Structural and tribological behaviours of silicon doped amorphous carbon films

We report on the structural evolution and low-friction behaviour of silicon (Si) doped amorphous carbon (a-C) films. Si-doped a-C films were fabricated by controlling the magnetron Si target currents under an electron cyclotron resonance (ECR) plasma system. Structural analysis indicated that Si atoms doped into the a-C lattice preferentially replaced certain sp3 C atoms, and subsequently formed bonds with other sp3 C atoms. Nanoindentation and nanoscratch tests showed that the Si-doped a-C films had hard surfaces with a hardness of approximately 24 GPa and a scratch depth of 40–60 nm. Ball-on-disk tribological evaluations further showed that these films exhibited low-friction behaviour, with a minimal friction coefficient of 0.02. Detailed transfer film characterisation revealed the formation of rich oxide and graphene structures within the stable contact-sliding interface of the Si-doped a-C film. The low friction mechanism was summarized as H–H interactions from the surface terminal passivation of Si dangling bond with hydroxide (-OH) or hydrogen (-H) groups and π*-π* interactions occurring between the graphene layers co-reducing the friction force. These findings shed light on the significance of doped Si atoms in the structural design and low-friction applications of carbon films.