Ga Zhang

Enhancing the tribological performance of PEEK exposed to water-lubrication by filling goethite (α-FeOOH) nanoparticles

The formation of a tribo-film on the counterface plays an important role on the tribological performances of polymer subjected to mixed and boundary lubrication conditions. However, when freshwater is used as a lubricant, the formation of a tribo-film usually is hindered. In order to overcome this disadvantage, PEEK/α-FeOOH nanocomposites were developed and their tribological performances were studied under water lubrication conditions in the present work. It was demonstrated that the inclusion of α-FeOOH nanoparticles (NPs) into the PEEK matrix improves significantly the tribological performance of the matrix. The nanostructures and properties of the tribo-films formed on the steel counterface were comprehensively studied. It was identified that the addition of α-FeOOH NPs promotes the formation of a lubricating tribo-film which covers the entire counterface. Based on the investigations on tribo-films, we deem that the α-FeOOH NPs act as precursors for the dehydration reaction promoting the formation of a tribo-film which consists of α-Fe2O3 and transferred PEEK. It is assumed that the enhanced tribological performance is related to the possibly high load-bearing capability and “easy-to-shear” characteristic of the tribo-film.

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.

Tribological Behaviors of Carbon Fiber Reinforced Epoxy Composites Under PAO Lubrication Conditions

In order to avoid occurrence of severe seizure of motion components exposed to mixed and boundary lubrication, e.g., in engine and transmission systems, to replace metal–metal friction pairs by polymer–metal pairs provides a potential solution. In the present work, the tribological behaviors of short carbon fiber (SCF)-reinforced epoxy (EP) composites when lubricated with polyalphaolefin base oil were investigated. It was identified that the running-in tendency and friction coefficient of the composites show a close dependence on the lubrication regimes. SCF improves greatly the wear resistance of EP although they increase slightly the friction coefficient. It was demonstrated that further addition of monodispersed SiO2 nanoparticles into the EP composite filled with SCF and graphite leads to higher wear resistance. The reinforcement of SCF and the formation of a high-performance tribofilm on the surface of steel counterpart play an important role on the tribological properties of EP-based materials.

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.

Friction and wear behavior of PEEK and its composite coatings

Abstract This chapter describes a general approach to the friction and wear behavior of poly-ether-ether-ketone (PEEK) coatings under dry sliding conditions. First of all, two efficient coating techniques, thermal spraying and painting, are introduced. Then, the crystalline structure and adhesive property of PEEK coatings are characterized. The correlation between the crystalline structure and the tribological characteristics of PEEK coatings is clarified. In addition, the effects of the sliding conditions, such as sliding velocity, applied load and ambient temperature, on the tribological behavior of an amorphous PEEK coating are studied. Based on 3D morphology analyses of worn tracks and scanning electron microscope (SEM) observations of worn surfaces, the friction and wear mechanisms of the amorphous PEEK coating are discussed. Finally, the tribological behavior of nano-SiC (7 wt%)-filled PEEK coating is described. The roles of the nano-SiC particles on the coating tribological behavior are discussed.

High lubricity and electrical responsiveness of solvent-free ionic SiO2 nanofluids

Solvent-free ionic nanofluids with a core–shell structure were successively synthesized via functionalizing silica nanoparticles with a charged corona and ionically tethering with oligomeric chains as a canopy. It was demonstrated that the as-synthesized nanofluids were endowed with outstanding dispersion stability and fluidity. Importantly, when used as a lubricant and additive, the nanofluids showed excellent friction-reducing and antiwear properties. That is, blending even a small fraction of nanofluids into polyethylene glycol base oil dramatically reduced the friction and wear of a steel–steel contact. Depending on the specific shell structures, the lubricity of the nanofluids exhibited a responsive characteristic to electrical potential. The friction coefficients can be adjusted by loading or unloading the external power and changing the direction of the electrical potential. Formation of a unique double electric layer consisting of both organic and inorganic materials during the friction was proposed, which can be essential for yielding robust lubrication of the adsorption film. Moreover, a nanostructured tribofilm comprising a significant fraction of silica and tribo-chemical products of the organic layers was identified, and it was believed to be significant in improving the boundary lubrication performance.

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.

Soft/Hard-Coupled Amphiphilic Polymer Nanospheres for Water Lubrication

Amphiphilic polymer nanospheres of poly(3-sulfopropyl methacrylate potassium salt- co-styrene) [P(SPMA- co-St)] were prepared by a simple soap-free emulsion polymerization method and used as efficient water lubrication additives to enhance the antiwear behaviors of the Ti6Al4V alloy. The monodisperse and flexible P(SPMA- co-St) bicomponent copolymer nanospheres were synthesized with a controllable manner by adjusting the mass fraction ratio of the monomers, with the hydrophobic polystyrene (PSt) as the hard skeletal carrier component and the hydrophilic PSPMA with a hydration layer structure as the soft lubrication layer in the course of friction. The influences of the monomer concentration, the copolymer nanosphere additive content, the load, and the frequency of the friction conditions on their tribological properties were studied in detail, and a probable antiwear mechanism of the soft/hard-coupled copolymer nanospheres under water lubrication was also proposed. The results show that compared with pure PSt, the P(SPMA- co-St) polymer nanospheres exhibited better antiwear property as an additive for water lubrication, and the friction coefficient and the wear volume first decreased and then increased with the increase of the SPMA content, indicating that the hydrophilic SPMA has a significant effect on lubrication properties owing to its hydration performance. Furthermore, with the increase of polymer nanosphere concentration, the friction coefficient and wear amount also decreased to a stable and low value at a saturation concentration of 1 wt %. The flexible polymer nanospheres with a hydrophilic soft SPMA shell and a rigid PSt core exhibited good friction-reduction and antiwear performance as lubrication additives, indicating their promising and potential applications in water lubrication and biological lubrication.

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.