Mechanical properties and coordinations of amorphous carbons

The elasticity and hardness of a-C and a-C:H are related to their mean network coordinations. It is shown that ${\mathit{sp}}^{2}$ sites contribute no rigidity if they form graphitic clusters, as suggested by experiment and electronic structure calculations, despite their having coordinations above the critical value 2.4. Hence both ${\mathit{sp}}^{2}$ sites and polymeric =${\mathrm{CH}}_{2}$ groups can severely lower the hardness of amorphous carbons.