Theoretical And Experimental Research On The Frictional And Tensile Behaviour Of Graphene

Graphene, a quasi-two-dimensional material with extraordinary physical, electric, optical and thermal properties, has been a hot area of research for several years since it had been found. Its superlubricity can be used in ultra-presion mechanical systems such as micro-electro-mechanical systems(MEMS) which consist of moving parts. How to control graphene to realize its superlubricity state is currently an important research topic. Meanwhile, the best way of mass preparation for high quality graphene is the CVD method, but the CVD-grown graphene always contain grain boundary. So a deeply understood of the effects of the grain boundary on the frictional and mechanical properties is one of the basic research topics for the design and manufacturing of graphene-based device.Based on the Brownian Dynamics(BD), the frictional behaviour between the diamond tip and suspended graphene has been studied by the means of the number of contacted atoms, the height and the asymmetry of the contact region. Through the analysis of the friction force under different loads and strain rate, a theoretical formula of the fiction coefficient can be figured out. The results indicated that friction is the combined effect of adhesion and out-of-plane, and the contribution of these two factors can be changed by varying the vertical loads and pre-strain. The asymmetry of the contact region which is the main factor to friction can be depressed by pre-strain, and then friction coefficient could be reduced significantly. Meanwhile, the pre-strain can reduce the fluctuations and improve the stationarity of the friction process. To polycrystalline graphene, unbalanced stress caused by defect in gain boundary could change graphene’s local stiffness which will change the contact state between AFM tip and graphene, and this effects can also be depressed by pre-strain. Furthermore, the theoretical analysis has been verified to some extent by experimental study that is called nano-scratching of AFM probe on suspended graphene.By the means of the Molecular Dynamics(MD) method, the mechanical properties of grain boundary has been studied from the viewpoint of energy. A new variable—single defect energy(Esingle), which is the total energy per defect, has been defined to characterize the energy properties of the grain boundary. The relationship between formation energy, single defect energy and maximum energy of atoms, misorientation angles, density of the defect were also be found, furthermore the dependence between the single defect energy and misorientation angles can be fitted into a linear empirical formulae. The effect of the grain boundary on the Young’s modulus can not be neglected at the beginning of the strain. The tensile strength that is greatly affected by the grain boundary depends linearly on the maximum energy of atoms, and that is because of the loading unevenness caused by the non-uniform of the local stiffness. The different crack growth orientation has been observed: the crack will growth along the armchair or zigzag directions in high strength grain boundary, otherwise it will growth along the grain boundary.