Microstructure, mechanical properties and stability of nitrided Y-TZP

The mechanical properties and microstructure of nitrided tetragonal zirconia polycrystals doped with yttria (Y-TZP) have been evaluated. It has been established that a surface layer is formed during nitriding at 1650 °C for 5 min, 1 and 2 h. Such layer exhibits a gradient in grain size, which results in a variation in the capacity of the material to undergo a stress assisted tetragonal to monoclinic (t–m) phase transformation together with a build up of residual stresses. After nitriding for 1 h, the outer surface forms non-transformable t′ phase, which protects the material from low temperature ageing degradation. It is shown that, in contrast with the original Y-TZP, the nitrided surface does not transform in the presence of water at 100 °C during time periods of up to 900 h. By contrast, nitrided Y-TZP is not stable after exposure in air at temperatures of 600 and 800 °C, since t–m transformation occurs in a subsurface layer of the nitrided rods, and this induces high internal stresses.