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 2 O 3 ) from the reaction of aluminum chloride (AlCl 3 ) with hydrogen/carbon dioxide gas mixtures in the temperature range of 954°–1100°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 <1000°C, transition theta, kappa, and delta modifications were codeposited with alpha‐Al 2 O 3 , whereas single‐phase alpha‐Al 2 O 3 was deposited at higher temperatures. At high AlCl 3 partial pressures, nickel aluminide phases were sometimes codeposited with Al 2 O 3 , which was attributed to the reaction of AlCl 3 with the nickel substrate in the presence of hydrogen gas.