Plasma spraying of mullite and pore formers for thermal insulating applications

Abstract

Porous materials are widely used in various fields requiring thermal insulation properties, such as the aircraft industry, with thermal barrier coatings. In such applications, plasma-sprayed yttria-stabilized zirconia proved to be a highly effective insulating coating. With higher void content, materials such as aluminum silicate are used in space industry (Intermediate eXperimental Vehicle, for instance) due to their low density and thermal conductivity.In this study, several thermal sprayed coatings were evaluated as candidates for low thermal conductivity applications. Porous deposits were produced thanks to the pore former introduction into the plasma plume, and so integrated inside coatings. Two pore formers were used, aromatic polyester and a mineral agent, leading to different microstructures. Porous mullite microstructures, with 80% porosity due to polyester removal by thermal treatment, were carried out, while layered microstructures were obtained with the mineral agent and low enthalpy plasma. Furthermore, thick coatings were sprayed with different plasma spraying configurations. A static spray set-up allowed porous one centimeter thick coating production without any manufacturing defects before and after thermal treatment. This was made possible throughout a deposit temperature control, via a cryogenic carbon dioxide cooling system.