The effects of Si incorporation on the electrochemical and nanomechanical properties of DLC thin films

Abstract

Silicon-doped diamond-like carbon (DLC) films with a Si content of up to 20.2 at.% were grown on Al2O3–TiC substrate by plasma-enhanced chemical vapour deposition. The influence of Si addition on the bonding structure, nanomechanical and corrosion behaviour of the DLC films was investigated by Raman and X-ray photoelectron (XPS) spectroscopy, nano-indentation, potentiodynamic and electrochemical impedance spectroscopy (EIS). Silicon addition promoted the formation of sp3 bonding and reduced the hardness. The deterioration of the nanomechanical properties is related to the increased hydrogen content in the films, leading to the formation of a polymeric sp3 CHn structure. The high hydrogen concentration in the Si-containing DLC samples was established by the increased Raman background slope. The EIS were analysed within the context of an equivalent circuit, which incorporated two time constants representing the DLC coating and the solution/Al2O3–TiC interface. Introduction of Si in the DLC led to significant improvements in the corrosion resistance of DLC, as revealed by increase in the charge transfer resistance and reduction in the anodic current of the polarisation curves. Films with a thickness of 20 nm remained intact after the polarisation scan when the Si concentration was increased above 11.8 at.%. The improvements in corrosion resistance are related to the formation of a passivation layer, which fills the pores present in the films.