Coatings to Resist High Temperature Corrosion

Wednesday, April 12, 2000 8:30 AM in Room Royal Palm Salon 1-3

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8:30 AM A1-1 Oxidation and Corrosion Protection for SiC Fibers with CVD Mullite Coatings
S. Varadarajan, A. Pattanaik, V.K. Sarin (Boston University)
Presently the use of SiCf/SiC composites is being limited by the degradation of the SiC fibers. This degradation can occur either during consolidation or during subsequent high temperature service. To overcome this limitation a chemical vapor deposition (CVD) process to deposit protective mullite (3Al2O3.2SiO2) interfacial barrier coatings has been developed. These coatings were deposited using the AlCl3/SiCl4/H2/CO2 system. Process parameters such as pressure, input gas velocity, partial pressure of reactants and the input Al to Si ratio were all found to have a strong influence on the coating structure, thickness, and morphology. Thus this process allows for structure/properties tailoring for specific applications. Preliminary test results indicate that CVD mullite coated SiC fibers have significantly improved oxidation and corrosion resistance as compared to uncoated SiC fibers. The development of the process and the effect of structure/properties of the mullite coatings on the oxidation and corrosion behavior of SiC fibers will be presented and discussed.
8:50 AM A1-2 Current Status of Environmental Barrier Coatings for Si-based Ceramics
K.N. Lee (NASA Glenn Research Center)
Silicon-based ceramics are the leading candidates for high temperature structural components in next generation gas turbine engines. One key drawback of silicon-based ceramics for such an application is the volatilization of protective silica scale in water vapor and the resulting rapid recession. Therefore, the realization of Si-based ceramics components in advanced gas turbine engines depends on the developmentt of protection schemes from water vapor attack. Currently plasma-sprayed external environmental barrier coating (EBC) is the most promising approach. In late 80's and early 90's a wide range of refractory oxide materials were tested as coatings on Si-based ceramics to provide protection from hot corrosion. After the discovery of silica volatilization in water vapor in early 90's, the focus of EBC development research has been shifted towards the protection from water vapor attack. Experience learned from the earlier coating developmental effort provided the foundation upon which more complex and advanced EBC coatings have been developed. This paper will discuss the brief history and the current status of EBC development for Si-based ceramics with the main focus on water vapor protection.
9:10 AM A1-3 Coating of Carbon Ceramics via Reaction with Metal Powders
E.Y. Gutmanas, I. Gotman (Technion, Israel)
A recently developed Powder Immersion Reaction Assisted Coating (PIRAC) method was used to modify the surface properties of carbon ceramics. Graphite or carbon-carbon composite plates and diamond particles were immersed into different carbide-forming metal powders (Cr, V, Ti, Mo, Nb, etc.) and annealed at 900 to 1200@super o@C in vacuum. In some cases, a crystalline halogen was admixed to the metal powder in order to accelerate the transfer of metal atoms onto the ceramic surface. As a result of the interaction between carbon and the metal, continuous coatings were formed on the carbon ceramic surface. The microstructure of the modified surface was investigated employing X-ray diffraction, SEM/EDS and high resolution SEM. The coatings contained different carbides of the corresponding metal, and exhibited a good adhesion to the substrate. Selective diffusion of atmospheric nitrogen through the reaction chamber walls provided an additional possibility of nitride or carbonitride formation on the carbon ceramic surface. The coatings obtained were characterized in terms of their oxidation resistance in open air and in terms of thermal stability in contact with Fe- and Co-based alloy matrices.
9:30 AM A1-4 Wettability of NiAl, Ni-Al-N, Ti-B-C, and Ti-B-C-N Coatings by Glass at High Temperature
D. Zhong, J.J. Moore (Colorado School of Mines); S. Thiel, J. Disam (Schott Glas, Germany)
In practice, sticking/adhesion of glass to glass molding dies and forming tools is a critical problem which limits the quality of glass products and, the performance and reliability of molding dies and forming tools. Depositing NiAl, Ni-Al-N, Ti-B-C, and Ti-B-C-N coatings and characterizing their wettability by glass at high temperature are part of an overall program that is being conducted to develop a non-sticking, oxidation resistant, and wear resistant coating system for glass molding dies and forming tools. Within this paper, the use of contact angle analysis for evaluation of wettability is described. The contact angles were measured by the sessile drop technique and analyzed by LECO Image Analyzer. The influences of microstructure as well as surface/interface reaction on contact angles were investigated. The results confirmed that NiAl, Ni-Al-N, Ti-B-C, and Ti-B-C-N are potential non-sticking coatings by glass at high temperature. The results revealed the relationship among wettability, microstructure, and oxidation resistance of those coating materials.
9:50 AM A1-5 Performance of Stabilised Dicalcium Silicate Coatings in Hot-Corrosive Environments
F. Jansen, J.A. Peters (Sulzer Innotec Ltd., Switzerland); X. Wei, M. Dorfman (Sulzer Metco)
New thermally sprayed dicalcium silicate coatings have been developed for increased stability in highly corrosive sulphur- and vanadium-containing environments at high temperatures (up to 1000°C). In this study, the performance of three specially stabilised dicalcium silicate based coatings were compared with a standard 6-8wt% yttria-stabilised zirconia coating after exposure to sulphate-vanadate slag mixtures at temperatures between 700-900°C. In addition, pure sulphidation tests were carried out at 1000°C in a 5 vol.% sulphur dioxide air atmosphere. The thermoshock behaviour as well as the thermal barrier efficiency of the coatings were assessed using a propane-oxygen burner-rig test. The determination of phase stability and the mechanisms of corrosive attack were investigated using x-ray diffractometry and scannning electron microscopy. @paragraph@The results indicate that the dicalcium silicate coatings exhibited superior resistance to hot-corrosion. Furthermore, the thermomechanical properties as well as the thermal barrier efficiency of these materials during thermal cycling compared favourable with the standard zirconia-based coatings. The mechanism of protection against corrosive attack appeared to be related to the formation of stable calcium vanadate compounds in the presence of molten vanadates. These calcium vanadate compounds prevent the penetration of the aggressive molten vanadates by forming a continuous layer at the coating surface. Selected dicalcium silicate coatings may find increasing application on metallic substrates requiring both thermal insulation and corrosion protection in aggressive sulphur- and vanadium-containing environments.
10:30 AM A1-7 Effect of Composition on the Oxidation and Hot Corrosion Resistance of NiAl Doped with Precious Metals
C. Leyens (DLR-German Aerospace Center, Germany); B.A. Pint, I.G. Wright (Oak Ridge National Laboratory)
Improvements in oxidation resistance, in terms of scale spallation and oxide growth rate, are required to provide the service lifetimes desired from gas turbine materials, in particular when thermal barrier coatings are involved. Reactive element-doped nickel aluminides have demonstrated superior spallation resistance and slower growth rates than MCrAlY-type coatings and are therefore considered to have significant potential for these applications. Although it is anticipated that the service temperatures will be higher than current engines, there will likely be locations where component temperatures will be in the regime where hot corrosion attack might be expected, so that both oxidation and hot corrosion must be considered. In the present study, cast NiAl alloyed with Cr, Pt, Pd, Ir and Ru was tested in 1-h cycles at 950@super o@C under hot corrosion conditions and at 1150@super o@C in oxygen. For comparison, Hf-doped NiAl variants and a cast NiPtAl alloy resembling the composition of commercial aluminide coatings were included. Cr was the only element that reduced hot corrosion attack of NiAl significantly. However, at higher temperatures, addition of Cr accelerated growth rate and promoted spallation of the oxide scale relative to Hf-doped NiAl. The results from initial detailed characterization indicate that rejection of chromium at the metal-oxide interface gives rise to the formation of chromium-rich precipitates in the alloy, which apparently modify its oxidation behavior. This suggests that for NiAl-based substrates, hot corrosion resistance and exceptional scale spallation resistance may be mutually incompatible goals.
11:10 AM A1-9 A Study of the Diffusional Response of Refractory and Other Elements in Multi-Component Systems Prior to and During CVD Aluminizing
A.L. Purvis, B.M. Warnes, J.E. Schilbe (Howmet Corporation)
Samples of commercially pure nickel and two common superalloys were prepared by electroplating a thin layer of platinum on the surface, then exposing the samples to temperatures of 950C and 1080C for periods of 2 and 6 hours. Using electron probe micro analysis (EMPA), elemental composition profiles were obtained from the samples following the diffusion steps. The relative diffusion coefficients for a number of elements were determined using both classical Boltzmann-Matano and other selected methods. It was discovered that elements such as molybdenum, cobalt rhenium, chromium, titanium and tantalum displayed a significant diffusional response in this relatively short time, while tungsten diffused to a lesser degree under these conditions. Samples were then high activity chemical vapor deposition aluminized (CVD), and profiles were repeated. It was discovered that the presence of additional aluminum in these systems showed a tendency toward significant interactions between diffusional phenomena, as expected. The limitations of a study such as this are revealed and discussed, along with an extensive discussion on the ramifications of coating service life. During oxidation testing of an applied thermal barrier coating (TBC), the presence of these detrimental elements in the applied TBC was revealed after an extended exposure. The presence of these elements are further discussed as a possible failure mechanism for the TBC.
11:30 AM A1-10 Influence of Surface Treatment on the Oxidation of MCrAlY Coatings
N. Czech (Siemens AG, Power Generation Group (KWU), Germany); H. Fietzek, M. Juez-Lorenzo (Fraunhofer Institut für Chemische Technologie (ICT), Germany); V. Kolarik (Fraunhofer-Institut für Chemische Technologie (ICT), Germany); W. Stamm (Siemens AG, Power Generation Group (KWU), Germany)
The oxidation resistance of the MCrAlY coatings is crucial for the life time of the blades in modern stationary gas turbines for electric power generation. In the case of protective overlay coatings the surfaces are generally corundum blasted, whereas bond coats for EB-PVD thermal barrier coatings need a smooth surface. For a better understanding of the influence of the surface treatment on the oxide scale formation in situ studies using high temperature X-ray diffraction were performed on a Re containing MCrAlY coating with 12% Al, and an MCrAlY coating with 8% Al for the use as overlay and as bond coat. The oxidation of the coatings with polished and with corundum blasted surfaces was investigated at 950 and 1000@super o@C in air. The coatings form an @alpha@-Al@sub 2@O@sub 3@ scale with spots of YAlO@sub 3@. The possibility of the formation of metastable Al@sub 2@O@sub 3@ phases at lower temperatures is discussed. The parabolic rate constants k@sub p@ are determined from the series of X-ray diffraction patterns and compared.
11:50 AM A1-11 The Effect of Ni-Re Interlayers on Interdiffusion from MCrAlY Overlay Coatings
R.A. Page, G.R. Leverant (Southwest Research Institute)
The life of MCrAlY coatings is controlled by maintaining a reservoir of Al that forms a protective alumina surface scale. Loss of Al from the coating occurs by spallation of the alumina due to thermal cycling and, also, by interdiffusion of the coating and substrate. It would be desirable to suppress interdiffusion that leads to loss of Al to the substrate and ingress of Ni from the substrate. This could be accomplished by the presence of a diffusion barrier that locally slows down diffusional processes. Initial measurements have shown that the presence of a thin Ni-Re layer between the MCrAlY and the substrate can lead to a reduction in the growth rate of the inner beta-NiAl depleted layer and, hence, a reduction of interdiffusion. Recent experiments have examined the effect of the Ni-Re layer thickness on interdiffusion. The results of these experiments will be presented. The effect of the interlayer on spallation during cyclic oxidation exposures will also be discussed.
12:10 PM A1-12 Depth Profile and Quantitative Trace Elemental Analysis of Diffusion Aluminided Type Layers on Ni-base Superalloys Using High Resolution Glow-Discharge Mass Spectrometry
I.T. Spitsberg (General Electric Aircraft engines); K. Putyera (Shiva Technologies)
High-resolution glow discharge mass spectrometry (GDMS) was used to quantify concentrations of trace elements in platinum aluminide (PtAl-type) diffusion coatings on nickel-base superalloy substrates. Concentration profiles for select trace elements were determined from the near-surface regions to a depth of more than 100 micrometers. The conventional procedure of converting ion intensity versus sputtering time data into concentration versus depth was performed utilizing appropriate correction factors. To evaluate the depth resolution error and accuracy of the concentration measurements, the roughness of the sputtered craters and crater walls was measured and the re-deposited materials on the crater periphery analyzed for number of specimens. A normalization procedure for calculating average impurity concentration in the diffusion coating layer was introduced and validated. It is shown that quantitative depth profile information can be obtained for the concentration of trace elements with complex distribution in the coating.