Genesis and role of wear debris in sliding wear of ceramics

We survey the formation, appearance and properties of wear debris in ceramics and their influence on the wear of these materials. These differ from those of metals, in accord with the different mechanical and chemical properties of ceramics. Ceramics, in contrast to metals, do not form hard and cohesive mechanically alloyed surface layers. The formation of wear debris is very sensitive to the environment (humidity), it occurs mostly by fracture on different scales: microfracture at low loads, grain boundary fatigue at intermediate loads and macroscopic fracture at high loads. The wear debris that remain in the wear track are ground to a very fine powder by continued rubbing. In dry ambient, this powder has low mechanical strength and has very little influence on wear. In humid ambient, the phenomena depend on the material. Tribochemical reactions of non-oxide ceramics form relatively large amounts of compact hydrated oxide. On oxide ceramics such as alumina and silica, interaction with water vapor has been observed to create thin hydroxide layers, which can act as lubricants, as observed on alumina. The tribochemical layers often form rolls on the surface. These rolls do not act as 'rolling bearings' and do not reduce friction or wear. Ambient humidity causes adhesion between the wear debris which are compacted into layers that have sufficient cohesion to reduce wear by distributing the contact stresses. In water and some aqueous solutions, silicon nitride and silicon carbide dissolve in water and do not form wear debris. (C) 2000 Elsevier Science S.A. All rights reserved.