Tribological characteristics of A356 Al alloy–SiCP composite discs
Highlights
► Tests have been done with Pin of brake pad material and disc of Al MMCs. ► Presence of copper the in tribo layers is responsible for significant reduction in wear rates.
Introduction
SiC particle reinforced aluminum matrix composites (Al MMCs) have been targeted for a variety of structural and tribological applications because of their superior properties over monolithic aluminum alloy. Al MMCs possesses high specific strength, hardness and excellent wear resistance. There has been an increased use of these Al MMCs as a substitute for cast iron as brake rotors/discs in ground transportation systems including passenger cars and trains [1], [2], [3], [4], [5], [6], [7], [8]. Further, efforts are also under way to optimize the properties and chemical formulation of both the disc and brake pad in order to achieve superior performance during braking.
In general, most of the friction and wear data on Al MMCs have been generated from pin-on-disc tests. In all those tests, pins of Al MMCs are slid against hardened steel discs or cast iron disc [1], [2], [3], [4], [9]. Information and data obtained from those tests [10] are of very limited use, since conditions prevailing are very much different compared to those in MMC discs/brake pad tribocouple. Further, fundamental issues concerning the tribology of Al MMCs disc/brake pad tribocouple need to be better understood, in order to take maximum advantage of Al MMCs [6], [7]. Recently Uyyuru et al. [11], [12] have carried out wear tests where Al MMCs have been used as disc materials. However, in these discs SiC particles were less than 23 μm size. But from the point of view of reducing the cost of Al MMC disc, use of coarse SiC particles are preferred compared to fine size particles which are more expensive. Hence, the present study is concerned with tribological characteristics of A356 Al SiCP composites containing 10 and 20 vol.% SiC particles of 40 μm size and understanding the wear mechanisms during sliding of brake pads against Al MMCs discs. More specifically, the effect of sliding speed and load on formation of complex tribolayers on wear and friction during sliding wear test.
Section snippets
Experimental
In this study, cast unreinforced A356 Al (Al–7 wt.% Si and 0.35 wt.% Mg) alloy is referred as A1 and A356 Al–10 and 20 vol.% composites are referred as C1 and C2 respectively. Both C1 and C2 composites have been processed by stir cast technique [2], [3]. A1, C1 and C2 had hardness (VHN) of 48, 65 and 85 respectively. In the present study, a commercial available polymer based brake pad, which is intended to use against cast iron disc brakes has been used as pin. Brake pad materials were
Results and discussion
After wear tests, C1 and C2 discs and respective pins had Ra values in the range of 0.9–1.3 μm and 1.1–2 μm respectively. However, for the case of A1 discs and pins, measured Ra values were in the range of 5–11 μm and 5.4–14 μm respectively. Magnitude of wear rates of A356 Al alloy disc and the pin are significantly higher; hence the data obtained from weight loss method is shown in Fig. 1. Here, it is appropriate to mention that pin wears out rapidly when slid against A356 Al alloy disc.
Conclusions
Wear and friction characteristic of A356 Al alloy and it composites having 10 and 20 vol.% SiCP have been evaluated using pin-on-disc set up at sliding speed in the range 1–5 m/s and a constant load 192 N. The following conclusions are drawn from the work:
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Wear rate of discs of A356 Al alloy–20 vol.% SiCP composite/brake pad tribo systems is extremely low and under most conditions wear rates are negative and the corresponding pin wear rates are always higher than the disc wear rates at all
Acknowledgements
The authors wish to thank Department of Science and Technology (DST), Government of India, New Delhi, for financial support. The author Mr. R.C. Shivamurthy would also like to thank Mr. Gurulinga, Senior Technical Assistant, Department of Materials Engineering, Indian Institute of Science, Bangalore, India, for his constant support in carrying out SEM analysis of worn surfaces.
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Present address: Advanced Manufacturing Engineer, GE Healthcare, Bangalore 560 066, Karnataka, India.