Micro-tribological study of metal surfaces using scanning probe microscope

微观摩擦学研究使用扫描探针显微镜(SPM)，它可以测量侧向力和法向力图像。将SPM置于气氛控制装置中，将尖端样品周围的相对湿度控制在30%~70%。测量也是在高纯度水浴中进行的。所研究的样品是云母、HOPG和MoS_2的解理表面，以及镍、铜和铝的抛光金属表面。为了讨论吸附水分子的微观摩擦学效应和水的毛细凝聚，定量地考察了样品表面法向载荷的函数关系的侧向力图像。对于云母、HOPG和MoS_2表面，用与原子尺度地形图像相同的方法获得了具有晶格周期的侧向力图像。侧向力的大小对法向力的依赖关系被发现受到针尖周围的大气和样品的影响。在相对湿度为30%的低相对湿度下，原子尺度成像中的侧向力大小几乎与法向载荷无关。在相对湿度高达70%的情况下，侧向力的大小依赖于法向载荷。在水中，侧向力的大小与法向载荷大致成正比。对于金属表面，侧向力的平均值几乎相同。我们认为，由于水的存在而产生的粘附力对侧向力有很大的影响。通过对赫兹分析、弹流润滑和水的毛细管凝聚等观点的讨论，揭示了针尖-样品的接触机理。

Micro-tribological studies are made using a scanning probe microscope (SPM) which can measure the lateral force as well as the normal force images. The SPM is placed in an atmosphere control apparatus and the relative humidity around the tip-sample is controlled from 30% to 70%. Measurements are also performed in a high purity water bath. Samples investigated are the cleaved surfaces of mica, HOPG and MoS_2, and the polished metal surfaces of Ni, Cu and Al. In order to discuss the micro-tribological effects of adsorbed water molecules and the capillary condensation of water, the lateral force images are examined quantitatively as a function of the normal load on the sample surface. For surfaces of mica, HOPG and MoS_2, lateral force images with the lattice periodicity are obtained in the same manner as atomic scale topography images. The dependence of lateral force magnitude on the normal force is found to be affected by the atmosphere around the tip and the sample. At low relative humidity, 30%, the lateral force magnitude in atomic scale imaging is almost independent of the normal load. At high relative humidity, 70%, the lateral force magnitude exhibits dependence on the normal load. In water, the lateral force magnitude is approximately in proportion to the normal load. For surfaces of metal almost same behavior is obtained in the average value of lateral force. We conclude that the adhesive force due to the presence of water has severe influence upon the lateral force. From discussion on the viewpoints of Hertz analysis, Elasto-Hydrodynamic Lubrication and capillary condensation of water, the contact mechanism of the tip-sample is revealed.