Hydrothermal degradation of zirconia ceramics

The surface degradation of yttria doped tetragonal zirconia polycrystals (Y - TZP) subjected to steam at a temperature of 200°C for various periods of times was investigated. The samples were obtained from two powders having different specific surface areas and mean particle sizes. The powders were sintered at three temperatures 1350, 1400 and 1450°C. The meas-urements of the depth of the degraded 1ayer and an analysis of its phase composition were carried out using SEM and X-ray diffraction. SEM analysis revealed that the degradation was much more advanced on samples sintered at the higher temperatures. The results revealed that in the material sintered at higher temperatures (l400°C and l450°C) the near surface zone had trans-formed into the monoclinic phase. This occurred even after the shortest time of exposure (6 hours). Additionally, it was found that samples prepared from finer powder were less resistant to phase transformation and surface degradation. The role of residual stress in the process of Y - TZP degradation is discussed.