In this work, a low-temperature melting composition located within the glass-forming region of the Y 2 O 3 -Al 2 O 3 -SiO 2 (YAS) system is proposed and tested as a protective coating for SiC ceramics. Glassy coatings 197 µm thick were obtained by flame spraying YAS granules on SiC substrates that had been grit blasted and coated with a Si bond layer. Bulk glasses of the same composition were also produced for use as a reference material. The hardness, elastic modulus, and thermal conductivity of the coatings and bulk specimens were evaluated and compared and the effect of heat treatment was investigated. Crystallization occurred in both the bulk glass and coating during isothermal treatments in air at 1100-1350 °C, but it did not compromise system integrity due to crack healing.

Compositionally graded mullite/ZrO 2 coatings, have been tested as environmental barrier coatings (EBCs) for protection against water vapor corrosion of Si-based ceramic components intended for application in turbine engines. Four and five layered systems were engineered by plasma spraying over SiC substrates consisting of a Si bond coat layer, 2 or 3 mullite/ZrO 2 composite graded layers as middle layers and a nanostructured YSZ topcoat. These coatings were heat treated at 1300 °C in both stationary and thermal cycling conditions in a controlled water vapor environment. The effect of these ageing conditions on the coatings was comparatively investigated. Crystallization of the composite coatings and sintering of the YSZ topcoat was perceived. A reduction of SiO 2 content was detected in the composite layers before aging. The porosity of the coating did not change appreciably with the ageing tests and only the evolution of the pre-existing cracks and the growing of a thermally grown oxide layer can be highlighted as the major effect of the ageing tests.

Mullite (Al 6 Si 2 O 13 ) is the basis of efficient environmental barrier coatings (EBCs) for protecting Si-based ceramic matrix composites (CMCs) selected to replace specific hot-section metallic components in advanced gas turbines. Furthermore, YSZ-mullite multilayer architectures with compositional grading between the bond coat and YSZ top coat were envisioned as solutions to ease their coefficient of thermal expansion (CTE) mismatch induced stress. Consequently, a proper understanding of the mechanical properties such as the elastic modulus, hardness or plastic/elastic recovery work serve for an efficient design of such refractory oxide multilayers. In this work, three different mullite powder morphologies (fused and crushed, spray-dried and freeze-granulated) were employed. Using depth-sensing indentation with loads in the range 100 – 500 mN, the role of the microstructure and morphology of the powder feedstock on the mechanical behaviour of air plasma sprayed mullite bond coats deposited on SiC Hexoloy substrates was investigated. Fully crystalline as-sprayed mullite coatings were engineered under controlled deposition conditions. Mechanical properties were measured for the as-sprayed coatings as well as for coatings heat-treated at 1300°C, in water vapour environment, for periods up to 500 h. Both E and H values of the coatings are found to be highly dependent on the morphology of the starting powders.

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.