Mechanisms of room temperature metastable tetragonal phase stabilisation in zirconia

Mechanisms of tetragonal phase stabilisation, at room temperature, in nanocrystalline (<100 nm), submicrometre-sized (100 nm(-1) mu m), and bulk zirconia (ZrO2) (>1 mum) are reviewed in detail. The merits, demerits and scope of each individual model are outlined. The analysis of the literature shows that, although the mechanism of tetragonal phase stabilisation in bulk ZrO2 is well understood, the room temperature tetragonal phase stabilisation mechanism in undoped, nanocrystalline ZrO2 is controversial. Various proposed models, based on surface energy (nanocrystallite size), strain energy, internal and external hydrostatic pressure, structural similarities, foreign surface oxides, anionic impurities, water vapour and lattice defects (oxygen ion vacancies), are discussed in detail. It is proposed that generation of excess oxygen ion vacancies within the nanocrystalline ZrO2 is primarily responsible for the room temperature tetragonal phase stabilisation, below a critical size. Hence, the mechanism of tetragonal phase stabilisation in nanocrystalline ZrO2 appears to be the same as that in doped ZrO2 (at room temperature) and undoped ZrO2 (at higher temperature).