Friction and wear behavior of PEEK and its composite coatings

CHAPTER 19

Friction and wear behavior of PEEK and its composite coatings Ga Zhang, Hanlin Liao and Christian Coddet

Contents 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8

Introduction Coating procedures Characterization of the coating crystalline structure Adherence of Coatings Correlation between the coating crystalline structure and its tribological behavior Effects of sliding condition on the tribological behavior and Mechanisms of amorphous PEEK coating Tribological behavior of nano-SiC (7 wt%)-filled PEEK coatings Conclusions References

458 459 460 462 463 465 475 479 480

Abstract This chapter describes a general approach to the friction and wear behavior of poly-ether-ether-ketone (PEEK) coatings under dry sliding conditions. First of all, two efficient coating techniques, thermal spraying and painting, are introduced. Then, the crystalline structure and adhesive property of PEEK coatings are characterized. The correlation between the crystalline structure and the tribological characteristics of PEEK coatings is clarified. In addition, the effects of the sliding conditions, such as sliding velocity, applied load and ambient temperature, on the tribological behavior of an amorphous PEEK coating are studied. Based on 3D morphology analyses of worn tracks and scanning electron microscope (SEM) observations of worn surfaces, the friction and wear mechanisms of the amorphous PEEK coating are discussed. Finally, the tribological behavior of nano-SiC (7 wt%)-filled PEEK coating is described. The roles of the nano-SiC particles on the coating tribological behavior are discussed.

19.1 Introduction In order to improve the surface performance of metallic parts, e.g., erosion resistance, friction coefficient and wear resistance, polymer coatings are nowadays more and more studied. Lots of smart coating designs have been proposed and many coating techniques such as thermal spraying [1–4], painting [5], spinning [6], suspension spraying [7] and electroplating [8] etc., were used to deposit polymer-based coatings on metallic substrates. According to the apparent forms of feedstock materials, these coating methods can be categorized into two types: solid feedstock such as powders and liquid feedstock such as suspensions or solutions. Using polymers, lots of remarkable results were achieved, in which the coated substrate showed a significant improvement in surface performance. As a result, polymer coatings attract more and more

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industrial interests, especially in the automotive fields. For example, polyamidimid/graphite coatings [8] deposited on engine pistons permit tight clearance, noise and friction reduction and thereby fuel economy. It can be predicted that long-term efforts will be carried out in the future on polymer coatings, in which the coating method and coating composition will be the most important issues. PEEK is a thermoplastic with an excellent toughness–stiffness combination and therefore it becomes one of the most attractive thermoplastics [9]. It is being increasingly used as bearing and sliding material in industrial applications due to its excellent thermal stability and good tribological performance, especially high wear resistance [10–12]. The good tribological performance of PEEK promotes the development of PEEK coatings on metallic substrates, especially lightweight alloy that generally presents poor tribological performances. In recent years, PEEK and its composite coatings were prepared on such substrates using thermal spraying and painting techniques etc. [5, 13–17]. PEEK and PEEK/SiC coatings deposited on aluminum were shown to reduce significantly the friction coefficient and wear rate of the substrate [5, 17]. Previous studies showed that thermally sprayed PEEK coatings deposited on engine piston skirts led to a reduction in fuel consumption of about 3% [18]. In this chapter, the preparation, crystalline structure and adhesive strength of PEEK coatings are introduced. It is shown how the crystalline structure and the sliding conditions, such as applied load, sliding velocity and ambient temperature, influence the dry sliding behavior of PEEK coatings. There is also an attempt to discuss the friction and wear mechanisms of an amorphous PEEK coating using the morphological information of the worn tracks. Finally, the behavior of nano-SiC (7 wt%)-filled PEEK coating is described.

19.2 Coating Procedures Thermal spraying and painting are two effective means for depositing PEEK coatings. In a previous study [1], the preparation of PEEK coatings by atmospheric plasma spraying (APS), high-velocity oxygen-fuel (HVOF) and flame spraying (FS) was studied. The results showed that the APS process led to a degradation of the polymer owing to the high temperature of the plasma jet. PEEK coatings deposited using HVOF had an enormous roughness due to the high blowing pressure of the high-velocity flame. FS was observed to be more suitable to the preparation of PEEK coatings as homogenous and dense coatings could be obtained. In order to obtain a dense coating, before spraying, the substrate was preheated up to a temperature higher than the melting point of PEEK, e.g., 400 ◦ C. During spraying, the PEEK powder was injected into the flame jet, where it was melted (or partially melted) and propelled to the substrate surface to build the coating. After spraying, the coating was quenched into cold water. The corresponding coating exhibited an amorphous structure and a good adherence to the substrate [16]. The infrared spectra (IR) indicated that very little degradation of PEEK occurred after flame spraying [19]. Due to the poor fluidity and agglomeration nature of the thin fillers required for preparing composite coatings, it is difficult to homogenize them in the feedstock powder. As a result, a homogeneous composite coating is difficult to be achieved. The homogeneity of the composite coating could be improved by a mechanical milling of the mixed powders before spraying [20]. With optimized milling parameters, the thin fillers, typically ceramic particles could be embedded in the surface layer of polymer grains. Painting is also an economic and effective method for depositing PEEK-based coatings on a substrate with regular configuration [21]. Employing this procedure PEEK, or PEEK and fillers, in case of composite coatings, were firstly dispersed into an aqueous medium. The mixture was then continuously stirred and then subjected to an ultrasonic vibration for reaching a uniform dispersion. Then, the slurry was applied evenly on the substrate. After being dried, the substrate-coating system was heated up to 400 ◦ C and held at this temperature for 5 min, and then quenched into cold water. Figure 19.1 shows examples of cross-sectional structures of the obtained PEEK, PEEK/micron-SiC and PEEK/microngraphite coatings. Dense coatings were obtained using this process.

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Resin

(a)

Resin

(b)

PEEK PEEK

SiC 40 m

Substrate

40 m Substrate

Resin

(c)

Graphite PEEK Substrate

40 m

Fig. 19.1. Cross-sectional structures of (a) PEEK, (b) PEEK/SiC and (c) PEEK/graphite coatings.

19.3 Characterization of the Coating Crystalline Structure The crystalline structure of PEEK coatings was shown to be of great significance for their mechanical properties [16]. Therefore, an investigation on the effect of the crystalline structure on the tribological behavior is important for understanding this behavior. However, it seems not possible to prepare amorphous bulk PEEK material because of its rapid crystallization speed and its small thermal conductibility [16, 22]. Meanwhile, amorphous PEEK samples, with thicknesses less than 1,000 μm, could be obtained using coating techniques, in which a quenching from the molten state of PEEK is performed [16]. Moreover, the crystalline structure of these samples could be controlled by general annealing treatments of these amorphous coatings. Thus, PEEK coatings with different crystalline structures were obtained by a combination of flame spraying process and thermal treatment procedures. PEEK coatings, with final thicknesses of 500 μm, were prepared by a flame spraying process. The PEEK feedstock powders had a mean diameter of 25 μm. The final coating roughness, Ra was about 0.8 μm. In order to obtain different crystalline structures, the as-sprayed coating, referenced as T1, was subjected to heat treatments. The annealing temperatures and the holding times are listed in Table 19.1. The treated coatings are referenced from T2 to T7 correspondingly. The coating structures were characterized by differential scanning calorimetry (DSC) measurements and X-ray diffraction (XRD) analyses. It is well known that in an amorphous polymer, the molecules range themselves randomly. When the polymer is heated at a temperature higher than its glass transition temperature (Tg ), some of the molecules organize themselves to form crystalline grains, for example, spherulites in case of PEEK [23–25]. Figure 19.2(a) shows the DSC scan of the as-sprayed coating (T1). A marked glass transition is apparent near the Tg (143 ◦ C) and it is followed by a marked exothermic crystallization peak. This confirms that the as-sprayed coating exhibits an amorphous structure [23]. Figure 19.2(b) and (c) show the DSC scans of coatings heat treated at different temperatures (T2–T5) and for different holding times (T4, T6–T7). Their glass transitions, compared to that of the as-sprayed coating, become less pronounced. These coatings do not exhibit any exothermic peak above Tg . Accordingly, it can be deduced that coatings T2–T7 exhibit semi-crystalline structures. On the DSC scans of the heat-treated coatings, in addition to the main melting peak, a minor peak appears a few degrees below the main peak temperature. The location and the shape of the main melting peak remain unchanged, while those of the smaller peak change. With an increase in the annealing temperature, this small endothermic peak shifts to a higher temperature and exhibits a larger enthalpy. With an annealing temperature of 260 ◦ C, an increase in the holding time leads to a similar behavior.

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Table 19.1. Annealing temperatures and holding times of PEEK coatings. Sample code

Annealing temperature (◦ C)

Holding time (min)

T2 T3 T4 T5 T6 T7

180 220 260 300 260 260

30 30 30 30 1 10

(b)

(a)

T2 (180 C, 30 min) Relative heat flow

Relative heat flow

T3 (220 C, 30 min) T4 (260 C, 30 min) T5 (300 C, 30 min)

0

50

100 150 200 250 300 350 400

0

50

Temperature (C)

100 150 200 250 300 350 400 Temperature (C)

(c)

Relative heat flow

T6 (260 C, 1 min) T7 (260 C, 10 min) T4 (260 C, 30 min)

0

50

100 150 200 250 300 350 400 Temperature (C)

Fig. 19.2. DSC scans of (a) as-sprayed coating, (b) coatings annealed at different temperatures T2–T5 and (c) coatings annealed with different holding times T4, T6–T7.

According to the stepwise crystallization model [24–27], two crystalline entities are produced in the spherulites of semi-crystalline thermoplastics during an isothermal annealing. In the early stage of crystallization, lamellar stacks consisting of thick lamellae appear in the amorphous region and they constitute the frame of the spherulites. However, the regions between these thick lamellae stay amorphous. Later, lamellar stacks with thinner lamellae form in these amorphous regions. The coexistence of these dual lamellae could explain the double melting behavior of coatings T2–T7. The low-temperature peak on the DSC diagram would result from the melting of the thinner lamellae in

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Tribology of Polymeric Nanocomposites (b)

Intensity (CPS)

30,000

T1 (without treatment) T2 (180 C, 30 min) T3 (220 C, 30 min) T4 (260 C, 30 min) T5 (300 C, 30 min)

25,000 20,000 15,000 10,000

T6 (260 C, 1 min) T7 (260 C, 10 min) T4 (260 C, 30 min)

25,000 20,000 15,000 10,000 5,000

5,000 0 10

30,000 Intensity (CPS)

(a)

15

20

25 20 (C)

30

35

40

0 10

15

20

25 30 20 (C)

35

40

Fig. 19.3. XRD patterns of the coatings (a) T1–T5 and (b) T4, T6–T7.

the semi-crystalline coatings. Thus, the increase of the temperature and the enthalpy of this peak with the annealing temperature and the holding time would be related to the thickening of the lamellar stacks consisting of thin lamellae. A structural evidence of this is provided by the XRD patterns of the coatings T1–T7 (Fig. 19.3). The X-ray patterns demonstrate that the as-sprayed and the annealed coatings possess amorphous and semi-crystalline structures, respectively. Increasing the annealing temperature and holding time increase slightly the diffraction intensity. Thus, it can be concluded that the increase of the two factors, in the studied range, increases slightly the coating crystallinity.

19.4 Adherence of Coatings In industrial applications, the coating adhesive strength is one of the most important parameters. Amorphous PEEK coatings exhibit a good adherence to the substrate [28]. Annealed coatings adhere poorly to the substrate. This feature is more enhanced by the coating thickness. Figure 19.4(a) shows the cross-section of the interface between an amorphous PEEK coating and an aluminum alloy substrate. It can be noticed that the coating has a good contact with the substrate. After heat-treating (180 ◦ C, 30 min), the coating separates from the Al substrate in certain zones and cracks are observed at the interface. Figure 19.4(b) shows the coating/substrate interface obtained after a thermal treatment at 220 ◦ C. In order to observe the contacting features of the treated coating to the substrate, the coating was peeled off from the substrate and the contacting surface was observed with a stereoscopic microscope (Fig. 19.4(c)). Besides the interfacial cracks, some spherical pores are observed at the interface. These pores are open to the interface and therefore the liquid resin filled them during the preparation of the cross-sectional samples (Fig. 19.4(b)). The reduction in the coating adhesive strength, after annealing, seems to be related to the residual stresses produced during the isothermal treatment. The densities of amorphous and semi-crystalline PEEK are 1.26 and 1.32 g/cm3 , respectively [22]. Therefore, crystallization will provoke a volume contraction of the coating. With this contraction, tensile residual stresses will appear in the coating. Meanwhile, the mechanical resistance of PEEK, for example the shear strength, decreases enormously above the glass transition temperature [29]. Accordingly, these residual stresses are large enough to break the bonding between the coating and the substrate. Thus, a relative movement occurs at the interface. This might explain the cracks observed at the coating/substrate interface. When increasing the annealing temperature above 220 ◦ C, the mechanical resistance of the coating is significantly reduced. At the early stage of annealing, the residual stresses might break the bonding between the coating and the substrate at some locations (Fig. 19.5(a)). Then, these zones might become free surfaces for the

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(b)

(a)

Porosities

Coating Coating

50 m

Substrate

Resin

50 m

Substrate

(c)

100 m

Fig. 19.4. Coating/substrate interfaces of (a) amorphous coating (T1), (b) semi-crystalline coating (T4) and (c) contacting surface of semi-crystalline coating (T4). (a)

(b) Coating

Coating

Substrate

Substrate

Fig. 19.5. Schematic representation of the interfacial pore growth.

following volume contraction. As illustrated in the schematic representation (Fig. 19.5(b)), under an isotropic residual stress, the small debonded zones might develop into spherical pores at the end of the crystallization process due to the reduced mechanical properties.

19.5 Correlation between the Coating Crystalline Structure and its Tribological Behavior The friction tests were performed on a Ball-On-Disc (BOD) tribometer under a room environment (temperature about 20 ◦ C and humidity about 70%). The counterpart consisted of a 100 Cr6 ball with a 15 mm diameter and a mirror-finished surface (Ra = 0.02 μm). The applied load and sliding velocity were, respectively, 15 N and 0.2 m/s. A 500-m relative sliding distance was performed, during which the friction force was measured with a sensor and was dynamically recorded into a computer. The friction coefficient was computed as the friction force divided by the applied load. The wear rate is defined as the worn volume per unit of applied load and sliding distance [9]. The inverse of wear rate is wear resistance. In this chapter, the cross-section of the worn tracks was obtained with a TaylorHobson Surtronic 3P profilometer (Rank Taylor Hobson Ltd., UK) after the completion of frictional

Tribology of Polymeric Nanocomposites

T1: without treatment; T2: 180 °C, 30 min; T3: 220 °C, 30 min; T4: 260 °C, 30 min; T5: 300 °C, 30 min; T6: 260 °C, 1 min; T7: 260 °C, 10 min

Friction coefficient

(a) 0.39 0.36 0.33 0.30 0.27 T1

T2

T3 T4 T5 Coating code

T6

(b) 25.0 Wear rate (106  mm3/N/m)

464

T1: without treatment; T2: 180 °C, 30 min; T3: 220 °C, 30 min; T4: 260 °C, 30 min; T5: 300 °C, 30 min; T6: 260 °C, 1 min; T7: 260 °C, 10 min

22.5 20.0 17.5 15.0 12.5 10.0 7.5 5.0 T1

T7

T2

T3

T4

T5

T6

T7

Coating code

Fig. 19.6. (a) Friction coefficient and (b) wear rate of studied coatings. (b)

21

23.0

18

22.5

Hardness (HK)

Hardness (HK)

(a) 24

15 12 9 6

22.0 21.5 21.0 20.5

3 0 T2 T1 No treatment 180 °C

T3 220 °C

T4 260 °C

Coating code

T5 300 °C

20.0 0

5

10

15

20

25

30

Holding time (min)

Fig. 19.7. Evolutions of coating hardness versus (a) the annealing temperature and (b) the holding time.

sliding. The cross-section area of the wear tracks timing the perimeter permitted to obtain the total worn volume. At least 10 measurements were performed for computing the mean value of the wear rate. Figure 19.6(a) and (b) show the friction coefficient and the wear rate of the studied coatings. The error bars represent the standard deviation of the obtained data. Compared with the amorphous coating, the semi-crystalline coatings exhibit a slightly lower friction coefficient (0.325 in average vs. 0.358) and a significantly reduced wear rate (11.53 in average vs. 19.5×10−6 mm3 /N/m). However, in the studied range, for the heat-treated coatings, the friction coefficient and wear rate show little dependence on the annealing conditions. Of course, an increase in the coating hardness, as a result of crystallization, could contribute to the improvement in the tribological performances. Figure 19.7(a) and (b) show the hardness of the as-obtained and the annealed coatings. Heat-treating significantly increases the coating hardness. However, in the studied range, the increase in annealing temperature and holding time only leads to a slight increase in the coating hardness. Correlated with the analysis on the coating crystalline structure, it can be inferred that semi-crystalline coatings exhibit a significantly higher hardness than the amorphous one. However, in the studied range, the increase in the crystallinity gives only a slight increase in the coating hardness. The semi-crystalline PEEK coatings could exhibit the mechanical properties of a

Friction and Wear Behavior of PEEK

(a)

465

(b)

Fig. 19.8. SEM micrographs of worn surfaces of (a) T1 and (b) T4.

composite material consisting of an amorphous matrix and spherulites [30]. The dispersed spherulites in the amorphous matrix could restrict the motions and slippages of the polymer chains [9]. Figure 19.8(a) and (b), respectively, show the worn surfaces of T1 and T4 coatings. Obvious plastic deformation and serious plows are observed on the worn surface of the amorphous coating (Fig. 19.8(a)). For the semi-crystalline coatings, the plastic deformation is reduced and a relatively smooth worn surface is observed (Fig. 19.8(b)). For a given load, compared with the amorphous coating, the semicrystalline coatings exhibit lower perpendicular deformations. The shear force between the two contact bodies and the plows on coating surface are decreased [31–33]. Moreover, the tribological behavior of a polymer can be related to its viscoelastic property [34, 35]. As aforementioned, semi-crystalline PEEK coating may be considered as a composite material. The coating stiffness can thus be increased and the mechanical loss module and the loss factor of polymers can be reduced after crystallization [30]. Under a shear force, during the relative sliding, the spherulites embedded in the amorphous matrix would restrict the motions and slippages of the polymer chains and thus decrease the internal energy dissipation in the semi-crystalline surface layer involved in the frictional process.

19.6 Effects of Sliding Condition on the Tribological Behavior and Mechanisms of Amorphous PEEK Coating The tribological behavior of a friction couple cannot be characterized just by intrinsic materials properties. The properties as well as the interactions of the system as a whole have to be taken into account [36]. Besides the material natures of the friction pairs, the frictional conditions could play an important role on the tribological behavior. Friedrich’s group [10, 37] also found that the dynamics associated with the tribological testing was very important for the tribological behavior of polymers. The product of applied pressure and sliding velocity, pv, was thus highlighted as an important factor in evaluating the tribological performance of polymer. In addition, a number of factors, such as adhesion between the sliding pairs, the interfacial temperature, the strain rate at which the polymer surface layer is deformed etc., may affect the frictional work and the material loss mode [31]. A great diversity, with respect to the results obtained by several researchers, in investigating the effect of velocity on the tribological behavior of polymers, is observed [5, 38–40]. Moreover, in polymer tribology, the relationship between the friction force and the applied load often deviates from a linear relationship [5, 10]. Temperature is also an important factor in determining polymer tribological behavior. The results of Lu and Friedrich [10] indicate that PEEK (semi-crystalline) exhibits the largest friction coefficient and wear rate near its Tg . Hanchi et al.

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(a)

(b) 180 Wear rate (106  mm3/N/m)

Friction coefficient

0.44 0.40 0.36 0.32 0.28 0.24 5N

1N

0.20 0.2

0.5

9N 0.8

150 120 90 60 30 0

1.1

Sliding velocity (m/s)

1.4

1N 5N 9N

0.2

0.5

0.8

1.1

1.4

Sliding velocity (m/s)

Fig. 19.9. Dependences of the (a) friction coefficient and (b) wear rate on the applied load and sliding velocity.

[40] also reported that PEEK and short carbon fiber-reinforced PEEK presented the highest friction coefficients near the Tg of PEEK. Therefore, the effects of the sliding conditions, i.e., sliding velocity, applied load and ambient temperature, on the tribological characteristics of PEEK coating were studied in this section.

19.6.1 Experimental details The amorphous PEEK coatings considered in this section were obtained using the painting technique. The feedstock powders (supplied by Victrex Scales Ltd.) had a mean diameter of 10 μm. The coatings had a 40-μm thickness. The mean surface roughness, Ra, of the coating was 0.3–0.4 μm. Friction tests were performed on a BOD CSEM tribometer (CSEM, Switzerland). The counterpart consisted of a 6 mm diameter 100 Cr6 steel ball with a mirror-finished surface (Ra = 0.02 μm). For investigating the influences of the sliding velocity and applied load on the tribological behavior of the amorphous PEEK coatings, the frictional tests were performed under laboratory environment (temperature about 20 ◦ C and humidity about 70%). The applied load and sliding velocity varied from 1 to 9 N and from 0.2 to 1.4 m/s, respectively. The sliding distance of all frictional tests was 2,000 m. When investigating the effects of temperature on the tribological behavior of the PEEK coatings, the applied load and sliding velocity were 9 N and 0.2 m/s, respectively. The frictional tests were performed at temperatures ranging from 20 to 200 ◦ C. The sliding distance of frictional tests was 1,000 m. Before the tests, the samples were heated to the prescribed temperature in a furnace attached to the tribometer.

19.6.2 Dependence of the tribological behavior on the sliding velocity and applied load Figure 19.9(a) and (b) illustrate the dependences of the friction coefficient and wear rate on the applied load and velocity, respectively. The tribological characteristics of amorphous PEEK are sensitive to the variations of sliding velocity and applied load. The sliding velocity and applied load play coupled roles on the tribological characteristics of the PEEK coating. Under 1 N, the increase of the sliding velocity, from 0.2 to 1.4 m/s, results in a practically monotonic decrease of the friction coefficient. Under 9 N, the increase of the velocity from 0.2 to 1.1 m/s induces an increase of the friction coefficient. On the other hand, a sharp drop of the friction coefficient occurs when the sliding velocity is further increased from 1.1 to 1.4 m/s. Comparatively, under 5 N, the friction coefficient exhibits a maximum value at 0.8 m/s. From Fig.19.9(b), it can be seen that the wear rate does not follow completely the

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same tendencies as those of the friction coefficient. In contrast with the friction coefficient, under 1 N, the wear rate monotonically increases when the sliding velocity is increased from 0.2 to 1.4 m/s. On the contrary, under 9 N, the wear rate presents a sharp drop when the sliding velocity is increased from 1.1 to 1.4 m/s, in correlation with the evolution of the friction coefficient. As mentioned above, the sliding velocity and applied load seem to play coupled roles on the tribological characteristics. In order to clarify the role of each factor, the other has to be considered simultaneously.

19.6.2.1 Tribological mechanism under 9 N Under a high pressure, the tribological behavior of amorphous PEEK is closely related to its viscoelastic behavior. The increase of sliding velocity leads to distinct modifications of tribological mechanisms. Figure 19.10(a) shows the 3D morphology of the worn track (indicated by arrows) generated under 9 N at 0.2 m/s (expressed hereafter as (9 N, 0.2 m/s)) after completion of 2,000 m sliding. The sliding direction of the counterpart ball is indicated on the image. It can be clearly seen that the worn track is not continuous and that some periodic material stacks with parabolic shapes appear along the sliding direction. Figure 19.10(b) shows the SEM observation of the ridge of a stack. Figure 19.10(c) shows the top-view of the worn track and the geometric profile of the track center along the sliding direction (along the dashed line). It can be noticed that the distance between two stack ridges (expressed hereafter as “period”) is about 1.4 mm (7/5 mm). These morphological features suggest that viscous flow of PEEK, as a result of creep, constitutes the main characteristic of the worn track. Following the two-term non-interacting friction model proposed by Briscoe [41], the surface layer, involved in the frictional process, was classified into two terms: the interface zone and the cohesive zone (Fig. 19.11). The former, with a depth of about 100 nm, could mainly determine the friction work dissipation caused by the adhesion force between the two friction pairs. The dissipation caused by the

(a)

(b)

70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

Worn debris

Worn track Sliding direction

5 m

m

(c)

15 0 0 5

10

WT1

15 0

0.5

0

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7 mm

Fig. 19.10. Characterization of a worn track generated at (9 N, 0.2 m/s): (a) 3D morphology of the work track; (b) SEM observation of a stack ridge, in which the arrows indicate the climbing and descending directions of the counterpart ball and (c) top-view and geometric profile of the center of the track along the sliding direction.

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Tribology of Polymeric Nanocomposites P

Material accumulation

Interface zone

V

Cohesive zone

Fig. 19.11. Schematic representation of material accumulation ahead of the sliding ball. The classification of surface layer is done according to Briscoe’s model [41].

plastic plow and hysteresis loss could be rather related to the cohesive zone, with a depth comparable with the contact length. The intermittent periodic nature of the worn track, indicated in Fig. 19.10, can be described as the result of a stick-slip motion between the two sliding counterparts, and a strainhardening effect of the polymer. For most of the polymers, the Van der Waals and hydrogen bonds are typical factors for the junctions occurring between the two counterparts [42]. Formation and rupture of these junctions control the adhesion component of friction. In the stick-slip process, due to the adhesion force in the interface zone, the PEEK surface presents a larger motion than the sub-surface material and, thereby material accumulation ahead of the counterpart occurs (Fig. 19.11). Meanwhile, the stress applied by the ball on the PEEK surface is such that the sub-surface material is also deformed. During the stick stage, the tangential stress is smaller than the critical stress, which is related to the adhesion between the sliding pairs and the contact configuration, but it increases with time. Once the stress applied on the polymer surface exceeds the critical stress [43, 44], the slip stage initiates and runs until the stress decreases to below the critical stress, when the ball and the PEEK surface stick again. The piling-up of the material ahead of the ball is completed during the slip stage. The material piling-up leads to a strain-hardening effect on the polymer by the entanglements of the molecular segments, which in turn increase the tensile strength of the polymer in the stacking zone [43]. During a 2,000 m sliding, the repeated stick-slip and strain-hardening effects are the determining parameters for the track morphology. In this case, the viscous resistance could determine the friction force. In molecular scope, at a low strain rate, the long molecules in the sub-surface zone could, to some degree, harmonize themselves to the horizontal deformation of the surface. The low hysteresis loss could explain the low friction coefficient in this condition. The adhesion between the two counterparts and the piling-up of the polymer might be the dominant friction mechanisms. For the wear mechanism, it might be described as a “transfer” mode in which case the interfacial bonding between the two counterparts is stronger than the cohesive strength of the PEEK [31]. The polymer material might be transferred to the counterpart surface progressively in the form of very small fractions. Finally, the accumulated material on the counterpart surface could fall off in large pieces (wear debris marked in Fig. 19.10(a)). When the sliding velocity is increased, the periodical nature of the worn track becomes less obvious. For (9 N, 0.8 m/s), the worn track becomes practically continuous. Figure 19.12(a) and (b) illustrate the 3D morphology of the worn track generated under these conditions and the distinct difference of the top-views of this worn track (WT2) with that obtained for (9 N, 0.2 m/s) (WT1). The worn surface of PEEK generated at (9 N, 1.1 m/s) is presented in Fig. 19.13(a). Figure 19.13(b) presents the framed zone with a higher resolution. Besides plows, rippled-liked folds are observed on the worn surface (Fig. 19.13(b)). These folds are assumed to be formed by the significantly superior deformation of the surface parallel to the sliding, compared with that of the sub-surface material.

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m

(a)

(b)

50 45 40 35

WT2

WT1

30 25 20 15 10 5 0

Fig. 19.12. (a) 3D morphology of the worn track generated under the conditions (9 N, 0.8 m/s); (b) comparison of top-views of the worn track generated for (9 N, 0.8 m/s) (WT2) with that generated for (9 N, 0.2 m/s) (WT1). (a)

(b)

Fig. 19.13. (a) Morphology of a worn surface generated at (9 N, 1.1 m/s); (b) observation of the framed zone in (a) with a higher resolution.

It can be inferred that, in the friction process, the motion of the sub-surface material is much smaller than that of the surface. When considering the aforementioned analyses, it can be deduced that, under 9 N, in the velocity range of 0.2–1.1 m/s, the speed dependence of the friction coefficient can be closely related to the viscous resistance of the polymer. At an increased stress rate, the higher viscous resistance gives rise to the friction coefficient. In the molecular scope, the high friction coefficients at intermediate velocities could be related to a less accordant motion of the molecules, in which case the entanglements and cuttings between the molecule segments could be significant. In such a case, the hysteresis loss could be important. In other words, this might be consistent with the higher loss modulus and loss tangent of the polymer at intermediate frequency [45–47]. To conclude, at intermediate velocities, plow and hysteresis loss could be the dominant friction mechanisms [31, 41]. Correspondingly, the combination of plow and peeling effects could be the dominant wear mechanism. Figure 19.14(a) shows the worn surface generated at (9 N, 1.4 m/s) and Fig. 19.14(b) presents the framed zone with a higher resolution. The well-defined micro-plows on the worn surface imply that micro-cutting seems to be the dominant friction and wear mechanism. In the range of high velocities, elastic behavior is prevalent in the contact zone [31, 48, 49]. In this case, the shear strength contributes mainly to the friction force. Moreover, it could be assumed that the increase in the sliding velocity might diminish the thickness of the cohesive zone by reducing the depth of stress distribution.

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(a)

(b)

Fig. 19.14. (a) Morphology of worn surface generated at (9 N, 1.4 m/s); (b) observation of the framed zone in (a) with a higher resolution.

Fig. 19.15. Morphology of a worn surface generated at (1 N, 0.2 m/s).

The above discussions suggest that in polymer tribology, from a certain pressure, there could be two critical sliding velocities. The first critical value corresponds to a velocity lower than that for which serious viscous flow can take place; the second corresponds to a velocity higher than that for which the hysteresis loss is no longer a significant component contributing to the friction force. The critical speed could be related to the material nature, e.g., molecule feature, crystalline structure. Between these two critical values, there could be a speed at which the polymer exhibits a high friction coefficient and wear rate.

19.6.2.2 Tribological mechanism under 1 and 5 N Under a 1 N load, no obvious material stacking was observed on the worn track. Figure 19.15 presents the morphology of the center of the worn track, after 2,000 m sliding for (1 N, 0.2 m/s). Interestingly, many little holes are noticed on the worn surface, some of which are indicated by black arrows. Under these conditions, the “ironing” effect could be a determinant factor for the tribological behavior [21, 50]. During the sliding process, the asperities on the coating surface are shocked by the counterpart. With repeated effects, fatigue could occur near these asperities and finally, the corresponding microzones are eliminated from the matrix. Accordingly, debris and holes develop on the worn surface.

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Fig. 19.16. Morphology of a worn surface generated at (1 N, 1.1 m/s).

Fig. 19.17. Morphology of a worn surface generated at (1 N, 1.1 m/s).

Consequently, the debris could contribute to a three-body abrasion in the following sliding process. Squeezed by the sliding ball under a low applied load, the debris can roll or be pushed to the center of the contact zone, where the deformation of PEEK perpendicular to the surface is the most significant [21]. Finally, the debris could be crushed in the plows. When increasing the sliding velocity, more serious plows are observed on the worn surface (Fig. 19.16). It is assumed that, under a small load, the interfacial temperature is a crucial factor determining the tribological characteristics [21]. As a result of the low thermal conductivity of PEEK, friction-induced heat surely provokes an increase of the contact temperature; thus, an increase of the sliding velocity can result in a higher contact temperature [32]. Due to the effect of thermal softening caused by friction-induced heat, the friction coefficient decreases as a result of the increased velocity, while the wear rate is increased. Under 5 N, both the viscoelastic behavior and the contact temperature can play important roles simultaneously on the tribological behavior [21]. Figure 19.17 presents the SEM observation of a worn surface generated at (5 N, 0.2 m/s). Above a critical pressure, the worn debris adheres well to the counterpart and is pushed together with the sliding counterpart (Fig. 19.18). A prow is pushed ahead of the debris and the material on the polymer surface is continually displaced sideways to form ridges adjacent to the developing groove [31]. The increase of the sliding velocity leads to a higher

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Fig. 19.18. Schematic illustration of the formations of prows and plows. (a) 0.45

(b) 240 210 Wear rate (106  mm3/N/m)

Friction coefficient

0.42 0.39 0.36 0.33 0.30 0.27

180 150 120 90 60 30 0

0

25

50

75 100 120 150 175 200 225 250

Temperature (°C)

20

50

80

120

160

200

Temperature (°C)

Fig. 19.19. Dependences of the (a) mean friction coefficient and (b) wear rate on ambient temperature.

interfacial temperature and in turn, aggravates the surface plows. Moreover, the hysteresis loss could also contribute to the variation of the friction force, to some degree, depending on the sliding velocity. From the above discussions, it can be inferred that there is a critical applied load, lower than that for which the “ironing” effect dominates the sliding process. In this case, no relative motion between the surface and sub-surface materials occurs and hence, no obvious viscous flow takes place. The interfacial temperature plays an important role on the tribological performance. Above a critical value, the viscoelastic behavior of the polymer surface layer plays an important role on the tribological behavior. This critical value could be closely related to the contact geometry and the in-depth deformation of the polymer. Such aspects as polymer surface morphology, hardness, elastic modules and Poisson’s ratio could affect this critical value.

19.6.3 Dependence of the tribological mechanism on the ambient temperature Figure 19.19(a) and (b) show the dependences of the mean friction coefficient and wear rate on the temperature. It can be seen that the temperature has a significant effect on the coating tribological performance. The friction coefficient increases with increasing ambient temperature and reaches a maximum value at 160 ◦ C (near Tg ). A further increase in the temperature, from 160 to 200 ◦ C, leads to a slight decrease of the friction coefficient. Like the friction coefficient, the preliminary increase in ambient temperature increases the wear rate. However, when the temperature is increased from 120 ◦ C to 160 ◦ C, a sharp drop of the wear rate was noticed. When the ambient temperature is further increased to 200 ◦ C, the coating exhibits a maximum wear rate. The comparison of the 3D morphologies of the worn tracks obtained at 20 ◦ C (WT3) and 50 ◦ C (WT4) are presented in Fig. 19.20. It can be noticed that the increase of temperature enlarges the distance between the periodic material stacks (as an average from ∼1.4 mm to ∼2.2 mm). According to the stick-slip motion feature, the increase in the distance between two periodic material stacks is indicative

Friction and Wear Behavior of PEEK

WT4

WT3

473 m 45 40 35 30 25 20 15 10 5 0

Fig. 19.20. 3D morphologies of the worn tracks obtained under 20 ◦ C (WT3) and 50 ◦ C (WT4). (a)

(b)

Fig. 19.21. (a) SEM observation of the worn track T4; (b) ridge zone framed in (a) with a higher resolution.

of the greater time required for the critical stress to be reached before slip occurs [43]. This could be attributed to an increased adhesion, between the two sliding pairs, and to a decrease in coating stiffness. Figure 19.21(a) shows the SEM observation of the worn track WT4, generated at 50 ◦ C. It can be observed that the ridge of the material stack exhibits a narrower width. Figure 19.21(b) shows the zone framed in Fig. 19.21(a) with a higher resolution. In this zone, scale-like parabolic folds, perpendicular to the sliding direction, are observed (indicated by dashed arrows). These folds might be formed by a significantly superior parallel deformation of the interface zone than that of the sub-surface. Strain hardening occurring in this zone, which increases the tensile strength of the polymer, is an indispensable factor for the formation of such folds. It should be noted that, outside the ridge zone, no fold is observed along the worn track. In molecular scope, the molecular entanglement occurring in this zone might limit the motions of the molecules. As a result, when the PEEK surface is deformed along the direction of the sliding ball, the sub-surface presents a much smaller deformation. When the temperature is further increased to 120 ◦ C, the distance between two material stacks is increased to 3.8 mm, averagely. The increase of temperature also results in serious plows on the worn surface. The thermal softening of PEEK caused by the temperature elevation is responsible for this characteristic. At 160 ◦ C, the coating exhibits the largest friction coefficient, but the smallest wear rate. The XRD analysis, after frictional test, indicates that the coating possesses a semi-crystalline structure [21]. Above Tg , at the beginning of the sliding process, the coating crystallizes rapidly following a stepwise mode [16]. As mentioned in the last section, crystallization is of great significance for the frictional

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(b)

(a)

(c)

3 m

Fig. 19.22. (a) SEM observation of a worn surface obtained at 160 ◦ C; (b) SEM observation of the worn surface with higher resolution and (c) SEM observation of the framed zone in (b).

process. The stick-slip effect is alleviated and the distance between the periodical material stacks is reduced (averagely, 1.2 mm), due to the increased coating stiffness. Figure 19.22(a) and (b) show the SEM observations of the worn surface at 160 ◦ C and Fig. 19.22(c) illustrates the framed zone in Fig. 19.22(b) with a higher resolution. White lines, perpendicular to the sliding direction and corresponding to fatigue damages, are observed along the worn track. In addition, parallel compact folds are noticed along the worn surface. From Fig. 19.22(c), it can be seen that friction fatigue initiates from the brim of the compact fold (indicated by arrow). Fatigue is known to be a change in the material state due to repeated stressing which results in progressive fracture. Its characteristic feature is an accumulation of irreversible changes, which gives

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Fig. 19.23. SEM observation of worn surface obtained at 200 ◦ C.

rise to generation and development of cracks [31]. For semi-crystalline PEEK, the increased stiffness could reduce the material deformation in the sub-surface zone. In molecular scope, the spherulites could restrain the movements of polymeric chain segments; thereby stress concentration might develop around the spherulite. The shearing stress in the surface layer and the structure feature of the spherulite could be responsible for the form and direction of the fatigue crack. Under repeated effects, friction fatigue occurs from the brim of the folds, where the maximum tangential stress or the tensile strain might take place (Fig. 19.22(c)). At 160 ◦ C, the highest friction coefficient could be ascribed to the high hysteresis loss of PEEK. Near Tg , the high mobility of the polymer segments, in the layer involved in the frictional process, could lead to a pronounced increase in the energy dissipation. This is consistent with the claim that polymer exhibits the highest loss tangent at the vicinity of Tg [51]. The increased mechanical strength caused by crystallization decreases sharply the wear rate. In this case, the “transfer” speed of PEEK on counterpart is decreased. Fatigue damage and mild plow constitute in this case the main wear characteristic. The increase in ambient temperature from 160 to 200 ◦ C decreases slightly the friction coefficient, while it increases enormously the wear rate. Above Tg , an increase in the temperature degrades sharply the mechanical strength of PEEK [22]. The SEM observation of the worn surface gives evidence that enormous deformation occurs during the sliding process (Fig. 19.23). This characteristic permits to conclude that, under a load of 9 N, a temperature of 200 ◦ C is out of the service temperature range.

19.7 Tribological Behavior of Nano-SiC (7 wt%)-Filled PEEK Coatings In order to improve the friction and wear behavior of polymeric materials, a typical concept is to reduce their adhesion to the counterpart material and to enhance their hardness, stiffness and compressive strength [9]. This can be achieved quite successfully by using fillers. Several studies have already been conducted on the tribological performance of PEEK-based composite materials, aiming at developing the composite concept and at understanding the roles of the fillers in the polymer matrix. It was thus proved that the incorporation of various fillers, especially nano-sized fillers, in a PEEK matrix might improve its tribological properties, especially the wear resistance [10–12]. Some delicate compositions of PEEK-based composites [9–11, 34, 40, 52, 53] were tailored. Xue et al. [51, 52] studied the

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(a)

(b)

Fig. 19.24. Morphology of the nano-SiC particles with (a) low and (b) high resolution.

tribological behavior of SiC-filled PEEK coatings. Their results indicate that the incorporation of nanoSiC particles in the PEEK matrix reduces the friction coefficient and increases the wear resistance. With a concentration of 7–10 wt% nano-SiC, the composite exhibits the best tribological performance. This was confirmed in our own work [5]. More generally, polymer-based composite coatings were intensively studied in the recent past. For example, the incorporation of MoS2 , graphite, SiC, SiO2 and polyfluo wax (PFW) appears beneficial to the tribological behavior of polyurethane (PU) coatings [7, 54], while the addition of nano-alumina particles into nylon-11 coating improves the scratch resistance of the polymer coating [20]. In this section, the tribological behavior of 7 wt% nano-SiC-filled PEEK coating was described and the roles of the nano-SiC particles were discussed.

19.7.1 Experimental details The coatings were prepared by the painting technique using PEEK powders with a mean particles diameter of 10 μm. Because of the quenching process, the composite coatings exhibited an amorphous structure [21]. The SiC particles had a mean diameter of 50 nm. Figure 19.24(a) and (b) show the morphologies of these particles with different resolutions. It can be clearly seen that these nano particles present an agglomerated nature. The frictional test conditions were identical to those described in Section 6.2.

19.7.2 Results and discussions 19.7.2.1 Coating structure Figure 19.25 shows the cross-section of a nano-SiC-filled PEEK coating. The white spots correspond to the SiC-rich zones. Even after mechanical mixing and ultrasonic bathing, the nano-SiC particles were not perfectly dispersed in the coating. During the coating preparation process, the nano-SiC particles are distributed on the surface of the PEEK particles and particularly in their interspaces. This occurred even at temperatures higher than the melting point of PEEK, due to the high viscosity of PEEK and the agglomerated nature of the SiC particles. A reduction in the polymer particle dimension would probably improve the dispersion state due to the increased specific surface area and the decreased interspace dimension. The dispersion state of the nanofiller particles in the PEEK matrix is not reported in most references. However, it can be assumed that this dispersion state is of great significance for the coating properties including its tribological behavior. Further efforts to improve the dispersion of nanofillers in thermoplastics and epoxies are still a subject of current and future studies [9].

Friction and Wear Behavior of PEEK

477

5 m

Fig. 19.25. Cross-sectional structure of a nano-SiC-filled PEEK coating. (a) 0.50

(b) Wear rate (106  mm3/N/m)

Friction coefficient

0.45 0.40 0.35 0.30 1N 5N 9N Regression results

0.25 0.20 0.15 0.2

0.5

0.8

1.1

Sliding velocity (m/s)

1.4

180

1N 5N 9N

150 120 90 60 30 0

0.2

0.5

0.8

1.1

1.4

Sliding velocity (m/s)

Fig. 19.26. Dependence of the (a) friction coefficient and (b) wear rate of PEEK/nano-SiC coating on the applied load and sliding velocity.

19.7.2.2 Tribological behavior The dependence of the friction coefficient and wear rate of PEEK/nano-SiC coating on the applied load and sliding velocity is shown in Fig. 19.26. Compared with a pure PEEK coating, the composite coatings present lower friction coefficients under combined conditions of high applied load and intermediate sliding velocity. Outside these ranges, the friction coefficient may become slightly higher than that of the pure coating. Meanwhile, the dependence of the friction coefficient on the sliding velocity is less prominent than that of the pure PEEK coating. The increase in the sliding velocity leads to a slight and linear decrease of the friction coefficient. The increase of the applied load also reduces the friction coefficient. Contrarily, an increase in the sliding velocity and the applied load enhances monotonously the wear rate. However, the incorporation of SiC particles always improves significantly the wear resistance. When ceramic particles are incorporated into the coating, the tribological behavior becomes more complicated. As a hard phase in the soft matrix, the SiC particles improve the coating stiffness and creep resistance by fixing the polymer molecules at the polymer/ceramic interface. Figure 19.27 shows the difference in hardness for a pure PEEK and a PEEK/SiC coatings prepared by the painting technique. The 7 wt% of SiC particles enhance by over 30% the coating hardness. Figure 19.28 shows a worn surface obtained under the condition (1 N, 1.1 m/s). When compared with the pure PEEK coating case

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16 Hardness (HV10 g)

14 12 10 8 6 4

0

PEEK

PEEK/SiC Coating type

Fig. 19.27. Hardnesses of PEEK and PEEK/SiC coatings.

Fig. 19.28. Worn surface of a PEEK/SiC coating produced under the condition (1 N, 1.1 m/s).

(Fig. 19.16), it clearly appears that the plows created on the surface of the composite coating are significantly reduced. Due to the agglomerated status of the SiC particles in the coating, some of them are probably torn out by the ball. Subsequently, they are spread on the surface of the worn track by the sliding ball. Thus, the direct contact and the adhesion between the ball and the coating are probably reduced. Under high loads, the severity of material accumulation, occurring ahead of the sliding ball, is alleviated by incorporating nanofillers. For example, under the sliding condition (9 N, 0.2 m/s), the period of the stacks is about 1.1 mm for the PEEK/SiC coating versus 1.4 mm for the pure coating. Based on the previous analysis, the decrease in the stack period corresponding to the incorporation of nano-SiC particles can be attributed to the decrease in the adhesion force between the sliding pairs and to the improvement of the coating stiffness. Figure 19.29 shows the worn surface of a PEEK/SiC coating produced under the condition (9 N, 1.1 m/s). The plows constitute the main surface characteristic. When comparing the surface morphology with that of a pure PEEK coating (tested in the same conditions, Fig. 19.13), it can be seen that ripple-like folds have disappeared from the worn surface. This indicates that the motion of the coating surface layer, versus the subsurface, was thus avoided. At higher sliding velocities, however, the plows on the worn surface are significantly alleviated. This situation corresponds to a high stress rate and thus a high stiffness in the surface layer involved in the friction process. Figure 19.30 shows a worn surface produced

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Fig. 19.29. Worn surface of a PEEK/SiC coating produced under the condition (9 N, 1.1 m/s).

Fig. 19.30. Worn surface of a PEEK/SiC coating produced under the condition (9 N, 1.4 m/s).

under the condition (9 N, 1.4 m/s). The worn surface exhibits a typical morphology of mild abrasion effect. The grinding effect produced by the SiC particles torn out from the matrix, and the increase in the stiffness of coating surface layer, become the dominant parameters of the tribological behavior.

19.8 Conclusions Being an effective solid lubricant, PEEK coatings are of great significance for improving the tribological performance of metallic parts. According to the works conducted in the authors’ laboratory, two efficient coating techniques, i.e. flame spraying and painting, were described. Using the existing techniques, dense PEEK coatings with amorphous structures were prepared. Semi-crystalline coatings were obtained when the amorphous coatings were subjected to an annealing treatment. The amorphous coatings present a good adherence to the substrate. Semi-crystalline coatings adhere badly to the substrate. An attempt has been made to characterize the tribological behavior of PEEK coatings under dry sliding conditions. Firstly, the correlation between the coating crystalline structure and its tribological behavior was investigated. Compared with the amorphous coatings, the semi-crystalline coatings exhibit lower friction coefficient and wear rate. After crystallization, the increased coating hardness and stiffness contributes to the improvement of the tribological performance. In addition, the effects

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of the sliding conditions, such as sliding velocity, applied load and ambient temperature, on the tribological behavior of an amorphous PEEK coating were investigated. The tribological behavior of the amorphous PEEK coating shows prominent dependence on the sliding conditions. A hypothesis was suggested in this chapter that the tribological behavior of the amorphous coating is closely related to its viscoelastic behavior. Finally, the tribological behavior of an amorphous nano-SiC (7 wt%)-filled PEEK coating was described. The incorporation of the nano-SiC particles decreases significantly the wear rate of the coating. The SiC particles could reduce the direct contact and thereby adhesion between the polymer and the counterpart. Moreover, at high sliding velocities, a grinding effect produced by the SiC particles could also be important for the tribological behavior of the coating.

List of Symbols and Abbreviations pv Ra Tg APS DSC FS HVOF IR MoS2 PEEK PFW PU SiC SiO2 SEM XRD

product of applied pressure and sliding velocity mean roughness glass transition temperature atmospheric plasma spraying differential scanning calorimetry flame spraying high-velocity oxygen-fuel infrared spectra molybdenum sulfide poly-ether-ether-ketone polyfluo wax polyurethane silicon carbide silicon dioxide scanning electron microscope X-ray diffraction

Acknowledgment The authors would like to acknowledge the company PSA Peugeot Citroën for the financial support. Thanks are due to Dr. H. Yu for his assistance in the DSC and XRD analyses and to Ms. P. Hoog for carefully checking the manuscript. One of the authors, G. Zhang, would like to thank his colleagues in LERMPS, University of Technology of Belfort-Montbéliard, for their help in experimental work.

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