Influence of Internal Lubricants (PTFE and Silicon Oil) in Short Carbon Fibre-Reinforced Polyimide Composites on Performance Properties

Short carbon fibre-reinforced thermoplastic polyimides (30 wt%) often show high and unstable coefficients of friction. In this study, the effects of internal lubrication by polytetrafluoroethylene (PTFE) (15 wt%) or silicon oil (15 wt%) have been investigated using a reciprocating cylinder-on-plate test under 50–100 N normal loads and 0.3–1.2 m/s sliding velocities. The sliding mechanisms are discussed by considering mechanical and thermochemical modifications. The PTFE additives provide lowest coefficients of friction completely stabilising after a transition from mechanically into thermally controlled sliding at 120 °C. The sliding mechanisms and homogeneous transfer films are mainly controlled by plasticisation rather than easy-shear of its lamellar structure that is hindered by fibre reinforcement. Thermoplastic lubricants decrease the mechanical strength and therefore cause deformation and highest wear rates under 200 N. Internal oil lubricants do not reduce coefficients of friction at mild to intermediate normal loads and sliding velocities, while they become most efficient at severe sliding conditions, augmenting the pv-limit. While coefficients of friction match uniquely to the pv-conditions, the wear rates are mainly influenced by the load level: the lowest wear rates are provided by PTFE at low to intermediate pv-conditions and by oil lubricants under high normal loads.