Evaporated-gas-induced splashing model for splat formation during plasma spraying

Abstract

The effect of surface adsorbates on splat formation during thermal spraying is examined by controlling substrate adsorption. Splats are formed on a polished flat stainless steel substrate surface by plasma spraying. The adsorption state of the substrate is controlled with different organic substances of different boiling points and different preheating temperatures. The droplet materials used are aluminum, nickel, and Al 2O 3. Three kinds of organic substances used are xylene, glycol and glycerol, which are brushed on the surface of the substrate before spraying. It is revealed that when preheating temperature of the substrate is lower than the boiling point of the organic adsorbates on the substrate surface, the splashing occurs during droplet flattening which results in the formation of the splats with an irregular morphology. However, when the preheating temperature exceeds approximately 50 °C over the boiling point of the organic adsorbates and evaporates all organic substance adsorbed on the surface, the regular disk type splats are formed which corresponds to no occurrence of the splashing during splatting. It is clearly shown that the existence of the evaporable adsorbates on the substrate surface influences significantly the splatting and consequently the splat morphology. On the basis of the result, the evaporated-gas-induced splashing model is proposed to explain the occurrence of splashing during splatting of a spray molten droplet and the formation of the reduced splat. The effect of preheating temperature of a substrtae surface on water adsorption and desorption characteristics in ambient atmosphere is disscussed from the point of view to create a clean surface and regular disc splat.