PROTECTIVENESS OF CVD AL2O3 FILMS ON TIN AGAINST HIGH-TEMPERATURE OXIDATION

TiN coupons coated with Al2O3 films of about 2.6 μm formed by a chemical vapor deposition technique were oxidized in a purified oxygen flow at atmospheric pressure in a temperature range 1073 to 1273 K. The film is protective at 1073 K for at least up to 185 ks. Though the film decreases the oxidation rate at the other temperatures, it shows breakaway kinetics. The time before breakaway, tb, increases as the film thickness increases, while it decreases as the oxidation temperature increases. During the oxidation a small number of rutile aggregates grow on the film, while porous massive rutile grows under the film. The mass gain due to the oxidation after tb corresponds mainly to the growth of the massive rutile, implying enhanced transport of oxygen into the substrate. The oxidation rate is higher than that of alloys which form α-Al2O3 scales as a rate-limiting layer. This difference is explained in terms of structural changes. A possible reason for the breakaway is also given in terms of the formation of gaseous products, such as nitrogen and nitrogen oxide, which can introduce mechanical defects in the film. © 1990, The Japan Institute of Metals. All rights reserved.