Composites Science and Technology 53 (1995) 21-25 0 1995 Elsevier Science Limited

ELSEVIER 0266-3538(94)00061-l

Printed in Northern Ireland. All rights reserved 0266-3538/95/$09.50

THE EFFECTS OF AGEING AND WHISKER-ORIENTATION ON WEAR BEHAVIOUR OF SiC,/Al COMPOSITES UNDER

UNLUBRICATED SLIDING FRICTION

D. Z. Wang, H. X. Peng, J. Liu, C. K. Yao

Department of Metals and Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China

&

Huan Han Center for Computer Application Development and Research of Heilongjiang Province, 150036, People’s Republic of China

(Received 13 May 1994; revised version received 7 September 1994; accepted 27 September 1994)

Abstract The efiects of ageing and whisker-orientation on the wear behaviour of SiC,,,/Al composites under unlubri- cated sliding friction have been investigated. The surface profiles of 52100 steel rings against MMC samples with different ageing treatments were also studied. The results show that the whisker orientation in an as-extruded SiC,,,/Al composite exerts a significant influence on the wear resistance, the lowest wear rate having been observed on the specimen in which the axis of the Sic, is normal to the wear surface. Appropriate over-ageing provided the best wear resistance and resulted in milder damage to the counter ring as a consequence of reduced loss and fracture of the Sic whiskers.

Keywords: SiCJAl composites, ageing, whisker ori- entation, dry sliding friction

1 INTRODUCTION

Sic-whisker-reinforced aluminium (SiCJAl) compos- ites possess good mechanical and tribological pro- perties and are therefore considered as potential engineering materials for various tribological applications.’ They have been employed, for example, in internal combustion engine pistons to improve their wear resistance. At present, most tribological investigations of metal-matrix composites (MMCs) have reported that the composites exhibited superior wear resistance over unreinforced alloy.‘A Although under severely abrasive wear, the wear resistance of the composite is two orders of magnitude greater than that of the matrix alloy.

Different investigations on the effects of whisker-

orientation on the wear behaviour of SiCJAl composites resulted in different conclusions. For example, Asanuma’ concluded that the composite with whiskers normal to the wear surface exhibited superior wear resistance over those with parallel- oriented whiskers under lubricated sliding conditions, whereas Wang6 concluded that the initial wear rate of SiCJ7091 Al composites depend strongly upon whisker orientation, the highest wear rates being observed on perpendicularly-oriented Sic, compos- ites. The steady-state wear rates of the composites were, however, generally independent of reinforce- ment orientation, with the exception of wear at 3.6 m/s where the parallel-oriented Sic, composite was superior.

Different conclusions on the effects of heat- treatment on the wear resistance of SiC-particle- reinforced aluminium composites were also obtained.‘-’ The better wear resistance on an over-aged Sic,/2124 Al composite was observed under lubricated conditions’ while the best abrasive wear resistance of peak-aged SiC,/SiC,/6061 Al composites was observed under unlubricated conditions.’ Furthermore, Su’ indicated that a decrease in hardness of peak-aged SiC,/Al-Zn-Mg composites had little influence on the wear rate. Apart from these results, investigations on the effect of heat-treatment on the wear resistance of SiC-whisker- reinforced aluminium composites are few.

All of the above different studies based on the microstructural aspects of the composites with regard to their wear behaviour may result in different conclusions. This is caused by the complex condition of the microstructure in the materials and wear test. SiCJAl composites prepared by the squeeze casting method possess superior mechanical properties and

71

22 D. Z. Wang et al.

strong interfacial bonding.“’ Studies based on the effects of microstructure and heat-treatment of the composites on the wear behaviour will have important significance in practical applications.

2 EXPERIMENTAL METHODS

The 20~01% SiCJ6061 Al and 25 ~01% SiC,/ZL109 composites used in this study were fabricated by the squeeze casting method, P-Sic whiskers, 0.1-l km in diameter and 30-100 pm in length, being used as reinforcement. Tables 1 and 2 give the chemical compositions of 6061 Al and ZL109 matrix alloys, respectively. The SiCJ6061 Al composites were extruded at 400°C with an extrusion ratio of 17 : 1. In as-extruded composites, the Sic whiskers aligned closely along the extrusion direction and the average aspect ratio (f/d) was 7. Figure 1 shows the distribution of Sic whiskers and a TEM image of the whisker/matrix interface in an as-extruded SiCJ6061 Al composite.

Pin-on-ring wear specimens were prepared from the as-cast and as-extruded composites. Figure 2 shows how the wear specimens were cut from an extruded composite plate. Prior to wear testing, all specimens were solution treated at 520°C for 1 h, water quenched, and then aged at 180°C for l-18 h. Microhardnesses were determined by a micro- sclerometer with a load of 50 g and a dwell time of 15 s. Each datum point is the average of more than six separate experimental results.

Unlubricated sliding wear tests were carried out on a pin-on-ring wear test machine under ambient conditions. The dimensions of the pin specimen are 3 mm in width, 5 mm in height and 15 mm in length. The material of the counter ring with a diameter of 32 mm is 52100 steel with a Rockwell C hardness of 64, which was achieved by water quenching from 840°C and tempering at 150°C for 2 h. The wear tests were carried out under normal loads of 20 N and 30 N, a sliding speed of 1*34m/s and a sliding distance of 480m. The surface roughnesses (R,) of the samples and counter rings before testing were 0.28 and 0.26 pm, respectively. A tool microscope with a precision of 0.01 mm was used to measure the widths of the wear tracks on the specimens. The volume loss and wear rate? were calculated from the width of the wear area.

The morphology of the worn surfaces, the cross-sections of the tested specimens, and the wear debris were observed by scanning electron micros- copy. The damage to the steel ring was evaluated by surface profile measurement with a digitized stylus profilemeter.

t W = VI(PL) (mm’/Nm): V, volume loss (mm’); P, normal load (N); L, sliding distance (m).

Table 1. Chemical composition of 6061 Al alloy (WV%)

Mg Si Cu Fe Mn Zn Al

1.12 0.92 0.23 0.32 0.012 0.0062 remainder

Table 2. Chemical composition of ZL109 alloy (ti%)

W Si Cu Fe Mn Zn Ni Ti Al

1.10 12.0 0.96 0.60 0.35 0.35 2.7 0.20 remainder

3 EXPERIMENTAL RESULTS

3.1 The effects of ageing condition and whisker- orientation on the wear behaviour Table 3 shows the effects of whisker-orientation and ageing condition of Sic,/6061 Al composites on the wear rate. Each datum point is the average of more than three experimental values. The lowest wear rate was observed on the over-aged composites with three kinds of whisker orientations. The Al-w specimen with the whisker orientation normal to the wear surface exhibited superior wear resistance to Aw-1, At-w and as-cast specimens, the wear rate of as-cast specimen being less than that of Aw-1 and At-w with the exception of Aw-I (over-ageing).

The relationship between the microhardness and volume loss of the SiC,/ZL109 Al composite is shown in Fig. 3. It is considered that there is no simple relationship between the microhardness and volume loss for this composite. The lowest volume loss was observed in the over-aged composite, despite the low hardness.

3.2 Morphology of worn surface and wear debris Figure 4 shows scanning electron micrographs of a worn surface and debris of Al-w and Aw-1 specimens after sliding for 480 m at 1.34 m/s. The Al-w sample showed a mild-wear-type surface that was slightly damaged, and the worn debris particles were very fine. Further detailed study of this debris indicated that no Sic whisker was found in them.

II_____-- _ --

Fig. 1. (a) The distribution of SIC whiskers and (b) TEM image of the whisker/matrix interface in an as-extruded

SiCJAl composite.

Composiies Scrence and Technology 53 (1995) 21-25 0 1995 Elsevier Science Limited

ELSEVIER 0266-3538(94)00061-l Printed in Northern Ireland. All tights reserved

0266-3538/95/$09.50

THE EFFECTS OF AGEING AND WHISKER-ORIENTATION ON WEAR BEHAVIOUR OF SiC,/Al COMPOSITES UNDER

UNLUBRICATED SLIDING FRICTION

D. Z. Wang, H. X. Peng, J. Liu, C. K. Yao Department of Metals and Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China

&

Huan Han Center for Computer Application Development and Research of Heilongjiang Province, 150036, People’s Republic of China

(Received 13 May 1994; revised version received 7 September 1994; accepted 27 September 1994)

Abstract The effects of ageing and whisker-orientation on the

wear behaviour of SiC,,,/Al composites under unlubri-

cated sliding friction have been investigated. The surface profiles of 52100 steel rings against MMC samples with different ageing treatments were also studied. The results show that the whisker orientation in an as-extruded SiC,/Al composite exerts a significant influence on the wear resistance, the lowest wear rate having been observed on the specimen in which the axis of the Sic, is normal to the wear surface. Appropriate over-ageing provided the best wear resistance and resulted in milder damage to the counter ring as a consequence of reduced loss and fracture of the SIC whiskers.

Keywords: SiCJAl composites, ageing, whisker ori- entation, dry sliding friction

1 INTRODUCTION

SiC-whisker-reinforced aluminium (SiC,/Al) compos- ites possess good mechanical and tribological pro- perties and are therefore considered as potential engineering materials for various tribological applications.’ They have been employed, for example, in internal combustion engine pistons to improve their wear resistance. At present, most tribological investigations of metal-matrix composites (MMCs) have reported that the composites exhibited superior wear resistance over unreinforced alloy.14 Although under severely abrasive wear, the wear resistance of the composite is two orders of magnitude greater than that of the matrix alloy.

Different investigations on the effects of whisker-

orientation on the wear behaviour of SiCJAl composites resulted in different conclusions. For example, Asanuma’ concluded that the composite with whiskers normal to the wear surface exhibited superior wear resistance over those with parallel- oriented whiskers under lubricated sliding conditions, whereas Wang6 concluded that the initial wear rate of SiCJ7091 Al composites depend strongly upon whisker orientation, the highest wear rates being observed on perpendicularly-oriented Sic, compos- ites. The steady-state wear rates of the composites were, however, generally independent of reinforce- ment orientation, with the exception of wear at 3.6 m/s where the parallel-oriented Sic, composite was superior.

Different conclusions on the effects of heat- treatment on the wear resistance of SiC-particle- reinforced aluminium composites were also obtained.7-’ The better wear resistance on an over-aged Sic,/2124 Al composite was observed under lubricated conditions7 while the best abrasive wear resistance of peak-aged SiC,/SiC,/6061 Al composites was observed under unlubricated conditions.” Furthermore, Su’ indicated that a decrease in hardness of peak-aged SiC,/Al-Zn-Mg composites had little influence on the wear rate. Apart from these results, investigations on the effect of heat-treatment on the wear resistance of SiC-whisker- reinforced aluminium composites are few.

All of the above different studies based on the microstructural aspects of the composites with regard to their wear behaviour may result in different conclusions. This is caused by the complex condition of the microstructure in the materials and wear test. SiCJAl composites prepared by the squeeze casting method possess superior mechanical properties and

21

24 D. 2.

Fig. 5. The morphologies of wear surface produced from SiCJZL109 Al composite under different ageing conditions:

(a) peak-aged (2.5 h); (b) over-aged (18 h).

samples after wear testing shows a relatively rougher surface with R, values of 0.59 and 0.35 pm, respectively.

4. DISCUSSION

4.1 Effects of ageing on the wear of SiCJAI composites As stated above, the over-aged composite exhibits superior wear resistance to the peak-aged composite. Long and deep cracks were observed on the worn surface of the peak-aged composite and this may cause large flaking of material, resulting in a high wear rate (Fig. 5(a)). By contrast, few cracks and a smooth surface were observed on the abraded surfaces of over-aged composites. This indicated that the hardness of the peak-aged composite is high and hence its toughness is low, so that the subsurface cracks initiated and propagated more easily and led to a high wear rate. In the over-aged composite, owing to its low hardness, better plasticity and toughness, the subsurface crack is difficult to initiate and propagate, and thus resulted in an improvement in wear resistance.

In addition, previous studiesY*i’ have already determined that ageing precipitates at the interface between Sic, and the matrix are expected to be

Fig. 6. Scanning electron micrographs of longitudinal cross-sections of as-cast SiCJ6061 AI specimens (peak- ageing) with different whisker orientations: (a) Al-w; (b)

Aw-I.

I.6 lb Ra=o. 35 ~1m

-2.6

I. 1

1

d Ra-0.20 ~1 l

-n 9

Trciverse Distance. mm

Fig. 7. The surface profiles for 52100 rings against different ageing treated Al-w samples and that of the unworn steel ring (20 N, 1.34 m/s, 480 m): (a) unworn; (b) under-aged; (c)

peak-aged; (d) over-aged.

anchors for the SIC particles. They will provide resistance to the loss of SIC particles or cracked fragments from the matrix because of the improved interfacial bonding in the over-aged composite. This may be another reason why the over-aged SiCJAI composite exhibits superior wear resistance. Simply because of the reduced loss of Sic whiskers and hence the low wear rate, the damage to the counter ring is mild, as shown in Fig. 7. It can be observed that the width and depth of the grooves on the abraded surface of the steel ring worn against the over-aged composite are small. The value of R, is small.

4.2 The effects of whisker orientation on the wear of SiCJAI composite As the results of the experiment have shown, the best wear resistance was observed in the Al-w composite sample with the whiskers perpendicular to the wear

Effects of age&g and whisker-orientation on wear behaviour 25

surface because of the strong interfacial bonding between the whisker and matrix.“b” As a hard phase, Sic whiskers protect the matrix from deformation and help to support the applied load. Particularly, when the whisker orientation is normal to the wear surface, a large part of the whisker is buried in the matrix. The deformation along the sliding direction is limited by the whisker, cracks are difficult to initiate and propagate, and the whisker is difficult to detach from the matrix. As a result, the wear surface is relatively smooth, wear debris particles are small in dimension. and no whiskers are observed in the debris (Fig. 4).

When the axes of the whiskers are parallel to the wear surface, the low plasticity caused long cracks to initiate and propagate along the direction of Sic, alignment (Fig. 6(b)). Large amounts of Sic whiskers were then detached from the matrix during sliding friction and wide and deep grooves were observed on the severely damaged worn surface. Many Sic whiskers were found in the debris. (Fig. 4(b) and (d)).

5 CONCLUSIONS

(1) Solution ageing treatment explicitly affects the wear of SiCJAl composites under unlubricated sliding friction. The composite exhibits superior wear resistance when suitably over-aged, and the damage to the counter ring is also mild.

(2) There is no direct relationship between the

wear resistance and the hardness. Better plasticity and toughness improve the abrasion resistance.

(3) In the as-extruded SiCJ6061 Al composite, the best wear resistance was observed when the whisker orientation is perpendicular to the wear surface.

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