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The occurrence of monoclinic zirconia phase has an important impact on the performance of thermal barrier coatings (TBC) of yttria-stabilized zirconia (YSZ). Therefore, a reliable method is needed to detect its contents and to investigate also its spatial distribution within the parent microstructure. This was the motivation to apply cathodoluminescence (CL) spectroscopy. YSZ coatings with different porosities were manufactured by atmospheric plasma spraying. CL analysis yielded monoclinic phase contents of 5.2 ± 1.6% for the high-porous sample and 3.4 ± 0.5% for the low-porous sample. The results were qualitatively confirmed by x-ray diffraction (XRD). However, due to its lower detection sensitivity the XRD results are quantitatively on lower level. Owing to its synthesis method, the applied powder feedstock showed a considerable content of monoclinic phase. The lower the particle temperatures were the larger fraction of monoclinic phase remained untransformed. This has to be considered when spraying high-porous TBCs. [ABSTRACT FROM AUTHOR]

In an effort to improve the performance of heat engines at high temperatures, advanced surface coatings have been developed from complex perovskites. Materials of Ba(Mg1/3Ta2/3)O3 and La(Al1/4Mg1/2Ta1/4)O3 composition were synthesized and applied as ceramic topcoats of thermal barrier coating (TBC) systems by atmospheric plasma spraying (APS) in single layer and in double-layer combination with conventional yttria stabilized zirconia (YSZ). Microstructural and phase analyses reveal that plasma spraying of complex perovskites is accompanied with the formation of vertical crack networks and secondary oxide phases which influence the failure mechanism of the TBCs. The low value of fracture toughness for the complex perovskites and the thermally grown oxide at the topcoat-bondcoat interface of the TBCs are, however, the major factors which lead to the coating failure on thermal cycling at about 1250 °C. [ABSTRACT FROM AUTHOR]

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat-isolative topcoat. Several recent research activities are concentrating on developing improved bondcoat or topcoat materials; for the topcoat especially, those with reduced thermal conductivity are investigated. Using advanced topcoat materials, the ceramic coating can be further divided into layers with different functions. One example is the double-layer system in which conventional yttria-stabilized zirconia (YSZ) is used as bottom and new materials such as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. In addition, new functions are introduced within the TBCs. These can be sensorial properties that can be used for an improved temperature control or even for monitoring remaining lifetime. Further increased application temperatures will also lead to efforts for a further improvement of the reflectivity of the coatings to reduce the radiative heat transfer through the TBC. [ABSTRACT FROM AUTHOR]

High-performance solid oxide fuel cells require a thin and gas-tight electrolyte membrane that must be coated on a porous and relatively rough support. A pretreatment of the delivered submicronmeter electrolyte powder of 8 mol%-yttria-stabilized zirconia (8YSZ) yielded a reduced sintering mismatch between the anode substrate made from NiO/8YSZ and the electrolyte coating. Furthermore, it also enhanced the powder packing inside the green film. Constrained sintering usually leads to inadequate film density and an unfavorable pore deformation and orientation. It was demonstrated that these limitations can be resolved by using a coshrinking substrate in a planar cell design. Relative densities of >97% were achieved, which are higher than those for free-standing layers. Additionally, the camber behavior was investigated in dependence of the temperature program with and without gravity effects, giving an overall suggestion for the cofiring parameters of the electrolyte. [ABSTRACT FROM AUTHOR]

Perovskite-type SrZrO3 was investigated as an alternative to yttria-stabilized zirconia (YSZ) material for thermal barrier coating (TBC) applications. Three phase transformations (orthorhombic↔pseudo-tetragonal↔tetragonal↔cubic) were found only by heat capacity measurement, whereas the phase transformation from orthorhombic to pseudo-tetragonal was found in thermal expansion measurements. The thermal expansion coefficients (TECs) of SrZrO3 coatings were at least 4.5% larger than YSZ coatings up to 1200°C. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense SrZrO3 showed lower Young's modulus, hardness, and comparable fracture toughness with respect to YSZ. The “steady-state” sintering rate of a SrZrO3 coating at 1200°C was 1.04 × 10−9 s−1, which was less than half that of YSZ coating at 1200°C. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner showed that at operating temperatures ∼1250°C the cycling lifetime of SrZrO3/YSZ double-layer coating (DLC) was more than twice as long as SrZrO3 coating and comparable to YSZ coating. However, at operating temperatures >1300°C, the cycling lifetime of SrZrO3/YSZ DLC was comparable to the optimized YSZ coating, indicating SrZrO3 might be a promising material for TBC applications at higher temperatures compared with YSZ. [ABSTRACT FROM AUTHOR]

Abstract: The interdependence between plasma spray process parameters and porosity of YSZ coating microstructures was investigated with simultaneous consideration of the deposition efficiency. Based on a factorial experimental plan, the argon plasma gas flow, the current, the interaction of argon flow and current, and the spray distance for the Triplex II plasma gun were found to yield the main contributions to porosity as well as to deposition efficiency. Each of these three process parameters has a significant individual effect on the in-flight particle velocities and temperatures. The contribution to the effects on porosity arises almost exclusively from the particle temperature. Regarding the deposition efficiency, the larger contribution originates from the particle velocity. To achieve a targeted high porosity at reasonable deposition efficiency a simple linear regression model was applied yielding an argon flow of 50slpm and a current of 470A at a spray distance of 200mm as the optimum parameter set. The average particle temperature estimated for this optimum is just above the melting temperature. At this setting, a porosity of 17.7% and a deposition efficiency of 32.5% may be expected. At a greater spray distance and lower power density (lower current and/or higher argon plasma gas flow) the deposition efficiency was observed to drop considerably. The cooling of the particles here becomes critical, i.e. the particles are only partly molten. This was verified by an analysis of the density distributions of measured in-flight particle temperatures. [Copyright &y& Elsevier]

Abstract: The recently introduced three-cathode TriplexPro™-200 atmospheric plasma spray gun was tested with yttria-stabilized zirconia. The effects on the particle characteristics (velocity and surface temperature) were measured by time-of-flight measurement and two-color pyrometry, respectively (DPV-2000 diagnostic system) while the arc current intensity, the plasma gas flow, and its composition were systematically varied. With typical spray parameters of the Triplex® II, using the TriplexPro™-200 with the 9.0 mm nozzle the particle characteristics were found to be almost the same while the nozzle still provides power reserves. In this case, helium may be dispensed with entirely. Using the 6.5 mm or the 5.0 mm nozzle operation is limited to a maximum 500 A current, which significantly increased particle velocities, but it was not possible to keep the particles fully molten. The investigated range of powder grain size (d 50 =58 μm) is to be considered with these parameters. Further possibilities are to be expected with the 6.5 mm and 5.0 mm nozzle using smaller-sized or lower-melting powders. Operating the TriplexPro™-200 gun with the 11.0 mm nozzle and/or nitrogen as secondary plasma gas instead of helium, increased particle temperatures and velocities are achievable. This offers extended possibilities to spray high-melting oxide ceramics like yttria-partially stabilized zirconia. [Copyright &y& Elsevier]

Abstract: The application of a thin film electrolyte layer with a thickness in the micrometer range could greatly improve current solid oxide fuel cells (SOFCs) in terms of operating temperature and power output. Since the achievable minimal layer thickness with conventional powder coating methods is limited to ∼5μm, a variety of thin film methods have been studied, but results on regular large-scale anode substrates are still lacking in the literature. In this paper, a wet coating process is presented for fabricating gas-tight 1–2μm thick 8YSZ electrolyte layers on a regular NiO/8YSZ substrate, with a rough surface, a high porosity and a large pore size. These layers were deposited in a similar way as conventional suspension based layers, but the essential difference includes the use of coating liquids (nano-dispersion, sol) with a considerably smaller particle size (85nm, 60nm, 35nm, 6nm). Successful deposition of such layers was accomplished by means of an innovative coating process, which involves the preparation of a hybrid polyvinyl alcohol/8YSZ membrane by dip-coating or spin-coating and subsequently burning out the polymer part at 500°C. Results from He leak tests confirmed that the sintered layers posses a very low number of defects and with values in the range 10−4–10−6 (hPadm3)/(scm2) the gas-tightness of the thin film layers is satisfactory for fuel cell operation. Moreover, preliminary results have also indicated a potential reduction of the sintering temperature from 1400°C to the range 1200–1300°C, using the presented coating process. [Copyright &y& Elsevier]

Abstract: Benefits and limitations of process diagnostics are investigated for the suspension plasma spraying of yttria-stabilized zirconia thermal barrier coatings. The methods applied were enthalpy probe measurements, optical emission spectroscopy, and in-flight particle diagnostic. It was proved that the plasma characteristics are not affected negatively by the injection of the ethanol based suspension since the combustion of species resulting from ethanol decomposition achieves a gain of plasma enthalpy. Furthermore, the conditions of the suspension injection into the plasma were found to be optimum as a significant content of evaporated powder material could be detected. Regarding the void content and segmentation crack density of the coatings, the in-flight particle diagnostic showed that the spray distance should be dimensioned in a way that the molten particles reach the substrates just before solidification starts. [ABSTRACT FROM AUTHOR]

Abstract: During the last decade a number of ceramic materials, mostly oxides have been suggested as new thermal barrier coating (TBC) materials. These new compositions have to compete with the state-of-the-art TBC material yttria stabilized zirconia (YSZ) which turns out to be difficult due to its unique properties. On the other hand YSZ has certain shortcomings especially its limited temperature capability above 1200°C which necessitates its substitution in advanced gas turbines. In the paper an overview is tried on different new materials covering especially doped zirconia, pyrochlores, perovskites, and aluminates. Literature results and also results from our own investigations will be presented and compared to the requirements. Finally, the double-layer concept, a method to overcome the limited toughness of new TBC materials, will be discussed. [ABSTRACT FROM AUTHOR]