Role of Third Bodies in Friction Behavior of Diamond-like Nanocomposite Coatings Studied byIn SituTribometry

Abstract

The friction and wear behavior of amorphous diamond-like nanocomposite (DLN) coatings (C:H:Si:0) in low speed, dry sliding contact has been investigated using a home-built in situ tribometer. Tests were performed under reciprocating sliding against sapphire and steel hemispheres in dry (∼4% RH) and moist (35 to 52% RH) air at contact stresses of 0.7 and 1.1 GPa. In situ visual observations identified how third body processes affected the friction behavior of DLN coatings. For all test conditions, a transfer film began to form on the sapphire hemisphere during the first cycle, and the coefficient of friction dropped during further buildup of the transfer film. Video analysis showed most of the sliding motion took place between the transfer film on the hemisphere and the DLN coating. This velocity accommodation mode, interfacial sliding, was responsible for steady-state friction coefficients between 0.03 and 0.05 with lower values obtained at high stress and low RH. When the transfer film broke up the friction began to spike (0.1 ≤ μ ≤ 0.5); during the spikes, shearing of third bodies also contributed to velocity accommodation, as debris attached, detached and recirculated between the hemisphere and the track. Subsequently, the friction coefficient fluctuated between low and high values due to reformation and breakup of the transfer film, respectively.