Nanoscale Tribological Mechanism And Experimental Study Of Graphene

In manufacturing and social life,the energy loss caused by friction and wear accounts for about 23%of the world’s energy consumption,of which more than 80%of the mechanical components fails because of friction and wear,and the economic losses account for about 2%-7%of GDP every year.If the friction between the contact interfaces can be greatly reduced,it will largely reduce machine wear,energy loss and economic loss.At present,superlubricacity in which the friction of the contact interface tends to "zero" is considered to be one of the important ways to reduce frictional loss.Graphene is the thinnest solid lubricating material in existence,Because of its excellent mechanical and tribological properties,it has become an ideal lubricant for superlubricating contact interfaces.However,the existing experimental methods are difficult to capture the structural deformation at nanoscale and energy dissipation of the contact interface.Molecular dynamics simulation methods can directly reveal the friction details of the contact atoms at nanoscale,and facilitately and effectively study the contact.The friction state of the interface and design the superlubrication model.At present,there are still many problems in the research of frictional mechanism of graphene at nanoscale.For example,the theoretical models contain mostly rigid substrates,the superlubrication contact load is low,the environment requirements in the experiment are harsh,and the superlubrication state is unstable.In response to the above problems,this article has conducted the following research:Firstly,molecular dynamics simulation is used to study the nanoscale friction mechanism of graphene based on non-rigid quartz substrate,and analyze the surface and interlayer friction behavior of graphene with different loads and different layer thicknesses.The research results show that the friction behavior of graphene surface is dependent on load and layer thickness;while the friction behavior between layers is affected by load,layer thickness and wrinkle competition.The wrinkles between layers also make graphene easy to wear.When the number of surface layers is increased to 4,the friction can be significantly reduced.This is because increasing the number of graphene layers can increase the in-plane deformation stiffness of graphene,thereby reducing the contact area,reducing friction and improving wear resistance.Wrinkles increase the degree of out-of-plane fluctuations of graphene,leading to increased interlayer friction,increased local stress,and fracture.Secondly,the friction behavior of adding nanoparticles on the contact interface with graphene on both sides is discussed.It can be obtained from the movement mode and mechanical properties of nanoparticles with different axis-to-diameter ratios that the complete spherical nanoparticles have better rolling characteristics.Increasing the size of nanoparticles can reduce the out-of-plane deformation of graphene,thereby reducing friction.Covering the surface of the nanoparticles with graphene can significantly reduce the friction of the system,and can achieve superlubricity state with a friction coefficient of 0.0084 under a load of 500 nN.When the load increases to a certain extent,the interface graphene is squeezed by the graphene folds on the surface of the nanoparticles and undergoes severe plastic deformation,forcing the friction force to increase,resulting in superlubricity failure.Finally,the basic friction model and superlubricity model of graphene is experimentally verified.Using a quartz plate and a quartz ball with graphene grown on the surface,the effect of different layer thickness on the friction properties of graphene is consistent with the simulation prediction.For the friction of the system with micro/nano particles added between two quartz sheets where graphene is grown,the results show that the influence of the size of the micro/nano particles without graphene growth on the friction coefficient is non-linear.By growing graphene on particles with a particle size of 8 μm,the system achieves super lubrication under 300 mN and 500 mN loads,with friction coefficients of 0.0017 and 0.0026,respectively,verifying the results of molecular dynamics simulations.