Abstract In previous studies at McGill University, tin was successfully cold sprayed onto carbon fiber reinforced polymers (CFRPs). A “crack-filling” mechanism was described as the deposition mechanism that allowed deposition of tin onto the CFRP. Improving the coating conductivity for lightning strike protection (LSP) purposes was achieved by adding other metal powders (aluminum, copper, zinc) to tin and cold spraying on the CFRP. At the same time, it was noticed that the addition of this secondary component (SC) provided an increase in deposition efficiency (DE), tamping was initially hypothesized to explain this improvement, thus prompting a study solely on the effect of SC hardness, which is reported elsewhere in this conference. However, it is recognised that other powder characteristics may also be influencing the DE. Thus, in this study, SCs with a wider variety of particle sizes, morphologies, densities and hardness values were mixed with tin and sprayed on CFRPs. The effect of SC properties on tin deposition is discussed and an optimal combination of SC properties for cold spraying of tin is suggested.

Abstract In this paper, the phenomenological behaviour of gas flow and particles motion during cold spraying has been studied. Observations of particles behaviour show two features: a uniform jet over a short distance ahead of the nozzle exit and then, a progressive dispersion. These behaviours are explained using a computational analysis based on a direct numerical simulation of the gas flow and the kinematic interactions with the particles. The CFD computation demonstrates that the gas stream starts to be unstable inside the nozzle with more turbulence as it moves towards the exit of the nozzle. The flow is self-oscillated along the flow direction and drives the motion of the Cu particles outside the nozzle. The zone of gas flow instability does correspond to the zone of experimental particle dispersion. Outside the nozzle, the particles form a straight jet over a certain distance that corresponds to the zone of the experimental uniform particles jet. Then, they are deviated and become more and more dispersed towards a very sparse jet along the flow direction. This phenomenon is explained by a Magnus lift force that deviates the particles trajectory when the gas flow becomes highly turbulent while developing a vorticity shedding.

Abstract Cold spray process was chosen as a good candidate for dimensional restoration and protection of components. Commercially pure aluminum, aluminum-alloy or titanium were recommended for different applications. This paper investigates laser surface texturing association to enhance durability of sprayed coatings. Laser is easy automated, localized and reliable process. It was applied for prior-surface treatment. Textured surfaces were produced and compared to conventional treatments, such as grit-blasting, in terms of deposition efficiency and adhesion bond strength. Patterns promoted direct particle embedment. Particle-substrate interface exhibited significant temperature rate and strain in cavities. Intimate contacts and particle compressive states were assumed responsible for improvement. The particle deformation and bonding behaviors were evaluated and discussed for the different configurations. Thus, window of deposition was increased with laser surface texturing. Anchoring mechanisms increased two fold the adhesion strength compared to conventional pre-treatments. In one case, the interface was stronger than the coating cohesive strength.

Abstract In cold spraying, coatings are formed by a high velocity impact of solid particles. The particles are accelerated in a supersonic gas jet at temperatures of only a few hundred degrees centigrade. In contrast to thermal spray processes no melting of the particles and negligible heating of the substrate occurs. A series of spray experiments with copper powders of different particle size ranges were performed to study the effect of various process parameters on microstructure and properties of the coatings. The coatings have been evaluated for their microstructure, density, oxygen content, hardness and bond strength. With nitrogen as process gas and a -25 +5µm powder, dense coatings were obtained within a broad range of gas inlet pressure and gas inlet temperature.

Abstract In the cold spray process, cross-sectional shape of the nozzle has a significant effect on spray pattern of coatings. The circular exit nozzle is parabolic in shape. So, spray pattern with the rectangular nozzle is wider than that with the circular spray nozzle. The goal of this investigation is to establish a design for the cold spray gun nozzle to gain more uniform spray profile of coatings. We have investigated the influence of expansion ratio, nozzle total length and the ratio of nozzle length of divergent section and parallel section of rectangular nozzle on behaviors of gas and particle by the computational fluid dynamics (CFD) in high pressure cold spraying. We have studied copper particles so far. In this study, we will examine aluminum particles. First, we investigate the influence of the size and shape of the rectangular section nozzle on the velocity, temperature, and particle distribution of aluminum particles by CFD. After that, the rectangular section nozzles were fabricated and coating formation experiments were conducted, spray patterns and coating cross-sectional structures were observed, and coating adhesion was also evaluated. The nozzle material was polybenzimidazole resin, which is difficult for aluminum particles to attach to nozzle walls.

Abstract FeMnCrSi and 316L alloy coatings were deposited on carbon steel substrates via high-pressure cold gas spraying and their microstructure, hardness, and wear resistance were obtained. Ball-on-disk testing (ASTM G99) was used to measure sliding wear behaviors. The mechanism of wear was found to be the same for both coatings, although FeMnCrSi had a higher coefficient of friction while 316L had less volume loss.

Abstract Grinding wheels are usually manufactured by powder metallurgical processes, i.e. by moulding and sintering. Since this requires the production of special moulds and the sintering is typically carried out in a continuous furnace, this process is time-consuming and cost-intensive. Therefore, it is only worthwhile for medium and large batches. Another influencing factor of the powder metallurgical process route is the high thermal load during the sintering process. Due to their high thermal sensitivity, superabrasives such as diamond or cubic boron nitride are very difficult to process in this way. In this study, a novel and innovative approach is presented, in which superabrasive grinding wheels are manufactured by thermal spraying. For this purpose, flat samples as well as a grinding wheel body were coated by low-pressure (LP) cold gas spraying with a blend of a commercial Cu-Al2O3 cold gas spraying powder and nickel-coated diamonds (8-12 μm). The coatings were examined metallographically in terms of their composition. Afterwards, the grinding wheel was conditioned for the grinding application and the topography was evaluated. This novel process route offers great flexibility in the combination of binder and hard material as well as a costeffective single-part and small-batch production.

Abstract This present work investigates the effect of electromagnetic fields on cold spray processes by means of an induction-heating cold spray (IHCS) system. Aluminum powder was cold sprayed onto inductively heated Ti6Al-4V (Ti64) substrates. These materials were selected to minimize the mechanical contribution to coating adhesion. As a result, changes in coating adhesion strength can be attributed to improved metallic bond formation due to the effect of the electromagnetic field. Four different initial substrate surface temperatures were used in the study to assess the role of initial temperature as well. Deposition efficiency and adhesion and tensile strength measurements were recorded and are used to characterize the hybrid coating process and compare it with traditional techniques.

Abstract 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.

Abstract The hot-section components of modern gas turbines (e.g., turbine blades and vanes) are typically manufactured from Ni-base superalloys. To develop the γ/γ' microstructure that imparts superior thermomechanical and creep properties, Ni-base superalloys usually require three distinct heat treatments: first a solution heat treatment, followed by primary aging, and finally secondary aging. To achieve oxidation resistance, MCrAlY coatings are applied on the superalloy components as either environmental coatings or bond coats for thermal barrier coatings. In this study, the effects of different processing sequences on MCrAlY coating characteristics and short-term isothermal oxidation performance were investigated. Specifically, cold spray deposition of NiCoCrAlTaY coatings was carried out on single-crystal Ni-base superalloy substrates that underwent various degrees of the full heat treatments prior to being coated. The remaining required heat treatments for the superalloy substrates were then performed on the coated samples after the cold spray deposition. The microstructures of the CMSX-4 substrates and NiCoCrAlTaY coatings were characterized after each heat treatment. Isothermal oxidation performance of the coated samples prepared using different sequences was evaluated at 1100°C for 2 hours. The results suggested a promising procedure of performing only solution heat treatment on the superalloy substrate before coating deposition and then primary aging and secondary aging on the coated samples. This processing sequence could potentially improve the oxidation performance of MCrAlY coatings, as the aging processes can be used to effectively homogenize coating microstructure and promote a thin thermally grown oxide (TGO) scale prior to actual isothermal oxidation.

Abstract Because of their high specific strength, carbon fiber reinforced plastics (CFRPs) are widely used in the aerospace industry. Metallization of CFRP by cold spraying as a surface modification method can improve the low thermal resistance and electrical conductivity of CFRP without the need for high heat input. Herein, we cold spray a Sn coating on cured CFRP substrates and examine the Sn/epoxy interface. The results suggest that the Sn coatings are successfully obtained at a gas temperature of 473 K and indicate no severe damage to the CFRP substrates. The stress and plastic strain distributions at the cross-section of the Sn/CFRP interface when a Sn particle is impacted onto the CFRP substrate are obtained using the finite element method.

Abstract Unlike their metal counterparts, composite structures do not readily conduct away the electrical currents generated by lightning strikes. Cost reduction and expected production growth of the next middle range airplanes require automated manufacturing process of polymer components. The development of an automated technology to metallize polymer based composite for lightning strike protection is the aim of the CO3 project (EU Grant agreement: ID831979). In this study, thermal and electrical conductivities of composites were achieved by cold spray deposition of Cu or Al coatings. Critical points to be addressed were substrate erosion during cold spray, lack of polymer-metal adhesion and poor deposition efficiency. Several strategies were tested: i) a thin polymer film was cocured at the substrate surface before cold spraying, to enable implantation of metallic particles in the film, helping coating build-up and protecting the fibers of the composite. ii) Cold spraying a mix of metal and polymer powders to improve coating adhesion and prevent fiber damage. iii) Supercritical Nitrogen Deposition technology, prior to cold spray, to mechanically anchor metallic particles into the polymer. Subsequent cold spraying of purely metallic coatings was more efficient and showed better adhesion. All coatings were tested in terms of adhesion strength and electrical conductivity.

Abstract Diamond-reinforced composites prepared by cold spray are emerging materials simultaneously featuring outstanding thermal conductivity and wear resistance. Their mechanical and fatigue properties relevant to perspective engineering applications were investigated using miniature bending specimens. Cold sprayed specimens with two different mass concentrations of diamond 20% and 50% in two metallic matrices (Al – lighter than diamond, Cu – heavier than diamond) were compared with the respective pure metal deposits. These pure metal coatings showed rather limited ductility. The diamond addition slightly improved ductility and fracture toughness of the Cu-based composites, having a small effect also on the fatigue crack growth resistance. In case of the Al composites, the ductility as well as fatigue crack growth resistance and fracture toughness have improved significantly. The static and fatigue failure mechanisms were fractographically analyzed and related to the microstructure of the coatings, observing that particle decohesion is the primary failure mechanism for both static and fatigue fracture.

Abstract The low-pressure cold spray (LPCS) technique could be an attractive method for copper metallization of ceramic substrates to power module applications due to its one-step quick and low-temperature process. However, manufacturing pure copper coating on a ceramic substrate by LPCS is still challenging due to its low deposition efficiency and poor adhesion strength. Our previous study successfully demonstrated the possibility of obtaining a zirconia substrate's metallization by using a feedstock powder mixture of copper and aluminum. However, the copper content in the coating was not high enough for power module applications. Therefore, in this study, we aim to improve the copper content in the coating layer composed of the composite powder deposited by LPCS on alumina and zirconia substrates. The influence of the gas pressure and standoff distance on the copper content and coating thickness are evaluated. The coating build-up with a high copper content and thickness is highly dependent on the kinetic energy of particles, enhanced by high gas pressure and short stand-off distance.

Abstract This study investigates the potential of cold-sprayed tungsten coatings for use in nuclear fusion reactors. Three commercially available tungsten powders were selected from which six series of feedstock were prepared. The feedstocks were deposited on aluminum, steel, and stainless steel substrates using high-pressure nitrogen cold spraying. The coatings produced were characterized based on SEM, EDX, and XRD analysis and were found to be free of oxides with levels of tungsten that were previously unachieved. The results indicate that cold spraying is a viable technology for applying tungsten-base coatings to critical components in nuclear fusion equipment.

Abstract Light water reactors (LWR) use zirconium-alloy fuel claddings, the tubes that hold the uranium-dioxide fuel pellets. Zr-alloys have very good neutron transparency, but during a loss of coolant accident or beyond design basis accident (BDBA) they can undergo excessive oxidation in reaction with the surrounding steam environment. Relatively thin oxidation-resistant coatings on Zr-alloy fuel cladding tubes can potentially buy coping time in these off-normal scenarios. In this study, cold spraying, solid-state powder-based materials deposition technology has been developed for deposition of oxidation-resistant Cr coatings on Zr-alloy cladding tubes, and the ensuing microstructure and properties of the coatings have been investigated. The coatings when deposited under optimum conditions have very good hydrothermal corrosion resistance as well as oxidation resistance in air and steam environments at temperatures in excess of 1100 °C, while maintaining excellent adhesion to the substrate. These and other results of this study, including mechanical property evaluations, will be presented.

Abstract In this work, Inconel 718 gas-atomized powder was successfully heat treated over the range of 700-900°C. As-atomized and as-heat treated powders were cold sprayed with both nitrogen and helium gasses. Cold spray of high strength materials is still challenging due to their resistance to particle deformation affecting the resulting deposit properties. Powder heat treatment to modify its deformation behavior has recently been developed for aluminum alloy powders, however, there is no literature reported for Inconel 718 powders. The microstructural evolution of the powder induced by the heat treatment was studied and correlated with their deformation behavior during the cold spray deposition. Deposits sprayed with heat-treated powders at 800 and 900 °C and nitrogen showed less particle deformation and higher porosity as compared to as-atomized deposit associated to the presence of delta phase in the powders precipitated by the heat treatment. In contrast, deposits sprayed with helium using both powder conditions, as-atomized and as heat-treated powders, showed high particle deformation and low porosity indicating that the type of gas has a greater effect on the particle deformation than the delta phase precipitated in the heat-treated powders. These results contribute to understanding the role of powder microstructure evolution induced by heat treatment on the cold spray deposits properties.

Abstract Industries developing cold-spray processes aim at producing dense and resistant coatings. Controlling microstructure and inter-particular fracture characteristics of sprayed coatings is essential to improve their properties. To do so, post-spraying heat treatment is a promising approach. This work addresses the development of such heat treatments and focuses on the analysis of recovery and recrystallization. Different heat treatment parameters were explored, namely, holding temperature and time, heating rate, and heating method. This approach revealed a competition between recrystallization and other microstructural evolution mechanisms, such as precipitation and porosity coalescence. An optimized heat treatment, allowing microstructural softening and adequate mechanical properties, was sought after. First, differential scanning calorimetry measurements applied to as-sprayed coatings enabled to identify recovery and recrystallization temperature ranges. Then, a variety of heat treatments was applied, involving long-time isothermal holdings as well as shorter cycles. Microstructure analysis and hardness measurements allowed making a first selection of treatment conditions.

Abstract Residual stress can be developed in most thermally sprayed coatings due to the momentum of molten particles during impact, and heat transfer during solidification of the splats. Another reason for residual stress built-up in thermally sprayed coatings is due to splat curl-up during solidification and the differences in thermal expansion coefficients between the coating and the substrate. However, in the cold spraying process, it is believed that the main reason for residual stress formation is plastic deformation during impact and flattening of solid particles. Residual stresses can drastically influence coating quality and reduce its service time. In this study, residual stress is measured for two well-known nickel based super alloys (Inconel 625 and Inconel 718) deposited on 7074 aluminum alloy substrates by the cold spraying technique. Residual stress in Inconel 625 was found to be highly tensile on the surface and compressive on the subsurfaces. After heat treatment the residual stress was relieved and was compressive in nature. Whereas for Inconel 718, residual stress was compressive on the surface and tensile on the subsurfaces in the as-sprayed condition. After heat treatment, the residual stress was compressive with increased magnitude. The heat treatment at 800°C made the residual stress more compressive. The porosities of both Inconel 625 and Inconel 718 were reduced after heat treatment.

Abstract Low pressure cold spraying is an attractive technique for onsite metal coating fabrication due to its compactness and portability. However, the bonding strength of the coating prepared by low pressure cold spraying is generally low, which restricts the further applications in engineering and industrial fields. To improve the bonding strength, pre-treatment on substrate surface can be an effective procedure. In this study, a low-temperature plasma treatment was applied to a pretreatment technique, and the effect of the treatment on particle bonding was compared with that of a laser treatment. Copper coatings on aluminum and copper substrates were selected and studied as basic metal materials. The SEM observation results show that the particle adhesion rate significantly increases by the laser and plasma treatments, due to the removal of the native oxide films on the substrates. The particle bonding on the plasma-treated substrate reveals better interfacial adhesion with less gap compared with the laser-treated one. The pre-treatment by low-temperature plasma can be an attractive technique to assist the cold spraying process due to the oxide removal ability and no thermal effect which can apply a wide range of materials.