**1. Introduction**

Cold gas dynamic spraying (CGDS) is a modern additive manufacturing approach and a promising technique in the field of materials processing that recently has been implemented for several industrial applications. The CGDS process is mainly a powder deposition process, which uses the ability to self-consolidate the solid particles that bond together at their solid-state. Such strong bonding self-consolidation ability resulted from a high-velocity (supersonic-velocity) impact [1–7]. Thurston developed this technique at the beginning of the twentieth century [8]. Afterwards, a pressurized or blast gas to propel metal powders up to a maximum speed approximately 300 m/s and then create a deposit via a high-velocity collision with a substrate. In the 1950s, the modern Rocheville technology with the gas flow across the De-Laval nozzle was an important breakthrough that, at that time, allowed the speeds to be higher than those of current methodologies and created a consistent thin coating. The cold gas dynamic spray process phenomenological behaviour was studied by the Russian Academy, Institute of Applied and Theoretical Mechanics further in the 1980s [8]. Their discoveries resulted in the creation of novel patents of cold gas-dynamic spraying devices and

experimental processes for the production of cold gas dynamic sprays which eventually resulted in reliable additive manufacturing processes. While several viability studies show the feasibility of cold gas dynamic spray, deposit development and bonding mechanisms are continuously being studied to extend the materials concerned.

Two phases, including particle/substrate adhesion and the deposit growth, rule the deposition process during CGDS. The distinct phenomena of bonding mechanisms characterize every phase. Interparticle cohesion is recommended for ductile materials (metals in terms of deposit growth due to plastic deformation) such as copper (Cu), aluminum (Al), nickel (Ni), and silver (Ag), etc. The cohesive effect of interfaces is considered to occur through atomic interactions, because of intimate metallurgical interaction during the transformation of the phase, while the interfacial zone is subjected to high impact collision and experiences an extreme plastic deformation rate [9–12]. By comparison, it was also possible to recognize the self-compaction, fragmentation, and the final deposit consolidation due to the interlocking and stacking of fragments, particularly for non-ductile materials e.g., ceramics.

Researchers have demonstrated numerical analysis and experimental findings of the bonding mechanisms, which primarily occur by mechanical anchoring, metallurgical bonding, interfacial mixing, or mechanical locking, in literature for CGDS. Metallurgical bonding is possible because of a dynamic recrystallization phenomenon [13], a hyper-quenching phenomenon caused by a substantially large plastic strain (adiabatic shearing) in the interface of interfacial confinement and the formation of an amorphous middle layer covering intermetallic region [14]. Mechanical anchoring is instigated by the slight indentation on the substrate by the particles, which ensures that the particles are anchored, and which is primarily seen in combinations of the metallic component with the substrates made of ceramic [15–17]. Mechanical interlocking means particle/substrate integration as a result of in-depth penetration of particle into the substrates combinations as follows: metal/metal [18,19], ceramic/metal [20], oxide/polymer [21], and metal/polymer [22]. In the case of mechanical particle deformation within a geometrical imperfection of the substrate surface, the concept of inter-locking can also be applied [23,24]. This is also an understanding of the material consistency through the surface, produced during the soft particle deposition on a hard substrate. Such instances of these occurrences are soft polymer/metal [25,26], metal/ceramic [17] and metal/polymer [1,23,24,27]. The adhesion mechanism also regulates the production of interfacial vortices during interfacial mixing which permits the intermix of particles and substrate across the interface [24,28,29]

Since a broad range of new and progressive materials can be deposited with CGDS, academics and industry are increasingly interested in the CGDS technology. The CGDS approach provides di fferent functional features for several obtainable industrial applications and significant progress is also anticipated in the coming decades. Several deposits of material can now be achieved [30,31]. They can be categorized according to their deposition technique and materials type. This comprises three distinct categories: (1) single material deposits, (2) a mixture of di fferent particles, composite-based deposits, and (3) a nanomaterials deposit (i.e., a deposit creating nanosized characteristics). Additionally, the adhesion mechanisms versatility of the CGDS method suggests an additional category of the deposit as material hybridization among particles and substrates. The specific form of deposit also takes into account the possibility of hybridization and is called "hybrid particle/substrate assembly."

The study into the processes of cold gas dynamic spray mechanism of surface generation focuses primarily on the peening e ffect [32], localized softening [33], pressure waves [34], recrystallization [35], size e ffect [36,37], localization deformation [38,39], bonding [28,40], adhesive strength [41–43], oxide destruction [44], crystal orientation e ffect [36,45], evolution of microstructure [46,47] in stress/strain and nanoindentation [48], and so on. The result of the material combination and initial impact velocity on the dislocation plasticity and cold gas dynamic spray coating surface microstructural processes is barely explored. Since deformation of particles takes place within a quite short duration of 10−<sup>9</sup> s order, it is very complicated to find the answers in-situ investigations [49]. Generally, detailed observations of the experimental study in a CGDS can usually be performed only after the spraying is completed and typically relies on particle cross-section microscopy observations

in the as-deposited condition [50–52]. Simulations of molecular dynamics (MD) provide us with an effective means of controlling and examining the complex nanoscale atomically structure and behaviour [53].

Cu, Al, Ni, and Ag, a ductile face-centered cubic transition metal, is prototypical in both CGDS multiscale models and particle deposition experiments. In this paper, we present atomic research with MD on the single supersonic-speed impact of Al, Cu, Ni, and Ag particles on a copper substrate. To describe at the fundamental level the deformation process of ductile material particles, our research centered on the basic mechanisms of metal hardening and dislocation plasticity. The mechanisms of plastic deformation are studied from the perspective of the evolution of atomic structure through jet initiation, dislocation density, and the evolution of microstructural transformation. The dislocation density at the interface of particle/substrate interfacial region is observed to grow much more quickly during the impact phase of Ni and Cu particles and the evolution of the microstructure for particles at varying initial impact speeds is very different.
