Synthesis of alpha-Al_2O_3 Template on Ni Superalloy Surface by Chemical Vapor Deposition

摘要

The life of thermal barrier coatings (TBCs) used in aircraft engine and power generation turbines is largely dictated by: (1) the ability of a metallic bond coating to form an adherent thermally grown oxide (TGO) at the metal-ceramic interface and (2) the rate at which the TGO grows upon oxidation. It is postulated that a thin alpha-Al_2O_3 template, if appropriately synthesized on a Ni-based alloy, will guide the alloy surface to form a TGO that is more tenacious and slower growing than what is attainable with state-of-the-art bond coatings. The feasibility of preparing such a template was examined using a chemical vapor deposition (CVD) process previously developed for cutting tool applications. A coating directly deposited by this method on a single crystal Ni superalloy substrate consisted of approx 1 mum alpha-Al_2O_3 crystals in a matrix of amorphous Al_2O_3. The coating, although not entirely alpha-Al_2O_3, promoted the formation of a highly stressed and adherent TGO layer (-6.0 GPa) on the alloy surface upon subsequent oxidation. When the alloy surface was modified with an electroplated Pt layer, the coating was entirely alpha-Al_2O_3, but with the presence of fine microcracks on the coating surface. In comparison to results observed for pure Pt and Ni substrates, it appeared that the role of the Pt interlayer was to promote the formation of kappa-Al_2O_3 nuclei, which subsequently transformed to alpha-Al_2O_3 during the CVD growth step. These results suggested that the nucleation and growth of a fully alpha-Al_2O_3 layer on the Ni superalloy surface, without forming microcracks, would be difficult through simple adaptation of the CVD process previously developed for the cutting tool industry.