The document summarizes friction and wear test results for several OX-PEKK polymer grades conducted using a block-on-ring tribometer. OX-PEKK C showed lower friction and wear than PEEK in tests against steel. Carbon fiber reinforcement significantly reduced OX-PEKK wear rates, with the 30% fiber grade performing better than the 40% grade. A bearing grade formulation had extremely low 8.5 μm/h wear despite a high 0.4 coefficient of friction, due to PEKK's strength and stiffness. Test methods and results are discussed to evaluate OX-PEKK composites for tribological applications.

1. Friction and Wear of Selected OX-PEKK Grades
In cooperation with Report on Project 10-2042
François Minec and
Kay Fritsche
(VELOX GmbH) 06/22/2010
Stefan Sutor
Andreas Gebhard
NanoProfile GmbH
Sauerwiesen 2
67661 Kaiserslautern
Germany
Tel +49 (0) 6301 703-300
Fax +49 (0) 6301 703-119

2. Friction and Wear of Selected OX-PEKK Grades
Table of contents
1 Introduction ..................................................................................................................................... 1
2 Friction and Wear Measurements.................................................................................................... 1
2.1 Testing equipment: The Atlas TriboTester ............................................................................ 1
2.2 Wear Test Setup and Loading Parameters ............................................................................. 2
2.3 Wear Test Evaluation and Computation of the Wear Rate .................................................... 3
3 Results and discussion..................................................................................................................... 4
4 Summary and Conclusions .............................................................................................................. 5

3. Friction and Wear of Selected OX-PEKK Grades
1 Introduction
NanoProfile GmbH traditionally develops its low friction and highly wear resistant materials based on
PEEK. Although PEEK has excellent thermal properties (high glass transition and melting tempera-
ture), an excellent chemical resistance and an exceptionally good wear resistance, there are certain
applications that require even higher wear and temperature resistance than is currently possible with
PEEK.
NanoProfile has therefore decided to assess OX-PEKK C as a possible substitute for PEEK in such
ultra high demanding applications. For this purpose Velox kindly provided us with samples of their
various OX-PEKK grades. This report covers the friction and wear measurements that were made on
these samples.
Table 1: Tested OX-PEKK grades.
PEKK grade Reinforcement
C None
IG 220 BS Unknown
BG (“bearing grade”) „Bearing Grade“, most likely
10 % CF, 10 % PTFE, 10 % Graphite
IG 230 C Unknown
C 30 C 30 % Carbon fiber
IG 240 C Unknown
C 40 C 40 % Carbon fiber
2 Friction and Wear Measurements
2.1 Testing equipment: The Atlas TriboTester
Figure 1 shows NanoProfile’s Atlas TT tribometer, where unidirectional sliding wear tests have been
conducted on all OXPEKK materials using the block-on-ring configuration. The Atlas TT is a fully
featured tribometer that is equipped with state-of-the-art sensors that enable the time resolved tracking
of friction, wear and counter body temperature. It is designed and constructed in compliance with
ASTM G 1371.
1
The American Society for the Testing of Materials: ASTM G137 - 97(2009): Standard Test Method for Rank-
ing Resistance of Plastic Materials to Sliding Wear Using a Block-On-Ring Configuration
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4. Friction and Wear of Selected OX-PEKK Grades
Figure 1: Block-on-ring wear test on the „Atlas TT 1“ tribometer.
2.2 Wear Test Setup and Loading Parameters
Figure 2 shows a schematic of the experimental setup including the position of the sensors for the de-
tection of friction, wear and counter body temperature.
Figure 2: Schematic of the block on ring contacting geometry (left) and of the overall test setup
(right).
Table 2 shows the testing parameters of the conducted wear tests.
Table 2: Wear testing parameters on Atlas TT.
Parameter Value
Testing geometry Block-on-ring according to ASTM G 137
Loading 5.00 MPa
Sliding speed 1.00 m/s
Lubricant None
Counter body 100Cr6 (1.3505, bearing steel),
60 HRc, Ra = 0,1 µm
Wear measurement optical displacement
measurement (time resolved)
Friction force measurement Unidirectional (time resolved)
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5. Friction and Wear of Selected OX-PEKK Grades
Wear test specimens were machined from samples of tensile tests bars kindly provided by Velox. As
counter bodies hardened and ground 100Cr6 bearing steel rings were used. Surface hardness was 60
HRc and surface roughness (Ra) was 0.1 – 0.2 µm. All tests were conducted at standard ambient tem-
perature, i.e. at 23 °C, and at a relative humidity of 50 %. All tests were conducted without lubricant
and after test specimens and counter bodies were cleaned according to ASTM G 137.
Eight separate tests were conducted per material in order to obtain a sufficient statistic reliability of the
test results. Each of these 8-test-samples is subjected to statistical analysis according to ASTM E 1782
in order to identify outliers, i.e. results that deviate markedly from the other results in the sample. Out-
lying results are eliminated and the corresponding tests are repeated until a valid sample containing at
least four separate tests is obtained. Wear rates are then reported as arithmetic means and confidence
intervals of the valid samples.
2.3 Wear Test Evaluation and Computation of the Wear Rate
The Atlas TT tribometer detects wear by tracking the height loss of the test specimens by means of
optical displacement measurement. The most important advantage of this time resolved wear tracking
is the possibility to clearly see the running-in phase of the materials and the steady state wear during
the experiment. In this steady state the differential wear rate w = ∂h / ∂t is computed. It represents the
specimen’s height loss over time; measured in µm/h. Due to its mathematical definition it is equal to
the slope of a linear regression of the time resolved height loss data (see Figure 3).
Figure 3: Time resolved height loss data of the specimen (red) and wear rate w as slope of the linear
regression (blue) of the height loss’s steady state.
During the experiment the friction force FF is also recorded. Division by the applied normal force FN
(= loading · sample cross section), yields the coefficient of friction µ:
FF
µ= .
FN
2
The American Society for the Testing of Materials: ASTM E178 – 08: Standard Practice for Dealing With
Outlying Observations
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6. Friction and Wear of Selected OX-PEKK Grades
3 Results and discussion
The wear rates (shown in Figure 4) and the coefficients of friction (shown in Figure 5) that are listed
in Table 3 (see the Appendix) have been determined in the steady states of the corresponding wear
tests as described in section 2.3. The numbers represent the arithmetic means of homogenous, i.e. out-
lier-free, samples. The stated ranges represent the corresponding confidence intervals. This means for
example, that the mean wear rate of ‘C 30 C’ has a 95 % probability of lying between 30 and 34 µm/h.
Figure 4: Wear rates of the tested OX-PEKK materials.
Most interestingly, the neat PEKK/steel pairing shows a coefficient of friction of only 0.35. This is
about 0.05 – 0.10 lower than for neat PEEK/steel. With 256 ± 60 µm/h PEKK’s wear rate is also sig-
nificantly lower than the one neat PEEK, which amounts to 384 ± 60 µm/h.
Compared to the neat PEKK, carbon fiber reinforcement yields a reduction of the wear rate of almost
an order of magnitude. At the same time wear is reduced, when the fiber content is 30 % and increased
when it is 40 %. Overall, the ‘C 30 C’ has better tribological properties than ‘C 40 C’. Of the two ‘IG
C’ grades the ‘IG 230 C’ has a lower wear rate and hence a higher wear resistance and about the same
coefficient of friction than its ‘IG 240 C’ counter part. The wear rate of ‘IG 220 BS’ is about four
times the wear rate of neat PEKK. What ever its fillers are considered to do, they do not make ‘IG 220
BS’ a composite suitable for tribological applications.
The ‘BG’ grade (= bearing grade) has an extremely low wear rate but the highest coefficient of friction
of all tested materials. According to our knowledge, this grade contains 10 % graphite, 10 % polytetra-
fluorethylene and 10 % carbon fibers. This combination of fillers is typical for standard bearing grades
not only in polyaryletherketones but also in many other performance polymers. The results on the
PEKK bearing grade, i.e. very low wear despite the coefficient of friction being higher than the neat
polymer’s, is a common observation which we recurrently make on many commercial bearing grades
containing this combination of fillers. The reason for this is yet unknown.
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7. Friction and Wear of Selected OX-PEKK Grades
Figure 5: Coefficients of friction of the tested materials.
Overall, it must however be stated that ‘OX-PEKK BG’ is amongst the best polymer based bearing
materials that we have ever tested.
4 Summary and Conclusions
NanoProfile has assessed the tribological performance of OX-PEKK and of some of its composite
materials by conducting friction and wear tests following ASTM G 137. Overall, OX-PEKK C showed
significantly lower friction (- 20 %) and less wear (- 30 %) than its more established relative PEEK.
Carbon fiber reinforcement of PEKK yielded a significant reduction of wear. Friction is reduced, but
only at a carbon fiber content of 30 %. At 40 %, i.e. for ‘C 40 C’ friction is slightly higher than for
neat PEKK. Consequently, from a strictly tribological point of view ‘C 30 C’ is the better material
than ‘C 40 C’. If mechanical requirements are taken into account, ‘C 40 C’ can be preferred over ‘C 30
C’.
‘OX-PEKK BG’, which is a PEKK-based bearing grade, features amazingly low wear. In dry sliding
against steel it exhibited a height loss rate of only 8.5 µm which is amongst the lowest values that have
yet been observed on our tribometers. Its coefficient of friction amounts however to 0.4, which is very
high for a tribologically optimized material but not unusual for a material containing 10 % of graphite,
polytetrafluorethylene (PTFE) and carbon fibers. We attribute the nonetheless extremely low wear rate
of the ‘BG’ material to PEKK’s outstanding combination of thermal resistance and high mechanical
strength and stiffness.
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8. Friction and Wear of Selected OX-PEKK Grades
Appendix: Numerical results friction and wear tests.
Table 3: Numerical results of the block-on-ring friction and wear tests conducted on the OX-PEKK
materials.
wf
PEKK grade ID* [µm/h] µ
C 451 256 ± 60 0.35 ± 0.02
IG 220 BS 450 1046 ± 162 0.30 ± 0.02
BG (“bearing grade”) 449 8.5 ± 0.9 0.40 ± 0.03
IG 230 C 452 27.3 ± 0.7 0.30 ± 0.02
C 30 C 453 32 ± 2 0.29 ± 0.03
IG 240 C 454 39 ± 2 0.28 ± 0.06
C 40 C 455 43 ± 12 0.37 ± 0.03
*
for internal reference only
A-1
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