Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84 ; DOI: 10.3221/IGF-ESIS.22.08 76 strength of aluminum and copper may be one of the reasons of highly dense and low porosity of their cold spray coating [10]. Effect of particle diameter The particle velocity is inversely related to particle diameter as: Vp = k/d n , where Vp is the particle velocity, k and n are the coefficients related to driving gas conditions for a certain material [15]. The dependence of Vp on the particle diameter under different spray conditions of temperature, pressure for nitrogen (C1, 2, 3) and helium (C4) gas is shown in Fig.8 for copper powder [16]. The converging-diverging de Laval nozzle with throat diameter 2mm, expansion ratio 9 and downstream length 100 mm is used with same carrier and main gas. It is shown that particle velocity decreases for all conditions with increase of particle diameter and decrease is remarkable when particle size is small particularly in the range of 20µm [15]. Figure 8 : Particle velocity vs. Particle diameter for different gas conditions [15]. Nature of carrier gas Regarding carrier gas type, though pre-heated nitrogen gas is used for a wide diversity of materials, but other hard materials cannot be deposited with nitrogen gas because higher velocity is required. Helium provides therefore a solution since it is inert and allows reaching the highest particle velocity. Yoon et al. [17] reported the enhancement of deposition efficiency when process gas changed from nitrogen to helium during cold spraying of NiTiZrSiSn amorphous powder. It is reported by Li et al. [16] that under all conditions the particles accelerated to higher velocity using helium as compared to nitrogen as driving gas. Helium is however 10 times more expensive than nitrogen, making it economically unviable for many applications unless recycled. However, a Helium Recovery System (HRS) installed at Canada, recovers helium from the cold spray chamber with sufficient purity (>99%) allowing for a cost-effective operation by insuring a recovery rate of above 85% [18] . In some applications mixture of helium (He) and nitrogen (N 2 ) is used as carrier gas. Nitrogen (N 2 ), being a diatomic gas, and its addition into He increases the enthalpy of the carrier gas for better heat-transfer with spray particles, but it also reduces the velocity due to the heavier atomic mass resulting in coatings with reduced density and hardness[19]. However, high corrosion resistance is reported by Balani et al. [19], for cold spray of 1100 Al onto 1100 Al substrate using He–20 vol.%N 2 as carrier gas compared to 100 vol.% He processed coating, though both the cold-sprayed coatings were more corrosion resistant compared to the 1100 Al substrate and coating by 100 vol.% He is more hard and dense. Effect of temperature The velocity of the gas at the throat (Vt) of the Laval nozzle is also a function of its temperature as: Vt = (γ RTt ) 0.5 , where γ is the ratio of gas specific heats, R is the specific gas constant, Tt is the gas temperature at the throat, respectively[10]. Hence the particle velocity increases with an increase in gas temperature. Though, gas pre-heating provides higher particle velocity but it also raises the risk of oxidation and/or nitridation which in turn can be detrimental for the design functionality of applied coatings[18] . However, it is also reported that at higher temperatures the gas