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Functionally Graded CVD Mullite Coatings

Published online by Cambridge University Press:  17 March 2011

Soumendra N. Basu  and
Vinod K. Sarin
Soumendra N. Basu
Affiliation:
Department of Manufacturing Engineering Boston University, Boston MA 02215.
Vinod K. Sarin
Affiliation:
Department of Manufacturing Engineering Boston University, Boston MA 02215.
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Abstract

Functionally graded mullite (3Al2O3.2SiO2) coatings were deposited on SiC substrates by Chemical Vapor Deposition (CVD) using the AlCl3-SiCl4-CO2-H2 system. It was found that due to preferential adsorption, coatings on SiC started off being Si-rich, while coatings on alumina substrates started out being Al-rich. In either case, if the coating composition was not close to stoichiometric mullite, the microstructure consisted ofg-Al2O3 nanocrystallites imbedded in a vitreous SiO2-rich matrix (nanocrystalline microstructure). On grading the composition, mullite grains nucleated when the composition of the growing nanocrystalline coatings reached a narrow surface composition range of Al/Si molar ratio of 2.9-3.4. Once nucleated, columnar mullite grains could be graded to highly nonstoichiometric Al-rich compositions. However, if the nucleated mullite grains were graded to be Si-rich, the mullite structure could not be sustained, and the coating reverted back to the nanocrystalline microstructure. This phenomenon is explained on the basis of the linkage of coordination polyhedra in the atomic structure of mullite.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 697: Symposium P – Surface Engineering 2001--Fundamentals and Applications , January 2001 , P7.2
Copyright
Copyright © Materials Research Society 2002

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