Investigation on tensile behaviors of diamond-like carbon films

• The deformation of DLC films can induce their graphitization.
• Strain-localized regions of DLC films are dominated by their sp2 clusters.
• The sp2 clusters are responsible for the relaxation of DLC films at small strains and film failures at large strains.

The lubrication performance of diamond-like carbon films is significantly influenced by their deformations under loading. However, their deformation mechanisms are unclear so far due to their nanoscale thicknesses and complex microstructures. In this study, these mechanisms are explored by investigating the tensile response of the DLC films via molecular dynamics simulations. The atomic strain localizations are observed, and the regions where they occur are dominated by sp2 clusters. These clusters relax the film at small tensile-strains by releasing its residual energies. The sp3-sp2 transitions are present at large tensile-strains and prefer to occur in the strain-localized regions. This preference significantly improves the graphitization level in these regions and thus promotes the sp2 clusters to propagate. The propagation severely damages the sp3 networks and leads to the failure of the film. This research suggests that reductions of heterogeneities such as existences of large-sized sp2 clusters may be useful to delay the film failure by suppressing the initial strain localizations. It is demonstrated that the propagation of sp2 clusters for the DLC films can be induced by their deformation besides the high friction temperature in their wear tests. This demonstration can help to improve the understanding of their trigological mechanisms.