Thermal transitions in polyimide transfer under sliding against steel, investigated by Raman spectroscopy and thermal analysis

Polyimides (PI) are known for their extremely high thermal stability and lack of a glass transition temperature below their decomposition point. Therefore, they are frequently used in high-demanding tribological applications. The tribological characteristics of sintered polyimide (SP-1) are presently investigated as a function of the sliding temperature that is artificially varied between 60°C and 260°C under fixed load in counterformal contact with a steel plate. For obtaining low friction and wear, a transfer film needs to develop onto the sliding counterface, induced by viscous polymer flow. As surface plastification is more difficult for high-performance materials, for example, polyimide, a transition towards low friction and stabilized wear rates is observed at temperatures higher than 180°C in accordance with the occurrence of plate-like transfer particles, while high friction and no transfer was observed at lower temperatures. This transition is correlated to a peak value in both friction and wear at 180°C and is further explained by Raman spectroscopy performed on the worn polymer surfaces and temperature-modulated differential scanning calorimetry. It is concluded that the intensity of C-N-C related absorption bands is minimal at 180°C and is complementary to the intensity of the CC phenylene structure that is maximal at 180°C. The orientation of the C-O-C structure slightly decreases relative to the sliding surface at higher bulk temperatures. The amount of CO functional groups is the lowest at 140°C, while its orientation progressively enhances at higher bulk temperatures. At 140°C also, the lowest wear rates were measured. The 180°C transition temperature with a peak value in both friction and wear corresponds to a secondary transition measured in the specific complex heat capacity, pointing out that the overall bulk temperature is presently more important than local flash temperatures for causing transitions in tribological behavior. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1407–1425, 2006

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