Recent Advances in Multiscale Simulation of UNCD Strength

Due to its outstanding mechanical, tribological, electronic transport, chemical and biocompatibility properties, the ultrananocrystalline diamond (UNCD) film grown by a microwave plasma chemical vapor deposition method under hydrogen-poor conditions has become the subject of intense research interests over the recent years [1–3; among others]. As can be found from the open literature, the potential of UNCD has been demonstrated for the applications in nano-technology. To better understand the responses of UNCD films under different conditions, a numerical study has been performed of the specimen size, loading rate, temperature and loading path effects on the material properties of single crystal diamond. A combined kinetic Monte Carlo (KMC) and molecular dynamics (MD) procedure is then developed for large-scale atomistic simulation of the responses of pure and nitrogen-doped UNCD films. In the proposed 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 box to create a polycrystalline UNCD film with certain grain boundary (GB). The mechanical responses of resulting UNCD film have been investigated by applying displacement-controlled loading conditions in the MD simulation box, and compared with those of single crystal diamond. By randomly adding different numbers of nitrogen atoms into the GBs of these polycrystalline UNCD films, the effects of nitrogen atom number density and GB width on the responses of UNCD have also been studied. Recently, efforts are being made to explore the combined size, rate and thermal effects on the strengths and failure patterns of both pure and nitrogen-doped UNCD films so that a hyper-surface could be formulated for designing UNCD-based MEMS devices. In this presentation, recent advances on multiscale (both spatial and temporal) model-based simulation of the pure and nitrogen-doped UNCD responses under various temperatures will be discussed, which provide a better understanding on the performance of UNCD-based MEMS devices in an extreme loading environment.