Available online at www.sciencedirect.com
Wear 264 (2008) 359–367
Dry friction and sliding wear of EPDM rubbers against
steel as a function of carbon black content
J. Karger-Kocsis a,∗ , A. Mousa b , Z. Major c , N. Békési d
a
Institut für Verbundwerkstoffe GmbH, Kaiserslautern University of Technology,
Erwin Schrödinger Str. 58, D-67663 Kaiserslautern, Germany
b Department of Materials and Metallurgical Engineering,
Balqa Applied University, Salt 19117, Jordan
c Institut für Werkstoffkunde und -prüfung der Kunststoffe, Montanuniversität Leoben,
Franz-Josef-Str. 18, A-8700 Leoben, Austria
d Institute of Machine Design, Budapest University of Technology and Economics,
Müegytem rkp. 3, H-1111 Budapest, Hungary
Received 24 August 2006; received in revised form 16 February 2007; accepted 22 March 2007
Available online 2 May 2007
Abstract
The dry friction and sliding wear of ethylene/propylene/diene rubbers (EPDM) were studied against steel as a function of the carbon black (CB)
content using various testing configurations, such as pin(steel)-on-plate(rubber) (POP) and ring(steel)-on-plate(rubber) (ROP). The EPDM rubbers
were characterized using tensile, compression tests and dynamic-mechanical thermal analysis (DMTA). The coefficient of frictions (COF) and
specific wear rates of the EPDMs were determined. It was found that with increasing CB content the specific wear rate was reduced. A similar
tendency was found for the COF in ROP tests. Both COF and wear rates of the EPDM mixes strongly depended on the test configurations. The
wear mechanisms were concluded by inspecting the worn surfaces in scanning electron microscopy (SEM) and discussed. Albeit several rubber
characteristics follow the same trend as the COF and wear rate, at least for this EPDM formulation, further investigations are needed to deduce
eventual correlations between them.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Dry sliding; EPDM; Friction; Sliding wear; Wear mechanisms
1. Introduction
Rubber parts are widely used in different application fields
such as automotive (sealing, bushing), construction (roofing,
dilatation joints) and electric/electronic (insulators). It has been
early recognized that the wear and abrasion behaviors of rubbers can be tailored upon request. To highlight this issue one may
compare the abrasion performance of a tire with that of an eraser.
The use of rubber products is inevitable in numerous applications when they fulfill sealing functions under dry and lubricated
sliding conditions. Although wear is considered as a system
characteristic (i.e. wear behavior depends on the configuration of
the tribotesting rig, loading conditions, surface characteristics of
∗
Corresponding author. Tel.: +49 631 2017203; fax: +49 631 2017198.
E-mail address: karger@ivw.uni-kl.de (J. Karger-Kocsis).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.wear.2007.03.021
the counterpart, etc.), efforts were always devoted to trace those
material parameters, which control the wear. As a consequence
some rules of thumb were deduced on how the wear characteristics of metals, ceramics and even polymers depend on selected
material variables (e.g. [1,2] and references therein).
The friction and wear of rubbers differ, however, substantially
from those of other solids (e.g. [3–6] and references therein).
This is mostly due to their very low elastic modulus and significant internal damping in a broad frequency range. The physical
processes occurring during the sliding wear of rubber are of
adhesive and hysteretic nature. The latter reflects the internal
friction of the rubber owing to the oscillating load generated by
the asperities of the counterpart. Adhesion between the rubber
and counterpart influences the loading history during sliding.
Albeit the sliding conditions (load, speed, duration, lubrication,
etc.) strongly affect the wear behavior, the latter should depend
in the first approximation on the inherent structure of the rubber
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(crosslinking type and degree, filler type and amount affecting
the filler/filler interactions). In order to shed light on this issue
we report in this work on the effect of carbon black (CB) content on the dry sliding behavior of an ethylene/propylene/diene
(EPDM) rubber (M denotes according to ISO 1629 the saturated
nature of the backbone of the related rubber). Further aim of this
work was to investigate on how the friction and wear properties
of the EPDM rubbers depend on the configuration of the testing
rigs.
2.2. Testing
2. Experimental
2.2.2. Hardness and density
The Shore A hardness of the rubbers was determined according to the ISO 868 standard. For the density determination the
Archimedes principle (buoyancy method) was adopted using
methanol.
2.1. EPDM rubbers
The rubber stocks were prepared in a laboratory internal
mixer and the curatives were introduced on a laboratory open
mill. The recipe used was as follows—EPDM (Keltan® 512
of DSM Elastomers, Sittard, The Netherlands): 100 parts; carbon black (N500): 0, 30, 45 and 60 parts; ZnO: 5 parts;
N-cyclohexyl-2-benzothiazole sulfenamide (CBS, Vulkacit CZ
of Bayer, Leverkusen, Germany): 0.6 part; 2-mercapto benzothiazole (MBT, Vulkacit Mercapto by Bayer): 0.6 part; zinc
dicyanatodiamine (Rhenogran Geniplex 80 of Rhein Chemie,
Mannheim, Germany): 0.6 part; zinc dibenzyl dithiocarbamate
(Rhenogran ZBEC-70 of Rhein Chemie): 1.5 parts. Recall that
the CB content was varied between 0 and 60 parts per hundred parts rubber (phr). Rubber sheets (ca. 2 and 4 mm thick)
were produced by compression molding at 160 ◦ C and 7 MPa
pressure using a Satim Press (Rion des Landes, France). The
vulcanization time was adjusted by considering the time needed
for the 90% crosslinking at T = 160 ◦ C. The related time was
read from the Monsanto moving die rheometer (MDR 2000
EA-1) curves. In addition to the above “home-made” EPDM
rubbers, a sulfur-cured EPDM rubber of unknown composition,
containing slightly more than 60 phr CB (not disclosed grade)
was also involved in this study. As this rubber was delivered
by TRW Automotive (Navarra, Spain), it is referred to TRW
further on.
2.2.1. Dynamic-mechanical thermal analysis (DMTA)
DMTA spectra were recorded on rectangular specimens
(length × width × thickness = 20 mm × 10 mm × ca. 2 mm) in
tensile mode as a function of temperature (from −100 ◦ C to
+100 ◦ C) and a frequency of 1 Hz using a Q800 device of TA
Instruments (New Castle, DE, USA). The conditions set were:
strain 0.01%, heating rate: 2 ◦ C/min.
2.2.3. Mechanical properties
Tensile tests were carried out on 2 mm thick dumbbells (type:
S1 according to DIN 53504) on a Zwick 1445 (Ulm, Germany)
universal testing machine at a deformation rate of 500 mm/min.
From the related stress–strain curves apart of the ultimate properties, the stress values at 100 and 300% elongations, termed
M-100 and M-300, respectively, were also read (ISO 37). To
determine the tear strength at 500 mm/min deformation rate the
recommendation of the ISO 34-1 standard (angle-type specimen with cut) was followed. The compression set of the EPDM
samples was assessed by ISO 815 by keeping the circular disk
specimens, compressed to 75% of their initial thickness, at 70 ◦ C
for 22 h.
2.2.4. Sliding friction and wear
Friction and wear characteristics were determined in pin
(steel)-on-plate (rubber) (POP) configuration using two different devices. In the Wazau device (Berlin, Germany) a steel pin
(316L; arithmetical roughness, Ra , less than 0.1 ␮m) with a
hemispherical tip of 3 mm diameter rotated along a circular path
(diameter: 33 mm). The steel pin was pushed against the rubber
plate with a given load. The following parameters were selected
Fig. 1. Scheme of the sliding wear test rigs.
J. Karger-Kocsis et al. / Wear 264 (2008) 359–367
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for this configuration – denoted further on as POP-C – load: 3 N,
sliding speed: 50 and 250 mm/s, duration: 6 h (except the neat
EPDM which was worn for 10–15 min). In the other tribometer
(UMT-2 of Center of Tribology, Campbell, CA, USA), the steel
hemisphere tip (of 5 mm diameter) moved reciprocating along
a line (20 mm). For this configuration, referred as POP-L, the
following parameters were selected—load: 5 N, speed: 5 mm/s,
duration: 1 h.
Measuring simultaneously both the normal and the friction force components during the tests the COF values were
computed and registered on-line. The specific wear rate was
calculated by:
Ws =
m
ρFL
(1)
where m is the mass loss recorded gravimetrically, ρ the
density, F the normal force and L is the overall sliding
distance.
To study the sliding wear a further test, termed ring (steel)on-plate (rubber) (referred to ROP), was also used. A rotating
steel ring (100Cr6, diameter: 60 mm, width: 20 mm, Ra between
0.1 and 0.2 ␮m) pressed against a rubber strip of 15–20 mm
width on a home-built device. The frictional force induced by
the torque was measured online and thus the COF was registered during the test. The test parameters were—load: 10 and
15 N; sliding speed: 50 and 250 mm/s; duration: maximum 1 h.
The specific wear rate was determined by Eq. (1). The various sliding test configurations are depicted schematically in
Fig. 1.
Fig. 2. E vs. T (a) and tan δ vs. T (b) traces for the EPDM rubbers studied.
2.3. Wear mechanisms
3. Results and discussion
The worn surfaces of the specimens were inspected in
a scanning electron microscope (SEM; JSM5400 of Jeol,
Tokyo, Japan). Prior to SEM investigation the specimens
were sputtered with an Au/Pd alloy using a device of Balzers (Lichtenstein). The wear development in the TRW rubber
was also followed by white light profilometry (MicroProf
from the Fries Research & Technology, Bergisch Gladbach,
Germany).
3.1. DMTA response
Fig. 2 displays the courses of the storage modulus (E ) and
mechanical loss factor (tan δ) as a function of the temperature
(T = −100 to 100 ◦ C). Note that with increasing CB reinforcement the stiffness increases in the whole temperature range.
The reinforcing effect of the CB is also well reflected by the
Table 1
Basic network-related and mechanical properties of the EPDM rubbers studied
Properties
EPDM-0
EPDM-30 (30 phr CB)
EPDM-45 (45 phr CB)
EPDM-60 (60 phr CB)
TRW
Mc [g/mol]
υc [×10−26 m−3 ]
tan δ [1] at the Tg peak
Density [g/cm3 ]
Shore A hardness [1]
M-100% [MPa]
M-300% [MPa]
Ultimate tensile strength [MPa]
Ultimate tensile strain [%]
Compression set [%]
Tear strength [kN/m]
2178
2.6
1.305
0.941
52
1.1
–
1.7
195
9
5.2
1063
5.8
0.950
1.023
62
2.2
6.0
6.4
314
8
15.3
611
10.4
0.798
1.052
71
3.9
–
9.2
240
7
12.6
315
21.1
0.626
1.103
81
5.7
–
12.6
250
5
18.1
250
27.4
0.701
1.137
79
3.3
11.2
19.0
530
11
16.5
–: not applicable.
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between crosslinks (Mc ):
Epl =
3ρRT
Mc
(2)
where Epl is the modulus at T = 296 K, ρ the density, R the
universal gas constant (8.314 J/(K mol)), and T is the absolute
temperature (i.e. T = 296 K).
Fig. 2 shows that with increasing CB content the apparent
Mc decreases. On the other hand, it is more straightforward to
consider the apparent network density (υc ) given by:
υc =
Fig. 3. Initial and steady state COFs for the EPDM rubbers in the POP-C
(v = 50 mm/s) and POP-L (v = 5 mm/s) configurations.
monotonous decrease of the tan δ with increasing CB amount.
On the other hand, the glass transition temperature (Tg ) was not
influenced by the CB loading. According to the rubber elasticity theory the inverse of the plateau modulus (1/Epl ) at a given
temperature above Tg correlates with the mean molecular mass
Fig. 4. Initial and steady state COFs for the EPDM rubbers in ROP tests at
various loads and sliding speeds, v = 50 mm/s (a) and v = 250 mm/s (b).
Nρ
Mc
(3)
where N is the Avogadro or Loschmidt number (6.023
× 1023 mol−1 ).
It has to be underlined that both Mc and υc are apparent values.
They reflect not only the chemical crosslinking (which is likely
constant) but also the rubber–CB and CB–CB interactions. The
network-related data are summarized in Table 1.
3.2. Mechanical properties
The mechanical properties, along with further characteristics,
are listed in Table 1. One can recognize that with increasing
CB content the hardness, density, M-100 and ultimate tensile strength values increase. On the other hand, the opposite
Fig. 5. Specific wear rates of the EPDM rubbers measured in POP (a) and ROP
(b) configurations.
J. Karger-Kocsis et al. / Wear 264 (2008) 359–367
tendency holds for the tan δ values at the Tg peak. The ultimate tensile strain goes through a maximum as a function of
the CB content. The tear strength displayed some scatter from
the expected monotonous increase with increasing CB content.
Comparing the results achieved for EPDM-60 and TRW one
can notice how large the influence of the recipe formulation
may be (cf. Table 1). It is also noteworthy that the stress–strain
behavior of the EPDM series was affected by the specimen type
and deformation rate. So, S3A specimens (DIN 53504), fractured at v = 200 mm/min crosshead speed, showed better tensile
performance than the reported results in Table 1.
3.3. Friction
Attempt was made to determine both initial (within the first
minute in the running-in phase) and steady-state coefficients
of friction (COF) (mean value in the second half of the test)
during the tests in the different test rigs. Fig. 3 shows the change
in the COF of the EPDM rubbers as a function of the POP
configurations. Comparing the POP-C and POP-L results one
Fig. 6. SEM pictures at different magnifications showing the characteristic
failure of EPDM-0 after POP-C (v = 50 mm/s) test. Note: sliding direction is
downwards.
363
may get the impression that the difference between the initial
and steady-sate COFs is the larger the smaller the diameter of
the steel indenter is. The large initial values for the POP-C test are
probably owing to ploughing of the small pin that penetrates into
the rubber having locally different heights. The latter is caused
by changes in the glue thickness through which the rubber plate
was fixed on a steel plate fixture. This assumption is supported
by the tendency of the COF as a function of the CB content (the
resistance to ploughing should increase with the CB content).
The steady-state COFs are closely matched with each other in
POP-C and POP-L tests. This is due to the similarity of the
related tests. Accordingly, the wear mechanisms should be also
similar. The steady-state COFs show a marginal increase with
the CB content (cf. Fig. 3).
The scenario is different for the ROP tests. Fig. 4 shows the
COF data at two different sliding speeds and normal loads. Note
that the initial COF decreases with increasing CB content. On
the other hand, the course of the steady-state COF underlies
some scatter as a function of CB content. The most interesting
observation is that the steady-state COF values of ROP are below
those of the POP tests. The most reasonable explanation for that
Fig. 7. SEM pictures taken from the wear track of EPDM-30 at various magnifications after the POP-C (v = 50 mm/s) test. For note, cf. Fig. 6.
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is that the contact surface is larger and thus the contact pressure
smaller for ROP than for POP. Further, the debris are better
removed from the contact area in ROP than in POP testing. It
is the right place to mention that an ideal line contact between
the EPDM strip and the steel ring could be only exceptionally
achieved on this rig, which was constructed for far more stiff
materials. This might have contributed to a pronounced scatter
in the related friction and wear data.
3.4. Specific wear rate
Fig. 5 collates the specific wear rates of the EPDM systems
at different testing conditions for the POP (Fig. 5a) and ROP
configurations (Fig. 5b). One can recognize that incorporation
of already 30 phr CB radically reduces the specific wear rate
compared to the neat EPDM. Increasing CB content is accompanied with further, however, less dramatic reduction of the wear
rate. This note holds for all testing configurations. The specific
wear rates under POP-C and POP-L configurations agree fairly
with one another (cf. Fig. 5a) although the sliding speeds were
strongly and the diameters of the indenters only slightly different between these tests. This is a clear hint that the wear
Fig. 8. SEM pictures showing the wear behaviour of EPDM-60 (a) and TRW
(b) rubbers after the POP-C (v = 50 mm/s) test. For note, cf. Fig. 6.
mechanisms including the transportation of the detached debris
should be similar. It is noteworthy that the specific wear rate of
10 phr organoclay modified EPDM rubbers under POP-C conditions agreed fairly with those of EPDM-45 and EPDM-60 in
the present study [7].
The fact that the specific wear rate in ROP (cf. Fig. 5b) was
considerably lower than in POP is linked with the type of loading (permanent versus repeating), difference in contact pressures
(low versus high), differences in the transportation of the debris
(more versus less efficient) and temperature development in the
contact area (high versus low for the ROP and POP configurations, respectively). To shed light on the related differences
the failure mode (wear mechanisms) of the EPDM rubbers have
been studied.
3.5. Wear mechanisms
3.5.1. POP configuration
SEM pictures taken from the worn track of EPDM-0 show
a less characteristic failure mode for rubbers, which is due to
the missing CB reinforcement (cf. Fig. 6). The unfilled EPDM
failed not by roll formation but by massive material detachment
Fig. 9. SEM pictures taken from worn surface of EPDM-0 after the ROP test
(F = 15 N and v = 250 mm/s). Sliding direction is downwards.
J. Karger-Kocsis et al. / Wear 264 (2008) 359–367
in small fragments which were partly agglomerated. This type
of failure is favored by the low tensile and tear strength values of
this EPDM-0 (cf. Table 1). Incorporation of 30 phr CB changes
the wear mechanisms fundamentally—a Schallamach-type [8]
wavy pattern appeared. The wave fronts are transversely oriented
365
to the sliding direction (cf. Fig. 7a). At higher magnification
it is seen that the wear mechanism in this EPDM-30 is rather
complex (cf. Fig. 7b). It consists of roll formation, cutting, and
fragmentation events. Note that the driving force for the Schallamach wave formation is a tangential stress gradient, which
emerges in POP. Further incorporation of CB in EPDM does
not change the basic wear mechanisms. However, the height of
the Schallamach waves become markedly smaller and the overall surface shows some smearing, “ironing” (cf. Figs. 7 and 8).
This smearing effect becomes even more pronounced for the
TRW rubber—see Fig. 8b (which may be caused also by its
undisclosed ingredients). Next, however, it will be argued that
the smearing is due to thermal and tribochemical effects. The
above shown wear mechanisms are in harmony with the low
wear rates measured experimentally for the EPDM stocks with
high CB contents (cf. Fig. 5a).
3.5.2. ROP configuration
The wear mechanism of EPDM-0 differs considerably from
POP under ROP condition—cf. Figs. 6 and 9. Torn tongues of
Fig. 10. SEM pictures taken from worn surface of EPDM-30 (a), EPDM-60 (b)
and TRW (c) after the ROP test. For the testing conditions and sliding direction
cf. Fig. 9.
Fig. 11. High magnification SEM pictures showing the formation of a fibrillar
network in EPDM-0 (a) and EPDM-60 (b) after the ROP test. For the testing
conditions and sliding direction cf. Fig. 9.
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Fig. 12. Profilometric scans taken from the bottom of the wear track of the TRW rubber as a function of time during POP-C test. Designation—wear time: 1 min
(top left); 50 min (top right); 90 min (bottom, left); 330 min (bottom right). Notes: steel pin diameter 10 mm, v = 50 mm/s.
waveform (Schallamach-type) appear in the run-in side under
ROP. They become easy fragmented due to the low tensile and
tear strength of this EPDM-0 in the subsequent contact area (cf.
Fig. 9). At high magnification one can see the onset of tearing
and microcutting events (cf. Fig. 9b). The worn surfaces are
smoother in ROP than in POP (cf. Figs. 10 and 7) for EPDM-30.
It was expected based on the experimental COF (cf. Figs. 3 and 4)
and wear data (cf. Fig. 5a and b) by assuming that a smoother
worn surface indicates for lower COF and wear. The difference in
the roughness of the worn surfaces was confirmed and quantified
by white light profilometry, the results of which are not reported
here. Recall that the actual contact pressure is smaller in ROP
than POP. On the other hand, the temperature rise in the contact
area is higher for ROP than for POP as assessed by infrared
thermography (not reported here). They are the major reasons
for the lower COFs and specific wear rates in ROP compared to
POP.
With increasing CB content the wear mechanisms changed
basically. In EPDM-30 cratering (pitting) with fragmentation
could be resolved (cf. Fig. 10a). At even higher CB content
thermal effects (causing the development of fibrils) with some
limited pitting became dominant (cf. EPDM-60 in Fig. 10b). The
appearance of such fibrils can be traced to tribochemical effects
(thermooxidative degradation of the EPDM). The TRW rubber
of unknown composition failed mostly by surface cracking (cf.
Fig. 10c). Surface cracking (as it was definitely not an artifact
due to the Au/Pd coating to avoid charging in SEM) suggests
again tribochemical degradation. High magnification SEM pictures demonstrate that for the smearing thermal effects (high
flash temperature accompanied with thermooxidative degradation) should be responsible. This is obvious in Fig. 11, where
thermally induced fibrils and fibril networks are visible on the
surfaces of EPDM-0 and EPDM-60, respectively. This arguing
is in line with literature data (e.g. [9] and references therein),
reporting that ethylene/propylene-based rubbers are susceptible
to decomposition resulting in “oily debris”.
We have got a further experimental support for tribochemical effects by inspecting the wear track as a function of time
exactly at the same place during POP-C for the TRW rubber.
Fig. 12 displays the topography assessed by white light profilometer. This technique was used to determine small volume
losses due to wear tests recently [10]. One can see how the initial
smooth surface became Schallamach-waved and “ironed” but
surface-cracked again. The formation of the latter is assigned
to chemical attack, which is likely supported by the sulfur
curing of the EPDM rubber. As we failed to get a useful
estimate for the flash temperature, X-ray photoelectron spectroscopy will be adopted next to detect the chemical changes
supposed.
The mechanisms described above are in line with the experimental findings. Albeit several characteristics of the EPDM
rubber may correlate with the measured sliding wear data, further tests are needed to check whether or not such correlations
exist, and if yes, in what form. Note that correlations between the
specific wear and fracture mechanical parameters (and via the
latter with network parameters) ([11] and references therein) and
specific wear and tensile/hardness characteristics ([12] and references therein) have been already proposed. Considering the
complex wear mechanism, which apart of the test configurations depend even on the variation of a single parameter of the
rubber composition (viz. CB content in this case), a general relationships between material and wear characteristics can hardly
be validated. This is likely the reason why advanced methods
focus on the assessment of the viscoelastic properties of each
rubber composition in order to describe/model its sliding wear
performance (e.g. [13]).
J. Karger-Kocsis et al. / Wear 264 (2008) 359–367
4. Conclusions
Based on this work devoted to determine the dry friction and
sliding wear behavior of EPDM rubbers with various carbon
black (CB) content against steel counterpart using different test
rigs (POP and ROP), the following conclusions can be drawn:
• increasing CB content results in reduced specific wear rate
irrespective to the testing configurations;
• the coefficient of friction (COF) depends on the rubber composition, test duration and type of the testing rig. The latter
affects the COF mostly via the contact pressure, flash temperature and transport of the debris from the contact area.
Nevertheless, the steady-state COF seems to be unaffected or
decrease with the CB content;
• the observed change in the friction and wear behaviors may
correlate with rubbers characteristics, at least for a given rubber composition with a single variable in the recipe (CB in
case of this EPDM formulation). Mapping of the wear mechanisms for different test configurations and parameters as a
function of time should contribute to a deeper understanding
of the friction and wear behaviors of rubbers.
Acknowledgements
The authors are thankful to Mr. J. Hamann (German Rubber
Institute, Hannover, Germany), Ms. R. Kovács (PCC, Leoben,
Austria), Mr. M. Herrera Bugeiro and Dr. B. Andersen (IVW,
Kaiserlautern, Germany) for their involvement in the experimental and art works. This work was performed in the framework
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of an integrated project of the EU (KRISTAL; Contract Nr.:
NMP3-CT-2005-515837; www.kristal-project.org).
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