Effect of nitridation on hydrothermal aging of Y-TZP

The effect of nitridation at 1650degreesC during 2 hours on hydrothermal aging of Y-TZP stabilized with 2.5 % moles of yttria has been studied. It has been shown that the superficial layer of nitrided samples was resistant to t-->m phase transformation in water at 100degreesC during the investigated time period (850 hours). Conversely, the non-nitrified material transformed at the surface but no significant reduction in biaxial strength was observed. However, the biaxial strength of nitrided samples diminishes with hydrothermal aging time, which is thought to be due to the presence of an important amount of monoclinic phase inside the sample which is due to the high transformability of tetragonal grains. It appears that water does not play any important role in degradation of nitrided samples. The observations has been associated to changes in crystalline structure of a surface layer and to the high transformability of the tetragonal phase to monoclinic one inside the nitrified samples.