Papers

ICMCTF2000 Session A4: Coating Processes Technology Advancement

Monday, April 10, 2000 12:30 PM in Room Royal Palm Salon 1-3

Monday Morning

Start	Invited?	Item
12:30 PM		A4-7 Innovative and Cost-Effective Deposition of High Temperature Coatings Using Electrostatic Spray Assisted Vapour Deposition Method K.L. Choy (Imperial College of Science, Technology, and Medicine, United Kingdom) This contribution presents an overview of our activities principally concerned with strategic investigation on the development of zirconia based coatings, and the exploitation of a novel and cost-effective Electrostatic Spray Assisted Vapour Deposition (ESAVD) for the manufacture of high temperature coatings. This novel ESAVD coating technology involves spraying atomised precursor droplets across an electric field where the droplets undergo chemical reaction in the vapour phase near the vicinity of the heated substrate. This produces a stable solid film with excellent adhesion onto a substrate in a single production run. This process is capable of producing thin or thick strongly adherent coatings with well controlled stoichiometry, crystallinity and texture. The deposition process occurs in an open atmosphere without the need of sophisticated reactor and/or vacuum chamber. This innovative technology is a rapid, flexible and cost-effective materials coating method. Existing coating technologies such as CVD and PVD are expensive, time consuming, requiring large capital input and offering little flexibility of use. However, ESAVD costs a fraction of current equipment due to its simple set up, and is capable of coating complex shapes with multiple and compositionally gradient layers making it extremely flexible. The ESAVD coating method has applications in almost all industrial fields. These include aerospace, power generation, electrical and electronics, automotive, oil industry, biomedical and consumer sectors. The use of ESAVD to produce protective coatings such as zirconia based coatings for high temperature applications will be highlighted. Thick thermal barrier coatings (TBCs > 250 micron thick) with columnar-like structures have been successfully deposited using the ESAVD method at high deposition rate (e.g. 1-5 micron per min) and at relatively low deposition temperatures (550-850°C). Moreover, this method has the capability for molecular tailoring of microstructure and composition to produce stress-strain columnar with careful engineered microporosity/microcracks, multilayer and graded structures to improve the coating adhesion, erosion resistance, and lower the thermal conductivity of the TBCs. YSZ with preferred orientation is possible using this deposition technique. The thermal conductivity property of the TBCs produced by ESAVD is lower than EBPVD, and as low as plasma spraying. The erosion and thermal cycling properties of the TBCs produced using ESAVD are better than those produced using a more costly plasma spraying method, and are comparable to those deposited using the very expensive EBPVD method. In addition, ESAVD is a non-line-of-sight process with high throwing power. Therefore, it can be used to fabricate a wide range of protective coatings for components in conventional boiler, gasification, steam turbine and gas turbine systems and for solid oxide fuel cell applications.
1:10 PM		A4-9 A Low Temperature CVD Process for Aluminum and Aluminide Coatings J. Liburdi, P. Lowden, V. Moravek (Liburdi Engineering Ltd., Canada) A novel, low temperature Organometallic Chemical Vapor process (LOM) has been developed by LIBURDI Engineering. The process which is widely used in the electronics industry to apply thin layers of pure aluminum has been successfully scaled from a 3" (75 mm) diameter tube reactor to a production hot wall metal retort with an internal diameter of 18" (0.45m) and a height of 60" (1.5m ). The LOM process is ideal for simultaneous coating external and internal surfaces, and can be used for low temperature atmospheric corrosion protection in place of IVADIZING or diffusion heat treated to produce a high temperature oxidation resistant aluminide coating for superalloys. The aluminum coating can also be alloyed with modifying elements such as platinum to further enhance its high temperature oxidation resistance, and can be used in conjunction with thermal barrier coatings, since it is free from contaminating halides. Potential applications range from coating of complex internal surfaces of a heat exchanger, or an automotive catalytic converter, to the coating of industrial and aero turbine blades with internal cooling passages.
1:30 PM		A4-10 Design and Operation of a Laboratory Scale CVD Reactor for Reproducible Synthesis of Hf-Doped Aluminide Coatings G.Y. Kim, L. He, J.D. Meyer, W.Y. Lee (Stevens Institute of Technology) The dynamic versatility of aluminizing Ni alloys by CVD offers an avenue of uniformly doping the resulting aluminide coating with a reactive element for enhanced oxidation performance. However, with the apparent lack of reproducible experimental results in the high-temperature coatings community, considerable uncertainties exist to properly project the viability of the doping approach. In this study, a laboratory scale CVD reactor was specifically designed for Hf doping with HfCl4 as the dopant precursor. Process parameters, doping procedures, and reactor conditions were systematically investigated to observe how Hf incorporates into the coating matrix, while carefully assessing the reproducibility of our experimental results.
1:50 PM		A4-11 Plasma CVD of Thin Al₂O₃ Films on Powders in a Circulating Fluidized Bed M. Karches (Institute of Process Engineering, ETH Zurich, Switzerland); M. Morstein (Laboratory for Surface Science & Technology, ETH Zurich, Switzerland); R. von Rohr (Institute of Process Engineering, ETH Zurich, Switzerland) Easy handling and high surface area brought powders into a dominant position among industrial goods. Bulk properties and surface properties can be controlled independently by coating the particles with a thin film. Aluminum oxide films are known to have excellent properties as diffusion barriers, even at high-temperatures, and protect particles during high-temperature applications such as thermal spray coating. Among the various film deposition techniques, Plasma CVD is particularly promising, since low-temperature plasmas provide a powerful source of reactive species at relatively low temperatures. Such plasma processes have mainly been applied to flat substrates so far, whereas no satisfactory process is currently available for the coating of powders. We have recently shown that a circulating fluidized bed, operated under vacuum conditions, can be used to expose the powder to plasma in a very efficient way¹. The coating reactor is a vertical tube (40 mm I.D.), where the particles are blown through by the reaction gas. The plasma is generated in situ in the upper part of this riser tube. The entrained particles are separated from the gas phase in a cyclone, recirculated to the reactor and enter a new deposition cycle. 1 kg of powder can be coated per batch. Circulating the powder enables efficient heat dissipation out of the plasma zone and improves the uniformity of the coating. SiC particles of 150 µm diameter were coated with about 1 µm aluminum oxide films using aluminum sec.-butylate as precursor and an argon/oxygen-mixture as process gas. The aluminum precursor was mixed with sec.-butanol for flowability enhancement and subsequently vaporized in a heated injection nozzle. Coating time was 3 - 5 hours. Chemical surface analysis of the coated SiC powder was performed using X-ray photoelectron spectroscopy (XPS). The diffusion barrier effect was investigated with a high temperature treatment of the coated powder in oxygen. It could be shown by electron microscopy that the coatings are dense and uniform. Variations of film thickness are small and correspond to the calculated variation of residence time in the reactor. The deposited material consists mainly of Al₂O₃, and low carbon impurity levels were reached. High-temperature oxidation of the SiC bulk material was reduced by an order of magnitude. These results demonstrate that uniform, dense, thin coatings can be obtained by combining PECVD and circulating fluidized bed technology. ¹M. Karches, C. Bayer, Ph. Rudolf von Rohr, in: Circulating Fluidized Bed Technology VI (Ed.: J. Werther) p. 537, DECHEMA, Frankfurt a.M. 1999
2:10 PM		A4-12 Advanced Protective Coatings for U.S. Navy Engines H.G. Simpson (Naval Aviation Depot) U.S. Navy gas turbine engines traditionally operate in extremely harsh conditions including environments of oxidation, sulfidation, corrosion, and erosion. When operating from military ships, fighter aircraft and helicopters experience constant exposure to salt water. The same aircraft are exposed to sand perhaps only a few hours later when operating on beaches and in the desert. The sand and salt have one effect at ambient temperature but have quite a different effect when ingested into the turbines operating in excess of 1000ºC. Unprotected compressor and turbine alloys are constantly at risk, and the addition of coatings to these materials has slowly become the norm. This presentation will discuss the coatings currently being used as well as the more advanced coatings being evaluated for use on military aircraft engines in the future.