On the sliding wear of nanoparticle filled polyamide 66 composites

COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 66 (2006) 3188–3198 www.elsevier.com/locate/compscitech

On the sliding wear of nanoparticle filled polyamide 66 composites L. Chang a, Z. Zhang a

a,b,*

, H. Zhang a, A.K. Schlarb

a

Institute for Composite Materials, University of Kaiserslautern, 67663 Kaiserslautern, Germany b National Center for Nanoscience and Technology, 100080 Beijing, China

Received 18 December 2004; received in revised form 18 February 2005; accepted 22 February 2005 Available online 9 September 2005

Abstract The tribological properties of polyamide 66 (PA66) composites, filled with TiO2 nanoparticles, short carbon fibres, and graphite flakes, were investigated. Sliding tests were performed on a pin-on-disk apparatus under different contact pressures, p, and sliding velocities, v. It was found that nano-TiO2 could effectively reduce the frictional coefficient and wear rate, especially under higher pv (the product of p and v) conditions. In order to further understand the wear mechanisms, the worn surfaces were examined by scanning electron microscopy and atomic force microscopy. A positive rolling effect of the nanoparticles between the material pairs was proposed which contributes to the remarkable improvement of the load carrying capacity of polymer nanocomposites.  2005 Elsevier Ltd. All rights reserved. Keywords: Polymer nanocomposites; Polyamide 66; TiO2 nanoparticle; Short carbon fibre; Graphite flake; Wear; Friction

1. Introduction Over the past decades, polymer composites have been increasingly applied as structural materials in aerospace, automotive and chemical industries, providing lower weight alternatives to traditional metallic materials. Numerous these applications are concentrated on tribological components, such as gears, cams, bearings and seals, etc., where the self-lubrication of polymers and polymer composites is of special advantage. The features that make polymer composites so promising in industrial applications are opportunities to tailor their properties with special fillers. For instance, short fibre reinforcements, such as carbon and glass fibres, have been successfully used to improve the strength and therefore the load carrying capacity of polymer composites [1–3]. Solid lubricants, e.g., polytetrafluoroethylene (PTFE) and graphite flakes, were proved to be generally helpful on * Corresponding author. Tel.: +49 631 2017213; fax: +49 631 2017196. E-mail addresses: [email protected], [email protected] (Z. Zhang).

0266-3538/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.02.021

developing a continuous transfer film between the two counterparts and accordingly reduce the frictional coefficient [4–6]. Recently, nano-scale inorganic particles are under consideration. Attempts give special hints that this method is promising for new routes of wear resistant materials even at very low filler content (about 1–4 vol%) [7,8]. However, the mechanisms by which nanoparticles in modifying the tribological performances in polymer composites are not fully understood. While, most polymers with special fillers can provide low frictional coefficient and low specific wear rate, their applications are generally limited to relatively lower contact temperatures and consequently lower pv (the product of p and v) conditions [9]. For instance, as shown in Fig. 1 measured by a dynamic mechanical thermal analyser (DMTA), the glass transition temperature of polyamide 66 (PA66) is around 75 C. PA66 becomes soften when service temperature reaches up to about 50 C, which could possibly happen in dry sliding wear under higher pv conditions. Correspondingly, the wear rate of material would be seriously increased with the degraded mechanical properties [3]. In order to evaluate the durability of materials under different

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2. Experimental

0.15

PA 6,6 5vol.% nano-TiO2/PA6,6

2.1. Material

0.12

0.09

Tan δ

E* [MPa]

3000

2000 0.06 1000 0.03

0 0

30

60

90

120

0.00 150

o

Temperature [ C] Fig. 1. Dynamic mechanical properties of the neat PA66 and PA66based nanocomposite with 5 vol% nano-TiO2 for the temperature range 110 to 230 C.

sliding conditions, the time-related depth wear rate, wt, was frequently introduced wt ¼ k   pv ¼

Dh ðm=sÞ t

ð1Þ

in which k* is called the basic wear factor of the material, p is the normal pressure, v is the sliding velocity, t is the test time, and Dh is the height reduction of the specimen. In this equation, the wear factor, k*, is supposed to be a material parameter, which is dependent on sliding conditions applied, e.g., p and v. Based on Eq. 1, the pv factor could be considered as a tribological criterion of the load-carrying capacity for bearing materials, which results in two evaluation parameters [1]: (i) the basic wear factor k*, which keeps nearly constant in a certain range of the pv product; (ii) the ‘‘limiting pv’’, above which the increase of the wear rate of materials is much rapid, so that the material cannot be used any more. To reduce the basic wear factor k* and to enhance the ‘‘limiting pv’’ value are the general targets of the design of wear resistant polymer composites. In our previous study [10,11], wear behaviours of epoxy-based nanocomposites were carried out under different normal pressures and sliding velocities. It was found that the addition of nano-TiO2 could apparently reduce both the frictional coefficient and the wear rate, especially under high pv conditions. In order to promote this effort, the current study concentrated on the thermoplastic PA66 matrix composites. PA66 is selected as an important thermoplastic and being widely used in injection moulded components [12–15], with strong commercial advantages of lower manufacturing cost. The tribological properties of the addition of naonTiO2 particles, containing short carbon fibre (SCF) and graphite flakes, were studied under various sliding conditions. It is expected that this study will be helpful towards a better understanding of the role of nanoTiO2 in reducing of wear rate of polymer composites.

A commercially available PA66 (Zytel 101L) was considered as matrix material in this study. Pitch-based short carbon fibres (Kureha M-2007S) and graphite flakes (Superior 9039) were selected as conventional reinforcements. The average diameter of short carbon fibres is approximately 14.5 lm with an average fibre length of about 90 lm. The size of graphite flakes is about 20 lm. TiO2 (Kronos 2310) with an average diameter of 300 nm was applied as an additional reinforcement. The mixture of PA66 with different fillers was achieved by a twin-screw-extruder with well-designed screw configurations at a melting temperature of 292 C. Thereafter, the specimens were produced using an Arburg Allrounder injection moulding machine with an injection pressure of 500 bar. The screw speed was set at 80 cm3/s and the mould temperature was 70 C. Wear specimens were cut from injection moulded plates (80 · 80 · 4 mm3) with a size of 4 · 4 · 12 mm3. Two compositions were involved in this work, i.e., one is 5 vol%graphite + 15 vol%SCF/PA66 and another one with additional 5 vol% of TiO2 particles. The compositions are designed for high wear resistance materials referring to earlier results from series of epoxy-based compositions [10,11]. 2.2. Wear test The wear test was performed on a Wazau pin-on-disc apparatus according to ASTM D3702. The specimen pins were rotated against a polished steel disc with the initial surface roughness of 220 nm. The tests were conducted for 20 h each in a wide range of wear conditions, i.e., the normal pressure in a range from 1 to 8 MPa and the sliding velocity from 1 to 3 m/s. During the test, the temperature of the disc was monitored by an iron-constantan thermocouple contacted to the edge of the disc. The frictional coefficient was recorded and calculated by a ratio between the tangential force and normal load. The mass loss of the specimen was measured in order to calculate the specific wear rate by the following equation, which is the same as the wear factor k* mentioned in Eq. (1):   Dm mm3 ws ¼ ð2Þ qF N L Nm in which FN is the normal load applied on the specimen during sliding, Dm is the specimenÕs mass loss, q is the density of the specimen, and L is the total sliding distance. All the test results were summarised in Table 1. Both the frictional coefficient and the contact temperature given in this table were mean values during the steady state of the sliding process. After wear tests, worn

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Table 1 Tribological properties of PA66 and PA66 matrix composites under various wear conditions pv factors

Frictional coefficient

Neat PA66

1 MPa, 1 m/s

1.16

45.09

6.70

Graphite + SCF/PA66

1 MPa, 2 MPa, 4 MPa, 8 MPa, 2 MPa, 2 MPa,

1 m/s 1 m/s 1 m/s 1 m/s 2 m/s 3 m/s

0.60 0.57 0.69 0.35 0.78 0.62

30.57 46.07 94.98 97.37 97.67 115.80

0.53 0.80 2.92 16.93 4.87 5.73

Nano-TiO2 + graphite + SCF/PA66

1 MPa, 2 MPa, 4 MPa, 8 MPa, 2 MPa, 2 MPa,

1 m/s 1 m/s 1 m/s 1 m/s 2 m/s 3 m/s

0.44 0.34 0.26 0.22 0.34 0.38

26.66 33.21 45.55 64.69 45.94 62.36

0.50 0.72 0.80 3.26 0.63 3.35

surfaces of specimens were examined by a JEOL-5400 scanning electron microscopy (SEM). For more exact detailed information, single fibres in the worn surface were examined by atomic force microscopy (AFM) (Digital Instruments), of which the lateral and vertical resolutions are 2 and 0.2 nm, respectively.

3. Experimental results 3.1. Standard wear condition – 1 MPa and 1 m/s

Temperature [oC]

Fig. 2 shows typical variations of the frictional coefficient and contact temperature against sliding time of neat PA66 in comparison with other two compositions

Contact temperature (C)

Specific wear rate (106 mm3/Nm)

Composition

under 1 MPa and 1 m/s. It was distinct that the two parameters were closely correlated with each other and the tendencies were well coincided, which has been reported as well in a previous study by Chang et al. [11]. The frictional coefficient of neat PA66 was apparently increased during initial wear stage. This was probably caused by the increase of real contact area and contact temperature owing to frictional heating [12,13,16]. Thereafter, it stabilises at a value, which was even higher than 1.0. This resulted in a rather high specific wear rate. Filled only with SCF and graphite, the frictional coefficient and the contact temperature were remarkably reduced. With the further addition of nano-TiO2, the stable frictional coefficient was reduced to a mean value of about 0.4 after the initial wear stage (cf. Fig. 2).

50 neat PA 6,6 40 graphite+SCF/PA 6,6 30 nano-TiO2+graphite+SCF/PA 6,6

Frictional Coefficient

20 1.2

neat PA 6,6

0.8

graphite+SCF/PA 6,6

0.4

nano-TiO2+graphite+SCF/PA 6,6 0

5

10

15

20

Sliding Time [Hour] Fig. 2. The typical sliding process curves of frictional coefficient and contact temperature against sliding time of neat PA66 and without and with nano-TiO2 under a standard wear condition of 1 MPa and 1 m/s.

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Consequently, the wear rates of two composites were remarkably reduced, which were more than ten times lower compared to that of neat PA66 (cf. Table 1). The worn surface of neat PA66 is given in Fig. 3. Grooves paralleled to the sliding direction are clearly observed which suggest the wear process governed by abrasive wear mechanism. In this case, the polymer generally obtains a high wear rate depending on the original roughness of harder counterpart and the contact pressure [13,17]. With a magnified view, viscous flow of PA is observed in a micron scale due to the high flash temperature occurring in real contact area. Owing to the low load carrying capacity, the soften PA66 was rapidly removed by the hard asperities of the metallic counterpart surface. Figs. 4 and 5 present the worn surfaces of two composites without and with nano-TiO2, respectively. It is clear that the surfaces of both composites were quite smooth resulted from adhesive wear. With a magnified view, local matrix micro-cracks may occur (Fig. 4(b)),

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which were probably caused by the ‘‘fatigue wear’’ of adhesive contact [9]. A polymer layer may transfer onto the metallic counterpart surface during running-in stage, and then results in this adhesive wear mechanism. For the nanocomposite, slight nano-grooves were observed on the carbon fibres, which are parallel to the sliding direction (Fig. 5(b)).

3.2. High pv factor With the improved load-carrying capacity by fillers, the composites could be subjected in much higher pv conditions than neat PA66. Fig. 6 presents the sliding processes of the two composites under 4 MPa and 1 m/s. In comparison to Fig. 2, it can be seen that the duration of running-in stages of both composites was apparently reduced. This reduction was caused by the more rapid formation of stable transfer films, due to the accelerated generation of wear debris and easier

Fig. 3. (a) SEM micrographs of the worn surfaces of neat PA66 measured at 1 MPa and 1 m/s, and (b) the magnified view of viscous flow.

Fig. 4. (a) SEM micrographs of the worn surfaces of the composition without nanoparticles at 1 MPa and 1 m/s, and (b) the magnified view of matrix ploughing damage.

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o

Temperature[ C]

Fig. 5. (a) SEM micrographs of the worn surfaces of the composite nano-TiO2 + graphite + SCF/PA66 with nanoparticles at 1 MPa and 1 m/s, and (b) the magnified view of nano-grooves on the fibre.

graphite+SCF/PA

100 80 60

nano-TiO2+graphite+SCF/PA

Frictional Coefficient

40 graphite+SCF/PA

0.8

0.4 nano-TiO2+graphite+SCF/PA

0.0 0

5

10

15

20

Sliding Time [Hour] Fig. 6. Comparisons of typical sliding process curves against sliding time of the composites without and with nano-TiO2 at a high sliding velocity of 3 m/s and a contact pressure of 2 MPa.

formation of transfer film under a high compact pressure. After the running-in stage, the frictional coefficient of the composite without nano-TiO2 became stable and obtained a high contact temperature. However, for nanocomposite, it was then sharply decreased and achieved a much lower value. As a result, the wear resistance of nanocomposite was remarkably enhanced. Fig. 7 shows the SEM micrographic of the worn surface of the composition without nano-TiO2 at 4 MPa and 1 m/s. Due to the high contact temperature during sliding, PA66 matrix was seriously soften. Associated with the increase of friction force, the fibre removal was seriously aggravated since the soften polymer ma-

trix could not effectively protect SCF from peeling-off (Fig. 7(b)). In comparison to that of brittle epoxy composite (cf. Fig. 2 in [18]), it was noticed that the breakage in interfacial region of SCF/PA66 was much limited because of the ductile of the polyamide, which deformed throng elongation rather than breakage and thus a roll formation of debris [14]. Additionally, Fig. 8 shows the typical roll debris on the counterpart surface formed by the composite. The worn surface of the nanocomposite was given in Fig. 9. Because of the reduction of friction and contact temperature, the fibres could effectively undergo the load and were gradually removed within a normal

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Fig. 7. (a) SEM micrographs of the worn surfaces of the composition without nanoparticles at 4 MPa and 1 m/s, and (b) the magnified view of fibre removal.

3.3. Time-related depth wear rate

Fig. 8. SEM micrographs of roll debris on the metallic counterpart surface.

process, i.e., fibre thinning, fibre fracture, and finally fibre peeling-off [18]. Fig. 9(b) shows the patterns of the fibre fracture of nanocomposite with good interfacial bonding even under high pressure.

The time-related depth wear rate calculated by Eq. 1 is given in Figs. 10 and 11 as a function of the pv factor. It is clear that the basic wear factors, k*, of the two composites were very close to each other under low pv conditions, e.g., lower than 2 MPa m/s. However, for the composition without nanoparticles, the slope change occurred between 2 and 4 MPa m/s, whereas for the nanocomposite it occurred between 4 and 6 MPa m/s. And, a huge wear rate of the composite without nanoparticles was obtained at 8 MPa m/s, i.e., 135 nm/s of depth wear rate, which is almost five times of that of nanocomposite at the same condition. This means that the ‘‘pv limiting’’ for nanocomposite is apparently improved. It was noticed that for both composites, the slope change occurred when the contact temperature was above 50 C (Table 1), which was in the glassy region of the PA66 matrix (Fig. 1). However, the composite with nanoparticles achieved this value under higher

Fig. 9. (a) SEM micrographs of the worn surfaces of the composition with nanoparticles at 4 MPa and 1 m/s, and (b) the magnified view of fibre fracture.

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neat PA 6,6 graphite+SCF/PA 6,6 nano-TiO2+graphite+SCF/PA 6,6

40

tection on the matrix material in interfacial region, which consequently protected on SCF from serious wear mechanisms [18].

135nm/s

Time-Related Depth Wear Rate[nm/s]

3194

30

4. Discussions 4.1. Mechanism considerations based on AFM observations

20

10

0 0

2

4

6

8

pv [MPa· m/s] Fig. 10. The time-related depth wear rate as a function of pv factor of the composites filled with and without nano-TiO2.

pv condition than that without nanoparticles due to the reduction of the frictional coefficient. Therefore, even though the addition of nanoparticles little affects the value of the Tg of PA66 (Fig. 1), the nanocomposite could be used for higher duties with lower heat generation due to the remarkable reduction of the frictional coefficient. It can be drawn that the enhancement on the wear resistance of PA66 nanocomposite was mostly caused by the reduction of contact temperature and thus abating the degradation of the mechanical properties of thermoplastic matrix. However, for thermosetting material, e.g., epoxy based nanocomposite, the addition of nanoparticles effectively improve the wear performances of material mostly by the pro-

In order to understand the role of nanoparticles for improving the wear performance of polymer composites, AFM was employed to investigate the worn surfaces, especially focused on single carbon fibre with and without nanoparticles. Fig. 11(a) shows an AFM image of an almost normal directional carbon fibre of the composition without nanoparticles tested at 1 MPa and 1 m/s. A cross-sectional measurement was preformed in Fig. 11(a) (as shown in Fig. 11(b)) and the results were summarised in Table 2. The height of the exposed fibre, i.e., the difference between ÔAÕ and ÔBÕ (0.26 lm), is close to the initial roughness of the counterpart surface (about 0.22 lm). It can be seen that the fibre surface is relatively smooth, but apparently tilted to the worn surface. Therefore, during friction process, the exposed fibre underwent most of load and was impacted by the asperities of counterpart. In comparison with that of epoxy-based composite (cf. Fig. 11 in [18]), the interfacial damage between SCF and matrix did not occur due to the better interfacial bonding of thermoplastic matrix and carbon fibre. As a result, the worn surface of the material was quite smooth and no serious fibre removal observed (cf. Fig. 4). Fig. 12(a) provides an AFM image of a carbon fibre of the nanocomposite in the normal direction tested at 1 MPa and 1 m/s. A cross-sectional measurement is

Fig. 11. (a) AFM image of the fibre in the worn surface of the composite without nanoparticles at 1 MPa and 1 m/s, and (b) a cross-sectional measurement. The arrow line represents the sliding direction.

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Table 2 Summary of the cross-sectional measurement results by AFM Fig. 11(b) a

Surface-distance (lm) Horizontal-distanceb (lm) Vertical-distancec (nm) Angled () a b c d

The The The The

Fig. 12 (b)

Fig. 12(c)

A–B

B–C

A–B

B–C

A–A 0

B–B 0

3.66 3.63 263.8 4.15

14.30 14.30 193.5 0.78

3.33 3.28 215.9 3.76

13.60 13.59 6.56 0.03

0.07 0.07 3.99 3.40

0.10 0.10 7.82 4.65

Ôsurface-distanceÕ refers to the surface trace between two points. Ôhorizontal-distanceÕ refers to the linear length in horizontal direction. Ôvertical-distanceÕ is the linear height in vertical direction. ÔangleÕ indicates that of the horizontal direction and the line determined by the measured two points.

Fig. 12. (a) AFM image of the fibre in the worn surface of the composite with nanoparticles at 1 MPa and 1 m/s, (b) a cross-sectional measurement, and (c) AFM sectional measurement of the nanoscale grooves with a magnified view as shown in (a). The arrow line represents the sliding direction.

presented in Fig. 12(b) and the results were summarised in Table 2 as well. The height of the exposed fibre was about 0.22 lm, which is again coincided with the roughness of the counterpart. However, the fibre surface is almost parallel to the worn surface, whereas tilted for the composite without nanoparticles (Fig. 11). This can be explained by a rolling effect of nanoparticles proposed in our early study on epoxy-matrix nanocomposites [18–20], which reduced the shear stress especially at edge of SCF and protected fibre from the impact of the asperities of counterpart (cf. Fig. 14 in [18]). In addition, Fig. 12(c) gives a perpendicular sectional measurement of nano-grooves by AFM with a magnified view as marked in Fig. 12(a). The width of the grooves is in a

range of 150–200 nm (Table 2), which is in the same order with the nanoparticle size (300 nm). However, the depth of the grooves was only around 5 nm. It is, therefore, considered that some of the individual hard nanoparticles were possibly embedded in the transfer film layer on the counterpart, whilst slightly scratched on fibres. The very shallow grooves suggest that the friction produced by particle grooving or sliding is quite slight. In this case, the size of the nanoparticles is very close to the gap between two counterparts. For hard particles, the sliding and rolling commonly occur together. Therefore, according to these phenomena, two competitive effects of nanoparticles are proposed. On the one hand, nanoparticles tend to enhance the wear resistance of

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Temperature[oC]

3196

120

graphite+SCF/PA6,6

90 nano-TiO2+graphite+SCF/PA6,6 60

Frictional Coefficient

30 graphite+SCF/PA6,6 0.6 nano-TiO2+graphite+SCF/PA6,6

0.3 0.0

0.5

1.0

1.5

2.0

Sliding Time [Hour] Fig. 13. Comparisons of the sliding process of the two compositions, i.e., graphite + SCF/PA66 and nano-TiO2 + graphite + SCF/PA66, with different testing durations, i.e., 10, 30, 60 and 120 min.

composite by reducing friction through a proposed rolling effect. On the other hand, the mild abrasive was simultaneously induced by the hard particles acting as the third bodies, which counteracted the beneficial effect of the reduction of the frictional coefficient in certain extent. As a whole, the specific wear rates of the composites with and without nanoparticles were almost similar under low pv conditions, even though the frictional coefficient of nanocomposite was always apparently lower. When pv factor increases, for the composition without nanoparticles, the contact temperature was apparently increased due to higher frictional work. In this case, the wear resistance of nanocomposite was remarkably improved. 4.2. ‘‘Running-in’’ stage In order to further understand the contribution of nano-TiO2 during friction process, the running-in stages of two composites were compared at various sliding durations, e.g., 10, 30, 60 and 120 min, respectively. Fig. 13 gives the dependence of the frictional coefficient and the contact temperature on sliding time. The correlated SEM worn surfaces are given in Fig. 14. As shown in the figures, the patterns of the surface damages were very well agreed to the performances of the frictional coefficient for both compositions. During the first 10 min, two compositions performed a similar frictional coefficient and contact temperature. Accordingly the worn surfaces are also very similar (cf. Figs.

14(a) and (b)). After 30 min, both compositions reached almost the highest contact temperature and serious fibre removals were observed correspondingly (cf. Figs. 14(c) and (d)). Thereafter, the sliding process of the composition without nano-TiO2 became stable and serious fibre removal remained (cf. Figs. 14(e) and (g)). However, for the composition with nanoparticles, the surface became much smoother with reduced fibre broken and removal (cf. Figs. 14(f) and (h)) due to the reduction of the frictional coefficient and the contact temperature, once the nanoparticles commence to function. According to the sliding processes of the two composites under various conditions (Figs. 2, 6, and 13), it can be found the reduction of friction by nanoparticles occurs always after the initial wear stages, during which the sliding processes for two compositions are very similar. Therefore, nanoparticles were gathered and distributed on the counterpart surface and greatly reduced the fiction with the rolling effect. In respect that the transfer films were generally developed during the running-in stage and produced adhesive wear mechanism, another assumption was also proposed. With the addition of nanoparticles, the real contact area was reduced by three bodies contact instead of surface contact and thus lower adhesive force. As a result, the frictional coefficient was effectively reduced by nano-TiO2 in all test conditions and restricted the elevation of the contact temperature, especially under very high pv conditions. This reduction consequently enhances the load carrying capacity of the composite.

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Fig. 14. Comparisons of the worn surfaces of graphite + SCF/PA66 after (a) 10 min (c) 30 min, (e) 60 min (g) 120 min, and that of nanoTiO2 + graphite + SCF/PA66 after (b) 10 min (d) 30 min (f) 60 min (h) 120 min, respectively.

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5. Conclusions

References

In the present paper, tribological properties of nanoTiO2 filled PA66 matrix composites, incorporated additionally with short carbon fibres and graphite flakes, were systematically studied under different sliding conditions. The following conclusions can be drawn:

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1. It was found that conventional fillers, e.g., SCF and graphite flakes, could effectively reduce the frictional coefficient and wear rate of PA66 at lower pv condition, e.g., 1 MPa and 1 m/s. 2. The serious fibre removal and rapid increase of wear rate were occurred at the contact temperature increased up to the Tg of matrix. In comparison with short fibre reinforced epoxy composites [10,11], the wear damage in the interfacial region of fibre/PA66 was slight due to the ductile nature of the matrix material. 3. With the addition of nano-TiO2, the frictional coefficient and contact temperature of the composite were further decreased, especially with very high pv products. As a result, the ‘‘limiting pv’’ of nanocomposite was clearly improved, although the basic wear factor k* was only a little bit enhanced. 4. Based on SEM and AFM observations, a positive rolling effect of the nanoparticles between the material pairs was proposed. This rolling effect helps to reduce the shear stress and consequently contact temperature, especially at high sliding pressure and speed situations. 5. With further investigations at the running-in stage, it was found the fibre removal was consequently changed with the change of frictional coefficient. The increase of frictional coefficient caused the aggravated fibre removal and thus resulted in a higher specific wear rate.

Acknowledgements Z. Zhang is grateful to the Alexander von Humboldt Foundation for his Sofja Kovalevskaja Award, financed by the German Federal Ministry of Education and Research (BMBF) within the German GovernmentÕs ‘‘ZIP’’ program for investment in the future. The authors appreciate Prof. Dr.-Ing. Dr. h.c. K. Friedrich, IVW, for his valuable discussions during the course of this work and the preparation of this paper.