Xu Rong-Guang, Zhang Gunan, Xiang Yuan, Garcia Jonathan, Leng Yongsheng
Department of Mechanical and Aerospace Engineering, The George Washington University, Washington, DC, 20052, United States.
Langmuir. 2022 Jun 7;38(22):6808-6816. doi: 10.1021/acs.langmuir.1c03268. Epub 2022 May 26.
Friction measurements by an atomic force microscope (AFM) frequently showed regular stick-slip friction signals with atomic-scale resolutions. Typically, for an AFM metal tip sliding on a metal crystal surface, the microstructure of the tip made from the thermally evaporated metal coating on a silicon cantilever was polycrystalline. Our detailed molecular dynamics(MD) simulations of a polycrystalline Pt tip ( = 10 nm in radius) sliding on an Au(111) surface revealed how the geometry of the polycrystalline tip took effect on the friction behavior at the contact interface. We found that the apex of the Pt tip with multiple grains near the edge of contact could induce severe plastic deformations of the gold substrate, leading to irregular stick-slip frictions upon sliding. Simulation results showed that in order to achieve a clear stick-slip friction signal with single atomic slips, the apex of the Pt tip must adopt a single crystalline protrusion without any neighboring grains involved in the metal contact. We showed that such a single crystalline protrusion, which presumably could be achieved during initial run-in or wear-out of high-energy Pt atoms in the neighboring grains, was passivated by a large number of gold atoms due to metal adhesion in the contact periphery. Using such a crystalline protrusion tip, we demonstrated that the stick-slip friction produced was very "tolerant" to the adhesion of a large number of gold atoms on the tip apex. We further showed that AFM tip mass used in MD simulations also played an important role in determining the transition between friction regimes, which could be well explained by the Prandtl-Tomlinson thermal activation model.
通过原子力显微镜（AFM）进行的摩擦测量经常显示出具有原子尺度分辨率的规则粘滑摩擦信号。通常，对于在金属晶体表面滑动的AFM金属尖端，由硅悬臂上热蒸发金属涂层制成的尖端微观结构是多晶的。我们对在Au(111)表面滑动的多晶Pt尖端（半径 = 10 nm）进行的详细分子动力学（MD）模拟揭示了多晶尖端的几何形状如何影响接触界面处的摩擦行为。我们发现，在接触边缘附近有多个晶粒的Pt尖端顶点会导致金基底发生严重的塑性变形，从而在滑动时产生不规则的粘滑摩擦。模拟结果表明，为了获得具有单原子滑移的清晰粘滑摩擦信号，Pt尖端的顶点必须采用单个晶体突出部分，且在金属接触中不涉及任何相邻晶粒。我们表明，这样一个单个晶体突出部分，大概可以在相邻晶粒中高能Pt原子的初始磨合或磨损过程中实现，由于接触周边的金属粘附，会被大量金原子钝化。使用这种晶体突出尖端，我们证明了产生的粘滑摩擦对大量金原子粘附在尖端顶点非常“耐受”。我们进一步表明，MD模拟中使用的AFM尖端质量在确定摩擦状态之间的转变中也起着重要作用，这可以通过普朗特 - 汤姆林森热激活模型得到很好的解释。
