Structural properties of ${\mathit{sp}}^3$-bonded hydrogenated amorphous carbon

Hydrogenated amorphous carbon (a-C:H) is important as a material for hard coatings. Here, we describe the structural properties of computer-generated models of a-C and a-C:H with nearly ideal ${\mathit{sp}}^3$ bonding. Without H, the material is very hard, but it is highly strained compared with ordinary ${\mathit{sp}}^2$-bonded a-C. Increasing the hydrogen content reduces the internal strain, facilitating ${\mathit{sp}}^3$ bonding, but also softens the material. In addition, the number of primitive rings in the structure decreases, and the size of the rings increases, indicating a transition to a polymeric form at high hydrogen content. Implications for very hard ion-beam-deposited a-C are discussed.