Mullite based compositions have interest for thermal barrier coatings because they have thermal expansion coefficients close to those of silicon ceramic substrates. In this work, mullite-zirconia coatings are obtained by flame spraying and characterized based on microstructure, crystal phases, hardness, elastic modulus, and thermal conductivity. Crystallinity is improved by in-situ heating with a flame torch, which is also shown to increase hardness and elastic modulus. Thermal diffusivity measurements show that the thermal properties of mullite-zirconia coatings are relatively stable over a wide temperature range and adequate for many thermal barrier applications.

This study investigates the influence of powder morphology and spray processes on the microstructure, crystallinity, hardness, and elastic modulus of mullite coatings. Coatings produced from mullite powders and suspensions are deposited by plasma spraying while measuring in-flight particle temperature and velocity. Powder morphology and spraying conditions are correlated with measured coating properties, creating a process map for engineering mullite coatings for specific applications. It is shown that coating crystallinity, microstructure, and mechanical properties vary widely depending on powder morphology, processing, and in-flight particle characteristics.

In this study, two processing routes are used to produce mullite powders for thermal spraying and the influence of each method on particle morphology and microstructure is investigated. Different thermal treatments are performed to improve grain cohesion and powder flow and their effect on the crystal structure of the powder is assessed as well. The powders are plasma sprayed, in-flight characteristics are measured, and splats are collected and analyzed. A correlation among powder morphology, in-flight particle properties, and splat morphology is established to better understand the influence of powder processing route on coating formation.

Titania (TiO 2 ) coatings are candidates for high-temperature applications in the fields of wear, corrosion, and environmental barrier coatings (EBCs); however, at temperatures at or above 540 °C, titania coatings are not pursued due to the usual presence of the anatase phase in the as-sprayed TiO 2 coatings. This phase tends to impede the applications of these materials at high temperatures due to the stresses provided by the critical anatase-to-rutile phase transformation at temperatures higher than 540 °C; such stresses tend to generate cracks in the coating microstructure, leading to premature coating failure. It has been hypothesized that this barrier could be overcome by the use of nanostructured TiO 2 coatings, due to their known high toughness and resilience levels. Nanostructured TiO 2 powders were HVOF-sprayed. The high velocity levels of the HVOF-sprayed particles generated a gas-tight microstructure (i.e., no through-thickness porosity). SEM pictures of the as-sprayed and heat-treated (800 °C for 1 h) coatings did not show any significant signs of crack network formation, which may have been prevented by the high toughness and resilience of these coatings. These coatings were also HVOF-sprayed on SiC substrates and did not exhibit macroscopic signs of delamination after a 1400 °C exposure for 1 h in air.

CaZrO 3 coatings were alternatively prepared by air plasma spray and flame spray processes. The microstructural characteristics and crystalline phases of the coatings were comparatively studied as a function of the spraying temperature achieved with each technique and the stand off distance. Image analyses were used to estimate their porosity. Thermal diffusivity was measured on free-standing thick coatings using the laser flash technique. Thermal conductivity was obtained from the experimental thermal diffusivity and density data. The hardness of the coatings was evaluated by Vickers indentation tests. Finally, different thermal treatments were carried out to evaluate the evolution of the crystalline phases and the properties of the coatings.