Unravelling ultrafast deformation mechanisms in surface deposition of titanium nanoparticles

Deposition of metallic nanoparticles by Cold Spray (CS) method, has gained considerable attention from the research community. However, the process of deposition at the atomic scale is poorly understood. To gain insight in molecular mechanisms and to optimize practical coating processes, molecular dynamics simulations have been used to understand the influence of deposition velocity and size of the particles on deformation, and resulting stresses and temperatures during impact. We report molecular dynamics simulation of titanium nanoparticles deposition on a titanium substrate. Here local stress, temperature and particle deformation behaviour are studied in detail during the impact. Ultra high strain rates in the order of 10(10) s(-1), and very high temperatures lead to ultrafast deformation behaviour. The simulations are conducted for three different particle velocities, which are achievable in real experimental settings. Realistic potentials are used to allow for the deformation of the substrate and nano-particle upon impact. We show that there is a direct relationship between the deposition velocity and temperature. For a small particle of 2 nm size, the maximum temperature reaches to around 842 K at the highest impact velocity of 700 m/s. However for the 20 nm particle temperature locally and globally rises further up to 1369 K. We show at higher velocities (> 500 m/s) deformation of the particle in the elastic regime is followed by strong plastic behaviour, and attribute this to very high temperatures. The calculated Young's modulus of titanium in the elastic regime is E = 95.7-194.3 GPa and are comparable to those reported in the literature for single crystal titanium. There is evidence of temperature and strain rate effects that leads to softening of the particle at higher velocities. We also show the local transformation of Ti into an amorphous state that could be due to high local temperatures.