Abstract
A chemical vapor deposition (CVD) procedure was developed for preparing a high-quality α-Al2O3 coating layer on the surface of a single-crystal Ni-based superalloy using AlCl3, CO2 and H2 as precursors. A critical part of this procedure was a short-time preoxidation step (1 min) with CO2 and H2 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 (∼50 nm) on the alloy surface. The outer part of this layer (∼20 nm) appeared to contain mixed oxides, whereas the inner part (∼30 nm) mainly consisted of α-Al2O 3 grains with θ-Al2O3 as a minor phase. We observed that the nucleation of α-Al2O3 in the preoxidized layer was promoted by (1) rapid heating (10 seconds) of the alloy surface to the temperature region where α-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.
Original language | English |
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Pages (from-to) | 1113-1124 |
Number of pages | 12 |
Journal | Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science |
Volume | 35 |
Issue number | 13 |
DOIs | |
State | Published - 2004 |
Funding
This research was sponsored by the United States Office of Naval Research (ONR) through Grant No. N00014-99-1-0281 and in part by the National Science Foundation through Grant No. NSF-GOALI DMR9801042. We are grateful to Dr. Steven Fishman, ONR, for his support and encouragement. The FIBs, HRTEM, and luminescence spectroscopy studies were sponsored by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory, managed by UT– Battelle LLC for the United States Department of Energy under Contract No. DE-AC05-00OR22725. We also thank D.W. Coffey, ORNL, for the TEM sample preparation.