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.

In cold spray (CS) additive manufacturing process, micrometer scale particles accelerated through a supersonic nozzle are targeted on a surface with velocities in the rage of 300-1500 m/s in solid state. The impact energy of the particles leads them to deform plastically with high shear energy near the impact interface and adhere to the surface metallurgically, mechanically, and chemically. Using CS, deposition of metals, metal matrix composites, and polymers are achieved with high adhesive/cohesive strength and low porosity. Sensitivity of the CS additive manufacturing process to the variabilities in the process parameters are still being understood. Among the process parameters, particle morphology can have significant implications on drag forces, and therefore, on the particle impact velocity. This in turn affects the deposition efficiency (DE) and the quality of products. In this work, a new approach is introduced for computing DE by incorporating particle sphericity and its variation into one-dimensional numerical models. Size, sphericity, and the variability of size and sphericity of aluminum, copper, titanium, and tantalum particles are measured from static optical microscope images. The data is used for predicting impact velocity, temperature, and DE. The model results are then compared with particle velocity measurements.

Metals were deposited on components made by 3-D printing with polyvinyl alcohol (PVA), a water-soluble polymer. The polymer was then dissolved, leaving a metal layer whose surface topography was the negative of that of the polymer. This is a rapid and low-cost alternative to 3D printing directly using metal, but to succeed it is essential for the sprayed metal to adhere to the polymer substrate. Tests were done in which aluminum and copper were sprayed using a twin-wire arc spray system onto 3D printed coupons, 50 mm x 50 mm in size, made from polylactic acid (PLA), PLA mixed with metal (aluminum, copper) or carbon fiber, and PVA. Adhesion depended on substrate roughness (minimum 1-2 μm) and substrate temperature (above the glass transition temperature but below the melting temperature of the polymer). It was shown that surface features could be made with high resolution on metal components using this technique.

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.

Metal surface characteristics play a significant role in interacting with their biological environment. Copper surfaces have been identified for their antimicrobial properties. Improvement of antibacterial and antiviral performances can be tailored by surface microstructure modification. Severe plastic deformation is an effective surface modification procedure to improve the mechanical performance of metal surfaces. This technique can be adapted to obtain surface grain refinement and induce surface roughness. In this work, cold spray shot peening is used to modify copper substrate surfaces and study the effects on their antibacterial properties. To modify the grain structure of copper, different shot-peening parameters were examined. The surface roughness and microstructure were investigated by employing optical and scanning electron microscopy. The bactericidal activity of copper substrates after shot peening treatment is discussed and a comparison between the bacterial load on treated (shot-peened surface and cold sprayed copper coating) and untreated surfaces (as-received) is provided. Testing of the surfaces after their exposure to the biological environment demonstrated improved microbial inactivation performances for surfaces that had undergone grain refinement without exceeding a certain roughness value.

In cold spray, high adhesion of soft materials on hard substrates is commonly achieved by using helium as the propelling gas. This is the case of copper coatings on steel where adhesion may reach values as high as 60 to 80 MPa (glue failure), however, helium is a limited, expensive natural resource, and the use of more abundant nitrogen gas is preferred in an industrial setting. Unfortunately, when using nitrogen gas, little to no adhesion is obtained. In order to eliminate the use of helium gas we studied how laser assisted cold spray could lead to an improvement in adhesion of nitrogen sprayed copper coatings. In this work, several laser parameters (e.g., power and spot size) and process parameters (traverse speed, relative position laser spot vs. gas jet) were varied at a coupon level. Upon optimization, an equivalent adhesion to the coatings prepared with helium was obtained. Furthermore, the cross section of the coatings showed that the copper particles penetrated the steel, similar to what is observed when using helium gas. Optimization of these parameters for application to large diameter (~559 mm) cylinders was also performed. A discussion on the mechanisms which contribute to achieving high adhesion considering the use of helium versus laser assistance is provided.

Cold spray (CS) is characterized as a solid-state process of high deposition efficiency for metallic coatings as well as additive manufacturing of metals. However, due to high velocity impact and extensive deformation of particles during CS, the as-received coatings or deposits may present anisotropic characteristics which could influence the performance of deposits. Hence this study aims to investigate the anisotropic behaviors of CS copper deposits in a systematic way. The microstructure and micromechanical properties of the deposits both in the cross-section (v-face) and in the parallel plane to the surface (p-face) were characterized. Tensile tests were performed at various loading angles with respect to the nozzle moving direction in the p-face. It is revealed that there exist strong microstructural and mechanical anisotropies in CS deposits. Different interparticle interaction results in more severe particle impact deformation in v-face than p-face, with larger elastic modulus and microhardness values. The tensile tests show an unexpected anisotropy in both ultimate tensile strength and elongation, with the highest performance occurring at the angle of 20°. The in-plane tensile anisotropy could be attributed to the parallel multiple passes. Therefore, a novel weave-spraying method was proposed, which can greatly reduce the tensile anisotropy of CS deposits.

To understand the adhesion mechanism of cold spraying, the characteristics of a newly formed cold spray surface are essential. This surface is formed by the dynamic plastic deformation of the substrate and particles during cold spray impact. Over the surface, the amount of newly generated surface, bonding state, and strength can differ. Even within an individual attached particle, the amount of plastic deformation also differs. To determine the relationship between the coating deposition mechanism, microstructure, and adhesion strength, tensile adhesion strength tests of cold sprayed copper coatings on an aluminum substrate were carried out. Then, using an Auger electron spectroscopic analyzer, the remained oxide film at the fracture surface, which is the bonding interface, was analyzed. The natural oxide film that covers the surface of the substrate before the impact, which is broken by plastic deformation during the spray process. However, the results show that it is not broken at the center of the collision crater, where the amount of plastic deformation of the substrate material is small. Hence, at the center of the collision crater, the oxide film still covers the substrate. Moreover, the results reveal that the adhesion strength is not uniform but is strong at the edge of the crater, where the oxide film has been removed by the colliding particle. These results reveal insights that will be valuable for future improvements in the adhesion strength of cold spray coatings.

Polymer metallization using cold spray method, due to low process temperature, is a potential candidate to form electrically conducting polymers as well as composites and improve the mechanical properties of their surface (abrasion, corrosion, etc.). Low Pressure Cold Sprayed copper coatings on PEEK (Poly-Ether-Ether-Ketone) based composites reinforced by carbon fibers have been investigated. Cold Spraying involves high erosion on composite materials due to solid state and high velocity particles thus a new way has been developed. Based on the elastic behaviors of organic materials, pure PEEK matrix has been added on the composite surface to behave as an interfacial layer between the composite and the coating. Optimization of the LPCS parameters has then been carried out using a careful choice of powder size distribution in order to avoid substrate destruction, erosion and delamination of the coating. Consequently, dense thick copper coatings have been obtained and analyzed in terms of microstructure implementing SEM observations. Finally, electric measurements have been performed in order to check the efficient metallization of the composites. A new way for metallic coating on organic composites using Low Pressure Cold Spraying is then demonstrated.

In this work, the bonding mechanism between Cu particle and substrates of Mg, Cu and stainless Steel (SS) was investigated by the direct observation of bonding interface on detached particle and substrate crater. In the cases of Cu/Cu and Cu/SS, dimple-like fractures were found on the detached Cu particle and substrate crater for the first time. Accompanying with EDS line scan and mapping results, such dimple fractures can be considered as the signs of strong metallurgical bonding. However, the bonding interface in case of Cu/Mg is smooth without signs of metallurgical bonding. Finite element analysis results revealed a ring of high contact pressure zone on the surface of particle and substrate, which is exactly the place where metallurgical bonding was observed. It can suggest that the high contact pressure zone is the dominant factor for the formation of metallurgical bonding on the oxide-free interface. The evolution of maximum contact pressure in different cases shows that the substrate hardness plays an important role during the single particle bonding. The present study provides a profound insight into the bonding mechanism of a single cold sprayed particle, which can give the guidance to the full deposition of cold sprayed coating.

In the cold spray process, cross-sectional shape of the nozzle has a significant effect on spray pattern of coatings. There is a rectangular and a circular cross-sectional shapes on the cold spray nozzle. It has been reported that the rectangular nozzle provide a more uniform particle velocity across the exit of the nozzle. 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 in order 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 behaviours of gas and particle by the computational fluid dynamics (CFD) in high pressure cold spraying. In this study, the spray pattern and microstructure of copper coatings with the rectangular cross-section nozzle optimized by CFD analysis in high-pressure cold spraying are reported. The optimized rectangular nozzle provide a more uniform spray profile of coatings. Relatively finer powder tended to deposit on the end of the coating.

Adhesion strength is one of the most important characteristics when discussing the reliability of a cold spray. A focused ion beam (FIB) was used to conduct ultra-micro tensile tests for micro-scale adhesive strength evaluations of high-pressure type cold-sprayed copper deposition on an aluminum substrate. It was also used to determine the essential factor of adhesion strength and the coating formation mechanism. The micro-scale local adhesion strength of cold-sprayed copper deposits on an aluminum substrate was successfully evaluated by FIB microtensile tests. The average local adhesion strength of this deposit was 223 MPa. The variations in adhesion strength between deposit and substrate were smaller than the interfacial strength of cold-sprayed deposits. This was caused by the repeated collision of these subsequent particles.

By means of In-Mold-Metal-Spraying (IMMS), wire arc sprayed metal coatings are transferred onto plastic parts during the injection molding process for the efficient production of metallized plastic parts. One potential field of application of IMMS parts are electrical applications such as electrically conductive tracks or electromagnetic shielding. In the current study, the properties of the transferred coatings, especially the electrical resistivity, are determined. Different feedstock materials are used for the application of the coatings. In the first investigation, pressurized air is used as atomizing gas for wire arc spraying. In contrary to Zn coatings, Cu coatings applied with pressurized air have a significantly higher electrical resistivity in comparison to massive copper. One possible reason for this is the oxidation of the Cu particles during the spraying process. Therefore, N 2 and a mixture of N 2 and H 2 are used as atomizing gas to reduce the oxidation of particles. Consequently, the electrical resistivity of IMMS parts can be significantly reduced. Furthermore, spraying distance, current and pressure of the atomizing gas are varied to investigate the influence of these process parameters on the coating properties.

The Nuclear Waste Management Organization (NWMO) has proposed the concept of a deep geological repository (DGR) for the storage of Canada’s used nuclear fuel. A major engineered component is the used fuel container (UFC) consisting of a steel core coated with copper for corrosion resistance. The copper coating is required to have sufficient ductility and adhesion strength to the steel substrate for loading requirements under DGR conditions. The NWMO has identified two coating technologies for the application process: electrodeposition and cold spray. Electrodeposition is utilized to coat the bulk of the UFC components (i.e., hemi-spherical head and lower assembly). A portion of the hemi-spherical head and the lower assembly openings remain uncoated in order to facilitate the final assembly closure weld process after fuel loading. This area is then cold sprayed with copper to complete the coating on the steel. Since the cold sprayed coating is highly strained in the as-sprayed state, it requires a heat treatment to impart ductility. The ductility is assessed indirectly by measuring the hardness of the material before and after the heat treatment. A recent advancement on this front includes the implementation of an optimized band heat treatment method to prototype UFC’s.

Polyimide-copper layers consisting of individual capsule-like splats were one-step fabricated by solution precursor flame spray through controlling the reaction between dianhydride and diamine dissolved in copper nanoparticles containing dimethylformamide solvent. The polyimide splat exhibited hollow structure with an inner pore of 10-15 µm and a tiny hole of 1-5 µm on its top surface. Transversal cut by focused ion beam milling of the individual splats and scanning electron microscopy characterization further revealed unique dispersion of the copper nanoparticles inside the polyimide shell. After 1000 h exposure to the testing synthetic seawater, continuous release of copper from the coatings containing up to 30wt.%Cu kept remarkable. Antifouling performances of the constructed layers were assessed by examining colonization behaviors of typical bacteria Bacillus sp. and marine algae Phaeodactylum tricornutum and Chlorella on their surfaces. Distribution of the inorganic nanoparticles endows the polyimide coatings with special capsule structure and exciting hydrophobicity and antifouling performances. The liquid flame spray route and the encapsulated structure of the polyimide-Cu coatings would open a new window for designing and constructing environment-friendly marine antifouling layers for long-term applications.

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.

Cu and Cu-MoS2 coatings were fabricated by cold gas dynamic spray and the fretting wear performance of the two coatings was compared. A mixture (95 wt.% Cu + 5 wt.% MoS2) was used as feedstock for the composite coating. Coatings were sprayed with identical gas flow conditions on the substrates preheated to approximately 170°C. The cross section of the coatings was analyzed by scanning electron microscope (SEM) and MoS2 concentration was measured, as well as coating microhardness. Fretting tests were carried out under gross slip conditions in ambient environment. SEM observation on wear scars and counterspheres revealed the development of third bodies, by which the sliding was accommodated. For the Cu-MoS2 coating, solid lubrication effects in the form of friction drops occurred in early cycles (< 5k), but eventually (> 5k) the coating's friction behavior was similar to the pure Cu coating. Third body morphology and wear of the two coatings were distinctly different, which could largely be attributed to the hardness reduction of the Cu-MoS2 composite due to poorly bonded interfaces induced by the effect of MoS2 during particle impact and coating formation.

Surface preparation is very important for reliable adhesive bonding of cold sprayed coatings to the substrate. In this work, the grit blasting of low-carbon A516 steel substrates with Al2O3 particles was studied and the roughness parameters Ra and Rt of the grit blasted surfaces were then measured. The influence of alumina grit size on the roughening of the A516 steel substrate, and the resulting effect on the roughness of the Cu coating – steel interface were studied. The results showed that variations of the grit blast size had significantly affected the resultant surface roughness of the substrate. The adhesive strength of the formed copper coatings on A516 steel substrates depends on the surface roughness and hardness of the base material. The adhesive strength about 110-200MPa was achieved. The specific features of the Cu coating-A516 steel interface topography were examined and discussed.

In the present work, we have examined the role of active elements in controlling the extent of splashing of plasma sprayed splats. Splats of Copper (Cu) and Copper alloyed with Al, Zr were deposited by plasma spray or as free-falling droplet. The splat formation and splat-substrate interfaces were characterised using SEM and FIB. It was found that the presence of Zr, either in the splat material or the substrate, enhanced disk splat formation in free falling droplets. Similarly, the presence of aluminium in plasma sprayed Cu- 10%Al increased the portion of favourable disk-shaped splats compared to pure copper. It was also found that the disk-shaped splat proportion of Cu-10%Al at room temperature is nearly identical to that of Cu splat at 300ºC, indicating that the addition of Al in the alloy leads to the improvement of splat formation, correlating with the improvement in the interface bonding with the substrate.

The main purpose of this study was to form cold sprayed copper coatings on A 516 low carbon steel, which is considered a prospective material for manufacturing used nuclear fuel containers. The 3 mm-thick Cu coatings were formed using the high pressure cold spray method with N2 as the propellant gas. To increase the adhesion strength of the deposited coatings a copper sublayer was formed first, using He as the propellant gas. The deformation of copper particles during the deposition process was studied. The obtained SEM images of the Cu layer-A 516 low carbon steel substrate cross-sections demonstrated that the Cu sublayer had a dense microstructure, and local jet-metallic mixing areas. The Cu particles were deformed considerably more severely in the sub-layer than in the following layers. The steel substrate underwent severe deformation due to the impact of Cu particles. The mutual severe deformation of Cu particles and steel substrate resulted in a considerable increase of adhesion strength up to 120MPa. The structure of coatings and coating-substrate interface was studied by OIM, SEM and EDS.