Effect of solution concentration on splat formation and coating microstructure using the solution precursor plasma spray process

The Solution Precursor Plasma Spray (SPPS) process had been successfully used to deposit 7YSZ thermal barrier coatings. In this research, the effects of solution precursor concentration on 7YSZ splat formation and coating microstructure are studied. With increasing solution concentration, solution viscosity increases and surface tension decreases. Solution concentration has no effect on precursor pyrolysis and crystallization temperatures. The average atomized droplets size of ∼ 35 μm is the same for low and high concentration solutions. By contrast, splat formation is greatly dependent on precursor concentration. Low concentration precursors experience surface precipitation and lead to shell formation. The deposits consist of semi-pyrolyzed material and result in a soft, porous coating. When the substrate temperature is raised to 450 °C, spongy deposits are formed. High concentration solutions are beneficial for volume precipitation within droplets. Solid particles are formed, melted and form splats on contact with the substrate. The build-up of dense splats provides a dense coating.