Formation of α-Al2O3 on 316L stainless steel

Abstract

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.