Effect of Reynolds Number of Molten Spray Particles on Splat Formation in Plasma Spraying

Abstract

Abstract A thermal spray coating is formed through successive impact, flattening, rapid cooling and solidification processes of a stream of spray droplets. Splashing may occur during droplet flattening process. Recent studies suggested that splashing can be suppressed when a molten droplet impacts on a preheated flat substrate. In this study, the splatting behavior in plasma spray is examined using molten spray droplets of different Reynolds number. Splats are deposited on preheated flat stainless steel surface. The morphology of splats is examined using optical microscopy and scanning electron microscopy. To adjust Reynolds number of spray droplets, copper droplets are produced using both Ar-H2 and Ar-He-H2 plasma jets under different operating conditions. As a result, the Reynolds number of spray droplets have been varied from about 18,000 to 90,000. It has been found that Reynolds number will influence splashing phenomena during splatting and consequent splat morphology. At low Reynolds number, splats present a regular disc morphology. However, when Reynolds number was increased up to about 5x104, the severe splashing around periphery of splat droplet was clearly observed despite the preheating of substrate. Based on the morphology of splats, a model for the spreading of molten droplet is proposed to explain the effect of Reynolds number on the flattening behavior of molten spray droplet.