Analysis of kidney-shaped indentation cracks in 4Y-PSZ

Abstract

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 observed geometrical features with residual stress considerations. The crack shape is affected by a radial decreasing hydrostatic stress field originating in the plastic deformation zone underneath the impression and with its centre at a certain depth from the surface. The hydrostatic stresses modify the generally assumed point force residual stress field. The model provides a self-similar description of the residual stress intensity factor for kidney cracks from different indentation loads. Furthermore, the experimental observation that one single half-penny crack rather than the two kidney cracks forms at higher indentation loads is explained by the model. For 4Y-PSZ a reasonably good agreement between the theoretically calculated and the experimentally observed 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∗ ∼ K 4 R H 3 , indicating the relative influence of deformation and fracture characteristics on crack morphology. It must be concluded that the kidney-shaped contour is the dominant geometry of elastic/plastic contact cracks in tough zirconia-containing ceramics.