Effects of preoxidation on the nucleation and growth behavior of chemically vapor-deposited α-Al2O3 on a single-crystal Ni-based superalloy

A chemical vapor deposition (CVD) procedure was developed for preparing a high-quality alpha-Al2O3 coating layer on the surface of a single-crystal Ni-based superalloy using AlCl3, CO2 and H-2 as precursors. A critical part of this procedure was a short-time preoxidation step (1 min) with CO2 and H-2 in the CVD chamber, prior to introducing the AlCl3 precursor. Without this preoxidation step, extensive whisker formation was observed on the alloy surface. Characterization results showed that the preoxidation step resulted in the formation of a continuous oxide layer (similar to50 nm) on the alloy surface. The outer part of this layer (similar to20 nm) appeared to contain mixed oxides, whereas the inner part (similar to30 nm) mainly consisted of alpha-Al2O3 grains with theta-Al2O3 as a minor phase. We observed that the nucleation of alpha-Al2O3 in the preoxidized layer was promoted by (1) rapid heating (10 seconds) of the alloy surface to the temperature region where alpha-Al2O3 was expected to nucleate; (2) the low oxygen pressure environment of the preoxidation step, which kept the rate of oxidation low; and (3) contamination of the reactor chamber with HfCl4. The preoxidized layer served as an effective diffusion barrier for mitigating the interaction with some of the alloying elements such as Co and Cr with the CVD precursors and eliminating whisker formation on the alloy surface.