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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]

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]

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]

Abstract: Perovskite Ba(Mg1/3Ta2/3)O3, BMT has promising bulk properties (thermal conductivity ~2W/m-K and coefficient of thermal expansion ~11×10−6/K at 1473K) for thermal barrier coating (TBC) applications at high temperature. However, during atmospheric plasma spraying (APS), such material was found to lose constituents due to the differences of vapor pressures resulting to non-stoichiometric composition of deposited coatings. To investigate the extent of phase decomposition at spray distance and varying electric arc current, different feedstock powders were plasma sprayed into water and collected for chemical, microstructural and phase analyses. When the electric arc current was decreased from 500A to 300A, the decomposition of the powders was reduced and the microstructure of the deposited coatings was improved. The thermal cycling lifetime of the deposited coatings at ~1250°C surface temperature is also higher. [Copyright &y& Elsevier]

Abstract: Plasma-sprayed ceramic coatings, used as thermal barrier or abradable coatings in high-pressure stages of gas turbines, are exposed to high thermo-mechanical loading due to harsh operating conditions. Under certain conditions they also have to withstand attack by calcium-magnesium-alumino-silicate (CMAS) deposits resulting from the ingestion of siliceous minerals with the intake air. Resistance to this kind of attack becomes more important at higher temperatures, when the melting temperature of the CMAS deposits is exceeded and a penetration into the coating microstructural features (cracks and pores) can take place. During cooling the CMAS solidifies and the coating loses its strain tolerance, which can lead to coating failure. Although the basic principles of failure seem to be understood, a detailed analysis of the mechanisms and the possibilities of avoiding delamination are still lacking, especially because there are as yet no adequate test beds. This paper investigates the possibility of testing such coatings in a burner rig test facility under thermal gradient cycling conditions and at the same time CMAS deposition. This novel and unique test approach promises a coating screening and characterization test under service conditions. The CMAS rig was established because the test conditions simulated here are closer to actual engine conditions, as compared to previous tests with primarily CMAS deposition and subsequent thermal furnace testing. The experimental setup of this new test approach is described and the applicability is confirmed. Furthermore, a first evaluation for plasma-sprayed coatings is presented. A significantly reduced lifetime was found for samples tested with CMAS attack in comparison to tests with water deposition only. The lifetime was also reduced compared to older results without any corrosive media. A decrease of nearly two orders of magnitude was found. A microstructural evaluation of the coatings is presented and the mechanisms and reasons for the very early failure are discussed. [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]