SiC-composite coatings

SiC-composite coatings

Wear 260 (2006) 594–600 On dry sliding friction and wear behaviour of PEEK and PEEK/SiC-composite coatings G. Zhang a,∗ , H. Liao a , H. Li a , C. Ma...

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Wear 260 (2006) 594–600

On dry sliding friction and wear behaviour of PEEK and PEEK/SiC-composite coatings G. Zhang a,∗ , H. Liao a , H. Li a , C. Mateus a , J.-M. Bordes b , C. Coddet a a

Laboratoire d’Etudes et de Recherches sur les Mat´eriaux, les Proc´ed´es et les Surfaces, Universit´e de Technologie de Belfort Montb´eliard, Site de sevenans, 90010 Belfort cedex, France b Technical Center of Belchamp 25420 Voujeaucourt, PSA Peugeot Citroen, France Received 3 June 2004; received in revised form 3 February 2005; accepted 11 March 2005 Available online 11 May 2005

Abstract In this work, polyetheretherketone (PEEK) and PEEK/SiC-composite coatings were deposited on Al substrates using a printing technique to improve their surfaces performance. The objective of this work was to investigate coatings friction and wear behaviour. Especially, the effect of sliding velocity and applied load on coatings friction coefficient and wear rate was evaluated in range of 0.2–1.4 m/s and 1–9 N, respectively. Compared to Al substrate, the coated samples exhibit excellent friction coefficient and wear rate. For PEEK coating, under an applied load of 1 N, the increase in sliding velocity can result in decreasing of friction coefficient at a cost of wear resistance. Under a load of 9 N, however, PEEK coating exhibits the highest friction coefficient and wear rate at an intermediate velocity. These influences appear to be mainly ascribed to the influence of contact temperature of the two relative sliding parts. In most test conditions, the composite coating exhibits better wear resistance and a little higher friction coefficient. SiC reinforcement in composite coating plays a combined role. First of all, it might lead to energy dissipation for activation of fracture occurred on the interface of PEEK and the powders. Moreover, it can reduce coating ploughs and the adhesion between the two relative sliding parts. © 2005 Elsevier B.V. All rights reserved. Keywords: PEEK; SiC; Composite; Coating; Coefficient of friction; Wear rate; Ball-on-disc

1. Introduction Polyetheretherketone (PEEK) becomes one of the most attractive polymer materials and is more and more used as bearing and sliding material in industrial applications due to its excellent thermal stability, good friction and wear resistance properties. A fair number of articles about the friction and wear behaviour of bulk PEEK and PEEK-based composites prepared by means of hot pressing, injection moulding, etc. have been published [1–6]. These studies were focused on the effect of sliding condition and reinforcement on the friction and wear behaviour. In order to meet the demand of engineering and design driven by ecological and economical reasons, some researches contributed to prepare PEEK coatings ∗

Corresponding author. Tel.: +33 3 84583243; fax: +33 3 84583286. E-mail address: [email protected] (G. Zhang).

0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.03.017

on lightweight metallic substrates recently [7,8]. Authors used thermal spraying and electrophoretic deposition methods to prepare PEEK coatings. However, it should be noted that homogenizing the reinforcements (in form of particles and especially fibers) in PEEK matrix seems very difficult in coating deposition process by these conventional coating technologies. Up to now, no published article of PEEK-based composite coatings can be found. In this work, pure PEEK and SiC (7 wt.%) reinforced PEEK coatings have been prepared by printing technique. Coatings tribological performance under dry sliding condition was evaluated using a ball-on-disc tribometer. The influence of sliding velocity and applied load on coatings friction and wear behaviour was investigated. The worn surfaces of the coatings were analyzed with scanning electron microscopy (SEM) after gold sputtering and the friction mechanism was investigated.

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Table 1 Physical properties of Al substrate Microhardness (HV 0.3)

Young’s modulus (GPa)

Roughness, Ra (␮m)

Friction coefficient

Wear rate (10−6 × mm3 /Nm)

30

69

0.6

0.54

1001

2. Experimental details

Table 2 Test trials with different sliding velocities and applied loads

2.1. Coating procedure

Test trial

The substrates were 75 mm diameter and 5 mm thick Al discs. The physical properties of the substrate including the coefficient of friction, wear rate were listed in Table 1. The friction coefficient and wear rate were obtained under 2 N at a sliding velocity of 0.2 m/s by friction tests as introduced in following part. The 10 ␮m diameter PEEK powder used in present study was supplied by ICI (Victrex Scales Ltd.). The 2–3 ␮m SiC powder (supplied by Shanghai institute of ceramics) was used as filler in the composite coating. The PEEK and SiC reinforced PEEK coatings were deposited by printing technique. In this method, powders were mixed in water for forming homogenous slurry. For mixing SiC and PEEK powders, an ultrasonic bath was necessary. The Al discs were placed in a cylindrical mould. The slurries were put onto the surface of the substrate discs then homogenously applied on the substrate with a scraper. After being dried, specimens were heated up to 370 ◦ C, held at this temperature for 5 min and then quenched into water at room temperature. The thickness of the coating was fixed at 40 ␮m. 2.2. Friction tests Friction and wear tests were performed using a ballon-disc arrangement on a CSEM tribometer (CH-2000 neuchˆatel7, CSEM). The schematic of the tribometer is shown in Fig. 1. The counterbody consisted of a 6 mm diameter 100C6 steel ball. During the test, the friction force was measured with a sensor and fed into computer dynamically. The friction coefficients were obtained when the measured forces were divided by the applied load. The tests were conducted at room temperature and lab air environment. As listed in Table 2, different applied loads ranging from 1 to

Sliding velocities (m/s)

Applied load (N)

1 2 3 4 5

0.2 0.5 0.8 1.1 1.4

1

6 7 8 9 10

0.2 0.5 0.8 1.1 1.4

5

11 12 13 14 15

0.2 0.5 0.8 1.1 1.4

9

9 N and sliding speeds from 0.2 to 1.4 m/s were selected to investigate their influences on the friction and wear behaviour. In every test trial, the ball slid 2000 m on the coating surface. Profilometry traces were taken across the wear tracks generated on the polymer coating after test, using Taylor–Hobson surtronic 3P profilometer (Rank Taylor Hobson Ltd.). The determination of volume loss of the coating was performed by the multiplication of the cross-sectional area and the perimeter of the worn circular track. The wear rate was calculated from the following equation: w=

V AL = (mm3 N−1 m−1 ) SF SF

where V is the volume loss (mm3 ), S is the sliding distance (m), F is the applied load (N), A is the cross-sectional area of the worn track (mm2 ) and L is the perimeter of the circular track.

3. Results and discussion 3.1. Friction coefficient

Fig. 1. Schematic of ball-on-disc tribometer.

Presented in Fig. 2 are the typical evolutions of the friction coefficient of PEEK and PEEK + SiC coatings as a function of the sliding distance at 0.5 m/s under 1 N applied load. It can be seen that the friction coefficients significantly follow a parabolic pattern: the friction coefficients increase with respect to sliding distance and arrive at a relatively stable value at last. In this study, the mean value of friction coefficients of the last 1000 m sliding distance was taken for evaluating the

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Fig. 2. Typical evolutions of friction coefficients vs. the sliding distances of (a) PEEK coating and (b) PEEK + 7% SiC coating. Applied load: 1 N; sliding velocity: 0.5 m/s.

friction coefficient of the PEEK and SiC filled PEEK coatings. Fig. 3 shows the sliding velocities dependence of friction coefficients of the two coatings under different loads. At sliding speed of 0.2 m/s, the friction coefficients of both PEEK and PEEK + SiC coatings are less dependent on applied loads. When the sliding velocities are increased, the friction coefficient evolves differently under different applied loads. Under load of 1 N, the friction coefficient of PEEK coating decreases while increasing the sliding velocity. Under load of 5 N, comparatively, the friction coefficient increases with increasing sliding velocity and arrives at a steady-state value finally. Under 5 and 9 N, the friction coefficient is almost on the same level at low sliding velocities. Under 9 N, however, the sliding velocity does not affect the friction coefficient in a monotonic way. A sharp drop of friction coefficient is observed with increasing sliding velocity from 1.1 to 1.4 m/s. Filled with SiC, the friction coefficient is increased somewhat at low load. However, under 9 N, the friction coefficients of the composite coating are on the same level as those of pure coating. The friction coefficient reaches maximum

Fig. 3. Variation of friction coefficients as function of sliding velocity under different loads of (a) pure PEEK coating and (b) PEEK + SiC coating.

value at intermediate velocities for each load. The peak value of friction coefficient tends to shift to 0.5 m/s under 9 N. The composite coating exhibits generally lower friction coefficient under higher load. 3.2. Wear rate Variations of wear rate with sliding velocity for PEEK and SiC filled coatings under various loads are shown in Fig. 4a and b, respectively. For PEEK coating, the difference of wear rate under 1 and 5 N becomes more significant while sliding velocities are increased. However, under these two loads, the evolutions of the wear rate of PEEK coating follow the same tendency: the wear rate tends to increase with increasing sliding velocities and eventually reaches a steady-state wear rate. Under a load of 9 N, the variation of wear rate with sliding velocities follows a similar tendency as that of friction coefficient. With increasing sliding velocities, the wear rate was firstly augmented; nevertheless, it exhibited a sharp drop at sliding velocity of 1.4 m/s. Fig. 4b shows clearly that in most test conditions, SiC incorporated PEEK coating exhibits a lower wear rate in

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Fig. 5. SEM micrographs of the worn surfaces of PEEK coating under load of 1 N at sliding velocity of (a) 0.2 m/s and (b) 0.8 m/s. Fig. 4. Variation of wear rate of PEEK and PEEK + SiC coating as function of sliding velocity under different loads of (a) pure PEEK coating and (b) PEEK + SiC coating.

comparison to the unfilled one. The increase of sliding velocities under low loads does not provoke evident varieties of wear rate. However, under the load of 9 N, the sliding velocity plays a very important role on the wear rate of the composite coating as in case of PEEK pure coating. At intermediate velocity of 0.5 m/s, the composite coating exhibits a highest wear rate. 3.3. SEM analysis In order to investigate the effects of sliding condition as well as SiC incorporation on friction and wear behaviour of the coating, SEM (JSW-5800LV, JOEL) observations of the worn surfaces after 2000 m sliding were carried out. Fig. 5 shows the morphologies of worn surfaces of pure PEEK coating after sliding under a load of 1 N. The observation reveals that plastic deformation and plough constitute predominant surface modifications and they become more important when sliding velocity is increased. The dependence of the worn surface morphologies on sliding velocity can be attributed to the friction-induced heat effects. Most energy

dissipated during sliding is transformed into heat and a high temperature gradient develops in the normal direction to the surface [9,10]. As a result of the low thermal conductivity of PEEK coatings, frictional heat being generated during sliding surely provoked an increase of the contact temperature and the increase of sliding velocities can be quantitatively correlated to the increase of sliding temperature [7,11,12]. Thus, the deterioration of coating mechanical properties such as hardness and shearing strength occurs as a result of interfacial temperature rising caused by friction-induced heat. Accordingly, the applied load can lead to more severe deformation of the coating under higher sliding velocity. The decrease in shear strength of the coating might explain the reduction of friction coefficient with sacrifices to wear resistance with increasing sliding velocity. Under higher load, more severe ploughs can be detected and the morphologies of worn surface vary significantly with sliding velocities. Generally, wear debris produced during sliding process can be attributed to an abrasive microcutting effect. The asperities of the harder surface of the steel ball exerted a ploughing action on the softer PEEK coating surface. As a result, the granular debris was produced [13]. Improvement of polymer debris hardness can occur during reciprocating sliding process as a result of work hardening

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Fig. 6. SEM micrographs of the worn surfaces of PEEK coating under load of 9 N: (a) at sliding velocity of 0.2 m/s; (b) at sliding velocity of 0.5 m/s; (c) at sliding velocity of 1.1 m/s; (d) at sliding velocity of 1.1 m/s; (e) at sliding velocity of 1.4 m/s.

caused by repeated plastic deformation [11]. Thus, it can act as abrasive grain in the following sliding process. Fig. 6 shows the worn surface of PEEK coating under load of 9 N. At sliding velocity of 0.2 m/s, except more severe plough, the worn surface is similar with those obtained under 1 N in which cases the ploughs exhibit clear-cut contours. On the top of these ploughs, there appear well-defined (acuate) peaks. Therefore, it can be concluded that under this condition, the predominant frictional mechanism can be explained in terms of shearing and ploughing of the asperities

or hardened debris on PEEK coating. As the sliding velocity is increased, acuate peaks on ploughs become less evident. It seems that some well-defined peaks are eliminated by the lug and separation of the adhesive junctions between the two sliding bodies (Fig. 6b). This indicates that adhesion force plays a more important role at higher sliding velocities. At velocity of 1.1 m/s, massive peeling (tearing away of coating surface) caused by a continuous lug of adhesive junction is observed in the middle of the wear trace (Fig. 6c and d). It can be inferred that the adhesive force is larger than coating’s shear strength

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in the junction region. Associated with the above results, with increasing sliding velocity, the increased adhesion between the two sliding bodies appears to be the main influencing factor giving rise to friction and wear during sliding. Worth noticing, however, when the velocity is further increased to 1.4 m/s, both the friction coefficient and the wear rates of the coatings exhibit sudden declines. In the study of Z. P. Lu [1], the sliding friction of PEEK and its composites were studied with different contact temperatures. Their results showed that for the friction coefficient of pure PEEK, a maximum value existed near the glass transition temperature (Tg ). In [3], the author also found that the friction coefficient and wear rate of PEEK reach maximum values in the region of transition from glassy to rubbery state. From Fig. 6e, periodical deformation perpendicular to the plough can be found on the worn surface. The slightly white lines periodically appear in a direction perpendicular to the ploughs in Fig. 6e. They correspond to the areas with more severe plastic deformations along the ploughs. These periodic deformations seem to be formed as a result of viscous fluid flow of the coating surface where the temperature is higher than Tg [11]. Therefore, it can be inferred that the peaks of the curves of friction coefficients and wear rate in this study correspond to the sliding velocity under which the contact temperature arrives at vicinity of glass transition temperature Tg . In this region, the adhesion between the two sliding surfaces reaches a peak value. Further augmentation of contact temperature above Tg caused by higher sliding velocity can benefit the friction and wear behaviour in two influencing aspects. Firstly, the augmented molecule chain flexibility reduces the chain scission occurrence [14]. Secondly, when the contact temperature surpasses the glass transition point, the rapid decline of polymer viscosity can reduce the adhesion and thus decrease the friction [11]. Under load of 5 N, there is no pronounced drop of friction coefficient and wear rate found when the sliding velocities were increased. As has been indicated in [1,7], the applied load is another positive influencing factor to the contacting temperature. Therefore, it can be inferred that the contact temperature has not surpassed the Tg region in the range of the velocities studied under load of 5 N. Filled with SiC powders, the composite coating exhibits higher friction coefficient under lower load compared with pure PEEK coating. From Fig. 7a, there are some holes observed which were formed after SiC powders being pulled out from the coating matrix. There is certain energy dissipated for producing fracture on the interface of the PEEK matrix and the SiC powders. Interfacial fractures were also found on worn surface of a short carbon-fiber-reinforced bulk PEEK, the higher coefficients of the friction observed in those cases appear to be derived from the activation of fracture in the interface of reinforcing powders and PEEK matrix as interfacial energy dissipation mechanism during the sliding process [4]. As a hard phase in the soft polymer matrix, SiC powders can reduce the coating deformation and true contact area with

599

Fig. 7. SEM micrographs of the worn surfaces of PEEK + SiC coating: (a) applied load: 1 N, sliding velocity: 0.5 m/s; (b) applied load: 9 N, sliding velocity: 1.1 m/s.

the counterbody under certain load [11]. As a result, it exhibits an important influence on reducing the plough and the adhesion between the relative sliding parts. In conclusion, the SiC powders influence the coating’s tribological behaviour in two aspects: firstly, it could lead to fractures in the interface of the two coating constituents; moreover, it could reduce the plough and the adhesion between the two sliding bodies. These two roles simultaneously influence the friction and wear behaviour according to the sliding conditions. For example, under high load and high sliding velocity, the composite coating exhibits better friction behaviour since the reinforcing powders can reduce effectively the adhesion force and the plough which can be seen from the comparison of Figs. 6c and 7b.

4. Conclusions (1) Compared with Al substrate, the polymer coatings exhibit excellent friction coefficient and wear rate. (2) The differences in friction and wear behaviour of the PEEK coating under different sliding conditions appear to be mainly ascribed to the influence of the contact

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temperature of the two relative sliding parts. Under low load, the friction coefficients decrease slightly at cost of wear resistance when the sliding velocities are increased. Under high load, first increase of sliding velocities can lead to augmentations of the friction coefficients and wear rates because of the increased adhesion between the two sliding parts. Further increase of velocity can inversely benefit the friction and wear behaviour. The peak values of friction coefficient and wear rate in this case appear to correspond the point in which the contact temperature arrived in vicinity of glass transition temperature of PEEK (Tg ). (3) When the coating was filled with SiC powders, the wear resistances of coating could be much increased in most sliding conditions with a little sacrifice in the friction behaviour at low load. The roles of the ceramic phases on coating’s friction and wear behaviour could be concluded in the following aspects. Firstly, it could lead to energy dissipation for the activation of fracture occurred in the interface of the PEEK matrix and the powders; secondly, it can effectively reduce the plough and the adhesion between the two relative sliding parts. These influencing factors play combined roles according to the different sliding condition.

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