To increase the efficiency of turbines for the power generation and the aircraft industry, advanced thermal barrier coatings (TBCs) are required. They need to be long-term stable at temperatures higher than 1200°C. Nowadays, yttria partially stabilized zirconia (YSZ) is applied as standard TBC material. But its long-term application at temperatures higher than 1200°C leads to detrimental phase changes and sintering effects. Therefore, new materials have to be investigated, for example, complex perovskites. They provide high melting points, high thermal expansion coefficients and thermal conductivities of approx. 2.0 W/(m K). In this work, the complex perovskite La(Al 1/4 Mg 1/2 Ta 1/4 )O 3 (LAMT) was investigated. It was deposited by the suspension plasma spraying (SPS) process, resulting in a columnar microstructure of the coating. The coatings were tested in thermal cycling gradient tests and they show excellent results, even though some phase decomposition was found.

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Emerging new applications and growing demands of plasma-sprayed coatings have initiated the development of new plasma spray processes. One of them is suspension plasma spraying (SPS). The use of liquid feedstock such as suspensions yields higher flexibility compared to the conventional atmospheric plasma spray processes as even submicron-to nano-sized particles can be processed. This allows achieving particular microstructural features, e.g., porous segmented or columnar-structured thermal barrier coatings. To exploit the potentials of such novel plasma spray processes, the plasma-feedstock interaction must be understood better. In this study, decomposition and evaporation of feedstock material during SPS were investigated, since particular difficulties can occur with respect to stoichiometry and phase composition of the deposits. Plasma conditions were analyzed by optical emission spectroscopy (OES). Experimental results are given, namely for gadolinium zirconate and for lanthanum strontium cobalt ferrite deposition. Moreover, the applied OES approach is validated by comparison with the simpler actinometry method.

In this study, a suspension containing Mg-Al-spinel nanopowder was deposited on bond-coated IN738 and stainless steel disks by suspension plasma spraying with and without substrate cooling. Coating surfaces and cross-sections were examined by SEM, EDS, and XRD analysis and thermal cycling tests were performed. SEM images of coatings obtained on cooled stainless steel show a unique columnar microstructure with a cauliflower-like surface. XRD spectra of the nanopowder and coatings revealed evidence of phase changes in the material deposited on cooled substrates. In preparing samples for thermal cycling tests, a YSZ layer was deposited on bond-coated IN738 prior to spraying the suspension. Double-layered Mg-Al-spinel/YSZ thermal barrier coatings produced on cooled substrates exhibited a thermal cycling lifetime of 2000 cycles at 1390°C, compared to 101 cycles for the TBCs sprayed without substrate cooling. The superior performance of the TBCs sprayed with substrate cooling is attributed to the densification of the coatings, revealed by SEM images, and possibly the formation of CaO-6Al2O3 needles and Al2O3 precipitates as identified by EDS measurements.