Second phases and material transfer in alumina ceramics sliding systems

To explore the effects linked to the presence of second phases in the wear mechanisms and performances of alumina, some low-purity alumina ceramics with an alumina content ranging from 79.0 to 95% were investigated. The tribological behaviour of these materials was evaluated using a pin-on-disc apparatus, coupling them with a reference material, a high-purity alumina, 99.6% alumina content. Extensive SEM observations at normal incidence and on the cross sections of the worn surfaces of specimens showed fairly close relationships among chemical–mineralogical composition, material transfer and friction and wear responses. Particularly interesting are some behaviours directly related to the presence of the transferred layers: (i) adhesion and sticking phenomena, due to the softened low-melting layers at the interfaces and (ii) an increasing wear rate of the high-purity alumina pins as the alumina content of the coupled discs decreases.