The particle critical impact velocity is a key parameter in cold spraying. The appropriate estimation of this critical velocity benefits the optimization of coating deposition in terms of deposition efficiency. The critical velocity can be estimated experimentally through measuring the relation between deposition efficiency and particle velocity. A recent study has revealed that the critical velocity can be estimated by the velocity resulting in the onset of adiabatic shear instability through numerical simulation of particle impact. However, it was found that the critical velocities of a pure metal material obtained by different investigators through simulation were significantly different. No reasonable explanation was provided to such difference. In the present paper, the critical velocities reported in the literature for Cu powder as a typical material were reviewed. The factors influencing the critical velocity were examined. The numerical simulation methods employed in the limited literature were also examined for a better understanding of the factors that influence the simulated results. A reasonable estimation method was proposed by using the numerical critical velocity.

The photocatalytic TiO 2 coatings on bio-degradable plastic(polybutylene succinate:PBS) were prepared by HVOF and plasma spraying techniques using three kinds of agglomerated powders (P200: 200nm, P30: 30nm, P7: 7nm). The microstructures of coatings were characterized with SEM and XRD analysis, and the photocatalytic efficiency of coatings was evaluated through the photo degradation of gaseous acetaldehyde. Furthermore, for the estimation of the mechanical characteristics, wear resistance of coatings was estimated by weight loss obtained from the abrasion wear test. The results indicated that with respect to both the HVOF sprayed P200 and P30 coatings, the high anatase ratio of 100% can be achieved regardless of fuel gas pressure. On the other hand, the HVOF sprayed P7 coating exhibited largely decreased anatase ratio (from 100% to 49.1%) with increasing the fuel gas pressure, which may be attributed to the much higher susceptibility to heat of 7nm agglomerated powder. In terms of photocatalytic efficiency, HVOF sprayed P200 and P30 coatings seem to predominate as compared to that of plasma sprayed P200 coatings owing to the higher anatase ratio. However, the HVOF sprayed P7 coatings didn’t show the photo catalytic activity, which may result from the extremely small reaction surface area to the photo-catalytic activity and low anatase ratio. In addition, it was found that the thermal sprayed TiO 2 coatings improve the wear resistance of PBS. Therefore, in the present study, possibility of production of further functional PBS plastic with photocatalytic performance and wear resistance by spraying of ceramics such as TiO 2 , was found.