The Science and Technology of Thermal Spray Coatings

Monday, April 30, 2001 1:30 PM in Room Sunrise

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1:30 PM T1-2-1 Real-Time Thermal Conductivity and Delamination Mechanisms of Plasma-Sprayed Thermal Barrier Coatings under High-Heat-Flux Conditions
D. Zhu, A.M. Calomino, R.A. Miller (NASA Glenn Research Center)
Ceramic thermal barrier coatings (TBCs) will play an increasingly important role in gas turbine engines because of their ability to further raise engine temperatures. However, the issue of coating durability is of major concern under high-heat-flux conditions. In particular, the accelerated coating delamination crack growth under the engine high heat-flux conditions is not well understood. In this paper, a laser heat flux technique is used to investigate the coating delamination crack propagation under realistic temperature-stress gradients and thermal cyclic conditions, by monitoring the coating real-time thermal conductivity variations. The coating delamination mechanisms are studied under various thermal loading conditions, and are correlated with coating dynamic fatigue, sintering and interfacial adhesion test results. A coating life prediction framework may be realized by examining the crack initiation and propagation driving forces for coating failure under high-heat-flux test conditions.
2:10 PM T1-2-3 Microstructural Characterization Studies to Relate the Properties of Thermal Spray Coatings to Feedstock and Spray Conditions
A.J. Allen, G.G. Long (NIST); H. Boukari, J. Ilavsky (University of Maryland); A. Kulkarni, S. Sampath, H. Herman (State University of New York at Stonybrook); A. Goland (Brookhaven National Laboratory)
Many of the technologically important properties of thermal spray coatings are governed by their strongly anisotropic void microstructures, which are in turn determined by the feedstock and spray conditions. While advances have occurred in controlling the feedstock condition and spray parameters, characterization of the resultant sprayed microstructures has proven more difficult. However, in an extensive series of small-angle scattering studies, the porosities, sizes and surface area distributions of the intrasplat cracks, intersplat lamellar pores and globular pores, together with their approximate orientation distributions, have been resolved and quantified for thick thermal-spray ceramic deposits, for the first time. Some of the complex relationships have been explored between these component microstructures and the feedstock and spray process conditions, and also between the component void populations and the measured deposit properties. Furthermore, advanced small-angle scattering techniques have been developed and applied to obtain some of this microstructural information for thinner coatings with thicknesses more representative of those used in technological applications. In this paper, the state of the art in this field will be briefly reviewed and the most recent results will be presented.
2:50 PM T1-2-5 Electrical Properties of Pressure Quenched Silicon Prepared by Thermal Spraying
S.Y.T Tan, R.J.G. Gambino, R.G. Goswami, S. Sampath, H. Herman (SUNY at Stony Brook)
High velocity thermal spray deposition of silicon particles onto single crystal substrates, yields metastable high pressure forms of silicon in nanocrystalline form within the deposit. The phases observed in the deposit include hexagonal diamond-Si, R-8, BC-8 and Si-IX. The peculiar attribute of this transformation is that it occurs only on <100> orientation silicon substrate. The silicon deposits containing the high pressure phases display a substantially higher electrical conductivity. Furthermore the nanocrystalline silicon deposit on a type silicon substrate forms a rectifier device. The current-voltage characteristics exhibit a barrier potential, and photoconductivity measurements indicate that the device can detect optical signals. The significance of this work lies in the fact that the versatility of thermal spray may enable applications of these high pressure forms of silicon.
3:30 PM T1-2-7 Part I: Characterization of Cr@sub3@C@sub2@-25% NiCr Plasma Sprayed Coatings Produced at Different Pressures
M.H. Staia (Universidad Central de Venezuela); T. Valente, C. Bartuli (Rome University La Sapienza, Italy); D.B. Lewis, P.C. Constable (Sheffield Hallam University, United Kingdom)
The present work was performed with the aim of characterizing Cr@sub 3@C@sub 2@-NiCr plasma sprayed coatings produced with a CAPS system, which are suitable for high temperature tribological applications. A nitrogen based plasma gas in a nitrogen filled spraying chamber at various pressures levels, ranging from 300 mbar to 1200 mbar, was adopted to verify eventual formation of nitride hard phases suitable to enhance wear resistance properties. To this purpose different combinations of spraying parameters (pressure, power input, spraying distance, substrate cooling) were selected. The powder used was fully characterized before spraying in terms of shape, size and composition.@PARAGRAPH@X-Ray diffraction studies were carried out for all coatings by using Bragg-Brentano geometry. An apparent volume fraction of both Cr@sub 3@C@sub 2@ and NiCr has been determined by using both the ratio of average integrated intensity from the diffraction results and image analysis performed on the cross sections of coatings.@paragraph@The major phases were determined to be similar to those present in the initial powder, which has been also analyzed by X-ray diffraction technique. The presence of graphitic carbon could be identified in all the specimens by X-ray diffraction and Raman spectroscopy, but was more clearly detectable in the specimens where the apparent volume fraction of Cr@sub 3@C@sub 2@ was the greatest. For experiments performed at atmospheric pressure it was a clear evidence of fcc Cr @sub 23@C@sub 6@ present in the coatings. @paragraph@Cross section of samples have been characterized by SEM and elemental mapping. The integrity of the interface and samples porosity was determined by using standard metallographic techniques. Vickers microhardness measurements were also carried out on all sprayed samples and the results have shown a significant correlation with spraying parameters, due to coatings different microstructural properties and carbide stability in the plasma flame. The results have been related to the processing conditions with the aim of process optimization.
3:50 PM T1-2-8 Part II: Tribological Performance of Cr@sub3@C@sub2@-25% NiCr Plasma Sprayed Coatings at Different Pressures
M.H. Staia (Universidad Central de Venezuela); T. Valente, C. Bartuli (Rome University La Sapienza, Italy); D.B. Lewis (Sheffield Hallam University, United Kingdom); A. Roman, J. Lesage, D. Chicot, G. Mesmacque (UST Lille, IUT A, France)
Cr@sub3@C@sub2@-NiCr thermal sprayed coatings have found extensive application for protection of components which resist at high temperature wear and corrosion degradation and, in general, their performance is a strong function of the processing parameters independent of the method used. They are proposed for continuos service in the range 550-815ºC, with a maximum of 900ºC for discontinuous service, due to their oxidation resistance. The variation of coatings response to wear and corrosion constitutes a measure of the degree to which these factors have been optimized.@paragraph@The present work has been undertaken in order to determine the response to the sliding wear behavior of reactively plasma spray coatings obtained in different pressures conditions, between 300mbars to 1200mbars, in nitrogen atmosphere by using a controlled atmosphere CAPS system. Some other parameters as plasma power, spraying distance and gas cooling during deposition have been also varied.@paragraph@Results in terms of coating thickness and roughness are also reported. Sliding tests have been performed by using pin-on-disc tribometer. The static partners were WC-6%Co steel balls. Average friction coefficients as low as 0.4 have been determined. Surface topography of the wear scars has been analyzed by using Zygo NewView 200 profilometer. The wear scars on both discs and pins have been studied using SEM and X-ray elemental mapping in order to determine the wear mechanism. The mechanical properties of coatings are discussed in relation to previous microstructural characterization.
4:10 PM T1-2-9 The Effect of Humidity on the Sliding Wear of Plasma Transfer Wire Arc Thermal Sparayed Low Carbon Steel Coatings
A. Edrisy (University of Windsor); T. Perry, Y.-T. Cheng (General Motors); A.T. Alpas (University of Windsor, ONTARIO,CANADA)
Low carbon steel coatings were fabricated on 319 AI alloy substrates using a plasma transferred wire arc (PTWA) type thermal spraying process. The structure of the coating consisted of iron splats with major axes parallel to the coating surface. The iron splats were separated by thin veins of iron oxide. Wear tests were performed using a pin on disc type wear tester equipped with an environmental test chamber, in argon, dry air (7-10 % RH), ambient humidity (40-50% RH) and high humidity (80-90% RH) atmospheres. A load range of 1- l00 N and a sliding speed range of 0.2- 2.5 m/s were used. It was shown that the wear rates of the coatings were strong functions of the testing environment and applied load. At low load and sliding velocity tests, an Fe2O3 rich compound formed on the worn surfaces. As the atmospheric humidity increased, the Fe2O3 became hydrided and triggered a tribo-polishing process so that the wear rates decreased with increasing humidity. At high loads the oxide thickness (Fe2O3 and possibly Fe3O4) exceeded 1 mm and formed a continuous layer on the wear track. SEM metallography indicated that the high wear rates were generally associated with the formation and propagation of subsurface cracks within the coatings, resulting in removal of iron splats during the sliding process. The hardness of the wear tracks in samples tested in argon increased to about 900 kg/mm2, from 300 kg/mm2 for the unworn surfaces. As a result, wear tests in argon atmosphere consistently resulted in lower wear rates. The relationships between the microstructural basis of wear processes and the mechanical and chemical properties of the coatings will be discussed to assess the suitability of these coatings for wear protection of cast aluminum surfaces. +ˇ. µ.
4:30 PM T1-2-10 Indentation Testing of HVOF Sprayed WC-Co
H.R. Alves (Fundação Centro Tecnológico De Minas Gerais- Cetec, Brazil); S. Sampath (SUNY at Stonybrook); J.R.T. Branco (Fundação Centro Tecnológico De Minas Gerais- Cetec, Brazil)
Indentation testing of HVOF sprayed WC-Co H.R. Alves, S. Sampath and J.R.T. Branco Ceramic and cermet thermal sprayed coatings find wide used in the aero-space and industrial segments to provide for high wear resistance, which depends on hardness, microstructure and toughness. The need for lower cost and higher performance has motivated this work. HVOF sprayed WC-Co, with three levels of feed stock carbide size were tested under micro-abrasion and the results analysed in terms of hardness, microstructure and toughness. Indentation and transversal scratching test were investigated as a means to measure toughness. The relationship between crack dimensions and acoustic emission parameters were also investigated. The variability of data is discussed and the results are compared with predictions by fracture mechanics models.
4:50 PM T1-2-11 Thermal Spray Coatings for Molten Carbonate Fuel Cells Separator Plates
A. Agüero, F.J. García de Blas, M.C. García, R. Muelas, A. Román (Instituto Nacional de Técnica Aeroespacial, Spain)
Molten salt corrosion at the wet seal of separator plates is one of the principal life-limiting factors of molten carbonate fuel cells (MCFC). The wet seal is therefore coated with an aluminide layer produced by Al Ion Vapour Deposition (IVD) followed by heat treatment; however, this coating only lasts approximately 20,000 h and not the expected 40,000 h. The Al IVD coating is also very expensive since only a few plates can be coated simultaneously due to size limitations of the existing commercial vacuum chambers employed in IVD. The need of heat treatment further increases costs, particularly since it requires long heating and cooling cycles in order to minimize distortion of the 0.25-0.30 mm thick stainless steel plates. Thermally sprayed coatings constitute an alternative which requires neither containment nor heat treatment, and also provides the possibility of depositing materials more resistant to molten carbonate than plain aluminides. However, this technique has not received much attention for this application since it usually requires sand blasting prior to coating to ensure good adhesion and moreover it can cause distortion during coating due to thermal shock. In this work, several commercially available coatings have been applied by plasma spray and HVOF and alternative deposition surface preparation methods were examined as alternatives to sand blasting. Moreover, substrate preheating and/or cooling during deposition were examined in order to eliminate substrate distortion. FeCrAl, FeAl and NiAl as well as the cuasicrystalline alloy AlCoFeCr were deposited on AISI 310 foils with a thickness of 0.25 mm, and after optimization and characterizaton by means of SEM-EDS, coated foils were tested by inmersion in a 62 mol % Li2CO3/ 38 m% K2CO3 molten carbonate mixture at 650°C and by electrochemical impedance spectroscopy. IVD Al coatings were also tested for comparison purposes and the results indicated that both FeCrAl and AlCoFeCr exhibit a much higher molten salt corrosion resistance than IVD aluminide coatings whereas AlFe and NiAl were attacked shorltly after the beginning of the test. Grinding of the substrate resulted in good coating adhesion and substrate distortion was minimized by cooling with Ar during deposition.