Cold spraying – A materials perspective

Abstract

Cold spraying is a solid-state powder deposition process with several unique characteristics, allowing production of coatings or bulk components from a wide range of materials. The process has attracted much attention from academia and industry over the past two decades.The technical interest in cold spraying is twofold: first as a coating process for applications in surface technology, and second as a solid-state additive manufacturing process, offering an alternative to selective laser melting or electron beam melting methods. Moreover, cold spraying can be used to study materials behaviour under extremely high strain rates, high pressures and high cooling rates. The cold spraying process is thus considered to be relevant for various industrial applications, as well as for fundamental studies in materials science.This article aims to provide an overview of the cold spray process, the current understanding of the deposition mechanisms, and the related models and experiments, from a materials science perspective. IntroductionCold spraying (CS) is a solid-state material deposition technique, where micron-sized particles of a powder bond to a substrate as a result of high-velocity impact and the associated severe plastic deformation. Acceleration of particles to high velocities is obtained via expansion of a pressurised and (ironically) 'hot' gas through a diverging-converging nozzle.Despite heating the process gas -which is to provide higher acceleration of the gas and also to facilitate particle deformation through thermal softening -the feedstock remains in the solid state throughout the entire process; hence the name 'cold' spraying. This is to underline the contrast to conventional thermal spraying where particles are completely or partially molten upon impact onto the substrate. Alternative terminology includes cold gas spray, micro cold spray, cold gas dynamic spray, kinetic spray, supersonic particle deposition and metal powder application (MPA), all of which refer to the same principle of solid-state powder deposition as described above. CS is used for coating and repair. It is also used for additive manufacturing (AM) at relatively high deposition rates as compared to the methods based on selective laser or electron beam melting. The main advantage of CS is that it alleviates the problems associated with high temperature processing of materials, such as oxidation and unfavourable structural changes. It is possibly the only continuous method to produce bulk components of metastable materials that are available only in the powder form, e.g. as obtained from mechanical attrition or gas atomisation. The aim of the present paper is to provide an overview of CS from a materials perspective, particularly for a broader audience in materials research, e.g. with interest in dynamic phenomena, plasticity and phase transformations. In terms of application and properties, CS is perhaps best to be compared to thermal spraying processes, as in [24,25]. In view of relevant physical phenomena, however, CS is mos...

Development of Sustainable Cold Spray Coatings and 3D Additive Manufacturing Components for Repair/Manufacturing Applications: A Critical Review

“…According to adiabatic shear instability theory when a particle impacts in cold spray, the particle/particle or the particle/substrate interfacial areas experience an extensive localized deformation (i.e., shear deformation) during impact which leads to disruption/instability of the thin oxide surface films. This empowers an intimate conformal contact between the particle and the substrate or the previous deposited layer, and thus forms bonding [28][29][30][31]. Better illustration of this theory can be understood with the help of the following points: (1) plastic strain energy from the collision is retained in the system as heat increases the temperature of the substrate and softens the material.…”

“…At high impact velocities, the outer region of the particle that impacts the substrate experiences severe plastic deformation and as a result a large temperature increase. It is thought that this temperature spike results in local melting of the particle, where the melted material forms a strong bond with the substrate [28,29]. The particle-particle and particle-substrate interaction due to thermal softening is hypothesized to give rise to local adiabatic shear instabilities [30].…”

“…The erosion velocity must be less than the particle velocity so as to produce adhesion action. Many researchers have worked on developing and modifying the theoretical models for predicting critical velocity in the cold spray process in terms of different material properties such as particle density, impact temperature, melting temperature, ultimate tensile strength, and hardness of the particle [28][29][30][32][33][34][35][36][37][38].…”

“…Bonding in cold spray is considered to identify with such a type of interfacial deformation as shown in Figure 5, which prompts the breakup of surface oxide layers and enables the contact of metallic surfaces at atomic level. Thus, the oxide layer should be considered as a geometrical constraint and not an "energy barrier" to metallurgical bonding [28]. It is generally accepted that the adhesion strength of the particles in cold spraying is only due to their kinetic/mechanical energy at impact, that is often abundant but not up to the energy needed to melt the particle (as such a "solid-state" deposition method).…”

“…Bonding in cold spray is considered to identify with such a type of interfacial deformation as shown in Figure 5, which prompts the breakup of surface oxide layers and enables the contact of metallic surfaces at atomic level. Thus, the oxide layer should be considered as a geometrical constraint and not an "energy barrier" to metallurgical bonding [28]. In actual fact, the surfaces of metallic parts are rarely flat and are generally covered with an oxide layer.…”