Size-driven phase transformation and microstructure evolution of ZrO2 nanocrystallites associated with thermal treatments

Thermal stability of nanocrystallites plays an important role in the manufacturing and application of nanocrystalline ceramics/glass-ceramics. This study explores the effects of thermal treatments on the microstructures of two ZrO2-SiO2 nanocrystalline glass-ceramics (NCGCs), an undoped one and a yttria-doped one. The two as-sintered NCGCs were composed of tetragonal (t) and monoclinic (m) ZrO2 nanocrystallites, and SiO2 component was amorphous. t-ZrO2 and m-ZrO2 nanocrystallites were metastable during thermal treatment. The content of m-ZrO2 in the undoped ZrO2-SiO2 NCGC first increased after thermal treating at 850 °C for 5 h, then decreased after thermal treating at and above 950 °C. After thermal treating at 1250 °C for 5 h, t-ZrO2 nanocrystallites experienced a rapid phase transformation during cooling, resulting the formation of 88.6 vol% m-ZrO2. Size-driven phase transformation was utilized to explain the metastability of t-ZrO2 and m-ZrO2 nanocrystallites. In contrast, the content of m-ZrO2 in the yttria-doped ZrO2-SiO2 NCGC continuously decreased with the increase of thermal treatment temperature. The addition of yttria improved the phase stability of t-ZrO2 up to at least 1250 °C. Crystallite size of both t-ZrO2 and m-ZrO2 nanocrystallites increased with the increase of thermal treatment temperature in the two NCGCs. The presence of residual thermal stress in the as-sintered NCGCs changed the lattice spacing of t-ZrO2 and m-ZrO2 nanocrystallites, and the stress can be released after thermal treatment. Thermal treatment exerts significant influences on the microstructure of ZrO2-SiO2 NCGCs.