Li Xiaowei, Zhang Dekun, Xu Xiaowei, Lee Kwang-Ryeol
School of Materials and Physics, China University of Mining and Technology, Xuzhou 221116, P. R. China.
Computational Science Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea.
ACS Appl Mater Interfaces. 2020 Sep 23;12(38):43320-43330. doi: 10.1021/acsami.0c12890. Epub 2020 Sep 10.
Graphene exhibits great potential as a lubricant additive to enhance the antifriction capacity of moving mechanical components in synergism with amorphous carbon (a-C) as a solid lubricant. However, it is particularly challenging for experiments to accurately examine the friction dependence on the physical nanostructure of the graphene additive and the corresponding interfacial reactions because of the inevitable complexity of the graphene structure fabricated in experiments. Here, we address this puzzle regarding the coeffect of the size and content of the graphene additive at the a-C interface using reactive molecular dynamics simulations. Results reveal that the friction-reducing behavior is more sensitive to graphene size than content. For each graphene structure, with increasing content, the friction coefficient always decreases first and then increases, while the friction behavior exhibits significant dependence on the graphene size when the graphene content is fixed. In particular, the optimized size and content of the graphene additive are suggested, in which an excellent antifriction behavior or even superlubricity can be achieved. Analysis of the friction interface indicates that with increasing graphene size, the dominated low-friction mechanism transforms from the high mobilities of the base oil and graphene additive in synergism to the passivation and graphene-induced smoothing of the friction interface. These outcomes disclose the roadmap for developing a robust solid-liquid synergy lubricating system.
石墨烯作为一种润滑添加剂具有巨大潜力，可与作为固体润滑剂的非晶碳（a-C）协同增强移动机械部件的减摩能力。然而，由于实验制备的石墨烯结构不可避免地具有复杂性，准确研究摩擦力对石墨烯添加剂物理纳米结构及相应界面反应的依赖性对实验来说极具挑战性。在此，我们使用反应分子动力学模拟解决了关于石墨烯添加剂在a-C界面处尺寸和含量协同效应的这一难题。结果表明，减摩行为对石墨烯尺寸的敏感性高于对含量的敏感性。对于每种石墨烯结构，随着含量增加，摩擦系数总是先降低后升高，而当石墨烯含量固定时，摩擦行为对石墨烯尺寸表现出显著依赖性。特别是，我们提出了石墨烯添加剂的优化尺寸和含量，在此条件下可实现优异的减摩行为甚至超润滑性。对摩擦界面的分析表明，随着石墨烯尺寸增大，主导的低摩擦机制从基础油与石墨烯添加剂协同作用下的高迁移率转变为摩擦界面的钝化和石墨烯诱导的平滑作用。这些结果揭示了开发强大的固液协同润滑系统的路线图。
