The Bond Strength and Ageing of Plasma-Sprayed Coatings of Alumina on Metal Substrates

Abstract

Changes in the bond strength of alumina coatings after soaking in deionised water have been studied in conjunction with investigations of the wear characteristics of ultra-high molecular weight polyethylene against ceramics coated onto metal substrates. The results reported in this paper concern plasma sprayed alumina coatings, applied both directly and using an intermediate bonding layer, onto medical grade stainless steel or titanium alloy. The bond strength was measured in tension both before immersion and at various intervals over a period of up to eighteen months (13 000 hours) soaking in water. After about four months a significant deterioration in bond strength was noted. This deterioration continued for the duration of the investigation and the reductions in the bond strengths observed were often greater than 75 per cent of the original values. A white powdery deposit was found in the containers in which the ageing was carried out and a similar deposit was also seen on some of the fracture surfaces. Analysis identified Al(OH)3 and an amorphous phase of alumina. A complex diffusion mechanism appeared to have taken place which resulted in a deterioration in the properties of the ceramic coatings.