ANALYSIS OF KIDNEY-SHAPED INDENTATION CRACKS IN 4Y-PSZ

Vickers indentation cracks in zirconia containing ceramics very often exhibit two non-connected mirror symmetric branches with kidney-shaped morphology. Kidney cracks, generated with different indentation loads (98-490 N) in 4 mol.% yttria-partially-stabilized-zirconia (4Y-PSZ), are described and analysed by proposing a model which combines the obs erved geometrical features with residual stress considerations. The cr ack shape is affected by a radial decreasing hydrostatic stress field originating in the plastic deformation zone underneath the impression and with its centre al a certain depth from the surface. The hydrostat ic stresses modify the generally assumed point force residual stress f ield. The model provides a self-similar description of the residual st ress intensity factor for kidney cracks from different indentation loa ds. Furthermore, the experimental observation that one single half-pen ny crack rather than the two kidney cracks forms at higher indentation loads is explained by the model. For 4Y-PSZ a reasonably good agreeme nt between the theoretically calculated and the experimentally observe d critical transition load from kidney to half-penny geometry, P, is obtained. The transition load varies with toughness, K-R, and hardness , H, according to P similar to K-R(4)/H-3, indicating the relative in fluence of deformation and fracture characteristics on crack morpholog y. It must be concluded that the kidney-shaped contour is the dominant geometry of elastic/plastic contact cracks in tough zirconia-containi ng ceramics. Copyright (C) 1996 Acta Metallurgica Inc.