Structural properties of sp3-bonded hydrogenated amorphous carbon

Hydrogenated amorphous carbon (a-C) is important as a material for hard coatings. Here, we describe the structural properties of computer-generated models of a-C and a-C with nearly ideal sp3 bonding. Without H, the material is very hard, but it is highly strained compared with ordinary sp2-bonded a-C. Increasing the hydrogen content reduces the internal strain, facilitating sp3 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. © 1991 The American Physical Society.