Optimal conditions for alumina coating formation on the MA956 superalloy for prosthetic bearing applications

An experimental study of the oxidation treatment at high temperature of the ODS MA956 superalloy was conducted in an attempt to achieve a protective alumina scale for biomedical applications. A quadratic response-surface model was developed in order to study the effects of treatment time and temperature (in the range of 1000°C to 1250°C) on scale thickness. The obtained model adequately represents the experimental response and shows that the thickness gradients of the layer increase with the temperature for each exposure time and decrease steadily to zero as the treatment time increases. The microstructural characterization reveals that the alumina scale formed at or above 1000°C consists of an α-alumina phase. Treatments at temperatures above 1150°C give rise to an alumina scale with some defect probability. An increase in the temperature up to 1200°C gives rise to the appearance of some blistering of the superficial scale. An oxidation treatment of 100 h at 1100°C was found to be the best for guaranteeing the formation of a defect- free, compact, adherent, and continuous α-alumina scale thick enough to support satisfactory wear and biological conditions.