Crack-tip Plasticity and Intrinsic Toughening in Nano-sized Brittle Amorphous Carbon

TL;DR

This study reveals that nano-sized amorphous carbon exhibits intrinsic toughening through atomic plasticity near crack tips, a phenomenon enabled by reduced sample dimensions and high local stresses.

Contribution

It demonstrates for the first time that intrinsic toughening occurs in monolithic brittle ceramics like diamond-like carbon at nanoscale dimensions.

Findings

Intrinsic toughening arises in nano-sized DLC due to atomic plasticity.

Atomic plasticity involves sp3 to sp2 rehybridization.

Reduced sample size decreases crack driving force, enabling plasticity.

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, 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.