Improvement in the performance of thermal barrier coating systems (TBCs) is one of the key objectives for further development of gas turbine applications. The material most commonly used as TBC topcoat is yttria stabilised zirconia (YSZ). However, the usage of YSZ is limited by the operating temperature range which in turn restricts the engine efficiency. Materials such as pyrochlores, perovskites, rare earth garnets, etc. are suitable candidates which could replace YSZ as they exhibit lower thermal conductivity and higher phase stability at elevated temperatures. The objective of this work was to investigate different multi-layered TBCs consisting of advanced topcoat materials fabricated by Suspension Plasma Spraying (SPS). The investigated topcoat materials were YSZ, dysprosia stabilised zirconia, gadolinium zirconiate, cerium doped YSZ and yttria fully stabilised zirconia. All topcoats were deposited with TriplexPro-210 plasma spray gun and radial injection of suspension. Lifetime of these samples was examined by thermal cyclic fatigue and thermal shock testing. Microstructure analysis of as-sprayed and failed specimens was performed with scanning electron microscope. The failure mechanisms in each case have been discussed in this article. The results show that SPS could be a promising route to produce multilayered TBCs for high temperature applications.

NiCoCrAlY coatings are widely used as bond coats for ceramic thermal barrier coatings (TBCs) and oxidation and corrosion protective overlay coatings in industrial gas turbines. High temperature oxidation behaviour of NiCoCrAlYs has a great influence on the coating performance and lifetime of TBCs. A promising route to decrease the oxidation rate of such coatings is post-coating surface modification which can facilitate formation of a uniform alumina scale with a considerably slower growth rate compared to the as-sprayed coatings. In this work, the effect of surface treatment by means of shot peening and laser surface melting (LSM) on the oxidation resistance of high velocity air-fuel (HVAF) sprayed NiCoCrAlY coatings was studied. Isothermal oxidation was carried out at 1000 °C for 1000h. Results showed that the rough surface of as-sprayed HVAF sprayed coatings was significantly changed after shot peening and LSM treatment, with a compact and smooth appearance. After the exposure, the oxide scales formed on surface-treated NiCoCrAlY coatings showed different morphology and growth rate compared to those formed on as-sprayed coating surface. The oxidation behaviour of surface treated HVAF-sprayed NiCoCrAlY coatings were revealed and discussed.

Superhydrophobic surfaces are of great importance in many industrial applications, especially where components are exposed to wet environments and low temperatures. Texturing of surfaces to reach superhydrophobicity can be achieved by thermal spraying technology, which is an attractive coating method as it is cheap, flexible and can employ a large range of feedstock materials. In this study, ceramic reinforced metal matrix composite (WC-CoCr) powders were sprayed using High Velocity Air Fuel method. They were varied based on their powder parameters such as carbide grain size, binder grain size and powder strength. The purpose was to investigate their hydrophobic characteristics and how these are influenced by different roughness profiles. The wetting properties such as contact angle and contact angle hysteresis were first investigated for the as-sprayed coatings. The roughness properties and Hausdorff Dimension were then related to the wetting properties. Aside from as-sprayed coatings, the effect of roughness and inherent wetting characteristics were studied by investigating the coating surface after grit blasting and polishing. Results show that powder parameters can lead to designing surfaces with higher surface roughnesses and thus having higher contact angles. It was also shown that surface composition of cermets has an impact on wettability, with the binder accounting for wetting characteristics and carbides accounting for roughness.

Reducing CO 2 emissions from power generation plants is intimately related to enhancing their thermal efficiency, which can be achieved by increasing the temperature/pressure of steam. However, any increase in steam temperature is inevitably accompanied by accelerated oxidation of boiler components. The use of renewable fuels such as biomass increases the problem by introducing a number of corrosive compounds into the boiler environment, resulting in more rapid degradation of components. Although thermal sprayed coatings are technocommercially attractive solutions for augmenting the durability of degradation-prone boiler components and are already used, further improvements in their performance are continuously sought. High-velocity air fuel (HVAF) coatings are promising in this context. In the present work, isothermal oxidation behavior of a candidate HVAF-sprayed Ni21Cr was studied in N 2 + 5% O 2 + 20% H 2 O at 600°C for 168h. The oxide scale growth mechanisms were studied by BIB/SEM/EDX to evaluate the effectiveness of the coatings. It was found that the water vapor effect is insignificant due to the Cr reservoir in the Ni21Cr coating, which yielded enhanced oxidation protection by forming nano-scale Cr 2 O 3 .

Development of TBCs allowing higher combustion temperatures in gas turbines is of high commercial interest since it results in higher fuel efficiency and lower emissions. It is well known that TBCs produced by suspension plasma spraying (SPS) have lower thermal conductivity as compared to conventional systems due to their very fine porous microstructure. Moreover, columnar structured SPS TBCs are significantly cheaper to produce as compared to the conventionally used electron beam – physical vapour deposition (EB-PVD). However, SPS TBCs have not yet been commercialised due to low reliability and life expectancy of the coatings. Lifetime of a TBC system is significantly dependent on topcoat-bondcoat interface topography. The objective of this work was to study the effect of topcoat-bondcoat interface in SPS TBCs by changing bondcoat spray parameters and bondcoat surface heat treatment. High velocity air fuel (HVAF) spraying was used for bondcoat deposition while axial-SPS was used for topcoat deposition. Same topcoat spray parameters were used for all samples. Lifetime was examined by thermal cyclic fatigue and thermal shock testing. The influence of surface roughness on lifetime has been discussed. The results show that HVAF could be a suitable process for bondcoat deposition to achieve long lifetime SPS TBCs.

In a variety of engineering applications, components are exposed to corrosive/erosive environment. Protective coatings are essential to improve the functional performances and/or extend the lifetime of the components. Thermal spraying as a cost-effective coating deposition technique offers high flexibility in coatings’ chemistry/morphology/microstructure design. However, the pores formed during spraying inherently restrict the use of coatings for corrosion protection. In view of the above gap to have a high quality coating, bi-layer coatings have been developed to boost the corrosion performance of the coatings. In a bi-layer coating, an intermediate layer is deposited on the substrate before spraying the coating. The electrochemical behavior of each layer is critical to ensure a good corrosion protection. The corrosion behavior of the layers strongly depends on coating composition and microstructure, which are affected by feedstock material and spraying process. In the present work, Cr 3 C 2 -NiCr top layer with different intermediate layers (i. e., Fe-, and Ni-based) were sprayed by high-velocity air fuel (HVAF) process. Microstructure analysis, as well as electrochemical tests, e.g., open-circuit potential (OCP) and polarization were performed. The results showed a direct link between the OCP of each layer in a bi-layer coating and corrosion mechanisms. It was found that the higher corrosion resistance of Ni-based intermediate layers than Fe-based coatings was due to higher OCP of the coating in the galvanic couple with top layers. Splat boundaries and interconnected pores reduced the corrosion resistance of the intermediate layers, however a sufficient reservoir of protective scale-forming elements (such as Cr or Al) improved the corrosion behavior.

It is well known that the presence of KCl deposited on superheater tubes in biomass- and waste-fired boilers leads to a severe corrosion and premature damage. In order to protect such critical components which are routinely exposed to aggressive environments, thermal sprayings are frequently proposed as a potential solution. By virtue of the techno-commercial benefits that provides as a direct outcome of its ability to cost-effectively deposit coatings virtually free of porosity and in situ formed oxides, the high velocity air-fuel (HVAF) process offers a particularly attractive approach. In the present work, the influence of KCl on the oxidation behavior of four HVAF-sprayed Ni-based coatings (Ni21Cr, Ni5Al, Ni21Cr7Al1Y, and Ni21Cr9Mo) has been investigated. The coatings were deposited onto specimens of 16Mo3 steel, a widely used boiler tube material. High temperature corrosion tests were carried out in ambient air at 600°C, with 0.1 mg/cm2 KCl being sprayed onto the samples prior to the exposure. Uncoated substrates and an identical test environment without KCl were used as reference. SEM/EDS and XRD techniques were utilized to characterize the as-sprayed and exposed samples. The results showed that the small addition of KCl significantly accelerated damage to the coatings. It was further revealed that the alumina-forming NiAl coating was capable of forming a more protective oxide scale compared to other chromia and mixed-oxide scale forming coatings. In general, the oxidation resistance of the coatings based on the kinetic studies had the following ranking (from the best to the worst): NiAl >NiCr> NiCrAlY> NiCrMo.

Development of thermal barrier coatings (TBCs) manufactured by suspension plasma spraying (SPS) is of high commercial interest as SPS has been shown capable to produce columnar microstructures similar to the conventionally used electron beam – physical vapour deposition (EB-PVD) process. Moreover, SPS is a significantly cheaper process and can also produce more porous coatings than EB-PVD. However, lifetime of SPS coatings needs to be improved further for them to be applicable in commercial applications. The bondcoat microstructure as well as topcoat-bondcoat interface topography affect the TBC lifetime significantly. The objective of this work was to investigate the feasibility of different bondcoat deposition process for SPS TBCs. In this work, a NiCoCrAlY bondcoat deposited by high velocity air fuel (HVAF) was compared to commercial NiCoCrAlY and PtAl bondcoats. All bondcoat variations were prepared with and without grit blasting the bondcoat surface. SPS was used to deposit the topcoats on all samples using the same spray parameters. Lifetime of these samples was examined by thermal cyclic fatigue and thermal shock testing. The effect of bondcoat deposition process and interface topography on lifetime in each case has been discussed. The results show that HVAF could be a suitable process for bondcoat deposition in SPS TBCs.

This study investigates the corrosion resistance Gd 2 Zr 2 O 7 /YSZ coatings and a YSZ layer of similar thickness. All coatings were produced by suspension plasma spraying, resulting in a columnar structure. Corrosion tests conducted at 900 °C for 8 h in a molten salt bath show that Gd 2 Zr 2 O 7 is not as corrosion resistant as YSZ. Molten salts react with Gd 2 Zr 2 O 7 producing GdVO 4 along the surface as well as between the columns of the coating. The formation of GdVO 4 between the columns, in combination with the low fracture toughness of Gd 2 Zr 2 O 7 , is likely responsible for the lower corrosion resistance. Furthermore, the presence of another layer of Gd 2 Zr 2 O 7 on top of the Gd 2 Zr 2 O 7 /YSZ coating, to prevent salt infiltration, did not improve corrosion resistance.

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.

Dysprosia stabilized zirconia coatings with large globular pores have good potential as TBC topcoats. In previous work, such coatings have been produced by air plasma spraying with the aid of a polymer pore former. The aim of this work is to optimize the spraying parameters. A design of experiments approach was used to create a two-level full factorial test matrix based on spray distance, powder feed rate, and hydrogen flow. An agglomerated and sintered dysprosia stabilized zirconia (DySZ) powder mixed with polymer particles was sprayed on Hastelloy X substrates that had been prepared with NiCoCrAlY bond coats. The coatings obtained were evaluated based on thermal conductivity, thermocyclic fatigue life, and morphology, which are shown to correlate with spray parameters and in-flight particle properties.

Dysprosia stabilized zirconia (DySZ) is a promising candidate to replace yttria stabilized zirconia (YSZ) as a thermal barrier coating due to its lower inherent thermal conductivity. It is also suggested in studies that DySZ may show greater stability to high temperature phase changes compared to YSZ, possibly allowing for coatings with extended lifetimes. Separately, the impurity content of YSZ powders has been shown to influence high-temperature sintering behavior. By lowering the impurity oxides within the spray powder, a coating more resistant to sintering can be produced. This study evaluates high purity and standard purity dysprosia and yttria stabilized zirconia coatings and their performance after extended heat treatment. Coatings were produced using powders with the same morphology and grain size; only the dopant and impurity content were varied. Samples were heat treated for up to 400 hours at 1150 °C and thermal conductivity measurements were plotted to show the evolution of thermal properties with respect to time. Thermal conductivity is compared to coating microstructure and porosity in order to track structural changes due to sintering.

The thermo-mechanical properties of a thermal barrier bond coat (BC) play an important role in governing the life-time of a coating system. The presented work aims to determine these properties for NiCoCrAlY coatings sprayed on Hastelloy X substrates sprayed under different process conditions. Temperature dependent Young’s modulus values are determined for both Atmospheric Plasma Sprayed (APS) and HVOF sprayed coatings using the four-point bending test. Particular attention is paid to microstructure-property relationships during heating. Young´s modulus was determined up to 950°C and evaluated for coatings loaded in both tension and compression. Results are discussed in the context of the effect of feedstock material, process conditions and microstructure characteristics. The methods and results presented are attractive, particularly for the thermal spray industry, since these properties are a prerequisite when the BC is to be considered in component design.

The aim of the study presented in this paper was to develop the next generation of production ready air plasma sprayed thermal barrier coating with a low conductivity and long lifetime. In order to achieve these goals; a number of coating architectures were produced using commercially available plasma spray guns. Modifications were made to powder chemistry including; high purity powders for sintering resistance, Dysprosia stabilised Zirconia powders and powders containing porosity formers. Agglomerated & Sintered (A&S) and Hollow Oven Spherical Powder (HOSP) morphologies were used to attain beneficial microstructures. Finally, dual layer coatings were produced using the different powder morphologies. Evaluation of the thermal conductivity of the coating systems from room temperature to 1200°C was conducted using laser flash technique. Tests were done on as-sprayed samples and samples heat treated for 100 hours at 1150°C in order to evaluate the first stage sintering resistance of the coating systems. Thermal conductivity results were correlated to coating microstructure using image analysis of porosity and crack content. The results show the influence of beneficial porosity on reducing the thermal conductivity of the produced coatings.

Plasma spraying of thermal barrier coatings (TBCs) on gas turbine parts is widely used today either to enable higher turbine inlet temperatures with consequent improvement of combustion efficiency or to reduce the requirements for the cooling system and increase components life-time. Development of low conductivity TBCs, which allows us to further increase gas turbine efficiency and availability, is an ongoing challenge. In order to get low thermal conductivity values an experimental program was conducted. Two zirconia powders were used for coating deposition: yttria partial stabilised zirconia (YPSZ) and dysprosia partial stabilised zirconia (DyPSZ). Microstructure evaluations were performed to evaluate the influence of the spraying parameters on the coating morphology and porosity level. Two methods were utilised to evaluate the coatings thermal conductivity: Laser Flash (LF) and Transient Plane Source (TPS). A comparison between the two methods was made as well as a correlation study between coating microstructure/composition and thermal conductivity (TC).

New technologies as Near Net-Shape Spray Forming offer good possibilities to obtain quickly and easily nozzle type refractory components. Excessive material utilisation and laborious machining can be avoided. Cylindrical shape components have been obtained by plasma spraying of yttria partially stabilized zirconia powder on a preshaped mandrel which was subsequently removed. Residual stress analysis through the coatings has been performed using three evaluation methods. A mathematical model able to predict the levels of residual stress is presented and used. A finite element analysis is developed in order to calculate the residual stress field in the near-net shape formed coatings. The validation of the theoretical analysis and mathematical models were performed by neutron diffraction measurements. The experimental results are in good agreement with those obtained by analytical methods and reveal low levels of residual stresses within the sprayed coatings.

The aim of this work is to develop a finite element model that predicts the residual stresses in plasma sprayed parts formed on conical mandrels and removed by various means. The theoretical analyses and mathematical models are confirmed by X-ray diffraction measurements on plasma-sprayed zirconia samples freed from shaped mandrels by means of cooling, melting, and chemical dissolution. Based on test results, the mandrel-cooling removal method produced the highest internal stresses. Paper includes a German-language abstract.