ICMCTF2010 Session A1-2: Coatings to Resist High Temperature Oxidation

Tuesday, April 27, 2010 1:30 PM in Room Sunrise
Tuesday Afternoon

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Start Invited? Item
1:30 PM A1-2-2 NiAl(CrZrHfSi) High Temperature Overlay Coating for Gas Turbine Engines
Brian Hazel, Ram Darolia, Michael Rucker (General Electric)
As jet engine high-pressure turbine (HPT) inlet temperature increase, new designs and materials are used to achieve the desired durability and reliability. Protective coatings on HPT blades have evolved from the use of simple aluminides to noble-metal aluminides as performance demands have increased. In the most demanding applications, insulating ceramic thermal barrier coatings (TBC’s) are applied over the diffusion aluminides to further reduce the bulk and surface temperatures of the component. The use of advanced environmental coatings and TBC systems in turbine engines has become necessary for protection of turbine airfoils and other gas turbine components. The bond coat plays a significant role in the material system environmental resistance, adhesion of the ceramic TBC and the overall life and capability of the TBC system. Recent GE Aviation bond coat development has focused on reactive element modified beta phase NiAl overlay coatings to further reduce oxide growth rates. The result is longer TBC adherence lives and greater environmental resistance for turbine airfoil operation.
1:50 PM A1-2-3 Effects of Substrate Surface Roughness and Microstructure on the Properties of dc Magnetron Sputter Deposited NiAl-Hf and NiAl-Cr-Hf Coatings
Mark Weaver, Michael Bestor, Mathew Kirsch, Joel Alfano (The University of Alabama)
This paper describes the influences of substrate roughness and processing variables on the properties of NiAl-Hf and NiAl-Cr-Hf overlay bond coatings deposited via dc magnetron sputtering. Coatings were deposited onto single crystal René N5 and CMSX-4 Ni-based superalloys followed by post deposition annealing at 1000°C for in flowing Ar+5%H2. The as-deposited coatings were found to consist of a single phase, b -NiAl, with columnar zone 2 microstructures. All of the coatings were found to contain leader defects, the number of which could be significantly decreased by reducing substrate surface roughness. However, surface roughness was observed to have no significant influence on coating adhesion. Post-deposition annealing was conducted at 1000°C to induce interdiffusion and to bring solutes out of solid solution. This resulted in the formation of a series of nanometer-scale precipitates within the coating and in the formation of a thin interdiffusion zone. The influences of these post-deposition heat treatments and of post-deposition peening prior to heat treatment on the isothermal and cyclic oxidation performance at 1050°C will be discussed.
2:10 PM A1-2-4 Formation of High Temperature Multifunction Coatings on the Basis of Spherical Al Particles
Vladislav Kolarik, MariadelMar Juez Lorenzo, Petia Kodjamanova, Harald Fietzek (Fraunhofer ICT, Germany)

Spherical Al particles in the size range of 2 to 10 µm oxidise at high temperatures forming hollow spheres from alpha-Al2O3. Depositing such Al particles on a substrate alloy by brushing or spraying and subjecting them to a heat treatment, coating structures are obtained that consist of an aluminised diffusion zone and an adherent quasi-foam top coat from sintered hollow alumina spheres, which has the potential to effectuate as a thermal barrier by gas phase insulation. The present work aims at understanding the mechanisms and investigating the appropriate parameters for the design of multifunction coatings.

Spherical Al particles sized in the range of 2 to 10 µm were deposited with different binder/plastisizer systems by brushing on Alloy 347H and IN738 obtaining a coating with an average thickness of 60 µm. For understanding the influence of temperature on the oxidation of the particles, the samples were subjected to a step-wise heating from room temperature up to 1400°C with steps of 50°C. At each step an XRD pattern was recorded in situ. The results show a temperature range, in which transient alumina phases with an amorphous structure are formed, followed by the formation of alpha-alumina on further heating.

For investigating the heat treatment for the coating formation, the samples were subjected to 1 to 4 h at 400°C and 500°C for curing and then to 5 h at 700°C to 900°C for forming the coating structure. Coating structures are encountered that consist of a quasi-foam top coat from hollow alumina spheres and a diffusion layer below that forms a protective alumina scale. Depending on the parameters of the thermal treatment, the sintering of the hollow alumina spheres to each other and the adherence of the top coat to the substrate is more or less pronounced. The formation of the diffusion zone and the adherence is influenced by the binder.

The structure stability and oxidation behaviour of the system was studied up to 2000 h. The adherence of the top coat depends strongly on a good and homogeneous contact during the thermal treatment. This can be controlled by the selection of an appropriate binder system. Only small amounts of brittle aluminide phases were found in the diffusion zone and no cracking was observed. A protective alumina scale is formed on the diffusion zone surface.

It can be concluded that the approach by particle size processing opens a potential for obtaining a complete thermal barrier coating system in one manufacturing step.
2:30 PM Invited A1-2-5 Platinum Group Metal-Containing Bond Coats
Tresa Pollock, D. Widrevitz, R. Adharapurapu, J. Zhu (University of Michigan); Don Lipkin (GE Global Research Center)
Platinum group metal (PGM) additions and their influence on bond coat properties have been investigated in B2 and γ – γ’ bond coats. Overlay B2 coatings with Cr and varying amounts Hf and Pt or Pd on a superalloy substrate have been studied using a combinatorial technique. While Pd is diffusionally depleted to a greater degree than Pt during oxidation, the cyclic oxidation kinetics of Pd and Pt-containing coatings were similar in the temperature range 1100˚C – 1150˚C. During cyclic oxidation, the B2 coatings evolve to a γ’ structure. A synergistic, beneficial effect of Pt+Hf and Pd+Hf was observed. In two-phase γ + γ‘ materials, platinum additions as low as 2.5at% improve oxidation. Large changes in elemental partitioning between the γ and γ‘ phases occurs with additions of Pt, Ir and Ru. Changes in partitioning strongly influence precipitate-matrix misfit and the resultant creep resistance. Tradeoffs between creep resistance and oxidation in PGM-containing systems will be discussed.
3:10 PM A1-2-7 High-Temperature Behavior of Magnetron Sputtered Si-B-C-N Materials Far Above 1000°C
Petr Zeman, Sarka Proksova, Radomir Cerstvy, Jaroslav Vlcek (University of West Bohemia, Czech Republic)

Operating a large number of construction materials in very aggressive environments of diverse industrial processes demands to develop new functional coatings protecting effectively the surface of these materials against oxidation and corrosion at temperatures exceeding 1000°C. Such coatings have to be thermally stable without any degradation of their structure and properties. Recently, novel amorphous Si-B-C-N coatings, which may accomplish all of these criteria, have been successfully prepared by magnetron sputtering in our laboratories. Their excellent oxidation resistance, due to the formation of an amorphous surface SiO2-based layer containing boron [1‑2], even above 1500°C during dynamical heating in flowing air, has stimulated the present study extending our knowledge about thermal stability and oxidation kinetics of these coatings. The Si-B-C-N coatings magnetron sputtered from a single B4C-Si target in a nitrogen‑argon gas mixture at optimum process parameters were investigated by means of differential scanning calorimetry (Labsys DSC 1600) and symmetrical thermogravimetry (TAG 2400). The structure of the coatings was characterized primarily by X‑ray diffraction. Freestanding coating fragments without a substrate were used as specimens for the experiments in order to eliminate an influence of a substrate. It was found that the magnitude of the N/(Si+B+C) concentration ratio is of a key importance to achieve ultra-high thermal stability of the coatings. When this ratio is sufficiently high, the Si-B-C-N material retains its amorphous structure in inert gases up to 1600°C . Analysis of oxidation kinetics revealed concurrently that the oxidation rate constants at temperatures ranging from 1100 to 1300°C are very low, which is caused especially by a low number of diffusion channels in the protective oxide layer being formed on the surface of the Si-B-C-N coatings.

[1] J. Vlcek, S. Hreben, J. Kalas, J. Capek, P. Zeman, R. Cerstvy, V. Perina, Y. Setsuhara, J. Vac. Sci. Technol. A 26, 1101 (2008).

[2] J. Capek, S. Hreben, P. Zeman, J. Vlcek, R. Cerstvy, J. Houska, Surf. Coat. Technol. 203, 466 (2008).
3:30 PM A1-2-8 Oxidation Behavior of Si-Doped Nanocomposite CrAlSiN Coatings
Hsien-Wei Chen, Jenq-Gong Duh (National Tsing Hua University, Taiwan)
Recently more and more hard coatings greatly emphasize the importance of oxidation characteristics, and this study thus attempts to dope Si into conventional CrAlN to form CrAlSixN coatings by RF magnetron sputtering on a silicon wafer to investigate how Si content affects oxidation behavior. The oxidation resistance of the CrAlSixN coatings was evaluated after annealing with temperature ranging from 800 to 1000 ℃ . The X-ray diffraction pattern revealed that CrAlSixN (x = 0.3~11.5) coatings exhibited better oxidation resistance than that of traditional CrAlN coatings. Observed from SEM, the CrAlSixN coatings presented denser structure than CrAlN coatings. The columnar structure, typically existing in CrAlN coatings and harmful to oxidation behavior, was also eliminated. Doping certain Si content could indeed help CrAlN coatings prolong diffusion paths due to their reduced gain sizes, thereby effectively inhibiting outside oxygen from penetrating into the coatings. Additionally, two fine Al2O3 and SiO2 layers given by the CrAlSixN coatings when oxidized could also serve as protective layers to enhance oxidation resistance by slowing oxygen diffusion. To sum up, the overall antioxidative capability of the CrAlSixN coatings after doping Si was significantly improved at elevated temperature due to the barriers resulted from dense structure and the said two fine protective layers prevented outside oxygen from diffusing into the coatings.
3:50 PM A1-2-9 TaNx Diffusion Barrier on Cobalt Cemented Tungsten Carbide
Yung-I Chen, Bo-Lu Lin, Yu-Chu Kuo (National Taiwan Ocean University, Taiwan)

To prevent the diffusion out of cobalt from cemented carbides at high working temperature, TaNx coatings were prepared as a diffusion barrier by direct current magnetron sputtering using a Ta target in an argon-nitrogen atmosphere. The nitrogen flow ratio, N2/(Ar+N2), in the sputtering process varied from 0.1 to 0.4. The deposition rate reduced as the nitrogen flow ratio increased. Silicon wafers and 6 wt.% cobalt cemented tungsten carbide were used as the substrates. Effects of nitrogen flow ratio on crystalline characteristics and mechanical properties of the TaNx coatings were examined by X-ray diffraction and nano-indenter, respectively. The TaNx coatings were annealed at 500, 600, 700, and 800oC for 4 hours in air, respectively. The diffusion barrier performance was evaluated by Auger electron spectroscopy depth profiles and X-ray diffraction. Oxidation resistance of the TaN coatings was also investigated. Orthorhombic Ta2O5 was observed after annealing above 700oC. The hardness declined with increasing annealing temperature due to the formation of the oxide compound with a lower hardness.

4:10 PM A1-2-10 Advanced High Temperature Coatings for Turbine Blade Applications: A REVIEW
Farzin Ghadami (University of Tehran, Iran)
Within all coating systems, for protection tasks, advanced high temperature coatings are the most successful and utilizable because of the very wide range of coating materials and substrates that can be processed specially in turbine component application. Surface science and engineering is now a key materials technology in the design of future advanced turbine Engine parts especially for blades and vans. In this literature, the scope of turbine blade coating is described in terms of the various processes, requirements, coating properties and new applications. Besides, the variety of coating materials including diffusion and overlay coatings are presented. In addition, turbine blade deposition, coating deposition methods such as vapor deposition and plasma spray and also miscellaneous applications are investigated.
Time Period TuA Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF2010 Schedule