Huimin Qi

Comparative study on tribological mechanisms of polyimide composites when sliding against medium carbon steel and NiCrBSi

Tribological behaviors of various polyimide (PI) composites when rubbing with medium carbon steel (MCS35) and NiCrBSi, were comprehensively investigated. When the conventional PI composite filled with carbon fibers and graphite was concerned, the carbon-based tribofilm formed on NiCrBSi surface resulted in obvious friction- and wear-reduction. However, no lubricating tribofilm was formed on MCS35 surface. Chelation of polymeric molecular radicals with the metallic counterparts was identified on the worn surfaces. Theoretical calculations corroborated that the Ni-based metal-organic compound showed a higher stability than the Fe-based one. With respect to the sliding of the hybrid nanocomposites containing silica or h-BN nanoparticles, the nanoparticles released onto the interface significantly mitigated tribo-oxidation of metallic counterparts, and were finally tribo-sintered into a compact layer after being mixed with remnant polymer and tribo-oxidation products. The effect of counterpart material was less pronounced for the tribological mechanisms of the nanocomposites than for the conventional composite.

Significance of an in-situ generated boundary film on tribocorrosion behavior of polymer-metal sliding pair

Polymer composites have a high potential for applications as tribo-materials exposed to sea water owing to their self-lubrication characteristic and high chemical stability. In the present work, tribological behaviors of polyetheretherketone (PEEK) composites rubbing with stainless steel in sea water were explored using a pin-on-disc tribometer integrated with a potentiostat for electrochemical control. It was demonstrated that further adding 5 vol% hexagonal boron nitride (h-BN) nanoparticles into PEEK reinforced with short carbon fibers (SCF) significantly enhanced the wear resistance. Moreover, the stainless steel exhibited significantly enhanced tribocorrosion resistance when rubbing with the hybrid nanocomposite, in comparison to the sliding against PEEK filled only with SCF. Nanostructures of the boundary films formed on the steel surface were comprehensively investigated. It was manifested that tribo-chemistry products of h-BN, i.e. H3BO3 and B2O3, were arrayed in a closely packed boundary film. It seems that inclusion of layer-structured H3BO3 and B2O3 improved the resilience of the boundary film. The continuous boundary film covering the steel surface provided a lubrication effect and strengthened the passivation layer. A new route for enhancing simultaneously tribological and corrosion resistance of polymer-metal pairs by controlling in-situ tribo-chemistry was thus proposed.

Switching Brake Materials To Extremely Wear-Resistant Self-Lubrication Materials via Tuning Interface Nanostructures

Tribological performance of motion components is one of the key aspects that must be considered in a wide range of applications such as vehicles, aircrafts, and manufacturing equipment. This work demonstrates that further addition of only low-loading hard nanoparticles into a formulated nonasbestos organic brake material directly switches its functionality to a self-lubrication material. More importantly, the newly developed nanocomposites exhibit an extremely low wear rate. Comprehensive investigations on the friction interface reveal that the great friction and wear reduction are due to the formation of a nanostructured lubricious tribofilm. Tribofilm formation is continuously fed by complex molecular species released from the bulk nanocomposites, for which nanoparticles digested within the tribofilm greatly enhance its robustness and lubricity. This work gains insight into the crucial role of the interface nanostructure and paves a route for developing extremely wear-resistant self-lubrication composites for numerous applications.

Comparative study of tribochemistry of ultrahigh molecular weight polyethylene, polyphenylene sulfide and polyetherimide in tribo- composites

HYPOTHESIS Tribochemical reactions of polymer matrix with steel counterpart can exert an important role in tribofilms structure, and thereby affect the tribological performance of its composites. EXPERIMENTS In the present work, tribochemistry of ultrahigh molecular weight polyethylene (UHMWPE), polyphenylene sulfide (PPS) and polyetherimide (PEI) in tribo-composites was comparatively studied. Two kinds of formulations, i.e. conventional composites filled with carbon fibers and graphite and hybrid nanocomposites containing additional silica (SiO2) nanoparticles, were investigated. FINDINGS It was revealed that when rubbing with steel, molecular chains of UHMWPE were broken and free radicals finally chelated with the counterface. Whereas, PPS molecules underwent thermal decomposition, oxidation and finally ferrous sulfide (FeS) and ferric sulfate [Fe2(SO4)3] were generated. Chelating reaction of PEI was identified only for sliding of PEI-based nanocomposite. Tribochemistry of polymer molecules played an important role in tribofilm formation and tribological performance of the conventional composites. When sliding took place with the hybrid nanocomposites at low pv conditions, tribochemistry of polymer molecules played a similar role as for the conventional composites. Nonetheless, at high pv conditions, independent on the polymer matrices, robust tribofilms containing high fraction of silica were generated on the steel counterface, minimizing direct rubbing of the friction pair.