Deposition and mechanical properties of polycrystalline Y₂O₃/ZrO₂ superlattices

Polycrystalline Y₂O₃/ZrO₂ superlattice thin films were deposited using opposed-cathode reactive magnetron sputtering. Pulsed direct-current power was used to eliminate arcing on the metallic targets. Radio-frequency power was applied to the substrates to achieve ion bombardment of the growing film. In order to reproducibly deposit at high rates in Ar-O₂ mixtures, the Y target voltage was used to indirectly feedback-control the O₂ partial pressure. Deposition rates as high as approximately 70% of the pure metal rates were achieved, typically 3.5 μm/h. Superlattices with periods ranging from 2.6 to 95 nm were deposited. Y₂O₃ layer thicknesses were either 75% or 50% of the superlattice period. X-ray diffraction and transmission electron microscopy studies showed well-defined superlattice layers. The ZrO₂ layers exhibited the high-temperature cubic-fluorite structure, which was epitaxially stabilized by the cubic Y₂O₃ layers, for thicknesses ≤7 nm. The equilibrium monoclinic structure was observed for thicker ZrO₂ layers. Nanoindentation hardnesses ranged from 11.1 to 14.5 GPa with little dependence on period. The hardness results are discussed in terms of current superlattice hardening theories.