1997 TMS Annual Meeting: Tuesday Abstracts

MATERIALS, COATINGS AND PROCESSES FOR IMPROVED RELIABILITY OF HIGH TEMPERATURE COMPONENTS: Session III: Coatings and Oxidation Coatings

Session Chairperson: Dr. K. Dannemann, GE Power Generation Engineering, One River Road, Schenectady, NY 12345

8:30 am

THE EFFECT OF THICKNESS AND TOTAL SULFUR CONTENT ON PWA1480 OXIDATION: A SCALE ADHESION MAP: J.L. Smialek, NASA Lewis Research Center, Cleveland, OH 44135

The cyclic oxidation resistance of advanced superalloys is dramatically improved when the typical sulfur impurity levels are reduced from about 3-10 ppmw to below 1 ppmw. Optimum scale adhesion can be obtained when the sulfur content is reduced below a discreet critical level. To demonstrate this phenomenon, coupons of PWA1480 were desulfurized by a hydrogen annealing process. The degree of desulfurization was controlled by sample thickness and annealing time and temperature, but did not strictly follow a Dt/L2 diffusion parameter dependency. From an initial sulfur value of 6 ppmw, 15 different sulfur levels were produced ranging from 0.05 to 4 ppmw. Cyclic oxidation resistance in 1100°C tests improved directly with the degree of desulfurization, reaching an optimum below about 0.2 ppmw. Furthermore, at equivalent sulfur levels, thick samples were prone to greater weight loss than thin samples. The results are discussed in terms of an adhesion map based on the equivalent number of interfacially segregated monolayers of sulfur.

8:50 am

ENVIRONMENTAL DEGRADATION OF GAS TURBINE MATERIALS IN STEAM: Y. Patel, D.C. Tamboli, V. Desai, Mechanical Materials and Aerospace Engineering Department, University of Central Florida, Orlando, FL 32816; N.S. Cheruvu, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228

Steam has been proposed as an efficient cooling medium for the hot section components in the advanced gas turbines as an option for more efficient cooling allowing them to operate at higher efficiency. However, the effects of steam and its impurities such as NaCl and Na2SO4 on the environmental degradation of gas turbine superalloys is little known. Three common gas turbine superalloys; X-45, IN-738 and Inconel-617 were exposed to steam at higher temperature. Cylindrical specimens with central bore were kept at elevated temperature and exposed to three steam environments flowing through the bore. The three environments are (a) steam generated from deionized water, (b) steam generated from deionized water with 5ppm each of NaCl and Na2SO4 and (c) steam generated from deionized water with 50ppm each of NaCl and Na2SO4. The respective exposure times were 1450, 2950 and 3900 hours. IN-738 showed severe internal oxidation in steam without contaminants. In contaminated steam the hot corrosion damage was maximum in Inconel-617. X-45 showed less oxidation damage than IN-738 and less hot corrosion than Inconel-617.

9:10 am

THE OXIDE LAYER PHASE STRUCTURE OF MCRALY-COATINGS: N. Czech, W. Stamm, Siemens AG Power Generation Group, and B. Kolarik, Fraunhofer-Institute for Chemical Technology

During service blades and vanes of stationary gas turbines are subjected to high temperatures. The increase in surface temperatures causes a more severe attack on the MCrAlY-coatings. These coatings are taken for bond coats for thermal barrier coatings as well. The production of a homogeneous oxide layer during service is necessary for a good oxidation protection and for a good adhesion to the ceramic. We will present in-situ X-ray measurements on 8% and 12% Al containing MCrAlY-coatings with and without Re at three different temperatures versus time. At lower temperatures mainly a two phase mixture of Cr2O3 and Al2O3 is detectable with different size distributions for the coatings with 8% and 12% Al. On microsections the -phase depletion is analyzed. At 1050°C a pure Al2O3 oxide layer is established. No Re can be found in the top layer.

9:30 am

THE EFFECT OF RHODIUM WEIGHT PERCENT ON THE OXIDATION RESISTANCE OF PLATINUM RHODIUM MODIFIED ALUMINIDE COATINGS ON MAR M-509 AND FSX-414 COBALT BASE ALLOYS: J. Kimmel, Z. Mutasim, Solar Turbines, Inc., 2200 Pacific Highway, San Diego, CA 92186-5376

Cobalt based alloys such as Mar-M 509 and FSX-414 are coated with a precious metal aluminide (platinum-rhodium-aluminide) to provide enhanced oxidation and hot corrosion resistance in gas turbine applications. The role of rhodium in the aluminide coating is not well understood. There is limited data suggesting that it provides added oxide adherence and reduced refractory element diffusion from the base alloy too the coating interface, thus reducing the tendency of spallation during high temperature exposure. However, too high of a rhodium content decreases the ductility of the coating and, after cyclic oxidation, may result in coating cracking and spallation. The objective of this study is to evaluate platinum-rhodium-aluminide optimum rhodium weight percentage on Mar-M 509 and FSX-414 cobalt based alloys. Metallurgical evaluation was performed on platinum rhodium aluminide coatings with varying rhodium weight variation between 0% and 10%. Oxidation tests were performed at 2000°F/1000 hours, 2100°F/1000 hours (100 hour cycles). Characteristics such as coating thickness, composition, microstructure, and weight changes were evaluated for the as-coated and oxidized specimens using SEM and EDX analysis.

9:50 am BREAK

10:20 am

HIGH TEMPERATURE OXIDATION OF NICKEL BASED SUPERALLOYS IN STEAM: V. Desai, D.C. Tamboli, Mechanical Materials and Aerospace Engineering Dept., University of Central Florida, Orlando, FL 32816; N.S. Cheruvu, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228

An improvement in the cooling of hot gas path components of the has turbines has been sought by replacing air with steam as a cooling medium. However the materials problems associated with steam at high temperature are unknown for gas turbine superalloys. This information is very crucial for the reliability and life prediction of the gas turbine components. In this study, oxidation in steam has been studied for four nickel based superalloys commonly used in the gas turbines, IN738, Inconel 617 and CMSX-4. The study was carried out at four different test temperatures within the range of 800°C to 950°C for times up to 1400 hours. The oxidation tests were also carried out in air to provide a comparison between the oxidation in steam and air. The results indicated higher internal oxidation in IN738 compared to other two alloys. The oxidation attack observed in IN738 was higher in steam compared to air, whereas in other alloys reverse was the case.

10:40 am

NITRADATION AND MECHANICAL DEGRADATION OF COATINGS IN GAS TURBINE BLADES: J. Kameda, T.E. Bloomer; Ames Laboratory & CATD, Iowa State University, Ames, IA 50011; S. Sakurai, Mechanical Engineering Research Laboratory, Hitachi Ltd., 3-1-1, Saiwai Hitachi 317, Japan

In-service environmental attack and mechanical degradation of CoNiCrAlY coatings in gas turbine blades have been investigated using a scanning Auger microprobe (SAM) analysis and small punch (SP) testing technique. Coating degradation was found to strongly depend on the location of blades operated for 21000h under liquefied natural gas. SAM analyses revealed extensive formation of AlN and Ni enriched in inner (concave) coatings but not in outer (convex) ones. The oxidation and carbonization also occurred more substantially in near surface regions of the inner coating. SP specimens were prepared from the inner and outer coatings in order that the specimen surface could be located near surface or interface coatings. SP tests demonstrated that the oxidation and nitridation in the inner coating produce significant mechanical degradation. Brittle cracking in the inner coating occurred at low strains (<3%) up to 950°C though near interface coatings showed a little higher ductility. The outer coatings near the surface and interface showed higher RT ductility (>6%) than the substrate and a rapid increase in the ductility above 800°C. The coating degradation mechanism is discussed in light of the distribution of operating temperatures.

11:00 am

DIFFUSION AND OXIDATION BEHAVIOR OF ELECTRODEPOSITED NI-AL PARTICLE COMPOSITE COATINGS: D.F. Susan, K. Barmak, A.R. Marder, Lehigh University, Department of Material Science and Engineering, Bethlehem, PA 18015

The high temperature diffusion and oxidation behavior of electro-deposited Ni matrix/Al particle coatings was studied to determine the relationships between coating microstructure and high temperature behavior. The coatings, containing up to approximately 20 vol. % Al particles, were deposited on nickel substrates and heat treated at 635, 800, and 1000°C for up to 100 hours. Upon heat treatment at 635 and 800°C, a two phase structure of Ni3Al (1) and solid solution develops with small Kirkendall voids also present. At 1000°C, the coating contains only a solid solution in agreement with the Ni-Al phase diagram. The microhardness of the coating was found to increase with increasing Nail content. Both as-plated and heat treated coatings were oxidation tested in air at 800, 900, and 1000°C for up to 900 hours. Results indicate that the Al content is sufficient to form a protective Al2O3 scale on the coatings during isothermal exposure at these temperatures.

11:20 am

HIGH TEMPERATURE OXIDATION BEHAVIOR OF NIOBIUM ALUMINIDE INTERMETALLICS WITH MOLYBDENUM SILICIDE AND NICKEL ALUMINIDE PROTECTIVE COATINGS: Y. Li, W.O. Soboyejo, R.A. Rapp, Ohio State University, Department of Material Science and Engineering, Columbus, OH 43210

Niobium aluminide intermetallics can exhibit attractive combination of high-temperature strength retention and room-temperature ductility/damage tolerance. Preliminary studies have also shown that niobium aluminides have moderate oxidation resistance at temperatures up to ~750°C in the uncoated condition. However, their oxidation kinetics are not fully understood. In this study, the high-temperature oxidation resistance of niobium aluminide intermetallics (Nb-15Al-10Ti, Nb-15Al-25Ti, Nb-15Al-40Ti and Nv-12.5Al-41Ti-1.5Mo) was studied using uncoated and coated alloys. Protective coatings were formed via in-situ chemical reactions designed to promote the siliciding an aluminizing of Mo and Ni substrate layers, respectively. The isothermal and cyclic oxidation behavior of coated and uncoated materials in air were then studied in the temperature range between 650 and 850°C. The oxide scales and internal layers were analyzed using XRD, SEM and EDX. The implications of the results are assessed for potential high-temperature applications of niobium aluminide intermetallics.

11:40 am

ELECTRO-SPARK ALLOYED HEAT RESISTANT COATINGS ON THE WC-CO BASE: A.D. Verkhoturov, S.V. Nikolenko, N.V. Lebukhova, Institute of Materials of Russian Academy of Sciences, Khabarovsk, Russia

Electro-spark alloying of the metallic surfaces is one of the most perspective coating technologies, among the many other current coating processes. The main advantages of this method include the possibility of depositing any current conducting materials on the surface; high adhesive strength of the layer, and the simplicity of the process. Electro-spark coatings usually have a relatively low continuity of 60-90%; defects (pores, microfractures), heterophase structure. In spite of that, it is possible to achieve, by using refractory alloys as electrodes, a stable effect of increasing heat-resistance. The research conducted by the Institute of Material Science of the Russian Academy of Sciences (Khabarovsk, Russia) showed that electrode materials which examine increasing heat-resistance, can be put in the following order (based on the main component): boride-carbide-nitride. The highest hardness (20-24 GPa) and wear resistance among coatings on steel, is achieved with a hard alloy with tungsten carbide-cobalt base (WC-92%, Co-8%). For increasing Wc-Co alloy electro-spark coatings', deposited on the parts working in high temperature and wear conditions, resistance to high-temperature oxidizing, we propose including self-fluxing materials or heat-resistant intermetallics in the alloy. The research in new materials, that include additives of Ni-Cr-B-Si and Ni3Al (0, 5-25%) to the Wc-Co alloy, was also conducted. We have studied the characteristics of macro- and microstructure of materials and coatings, as well as their chemical compositions, electrode material transfer kinetics, and the optimal conditions of alloying the surfaces. Kinetics of high-temperature oxidizing and phase composition of the interaction products, were examined using a differential thermal analysis in non-isothermal conditions method.