| As an emerging material in the 21st century,graphene is gradually making its mark in the field of aerospace materials due to its excellent mechanical,thermal,optical,and tribological properties.Its ultra-low friction between layers and atomic-level smooth surface can be used as a lubricant in electronic components and Micro Electro Mechanical Systems(MEMS)of aerospace devices;its ultra-high light transmission and electrical conductivity can be a preferred photoconductive material for solar cells.However,the harsh environment of high vacuum,high radiation,and high-temperature difference in space will have a qualitative impact on the performance of graphene,which will lead to the reduction of the efficiency of the relevant devices or even failure.Therefore,studying the frictional properties of graphene in extreme environments and the mechanism behind will be beneficial for better application of graphene and other two-dimensional materials in aerospace.In this paper,we use the mechanical exfoliation method to obtain graphene films from single-crystal graphite,and modify graphene based on ultraviolet(UV)irradiation and high-temperature annealing,and characterize them by optical microscopy and atomic force microscopy(AFM)to study the changes of graphene frictional properties after UV irradiation modification and analyze the intrinsic mechanisms affecting friction.The results of the study showed that:1.The surface properties and structure of graphene were changed after short-time irradiation treatment of graphene surface by ultraviolet(UV)light(low-pressure mercury lamp,working wavelength 185 nm and 254 nm)under vacuum conditions for different time periods.It was found that after short-time UV vacuum irradiation,the roughness,adhesion,and friction of graphene surface gradually decreased with the increase of irradiation time,and the friction enhancement phenomenon was significantly suppressed,but these phenomena gradually weakened with the increase of graphene thickness;when the irradiation time continued to increase,cavity defects were generated on the graphene surface,and the friction force increased,while the structural strength of graphene decreased,and under the action of a certain load,the needle tip contact area graphene can be removed as a whole.The analysis suggests that the two-way regulation of graphene friction properties can be achieved by controlling the UV vacuum irradiation time.2.Like UV irradiation-modified graphene,the high-temperature environment also has an important effect on the frictional properties of graphene.Through the high-temperature annealing study of graphene,it was found that the surface structure of graphene would not be destroyed and its hydrophobicity would not be changed by vacuum annealing at 150 ℃.Therefore,the short-time UV vacuum irradiation modified graphene was subjected to 150°C vacuum annealing,and the adhesion and friction were found to increase to some extent,but still lower than the original graphene samples.As for the UV ozone-modified graphene,150°C vacuum annealing can remove some of the surface adsorptions,but the structural defects are intensified and the friction force is still much higher than that of the pristine graphene.It is analyzed that vacuum annealing will reduce the residual stress and reduce the impurities such as surface organic gum during the mechanical transfer of graphene,but it will also remove the sp3-like bond formed between graphene-substrate interface by short-time UV vacuum irradiation and change the out-of-plane stiffness of graphene,and the combined effect of both can achieve the dynamic regulation of graphene frictional properties.3.Based on UV long-time vacuum irradiation,irregular mesh defects appear on the graphene surface,the perfect ortho-hexagonal lattice structure of graphene is destroyed,and the C-C bond strength is reduced.When the needle tip slides on the sample surface with a certain lateral shear force,it adheres to some graphene fragments,causing the initial needle tip-graphene contact pair to transform into graphene-graphene contact pair.Based on this,this study proposes a UV vacuum irradiation aging graphene friction transfer mechanism,i.e.,the graphene sample is aged by UV vacuum irradiation,force fracture,and transfer process,and the experimental results find that when the needle tip has adhered with graphene,the graphene-graphene contact pair makes the friction force,adhesion force and friction coefficient lower than normal,and the friction coefficient reaches 0.006 to achieve ultra-low friction. |
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