Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84; DOI: 10.3221/IGF-ESIS.22.08 77 density and viscosity will decrease and hence the drag force of the gas, which is the force responsible for particle acceleration should decrease at higher gas temperatures and hence this area needs to be further explored[10]. It is also reported that critical velocity (Vc) decrease with the increase in the particle temperature by about 14m/s with a temperature increment of 100 o C, due to the thermal softening effect, shown in Fig.9, [16]. Figure 9 : Critical velocity vs. Mean particle temperature [16] Effect of oxidation condition Li et. al.[14], has reported the dependence of critical velocity on the particle oxidation conditions. It is reported that the large discrepancy among the critical velocities for copper particle can be attributed to the difference in oxygen content of the copper powder. The results as shown in Fig.10 showed that with copper powder the critical velocity(Vc) changed from about 310 m/s at an oxygen content of 0.02 wt.% to 610 m/s at an oxygen content of 0.38 wt.% and with nickel- based Monel alloy, the critical velocity was increased from 583 to 632 m/s as the oxygen content was changed from 0.016 to 0.108 wt.%. The study revealed that at high oxygen content, sprayed particles need to break and extrude the oxide scale on impact, therefore the critical velocity is dominated by oxide on the powder and is independent of the material properties as compared to low oxygen content materials. Figure 10 : Effect of oxygen content on the critical velocity of different spray materials [14]. Effect of Nozzle Design However, improvements in nozzle design using gas dynamic models have lead to higher deposition velocities and the ability to deposit larger particles, which results in denser coatings and higher deposition efficiency. So the particle velocity is also influenced by the nozzle design like its nozzle inlet diameter, throat diameter, exit diameter or expansion ratio (i.e., the ratio of the area of the exit to the throat), the entrance convergent section length (upstream length)and the divergent exit length(downstream length). It is found that increasing the length of the nozzle has a significant effect on particle