Columnar-structured thermal barrier coatings deposited via the water-based suspension plasma spray process

Suspension plasma spray (SPS) has been developed to be a rapid, facile and cost-effective process to deposit columnar-structured thermal barrier coatings (TBCs). Different than the most commonly used ethanol-based suspensions, water-based suspensions have not been used in the SPS process to deposit columnar-structured TBCs due to their high surface tension, although they are much cheaper and safer. In this work, a new water-based SPS process was prepared by adding surfactant to lower the surface tension. The optimum content of dispersant and surfactant added into the suspension was determined via the measurements of viscosity, particle size, surface tension, contact angles, and atomized droplet size. Coatings deposited using suspension with and without surfactant showed typical columnar-structured microstructures and vertically-cracked microstructures, respectively. The coatings deposited using suspension with surfactant also showed an evolution from columnar-structured microstructures to mixed microstructures of columns and cracks, and to homogeneous microstructures with the increase of standoff distance. The formation of different coating microstructures were correlated to the size of droplets after aerodynamic breakup and the Stokes number of in-flight particles. The new water-based suspension together with the water-based SPS process show high potential to be a cheap and effective alternative to the ethanol-based SPS process.