Tribological Performance of Attapulgite Reinforced Polyimide Composites under Dry Friction Condition

Operating environments of mechanical motion components become increasingly harsh with rapid development of modern transportation and other industrial activities. Friction and wear are main causes of shorten service life and weaken safety and reliability of equipment, thus causing safety accidents in serious cases. Polymer-based composites have been developed increasingly for self-lubricating materials by virtue of their lightweight, superior tribological performance, high specific strength, improved corrosion resistance and thermal stability. Numerous high-performance self-lubricating composites were formulated for different sliding contact conditions. Nevertheless, the majority of self-lubricating polymer composites were not suitable for operation at higher service temperature, i.e. even polyether ether ketone-based composites only service at 260 ℃. Polyimide (PI) is an outstanding engineering material with service temperature of 300~500 ℃. While, the poor wear resistance and anti-friction properties of pristine PI severely limit its further applications in harsh motion conditions. Attapulgite is a hydrous layer-ribbon magnesium aluminum silicate clay mineral having a fibrous morphology. Components and crystal structures of attapulgite are very similar to that of serpentine, whereas the cost of attapulgite is much lower because of its natural abundance and simplicity for refining. Moreover, researchers reported modified attapulgite as oil additives exhibit excellent lubricating performance, which ascribed to growth of a robust tribofilm on steel surface during sliding process. Although tribological performance of polymer composites reinforced with attapulgite have been investigated in several pioneering works, effect of attapulgite on tribo-performance of conventional carbon fiber (CF)-reinforcing PI composites has not been studied yet. In particular, comprehensive understanding of complex physio-chemical actions of attapulgite and PI at dry sliding interface is still lacking.Herein, three different polyimide (PI)-based composites (PI+10CF/Gr, PI+10CF/Gr+5ATP and PI+10CF/Gr+5SiO2) were prepared and their tribological properties under dry friction conditions were investigated using a Pin-on-Disk (POD). The microscopic morphologies and chemical compositions of tribofilms and worn composites’ surfaces were further characterized with scanning electron microscope (SEM) and optical microscope (OM). Friction experiments demonstrated that compared with conventional PI+10CF/Gr composite, the friction and wear of PI+10CF/Gr+5SiO2 and PI+10CF/Gr+5ATP were greatly reduced under higher PV valves (PV value=3, 5 and 7 MPa·m/s). In particular, PI+10CF/Gr+5ATP improved wear resistance by up to 69% and 50% compared to PI+10CF/Gr and PI+10CF/Gr+5SiO2 under PV value of 3 MPa·m/s. SEM graph of worn steel surface demonstrated that the entire surface areas of the metallic disc including of original grooves were covered by transferred material after being rubbed against PI+10CF/Gr+5ATP. The tribo-oxidation of steel surface was inhibited with incorporation of ATP filler. X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared spectroscopy (ATR-FTIR) and Raman spectrometer (Raman) analyses revealed that ATP undergoes tribo-chemical reactions under friction heat and shear stress, and its tribo-chemical products, i.e MgO, SiOx and Al2O3 nanocrystals tribo-sintered with PI molecular chain segments and graphitic carbon, forming a“high quality”tribofilm with high load-carrying ability, which significantly improved the friction and wear resistance of PI composites. This study will provide a foundation for future researches of high temperature-resistant and long-life tribo-materials. © 2023 Science Press. All rights reserved.