Effects of Processing Parameters on Alumina Coatings Deposited on Nickel Substrates by Reacting Aluminum Chloride and Hydrogen/Carbon Dioxide Gas Mixtures

Pure nickel coupons were used as substrates in the deposition of alumina (Al{sub 2}O{sub 3}) from the reaction of aluminum chloride (AlCl{sub 3}) with hydrogen/carbon dioxide gas mixtures in the temperature range of 954--1,100 C and system pressures of 2.7--13.3 kPa. The apparent activation energy estimated from the coating growth rate averaged 320 kJ/mol at 13.3 kPa. At temperatures <1,000 C, transition {theta}, {kappa}, and {delta} modifications were codeposited with {alpha}-Al{sub 2}O{sub 3}, whereas single-phase {alpha}-Al{sub 2}O{sub 3} was deposited at higher temperatures. At high AlCl{sub 3} partial pressures, nickel aluminide phases were sometimes codeposited with Al{sub 2}O{sub 3}, which was attributed to the reaction of AlCl{sub 3} with the nickel substrate in the presence of hydrogen gas.