GRAIN-SIZE EFFECT ON ABRASIVE WEAR MECHANISMS IN ALUMINA CERAMICS

Despite the wealth of literature on the friction and wear behaviour of ceramics, their wear mechanisms are far from well understood. The need to develop a precise understanding of the wear mechanisms is imperative for the development of better wear-resistant ceramics. Particularly in alumina ceramics, grain coarsening has been found to improve the crack growth resistance characteristics. However, if the same would be beneficial for wear resistant applications has not been systematically investigated. To evaluate if there exists a strong grain size effect on abrasive wear mechanisms in ceramics, a series of tests were conducted under severe sliding contact conditions in an alumina ceramic. Unlubricated sliding wear tests were performed on alumina of three different grain sizes, 0.7, 5 and 25 mum, using a blunt conical diamond indenter of about 100-muM tip radius as the slider at room temperature under normal loads of 8-40 N and sliding distances up to 120 mm. The abrasive wear behaviour was characterised by measuring the wear volume and wear rate as a function of normal load and sliding distance. In addition, the friction behaviour was also studied simultaneously as a function of these two variables. The wear damage mechanisms were examined using both optical and electron microscopy as well as energy dispersive X-ray spectroscopy. For a given grain size and sliding distance, the width and depth of the worn track (hence wear volume) and the frictional force all increased with normal load. These observations were more markedly displayed in the coarse-grained than in the fine-grained alumina. In the 0.7-mum fine-grained alumina, the wear process appeared to be mainly controlled by plastic deformation especially at low loads (e.g. < 10 N). At higher loads, however, both plastic deformation and grain boundary fracture processes seemed to be involved. In the 5-mum and 25-mum coarse-grained aluminas, the predominant material removal process was controlled by both intergranular and transgranular fractures.