Microstructure of Environmental Barrier Mullite and Erbium Silicate Coatings on SiC-Fiber Bonded Composites

Abstract

SiC-composites are candidate structural materials for high temperature applications such as gas turbines. For this purpose, a suitable coating against high temperature oxidation is essential. The coating material selection was made by phase stability/durability under service environment, CTE matching, low elastic modulus and chemical compatibility with SiC. This work is to evaluate the microstructural quality of mullite and erbium silicate coatings on SiC fiber bonded composites. In this work, SiC-fiber bonded 2D-composite (TyrannohexTM) by Ube Industries Ltd., were coated with mullite and erbium silicate by plasma spraying. The thickness of mullite and erbium silicate coatings was determined to be 120 microns and 220 microns, respectively. The fabricated samples were diamond polished for cross-sectional analyses by optical and scanning electron microscopy. The coating-substrate interfaces have been found bit undulating. The bonding between mullite and the substrate was found insufficient as compared to that observed in the erbium silicate coated substrate. The cross-sectional analyses of erbium silicate coating revealed the presence of throughthickness micro- and macro-cracks. The width of the macrocracks was found in the range of 1-3 microns. It is conceived that the evolution of large concentration of through-thickness-cracks is due to the stresses in the coating. The most probable sources of stresses are the precipitation of various second phases in the coating materials and the thermal expansion mismatches that arise during cooling. The mullite coating on the other hand is not found with macrocracks and on the whole the cohesion within the mullite coating is rather strong. The cross-sectional examination also revealed the irregularly distributed pores and cavities in both coating systems that are typical of thermal sprayed coatings. The surfaces of the coatings are characterized of typical lamellar microstructure formed by the flattened platelets of individual particles. The size of these droplets ranges from submicrometer to several micrometers.