Investigation of the effect of stainless-steel grain size on the continuity of graphene frictional behavior using molecular dynamics (MD) simulation

WM Guo and QS Bai and YH Dou and HF Wang and SD Chen, COMPUTATIONAL MATERIALS SCIENCE, 226, 112210 (2023).

DOI: 10.1016/j.commatsci.2023.112210

Nanograin refinement can improve the strength of stainless steel, but the substrate grain boundaries will cause graphene bulging, so it will affect the lubrication effect of graphene. In this study, the effect of stainless-steel grain size on the frictional strengthening behavior of graphene is investigated, and the mechanism of grain boundary distribution and grain boundary crossing on graphene friction strengthening behavior was revealed. A normal load constraint of 0.8 nN is set for the nano-friction process. Stainless steel grain size greater than or equal to 10 nm, the maximum friction force of graphene does not vary with grain size is about 6 nN. However, the friction strengthening behavior is discontinuous, and the friction force fluctuates significantly, which is not conducive to micromechanical structure lubrication. When the grain size of stainless steel is less than 10 nm, the frictional strengthening behavior is continuous and the friction force after strengthening will maintain small fluctuations. Since the small fluctuations of this stick-slip behavior are material properties of graphene and cannot be avoided, this state is already the ideal result for graphene lubrication. However, as the stainless-steel grain size continues to become smaller, the maximum friction of graphene will increase, which is not good for graphene lubrication. This means that after the grain size is less than 10 nm, the continuation of the grain refinement process for stainless steel will improve the strength of stainless steel, but will weaken the graphene lubrication efficiency. Moreover, the difficulty of the nanograin refinement process will be significantly increased. Therefore, the stainless-steel grain size of 7 nm by grain refinement is the best solution to balance the stainless- steel strength and graphene lubrication efficiency. With a grain size of 7 nm, the stainless-steel grain boundaries are densely distributed, and the graphene bulge area will occupy 65% of the total area, allowing for continuous frictional strengthening behavior. The maximum height of graphene bumps is 2.8 angstrom and the maximum friction force is about 7 nN. Meanwhile, the relationship between the grain size of stainless steel and the continuity of graphene frictional strengthening behavior was explored, and the mechanism of action of grain boundary crossing by modulating the height of graphene bulge and thus affecting the frictional maximum was revealed. The nano-friction simulation of stainless steel/graphene can clearly capture the details of the nanofriction process and the variation of friction values, providing a basis for nano-friction studies of graphene on polycrystalline material surfaces. The simulation can determine the range of stainless-steel grains that exhibit the best friction state with graphene. The simulation results also provide guidance for the refinement of stainless steel nanograins and insights for the application of nanocrystalline stainless steel/graphene in MEMS.

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