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 sp2 sites contribute no rigidity if they form graphitic clusters, as suggested by experiment and electronic structure calculations, despite having coordinations above the critical value 2.4. Hence both sp2 sites and polymeric = CH2 groups can severely lower the hardness of amorphous carbons.