Reactive sputter deposition of ceramic oxide nanolaminates: Zirconia-alumina and zirconia-yttria model systems

We review the growth of ceramic nanolaminate films by reactive sputter deposition. We investigate phase formation in nanolaminates with the same nominal architecture but with different chemical constituents. Two model systems, zirconia-alumina and zirconia-yttria, allow us to compare the effects of chemical reactivity between constituents at their interface. In zirconia-alumina nanolaminates, each component is a separate entity and their interface is incoherent. Phase evolution in zirconia layers of decreasing thickness is predicted by the finite crystal size effect. Tetragonal zirconia is produced in layers whose thickness is less than the critical thickness for stabilization of monoclinic zirconia, the STP phase. The amorphous structure of alumina is a consequence of its structural flexibility. Overall morphological roughness of the film arises from the polycrystalline nature of the zirconia layers. In zirconia-yttria nanolaminates, an interfacial reaction between components completely obliterates yttria as a separate entity. The reaction product, cubic zirconia-yttria, forms needle-like crystallites and accentuates the overall morphological roughness due to the polycrystalline nature of the deposit. As zirconia layer thickness increases, monoclinic zirconia is produced along with interfacial cubic zirconia-yttria. This research clearly demonstrates the ability to form interfacial oxide solid solutions at low temperature in a system in which the bulk equilibrium phase diagram predicts reaction between oxide components. Thus, thin films consisting entirely of interface reaction products can be fabricated if the bilayer spacing is small enough.