Papers

ICMCTF1999 Session A4: Coating Processes Technology Advancement

Wednesday, April 14, 1999 8:30 AM in Room Council/Chamber/Cabinet

Wednesday Morning

Start	Invited?	Item
8:30 AM		A4-1 Intelligent Processing of Materials for Thermal Barrier Coatings Y.C. Lau, J. Ruud, D.M. Gray, P. Houpt, N. Kapoor, A. Narendra, R. DeMuth, C.M. Penney, P. DiConza, H.P. Wang (GE Corporate Research and Development) Ceramic thermal barrier coatings have been developed to protect the hot-path turbine components, but most of these coatings do not have the consistent quality and durability needed to assure satisfactory performance in advanced gas turbines. The goal of this work is to develop a more knowledge-based and data-intensive "intelligent process" for air plasma spray that will improve the reproducibility, quality and reliability of the thermal barrier coatings. Particular challenges include development of a suite of process sensors, accurate process models, on-line feedback to make real-time adjustments as the coating is being applied, and integration of the overall system into a manufacturing environment. The project employed the full Design for Six Sigma tool kit as a systematic way to design for robustness, with special emphasis on transfer functions and Design of Experiments. Results of using the new closed-loop controller included improving the process capability of one of the key coating properties from an estimated three sigma up to six sigma.
9:10 AM		A4-3 On the Reproducible Production of Precisely Specified Coatings at Minimal Cost R.C. Tucker, W.A. Payne, M.R. Holdsworth (Praxair Surface Technologies, Inc.) The reproducible production of a precisely specified coating is necessary to its utilization in industry, and the breadth of the applications which a given coating can economically address is a function of its cost. To some degree, the use of closed feed back loop sensing systems can help to control a coating process to achieve a consistent, reproducible coating. This approach requires both a knowledge of the critical parameters in a process, appropriate sensors to measure these parameters, and adjustable (usually with computer control) valves or other controls on the process parameters. These measurements and control systems are useful in the development of coating processes and new coatings, but all of these result in added capital and operating expense. Most of these costs can be avoided if, with an adequate knowledge of the process, the equipment used to operate the process and the direct control of the process parameters such as gas flows, electrical power, powder flow, etc. are designed, manufactured, operated, and maintained with precision and accuracy. These considerations are particularly important when multiple coating sites on an international basis are to be used to produce a given coating. This paper will address these issues and provide illustrative examples from thermal spray, physical vapor deposition, and chemical vapor deposition processes and coatings.
9:50 AM		A4-5 Improved Diffusion Coatings Through Advanced Manufacturing Techniques B.M. Warnes, D.C. Punola (Howmet Corporation) Improvements in diffusion coatings for gas turbine blades and vanes have been achieved through the introduction of a number of new or improved manufacturing techniques. These advancements, for simple aluminide and platinum modified aluminide systems, have been achieved in both the platinum electroplating and aluminzing areas. Improvements include items such as the use of an advanced platinum electroplating process utilizing a clean bath chemistry and the further refinement of the low activity, Chemical Vapor Deposition aluminizing process. The use of these advanced manufacturing techniques has been demonstrated to improve the life of coatings in exposure to high temperature, oxidation environments. In addition, to the coating life improvements, these improved manufacturing techniques have allowed the manufacturing sequence of the gas turbine hardware to be improved. Specifically, the use of the low activity, CVD produced platinum aluminide system has allowed the coating process to be performed on cast, advanced gas turbine blades before most, if not all, machining operations are completed. This improved manufacturing sequence, known as Cast Coat, has reduced manufacturing cost, improved product turn times and reduced over all supply chain inventory levels.
10:10 AM		A4-6 Vapour Aluminide Coating of Internal Cooling Channels in Turbine Blades and Vanes A.B. Smith, A. Kempster, J. Smith (Diffusion Alloys Limited, England) A production-scale vapour aluminising process has been developed which facilitates the protective aluminide coating of the surface of internal cooling channels/passages in superalloy turbine blades and vanes, without recourse to either reduced pressure or pressure pulsing. The plant employed to apply this process industrially is briefly described. Vapour aluminide coatings thus formed on a nickel-base superalloy have been subjected to a detailed characterisation. The techniques employed for this purpose include X-ray diffraction, Auger electron spectroscopy, optical microscopy - Nomarski differential interference contrast and bright field illumination, microhardness testing, profilometry and scanning electron microscopy. It has thus been determined that the characteristics of such aluminide coatings largely parallel those exhibited by outward-diffusion-type NiAl coatings formed on nickel-base superalloys by low-aluminium-activity pack aluminising. Two illustrative examples of the industrial application of this vapour aluminising process, to coat the linear internal cooling channels in a turbine blade and the serpentine channels in a vane, are presented. In both cases it has been established that all the internal surfaces have been coated with an aluminide layer that exhibits good uniformity of thickness throughout. The coated internal cooling channel surfaces have additionally been shown to be clean and free from any extraneous material.
10:30 AM		A4-7 The Control of the Composition and Structure of Aluminide Layers Formed by Vapour Aluminising A. Squillace (Alfa Romeo Avio SpA, Italy); R. Bonetti (Berna AG, Switzerland); N.J. Archer, J.A. Yeatman (Archer Technicoat Ltd., United Kingdom) Vapour aluminising or out-of-pack aluminising is a major production technique for producing aluminide coatings on gas turbine parts. The aluminide layers enhance the oxidation resistance of hot-section components. The layers are formed by the inter-diffusion of aluminium from the vapour phase and elements from the base alloy. The final composition of the layer is dependent on the method of its formation and the substrate alloy. This paper presents a comparison between two vapour aluminising processes widely used in production. One produces an initially high concentration of aluminium at the surface of the part with predominantly inward diffusion of aluminium, and needs subsequent heat treatment. The other produces a lower concentration of aluminium at the surface of the part with substantial outward diffusion of nickel from the substrate alloy. Both processes achieve an aluminide layer with acceptable oxidation resistance for specific applications.
10:50 AM		A4-8 Thin, Two-Phase Glass Coating Protect Against High-Temperature Corrosion. K.E. Wiedemann, P.D. Beesabathina (Analytical Services & Materials, Inc.) Thin, two-phase-glass coatings that have been recently developed and their method of application are described. These coatings provide effective protection from high-temperature corrosion. They can be used alone and they can be used with existing coating systems to provide additional protection and extend life. The protection stems from the sealant and barrier qualities that the two-phase-glass morphology imparts. Single-phase glasses act as barriers below their softening point because they are rigid and restrict the transport of corrosive ions and molecules to solid-state mechanisms. Above their softening point, single-phase glasses act as sealants because they can flow to fill gaps and repair cracks, but they no longer act as barriers. In two-phase glasses that have strong immiscibility, there are two softening points: one for each phase. If the majority phase has the higher softening point then at temperatures between the two softening points the glass will act as both a sealant and a barrier because the minority phase can flow to fill gaps and repair cracks but the majority phase remains rigid. The difference between the two softening temperatures can be as much as 800°C enabling the glass to provide effective protection over a broad temperature range. Dense glass coatings where the majority phase has a high softening point are difficult to make using traditional coating application methods, but are easy to make using sol-gel methods. Such coatings effectively protect nickel-based alloys typically used in gas-turbine-engine applications against isothermal oxidation, thermal-cycling oxidation, and molten-sodium-sulfate hot corrosion.
11:10 AM		A4-9 Electrostatic Assisted Vapour Deposition of Y₂O₃ ZrO₂ Coatings for Gas Turbines K.L. Choy (Imperial College of Science, Technology and Medicine, London, 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 thermal barrier coatings (TBCs). 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. Thick TBCs (> 250 micron thick) with columnar-like structures have been successfully deposited using the ESAVD method at high deposition rate (1-5 micron per min) at relatively low deposition temperature (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 or the TBCs. YSZ with preferred orientation is possible using this deposition technique. The simplicity and flexibility of ESAVD in tailoring the microstructure of the TBCs are unrivalled by the existing Electron Beam Physical Vapour Deposition (EB-PVD) and plasma spraying methods. 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.
11:30 AM		A4-10 Investigation of Dominating Factors on Flattening Behavior of Plasma Sprayed Ceramic Particles Y. Tanaka, M. Fukumoto (Toyohashi University of Technology, Japan) Recently, we have shown that the splat morphologies of most metallic materials thermally sprayed strongly depend on the substrate temperature and usually the change occur drastically near the transition temperature:Tt. In the present study, the effects of substrate material, powder material and PVD coating material on the flattening of the plasma sprayed ceramic particles were investigated. Commercially available several kinds of powders, that is, Al₂O₃, TiO₂, YSZ and Al₂O₃-TiO₂, were plasma sprayed and collected on the mirror polished substrate surface. The substrate materials are AISI304 stainless steel, brass and glass, and those were held at a designated temperature before spraying. To investigate the effect of wetting on the flattening of the particle, Au, Ti and Al coated substrates by PVD were also prepared. The splat morphology collected on the room temperature substrate was splash type. As the substrate temperature increased, the central solidification area of the splash splat gradually enlarged, and finally the splat morphology changed to the disk type over Tt range. This transition is maybe caused by both the improvement of wettability at splat/substrate interface and suppression of rapid solidification with the substrate temperature increasing. The different transition behavior was observed on the common substrate material with each PVD coating. It is well known that the standard formation free energy of oxide can be closely related to the static wetting of melt material on the substrate. If this relation can be applicable to the dynamic wetting like thermal sprayed particles on the substrate, it can be estimated from the results obtained that the better wetting promotes the occurence of the disk splat. The observation results of the bottom surface microstructures of the splats supported well this hypothesis. Consequently, it was clarified that the wetting played an important role on the flattening behavior of the plasma sprayed ceramic particles.
11:50 AM		A4-11 Flattening and Solidification Behavior of Metal Droplet on Flat Substrate Surface Held at Various Temperatures. M. Fukumoto, E. Nishioka, T. Matsubara (Toyohashi University of Technology, Japan) A freely falling experiment, in which a metal droplet fell freely and impinged on a flat substrate, was conducted as a simulation of the thermal spray process. The effect of substrate temperature on the flattening and solidification of the droplet was mainly investigated in this study. The transition of the splat pattern was recognized in the experiment, that is, the splat morphology of Ni and Cu droplet on the room temperature substrate was splash-type, while that on the high temperature substrate was disk-type. The cross section microstructure of the splat on the room temperature substrate was composed of an isotropic coarse grain, while that on the high temperature substrate was quite fine columnar structure. As the mean interparticle spacing in the splat changed transitionally with the substrate temperature, the solidification rate in the splat on the high temperature substrate was higher than that on the room temperature substrate. The unique porous microstructure and flowing pattern were observed in the bottom surface of the splat on the room temperature substrate, while the flat microstructure without pore was recognized in that on the high temperature substrate. The difference of the solidification rate between these two kinds of splats seems to be attributed to the interface microstructure between splat and substrate. The flat microstructure can transfer the heat of splat to substrate better because the practical contact area is larger. Then it can be concluded that the quite rapid solidification occurred at the interface between splat and substrate, and the inside of the splat solidified slowly. Furthermore, from the observation results by a high-speed camera, the droplet on the room temperature flattened rapider considerably than that on the high temperature substrate. According to Rayleigh-Taylor instability, the rapider the droplet flattens, the more complex its shape is. The splashing occurred on the room temperature can be explained by this instability.