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Lanthanum zirconate (LaZrO) has been proposed as a promising material for thermal barrier coatings. During atmospheric plasma spraying (APS) of LaZrO a considerable amount of LaO can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the LaO-ZrO system, in the composition range of pyrochlore structure, the stoichiometric LaZrO has the highest melting point and other compositions are eutectic. APS experiments were performed with a TriplexPro™-200 plasma torch at different power levels to achieve different degrees of evaporation and thus stoichiometry. For comparison, some investigations on gadolinium zirconate (GdZrO) were included, which is less prone to evaporation and formation of non-stoichiometry. Particle temperature distributions were measured by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected at specific torch input powers indicating evaporation and solidification processes. Based on this, process parameters can be defined to provide stoichiometric coatings that show good thermal cycling performance. [ABSTRACT FROM AUTHOR]

The thin film low pressure plasma spray process (LPPS-TF) has been developed with the aim of efficient depositing uniform and thin coatings with large area coverage by plasma spraying. At high power input (~150 kW) and very low pressure (~100 Pa) the plasma jet properties change considerably and it is even possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition. In addition, the resulting microstructures are unique and can hardly be obtained by other processes. In this paper, microstructures made by LPPS-TF are shown and the columnar layer growth by vapor deposition is demonstrated. In addition to the ceramic materials TiO, AlO or MgAlO, the focus of the research was placed on partially yttria-stabilized zirconia. Variations of the microstructures are shown and discussed concerning potential coating applications. [ABSTRACT FROM AUTHOR]

In the field of thermal barrier coatings (TBCs) for gas turbines, suspension plasma sprayed (SPS) submicrometer-structured coatings often show unique mechanical, thermal, and optical properties compared to conventional atmospheric plasma sprayed ones. They have thus the potential of providing increased TBC performances under severe thermo-mechanical loading. Experimental results showed the capability of SPS to obtain yttria stabilized zirconia coatings with very fine porosity and high density of vertical segmentation cracks, yielding high strain tolerance, and low Young's modulus. The evolution of the coating microstructure and properties during thermal cycling test at very high surface temperature (1400 °C) in our burner rigs and under isothermal annealing was investigated. Results showed that, while segmentation cracks survive, sintering occurs quickly during the first hours of exposure, leading to pore coarsening and stiffening of the coating. In-situ measurements at 1400 °C of the elastic modulus were performed to investigate in more detail the sintering-related stiffening. [ABSTRACT FROM AUTHOR]

Thermal spray coatings from liquid feedstock such as suspensions and solution precursors have received increasing interest due to the unique coating properties obtainable by these processes. Several research groups are working on the basis of plasma as well as on high-velocity oxy-fuel approaches to manufacture advanced nanostructured and nanophased materials. These activities are reflected in various recent publications and conference presentations about feedstock preparation, equipment and process design, modeling techniques, in-process diagnostics, coating characterization, and emerging applications. This article will review these recent developments to give an up-to-date overview and to trace the current trends. [ABSTRACT FROM AUTHOR]

Highly porous NiTi alloys with pseudoelastic properties are attractive candidates for biomedical implants, energy absorbers, or damping elements. Recently, a new method was developed for net-shape manufacturing of such alloys combining metal injection molding with the application of suitable space-holder materials. A comprehensive study of mechanical properties was conducted on samples with a porosity of 51% and a pore size in the range of 300-500 μm. At low deformations <6%, fully pronounced pseudoelasticity was found. Even at higher strains, a shape recovery of maximum 6% took place, on which the onset of irreversible plastic deformation was superposed. Results of static compression tests were also used to calculate the energy-absorbing capacity. Fatigue of porous NiTi was investigated by cyclic loading up to 230,000 stress reversals. The failure mechanisms responsible for a reduction of shape recovery after an increased number of load cycles are discussed. [ABSTRACT FROM AUTHOR]

Growing demands on the quality of thermally sprayed coatings require reliable methods to monitor and optimize the spraying processes. Thus, the importance of diagnostic methods is increasing. A critical requirement of diagnostic methods in thermal spray is the accurate measurement of temperatures. This refers to the hot working gases as well as to the in-flight temperature of the particles. This article gives a review of plasma and particle temperature measurements in thermal spray. The enthalpy probe, optical emission spectroscopy, and computer tomography are introduced for plasma measurements. To determine the in-flight particle temperatures mainly multicolor pyrometry is applied and is hence described in detail. The theoretical background, operation principles and setups are given for each technique. Special interest is attached to calibration methods, application limits, and sources of errors. Furthermore, examples of fields of application are given in the form of results of current research work. [ABSTRACT FROM AUTHOR]

solid oxide fuel cell (SOFC) with a thin-film yttria-stabilized zirconia (YSZ) electrolyte was developed and tested. This novel SOFC shows a similar multilayer set-up as other current anode-supported SOFCs and is composed of a Ni/8YSZ anode, a gas-tight 8YSZ electrolyte layer, a dense Sr-diffusion barrier layer and a LSCF cathode. To increase the power density and lower the SOFC operating temperature, the thickness of the electrolyte layer was reduced from around 10 μm in current cells to 1 μm, using a nanoparticle deposition method. By using the novel 1 μm electrolyte layer, the current density of our SOFC progressed to 2.7, 2.1 and 1.6 A/cm at operation temperatures of 800, 700 and 650°C, respectively, and out-performs all similar cells reported to date in the literature. An important consideration is also that cost-effective dip-coating and spin-coating methods are applied for the fabrication of the thin-film electrolyte. Processing of 1 μm layers on the very porous anode substrate material was initially experienced as very difficult and therefore 8YSZ nanoparticle coatings were developed and optimized on porous 8YSZ model substrates and transferred afterwards to regular anode substrates. In this paper, the preparation of the novel SOFC is shown and its morphology is illustrated with high resolution SEM pictures. Further, the performance in a standard SOFC test is demonstrated. [ABSTRACT FROM AUTHOR]

Powder injection parameters such as gas flow, injection angle, and injector position strongly influence the particle beam and thus coating properties. The interaction of the injection conditions on particle properties based on DPV-2000 measurements using the single-cathode F4 torch is presented. Furthermore, the investigation of the plasma plume by emission computer tomography is described when operating the three-cathode TriplexPro™ torch. By this imaging technology, the three-dimensional shape of the radiating plasma jet is reproduced based on images achieved from three CCD cameras rotating around the plume axis. It is shown how the formation of the plasma jet changes with plasma parameters and how this knowledge can be used to optimize particle injection. [ABSTRACT FROM AUTHOR]

The very low-pressure plasma Spray (VLPPS) process has been developed with the aim of depositing uniform and thin coatings with coverage of a large area by plasma spraying. At typical pressures of 100-200 Pa, the characteristics of the plasma jet change compared to conventional low-pressure plasma-spraying processes (LPPS) operating at 5-20 kPa. The combination of plasma spraying at low pressures with enhanced electrical input power has led to the development of the LPPS-TF process (TF = thin film). At appropriate parameters, it is possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This technique offers new possibilities for the manufacturing of thermal barrier coatings (TBCs). Besides the material composition, the microstructure is an important key to reduce thermal conductivity and to increase strain tolerance. In this regard, columnar microstructures deposited from the vapor phase show considerable advantages. Therefore, physical vapor deposition by electron beam evaporation (EB-PVD) is applied to achieve such columnar-structured TBCs. However, the deposition rate is low, and the line-of-sight nature of the process involves specific restrictions. In this article, the deposition of TBCs by the LPPS-TF process is shown. How the evaporation of the feedstock powder could be improved and to what extent the deposition rates could be increased were investigated. [ABSTRACT FROM AUTHOR]

This article summarises recent developments in solid oxide fuel cell research regarding materials, processing and microstructure–property relationships. In the materials section, the various cell and stack materials are briefly described, i.e. electrolytes, electrodes, contact and protective layers, interconnects and sealing materials. The section on processing gives an overview of manufacturing technologies for cells including a view of different substrate materials and designs. Besides the widely used planar cell designs, the technologies for tubular designs are also described. In addition, the technologies are grouped with respect to the support, e.g. metal- or ceramic–metal (cermet anode substrate)-supported SOFCs. Finally, special emphasis is laid on the microstructure of functional layers which primarily govern the power output of the SOFC. [ABSTRACT FROM AUTHOR]

Abradable coatings in compressor and high-pressure stages of gas turbines must provide specific hardness and porosity values to achieve an optimal cut-in of the blade tips. A fractional factorial experimental plan was designed to investigate the influence of the plasma spraying parameters argon flow rate, current, spraying distance and powder feed rate on these properties of magnesia spinel. Based on the results, magnesia spinel coatings with low (~400 HV0.5), medium (~600 HV0.5) and high hardness (~800 HV0.5) could be reliably manufactured. Further incursion rig tests confirmed the dependence of the rub-in behavior and abradability on the coating characteristics and process parameters, respectively. Process monitoring was also applied during plasma spraying of magnesia spinel abradables on batches of turbine components. The recorded particle characteristics and coating properties showed a good reproducibility of the spraying process. [ABSTRACT FROM AUTHOR]

Suspension plasma spraying (SPS) offers the manufacture of unique microstructures which are not possible with conventional powdery feedstock. Due to the considerably smaller size of the droplets and also the further fragmentation of these in the plasma jet, the attainable microstructural features like splat and pore sizes can be downsized to the nanometer range. Our present understanding of the deposition process including injection, suspension plasma plume interaction, and deposition will be outlined. The drawn conclusions are based on analysis of the coating microstructures in combination with particle temperature and velocity measurements as well as enthalpy probe investigations. The last measurements with the water cooled stagnation probe gives valuable information on the interaction of the carrier fluid with the plasma plume. Meanwhile, different areas of application of SPS coatings are known. In this paper, the focus will be on coatings for energy systems. Thermal barrier coatings (TBCs) for modern gas turbines are one important application field. SPS coatings offer the manufacture of strain-tolerant, segmented TBCs with low thermal conductivity. In addition, highly reflective coatings, which reduce the thermal load of the parts from radiation, can be produced. Further applications of SPS coatings as cathode layers in solid oxide fuel cells (SOFC) and for photovoltaic (PV) applications will be presented. [ABSTRACT FROM AUTHOR]

The very low pressure plasma spray (VLPPS) process operates at a pressure range of approximately 100 Pa. At this pressure, the plasma jet interaction with the surrounding atmosphere is very weak. Thus, the plasma velocity is almost constant over a large distance from the nozzle exit. Furthermore, at these low pressures the collision frequency is distinctly reduced and the mean free path is strongly increased. As a consequence, at low pressure the specific enthalpy of the plasma is substantially higher, but at lower density. These particular plasma characteristics offer enhanced possibilities to spray thin and dense ceramics compared to conventional processes which operate in the pressure range between 5 and 20 kPa. This paper presents some examples of gas-tight and electrically insulating coatings with low thicknesses <50 μm for solid oxide fuel cell applications. Furthermore, plasma spraying of oxygen conducting membrane materials such as perovskites is discussed. [ABSTRACT FROM AUTHOR]

Atmospherically sprayed thermal barrier coatings (TBCs) are nowadays an essential part in modern gas turbines. However, a design integrated use of these coatings is only possible with reliable lifetime models. In this paper, a model is outlined which describes the major failure in TBCs associated with the growth of a thermally grown oxide (TGO) on the bond coat (BC). An essential part of the model is a simplified description of the crack growth as a result of thermal cycling and TGO growth. In addition, the energy release rate for the system is calculated and compared to an estimated critical energy release rate reduced by the crack growth. If both are equal, failure is assumed. The results of the modeling are compared to thermal cycling experiments partly applying a thermal gradient. BC temperatures and also microstructures of the ceramic topcoat have been varied and the influence on the cyclic life studied. [ABSTRACT FROM AUTHOR]

Perovskite-type SrZrO3 has been investigated as a candidate material for thermal barrier coating application. During plasma spraying of SrZrO3, SrO volatilized more than ZrO2 and the coating composition deviates from initial stoichiometry. In this investigation, partial evaporation was investigated by spraying SrZrO3 powders into water. The influences of spraying current, distance and particle size of the powder on the partial evaporation were also investigated in a quantitative way. With optimized spraying parameters, stoichiometric SrZrO3 coating was produced by adding an excess amount of Sr in the precursors before plasma spraying to compensate for the volatilized component. [ABSTRACT FROM AUTHOR]

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]

Wear at the electrode surfaces of a one-cathode plasma torch changes the characteristic fluctuation pattern of the plasma jet. This affects the trajectory of the particles injected into the plasma jet in a non-controllable way, which degrades the reproducibility of the process. Time-based voltage measurements and Fourier analysis were carried out on a one-cathode F4 torch at different wear conditions to determine the evolution of wear dependant characteristics. A significant correlation is observed between increasing torch wear and decreasing voltage roughness and high frequency noise. Furthermore, by means of particle diagnostic systems, the change in the particle velocity and temperature has been measured. The variations of the particle characteristics are significant and thus an influence on the sprayed coating microstructure is to be expected. [ABSTRACT FROM AUTHOR]

Comparative in-flight measurements of particles plasma sprayed by F4 and Triplex II guns were carried out using the diagnostic systems DPV-2000 and Accuraspray-g3. The comparison of mean particle velocities and temperatures as well as intensity profiles of the plume show a good agreement confirming their accuracy. However, the varying operating principles especially the deceased dimensions of the measurement volumes have to be considered carefully when evaluating the results. Furthermore, some applications of the diagnostic systems DPV-2000 and Accuraspray-g3 are shown. Finally some application limits, which were identified for certain powder compositions at higher plasma power levels will be discussed. [ABSTRACT FROM AUTHOR]

It is generally known that the porosity of thermal barrier coatings is essential to guarantee a sufficiently high strain tolerance of the coating during thermal cycling. However, much less is known about the influence of the specific morphology of porosity, such as microcracks and typically larger pores, on the performance of the coatings. Both features are usually formed during plasma spraying of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). In this investigation, the influence of microcracks on the thermal cycling behavior was studied. The amount of microcracks within YSZ thermal barrier coatings was changed by changing the powder-feeding rate. Only small changes of the total porosity were observed. Mercury porosimetry served as a tool to investigate both the amount of microcracks and pores in the coating. Additionally, microcrack densities were determined from metallographical investigations. A linear dependence between the amount of fine pores determined by Hg porosimetry and the crack density was obtained for one set of coatings. Thermal cycling TBC specimens with different microcrack densities were produced and tested in a gas burner test facility. At high surface temperatures (above 1300 °C), failure occurred in the ceramic close to the surface. Under these conditions, the samples with increased horizontal microcrack densities showed a significant increase of thermal cycling life. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including the use of sensors in thermal spraying, the effectiveness of high velocity oxygen fuel (HVOF) to produce high performance bond coats and the developments of thermal barrier coatings.