Papers

ICMCTF1999 Session A1: Coatings to Resist High Temperature Corrosion

Monday, April 12, 1999 1:30 PM in Room Council/Chamber/Cabinet

Monday Afternoon

Start	Invited?	Item
1:30 PM		A1-1 Beneficial Effects of Pt Incorporation on the Scale Adhesion Behavior of Aluminide Coatings W.Y. Lee (Stevens Institute of Technology); Y. Zhang (University of Tennessee); J.A. Haynes, I.G. Wright, B.A. Pint (Oak Ridge National Laboratory) Since the early 1970's, it has been a common industrial practice to incorporate Pt in various aluminide diffusion coatings used in aircraft and land-based gas turbines for enhanced oxidation resistance. More recently, some Pt-aluminide coatings have been used as premier bond coatings for strain-tolerant thermal barrier coatings (TBCs) made by electron beam physical vapor deposition (EBPVD). As we have begun to recognize that the EBPVD-TBCs ultimately fail by spallation at the metal-scale interface upon high-temperature oxidation, interest has been renewed in the mechanism(s) by which Pt exerts its beneficial effects on scale adhesion. In this presentation, we will briefly review prior knowledge and will report new experimental observations, which demonstrate the ability of Pt to retard localized scale spallation over grain boundaries, mitigate the deleterious effect of sulfur impurity segregation at the metal-scale interface, and inhibit void formation at the interface in a single-phase (Ni,Pt) Al coating produced by chemical vapor deposition. Implications of these observations will be discussed in the context of exploring other possible avenues of improving the scale adhesion behavior, namely desulfurization, reactive element doping, and microstructure control, for advanced TBC applications. Research sponsored by Advanced Gas Turbine Systems Program, DOE Office of Industrial Technologies, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation.
2:10 PM		A1-3 Alumina Scale Failure Resulting from Stress Relaxation V.K. Tolpygo (University of California at Santa Barbara.); D.R. Clarke (University of California at Santa Barbara) Relaxation of the stresses, produced by oxidation, is analyzed in terms of its influence on the morphology and spalling resistance of the alumina scales on Fe-Cr-Al heat-resistant alloys. Despite a general belief that stress relaxation is a beneficial factor in oxide adherence, several types of scale failure are shown to occur as a direct result of stress relaxation. These failure modes include: "sub-critical" buckling and spalling of alumina scale during cooling; cracking under thermally-induced tension during heating; spalling due to the stress-driven morphological changes in the scale. The importance of the rate of temperature change, the substrate compliance, and the oxide-metal interface morphology is emphasized.
2:30 PM		A1-4 Oxidation Behaviour of MCrAlY Coatings with Different Al-Contents and Surface Morphologies N. Czech (Siemens AG, Power Generation Group (KWU), Germany); H. Fietzek, M. Juez-Lorenzo, V. Kolarik (Fraunhofer Institut für Chemische Technologie (ICT), Germany); W. Stamm (Siemens AG, Power Generation Group (KWU), Germany) In modern stationary gas turbines for electric power generation the oxidation resistance of the MCrAlY coatings is of essential importance for the life times of the blades. The Al-content has a significant influence on the phase distribution in the coating and thus on its oxidation behaviour. The coating surfaces are in the case of stand-alone protective overlay coatings generally shot peened, whereas bond coats for EB-PVD thermal barrier coatings need a smooth surface. For a better understanding of the influence of the surface morphology and the Al-content on the oxide scale formation in situ studies using high temperature X-ray diffraction were performed on an MCrAlY coating with 12% Al and 3% Re, and an MCrAlY coating with 8% Al. The measurements were performed in air in a temperature range from 900 to 1000 C on samples with shot peened and polished surfaces. The coatings form an α-Al₂O₃ scale with spots of YAlO₃. The possibility of the formation of metastable Al₂O₃ phases at lower temperatures is discussed. The parabolic rate constants k_p are determined from the series of X-ray diffraction patterns and compared.
2:50 PM		A1-5 The Effect of Water Vapor on the Cylic Oxidation of Alloys that Develop Alumina Scales for Protection R. Janakiraman, G.H. Meier, F.S. Pettit (University of Pittsburgh) Cyclic oxidation tests have been performed at 1000 and 1200°C in wet ( ) and dry air on a number of alloys and coatings that form - scales upon exposure to oxidizing conditions. The alloys that were investigated included the superalloys PWA 1480, PWA 1484, CMSX4, diffusion aluminide coatings on PWA 1480 and PWA 1484, Co-24 wt% Cr-10.5 wt% Al-0.3 wt% Y. In cases where some cracking and spalling of the alumina scales occurred in dry air, the presence of water vapor caused the degradation rate, as determined via weight change versus time measurements, to be increased by a factor of two. When no cracking or spalling of the alumina occurred in dry air, as was the case for alloys with low sulfur concentration (e.g.<1ppm), water vapor had no effect on the oxidation behavior. Not any differences in the residual stresses in the alumina scales were detected for oxidation in wet or dry air. It is proposed that water vapor causes stress corrosion cracking at the - alloy interface during cyclic oxidation. The mechanism consists of water vapor having access to the - alloy interface via cracks in the alumina and affecting the interfacial toughness of this interface by altering the nature of the bonds at this interface. It is proposed that water vapor affects the cyclic oxidation of alloys when the stored elastic energy driving oxide spalling has values close to that of the toughness of the
3:10 PM		A1-6 Studies on the Transient State of Oxidation of VPS and HVOF MCrAlY Coating D. Toma, W. Brandl (University of Applied Science Gelsenkirchen, Germany); U. Köster (University of Dortmund, Germany) Thermal barrier coating (TBC) systems protect turbine blades against high temperature corrosion. They consist of a MCrAlY bond coating and a ceramic top layer. The oxidation resistance of MCrAlY coating is based on the formation in the steady-state stage of oxidation of α-alumina, which growns between the metallic bond coating and the ceramic top coating. α-alumina is thermodynamically stable and grows very slowly. The high temperature oxiadtion experiments with vacuum plasma sprayed (VPS) and high velocity oxygen fuel (HVOF) sprayed MCrAlY coatings had shown that the transient stage of oxidation for HVOF coatings is very short. Moreover, in contrast to the VPS coatings on which in the early stage of oxidation Cr₂O₃, NiO or CoO are identified, on HVOF coatings only metastable modifications of alumina were observed. The transformation of metastable alumina modifications to α-alumina is very fast. In situ x-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to examine the morphology and the microstructure of the formed oxide scale.
3:50 PM		A1-8 Chemical Vapor Deposition of Smooth α-Al₂O₃ Films on Nickel Base Superalloys as Diffusion Barriers J. Müller, M. Schierling, E. Zimmermann, D. Neuschütz (RWTH Aachen, LTH, Germany) With rising operation temperatures of gas turbines interdiffusion may occur between the structural material of the blades (superalloy) and the corrosion-resistant layer (MCrAlY) leading to premature coating failure. Depositing a diffusion barrier between the base material and the layer is a conceivable means to suppress this diffusion. α-Al₂O₃ appears to be well suited as diffusion barrier since it passes through no phase transition at high temperatures and possesses a hcp crystal structure without any defects. However, chemical vapor deposition of α-Al₂O₃ is known to be difficult because of preferred whisker formation due to the high contents of Ni, Cr or Co. In this paper two ways to suppress the growth of whiskers and to produce smooth α-Al₂O₃ films by thermal CVD are presented. The α-Al₂O₃ films were deposited in a vertical hot wall reactor at 1050°C and a total pressure of 100 mbar from a feed gas mixture of AlCl₃, H₂ and CO₂. Single crystal heat-treated CMSX4 samples were used as substrates. The first method of whisker growth suppression involved the deposition of a CVD-TiN interlayer at 835°C and 1 bar total pressure from a gas mixture of TiCl₄, H₂ and N₂. The TiN layer formed a dense cover on the superalloy and also served as a buffer to reduce thermally induced stress due to the CTE mismatch between CMSX4 and α-Al₂O₃. To test the efficiency of this layer system as a diffusion barrier, NiCoCrAlY was deposited on the α-Al₂O₃ film and the resulting structure annealed for 100 h at 1100°C and subjected to thermocycle experiments. The results proved that the α-Al₂O₃ film remained dense and very effective as diffusion barrier. The second method presently investigated into detail involves a special pretreatment of the substrate surface that allows the smooth deposition of α-Al₂O₃ without any interlayer.
4:10 PM		A1-9 Deposition of NiAl Thin Films from NiAl Compound Target Fabricated via Combustion Synthesis D. Zhong (Colorado School of Mines); J. Disam, S. Thiel (Schott Glas, Germany); J.J. Moore (Colorado School of Mines) A dense and homogeneous NiAl compound target has been fabricated via combustion synthesis. NiAl thin films have been produced by DC and RF magnetron sputtering from this NiAl target. The films were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, simultaneous thermal analysis, scanning electron microscope, transmission electron microscope, microhardness test, and scratch testing to study the relationships among process, microstructure, and properties. By successfully manipulating the synthesis parameters of the target and the deposition parameters of the film, a NiAl thin film can be designed to meet the needs for various high temperature applications. One such application, which is currently being pursued with ACSEL's industrial partners, is development of a coating system for glass molding dies.
4:30 PM		A1-10 Protective Si-Al-C-N Coatings for g-TiAl Turbine Blade Materials to Ensure Long-Time Oxdation Protection C.W. Siry (Lufthansa Technik AG, Germany); E. Lugscheider (RWTH Aachen University of Technology, Materials Science Institute, Germany); S.R.J. Saunders (National Physical Laboratory - Teddington, United Kingdom) Thin dense Si-Al-based ceramic coatings were deposited by PVD on Ti-based substrate materials such as TiAl6V4 that are currently used for compressor blades and g-TiAl that is being developped for future applications. Physical Vapour Deposition allows generation of novel coating systems that cannot be produced with conventional techniques . By use of Si/Al-targets during sputtering in reactive mode it has been possible to achieve highly protective Si-Al-C-N coatings. The selected process gas were argon. In the reactive operation mode ceramic coatings have been produced by admitting nitrogen and methane during sputtering from solid Si-Al targets. The properties, mophology and composition of the coatings are described. The thermal behaviour regarding crystallisation effects was investigated using high temperature X-ray diffractometry. The results of varying the composition with regard to mechanical surface properties as well as the protective effects concerning oxidation and sulphidation resistance have been demonstrated. Despite the observed cracking protection was maintained. A microstructural examination was undertaken to investigate this behaviour. Interdiffusion into Ti-based alloys is believed to lead to these beneficial e fects. Moreover, the performance against high temperature impact as well as the long time oxidation protective behaviour up to 4000 h is demonstrated.
4:50 PM		A1-11 Metallic Diffusion Coatings for Corrosion-Protection of Low-Cost Alloys in Aggressive Environments P. Jayaweera, K. Lau, N. Jiang, D.M. Lowe, A. Sanjurjo (SRI International); K. Krist (Gas Research Institute); S. Kujak (The Trane Company) Coatings on low-cost alloys provide an economical alternative to expensive super alloys for applications in aggressive environments. The required corrosion protection can be obtained with coatings based on organic, ceramic or metallic constituents depending on the application environment. For heat transfer applications, metallic coatings offer significant advantages over other types of coatings such as ductility, thermal stability, proximal thermal expansion coefficients between substrate and the coating, and high thermal conductance. A number of metallic coatings prepared by chemical vapor deposition (CVD) or modified CVD techniques were tested for their corrosion resistance and long term stability in a bromide based high temperature chemical heat pump environment. Some of these metallic coating formulations provided excellent corrosion resistance to low cost steels in this extremely aggressive, high temperature environment. In this paper, these potential coatings and their corrosion rate measurements under simulated chemical heat pump conditions will be presented.