ICMCTF2007 Session A1-1: Coatings to Resist High Temperature Oxidation and Wear

Monday, April 23, 2007 1:30 PM in Room Sunrise

Monday Afternoon

Start	Invited?	Item
1:30 PM		A1-1-5 Creep in α-Al₂O₃ Thermally Grown on β-NiAl and NiAlPt Alloys B.W. Veal, A.P. Paulikas (Argonne National Laboratory); B. Gleeson (Iowa State University); P.Y. Hou (Lawrence Berkeley National Laboratory) We have measured creep relaxation in α-Al₂O₃, thermally grown on [100] single crystals of stoichiometric β-NiAl, and on β- and γ'- phase alloys of NiAlPt at temperatures between 950 - 1100°C. Creep is monitored using in-situ measurements of strain relaxation in the oxide following the sudden imposition of a stress. A stress is imposed by abruptly changing the sample temperature, exploiting the thermal expansion difference between oxide and substrate. The in-plane elastic strain is obtained using a sin²ψ x-ray diffraction technique exploiting synchrotron radiation. For β-NiAl, we find that strain relaxation rates are comparable to those observed in fine grain α-Al₂O₃ ceramics, when ceramic results are extrapolated to the lower temperatures examined here. The dependence of creep rate on grain size is considered. Creep rates at stress levels of 100 MPa, or less, are proportional to σⁿ, with n ≤ 2, consistent with a diffusional creep mechanism. Research was primarily sponsored by the U.S. Department of Energy, BES, Materials Science, under contract Nos. W-31-109-ENG-38 (ANL) and DE-AC03-76SF00098 (LBNL). Additional support was received from the Office of Naval Research.
1:50 PM		A1-1-6 Confocal Photo-Stimulated Microspectroscopy: A Technique to Map Residual Stresses in Thermally Grown Oxides in 3D D.B. Hovis, A.H. Heuer (Case Western Reserve University) Confocal Photo-Stimulated Microspectroscopy (CPSM) can measure residual stresses in Al₂O₃ with high spatial resolution by taking advantage of the fluorescence caused by small quantities of Cr3⁺ ions. By combining a Laser Scanning Confocal Microscope (LSCM) with a high precision spectrometer, it is possible to simultaneously image the sample while measuring stresses. Our new instrument allows diffraction limited imaging with high-speed stress measurement. This has revealed much larger residual stress variations than are typically predicted in TGOs.
2:10 PM		A1-1-9 Gas Phase Aluminizing of Nickel Alloys with Hydrogen Chloride J. Kohlscheen, H.R. Stock (Stiftung Institut für Werkstofftechnik, Germany) Aluminizing of novel nickel alloy Haynes 282 and alloy 201 was performed by chemical vapor deposition. Aluminum chloride served as precursor and was generated in-situ by feeding gaseous hydrogen chloride (HCl) into the retort reacting with a solid high-melting aluminum alloy at about 1350 K. Either continuous HCl flow or discontinuous addition was applied. Hydrogen (H₂) to HCl gas flow ratios from 0 : 1 up to 9 : 1 (total flow of 50 l/h in every case) were tested at atmospheric pressure. Coating thickness was determined from glow discharge spectroscopy and metallographic cross sections. It is shown that formation of the desired β-NiAl phase is relatively insensitive to the gas flow ratio, yet forms best at hydrogen excess with a H₂ to HCl flow ratio of approx. 4 : 1 (40:10 l/h). Discontinuous addition of HCl does not lower coating growth rate significantly in either case because the chloride introduced into the chamber is not consumed in the aluminum deposition process. The results are explained by thermo-chemical calculations showing that an excess of hydrogen chloride reduces formation of NiAl on the part to be coated. Isothermal oxidation tests revealed a highly improved oxidation resistance of the aluminized surfaces.
2:30 PM		A1-1-10 Development of Internal Porosity in Platinum-Aluminide Coatings During Cyclic Oxidation V.K. Tolpygo (University of California, Santa Barbara) The microstructural evolution of platinum-modified nickel-aluminide coatings, which takes place in the course of high temperature cyclic oxidation, results in two major types of coating degradation. One type, extensively studied and reported over the recent years, is the development of surface undulations, also known as rumpling. The other type of degradation is the formation of internal pores in the coating, which occurs concurrently with rumpling. Unlike the pores that are commonly observed at the oxide-metal interface in many alumina-forming alloys, the pores in NiPtAl coatings nucleate and grow entirely within the coating. Examination of the coating microstructure after various exposures shows that both the size and the number of pores increase with oxidation time. It is demonstrated that internal porosity in some coatings may lead to complete disintegration after prolonged cyclic oxidation at 1150°C. In this work, various aspects of this phenomenon are examined, and the role of chemical composition and the effect of temperature regime on the propensity of pore formation are discussed.
2:50 PM		A1-1-11 Examination of the Platinum Effect on the Oxidation Behavior of Nickel-Aluminide Coatings P.Y. Hou (Lawrence Berkeley National Laboratory); V.K. Tolpygo (University of California, Santa Barbara) Oxidation resistant nickel-aluminide coatings are designed to develop a protective alumina scale during high temperature exposure. It is well established that platinum additions, typically about 6-8 at%, provide further improvements in oxidation resistance of such coatings, yet the nature of the platinum effect is still not understood. In this work, the oxidation behavior of two commercial NiAl and NiPtAl coatings deposited on the same Ni-base single crystal CMSX-4 alloy is analyzed. Cyclic and isothermal oxidation tests are conducted at 1150°C in air. The bond coat, TGO and TGO/bond coat interface microstructures, as well as the TGO/bond coat interface chemistry are studied as a function of oxidation time. Although both coatings form protective Al₂O₃ scales, the oxide is extremely adherent to the NiPtAl coating but exhibits extensive spallation on the NiAl. Numerous pores develop at the Al₂O₃/NiAl interface, and sulfur is found to segregate at the pore surfaces and at the interfaces. The lower fracture toughness of the Al₂O₃/NiAl interface, due to sulfur segregation and the presence of pores, leads to spallation of the scale upon cooling from the oxidation temperature, especially over the convex areas along ridges on the coating surface. On the other hand, the presence of platinum prevents sulfur segregation and pore formation at the Al₂O₃/NiPtAl interface. As a result, the alumina scale on the NiPtAl coating remains intact and virtually no spallation is observed even after prolonged cyclic oxidation.
3:10 PM		A1-1-12 Effects of γ-Ni+γ'-Ni₃Al-Based Coating Composition on Oxidation Behavior and Superalloy Compatibility T.I. Izumi, N.M. Mu, L.Z. Zhang, B. Gleeson (Iowa State University) Pt-modified γ+γ'-based coatings have been shown to have excellent oxidation resistance and to be viable alternatives to β-NiAl-based bondcoats in thermal barrier coating systems. In this study, the 1150°C oxidation behavior of γ+γ'-based alloys containing up to 20 at.% Pt and 15~22 at.% Al was investigated in order to ascertain optimum target coating compositions. It was found that the co-addition of Pt and Hf was extremely beneficial to oxidation resistance, to the extent that Ni-20Al-20Pt-Hf and Ni-20Al-10Pt-Hf alloys oxidized at significantly slower rates than that of a Ni-50Al-15Pt β-NiAl alloy. Interdiffusion experiments were also conducted to assess compatibility between γ+γ' alloys and Ni-based superalloys. The γ+γ' alloys showed excellent compatibility with the various superalloy substrates. A feasible method to deposit Pt+Hf-modified γ+γ' coatings having overall compositions closely approaching those targeted (Ni-Al_15-20-Pt_15-20-Hf) on the basis of the bulk-alloy study was subsequently developed. Cyclic oxidation testing of these γ+γ' coatings at 1150°C in air revealed the absence of surface rumpling and excellent compatibility with the superalloy substrate as major advantages compared to traditional Pt-modified β-NiAl coatings.
3:30 PM		A1-1-13 Synthesis and Oxidation Performance of Al-Modified γ+γ' Bond Coatings on Ni-Based Superalloys J.P. Stacy, Y. Zhang (Tennessee Technological University); B.A. Pint, J.A. Haynes (Oak Ridge National Laboratory); B.T. Hazel, B.A. Nagaraj (GE Aircraft Engines) Simple Pt-enriched γ+γ' coatings were synthesized on René 142 and single-crystal N5 Ni-based superalloys by electroplating a thin layer of Pt followed by a diffusion treatment in vacuum at 1150-1175°C. For the superalloys containing 13-14 at.% Al, the typical Al content in the "simple" γ+γ' coating was in the range of 16-19 at.%. In general, the alumina scale adherence to these γ+γ coatings was not as uniform as to β-(Ni,Pt)Al coatings on the same superalloy substrate. Modified γ+γ coatings with ~22 at.% Al have been developed by introducing a secondary short-term aluminizing step via pack cementation. Integration of reactive elements, such as Hf, into the γ+γ coating during the aluminizing process also is being explored. The cyclic oxidation performance of the modified γ+γ coatings is being evaluated at 1100-1150°C to determine whether the higher Al content will improve the oxidation behavior.