A proposed process control chart for DC plasma spraying process. Part II. Experimental verification for spraying alumina

The role of particle injection velocity in influencing the nature of alumina coatings obtained by plasma spraying was studied. Previously reported process chart obtained by computational fluid dynamics (CFD) study on the particle states of alumina with respect to particle injection velocity and size was verified experimentally. For this purpose, alumina particles of three different size ranges with a mean size of 25, 40, and 76 mum were subjected to different injection velocities. The coating obtained was analyzed for cross-sectional microstructure and thickness by optical microscopy. In addition, the role of particle injection velocity and size in influencing the coating-deposition efficiency was studied. The experimental results agreed well with the CFD results, which had indicated the dependence of particle trajectory in the plasma plume on the particle injection velocity and size leading to the changes in the extent of melting, While a higher coating thickness and deposition efficiency was obtained with 25-mum particles, with further increase in particle size, a reverse trend, was observed. This was attributed to the changes in heat-transfer characteristics of the particles with size, which governed the coating buildup and deposition efficiency.