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 alumi na (Al2O3) from the reaction of aluminum chloride (AlCl3) with hydroge n/carbon dioxide gas mixtures in the temperature range of 954 degrees- 1100 degrees C and system pressures of 2.7-13.3 kPa, The apparent acti vation energy estimated from the coating growth rate averaged 320 kJ/m ol at 13.3 kPa, At temperatures <1000 degrees C, transition theta, kap pa, and delta modifications were codeposited with alpha-Al2O3, whereas single-phase alpha-Al2O3 was deposited at higher temperatures. At hig h AlCl3 partial pressures, nickel aluminide phases were sometimes code posited with Al2O3, which was attributed to the reaction of AlCl3 with the nickel substrate in the presence of hydrogen gas.