No abstract

No 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.

A ball-milled mixture of glass and alumina powders has been plasma sprayed to produce alumina-glass composite coatings. The coatings have the unique advantage of a melted ceramic secondary phase parallel to the surface in an aligned platelet composite structure. The alumina raises the hardness from 300HV for pure glass coatings to 900HV for a 60wt% alumina-glass composite coating. The scratch resistance increases by a factor of three and the wear resistance by a factor of five. The glass wears by the formation and intersection of cracks. The alumina wears by fine abrasion and supports most of the sliding load. The wear resistance reached a plateau at 40-50vol% alumina, which corresponds to the changeover from a glass to a ceramic matrix. Keywords: glass composite coatings, wear, thermal spraying