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 their having coordinations above the critical value 2.4. Hence both sp² sites and polymeric =CH₂ groups can severely lower the hardness of amorphous carbons.