Formation of α-Al₂O₃ on 316L stainless steel

Thin films of alpha-phase aluminium oxide (Al₂O₃) are fabricated on 316L stainless steel substrates by radio frequency (RF) magnetron sputtering in two steps. First, the Al/α-Al₂O₃ films are deposited at different concentrations of α-Al₂O₃ (10, 50 and 100%) in a vacuum chamber at a temperature of 400°C. Next, the samples are heat-treated at different temperatures for a period of 2 h. The authors measured the phase, microstructure, thickness and 3-D topography by X-ray diffraction, field-emission scanning electron microscopy ADE Phase-Shift, field-emission transmission electron microscopy and high-resolution transmission electron microscopy. It was found that using the 100% α-Al₂O₃ condition yielded oxidised coatings with 61·4% α-Al₂O₃ at 800°C, but these films were very thin. In order to minimise the effect of thermal expansion mismatch between the substrate and the deposited coatings and increase the thickness of the coatings, Al₂O₃/Al/Al₂O₃ multilayers were generated. These coatings are compact and have smooth, crack-free surfaces. The oxidised coatings have good bonding strength with the substrate and are very dense. After oxidation at 800°C, the coatings mainly consist of α-Al₂O₃ and θ-Al₂O₃ phases, indicating that the addition of seeds reduced the transformation temperature.