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 a lumina scale for biomedical applications. A quadratic response-surface mode l was developed in order to study the effects of treatment time and tempera ture (in the range of 1000 degrees C to 1250 degrees C) on scale thickness. The obtained model adequately represents the experimental response and sho ws 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 sc ale formed at or above 1000 degrees C consists of an alpha-alumina phase. T reatments at temperatures above 1150 degrees C give rise to an alumina scal e with some defect probability. An increase in the temperature up to 1200 d egrees C gives rise to the appearance of some blistering of the superficial scale. An oxidation treatment of 100 h at 1100 degrees C was found to be t he best for guaranteeing the formation of a defect-free, compact, adherent, and continuous oc-alumina scale thick enough to support satisfactory wear and biological conditions. (C) 1999 John Wiley & Sons, Inc.