Abstract-Solution precursor plasma spray (SPPS) is a relatively new thermal spray process in which chemical precursors are injected into DC-arc plasma spray torch in place of powder. This process is able to make relatively porous (15-25% porosity) thermal barrier coatings with through-thickness cracks that enhance their thermal strain resistance. The SPPS process can also make dense titania and alumina zirconia coatings. The process can make thin and thick coatings, dense and porous coatings, structural and functional preforms, new compositions, and metastable materials. Metastable materials arise because the solution is molecularly mixed and the cooling rate in thermal spray is high. The process has the disadvantage of needing to provide energy to evaporate the solvent and of being a new process where less extensive empirical knowledge and modeling insights exist, compared to air plasma spray with powders. The microstructure process parameter relation is explored. Results from modeling studies concerning evaporation of droplets and related solute concentration gradients, modeling studies of aerodynamic break-up and experimental studies of non-aero-based droplet break-up and of the effects of solution concentration will be described. These studies will be related to experimental results for making dense coatings.

The Solution Precursor Plasma Spray process allows the creation of coatings directly from chemical precursors, thus avoiding the task of making sprayable powders. To date, our research has been based on injecting chemical precursors into a DC plasma torch. The process has proven to be useful in making vertically cracked thermal barrier coatings and has shown special advantages for making thick thermal barrier coatings (up to 4 mm). More recently, the process has been modified to produce dense, crack free coatings. This development was enabled by an improved understanding of the process, including making a coating almost exclusively from ultra-fine splats and avoiding the formation of vertical cracks. A crack free, dense alumina-yttria stabilized zirconia coating has been produced which is 98% dense and has an average Vickers hardness (300 gf) of 1177.

Real-time control offers the potential to reduce plasma spray variations that affect yield and coating quality. Important factors for designing such controllers are discussed including dominant nonlinearities, cross-coupling interactions, and sensor issues. The performance of several alternative strategies to achieve better coating thickness control are evaluated.