Papers

ICMCTF2004 Session A1-2: Coatings to Resist High Temperature Corrosion and Wear

Thursday, April 22, 2004 1:30 PM in Room Sunrise

Thursday Afternoon

Start	Invited?	Item
1:30 PM		A1-2-1 Effect of Surface Impulsive Thermal Loads on Fatigue Behavior of Constant Volume PropulsionEngine Combustor Materials D. Zhu, D.S. Fox, R.A. Miller, L.J. Ghosn, S. Kalluri (NASA Glenn Research Center) The development of advanced high performance constant-volume-combustion-cycle engines (CVCCE) requires robust design of the engine components that are capable of enduring harsh combustion environments under high frequency thermal and mechanical fatigue conditions. In this study, a simulated engine test rig has been established to evaluate thermomechanical fatigue behavior of candidate engine combustor materials, Haynes 188 and Haynes 230, under superimposed CO₂ laser surface impulsive thermal loads (30-100 Hz) in conjunction of the mechanical fatigue (10-20 Hz) loads. The high cycle mechanical fatigue testing of some laser pre-exposed specimens has also been conducted under a frequency of 100 Hz to determine the laser surface damage effect. The test results have indicated that material surface oxidation and creep-enhanced fatigue is an important mechanism for the surface crack initiation and propagation under the simulated CVCCE engine conditions. The future surface protection approaches and coating development needs will also be discussed for improving the materials fatigue resistance.
1:50 PM		A1-2-2 Microstructures and Hot Corrosion Behaviors of Aluminized Austenitic Fe-Mn-Al and Inconel 718 Alloys J.W. Lee (Tung Nan Institute of Technology, Taiwan); C.H. Ching (Tung Nan Institute of Technology, Taiwan, R.O.C.); J.G. Duh (National Tsing Hua University, Taiwan, R.O.C.); J.S. Huang, Y.C. Chen (Tung Nan Institute of Technology, Taiwan, R.O.C.) Aluminization has been employed on alloys to form dense and adhesive Al₂O₃ surface layers to prolong their service life at high temperature. Austenitic Fe-Mn-Al alloy and superalloy Inconel 718 were used to deposit high aluminum containing surface layers at 900°C for 9 hours. The phase transformations and microstructure phenomena of aluminized coating layers and matrix of alloys were studied with X-ray diffractometer and electron probe microanalyzer. The hot corrosion tests of two aluminized alloys were conducted in 90% Na₂SO₄ and 10% NaCl mixture molten salt at 900°C for 1 to 100 hours. It is found that the iron aluminide and nickel aluminide phases ranging 80-100 µm in thickness were achieved by pack aluminization processes on the Fe-Mn-Al and Inconel 718 alloy surfaces. Dense alumina surface layers were produced on the aluminized alloys after pre-oxidation treatment at 1100°C for 1 hours. It was observed that hot corrosion performances of two alloys were enhanced by aluminizing processes. Similar hot corrosion behaviors were revealed on two aluminized alloys. However, the inward and outward diffusion of aluminum on the surface layer showed detrimental effect to the hot corrosion resistance of alloys tested for long time.
2:10 PM		A1-2-3 The Effect of Specimen Treatment and Surface Preparation on the Isothermal Oxidation Behaviour of HVOF- Sprayed MCrAlY-Coatings W. Brandl, G. Marginean, D. Maghet, D. Utu (University of Applied Sciences Gelsenkirchen, Germany) Blades and vanes of stationary gas turbines are subjected during service to high temperatures and corrosive media. The turbine manufacturers have applied protective coatings to increase the durability and field performance especially of turbine blades. Porosity, oxide stringers and clusters, fissures and cracks permit corrosion paths through the coating to the substrate. Because of that it is important to obtain coatings with low porosity and oxide content. High quality (low porosity and oxides) coatings are required to provide optimum protection against hot corrosion and oxidation. Alloys and coatings designed to resist oxidising environments at high temperatures should be able to develop a surface oxide layer, which is thermodynamically stable, slowly growing and adherent. During oxidation in the temperature range 850 - 1300°C, MCrAlY- coatings form Al₂O₃ rich scales, which are reasonably effective for long-term applications involved in either isothermal or thermal cyclic oxidation. This study is concerned with the isothermal oxidation behaviour of High Velocity Oxygen Fuel sprayed CoNiCrAlY coatings, with different aluminium contents (12 and 15 wt.% Al). Differently treated specimens (grinding, polishing and EB-remelting) were oxidized at 950°C in synthetic air for different periods of time. Oxidation studies include thermogravimetric weight gain measurements under isothermal conditions in order to quantify the parabolic oxidation rate constant. The microstructure and morphology of as-sprayed and of the oxidized coatings were characterised by SEM and XRD. Comparing the oxidation rate constants of the samples it can be observed that a finer structure leads to a better quality of the formed oxide layer and to higher oxidation resistance.
2:30 PM		A1-2-4 Advanced Transmission Electron Microscopy Investigations on the Oxidation of Metallic Alloys M. Rühle (Max Planck-Institut fuer Metallforschung, Germany); A. Catanoiu, P. Bellina (Max Planck-Institut für Metallforschung, Germany); C.G. Levi (University of California, Santa Barbara) The adhesion of an oxide scale on an oxidized metal (metal alloy) depends critically on the composition and structure of the interface between the metal oxide and the metal substrate. Information on both, structure, and composition, can be obtained on a (nearly) atomic level by advanced transmission electron microscopy (TEM) techniques. Within the last five years TEM improved dramatically with the availability of a new generation of instruments. In this presentation the different TEM techniques (Q-HRTEM, Q-AEM, ESI and ELNES) will be described. Then TEM investigations will be presented for the oxidation of NiAl, Ru-alloy, and on interfaces in thermal barrier coatings. The results will be critically discussed. Implications of the results will be presented.
3:10 PM		A1-2-6 Long Term Oxidation and Phase Transformations in Aluminized CMSX-4 Superalloys N. Mu, J. Liu, Y.H. Sohn (University of Central Florida); Y.L. Nava (Solar Turbines) Twenty disk-shaped aluminized CMSX-4 single crystal superalloy specimens were oxidized in air at 788º, 871º, 954º and 1010ºC for 1000, 2500, 5000, 7500 and 10000 hours. Phase constituents and residual stress of thermally grown oxide (TGO), as well as microstructural evolution in the aluminized layer of CMSX-4 were examined by using photo-stimulated luminescence spectroscopy (PSLS), optical microscopy and electron microscopy. The compressive residual stress arising from thermal expansion mismatch increased with increasing oxidation temperature and time. A large scatter in the residual stress was observed with the spallation of TGO scale. Whisker-shaped metastable Al₂O₃ phases were detected by both PSLS and transmission electron microscopy. Presence of the metastable Al₂O₃ phases played an important role impacting the spallation modes of TGO involving grain boundary ridges of β-NiAl. Protective diffusion aluminide coatings, which initially consisted of β-NiAl, transformed into Al-depleted γ" and γ solid solution with increasing oxidation temperature and time. A finite difference method was used to simulate and predict the phase constituents and Al concentration in the diffusion aluminide coatings to predict the lifetime of coatings as a function of temperature.
3:30 PM		A1-2-7 Effect of Cycle Length on the Oxidation Performance of Iron Aluminide Coatings Synthesized by Chemical Vapor Deposition Y. Zhang (Tennessee Technological University); B.A. Pint, G.W. Garner, K.M. Cooley, J.A. Haynes (Oak Ridge National Laboratory) One of the lifetime issues of particular concern for application of iron aluminide coatings is the possible compatibility problems between Fe-Al coatings and substrates which have substantially different coefficients of thermal expansion (CTE). In order to explore the role of CTE on coating performance, coatings made by chemical vapor deposition on Fe-9Cr-1Mo and type 304L stainless steel were tested in humid air at 700°C with different cyclic oxidation frequencies. A high cycle frequency (1h cycle time) was used as an accelerated test and after 2000, 1h cycles coatings on both substrates cracked allowing accelerated attack of the substrate in this environment. When the cycle time was increased to 100h, similar coatings have shown excellent performance for 40, 100h cycles (4000h) without degradation. Characterization of the Al content of coatings after a 2000h isothermal test showed that minimal back diffusion of Al occurred at 700°C. The failure of the coatings in 1h cycles suggests a long-term durability problem due to the CTE mismatch between coating and substrate.
3:50 PM		A1-2-8 Dilatometer Studies of MCrAlY Coatings T.A. Taylor (Praxair Surface Technologies) MCrAlY coatings applied by plasma spray deposition were studied in the dilatometer for first-cycle sintering and in the thermally stabilized state for thermal expansion. Several different compositions were tested. Comparisons were made to previous expansion curves of other MCrAlY coatings.
4:10 PM		A1-2-9 MOCVD Processed Platinum Aluminide Coatings on Titanium Alloys M. Delmas (CIRIMAT, France); M. Pons, C. Vahlas (CNRS, France) Titanium alloys present very good mechanical properties, especially in view of their low density. However, their reduced oxidation and corrosion resistance at high temperature are limiting factors to their application in aero gas turbine engines. If adapted protective coatings are applied on their surface then, titanium could replace heavier alloys such as steels that are used nowadays in those parts of turbines operating at temperatures exceeding 450°C. Platinum aluminides coatings have already been tested on titanium alloys with a remarkable improvement in oxidation resistance compared with unprotected and even with plain aluminium coated alloys. Usually, PtAl coatings are processed by electroplating several micrometers thick platinum layer followed by aluminization by pack cementation. In the present contribution, an alternative route for the processing of such a coating is proposed, based on the simultaneous application of both elements by metalorganic chemical vapor deposition (MOCVD). Such a route presents significant advantages such as simplicity and contamination (namely sulfur) free coatings. Deposition is performed in a cold wall reactor on all sides of a 15 mm diameter and 1 mm thick Ti6242 coupon in one run. Deposition of pure aluminum from tri-isobutyl aluminum in the temperature range 300° - 400°C is in progress in view to optimize coating uniformity by computational fluid dynamics simulation. Tuning of the composition and microstructure of the coatings in view of the aimed application will be performed through isothermal and cyclic oxidation tests of the as processed samples.
4:30 PM		A1-2-10 Bond Coat Development for Thermal Barrier Coatings on Gamma Titanium Aluminides C. Leyens, R. Braun (German Aerospace Center (DLR), Germany) With increasing elevated temperature capabilities of gamma titanium aluminides, oxidation/ hot corrosion protection as well as thermal protection might become important to further push these outstanding materials beyond their cur-rent limits. While thermal barrier coatings (TBCs) based on yttria stabilized zirconia are state-of-the-art for high pressure turbine hardware, thermal protection of gamma titanium aluminides has not yet been explored in greater detail. In the present study EB-PVD TBCs were deposited onto gamma titanium alloy Ti-45Al-8Nb which was protected by chromia- and alumina-forming nitride and diffusion coatings, respectively. The paper will discuss results of cyclic oxidation tests in the temperature range between 850 and 950°C. Post-oxidation SEM/EDX analysis was used to characterize coating microstructure, correlate it to different lifelines of the TBC, and finally assess the potential of the alumina coatings for bond coat applications on gamma TiAl.
4:50 PM		A1-2-11 Analysis of the SiO₂ Layer Grown on HVOF-Sprayed MOSi₂ Coatings during Oxidation at High Temperatures G. Reisel (Chemnitz University of Technology, Germany) Molybdenum disilicide (MoSi₂) shows a great potential as a structural material as well as a protective coating for applications in the temperature range above 1300°C caused by its unique combination of properties like the moderate density, the high melting point, the low coefficient of thermal expansion and the excellent oxidation resis-tance. MoSi₂ was sprayed onto steel substrates by high velocity oxy-fuel spraying and then solved from the substrate, so that the coating could be tested as a free-standing body. The coat-ings as sprayed were characterized by SEM, XRD and EDX. Oxida-tion tests were done in a simultaneous thermoanalytical equipment by means of thermogra-vimetric measurements at temperatures between 1100°C and 1500°C. The grown SiO₂ layers was examined by means of SEM, TEM, SAED, EDX and XRD. Below test temperatures of 1300°C the oxide layer appears thin and not dense. XRD confirms the existence of a low amount of cristobalite. At tempera-tures equal or higher 1300°C the appearance of the SiO₂ layer changes significantly. The oxide layers are now dense with a thickness about 10 µm. It seems to be glassy with inclu-sions. TEM investigations show that it consists of cristobalite crystallites which are embedded in an amorphous membrane like matrix of silicon oxide.