CVD COATINGS FROM THE AL-B-SI SYSTEM ON CARBON

Borides of aluminum and silicon have previously shown superior resista nce to oxidation at high temperature and other useful properties. In t his paper, carbon substrates were coated using chemical vapor depositi on (CVD) from the Al-B-Si-H-Cl-Ar system in a cold-wall reactor. B-Si compounds such as SiB4+/-x and SiB6 were deposited as thick coatings b y varying the B/Si molar ratio in the gas phase and the deposition tem perature. A transition from nodular to faceted grain growth was observ ed between 1150 degrees C and 1250 degrees C. Al-B coatings were also deposited, but consisted of a boron-rich phase with faceted grains irr espective of the B/Al ratios or deposition temperatures used in this i nvestigation. Although aluminum acted as a growth modifier to improve the quality and increase the size of grains, Al-B and B-Si compounds c ould not be deposited simultaneously from the gas phase and no ternary compounds from the Al-B-Si system were observed. (C) 1997 Elsevier Sc ience S.A.