Deposition mechanisms of thermal barrier coatings in the solution precursor plasma spray process

In an effort to understand the deposition mechanisms in the solution precursor plasma spray (SPPS)—a promising method for the deposition of highly-durable thermal barrier coatings—some model spray experiments were performed. In the SPPS process, an aqueous chemical precursor feedstock, which results in a ZrO2–7wt.%Y2O3 ceramic solid solution coating, is injected into the plasma jet and the coating is deposited on a metal substrate. The model experiments performed here entailed holding the plasma torch stationary. The deposits collected in these experiments were then characterized to identify the deposition mechanisms. In addition, actual coatings deposited by multiple-pass scanning plasma-torch experiments were also characterized to verify the identified deposition mechanisms. Striking resemblances were found between the morphologies of the microstructural features in the actual coating and in the deposits from the model experiments. Melting and solidification, resulting in ultra-fine ‘splats’, were identified for the first time in the SPPS process.