Emergence of intrinsic toughening in brittle amorphous carbon at nanoscale enabled by crack-tip stress concentration

Abstract

Most monolithic brittle materials are vulnerable to the failure by cracks because of a lack of intrinsic toughening mechanisms, such as the plasticity in the vicinity of the crack front. As a result, most of the efforts to mitigate the sudden failure of brittle ceramics have been focused on developing the extrinsic toughening mechanisms that hinder crack propagation behind the tip, such as the fiber bridging. In this work, it is experimentally demonstrated that the intrinsic toughening arises even in the brittle monolithic ceramic material such as diamond-like carbon (DLC) when its external dimension reduces down to sub-micron scales. This unique phenomenon owes its origin to the decrease of the crack driving force in the small samples, which in turn enables them to bear high enough stresses to activate the local atomic plasticity. Through nanomechanical tensile and bending experiments, electron energy loss spectroscopy analysis, and finite element method for stress distribution calculation, it was confirmed that the local atomic plasticity was dominantly carried by $sp^3$ to $sp^2$ carbon transformations dissipating the fracture energy, which is responsible for the intrinsic toughening. Additionally, the application of this unique phenomena at nanoscale is suggested to control the friction coefficient of DLC by surface nanopatterning.