Synthesis of alpha-aluminum oxide and hydrogen fluoride-doped beta-nickel aluminide coatings on single crystal nickel-based superalloy by chemical vapor deposition.

摘要

Thermal barrier coatings (TBCs) are widely used for air-cooled turbine components in advanced aircraft engines and power generation systems. The dominant failure mode observed in TBCs is progressive fracture of the metal-oxide interface upon oxidation and thermal cycling. Two potential coating methods for improving TBC performance were studied: (1) preparing a high-quality α-Al ₂O₃ coating layer on the surface of a single crystal Ni-based superalloy (René N5) to extend the oxidative stability of the interface and (2) doping β-NiAl bond coating with a small amount of Hf to improve the adhesion of thermally grown oxide (TGO) at the interface.; In the first coating method, a novel chemical vapor deposition (CVD) procedure was developed using AlCl₃, CO₂ and H ₂ as precursors. A critical part of this procedure was a short-time pre-oxidation step (1 min) with CO₂ and H₂ in the CVD chamber, prior to introducing the AlCl₃, vapor. Without this pre-oxidation step, extensive whisker formation was observed on the alloy surface. Characterization results showed that the pre-oxidation 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) consisted of α-Al₂O₃ as a dominant major phase and &thetas;-Al₂O₃ as a minor phase. It appeared that the preferential nucleation of β-Al₂O₃ in the pre-oxidized layer was promoted by: (1) rapid heating (∼10 sec) of the alloy surface to the temperature region, where α-Al ₂O₃ was expected to nucleate instead of metastable Al ₂O₃ phases, (2) the low oxygen pressure environment of the pre-oxidation step which kept the rate of oxidation low, and (3) contamination of the CVD chamber with HfCl₄. It appeared that the role of HfCl ₄ was to enhance the preferential nucleation of α-Al₂O ₃ in the pre-oxidized layer.; In our second coating method, we utilized the dynamic versatility of CVD as an avenue to proactively control the concentration and distribution of Hf as a beneficial dopant in the aluminide coating matrix. A laboratory-scale, hot-wall CVD reactor was specifically constructed to perform critical Hf doping experiments through “continuous” and “sequential” doping procedures. The continuous doping procedure, in which HfCl₄ and AlCl₃ were simultaneously introduced with H₂, required a relatively high HfCl₄ concentration in the gas phase to incorporate an average Hf concentration of ∼0.1 wt% in the β-NiAl coating matrix. (Abstract shortened by UMI.)