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Cold spray (CS) is an emerging coating technology that makes use of a converging/diverging nozzle, a high pressure and a heated gas source to create a high‐velocity gas flow. Coating deposition occurs at relatively low temperatures compared to other spray technologies, therefore the sprayed particles remaining in the solid-state. The process can produce dense coatings when the process parameters are optimized. Despite the incredible advantages of CS and the established knowledge of CS considering metal substrates, some questions like the bonding mechanism and the adhesion/cohesion strength of the coating with polymeric substrates remain an open research topic. In this paper, a metal coating (AlSi10Mg) was deposited on a plate in polypropylene by cold spray in order to increase the properties of the surface. Metallic particles, in the range of 20-40 µm, were injected into gas flow and propelled to supersonic velocities. Polymers require a deposition technique in which the substrate remains at relatively low temperatures during the whole process in order to avoid plastic deformation or degradation. Determination of the mechanical properties of thin films on substrates by indentation has always been difficult because of the influence of the substrate on the measured properties. In order to evaluate the local properties of the deposited coating, microindentation tests and DMA were carried out. In particular hardness values by microindentation tests and values of elastic modulus by DMA were calculated.

Polymer matrix composites (PMCs) are the most used in aerospace fields due to their special mechanical properties coupled with reduced weight. Metallic structures can be manufactured on the surface of PMCs producing hybrid constructions with intriguing characteristics. The cold spray technology (CS) seems to be the best solution for the manufacturing of metallic coatings on polymer-based substrates. However, the last researches on PMCs metallization through CS were carried out without taking into account a proper manufacturing of the substrates. Therefore, the customized manufacturing of PMCs used as substrates for CS deposition represents a crucial aspect to be considered, the target surface should be designed and manufactured to fit better the CS requirements. In this scenario, the Fused Filament Fabrication method (FFF) is the best solution for tailored composites manufacturing. Therefore, this work aims to develop a novel manufacturing route to produce innovative composite structures by using the FFF technology for the 3D-printed polymer-based substrate and the cold spray technology for the 3D-printed metal deposition. The novelty is that, for the first time, the polymer-based substrates will be designed and manufactured “ad hoc” for the cold spray metallization by using the rapid prototyping 3D-printing process successfully integrated with CS technology.

Cold spray deposition, which can be framed in the wider family of additive manufacturing, is a manufacturing technique that is able to produce coatings on diverse types of substrates through the deposition of feedstock powder. As a low-temperature process, cold spray represents a potential solution for the metallization of temperature-sensitive materials, i.e. polymers and polymer matrix composites. The study of the cold spray technology for the metallization of polymers is still in its early stage, and the deposition mechanisms of metals on polymer-based materials are not thoroughly understood yet. On these premises, a review on this topic is needed to systematically depict the actual state of the art and to provide a reliable and well-organized overview discussing all the theories arisen in these years. In summary, this review aims: i) to collect all the available literature and enucleate the most discussed and interesting points (the most prevailing theories regarding the bonding mechanisms, the influence of the different process parameters and the main characteristics of cold-sprayed coating), providing a reliable and well-organized state of the art; ii) to define the open questions and to delineate the directions of future work.

In this work, the manufacturing through cold gas dynamic spray (cold spray or CS) of metallic composite coatings of Al-Al2O3 on organic composite substrates with thermoplastic PLA matrix and hemp fibers was studied. Alumina powders, with a mean diameter of 50 μm, were used blended with aluminum powders in three different weight concentration percentages (0, 15, 20, and 45%) as feedstock material in order to highlight and discuss the variations of the coating surface properties depending on the alumina percentage. The coatings were produced by using a low-pressure cold spray equipment. A detailed experimental campaign, including microstructural observations and confocal microscopy, was carried out to study the structure of the coatings. Moreover, the tribological behavior of the coatings was studied through both scratch test and pin-on-disk test. The experiments showed that a small addition of alumina improves the compactness of the coating and its resistance to scratch and wear behavior.

International audience; Composite materials are widely used as main parts and structural components in different fields, especially for automotive and military applications. Although these materials supply different advantages comparing to the metals, their implementation in engineering applications is limited due to low electrical and thermal properties and low resistance to erosion. To enhance these above-mentioned properties, the metallization of composite materials by creating a thin metal film on their surface can be achieved. Among different coating deposition techniques, Cold Spray appears to be the most suitable one for the metallization of temperature-sensitive materials such as polymers and composites with a thermoplastic matrix. This process relies on kinetic energy for the formation of the coating rather than on thermal energy and consequent erosion and degradation of the polymer-based composite can be avoided. In the last years, a new method to produce composite materials, as known as Fused Filament Fabrication (FFF), has been developed for industrial applications. This technique consists of a 3D printing process that involves the thermal extrusion of thermoplastic polymer and fibers in the form of filaments from a heated mobile nozzle. The implementation of this new technique is leading to the manufacturing of customized composite materials for the cold spray application. In the presented experimental campaign, Onyx material is used as a substrate. This material is made of Nylon, a thermoplastic matrix, and chopped carbon fibers randomly dispersed in it. Aluminum powders were cold sprayed on the Onyx substrate with a low-pressure cold spray (LPCS) system. This study aims to investigate the possibility of the metalizing 3D-printed composite material by cold spray technology. For this purpose, optical and microscopical analyses are carried out. Based on the results, the feasibility of the process and the influence of the morphology of the substrate are discussed, and optimal spraying conditions are proposed.

Polymer matrix composites are finding never-ending widespread uses in the last decades, one recent tendency is to metallize their surface to further widen their field of application. Cold-spray deposition is one of the most promising techniques that can be adopted to this aim. Cold-spray deposition on polymers is in its early stage and more experimental work is required to fully understand the phenomena ruling the deposition. In this paper, the results of nanoindentation measurements on cold-spray coatings on various substrates will be presented and discussed. Polypropylene was used as matrix while carbon and glass fibers have been used as reinforcement, both steel and aluminum have been used as feedstock material for the cold-spray deposition. Nanoindentations tests have been then carried out on all the different samples, the influence of the fibers and of the powders sprayed on the behavior of the coatings is discussed in light of the experimental outcomes.

The deposition of steel powders on glass fiber reinforced polymer (GFRP) with the Cold Spray technique is investigated. Metallization of composite materials is still challenging when high-strength and dense powders, such as steel, are used due to excessive erosion of the polymer or disruption of the fibers during the deposition. To overcome these issues, low-pressure cold spray equipment has been used for the experimental campaign shown in this paper. Different coatings were obtained by using nitrogen as the carrier gas and different sets of pressure and standoff distance. Optimization of the process parameters, including the stratification of the laminate, has been carried out. Results of an initial study of the feasibility of the process and the influence of the different chosen spray conditions are exposed. In particular, the bonding mechanisms of steel powders and their morphology are discussed.

Cold spray deposition of aluminum powders on polyethylene reinforced with glass fibers has been studied. Different processing conditions, including different stratifications of the fiber-reinforced polymer used as a substrate, have been investigated to better understand the deposition mechanism. The experimental campaign has been conceived and tailored to highlight the influence of several process parameters on the deposition of the first layer of particles. The results obtained allowed to highlight and clarify the influence of standoff distance and gas pressure and also gave some insights concerning the effect of the substrate stratification. Guidelines regarding the design of the manufacturing process have been also released.

The aim of this chapter is to bring together, summarize and explain the latest development in the field of the cold spray deposition on polymeric and composite substrates. Although the deposition on metallic substrates has been widely studied and there are interesting books reporting the phenomena occurring and describing the process in detail, the deposition on polymeric substrates is a relatively new branch of cold spraying and to date there are no review papers or books focused on this topic. Several original research papers dealing with this topic have been published in the last years so it can be assumed that the available material is enough to allow the writing of a book chapter that summarizes what was done and highlights what more needs to be done. First it is worth to notice why this topic is of interest. Polymer matrix composites (PMCs) are widely used in the aerospace industry and in the military because of their low density, high specific strength and stiffness, and other unique properties such as ease formation and machining, well size stability. At the same time the use of polymeric components in aircraft, automobile, and power sector has gradually increased due to the development of new polymers with enhanced mechanical and physical properties. Some special characteristics such as electrical conductivity, thermal conductivity, electromagnetic shielding, erosion and radiation protection have to be improved to further widen the fields of application of these materials. Therefore, the surface metallization of a PMC or polymeric substrate is considered to be an effective technique to enhance the above-mentioned surface properties. To date there are several conventional techniques for the polymer metallization. The most common metallization techniques include vapor phase deposition, thermal spray deposition, electro deposition, electroforming and electroless deposition. However, these techniques have disadvantages. High equipment and processing costs, size limitations of workpiece produced by vapor phase deposition-based techniques (deposition in atomic dimension and workpiece should be placed in a limited cavity). Low adhesive force, poor stability in the electroplating case and the consequent pollution it generates, long production cycle and high cost of the mold for electroforming, distortion of the substrate surface resulting from the molten particles, and high-temperature flames in thermal spraying (TS). Recently, the cold spray technique, with suitable modification, has been increasingly used for the metallization of polymers and PMCs, especially for the advance polymer composite such as carbon fiber-reinforced polymer (CFRP). Compared to the techniques mentioned in the previous indent the cold spray offers some intriguing advantages when spraying metallic particles on polymeric substrates: (i) the deposition is made possible by the significant plastic deformation of the particles and of the substrate, some polymers (the thermoplastic ones) can experience an huge amount of plastic deformation so this type of interaction easily occurs; (ii) no chemical reaction are required to take place between the particles and the substrate, so it is not a problem the fact that due to their different chemical nature it would be almost impossible to observe a chemical bonding between a metal and a polymer; (iii) compared with thermal spray processes a less heat input is required in cold spray, therefore, heat effects such as surface distortion, oxides, void, phase transformation, and residual stresses are considerably reduced in the coatings; on this premise cold spray appears to be a suitable technique to process temperature-sensitive materials, such as polymers; (iv) in addition, CS has a technical advantage (similar to the TS)—the spraying gun can be held by a robot arm or the workpiece can be installed onto a numerical control working platform to prepare large-sized or complex surface parts with flexibility and compatibility, and this can be a big competitive advantage in the realm of additive manufacturing. In this chapter it will be provided a comprehensive overview of the deposition of metallic coatings on polymeric substrates through cold spray. The deposition behavior of the coatings and the adhesion mechanisms between the substrate and the particles will have discussed to set a foundation for the practical application of cold spray on polymers. The microstructure and the properties of the coatings will be as well reported to provide a useful guideline for practical applications. Moreover, a section will be devoted to the study of the tailored design of the PMCs substrates and to the study of proper chemical formulations of the polymers to facilitate the cold spray deposition. The main topics that are covered in this chapter are below reported: Different experimental set up adopted; Various substrate/coating configurations that can be produced; Mechanisms that rule the adhesion between the particles and the substrate; Deposition behavior and influence of the primary process parameters; Mechanical, tribological, and physical properties of the coatings, including microstructure; Modeling of the process; Tailored design and production of the PMCs to facilitate the deposition; Tailored chemical composition of the polymers to facilitate the deposition; and Direction of further investigation.

The low-velocity impact behaviour of novel composite structures, made of a polymer matrix composite with a superficial 3D printed metallic layer, was studied. The metallic layer was produced through cold spray deposition which is a relatively new additive coating technique allowing for the deposition of metallic coatings on thermal-sensitive substrates, such as the polymeric materials. Micron-sized aluminium particles were used to metallise the surface of basalt-nylon 6 (PA6) laminates manufactured by the compression moulding technique. Three different energy levels (10 J, 20 J, 30 J) were set for the impact tests. The results obtained pointed out the beneficial contributions of the metallic coating on the energy absorbing modes of the polymeric substrates; the positive effects on the damage mechanisms as well as the crack propagation were detected by plastic deformation analyses and ultrasonic damage controls.