Boride-based ultra-high temperature ceramic coatings deposited via controlled atmosphere plasma spray

This study investigates the capability of a custom-built controlled atmosphere plasma spray (CAPS) system for deposition of zirconium boride (ZrB2) and hafnium boride (HfB2) thermal protection system (TPS) coatings. Due to poor oxidation resistance these materials must be deposited in the absence of oxygen to avoid high inclusions of oxides that detrimentally effect TPS coating properties. A low cost, simple CAPS system was developed to allow for the deposition of these coatings within an inert atmosphere, in a typical lab environment. The CAPS system has demonstrated a 46% reduction in oxide inclusions for ZrB2 coatings with a 40% reduction in elemental oxygen and facilitated the deposition of a HfB2 coating with low oxide inclusions. Weibull analysis on the variability in the hardness of ZrB2 samples revealed a tri-modal distribution of hardness that was attributed to regions of differing oxide inclusions. The CAPS deposition of HfB2 did not achieve a deposited microstructure that was considered effective as a TPS. A Weibull analysis of the hardness of the HfB2 coating demonstrated a bimodal distribution that corresponded to low oxide regions and locations composed of a similar mix of boride and monoclinic oxide. The heterogeneous microstructure was attributed to the pre-processing of the HfB2 powder.