Characterisation of polyimides under high-temperature sliding

Polyimides are used in friction and wear applications because of their high strength and thermal stability up to 310 (short term) or 450 °C (long term). After sintering, secondary transition temperatures do not occur up to the degradation point. The tribological characteristics of sintered polyimides are presently investigated for counterface bulk temperatures between 100 and 260 °C. Through the high stiffness and lack of softening, transfer mechanisms are different than those of traditional engineering polymers. No transfer film was observed at low temperatures, resulting in high friction (μ > 0.30). For bulk temperatures above 180 °C a sudden drop in friction was found towards μ < 0.30, in accordance with the formation of a non-homogeneous transfer film consisting of plate-like particles. Wear rates are minimum for a bulk temperature of 140 °C and progressively increase at higher temperatures. The evaluation of two temperature models for calculating bulk and flash temperatures indicates that the wear transition corresponds to flash temperatures above 180 °C. Thermal analysis reveals that the 180 °C transition temperature is explained by dehydration effects. The orientation of the polymer chains at the sliding surface is investigated by post-mortem Raman spectroscopy, showing that the C–N–C bonds in the molecular structure are progressively stretched along the sliding direction with higher sliding temperatures. However, at 180 °C there is a reorganisation between the axial and the transverse orientation.