Alkaline water electrolysis is currently the most promising approach to produce hydrogen. However, a main limitation for large-scale application originates from the significant energy loss caused by the coverage of bubbles on the electrode surface. Here, pore-graded Ni electrodes with a positive and negative gradient porous structure that boosts the desorption and release of gas bubble are reported, resulting in a greatly advanced mass transference. The electrodes are obtained from a blend of Ni and Al via high-pressure cold spray. The gradient porosity is realized by varying the addition of Al and chemical etching. As-sprayed electrodes are annealed to eliminate the residual stress and strengthen the adhesion of layers, hence improving their durability. As a result, the electrode with a positive pore-graded structure exhibits a better HER/OER performance when tested with a carbon rob counter electrode. Notably, when tested with an annulus counter electrode of Nickel foam, the electrode with a negative pore-graded structure achieves minimal HER/OER overpotential, outperforming other porous electrodes. This is benefited from improved bubble removal and mass transference capability. All prepared electrodes showed an excellent stability that after 500 cycles of HER/OER test without a large potential fluctuation.

Metallization of polymers and fiber-reinforced polymer composites is gaining attention due to the widespread application of these components in various industries, such as wind energy, aerospace, and automotive industries. Cold spray is a promising new technique to achieve the metallization of polymer and fiber-reinforced polymer composites. This work investigates the deposition mechanisms of polymer-coated metallic particles on polymer-based substrates by finite element analyses. Impact mechanics of PEEK-coated nickel particles impacting PEEK and carbon fiber-reinforced PEEK substrates are modeled. Results show the prominence of mechanical interlocking of metallic particles in the substrate, which occurs due to their entrapment inside the substrate, caused by the high energy impact-induced welding of scraped PEEK coating. The PEEK coating acts as a cushioning component, effectively mitigating the impact energy of the metallic component. The scraped PEEK coating also accumulates on the upper half of the particle, forming a cap welded to the substrate and sealing the metallic particle inside. It is observed that the depth of the carbon fiber mat in the substrate affects the mechanism and the success of deposition.

Thermal spray processes benefit from workpiece cooling to prevent overheating of the substrate and to retain metallurgical properties (e.g., temper). Cold-gas “plume quenching” is a plume-targeting cooling technique, where an argon curtain is directed laterally above the substrate surface to re-direct high temperature gases without impacting particle motion. However, there has been little investigation of its effect on the molten particles and the resulting coating properties. This study examined high- and medium- density tantalum and nickel coatings, fabricated by Controlled Atmosphere Plasma Spray with and without plume quenching on aluminum and titanium substrates. To compare the effect of plume quenching, the deposition efficiency was calculated through coating mass gain, and the coating density, stiffness, and adhesion were measured. The tantalum and nickel coatings were largely unaffected by plume quenching with respect to deposition efficiencies, coating density, adhesion, and stiffness. These results indicate that a plume quench could be used without affecting the coating properties for high- and medium-density metals while providing the benefit of substrate cooling that increases with higher plume quench gas flow rates.

During the impact and solidification of thermal spray droplets on a substrate, the density increases when the droplet solidifies. Depending on the material, the changes in density could be significant. For example, aluminum oxide's density changes by 66%, while the changes are 12% and 19% for nickel and copper, respectively. For zirconia, this change is 24%. The effect of such densification on the dynamic of the droplet impact and the formation of porosity could be dramatic. In this study, the effect of shrinkage of a molten droplet during solidification on droplet impact is numerically investigated for several materials. Results for the impact of molten alumina, nickel, copper, and zirconia droplets on both smooth and rough surfaces are presented. The results of variable density cases are compared with those assuming constant density. The effect of thermal shrinkage is particularly vital in the interaction of two impacting droplets. The shrinkage promotes the formation of additional pores.

This paper discusses the challenges of constructing mathematical models of physicochemical and heat-mass transfer processes associated with reactive heterogeneous materials used in laser additive manufacturing. The results of calculations of thermocapillary convection induced by laser heating in an aluminum melt with an admixture of nickel particles are presented. Models of interphase and chemical interactions with the formation of intermediate phases and intermetallic compounds on nickel particles added to the melt during laser alloying or cladding are proposed, which make it possible to calculate the composition of intermetallic phases in the trace of the beam after crystallization and cooling.

Chemical composition differences between the feedstock powder and the final coating of a series of composite abradable coatings were investigated. Graphite filler material mass distributions in the coating, overspray powder and burned power is calculated. A preliminary mechanism of graphite loss during the deposition process is established. It is found that the graphite content in the coating is significantly lower than that in the feedstock powder. Over 70% graphite in the feedstock powder is lost during the deposition process. Melting and shrinkage of the nickel shell of the nickel cladded graphite particle as flying through the flame, which resulted in the exposure of the graphite core to the flame and substrate, is the main reason for graphite loss and chemical composition change between the feedstock powder and the final coating. A random manner of particle structure transformation in the flame and its reactions with the spray environment is concluded as an important reason for the poor process repeatability of abradable coatings.

In the course of this investigation, thermal spraying with different fuel and shroud gas combinations was investigated in terms of its effect on the in-flight particle properties (temperature, velocity) and on the final coating properties (coating thickness, porosity, oxygen content and corrosion behaviour). Independent on the shroud gas, the particle in-flight temperature and velocity were highest when using ethylene as fuel gas and lowest when using propane. Methylene resulted in intermediate properties. The change in the shroud gas from air to nitrogen generally resulted in lower in-flight particle temperatures and also lower velocity. The coating properties in terms of porosity and oxygen content directly correlated to the particle in-flight properties. With decreasing velocity and increasing temperature, the porosity and the oxygen content increased, respectively. The corrosion behaviour of the nickel coatings was studied in 0.5 M sulfuric acid media by means of potentiodynamic polarization curves. Good corrosion properties were observed when methylene and air served as fuel gas and shroud gas, respectively.

B 4 C-Ni powders ranging in content from 5-60 wt% Ni were fabricated by pressurized hydrogen reduction and deposited on mild carbon steel substrates by air plasma spraying. The microstructure, morphology, and phase composition of the powders and coatings were evaluated by means of SEM and XRD analysis. The influence of Ni content on coating microstructure, fretting wear resistance, hardness, and adhesive strength was investigated in detail. The results show that Ni affects fretting wear resistance, which was found to be highest in the coating with 40 wt% nickel. The B 4 C-40Ni coating also proved superior in terms hardness, porosity, and friction coefficient, although its adhesive strength was the lowest.

In this study, cold sprayed Ni is deposited on Al substrates using different gas pressures. Spherical Ni powder was sprayed on cylindrical substrates using argon as the powder carrier and compressed air as the propellant. Coating and splat surfaces and cross-sections were examined, adhesion strength was measured, and particle velocity and temperature were determined through CFD simulations. The results show that denser, more well adhered coatings were obtained under higher propellant pressure. Higher gas pressure increases particle velocity, which intensifies material deformation and the disruption of surface oxides in the impact area, resulting in greater metallurgical bonding between the splats and the substrate. The formation of Ni-Al intermetallic phase at the interface region due to heat treatment was confirmed and its effect on bonding strength is discussed.

Resonant ultrasound spectroscopy was applied to analyze the elastic anisotropy of thick copper, aluminum, titanium, and nickel coatings prepared by cold spraying and to determine the respective elastic moduli. The results show that the coatings exhibit only weak deviations from perfect isotropy, and the obtained elastic moduli are comparable with those of the corresponding polycrystalline bulks. The increased internal friction observed in some of the studied coatings may indicate grain refinement and consequent grain boundary sliding.

This work studies the thermal and hydrodynamic performances of pyramidal fin arrays produced using the cold spray process as an additive manufacturing process. Near-net shaped pyramidal fin arrays of various materials were manufactured (pure aluminum, pure nickel and stainless steel 304). Fin array characterization such as fin porosity level and surface roughness evaluation was performed. The nickel pyramidal fin array is shown to be rougher compared to the two other materials used in this study. The results obtained show a lower thermal efficiency for stainless steel 304 whereas the performances of the aluminum and nickel fin arrays are similar. The multi-material sample has a better thermal efficiency than stainless steel 304, which constitutes the proof of concept of using a streamwise anisotropic fin array.

To assess workplace exposure to nickel during thermal spraying, an air quality study was conducted at a thermal spray facility between 2007 and 2010 and 22 thermal spray operators were monitored. Results show that certain activities and operations have the potential for high exposure to nickel compounds. It is also shown that the biological agent tolerance value (BAT-value) of water-insoluble nickel compounds can be respected if specific technical, organizational, and personal-protective measures are consistently implemented.

In this study, a nickel-coated Al 2 O 3 powder, produced by hydrothermal hydrogen reduction, is cold sprayed with the aim of increasing the volume fraction of alumina particles in the nickel matrix. Spray trials were conducted to investigate the effect of oxidation temperature on the microstructure and hardness of as-sprayed as well as heat treated Ni-Al 2 O 3 composite coatings. Results show that oxidizing has a significant effect on microstructure and hardness and that the morphologies of oxide films differ depending on the heat treatment.

Even though cold spraying is well established today, the handling of some materials, as for example nickel, is still challenging because of nozzle clogging at higher process gas temperatures. Due to these process limitations in the past, it was not possible to produce coatings of sufficiently high quality. These problems can be solved by nozzle modifications. By a more effective nozzle cooling system, nozzle clogging during cold spraying of nickel can be avoided. This enables the use of high-end spray conditions, attainable by a modified cold spray gun heating system. For the presented study, the coatings were produced by varying the spray parameters up to a process gas temperature of 1000 °C at a pressure of 5 MPa with a gas flow of about 160 m 3 /h - conditions that were not within reach in cold spraying so far. Coating microstructures are nearly fully dense, with negligible porosity below 0.1%. The coatings processed under these high-end parameters show high tensile strength of 570 MPa, similar to that of highly deformed bulk material and a low hardness of 200 HVIT similar to that of pure nickel. The results demonstrate that the quality of cold-sprayed nickel coatings significantly improves with higher process gas temperatures and pressures, pushing cold spraying to new limits. This may also apply to other high-strength metals and alloys, opening the opportunity to meet the quality requirements for many new applications.

A series of Ni-based cored wires with different boron contents were designed to prepare corrosion-resistant coatings by two-roll wire-arc spraying. These coatings were evaluated for their potential to provide added protection and reduced maintenance for applications in waste-to-energy (WTE) plants. The as-deposited coatings, which primarily are composed of nanocrystalline particles, exhibit uniform and dense layered structures with porosity of about 3%. The investigators selected thermo-gravimetric techniques to evaluate the high-temperature corrosion behavior of the coatings in molten salt environment (Na 2 SO 4 -10 wt% NaCl) at 800°C. The coated surfaces exhibited significantly reduced corrosion rates in comparison to those of the SA 213-T 2 substrate during all tests. These results were due to the formation in the coatings of composite surface oxide films, including Cr 2 O 3 and NiCr 2 O 4 , which serve to prevent the diffusion or penetration of corrosive species. Furthermore, the boron content appears to have a significant influence on the corrosion behavior of the designed coatings: the coating with the best performance had 16 at. % B added. The wire-arc sprayed Ni-based coatings could be an effective and economical treatment to prevent corrosion and extend the lifetime of super-heater tubes in WTE plants.

Nickel – yttria stabilized zirconia coatings and nickel – samaria doped ceria coatings were fabricated by solution precursor plasma spraying using the Northwest Mettech Axial III plasma torch. Three sets of plasma spray parameters were used resulting in comparatively low, intermediate, and high plasma powers of 63 kW, 102 kW, and 152 kW, respectively. The high and low power conditions resulted in powdery type coatings with poor adhesion to the substrate and between particles. The intermediate power conditions resulted in harder coatings with improved adhesion and electrical conductivity.

In rocket engine combustion chambers the cooling channels are subjected to extremely high temperatures and environmental attack. Because of the good heat conduction the inner combustion liner is made of copper. Thermal and environmental protection can be provided by Thermal Barrier Coating Systems. The performance of an APS-sprayed standard coating system for nickel based substrates (NiCrAlY and YPSZ) on copper substrates is investigated. Because mechanical and thermal properties (e.g. the coefficients of thermal expansion) of the two substrates are different, known failure mechanisms for nickel based substrates can not be directly transferred to the new application. Thermal cycling and laser shock testing is performed to identify possible failure mechanisms. The laser shock setup consists of a high-power diode laser (3kW) and realizes surface temperatures of up to 1500°C. Furthermore, it is possible to realize high thermal gradients inside the specimen, similar to those in real service. Delamination of the thermal barrier coating at the interface between bond coat and substrate is observed. Usually, this interface is not failing in standard applications, which gives an important hint for further research. Furthermore, FEM analysis confirms that stresses are maximal at this interface.

Coldspray is becoming a consolidated technology in deposition of several metallic coatings such as aluminium, titanium, nickel, copper and their alloys. However, increasing attention is paid to the capabilities of coldspray to deposit composite Cer-Met coatings starting from powders blends. High ceramic content coatings are difficult to get by Cold-Spray technique due to the low ductility of the ceramic phase, but as for metal coatings the low deposition temperature reduce the residual stresses, coating porosity and oxidation. Furthermore, using powder blends, obtained by quickly mixing the pure components, leads to a great versatility on materials usable for deposition. In present work, the influence of alumina content on coldspray deposition of Ni-Al 2 O 3 coatings was studied. Five different powder blends were prepared, the ceramic content was set as 10, 25, 50, 75 and 90 weight percent of alumina. Ni-pure coatings have been prepared and compared with composites coatings. Characterization techniques were employed in order to determine the coating morphology and microstructure , with special attention to determine the real alumina content in the coating with respect to initial powder feedstock. The final coating porosity and the matrix hardness were also evaluated. Finally, the performance of the coatings in terms of oxidation resistance was determined and discussed.

In order to explore wear properties of Ni-based coating on the copper substrate, the coatings with different composition were designed. Ni-based coating, Ni-based/Al 2 O 3 and Ni-based/Al 2 O 3 /WC coatings were sprayed by HVAF on the copper substrates. Wear properties of different composite coating were measured at different loads at room temperature. The experimental results indicate that all the coatings have high wear resistance. Adding Al 2 O 3 in sprayed powders has little effect on the microstructures and wear properties of coating. The wear resistant is improved by the addition WC-12Co obviously. The best wear resistant is obtained when optimum WC-12Co content is added.

In this study, nickel-zirconia cermet layers are produced by solution precursor plasma spraying (SPPS) and compared with suspension plasma sprayed (SPS) coatings of similar content. Although nickel is uniformly distributed in both coatings, its presence in the suspension caused problems with the SPS process. With the SSPS process, precursor solutions are fragmented into droplets in which Ni, Zr, and Y are intimately mixed, resulting in very fine microstructure without the problems encountered with the SPS process. It was also found that plasma gas enthalpy and spray distance have predominant effects on in-flight pyrolysis of the elements, and that plasma gas mixture has an impact on porosity as well as the oxidation state of the nickel.