Phase transformation of nanosized ZrO₂ upon thermal annealing and intense radiation

The phase stability and microstructure evolution of zirconia nanofilms on Si substrates prepared by ion beam assisted deposition (IBAD) upon thermal annealing and intensive radiation have been studied by in situ transmission electron microscopy (TEM), ex situ X-ray diffraction, and Raman spectroscopy. For as-prepared amorphous-dominant ZrO₂ thin films, a phase transformation sequence of amorphous-to-tetragonal and tetragonal-to-monoclinic has been identified upon increasing annealing temperature from 500, 850, to 1000 °C. This phase transformation sequence varying with annealing temperature is accompanied by concomitant grain growth from ∼5 to ∼50 nm, consistent with the grain-size-controlled phase stability as a result of total energy crossover among different zirconia polymorphs. Upon ion bombardments of 350 KeV O⁺ and 1 MeV Kr²⁺ at room temperature, a monoclinic-to-tetragonal phase transformation was observed in the monoclinic-dominant ZrO₂. This monoclinic-to-tetragonal phase transformation may be attributed to the oxygen vacancy accumulation in ZrO ₂ upon irradiation. Furthermore, both 1 MeV Kr²⁺ and 350 KeV O⁺ bombardments on the amorphous-dominant ZrO₂ lead to an amorphous-to-tetragonal phase transformation as a result of radiation-induced recrystallization process. Thermodynamically metastable tetragonal ZrO₂ phase can be stabilized at room temperature under intensive radiation by relatively low-energy ion bombardments. These results suggest a method of combining both thermal annealing and ion beam technique for controlling ZrO₂ phase stability and thus tailoring materials properties for many engineering applications including actinide host matrix for advanced nuclear energy systems.