The measurement of surface contact fatigue and its application to engineering ceramics

Contact fatigue deformation and fracture in three zirconia ceramics has been induced by pressing 120 degrees hardened silver steel conical tips against flat polished ceramic surfaces. The repeated point contact loading method that has been developed is described. It has been shown that it is possible for a softer material to induce cumulative plastic deformation, and subsequent fracture, in a harder material. The performance of single crystal cubic zirconia, tetragonal toughened zirconia and partially stabilized zirconia have been compared. In all cases, the ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the zirconia substrate. Cracking arising from fatigue damage under cyclic loads of 19.6 +/- 9.8 N was observed to occur in (001) calcia stabilized zirconia much earlier than it appeared in the (111) plane. In both cases, the formation and progression of a 'perimeter' crack was visible at the edge of the contact zones as the number of cycles was increased. It has been proposed that the cracking is initiated by a dislocation interaction mechanism. The subsequent crack propagation is primarily conchoidal in form, rather than crystallographically oriented. A tetragonal to monoclinic martensitic transformation occurred in the ceria stabilized zirconia, beneath and adjacent to the contact zone. The transformation zone increased in size as the number of cycles increased. The volume expansion associated with this transition caused granular lifting, first visible around the periphery of the contacting areas, then intergranular fragmentation, followed by spalling of the substrate. The fatigue mechanism observed in the magnesia partially stabilized zirconia was a combination of the two above, resulting in transgranular fracture. No material transfer, i.e. metal onto ceramic, or vice versa, was generally observed until after the fracture zones had become well extended and fragmented.