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 (Al₂O₃) from the reaction of aluminum chloride (AlCl₃) 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 θ, κ, and δ modifications were codeposited with α-Al₂O₃, whereas single-phase α-Al₂O₃ was deposited at higher temperatures. At high AlCl₃ partial pressures, nickel aluminide phases were sometimes codeposited with Al₂O₃, which was attributed to the reaction of AlCl₃ with the nickel substrate in the presence of hydrogen gas.