Phase-Stability in Nanostructured and Coarse-Grained Zirconia at High-Pressures

Zirconia powders with grain sizes from the nanometer (5 - 30 nm) to the micrometer (similar to 1 mu m) regime were investigated by in-situ high-pressure X-ray diffraction at pressures up to 14 GPa. In micrometer sized ZrO2, a broad transition is observed at increasing pressure from the monoclinic to the orthorhombic polymorph. This first-order phase transition is, therefore, concluded to be martensitic in character. In the nanosized material, a metastable tetragonal polymorph exists and the stabilization is interpreted as a result of the surface stress of the nanocrystalline grains. An effective internal pressure in the particles above 2.5 GPa is estimated. The critical particle radius for the stabilization of the metastable phase in nanosized ZrO2 is calculated to be 4-6 nm.