Mechanical properties of copper/carbon nanocomposite films formed by microwave plasma assisted deposition techniques from argon-methane and argon-acetylene gas mixtures

Nanostructured copper/amorphous hydrogenated carbon (a-C:H) composite films have been deposited on silicon substrates by a hybrid technique combining microwave plasma-enhanced chemical vapor deposition and sputter-deposition processes from argon-methane and argon-acetylene mixtures of various compositions. The size of crystallites, ratio between sp2 and sp3 types of carbon bonds, hardness, friction coefficient, and wear resistance of composite films were investigated as functions of the carbon content in the films expressed by the atom number ratio C/(C + Cu). The size of crystallites decreased down to 2 nm in films having high carbon contents (60%, C/(C + Cu)). Composite films formed from Ar-C2H2 mixtures with a carbon content of 78% (C/(C + Cu)) exhibited relatively high hardness (6.1 GPa) and very high elastic recovery upon unloading (90%). However, the wear resistance of hard Cu/a-C:H films was very low ((5.7 ± 0.6) × 10−6 mm3/Nm). During the friction tests, a very low resistance to crack formation and cross-sectional propagation was observed. Cu/a-C:H films formed from Ar-CH4 mixtures and containing 75% of carbon (C/(C + Cu)) possessed low friction coefficients (0.02-0.04) and volume wear coefficients ((0.11 ± 0.02) × 10−6 mm3/Nm). The intensity Raman peak ratio ID/IG (0.71) proved to be much lower for the films having a low value of volume wear coefficient. This is the result of the presence of nanosized carbon clusters in the polymer-like matrix and copper crystallites that provides very low shear stresses during friction tests and forms nanosized wear debris.