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

The behaviour of glass coatings during thermal spray deposition is fundamen tally different from that of metal and ceramic coatings because quench stre sses cart be relaxed during spraying. However, the expansion coefficient mi smatch between the coating and the steel substrate remains an important sou rce of residual stress and this paper investigates the effect of incorporat ing an alumina second phase into the glass to reduce the mismatch. Real tim e measurements on the deflection of coated specimens during spraying show t hat the residual stress can be reduced to zero by controlling the second ph ase content. The results also show that substantial temperature variations may be developed across large substrates during spraying as a consequence o f the scanning action. These variations can affect the residual stress owin g to the thermally activated nature of the stress relief and the marked sen sitivity of glass properties to temperature. This research indicates that t he pattern of residual stress developed in glass coatings is complex and wi ll depend upon the substrate dimensions and the process operating condition s.