Influence of normal velocity on microstructure and density of films produced by nanoparticle impact

We describe the use of the laser ablation of microparticle aerosol process to deposit patterned thick films of Ag by impacting nanoparticles with a mean size of 6 nm at high velocities and systematically study the effects of varying the nanoparticle impact velocity, while fixing the nanoparticle size. A positive relationship between impact velocity and relative density of the deposits is observed, but it is shown that the geometry of the deposits also influences the deposit density, with taller, narrower deposits resulting in lower relative densities. Scanning electron microscopy of film cross-sections shows that there are spatial variations in the relative density, with lower densities near the deposit surfaces. Using molecular dynamics simulations of the impact process for 6 nm particles, it is shown that a critical normal impacting velocity of 600-750 m/s exists for obtaining dense deposits. Normal velocities less than the critical velocity do not result in sufficient plastic deformation to fill interstices between the impacted particles. A geometric model based on this critical normal velocity and the relative deposit height is developed to explain the influence of relative heights and impacting velocity on porosity. Good qualitative agreement between the geometric model and experiments is demonstrated.