The authors wish to thank the participating communities, the Laboratory of  Mechanics surfaces and materials processing, University Lille for the access and 84  usage of its research facility. Present research is financially sponsored by Found  for Scientific Research of the Flemish Community (FWO) and the Ghent  University Research Board.The scratching process is a well know concept and is usually defined as a kind of  surface abrasion, where plastic deformation is promoted by relative friction  between soft phase and a hard intender. It is necessary to reduce material loss to minimum or even to reach zero to have an efficient and effective functionality of the materials. Polymers being highly sensitive to wear and scratch damage, their various modes of deformation such as, tearing, cracking, delamination, abrasive  and adhesive vary with a narrow range of contact variables like applied normal load, sliding velocity, interfacial lubrication and testing temperature. This is particularly important when these materials are used to improve the tribological performance by adding various types of fillers such as, carbon fibers, graphite,PTFE, TiO2, and ZnS are added. The polymers with nanocomposites have the advantages over micro- composites from the viewpoint of wear and scratch damage, the underlying mechanism of damage in the single asperity mode is still unclear. The goal of this study is to experimentally evaluate the deformation modes and the friction processes involved during the scratching of polymer reinforced with nanocomposites. The scratches were produced on the semicrystalline polyetheretherketone (PEEK) surface using a Rockwell C diamond indenter was pressed onto the flat surface of each sample, until a complete loadindentation  depth-curve was achieved. These scratched surfaces were assessed  with optical microscope and scanning electron microscope (SEM) for prevailing deformation mechanism and the geometry of damage

DE BAETS, Patrick

IOST, Alain

PEREZ DELGADO, Yeczain

RODRIGUEZ, Vanessa

STAIA, Mariana

SUKUMARAN, Jacob

English

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Vanessa RODRIGUEZ, Jacob SUKUMARAN, Yeczain PEREZ DELGADO, Mariana STAIA, Alain
IOST, Patrick DE BAETS - Scratch evaluation on a high performance polymer - Mechanical
Engineering Letters - Vol. 9, p.76-84 - 2013
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Scratch evaluation on a high performance polymer 
Vanessa RODRIGUEZ1, Jacob SUKUMARAN1, 
Yeczain PEREZ DELGADO1, Mariana STAIA1, Alain IOST2, 
Patrick DE BAETS1 
1Department of Mechanical Construction and Production, Laboratory Soete, Ghent University 
2Laboratory of Mechanics, Surfaces and materials processing, Université Lille,  
Abstract 
The scratching process is a well know concept and is usually defined as a kind of 
surface abrasion, where plastic deformation is promoted by relative friction 
between soft phase and a hard intender. It is necessary to reduce material loss to 
minimum or even to reach zero to have an efficient and effective functionality of 
the materials. Polymers being highly sensitive to wear and scratch damage, their 
various modes of deformation such as, tearing, cracking, delamination, abrasive 
and adhesive vary with a narrow range of contact variables like applied normal 
load, sliding velocity, interfacial lubrication and testing temperature. This is 
particularly important when these materials are used to improve the tribological 
performance by adding various types of fillers such as, carbon fibers, graphite, 
PTFE, TiO2, and ZnS are added. The polymers with nanocomposites have the 
advantages over micro- composites from the viewpoint of wear and scratch 
damage, the underlying mechanism of damage in the single asperity mode is still 
unclear. The goal of this study is to experimentally evaluate the deformation 
modes and the friction processes involved during the scratching of polymer 
reinforced with nanocomposites. The scratches were produced on the semi-
crystalline polyetheretherketone (PEEK) surface using a Rockwell C diamond 
indenter was pressed onto the flat surface of each sample, until a complete load-
indentation depth-curve was achieved. These scratched surfaces were assessed 
with optical microscope and scanning electron microscope (SEM) for prevailing 
deformation mechanism and the geometry of damage.  
Keywords 
scratch, deformation, semicrystalline PEEK, tribological performance, 
nanocomposites. 
1. Introduction
When two materials are sliding against each other under the influence of a 
normal force, usually, the surface of the sharper material loses mechanical 
cohesion and debris is formed that is dislodged from the contact zone resulting 
in wear or scratch damage. It is necessary to reduce material loss to minimum 
amount or even to reach zero to have an efficient and effective functionality of 
the materials. However, polymers are highly sensitive to wear and scratch 
damage, their exhibit various modes of deformation such as, tearing, cracking, 
delamination, abrasive and adhesive even with a narrow range of contact 
variables like applied normal load, slider velocity, interfacial lubrication and 
testing temperature. This is particularly important when these materials are used 
to improve the tribological performance of bearings, coatings, optical, and 
plastics engineering applications for consumer products.  
The advantages to use scratch damage in polymers is because their usage can 
be expanded to other applications such as  electronic, household and automotive, 
where long term esthetics is important of scratch. Another advantages is that 
they can obtain the deformation characteristics for a range of imposed conditions 
(load, speed, temperature, etc) by a simple test (Briscoe, Evans et al. 1996; 
Jardret, Zahouani et al. 1998) and still to understand the friction models such as 
plowing and sliding contributing to friction (Briscoe and Sinha 2003).  
A scratch damage is a mark that forms visible grooves and/or surface damage; 
this is the typical damage mode for surfaces that withstand heavy moving loads 
by swivels or ball bearings. The complexity of the subject is underlined by the 
numerous others factors that influence the material response of polymers to 
scratches; these include scratch loads and speed, coefficients of friction, 
geometry, and number of scratch tips, amount and types of fillers or additives 
(Wong, Lim et al. 2004).  
In this investigation, various types of nanocomposites are added in a high 
performance polymer to improve the tribological properties and the effects of 
nanocomposites in the scratch damage.  These nanocomposites often have a 
deleterious effect on the surface appearance of the polymer due to the poor 
scratch resistance it imparts. The reason for such an effect is still poorly 
understood and it will be shown in this work the usefulness of the current 
method in investigating this effect (J. Jancar 1999). Previous researches (R.A. 
Vaia 2002; A. Sviridenok 2007; Z.Z Yu 2007) generally defined three major 
characteristics and form the basis of performance of polymers fillers such as: 
nanocospically confined matrix polymer chains; nanoscale inorganic 
constituents, and nanoscale arrangement of these constituents. The full use of 
these fundamental characteristics of nano-reinforcements in polymers facilitates 
the achievement of enhanced properties in polymer nanocomposites, which are 
not displayed by their macro- and microcomposites counterparts. Furthermore, 
interfaces between nanofillers and matrix in polymers nanocomposites constitute 
a very high-volume fraction of the bulk material, which is important for bonding 
of filler to matrix. From the tribological aspect, the benefit of polymer 
nanocomposites is that the material removal is expected to be less than the 
micro-sized particle composites since the nano-additives have similar sizes to 
the segments of the surrounding polymers chains. However, for polymers 
nanocomposites, there are diver’s parameters that may control the friction, wear 
and scratch damage and include size, aspect ratio, hardness, nature of 
polymer/particle interface and transfer films that may arise due to the interaction 
of the particles and counterface, leading to the complexity in understanding the 
tribological behaviour of these materials. In this work we are investigated one of 
the previous parameters mentioned to understand the scratch behaviour of a 
polyetheretherketone (PEEK) polymer filled with nanocomposites, such as, 
carbon fibers, graphite and PTFE and to determine if the effects of the those 
nanocomposites affect the scratch behaviour and the material surface, using 
simple scratch test with a progressive load and assessed with optical microscope 
and scanning microscope (SEM) for prevailing deformation and geometry 
damage.  
2. Experimental thecniques
Scratch test device 
Scratch experiments were performed in a scratch tester Millennium developed 
by Tribotechnic with the standard ISO/EN 1071-3, ASTM G171, this test 
method consists of scratching on the surface with a diamond tip on which is 
applied a constant or progressive load. The main criteria are that the scratching 
process produces a measurable scratch in the surface being tested without 
causing catastrophic fracture, or extensive delamination of surface material. It is 
applicable to a wide range of materials. These include metals, alloys, and some 
polymers. Because the degree and type of surface damage in a material may vary 
with applied load, the applicability of this test to certain classes of materials may 
be limited by the maximum load at which valid scratch width measurements can 
be made. When the scratch is concluded, the sample moves under the video 
system, to examine the different finds of damage done by the tip and correlate it 
with load applied.  A Rockwell C diamond tip indenter with a radius of 0.2 mm 
was pressed onto the flat surface of each sample  until a complete load-
indentation was achieve as it can be observed in the figure 1. The scratcher 
moves across the samples with a scratching speed of 10mm/min while 
simultaneously applying a progressive load of 50N. After testing, an optical 
microscopy (OM) and scanning electron microscopy (SEM) was used to 
investigate the scratch surface characteristics.   
Figure 1. Scratch test device 
Materials 
The semi-crystalline PEEK has found a special interest, as it is characterized by 
a comparatively good processability as well as outstanding mechanical 
properties such as toughness and strength. Whereas PEEK has been identified as 
a good tribological polymer in general, have clearly highlighted the influence of 
morphological parameters such as crystallite size and degree of crystallinity as 
well as orientations on the resulting friction and wear performance. In addition, 
few fibers are incorporated in the PEEK matrix to increase the mechanical 
properties, reduced the friction and providing significant enhancement of 
stiffness and strength.  
We have chosen this polymer in the present study to determine the effect of 
the fibers in the evaluation of scratch deformation, two different PEEK semi-
crystalline polymers commercially supplied by INM- Leibniz Institute in 
Germany proprietary materials were selected based on their characteristics: (i) a 
wear grade, 10%vol short carbon fiber reinforced PEEK + 10%vol graphite as 
solid lubricants identified by the name of PEEK-S01 and; a self-lubricating 
grade PEEK with 10%vol PTFE+10%vol graphite + 10%vol short carbon fiber 
identified with the name of PEEK-E02. The mixture of the PEEK with various 
fillers was achieved by twin-screw-extruders by injection moulding with 
standard screw configurations, the polymers samples were presented in sizes of 
70x70x4mm plates and it has been cut to the size of 40x40x4mm in a square 
shape. The average diameter of the particles was 300nm. In the Table 1 presents 
mechanical properties of the studied materials.  
Table 1. Mechanical and thermal properties of the materials used in this work. 
Materials properties 
PEEK+CF+Gr   
(PEEK-S01) 
PEEK+CF+Gr+PTFE 
(PEEK-E02) 
Tensile strength (MPa) 126.8 145 
E- modulus (MPa) 4696.8 11500 
Impact strength (KJ/m2) 7.5 6
Fracture toughness (MPa *m 1/2) 4.6 2
Density (g/cm3) 1.36 1.45
Melting point (°C) 343 343 
3. Results and discussion
Various properties were observed in this study such as penetration depth, 
tangential force, surface damage as a function of load, there are plotted in the 
figure 2 and 5 for the two PEEK filled with composites were shows the 
experimental results of scratching test. In both figure, it can be observed that the 
scratch force is increased as the load increased. From the Coulomb’s law, it is 
evident that there is a linear relationship between scratch force and applied load. 
In previous investigations by Sujeet Sinha (Sinha and Lim 2006) founds that the 
scratch forces for all polymers that their investigated such as polypropylene, 
polycarbonate, polyvinylchloride, polyetheretherketone etc, were very close to 
each other for lower loads, but friction forces increases as the normal load (or 
scratch depth is increased) similar to our results. And they concluded that the 
interfacial friction effects are larger for softer polymer as the depth of scratch 
and hence the real contact will be greater for softer polymer than for harder 
polymer. Thus, the scratch force is adjusted by the indenter tip based on the 
interfacial friction and yield properties of the polymer. This is applicable to all 
polymers which deform in ductile manner. 
The figure 2 is a typical testing curve for polymers under progressive load 
scratch test of the PEEK-E02 sample, where the tangential force curves show 
small magnitude of fluctuations at the beginning and at the middle of the test due 
to the inertial effects of instantaneously accelerating the scratch head to the 
designated scratch speed. But after this, the tangential force increase slightly and 
in a digressive manner to its final level, influences at the end by the movement 
of the indenter. 
Figure 2. Typical course of the scratch loads versus scratch length measure 
in PEEK-E02 with a progressive load of 50N 
Figure 3. Profilometer results of a typical scratch on a PEEK-E02 surface: (a) depth profile with 
clear evidence of the scratch with different positions of the scratch; beginning, middle and end (b) 
3D view of the scratch test 
An evaluation of the scratch produced using profilometry leads to the profiles 
shown in figure 3 (a) the penetration depth with less material deformation at the 
beginning and a progressively increasing residual scratch towards the end of the 
scratch. At this position the scratch depth is also slightly deeper from the normal 
scratch course, due to the back-movement of the scratch head. In the figure 3(b) 
a corresponding a 3D view of the scratch following the original scratch 
direction, so that the deeper valley and the pushed-up hills at the rim of the 
scratch are more visible.  
Additionally a few analysis of scanning electron microscopy images for the 
PEEK-E02 polymer figure 4 and figure 7 for PEEK-S01, were performed to 
understand how material deformation and removal processes take place during 
scratching. This has indications in the understanding of abrasive wear process 
for polymers because abrasive wear mechanism takes places by hard asperities 
or loose debris particles.  
In figure 4, the sample of PEEK polymer filled with carbon fibers, graphite 
fibers and PTFE shows that the surface deforms with the formation of noticeable 
cracks, microcracks, materials removal and some detached debris particles 
within the scratch zone for PEEK-E02 material. In some cases depending upon 
the type of the materials and the severity of the contact, the material deformation 
can take place around the scratch tip with or without wear debris formation 
(Rajesh and Bijwe 2005). 
Figure 4. The scratch surface damage observed using scanning electron micrographs on PEEK-
E02 under the scratching load of 50N and scratching velocity of 10mm/min 
For the PEEK-S01 material filled with carbon fibers, graphite fibers shows 
smaller differences in the curve under the progressive load of the scratch test 
compared with the PEEK-E02 polymer as it can be seen in the testing curve 
under load scratch test figure 5, the appearance of the scratches on the both 
materials is rather similar but in this case with less fluctuations and smooth 
changes during the scratching test.  The scratch frictional force, and contact zone 
size increased almost linearly with applied load.  
Figure 5. Typical course of the scratch loads versus scratch length measure  
in PEEK-S01 with a progressive load of 50N. 
The depth profilometry allows obtaining more precise information on the 
remaining penetration depth during scratching test. In the figure 6 (a) we can see 
the variation of the penetration in the bulk of polymer for the PEEK-S01 filled 
with carbon fibers and graphite it can be seen how is slightly deeper and 
smoothly in the beginning until the end of the scratch test. And it can be notice 
in the figure 6 (b) the 3D images and the penetration depth and the grooves 
formation during the scratching test in the material tested. 
Figure 6. Profilometer results of a typical scratch on a PEEK-S01 surface: (a) depth profile with 
clear evidence of the scratch with different positions of the scratch; beginning, middle and end (b) 
3D view of the scratch test. 
Also, by scanning electron microscopy (SEM) of the PEEK-S01 polymer in 
the figure 7 we can see the contour of the scratched surface when the load was 
further increased. The PEEK-S01 surface deforms with the formation of 
noticeable cracks within the scratch. A regular crack formation was seen, which 
can be due to surface stresses that the microstructure in the semi-crystalline 
polymer were subjected. Comparing SEM images of the PEEK-S01 with the 
material PEEK-E02 it can be observed there is not clear formation of debris of 
the PEEK-S01 than in the SEM images of the PEEK-E02. A well-defined edge 
on the two sides of the scratch groove and a smooth scratch surface are formed 
after scratch pattern.  
Figure 7. The scratch surface damage observed using scanning electron micrographs on PEEK-
S01 under the scratching load of 50N and scratching velocity of 10mm/min 
Conclusions 
In this paper a scratch test method has been evaluated into a polymers filled with 
nanocomposites to determine the effect of the filler into the scratch resistance. 
The progressive load that was applied was 50N at 10mm/min of scratching 
speed. The scratch force was study the trends with increasing normal load and 
the penetration depth of scratch. Further scanning electron microscopic was 
carried out on the scratched polymer surface to investigate the deformation 
material and the removal characteristics. From this investigation we can draw 
the following conclusions:  
Adding different fillers in the matrix of PEEK polymers it do not have 
significantly change in the scratching test and the penetration depth is slightly 
more deeper for the PEEK-S01 than PEEK-E02 this can be attributed that the 
PTFE the solid lubricant its accumulated around the carbon and the graphite 
fiber acting as a smoothing material.  
Observing the SEM micrographs of the scratches on the material illustrates 
cracks, microcraks, material removal and debris in the contour of the scratch 
zone for both types of polymers. 
Acknowledgements 
The authors wish to thank the participating communities, the Laboratory of 
Mechanics surfaces and materials processing, University Lille for the access and 
usage of its research facility. Present research is financially sponsored by Found 
for Scientific Research of the Flemish Community (FWO) and the Ghent 
University Research Board.  
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Scratch evaluation on a high performance polymer

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Scratch evaluation on a high performance polymer

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