A model for oxidation of molybdenum particles during plasma spray deposition is developed. The diffusion of metal an-ions or oxygen cat-ions through a thin oxidized film, chemical reactions on the surface, and diffusion of oxidant in gas phase are considered as possible rate-controlling mechanisms with controlling parameters as the temperature of the particle surface, and local oxygen concentration and flow field surrounding the particle. The deposition of molten particle and its rapid solidification and deformation is treated using a Madejski-type model, in which the mechanical energy conservation equation is solved to determine the splat deformation and one-dimensional heat conduction equation with phase change is solved to predict the solidification and temperature evolution. Calculations are performed for a single molybdenum particle sprayed under the Sulzer Metco-9MB spraying conditions. Results show that the mechanism that controls the oxidation of this droplet is the diffusion of metal/oxygen ions through a very thin oxide film. A higher substrate temperature results in a larger rate of oxidation at the splat surface, and hence, a larger oxygen content in the coating layer. Compared to the oxidation of droplet during m-flight, the oxidation during deposition is not weak and can become dominant at high substrate temperatures.

A numerical model is developed to study the effects of the contact resistance, droplet impacting droplet temperature, and substrate temperature on the droplet solidification rate and temperature of the droplet under the condition when the substrate can melt and re-solidify. Two-dimensional simulations show that the interface velocity is small in the area of poor contact with an irregular solidification interface shape. During the impact of Molybdenum on a steel substrate, Mo solidifies while the steel substrate melts.

Molybdenum splats were produced at three plasma conditions on steel substrates preheated to three temperatures. Morphology of splats and corresponding craters formed on substrates were observed; dimensions of splats and craters were measured with an optical non-contact interferometer. It is found that substrate is significantly melted and deformed upon impact of the droplet, which leads to the formation of flower like splats and craters. On average, only about 36 to 53 % of the areas covered by splats were in good metallurgical/mechanical contact with substrate. Normalized crater volume increases with droplet size and the contact is improved for the high particle energy/high substrate temperature condition as compared with low particle energy/medium substrate energy condition. Splat morphology and crater formation is explained based on impinging jet heat transfer model.