CRACK GROWTH BY THE ISOTHERMAL MARTENSITIC PHASE TRANSFORMATION IN TETRAGONAL ZIRCONIA POLYCRYSTALS

In this study, we investigated the crack nucleation, growth and propagation on the surface of Y-TZP (yttria-stabilized tetragonal zirconia polycrystals, noted by Y-TZP) during isothermal phase transformation by low temperature ageing. Crack initiation and growth on the surface of Y-TZP specimen was dependent on the sintered microstructure, i.e, sintered density, grain size, pore structure, and residual stress etc.. In the case of Y-TZP with 2mol % yttria content, the phase transformation of tetragonal to monoclinic phase began on the surface and it induced a crack nucleation in the specimen at the initial stage of low temperature ageing. Most of cracks in 2Y-TZP by low temperature ageing were firstly formed on the surface of specimens (free surface, weak bonding grains, etc.) where the change of strain free energy for a tetragonal to monoclinic transformation was small, and surface cracks grew into the bulk interior through the grain boundaries.