Correlation of friction and wear across length scales for PEEK sliding against steel

Correlation of friction and wear across length scales for PEEK sliding against steel

Accepted Manuscript Correlation of friction and wear across length scales for PEEK sliding against steel Xian-Qiang Pei, Leyu Lin, Alois K. Schlarb, R...

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Accepted Manuscript Correlation of friction and wear across length scales for PEEK sliding against steel Xian-Qiang Pei, Leyu Lin, Alois K. Schlarb, Roland Bennewitz PII:

S0301-679X(19)30195-1

DOI:

https://doi.org/10.1016/j.triboint.2019.04.001

Reference:

JTRI 5717

To appear in:

Tribology International

Received Date: 15 February 2019 Revised Date:

25 March 2019

Accepted Date: 1 April 2019

Please cite this article as: Pei X-Q, Lin L, Schlarb AK, Bennewitz R, Correlation of friction and wear across length scales for PEEK sliding against steel, Tribology International (2019), doi: https:// doi.org/10.1016/j.triboint.2019.04.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Correlation of friction and wear across length scales for PEEK sliding against steel Xian-Qiang Pei1,2*, Leyu Lin2, Alois K. Schlarb2,3,4, Roland Bennewitz1

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INM-Leibniz Institute for New Materials, 66123 Saarbrücken, Germany

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Chair of Composite Engineering, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany 3

Research Center OPTIMAS, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany Qingdao University of Science and Technology, Qingdao 266042, P.R. China

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Abstract

The tribological properties of poly(ether-ether ketone ) (PEEK) were investigated at different length scales in order to elucidate commonalities and differences in friction and wear. To achieve this goal, the PEEK/steel tribo-system was studied by block-on-ring , block-on-disc, cone-ondisc, and cylinder-on-disc tests as well as by asperity scratching. For better comparability,

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asperities were prepared from the counter-body steel of the macroscopic experiments. Friction and wear properties were compared on the basis of the pv level. Friction coefficients in macro

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sliding can be related to the interfacial shear strength in asperity scratching by material’s yield pressure. The study confirms that friction and wear of PEEK at different scales can be correlated,

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despite differences of characteristic velocity and pressure in different experiments. Keywords: Multi-scale tribology; Asperity scratching; Shearing; PEEK 1. Introduction

Advantages of polymers, such as low density, high strength-to-weight ratio, self-lubrication, or ease of processability, have made them excellent candidates in solving tribological problems in industry [1]. However, friction and wear are not intrinsic properties of materials, but depend 1

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on the contact configuration, applied load, running velocity, temperature, and lubrication conditions. Prior to application, the tribological performance of polymers is usually studied in different test configurations, e.g. pin-on-disc[2], block-on-ring[3], and thrust bearing

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configuration [4].With these test rigs, the applied load and sliding velocity can be easily varied. It has been recognized that in macroscopic sliding contacts interaction of microscopic asperities plays a significant role for the overall tribological performance [5]. This observation is the

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motivation for investigating tribology of materials also in single asperity experiments [6].

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The increasing importance of polymer tribology has led to the publication of an abundance of tribological data on different polymers. Attention has to be paid that data recorded in millimeterscale contact configurations may not be a valid resource in designing micrometer-scale tribosystems [7]. Such a discrepancy between length scales was reported by Han et al.[8], who registered a micrometer-scale friction coefficient of 0.012 for PEEK using a sharp Si3N4-tip in

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contrast to a friction coefficient of 0.4 registered in macro-scale sliding. Our previous research revealed comparable friction coefficients in macro-scale sliding and single-asperity sliding when a blunt diamond indenter (apex angle 120°, and radius of 200 um) was pressed into and moved

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against PEEK [9]. In a study of the tribology of Ti–MoS2 coatings, Stoyanov et al. demonstrated

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that the micro-scale tribo-performance deviated only slightly from that of macroscopic behavior [10], where the difference was attributed to differences in transfer film thickness. The counterbody material was different for micro- and macro-scale tribological testing in the above mentioned comparison. Since the counter-body material affects the tribological performance of polymers drastically [11], it is usually not adequate to compare tribological properties at different length scale in experiments with different counter-body materials.

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The development of high performance polymeric materials requires a comprehensive understanding of their properties at different length scales [12]. Unfortunately, systematic multiscale studies on polymer tribology are rare. Towards this goal and excluding the effects of

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different counter-body materials, we have produced steel indenters for micrometer-scale scratch experiments from the same steel which was used for macro-scale sliding tests. The selection of PEEK as target polymer was based on its widespread application in components which have to

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withstand macro-scale tribo-loadings [13]. PEEK materials have received increasing attention as candidates for the replacement of titanium materials in dentistry [14], where micrometer-scale

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tribological processes are of great importance [15]. Stimulated by the importance of PEEK for practical tribo-application, we used PEEK as model polymer and investigated its tribological properties at different scales, from millimeter-scale block-on-ring contacts to single-asperity sliding. It is important to point out that the same steel was used for the counterbody (disc, ring

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and indenter) in all tribological tests in the present study, excluding the influence of material differences on friction and wear in our studies on different scales [11, 16]. The goal of our study was as follows: (1) To reveal possible scaling effects in friction and wear of PEEK; (2) To

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elucidate the tribological mechanisms of PEEK associated with each length scale; and (3) To

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establish a correlation of tribological properties across different scales. 2. Experimental

Commercially available poly (ether ether ketone) (PEEK, VESTAKEEP® 2000G) plates were provided by Evonik Industries AG, Germany. The plates were cut into different sizes for different testing configurations, which are listed in Table 1 and shown in Fig.1.

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In parallel to standard macro sliding tests in the configuration of block-on-ring [17] and blockon disc [18], macro sliding tests were also conducted at low pv conditions of 2 MPa and 0.1 m/s as described in Table 1. The total sliding distance for these measurements was 10 km, within

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which a steady state was reached according to a preliminary study. Similar low pv tests were also done with small PEEK samples (cylindrical pins or conical pins with flat ends), which were tested on a pin-on-disc machine (CSEM, Switzerland) under dry sliding conditions against

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100Cr6 axial needle bearing washers (Schaeffler Technologies AG & Co. KG, Germany). The friction forces were divided by the normal force to calculate the friction coefficient. In the case

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of conical pins with flat-ends, the contact pressure decreased due to the increased nominal contact area. The radius of the sliding circle was set to 16 mm, which was the same as in standard block-on-ring tests. After tribo-tests, the PEEK worn surfaces were observed using a Keyence VHX-2000D optical microscope (Keyence, Japan). The disc surfaces were inspected in

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a scanning electron microscope (Quanta 400 FEG ESEM, FEI) in order to characterize the morphology of transfer films.

Table 1: Information on tribological testing configurations for PEEK (yellow color) against

Description

ring

Schematic

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block-on-

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100Cr6 steel in the form of ring, disc and indenter. Instrument

Range of p and v

nominal contact

home-built

1-4 MPa,

square 4×4 mm2

tribometer

0.5-4 m/s

4

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square 4×4 mm2

home-built

1-4 MPa,

disca

tribometer

0.5-4 m/s

cylinder-on-

CSEM

2 MPa,

disc

tribo-meter

0.1 m/s

cone-on-

CSEM

Initial pressure 2

1 mm diameter,

disc

tribo-meter

MPa,

which increased

0.1 m/s

to ca. 1.6 mm

CSEM scratch 1 MPa,

flatb

tester

micro

square 4×4 mm2

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CSEM scratch 100-200 MPa,

Apex

tester

120.2°

0.0001-0.01 m/s

angle:

Radius:124.4 µm Intermittent

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scratching

afterwear

0.01 m/s

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asperity

1 mm diameter

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block-on-

single

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block-on-

multi-pass sliding

Note: Data for block-on-disca and block-on-flatb were previously published in [18] in a study on velocity dependence. In the current manuscript, friction coefficient data was used for correlation across length scales as a function of the pv level.

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(b)

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(a)

1000µm

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100 µm

250 µm

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200 µm

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(d)

(c)

Fig.1 PEEK samples with different size and geometry and the indenter for single asperity micro scratching: (a) for block-on-ring or block-on-disc tests: sample with cross-sectional area of 4

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mm×4 mm, (b) 100Cr6 steel indenter, (c) for cylinder-on-disc tests: small sample with circular

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cross-section of diameter ca. 1 mm, and (d) for cone-on-disc tests: 90o cone shaped sample with circular cross-section of initial diameter ca. 1 mm. Single asperity micro scratching tests were conducted on a micro scratch tester (CSEM, Switzerland). The indenter was a 100Cr6 spherical tip with half apex angle of 120.2° and radius of 124.4 µm (Fig.2). Friction and wear were measured by multiple-pass uni-directional scratching in the same track, which was repeated for 100 cycles. Constant loads of 1 N, 2 N and 6

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3 N and velocities between 100 µm/s and 1000 µm/s were applied. During each stroke of scratching, the lateral force and penetration depth were recorded. The residual depth was measured in an additional stroke at low load. The scratch friction coefficient was calculated as

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the ratio between lateral force and normal load. The residual width of wear tracks was measured using an optical microscope (Keyence VHX-2000D, Japan). After sputter-coating with a gold layer, the scratched PEEK surfaces were observed in a scanning electron microscope (Quanta

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400 FEG ESEM, FEI) to reveal the tribological mechanisms.

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3. Results and discussion

3.1 Effects of sample size and geometry on the friction and wear of PEEK Fig.2 presents the effects of sample size and geometry on the friction and wear properties of PEEK under a pv level of 0.2 MPa m/s. Under such a low pv factor, the influence of frictional

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heating can be neglected [19]. As can be seen from the evolution of the friction coefficient with increasing sliding cycles (Fig.2a), sample size and geometry did influence the friction coefficient of PEEK against steel. The most prominent difference was observed between the block-on-ring

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configuration and the other three configurations. The friction coefficient was 0.46 for the blockon-ring and 0.36 for the other configurations. It is interesting to note two distinctions of the

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block-on-ring configuration: the curved contact interface which is associated with a larger geometric contact area and the absence of movement in a fixed plane, which is a characteristic of the three block-on-disc experiments. The increase of geometric contact area for the curved contact in the block-on-ring configuration was only 1.2% of the 4 × 4 mm2 cross-section, we therefore suggest that movement induced material smearing along a circular track was responsible for the different friction coefficient in block-on-disc tests. Effects of the sample size 7

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were observed in the three block-on-disc tests with different contact areas only during the first 60000 sliding cycles, after which the friction coefficient reached a steady state and its value did not differ between the sample sizes. The steady friction coefficient was 0.36, which is similar to

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that reported for PEEK sliding against steel in reciprocating tests of a cylinder-on-disc

(a)

4.0

Height loss rate (nm/cycle)

2 MPa, 0.1 m/s

0.5

0.4

0.3

block-on-disc block-on-ring cylinder-on-disc cone-on-disc

0.2

0.1

3.5 3.0 2.5

(b)

2 MPa, 0.1 m/s

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Friction coefficient

0.6

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configuration [20].

2.0 1.5 1.0 0.5 0.0

0

20000

40000

60000

80000

Number of sliding cycles

100000

block-on-disc

block-on-ring cylinder-on-disc cone-on-disc

Contact configuration

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Fig.2 Friction and wear of PEEK tested in different configurations and for different sample sizes. The dependence of height loss rate on experimental configuration and sample size is more

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complicated (Fig.2b). The block-on-ring configuration gave the lowest wear rate and the cylinder-on-disc test led to the highest one. Similar wear rates were found for block-on-disc and

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cone-on-disc contact configurations. This distinct difference in wear rate is reflected in the different morphologies of the worn PEEK surfaces and of the corresponding transfer films (Fig.3). For the block-on-disc tests at low pv level, parallel ploughing furrows covered the worn surface of PEEK, in which wear debris was compacted and stuck (Fig.3a). Correspondingly, patches of layers were formed on the counterpart surface (Fig.3b).

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With same sample size but different contact geometry, block-on-ring tests resulted in peeling off of large patchy films from the worn surface due to fracture of PEEK material (Fig.3c). In contrast, the dominant mechanism for developing the morphology in the block-on-disc

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configuration was shearing (Fig.3a). The peeling off of surface layers was initiated from crazing and occurred only after the accumulated strain reached a critical level [21]. In this process, more material was kept in the interface for a longer time and less was transferred to the counterbody

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surface (Fig.3d). As a result, the block-on-ring configuration exhibited higher wear resistance of and a higher friction coefficient as stated before (Fig.2a). The different morphology of the worn

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surfaces leads us to the conclusion that different friction mechanisms were associated with the different contact configurations, i.e. tearing and fracture with block-on-ring and shearing with block-on-disc configurations. According to the provider of the PEEK material, the tensile yield stress is 100 MPa, which is much higher than its interfacial shear strength in sliding, which was

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reported to be 15 MPa by Briscoe et al [22] and 40 MPa as we deduce in the following part. This difference explains the higher friction coefficient in block-on-ring tests compared to block-ondisc tests.

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Bearing in mind that rotating contact is the common aspect among block-on-disc, cylinder-ondisc and cone-on-disc configurations, the influence of sample size on wear performance can be

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discussed based on results from these three configurations. When block samples were replaced with cylinders or cones, the worn surfaces became smoother with fine scratches aligned in parallel (Fig.3e and g), and wear debris was expelled outside of the interface as can be inferred from the pieces left on the worn surface. On the counter disc surface, transfer layers from the smaller sized samples (cylinder and cone) were also patchy (Fig.3f,h). Careful comparison (e.g. area enveloped by the black ellipses with same area in Fig.3) between the three sample 9

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geometries with nominal flat contact surface revealed that more fine particulate material stuck to the steel surface between the patchy layers in the case of block samples (Fig.3b) compared to those of cylinder and cone specimens (Fig.3f,h). Considering the fact that wear particles could be

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retained in the sliding interface before they were ejected out of contact [23], there would be a longer distance for debris to move for big sample size than smaller ones. Conclusively, effects of sample size on wear performance were associated with the dynamics of wear debris migration in

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the sliding interface. In the presents study, the smaller sample size (cylinder and cone) led to less interaction between debris and counterpart surface and, as a result, fewer fine particle attachment

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as stated before.

(b)

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(a)

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(c)

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50 µm

20 µm

(d)

100 µm

20 µm

10

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(e)

(h)

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20 µm

100 µm

(g)

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(f)

100 µm

20 µm

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Fig.3 Morphology of worn surfaces (left column) and of corresponding transfer films formed on the counter steel surfaces (right column) for different contact configurations: (a,b) block-on-disc, (c,d) block-on-ring, (e,f) cylinder-on-disc and (g,h) cone-on-disc. Tests were performed with a

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contact pressure of 2 MPa and a sliding velocity of 0.1 m/s to a total sliding distance of 10 km.

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3.2 Friction and wear of PEEK in single asperity micro scratching Fig.4 presents the friction and wear characteristics of PEEK against a single steel asperity. The steady-state friction coefficient was constant and exhibited no significant variation with load. At a velocity of 100 µm/s, the steady friction coefficient was 0.33, similar to the value reported for macro-scale sliding in the foregoing part. Constant friction coefficients for varying normal load were also reported for continuous sliding of PEEK against steel [24]. The friction coefficient 11

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decreased slightly with the increasing velocity. A dependence of PEEK/steel friction coefficients on velocity rather than on applied load has been reported before [25]. While the friction coefficient was practically constant for the applied loads and velocities, the height loss rate

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increased with increasing normal load (Fig.4b), consistent with macro-scale sliding tests [26]. No significant dependence of the wear rate on the velocity was found in the studied normal load

100 um/s

500 um/s

1000 um/s

40

(a)

Height loss rate (nm/cycle)

0.35

30

0.25 0.20

(b)

100 um/s 500 um/s 1000 um/s

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Friction coefficient

0.30

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range.

20

0.15 0.10

10

0.05 0.00

0

2

Normal load (N)

3

1

2

Normal load (N)

3

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1

Fig.4 Tribological properties of PEEK in single-asperity micro scratching: (a) Friction coefficient and (b) Height loss rate for increasing normal load at different velocities.

was

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To analyze the micro-scale wear mechanisms of PEEK, the morphology of scratched surfaces analyzed and is shown in Fig.5. After sliding at a low speed of 100 µm/s, the scratch

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grooves exhibited fine scratches parallel to the testing direction and wear debris was stuck to the worn surfaces in form of aggregates (Fig.5a,c). When the velocity was increased to 1000 µm/s, no wear debris aggregation was observed at 1 N load (Fig.5b vs. Fig.5a), while small patches started to detach and leave the contact interface (Fig.5d vs. Fig.5c).

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(a)

50 µm

(d)

20 µm

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(b)

50 µm

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50 µm

Fig.5: Morphology of worn surfaces of PEEK after 100 cycles of single-asperity scratching

µm/s.

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under 1 N (upper row) and 3 N (lower row) at different velocities: (a,c)100 µm/s and (b,d)1000

The morphology of the worn PEEK surface is correlated to the characteristics of the transfer films formed on the steel indenter shown in Fig.6. Hardly any transfer films were formed on the asperity surface at the lowest velocity (100 µm/s, Fig.6a and Fig.6c). With reference to Fig.5a and Fig.5c, it can be concluded that the wear debris stuck on the PEEK surface and that only 13

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little debris was transferred to the asperity surface. Consequently, the friction process took place between PEEK and steel asperity even in repeated sliding. This situation changed with increase of velocity. A ten times higher velocity led to significant formation of transfer films on the

(Fig.5b,d).

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asperity (Fig.6b,d), corresponding to the removal of debris from the PEEK worn surface

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(d)

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(c)

100 µm

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100 µm

100 µm

100 µm

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Fig.6 Morphology of PEEK transfer films on the asperity surface after 100 cycles of scratching under 1 N (upper row) and 3 N (lower row) at different velocities: (a,c) 100 µm/s, (b,d) 1000

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µm/s. The real, elliptical contact area between single-asperity contact and groove was calculated by applying a method we established in a previous study [27]. Fig.7a plots the contact area as

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function of the normal load for different velocities. The contact area increased with increasing normal load, a trend observed in continuous sliding [11] and single asperity scratching

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measurements [28]. An increase of velocity from 100 µm/s to 500 µm/s had no influence on the contact area, a minor decrease was observed for higher sliding velocity of 1000 µm/s. The resulting contact pressure is shown in Fig.7b. Between 100 µm/s and 1000 µm/s, the variation of contact pressure with normal load was marginal. A constant pressure of 100 MPa was representative for these testing parameters. This value is close to the yield strength of PEEK

Fig.5.

100 um/s 500 um/s 1000 um/s

0.020 0.015 0.010 0.005

1.0

1.5

2.0

Contact pressure (MPa)

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0.025

140

(a)

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Contact area (mm2)

0.030

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[29], which is consistent with the appearance of scratch grooves with well-defined boundary in

130

(b)

100 um/s 500 um/s 1000 um/s

120 110 100 90 80

2.5

1.0

3.0

1.5

2.0

2.5

Normal load (N)

Normal load (N)

15

3.0

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1.2

0.8 0.6

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Lateral force (N)

1.0

(c)

100 um/s 500 um/s 1000 um/s

0.4 0.2 0.010

0.015

0.020

0.025

Contact area (mm2)

0.030

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0.005

Fig.7. (a) Calculated contact area and (b) contact pressure in single-asperity/PEEK contact as a

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function of normal load for different sliding velocities. (c) Lateral force versus contact area at different velocities for single-asperity scratching measurements.

Fig.7c shows the relation between lateral force and contact area in single-asperity sliding studies. According to Bowden and Tabor [30], a linear relation between lateral force and contact

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area indicates a shearing friction mechanism and the slope of this plot can be considered as the shear strength of the tribo-system. In the velocity range between 100 µm/s and 1000 µm/s, the resulting shear strength is 40.3 MPa.

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3.3 Correlation of tribological properties of PEEK in different scale

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The tribological properties of polymers depend on the pv level, i.e. the product of normal pressure and velocity, under which the tribo-system is running [1]. A potential way of correlating tribo-data recorded at different length and load scales is an analysis as function of pv conditions. Fig.8 compares the friction coefficient of PEEK sliding against steel at different scales. Considering the lower limit of pv level in micro-scale scratching tests (0.01 MPa m/s), friction value at the same pv factor in block-on-flat configuration is taken as a reference for comparison, 16

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which was tested at 0.01 m/s and can fill the velocity gap from micro-scale ( till 1000 µm/s ) to macro-scale ( from 0.1 m/s ) tribo-tests. Below this reference friction lie data points for asperity scratching and block-on-disc measurements. The single-asperity level friction coefficient was

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around 0.32 in a pv range between 0.01 MPa m/s and 0.1 MPa m/s, and the block-on-disc configuration exhibited friction coefficient value around 0.37 for pv levels ranging from 0.2 MPa m/s to 4 MPa m/s. It is also worthy of noting that the friction coefficient did not depend on the

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nominal contact size at the tested pv level of 0.2 MPa m/s for block-on-disc, cylinder-on-disc and cone-on-disc configurations. When the contact configuration was shifted from block-on-disc to

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block-on-ring, all the friction coefficients were increased to values above the reference, which

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was around 0.5 for pv levels ranging from 0.1 MPa m/s to 4 MPa m/s.

Fig.8 Summary of friction coefficients as a function of pv level tested at different length scales. For the experimental details refer to Table 1. The overview in Fig. 8 demonstrates the small but characteristic differences between experimental configurations. In particular, friction coefficients for single-asperity scratching are a little lower and friction coefficients for the block-on-ring configuration are a little higher than 17

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those for the different block-on-disc configurations, which exhibit a constant friction coefficient over the whole range of pv levels. The lower friction value for single-asperity scratching can be explained by the elastic recovery at the trailing edge of the sliding contact, which is probably less

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effective for the roughness asperities of an extended steel contact because of the interaction of adjacent asperities and because of the stress exerted by the smooth surface around asperities. For an explanation of the higher friction coefficient in the block-on-ring configuration, we refer to

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two additional observations. In contrast to all other configurations, the friction coefficient does not approach a stable value even in long experiments (see Fig. 2a). The observed fluctuations are

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believed to reflect the instability of a dense tribofilm (see Fig. 3d) which causes the intermittent increase in friction.

The value of the friction coefficient can be understood in terms of a relation between mechanical material properties suggested by Shooter and Tabor [31] for the frictional properties

μ=

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of polymers:

shear strength 40.3 MPa = ≈ 0.4 (1) effective yield pressure 100 MPa

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For the single-asperity configuration, we have extracted the shear strength of 40.3 MPa in the

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previous section. The value of 100 MPa for the effective yield pressure was provided by the material manufacturer. The resulting friction coefficient is close to the values measured in our experiments, not only in single-asperity scratching but also in experiments with extended contacts of blocks. The similarity in friction coefficients indicates similar mechanisms. In the case of steel sliding against PEEK, the dominant mechanism appears to be asperity friction. This conclusion is further supported by the similarity of transfer film structure on and around asperities for scratching experiments and experiments with extended contacts. Fig.9 shows the 18

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morphology of transfer films on steel counterparts in more detail. As already observed in Fig. 3, the transfer films were composed of aggregated debris protruding out of the surface surrounded by thinner layers. The detailed images in Fig. 9 reveal a close similarity in the micro structure of

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the transfer film patches from different experiments, namely stacks of flake-like debris (Fig.9). This similarity indicates that roughness asperities in continuous sliding function in the same way as the single asperity in scratching experiments. Consequently, the PEEK material is exposed to

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similar loading and shearing conditions microscopically within the real contact area and thus exhibits very similar friction coefficients for all experiments. The interaction of steel asperities

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with PEEK constitutes the mechanism of correlation between the friction characteristics at different length scales.

(b)

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(a)

5 µm

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5 µm

Fig.9: Details of the transfer film morphology on the steel counter surface in asperity scratching and continuous sliding: (a) steel indenter-on-PEEK, 1 N, 1000 µm/s, 100 cycles; (b) PEEK cylinder-on-disc, 2 MPa, 0.1 m/s, 10000 m. The wear of PEEK in different testing configurations is compared by plotting its height loss rate as function of pv levels in Fig.10. The height loss rate tended to increase with increasing pv 19

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factor for the different configurations, also in agreement with reports for continuous sliding of PEEK [24]. Interestingly, the wear rates can be divided into two groups as indicated by the line in Fig.10 according to path for debris movement during sliding. For linearly confined paths

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(block-on-ring and block-on-flat), the height loss rate was lower than for those with circular paths which increased the possibility of debris expelling (block-on-disc, cylinder-on-disc and cone-on-disc). Under the latter configurations, the wear debris moved more along and out of the

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track on the counter-body surface, which led to a higher wear rate. When the effects of sample size is considered, cylinders with the smallest nominal contact area exhibited much higher height

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loss rate compared to cones and blocks with larger contact area under the same pv conditions. This is understandable because wear debris can leave smaller contacting interfaces on a shorter path, which results in the higher wear rate. These tendencies observed for the wear rate as function of contact size and of path for wear particle movement are still the result of a complex

roughness asperities.

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interplay between debris migration, shear stress in the contact, and interaction with adjacent

Different wear rate values can be registered for the same pv levels depending on pressure and

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velocity combinations. For example, the highest wear rate value for the block-on-disc configuration at 2 MPa m/s was caused by the highest pressure (4 MPa) compared to only1 MPa

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for the other data points. Finally, we mention that the rate of increase in scratch depth for the single-asperity micro-scratching experiments is about ten times larger than the corresponding height loss rate of extended contacts in Fig.10. The larger height loss rate in single-asperity scratching fits well to the observation of scratches within the wear tracks on PEEK shown in the left panels of Fig. 3.

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Fig.10 Effects of pv level on the height loss rate measured in different configurations. 4. Conclusions

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Friction and wear of PEEK sliding against steel were studied at different length scales and configurations with different nominal contact size, and even including single-asperity contact. Agreement in friction coefficient between macro-scale sliding and single asperity contacts was

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achieved by preparing the microscopic indenter from the same steel as used in all experiments and by running the experiments for a high number of scratching cycles. The overall friction

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coefficient equals the shear strength, extracted from single-asperity results, divided by the material’s effective yield pressure. The height loss rate increased with increasing pv level, linking the regimes of the different experimental configurations. Regarding wear performance, rotating contact and smaller contact area led to higher height loss rate at otherwise comparable conditions, which can be ascribed to the role of wear debris mobility at the interface. 21

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Acknowledgments The authors acknowledge financial support of the German Research Foundation (Deutsche

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Forschungsgemeinschaft) on the projects BE 4238/7-2 and SCHL 280/22-2, Evonik Industries AG, Germany, for the donation of the experimental materials, and thank Eduard Arzt for the continuous support of this project. The authors are also grateful to Karl-Peter Schmitt of INM for

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ACCEPTED MANUSCRIPT Highlights 1. Integral study on friction and wear of PEEK was conducted across length scales from macro sliding to asperity scratching.

scratching by the material’s yield pressure.

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2. Macro-scale friction coefficients can be related to the interfacial shear strength in asperity

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3. Mobility of wear debris was revealed to play a critical role in the wear properties of PEEK.