Showing papers in "Journal of Thermal Spray Technology in 2000"

Thermal Conductivity and Elastic Modulus Evolution of Thermal Barrier Coatings under High Heat Flux Conditions

Abstract: Laser high heat flux test approaches have been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems. Thermal conductivity change kinetics of a thin ceramic coating were continuously monitored in real time at various test temperatures. A significant thermal conductivity increase was observed during the laser-simulated engine heat flux tests. For a 0.25 mm thick ZrO2-8% Y2O3 coating system, the overall thermal conductivity increased from the initial value of 1.0 W/m K to 1.15, 1.19, and 1.5 W/m K after 30 h of testing at surface temperatures of 990, 1100, and 1320 °C, respectively, Hardness and elastic modulus gradients across a 1.5 mm thick TBC system were also determined as a function of laser testing time using the laser sintering/creep and microindentation techniques. The coating Knoop hardness values increased from the initial hardness value of 4 GPa to 5 GPa near the ceramic/bond coat interface and to 7.5 GPa at the ceramic coating surface after 120 h of testing. The ceramic surface modulus increased from an initial value of about 70 GPa to a final value of 125 GPa. The increase in thermal conductivity and the evolution of significant hardness and modulus gradients in the TBC systems are attributed to sintering-induced microporosity gradients under the laser-imposed high thermal gradient conditions. The test techniques provide a viable means for obtaining coating data for use in design, development, stress modeling, and life prediction for various TBC applications.

Characterization of copper layers produced by cold gas-dynamic spraying

Abstract: The cold gas-dynamic spray method produces coatings or deposits by introducing solid feedstock particles into a supersonic gas stream developed through the use of a converging-diverging (de Laval) nozzle. The particles thus accelerated impact on a substrate surface and develop into a dense deposit through a process believed to be similar to cold compaction. The work reported here explores the internal nature and physical characteristics of copper deposits produced by the cold gas-dynamic spray method using two vastly different starting powders: in one case, a “spongy” copper obtained by a direct-reduction process, and in the second, a denser, more spheroidal particulate produced by gas atomization. Optical and electron microscopies (scanning electron microscopy [SEM] and transmission electron microscopy [TEM]) were used to observe details of microstructure in the feedstock particles and deposits. Young’s modulus and residual stress measurements for the deposits were obtained through mechanical means, and measurements of hardness and electrical conductivity are reported. The internal structure of the cold-spray deposit was influenced by the surface purity of the feedstock material.

X-ray diffraction characterization of crystallinity and phase composition in plasma-sprayed hydroxyapatite coatings

Abstract: Orthopedic and dental implants consisting of a metallic substrate plasma spray coated with hydroxyapatite (HA) are currently used in reconstructive surgery. The crystalline phases present in the calcium phosphate ceramic and the degree of crystallinity must be controlled for medical applications. X-ray diffraction (XRD) is routinely employed to characterize the phase composition and percent crystallinity in both biological and sintered HA. However, application of the same XRD methods to plasma-sprayed coatings is complicated by the potential presence of several crystalline contaminant phases and an amorphous component.

Thermal cycling behavior of plasma-sprayed thermal barrier coatings with various MCrAlX bond coats

Abstract: The influence of bond coat composition on the spallation resistance of plasma-sprayed thermal barrier coatings (TBCs) on single-crystal Rene N5 substrates was assessed by furnace thermal cycle testing of TBCs with various vacuum plasma spray (VPS) or air plasma-spray (APS) MCrAlX (M=Ni and/or Co; and X=Y, Hf, and/or Si) bond coats. The TBC specimens with VPS bond coats were fabricated using identical parameters, with the exception of bond coat composition. The TBC lifetimes were compared with respect to MCrAlX composition (before and after oxidation testing) and MCrAlX properties (surface roughness, thermal expansion, hardness, and Young’s modulus). The average TBC spallation lifetimes varied significantly (from 174 to 344 1 h cycles at 1150 °C) as a function of bond coat composition. Results suggested a relationship between TBC durability and bond coat thermal expansion behavior below 900 °C. Although there were only slight differences in their relative rates of cyclic oxidation weight gain, VPS MCrAlX bond coats with better oxide scale adhesion provided superior TBC lifetimes.

Effect of heat treatment on the thermal conductivity of plasma-sprayed thermal barrier coatings

Abstract: The effect of heat treatment on the thermal conductivity of plasma-sprayed Y2O3 stabilized ZrO2 (YSZ) and Al2O3 coatings was investigated. A heat treatment of 1300 °C in flowing argon for 50 h was found to significantly increase the thermal conductivity of the coatings when compared to measurements in the assprayed condition. Transmission electron microscopy (TEM) examination of the microstructures of the coatings in the as-sprayed and heat-treated conditions revealed that sintering of microcracks at the splat interfaces was the main cause for the increase in thermal conductivity. In the YSZ coatings, complete closure of microcracks was frequently observed. In contrast, microcrack closure in the Al2O3 coatings was characterized by the isolated necking of particles across a microcrack rather than complete closure. A model for thermal conductivity in a solid containing oriented penny-shaped cracks was used to explain the observed increase in thermal conductivity after heat treatment.

Plasma-Sprayed Nanostructured Al 2 O 3 /TiO 2 Powders and Coatings

Abstract: Air plasma spray has been used to produce metastable oxide-ceramic powders and coatings, starting with commercially available Al2O3/13TiO2 powder feed. The feed material undergoes rapid melting and homogenization in the high-temperature zone of the plasma jet. A metastablex-Al2O3·TiO2 phase is formed when the molten droplets are quenched on a chilled substrate. The metastable phase has a defect spinel structure and a nanocrystalline grain size. When heated, it decomposes into an equilibrium two-phase structure, consisting ofα-Al2O3 andβ-Al2O3·TiO2. Both types of ceramic materials have potential as hard, wear-resistant coatings.

Structure and phases in nickel-base self-fluxing alloy coating containing high chromium and boron

Abstract: The structure of a nickel-base, self-fluxing alloy coating, containing chromium and boron thermal sprayed and fused, was investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). A lumpy M6C carbide, a rodlike M3B2 boride of tetragonal structure, a rodlike M7C3 carbide of hexagonal structure, and a Ni-Ni3B eutectic phase formed in the coating after fusing. Metals of M6C, M3B2, and M7C3 phases are composed of chromium, molybdenum, and nickel; chromium and molybdenum; and mainly chromium, respectively. The nickel phase in the coating has the L12 type superlattice structure.

Oxidation of stainless steel in the high velocity oxy-fuel process

Abstract: The high velocity oxy-fuel (HVOF) spray process has been primarily used for the application of wear-resistant coatings and, with the introduction of new, more powerful systems, is being increasingly considered for producing corrosion-resistant coatings. In this study, the influence of various spray parameters for the JP-5000 and Diamond Jet (DJ) Hybrid systems on the oxidation of stainless steel 316L is characterized. Experimental results reveal that coating oxygen contents of less than 1 wt.% can be more easily attained with the JP-5000 than the DJ Hybrid systems because of the former’s design. In both cases, however, the low particle temperatures necessary for low oxygen content coatings may impair bond and cohesive strength. Heat treating the coatings after processing reduces hardness, metallurgically enhances bond strength, and enables the spheroidization of oxide layers surrounding unmelted particles.

Properties of Cr3C2-NiCr cermet coating sprayed by high power plasma and high velocity oxy-fuel processes

Abstract: The structure, hardness, and shear adhesion strength have been investigated for Cr3C2-NiCr cermet coatings sprayed onto a mild steel substrate by 200 kW high power plasma spraying (HPS) and high velocity oxy-fuel (HVOF) processes. Amorphous and supersaturated nickel phases form in both as-sprayed coatings. The hardness of the HVOF coating is higher than that of the HPS coating, because the HVOF coating contains more nonmelted Cr3C2 carbide particles. On heat treating at 873 K, the amorphous phase decomposes and the supersaturated nickel phase precipitates Cr3C2 carbides so that the hardness increases in the HPS coating.

Evaluation of four high velocity thermal spray guns using WC-10% Co-4% Cr cermets

Abstract: Four high velocity thermal spray guns were evaluated in the production of 10% Co-4% Cr tungsten carbide (WC) cermets. Three high velocity oxygen fuel guns (JP-5000, JP-5000ST, and Diamond Jet [DJ]-2700) and one plasma gun (Axial III) were used to spray the same angular, agglomerated, and crushed WC-10Co-4Cr powder. The DPV-2000 was used to monitor the in-flight velocity and temperature of the WC cermet-sprayed particles. From those values, spray conditions were selected to produce coatings that were evaluated in terms of porosity, hardness, and deposition efficiency. Results show that the plasma Axial III provides the highest particle temperature, between 2000 °C and 2600 °C, depending on the spray conditions. The JP-5000 imparts the highest velocity to the particles, between 550 and 700 m/s, depending on the spray conditions. The ST version of the JP-5000 provides the same velocity as the standard version but with lower particle temperature. The DJ-2700 sprays particles with temperature and velocity between those of the JP-5000 and the Axial III. Minimum porosity values of 2.1%, 3.7%, and 5.3%, respectively, were obtained for the JP-5000, the DJ-2700, and the Axial III guns. The porosity and carbide degradation are found to depend mostly on the particle velocity and temperature, respectively. The values for the Vickers microhardness number (200g) ranged from 950 to 1250. Measurements of the deposition efficiency indicated a variation between 10% and 80%, depending on the spray conditions and the gun used.

Thermal sprayed nanostructured WC/Co hardcoatings

Abstract: Hardcoatings of WC/Co, produced by high-velocity oxy-fuel (HVOF) and air plasma spray (APS) deposition, have been studied. During HVOF deposition, nanostructured powder experiences more decarburization than conventional powder, whereas in APS deposition, just the opposite effect occurs This is explained in terms of the influence of the highly porous, spherical-shell morphology of the nano-WC/Co particles on their melting characteristics and reaction kinetics. In particular, heterogeneous melting and localized superheating of the high-surface-area powder is considered to be a controlling factor in decarburization. The situation is further complicated in APS deposition by high-temperature vaporization of Co and C.

Three-dimensional simulation of thermal plasma spraying of partially molten ceramic agglomerates

Abstract: Thermal plasma spraying of agglomerated nanostructured ceramic particles has been studied using computational fluid dynamics. The plasma jet is modeled as a mixture of Ar-H2 plasmas issuing into a quiescent atmosphere. The particles, modeled as micron-sized spheres, are introduced into the jet outside the plasma gun exit with radial injection. The existence of a simple target in front of the plasma gun is taken into account. The trajectories and state histories of particles of various sizes during their flight through the jet are presented. Moreover, the solid-liquid interface within the particles is tracked in an attempt to predict the amount of unmelted material retained in these particles at various axial distances from the gun exit. The effects of turbulence in the jet on these particle histories are accounted for. It is shown that, for the range of particle sizes and the plasma gun operating conditions studied, both the deposition location and the retained unmolten fraction are strongly affected by the size of the particles. The predictions are significant in terms of showing general trends, which will be useful in identifying processing windows for producing optimally nanostructured coatings.

Application of plasma spraying to tubular-type solid oxide fuel cells production

Abstract: Solid oxide fuel cells (SOFCs) feature the highest energy conversion efficiency of any type of fuel cell yet developed. This article describes SOFC production by means of plasma spraying and presents the resulting SOFC performance. The application of plasma spraying to tubular SOFC production has realized good performance on the order of 40 W or greater in terms of electricity generation per cell stack under standard conditions of 200 mA/cm2.

Reliability of laser flash thermal diffusivity measurements of the thermal barrier coatings

Abstract: The thermal diffusivity of free standing thermal barrier coatings (TBCs) was measured by the laser flash technique. The combination of low thermal conductivity (1 to 2 W/m K) and small TBC thickness (300 to 600 µm thick) can cause errors in the measurements. Back surface (opposite the laser) temperatures of free standing plasma-sprayed TBCs were measured as a function of time and laser power. The front surface temperatures were calculated using thermal transport equations. In the high power region, thermal diffusivity decreased significantly with increasing laser power. In the moderate power region, thermal diffusivity remained constant. In the low power region, measurement became unreliable because of noise. The detector nonlinearity was believed to be a possible cause of deviation in the high power region. Measurements at different laser power levels should be conducted in order to obtain reliable thermal diffusivity values for TBCs.

Anisotropic thermal conductivities of plasma-sprayed thermal barrier coatings in relation to the microstructure

Abstract: Anisotropic thermal conductivities of the plasma-sprayed ceramic coating are explicitly expressed in terms of the microstructural parameters. The dominant features of the porous space are identified as strongly oblate (cracklike) pores that tend to be either parallel or normal to the substrate. The scatter in pore orientations is shown to have a pronounced effect on the effective conductivities. The established quantitative microstructure-property relations, if combined with the knowledge of the processing parameters-resulting microstructure connections, can be utilized for controlling the conductivities in the desired way.

Droplet splashing during arc spraying of steel and the effect on deposit microstructure

Abstract: The mechanism by which droplet deposition occurs is important when filling substrate features for the electric arc spray forming of steel tooling. Particle image velocimetry and high-speed video imaging techniques have been used to observe droplet deposition, particularly with regard to the behavior of droplets originating from splashing. Droplet splashing on deposition has been seen to be significant, and splash droplets form a large proportion of the overspray. The splash droplets are smaller and, when first created, move slower than the parent droplet. When spraying into deep features, the lateral and upward movement of splash droplets acts as a mechanism for deposit formation onto surfaces in shadow from the main spray. Microstructural study has shown that oxidation of the splash droplets before redeposition leads to a deposit with a high fraction of oxide. Simultaneous growth of deposit formed directly from the spray, and from splash droplets, results in a banded microstructure containing elongated macropores. A mechanism for such microstructural evolution is proposed.

Development of thermal spraying-sintering technology for solid oxide fuel cells

Abstract: A thermal spraying-sintering process has been developed for an electrolyte and interconnect layer, which results in improved gas tightness, a thinner layer, and higher electric conductivity as required for solid oxide fuel cells (SOFCs). The process is characterized by the heat treatment of composition-controlled plasma-sprayed layers. For the electrolyte, the addition of MnO2 to zirconia powder is effective for reducing the sintering temperature to obtain gas tightness and for suppressing the reaction between zirconia and air electrode material. An electrolyte layer of 60 µm thickness with sufficient gas tightness and high ionic conductivity was obtained by this process. For the interconnect, chromium-rich lanthanum chromite powder, La0.8Ca0.2Cr1.10O3, is optimum for both gas tightness and high electric conductivity of the layer. In addition, a single cell with a 60 µm electrolyte was successfully fabricated using the thermal spraying-sintering process. As a result of an operating test using O2 and humidified H2 at 1000°C, a power density of 0.73 W/cm2 was obtained. It was demonstrated that the thermal spraying-sintering technology is effective for the fabrication of a thin gas tight layer for SOFCs.

Formation of Amorphous Fe-Cr-Mo-8P-2C Coatings by the High Velocity Oxy-Fuel Process

Abstract: Alloy powders of Fe-10%Cr-8%P-2%C(10Cr), Fe-20%Cr-8%P-2%C(20Cr), and Fe-10%Cr-10%Mo-8%P-2%C(10Mo) compositions (in mass%) were sprayed by the high velocity oxy-fuel (HVOF) process under different conditions. The as-sprayed coatings of 10Mo alloy were composed of only an amorphous phase under all the spray conditions, while the as-sprayed coatings of the 10Cr and 20Cr alloys consisted of an amorphous phase with a small amount of crystalline material. The volume fraction of the crystalline material increased slightly with the rise of the flame temperature. The hardnesses of the as-sprayed coatings of the 10Cr and 20Cr alloys were 600 to 700 DPN, respectively, while the 10Mo coating composed of an amorphous phase revealed 560 DPN. The corrosion resistance of the as-sprayed coating of the 10Mo alloy was the best among three amorphous coatings and also superior to the nickel-base self-fluxing alloy and SUS316L stainless steel coatings in 1N H2SO4 and 1N HCl solutions.

Nanopowder deposition by supersonic rectangular jet impingement

Abstract: With a view toward developing the next generation of coatings using nanopowders, a cold gas dynamic spray (CGDS) technique has been investigated. In this method, a powder feeder is used to inject nanopowder agglomerates into a supersonic rectangular jet, with a design Mach number of 3.2. The powder particles gain speeds of up to 700 m/s through momentum transfer from the jet and bond to the substrate surface due to kinetic energy dissipation. Coatings of copper and nano-WC/10% Co on steel and aluminum substrates (3 to 5 µm in thickness) have been produced. The benefit of this process is that the material does not undergo any chemical changes during coating formation. To improve the quality of the coatings produced, the flapping motions produced by supersonic jet impingement were studied. Powder particle velocities and the jet impingement flow field were quantified using particle image velocimetry (PIV).

Development of metallic substrate supported planar solid oxide fuel cells fabricated by atmospheric plasma spraying

Abstract: A planar solid oxide fuel cell (SOFC) consisting of a cell supported with a porous metallic substrate and a metallic separator has been developed. In the fabrication of the cell, anodes and electrolytes were formed on sintered Ni-felt substrates using flame spraying (FS) and atmospheric plasma spraying (APS), respectively. The APS is also applied to form (LaSr)MnO3 protective coatings on the metallic separators. With these metallic cells and separators, a 3 kW-class stack, which consisted of 30 cells (15-cell block×2) was constructed and operated. The active electrode area of the cell was 600 cm2. The stack generated 3.3 kW at 970 °C when the current density was 0.3 Acm−2 and the fuel utilization 50%. It did not show any degradation for the initial 2100 h, but a few cells in the lower 15-cell block became unstable after 2100 h. On the other hand, the upper 15-cell block was stably operated for 3200 h.

Characterization of plasma-sprayed hydroxyapatite/TiO 2 composite coatings

Abstract: To enhance the bonding between hydroxyapatite (HA) coating and titanium alloy substrate, HA/TiO2 composite coatings have been fabricatedvia plasma spraying. Bonding strength evaluation, simulated body fluid tests, and cell culturein vitro were carried out to characterize the composite coatings. The results obtained showed that the addition of TiO2 to HA coating improved the bonding strength of the coating significantly. After being immersed in simulated body fluid (SBF) for a period, the surfaces of HA/TiO2 composite coatings were completely covered by carbonate-containing apatite, which indicated that the coatings possess good bioactivity. Thein vitro cell culture indicated good cytocompatibility for HA/TiO2 composite coatings.

Improving corrosion properties of high-velocity oxy-fuel sprayed inconel 625 by using a high-power continuous wave neodymium-doped yttrium aluminum garnet laser

Abstract: Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as high-velocity oxy-fuel (HVOF) and plasma spraying, these coatings may in certain service conditions show inadequate performance,e.g., due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermalsprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal-sprayed coating material. To overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coatings was studied in the present work. The coating material was nickel-based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by HVOF spraying onto mild steel substrates. High-power continuous wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical microscopy and scanning electron microscopy (SEM). Laser remelting resulted in homogenization of the sprayed structure. This strongly improved the performance of the laser-remelted coatings in adhesion, wet corrosion, and high-temperature oxidation testing. The properties of the laser-remelted coatings were compared directly with the properties of as-sprayed HVOF coatings and with plasma-transferred arc (PTA) overlay coatings and wrought Inconel 625 alloy.

Study of plasma- and detonation gun-sprayed alumina coatings using Taguchi experimental design

Abstract: Atmospheric plasma spraying (APS) is a most versatile thermal spray method for depositing alumina (Al2O3) coatings, and detonation gun (D-gun) spraying is an alternative thermal spray technology for depositing such coatings with extremely good wear characteristics The present study is aimed at comparing the characteristics of Al2O3 coatings deposited using the above techniques by using Taguchi experimental design Alumina coating experiments were conducted using a Taguchi fractional-factorial (L8) design parametric study to optimize the spray process parameters for both APS and D-gun The Taguchi design evaluated the effect of four APS and D-gun spray variables on the measured coating attributes The coating qualities evaluated were surface roughness, porosity, microhardness, abrasion, and sliding wear The results show that the coating quality is directly related to the corresponding coating microstructure, which is significantly influenced by the spray parameters employed Though it is evident that the D-gun-sprayed coatings consistently exhibit dense and uniform microstructure, higher hardness, and superior tribological performance, the attainment of suitable plasma-sprayed coatings can be improved by employing the Taguchi analysis

Plasma spraying of functionally graded yttria stabilized zirconia/NiCoCrAlY coating system using composite powders

Abstract: Pre-alloyed and plasma spheroidized composite powders were used as the feedstock in the plasma spraying of functionally graded yttria stabilized zirconia (YSZ)/NiCoCrAlY coatings. The ball milling parameters of the composite powders and the plasma spraying parameters for preparing functionally graded materials (FMGs) coatings were optimized to obtain the best performance for the thermal barrier coatings (TBCs). Microstructure, physical, mechanical, and thermal properties of YSZ/NiCoCrAlY FGMs coatings were investigated and compared with those of traditional duplex coatings. Results showed that the advantages of using pre-alloyed composite powders in plasma spraying were to ensure chemical homogeneity and promote uniform density along the graded layers. Microstructure observation showed the gradient distribution of YSZ and NiCoCrAlY phases in the coating, and no clear interface was found between two adjacent different layers. Oxidation occurred during plasma spray and the resultant aluminum oxide combines with YSZ in a wide range of proportions. The bond strength of functionally graded coatings was about twice as high as that of the duplex coatings because of the significant reduction of the residual stresses in the coatings. The thermal cycling resistance of functionally graded coating was much better than that of duplex coating.

Multimodal coatings : A new concept in thermal spraying

Abstract: Recently, considerable emphasis has been placed on HVOF thermal spraying of nanostructured WC/Co with the intention of achieving high hardness combined with excellent wear resistance. However, depositing dense coatings and simultaneously preventing decarburization has remained a challenge. We have approached the problem by developing a novel feedstock material that consists of a mixture of coarse and fine particles of WC/Co. Particles of different sizes have a different response in the combustion flame. The resultant coating is dense and has no decarburized phases. The abrasion wear resistance is at least 50% better than that of a pure coarse-grained WC/Co coating.

Evaluation of thermal barrier coating systems on novel substrates

Abstract: Testing was conducted on both plasma-sprayed (PS) and electron beam-physical vapor deposited (EB-PVD) Y2O3-stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) applied directly to oxidation-resistant substrates such as β-NiAl, oxide-dispersed FeCrAl, and NiCr. On an alloy that forms a very adherent alumina scale, β-NiAl+Zr, the coating lifetime of YSZ in furnace cyclic tests was 6 or more times longer than on state-of-the-art, YSZ coatings on single-crystal Ni-base superalloys with MCrAlY or Pt aluminide bond coats. Coatings on FeCrAl alloys appear to be a viable option for applications such as the external skin of the X-33, single stage to orbit, reusable launch vehicle. Model chromia-forming bond coat compositions also show promise for power generation applications at temperatures where hot corrosion may be a major problem. In general, while this work examined unique materials systems, many of the same fundamental failure mechanisms observed in conventional TBCs were observed.

Diagnostics and modeling of an argon/helium plasma spray process

Abstract: The natural instability of the are in direct current (DC) plasma torches used in spray processing is one of the most important causes for variations in heating of sprayed particles, leading to inconsistencies in the final coating quality. A relatively simple diagnostic system has been set up to monitor the plasma jet instability, as well as some important process characteristics. Effects of the operating parameters and the anode condition on properties of plasma jets, particle properties, and coatings have been measured. These results show that the inconsistency caused by the jet instability influences the plasma spray process in several ways. The coating porosity and the deposition efficiency can be correlated to an average jet length obtained from a series of high speed images. Selected frequency peaks in the power spectrum of the acoustic signal are correlated with the average jet length, and these results are used to derive a simple control scheme, which adopts a fuzzy look-up model indicating the condition of the anode. Increasing the are current is the most effective way to counteract the negative effects of anode erosion.

Microstructure and Polarization Resistance of Thermally Sprayed Composite Cathodes for Solid Oxide Fuel Cell Use

Abstract: Porous composite cathode coatings containing (La0.8 Sr0.2)0.98MnO3 (LSM) and ZrO2-12% Y2O3 (YSZ) were prepared by vacuum plasma spraying (VPS) and flame spraying (FS) on prefabricated substrate-based planar solid oxide fuel cells (SOFC) with 60 mm in diameter. Microstructural observations reveal the open porosity of the cathode coatings and prove qualitatively the compositional gradient from YSZ-LSM composite to pure LSM. The electrochemical behavior was investigated by impedance spectroscopy. The results of graded cathodes compared with nongradient and bilayered ones are discussed with respect to the cathodic polarization resistance between 750 and 950°C. Bilayered cathodes indicate the lowest cathodic losses followed by the graded ones and the conventional composite. Flame spraying as a rarely used processing tool for SOFC components can provide cathodes of high electrochemical performance.

Development of refractory silicate-yttria-stabilized zirconia dual-layer thermal barrier coatings

Abstract: Development of advanced thermal barrier coatings (TBCs) is the most promising approach for increasing the efficiency and performance of gas turbine engines by enhancing the temperature capability of hot section metallic components. Spallation of the yttria-stabilized zirconia (YSZ) top coat, induced by the oxidation of the bond coat coupled with the thermal expansion mismatch strain, is considered to be the ultimate failure mode for current state-of-the-art TBCs. Enhanced oxidation resistance of TBCs can be achieved by reducing the oxygen conductance of TBCs below that of thermally grown oxide (TGO) alumina scale. One approach is incorporating an oxygen barrier having an oxygen conductance lower than that of alumina scale. Mullite, rare earth silicates, and glass ceramics have been selected as potential candidates for the oxygen barrier. This paper presents the results of cyclic oxidation studies of oxygen barrier/YSZ dual-layer TBCs.

Integrated fabrication process for solid oxide fuel cells in a triple torch plasma reactor

Abstract: This paper describes an approach for an integrated manufacturing process for solid oxide fuel cells. The approach is based on successively depositing the different layers of the cell using plasma deposition processes in a controlled-atmosphere chamber. Cells have been manufactured following this approach with minimal changes in process conditions for the different processes. The cells have been evaluated with regard to their materials characteristics and with regard to their electrical performance. The cell performance has been acceptable, with open circuit voltages of about 1 V and power densities between 325 and 460 mW/cm2. Process modifications to improve the performance further are possible. The described process has the potential for being easily automated.