Papers

ICMCTF2002 Session A1-2: Coatings to Resist High Temperature Corrosion

Monday, April 22, 2002 1:30 PM in Room Esquire/Towne

Monday Afternoon

Start	Invited?	Item
1:30 PM		A1-2-1 Thermal Expansion in MCrAlY Alloys T.A. Taylor, P.N. Walsh (Praxair Surface Technologies, Inc.) A range of common MCrAlY coatings were made by argon-shrouded plasma spray deposition. The samples were hollow tubes, 0.5" diameter, 1" high with 0.025" wall thickness. Sintering effects were removed by heat treatment in vacuum for 4 hours at 1080°C. Then the thermal expansion curves were determined by heating in argon from room temperature to 1100°C at 5°C per minute. The resulting thermal expansion coefficients were compared in terms of the chemical composition of the coatings.
1:50 PM		A1-2-2 Microstructural Evolution of Platinum Aluminide Bond Coats during Thermal Cycling M.W. Chen, R.T. OTT, K..J.T. Livi, T.C. Hufnagel, K.J. Hemker (Johns Hopkins University) Microstructural evolution induced by thermal cycling in platinum aluminide bond coats for thermal barrier coatings (TBCs) has been investigated with optical microscopy, transmission electron microscopy, micro-probe analysis and in situ elevated temperature X-ray diffraction. Before thermal cycling, the structure of the as received bond coat was confirmed to be an ordered B₂ phase, but significant lattice strains were found which were associated with the formation of a modulated structure and small scale fluctuations in chemical composition. Thermal cycling resulted in significant changes in the composition and morphology of the bond coat. The compositional development assisted by chemical diffusion during thermal cycling has been related to the transformation of the bond coat from its original Al-rich B₂ structure to a Ni-rich L10 martensite. In situ TEM observations and x-ray diffraction measurement indicate that this martensitic transformation in the thermally cycled bond coat is reversible with thermal cycling. The L1₀ martensite was found to be stable at lower temperatures and the B2 parent phase at elevated temperatures. Transformation temperatures in the range of 650-800°C have been identified. Quantitative x-ray measurements indicate that the atomic volume of the B2 phase is approximately 2% smaller than the atomic volume of the martensite. The resulting transformation strain is thought to play an important role in governing the stress and strain distributions in thermally cycled bond coats. Possible connections to TBC performance and life will be outlined. The support of General Electric Aircraft Engines and the National Science Foundation (Grant No. DMR9986752) are gratefully acknowledged.
2:10 PM		A1-2-3 Oxidation and Hot Corrosion of a HVOF-Sprayed MCrAlX Coating S. Pahlavanyali (Tarbiat Modares University, Iran); A. Sabour (Tarbiat Modares Universiy, Iran) During operation, the hot-section components of gas turbines are subjected to oxidation and hot corrosion as well as high thermal and mechanical stresses. Turbine blades are protected against these environmental degradations by high temperature coating systems. Both isothermal and cyclic oxidation behavior of a NiCoCrAlYTa HVOF-sprayed coating were carried out at 1000° C and 1050 respectively, in still air. Sodium sulfate induced hot corrosion has also been studied under a simulated gas atmosphere at 900 °C. The microstructures of the scales were examined by x-ray diffraction and scanning electron microscopy. It is not detected any considerable amount of internal oxidation or sulfidation on the specimens after tests. A continuos alumina layer was formed on the surface of coating during these tests.
2:30 PM		A1-2-4 Corrosion Resistant Metallic Coatings for Applications in Highly Aggressive Environments N. Priyantha, P. Jayaweera, A. Sanjurjo, K. Lau, F. Lu (SRI International); K. Krist (Gas Technology Institute) Surface modification to improve the corrosion resistance of low-cost alloys is an economically attractive alternative to the use of expensive corrosion resistant alloys. A low-grade steel surface modified by a metallic diffusion coating can provide excellent corrosion resistance similar to that obtained with an expensive super alloy. The metallic diffusion coatings can be designed with optimum corrosion resistant properties for the desired environment of exposure using fluidized bed reactor chemical vapor deposition (FBR-CVD) techniques. We have used FBR-CVD for the preparation of diffusion coatings containing Cr, Ni, Si, and Ti on carbon and low-grade stainless steels. Several formulations of diffusion coatings on 409 SS have shown corrosion resistance similar to that of SeaCure or 29-4C in chemical heat pump and condensing heat exchanger applications. In this paper, we describe the preparation, characterization, and the performance evaluation of a number of corrosion resistant metallic diffusion coatings.
2:50 PM		A1-2-5 Effect of PLD Ti and Al Oxide Coatings on inconel 718 Against Carbon and Sulfur Deposition from JP-8 Fuel Decomposition O. Altin, S. Eser, A.R. James, X. Xiaoxing (The Pennsylvania State University) Pulsed laser deposition (PLD) technique has been used to deposit titanium oxide and aluminum oxide on the superalloy Inconel 718 substrates. Characterization of the coatings was carried out using Field Emission SEM, AFM, and grazing incident X-ray diffraction. The coating thickness of the oxides on Inconel 718 was 1 µm. The performance of the substrate and coatings against carbon and sulfur corrosion from JP-8 fuel was tested in flow reactor at 470°C and 34 atm fuel temperature and pressure for 24h. The results showed that the coating of Ti and Al oxides on Inconel 718 inhibited sulfur reactions and carbon collection significantly.
3:10 PM		A1-2-6 Corrosion Behaviors of Low Carbon Steel, SUS310 and Fe-Mn-Al Alloy with Hot-dipped Aluminum Coatings in NaCl-induced Hot Corrosion C.J. Wang (National Taiwan University of Science and Technology, Taiwan,R.O.C.); J.W. Lee (Tung nan Institute of Technology, Taiwan, ROC); T.H. Twu (National Taiwan University of Science and Technology, Taiwan, ROC) Aluminum coating is a promising protective layer for components used in the critical high temperature applications. The aluminum coated low carbon steel, SUS 310 and Fe-Mn-Al-Cr-Si-C alloy with 9 mg/cm² NaCl deposition were oxidized at 900⁰C for 1 to 144 hours to investigate the hot corrosion behaviors. The specimens were coated with aluminum by dipping in aluminum liquid at 700⁰C for 10 seconds. A preoxidation treatment of aluminum-coated specimens at 900⁰C for 2 hours were also employed. The hot corrosion resistance of as-dipped low carbon steel and SUS 310 were significantly improved due to the formation of aluminized layer. The self-healing ability of specimens was also enhanced. Owing to the high solubility of aluminum in Fe-Mn-Al-Cr-Si-C alloy substrate, complex phase transformations were induced by rapid diffusion of aluminum coating layer into matrix. Hot corrosion resistance of aluminum coated Fe-Mn-Al-Cr-Si-C alloy is thus reduced. The thickness of aluminized layer increased after preoxidation treatment. However, the hot corrosion behaviors of preoxidized specimens failed to exhibit better performance than the as-dipped specimens due to the microcracks formed during preoxidation treatment on the surface.
3:30 PM		A1-2-7 Thermal Oxidation of Ru-doped NiCoCrAlYTa Powders F. Juarez (CIRIMAT-ENSIACET, France); D. Monceau (CNRS-CIRIMAT-ENSIACET, France); D. Tetard (CNRS-SPCTS, Univ. Limoges, France); B. Pieraggi (CIRIMAT-ENSIACET, France); C. Vahlas (CNRS-CIRIMAT-ENSIACET, France) This paper introduces the spouted bed chemical vapor deposition, an original technique for the doping by platinum group metals, of commercial powders that are used as raw materials for the processing of bond coats in thermal barriers. In this frame, Praxair NI-482 powders were doped by ruthenium starting from ruthenocene. The doping level, purity, microstructure and distribution of ruthenium on the powders were established. Doped and undoped (reference) powders were sintered at 1200°C by uniaxial hot pressing. Both isothermal and cyclic oxidation tests, at temperatures ranging between 900°C and 1150°C, were performed on the obtained coupons. Isothermal oxidation tests were carried out in a thermogravimetry apparatus in order to determine the parabolic rate constants. Long term cyclic oxidation is still in progress to provide a more reliable simulation of the operating conditions of the material, and eventually to reveal the auto-repairing character of the scale products. Microscopic and spectrometric evaluation of the oxidized samples yielded information on the morphology of the scales, their nature and the distribution of their components over the surface. The addition of 0.5 at. % of ruthenium in Praxair NI-482 NiCoCrAlYTa powders yields a significantly lower mass gain of the doped coupons than the undoped ones. Consequently, corresponding oxidation kinetics are reduced, especially at high temperature. It is thus expected that the performance of ruthenium doped bond coats will be improved. Finally, this study reveals that spouted bed chemical vapor deposition is a convenient method for the systematic screening of the doping by different elements, of raw materials used in thermal barriers.
3:50 PM		A1-2-8 Composition and Oxidation Resistance of Ti-B-C and Ti-B-C-N Coatings Deposited by Magnetron Sputtering D. Zhong, J.J. Moore, B.M. Mishra, T. Ohno (Colorado School of Mines); E.A. Levashov (Moscow Institute of Steel and Alloys, Russia); J. Disam (Schott Glas Inc., Germany) In this work, Ti-B-C and Ti-B-C-N coatings have been deposited from a TiB₂-TiC composite target by using RF magnetron sputtering technique. In this paper, the composition and oxidation kinetics of Ti-B-C and Ti-B-C-N coatings will be reported and discussed. The film composition was characterized using x-ray photoelectron spectroscopy (XPS). Both dynamic and isothermal oxidation kinetics were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The oxide compositional depth profile, structure, and morphology were characterized by Rutherford backscattering spectrometry (RBS), x-ray diffraction (XRD), and scanning electron microscope (SEM), respectively. The film composition and the chemical states of elements in the films were altered by the sputtering gas atmosphere. The oxidation rates, activation energies, and mechanisms of Ti-B-C and Ti-B-C-N films will be presented in detail.
4:10 PM		A1-2-9 Advanced Nitride Coatings for Oxidation Protection of Gamma Titanium Aluminides C. Leyens (DLR-German Aerospace Center, Cologne, Germany); P.Eh. Hovsepian, W-D Münz (Sheffield Hallam University, United Kingdom) Ti aluminides are anticipated to be used at temperatures of the order of 750 to 800°C and for future generation alloys even well above that limit, where oxidation resistance becomes a major concern. Novel nitride coatings have been developed using a combined cathodic arc/sputtering process, where the substrates are metal ion etched, running a Cr target in a cathodic arc mode prior to coating deposition. The metal ion etching process generates an ion implanted subsurface zone on which, in a subsequent step, an 0.2-0.3 µm thick monolithically grown base layer is deposited, followed by a multilayer/superlattice structured main coating. The paper presents results on metal ion-etched gamma titanium aluminide Ti-45Al-8Nb overlaid with a fine layered TiAlCrN/TiAlYN as well as TiAlN/CrN superlattice coating. Specimens were tested under isothermal and cyclic oxidation conditions in air at 750°C and 850°C. Both coating systems demonstrated excellent oxidation resistance up to 1000 hrs under these conditions. Detailed investigation of the coating microstructure in the as-deposited condition and after oxidation testing will be presented to underline the great potential these coating systems appear to have.
4:30 PM		A1-2-10 Thermal Stability and Morphology of Cr_1-xAl_xN Thin Films Deposited by Reactive Magnetron Sputtering O. Banakh, P.E. Schmid (IPA-DP-EPFL, Switzerland); R. Sanjinés (Institute of Applied Physics, Switzerland); F. Lévy (IPA-DP-EPFL, Switzerland) Cr-Al-N films are believed to be promising nitrides for protective coatings due to the formation of both aluminium and chromium oxides, which suppress the oxygen diffusion. Cr_1-xAl_xN thin films with 0≤x≤0.5 were deposited by reactive magnetron sputtering from bulk Cr and Al targets in a mixed Ar/N₂ atmosphere at a substrate temperature of 573K. All the films crystallise in the rocksalt-type, cubic structure showing a (111) preferential orientation. Scanning Tunneling Microscopy examination reveals small (20 nm) crystallites shaped as triangular pyramids. With increasing aluminum content in the films a lattice shrinkage is observed and the oxygen contamination increases up to 8 at.% for x=0.5. The thermal stability of Cr_1-xAl_xN films has been investigated by annealing in air at various temperatures up to 1173K. The degree of film oxidation was evaluated by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Rutherford Backscattering Spectroscopy. XRD analysis in a grazing incidence configuration reveals the presence of a Cr₂O₃ surface layer. No peaks related to crystalline Al₂O₃ phases have been detected by this method. The results show that films rich in Al are more resistant to high temperatures compared to pure CrN and to films with low Al content. Actually the presence of Al progressively prevents the Cr₂O₃ formation. Films with the highest Al content (x=0.5) are stable up to 1173K. FTIR measurements performed in the range 400 to 1400 cm^-1 show the presence of Cr-O and Cr-N bonds. No clear evidence about the presence of Al-O bond has been found. The results obtained by FTIR confirmed the stability of the films with higher Al content.