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The thermodynamic stability of tetragonal (t-) ZrO2 nanocrystallites below the bulk stability temperature 1200 °C was studied through specially synthesized crystallites that exhibited an extremely slow coarsening rate. The nanocrystallites were mechanically transformed to the monoclinic (m-) structure, and, because the crystallite size was kept below approximately 20 nm, the t-structure was completely recovered solely by thermal treatments between 900 and 1100 °C. These results gave strong evidence to the notion that, for sufficiently small crystallite size, nanocrystalline t-ZrO2 is not just kinetically metastable but can be truly thermodynamically more stable than the mpolymorph in air below 1200 °C.

Tetragonal ZrO 2 nanocrystallites-with or without yttria (3 mol%) doping-have been synthesized via a precipitation process in which the hydrous oxide precipitate reacts with hexamethyldisilazane (HMDS) vapor before calcination. The nanocrystallites are formed and retain a tetragonal structure for hours after calcination at temperatures of 300°-1100°C. The enhanced structural metastability has been attributed to the combined effect of suppressed grain growth and reduced surface energy that results from the HMDS treatment.

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