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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 mm, theta-phase alumina was deposited at temperatures of approximately 850, 1050, and 1250°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°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°C). This reduction is attributed to both a decrease in the amount of NiO formed and to a Cr203 scale which thickened more slowly. Annealing studies were performed on coated Ni-2 OCr specimens to evaluate the effects of a vacuum heat-treatment (4 h at 1080°C, 6 x 10_B 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-A1202, as it was determined that uncoated, flame-treated samples did not form NiCr2O4 during the anneal.

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Abstractα-Alumina films are useful for high-temperature, wear, and semiconductor device applications because of their good oxidation resistance, high hardness values, and electrical properties. α-Alumina films have been previously synthesized using techniques such as chemical vapor deposition, sol-gel, physical vapor deposition, and plasma spraying. This paper presents an alternative approach for producing high quality dense α-alumina coatings using a flame-assisted process called combustion chemical vapor deposition (CCVD). This process is an open atmosphere technique that does not require the use of a reaction chamber. In this work alumina films were grown on YSZ at temperatures ranging from 900 to 1500°C. At lower temperatures only amorphous alumina was grown, but as the deposition temperature increased different alumina phases were formed. At 1100°C, a thin highly crystalline θ-Al2O3 coating was formed. At temperatures higher than 1100°C thick θ-Al2O3 coatings were deposited on the YSZ. Coatings were characterized by scanning electron microscopy (SEM) and x-ray diffraction (XRD).

Chromia thin films were deposited onto substrates of Ni and Ni-20Cr by gaseous fuel combustion chemical vapor deposition. The chemical precursor utilized to achieve chromia films was aqueous 0.27M chromium nitrate, and the depositions took place within the flame at temperatures between 200 and 550°C. Amorphous coatings were deposited between 200 and 375°C, whereas crystalline coatings of chromia (eskolaite) were deposited between 400 and 550°C. The eskolaite deposition rate was approximately five times faster than the amorphous chromia. Chromia coatings containing 2 wt% yttria were also formed. The deposition temperature necessary to form crystalline chromia/yttria coatings was slightly higher (∼50°C) than those containing chromia only. The ability of the various coatings to provide oxidation protection was characterized using thermogravimetric analysis (TGA). The eskolaite coatings that are at least 1 im thick reduce the oxidation mass gain/area of Ni-20Cr by nearly a factor of ten and the parabolic constant by an order of magnitude, whereas the amorphous chromia coatings that are 0.6 μm thick do not demonstrate such an improvement. Yttria-doped chromia coatings further reduce the parabolic constant by two orders of magnitude, consistent with expectation. Characterization of the coatings was accomplished using a variety of techniques, including SEM, TEM, EDS, and XRD.