ICMCTF1998 Session A1-2: Coatings to Resist High-temperature Corrosion (2)

Wednesday, April 29, 1998 1:30 PM in Room Council/Chamber/Cabinet

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		A1-2-1 Influence of the Surface Roughness on the Oxide Scale Formation on MCrAlY Coatings Studied In Situ by High Temperature X-ray Diffraction N. Czech (Siemens AG, Power Generation Group (KWU), Germany); 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) In modern stationary gas turbines for electric power generation the life time of the blades is mainly controled by the oxidation resistance of the coatings. In the case of protective overlay coatings the surfaces are generally rough, whereas bond coats for EB-PVD thermal barrier coatings need a smooth surface. For a better understanding of the influence of the surface roughness on the oxide scale formation in situ studies using high temperature X-ray diffraction were performed on an MCrAlY coating with 12% Al and 3% Re for the use as overlay and as bond coat. The measurements were performed in air in a temperature range from 900 to 1000°C.The results show differences in the oxide scale formation on the same coating with an as sprayed and a polished surface. On the as sprayed surface Cr₂O₃ and spinels were observed besides α-Al₂O₃ in the initial state and on the polished surface a coherent α-Al₂O₃ scale with spots of YAlO₃ was found. No Re was found in the oxide scale.
1:50 PM		A1-2-2 Molten Aluminum Corrosion Resistance of Ceramic Thermal Spray Coatings Y. Wang (General Motors Corp.) The purpose of this work was to develop ceramic thermal spray coatings on tools such as cast iron ladles, heat element protectors and steel molds to replace pure ceramic and ceramic slurry coated tools for extended life and lower aluminum casting operation cost. Seventeen different ceramic coating materials were selected for screen test based on thermodynamic and mechanical properties, as well as commercial availability and cost. The study included thermodynamic modeling, coating microstructure optimization of ceramic/metallic and bond layers on different substrates, as well as the thermal spray processes. Two coating materials offered much better corrosion resistance to molten aluminum than the rest. The optimized coating microstructures showed much better thermal shock resistance. The ceramic thermal spray coating coated tools had much longer life than ceramic slurry coated tools.
2:10 PM		A1-2-3 Oxide Scale Growth on MCrAly Coatings After Pulsed Electron Beam Treatment G. Múller, G. Schumacher, D. Straub (Forschungszentrum Karlsruhe GmbH, Germany) The surface of LPPS - MCrAlY coatings were restructured by large area pulsed electron beam treatment of some 10 µs pulse duration to influence the oxidation kinetics and the oxide scale morphology. The electron beam generated by the new developed facility GESA has a diameter of about 10 cm and sufficient power density to melt a thin surface layer of this area by just one electron beam pulse. After pulse breakdown the melt freezes quickly due to heat conduction into the bulk. The high cooling rates in the order of 106 K/sec at about 20 µm melt depth result in a nanometer grained layer with a smooth surface. Investigation of the oxidation behaviour at 950 °C in air shows the development of a monophase ?-alumina scale on the treated surface with a considerably diminished growth rate compared to the untreated coatings. Therefore it is expected that electron beam treatment improves the corrosion resistance of the MCrAlY coating and its suitability as a bond coat for EBPVD thermal barrier coatings. The kinetics of oxide growth and the layer structure is compared for electron beam treated and just polished coatings.
2:30 PM		A1-2-4 Relation Between the Oxidation Protection of Stabilised ZrO₂ Coatings at High Temperatures and Residual Coating Stress M. Andritschky, P. Alpuim (Universidade do Minho, Portugal); C. Funke, D. Stöver (Research Center Jülich, Germany) Thin stabilised zirconia coatings deposited by magnetron sputtering proofed to reduce the high temperature oxidation of the underlying metallic high temperature alloy. Within the frame of this work, the protection of single crystal alloy CMSX4 at temperatures of about 1000°C was studied. Three different zirconia based coatings were applied, ZrO₂ stabilised by 9 % Y₂O₃ , mixtures out of ZrO₂ with Al₂O₃ and multilayered coatings of ZrO₂ with Al₂O₃ with a periodicity of about 5 nm. These coatings were able to improve the oxidation resistance of CMSX 4 in laboratory atmosphere. Best results were obtained for the multilayered coating. paragrapfIn parallel with substrate corrosion varied the residual stress within the coatings. The coating residual stress was evaluated by RAMAN spectroscopy, both for the ZrO₂ PVD coating and the growing Al₂O₃ oxidation layer. These stress measurements are able to indicate the failure of the protective overlaying coatings.
2:50 PM		A1-2-5 CVD Mullite Coatings for Corrosion Protection of Si-Based Ceramics J.A. Haynes, K.M. Cooley (Oak Ridge National Laboratory); W.Y. Lee (Stevens Institute of Technology); D.P. Stinton (Oak Ridge National Laboratory) Sodium corrosion of the protective SiO₂ surface scale is a potentially serious problem for Si₃N₄ and SiC components in gas turbine engines. The objective of this program was to develop corrosion-resistant chemical vapor deposition (CVD) mullite coatings to protect Si-based ceramics from hot corrosion. Mullite was selected as a protective coating material due to its high temperature stability, good corrosion resistance, and thermal expansion match with Si₃N₄ and SiC. Chemical vapor deposition provides a means of depositing a thin, dense mullite (3Al₂O₃-2SiO₂) layer with good adherence. Mullite was deposited by CVD using AlCl₃ and SiCl₄ as precursors. The effect of reagent concentrations was found to be significant. With the liquid SiCl₄ source at ambient temperature, the minimum flow rate of SiCl₄ (in the Ar carrier gas) resulted in CVD mullite coatings with a Si content that was higher than the stoichiometric value. The mullite coatings with a high Si content displayed decreased corrosion resistance. After refrigerating the SiCl₄ source to temperatures of 0 and -10°C, mullite coatings with high Al contents were deposited. The results of coating development and hot corrosion testing will be discussed. Research sponsored by the Advanced Automotive Materials Program, DOE Office of Transportation Technologies, under contract DE-AC05-96OR22464 with Lockheed Martin Research Corporation.
3:10 PM		A1-2-6 Break
3:30 PM		A1-2-7 Deposition of Functional Oxide Layers for High Temperature Applications by Filtered Cathodic Arc Plasma Deposition F. Koch (Forschungszentrum Juelich, Germany); B. Bliznakovska (Univ. Skopje, Macedonia); W.J. Quadakkers, H. Bolt (Forschungszentrum Juelich, Germany) Modern gas turbine components which are exposed to hot combustion gas are coated with thermal barrier coatings in order to allow for increased combustion gas temperatures and, at the same time, for lower temperatures of the metallic base material which result in acceptable lifetimes. As thermal barrier coatings usually YSZ (Yttria Stabilised Zirconia) is applied onto a MeCrAlY interlayer which provides oxidation protection of the base metal. This occurs by the natural formation of an oxide scale at the MeCrAlY - YSZ interface. However, experiments with this material combination showed that delamination failure of the YSZ coating frequently occurs at this interface due to excessive scale growth. In order to improve the adhesion of the thick YSZ coatings on metal substrates, thin alumina coatings were deposited on Ni-base alloy specimens with high Al-content. For this coating process the cathodic arc plasma technique was adapted and an efficient macro-particle filter had been developed to minimize the particle contamination of the coatings. It was found that the particle coverage of the coatings could be reduced to less then 1%. Alumina coatings with thicknesses varying from 50 nm to 1 micron had been prepared. Subsequently these coatings were characterized and oxidation experiments were carried out. The protection efficiency, resp. the further oxidation behaviour of these layers was compared with the formation process of native alumina scales on unprotected substrates. The results of the post oxidation characterization allowed conclusions on the protection efficiency of the coatings as well as on the early stages of the growth process of alumina scales. On the basis of this characterisation further alumina coated specimens will subjected to the plasma spray deposition of thick YSZ thermal barrier coatings and subsequent oxidation experiments.
3:50 PM		A1-2-8 Thermal Stability of Al-O-N PVD Diffusion Barriers R. Cremer, M. Witthaut, D. Neuschuetz (RWTH Aachen, Germany) The use of Al-O-N coatings as a diffusion barrier for components with a high thermal load, e.g. in gas turbines, was investigated. Films with compositions along the quasibinary section Al₂O₃-AlN were deposited by physical vapor deposition (PVD), characterized with regard to their composition and structure and examined in view of their thermal stability. Ternary homogeneous Al-O-N phases can be deposited on austenitic steel (X 5 CrNi 18 12) by means of magnetron sputtering ion plating (MSIP). Their corrosion resistance and adhesion have proven to be superior to those of the binary subsystems. Al-O-N coatings were deposited onto Ni base superalloys (CMSX4) and characterized using X-ray photoelectron spectroscopy (XPS) regarding their composition and X-ray diffraction (XRD) concerning their structure. The phase stability of the films was examined by vacuum annealing at temperatures up to 1200° C and a subsequent XRD analysis. To investigate a possible application of these films as diffusion barrier between CMSX4 and MCrAlY, a MCrAlY coating was deposited onto selected samples. The MCrAlY coating serves as a corrosion protection for the CMSX4 material. With the aim of raising the operating temperature of turbine blades to values at or above 1100° C, the problem of noticeable interdiffusion between nickel base superalloy and MCrAlY layer is encountered. A possible solution for this problem is the deposition of a diffusion barrier between alloy and MCrAlY. The composite was then tempered at 1100 °C for 4 hrs. The interfaces CMSX4-AlON as well as AlON-MCrAlY were examined by means of high resolution Auger electron spectroscopy (AES), electron probe microanalysis (EPMA) and scanning electron microscopy (SEM). By SEM the stability of the AlON diffusion barrier within the composite was analyzed. Knowledge about the elemental distribution within the composite was gained by means of AES and EPMA. Through a comparison between the elemental distributions before and after the tempering the diffusing components and therefore the efficiency of the diffusion barrier coating was determined.
4:10 PM		A1-2-9 Life Time Modeling at the High Temperature Oxidation of Coatings and Alloys for Gas Turbine Blades E.S. Kartavova, P.G. Krukovsky (Institute of Engineering Thermophysics, Ukraine) Mathematical modeling of high temperature oxidation and diffusion is a purpose of this paper. The mathematical model of diffusion and oxidation is described by diffusion equations of alloying elements and oxidant with oxidation processes taking into consideration by mass source term. The diffusion and oxidation mathematical model is integrated by numerical methods (the finite difference method). It permits one to take into consideration the most general peculiarities of the problem formulation: - the multilayer, the relationships between coefficients and coordinate, time and concentration. To determination the main parameters of the model (diffusion, mass transfer, mass uptaking coefficients, etc.) was used the inverse problem solutions [1]. Inverse problem calculations were carried out with using experimental data from short-time tests. This is allowed to tuning of model parameters by means of experimental data. Determined coefficients were used for the modeling of high temperature oxidation and diffusion apply to few alloys, coatings and its system. The element concentrate distribution and oxide thickness were calculated using determined coefficients for experimental data. The experimental data and calculated results of concentration distribution are in good correspondence. paragraf The usage of model and inverse problem solution for tuning the model parameters allowed to do the prediction of the diffusion and oxidation states of specimen made from nickel based alloy with and without metal-ceramic coatings at the high temperatures during 50000 h treatment. 1. Krukovsky, P.G. A Universal Approach to Solution of Inverse Heat Transfer Problems (Method and Software). Proceedings of the 30th 1995 National Heat Transfer Conference, Vol 10, ASME, New York, 1995, pp. 107-112.
4:30 PM		A1-2-10 Corrosion Behaviour of Plasma-Spray Coated Functionally Gradient Materials A.S. Demirkiran (Sakarya University, Turkey); E. Celýk (Florida State University); E. Avci (Sakarya University, Turkey) Functionally gradient materials are a new concept for high temperature mateirals, which consist of ceramics on one side, metal on the other and an intermediate layer whose structure, composite and morphology chagne continuously from ceramic to metal in micro-scale. Hot salt corrosion was employed on functionally gradient materials. This kind of corrosion can cause serious problems in gas turbines and combustion chambers. In the present work, the lasma spray technique was used to deposite powders such as NiCrAl, NiCrAl + MgZrO₃ and MgZrO₃ on the substrate. We selected a certain compsotion for cermet layer FGM samples were dipped in Na₂So₄+K₂So₄, Na₂SO₄+NaCl, Na₂SO₄ + V₂O₅ containing environment at temperature of 600^oC. The microstructure of the coatings were evaluated by means of an optical microscope and SEM. The results show that corrosion rate presents a parabolic function depending on time. It is also obtained that the most corrosive media is V₂O₅ environment. Morever, the corrosion resistance of FGM is much higher than TBCs considerably.
4:50 PM		A1-2-11 Oxidation Behaviour of Plasma-Spray Coated Functionally Gradient Materials E. Celýk (Florida State University); A.P. Demýrkiran, M. Yargan, E. Avci (Sakarya University, Turkey) Functionally gradient materials are coating systems which are used to increase performances of high temperature components in the hot gas turbine section of gas turbine engines, diesel engines, in aerospace and aircraft applications. These coatings consiste of a transition from metallic bond layer to cermet and from cermet to ceramic layer. In this study, oxidation behaviour of plasma-spray coated functionally gradient materials on AISI 304 L stainless steel substrates were investigated. The plasma spray process was employed to deposit metallic, cermet and ceramic powders such as NiCrAl, niCrAl + MgCrO3 and MgZrO3 on the substrate. We selected a certain composition for each sample and cermet layer. Oxidation tests were carried out at temperature of 800 and 1000^oC. The microstructure of the powders, coatings and oxidized coating were examined by means of an optical microscope, SEM and X-Ray Diffractometry. The results show that the oxide film thickness and oxidation rate at 1000°C are higher than at 800°C considerably.