A Numerical Study of the Size and Rate Effects on the Mechanical Response of Single Crystal Diamond and UNCD Films

To better understand the mechanical response of ultrananocrystalline diamond (UNCD) and its grain boundary mechanism, a numerical study is performed of the specimen size and rate effects on the mechanical properties of single crystal diamond and UNCD films under uniaxial and shear loading paths, respectively. To compare with the UNCD films, single crystal diamond blocks of various sizes under tensile loading in the 100 hi direction and shear loading with the {100} 110 hi slip at different rates are first investigated via the molecular dynamics (MD) simulation. A combined kinetic Monte Carlo (KMC) and MD procedure is then developed for large-scale atomistic simulation of the mechanical response of UNCD films. In this numerical procedure, two single crystal diamond films, that are formed by the KMC method based on the mechanisms of UNCD growth from carbon dimers on the hydrogen-free (001) surface, are compressed along the [001] direction with two growth surfaces contacting each other at an elevated temperature in the MD simulation to create a polycrystalline UNCD film with certain grain boundary. The mechanical response of the resulting UNCD film is investigated by applying displacement-controlled shear loading in the MD simulation, and is compared with that of single crystal diamond. The preliminary results presented in this article provide a better understanding of the size and rate effects on the material properties of diamond and the role played by the grain boundary on influencing the mechanical response of UNCD films.