Coating Processes Technology Advancement
Thursday, May 3, 2001 8:30 AM in Room Royal Palm Salon 1-3
A4-1-1 Smart Overlay Coatings - Concept and Practice
J.R. Nicholls, N.J. Simms (Cranfield University, United Kingdom); M. Taylor, H.E. Evans (University of Birmingham, United Kingdom)
Smart overlay coatings are a functionally gradient coating system designed to provide high temperature corrosion protection over a wide range of operating conditions.@paragraph@ The "SMARTCOAT" design consists of an MCrAlY base, enriched first in chromium, then aluminium to provide a chemically graded structure. At elevated temperatures, above 900@super o@C (1650@super o@F), the coating oxidises to form a protective alumina scale. However, at lower temperatures this alumina scale does not reform rapidly enough to confer protection under type II hot corrosion conditions. The coating is therefore designed with an intermediate chromium rich interlayer, which permits the rapid formation of chromia healing for areas of type II corrosion damage.@paragraph@ Laboratory and burner rig tests have been carried out on a series of developmental smart overlay coatings. These have shown that the development of an intermediate chromium rich phase provides protection under low temperature hot corrosion conditions, while the aluminium rich surface layer provides resistance to high temperature oxidation and type I hot corrosion. Thus the single application of "SMARTCOAT" permit operation over a broad range of industrial and marine turbine conditions.
A4-1-3 Critical Issues in the Production of Advanced Modified Platinum Aluminide Coatings
C. Ramade, A. Malié, C. Cléach, Y. Jaslier (Snecma Services, France); G. Oberlaender, B. Da Silva (Snecma Moteurs, France)
Increased demand on the high temperature capability of high pressure turbine blades has driven the development of protective coatings as systems against corrosion, oxidation and bond coats for thermal barrier coatings. In this context, various types of platinum modified aluminide coatings have been used depending on the application. This paper addresses the criticalities of applying Pt modified aluminide coating technology to advanced design high pressure turbine blades and presents some of the latest improvements that have been brought to the technology to combine the requirement of an improved performance coating system with a robust process. Improved process control was achieved in terms of platinum plating optimisation, cooling hole obstruction, control of coating sulfur content and microstructure as well as post-coat surface conditioning
A4-1-4 Molten Metal Hydroxide Removal of Thermal Barrier Coatings
J.E. Schilbe, B.M. Warnes (Howmet Thermatech Coatings)
During both the manufacture of new engine components and the refurbishment of engine run airfoils, it is often necessary to strip a thermal barrier coating (TBC). Ceramic coatings are typically removed by grit blasting or HF cleaning or autoclave processes, and these procedures are expensive, labor intensive and time consuming. In addition, traditional TBC strip processes often cause damage to the bond coat and/or the substrate. A research program was undertaken to determine whether immersion of components with ceramic coatings in molten potassium hydroxide at atmospheric pressure could remove a TBC in a short time without damaging either the bond coat or the substrate. The process is described, and the effects of the process on a variety of alloy/bond coat combinations are presented and discussed.
A4-1-5 Engineering Qualification of Diffusion Coatings Used by GE Aircraft Engines
T. Grossman (GE Aircraft Engines)
GE Aircraft Engines employees a rigorous process to qualify production coatings for use in their jet engines. All coating sources must demonstrate both engineering performance and production performance capabilities prior to GE’s approval to coat production hardware. New suppliers and established suppliers that request modifications to previously approved processes are required to undergo qualification. The result is a robust supply chain with demonstrated process capability. The qualification process, performance standards and process capability tests are all discussed.}
A4-1-7 As-deposited Mixed Zone in Thermally Grown Oxide Beneath a Thermal Barrier Coating
K.S. Murphy (Howmet Castings); K.L. More, M.J. Lance (Oak Ridge National Laboratory)
Gas turbine designers are increasingly using Electron-Beam Physical Vapor Deposited (EB-PVD) Thermal Barrier Coatings (TBC) to meet the challenge of higher efficiency gas turbine engine requirements. A key feature for expanding the use of TBC's is increased spallation life and reduced spallation life variability. Such a coating system comprises a substrate (Ni-based single crystal alloy), a bond coat (Diffusion aluminides or MCrAlY), a ceramic (7 wt.% yttria stabilized zirconia), and a thin Thermally Grown Oxide (TGO) between the bond coat and the ceramic. The TGO is intended to be @alpha@-alumina but evidence suggests that in some cases the as-deposited TGO may not be entirely @alpha@-alumina.@paragraph@ The thin nature of the as-deposited TGO (<0.5 microns) makes analysis of the phases present and morphology difficult. Advancements in TEM sample preparation and Photo-stimulated Luminescence Spectroscopy (PSLS) have allowed higher quality and easier interrogation of the TGO.@paragraph@ EB-PVD TBC coatings were applied to platinum-aluminide bond coats on a Ni-based superalloy. One PVD process variable was selected and coatings were made at three levels of this variable. STEM and PSLS results are reported for each level of the process variable. An explanation for the creation of the mixed oxide zone found in some TGO morphologies is presented.
A4-1-8 Full-Scale Modelling of Thermal Spray Process
A.V. Zagorski (ALSTOM Power Technology, Switzerland); F. Stadelmeier (ALSTOM Power, Switzerland)
A complex approach to the simulation of the entire thermal coating process and modellinfg results are presented. The simulation tool consists of physical models and their computational realizations for all major elements of the process such as a plasma torch, sub-sonic and supersonic plasma flows, particle motion and heating, plasma-substrate and particle-substrate interaction, formation of the coating layer. Simulation results are compared with experimental data for VPS and APS. @paragraph@ Using a simplified thermo-mechanical splat model and statistical model of particle deposition some aspects of interface and microstructure formation are studied. Influence of particle size, temperature and speeds as well as spray angle and standoff distance on the coating structure and surface texture is investigated in detail. @paragraph@ Influence of the plasma torch design, gas composition, motion control and process parameters on the spray patterns, coating porosity and microstructure, substrate and coating temperature is discussed. Several examples of sensitivity analysis and coating optimization for those parameters are presented. @paragraph@ Advantages and drawbacks of the complex simulation of the spray process are discussed.
A4-1-9 Advanced Coatings for High Pressure Turbine Airfoil Repair
T.M. Gartner (Lufthansa Technik AG, Germany)
In aero engines many components undergo distortion, cracking, burning and material degradation during operation. The strategy of Lufthansa Technik AG is to reduce the costs per flight hour by developing appropriate repair methods for engine parts. A number of thermal and chemical processes are applied to improve the efficiency of repairs in terms of extended operation time. Initially the components are cleaned and the entire coatings are removed without attacking the substrate. The defects like cracks and material degradation can be either brazed or welded under special conditions described in detail. Advanced coating systems consisting of hot corrosion resistant coatings that are more frequently combined with thermal barrier top coatings are re-applied. In contrary to new parts manufacturing, specific problems may arise during the parts repair that have to be solved individually. Two examples of those problems including their successfully developed solutions are presented. The high performance repairs have shown significant life time extension leading to an overall cost reduction.
A4-1-10 Controlled TGO Formation During the EB-PVD Coating Process for Improving TBC Life Time
G. Marijnissen, E. Vergeldt, A. van Lieshout, W. Beele (Interturbine Coating Center, The Netherlands)
The adhesion of an EB-PVD thermal barrier coating is mainly caused by the thermally grown oxide TGO in between the bond coating and the ceramic top coat. Coating failures occur nearly always in this TGO. @paragraph@ For creating optimal properties the TGO has to be a dense thin hexagonal @alpha@-alumina layer. The formation of this interlayer is critical and proved to be dependent on the material combination, base metal and bond coat and all preparation and coating steps for the EB YPSZ ceramic top coating. @paragraph@ Especially on MCrAlY type of bond coatings the initial oxide is a chromium oxide, and the final oxide is always @alpha@-alumina. However if the initial chromium content in the TGO is high, this will influence the life time of the coating very negative. The change between these two oxides is created by a combination of chemical thermodynamics of the TGO - metal interface and reaction kinetics and diffusion processes in the metal and the TGO. This complex oxide formation process is very sensitive for all kind of process variations.@paragraph@ By careful control of all process parameters it is not only possible to improve the TGO formation, but also to increase the life time of a thermal barrier coating system, and to reduce the variation in the results dramatically.
A4-1-11 Conformable Eddy-Current Sensors for Condition Assessment of Gas Turbine Coatings
A. Washabaugh, Y. Sheiretov, V. Zilberstein, D. Schlicker, N. Goldfine (JENTEK Sensors, Inc.)
New advances in the use of conformable Meandering Winding Magnetometer (MWM@super TM@) eddy current sensors provide nondestructive inspection capabilities for condition monitoring of protective coatings on turbine blades. Metallic overlay coatings and thermal barrier coatings (TBCs), including a metallic bond coat and the ceramic topcoat, allow engine operation at higher temperatures and provide protection from severe oxidation and high-temperature corrosion for turbine blades and vanes. Effective condition monitoring and remaining life assessment is essential to address both safety and cost concerns; however, this monitoring requires accurate and practical nondestructive methods that provide relevant information about thickness and degradation of the coatings as well as degradation of the substrate.@paragraph@ A nondestructive method for inspecting these coatings using spatially periodic conformable eddy-current sensors, such as the MWM, and quantitative model-based inversion algorithms is described in this paper. For TBCs, multiple frequency measurements and data inversion methods provide simultaneous and independent determination of multiple unknowns, such as metallic bond coat thickness, metallic bond coat porosity, and ceramic topcoat thickness. This measurement method is suitable for manufacturing quality control and in-service inspection of non-magnetizable materials. Similar methods have been applied to thermally aged samples for estimation of depletion zone thickness and other representative features of coating degradation. This paper provides a brief review of the MWM technology, a description of improved multiple frequency quantitative inversion methods, and representative measurements on both as-manufactured and aged coating systems.