Synthesis of mullite coatings by chemical vapor deposition
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Synthesis of mullite coatings by chemical vapor deposition Rao P. Mulpuri and Vinod K. Sarin Department of Manufacturing Engineering, Boston University, Boston, Massachusetts 02215 (Received 7 August 1995; accepted 4 March 1996)
Formation of mullite on ceramic substrates via chemical vapor deposition was investigated. Mullite is a solid solution of Al2 O3 and SiO2 with a composition of 3Al2 O3 ? 2SiO2 . Thermodynamic calculations performed on the AlCl3 –SiCl4 –CO2 –H2 system were used to construct equilibrium chemical vapor deposition (CVD) phase diagrams. With the aid of these diagrams and consideration of kinetic rate limiting factors, initial process parameters were determined. Through process optimization, crystalline CVD mullite coatings have been successfully grown on SiC and Si3 N4 substrates. Results from the thermodynamic analysis, process optimization, and effect of various process parameters on deposition rate and coating morphology are discussed. I. INTRODUCTION
Silicon-based ceramics such as Si3 N4 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 considerably limit their utilization. For example, corrosion can occur at elevated temperatures as a result of the combined effect of the oxygen plus gaseous, condensed, or particulate impurities induced via the gas stream in a turbine engine.1 In addition, contact stress damage at ceramic/ceramic interfaces has been projected as a major failure mode for ceramic components in advanced heat engines.2 It has been established that the application of a coating on silicon-based substrates is an effective technique to minimize or overcome these problems.3,4 Several coating systems have been developed, but none to date has been able to meet all the requirements due to the severity of these applications.5,6 Mullite (3Al2 O3 ? 2SiO2 ), due to its unique properties such as thermal expansion match with Si3 N4 and SiC, stability, and significantly superior corrosion resistance at high temperatures,7,8 is considered a prime candidate coating material. Mullite is a solid solution of Al2 O3 and SiO2 and is the only stable crystalline compound in the Al2 O3 –SiO2 system. Although it is popularly quoted with an Al2 O3 : SiO2 ratio of 3 : 2, mullite is known to have a range of solid solubilities. After several years of research, there are still controversies regarding the melting behavior and the solid-solution range of mullite. However, it has been observed that the solid solubility is dependent on the method of synthesis. Attempts to J. Mater. Res., Vol. 11, No. 6, Jun 1996
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grow mullite coatings by various processing techniques have met with limited success.4 Deposition of plasmasprayed mul
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