Ageing of zirconia-toughened alumina ceramics under different hydrothermal conditions

Changes in the crystalline phases of ZrO2 in different zirconia-toughened alumina (ZTA) compositions were studied at lower temperature (150-250 degrees C) by exposing them in a stream of humid air for a period of 24 h. Fresh samples of these compositions were also aged in an autoclave with superheated water for 2.5 h in the same temperature range, and the degree of phase transformation was compared with data from the previous samples to establish the effect of pressure on this phenomenon. It was observed for all the ZTA compositions that phase transformation increased monotonically with ageing time, temperature and pressure, although for the compositions with a lower percentage (15 vol%) of zirconia (3 mol% Y2O3) ageing in autoclave was more detrimental particularly at lower temperature. As the ageing temperature increased in the composition with 15 vol% ZrO (3 mol% Y2O3), the influence of ageing duration on phase transformation of ZrO2 increased and the effect for 24 h of exposure time was found to be even more catastrophic than that of autoclave pressure at 250 degrees C. On the other hand for the ZTA compositions containing more than 15 vol% zirconia (3 mol% Y2O3), the damaging effect of ageing in the autoclave was greater and this difference increased with increase in temperature