Metastable tetragonal zirconia formation and transformation in reactively sputter deposited zirconia coatings

Zirconia coatings were produced by reactive d.c. magnetron sputter deposition using a system with multiple sputter sources and a biased substrate stage. Crystal structure and phase stability of the coatings were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Tetragonal zirconia with either a random orientation or a highly (111) preferred orientation was deposited when a substrate bias was applied, whereas coatings grown with no substrate bias had the equilibrium monoclinic structure. It was revealed that bias sputtering effectively decreased crystallite size in the as-deposited coatings, which resulted in room temperature stabilization of the metastable tetragonal phase. XRD analysis of annealed coatings showed that the volume fraction and stability of the tetragonal phase was strongly dependent on substrate bias and annealing temperature.