Effect of Deposition Parameters on the Microstructure and Growth Rate of CVD Mullite Environmental Barrier Coatings
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0890-Y01-06.1
Effect of Deposition Parameters on the Microstructure and Growth Rate of CVD Mullite Environmental Barrier Coatings Soumendra Basu, Tushar Kulkarni and Vinod Sarin Department of Manufacturing Engineering, Boston University, Brookline, MA 02446, USA ABSTRACT Si-based ceramics such as SiC require environmental barrier coatings to protect against hotcorrosion and recession in gas turbine applications. Dense, crystalline mullite coatings of uniform thickness have been deposited by hot-wall chemical vapor deposition (CVD) on SiC substrates, using the AlCl3-SiCl4-CO2-H2 system. The effects of the CVD deposition parameters such as temperature, total reactor pressure, and metal-chloride partial pressure on the coating microstructure and growth kinetics have been investigated, and are discussed in this paper. INTRODUCTION The operating temperature in advanced gas turbines has been increasing steadily for improved fuel efficiency and reduced emissions. To accommodate this increase, Si-based ceramic components have been introduced into the hot section of gas turbines. Although these ceramics have excellent high-temperature oxidation resistance, they are susceptible to hot corrosion [1] and recession [2] in corrosive atmospheres containing Na, S, and high-pressure steam. In order to overcome these limitations, environmental barrier coatings (EBCs) are being developed. Mullite (3Al2O3•2SiO2) has received considerable attention as a potential coating material for silicon-based ceramics due to its excellent corrosion resistance, creep resistance and high temperature strength. Mullite has a much better CTE match with the Si-based ceramics than alumina and zirconia (Table 1), thereby minimizing thermal stresses during temperature cycling. Table 1. Density and CTE Values of Selected Ceramics.
Mullite coatings have been deposited by plasma spraying [3] and flame spraying techniques [4]. The CVD mullite deposition process was developed [5] and patented [6] at Boston University. The CVD process is well suited for tailoring interfaces, controlling interfacial reactions, microstructure and composition during growth (as opposed to a post deposition process). It lends itself readily for control of coating uniformity and thickness and is not line of sight process enabling uniform deposition on complex parts with edges, corners and curvatures.
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GROWTH OF MULLITE COATINGS Mullite coatings were grown on SiC substrates using the AlCl3-SiCl3-CO2-H2 system in a hotwall CVD reactor [5], with the overall reaction: 6AlCl3(g) + 2SiCl3(g) + 13CO2(g) + 13H2 (g) 3Al2O3•2SiO2(s)+ 13CO(g) + 26HCl(g)
(1)
AlCl3 vapor was produced by passing Cl2 gas over heated Al chips, while SiCl4 vapor was produced by heating liquid SiCl4 and using Ar as a carrier gas. The metal chlorides (AlCl3 and SiCl4) were pre-mixed before being introduced into the hot zone of the CVD reactor. Detailed thermodynamic analysis of the AlCl3-SiCl4-CO2-H2 system was carried out to identify the parameters to be used for CVD mullite growth [5]. Based on th
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