Influence of temperature on the bonding mechanism of plasma-sprayed coatings

Abstract

Although considerable work has been carried out on the bonding mechanism of sprayed coatings, the relationship between the temperatures of substrate and particle and the adhesion of the coatings was not clearly pointed out. The purpose of this paper is to analyse the influence of temperature on the adhesive mechanism of thermally sprayed coatings. In this paper the importance of the temperature of both the substrate and the particle in the adhesive mechanism is emphasized. In order to clarify the adhesive mechanism at the boundary between the coating and the spreads on it is made. The model of a uniform layer of liquid (the particle) at initial temperature T p which is suddenly brought into contact at time zero with a substrate at initial temperature T s is considered. The diameter of the splat particle is assumed to be much larger than its thickness; edge effects are neglected and the problem is then considered as one-dimensional heat flow. Two phase changes are under consideration: the solidification of the spray droplet and the melting of the substrate. By analogy with the method of Weiner who obtained the solution for transient heat conduction in a multiphase media, the exact solution for this solidification- melting problem is found. The solution gives the pertinent information on the thermal behaviour of both the particle and the substrate. Moreover it gives the relationship between the initial molten droplet temperature and the initial solid substrate temperature for the onset of melting in the solid. The results of the analysis indicate the importance of physical properties of both the spray droplet and the substrate in the incipient melting of the substrate and emphasize the importance of the substrate because the temperature of the particle is usually difficult to control. Obviously, a particle-substrate system where the solidification-melting conditions are easily realized has greater bond strength than a system where these conditions are difficult to meet. Thus the reason why some materials stick generally to all surfaces is clarified, particularly if the solidification-melting conditions are favourable to the creation of a metallurgical bond.