About the 1996 TMS Annual Meeting: Tuesday Morning Sessions (February 6)

HIGH TEMPERATURE COATINGS II: Intermetallic/Ceramic Coatings I

Program Organizers: Narendra B. Dahotre, Center for Laser Applications, University of Tennessee Space Institute, Tullahoma, TN 37388; Janet M. Hampikian, School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, GA 30332; Jacob J. Stiglich, PO Box 206, Sierra Madre, CA 91025

Tuesday, AM Room: B1

February 6, 1996 Location: Anaheim Convention Center

Session Chairpersons: Peter F. Tortorelli, Metals and Ceramics Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6156; Janet M. Hampikian, School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, GA 30332

8:30 am Keynote Presentation

THE FORMATION OF PROTECTIVE SCALES DURING HIGH-TEMPERATURE OXIDATION: F.H. Stott, Corrosion and Protection Centre, UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, UK

Many high-temperature nickel-or iron-base alloys rely on the formation of slow-growing alumina-rich scales for protection against environmental degradation. Although Al2O3 scales can be developed on binary Ni-Al or Fe-Al alloys, the presence of a third element, usually chromium, promotes their establishment on alloys of lower aluminium concentration. Following establishment of the scale, the subsequent growth kinetics are controlled mainly by transport of reactants along short-circuit paths in the scale. Despite their slow growth rates, mechanical failure of such scales can cause enhanced rates of metal loss. This failure occurs when the strain arising from residual growth stresses and thermal-cycling stresses exceeds a critical value. However, the addition of small amounts of more reactive elements, such as yttrium, to the alloy can give a considerable improvement in mechanical integrity of the scale, resulting in a much improved overall oxidation performance.

8:55 am

EVALUATION OF TBC-COATED [[beta]]-NiAl SUBSTRATES WITHOUT A BOND COAT: B.A. Pint, Oak Ridge National Laboratory, PO Box 2008, MS 6156, Oak Ridge, TN 37831-6156; B. Nagaraj, General Electric Aircraft Engines, One Neumann Way, MD H85, Cincinnati, OH 45215-6301

[[beta]]-NiAl substrates with various alloy additions and oxide dispersions were coated with Y2O3-stabilized ZrO2 using electron beam-physical vapor deposition (EB-PVD). Cyclic oxidation experiments (1000-1200[[ring]]C) were conducted to study the effect of substrate dopants such as Y and Zr on the coating lifetime without the intermediate metallic bond coat layer used in conventional thermal barrier coatings. Oxidation kinetics and Al2O3 scale microstructures were compared to those for uncoated substrates. Degradation of the substrate-alumina interface by void formation was the primary mechanism leading to coating spallation. Coating lifetime was correlated to the alumina scale adhesion on the uncoated substrates.

9:15 am

HIGH-TEMPERATURE OXIDATION/SULFIDATION RESISTANCE OF IRON-ALUMINIDE COATINGS: P.F. Tortorelli, I.G. Wright, G.M. Goodwin, M. Howell, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831

Iron aluminides show excellent corrosion resistance in oxidizing and sulfidizing environments at temperatures well above those at which these alloys have adequate mechanical strength. Therefore, it is anticipated that appropriate compositions of iron aluminides may find application as coatings or claddings on materials which are less corrosion-resistant at high temperatures but have higher strengths. Toward this end, the behavior of weld-overlay iron-aluminide coatings on steel substrates and of co-extruded Fe3Al/stainless steel tubing in elevated temperature air and H2S-H2-H20 mixed gases is being studied. Results based on weight changes and corrosion product chemistry and morphology have shown that the corrosion resistance of these layers can be essentially equivalent to that of wrought alloys with similar aluminum and chromium concentrations when coating defectss are minimized by appropriat material selection and process control. The changes in corrosion behavior as a function of compositional modifications resulting from the coating process or from the choice of starting materials can be explained on the basis of what is known from previous high-temperature oxidation/sulfidation studies of bulk iron aluminides. Research sponsored by the U.S. Department of Energy, Fossil Energy AR&TD Materials Program under contract DE-AC05-840R21400 with Lockheed Martin Energy Systems.

9:35 am

THE EFFECT OF PLASMA SYNTHESIZED ALUMINA COATINGS ON THE OXIDATION BEHAVIOR OF IRON ALUMINlDES: P.Y. Hou, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720; K.B. Alexander, PO Box 2008, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376; Zhi Wang, I. G. Brown, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720

Plasma synthesized alumina films were deposited on two types of Fe3Al substrates with or without a small amount of Zr addition. After subsequent oxidation treatments at 1000[[ring]]C in air, the coating on the Zr-doped Fe3Al remained strongly adherent and suppressed the growth of the alumina scales from the alloy. The same coating on the undoped Fe3Al however, spalled off completely upon cooling to room temperature. These results suggested a strong substrate effect on coating adherence. Interface chemistry, studied by Auger Electron Spectroscopy, and coating microstructures, studied by TEM, are presented to explain the observed behavior. Results are discussed in terms of oxide adhesion properties in general.

9:55 am

PLATINUM ALUMINIDE DIFFUSION BARRIER FOR THE OXIDATION PROTECTION OF TITANIUM AND TITANIUM INTERMETALLIC ALLOYS: J.R. Nicholls, M.J. Deakin, Cranfield University, Cranfield, Bedford, UK; M. R. Winstone, S. Kerry, Structural Materials Centre, DRA Pyestock, Farnborough, Hampshire UK GUl4 OLS

Titanium alloys find wide applications in gas turbine engines, with creep resistant alloys operating to over 600[[ring]]C but the drive to reduce engine weight will lead to future engines using titanium aluminide alloys at temperatures between 700 and 900[[ring]]C. This will only be possible with oxidation resistant coatings. This paper presents results of the development of a PtAl2 intermetallic barrier coating for titanium and titanium aluminide alloys. A 3um coating is manufactured by multilayer sputter deposition, coupled with high temperature reaction synthesis. Oxidation studies have shown that the PtAl2 layer successfully protects titanium alloys against the ingress of oxygen for exposures up to l000h at 700[[ring]]C. Longterm exposure tests on aluminide alloys are in progress at 800[[ring]]C. Under creep conditions of 350MPa at 650[[ring]]C the coating extends the life of alpha-2 aluminide alloys by a factor of 2 by reducing environmentally assisted cracking.

10:15 am BREAK

10:25 am

A STUDY OF THE MECHANISM OF "PEST" IN NIOBIUM ALUMINIDE: R.J. Hanrahan Jr., Los Alamos National Laboratory, TA-3, MS-G770, Los Alamos, NM 87545; M. Puga-Lambers, E. S. Lambers, University of Florida, MAIC, Materials Science and Engineering Department, 121 Rhines Hall, Gainesville, FL 32611; S.P. Withrow, Oak Ridge National Laboratory, PO Box 2000, Bldg. 3003, MS-6048 Oak Ridge, TN 37831

The "pest" phenomenon of accelerated oxidation and disintegration is exhibited by numerous intermetallic compounds exposed at intermediate temperatures. We have studied this phenomenon in NbAl3 using a combination of ion implantation, SIMS, and AES. A continuous oxide layer is formed in the surface of one side of the specimen at room temperature using ion implantation. Using 018 as the implanted species a built in marker is provided for subsequent. SIMS analysis. The implanted specimens were exposed at 760[[ring]]C under Ar-50ppm O2 to induce pest. The implanted surfaces survived largely intact while the rest of the specimen disintegrated. Comparison of SIMS profiles from implanted and non-implanted regions of the same specimen show that oxygen transport through the implanted layer was dramatically reduced. This result shows that the controlling mechanism in pest of NbAl3 involves oxygen dissolution in the substrate.

10:45 am

THE DIFFUSIONAL AND MICROSTRUCTURAL PHENOMENA IN TWO DIFFERENT SUPERALLOYS WITH A NiCoCrAlYTa COATING UNDER CREEP AFTER DIFFERENT PRE-DAMAGE TREATMENTS: Alejandro Sanz, P. Bemadou, Laboratoire de Metallugrgie, 10 Avenue E. Belin, 31055 Toulouse cedex, France. L. Llanes, M. Anglada, M. Gomez, UPC ETSII, Dep de Ciencia de loc Materiales y Metalurgia, Av; Diagonal 647, 08028 Barcelona, Spain; M. B. Rapaccini, Polytecnico di Milano, Dipartimento de Ingenieria Aeroespaziale, Piazza Leonardo da Vinci, Milano, Italy

NiCoCrAlYTa coatings are frequently used to protect the components in the hot path of the gas turbines engines. A careful selection of the coating which offers sufficient chemical compatibility with the substrate and a low interdiffusivity is necessary for these high temperature systems. The effect of different pre-damage treatments on the creep behavior and in the diffusional phenomena of single crystals, [001] oriented, of the AM-3 and MC-2 superalloys coated with a Low Pressure Plasma Spray (LPPS) NiCoCrAlYTa coating is described. The pre-damage treatments comprise oxidation treatments at 1373 K (1100C) during eight hours as well as creep treatrnents during the same period of time and at the same temperature under stress of 100 and 140 Mpa. After the pre-damage treatments, all samples were submitted to high temperature creep tests at 1323 K (1050C) and 140 Mpa until fracture. The phenomenalogical description of multicomponent diffusion by extending the Fick's law to a n-components system requires the use on n-l independent diffusion couples for the determination of (n-1²) concentration-dependent interdiffusion coefficients at one composition. An alternative approach in the study of isothermal diffusion in multicomponent systems is to analyze the concentration profiles for interdiffusion fluxes of all components without invoking Fick's law. This approach allows to by-pass the need for (n²-l) interdiffusion coefficients and provides a direct insight into the direction as well as magnitudes of the fluxes at any section within the diffusion zone. Using the data obtained from Energy Dispersive microbrobe Spectroscopy (EDS) the diffusional phenomena between the coating and the substrate is characterized and correlated with the chemical nature of the substrate and to the different pre-damage treatments.

11:05 am

SILICIDE COATINGS FOR NIOBIUM ALLOYS: M. Vilasi H. Brequel, R. Podor, J. Steinmetz, Laboratoire de Chimie du Solide Mineral, URA CNRS 158, Faculte des Sciences. BP 239-54506 Vandoeuvre Les Nancy Cedex - France

A pack cementation process for the codeposition of Si, Cr, Fe and Ti has been adapted to pure niobium and niobium alloys. Through choice of a masteralloy rather than pure elements, and an adequate activator, it is possible to inexpensively produce complex silicide coatings in a single step which offer significant promise to protect niobium base materials in oxidizing environments to the highest temperatures of their application. A clear knowledge of the Nb-T-Si (T = Cr; Fe; Ti) ternary phase diagrams, previously studied, and of the Nb-Fe-Cr-Si quaternary phase diagram, partly determined in this study, allows the choice of a "diffusion path" leading to the formation of coatings with phases selected with regard to their oxidation resistance. Two different chromia former coatings have been particularly studied between 973 to 15,73 K in air. Finally, the results of a complete crystallographic characterization of the different silicides, showing possible Nb/Ti, Nb/Cr, Cr/Fe and Si/Fe exchanges, are useful to determine the additional elements which could improve both the physical and mechanical properties of these coatings.

11:25 am Invited

ALUMINIDES: YESTERDAY OR TOMORROW COATINGS TO PROTECT AGAINST HIGH TEMPERATURE CORROSION? Roland Streiff, Universite de Provence, 13331 Marseille Cedex 3, France

Aluminides were the first coatings used to protect turbine hot components against high temperature oxidation. However, in aggressive environment such as marine environments or burning lower grade fuel, they suffer hot corrosion, the result of the fluxing of the protective oxide scale by molten salts. Despite the development of the MCrYalY class of overlay coatings which have been especially designed to withstand hot corrosion attack, aluminide coatings are still widely and the most used high temperature protective coatings. This paper will review the status of current research on the aluminides used as coatings either on superalloys or on titanium alloys and titanium aluminides. Aluminide coatings modified by "reactive elements", such as hafnium, yttrium, cerium, tantalum or silicon have been developed. Very recent results lead to the conclusion that substrate hafnium addition might be the avenue of economical oxidation resistance improvement for tomorrow's aluminide coatings.

11:50 am

DEVELOPMENT OF OXIDATION RESISTANT COATINGS: B.D. Chattoraj, A. Garg, M.S. Mukhopadhyay, Research and Development Center for Iron and Steel, Steel Authority of India Limited, Ranchi - 834002, India

Surfaces of steel and graphite electrodes are corroded by oxidation at high temperatures. These can be protected by using suitable oxidation resistant coatings. For steel, the coat should be easily removable after heat treatment, whereas for the electrode, it should adhere even during cooling and be capable of withstanding a higher temperature. Accordingly, different materials were chosen for different substrates, but bonding components were similar. Coatings for steel were based on magnesia-alumina and on alumina-silicate for the electrode. Adherence and other properties may be controlled by using an appropriate sodium oxiae-potassium oxide-silica bond. The coefficient of thermal expansion (CTE) of the coating is an important criterion and may be controlled by adjusting the chemistry. The CTE of coating mass should be very close to that of graphite electrode and should be widely different from that of steel slab. Repeated experiments in the laboratory followed by evaluation through plant trials have given encouraging results. The scale formation and electrode consumption were reduced by 50- 70% and 20%, respectively.