Development of APS MCrAlY Dense Bond Coats 2006 UTSR Summer Fellowship Program Project Report

Transcription

1 Development of APS MCrAlY Dense Bond Coats 2006 UTSR Summer Fellowship Program Project Report Submitted by: Emmanuel Perez, University of Central Florida, Orlando Fl 2006 University Turbine Systems Research (UTSR) Summer Fellow General Electric Company (GE Energy) University Turbine Systems Research (UTSR) Introduction The objective of this study was to mimic the microstructure of MCrAlY coatings sprayed using HVOF (High Velocity Oxy-Fuel) processes by using an Air Plasma Spray (APS) process in order to improve oxidation and spallation life of MCrAlY APS coatings. Figure 1 shows the typical microstructures obtained when spraying bond coats using the (a) HVOF process, and (b) the APS process. The microstructures developed by each process are very different. The HVOF process produces a dense coating that protects the substrate material against oxidation and also serves a bond coat for the top Thermal Barrier Coating (TBC) coating. APS MCrAlY coatings typically contain more internal defects (pores, oxides, particle splat boundaries) and do not provide the same level of oxidation protection as a dense HVOF coating. APS coatings add-in thermal expansion mismatch and adhesion when used as a bond coat layer between the substrate and the TBC. Prior to this study no data had been collected to determine the optimal operating conditions for spraying dense MCrAlY bond coats using APS. The study determined the optimal spraying parameters for the gun/powder system in order to maximize coating density, minimize internal oxidation in the coating, and to optimize the surface roughness of sprayed coating. The coating surface roughness is a critical parameter for the bond coat [1-8]. It provides mechanical bonding between the TBC and the bond coat. This mechanical interlocking increases the adhesion of the TBC to the bond coat, and improves the spallation life of the TBC system. Figure 1. Typical microstructures of coatings sprayed by (a) HVOF and (b) APS processes.

2 Experimental Procedure A spray booth cell equipped with robotic systems for controlled plasma and HOVF thermal spraying was used to carry out the experiments. IN738 tabs were tack welded onto larger plates to hold the samples in place in the spray booth. One plate was prepared for each spray parameter. Before spraying, the plates were grit blasted to clean and roughen the surface. In order to begin the optimization process plasma gun, a Design of Experiment (DOE) methodology was adopted. A screening experiment was performed that consisted of a starting parameter obtained from the gun manufacturer, followed by nine separate spray runs with varying parameters. The secondary gas, current, and powder feed rates were chosen as variables because these were expected to have the most significant effects on the powder particle states (temperature and velocity) in the plasma plume [9,10]. The primary gas was held constant. The gun spray distance and the spray velocity were obtained from the gun manufacturer and were also held constant. The number of spray passes was determined from the expected deposition rate on the manufacturer specification. Time was devoted to the determination of the proper carrier gas flow rate and pressure. Although the carrier gas was expected to affect the particle states in the plasma plume, it was not considered as a variable for the screening runs. In order to relate the gun parameter to the particle states (average particle temperature and average velocity) the Sulzer Metco Accuraspray system was used [11-14]. Temperature and velocity data was collected to attempt to relate the particle deposition characteristics to the microstructural development of the coating. Higher particle temperature and velocity may produce a denser coating, but may result in higher internal oxidation density. Lower temperatures and velocities may result in poorly formed, porous coatings. The properties of the coating are heavily dependent process parameters, the state of the powder particles in the plume, as well as on deposition characteristics [15-27]. The experiment attempted to achieve an optimal combination between low particle temperatures and high velocities. To observe the developed internal microstructure of each coating, tabs were mounted in epoxy, and the cross sections were polished. Micrographs were collected using optical microscopy. To quantify the coating microstructure, the optical micrographs were analyzed by digital processing of the micrographs. The results from the screening experiment were used to determine the quality of the spray parameter and provide the direction in which the optimal spray parameter may be found in the next set of experimental spray trials. Based on the results, an optimization DOE, referred to here as OPE-I, was then designed. OPE-I further converged on the optimal spray parameter for proper deposition. A near optimal coating was obtained with this optimization. Based on these results, another optimization experiment, OPE-II was conducted. The methodology for these spray-runs deviated from the DOE methodology. Further optimization required reduction of the average particle size in the MCrAlY powder used. After spraying, the samples were again prepared for metallography and the coating microstructure analyzed in the same manner as the screening experiment.

3 Results and Discussion Screening Experiment: To start the optimization process of the plasma gun spray parameter with MCrAlY powder, ten separate spray runs were performed. Microstructural analysis of the coatings was performed to measure the levels of internal porosity and oxidation, as well as the coating developed thickness and surface roughness. Preferred coating microstructures were downselected based on the levels of oxidation, porosity and the developed coating thickness. All spray parameters for the screening experiment developed cold microstructures. All microstructures contained a large fraction of unmelted particles embedded in the coating, which resulted in highly porous coatings. Shown in Figure 2(a) is the microstructure developed with the plasma gun manufacturer starting spray parameter. The coating developed significant thickness, but porosity and internal oxidation are significant. Figure 2(b) shows the least desired microstructure that developed. The Accuraspray average particle temperature and velocity data indicated that this spray parameter produced the lowest average particle temperature and velocity obtained in these experimental runs. This indicated that the MCrAlY particles in the plasma plume did not achieve enough temperature and velocity to partially melt, deform upon impact and compact to produce a dense coating. Figures 2 (c) and (d) show the best microstructures that were developed during the screening experiment. Although cold, these runs developed coatings with uniform deposition and developed the desired thickness. The spray parameters for these two coatings were then used to design the next DOE for further optimization of the coating. Figure 21. Microstructural development of some of the MCrAlY coatings sprayed during the screening experiment: (a) using the manufacturer s starting spray parameter (b) the least desired microstructure developed, (c) and (d) the two better microstructures developed.

4 Optimization Experiment I (OPE-I): The best spray parameters from the screening experiment and the results its DOE were used to design a new DOE to further optimize the spray parameter. Based on the screening experiment, increasing the plasma energy was desired in order to achieve higher average particle temperatures and velocities for improved coating density. Microstructural analysis of the coatings was performed in the same fashion as in the screening experiment to measure the levels of internal porosity and oxidation as well as the coating developed thickness and surface roughness. As shown in figures 3 (a)-(d), increased energy plasma resulted in decreased levels of internal porosity and oxidation, higher thicknesses and even depositions. These levels of porosity and internal oxidation remained above the desired values for this study. Figure 3. Microstructural development of some of the coatings sprayed using OPE-I spray parameters. Optimization Experiment II (OPE-II): The coating microstructures developed up to this stage of the optimization process were relatively dense, but the levels of porosity and oxidation remained above the desired levels. At this stage spray parameters that produced the better coatings operated the plasma gun at maximum manufacturer recommended power levels for safe gun operation. To further optimize the coating, the MCrAlY powder average particle size was reduced. The smaller particles were expected to achieve higher temperatures with nearly the same resident time in the plasma plume as compared to larger particles. Further optimization of the spray process was conducted outside the scope of a classical DOE methodology. The manufacturer s starting parameter and the improved spray parameters obtained from the OPE-I optimization were sprayed with the new powder. As shown in figure 4

5 (a)-(d), the coatings developed denser microstructures than those of OPE-I. Porosity and internal oxidation were reduced. Coating thickness increased, indicating better deposition of the particles to the surface during the spray process, requiring less spray passes to achieve the desired coating thickness. Of the sprayed coatings, the coatings shown in figure 4c and 4d developed the best coating produced in this study. The coating porosity was almost eliminated. The coating density was nearly comparable to that of a typical HVOF coating. Thus the coating is expected to provide oxidation protection more similar to HVOF even though applied by air plasma spray. Internal oxidation was observed, but the concentration in the coating was deemed acceptable. Future oxidation comparison testing will show if this level of oxides in the coating is detrimental to oxidation protection. By comparison of figure 1 (a) with Figure 4(d), the coating microstructure closely resembled that of a typical MCrAlY coating sprayed by HVOF processes. As a result, the coating shown in figure 4(d) achieved the desired microstructure, and the spray parameter used was determined to be optimal for the process. Figure 3. Microstructural development the coatings sprayed using OPE-II spray parameters. Conclusion: A spray parameter optimization of an APS gun was conducted to spray MCrAlY powders to produce dense bond coats that mimic the microstructure of typical HVOF MCrAlY coatings. A screening experiment and an optimization experiment were carried out under a DOE methodology to converge onto an optimal spray parameter. Further spray runs were conducted outside the scope of the DOE to further optimize the coating s microstructure. Optimization led to the development of a coating microstructure that resembles that of a typical HVOF coating with very low porosity. Some internal oxidation remained, but was considered under acceptable

6 limits. Because of the low porosity, the optimized spray parameter should be expected to produce coatings that offer oxidation protection to the component s substrate material. Acknowledgements: This work was supported as a fellowship program by the General Electric Company, GE Turbines division, and the University Turbines Systems Research (UTSR) coordinated for the Department of Energy by the South Carolina Institute for Energy Studies (SCIES) at Clemson University. References: 1. J. Liu, J.W. Byeon, Y.H. Sohn; Surface and Coatings Technology, Volume 200, Issues 20-21, 22 May 2006, Pages L. Xie, Y. H. Sohn, E. H. Jordan, M. Gell; Surface and Coatings Technology, Volume 176, Issue 1, November-December 2003, Pages D. Zhang, S. Gong, H. Xu and Z. Wu; Surface and Coatings Technology, In Press, Corrected Proof, Available online 31 January F. Traeger, M. Ahrens, R. Vaßen and D. Stöver; Materials Science and Engineering A, Volume 358, Issues 1-2, 15 October 2003, Pages M. Ahrens, R. Vaßen and D. Stöver; Surface and Coatings Technology, Volume 161, Issue 1, 1 November 2002, Pages M. J. Pindera, J. Aboudi, S. M. Arnold; Materials Science and Engineering A, Volume 284, Issues 1-2, 31 May 2000, Pages A. M. Freborg, B. L. Ferguson, W. J. Brindley and G. J. Petrus; Materials Science and Engineering A, Volume 245, Issue 2, 1 May 1998, Pages Kh.G. Schmitt-Thomas, H. Haindl and D. Fu; Surface and Coatings Technology, Volumes 94-95, October 1997, Pages E. Pfender; Pure Appl. Chem., Vol. 60, No. 5, 1988, pp P. Fauchais; J. Phys. D: Appl. Phys. 37 (2004) R86-R F.Nadeau, L.Pouliot, J.Blain; " Practical Applications of Spray Plume Sensors in the Aeronautic and Automotive industries ", in Thermal Spray: surface Engineering via Applied Research, Proceedings of the ITSC 2000 Thermal Spray Conference, Montreal, Canada, May 2000, pp L.Pouliot, J.Blain, F.Nadeau, M.Lamontagne, J.F. Bisson, C.Moreau, "Significant Increase in the Sensitivity of In0Flight Particle Detector Through Improvements & Innovation", in Thermal Spray 2001: New Surfaces for a New Millennium, Proceedings of the ITSC 2001 Thermal Spray Conference, May 2001, Singapore, pp J.F.Bisson, M.Lamontagne, C.Moreau, L.Pouliot, J.Blain & F.Nadeau, "Ensemble In-Flight Particle Diagnostics Under Thermal Spray Conditions", in Thermal Spray 2001: New Surfaces for a New Millennium, Proceedings of the ITSC 2001 Thermal Spray Conference, May 2001, Singapore, pp

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