Stability of Y-TZP during hydrothermal treatment: neutron experiments and stability considerations

The degradation of yttria-stabilised zirconia in humid atmospheres at temperatures around 250 °C is influenced by a number of microstructural parameters. The corrosive attack starts from the surface and is associated with the tetragonal to monoclinic transformation. In order to investigate structural changes chemically homogeneous single phase t-powders had been hydrothermally treated in a D2O-atmosphere and subsequently investigated by neutron diffraction. Slight structural changes were observed after water treatment: a general contraction of both a and c parameters, hence, the “free” structural z(oxygen) parameter as well as the c/a-ratio are closer to the values of cubic zirconia. In this paper the structural changes due to the penetration of water radicals during hydrolysis are discussed. They give rise to stresses which then have an impact on the energy balance between the tetragonal and the monoclinic phase (martensitic transformations). Since the powder state can be assumed to be mostly free from macro-straining, the changes of the lattice constants were used to deduct a shape change tensor. Hence assuming linear elastic behaviour (isotropic elastic modulus of 200 GPa for both a surface grain and its environment) maximum stresses of 328 MPa for the a-axis and 488 MPa for the c-axis were found. Thus, water radicals basically lead to further tensile stresses in the surface which increase the energy difference between the t and m phase, i.e. a further destabilisation.