Functionally Graded CVD Mullite Coatings
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Functionally Graded CVD Mullite Coatings Soumendra N. Basu and Vinod K. Sarin Department of Manufacturing Engineering Boston University, Boston MA 02215. ABSTRACT Functionally graded mullite (3Al2O3.2SiO2) coatings were deposited on SiC substrates by Chemical Vapor Deposition (CVD) using the AlCl3-SiCl4-CO2-H2 system. It was found that due to preferential adsorption, coatings on SiC started off being Si-rich, while coatings on alumina substrates started out being Al-rich. In either case, if the coating composition was not close to stoichiometric mullite, the microstructure consisted of γ-Al2O3 nanocrystallites imbedded in a vitreous SiO2-rich matrix (nanocrystalline microstructure). On grading the composition, mullite grains nucleated when the composition of the growing nanocrystalline coatings reached a narrow surface composition range of Al/Si molar ratio of 2.9-3.4. Once nucleated, columnar mullite grains could be graded to highly nonstoichiometric Al-rich compositions. However, if the nucleated mullite grains were graded to be Si-rich, the mullite structure could not be sustained, and the coating reverted back to the nanocrystalline microstructure. This phenomenon is explained on the basis of the linkage of coordination polyhedra in the atomic structure of mullite.
INTRODUCTION Silicon-based ceramics such as SiC and Si3N4 are leading candidate materials for use as components in the next generation of advanced gas turbines, internal combustion engines and heat exchangers. Although, SiC and Si3N4 have excellent oxidation resistance due to the formation of a protective SiO2 layer at the surface during high temperature exposure, they have two major limitations. In complex combustion environments, the presence of elements such as Na, Va, and S, lead to the formation of corrosive oxides such as Na2O, V2O5, SO2 and SO3. These oxides react with the silica scales on the Si-based ceramics forming non-protective low melting temperature silicates, leading to severe pit formation, material loss and increased porosity [1]. Also, in the presence of high-pressure water vapor, the protective silica scale volatilizes to gaseous Si-O-H species, exposing the ceramic surface. This leads to an accelerated oxidation of the ceramic surface to SiO2, which in turn volatilizes. This repeated cycling of oxidation and volatilization leads to a rapid recession of the surface of the Si-based ceramic [2]. In order to avoid the problems of hot corrosion and recession, refractory environmental barrier coatings (EBC) on these Si-based ceramics have to be developed. These environmental barrier coatings need to satisfy several requirements [3]. The surface of the coating needs to be environmentally durable to the aggressive atmospheres it is exposed to. The coating has to act as an effective diffusion barrier, and must be mechanically tough to be free of cracks in order to prevent exposure of the substrate to the environment. The coating must be stable to avoid deleterious phase changes during long-term high temperature exposures. The coatings sho
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