Temperature-dependent mechanical and tribological behaviour of unfilled and graphite-filled polyimides

Polyimide (PI) is used in tribological applications under extreme conditions, e.g. at elevated temperatures where liquid lubrication is impractical. This study investigates the temperature-dependent mechanical and tribological behaviour of biphenyltetracarboxylic dianhydride (BPDA)-based PI, which remains understudied, using the well-studied pyromellitic dianhydride (PMDA)-based PI as a baseline for comparison. The effects of graphite fillers are also examined, including filler presence, graphite type, graphite concentration, and processing scale of the PIs. BPDA-based PI generally exhibits superior mechanical performance, with higher storage and loss moduli, strength, and toughness than PMDA-based PI. However, BPDA-based PI undergoes a glass transition at 300 °C, leading to a substantial reduction in storage and loss moduli to values below those of PMDA-based PI. Tribologically, both PIs demonstrate excellent wear resistance from 25 to 390 °C. Wear rates remain within the ultralow wear regime (<10 −6 mm 3 Nm −1 ) below 230 °C and increase slightly to the near ultralow wear regime (10 −6 mm 3 Nm −1 ) above this temperature, likely due to thermal degradation. Both PIs exhibit high friction at room temperature, attributed to unimidised PI with opened imide rings that restrict chain mobility and increase adhesion to metal the counterface. Friction decreases significantly for both PIs at 190 °C due to dehydration-induced imide ring closure, enhancing chain mobility and reducing adhesion. Above 190 °C, PMDA-based PI maintains stable friction, whereas BPDA-based PI shows increasing friction consistent with its glass-transition behaviour. Graphite fillers modify the mechanical properties of both PIs, but show minimal influence on friction and wear, except for a modest improvement at 390 °C and a detrimental effect at room temperature, particularly for PMDA-based PI. Notably, iron-containing back-transfer films occasionally form on graphite-filled PMDA-based PI at room temperature, substantially reducing friction. The presence of iron in these films suggests a potential pathway to promote their formation, mitigating room-temperature friction in PMDA-based PI.