FRICTION AND WEAR CHARACTERISTICS OF MULLITE, ZTM AND TZP CERAMICS

The friction and wear of self-mated mullite-zirconia-toughened mullite (ZTM)-TZP ceramics have been investigated using a block-on-ring tribo meter in different lubricants at varying loads. Load-dependent wear tr ansitions were observed for these ceramics. The wear transition was us ually accompanied by the abrupt change of friction coefficient and wea r rate. The main pretransition wear mechanisms are plastic deformation , ploughing and sometimes microfractures, while fracture is the domina nt wear mechanism of posttransition. Compared with mullite, TZP has th e distinctive feature of having various cracks on its worn surfaces, w hile no large crack was observed on the worn surfaces of mullite even when wear transition had occurred. Phase transformation of zirconia wa s expected to have a two-sided influence to the wear of ceramics: it n ot only restrains the propagation of preexited cracks, but also induce s microcracks and causes high residual stress. In ZTM composites, the wear rate is usually smaller than that of mullite at low loads and lar ge cracks occur more easily on the worn surfaces as the volume content of zirconia is increased. (C) 1998 Elsevier Science S.A. All rights r eserved.