Date of Award

2009

Degree Type

Thesis

Department

Chemical and Biomedical Engineering

First Advisor

Tewari, Surendra

Subject Headings

Ceramic materials, Silicon, Coating processes, Materials science

Abstract

Silicon carbide/silicon carbide (SiC/SiC) composites are the leading candidates for advanced high temperature structural components in gas turbine engine application. When SiC surface reacts with O2, protective SiO2 scales are formed, which are the basis for the corrosion resistance of SiC. Their main draw back in terms of usage in a gas-turbine engine is the volatilization of the protective silica scale because of the water vapor which is a byproduct of fuel combustion. The purpose of this project was to develop environmental barrier coatings (EBCs) based on ceramic powders to provide resistance to such moisture damage. In this thesis slurry-coating development has been pursued in terms of optimizing the preparation of powder-mixtures, the amount of polyvinyl butyral (PVB) to be added as a binder in the slurry, the degree of powder loading in the slurry, and coat sintering temperatures with different slurry compositions. The alpha-SiC ceramics were coated by the slurry dip process and sintered at different temperatures the coated samples were examined under scanning electron microscope. Coated samples were thermally cycled in 96.5 H2O vapor-balance O2 flowing gas environment from 1300oC (or 1350oC) to room temperature (RT) for 100 to 300 cycles (1 cycle = 1 h hot temperature and 15 min. RT) to examine the coating stability. Observations yielded the slurry containing 0.8gm of PVB and 40 gm of powder in 20gm of Ethanol, 0.12 gm of PE gave a good coating surface with no cracks. Presence of the boron oxide (B2O3) in the ceramic powder further reduced the sintering temperature by 15oC. Under combustion environment, the mullite/Gd2SiO5 EBCs had a better protection up to 1350oC for 100 h exposure. With further exposure to 300 h at 1350oC, most of the coating was delaminated from substrate with strong surface cracks. In addition, the interfacial oxidation damage thickness increased with thermal cycling exposure times. At 1350oC cycling temperature, the damage thicknesses were about 5 microns and 10 microns after 100

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