Grain growth and phase stability of nanocrystalline cubic zirconia under ion irradiation

Abstract

Grain growth, oxygen stoichiometry, and phase stability of nanostructurally stabilized cubic zirconia (NSZ) are investigated under 2 MeV Au-ion bombardment at 160 and 400 K to doses up to 35 displacements per atom (dpa). The NSZ films are produced by ion-beam-assisted deposition technique at room temperature with an average grain size of 7.7 nm. The grain size increases with irradiation dose to $\ensuremath{\sim}30\text{ }\text{nm}$ at $\ensuremath{\sim}35\text{ }\text{dpa}$. Slower grain growth is observed under 400 K irradiations, as compared to 160 K irradiations, indicating that the grain growth is not thermally activated and irradiation-induced grain growth is the dominating mechanism. While the cubic structure is retained and no new phases are identified after the high-dose irradiations, oxygen reduction in the irradiated NSZ films is detected. The ratio of O to Zr decreases from $\ensuremath{\sim}2.0$ for the as-deposited films to $\ensuremath{\sim}1.65$ after irradiation to $\ensuremath{\sim}35\text{ }\text{dpa}$. The loss of oxygen suggests a significant increase in oxygen vacancies in nanocrystalline zirconia under ion irradiation. The oxygen deficiency may be essential in stabilizing the cubic phase to larger grain sizes.