Effects of an α-Al2O3 thin film on the oxidation behavior of a single-crystal Ni-based superalloy

Abstract

A ϳ150-nm-thick coating layer consisting of ␣-Al 2 O 3 as the major phase with a minute amount of -Al 2 O 3 was deposited on the surface of a single-crystal Ni-based superalloy by chemical vapor deposition (CVD). Within 0.5 hours of oxidation at 1150°C, the resulting thermally grown oxide (TGO) formed on the coated alloy surface underwent significant lateral grain growth. Consequently, within this time scale, the columnar nature of the TGO became established. After 50 hours, a network of ridges was clearly observed on the TGO surface instead of equiaxed grains typically observed on the uncoated alloy surface. Comparison of the TGO morphologies observed with and without the CVD-Al 2 O 3 layer suggested that the transient oxidation of the alloy surface was considerably reduced. Also, the CVD-Al 2 O 3 layer significantly reduced the growth rate of the TGO and improved its spallation resistance, while slowing the internal oxidation of Ta-rich areas that were present in the superalloy as-casting defects. These results demonstrated that this thin ␣-Al 2 O 3 coating could be used as a means of favorably altering the TGO morphology and growth kinetics for no bond coat thermal barrier coating (TBC) applications.

Development of Protective Coatings for Single Crystal Turbine Blades

“…The low partial pressure of oxygen below the α-Al 2 O 3 will aid this seeding process, and reduce the formation of other mixed oxides and spinels. This type of seeding has been suggested by Su, et al [10], where they conducted oxidation tests on a Rene N5 superalloy with and without an α-Al 2 O 3 layer (~0.15 μm thick); no bond coat was utilized in their study. A CVD templated α-Al 2 O 3 layer exhibited only 0.7 μm of new TGO formation on isothermal exposure at 1150°C for 500 hours.…”

“…A CVD templated α-Al 2 O 3 layer exhibited only 0.7 μm of new TGO formation on isothermal exposure at 1150°C for 500 hours. In the absence of the α-Al 2 O 3 layer, the TGO spalled and the system exhibited weight loss characteristics [10]. One improvement to this approach is the incorporation of less than 1 weight percent Y, since the presence of Y has been found to reduce the grain boundary diffusion of oxygen in alpha alumina by more than one order of magnitude [11].…”

Effects of an Electroplated Platinum Interlayer on the Morphology and Phases of Chemically‐Vapor‐Deposited Alumina on Single‐Crystal, Nickel‐Based Superalloy

“…We have recently developed a novel CVD procedure for preparing a ϳ100 nm thick layer of fully crystalline ␣-Al 2 O 3 , directly on the surface of a singlecrystal, Ni-based superalloy, as described elsewhere. 19 This layer eliminated transient oxide formation and promoted the epitaxial growth of ␣-Al 2 O 3 grains underneath the ␣-Al 2 O 3 layer on oxidation at 1150°C. The CVD ␣-Al 2 O 3 layer apparently reduced the number of grain boundaries in the ␣-Al 2 O 3 scale, and consequently decreased the rate of alloy oxidation since grain boundary diffusion is the dominant transport mechanism at this temperature.…”

Effect of Pre-Oxidation and Thermal Exposure on the Microstructure of Superalloys and Thermal Barrier Coating Systems

“…δ, γ and θ) are developing before later transforming into stable α-Al 2 O 3[65,55,66]. Transformation from θ to α has been reported to result in a change in topography from whiskers to a network of ridges for both Ni-Al and (Ni,Pt)-Al systems due to growth stresses generated when θ-Al 2 O 3 transforms to α-Al 2 O 3[4,67]. The transformation is accompanied by a change in volume: the PBR value for θ-Al 2 O 3 on NiAl is 1.95 and is 1.78 for α-Al 2 O 3[55].…”

“…This growth stress produces microcracks along the ridges. As oxidation proceeds, the microcracks act as short-circuit paths for outward diffusion of Al 3+ , resulting in an increased growth of the ridges[4,67,68].…”