Crystallite and grain-size-dependent phase transformations in yttria-doped zirconia

In pure zirconia, ultrafine powders are often observed to take on the high-temperature tetragonal phase instead of the equilibrium monoclinic phase. The present experiments and analysis show that this observation is one manifestation of a much more general phenomenon in which phase transformation temperatures shift with crystallite/grain size. In the present study, the effect of crystallite (for powders) and grain (for solids) size on the tetragonal monoclinic phase transformation is examined more broadly across the yttria-zirconia system. Using dilatometry and high -temperature differential scanning calorimetry on zirconia samples with varying crystallite/grain sizes and yttria content, we are able to show that the tetragonal - monoclinic phase transformation temperature varies linearly with inverse crystallite/grain size. This experimental behavior is consistent with thermodynamic predictions that incorporate a surface energy difference term in the calculation of free-energy equilibrium between two phases.