Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84; DOI: 10.3221/IGF-ESIS.22.08 73 A DVANTAGES OF CS oating technology has rapidly advanced with the addition of cold spray coating techniques. The major advantage over thermal spray techniques are the low temperatures involved which minimize any potential phase change and keep the particles in their unmodified solid state. The difference between the Cold Spray Process(HVPC) and other thermal spray processes is illustrated in Fig. 3. In the thermal spray process, a coating is formed by melting the coating material and then quenching the molten droplets. Hence thermal sprayed coatings in general have microstructures with varying degrees of porosity, oxides and other inclusions, and low corrosion resistance characteristics [2]. However, CS process has advantage of metal deposition with low heat input, local deposition to limited area, and deposition free of oxides and other inclusions can be produced to any metal surface, and due to compressive stresses, the dense uniform deposit of any thickness with wrought-like microstructure are obtained. Moreover, the oversprayed expensive raw material can be collected for reprocessing [2, 6]. M ECHANISM OF C OLD S PRAY PROCESS he bonding mechanism in thermal spraying can be explained by the occurrence of local adiabatic shear instabilities,at particle-substrate and particle-particle interfaces due to thermal softening, however the true bonding mechanism in cold spray process is still poorly understood [7, 8]. By means of a so-far widely accepted model; during impact, the solid particles undergoes plastic deformation, disrupt thin surface films (oxides), and in turn, intimate conformal contact is achieved and combined with high contact pressure, promotes bonding with the target surface[9, 10]. The common phenomena that have been observed during spraying onto various substrates are substrate and particle deformation, and substrate melting as there is evidence for the formation of a metal-jet [7, 11] as shown in Fig. 4, in which 20 µm copper sphere impacting an aluminum plate at 650 m/s is modeled. It shows the material adjacent to the interface behave as a viscous fluid-like, results in the formation of interfacial waves, roll-ups, and vortices [11]. Figure 4 : Impact of a Cu particle on a Cu substrate at successive times: ( a) 5 ns, ( b) 20 ns, ( c) 35 ns, ( d) 50 ns [11]. It has been suggested that the adhesion strength of the particles in cold spraying is solely to their kinetic energy at impact, which is typically much less than the energy required to melt the particle and hence cold spray is a solid-state process [8, 9]. This concept is explained in the EDS image Fig.5 of cold sprayed copper deposit on aluminium substrate, examined by Champagne et al. [11]. It shows the forced mixing between the deposited copper (light area) and the aluminum substrate ( dark region), and that can be achieved through deep-impact penetration of the copper into the aluminium [11]. This theory would also explain the minimum particle velocity necessary to achieve deposition, because sufficient kinetic energy must be available to plastically deform the solid material [9, 10]. An empirical model by Champagne et al. [11] shows that interface mixing depends on the substrate hardness and coating material density, and the particle velocity (m/s) Vp needed for the attainment of interfacial mixing, as: Vp = [(7.5 x 10 4 )( B/ρ)] 0.5 , where B is the substrate Brinell hardness C T