The spreading process of an isothermal droplet impinging on flat substrate surface in plasma spraying is studied numerically in 2D cylindrical coordinate systems by using 'Marker-And-cell (MAC) Technique. The changes and distributions of the transient contact pressures upon substrate surface at flattening are calculated under different droplet conditions with different impacting velocities and densities. The simulated results show that the transient contact pressure is initially high and concentrates at a small contacting area, it then spreads and drops quickly while droplet flattens. The maximum pressure is located at the front of the droplet at early stage of deformation, which pushes the fluid moving quickly along substrate surface and results in lateral flow. The contact pressure is mainly related to the droplet density and impact velocity. The peak pressure reduces consistently along the substrate surface so that the splashing at the periphery of flattening droplet may occur to form a reduced disk like splat because of the falling of contact pressure in this region and the escaping of the evaporated gas from the droplet / substrate interface.

The flattening of droplet impacting on a substrate is one of the most important basic processes during the formation of a thermal spray coating. The structure of splat will determine the structure of coating, and consequently the adhesion and properties of coating. This paper investigates the dependency of the change of the morphologies of splats from irregularly complicated form to regular disk form by the preheating of substrate on the types of evaporable substances adsorbed on substrate surface in order to provide farther evidence for the evaporated gas induced splashing mechanism. Paper includes a German-language abstract.

The splashing usually occurs when a droplet impact on a substrate surface during thermal spraying, which results in the formation of splat with irregularly complicated morphology. In present study splats are formed on polished stainless steel substrate surface covered with different organic substances with different boiling points by plasma spraying under different preheating temperature of substrate in order to clarify the factors which control the splashing during droplet flattening in thermal spray process. The droplet materials used are aluminum, nickel, copper, Al2O3 and molybdenum. Three kinds of organic substances used are xylene, glycol and glycerol which are brushed on the surface of substrate before spraying. It is found that when the preheating temperature exceeds 50°C over the boiling point of organic substance brushed on substrate surface the regular disk type splats are formed in the case that no substrate melting occurs by molten droplet. When the flattening of droplet causes the melting of substrate such as the combination of Mo droplet with stainless steel substrate, the preheating of substrate has no influence on splat morphology. The evaporated gas induced splashing and substrate surface melting induced splashing models are proposed to interpret the formation of the annulus-ringed splat.

The structure and morphology of plasma sprayed splats are experimentally investigated using different droplet materials and substrate materials. Droplet materials include aluminum, copper, nickel and refractory metals such as molybdenum and tungsten, and substrate materials include aluminum, stainless steel, and molybdenum plates. The results show that the splashing occurs during the splatting of a completely molten droplet. Most splats formed by droplets molten completely are only central part of the ideal disk type ones, which are defined as the annulus-ringed disk-like splat. It is found that the morphology of such annulus-ringed disk-like splat is greatly influenced by the combination of droplet and substrate materials depending on whether substrate melting occurs. With the combinations of droplet and substrate materials which are of similar thermal properties the splashing of central area of splat tends to occur to present a honeycomb structure at the center of splat. When droplet impacting can cause melting of substrate annulus-ringed splat prefers to present a split type. The flattening ratio of an annulus-ringed disk splat is typically less than 2.