Papers

ICMCTF2012 Session A1-3: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

Thursday, April 26, 2012 8:00 AM in Room Sunrise

Thursday Morning

Start	Invited?	Item
8:00 AM		A1-3-1 Development of high-temperature oxidation resistant coatings by electrodeposition Xiao Peng (Institue of Metal Research,Chinese Academy of Sciences, China) A novel concept is proposed for designing high-temperature oxidation-resistant coatings using electrodeposition. The electrodeposited coatings have a nanocrystalline Ni matrix with nano length scale dispersion of nanoparticles of Cr or/and Al. The Ni nanostructured coatings are oxidation-resistant based on a model on the easily selective oxidation to form a protective scale of Cr₂O₃ or Al₂O₃ as proposed as follows. Numerous Cr or/and Al nanoparticles on or close to the surface act as the “diffusionless” sites for nucleating the corresponding oxides at the onset of oxidation and those in deeper areas simultaneously supplies sufficient flux of the Cr or/and Al along the abundant grain boundaries in the Ni matrix, toward the surface for a rapid linkage of the oxide nuclei through their lateral growth. The validity of the model is verified by characterization of the scales formed in the initial oxidation stage and comparison of the oxidation of these coatings with that of component-similar materials of two other types, composites electrodeposited using Cr or/and Al microparticles and coarse-grained alloys prepared by arc-melting.
8:40 AM		A1-3-3 Producing high temperature multifunction coatings on the basis of micro-sized spherical aluminum particles Raquel Roussel, MariadelMar Juez Lorenzo, Veronica Kuchenreuther, Vladislav Kolarik (Fraunhofer ICT, Germany) Micro-sized spherical Al particles in the range of 1 to 20 µm deposited as slurry by brushing and spraying on the surface of a Ni- or Fe-based alloy oxidize at high temperatures to a top-coat from sintered hollow alumina spheres while forming an aluminized diffusion zone in the substrate. The top-coat has the potential to effectuate as a thermal barrier coating by gas phase insulation and the diffusion zone forms a protective alumina layer. The adherence of the top-coat and the formation of the diffusion zone depend on the heat treatment and are influenced notably by the particle size of the aluminium. Samples from Alloy 321 and IN738 were coated with single size spherical aluminium particles with 2-3 µm and a multi-size mixture of 1-20 µm and subjected to a heat treatment. PEG was used as binder. For Alloy 321 the Al particles oxidize rapidly enough for forming sintered hollow alumina spheres with sufficient wall thickness at temperatures around the melting point. Simultaneously diffusion takes place. For the Ni-based IN738 the suitable heat treatment temperatures are higher. At 900°C and 950°C a homogeneous diffusion zone with an adherent top-coat was formed. For both alloys reproducibility was observed keeping the heat treatment parameters constant. The multi-size source particle powder seems to benefit the adherence of the top-coat. The results indicate that the smaller particles contribute more to diffusion while mechanically more stable alumina scales form on larger particles. Exposure experiments to 2000 h confirm the higher stability of the coating when using the multi-size Al particles.
9:00 AM		A1-3-4 Thermal barrier coatings on γ-TiAl protected by the halogen effect Simone Friedle, Michael Schütze (Dechema e.V., Frankfurt am Main, Germany); Nadine Nießen, Reinhold Braun (DLR - Deutsches Zentrum für Luft- und Raumfahrt, Germany) Nickel-based superalloys protected by thermal barrier coating (TBC) systems of yttria-stabilized zirconia (YSZ) are the existing standard high temperature material in aero-engines. A current research goal is to substitute these alloys to some extent with lightweight γ-TiAl, which have a low specific weight and high specific strength at elevated temperatures. The oxidation resistance of γ-TiAl, which is limited to approximately 750°C, is significantly improved by the halogen effect, affording an oxidation protection up to 1050°C. In this process, treatment of γ-TiAl surfaces with fluorine promotes the selective formation of a slow-growing α-alumina layer. A halogen-affected zone of only 1-3 µm provides a constant supply of halogen to maintain the oxidation resistance even under cyclic conditions in water-vapor and sulfur-containing atmospheres. With this well-established method, γ-TiAl alloys could potentially replace heavy-weight nickel-based superalloys in some industrial applications in the future. For increased turbine efficiency, application of a TBC is necessary in order to improve the lifetime of turbine blades and vanes. TBCs have therefore previously been tested on γ-TiAl by applying bond coats of aluminide and Ti-Al-Cr based intermetallic coatings as well as nitride coatings. These applications, however, suffered from problems such as the formation of brittle TiAl₂ and TiAl₃ phases, interdiffusion between the bond coat and the substrate, and an inefficient oxidation resistance at long-term exposures above 900°C. Here, we present a novel concept that combines TBCs with the halogen effect on γ-TiAl, eliminating the disadvantages that have been observed using traditional bond coats on these alloys. The halogen treatment is also a very simple and economical process showing great potential for industrial applications, representing an additional noteworthy advantage. For the first time, we demonstrate the successful application of EB-PVD TBCs on γ-TiAl protected by the halogen effect. The γ-TiAl based alloys were treated with fluorine by means of gas and liquid phase treatment and plasma immersion ion implantation. Subsequently, YSZ thermal barrier coatings were deposited using electron-beam physical vapor deposition at substrate temperatures of ~1000°C. The oxidation behavior of the specimens was determined under cyclic oxidation conditions at 900°C in air, revealing excellent adhesion of the TBC on oxide scales existing predominantly of α-alumina.
9:20 AM		A1-3-5 High Temperature Protection of Ferritic Steels by Nano-Structured Coatings: Supercritical Steam Turbines Applications Maria (M.) Mato, Maria (M.) Hierro, Saul Castañeda, German Alcalá, Maria (M.) Lasanta (Universidad Compultense de Madrid, Spain); Marta Tejero (Universidad Complutense de Madrid, Spain); Juan Sánchez (Instituto de Ciencia de Materiales de Sevilla, Spain); Marta Brizuela (Tecnalia, Spain); Francisco Pérez (Universidad Complutense de Madrid, Spain) In many applications at high temperature, micro-structured coatings have been applied in order to protect structural materials against a wide range of different environments: oxidation, metal dusting, sulphidation, molten salts, steam, etc… The resistance achieved by the use of different kind of coatings have been optimum, and with late design such as TBC´s and FGM´s coatings. Although, the lifetime of them are related with inter-diffusion, and different CET as main degradation mechanisms. In the case of supercritical steam turbines, may attemps have been made in terms of micro-structural coatings design, mainly based in aluminides, and another diffusion coating systems. In order to consider another alternatives to minimize those problems, nano-structured coatings, applied by PVD and HIPIMS-PVD based in Cr, Ti and Al design, have been applied onto high temperature structural materials in order to analyze their high temperature oxidation resistance in steam environments. The gravimetric results obtained have been analysed upto 2.000 hours, jointly with the evaporation behavior analysed by TG-Mass spectromnetry. Excellent results have been achieved for the nano-structured coatings tested. Those results are comparables with the results obtained for micro-sctructured coatings, and in some case better for nano-structured coatings. According to the results obtained, the nano-structured coatings have a potential application as protective systems in high temperature, for some applications will be proposed.