Due to their promising photocatalytic properties under visible light irradiation, thermally sprayed ZnO-TiO 2 coatings are of interest as substitute for TiO 2 for various industrial applications, like hydrogen production via water splitting or the reduction of organic pollutants in water. Suspension spraying is an effective method to produce coatings in the binary ZnO-TiO 2 system to form Zn 2 TiO 4 in-situ during the spraying process. Aqueous suspensions containing fine dispersed ZnO and TiO 2 particles are mixed at tailored composition and sprayed using the SHVOF and SPS spraying processes. Coatings with homogeneous distribution of elements and different surface structures and phase compositions are obtained. The phase composition is analyzed via XRD. UV-Vis spectroscopy measurements and photocatalytic tests of Rhodamin B degradation are performed. The potential to use appropriate binary suspension feedstock to produce ZnO-TiO 2 -Zn 2 TiO 4 coatings with different microstructures and photocatalytic properties is presented.

The need for sustainable use of resources requires continuous improvement in the energy efficiency and development of new approaches to the design and processing of suitable materials. The concept of high entropy alloys (HEAs) has recently been extended to more general compositional complex alloys (CCAs) and multi-principal element alloys (MPEAs). One of the major challenges on the way to application of these alloys is the extensive design and selection efforts due to the great variety of possible compositions and its consequences for workability and resulting material properties. The favorable high-temperature strength of Ni-based and Co-based superalloys is ascribed to a defined γ/γ’ structure consisting of a disordered FCC A1 matrix and ordered L 12 γ’ precipitates. In the current work we extended this design concept to CCAs, allowing disordered BCC A2 and ordered B2 phases in additions or in substitution of the original γ/γ’ structure. We used a high-throughput screening approach combining CALPHAD-based computational tools with in situ alloying by means of laser cladding. Wall-type specimens with gradient composition in the system Al-Co-Cr-Fe-Ni-Ti with varying Al, Ti and Cr content were analyzed. The combined modelling and experimental screening approach was demonstrated to be a powerful tool for designing new high performance AM-ready feedstock.

When compared with conventional thermal spraying processes, thermal spraying of suspensions allows to produce coatings with outstanding properties in terms of microstructure, surface topography, and phase compositions, as well as mechanical, electrical or tribological requirements. The use of suspensions as feedstock results in an almost unlimited flexibility in terms of chemical composition of the sprayed coatings. Moreover, thermal spraying of suspensions is a promising technique for processing expensive raw materials. Zn 2 TiO 4 coatings are only one example where the high costs of blended oxide powders as feedstock material hinders the market introduction, whereas outstanding electrical properties and photocatalytic activity of thermally sprayed Zn 2 TiO 4 coatings are of great interest for various industrial applications. In this work, single oxide ZnO and TiO 2 raw materials as well as a Zn 2 TiO 4 feedstock powder were used to develop tailored aqueous suspensions suitable for thermal spraying. To follow the formation of the compositions in the system ZnO-TiO 2 , differential thermal analysis (DTA) and thermal gravimetry (TG) measurements were performed. Preparation routes of stable suspensions with low sedimentation rates, low viscosity and good flowability are discussed. Exemplary microstructures and phase compositions of sprayed coatings are shown. In all sprayed coatings, the Zn 2 TiO 4 phase has been formed during Suspension High Velocity Oxygen Fuel Spraying (S-HVOF). This work demonstrates the potential to develop appropriate cost-efficient suspension feedstocks from single oxide raw materials to obtain Zn 2 TiO 4 coatings.

Additive Manufacturing (AM) processes offer geometrical freedom to design complex shaped parts that cannot be manufactured with conventional processes. This leads to new applications including aerospace propulsion systems where the Ni-superalloy based material has to withstand high operating temperatures. In this contribution, the influence of heat treatment and surface conditioning of the additively manufactured Inconel 718 substrates on the thermocycling performance of suspension sprayed YSZ coatings was investigated. The different surface conditions included as-built, sandblasted and milled substrate surfaces with and without heat treatment. YSZ coatings were applied using suspension plasma spraying (SPS) with commercial available suspensions. Thermal cycling tests (FCT) at 1100°C, 1300 °C, and 1500 °C were applied to coating systems until failure occurred. The microstructures of the samples were characterized before and after thermal cycling. The performance of the coatings was mainly influenced by the coating morphology and FCT test conditions and less by the state of the AM substrates. Columnar-like YSZ SPS sprayed coatings on AM Inconel 718 substrates seemed to be a promising candidate for rocket engine applications.

Boundary layers on surfaces will change from laminar to turbulent flow after a critical length. Due to the differing heat transfer coefficients of laminar and turbulent flow, the point of transition can be detected by heating the surface and measuring surface temperature by thermographic imaging. Locating the transition point is crucial for the aerodynamic optimization of components. In this study, fiber reinforced polymer composites (FRPCs) were chosen as the test substrate. Experiments were conducted using the flame spray process and NiCrAlY coatings. Multilayered coatings consisting of an aluminum bond coat, a layer of alumina as electrical insulation, and a heating layer of titania were fabricated by atmospheric plasma spraying. Free-flight tests were conducted with a functionalized winglet in order to assess the ability of thermally-sprayed heating elements to detect the location of transition of the flow regime. The results showed that the thermally-sprayed elements heat surfaces uniformly, with sufficient radiation losses for thermographic imaging. It was also shown that the change in temperature at the point of transition was readily observable using thermography.

Cr 2 O 3 coatings and Cr 2 O 3 -based binary coatings offer high hardness, excellent sliding wear performance and corrosion resistance. Therefore, they are widely applied in the paper and textile industry, as well as for pumps and mechanical sealing systems. Compared to the conventional spray processes, the technology of Suspension-HVOF (high velocity oxy-fuel flame spraying of suspensions, S-HVOF) using submicron-scaled raw materials allows the production of dense, finely-structured coatings with smooth surfaces and improved mechanical properties. This work investigates the microstructure and performance of Cr 2 O 3 and Cr 2 O 3 -15wt.%TiO 2 coatings obtained by SHVOF with water-based suspensions. For the development of the binary composition two routes were used to produce ready-to-spray suspensions: i) dispersion of an appropriate alloyed material in the solvent and ii) mixture of two stable suspensions in the desired ratio. In order to evaluate the potential of suspension spraying over the conventional APS and HVOF processes, the mechanical properties and the corrosion resistance of the S-HVOF Cr 2 O 3 and 85Cr 2 O 3 /15TiO 2 coatings were compared with the coatings produced from spray powders. From the experimental results it was observed that the suspension sprayed coatings showed in most of cases denser microstructure, good mechanical properties and superior corrosion performances.

Laser cladding technology is widely used in industry to precisely apply tailored surface coatings, as well as three-dimensional deposits for repair and additive layer-by-layer fabrication of metallic parts. However, the processing of larger components, like tools for oil and gas production, is economically challenging due to the conventionally low deposition rates. Consequently, industry is requesting more powerful technologies that maintain the quality advantages of the laser technology, but also make the process more productive and time effective. The modern highest power diode lasers offer practical solutions for applying of large-area laser cladding with significantly increased productivity. Using a fiber-coupled diode laser of 20 kW power and the accordingly developed laser cladding heads, real deposition rates of metal alloys, e.g. Inconel 625, could reach 14 kg/h. With the new-developed powder nozzles with rectangular profile of the powder jet allows at a laser power of 20 kW single tracks with 45 mm-width can be produced. Besides the laser source, the processing laser head is the key parameter for a high productivity and efficiency of the whole cladding procedure. The paper presents a new generation of high-performance laser cladding heads with integrated process sensors, which guarantee a stable long-time operation at highest power levels. The deposition rates achieved with this technology are equal or even exceed typical values of the common PTA technique. Current applications are large-area coatings on power plant components, hydraulic cylinders for off-shore equipment, and large metal forming tools for automotive bodies.

Dense sintered technical ceramics demand special surface preparation in order to be coated by thermal spraying. Sandblasting results in the damage of the interface region and leads to bonding defects. On the contrary, by varying the laser conditions, different laser structured surfaces were generated, which at Rz roughness values in the range of 40 μm, allowed to achieve thick and well bonding coatings. Therefore, laser ablation is proved to be the best method for surface preparation of ceramic substrates. In the case of porous ceramics (including pre-sintered ceramics) substrate preparation can be omitted, depending on their porosity level. Ceramics with porosity content up to 60 % can be coated by APS, whereas HVOF can be used on ceramics with porosity up to 30 %. On ready-to-get-coated pre-sintered ceramic parts (without substrate preparation), followed by co-firing of the substrates and coatings, the development of new ceramic components is possible.

HVOF spraying has established itself in the last decade as a very cost competitive method, increasingly replacing VPS/LPPS for the application of typical high temperature protective metallic coatings on IGT hot gas path components. In order to further improve this technology, ALSTOM recently developed a new design of a 4-injector-block for the K2-gun. This advanced design does not only significantly optimize the energy consumption and spraying time, but also enables the future application of new complex metallic coatings. A CFD simulation of the K2-gun was performed in order to identify the optimization potential. Based on the results, an improved prototype was manufactured, validated and successfully introduced into manufacturing. The aim of the study was also to investigate in detail the impact of the hardware modification on parameters such as spray spot geometry, deposition efficiency, fuel consumption and the corresponding coating quality (porosity, bonding etc.), which shall be discussed within this paper.

Wear protection is one of the major applications of thermally sprayed hardmetal coatings. This paper presents the latest results of a systematic study on the influence of Cr 3 C 2 -NiCr feedstock powder characteristics on coating microstructures and economic parameters like deposition rate and deposition efficiency. Four commercial Cr 3 C 2 -NiCr powders with spherically shaped particles but different structural features were characterized and deposited by a liquid-fueled and a gas-fueled HVOF and a HVAF process. Deposition rates and efficiencies were determined; all coatings were analyzed in as-sprayed condition and selected samples were heat-treated at 800 °C in argon atmosphere. The effects of the feedstock powders and spray processes on the coating characteristics (microstructure, hardness, Young’s modulus and diffusion processes during heat treatment) were studied.

With today’s continuously increasing demand for flexibility in heavy duty gas turbines operation, power plant owners are forced to change their operation regime from base load to cyclic operation. The required coating properties for the two regimes are contradicting and cannot be optimized with current MCrAlY systems. Furthermore, for each turbine component, as well as in individual part locations, the loading boundary conditions are differently weighted. For an overall optimized component protection it is therefore of interest to produce coatings with flexible and individually tailored properties. In this context, ALSTOM invested into the development of an Advanced Modular Coating Technology (AMCOTEC), which is based on several powder constituents and a new application method, allowing in-situ compositional changes. With this approach, coating properties, such as oxidation, corrosion, erosion resistance, cyclic lifetime etc. can be modularly adjusted for individual component types and areas. This also includes production of functionally graded coatings, without changing the chemistry of powder fractions.

This work demonstrates a new single-cathode, multi-anode plasma spray process and compares it with conventional APS and HVOF spraying. Alumina feedstock powders mixed with 13, 40, and 44 wt% titania were deposited under a wide range of spraying conditions following a design of experiments approach. Deposition rate and efficiency were measured and coating characteristics, including microstructure, phase composition, hardness, Young’s modulus, electrical resistivity, and cavitation wear, are compared. The results are presented and the advantages of each process are discussed.

This study assesses the potential use of thermally sprayed dicalcium diiron pentaoxide (Ca 2 Fe 2 O 5 ) for thermoelectric generators. Ca 2 Fe 2 O 5 coatings up to 2 mm thick were produced by atmospheric plasma spraying and examined. Compared to the bulk material, the coatings exhibit lower thermal and electrical conductivity. The Seebeck coefficient could not be measured, and the thermoelectric performance was inadequate. The limitations derive not only from the thermal spray process, but also the material itself.

The work presented in this paper addresses some of the challenges of manufacturing thermoelectric (TE) generators by thermal spraying. One of the main obstacles is achieving good coat-on-coat bonding between different types of materials. The coatings must also be mechanically stable and optimized for their respective function. At least four types of materials are required, including electrical insulators, conductors, and thermoelectrically active p- and n-type semiconductors. Four ceramic and three metal feedstock powers were deposited by APS, HVOF, and HVAF spraying using special masking systems, substrate pretreatments, and layer thickness monitoring. After process optimization for each material, multilayer TE generator modules were successfully produced.

Compounds of the material group known as MAX phases combine metallic and ceramic properties. In this work, MAX-phase coatings are deposited from modified Ti 3 SiC 2 and Ti 2 AlC commercial feedstock powders using HVOF and atmospheric plasma spraying (APS). Feedstock powders and coatings were studied by microscopy and XRD. Despite the use of unoptimized powders, well adhering and relatively dense coatings were produced. HVOF-sprayed layers had denser microstructures with higher amounts of MAX phases. Optimizing the shape and particle-size distribution of feedstock materials is expected to improve coating properties.