A key technology to minimize CO 2 -emissions is the production of hydrogen from water electrolysis. The proton exchange membrane water electrolysis (PEMWE) consists of a stacked system out of bipolar plates (BPP), porous transport layers (PTL) and a membrane electrode assembly (MEA). Research activities are ongoing to minimize material input, reduce costs and increase the performance. For example, the BPP on the anodic side of the stack is currently manufactured of bulk titanium and its substitution by a Ti-coated steel substrate is economically interesting. The main requirements for the BPP-coating are a high coating density, a low electrical resistance and a long lifetime in a harsh electrochemical environment. Coating application on substrates of s ≤ 0.5 mm thickness is conducted with three thermal spraying technologies: Cold Gas Spraying (CGS), High Velocity Air-Fuel (HVAF) spraying and High Velocity Oxy-Fuel (HVOF). Substrate preparation is examined as well. Coating development is conducted with regards to coating thickness, density and oxidation. The examination of coatings includes roughness analysis, structural and chemical analysis. The results allow an evaluation of the suitability of thermally sprayed Ti-coatings by the structural properties for the PEMWE application. Among the three tested processes, CGS is the most suitable for this type of application. The three chosen thermal spraying processes are examined for coating application on metal sheets in context of PEMWE for the first time.

In biomass boilers, corrosion is a prevalent concern that arises at high temperatures. This is mainly because the fuels consumed in these boilers have a high alkali, chlorine, and other molten salt content that has occasionally led to material depletion, leaks, and unforeseen plant shutdowns. Applying protective coatings using thermal spray techniques is a practical answer to this issue. The current work focused on applying powders of Inconel 625 and Inconel 718 to boiler steel using a high-velocity oxy-fuel spraying method. The samples after coating deposition were subjected to the conditions of a biomass-fired boiler for 15 cycles to study the performance of the coatings in a real environment. The decrease of thickness over time was used to evaluate the erosion-corrosion process. Various characterization techniques were used to examine the as-sprayed and eroded-corroded specimens. The X-ray diffraction (XRD) technique was utilized to analyze the phases, while the surface characteristics of powders, coatings, and samples exposed to erosion-corrosion were investigated through scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). When exposed to the actual boiler environment, the findings showed that Inconel 625-coated steel performed better than Inconel 718-coated steel.

Erosion-corrosion is a severe problem observed in the coal fired thermal power plant boilers which lead to premature failure of boiler tubes. Thermal spray coatings have been applied successfully to check the erosion-corrosion of boiler tubes. In the present research work NiCrTiCRe coating powders were successfully deposited on T22 boiler steel by two different coating processes i.e. high velocity oxy-fuel (HVOF) and cold spray process. The performance of the coatings in actual power plant boiler were investigated and compared. The uncoated and coated T22 boiler steels were subjected the superheater zone of the coal fired boiler for a total of 15 consequent cycles. The thickness loss data and weight change analysis were used to establish kinetics of the erosion-corrosion. X-ray diffraction, surface field emission scanning electron microscope/energy dispersive spectroscopy (FE-SEM/EDS) techniques were used in the present work for the analysis. The results of thickness loss data indicated that the cold sprayed coating performed better in thermal power plant boiler environment.

Titanium porous transport layers (PTL) are important components in proton exchange membrane water electrolysis (PEMWE) cells. The performance enhancement and the reduction of manufacturing cost of PTLs are of importance for market expansion of PEMWE. Vacuum plasma spraying (VPS) was used to prepare PTL or modify PTL of sintered titanium powders and the PTLs by VPS showed a high performance. Regarding the cost efficiency, it is of great interest to produce PTLs using more economical spray processes than VPS. In this study, high velocity oxy-fuel spraying (HVOF) was used to produce highly porous titanium coatings for this purpose. The spray process was developed to achieve a high porosity of up to Φ = 30 % using three titanium powders with size distributions of fA = -90 +45 μm, fB = -63 +20 μm and fc = -45 +11 μm. The coating structures were examined on the cross sections of the titanium coatings with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The porosity was determined using the image analysis system ImageJ. The deposition efficiency of the titanium powder fC = -45 +11 μm was determined. The results show that the coating structure significantly depends on the titanium powders. Highly porous titanium coatings of Φ = 24 - 40 % can be produced with the titanium powders of fB = -63 +20 μm and fc = -45 +11 μm. Titanium oxides are hardly visible on the cross-sections of the titanium coatings. A high deposition efficiency of approximately DP ≈ 70 % was measured for the titanium powder of fc = -45 +11 μm.

The Ti-6Al-4V alloy is widely used in aerospace applications for its excellent mechanical properties, however, it presents low wear resistance. It is often coated with a cermet using high-velocity oxy-fuel (HVOF) spraying to improve its wear performance. The Cr3C2-NiCr cermet becomes particularly interesting since it is non-carcinogenic, compared to traditional cermet coatings containing tungsten-cobalt compounds. While the improvement in wear resistance of Ti-6Al-4V with this coating has been demonstrated, its impact on the fatigue performance of the alloy remains to be studied. This is precisely the aim of this study, which focuses on the fatigue life of a Cr3C2-25NiCr-coated Ti-6Al-4V alloy. Among the various influencing factors, surface preparation represents a significant source of crack initiation, particularly in the case of sandblasted surfaces. Indeed, the inclusion of fragmented alumina particles can produce stress concentration zones. Thus, laser texturing, which is a method involving the creation of anchoring points through controlled ablation, can be considered today as a less harmful surface preparation technique. The results obtained from cyclic tensile fatigue tests with a stress ratio of 0.1 for these two surface preparation methods are presented in this paper.

The cavitation performance of wear resistant cermet coatings can deteriorate in a corrosive environment. This investigation therefore considered the cavitation resistance in seawater of thermally sprayed High Velocity Oxy Fuel (HVOF) WC-10Co-4Cr coatings deposited on two different substrate materials of carbon steel and austenitic stainless steel. Coatings were deposited using industrially optimised parameters. Cavitation tests were conducted following the ASTM G32 test method in indirect mode, where there was a gap of 0.5 mm between the sonicator and the test surface. A submersed copper cooling coil controlled the temperature of the seawater. The cumulative cavitation erosion mass loss and cavitation erosion rate results are reported. The eroded substrate and coating surfaces were analysed using Scanning Electron Microscopy (SEM) in combination with energy dispersive x-ray analysis (EDX) to understand the failure modes. Coating phases were identified using x-ray diffraction. Results are discussed in terms of the cavitation failure modes and cavitation erosion rates for both the substrate and coated surfaces.

As a supersonic solid-state deposition process, Cold Spray (CS) has a unique role among other thermal spray techniques as it uses compressed and heated gas to accelerate particles to a critical velocity. CS can be an expensive process, especially when helium is used as a processing gas. In recent thermal spray developments, High-Velocity Air Fuel (HVAF) has taken a specific place in terms of providing dense and strong coatings similar to CS, but also coatings with less oxidation than High- Velocity Oxy-Fuel (HVOF). In contrast to these techniques, HVAF uses a mixture of fuel and air, instead of pure oxygen as in HVOF, to accelerate particles. Therefore, HVAF appears as a relatively cheaper and environmentally friendly alternative for the deposition of a wide variety of materials. The aim of this research is to produce fully dense copper coatings with limited oxidation using an inner diameter (ID) HVAF system and to compare the microstructure with CS copper coatings. Coating microstructures, surface roughness, and microhardness are studied using different characterization methods such as Scanning Electron Microscopy (SEM). Through this paper, the influence of both spray processes, CS and ID-HVAF, on the deposition of copper coatings is discussed. Cross-sectional studies of different coatings show a fairly dense microstructure for CS and ID-HVAF coatings. Moreover, it is discussed how the copper coating properties can change upon modifying the spray parameters.

Driven by the search for an optimum combination of particle velocity and process temperature to achieve dense hard metal coatings at high deposition efficiencies and powder feed rates, the high velocity air-fuel spraying process (HVAF) was developed. In terms of achievable particle velocities and temperatures, this process can be classified between high velocity oxy-fuel spraying (HVOF) and cold gas spraying (CGS). The particular advantages of HVAF regarding moderate process temperatures, high particle velocities as well as high productivity and efficiency suggest that the application of HVAF should be also investigated for the manufacture of MCrAlY (M = Co and/or Ni) bond coats (BCs) in thermal barrier coating (TBC) systems. In this work, corresponding HVAF spray parameters were developed based on detailed process analyses. Different diagnostics were carried out to characterize the working gas jet and the particles in flight. The coatings were investigated with respect to their microstructure, surface roughness and oxygen content. The spray process was assessed for its effectiveness. Process diagnostics as well as calculations of the gas flow in the jet and the particle acceleration and heating were applied to explain the governing mechanisms on the coating characteristics. The results show that HVAF is a promising alternative manufacturing process.

Silicon coatings have been developed for environmental barrier coatings by thermal spraying. Until now, these coatings have been produced almost exclusively by Atmospheric Plasma Spraying (APS). High Velocity Oxy-Fuel (HVOF) spraying is commonly used to produce dense metallic and carbide-based coatings due to high particle velocities. However, there have been no scientific reports on HVOF-sprayed silicon coatings in the literature. This study was conducted to investigate the feasibility of fabricating silicon coatings by HVOF using a DJ2600 spray system. Both the spray powders and the parameters were varied. The coatings were investigated on their surfaces and cross-sections using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). The hardness and indentation modulus of the silicon coatings were also determined. The results show that the particle size distribution and the stand-off distance are important influencing factors. Dense coatings could be produced by HVOF spraying, confirming the feasibility.

Carbides are interesting materials for many wear resistant and high temperature applications, however, the production of coatings with these materials represents a significant challenge as they tend to oxidise or decompose into gaseous phases when they are exposed to extreme thermal spray conditions. An innovative method merging suspension and solution precursors was developed to allow the production of carbide composite coatings. Suspensions of carbides and borides were modified with the addition of oxide precursors to obtain composite coatings by high-velocity oxy-fuel (HVOF) thermal spray. The transformation of these oxides precursors and their subsequent melting during spraying contribute to protect the carbides from oxidising conditions, avoid their degradation during the spray process and support the development of dense coatings, as it was demonstrated by dispersive X-ray spectroscopy and X-ray diffraction analysis. The relationships between processing and microstructure were studied in terms of porosity phase distribution and mechanical properties, proving that this novel approach could be applied to obtain coatings of materials that are prone to decompose during thermal spraying.

Thermally sprayed WC-based hardmetal coatings offer high hardness, good sliding wear and abrasion performance and find large applications in mechanical engineering, valve construction, or offshore applications. WC-Co coatings are mainly produced by high-velocity oxy-fuel spraying (HVOF) from conventional spray feedstock powders. In our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. Significant work was devoted to the development of appropriate aqueous hardmetal suspensions starting from commercially available fine WC and Co raw powders feedstock. This contribution proposes a step forward in the development of the S-HVOF WC-12Co coatings and evaluation of their microstructural and tribological properties. Suspension spraying trials were carried out using gas-fuelled HVOF TopGun system. For comparison purposes, liquid-fuelled HVOF K2 was employed to spray WC-12Co coatings starting from commercial available spray powder. Microstructural characterization, X-Ray diffraction and microhardness of the coatings were evaluated. Oscillating sliding wear tests were conducted against sintered Al 2 O 3 and WC-6Co balls. The sliding wear performances of the WC-Co sprayed coatings were discussed in term of the microstructure, phase composition and coating-ball test couples.

Thermal spraying of fine and ultrafine powders is realised by a novel method based on highly filled filaments as feedstock material for high velocity oxy fuel flame spraying (HVOF). Hereby, the desired coating material is supplied as finely dispersed powder within a polymer filament. Thus, the polymer works only as a transport medium and is fully decomposed when entering the combustion zone instantly releasing the solid dispersion, similar to the liquid dispersion medium in a suspension. It can work as an appropriate method to process fine and ultrafine powders. The solid nature of the dispersion medium poses several benefits compared to liquids, especially from the manufacturing point of view, since the process is geared to a wire flame spraying method. This work focusses on the challenges and benefits of this novel approach. First experimental results of spraying different filaments are presented.

This paper evaluates the cavitation erosion wear rate and failure modes of WC-10Co-4Cr coatings. These coatings are used in various industrial applications to protect against erosive, abrasive, sliding and cavitation wear in corrosive environments. Cavitation erosion tests were performed using a modified ASTM G-32 cavitation test rig. Thermally sprayed High Velocity Oxy-Fuel (HVOF) WC-Co-Cr coatings were deposited using industrially optimised coating process parameters on carbon steel and stainless-steel substrate coupons. Coatings were tested to simulate the cavitation bubbles occurring in valves, pumps, and ship propellers. Indirect cavitation was used to impact the cavitation bubbles on the test specimen at a fixed offset distance from the vibrator end. Test specimens were immersed in natural seawater. A water circulation cooling system was used to control the temperature of the water. The cumulative mass cavitation erosion and erosion rate results were evaluated. The coating microstructure was analysed using Scanning Electron Microscopy (SEM) and x-ray diffraction. Post-test evaluations included SEM observation in combination with energy dispersive x-ray analysis (EDX) to understand the failure modes. Results are discussed in terms of the factors controlling the cavitation erosion rate.

The adaptation of medium-entropy alloys (MEAs) by minor alloying constituents allows a targeted modification of the property profile of this material class for surface protection applications. In the present work, the potential of BSiC additions in the MEA system CrFeNi as base for adapted feedstock materials for thermal spraying is investigated. The alloy development was carried out in an electric arc furnace. Compared with the initial alloy, a significant increase in the wear resistance of the castings was demonstrated for the adapted alloy composition. Subsequently, powder was produced and characterized by inert gas atomization, followed by processing via high velocity oxy-fuel (HVOF) spraying. The tribological behavior was evaluated comparatively for all manufacturing variants considered. A good agreement in the property profile was determined, confirming the basic alloy development approach based on metallurgical processes. The evaluation of the process-structure property relationships confirms the great potential of adapted alloy systems for complex alloys in the field of surface engineering.

The use of suspensions in the thermal spraying process, makes it possible to apply sufficiently thin (<30 μm), metallic coatings made of nickel-chromium alloy 2.4869 (NiCr8020). High velocity oxy-fuel suspension flame spraying (HVSFS) is used to manufacture these thin metallic coatings in order to be able to effectively use them as electric panel heaters. Area heating capacities of 25 W cm -2 are possible with them and heating rates of 15 K s -1 even outperform many ceramic heating elements. In addition, it provides a flexible way to apply the heating coatings directly to the components to be heated. The use of fine powders in the micron and sub-micron ranges allows a more precise adjustment of the coating thickness, compared to conventional thermal spraying techniques, even in the thickness range below 10 μm. Therefore, an adaption to customer needs is possible regarding the electric panel heater characteristics, like electric resistance, applied voltages and current range and heating rates.

The addition of refractory metals represents a promising development approach for future high-entropy alloys (HEAs). Niobium and molybdenum are particularly suitable for increasing hardness as well as wear and corrosion resistance. In the context of surface protection applications, eutectic alloys with their homogeneous property profile are of particular interest. In the present work, two eutectic HEAs (EHEAs) were developed for the starting Al 0.3 CoCrFeNi using electric arc furnace. Following mechanical and microstructural characterization, the two alloys Al 0.3 CoCrFeNiMo 0.75 and Al 0.3 CoCrFeNiNb 0.5 were identified. For thermal spray processing, powders were prepared by inert gas atomization. The coatings produced by high velocity oxy-fuel (HVOF) spraying were characterized and evaluated comparatively to the castings, allowing process-structure-property relationships to be derived. Based on the results, statements on possible application potential can be made.

MCrAlX powder compositions (M=Ni, Co and X=Y, Hf, Si or combination) are often thermally sprayed (TS) via vacuum plasma spray (VPS), low pressure plasma spray (LPPS) or high velocity oxy-fuel (HVOF) to produce high temperature oxidation and hot corrosion resistant bond coats (BC) for thermal barrier coatings (TBCs). Cold spray (CS) technology is currently considered as a promising alternative to the traditional TS solutions having the advantage of delivering oxide-free and very dense metallic coatings at relatively lower costs compared to VPS and LPPS. Here, we first present high-pressure CS deposition of NiCoCrAlY and NiCoCrAlYHfSi and discuss the influence of feedstock properties on the deposited BCs. CFD numerical simulation is employed to tailor the spray conditions based on the feedstock characteristics. Secondly, we present the laser assisted cold spray (LACS) deposition of NiCoCrAlYHfSi BCs using a low-pressure CS system. We show that LACS can be successfully used to deposit this particular powder while eliminating nozzle erosion and low deposition efficiency disadvantages observed during conventional CS. Lastly, high temperature isothermal oxidation of a TBC architecture having LACS BC is presented.

Thermal spray is a versatile process that produces high-quality coatings possessing diverse properties such as superhydrophobicity, wear resistance, corrosion resistance, dielectric properties etc. Conventionally, powder feedstock is used in thermal spray, and this process is commercialised in numerous industrial processes. However, liquid feedstock based thermal spray is still in its development phases, due to limited information available on process parameters. Various parameters such as plasma/fuel gas, plasma current, feedrate, feeding angle, type of feedstock (suspension or solution precursor), feedstock concentration, feedstock viscosity, solvent, etc. significantly influence the thermal and kinetic energy exchange between plasma/flame and feedstock material. Suspension plasma spray (SPS) and suspension high velocity oxy-fuel spray (SHVOF), once optimised, can give rise to coatings with multiscale features. An in-depth understanding of the complex interaction between feedstock solution/suspension chemical-physical properties and plasma/flame jet characteristics is essential to understand its specific impact on coating properties and their application. This paper presents comparisons between two different TiO2 coatings, deposited by SPS and S-HVOF, and obtained by varying some of the fundamental spray deposition parameters. The surface morphology and cross-sections of the as-deposited coatings were compared through SEM/EDX. Further, surface wetting properties were analysed through measuring the static and dynamic contact angles.

Thermal barrier coatings (TBCs) are one of the most promising applications for suspension thermal spraying. By using a liquid feedstock, coating morphologies close to the columnar-like structure of electron-beam physical vapor deposition (EB-PVD) are possible. Additionally, vertically cracked TBC coatings (VC-TBC) have shown a great potential for industrial applications. In this work, we will analyze the process window for TBCs produced by using suspension spraying. Commercially available alcohol- and water-based YSZ suspensions have been processed using suspension atmospheric plasma spraying (S-APS) and suspension high-velocity oxy-fuel (S-HVOF) to achieve both columnar-like and VC morphologies. The coatings have been analyzed and compared in terms of microstructure, phase composition, bond strength and thermal cycling performance. Additionally, the suitability of S-APS and S-HVOF TBCs for industrial applications will be discussed, as well as which actions are needed to increase its competitiveness.

A known family of rare-earth oxide (REO) ceramics have recently been found to exhibit intrinsic hydrophobicity, even after exposure to high temperatures and abrasive wear. In this study, thin CeO 2 coatings were developed for hydrophobic applications using suspension high velocity oxy-fuel (SHVOF) thermal spray. It is an efficient method to produce large superhydrophobic surfaces with a unique hierarchically textured structure on a variety of substrates. The use of suspension also enables the process of fine-grained powders to form nanostructured coatings with significant improvement of mechanical and chemical properties for numerous applications. An aqueous suspension with a solid concentration of 30 wt.% sub-micron CeO 2 particles (<200 nm) was used as suspension feedstock. The as-sprayed CeO 2 coating on a stainless steel significantly improved the substrate’s surface hydrophobicity from a low contact angle of 57° to nearly 150°. The surface chemistry of SHVOF thermal sprayed CeO 2 coatings was also investigated by X-ray photoelectron spectroscopy (XPS). It was confirmed that the near-super-hydrophobicity was mainly attributed to its unique hierarchically structured surface.