Development of substrate-coating adhesion in thermal spraying

Abstract

Different mechanisms for the development of substrate-coating adhesion during thermal spraying are considered. The most important is mechanical interlocking formed chiefly due to roughness of the substrate surface, high pressures developed on the droplet impact, and solidification of the lower part of the splat. Possible deformation of the substrate surface, rebounding of the impinging droplets, and influence of spraying at off-normal angles on the development of adhesion are also considered. Thermal mechanisms involving partial or complete melting and subsequent solidification in the substrate interfacial region are shown to be effective in creation of adhesive bonds. The roles of the diffusion processes and the influence of the splat morphology on adhesion are also discussed. The theoretical results agree well with the observed tendencies of behaviour of the thermal spray coatings and in the case of the thermal mechanisms of adhesion these results are in good agreement with the experimental data. The splashing of droplets impinging on to the substrate surface during thermal spraying is considered. Supercooling formed in the flattening droplet is shown to consist of the thermal supercooling and that which arises due to the high pressure developed on the droplet impact. Solidification starts when the supercooling exceeds some critical value. With a ‘cold’ substrate, when its temperature is less than a critical value, a marked contribution to the supercooling can be attributed to the high pressure effect. In this case a regular disc shaped splat will be formed in the central part of the flattening droplet and splashing will occur at the periphery. With a ‘hot’ substrate, when the pressure effect on the supercooling is reduced and the thermal effect is increased, no splashing occurs and a regular disc shaped splat is formed. The influence of the residual stresses on substrate-cooling adhesion is discussed.