Nucleation mechanisms in chemically vapor-deposited mullite coatings on SiC
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Nucleation mechanisms in chemically vapor-deposited mullite coatings on SiC Ping Hou, S.N. Basu,a) and V.K. Sarin Department of Manufacturing Engineering, Boston University, 15 St. Mary’s Street, Boston, Massachusetts 02215 (Received 7 December 1998; accepted 19 April 1999)
Dense, uniform, and adherent chemically vapor-deposited mullite coatings were deposited on SiC substrates using the AlCl3–SiCl4–H2–CO2 system. Typical coating morphology consisted of a thin interfacial layer of ␥–Al2O3 nanocrystallites embedded within a vitreous SiO2-based matrix. When a critical Al/Si ratio of 3.2 ± 0.29 was reached within this nanocrystalline layer, mullite crystals nucleated and grew as columnar grains. The thickness of the nanocrystalline layer decreased as the input AlCl3/SiCl4 ratio was increased. In all cases, the Al/Si composition in the coating increased from the coating/substrate interface to the coating surface. Critical factors leading to the nucleation and growth of mullite crystals are discussed in this article.
I. INTRODUCTION
Silicon-based ceramics (Si3N4 and SiC) are currently the leading candidate materials for high-temperature applications because of their excellent mechanical properties, such as high-temperature strength and low-creep rate. However, their susceptibility to high-temperature corrosion and contact-stress damage has led to extensive research to develop protective coatings for these ceramics. Mullite (3Al2O3 ⭈ 2SiO2), due to its thermal stability, superior corrosion resistance at high temperatures and its thermal expansion match with SiC, has been targeted as a prime candidate coating material. Traditionally, mullite has been produced by mixing precursors containing Al and Si and heating to temperatures higher than 1000 °C.1 The technique of producing mullite coatings by chemical vapor deposition (CVD) has only recently been developed.2,3 Using this technique, dense, uniform mullite coatings can be directly deposited on substrates of complex shapes at a relatively low temperature of around 950 °C. These mullite coatings show promise as protective barriers for Si-based ceramics in the corrosive high-temperature environments. Formation of bulk mullite is a complex process that strongly depends on the synthesis methods used and the nature of starting materials. These factors determine the mullitization temperature, compositional homogeneity of the mullite formed, and the distribution of point defects in mullite. This article examines the microstructures of CVD mullite coatings grown under conditions of varying
a)
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2952
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J. Mater. Res., Vol. 14, No. 7, Jul 1999 Downloaded: 04 Apr 2015
input AlCl3/SiCl4 gas ratio. The results are discussed in light of critical microstructural features needed for the nucleation of mullite. II. EXPERIMENTAL DETAILS
Mullite coatings were deposited by chemical vapor deposition using the AlCl3–SiCl4–H2–CO2 system on 3 × 4 × 20 mm3
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