Mechanical properties of nanocrystalline diamond/amorphous carbon composite films prepared by microwave plasma chemical vapour deposition

Nanocrystalline diamond films (NCD) have been deposited by microwave plasma chemical vapour deposition from CH₄/N₂ mixtures with varying methane content. They consist of diamond nanocrystallites with sizes of 3–5 nm embedded in an amorphous matrix with grain boundary widths of 1–1.5 nm. The CH₄ content in the gas phase has almost no influence on the microscopic structure but a strong effect on the macroscopic structure and morphology. The mechanical and tribological properties of these films have been investigated by nanoindentation, nano tribo tests, and nano scratch tests. The hardness of a 4-μm-thick film deposited with 17% methane was about 40 GPa, the indentation modulus 387 GPa, and the elastic recovery 75%. Ball-on-disk tests against an Al₂O₃ ball revealed, after initially higher values, a friction coefficient of ≤0.1. Tribo tests and scratch tests proved a strong adhesion and a protective effect on silicon substrates. Finally, the correlations between the macroscopic structure of the films and their mechanical and tribological properties are discussed.