Papers

ICMCTF2006 Session A1-2: Coatings to Resist High Temperature Corrosion and Wear

Tuesday, May 2, 2006 1:30 PM in Room Sunrise

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		A1-2-1 Development of Coatings for Protection in Specific High-Temperature Environments M. Schütze (Karl-Winnacker-Institut der DECHEMA e.V., Germany) Metallic and intermetallic coatings are widely used in jet engines and land-based gas turbines for oxidation and corrosion protection in the hotter parts of the engines. However there are a significant number of industrial processes where the use of protective coatings at high tem-peratures could contribute to a significant extension of life-time or an increase in operation temperature and thus efficiency. Examples of such industries are incineration and gasification of waste, biomass and coal, chemical process industries and petrochemical plants where highly aggressive environments are encountered containing species of e.g. carbon, chlorine, sulphur or vanadium. Since most of these process environments contain only very low oxygen partial pressures or exhibit high concentrations of extremely aggressive compounds, the conventional, uncoated materials come to their limits. In recent years in laboratory work a number of new types of coatings have been developed for high-temperature application which include diffusion coatings, overlay coatings and nanotechnological approaches for sealing porosity. In the paper the background of this development and the thermodynamic fundamentals will be discussed together with some more recent solutions based on synergistic effects of multi-element coatings. The results of performance tests of these coatings in sulphidizing, carburizing, chloridizing and vanadate environments will be presented. At the end conclusions can be drawn on the suitability of the different types of coatings for their specific applications.
2:10 PM		A1-2-3 Oxidation Resistant Cu-Cr Coatings for a Copper-Based Alloy GRCop-84 K.T. Chiang (Southwest Research Institute); D.L. Grimmett (Pratt & Whitney, Rocketdyne); J.A. Nesbitt (NASA Glenn Research Center) The effectiveness of Cu-Cr coatings in protecting a GRCop-84 (Cu-8at.%Cr-4at.%Nb) alloy was evaluated by high temperature exposure of coated specimens at 500-800°C in Ar-2%O₂ mixture. Three Cu-Cr coatings, Cu-17.1Cr, Cu-21.3Cr and Cu-25.6Cr (in wt.%), were deposited by Kinetic Metallization method using prealloyed -635 mesh powders. The oxidation kinetics was studied by thermogravimetric analysis. The oxide phases that formed were characterized by x-ray diffraction and oxide morphology was evaluated by scanning electron microscopy. All three coatings provide oxidation protection for the GRCop-84 alloy by reducing the rate of oxygen ingress. The Cu-25.6Cr coating provides the best oxidation resistance over the entire temperature range of 500-800°C. The mechanism of oxidation protection will be discussed.
2:30 PM		A1-2-4 Oxidation, Embrittlement and Fatigue of Coated Titanium Alloy Ti-6-2-4-2S C. Leyens, J. Lindemann, O. Schroeter (Technical University of Brandenburg at Cottbus, Germany); M. Froehlich (German Aerospace Center (DLR), Germany); O. Piotrowski (Titanium Engineering SA, Switzerland) Due to their high affinity to oxygen, titanium alloys have poor oxidation resistance and tend to embrittlement when exposed to oxidizing atmospheres. Protection of titanium alloys by coatings is a promising way to improve environmental resistance, however, this is often achieved at the expense of mechanical properties, particularly fatigue resistance. The fatigue debit is often higher than can be accepted for technical applications and this has hindered the use of coatings for this purpose as yet. Titanium alloy Ti-6-2-4-2S, an alloy currently used in aero engine applications at moderately elevated temperatures, was coated with two different types of metallic coatings from the Ti-Al-Cr and the Ti-Al-Ag system by magnetron sputtering and by a ceramic layer using an electrochemical coating process. Coated samples and uncoated reference samples were isothermally exposed to air at 600°C for 10, 100 and 1000h, respectively. Subsequently, room temperature fatigue tests were conducted using rotating beam loading at R=-1, f=100Hz. Cross-section micro-hardness measurements after testing as well as careful metallographic investigations using SEM revealed significant differences of the coatings in terms of oxidation behaviour and embrittlement as well as fatigue behaviour. The relevant mechanisms causing fatigue failure will be highlighted. Clearly, for practical applications, a fatigue debit due to the presence of coatings must be considered which presently appears to be 10% at best.
2:50 PM		A1-2-5 Creep and Corrosion Testing of Aluminide Coatings on Ferritic Substrates S. Dryepondt, B.A. Pint (Oak Ridge National Laboratory); Y. Zhang (Tennessee Technological University) The performance of iron aluminide coatings on commercial ferritic-martensitic Fe_-9Cr alloys (Fe-9Cr-1Mo and Fe_-9Cr_-2W) is being investigated in creep and corrosion testing. Coatings were manufactured by chemical vapor deposition (CVD) and pack cementation at 900-1050°C. Corrosion tests were conducted at 650-700°C in wet air (10%water vapor) which accelerated the oxidation rate of uncoated Fe_-9Cr alloys compared to dry air. With a CVD aluminide coating, a thin protective alumina layer was formed after more than 12,000h of testing in wet air. One concern about these high temperature coating processes is the effect on the creep strength of alloys with a martensitic microstructure. Initial creep testing at 650°C has shown an expected degradation due to Al interdiffusion with the substrate. However, the creep strength of an uncoated Fe_-9Cr_-2W specimen exposed to the pack coating time and temperature profile was not degraded. Preliminary results from combined creep and corrosion exposures also will be reported.
3:10 PM		A1-2-6 The Long-Term High Temperature Oxidation of Hot-Dipped Al-(Si) Coating on the 2.25 Cr-1 Mo Alloy C.-J. Wang, H.-C. Chen, C.-C. Wei, Y.-E. Wu (National Taiwan University of Science and Technology, Taiwan) Protective coatings are commonly used to extend the life of alloys in corrosive environments encountered in various industrial fields. Effect of Si content for the cyclic and static oxidation behaviors of hot-dipped Al-(x wt%) Si coatings on 2.25Cr-1Mo alloy was studied at 750°C in dry air. OM, XRD and SEM/EDX were utilized to characterize the element distributions phase composition, and morphology of the aluminized after hot-dipped treatment. The morphology and the phase compositions of the oxide scale were also studied. Experiment results indicate that the oxidation kinetics obey the parabolic rate law. A nearly linear relationship between the oxidation rate constants and time is observed. The composition of scales formal on 2.25Cr-1Mo alloy consists of Cr₂O₃, FeAl₃, Fe₂Al₅, and FeAl.
3:30 PM		A1-2-7 Plasma-Enhanced Planar Magnetron Sputtering of Ta and Cr Coatings on Gun Steel S.L. Lee (US Army ARDEC-Benét Labs); R. Wei, J. Arps, E. Langa (Southwest Research Institute); A. Welty, C.P. Mulligan (US Army ARDEC-Benét Labs) Ta and Cr coatings were deposited on gun barrel steel using plasma enhanced planar magnetron sputter techniques under various ion-assisted and biasing conditions. Prior to deposition, various sputter cleaning techniques including argon and argon plus hydrogen plasma, and nitriding process were performed to clean the substrate steel surface. Cr was then used as a bond layer for subsequent Ta coating deposition. The effect of the bond layer thickness on coating characteristics was also investigated. Auger electron spectroscopy showed complete removal of oxides from steel surfaces using Ar and Ar plus hydrogen plasma. XRD and Knoop hardness tests showed compressively stressed hard bcc Cr and predominately bcc phase Ta coatings. Micro-scratch test and groove adhesion test showed adherent bore coatings can be deposited using the plasma enhanced techniques. SEM and EDX were used to evaluate coatings morphology, microstructure, and adhesion characteristics. Optimum sputter deposition parameters are being identified, which can lead to an effective sputter deposition process of environmental friendly coatings for gun bore applications.
3:50 PM		A1-2-8 Synthesis of Titanium Oxide/Aluminum Oxide Multilayer Coatings by Reactive Pulsed D.C. Magnetron Sputtering with Enhanced Hardness C.A. Freyman, Y.W. Chung (Northwestern University) There are two important requirements for wear-protective coatings used at high temperatures under oxidizing conditions. One is their oxidation resistance. Another is their hardness properties at high temperatures. Maximal valence oxides, by definition, satisfy the first requirement. However, most oxides are relatively soft. Our approach is to use oxide nanolayers to improve the hardness. Specifically, titanium dioxide/aluminum oxide multilayers have been deposited by reactive pulsed d.c. magnetron sputtering with no deliberate substrate heating. Individual layers are on the order of 1-5 nm thick. Apart from the formation of Al₂TiO₅, titanium oxide and aluminum oxide are immiscible at temperatures up to 1600°C. This helps to stabilize the multilayer architecture. Our studies indicate that such multilayer coatings can be synthesized, with rutile TiO₂ and amorphous Al₂O₃ as the primary constituents. Structure and hardness properties were explored as a function of substrate bias and oxygen partial pressure. The hardness of these oxide multilayer coatings is enhanced over the rule-of-mixtures values. These results suggest that such oxide multilayers may have promise in high-temperature wear applications.
4:10 PM		A1-2-9 The Influence of CrN/AlN Thickness Ratios on the Microstructure and Oxidation Behavior in the Chromium Nitride/Aluminum Nitride Multilayer Coatings S.-K. Tien, J.G. Duh (National Tsing Hua Univerisity, Taiwan) Chromium nitride/aluminum nitride multilayer coatings with modulation period of 4nm and CrN/AlN thickness ratios of 0.33, 1.0 and 3.0 were fabricated on both the stainless steel 420 and Si substrates by RF magnetron sputtering. In order to investigate the oxidation behavior at elevated temperatures, the films were annealed at 700, 800, 900, and 1000°C for 1 hr in air. The existence of multilayer and phase transformation were analyzed by X-ray diffraction and transmission electron microscope (TEM). Oxides formed on the surface were identified by means of EDX. The hardness of CrN/AlN multilayer coatings with different thickness ratios was all above 30 GPa, which was much better than that of rule of mixture. After heat treatment, oxide that formed on the surface, contained both Al-rich and Cr-rich oxide, in which Cr-rich oxide was embedded in the Al-rich (Al_x,Cr_1-x)₂O₃. The comparisons of hardness and oxidation resistance for CrN/AlN coatings with different thickness ratios were also discussed.
4:30 PM		A1-2-10 Isotope Exchange Studies of Oxidation Mechanisms in Nickel-Base Superalloys Using FIB-SIMS Techniques T.N. Ooi, R.J. Chater, D.S. McPhail, B.A. Shollock (Imperial College London, United Kingdom) The thermally grown oxide layer is widely believed to be the root cause of failure of thermal barrier coating system used in nickel superalloy turbine blades. Understanding the mechanisms of oxidation of the bond coat that is applied to the superalloy is key to improving the performance of the thermal barrier coatings. FIB-SIMS (Focused Ion Beam-Secondary Ion Mass Spectrometry) techniques in conjunction with tracer diffusion experiments represent a powerful tool in characterizing this oxide layer. This paper presents the results of the oxidation studies on single crystal nickel-base superalloys/bond coat systems. In these studies, a two-stage oxidation experiment is used where ¹⁸O₂ serves as a tracer element during the second stage oxidation. The aluminium oxide grown in ¹⁶O₂ during the first stage oxidation represents a background oxide. Mass spectrum collected by FIB-SIMS reveals the counter mass transportation of inward diffusion of oxygen and outward diffusion of aluminium. New oxide formation during the second stage oxidation under ¹⁸O₂ enriched environment is observed at both the gas/oxide interface as well as oxide/superalloy interface. FIB-SIMS scanning enables high-resolution isotope maps, in particular ¹⁸O^-, to be captured. These confirm the existence of new oxide forming at the oxide/superalloy interface with clear indication of short circuit diffusion paths through the existing oxide. These data allow the diffusion mechanisms for different superalloy/base coat systems to be identified and contrasted, allowing the role of alloying additions to be elucidated.
4:50 PM		A1-2-11 AlGreen: An Environmentally-Friendly Diffusion Slurry Aluminide Coating for Gas Turbines L.B. Kool (General Electric Global Research); D. Carr (General Electric Energy); R DiDomizio, M.F. Gigliotti, A. Kogan (General Electric Global Research); B.S. Noel (General Electric Energy); R. Thompson (Thom-Tek, Inc.) Gas turbine airfoils are typically constructed from superalloys having the physical properties needed for high temperature operation. Under conditions of operation, however, these alloys would be quickly degraded by oxidation reactions that occur at high temperature. To prevent this problem, oxidation-resistant coatings are widely used in the industry. These coatings often include a diffusion aluminide outer layer, which imparts oxidation resistance by forming a tenacious oxide film. Diffusion aluminide coatings are often applied by means of an aluminum slurry that is applied as an overlay coating or paint, then subsequently diffused into the underlying base metal in a high temperature diffusion treatment. Aluminum slurries can be hazardous because they are composed of finely divided aluminum powder, which is notoriously reactive. This problem of instability is usually mitigated by including slurry components that stabilize the aluminum powder. One such component is hexavalent chromium. A major drawback of Cr(VI) is that it is a known carcinogen. Although they are widely used in the industry, processes utilizing hexavalent chromium are being phased out because of environmental concerns. In this paper we describe our development of an environmentally-friendly, Cr(VI)-free slurry aluminizing process and its implementation in the GE Gas Turbine business.