Effect of transfer layer on dry sliding wear behaviour of cast Al-based composites synthesized by addition of TiO2 and MoO3

Two types of composites have been developed by solidification processing by addition of 3, 4, and 5 wt% powders of oxide-TiO2 and MoO3, to molten Al-5 wt% Mg alloy. The oxide particles react with the molten alloy resulting in alumina and releasing alloying elements of Ti or Mo. Dry sliding wear behaviour of pins of cast composite, fabricated by solidification of melt-particle slurry in mold, has been determined by pin-on-disc wear tests carried out conventionally and while removing wear debris by camel brush. The accumulated volume loss in composites increases linearly with increasing sliding distance and the wear rate increases more or less linearly with increasing load. Increasing particle content decreases wear rate at a given load. The accumulated volume loss is considerably higher when wear debris is removed by camel brush during dry sliding wear. The nature of the wear debris has been confirmed to be oxidative. The relatively brighter compacted oxide transfer layer could be observed in the SEM micrograph of worn pin surfaces of the composites developed by addition of MoO3 and TiO2 respectively. Since the accumulated volume loss in wear is relatively more when the wear debris is removed during dry sliding wear test it may be inferred that wear debris is more beneficial for wear resistance through formation of transfer layer rather than its harmful role in enhancing volume loss through three body wear. At higher loads, the oxide debris are expected to get better compacted to form transfer layer, spread over a larger area of the sliding surface and thus, their removal causes a larger wear compared to that without removal of wear debris. However, a larger cover of transfer layer at higher load does not necessarily imply reduced accumulated volume loss because the wearing process is more aggravated at higher load. Apart from adhesion, micro-cutting and abrasion, the transfer layer also flakes off during dry sliding wear as indicated by the presence of chunky sheet of oxides in wear debris.