Influence of secondary phase on residual stress of plasma sprayed glass coatings

Abstract

The behaviour of glass coatings during thermal spray deposition is fundamentally different from that of metal and ceramic coatings because quench stresses can berelaxed during spraying. However, the expansion coefficient mismatch between the coating and the steel substrate remains an important source of residual stress and this paper investigates the effect of incorporating an alumina second phase into the glass to reduce the mismatch. Real time measurements on the deflection of coated specimens during spraying show that the residual stress can be reduced to zero by controlling the second phase content. The results also show that substantial temperature variations may be developed across large substrates during spraying as a consequence of the scanning action. These variations can affect the residual stress owing to the thermally activated nature of the stress relief and the marked sensitivity of glass properties to temperature. This research indicates that the pattern of residual stress developed in glass coatings is complex and will depend upon the substrate dimensions and the process operating conditions.