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

Abstract

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 degrees C to 1250 degrees 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 degrees C consists of an alpha-alumina phase. Treatments at temperatures above 1150 degrees C give rise to an alumina scale with some defect probability. An increase in the temperature up to 1200 degrees 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 the best for guaranteeing the formation of a defect-free, compact, adherent, and continuous alpha-alumina scale thick enough to support satisfactory wear and biological conditions.

Effect of environmental changes on the corrosion behaviour of Al2O3/MA 956 system

“…The nature of the alumina layer, a-alumina [9], its high compactness and adherence to the metallic substrate hinders a connection between the corrosive medium and the metallic substrate, so there is no possibility for the corrosive process to be allowed to develop. Fig.…”

Evaluation of the scale integrity in the Al2O3/MA956 system at different polarisation by using EIS

“…It is known that bulk a-alumina possesses a high chemical inertia in physiological media [2]. The oxidation kinetic study performed on MA956 made it possible to establish the optimal conditions to obtain an adherent a-alumina layer [6,7]. Therefore they can present defects such as pores, voids, channels or cracks which can act as easy paths for the entrance of aggressive medium.…”