| Friction generally involves the interaction between the surfaces of two objects in contact and widely exists in microelectromechanical devices and flexible electronics.When the size is gradually reduced to the micro-and nano-scales,the surface force becomes larger,causing serious tribological problems.The curved surface with complex shape and structure exacerbates the seriousness of the problem.In addition,the increase in indentation depth caused by the deformation of the flexible substrate produces more energy dissipation,which affects the reliable operation of flexible microdevices and reduces their service life.To achieve friction reduction for interfaces at micro-and nano-scales,this paper studies the interface construction,tribological properties,and mechanism: Firstly,the interfaces with different properties and structures at microand nano-scales were constructed;Then the regulation and mechanism of interfacial friction by surface modification of complicatedly curved surfaces at micro-and nano-scales were studied;The friction and underlying mechanism of complex interfaces at micro-and nano-scales on the soft elastic substrate were studied,and the design of flexible superlubricated interface based on the synergy of heterogeneous materials was proposed.The main research contents are as follows:Firstly,a variety of interfaces at micro-and nano-scales were constructed based on the surface modification of complicatedly curved surfaces at micro-and nano-scales and atomically thin graphene.The surface wettability and surface structure of the material are adjusted by modifying the complicatedly curved surfaces of tip at micro-and nano-scales with self-assembled monolayers based on covalent bonding.The fluorosilane self-assembled monolayers with lower surface energy can adjust the surface hydrophobicity.The surface properties and mechanical properties of the substrate were tuned using graphene with different thicknesses obtained by mechanical exfoliation.Based on surface modification of self-assembled monolayers and atomically thin graphene,micro-and nano-scale interfaces with different properties and structures were constructed,which provides the possibility to adjust the tribological properties of interfaces at different scales.Secondly,the regulation of friction at micro-and nano-scales by surface properties and structures of complicated curved surfaces was studied.Combined with the calculation using the Laplace equation,Derjaguin-Muler-Toporov(DMT)contact model,and Prandtl-Tomlinson model,the effects of surface hydrophobicity,elastic modulus,out-of-plane stiffness,and surface structure on interfacial adhesion,contact area,contact quality,and interfacial barrier,as well as their important roles in interfacial friction,was revealed in turn.The study found that the hydrophobic functionalized surface modification based on the self-assembled monolayers can inhibit capillary interaction and reduce interfacial adhesion.The high elastic modulus of graphene is beneficial to the formation of a small contact area with the self-assembled monolayers.The increase of the out-of-plane stiffness caused by increasing the number of graphene layers is beneficial to reducing the interfacial contact quality and energy dissipation.The incommensurate contact forms due to the natural lattice mismatch between self-assembled monolayers and graphene,resulting in the reduction of the interfacial barrier during sliding by 68%.Then the superlubricated interface at micro-and nano-scales on hard substrates with the friction of coefficient as low as 0.0010 was obtained based on the synergy of hydrophobic self-assembled monolayers and atomically thin graphene.Besides,the quantitative relationship between interfacial friction and size was obtained.Thirdly,the effect of the deformation of the flexible substrate on the interfacial friction at micro-and nano-scale was investigated.The underlying mechanism of friction on the flexible substrate was revealed in terms of indentation depth,interfacial adhesion,and out-of-plane deformation caused by contact quality.Studies have shown that surface modification based on self-assembled monolayers reduces the friction of graphene on flexible substrates by 73% through reducing interfacial adhesion and indentation depth.The increase in the elastic modulus of the flexible substrate causes a decrease in the indentation depth,resulting in the sub-linear reduction of the contact area.The increased out-of-plane stiffness reduces the interfacial contact quality,effectively reducing the out-of-plane deformation of graphene on the flexible substrate and reducing energy dissipation.Further verification was performed by calculating the theoretical friction under different substrate stiffnesses,graphene thicknesses,and normal loads from the three aspects: the contribution of van der Waals force to friction,friction due to normal contact deformation,and friction generated by radial and circumferential components of deformation near the contact area.Then the research scale was extended to other micro-and nano-scales,and the superlubricity of the flexible interface at micro-and nano-scales was obtained with the friction of coefficient as low as 0.0014.Finally,the effects of external conditions such as relative humidity,normal load,and scanning speed on tribological properties of the hydrophobic self-assembled monolayers/graphene interface as well as its wear resistance during 10,000 cycles of sliding were investigated.The tribological properties are not affected by environmental humidity and sliding speed;The interfacial friction shows the anisotropy caused by the out-of-plane deformation of graphene,but it has a low coefficient of friction at different rotation angles;Besides,this interface has a high load-bearing capacity,excellent wear resistance,and good superlubrication stability.The mechanism of friction stability is revealed from aspects of surface hydrophobicity,strong chemical interaction,and excellent mechanical properties. |
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