Mechanisms of Hf dopant incorporation during the early stage of chemical vapor deposition aluminide coating growth under
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ODUCTION
DIFFUSION aluminide coatings produced by pack cementation, gas-phase pack, and chemical vapor deposition (CVD) have been successfully developed and used over the past 50 years to protect Ni alloy components from hightemperature environments encountered in aircraft engines and land-based gas turbines.[1] Recently, aluminide coatings have been adapted for use as bond coatings for thermal barrier coatings (TBCs).[2] State-of-the-art TBCs consist of a strain-tolerant, thermally insulating, Y2O3-stabilized ZrO2 (YSZ) layer prepared by electron beam physical vapor deposition (EBPVD). A bond coating protects the underlying alloy from oxidation by forming an adherent thermally grown oxide (TGO) at the bond coating / YSZ interface. The dominant failure mode observed in EBPVD TBCs is progressive TGO damage and consequent TBC delamination upon oxidation and repeated thermal cycling.[3,4,5] Therefore, improving the scaleadhesion behavior of bond coatings is a critical issue in the development of more reliable TBCs. One potent way of improving the quality of a TGO, and, therefore, its adhesion, is to dope Ni-based alloys with a small amount of “reactive elements” such as Y, Hf, Zr, etc.[6,7,8] G.Y. KIM, Senior Process Design Engineer, Process Technology Group, is with Genus, Inc., Sunnyvale, CA. L.M. HE, Postdoctoral Researcher, is with the Chemical Engineering Department, Georgia Institute of Technology, Atlanta, GA. J.D. MEYER, Ph.D. Candidate, and W.Y. LEE, Professor, are with the Chemical, Biomedical, and Materials Engineering Department, Stevens Institute of Technology, Hoboken, NJ. Contact e-mail: [email protected]. A. QUINTERO, Associate Professor, is with the Universidad Central de Venezuela, Caracas, Venezuela. J.A. HAYNES, Staff Member, is with the Oak Ridge National Laboratory, Oak Ridge, TN. Manuscript submitted January 22, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Recent experimental observations with model bond-coating materials have shown that the cyclic oxidation life of an EBPVD-TBC layer significantly improves when cast NiAl doped with Zr or Hf is used as a substrate.[9] The beneficial effects of reactive elements in Ni and coating alloys have been extensively studied and discussed thoroughly.[6,10–13] Some coating work has been conducted to incorporate Y, Zr, and/or Hf during aluminizing by pack cementation or gas-phase pack.[11,14] Also, other hybrid methods have been pursued.[15,16] However, in actual manufacturing practice, these coating approaches are found to be unsuitable[17] because of inherent processing irreproducibility in terms of precisely controlling the dopant concentration and distribution in the coating matrix. Prior studies with bulk alloys and intermetallics have certainly shown that, in order for its beneficial effect to be operative, a reactive element must be present at a well controlled characteristic level and distribution in the matrix materials.[7,18,19] Recent manufacturing advances in aluminizing by CVD offer new processing opportunities to significantly improve the p
