Nanoindentation of Amorphous Carbon: a combined experimental and simulation approach

Amorphous carbon (aC), in particular diamond-like carbon (DLC), is one of the most studied and promising coating materials, but many of the atomic-scale mechanisms involved in their plastic deformation process are not fully understood. The mechanical response of non-hydrogenated DLC films with different sp${}^3$ concentrations is investigated using nanoindentation experiments, ab-initio simulations, and classical molecular dynamics simulations. Experimental characterization is carried out with Raman spectroscopy and Electron Energy Loss Spectroscopy (EELS). Ab-initio and classical simulations show good agreement for sp${}^1,$ sp${}^2$ and sp${}^3$ content of in-silico samples. Elastic modulus and hardness of DLC films increase with sp${}^3$ content, for sp${}^3$ between 10% and 55%, and excellent agreement is obtained between experiments and simulations. Simulated strain distributions are shown to be highly anisotropic, unlike continuum-scale predictions for the use of a perfectly spherical indenter in an amorphous solid. MD simulations also reveal two different plasticity modes depending on the sp${}^3$ level of the indented sample. For films with sp${}^3$ concentrations less than 40%, plasticity is mainly mediated by the sp${}^2$ to sp${}^3$ transition. For larger sp${}^3$ concentrations, DLC plastic deformation is attributed to densification due to bond rearrangement. All in all, our work offers a comprehensive study of DLC, revealing unexpected plastic deformation mechanisms that had not been considered before. Our study might help to the fundamental understanding of amorphous carbon coatings for both scientific and technological purposes.