Towards improving low-temperature degradation of zirconia/alumina ceramics via in-situ formation of an Al2O3 functional surface layer through sintering in the presence of graphite powder

Zirconia (ZrO2) and alumina (Al2O3) ceramics are used in technical and biomedical applications. The great challenge in ZrO2-based ceramics, to improve their mechanical properties, is to take control of the stress-activated t → m martensitic transformation, since this transformation causes accelerated low-temperature degradation (LTD) under ambient conditions, leading to premature failure by the presence of water or its vapor. Dispersion of Al2O3 in a ZrO2 matrix, the so-called alumina toughened zirconia (ATZ), can restrain LTD of mechanical properties of zirconia, however exposure of zirconia to moisture must be avoided to suppress the t → m transformation. In this paper, the in-situ formation of an alumina layer surrounding ATZ pieces, during sintering under the presence of graphitic compounds and a low oxygen atmosphere, is reported. The layer formation mechanism consists of the reaction of CO with Al2O3 during the heating of the specimens, where a volatile species of aluminum forms, which, on contact with the specimen surface and due to the presence of CO2, is oxidized to the non-volatile Al2O3, thus forming the surface layer. This approach improves the hydrothermal stability and enhances the possibility of applying ATZ in ceramic implants, solid oxide fuel cells, and thermal barrier coatings.