  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, we experimentally demonstrate 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, we confirmed that the local atomic plasticity associated with sp3
to sp2 rehybridization is responsible for the intrinsic toughening.
