Investigation On Current-Carrying Friction And Wear Mechaisms Of Pantograph Strip And Its Coating Protection

Rail transit is an important component of modern economic system in China and a powerful material guarantee for promoting the region connection and urbanization process.As the pantograph-catenary system is one of key elements in the power source of rail vehicles,its current-carrying stability plays an important role in the safety of rail vehicles and passengers.However,such safety is threantened when the current-carrying stability is greatly degraded induced by the surface wear failure of the pantograph strip.Therefore,the development of surface coating technology for improving the friction and wear performances of the pantograph strip becomes an effective method to solve the above safety problems.Following are our detailed works and results:（1）Relationships between the current-carrying stability of the pantograph-catenary system and the friction and wear performances of the pantograph strip were established.Simulation results indicate that the dynamic evolution process of contact pressure between the pantograph and the catenary undergoes two stages:initial vibration and final equilibrium.The statistical indicators of such pressure in its equilibrium stage are used to quantitatively evaluate the current-carrying stability of the pantograph-catenary system.When the friction coefficient of the pantograph strip increases two times,the root mean square and the non-uniformity coefficient of the contact pressure increase by 26%and45%,respectively.Similarly,these statistical indicators sharply increase after the surface wear of the pantograph strip occurs,which means its excellent friction and wear performances are fundamental elements for the stable current-carrying characteristics of the pantograph-catenary system.（2）Improvement of lubrication coatings on the current-carrying friction and wear performances of the pantograph strip was studied.Experimental results show that the friction coefficient and the wear rate of the electric carbon pantograph strip are measured to be about 0.196and 3.56×10^-3 mm³/min,respectively,and its surface wear failure occurs after 10⁴ sliding cycles.However,when the pantograph strip is lubricated by composite lubrication coatings,its friction coefficient and wear rate decrease by about 10%and 7%,respectively.Characterization analysis shows that the friction reduction is attributed to the in-situ formation of transfer films with the easy-shear capability,and the suppression of adhesive and abrasive wears of the pantograph strip is responsible for its wear resistance enhancement.In addition,the lubrication performance of the composite coatings is further improved by the addition of carbon-based nanoparticles,because the free movement of molecular chains in the composite coatings is prevented by these nanoparticles during the friction process.（3）Mechanisms of micro/nano-scale regulation strategies on the inherent surface properties of carbon-based nanoparticles were discussed.At the micro-scale,graphene has a relatively high surface friction force at the edge of grooves on its supporting substrate,and its wear failure is also easy to occur.When these grooves are filled with C60 molecules,the surface roughness of the substrate is significantly reduced,resulting in a decrease of about an order of magnitude in the friction force of graphene and an increase of about five times in its wear resistance.At the nano-scale,the friction force of graphene has a rapid increase by the presence of lattice defects in its atomic structure when such force is only dominated by non-bond interatomic interactions at contact interfaces.Once the load exceeds the critical value,the friction force of graphene is significantly increased due to the formation of interfacial covalent bond,and the possibility of such formation is highly improved by the introduction of lattice defects.（4）A cross-scale design of high-lubrication composite coating was proposed.Theoretical research pointed out that intrinsic and external defects are introduced into the atomic structure of graphene during its surface oxidation modification.These defects enhance the surface adsorption of graphene due to the interfacial adhesion effect,causing that its lubrication enhancement on the composite coating is expected to be improved.To verify this expectation,the current-carrying friction experiments were carried out.It was found that the friction force of oxidatively modified graphene reinforced coating is about 6%lower than that of unmodified one,showing a better lubrication effect after the oxidation modification on graphene.Obviously,the macroscopic experimental results show a good consistency with the microscopic simulation results,which verifies the expectation of the lubrication effect enhancement of composite coatings by oxidation modification on graphene.In conclusion,the oxidatively modified graphene reinforced coating can improve the current-carrying stability of the pantograph-catenary system by about 4%.