Effects of Preoxidation on the Nucleation and Growth Behavior of Chemically Vapor-Deposited -Al 2 O 3 on a Single-Crystal Ni-Based Superalloy L.M. HE, Y.-F. SU, L.F. ALLARD, M.J. LANCE, and W.Y. LEE A chemical vapor deposition (CVD) procedure was developed for preparing a high-quality -Al 2 O 3 coating layer on the surface of a single-crystal Ni-based superalloy using AlCl 3 , CO 2 and H 2 as precursors. A critical part of this procedure was a short-time preoxidation step (1 min) with CO 2 and H 2 in the CVD chamber, prior to introducing the AlCl 3 precursor. Without this preoxidation step, extensive whisker for- mation 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 -Al 2 O 3 grains with -Al 2 O 3 as a minor phase. We observed that the nucleation of -Al 2 O 3 in the preoxidized layer was promoted by (1) rapid heating (10 seconds) of the alloy surface to the tempera- ture region where -Al 2 O 3 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 HfCl 4 . 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. I. INTRODUCTION THERMAL barrier coatings (TBCs) offer a potential avenue to increase turbine inlet temperature and the ther- modynamic efficiencies of gas turbines used for aircraft propulsion and land-based power generation. State-of the art TBCs consist of a strain-tolerant Y 2 O 3 -stabilized ZrO 2 (YSZ) layer prepared by electron beam physical vapor deposition (EBPVD) and a metallic bond coat, which provides high- temperature oxidation protection. The overall performance of the EBPVD-TBC system is not only dependent on the strain tolerance of the YSZ layer, but also strongly dictated by the ability of the metallic bond coat to form and retain an adherent TGO upon oxidation and thermal cycling. [1–4] As previously reviewed by Lee et al., [3] a potential break- through in the TBC technology may be achieved by extend- ing the interface region’s oxidative stability beyond what is currently possible with traditional metallic bond coats. There are some interesting observations reported in the lit- erature about enhancing the oxidative stability of the metal- ceramic interface by incorporating a thin layer of high-quality Al 2 O 3 . [5,6] As reported in his patent, Strangman [5] observed that the presence of an Al 2 O 3 interlayer (1 m), prepared by chemical vapor deposition (CVD) between an NiCoCrAlY bond coat and an EBPVD-YSZ layer, increased the burner rig life of the TBC by 5 times. The increased oxidation resistance was attributed to the dense morphological quality and high chemical purity of the CVD-Al 2 O 3 layer. Another independent study by Sun et al. [6] showed that the presence of a CVD Al 2 O 3 layer (4-m thick) between a plasma-sprayed YSZ layer and a NiCoCrAlY layer substantially increased the cyclic oxida- tion life of the YSZ layer. It was reported that the formation of spinel was not observed at the YSZ-TGO interface. Both Strangman and Sun et al. used a CVD process, which uses AlCl 3 , CO 2 , and H 2 as precursors at a deposition tempera- ture of 1000 °C. This chloride-based CVD process was pre- viously developed, and is being widely used by the cutting tool industry. [7] The overall reaction for the CVD-Al 2 O 3 process is [1] WC/Co cutting tools are typically coated with a Ti(C,N) interlayer prior to the -Al 2 O 3 growth step. The Ti(C,N) interlayer is used as a diffusion barrier, since some of sub- strate elements and impurities (particularly Co and Cr) are found to cause the formation of metastable Al 2 O 3 phases or undesired morphological features such as whiskers. [7,8] Note that -Al 2 O 3 is the thermodynamically stable phase, whereas -Al 2 O 3 and -Al 2 O 3 are metastable phases. The formation of Al 2 O 3 whiskers during CVD is generally explained by the vapor-liquid-solid (VLS) mechanism. [8,9] For example, if Co (or Cr) on the substrate surface were chlori- nated to become a liquid chloride (i.e., CoCl 2 ), this liquid phase would then catalyze the surface reaction represented by Eq. [1] and subsequently Al +3 and O -2 ions initially dissolved into the liquid phase would precipitate as a solid Al 2 O 3 phase at the liquid/solid interface. The whisker morphology is pro- moted, because the rate of this VLS growth is much faster than that of thin film growth on the substrate surface. Another important processing feature is that air leaking into the CVD process must be tightly controlled in order to avoid the devel- Al 2 O 3 (s) + 6HCl (g) + 3CO (g) 2AlCl 3 (g) + 3H 2 (g) + 3CO 2 (g) → METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 35A, MARCH 2004—1113 L.M. HE, formerly Doctoral Candidate, Dept. of Chemical, Biomedical, and Material Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, is with School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30322. Y.-F. SU, Postdoctoral Researcher, formerly Doctoral Candidate, and W.Y. LEE, Professor and Department Director, are with the Department of Chemical, Biomedical, and Materials Engineering, Stevens Institute of Technology, Hoboken, NJ 07030. Contact e-mail: wlee@stevens-tech.edu L.F. ALLARD, Leader of the Materials Analysis User Center at High Temperature Material Labo- ratory, and M.J. LANCE, Research Staff Member, are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted November 15, 2002.