Stabilization of tetragonal and cubic phases of ZrO2 in pulsed sputter deposited ZrO2/Al2O3 and ZrO2/Y2O3 nanolayered thin films

Abstract

We have prepared nanolayered thin films of ZrO2/Al2O3 and ZrO2/Y2O3 using pulsed sputtering to study the feasibility of stabilization of various polymorphs of ZrO2. These films have been deposited at various substrate temperatures (350–700 °C), individual layer thicknesses (1.9–25.2 nm), and modulation wavelengths (Λ, 9.6–32.7 nm). The cross section of ZrO2 and Y2O3 films consisted of microcolumns, whereas the ZrO2/Al2O3 and ZrO2/Y2O3 multilayer systems exhibited a less columnar microstructure. X-ray diffraction (XRD) data showed monoclinic, cubic, and amorphous structures for ZrO2, Y2O3, and Al2O3 thin films, respectively. The tetragonal phase of ZrO2 (t-ZrO2) was stabilized for ZrO2/Al2O3 nanolayered thin films prepared at Λ≤13.8 nm. Our studies demonstrated that a critical ZrO2 thickness (≤10.5 nm at a substrate temperature of 700 °C) is required in order to stabilize the t-ZrO2 phase, which was in agreement with thermodynamically derived critical radius for the stabilization of the tetragonal phase. In the case of ZrO2/Y2O3 nanolayered thin films, the cubic phase of ZrO2 was stabilized for films prepared at 24.4 nm≥Λ≥9.9 nm. The nanolayered thin films prepared at Λ=9.9 nm exhibited satellite reflections of third order, suggesting the formation of superlattice structure. The formation of various polymorphs of ZrO2 was also confirmed by micro-Raman spectroscopy. XRD studies of nanolayered thin films heat treated in air demonstrated the stability of the tetragonal and the cubic phases of ZrO2 in the ZrO2/Al2O3 and ZrO2/Y2O3 multilayers, respectively, up to 1100 °C. ZrO2/Al2O3 and ZrO2/Y2O3 nanolayered thin films exhibited maximum nanoindentation hardness values of 30.5 and 28 GPa, respectively, which were very high as compared to the rule-of-mixture values. Detailed studies on elastic/plastic behavior of the multilayers indicated highest resistance to plastic deformation for the ZrO2/Al2O3 multilayers.