Differences in the tetragonal to monoclinic phase transformation rate in hot water of 3 mol% Y2O3-ZrO2 ceramics under different surface conditions

Hot-isostatic-pressed (HIP'd) tetragonal zirconia ceramic containing 3 mol% Y2O3 was soaked in hot water at 95-100 °C to evaluate the transformation rate from the tetragonal (T) to the monoclinic (M) phase. The transformation rate differed considerably between the polished surface and the surface cut by a diamond wheel. After about 1000 h of soaking in hot water, the volume of the M-phase was 50% in the cut surface and less than 20% in the polished surface. Tetragonal ZrO2 with polished and cut surfaces were annealed at 1000 °C for 2 h in air. When the annealed specimens were soaked in hot water, the transformation rate in the cut surface was the same as that before annealing; on the other hand, the M-phase volume in the polished surface increased to 70% after 1000 h of soaking. Surface roughness, a rhombohedral phase generated by cutting or machining and orientation of the T-phase crystal due to machining, are thought to be causes of the transformation rate change. However, not one of them on its own could explain all the phenomena without contradiction.