Alumina coatings applied via combustion chemical vapor deposition and their effects on the oxidation of a Ni-base chromia former

A novel deposition technique, combustion chemical vapor deposition, was used to deposit thin films of alumina on Ni-20Cr. Using a solution of 0.00135 M Al acetylacetonate in ethanol and a deposition time of 40 min, theta-phase alumina was deposited at temperatures of approximately 850, 1050, and 1250 degrees C, with some alpha alumina present at the latter temperature. The theta-phase grain size was found to vary with deposition temperature, with smaller grain sizes (2-8 nm) observed with lower deposition temperatures. The 850 degrees C deposition temperature was selected for kinetic studies in order to examine the effects of the alumina coatings on suppressing substrate oxidation. The coatings afforded some oxidation protection even during the deposition process itself, which occurs within the oxidizing environment of a flame. Kinetic measurements showed that the alumina coatings slowed oxidation at all test temperatures (800-1100 degrees C). This reduction is attributed to both a decrease in the amount of NiO formed and to a Cr2O3 scale which thickened more slowly. Annealing studies were performed on coated Ni-20Cr specimens to evaluate the effects of a vacuum heat-treatment (4 h at 1080 degrees C, 6 x 10(-6) Torr) on oxidation resistance in air. Alumina-coated and annealed specimens showed further reductions in the oxidation kinetics. This additional reduction in kinetics is associated with the presence of a layer of NiCr2O4 which contains some Al, and which is formed during the annealing process. The NiCr2O4 formation is facilitated by the theta-Al2O3, as it was determined that uncoated, flame-treated samples did not form NiCr2O4 during the anneal.