM images of (c) steel
counterface and (d) worn surface of neat PEEK after friction and wear sliding
test at 60°C.
Figure 8.4. (a) Steel counterface, (b) sample worn surface and SEM images of (c) steel
counterface and (d) worn surface of neat PEEK after firciton and wear sliding
test at 200°C.
96
The worn sample surface and steel counterface characteristics of the PTFE/PEEK
composites (C15, for example) after sliding at 60°C are shown in Figure 8.5. Thin
transfer films were observed on the steel counterface after sliding. On the sample worn
surface, PTFE particles (in light color) were seen embedded in the PEEK matrix. The
worn surface was mostly smooth with minimal scratches. Wear debris of the composite
were observed in fine power form. No polymer drawing was seen at the trailing edge of
the composite sample at the low temperature (60°C).
Figure 8.5. (a) steel Steel counterface, (b) sample worn surface and SEM images of (c)
steel counterface and (d) worn surface of C15 composite after friction and
wear sliding tests at 60°C.
97
Elevated temperature tribological characteristics of the PTFE/PEEK composites
were shown in Figure 8.6 (using C15 as an example). Transfer films were found on the
steel counterface similar to the one observed on the neat PEEK sample during a sliding
experiment. This indicated large plastic flow and transfer of the PEEK matrix. Polymer
extrusion and drawing were also observed at the trailing edge of the composite sample
pin, though to a less extent when compared with that observed for neat PEEK. This may
result from significant reduction in sliding friction due to PTFE lubrication.
Figure 8.6. (a) Steel counterface, (b) sample worn surface and SEM images of (c) steel
counterface and (d) worn surface of C15 composite after friction and wear
sliding tests at 200°C.
98
At 60°C, neat PTFE showed significant wear which may be attributed to rapid
destruction of banded crystalline PTFE structure and the large amount of material transfer
to the counterface. As shown in Figure 8.7 (c), a large amount of PTFE transfer films (in
dark color) was observed on the steel counterface. At the trailing edge of PTFE pins, thin
strips of PTFE were drawn from the bulk (Figure 8.7 (b)). The large amount of PTFE
debris accumulated at the leading edge of PTFE pins was due to breakage of PTFE
transfer films. At 200 °C, the similar drawing effect was seen on PTFE sliding pins.
However, the optical micrograph of the steel counterface showed thin and uniform
transfer films, which may be responsible for decreases of wear at elevated temperature.
Figure 8.7. Steel counterface, (a) and (d); PTFE sample worn surface, (b) and (e), and
micrographs of counterface, (c) and (f). (a), (b) and (c) from tests at 60°C, and
(d), (e) and (f), from 200°C.
99
8.3 Relationship between Friction and Wear at Elevated Temperature
The power-law relationship between friction and specific wear rate of neat PEEK
and PTFE/PEEK composites obtained in Chapter 5 is found also applicable to the
tribological results at elevated temperature, i.e.,
𝜇𝑐 (𝑇) 𝛽(𝑇)
ẇc (T) = (
)
⋅ ẇ𝑃𝐾 (𝑇) ,
𝜇𝑃𝐾 (𝑇)
(8.1)
where ẇc(T), ẇPK(T), μc(T) and μPK(T) are specific wear rates and friction coefficients of
the composite and neat PEEK, respectively. Note that the exponent, β, is a function of
contact-surface temperature depending on the temperature below or above the glass
transition temperature of the matrix material. Figure 8.8, the power law relationship
between friction and wear of neat PEEK and the composites are shown for the cases of
100 °C, 125 °C, 150 °C, 175 °C and 200 °C. Below the glass transition temperature
(152°C) of PEEK, the value of β is close to 4 and above the Tg it is about 1.
Experimental results of friction and wear of neat PEEK and C10, C15 and C20 and the
values of β at different temperatures are given in Table 8.2.
100
101
0.43
0.63
0.63
150
175
200
0.26
0.29
0.30
1.24×10-5
1.55×10-6
1.40×10-6
0.31
1.38×10-5
0.26
0.30
1.03×10-5
1.15×10-5
μ
PEEK
ẇ
(mm3/Nm)
5.16×10-7
6.05×10-7
1.68×10-6
1.36×10-6
3.18×10-6
3.08×10-6
C10
ẇ exp.
(mm3/Nm)
6.07×10-7
6.13×10-7
1.24×10-7
1.81×10-7
4.00×10-6
2.93×10-6
ẇc theo.
(mm3/Nm)
* Value of β is determined for each individual temperature.
0.42
0.42
100
125
0.42
μ
60
T
(°C)
0.24
0.23
0.22
0.22
0.27
0.27
μ
4.75×10-7
4.68×10-7
7.36×10-7
8.65×10-7
2.45×10-6
1.95×10-6
C15
ẇ exp.
(mm3/Nm)
4.79×10-7
4.70×10-7
6.65×10-7
8.42×10-7
2.49×10-6
1.83×10-6
ẇc theo.
(mm3/Nm)
0.21
0.20
0.24
0.23
μ
3.79×10-7
5.09×10-7
1.21×10-6
1.06×10-6
C20
ẇ exp.
(mm3/Nm)
4.21×10-7
4.18×10-7
1.46×10-6
9.86×10-7
ẇc theo.
(mm3/Nm)
1.1
1.2
3.8
3.9
3.8
3.9
β*
Table 8.2. Experimental and theoretical predictions of friction and wear of neat PEEK and PTFE/PEEK composites, and the exponent
β in power law relation.
10-4
ẇ (mm3/Nm)
10-5
ẇc=C1µβ
β≈4
100 °C
125 °C
10-6
150 °C
10-7
0.1
μ
(a)
1
10-5
175 °C
ẇ (mm3/Nm)
200 °C
10-6
ẇc=C2µβ
β≈1
10-7
0.1
μ
(b)
1
Figure 8.8. Power law relationship between friction and wear at various temperatures, (a)
below Tg of PEEK matrix (152 °C), (b) above Tg of PEEK matrix.
102
The specific wear rates, ẇc, obtained by the power-law relationship (Eq. 8.1) exhibited
good agreement with experimental data at all temperatures. The values of β at different
contact surface temperatures were determined and shown in Figure 8.9. Also shown in
Figure 8.9 is the storage modulus of neat PEEK. Variation of the exponent β with
temperature is seen to follow that of the neat PEEK storage modulus. So far, the value of
β is based on experimental observations. The physical meaning of β is not determined at
this time but is likely related to the mechanical properties of the PTFE/PEEK composites
as shown in Figure 8.9. For temperature below the PEEK glass transition temperature, β
is probably associated with the elastic properties of the PTFE/PEEK composites and for
temperature above the PEEK glass transition temperature, β in the power-law relationship
may be related to viscoplastic behavior of the composite. Exact relationship between β
and composite elastic and viscoplastic behavior is still unclear at this time and worth
further study. Friction and wear of the PEEK polymer at T>Tg behaved differently from
that of below Tg due to viscoelastic/plastic transition of the PEEK material.
8.4 Elevated Temperature Friction Theory
Experimental observations of the steel counterface after sliding wear test of the
PTFE/PEEK composite showed that the counterface was fully covered with transfer films
at temperature above Tg, independent of the volume fraction of PTFE in the composite.
Hence the friction theory (Chapter 7) based on transfer film area coverage ratio for room
temperature is not adequate to address the friction of composites with different amounts
of PTFE above Tg. In this chapter, a new friction theory is proposed to address the
PTFE/PEEK composite tribology at all temperatures.
103
5
4.5×109
4.0×109
4
3.5×109
β
2.5×109
2.0×109
2
E' (Pa)
3.0×109
3
1.5×109
β
1
1.0×109
E' of PEEK
5.0×108
0
0
50
100
150
200
0
250
T ( °C )
Figure 8.9. Values of β in the power-law relationship and the storage modulus of neat
PEEK.
An analytical theory that may be adequately capable of studying and predicting friction
of PTFE/PEEK composite with a wide range of PTFE volume fraction would be difficult
due to the complex mechanisms involved in friction and wear of the composites. The
composite microstructure and the PTFE lattice structure [19, 47, 100, 101] have been
noted to lead to friction and wear mechanisms that vary with the amount of PTFE in the
composite. In addition, the method and conditions used in a friction test could also
influence the friction coefficient of the composite. In view of the many factors that could
104
affect friction behavior of PTFE/PEEK composite, to develop a suitable friction theory
for the PTFE/PEEK composites with a PTFE volume content ranging from 0% to 100%,
proper assumptions need to be made on the composite microstructures and apparent
friction of PTFE phase in the composite as well as the friction test method.
Consider a PTFE/PEEK composite with low PTFE volume fraction in sliding
friction and wear. The PTFE particles that broke loose or detached from the PEEK matrix
during sliding functioned as solid-state lubricants. The lubricating PTFE particles were
∗
assumed to have a friction coefficient (𝜇𝑃𝑇
) lower than that of the neat PTFE (𝜇
̅̅̅̅̅)
𝑃𝑇 due
to (a) unique lattice structure of the PTFE crystallite as shown in Figure 8.10 [47], (b)
interlayer sliding of crystalline slices as shown in Figures 8.10 and 8.11 [47, 98], and (c)
the lack of sufficient bulk PTFE materials constraining the exfoliation of the crystalline
slices (Figure 8.1) [47].
Figure 8.10. PTFE crystallite structure (following the illustrations in [47]).
105
Figure 8.11. Schematic PTFE sliding mechanisms (following the illustrations in [98] and
[47]).
The apparent friction coefficient of the PTFE particles in the composite is
approximated by
∗ (𝑇)
𝜇𝑃𝑇
= 𝜉 ̅̅̅̅̅(T)
𝜇𝑃𝑇
where
composite, and
for (𝑉𝑓 ≤ 𝜑𝐶 ),
(8.2)
is the apparent friction coefficient of the lubricating PTFE particles in the
is the friction coefficient of the neat PTFE. The in Eq. 8.2 is a
parameter whose magnitude depends on the test system used and the PTFE content in the
composite. The physical meaning of ξ is a measure of the degrees of difficulty for the
PTFE particle within the PEEK matrix to exfoliate and is a reflection of the combined
effect of PTFE morphology, composite microstructure and testing method. The size of
PTFE crystals (micron level) may not significantly affect the value of ξ in view of the
106
size of crystalline slices (nanometer level) involved in the sliding mechanism and
microstructure of the PTFE/PEEK composite with low volume fraction of PTFE.
However, further work is needed to provide a clear understanding of the parameter ξ .
Based on the discussions above, the ξ is assumed unchanged below a critical PTFE
volume fraction (φc) in the composite.
As the volume fraction of PTFE in the composite increased above the critical
volume fraction (φc), the apparent friction coefficient of the PTFE in the composite
became high due to the constraint on exfoliation of crystalline slices of PTFE exerted by
the increasing amount of surrounding PTFE material. In this study, the apparent friction
coefficient of PTFE is assumed to increase linearly with the PTFE volume fraction until it
becomes the same as the friction coefficient of the neat PTFE (i.e., 100% PTFE volume
fraction in composite),
∗ (𝑇)
𝜇𝑃𝑇
= 𝜉 ̅̅̅̅̅(T)
𝜇𝑃𝑇
+
̅̅̅̅̅
𝜇
𝑃𝑇 (𝑇) (1 − 𝜉)
(𝑉𝑓 − 𝜑𝑐 )
(1 − 𝜑𝑐 )
for 𝑉𝑓 ≥ 𝜑𝐶 .
(8.3)
Hence in this engineering friction model, bilinear friction is approximated by the
lubricating PTFE during sliding. The values of φc and ξ chosen to reflect the influences of
the composite microstructure, morphology of PTFE lubricant, and experimental
conditions as shown in Figure 8.12.
107
μ*PT
μPT
ξ μPT
0
φc
0.2
Figure 8.12. Approximate relation of
0.4
0.6
Vf (PTFE)
0.8
1
and Vf of PTFE in PTFE/PEEK composite.
Based on the PTFE/PEEK composite microstructure considerations during slidng
contact of the composites with different PTFE volume fractions, the value of φc is chosen
to be 0.15. As discussed previously, the parameter ξ is assumed to vary with the type of
tribo-system in sliding experiment. For the test with a pin-on-disk or block-on-ring
(rotating), type tribo-tester, ξ is assumed to be 0.4, whereas for the tests with pin-on-plate
(reciprocating) type tribo-tester, the ξ is smaller and assumed as to 0.2. The smaller ξ for
the pin-on-plate type reciprocating test accounts for the larger amount of PTFE debris
being retained and accumulated on wear tracks during a sliding test than that are retained
on wear tracks during a spinning pin-on-disk (or the block-on-ring) test. Based on the
uniform-shear friction model given in Chapter 5, the friction coefficient of the
PTFE/PEEK composite (μc) may be expressed by
𝑉𝑓
1 − 𝑉𝑓
1
= ∗
+
,
𝜇𝑐 (𝑇) 𝜇𝑃𝑇 (𝑇) 𝜇𝑃𝐾 (𝑇)
108
(8.4)
where Vf is the volume fraction of PTFE phase in the composite. Eq. 8.4 is applicable for
all PTFE volume fractions in PTFE/PEEK composites. Note that only the friction
coefficients ̅̅̅̅̅
𝜇𝑃𝑇 and 𝜇𝑃𝐾 of neat PTFE, neat PEEK and the volume fraction Vf of PTFE
are needed for determining the composite friction coefficient 𝜇𝑐 . The validity of the
theory for PTFE/PEEK composite friction will be checked against the experimental
results on PTFE/PEEK composites obtained at room and elevated temperatures as well as
the test data reported in the literature.
8.4.1 Validation of Friction Theory with Room Temperature Experiments
In Figures 8.13 to 8.16, friction coefficients of PTFE/PEEK composites are determined
and compared with experimental results [19, 63, 98]. The relevant parameters used in Eq.
8.4 for PTFE/PEEK friction coefficient predictions are summarized in Table 8.3. Good
agreement between the predictions and the experimental results is observed for the full
range of PTFE volume content (0% to 100% by volume) at room temperature. The
predictions by Eq. 8.4 capture the characteristic changes in the PTFE/PEEK composite
friction coefficient with increasing PTFE volume content.
Table 8.3. Parameters used for Eq. 8.4 to determine composite friction coefficient.
Reference
Test Type*
This study
Pin-on-Disk
0.208
0.407
0.4
0.15
Vail et al. [63]
Pin-on-Plate
0.135
0.37
0.2
0.15
Burris et al. [19]
Pin-on-Plate
0.135
0.363
0.2
0.15
Onodera et al. [98]
Pin-on-Disk
0.208
0.288
0.4
0.15
* With different counterface materials.
109
Figure 8.13. Comparison between friction theory (Eq. 8.4) and experimental results
obtained in this study.
Figure 8.14. Comparison between friction theory (Eq. 8.4) and experimental results from
[63].
110
Figure 8.15. Comparison between friction theory (Eq. 8.4) and experimental results from
[19].
Figure 8.16. Comparison between friction theory (Eq. 8.4) and experimental results from
[98].
111
8.4.2 Validation of Friction Theory with Elevated Temperature Experiments
Friction and wear experiments on PTFE/PEEK composite conducted from 60 °C
to 200 °C were compared with the theoretical predictions (Eq. 8.4). The measured friction
coefficients at different temperatures are found in good agreement with theoretical
predictions (Figure 8.17). At elevated temperature, neat PTFE and PTFE/PEEK
composites exhibited a reduction in friction around 116 °C. This may attribute to the α
phase transition of the PTFE. DMA results discussed in Section 4.22 (b) showed that
above the α transition, loss modulus of the PTFE decreased. This suggested a lower
energy dissipation and a higher molecular mobility of PTFE polymer chains. Similar
observations of the temperature dependent friction behavior of PTFE are also reported in
[102]. Further increase in temperature above the glass transition temperature of neat
PEEK, resulted in higher friction coefficients of PTFE/PEEK composite. Friction of
PTFE/PEEK composite at elevated temperature was affected by both thermal transitions
of the PTFE and PEEK phase.
112
0.7
Experimental
Eq. 8.4, ξ=0.4
PEEK Test Results
PTFE Test Results
0.6
PEEK
0.5
μ
0.4
0.3
C10
C15
0.2
C20
PTFE
0.1
0
25
50
75
100
125
T (°C)
150
175
200
225
Figure 8.17. Theoretical and experimental friction coefficients of neat PEEK, neat PTFE
and PTFE/PEEK composites at elevated temperature.
8.5 Elevated Temperature Wear Theory
A new wear theory for PTFE/PEEK composite has also been developed in this
research based on the power-law relationship of friction coefficient. Combining Eqs. 8.1
and 8.4, we have
ẇc (T) =
ẇPK (𝑇)
𝛽(𝑇)
∗
𝜇𝑃𝑇
(𝑇) − 𝜇𝑃𝐾 (𝑇)
[𝑉𝑓 (
) + 1]
𝜇𝑃𝐾 (𝑇)
113
.
(8.5)
This equation provides a straightforward approach to determine the specific wear rate of
the PTFE/PEEK composite at elevated temperature provided that the friction and wear
properties of neat PEEK and PTFE are known. Input data to wear equation (Eq. 8.5) and
specific wear rate predictions for C10, C15 and C20 composite as shown in Table 8.4.
For simplicity, two values of the exponent β, 3.8 and 1.2, are used in the predictions, one
for below and one for above the Tg of PEEK. Different friction and wear characteristics
of the PTFE/PEEK composite were observed below and above the glass transition of
PEEK. The viscoelastic/plastic transition of the PEEK matrix altered the friction and
wear mechanisms of the composite. The change in friction mechanisms resulted in
different values of β. The theoretical predictions and experimental data obtained from the
current experiments (discussed in Section 8.1) are given in Figure 8.18.
T
(°C)
Table 8.4. Parameters used for Eq. 8.5 to determine composite wear rate.
Input Parameter
Predictions (Eq. 8.5)
ẇ PEEK
μ PEEK
μ PTFE
β
ẇ C10
ẇ C15
ẇ C20
60
1.03×10-5
0.42
0.20
3.8
2.69×10-6
1.58×10-6
1.12×10-6
100
1.38×10-5
0.42
0.20
3.8
3.37×10-6
1.95×10-6
1.37×10-6
125
1.15×10-5
0.42
0.14
3.8
1.67×10-6
8.38×10-7
5.54×10-7
150
1.24×10-5
0.43
0.14
3.8
1.45×10-6
6.92×10-7
4.48×10-7
175
1.55×10-6
0.63
0.15
1.2
6.93×10-7
5.33×10-7
4.29×10-7
200
1.40×10-6
0.63
0.14
1.2
5.97×10-7
4.55×10-7
3.64×10-7
* Units of specific wear rate, ẇ, are mm3/Nm.
114
10-5
Experimental
ẇ (mm3/Nm)
Eq. 8.5, ξ=0.4
10-6
C10
C15
C20
10-7
50
75
100
125
150
T (°C)
175
200
225
Figure 8.18. Specific wear rates of PTFE/PEEK composites (C10, C15, and C20) and
theoretical predictions.
With known friction and wear properties of the neat PEEK and the neat PTFE, the wear
theory developed in this study successfully predicts specific wear rates of the
PTFE/PEEK composites at all temperatures. The relationship established between friction
and wear of PTFE/PEEK composite reveals the dependence of wear on friction-induced
interface shear during sliding. For the composites, transfer films introduced by the PTFE
solid-state lubricant reduce the friction resistance. The reduction in friction then lowers
the shear stress along the contact surface, which in turn decrease the material damage and
loss. Friction and wear mechanisms of PTFE/PEEK at low and elevated temperatures are
115
complicated and different. The friction and wear theories proposed in this study are based
on experimental study and observations. Friction and wear mechanisms of PTFE/PEEK
composite at elevated temperature are discussed in detail in the next chapter.
116
Chapter 9
Mechanisms of Friction and Wear of
PTFE/PEEK Composite at Elevated Temperature
To date, friction mechanisms of polymer composites remian unclear due to
complex composite microstructure, plastic flow behavior and effects of thermal
environment. Early researchers [10, 13, 103, 104] attempt to investigate friction and wear
mechanisms of engineering materials from different perspectives. However, no universal
friction law has been established that can correctly describe tribological behavior of all
materials. Sliding friction and wear of polymeric materials involve plastic deformation,
distortion of intermolecular bonds, local material damage and fracture, interfacial
material transfer and other issues related to thermal effects. Thus, sliding friction and
wear of polymeric materials is a system response rather than a simple material property
[44]. In the study of tribology of PTFE/PEEK composite at high temperature, friction and
wear mechanisms may be more complex, critical experiments at different temperatures,
as discussed in previous chapters, are needed.
9.1 Friction
9.1.1 Neat PEEK
Friction of neat PEEK showed a clear transition from low to elevated
temperatures (Figure 8.17). When neat PEEK polymer slides on a smooth (Ra = 0.1 µm)
metallic counterfcae below its glass transition temperature, frictional force was originated
primarily from adhesion between the real contact area of the PEEK and the steel
counterface [104]. Bonding between the two surfaces in contact due to the adhesion was
mainly weak interactions forces (e.g., hydrogen bonding and van der Waals force). The
relative magnitude of the adhesion force may be expressed in terms of surface energies of
117
the polymer and the substrate. For neat PEEK, surface energy lies between 34- 38 mN/m
[105]. Friction of PEEK therefore originates from continuous formations and failures of
the adhesion junctions along the contact surface at temperatures below Tg. Experimental
results show that very limited PEEK was transferred to the steel counterface, indicating
breakage of adhesion junctions occurred at the contact interface between PEEK and steel.
The mechanism of friction for neat PEEK sliding on a smooth steel counterface at low
temperature was adhesive friction.
Figure 9.1. SEM image of the trailing edge of PEEK slid at 200 °C.
At elevated temperature, friction characteristics of neat PEEK were different from
those at low temperature. Large plastic flow of neat PEEK occurred above its glass
118
transition temperature. As shown in Figure 9.1, extrusions and drawing of neat PEEK at
200 °C were clearly seen. The friction mechanism at elevated temperature was dominated
by plastic deformation and flow of neat PEEK during sliding contact. Above its Tg,
intermolecular sliding of PEEK polymer chains was enabled, and the polymer became
easily to shear plastically. The very low modulus and yield stress at elevated temperature
enabled PEEK to flow plastically under normal pressure and frictional force with large
area in contact with the counterface, resulting in increased adhesion. The increased
intermolecular mobility at elevated temperature resulted in a considerable transfer of
PEEK to the steel counterface. The increase of adhesion at elevated temperature led to
high friction of neat PEEK.
9.1.2 Neat PTFE
At both low and elevated temperatures, PTFE exhibited low friction and high
wear. This is mainly resulted from its unique smooth molecules and lattice crystalline
structure. When PTFE slid against a steel counterface, due to mechanical shear and
temperature rise, its polymer chains may have chain scission [106]. PTFE radicals
generated by chain scissions then reacted with the metallic counterface and bonded
chemically to the surface. With continued deposition of PTFE transfer films on the
counterface, PTFE was then sliding on its own films instead of the bare steel counterface.
The friction shear was shifted from the PTFE-steel interface to the PTFE-PTFE interface.
The low cohesive energy of PTFE (4.19 kJ/mol [107]) and the ease of slippage between
PTFE molecular chains significantly reduced the friction coefficient.
Friction of PTFE was found decreased with increasing temperature from the
experiment. The temperature effect on PTFE friction has been related to its viscoplastic
119
behavior [99]. The temperature dependence of PTFE friction may be modeled by a
modified Arrhenius equation [51], which leads to a decrease in friction with increasing
temperature. Activation energy of the sliding of PTFE was determined experimentally by
Blanchet et al. [51] to be 9.2 kcal/mol and by Tanaka et al. [101] to be 7 kcal/mol. Such
low activation energies indicated that van der Waals bonds were broken in the friction
process of PTFE. Thus, the friction mechanism of neat PTFE is due to relative slippage
between its crystallites.
9.1.3 PTFE/PEEK composite
Friction of PTFE/PEEK composite at low temperature was affected by transfer
films formed on the steel counterface. The overall friction measured was a resultant of
friction from the film-covered area and the uncovered area. Within the transfer-film
covered area, XPS analysis revealed that the transfer film consisted of both PTFE and
PEEK. Onodera et al. [98] performed XPS with argon etching and molecular dynamic
simulation of the transfer film for PTFE/PEEK sliding on a metallic counterface, showing
that PTFE films formed on the top-most layer of the transfer films. This study provides a
physical foundation for the development of current new friction theory (Eqs. 7.11 and
7.12). The friction laws, Eqs. 7.11 and 7.12, assume the film covered area has the same
friction coefficient of PTFE since the top-most layer was the PTFE phase. The top layer
of PTFE films was also examined with a polarized optical microscope. Figures 6.4 (b)
and (c) clearly show a bright layer of PTFE on top of the transfer film for PTFE/PEEK
composite and neat PTFE. Hence, when PTFE/PEEK composite is sliding on a steel
counterface, transfer films formed first on the counterface with a PTFE layer on top of it.
The composite then slid through the film covered area and was lubricated, due to the low
120
shear flow stress of the PTFE layer. The resulting friction of the composite was reduced
and therefore was related to the amount of transfer film coverage (i.e., α).
At elevated temperature, PEEK matrix of the PTFE/PEEK composite slid with
large plastic deformation and flow. Friction mechanisms of PTFE/PEEK composite were
changed from that observed at low temperature. Transfer films fully covered the
counferface for the composites with all different compositions as shown in Figures 8.5(a)
and 8.7(a). Back-scattering SEM micrograph of the composite transfer film (Figure 9.2)
shows that PTFE (appears white in the image) were embedded in the film surrounded by
PEEK. The PTFE at elevated temperature lubricated the composite during friction and
wear. Accordingly, friction of PTFE/PEEK composite were reduced due to the
lubrication effects of PTFE.
Figure 9.2. SEM image (back-scattering mode) of transfer films of C15 slid on steel
counterface at 200°C.
121
9.2 Wear
9.2.1 Neat PEEK
Common mechanisms of sliding wear of polymers, such as thermoplastics,
thermosets and elastomers are discussed by in [104]. During sliding of a polymer on a
metallic counterface, surface and subsurface deformation of the polymer was caused by
passage of protuberances on the counterface. The protuberance could be either asperities
or debris retained in the sliding interface. Damages caused by the passing of
protuberances on the polymer surface may be adhesive and/or abrasive. These two wear
modes are not mutually exclusive and usually coexist depending on materials of the
sliding pair and sliding conditions [39]. In general, characteristics of abrasive wear are
scratching marks on the polymer surface and those of adhesive wear are scale-like wear
particles. In the case of neat PEEK sliding on steel counterface at low temperature, both
scratching marks and scale-like wear debris were seen (Figure 8.4), suggesting the wear
mechanism of neat PEEK at low temperature may be a combination of adhesive and
abrasive wear.
At elevated temperature, the specific wear rate of neat PEEK reduced
pronouncedly. Associated with the low wear, transfer films were seen on the counterface
(Figures 8.5 (a) and (c)). In addition, polymer drawing at the trailing edge of the PEEK
specimen indicated large plastic flow. As a result, PEEK was transferred to the steel
counterface during sliding and stayed. Instead of fracture and forming wear debris during
the sliding of PEEK at low temperature, it exhibited large deformation and plastic flow at
elevated temperature. The net material loss was minimal and resulted in a low wear rate.
122
The mechanisms responsible for its low wear was mainly by transfer and plastic flow of
PEEK.
9.2.2 Neat PTFE
Neat PTFE had high wear rate at both low and elevated temperatures. The wear
mechanism of PTFE is related to its unique lattice crystalline structure and smooth
polymer chain profile. Figure 8.8, long films can be seen generated at the trailing edge
during sliding of PTFE on a steel surface. The film formation and polymer drawing were
attributed to the destruction of lattice structure by slippage of crystalline slices. The
transferred PTFE on the counterface was then get scraped by the PTFE pins in the next
revolution of rotation and accumulated at the leading edge of the pin (Figures 8.7 (b) and
(e)). This transfer-scrape process repeated itself during the sliding of PTFE and
eventually led to high wear.
9.2.3 PTFE/PEEK Composite
Wear of PTFE/PEEK composite was reduced because of the presence of transfer
films. At low temperature, transfer films on the sliding counterface firstly prevented
direct contact of the steel counterface from the PTFE/PEEK composite sliding pin.
Though transfer films were thin and only partially covered the steel counterface, abrasion
of polymer composite surface is mitigated. Less scratching marks were seen on the
composite worn surface (Figure 8.6 (d)) than that on neat PEEK worn surface (Figure
8.4(d)). Also, the aforementioned PTFE layer on top of the transfer film reduced adhesion
and friction shear on the composite. The adhesive wear of the composite was not as
significant as that of the neat PEEK, which resulted in a lowered wear rate.
123
At elevated temperature, wear rates of PTFE/PEEK composite were similar as
shown in Figure 8.18. This may be attributed to plastic flows of both PEEK and PTFE.
Once the PEEK and the PTFE in the composite started to deform plastically and were
transferred to the counterface during sliding, the transferred materials tended to stay on
the counterface. Loose wear particles and debris were rarely seen during the sliding.
Therefore, net wear loss of the material was low and such a low wear rate was observed.
124
Chapter 10
Conclusions
A combined experimental and theoretical study on tribological behavior of
PTFE/PEEK composite at elevated temperature has been carried out. Based on the
results, the following conclusions may be drawn:
(1) Friction coefficients of PTFE/PEEK composite at low temperature decreased
with increasing PTFE volume fraction. This may attribute to transfer film
lubrication. The transfer film coverage ratio was found to relate to the volume
fraction of the PTFE phase by an error function.
(2) Below glass transition temperature of PEEK (152 °C), friction coefficients of
PTFE/PEEK composites decreased with increasing temperature due to
lubrication of PTFE. Above glass transition temperature of PEEK, friction
coefficients of the composite increased with temperature due to plastic
deformation and flow of the PEEK matrix and the PTFE phase.
(3) Specific wear rates of PTFE/PEEK composite at low temperature decreased
with PTFE volume fraction due to increased transfer film coverage, which
prevented direct abrasion of the composite surface and mitigated adhesion
between polymer composite and the counterface.
(4) Specific wear rates of PTFE/PEEK composites at elevated temperature above
glass transition of PEEK were low and their values were similar. At elevated
temperature, PEEK started to flow plastically and stayed on the counterface
instead of becoming wear debris and expelled.
125
(5) Friction and wear of PTFE/PEEK composite are inherently related. A power
law relationship between the two is established at both low and elevated
temperatures.
(6) At low temperature, a new friction theory is developed with the aid of solid
film lubrication and the rule of the mixtures for the PTFE/PEEK composite.
With the aid of the transfer film cover age ratio, the theory is shown to predict
friction coefficients of PTFE/PEEK composite at low temperature.
(7) Friction and wear mechanisms of PTFE/PEEK composite are fundamentally
different below and above the PEEK glass transition temperature. A new
mechanism-based friction model is introduced for developing PTFE/PEEK
composite friction and wear laws at elevated temperature. Theoretical
predictions show good agreement with elevated temperature experimental
results. This model also provides good correlation with test data at low
temperature. Thus, this model is more general and applicable to all
temperatures.
(8) Wear of PTFE/PEEK composite at both low and elevated temperatures are
modeled with a power law relationship. The new wear theory is used to
predict specific wear rates of the PTFE/PEEK composite at both low and
elevated temperatures.
The combined experimental and theoretical investigation carried out in this study
provides a new perspective to investigate tribological behavior of PTFE/PEEK composite
126
at both low and elevated temperatures. Merits and innovative contributions of this
investigation to the PTFE/PEEK composite tribological behavior include the following:
(1) Extending the friction and wear coefficient power-law relationship observed
in metal and non-metal to polymer composite materials with the use of
specific wear rate to characterize polymeric material wear behavior.
(2) Develop PTFE/PEEK composite friction and wear models to enable the use of
only constituent properties to predict friction and wear of PTFE/PEEK
composite.
(3) Identify differences in friction and wear behavior of PTFE/PEEK composite at
low and elevated temperatures.
(4) Provision of new research direction that includes both PTFE morphology and
composite microstructure in friction and wear predictions.
Critical issues that have not been fully addressed in this study require further
investigations including but not limited to:
(1) Physical interpretation and clear understanding of the parameters used in
friction and wear modeling, i.e., β and ξ.
(2) Effect of viscoplastic behavior of neat PEEK, neat PTFE and PTFE/PEEK
composites on friction and wear at elevated temperature.
(3) Rigorous quantitative analysis and evaluation of the relationship between
PTFE crystalline morphology and composite microstructure, such as crystal
size, orientation and degrees of crystallinity, and elevated temperature friction
and wear of PTFE/PEEK composites at both low and elevated temperatures.
127
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Appendix A
Numerical Simulation of Randomly
Distributed Spherical Particle Filled Composite
In this study, the cross-sectional area fraction of particles in a composite was
investigated by a numerical simulation method. For a given randomly distributed spherical
particles reinforced composite, the correlation between volumetric fraction of the filler and
the area fraction of particles on a cross section was studied. Models of composite with
different filler volume fraction was created and the cross-sectional area fractions were
calculated and compared with the volume fraction. The results showed that the crosssectional area fraction is approximately equal to the volume fraction if the particle size is
small and total number of particles is large.
To construct the model, following constrains were implemented.
1. Radii of particles are normally distributed with a mean and a standard deviation.
2. Locations of particle are randomly distributed such as the three coordinates (x, y,
and z) are totally random.
3. Particles are mutually exclusive.
Steps to construct the composite model were illustrated in the flowchart below in Figure
A.1.
142
Figure A.1. Flow chart of the particle reinforced composite model.
Once the model is constructed, 100 slices were taken along z-axis. The slicing
process is shown in Figure A.2 and an example of a slice of cross section is shown in
Figure A.3.
143
Figure A.2. Illustration of slicing the cross section.
Figure A.3. A cross section of 10% by volume particle filled composite.
144
For each slice of cross section, the area fraction is calculated and compared with
the composite volume fraction. The particle size effect was also investigated. Particle
mean radii ranging from 0.5 to 4 were tested for various volume fractions up to 20%. A
10% volume fraction example is shown in Figure A.4.
Figure A.4. Distribution of the area to volume fraction ratio of a 10% composite for
different particle mean radii.
As shown in in the figure above, with shirking the particle size and increasing particle
number, the distribution of the area to volume fraction ratio coverages to 1 gradually. The
area to volume fraction ratio of other volume fraction composites in this simulation study
exhibited the same converging behavior. For a particle radius to matrix length ratio of
1/100, the mean of area to volume fraction ratio was 0.9959 with a standard deviation of
0.0453. For PTFE/PEEK composites, the average PTFE particle radius was 50 microns
and the cubic pin was 6.35 mm each side. That yielded a particle radius to matrix length
145
ratio of 1/127, which is close to the value in the simulation. Therefore, according to the
numerical simulation, it is safe to claim that the cross-sectional area fraction is
approximately equal to the volume fraction of the reinforcing particle.
146
Appendix B
Friction Coefficients by the Rule of Mixtures
To include the effect of transfer films on the friction behavior of PTFE/PEEK
composites, the contact area (test pin cross-section area) between the composite test pin
and steel counterface is divided into two distinctly different areas – one area with and one
area without transfer films coverage. Assuming the shear stress (𝜏) across the contact area
(𝐴) is uniformly distributed, the total shear force (𝐹𝑐 ) acting on the contact area is equal
to 𝜏𝐴. The shear force (𝐹𝑇𝐹 ) acting on the contact area covered by transfer films is given
by
𝐹𝑇𝐹 = 𝜏𝛼𝐴 ,
(𝐵. 1)
where 𝛼 is the transfer films area coverage ratio (ACR). Similarly, in the contact area not
covered by transfer films, the shear forces carried by the PTFE particles (𝐹𝑃𝑇 ) and the
PEEK matrix (𝐹𝑃𝐾 ) are given by
𝐹𝑃𝑇 = 𝜏(1 − 𝛼)𝑉𝑓 𝐴
and
𝐹𝑃𝐾 = 𝜏(1 − 𝛼)(1 − 𝑉𝑓 )𝐴 ,
(𝐵. 2)
where 𝑉𝑓 is the PTFE volume fraction in the composite.
Now the total normal load (𝐿𝑐 ) acting in the contact area is given by
𝐿𝑐 = 𝐿𝑇𝐹 + 𝐿𝑃𝑇 + 𝐿𝑃𝐾 ,
(𝐵. 3)
where 𝐿𝑇𝐹 is the normal load on the contact area covered by transfer films, and 𝐿𝑃𝑇 and
𝐿𝑃𝐾 are the normal loads carried by the PTFE particles and the PEEK matrix in the contact
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area not covered by transfer films. Using the relationship between friction coefficient (𝜇),
shearing force (𝐹) and normal load (𝐿), Eq. (18) can be written as
𝐹𝑐 𝐹𝑇𝐹 𝐹𝑃𝑇 𝐹𝑃𝐾
=
+
+
.
𝜇𝑐 𝜇 𝑇𝐹 𝜇𝑃𝑇 𝜇𝑃𝐾
(𝐵. 4)
Substituting Eqs. (16) and (17) into Eq. (19), we have the following expression for the
friction coefficient (𝜇𝑐 ) of PTFE/PEEK composites
(1 − 𝛼)𝑉𝑓 (1 − 𝛼)(1 − 𝑉𝑓 )
1
𝛼
=
+
+
𝜇𝑐 𝜇 𝑇𝐹
𝜇𝑃𝑇
𝜇𝑃𝐾
(𝐵. 5)
or
𝜇𝑐 =
1
(1 − 𝛼)𝑉𝑓 (1 − 𝛼)(1 − 𝑉𝑓 )
𝛼
+
+
𝜇 𝑇𝐹
𝜇𝑃𝑇
𝜇𝑃𝐾
.
(𝐵. 6)
Equation (21) is the inverse rule of mixture (IROM) for the friction coefficient of
PTFE/PEEK composites. If a uniform pressure distribution on the contact area is assumed
instead of a uniform shear stress distribution for the determination of composite friction
coefficient, the following linear rule of mixture (LROM) is obtained for 𝜇𝑐 , i.e.,
𝜇𝑐 = 𝛼𝜇 𝑇𝐹 + (1 − 𝛼)𝑉𝑓 𝜇𝑃𝑇 + (1 − 𝛼)(1 − 𝑉𝑓 )𝜇𝑃𝐾 .
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(𝐵. 7)