Papers

ICMCTF2009 Session A1-1: Coatings to Resist High Temperature Oxidation

Tuesday, April 28, 2009 8:00 AM in Room Royal Palm 1-3

Tuesday Morning

Start	Invited?	Item
8:00 AM		A1-1-1 Platinum Nucleation Mechanisms and their Role in Platinum Aluminide Coatings M.A. Craig (Cranfield University, United Kingdom); L. Chirivi (Cranfiedl University, United Kingdom); K. Long, M. Robinson (Cranfield University, United Kingdom); D. Rickerby (Rolls Royce, United Kingdom); J.R. Nicholls (Cranfield University, United Kingdom) Platinum aluminide coatings are the bond-coat of choice for the aerospace industry in areas which are exposed to very high temperatures and corrosion products. Production methods for the deposition of the platinum vary; with industry favouring electroplating whereas academia favours sputtering by physical vapour deposition. Platinum aluminide coatings have been shown in the past to show exceptional resilience to high temperature oxidation and corrosion. Various types of platinum aluminide coating are commercially available (RT22, SS82A, CN91 and MDC 150L type) and are all produced with an initial platinum layer deposited followed by aluminising (either in pack (RT22), out of pack (SS82A) or vapour deposition (CN91 and MDC150L). This paper will show the importance of how the nucleation of the initial platinum layer affects the final structure of the platinum aluminide coating for both high and low temperature aluminide structures. Electrodeposition of platinum using the 5Q salt clearly shows that different nucleation mechanisms form different platinum aluminide coatings as the platinum, in some structures, acts as a diffusion barrier. The platinum deposited using PVD methods do not show as clearly that the microstructure has changed, however, cyclic oxidation tests are conclusive that nucleation affects the final quality of the coating.
8:20 AM		A1-1-2 Oxidation and Volatilization of Coatings for Power Generation: New Super Sultra Critical Steam Turbines F.J. Perez, S.I. Castaneda, M.P. Hierro, F.J. Bolivar (Universidad Complutense, Spain) The steels with chromium contents between 9 and 12% wt are used for power plants with advanced steam oxidation conditions. They present good creep properties as 9% Cr steels and good oxidation resistance, at the temperatures range between 500-600°C. In the last years numerous investigation in development of coatings has been realized with the aim the protected them against the oxidation in order to allow operation of steam turbines at 650°C. In this study, Al-Hf protective coatings were deposited by CVD-FBR on ferritic steel HCM-12A followed by a diffusion heat treatment, have shown to be protective at 650°C under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure. Morphology and composition of coatings were characterized by different techniques, such as scanning electron microscopy (SEM), electron probe microanalysis, and X-ray diffraction (XRD). On the other hand, volatile species by mass spectrometry (MS) and thermogravimetric mea surements (TG) during the initial stages oxidation of the P92 ferritic steel without and of Al and Al/Si coating by chemical vapour deposition in fluidized bed reactors (CVD-FBR) at 650ºC in Ar+20%H2O for 150h were studied. In order to go forward to coatings design the Thermocalc code as used as base for the MS-data. TG-MS experiments were conducted in a closed steam loop in order to obtain information about the oxyhydroxides formation as reaction between coatings and steam. From those results the role of the different coating element could be established and optimized for the coating durability. An oxidation mechanism based on the TG-MS results is given. The morphology/composition and structure of the oxidized samples were also studied using SEM/EDS and XRD techniques.
8:40 AM		A1-1-3 High Temperature Coatings for Gas Turbine Engines B.T. Hazel, J. Rigney, B. Boutwell, D. Wortman, R. Darolia, M. Gorman (GE Aircraft Engines) As jet engine high-pressure turbine (HPT) inlet temperature increase, new designs and materials are used to achieve the desired durability and reliability. Material advances have included use of single-crystal, Ni-based superalloys and protective coatings. Protective coatings on HPT blades at GE Aviation have evolved from the use of simple aluminides to noble-metal aluminides as performance demands have increased. In the most demanding applications, insulating ceramic thermal barrier coatings (TBC’s) are applied over the diffusion aluminides to further reduce the bulk and surface temperatures of the component. The use of advanced environmental coatings and TBC systems in turbine engines has become necessary for protection of turbine airfoils and other gas turbine components1. A more reliable and durable TBC system is necessary to increase the operating capability of turbine airfoils. The bond coat plays a significant role in the adhesion of the ceramic TBC and the overall life and capability of the TBC system reliability. Recent bond coat development has focused on advances to reduced oxide growth rates, reduced rumpling tendency, and stabilize the coating-substrate interface. The result is longer TBC adherence lives and a more durable TBC system for turbine airfoil operation. 1D.V. Rigney, R. Viguie, D.J. Wortman, D.W. Skelly, Journal of Thermal Spray Technology, 6 [2], 1997, 167-175.
9:20 AM		A1-1-6 Computational Investigation into the Phase Stability and Phase Distribution in the Ni-Cr-Al Ternary System K. Ma, J.M. Schoenung (UC Davis) The Ni-Cr-Al ternary system is of great significance in high temperature coatings. The stability and distribution of phases within the Ni-Cr-Al system are sensitive to the presence of a fourth element even if its content is not high. In this study, thermo-calc^® software was applied to calculate the effects of C, N, O and Fe on the phase stability and phase distribution in the Ni-Cr-Al system, especially at elevated temperatures. The mass fraction of each phase, including Ni (Cr) γ phase, NiAl β phase, Ni₃Al γ' phase and so forth, was calculated for select temperatures in the range of 200°C up to 1600°C while allowing the concentration of the additional element to be variable. In addition, the diffusion path of the fourth element in the material system was illustrated by evaluating its distribution in the different phases as a function of temperature. It was observed that minor change in Fe or O content has a significant influence on the mass fraction of the primary phases in the Ni-Cr-Al system. The computational results were compared with our previous experimental study and they are consistent with each other.
9:40 AM		A1-1-7 New Generation HIPIMS Nanostructured Coatings to Perform in Severe High Temperature Environment P.Eh. Hovsepian, A.P. Ehiasarian (Sheffield Hallam University, United Kingdom); C. Leyens (TU-Cottbus, Germany); R. Braun (German Aerospace Centre, Germany); F.J. Trujillo (Universidad Complutense de Madrid, Spain) CrAlYN/CrN were designed to meet the demands for highly oxidation resistant and wear resistant coatings for environmental protection of light weight materials such as γ-TiAl alloys for future applications in aero and automotive engines. CrAlYN/CrN coatings utilising nanscale multilayer structure with a typical bi-layer thickness of 4.2nm were successfully produced. The surface pre-treatment was carried out by bombardment with Cr⁺ ions generated by High Power Impulse Magnetron Sputtering, (HIPIMS) discharge, whereas the nanoscale multilayer CrAlYN/CrN coating was deposited by Unbalanced Magnetron Sputtering (UBM) or by HIPIMS. Scanning Transmission Electron Microscopy, (STEM) revealed that the coating/substrate interface was extremely clean and sharp. Large areas of coating grown epitaxially were observed. STEM-Energy Dispersive Spectroscopy (EDS) profile analysis further showed that during the HIPIMS ion bombardment Cr had been implanted into the substrate to a depth of 5 nm. In contrast to UBM coatings, HIPIMS deposited coatings showed extremely sharp interfaces between the individual layers in the nanolaminated material and almost no layer waviness. This improved structure resulted in further enhancement of the coatings barrier properties. CrAlYN/CrN showed potential for reliable protection of γ-TiAl alloys against wear and aggressive environmental attack. For coated γ-TiAl alloys, thermo gravimetric quasi-isothermal oxidation tests in air at 750°C after 1000 hours exposure showed four times smaller weight gain compared to the uncoated material. HIPIMS coatings were superior to UBM deposited coatings. Tested even at higher, 850°C temperature the HIPIMS coatings showed by factor of 2 lower mass gain as compared to the UBM deposited coatings. In sulphidation tests after 1000 hours exposure to aggressive H₂/H₂S/H₂O atmosphere the CrAlYN/CrN protected γ-TiAl alloys showed reduced weigh gain by factor of four as compared to the uncoated substrate. High temperature pin-on-disc tests revealed that CrAlYN/CrN reduces its friction coefficient from 0.56 at room temperature to 0.4 at 650°C, which demonstrates the excellent high temperature tribological behaviour of the coating. HIPIMS deposited coatings showed extremely low wear coefficient of KC= 1.83 E-17 m³ N^-1m^-1 at this temperature.
10:00 AM		A1-1-9 Investigation of an Oxidation Barrier for IN 617 at Low pO₂ E.A. Clark, J.Y. Yang (University of California, Santa Barbara); A.G. Evans (University of California Santa Barbara); C.G. Levi (University of California, Santa Barbara) Inconel 617 is a candidate material for He heat exchangers in the GenIV very high temperature nuclear reactor concept. However, impurities in the He may lead to degradation by oxidation, carburization or decarburization of the alloy. Alpha-alumina is attractive as a protective layer, but to assure that this phase forms (rather than transient aluminas) in the low pO₂ environment, at the use temperature (T≤1000°C), surface modification is required. This investigation focuses on the formation of alpha alumina at various temperatures and pO₂ environments using two surface modification approaches. (i) Diffusion aluminizing with subsequent heat treatment to the desired composition and phase constitution at the surface. (ii) Cladding (by diffusion bonding) with FeCrAl-Y/RE, an alloy, expected to readily form alpha alumina at low temperatures and low pO₂. The following concerns associated with these approaches are examined. The aluminized surfaces are prone to the formation of persistent transient aluminas. The clad materials exhibit rapid interdiffusion that may compromise the long-term durability of the barrier layer. Solution approaches are discussed in the context of recent research.
10:20 AM		A1-1-10 Stable Electrodes and Passivation Coatings for High Temperature Acoustic Wave Sensors B. Sturtevant, D.J. Frankel, G. Bernhardt, T. Moonlight, M. Pereira da Cunha, R.J. Lad (University of Maine) A critical need exists for sensors that can monitor temperature, strain, and corrosion in strategic hot spots (up to 900°C) within a turbine engine. We have developed surface acoustic wave (SAW) sensors based on langasite (La₃Ga₅SiO₁₄) piezoelectric crystals that remain stable to at least 900°C. A major challenge for this technology is to develop oxidation resistant metallic electrodes and ultra-thin passivation coatings that can mechanically protect the active sensor surface. We have been able to produce electrodes that are stable up to 900°C by synthesizing nanostructured ultra-thin (< 100 nm) Pt-10%Rh / ZrO₂ electrode films grown by e-beam co-evaporation. X-ray diffraction (XRD), resistivity, and electron microscopy (EM) studies indicate that the incorporation of a dispersed ZrO₂ phase into a Pt-10%Rh electrode nanocomposite architecture prevents recrystallization and dewetting of the electrode film from the langasite substrate, which rapidly occurs for pure Pt electrodes. We have also developed ultra-thin (<50 nm) SiAlON passivation coatings that mechanically protect the sensor surfaces, yet allow interaction with the environment. Different SiAlON stoichiometeries were produced by rf magnetron sputtering of Al and Si targets in O₂/N₂/Ar mixtures. The SiAlON films are amorphous and extremely smooth (< 1 nm rms) and remain so even after extended annealing at 900°C in air.
10:40 AM		A1-1-5 High Temperature Oxidation Behavior of RF Magnetron Sputtered NiCrAl Coatings on Superalloys R.A. Mahesh, R. Jayaganthan, S. Prakash (Indian Institute of Technology, India) Magnetron sputter deposition is one of the most important techniques to produce thin films. Sputter deposition processes have been applied to produce overlay coatings for turbine components to improve oxidation resistance. Ni-Cr-Al ternary alloys are widely used for turbine blades, engine parts and many high temperature resistant components in energy production, aerospace and chemical industries. Since the 1970s, over-lay coatings of these alloys have been effectively used to provide oxidation and corrosion protection for the components exposed to hot gases. Microstructures have also been found to affect the high temperature corrosion resistance of many alloys. Researchers have showed that a small grain size promoted the formation of Cr₂O₃ protective scales on stainless steels. Ni-Cr-Al alloys are the main components of certain coatings used on Ni-base superalloys, therefore, the study of the oxidation of Ni-Cr-Al alloys is beneficial to the understanding of oxide forma tion of the complex coated superalloys. Recently, nanocrystalline coatings prepared by the magnetron sputtering technique have attracted much attention. In the present work, the oxidation behaviors of NiCrAl thin film deposited by RF magnetron sputtering was studied at 900°C, which represents the average service temperature for turbine components. The oxide scale formed on the surface of the film was analysed by various techniques like XRD, FESEM/EDAX and X-ray mapping analysis. NiCrAl coated Superni 750 has provided better protection in the given environment.