ICMCTF2008 Session A1-1: Coatings to Resist High Temperature Oxidation and Wear
Tuesday, April 29, 2008 8:00 AM in Sunrise
A1-1-1 Development of High Temperature Coatings and TBC Bond Coats for Gas Turbine Engines
Y. Cadoret (SNECMA, France)
Important efforts to develop high temperature coatings are being carried out at SNECMA to increase turbine efficiency and performance in aeroengine. The protection of components against very high temperature and environmental degradation is a major issue in modern gas turbines. The extreme operating conditions lead to the development of various coating technologies. The deposition of aluminium on the surface of the Ni-based superalloy is one of the most commonly employed technique. The oxidation resistance of this intermetallic bond coat system (@beta@-NiAl) is based on its ability to form a protective alumina scale at high temperature. Platinum additions in aluminide coatings and incorporation of reactive element dopants (Y,Zr,Hf,Si) are known to improve both hot corrosion and oxidation resistance. The effect of various impurities, such as S or C, is also of particular interest because this could strongly affects the coating oxidation performances.@paragraph@ Thermal Barrier Coating systems (TBCs) have also proven their capability to protect the hottest section of gas turbine engines. This multilayer system allows to increase the component operating temperature, up to several hundreds of degrees, in the combustor and turbine sections. Considerations in deciding whether a part should be coated or uncoated include the choice of the substrate composition, the performance requirements and the cost target. The lifetime of the TBC is directly affected by the adhesion of the interfacial alumina scale. Spallation is particularly damaging because the turbine blade is no longer protected from very high temperature. The expertises of field returned parts contribute to improve the understanding of the unexpected degradation mechanisms (CMAS, Erosion, FOD, phase transformation, rumpling, voiding.....) and allow to promote the development of new TBC systems with higher performance and reliability.
A1-1-4 High Temperature Resistant Coatings Made by HVOF Spraying of Small-Size Alloy Particles in Suspension
X. Ma, J. Roth (Inframat Corporation); D. Gandy, G. Frederi (Electric Power Research Institute)
Coatings for use at high temperature require full density, high adhesion, minor oxide inclusions, and preferably fine grains. High velocity oxygen-fuel (HVOF) thermal spray processes have been applied extensively for making such coatings with the highest density and adhesion strength, which is not achievable in most other thermal spray processes. However, the existence of not or partially melted particles are usually observed in HVOF-formed coatings due to relative low flame temperature and short particle resident time in the process. This work has investigated the development of an innovative HVOF process using fine powders typically less than 10 µm, which has no flowability in a conventional thermal spray process. The advantages of using small and even nano-sized particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. Process investigations have proven the benefits for making alloy coatings with full density and high bond strength attributing to the better melting of the small particles and the very high kinetic energy of particles striking on the substrate. High temperature oxidation and hot corrosion tests at 800-850@super o@C have demonstrated that the alloy coatings made by the novel process have superior properties to conventional counterpart coatings in terms of oxidation rates and corrosion penetration depths. One of the targeted applications is for the protection of fossil-fired boilers that operate in a high temperature and corrosive environment.
A1-1-5 Microstructural Stability of fcc- γ+B2-ß Coatings on Substrate in Ni-Cr-Al System - A Phase Field Model Study
R. Mohanty, Y. Sohn (University of Central Florida)
Degradation of overlay coatings and bond coats for thermal barrier coatings for Ni-base superalloys occur by the depletion of Al content of the coatings either by means of oxidation to form an external protective Al@sub2@O@sub3@ layer and coating-substrate interdiffusion. The present work presents the phase-field simulation of interdiffusion process that takes place between the coating and the substrate with solid-to-solid diffusion couples of Ni-Cr-Al alloys containing fcc-γ vs. γ+ß (B2) phases. The phase field model was devised using the available thermodynamic and kinetic data for the Ni-Cr-Al system along with the concentration dependent chemical mobilities. Evolution of interdiffusion microstructures in these γ+ß/γ diffusion couples were simulated with alloys of varying compositions and volume fractions of the precipitate phase (i.e. ß). It was observed that the γ+ß two-phase region receded from the original couple interface due to the dissolution of ß phase. The rate of dissolution or the recession distance was found to be a function of concentration of the single-phase γ alloy and the volume fraction of the ß phase in the two-phase alloy of the couple, i.e. the higher the Cr and Al content and the volume fraction of ß, the lower is the recession rate. Composition profiles predicted from the simulation suggested that the interdiffusion occurs both in the single-phase γ and the two-phase γ+ß region, while the γ phase maintains local equilibrium with the ß phase. These results were compared and found to be in good agreement with the results of experimental works published in literature.
A1-1-6 Growth Strains in a-Al@sub 2@O@sub 3@ Thermally Grown on ’-Phase NiPtAl(Hf) Alloys
B.W. Veal, A.P. Paulikas (Argonne National Laboratory); B. Gleeson (University of Pittsburgh)
We have performed in-situ measurements of growth strains in a-Al@sub 2@O@!sub 3@ scales, grown on α-phase Ni-20Al-20Pt-Hf (at. %) alloys with Hf concentrations varied between 0 - 1 %. Strain measurements were obtained, using 21.6 keV synchrotron x-radiation exploiting a single-exposure sin2? Method. Samples were first heated to 1150 °C and were held isothermally until the ? -> a phase transition was complete. Kinetics of the ? -> a phase transition were monitored at 1150@super o@C by measuring the time dependence of ? And a-phase intensities and strains. The transformation rate systematically slows with substrate Hf concentration. We observe small or weakly compressive steady state stresses in scales formed on Hf-free alloys. Tensile stresses appear when alloys contain Hf. After the ? -> a transformation had occurred, the temperature was dropped to 1100@super o@C; that imposed a compressive stress on the oxide, a consequence of the CTE difference between oxide and substrate. Oxide creep relaxation was then monitored at 1100@super o@C. The creep rate, at 1100@super o@C, was reduced when Hf was added to the alloy. Scale growth and relaxation mechanisms will be discussed. @paragraph@Research was sponsored by the U.S. Dept. of Energy, BES, Materials Science, under contract DE-AC02-06CH11357 (ANL) and by the Office of Naval Research, contract No. N00014-02-1-0733.
A1-1-8 The Effect of Platinum Content on Gamma/Gamma-Prime Diffusion Coatings
J.A. Haynes, B.A. Pint (Oak Ridge National Laboratory); Y. Zhang (Tennessee Technological University); I.G. Wright (Oak Ridge National Laboratory)
Platinum diffusion coatings, comprised of Ni-Pt-Al compositions that form gamma and gamma-prime phases, are receiving increasing attention as a bond coating for thermal barrier coating applications. This study investigated the impact of Pt content on the initial coating composition and subsequent oxidation behavior of gamma/gamma-prime platinum diffusion coatings on various Ni-based superalloys. Platinum-containing gamma/gamma-prime diffusion coatings were fabricated by vacuum annealing electroplated Pt on single-crystal and directionally-solidified superalloy substrates. Specimens with 7 and 12 µm thickness electroplated Pt were annealed over the range 1050 - 1175@super o@C for 2h. Compositions of the resultant coatings were characterized by electron microprobe analysis to evaluate distribution of Pt and Al as a function of annealing temperature and Pt thickness. Selected gamma/gamma-prime coatings fabricated from 7 and 12 µm Pt were comparatively evaluated in cyclic oxidation at 1100 and 1150@super o@C. Coatings with higher Pt contents showed significant improvements in spallation resistance at both 1100 and 1150@super o@C, particularly on the directionally-solidified substrate which contained higher Hf and S contents.
A1-1-9 Interdiffusion Behavior of Pt-Diffused @gamma@+@gamma@' Coatings on Ni-Base Superalloys
Y. Zhang, L. Liu (Tennessee Technological University); B.A. Pint, J.A. Haynes (Oak Ridge National Laboratory); B.T. Hazel, B.A. Nagaraj (GE Aircraft Engines)
Platinum-diffused @gamma@ + @gamma@' coatings (16-19 at.% Al, ~18 at.% Pt) were synthesized on René 142 and René N5 Ni-base superalloys by electroplating the substrates with ~7 µm of Pt, followed by an annealing treatment in vacuum at 1150-1175°C. In order to study the compositional and microstructural evolutions of these coatings at elevated temperatures, interdiffusion experiments were carried out on coated specimens in the temperature range of 900-1100°C for various durations. After 1000h diffusion at 1000°C and 1050°C, the thickness of a 30-µm @gamma@ + @gamma@' coating increased to ~65 and 120 µm, respectively. Composition profiles of the alloying elements in the @gamma@ + @gamma@' coating before and after diffusion experiments were determined by electron probe microanalysis. A diffusion model was applied to predict the Pt content and the coating thickness after diffusion.
A1-1-10 Volatilisation Studies on Iron Aluminide Coatings for Supercritical Steam Turbine Applications by TG-Mass Spectrometry
F.J. Bolivar, S.I. Castañeda, L. Sánchez, M.P. Hierro, F.J. Pérez (UCM, Spain)
A way to achieve a good corrosion protective layer on 9-12%Cr ferritic/martensitic steels is by aluminizing it by means of CVD-FBR technology. An iron aluminide coating is formed on the steel surface which has excellent oxidation and corrosion resistance, with lower rates of attack than ferritic heat resistant materials. Iron-aluminide coatings may be improved by using reactive elements because it is well known that a small addition of reactive element (RE), to several materials improvement has resulted in their oxidation resistance at high temperatures and beneficial effects have been observed on most chromia and alumina forming alloys. There are two general methods for introducing reactive element in chromia/alumina forming alloys: a)internal doping, in which a RE is introduced directly to the alloy as a metallic element or a dispersed oxide; b) external doping using an oxide coating or by implantation of the RE in the upper alloy surface. This CVD-FBR coating technique, is very attractive to be applied on this kind of material, since the aluminization is possible below 714ºC, temperature that means important microstructural changes and loss in mechanical properties during the coating process could be avoided. In order to study the volatilisation process of coated ferritic/martensitic steels in SCT (supercritical steam turbines environments), TG-Mass spectrometry studies have been develop to know the main species, forming the coating, that volatilises preferentially. This study jointly with thermodynamic calculations have been done to establish steam oxidation mechanism based in validated thermodynamic calculations and esperimental results.
A1-1-11 Observation of High Temperature Phase Transformation in the Aluminide Cr-Mo Steel Using EBSD
W.-J. Cheng, C.-J. Wang (National Taiwan University of Science and Technology, Taiwan)
5Cr-0.5Mo steel was coated by hot-dipping into a molten bath containing pure Al. The phase transformation in the aluminide layer during diffusion at 750°C in static air was analyzed by Electron Backscatter Diffraction (EBSD). The results showed that the aluminide layer in the as-coated specimen consisted of an outer FeAl@sub3@ layer and an inner Fe@sub2@Al@sub5@ layer. The Fe@sub2@Al@sub5@ phase kept growing with increasing exposure at 750°C, but the FeAl@sub3@ phase didn't change significantly. The FeAl@sub3@ phase disappeared once the aluminum layer was consumed. After 60min of exposure at 750°C, FeAl@sub2@ phase formed at the inner and outer sides of the Fe@sub2@Al@sub5@ phase while FeAl phase formed at the interface between FeAl@sub2@ and the steel substrate. With prolonged exposure, the thickness of FeAl increased and the volume of FeAl@sub2@ expanded at the expense of Fe@sub2@Al@sub5@. Based on Crystal Orientation Map (COM), Fe@sub2@Al@sub5@ grew preferentially along the C-axis,  direction, of the crystal structure.
A1-1-12 Maxthal as Candidate for Oxidation Resistant Coatings
M. Sonestedt, K. Stiller (Chalmers University of Technology, Sweden); J.-P. Palmquist, M. Sundberg (R&D Kanthal AB, Sweden); J. Frodelius, H. Högberg, L. Hultman (Linköping University, Sweden)
Coatings have been fabricated by spraying Maxthal 211 (Ti@sub 2@AlC) powder with High Velocity Oxy-Fuel spraying on high temperature Ni-base alloys. Maxthal 211 is an alumina forming ternary carbide with a thermal expansion coefficient near that of alumina. These properties make this material suitable for high temperature applications because it produces a protective oxide that does not spall off during cycling. So far, Maxthal 211 has only been used in bulk form with good oxidation properties in air up to 1450@super o@C. @paragraph@ This work is an initial oxidation study of Maxthal 211 coatings. The coatings have been analyzed both prior to and after oxidation with different characterization techniques, mainly scanning electron microscopy and X-ray diffraction. Coating properties such as phase content, adhesion etc. will be presented and discussed. To show the properties of this material over a wider temperature range, the results will also include long-term exposures at lower temperatures (700-900@super o@C) for the bulk material.
A1-1-3 Characterisation and High Temperature Oxidation Behaviour of HVOF Sprayed NiCrAl Coatings
M.R. Ramesh, S. Prakash, S.K. Nath (Indian Institute of Technology Roorkee, India)
High Velocity Oxy-fuel (HVOF) spraying is one of the most versatile thermal spraying processes to deposit coating material for use at high temperature. In the present investigation, HVOF process with an oxygen and liquid petroleum gas as the fuel gas was used to deposit NiCrAl alloy powder on boiler tube steel substrate. The coatings were characterized with respect to coating thickness, porosity, microhardness and microstructure. The high temperature oxidation behaviour of the coatings subjected to cyclic conditions for 50 cycles at 900@super o@C has been studied. Thermogravimetric studies have been carried out to establish the kinetics of oxidation. The combined techniques of X-ray diffraction, optical microscopy, scanning electron microscopy and electron probe micro analysis have been used to characterize the oxidation product. Thermodynamically stable a-Al@sub 2@O@sub 3@ formed on the surface of the coating shows slow scale growth kinetics during oxidation. Preferential oxidation of Al and Cr along the nickel rich splat boundaries inhibits oxidation of the substrate steel by blocking the diffusion of reacting species. The formation of a-Al@sub 2@O@sub 3@ and mixed spinel oxide of Ni and Cr are reported as protective oxides against high temperature oxidation.