Abstract Additive manufacturing processes have been used to produce or repair components in different industry sectors like aerospace, automotive, and biomedical. In these processes, a part can be built by either melted particles as in selective laser melting (SLM) or solid-state particles as in the cold spray process. The cold spray has gained significant attention due to its potential for high deposition rate and nearly zero oxidation. However, the main concern associated with using the cold spray is the level of porosity in as-fabricated samples, altering their mechanical properties. These pores are primarily found in the regions where adiabatic shear instability does not occur. It is worth noting that the deformation of the impacted solid particle plays a vital role in reaching the shear instability. Therefore, for investigating the adiabatic shear instability region, an elastic-plastic simulation approach has been used. For this purpose, it is assumed that an elevated temperature solid Ti6Al4V particle impacts on a stainless-steel substrate surface at high velocity. The results show that increasing particle temperature will significantly enhance particle deformation because of thermal softening. Additionally, they illustrate that a material jet responsible for producing a bonding between particle and substrate by ejecting the broken oxide layer will be formed when the particle has a temperature above 1073 K and substrate remains at room temperature. In the end, it should be noted that increasing particle temperature up to 723 K will not have a significant effect on substrate deformation and final substrate temperature.

Abstract High-entropy alloys (HEAs) represent an innovative development approach for new alloy systems. These materials have been found to yield promising properties, such as high strength in combination with sufficient ductility as well as high wear and corrosion resistance. Especially for alloys with a body-centered cubic (bcc) structure, advantageous surface properties have been revealed. However, typical HEA systems contain high contents of expensive or scarce elements. Consequently, applying them as coatings where their use is limited to the surface represents an exciting pathway enabling economical exploitation of their superior properties. Nevertheless, processing conditions strongly influence the resulting microstructure and phase formation, which in turn has a considerable effect on the functional properties of HEAs. In the presented study, microstructural differences between high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) sprayed coatings of the alloy AlCrFeCoNi are investigated. A metastable bcc structure is formed in both coating processes. Precipitation reactions are suppressed by the rapid solidification during atomization and by the relatively low thermal input during spraying. The coating resistance to corrosive media was investigated in detail, and an improved passivation behavior was observed in the HVAF coatings.

Abstract In subzero conditions, atmospheric ice naturally accretes on surfaces in outdoor environments. This accretion can compromise the operational performance of several industrial applications, such as wind turbines, power lines, aviation, and maritime transport. To effectively prevent icing problems, the development of durable icephobic coating solutions is strongly needed. Here, the durability of lubricated icephobic coatings was studied under repeated icing/deicing cycles. Lubricated coatings were produced in one-step by flame spraying with hybrid feedstock injection. The coating icephobicity was investigated by accreting ice from supercooled microdroplets using an icing wind tunnel. The ice adhesion strength was evaluated by a centrifugal ice adhesion tester. The icing performance was investigated over four icing/deicing cycles. Surface properties of coatings, such as morphology, topography, chemical composition and wettability, were analyzed before and after the cycles. The results showed an increase in ice adhesion over the cycles, while a stable icephobic behaviour was retained for one selected coating. Moreover, consecutive ice detachment caused a surface roughness increase. This promotes the formation of mechanical interlocking with ice, thus justifying the increased ice adhesion. Finally, the coating hydrophobicity mainly decreased as a consequence of the damaged surface topography. In summary, lubricated coatings retained a good icephobic level after the cycles, thus demonstrating their potential for icephobic applications.

Abstract This study assesses the quality of flame-sprayed alumina coatings produced from recently developed alumina cord using argon and compressed air as atomizing gases. Coatings of different thicknesses were deposited on aluminum substrates and then analyzed using optical microscopy, X-ray diffraction, and resistivity measurements. The coatings, particularly those sprayed with argon, had fine microstructure and higher surface and volume resistivity than flame-spray coatings made from alumina cord in the past. They were also found to have higher alpha phase content than plasma-sprayed coatings, regardless of the atomizing gas used. The effect of humidity and the possible formation of aluminum hydroxides are also addressed.

Abstract Developing effective heating systems to prevent ice accretion on the surface of wind turbine blades and aircraft wings is of great significance for extreme cold environments. However, due to high velocity impingement of water droplets and solid particles on the surface of these components, an appreciable degree of surface material degradation may occur. In this study, nickel-chromium-aluminum-yttrium (NiCrAlY) was chosen as a metal matrix material for a coating-based heating system. Pure ceramic powders, namely, alumina and titania, and a cermet powder, tungsten carbide-cobalt (WC-12Co), were mechanically admixed with NiCrAlY powder and deposited to fabricate reinforced metal matrix composite (MMC) coatings. The powders were deposited on cylindrical low carbon steel bars by using flame spraying. The specimens were placed in a wind tunnel to conduct a comparative investigation of their erosive wear resistance under water droplet impact. A cold spraying unit was used for solid particle impact erosion tests. The erosive wear rates were quantified by measuring mass loss. The experimentally obtained results showed noticeably lower wear rate in NiCrAlY-WC-12Co and NiCrAlY-titania coatings compared to the other coatings. The results suggest that certain MMC coatings could be effectively employed to decrease the erosion rate of coating-based heating elements.

Abstract Titanium dioxide coating can be deposited by many processes such as thermal spray, Sol-Gel, vapor deposition, and so on. Among those coating deposition methods, thermal spray process is characterized by rapid formation of coatings easily and flexibly compared with other coating depositing processes. It is reported that nano-structured materials such as aluminum oxide, manganese(III) oxide, and zirconium dioxide could be produced by thermal spraying process with liquid feedstocks. With the process using liquid feedstocks, it was reported previously that nano-structured anatase titanium dioxide can be synthesized and the deposit mainly consist of anatase phase. In this paper, the effects of spray parameters on the grain size and phase structure of titanium dioxide deposits are systematically studied using liquid flame spraying system with butyl titanate as a liquid precursor. Paper includes a German-language abstract.

Abstract The novel High Velocity Combustion Wire Process (HVCW), which has a higher particle velocity compared to some other conventional spray processes, appears to be an alternative to improve the quality and corrosion resistance of the layers. In this paper, various electrochemical measurement methods are used to compare the electrochemical and corrosion behavior of thermal spray coatings based on Fe and Ni. For this purpose, the corresponding layers are produced not only by means of the HVCW process, but also by flame spraying, arc spraying, and high-speed flame spraying. The microstructure of these layers is investigated in order to understand the corrosion mechanisms of the layers sprayed under different manufacturing conditions. Paper includes a German-language abstract.

Abstract This paper aims to characterise the response of chromium carbide-based alloy-nickel base alloy thermal spray coatings under erosion-oxidation conditions. Chromium carbide-based alloy-nickel base alloy coatings have been extensively applied to components subject to combined erosion and high temperature corrosion. As a first step in understanding the mechanisms behind such behaviour the transient oxidation responses of coating-only samples were assessed under isothermal conditions between 700-900 deg C. The influence of splat microstructure and compositional degradation on this response was considered by examining coatings produced by HVAF and high-speed flame spraying techniques. It was found that compositional degradation during the coating plays a lesser role in the observed, unsteady oxidation. The influence of spray technique on compositional variation is thought to have played a lesser role in dictating the initial oxidation response. Paper includes a German-language abstract.

Abstract In this paper, various nickel-based coatings (NiCr, NiAl, and NiCrAlFe) are applied using flame and plasma spraying. The coatings are post-treated with a 6 kW carbon dioxide cw laser. Four different laser powers are used to fuse the layers together. The structure of the post-treated layers and the heat-affected area are examined with the aid of light and scanning electron microscopy. The microhardness profile of the layer and the depth of the heat affected area are measured. The wear resistance is tested by means of the rubber wheel wear test. It is observed that the plasma-sprayed NiCrAlMo-Fe layers, which have been post-treated with a laser power of 1000 W, have the highest wear resistance. Paper includes a German-language abstract.

Abstract This paper aims to develop a special nickel-chromium-chromium carbide cermet coating deposited by subsonic velocity flame spraying with excellent quality to resist high temperature oxidation, corrosion, and erosion in boiler environment and to explore the influence of chromium carbide fraction and sealing on the coating performance. Thermal spraying has offered a cost effective, reliable solution for boiler tube protection to extend boiler life. Plasma spraying has suffered a challenge from high velocity oxygen fuel (HVOF) technique due to its high velocity, strength cohesion, and low porosity when HOVF process has become one way to obtain high quality coatings. A subsonic velocity flame spraying can satisfy the requirements, and the coatings can be done inside the boiler or the workshop. Testing results show that various coatings exhibit different characteristics of erosion wear. Paper includes a German-language abstract.

Abstract A stronger integration of thermal spraying in industrial production lines is essentially prevented by insufficient options for process control. Emission spectroscopy is a passive characterization method that requires very short measurement times and is therefore suitable for in-situ applications. For many coating applications, contamination cannot be tolerated. The use of certified spray powder, for example, cannot avoid inclusions of tungsten, which can be traced back to the detachment of droplets from the electrodes of a DC plasma torch. Monitoring the emitted spectrum of a flame during the spraying process enables the detection of process or material-related impurities. In addition, emission spectroscopy offers great potential to be developed into a process control tool that uses the local intensity of characteristic emission lines. In this paper, investigations with different spraying methods are carried out, and the results are evaluated against the background of the process characteristics. Paper includes a German-language abstract.

Abstract The governments, in order to protect the environment, have increased the restrictions on products containing Volatile Organic Compounds. These products include the paints, which are usually used as protection of metallic structures. An alternative to these products are polymeric coatings obtained by thermal spray techniques. In this paper, the abrasion wear resistance and corrosion resistance of polyamide 11 and polyamide 11 + aluminum oxide coatings, flamesprayed, are compared. The electrochemical investigations are carried out to analyze the corrosion resistance in a marine environment. The structural examinations of such layers are carried out by means of optical and scanning electron microscopy. Paper includes a German-language abstract.

Abstract The microstructure of thermally sprayed layers depends on the characteristics of the process used. In addition to the machine settings, the spray additive has a significant influence, especially when using wires. As a rule, the use of wires allows higher application rates and efficiency levels compared to powders. However, the choice of materials is limited. Cored wires significantly expand the range of applicable materials. The sprayability depends heavily on the quality of the wire. This paper uses a high speed CCD camera for investigations on the process stability and melting off behavior of wires with different design in high velocity combustion wire and arc spraying processes. The analysed process characteristics are correlated to the resulting coating properties. Paper includes a German-language abstract.

Abstract The industrial flame spraying process has been analyzed by three-dimensional Computational Fluid Dynamics (CFD) simulation. The actual process is employed at the Volvo Aero Corporation for coating of fan and compressor housings. It involves the Metco 6P gun where the fuel, a mixture of acetylene and oxygen, flows through a ring of 16 orifices, while the coating material, a powder of nickel-covered bentonite, is sprayed through the flame with a stream of argon as a carrier gas by a central orifice. The gas flow was simulated as a multi-component chemically reacting incompressible flow. The standard, two equations, k-e turbulence model was employed for the turbulent flow field. The reaction rates appeared as source terms in the species transport equations. They were computed from the contributions of the Arrhenius rate expressions and the Magnussen and Hjertager eddy dissipation model. The particles were modeled using a Lagrangian particle spray model. In spite of the complexity of the system, the complex geometry and the numerous chemical reactions, the simulations produced fairly good agreement with experimental measurements. The powder size distribution was found to play a critical role in the amount of unmelted fraction of particles. The modeling approach seems to give a realistic description of the physical phenomena involved in flame spraying, albeit some model refinement is needed.

Abstract Three different types of polyethylene powders were flame sprayed onto pre-heated steel substrate previously coated by electrostatic spray system with a thin epoxy primer layer. Properties of the polyethylene (PE) powders, including powder density, particle size and melt flow rate (MFR) were measured in order to study their influence on the mechanical properties of the coating. The spray experiments started with optimization of spraying parameters. The main variables were pre-heating temperature of the substrate, temperature increase during spraying (influenced by the spraying distance), and thickness of the PE coatings. The laboratory tests performed for the coatings were coating characterization by microscopy and mechanical testing. Porosity and thickness of the coatings were determined by optical and stereo microscopy studies from polished cross-sectional samples. Hardness, impact strength, peel strength, and adhesive strength of the coatings were also investigated. Also some hot water sinking and heat cycling tests were performed. As a result from the present studies it can be concluded that powder properties have great influence on the mechanical properties of the final coating.

Abstract Improved understanding of microstructure-property relationship can help to shift from experiment-based to science-based development of thermally spray deposits. This should result in shorter and less expensive development as well as in higher functionality and reliability of the deposits. Significant amount of work has been done, however, nearly always studying deposits manufactured by only one of the thermal spray techniques. Results are therefore often spray technique specific. A broad study with samples manufactured by a number of different thermal spray techniques seems to be missing yet. Relationships valid across different techniques should provide better understanding of the generic relationships. This research employs number of different techniques - flame, HVOF, plasma (APS, VPS, WSP), to generate a wide range of microstructures. Various Ni-based alloys are studied starting from a simple chemistry (Ni) and ending with complex NiCrAlY alloys. Presented results were obtained with NiCr (80% Ni, 20% Cr) feedstock. Microstructures are characterized by various techniques-OM, SEM, XRD, small-angle neutron scattering (SANS) and others - to obtain the most comprehensive set of macro to micro structural parameters available today. The wear and corrosion properties of these deposits are measured together with internal coating stresses and the most generic microstructure-property relationships are sought.

Abstract High-velocity air fuel (HVAF) spraying was selected for spray trials of a Cr3C2-NiCr powder. To determine the effect of spray parameters on coating characteristics, particularly porosity and phase degradation, a statistical design of experiments was implemented. A wide range of statistical designs have been applied to the optimization of thermal spray coatings with a great deal of success. In this instance, a lack of prior knowledge and the need to assess many process-variable interactions efficiently led to the selection of a two-level full factorial design. High and low settings for each variable, including spray distance, traverse speed, and powder feedrate, were chosen based on the ranges typically used to spray similar materials. The resulting coatings were assessed for microhardness, porosity, residual stress, deposition efficiency, and phase transformation, after which several follow-up runs were conducted to explore trends brought to light by the initial factorial design.

Abstract Cored wires show a high potential for production of protective coatings for combined corrosion and wear applications. Iron and nickel based grooved cored wires without and with different reinforcing carbide fillers have been sprayed by arc- and high velocity combustion wire (HVCW) spraying with a Praxair Type 216 gun. Depending on the wear mechanism coatings with a similar abrasive or oscillating wear resistance like HVOF WC/Co/Cr 86/10/4 have been produced. For effective protection against oscillating wear wires with a large diameter and therefore a high content of reinforcing carbide filler have to be applied. All nickel based coatings with chromium addition show an improved corrosion resistance compared to HVOF-sprayed WC/Co/Cr 86/10/4. For coatings from wires with NiCr 80/20 velum no effect of severe sulphurous corrosion in the DIN 50018 test is observed. HVCW-spraying is especially suitable, when only a low degree of interaction between velum and filler material is wanted as for cermet-like coatings. Conventional arc-spraying rather meets the demands of a high degree of interaction between velum and filler necessary for the production of pure metallic coatings like NiCrBSi. All manufactured coatings show good machinability.

Abstract This work is devoted to the study of peculiarities of thermal sprayed coatings behavior under conditions of cavitation, as well as the elaboration of compositions of cavitation-resistant coatings and technology for their application with the aim of engine cylinders sleeves protection from cavitation-erosion destruction. The methods of arc metallization, flame and plasma spraying were used for coatings deposition. Powders of metal alloys and oxides, mechanical mixtures of nickel alloys with carbides, wires and flux-cored wires were applied as materials for thermal spraying. Method of magneto-striction vibration was used to determine the coatings cavitation resistance. A correlation between a bond strength of coatings and the character of their cavitation destruction was established. The best results were obtained in the case of using stainless steel wires and flux-cored wires using. Resistance of coated diesel engines sleeves was increased 1.6 times in comparison with sleeves without coatings. Semi-automatic line for arc metallization of diesel sleeves with a production of 600,000 sleeves per year was designed, build up and put into operation.

Abstract This paper presents the electrochemical characterization of a chromium carbide-NiCr coating applied using high-speed flame spraying. It examines the behavior of the complete system, the steel, the steel coating, and the coating immersed in NaCl solution alone. The paper discusses electrochemical measurement methods such as the measurement of polarization resistance, anodic polarization, and open circuit potential. The tests are compared with each other and with results from metallographic examinations. The structure was characterized by light and scanning microscopy. In addition, an analysis of the residual water was carried out using an ICP technique. The paper also includes a study of the various mechanisms that could affect the behavior of such coating types in a corrosive environment. Paper includes a German-language abstract.