High entropy alloys (HEAs) are classified as a new class of advanced metallic materials that have received significant attention in recent years due to their stable microstructures and promising properties. In this study, three mechanically alloyed equiatomic HEA coatings – AlCoCrFeMo, AlCoCrFeMoW, and AlCoCrFeMoV – were fabricated on stainless steel substrates using flame spray manufacturing technique. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Vicker’s microhardness were utilized to characterize the fabricated HEA coatings. Furthermore, Joule heating experiments using a modified version of a two-probe test was used to measure the electrical resistivity of the HEA coatings. To prevent short-circuiting of the metallic coatings, a thin layer of alumina was deposited as a dielectric material prior to the deposition of HEA coatings. The microstructure of the HEA coatings showed the presence of multiple oxide regions along with solid-solution phases. The porosity levels were approximately 2 to 3% for all the HEA coatings. The HEA coatings had a thickness of approximately 130 to 140 μm, whereas the alumina layer was 120 to 160 μm thick. The electrical resistivity values were higher for all the HEA coatings compared to flame-sprayed Ni-20Cr and NiCrAlY coatings and AlCoCrFeNi HEA thin film, which may be attributed to the characteristics of HEAs, such as severe lattice distortion and solute segregations. The combined interaction of high hardness and increased electrical resistivity suggests that the flame-sprayed HEA coatings can be used as multifunctional wear-resistant materials for energy generation applications.

The piezoresistivity of flame-sprayed NiCoCrAlTaY on an electrically insulated surface of a steel substrate was investigated through cyclic extension and compression cycles between 0 and 0.4 mm for 1000 cycles and uniaxial tensile test. The sprayed NiCoCrAlTaY was in grid form with grid thickness of 3 mm and grid length of 30 mm while the electrical insulation was fabricated by flame spraying alumina on the surface of the steel. During mechanical loading, instantaneous electrical resistance measurements were conducted to evaluate the corresponding relative resistance change. Images of the loaded samples were captured for strain calculations through Digital Image Correlation (DIC) technique. After consolidation of the pores within the coating, the behavior of the flame-sprayed NiCoCrAlTaY was consistent and linear within the cyclic compression and extension limits, with strain values of approximately -1000 με and +1700 με, respectively. The coating had a consistent and steady maximum relative resistance change of approximately 5% within both limits. The tensile test revealed that the coating has two gauge factors due to the bi-linearity of the plot of relative resistance change against strain. The progression of damage within the coating layers was analyzed from its piezoresistive response and through back-scattered scanning electron microscopy images. Based on the results, the nickel alloy showed high piezoresistive sensitivity for the duration of the loading cycles, with little or no damage to the coating layers. These results suggest that the flame-sprayed nickel alloy coating has great potential as a surface damage detection sensor.

In a novel approach for guiding elements of sawing machines wear resistant coatings were applied on open-porous AlSi 7 Mg substrates by means of high velocity suspension flame spraying (HVSFS). The challenge is to establish a wear resistant coating but simultaneously maintain the open-porous structure that is necessary to serve as a permeable structure for liquid cooling or lubricant media under operation. In a first approach, a water-based suspension containing a mixed Al 2 O 3 -TiO 2 powder for HVSFS was used to deposit dense and well adherent mixed-oxide coatings. As the substrates exhibit an open-porous structure and a well-defined pore size distribution, transpiration cooling through the pores is possible and even necessary in order to ensure a low thermal impact on the fragile pore structure, preserving the open-porosity of the substrate. The coatings are characterized and compared by the means of light microscopy and hardness indentation.

High entropy alloys (HEAs) constitute a new class of advanced metallic alloys that exhibit exceptional properties due to their unique microstructural characteristics. HEAs contain multiple (five or more) elements in equimolar or nearly equimolar fractions compared to traditional alloy counterparts. Due to their potential benefits, HEAs can be fabricated with thermal spray manufacturing technologies to provide protective coatings for extreme environments. In this study, the AlCoCrFeMoW and AlCoCrFeMoV coatings were successfully developed using flame spraying. The effect of W and V on the HEA coatings were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and micro-hardness testing. Furthermore, performance of the coating under abrasive loading was investigated as per ASTM Standard G65. Microstructural studies showed different oxides with solid-solution phases for all the HEA coatings. Hardness results were higher for the AlCoCrFeMoV coatings followed by AlCoCrFeMoW and AlCoCrFeMo coatings. Lower wear rates were achieved for the AlCoCrFeMoV coatings compared to AlCoCrFeMoW and AlCoCrFeMo coatings. The evolution of multiple oxide phases and underlying microstructural features improved the resistance to abrasive damage for the AlCoCrFeMoV coatings compared to other HEA coatings. These results suggest that the flame-sprayed HEA coatings can be potential candidates for different tribological interfaces while concurrently opening new avenues for HEA coating utilization.

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.

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.

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.

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.

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.

In this study, NbC coatings, 250 µm thick, were deposited by low-velocity flame spraying on stainless steel substrates and were laser remelted in a controlled argon atmosphere. Isolated passes transverse of the coatings were performed at different focal lengths at speeds of 10, 15, and 20 mm/min. Using the selected laser parameters, layers were recast with eight passes at 10% superposition. Erosion-corrosion tests were performed and coating surfaces and cross-sections were characterized via SEM, EDS, and XRD analysis. Modified surfaces of dense, 800-µm thick coatings with no defects and excellent metallurgical bonding with the substrate were obtained. It was found that dilution of the coating with the substrate formed a gradient of chemical composition and mechanical properties and that erosive-corrosive wear resistance was highest for an erodent impact angle of 90°.

The economic feasibility of using thermal-sprayed heat generating coatings for temperature control in steel pipes was investigated. A data-intensive model was developed to compare fabrication, installation, operation, and maintenance expenditures with those of conventional heating cables. The multi-layered coating consists of flame-sprayed Al 2 O 3 and NiCr layers and cold-sprayed copper. Scalability factors were incorporated in the model to estimate the total projected costs for fabricating the coatings as opposed to installing heat tracing. Although material costs for the coating and heat tracing were approximately the same, the cost of fabrication for the coating was higher due mainly to labor expenses. However, the coating-based system was found to be more energy efficient than heat tracing due to the good adhesion and reduced thermal contact resistance between the heating elements and pipe.

This study investigates the effect of incorporating different reinforcing particles on the microstructure, electrical resistance, and heating efficiency of flame-sprayed nickel-based coatings. Feedstock powders were prepared by adding Al 2 O 3 , TiO 2 , and WC particles to NiCrAlY powder, and the various combinations were applied to alumina-coated carbon steel substrates. A number of Joule heating experiments were conducted by creating voltage differences across the coatings and measuring temperature changes due to induced electron flow and associated resistive heating. It was found that the electrical properties of the ceramic particles have a major effect on heat generation and that there is considerable room for improvement.

In this study, icephobic polymer coatings were produced by flame spraying using different process parameters. Process optimization for low-density polyethylene (LDPE) coatings was achieved through design of experiments. The most icephobic coating was produced at a traverse speed of 900 mm/sec and a spraying distance of 250 mm. Although surface roughness affected ice adhesion, thermal effects proved to be the main factor influencing the performance of the coating. The higher the processing temperature, the smoother the surface and the greater the polymer degradation. It is also shown that coating degradation can be caused during post heating steps with similar consequences in the ice-shedding performance of the LDPE coatings.

For the engines used in small turboprop aircrafts, the introduction of abradable coatings represents a feasible way to reach higher levels of overall engine efficiency, specifically by improving the fuel consumption and increasing the inter turbine temperature margin. Abradable coatings on seals also contribute to improved hot restarts capability of an engine and lead to substantial extension of service life of the rotating counter bodies. In our contribution, we concentrate on flame sprayed nickel graphite abradable coating that can be used in turboprop engines both for seals and clearance control. The focus is the impact of spraying parameters on the physical and function properties of the abradable coating.

Minimizing ice adhesion and preventing ice accumulation on different surfaces has remained in scientific focus from the mid 1900’s. Decades after, scientists are combatting the same challenges, which only outlines the complex nature of ice and ice adhesion related research. One approach to exploit passive coating technology for low ice adhesion utilizes slippery surfaces that combine porous solid material with lubricating liquid. This study presents a novel method in creating functional slippery liquid infused porous surfaces, SLIPS, by exploiting flame spray technique. We demonstrate the functionality of these thermally sprayed SLIPS in ice adhesion and wettability. The ice adhesion results confirmed the potentiality of thermally sprayed SLIPS as ice repellent surfaces with an order of magnitude lower ice adhesion strength compared to polished stainless steel.

This study aims at evaluating the erosion resistance at temperature of several hard coatings, including: CrC-NiCr by HVOF, Fe-based alloy by Arc Spray, NiCrBSiFe by powder flame spraying. These coatings are to be used for the recovery of highly eroded walls (above 10 mm thickness) of gray cast iron in the exhaust ducts in heavy-fuel engines. The erosion test consists of erosive particles thrown through a high temperature gas jet, for 5 cycles of 5 minutes, according to ASTM G211-14 (modified). Coated samples are subjected to a fuel gas-torch reaching a front temperature of 450ºC and a back temperature of 90ºC (water cooled), simulating the actual application. The eroded samples are characterized using EDS, and SEM. The results show the erosion rate of each material/system, and their corresponding erosion mechanisms. Thus, the results allows for the selection of an optimum coating for this surface recovery application.

Marine biofouling has emerged as worldwide serious problems for artificial marine infrastructures. Among the measures taken so far to solve such problems, construction of an antifouling layer has been proven to be effective in offering long-term antifouling performances. Antifouling based on the use of biocides is the most important method in modern maritime industries. While tributyltin (TBT)-based self-polishing coatings are being replaced by other biocide-releasing coatings, the environmental toxicity of these compounds is also under scrutiny. Therefore, there is a significant interest in developing non-toxic technologies. Green biocides can also be extracted from many types of organisms including terrestrial plants, sea creatures and bacteria. In this study, flame sprayed polyethylene (PE)-capsaicin composite coatings were developed for marine antifouling applications. Capsaicin powder were fixed by polymer-based substrate and distributed evenly. Antifouling test indicated excellent antibacterial properties of PE-capsaicin composite coatings against adhesion of marine Bacillus sp. bacteria. Prohibited formation of biofilm on the surfaces of the thermal sprayed composite coatings gives clear insight into their potential applications as antifouling layers in the marine environment.

Niobium and its alloys have been used for several industrial applications including high strength low alloy steels (HSLA) in the automotive and aerospace industries, mainly because of being highly corrosion resistant in different media. Niobium pentoxide is an excellent option to reduce costs in repairing damages caused by corrosion, protection of industrial equipment or systems for being chemically inert to corrosive agents that cause severe corrosion and are normally present in refineries and other industrial environments. Whatever the application, mechanical properties of coatings define their effective durability and performance, mainly for tribocorrosion applications. In this work, these properties are studied and correlated for Nb 2 O 5 coatings applied by Flame Spray onto steels. Salt Spray corrosion (ASTM B117), abrasive wear resistance (ASTM G65-16) and scanning electron microscopy (SEM) microstructural analysis allowed the evaluation of the coatings.

Porous copper coatings, which act as wicks for liquid transport, were fabricated using a flame spraying process. Copper and aluminum powders were fed independently into the spray torch and deposited on copper substrates to form a composite coating. The aluminum was subsequently removed using chemical leaching leaving a porous copper coating behind. Varying the feed rate of aluminum powder allowed the coating porosity to be controlled. Channels to enhance liquid flow were made in some of the porous copper coatings by placing pieces of aluminum wire mesh on the copper substrate before spraying. During spraying the sprayed powders passed through the mesh opening and created pyramid shaped arrays on the substrates. The groove width was controlled by using different wire mesh sizes. Coatings were made with porosity varying from 2 to 44 %, and groove width ranging from 0.16 to 0.53 mm. The capillary performance of the coatings was evaluated experimentally by measuring the rate of rise of ethanol in the coatings. The rate of rise increased with coating porosity, and decreased with groove width.

Suspensions have shown a great potential for being employed as the spraying materials in flame spraying processes with the aim of producing thin and dense coatings. The internal axial injection of a suspension within processes like the high-velocity suspension flame spraying (HVSFS) offers the advantage of complete suspension entrainment within the gas stream, which therefore results in enhanced momentum and heat transfer to the particles. Experimental assessment of the achieved particle velocities and temperatures within the combustion chamber is nonetheless practically infeasible. A better insight into the process is attainable through employing computational simulations. Following a computational fluid dynamic (CFD) modelling approach for HVOF processes, combustion and gas flow turbulences were simulated for different combustion chamber geometries and ethene/oxygen ratios commonly used in the HVSFS process. Simulations were done with the commercial software ANSYS CFX 16.2. To account for the highly turbulent flow characteristics, the k-ε model and the Shear Stress Transport (SST) turbulence model were chosen, employing an eddy-dissipation-model for fuel gas combustion. Second-order upwind discretization was used to enable a good resolution of flow features like shock diamonds. The results of the simulation using different levels of detail of the combustion reaction were compared to experiments employing the modelled combustion chambers and gas flows. Chamber pressure and positions of the shock diamonds were monitored in order to allow a qualitative evaluation of the calculated values.