Microstructure and oxidation behavior of Al plus Si co-deposited coatings on nickel-based superalloys

Diffusion coatings have been widely used to metallic protective coatings or bond coats in thermal barrier coatings (TBCs) to protect the aeronautical turbine blades from detrimental oxidation and hot corrosion. However, degradation of these coatings caused by oxidation or hot corrosion can be particularly severe and rapid in the operating environment of the turbine. We present an experiment to modify gamma-Ni + gamma'-Ni3Al coating by co-deposition of Al and Si in order to improve the performance of coatings. In this study, microstructure and oxidation behavior of Al and Si co-deposited diffusion coatings were investigated. These coatings were deposited by Pt electroplating followed by aluminization and siliconization using the pack cementation process. The composition (in wt.%) of the packs was 2Al-4NH(4)Cl-xSi-(94 - x)Al2O3 with different Si contents (x = 4, 6, 8 and 10) and the corresponding microstructure of the coatings was delta-Ni2Si. The isothermal oxidation tests at 1000 degrees C in air indicated that the coating formed using the packs containing 10 wt.% Si had optimal oxidation resistance. After oxidation, the microstructure of the coatings was that of Cr2NiO4 + NiO or NiO + Al2O3. The mechanisms through which Si-rich coating forms and protects against oxidation are discussed. (C) 2014 Elsevier B.V. All rights reserved.