MANE 6960 - Friction & Wear of Materials
Wear Behavior of PTFE and PTFE-Based
Composites in Sliding Contact with Common
Titanium Alloys
Matthew Lessard
15 December 2009
1.0 Polymeric Materials
Polymers play a very important role in modern mechanical engineering and
machine design. The utility of engineering plastics and elastomers is realized
where applications require structural materials that possess a level of chemical
resistance or reduced component weight that can not be achieved by metals or
ceramics. It is the tribological properties of these materials, however that makes
them invaluable in applications where dynamic contact exists between machine
components.
Significant advances have been made in recent years in the field of polymer
tribology, enabling the use of these materials in systems with very demanding
operating conditions. Of particular interest is the usefulness of polymer materials
as solid lubricants in sliding bearing applications where boundary lubrication is
not feasible or cost-effective. In modern tribology, only a few select materials
have been identified as possessing the unique molecular properties necessary to
make for an effective bearing polymer.
Polytetrafluoroethylene (PTFE) is one such material that is frequently employed
in bearing applications due to its inherently low coefficient of friction. PTFE is a
linear fluorocarbon that is prone to slippage in the crystalline formation of its
molecular bond structure when subjected to shear loading. This ‘slipping’
phenomenon afforded by its unique molecular structure is the most likely reason
that PTFE maintains such a low coefficient of friction (as low as .02 under
specific operating conditions) [2], making it an excellent solid lubricant material.
When coupled with a suitable counter surface in dynamic contact, a thin layer of
PTFE is often transferred from the bulk material to the surface of the mating
component. This condition is commonly referred to as the transfer film layer in
the field of tribology. It is the subsequent PTFE-PTFE contact that results in a
low coefficient of friction and earns this material its ‘self-lubricating’ designation.
This low COF material property is typically attributed to the synergy of the PTFE
chains (shearing interface) of the deposited surface and the bulk PTFE surface.
Figure 1 presents a series of SEM micrographs showing the transfer film layer
and wear debris from pin-on-disk testing using a 1045 steel disk and PTFE /
PTFE composites (3N normal load, .15 m/min relative surface velocity) [13].
Figure 1.
Transfer Film on AISI 1045 Steel Plate (Left Column) and Wear Debris (Right
Column) of PTFE (A) Filled PTFE/15 vol.% MoS2 (B) Filled PTFE/15 vol.%
graphite (C) [13]
Despite its self-lubricating properties and relatively high temperature stability,
PTFE has been shown to exhibit relatively poor wear and abrasion resistance
when compared to similar polymer materials such as polyethylene and polyamide
[2]. The high rate of wear exhibited by this material tends to limit its effectiveness
as a bearing material in its virgin form. Several test studies have demonstrated
that a direct correlation exists between the bulk wear rate (volume material loss)
experienced by PTFE and both bearing pressure and sliding velocity. Increasing
the dynamic bearing pressure seen by a PTFE tribosystem, (typically coupled
with a metallic material of higher material hardness) results in an exponential
increase in the volume loss of PTFE as a function of distance traveled. A similar
relationship exists between wear and relative surface velocity. Increasing
velocity at the bearing interface also yields an exponential increase in volume
loss of the softer PTFE material due to the resultant increase in localized surface
heating, a phenomenon commonly referred to as flash heating. Elevated surface
temperatures in conjunction with a change in the crystalline shear mechanism of
PTFE at increased speed ultimately results in larger wear debris particles and
higher wear rates.
Figure 2. Wear Rate of a PTFE-Based Composite as a Function of Pressure and Velocity
[16]
To make for a useful tribological material, significant efforts have been made to
increase the wear resistance of PTFE, leading to the development of a number of
PTFE composite materials. These composites can typically be classified
according to two basic categories which describe the general method of
modification [2]; Bulk-modified composites and Interface-modified composites.
Bulk-modified composites employ ‘hard’ filler materials such as ceramics, metals
or synthetic fibers directly in the bulk self-lubricating material (i.e. PTFE). The
function of these ‘hard’ filler materials is essentially to strengthen the polymer
matrix, increasing its load-carrying capacity and wear-resistance. Some of the
more common filler materials used to increase bulk material strength are glass,
aramid and carbon (all usually in short fiber form).
On the other end of the composites spectrum, interface-modified composites
employ a softer self-lubricating material (i.e. PTFE, graphite, molybdenum
disulfide) as a ‘filler’ in a harder polymer matrix. The PTFE fillers (also commonly
in short fiber form) provide the composite with the desired low friction properties,
while the strength and toughness of the bulk matrix (some common polymers are
epoxy, phenolic and PEEK), is augmented by additional fillers. Alumina
(aluminum oxide) and titanium oxide are frequently employed as strengthening
fillers in interface-modified polymer composites and their particle size is central to
the level of wear resistance that can be achieved using these fillers. Recent
studies [2] have shown that using fillers with particle size on the nano-scale
(termed nanocomposites) can effectively reduce wear rates by one and in some
cases, two orders of magnitude when compared to micro-sized filler particles.
This is also true of bulk-modified composites.
There are two primary subcategories that exist for interface-modified composites,
thermoplastic materials, and those that employ a thermosetting matrix (such as
epoxy compound). The performance of thermoset composites is of particular
interest to the author of this paper and will be the focus of its content. The wear
performance of neat PTFE will also be examined as it is the primary constituent
in most polymer bearing composites and its relative performance is of great
interest.
2.0 Titanium and Titanium Alloys
The corrosion resistance, comparatively low weight and high strength of titanium
and its alloys makes it very desirable as a structural material in mechanical
design. Titanium alloys, however, have a tendency to gall and seize due to low
work hardening behavior in applications where bearing pressure and sliding
velocities are high [10]. As such, much work has been accomplished in research
and development of coatings and surface treatments aimed at increasing the
surface hardness (and wear resistance) of this material. Despite the progress
made through surface treatment of titanium with carbides, oxides, nitrides (etc),
the wear resistance of titanium is still considerably higher than most alloy steels
when evaluated in similar operating conditions [15].
The exceptional strength-to-weight ratio of titanium makes this an interesting
material for bearing applications in the aerospace industry, where weight savings
is often a critical factor to system performance. Aerospace components are
typically sized and designed to the upper strength limits of the respective material
substrate. Subsequently, bearing contact stresses are often significantly higher
than would be encountered in standard machine design practices. The poor
tribological behavior of titanium necessitates the use of surface coatings to
mitigate friction and wear resulting from dynamic contact between components
manufactured from these alloys. Given the strength and general material
properties of common titanium aerospace alloys, PTFE composites and
engineered plastics with their relatively lower material hardness and shear
properties would seem a plausible couple for titanium in bearing tribosystems.
3.0 Titanium-Polymer Tribosystems
Little work has been published in recent years concerning the wear performance
of titanium in sliding contact with PTFE and its composites, however, the work
that has been accomplished using varying operating conditions tend to show
significantly increased wear rates when compared to high hardness stainless
steel / PTFE couples. This general relationship was witnessed in experimental
work performed by Qu, Blau et. al. [3] concerning the friction and wear behavior
of two different titanium aerospace alloys against PTFE. Qu subjected Ti-6Al-4V
and Ti-6Al-2Sn-4Zr-2Mo titanium alloys to standard pin-on-disk sliding wear
experiments at different speeds (0.3 and 1.0 m/s) using a 10N normal load.
Additional slider materials examined during this testing were AISI 440C stainless
steel, silicon nitride (NBD200 Grade 5) and alumina (AFBMA Grade 25). The
results obtained by Qu are illustrated in Table 3 below.
Table 1. Friction and Wear Results Obtained by Qu [3] for Ti64 and Ti6242 in Sliding
Contact (Pin-On-Disk) with PTFE and Other Slider Materials
As is denoted by the N/Ma specific wear value reported for the titanium substrate
in Table 1, the neat PTFE slider demonstrated its characteristic self-lubricating
qualities during each of these sliding tests. In each instance, a thin film transfer
layer was deposited on the surface of each titanium sample at test’s end, leading
to lower COF values and higher wear rates than the other materials tested.
When comparing the results of this test to very similar pin-on-disk test results
using neat PTFE and a hardened 440C stainless steel counterface [11], PTFE
wear rates were found to be up to three orders of magnitude lower when using
440C over titanium alloys. Historically, surface finish and preparation has shown
to have a considerable effect on specific wear rates and could also have
contributed to the significant difference in wear rates; however most likely not to
the degree shown.
Probably the most interesting work encountered during the course of this
research in the field of polymer-titanium interface engineering actually involved
the wear performance of ultra-high molecular weight (UHMW) polyethylene as a
tribomaterial rather than PTFE. Researchers Li, Dong and Shi [12] took an
interest in the sliding wear behavior of Ti6Al4V when coupled with UHMW
polyethylene as this material combination is frequently employed in biomedical
applications. Their experimental examination originally initiated as a study into
potential titanium surface treatment methods aimed at increasing the wear
resistance of Ti64 [16]. Using diamond-like coatings, thermal-oxidation (TO) and
oxygen diffusion (OD) surface engineering techniques, Li was able to
demonstrate significant gains in the tribological performance of titanium when
coupled with UHMW in dynamic contact.
During the course of their testing (using standard 5Mpa pin-on-disk test
methods), a wear phenomenon was witnessed whereby the un-treated baseline
6-4 titanium sample consistently exhibited significant wear damage when
operated against the relatively softer UHMW slider. Deep, wide grooving was
witnessed in the wear scar on the untreated titanium, with a depth and pattern
indicating a severe abrasive wear mechanism, with relatively low wear displayed
by the UHMW slider. As this author can testify (as a result of numerous bearing
endurance tests), this is a fairly common condition when coupling untreated 6-4
titanium against epoxy-based PTFE composites. Where one would expect the
softer polymer material to display the bulk of the material wear in this couple, the
titanium substrate exhibits deeper and more pronounced wear scarring when
operated at high bearing pressures.
In their original work [16], Li, et. al. identified this abnormal wear phenomenon
and attempted to explain the wear as a function of poor adhesive attraction
between the UHMWPE and the un-treated titanium countersurface due to an
ionic character of the surface interface layers. It wasn’t until two years later that
this team developed an interesting hypothesis for this wear condition; hydrogeninduced wear.
This theory was originally introduced by a team of Russian
researchers in the late 1970’s [17] when examining the tribological performance
of polymers when operated against tungsten carbide-cobalt coatings, however
the theory was not pursued and developed after the journal article was published
in 1993. Li et. al. recognized the plausibility of this theory as an explanation for
the wear mechanisms displayed during their experimentation and they attempted
to prove this concept through additional experimentation using UHMW and
titanium alloys.
During testing, Li (and Zaitsev) noticed a trend wherein hard alloys experienced
higher than expected wear rates when coupled in dynamic contact with polymers
such as polycaproamide (PCA), phenolformaldehyde polymer (PF) and expoxy
compound (EDC), which all contain reactive groups such as amide, ether and
hydroxyl groups. The theory of hydrogen-induced wear assumes that the
excessive wear induced in the harder alloys was primarily attributed to a
chemical action of these reactive groups. The destruction of hydrogencontaining groups from the high localized heat developed by dynamic friction
leads to the inward diffusion of the excess hydrogen into the alloy materials
during operation, accelerating subsurface cracking and resulting in increased
specific wear rates. Titanium and its alloys are well known to be subject to
hydrogen embrittlement, which has shown to reduce the toughness and ductility
of this material.
Li conducted pin-on-disk testing using UHMW pins and disk samples
manufactured from both Ti64 and 316L in an effort to analyze this condition. Aspolished 6-4 titanium was evaluated as well as thermal-oxide treated 6-4 titanium
(a common method used to develop a hard titanium oxide layer on the surface of
the titanium disk). After testing was completed (5 Mpa effective pressure, 0.3 /
1.0 fpm relative surface velocity), a composition depth profile was generated
using glow-discharge spectrometry, and each wear surface was also
characterized using SEM and TEM microscopy. After sectioning, additional
spectro-analyses were performed using Fourier-transform infrared (FT-IR) and
electron spectroscopy for chemical analysis (ECSA) in an exhaustive review of
the wear composition.
As a brief summary of Li’s findings [12], the presence of TiH x, titanium hydride
was found to be present in significant concentrations both in the Titanium oxide
layer, and more importantly in the α-phase bulk below the TiO layer. The
presence of TiHx below the direct interface layer indicates that the diffusion of
hydrogen into the titanium bulk could well have contributed to the deep abrasive
wear experienced during this test. This condition was witnessed in both the aspolished titanium sample as well as the TO-treated sample. The percent
composition of TiHx found in the α-Ti of the TO-treated sample was significantly
lower than was witnessed in the as-polished titanium sample. This stands to
reason as testing has shown the Titanium oxide serves as an effective barrier
against hydrogen diffusion [18]. Figure 3 provides a graphical illustration of the
hydrogen-assisted wear mechanism as proposed by Li et. al.
Figure 3.
Illustration of Proposed Hydrogen-Assisted Wear Mechanism for the
Tribosystem UHMW/Ti6Al4V [12]
This finding shows promise in explaining the poor wear performance exhibited by
titanium alloys in PTFE composite tribosystems. Future development efforts with
PTFE composite bearing design should include consideration for the hydroxide
content of any epoxy-compound matrices employed in the liner system. Coupled
with the application of thermal-oxide surface treatments, significant gains could
potentially be achieved in the performance of self-lubricating composites coupled
with titanium alloys.
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