Deposition of alumina thin film by dual magnetron sputtering: Is it γ-Al2O3?

Alumina thin films were deposited by reactive dual magnetron sputtering at 550 °C on cemented carbide substrates. A Young's modulus of 315 GPa and a Vickers hardness of 2348 were determined by nanoindentation and were compared to reference materials. The crystal structure of such films is usually referred to as γ-Al2O3; however, the crystal structure of cubic γ-Al2O3 is not well defined, not even for bulk materials. The alumina grain size of the films was about 50 nm as measured by dark-field imaging in a transmission electron microscope. The energy-filtered electron diffraction patterns were segmented: one part showed an amorphous intensity distribution, not known for γ-Al2O3, the other part contained reflections arranged in rings, the brightest of which had lattice spacings of the (4 0 0) and (4 4 0) reflections of γ-Al2O3. Therefore, the structure of the thin films is referred to as pseudo γ-Al2O3. This nomenclature expresses that this phase is different from γ-Al2O3 but among the Al2O3 phases is most closely related to this phase. Differences between the two crystal structures are highlighted and discussed with respect to lattice spacings, intensities of the various reflections, chemical composition and other physical properties. The pseudo γ-Al2O3 films contained an Al/Ar mole fraction ratio of about 17 as determined by energy-dispersive X-ray spectroscopy.