| Silicon-based ceramics such as {dollar}rm Sisb3Nsb4{dollar} and SiC are currently the leading candidate materials for high temperature applications due to their unique combination of high strength and thermal conductivity, low thermal expansion, good high temperature stability and oxidation resistance. However, their susceptibility to high temperature corrosion and damage due to contact stress limit their usage in most applications. It has been established that substantial improvements can be made by the application of thin protective coatings. However, several coating systems developed earlier were unable to satisfy the stringent requirements due to their inadequate adherence and chemical instability at high temperatures for extended periods.; The objective of this dissertation was to grow CVD mullite {dollar}rm (3Alsb2Osb3.2SiOsb2){dollar} coatings on {dollar}rm Sisb3Nsb4{dollar} and SiC. High corrosion resistance, high thermal shock resistance, chemical stability at high temperatures, and a coefficient of thermal expansion close to that of {dollar}rm Sisb3Nsb4{dollar} and SiC make mullite an ideal candidate coating material. Multi-layers of {dollar}rm Alsb2Osb3{dollar} and SiO{dollar}sb2,{dollar} and monolithic mullite coatings were synthesized.; Equilibrium thermodynamic analysis utilizing minimization of Gibbs free energy was performed to obtain information about the chemical reactions at various process conditions. Binary and ternary CVD phase diagrams, concentration and theoretical efficiency curves were constructed and used in identifying the initial process conditions. Thermodynamic analysis, along with kinetic and flow considerations were used in establishing the rate-limiting steps. The effect of process conditions such as temperature, pressure, and parameters such as inlet gas stoichiometry on the coating growth rates, morphology, structure, and composition were studied. The coatings were characterized for structure and morphology using X-ray diffraction, SEM, and TEM, and tested for adhesion by micro- and macro-scratch tests, and corrosion protection against {dollar}rm Nasb2SOsb4{dollar} based salts.; Multi-layered {dollar}rm Alsb2Osb3{dollar}/SiO{dollar}sb2{dollar} coatings did not facilitate mullite formation. The coatings failed by delamination due to devitrification of SiO{dollar}sb2{dollar} prior to any considerable inter-diffusion of the layers. Codeposition with AlCl{dollar}sb3{dollar}-SiCl{dollar}sb4{dollar}-CO{dollar}sb2{dollar}-H{dollar}sb2{dollar} precursors yielded crystalline mullite coatings. The deposits were non-uniform in composition through the thickness of the coating. The composition of Al increased with coating growth, and resulted in enhanced growth rate. The coatings were oriented with the c-axis perpendicular to the coating/substrate interface. The coatings were adherent, and protected {dollar}rm Sisb3Nsb4{dollar} from hot corrosion. |
