Formation of a protective alumina layer after sintering for the deceleration of low temperature degradation in alumina-toughened zirconia ceramics

Abstract

Zirconia-based ceramics are prone to transformation from the tetragonal phase to the monoclinic phase in the presence of moisture at low temperatures. Here, we report the formation of a protective alumina layer after conventional sintering in air for alumina toughened zirconia (ATZ) composites at 1520°C using graphite as a powder bed. Alumina layers with thicknesses of 1.7 and 2.20µm were formed after sintering at 0h for ATZ+15wt% Al2O3 and ATZ+30wt% Al2O3, respectively. A sintering time of 10h increased the thickness of this Al2O3 layer, allowing its retention after the inevitable final polishing step in the production of real ceramic bodies. This could lead to a dramatic retardation of the t→m ZrO2 phase transformation via the presence of alumina grains that can isolate the zirconia grains in the lattice structure, which prevents the propagation process under the most hostile hydrothermal conditions by taking advantage of the combined high hardness of alumina with the high fracture toughness of zirconia.