Wear 262 (2007) 1038–1047
Scratching of polymers—Modeling abrasive wear
Sujeet K. Sinha ∗ , W.L.M. Chong, Seh-Chun Lim
Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
Received 10 May 2006; received in revised form 15 September 2006; accepted 17 October 2006
Available online 20 November 2006
Abstract
This paper aims to model abrasive wear for polymers using single-track and intersecting scratching techniques. Scratch and pin-on-disc wear
tests were conducted on five different commercial polymers. Wear debris generated by multiple-pass single-track and intersecting scratching tests
were compared and correlated with the specific wear rates of the same polymers in a pin-on-disk test using ground steel surface (Ra = 1.34 ␮m) as the
counterface. It is the purpose of this paper to establish a correlation between scratching phenomenon and abrasive wear of polymers. Confirming previous studies, for polymers, the traditionally computed scratch hardness does not follow any trend with the abrasive wear. However, if the amount of
wear debris in scratching is quantitatively measured and plotted against specific wear rate in pin-on-disc test, a reasonable linear relation is obtained.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Scratching; Abrasive wear; Polymer wear
1. Introduction
Scratch testing is a very useful tool for understanding material
deformation and removal mechanisms in abrasive wear and it is
commonly used for materials’ surface characterization. Abrasive wear can be defined as wear due to the penetration of hard
asperities into the softer surface of a solid in dynamic contact
and scratch test can be employed as a tool for understanding
material wear process in the presence of hard asperities. Many
studies have been carried out by adopting single-track scratch
test on many different types of materials where the consequences
of variable parameters, such as scratching velocity, normal load
and attack angle of indenter [1–3], are investigated. In a study
carried out by Xie and Hawthorne [4], scratch test is used to
measure the critical plastic strain to micro-fracture for metals,
which represents the resistance of a material to ploughing wear.
Thus, scratching has been used for modeling wear resistance of
metals. Scratching and deformation maps have been constructed
by Briscoe et al. for several polymers [5,6]. Scratch morphology
in the scratching of polymers, which investigated the important
stick–slip phenomenon, was presented by Zhang and Nishizoe
[7].
∗
Corresponding author. Tel.: +65 6874 4825.
E-mail address: [email protected] (S.K. Sinha).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.wear.2006.10.017
Tabor showed that the normal hardness of minerals, used
in Mohs’ scale, correlated linearly with their scratch resistance
(Mohs Number) [8]. Except for the hardest mineral, diamond, all
other minerals fall on a straight line when plotted against Mohs
Number. In a survey by the present authors in the literature, it
was found that the scratch hardness of metals correlates with the
normal hardness and hence will correlate with the wear property according to the Archard’s wear model which states that
the wear rate for metals is inversely proportional to the normal
hardness. However, when we relate scratch hardness to the wear
rate of polymers, there seems to be no relation [9]. In fact, unlike
for metals, normal hardness of polymers is also not related their
wear rate. For example, poly(methylemethacrylate) (PMMA)
is roughly 5–6 times harder than ultra-high molecular weight
poly(ethylene) (UHMWPE) in normal or scratch hardness test,
however PMMA shows approximately 85 times higher wear rate
than that of UHMWPE measured under the same experimental
conditions (data from the present study). This shows that scratch
hardness or normal hardness are unlikely to be used in modeling
abrasive wear performance of polymers. This could be the reason that most of the wear models for polymers proposed in the
literature do not incorporate hardness (normal or scratch) as one
of the variable parameters [10]. Moreover, for more ductile polymers, a one-pass single-track scratching often does not produce
wear debris and hence, despite being dynamic, a scratch test cannot be used in the present form for modeling wear of polymers.
S.K. Sinha et al. / Wear 262 (2007) 1038–1047
Therefore, it is important to understand the scratching process
in order to correlate scratch test data to wear rates.
In order to understand the wear debris generation mechanism
for UHMWPE, Wong et al. [11] conducted intersecting scratching test and concluded that intersecting scratching increases
wear rate significantly in comparison to single-track scratching even when the number of passes for the two are kept the
same. They discovered that the material at the corners of the
intersection undergoes severe plastic deformation in an intersecting scratching process and thus the wear debris is formed at
these corners due to protrusions and low-cycle fatigue.
Similar study [12] on many other polymers has led to the
conclusion that, for many polymers, multi-pass single-track or
intersecting scratching may provide a link to correlate scratching
process with wear performance. Thus, the objective of this paper
is to explore the possibility of using the scratching technique, the
single-track and the intersecting sequences, to model abrasive
wear of polymers.
2. Experimental
2.1. Test apparatus
All scratch tests in this project are conducted using a
laboratory-built scratch tester, which is capable of conducting
single-track and intersecting scratching, on planar specimen
using fixed dead weights. Detail of this apparatus is provided
in our previous work [13]. The specimen is placed on the base
plate, which is mounted onto the displacement stage, and the
specimen is secured by tightening the screw on the base plate.
Normal load is added directly on top of the indenter attached
to the cantilever arm. Scratching is conducted by moving the
stage holding the polymer sample along a single axis at a certain
velocity. The indenter used is a conical diamond indenter with
90◦ included cone angle having a tip radius of 2 ␮m.
Wear tests are conducted using a pin-on-disc setup. The disc
used in this experiment is made of DF3 tool steel of hardness 96
HRB and surface roughness of 1.34 ␮m. The disc attached to a
motor is capable of rotating to a speed of up to 1500 rpm. The
rotational speed set in the experiment is checked by a tachometer.
Strain gauges are attached to the cantilever arm holding the pin
for the friction force measurement during the wear test.
Optical images of the pin surface and the indenter tip are
taken using an optical microscope (Olympus). Post-scratching
and wear analyses are carried out using scanning electron microscope (SEM, model JEOL JSM-5800LV) and a non-contact laser
profilometer (manufactured by NanoFocus AG Germany).
2.2. Experimental procedures
The scratching test is conducted on five different types
of commercially available polymers. It is the purpose of this
project to study the effects of scratching over different classes
of polymers, such as thermoset and thermoplastics. PMMA is
an amorphous thermoplastic. Poly(ether ether keton) (PEEK),
poly(oxymethylene) (POM) and UHMWPE are semi-crystalline
thermoplastic. Epoxy is a thermoset. PMMA, POM and PEEK
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were obtained from a commercial supplier, Goodfellow Cambridge Limited, UK, and they came in extruded cylindrical
form with diameter ranging from 25 to 35 mm. UHMWPE
(GUR1050) was obtained from supplier Perplas Medical Limited, UK, and it came in extruded sheet form.
Epoxy was obtained in the laboratory by adding hardener to
the epoxy resin (Resinfusion 8603 R/H) with a mixing ratio of
15:100. After mixing, the solution was stirred to obtain uniform
mixing and placed into a vacuum chamber to remove air that was
trapped inside the solution. After air was completely removed,
the solution was left to cure at room temperature for 24 h.
All polymers were cut to thickness of 12–15 mm (which is
much greater than 10 times that of the maximum scratch depth)
from the bulk material obtained from supplier. After cutting the
polymer, the test surface was smoothened with abrasive paper
#1000 followed by a final polishing to near-mirror finishing
using alumina particle paste (0.3 ␮m). The sample was then
washed with water and excess water left on the surface was
removed using a clean cotton wool. After polishing and drying,
the sample was placed on the base plate of the scratch tester and
clamped using screws.
Before conducting the scratch test, the indenter tip is cleaned
with alcohol and inspected under optical microscope to ensure
indenter tip is free of any minute particles as unclean indenter
may affect scratching. After cleaning and inspection, the indenter is attached to the cantilever arm. The scratch bed is then
moved to the intended position for scratch test and the cantilever
arm is adjusted to a position that is parallel to the horizontal surface. Weights required are then added onto the cantilever arm.
After the scratch test, the indenter is carefully removed from the
arm and moved to an optical microscope for observation. Wear
debris attached to the tip is then collected using an adhesive
tape. The wear debris collected tape and the scratched polymer
sample are then gold-coated and viewed under SEM and laser
profilometer.
All scratch tests are conducted under a constant normal load
of 0.5 N, constant scratching velocity of 10 ␮m s−1 and total
scratch length of 1 mm. This normal load was necessary to introduce scratch depth which was at least 10 times (or more) the
tip radius (a tip defect) to avoid tip size effect. For intersecting scratch test, orthogonal omni-pass scratching sequence, as
shown in Fig. 1, is adopted as this sequence has been found to
give the maximum amount of wear debris [11,12].
For the pin-on-disc wear test, pins of diameters ranging from
6.35 to 7.2 mm are fabricated from the bulk polymers obtained
from the supplier and epoxy manufactured in our laboratory.
The pin is first weighed using an electronic micro-weighing
machine to obtain initial mass and then placed into the pin
holder and clamped by tightening the screws. The disc is cleaned
with ethanol and after drying, the disc is inspected under optical
microscope to ensure no foreign particles are left on the surface.
The disc is then mounted on the rotary stage. The rotational
speed of the disk is set at 150 rpm and a load of 20 N is applied
on the pin by a pulley system attached to the cantilever. The pin is
then lowered at a position 20 mm from of the center of the disc to
contact the surface of the disc. The linear speed of the disk at the
point of pin contact is 0.314 m s−1 . This was the lowest speed in
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S.K. Sinha et al. / Wear 262 (2007) 1038–1047
Fig. 1. Orthogonal omni-pass scratching sequence. Scratches were conducted by alternating path A and path B.
pin-on-disk that could be achieved in the current system. Further
reduction in the motor rpm did not give a smooth running of the
motor spindle affecting the wear results. Although the scratching speed is low by several orders compared to the sliding speed
in the pin-on-disk test, the comparison between these two test
results is still valid. The dependence of the abrasive wear on
speed becomes influential only for very high sliding speeds. For
the lower side as employed in this work, the frictional heat generated is still low enough to be quickly dissipated by the metallic
disk. On another note, if the sliding test was to be carried out
at the same speed as that of scratch test then measuring wear
could have become near impossible due to the extremely long
time taken for each test. The polymers required a sliding distance of at least ∼100 m or more to observe measurable weight
difference.
After completing the wear test by sliding, the pin is removed
from the pin holder and its surface is observed under optical
microscope. The pin is weighed to obtain its mass after the wear
test in order to calculate the mass difference and the specific wear
rate. The disc is removed from the rotary stage and observed
under SEM for the transfer film formation on the wear track.
The duration of wear test varied for different polymers and
the data are presented in Table 1. The test duration was varied
as the polymers wore at different rates, i.e. for the low wearing
polymers, it was essential to slide the pin longer in order to be
able to measure any mass change in the pin after wear test. Three
runs are conducted for each polymer and the specific wear rates
reported are average of the three runs for each case.
supported in the rear-half of the indenter. A general equation for
scratch hardness is given as follows [5]:
Hs =
4q × Fn
πd 2
(1)
where Hs is the scratch hardness, Fn the normal load applied, d
the scratch width after test and q is a factor which varies from 1,
for elastic materials, to 2 for plastic material (q is taken to be a
mean value of 1.5 in this study as most polymers are viscoelastic
viscoplastic in nature).
For ductile material, such as metals, the volume of material
removed during two-body abrasion by a conical abrasive particle
is given as [14]:
Vr
Fn × tan θ
=
l
π × Hn
(2)
where Vr is the volume of material removed, l the scratch distance, Fn the normal applied load, Hn the normal hardness and
θ is the attack angle.
In practice, the actual volume loss of material will be lower
than Vr given by Eq. (2). This is because only part of the volume
of the wear groove is detached from the bulk material as wear
debris; the rest of the volume loss in wear groove is plastically
deformed making edges of the groove, the so-called material
pileups. A model for abrasive wear was developed by Zum Gahr
[15] to overcome this inaccuracy associated with Eq. (2). This
model essentially defines a new parameter fab , which is given as
follows:
Av − (A1 + A2 )
Av
2.3. Analytical model
fab =
Scratch hardness is the resistance of a material to scratching by another material. The main difference between normal
hardness and scratch hardness is that normal hardness could be
considered as quasi-static where the indenter is uniformly and
symmetrically supported over the contact area. For scratching,
due to the dynamic nature, the indenter is only fully supported
by the material in the front-half of indenter, and, partially or not
where Av is the cross-sectional area of the wear groove after
scratching and A1 + A2 is the sum of the cross-sectional areas of
the material pushed to groove sides as pileups.
It should be noted that fab is equal to unity for ideal microcutting, equal to zero for ideal micro-ploughing and greater
than unity for micro-cracking [15]. Therefore, from Eq. (3), the
volumetric wear loss, Vr , may be given as:
Vr
= fab × Av
l
Table 1
Experimental parameters for the wear tests
Polymer
Speed (rpm)
Duration of test (s)
Sliding distance (m)
PMMA
Epoxy
PEEK
POM
UHMWPE
150
150
150
150
150
300
300
600
600
1200
94.25
94.25
188.50
188.50
376.99
(3)
(4)
Zum Gahr correlated the abrasive wear resistance of several metals with the ratio of the hardness of wear debris to the fab value
of the wearing material and found a linear trend (see p. 154 of
ref. [16]).
The above analyses explain some attempts in the past to relate
scratching with wear for ductile metals. However, for polymers,
no such analysis is available in the literature. This is probably
S.K. Sinha et al. / Wear 262 (2007) 1038–1047
because polymers’ response to scratching is very complex. The
deformation and energy dissipation in polymers involve plastic
deformation with viscous effect and fracturing by crack formation inside the scratch groove. Viscoelastic recovery in polymers
is also much greater than would be possible in the case of metals
[12]. The reason for no relation between scratch hardness (calculated using the width of the scratch groove) and abrasive wear
for polymer prompted us not to use the profile of the scratch
groove for wear volume calculation in scratching. The alternative approach, as adopted in this study, is to account for the wear
debris generated during scratching. Ideally, the volume of each
wear particle could be summed to obtain total wear volume.
However, owing to the extremely small (␮m scale) size of the
wear debris we have estimated the extent of wear by measuring
the surface area of the wear debris. Further details are explained
in the next section.
3. Results
3.1. Scratch test
Fig. 2(a) shows a SEM picture of a single-track scratch on
PMMA and Fig. 2(b) is a picture of the tip taken immediately
after conducting the single-track scratch. Twenty-pass scratching was conducted, meaning that the scratching was done on the
same groove repeatedly for 20 times by drawing the tip in the
same direction.
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Some wear debris, located around the scratch groove, could
be observed on the surface of the sample and some wear debris
is attached to the tip as can be observed from the optical image.
Fig. 3(a) is a SEM picture of a 20-pass intersecting scratch
on PMMA and Fig. 3(b) is the picture of the tip after scratching. Comparing Figs. 2(a) and 3(a), it could be observed that
intersecting scratching produces quantitatively larger amount of
wear debris than the single-track scratching for the same number of passes, i.e. 20 passes (10 passes in each direction in the
case of intersecting scratching). This is in accordance with the
previous studies [11,12].
The groove profile of PMMA at a particular location (for the
case of Fig. 2(a)) is shown in Fig. 4 and the depth of this groove
is estimated to be approximately 24 ␮m.
When comparing a single-pass intersecting scratch and a 20pass intersecting scratch on PMMA surface, as shown in Fig. 5(a
and b), respectively, “wall” formation is observed in Fig. 5(a) (as
pointed by the arrows) but not in Fig. 5(b). It was suggested that
the “walls” could be detached from the groove [12] as a result
of the change in the scratching direction for multi-pass test.
This detachment of the “walls” adds on to the amount of wear
debris from scratching when intersecting scratching sequence
is adopted. It is possible that further wear debris particles are
produced at the intersection as the tip widens the scratch groove
during successive passes by forming and detaching “walls”. The
wear debris formation is facilitated by the micro-cracks that
appear in the scratch groove of PMMA. Thus, toughness and
Fig. 2. (a) Twenty-pass single-track scratch on PMMA. Wear debris were observed on the surface around the scratch groove; (b) tip after 20-pass single-track scratch
on PMMA. Wear debris attached to the tip were observed. Magnification ×100.
Fig. 3. (a) Twenty-pass intersecting scratch on PMMA; (b) tip after 20-pass intersecting scratch on PMMA. More wear debris generated in intersecting scratching
as compared to single-track scratching.
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S.K. Sinha et al. / Wear 262 (2007) 1038–1047
Table 2
Estimated projected wear debris area from 20-pass single-track and intersecting
scratch tests
Fig. 4. Scratch profile of PMMA. The scales for x-axis and y-axis are in ␮m.
low-cycle fatigue properties play a significant role in minimizing
wear debris detachment from the bulk for a polymer as cracking
is delayed for a tough material.
Fig. 6 shows the effects of 20-pass single-track scratching
and 20-pass intersecting scratching on all the polymers used
in this project except for PMMA, which have been shown in
Figs. 2 and 3. From Fig. 6, it is observed that intersecting scratch
produces more wear debris than single-track scratch for all polymers, even though the number of passes for the two type of tests
are same. This may not strictly apply to epoxy, which wore heavily in comparison to other polymers for both types of scratching
sequences, i.e. single-track and orthogonal.
The computation of wear volume produced by the scratch
tests on polymers is extremely challenging because of the very
small quantity of wear debris particles involved. The commonly
practiced change of mass method is not applicable owing to
the high sensitivity required in mass measurement. It was not
possible to collect the wear debris particles and weigh them
separately as there could be large error due to contamination,
besides, the debris particles were clearly visible only on SEM.
Therefore, we have estimated the volume of the wear particle
by measuring the top projected area of each wear debris particle and then summing them up. The assumption here is that
the heights of the wear debris particles are same and hence the
total projected area would be proportional to the total volume
of the wear debris. It is noted here that the wear volume cannot
be estimated by measuring the volume of the scratch groove as
Polymer
Wear debris area from
single-track scratch
(×103 ␮m2 )
Wear debris area from
intersecting scratch
(×103 ␮m2 )
PMMA
PEEK
UHMWPE
POM
Epoxy
25.69
0.18
0.00
1.56
79.16
75.97
25.28
0.00
24.33
167.28
this quantity does not represent actual wear. By definition, wear
constitutes the part of the materials that has separated from the
bulk. The volume of the scratch groove is often partly accommodated as the pile-up on the sides of the scratch and partly as
viscoelastic recovery without the formation of any wear debris.
Moreover, as explained in Section 1, we have avoided the route
of groove geometry measurement for the wear volume estimation in scratching. Therefore, it is important to account for the
actual wear particles which are no more integral part of the bulk
of the polymer. The result of the estimated projected area of
the wear debris is shown in Table 2. From Table 2, it is found
that epoxy generated the largest amount of wear debris both
in the single-track and intersecting scratching. For PEEK, the
wear debris area increases for intersecting scratching by about
220 folds in comparison to that for single-track. Expectedly,
UHMWPE did not produce any wear debris in either of the two
scratching sequences.
The fab values and scratch hardness for all the polymers are
calculated using Eqs. (3) and (1), respectively, at three different locations on the scratch groove and the average values are
presented in Table 3. The fab value is calculated from 20-pass
single-track samples and the scratch hardness is calculated from
Table 3
fab values and scratch hardness of polymers
Polymer
fab
Scratch hardness (MPa)
PMMA
PEEK
UHMWPE
POM
Epoxy
0.440
0.358
0.052
0.374
0.953
346
339
152
332
600
Fig. 5. (a) One-pass intersecting scratch with “walls” visible at the intersection; (b) 20-pass intersecting scratch with walls detached forming wear debris for PMMA.
S.K. Sinha et al. / Wear 262 (2007) 1038–1047
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Fig. 6. Single-track scratch and intersecting scratch on four polymers after 20 cycles of scratching for each. Intersecting scratch generated more wear debris than
single-track scratch for all polymers tested except UHMWPE for which case no wear debris was generated for either of the two cases.
1-pass single-track samples. Epoxy shows high wear debris generation due to micro-cutting and has fab value close to unity after
20 passes. This proves that the concept of fab proposed by Zum
Gahr may be applicable to some polymers. All other polymers
including PMMA have low fab . UHMWPE, which deforms in
a ductile manner, gives fab which is close to zero. The value of
fab for Table 3 were computed after 20 passes of scratching on
a single track, however, if we compare these fab values with the
values obtained after only one pass of scratching, we find some
inconsistencies. The values of fab , after only one scratching pass
on polymers, are very low or negative for many polymers as
was observed in a previous study [12]. However, after 20 passes,
the value of fab increases. This is mainly because after the first
pass there is no further increase in the pileups (represented by
areas A1 and A2 in Zum Gahr’s relation), however, the value
of Av continues to increase due to the wearing actions on the
walls of the groove. Thus, the value of fab becomes positive
and steadily increases with the number of passes. Although the
fab values computed after 20 passes give some reasonable values with regards to their wear performance, it is not clear at this
point if fab can be used as a parameter for wear resistance of polymers. Thus, without further discussion, we may conclude for the
present that modeling wear for polymers using Zum Gahr’s proposed fab parameter is challenging and requires more study. This
parameter, which was originally proposed for metals for single
pass scratching, is not applicable to polymers in its present form
for single-pass scratching.
3.2. Specific wear rate
Three runs are conducted for each polymer to obtain the specific wear rate (mm3 N−1 m−1 ) and the average results of the
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S.K. Sinha et al. / Wear 262 (2007) 1038–1047
Fig. 7. Optical micrographs of the pin surfaces after three-body wear: (a) PMMA, (b) PEEK, (c) UHMWPE, (d) POM and (e) epoxy. Scratch grooves were observed
in all specimens which implies wear mode is of abrasive type. Magnification ×50.
Table 4
Specific wear rate of polymers
4. Discussion
Polymer
Specific wear rate
(×10−6 mm3 N−1 m−1 )
PV value
(Pa m s−2 )
Coefficient
of friction
PMMA
PEEK
UHMWPE
POM
Epoxy
1315.9
31.7
15.5
168.2
3506.6
145,560
149,690
187,138
149,690
153,997
0.48
0.32
0.19
0.32
0.45
three runs are presented in Table 4. Pressure–velocity (PV) values and coefficient of friction are also included in the table for
easy reference.
It is observed that epoxy has shown the highest wear rate,
followed by PMMA, POM, PEEK and UHMWPE. Coefficient
of friction was highest for PMMA followed by epoxy, POM,
PEEK and UHMWPE.
Fig. 7(a–e) shows optical micrographs of the individual polymer pin surfaces after the wear test under a magnification of ×50.
Groove lines are observed on the surfaces of all the pins and this
implies that the wear mode of the polymers is of an abrasive
type as groove formation on the surface is a characteristic of
abrasive wear. Epoxy shows grooves and much localized adhesive wear with flaky wear debris formation; wear rate for epoxy
is much higher than that for other polymers. Fig. 8 presents
the SEM pictures with two different magnifications of the wear
track after the wear test. Transfer film on the counterface was
formed for all polymers. For high wearing materials, such as
epoxy and PMMA, we see some accumulation of the wear
debris on the sides of the wear track in addition to the film
formation.
The scratching of polymers in single-track and intersecting
sequences has clearly shown that the intersecting scratching
gives considerably higher amount of wear debris than the singletrack sequence. The reason for this has been explained in our
previous works [11,12]. It is interesting to observe that the ratio
of the wear data obtained in the two types of scratching for
each polymer is not the same and it is difficult to conclude
any relation between them. However, we may observe that a
material which shows low wear in other types of sliding wear
tests also shows low wear volume in a repeated scratch test.
UHMWPE did not show the formation of any wear debris even in
the intersecting scratching sequence for 20 passes. This is quite
typical of this thermoplastic considering its high wear resistance.
The reason for this wear/scratch resistance is the ability of this
polymer to deform repeatedly without fracture (high toughness;
%elongation at failure ∼4001 ) and generally low friction coefficient (∼0.19). Studies have shown that the abrasion and wear
resistance of UHMWPE can decrease dramatically if the %elongation at failure is reduced, for example, by ␥-irradiation of
varying doses [17,18]. PEEK has also performed well in singletrack scratching, however, in the intersecting sequence, the wear
volume increased by many folds. Again, the influence of %elongation at failure on wear/scratch resistance is obvious. PEEK,
though stronger than UHMWPE in terms of ultimate tensile
strength (PEEK 70–100 MPa; UHMWPE 50 MPa), has much
low value of %elongation (100–150) than that for UHMWPE.
1 The %elongation and strength data for polymers were provided by the supplier.
S.K. Sinha et al. / Wear 262 (2007) 1038–1047
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Fig. 8. Wear debris and transfer film on disc surface after wear test for PMMA, PEEK, UHMWPE, POM and epoxy (SEM images).
There is also a large difference in the coefficient of friction for
UHMWPE (μ = 0.19) and PEEK (μ = 0.32) when tested against
steel surface on pin-on-disk apparatus. PMMA and epoxy have
shown very predictable result for both single-track and intersecting scratching. Both polymers are low ductility and high friction
materials and they wear heavily despite their high ultimate tensile strength compared to many semi-crystalline thermoplastics.
The wear volume is high compared to that for semi-crystalline
thermoplastics. Further point of note is that the polymer with
higher glass transition temperature (Tg ) is expected to behave in
less ductile manner and wear more. Tg is in some way related to
%elongation to failure. However, due to the lack of enough data
based on polymers of different Tg and their wear rates, detailed
discussion on this point is difficult. We do observe some exception if Tg is used as a criterion. For example, PEEK (Tg ∼ 157 ◦ C)
is a low wearing materials whereas POM (Tg ∼ −76 ◦ C) shows
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S.K. Sinha et al. / Wear 262 (2007) 1038–1047
if one takes the route of estimating actual wear debris produced in
a repeated scratch tests, the relative wear performance of several
polymers can be estimated just by conducting scratch test.
5. Conclusion
Multiple-pass single-track and intersecting scratch tests are
conducted on selected polymers. The wear debris area for each
polymer in scratching is compared with the specific wear rates
obtained in pin-on-disk wear test. Following conclusions are
drawn from the above study:
Fig. 9. Total projected wear debris area in scratch test after 20 passes as a
function of specific wear rate in pin-on-disk test. Open square symbols are
for intersecting scratching whereas closed square symbols are for single-track
scratching.
very high wear rate. Thus, without further discussion, we believe
that the glass transition temperature may not be a good indicator of the wear performance of the polymers. The traditionally
used %elongation before failure as one parameter contributing
to wear resistance of polymers in better suited than Tg . This
conclusion agrees well with many studies where abrasive wear
of polymers sliding against metals has been related inversely to
the work of fracture (toughness) [19,20]. In fact, both adhesive
and abrasive wear responses of polymers are fatigue-controlled
phenomena and hence depend upon the toughness property of
the polymers.
As the primary objective of the present paper is to correlate
abrasive wear performance of polymers to scratching characteristics, we present the relevant data in Fig. 9. The total wear debris
area for single-track and intersecting scratching is plotted as a
function of the abrasive wear rate measured in pin-on-disc wear
tests. We observe a direct linear relation between the wear debris
area and the abrasive wear rate. It is interesting to note that the
wear debris area for multi-pass single-track scratching is in better agreement than that for intersecting scratching when related
to abrasive wear. We may conclude that it is not the scratch
or normal hardness (which are calculated using the dimensions
of the scratch groove or indentation mark left behind, respectively) of polymers but rather the actual wear debris formation
in a repeated scratch test, as shown in this study, can provide a
link between the scratch and wear performances of many polymers. The above result confirms that the repeated scratch test
(single-track or intersecting) can be used as a model test for
wear performance of polymers if the total wear debris produced
during scratching is carefully traced and the wear debris areas
summed. The method of measuring wear debris area, as followed
in this study, provides a workable measure of wear in scratching,
however, a better tool for the measurement of wear volume of
the wear debris is required.
As we discussed in the very beginning of this paper, scratch or
normal hardness values do not provide any estimate of the wear
resistance of a polymer. In fact, data are contradictory. However,
1. The projected wear debris area in a 20-pass single-track and
intersecting scratch has shown a linear relation with the specific wear rates for polymers. This proves that scratching can
be employed as a means to model abrasive wear for polymers.
2. Scratch or normal hardness properties, computed in the traditional way for polymers, do not correlate with the wear
performance and hence it is important to devise a way to
measure the volume of wear debris produced in a multi-pass
scratch test (single-track or intersecting) in order to relate
scratching with abrasive wear performance.
3. The Zum Gahr parameter, fab , is not applicable to polymers if
calculated after a single pass scratch. However, some correlation may appear if fab is measured after multi-pass scratching.
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