Splat profile of impinging droplets on rough substrates: influence of surface roughness

A thermal spraying process contains various individual and fundamental processes. Among them, the flattening process is important and governs the quality of sprayed coatings. Therefore the flattening behaviors of spraying particles must be clearly understood. Ceramic and metal particles are generally sprayed onto the roughened substrates to enhance the adhesion strength of the coatings in plasma spraying technique. This report describes the flattening profile of the metal droplets impinging on the roughened substrates. Introducing the unevenness ratio, the droplet profile is evaluated with respect to surface roughness of the substrate surfaces, Re and We numbers. The experimental results show that the unevenness ratio increases with surface roughness of the substrates. Under the droplets of almost the same size the unevenness ratio has a bilinear relationship with the impinging velocity, Re and We numbers. Defining this corner point by a critical value of the droplets, the splat behaves the splash-like shape above the critical point. It was concluded that the critical value is influenced not only by the flattening parameters, but also by the surface roughness of the substrates.