Structure-property correlations in aluminum oxide thin films grown by reactive AC magnetron sputtering

Abstract

Crystalline alumina thin films were deposited on glass, silicon and WC-Co substrates by reactive inverted cylindrical AC magnetron sputtering technique both with and without aluminum precoating at two working pressures. Coatings prepared on Si wafers with Al precoating were found to be γ-phase of aluminum oxide. However alumina coatings prepared on Si wafer without Al precoating showed evidence for the presence of δ and θ-phases besides the γ-phase. Two coating samples grown on Si substrates with Al prelayer were annealed at 400 and 900 °C for 8 h under flowing oxygen conditions. Annealing at 400 °C increased the grain size for γ-alumina phase while annealing to 900 °C formed δ and κ-phases in the coatings. Scanning electron microscopy and energy dispersive X-ray microanalysis were carried out to investigate the thin film surface morphology and composition. While γ-alumina coatings were ultrafine grained, coatings containing δ- and κ-phases showed large size grains. Nanoindentation and nanoscratch tests were performed on all samples to determine the thin film hardness and scratch resistant properties. Higher working pressure during film deposition seemed to lower film hardness values. Significant differences were observed in the mechanical properties of alumina thin films grown with Al precoating on silicon substrates. While the Al precoating improves thin film adhesion and leads to less severe failure, it significantly decreased the thin film hardness as measured by nanoindentation technique.