Gadolinium zirconate (GZ) is considered as a promising top coat candidate for high temperature (>1200 °C) thermal barrier coating (TBC) applications. Suspension plasma spray (SPS) technique has shown the capability to generate a wide range of microstructures which includes the more desirable columnar microstructure. In this study, GZ single layer TBCs were deposited by axial SPS process. The variable parameters include the standoff distance, solid load content of the suspension and input power. The cross section and top surface of the as sprayed TBCs were analyzed by SEM. The phase content in the as sprayed TBCs was analyzed by XRD. The porosity content of the as sprayed TBCs was measured using image analysis. In the SEM analysis, it was observed that a lower solid load content in the suspension favored the formation of a columnar microstructure. Additionally, at lower solid load content, increase in standoff distance resulted in columnar microstructure with high porosity content in the TBC. However, with higher solid content suspension and alteration of input power, only a dense vertical cracked microstructure can be obtained.

Aluminum titanate (Al 2 TiO 5 ) is a congruently melting compound in the binary Al 2 O 3 -TiO 2 system, which decomposes below 1200 °C. Its properties (e.g. thermal conductivity, CTE) differ significantly from those of Al 2 O 3 and TiO 2 . Thus it is of special interest to study the stability of Al 2 TiO 5 in the spray process and its influence on the coating properties. A commercial fused and crushed Al 2 O 3 -40%TiO 2 powder, which was found to be substoichiometric, was selected as the feedstock material for the experimental work, as the composition is close to stoichiometric Al 2 TiO 5 . Part of that powder was heat-treated in air at 1150° and 1500°C in order to vary the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including metallographically prepared cross sections. A powder having Al 2 TiO 5 as the main phase was not possible to be prepared due to inhomogeneous distribution of Al and Ti in the original powder. Plasma spraying was performed with a TriplexPro-210 (Oerlikon Metco) using Ar-H 2 and Ar-He plasma gas mixtures with 41 and 48 kW plasma power. Coatings were studied by XRD, SEM of metallographically prepared cross sections, and microhardness HV1. Moreover, the results show a clear influence of the Al 2 TiO 5 content in the feedstock powder on the phase composition of the coatings.

Environmental barrier coatings (EBC) are currently being investigated to protect ceramic matrix composite (CMC) turbine engine components in water-vapor rich combustion environments. Dense, crack-free, uniform and well-adhered coatings are demanded for this purpose. This paper represents an assessment of different thermal spray techniques for deposition of Yb 2 Si 2 O 7 and silicon (Si) EBC layers. Plasma spraying of refractory silicates is known to be complicated by undesired glass transition due to rapid solidification as well as evaporation of Si-bearing species during spraying. Plasma spraying of low-density Si also requires careful optimizations as it tends to oxidize during spraying, particularly at atmospheric conditions. Bearing these problems in mind, the Yb 2 Si 2 O 7 coatings were deposited by atmospheric plasma spraying (APS), high-velocity oxygen-fuel spraying (HVOF), and plasma-spray physical vapor deposition (PS-PVD) techniques. As-sprayed microstructure, amorphous content and phase composition of the coatings were analyzed. Based on the findings, the advantages and disadvantages of each method over other techniques are discussed with respect to process parameters and material properties.

In this work, the effects of plasma spray process variables are systemized in various process schemes. On this basis, different approaches to improving process reliability are described and assessed paying particular attention to in-flight particle diagnostics. A new test applying spray bead analysis is introduced and the first results are presented.

A numerical model of a supersonic compressible plasma flow has been developed with the aid of CFD software to describe the thermodynamic and transport properties of a plasma jet in order to investigate the PS-PVD process and how to optimize it for thermal barrier coatings and, in particular, the formation of columnar microstructures. The required properties of the plasma gas mixtures were obtained as a function of temperature and pressure from thermodynamic calculations in chemical equilibrium with the effect of ionization. Two-dimensional Monte Carlo simulations were conducted to provide insight on the evolution of columnar microstructure, accounting for self-shadowing and vapor incidence angle but ignoring the effect of diffusion. Simulated structures and predicted values are presented and compared with actual images and measurements.

In this study, the current industry standard topcoat for thermal barrier coatings, 8YSZ, is deposited by suspension plasma spraying and its room-temperature erosion resistance is compared with that of SPS sprayed gadolinium zirconate/YSZ and triple-layered GZ dense /GZ/YSZ. A columnar microstructure was observed in both the single- and multi-layered TBCs. Single-layer 8YSZ had a higher erosion resistance than multi-layered GZ/YSZ despite of its higher porosity among the as-sprayed coatings. In the case of the triple-layer coating, the denser top layer helped to slightly improve erosion resistance over that of the GZ/YSZ double-layer TBC.

In this work, computational fluid dynamics (CFD) results confirm earlier calculations indicating that significant evaporation occurs in plasma torch nozzles. In addition, experimental work is performed, investigating the nature of ceramic deposits produced by plasma spray-physical vapor deposition (PS-PVD), particularly coatings composed of nanosized clusters. It was found that as the hot plasma jet comes close to the relatively cool substrate, a boundary layer is formed due to the rapid drop in temperature and velocity. In summary, coatings produced by PS-PVD are a mixture of nanocluster and vapor deposition.

La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF), deposited on a metallic porous support by means of plasma spray-physical vapor deposition (PS-PVD) is a promising candidate for oxygen-permeation membranes. However, after O 2 permeation tests, membranes show vertical cracks leading to leakage during these tests. In this work, a feature leading to crack formation has been identified. More specifically; Membrane residual stress changes during thermal loading have been found to be related to a phase transformation in the support. In order to improve the performance of the membranes, the metallic support has been optimized by applying an appropriate heat treatment. Additionally, it has been found that coatings deposited at lower oxygen partial pressures consist of 70% cubic and 26% rhombohedral perovskite phases. This increases the non-stoichiometry, which drives the formation of non-perovskite phases during annealing, affecting the membrane stability and the ionic conductivity. The amount of oxygen added during spraying can be used to suppress the cubic to tetragonal phase transformation.

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-6Al 2 O 3 needles and Al 2 O 3 precipitates as identified by EDS measurements.

This study deals with the deposition of coating materials that can be difficult to process by plasma spraying, including lanthanum and gadolinium zirconate, two pyrochlores of interest for thermal barrier applications, and lanthanum strontium cobalt ferrite (LSCF), a perovskite of interest for gas separation membranes. In addition to conventional atmospheric plasma spraying (APS), the feedstock powders were applied by suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD). The spraying processes are described in detail along with the characteristics of the powders and coatings and the effects of various spray parameters on splat behavior and coating composition and structure.

This study compares the deposition and oxidation behavior of two oxide-dispersed CoNiCrAlY powders, one commercially obtained, the other prepared in a high-energy attrition ball mill using CoNiCrAlY and nanosize α-alumina powders. The custom powder was deposited by HVOF spraying using two sets of parameters, one optimized for CoNiCrAlY powder, the other for fine alumina. Coatings produced under the latter conditions were found to be porous, which can be attributed to a low degree of melting in the dispersed alumina. Isothermal oxidation testing at 1373 K for up to 1000 h in air caused oxidation not only at the surface, but also inside the coatings due to the movement of oxygen through the pores. The coatings deposited under the other set of parameters, i.e., at higher power levels, were free of pores. Isothermal oxidation tests were also carried out on coatings produced from the commercial powder, in this case, by HVOF and as well as vacuum plasma spraying. The coatings obtained by HVOF spraying were found to have a thinner thermally grown oxide layer than not only the VPS coatings, but also conventional metallic bond coats. Internal oxidation in the HVOF coatings is due to insufficient cohesion of the spray particles. Furnace cycling tests were conducted on specimens with an additional ceramic thermal barrier coating. Specimens with VPS bond coats produced from commercial oxide-dispersed powder achieved almost same number of cycles to delamination as specimens with conventional metal bond coats.

In this work, plasma spray-physical vapor deposition (PS-PVD) is used to create oxygen transport membranes, consisting of gastight LaSrCoFeO thin films on porous MCrAlY metallic supports. During spraying, a protective layer of alumina forms at the interface between the membrane and support preventing interdiffusion. Surface roughness of the metallic support is shown to play a critical role in limiting microstructural defects. Phase composition, growth rate, and microstructure buildup are also investigated along with the annealing behavior of LSCF films at different temperatures. Initial results are promising and further improvements are expected by optimizing process parameters.

Atmospheric plasma spray parameters were developed for a three-cathode torch with a high-velocity nozzle and MCrAlY powders of different particle size fractions. The main objectives of the work are to achieve bond coats with low oxygen content and porosity. Other goals are achieving sufficient surface roughness at high deposition rates and efficiencies. The oxidation behavior of the sprayed coatings was characterized by thermal gravimetric analyses and isothermal heat treatments.

The durability of columnar TBCs produced by PS-PVD are strongly influenced by the compatibility of the metallic bond coat and ceramic topcoat. Studies have shown that a smooth bondcoat surface improves thermal cycling performance and that further improvements are possible by optimizing the formation of the thermally grown oxide layer. In this work, preheating and the deposition of the first coating layer are varied in order to adjust oxide growth. The results show that thermal cycling lifetimes can be more than doubled without a major increase in manufacturing time.

Plasma spraying at very low pressure (50-200 Pa) is significantly different from atmospheric plasma conditions (APS). Applying powder feedstock it is possible to defragment the particles into very small clusters or even to evaporate the material. As a consequence, the deposition mechanisms and the resulting coating microstructures could be quite different compared to conventional APS liquid splat deposition. Thin and dense ceramic coatings as well as columnar-structured strain-tolerant coatings with low thermal conductivity can be achieved offering new possibilities for application in energy systems. To exploit the potential of such a gas phase deposition from plasma spray-based processes, the deposition mechanisms and their dependency on process conditions must be better understood. Thus, plasma conditions were investigated by optical emission spectroscopy. Coating experiments were performed, partially at extreme conditions. Based on the observed microstructures, a phenomenological model is developed to identify basic growth mechanisms.

Lanthanum zirconate (La 2 Zr 2 O 7 ) was proposed as a promising material for thermal barrier coatings. At atmospheric plasma spraying (APS) of La 2 Zr 2 O 7 a considerable amount of La 2 O 3 can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the La 2 O 3 -ZrO 2 system, in the composition range of pyrochlore structure, the stoichiometric La 2 Zr 2 O 7 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 Gd 2 Zr 2 O 7 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 intended to show good thermal cycling performance.

Preparation of La 1-x Sr x Fe 1-y Co y O 3-δ (LSFC) coatings by plasma spraying at low pressure (200 Pa) using different plasma spray parameters is reported and discussed. Deposition with Ar-He plasma leads always to formation of coatings containing a mixture of LSFC perovskite, SrLaFeO 4 , FeCo and metal oxides. Coatings deposited at higher oxygen partial pressures by addition of oxygen into the vacuum chamber contain more than 85% perovskite and only few percent Fe 3 O 4 and/or CoO. The microstructures of the investigated LSFC coatings depend sensitively on the oxygen partial pressure, the substrate temperature and the deposition rate.

Oxide compounds basically composed of calcium, magnesium, aluminum and silicon cations also known as CMAS, can be deposited on the surface of thermal barrier coatings (TBC) of gas turbine blades. Under certain operation conditions these compounds have been found to aggressively degrade the TBC, hence affecting the thermo-mechanical properties of the underlying component. Detailed investigation on the interaction of CMAS and the atmospheric plasma sprayed (APS) yttria-stabilized zirconia (YSZ) TBC was performed 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. Variable exposure times at approximately 1250°C/1050°C surface/substrate temperatures were applied. The lifetime of the TBC was indicated by the number of thermal cycles until significant spallation occurred. X-ray spectroscopy and microstructural analyses were conducted on the cycled samples to determine the effect of thermo-chemical interactions. It was found that with extended heating period of 10 times the standard cycle, the number of sustainable load alternations heating/cooling was reduced. Interaction of CMAS and YSZ induces formation of glassy soda-silicate phase. Thermal cycling of thermo-physically mismatched TBC and glass melt causes crack formation and coating failure.

Plasma-sprayed ceramic coatings are often used as thermal barrier or abradable coatings in high-pressure stages of gas turbines. They are exposed to high thermo-mechanical loadings, due to the harsh operating conditions. Today, a material typically used in engines as thermal barrier coating material is yttria-stabilised zirconia (YSZ). This material has a low conductivity and a high thermal expansion coefficient, but a limited temperature capability of about 1200°C in long-term applications. For the use as abradable coatings, thicker coatings with a thickness above one millimetre are necessary. However with increasing coating thickness and limited cooling efficiency there is a risk of premature failure. As a result new ceramic materials have been developed. For the lifetime analysis they were tested by thermal gradient cycling tests. In the present work an APS ceramic double-layer topcoat composed of 7YSZ and a top layer of non-stoichiometric magnesia alumina spinel (Mg-Al-Spinel) was used. The layer was sprayed on disc-shaped IN738 superalloy substrates which were coated with a VPS bondcoat. Under specific thermal cycling conditions with temperatures above 1400°C, these samples showed a typical failure mechanism with exfoliation of thin coating lamellae, starting from the coating surface. This failure mechanism was analysed in detail, e.g. by scanning electron microscopy (SEM), X-ray diffraction, and chemical analysis. From these findings, a description of the failure mechanism was developed.

Fracture toughness and phase stability are crucial properties of thermal barrier coatings (TBC) during highly loaded thermomechanical operations in gas turbines. While several alternative TBC materials have exhibited excellent thermal resistance, their potential applicability has been limited due to poor endurance to cyclic stresses. The addition of TiO 2 to the non-transformable tetragonal t´-YSZ has been found to effectively enhance the fracture toughness and phase stability of YSZ at high temperature exposures. Thermal cycling tests in a burner rig were conducted on TBCs prepared from atmospheric plasma sprayed titania-doped YSZ to verify this phenomena. Exposure temperature was 1400°C at the surface and thermal gradient across the sample was provided by simultaneous back-cooling treatment. Cycling tests reveal that the slight increase in the tetragonality of the deposited coatings with increasing amount of dopant did not cause a significant effect to the lifetime of the TBCs. Moreover, increasing amount of Ti-substitution did not influence the fracture toughness of the bulk YSZ.