Identification of coating deposition mechanisms in the solution-precursor plasma-spray process using model spray experiments

In an effort to understand the deposition mechanisms in solution-precursor plasma-spray (SPPS)—a promising method for the deposition of highly-durable thermal barrier coatings (TBCs)—some model spray experiments were performed. In the SPPS process an aqueous chemical precursor feedstock, that results in ZrO2–7 wt.% Y2O3 (7YSZ) 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 and arranging the substrates in two different ways: (i) substrate placed at an offset below the plasma-torch center-axis and (ii) window-shielded substrate placed along the plasma-torch center-axis. These experiments allowed us to collect deposits from different regions of the plasma jet, which were then characterized, in an effort to identify the deposition mechanisms. The operative deposition mechanisms were found to depend critically on the trajectories of the precursor droplets in the plasma jet. Actual coatings deposited using scanning plasma-torch experiments were also characterized. Striking resemblance was found between the morphologies of the microstructural features in the deposits from the stationary and the scanning plasma-torch experiments. This is used to elucidate the coatings deposition mechanisms in the scanning plasma-torch SPPS process.