PHASE-STABILITY IN NANOSTRUCTURED AND COARSE-GRAINED ZIRCONIA AT HIGH-PRESSURES

Zirconia powders with grain sizes from the nanometer (5 - 30 nm) to th e micrometer (similar to 1 mu m) regime were investigated by in-situ h igh-pressure X-ray diffraction at pressures up to 14 GPa. In micromete r sized ZrO2, a broad transition is observed at increasing pressure fr om the monoclinic to the orthorhombic polymorph. This first-order phas e transition is, therefore, concluded to be martensitic in character. In the nanosized material, a metastable tetragonal polymorph exists an d the stabilization is interpreted as a result of the surface stress o f the nanocrystalline grains. An effective internal pressure in the pa rticles above 2.5 GPa is estimated. The critical particle radius for t he stabilization of the metastable phase in nanosized ZrO2 is calculat ed to be 4-6 nm.