| The next generation of gas turbine systems has been targeted to operate at temperatures as high as 1400°C for more efficient power generation. Si-based ceramics such as SiC have long been regarded as leading candidate materials for such high-temperature applications. However, these Si-based ceramics are susceptible to hot corrosion, and thus require a protective coating. Mullite (3Al2O3·2SiO2) is an ideal material for such a corrosion resistant coating.;Mullite coatings were deposited on SiC substrates by a Chemical Vapor Deposition (CVD) Process, using the AlCl3-SiCl4-CO 2-H2 system. The objective of this research was to understand the fundamental issues related to the microstructural development of CVD mullite coatings. This includes obtaining an understanding of the nucleation and growth processes that occur during the deposition of the coating, as well as studying the evolution of the coating microstructure during prolonged exposure to high temperatures.;The typical microstructure of a CVD mullite coating on SiC consists of a thin sub-micron layer at the coating/substrate interface, consisting of gamma-Al 2O3 nanocrystallites embedded in a vitreous SiO2-rich matrix. This layer is referred to as the nanocrystalline layer. It was shown that mullite grains nucleate when the surface composition of the growing nanocrystalline layer reaches a narrow critical composition range of Al/Si molar ratio of 3.2 +/- 0.3. The substrate plays a role in determining the microstructure of mullite coatings. Due to preferential adsorption, a coating on SiC starts off being Si-rich, while a coating on an alumina substrate starts out being Al-rich. In either case, the Al/Si concentration is outside the critical range and a nanocrystalline microstructure is formed. In both cases, mullite nucleates, when the surface composition is graded to fall within the critical range.;Once nucleated, the mullite grains serve as a structural template for further growth, leading to the formation of columnar mullite grains. The composition of these grains can be graded to highly nonstoichiometric Al-rich compositions. In this research, mullite grains with an Al/Si ratio as high as 15 were grown, which is among the most Al-rich mullite grown to-date. The Si-rich coating adjacent to the substrate allowed for excellent chemical bonding to SiC, while the Al-rich outer surface of the coating was responsible for the excellent corrosion resistance exhibited by the coatings. The effects of nonstoichiometry on the atomic structure of such highly Al-rich mullite are discussed. Interestingly, 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.;The evolution of the coating microstructure during high-temperature exposure was investigated for both nanocrystalline and crystalline layers. Mullite coatings annealed directly at 1400°C for 100 hours exhibited cracking. In contrast, coatings preannealed at 1250°C for 100 hours, exhibited no cracking after 100 hours exposure at 1400°C. This phenomenon was attributed to complete mullitization of the nanocrystalline layer during the preanneal. At 1400°C, twinning and precipitation of alpha-Al2O 3 occurred, leading to a mullite/alumina nanocomposite structure. |
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