Chemical Vapor Deposition of Polycrystalline Al2O3

Polycrystalline Al₂O₃ was chemically vapor-deposited onto sintered Al₂O₃ substrates by reaction of AlCl₃ with (1) H₂O, (2) CO:H₂, and (3) O₂ at 1000° and 1500°C and 0.5 and 5.0 torr. Although the thermodynamics of all these reactions predict the formation of solid Al₂O₃, the deposition rate of the first reaction was considerably greater than that of the second. The third reaction was so slow that no measurable deposit was formed in 6 h at 1500°C. Formation of dense deposits of α-Al₂O₃ was favored by increasing temperature and decreasing pressure. Microstructural examination of the dense deposits showed long columnar grains, the largest of which extended through the deposit from the substrate to the surface.