Growth-controlled cubic zirconia microstructure in zirconia–titania nanolaminates

Abstract

Zirconia microstructure selection (phase and crystallographic orientation) was studied in sputter-deposited multilayer films consisting of nanocrystalline ZrO2 and amorphous TiO2. The goal was to understand the mechanism for ZrO2 microstructure selection as a function of nanolaminate architecture (number of interfaces and ZrO2 and TiO2 layer thickness) in a growth regime of limited surface diffusion. The results show that there are two competing paths that ZrO2 microstructure can follow in a ZrO2–TiO2 nanolaminate. One path depends upon geometric shadowing resulting in the formation of a columnar morphology within a ZrO2 layer. This path leads to cubic (220) growth as an intermediate microstructure, and ultimately to the formation of cubic (200) crystallites spanning several bilayers. In this case, ZrO2 renucleation within a layer is suppressed. The second path depends upon renucleation within a ZrO2 layer, involving first tetragonal (111) formation, and then the transformation of tetragonal (111) to monoclinic(11-1)-(111), consistent with a finite crystal size effect. The establishment and maintenance of a columnar morphology (and hence, the suppression of renucleation events), ultimately leads to cubic (200) growth, and is consistent with a growth-controlled microstructure selection mechanism. This mechanism appears to strongly depend upon the existence and planar quality of the TiO2 growth–restart surfaces.