The sliding behaviour of sintered and thermoplastic polyimides investigated by thermal and Raman spectroscopic measurements

Polyimides are used in dry sliding applications due to their high strength and thermal stability. Friction and wear behaviour of sintered polyimide (SP) and thermoplastic polyimides (TP) are investigated for steel counterface temperatures between 100 and 260 °C. Afterwards, the tribophysical phenomena in the sliding interface are investigated by calorimetric and thermogravimetric analysis of the wear debris and Raman spectroscopy of the worn polymer surfaces. Spectroscopic analysis of SP shows that Csingle bondNsingle bondC bonds in the main molecular structure are progressively stretched along the sliding direction with a reorganisation between axial and transverse orientation at 180 °C. Stretching of the Cdouble bond; length as m-dashO and Csingle bondOsingle bondC side groups depends on the applied load. For TP there is degradation of the imide part under high temperatures while the sliding surface visually colours from transparent to opaque yellow. The observed chemical reaction of TP under sliding can be attributed to ring opening by chain scission induced by sliding and cross-linking between the adjacent polymer chains. From thermal analysis, a reversible endotherm reaction at 180 °C might be attributed to dehydratation of polyimides. This reaction depends slightly on the applied sliding temperature and is less prominent for 100 °C and the most prominent for the 260 °C sliding temperature, suggesting further imidisation under sliding for sintered polyimides. Thermoplastic polyimides show a clear shift in secondary glass temperature towards higher temperatures and a decrease of the melting peak and recrystallisation peak intensities, suggesting the formation of cross-links in the amorphous phase.