Ab initio and experimental study on the effect of Y additions on the phase formation and thermal stability of Al2O3 thin films deposited by filtered cathodic arc evaporation

The effect of 0.6 to 7.5 at.% Y addition on the phase formation and thermal stability of Al2O3 has been investigated using density functional theory and post-annealing of Y containing alumina thin films deposited by filtered cathodic arc evaporation. The calculations indicate the decomposition of the Y containing γ- and α-Al2O3 solid solutions into Y2O3 and the corresponding alumina phase. This prediction is consistent with experiments: The lattice parameters of the γ-Al2O3 thin films with and without Y are comparable and are hence inconsistent with the predicted expansion in equilibrium volume as Y is incorporated into the γ-(Al,Y)2O3 solid solution. The predicted metastable character of γ-(Al,Y)2O3 is also consistent with the formation of γ-Al2O3, Y2O3, and Y3Al5O12 in the as-deposited state as identified by X-ray diffraction. While the phase transition from γ-Al2O3 to α-Al2O3 phase takes place at T ≤ 1100 °C, the formation of traces of α-Al2O3 is restrained to T ≤ 1200 °C for alumina thin films containing Y and additionally the formation of Y3Al5O12 is observed. The restrained transition of the metastable γ-Al2O3 polymorph to α-Al2O3 may be explained by the segregation of Y at the metastable Al2O3 grain boundaries impeding mass transport and hence retard both formation and grain growth of α-Al2O3.