Atoms are colored according to their height in Y this website direction. (e,f,g,h) Cross-sectional views of the substrate after scratching with probe radiuses of 6, 8, 10, and 12 nm, respectively. Atoms are colored according to shear strain ranging from 0 JNK pathway inhibitors to 1. Figure 7 presents numbers of HCP and defect atoms generated within the substrate after penetration and scratching with the four probe radiuses. For each probe, there are more HCP and defect atoms generated in the scratching stage than that in penetration stage, because of the more complex plastic deformation associated with the multi-axial localized stress states. When the probe radius is not larger than 10 nm, there are more defect

atoms than HCP atoms in both penetration and scratching stages for each probe radius. However, the friction with the probe radius of 12 nm results in

more HCP atoms than defect atoms generated within the material. The formation of HCP atoms is associated with the activity of partial dislocations, while defect atoms are composed of not only dislocation selleck kinase inhibitor cores but also vacancies. Therefore, Figure 7 indicates that the dislocation activity plays more pronounced role in governing incipient plasticity for larger probe. In addition, the incipient plasticity shows strong dependence on probe radius: the larger the probe, the larger both the HCP and defect atoms. Figure 7 Influence of probe radius Fludarabine supplier on numbers of HCP and defect atoms generated within the substrate under friction. Conclusions In summary, we perform MD simulations to investigate the atomic scale origin of the minimum wear depth of single crystalline Cu(111) during single asperity friction. Simulation results show that scratching impression can only be made under a scratching depth at which there are permanent defects formed. It is indicated that the minimum wear depth is equivalent to the critical penetration depth associated with the first force-drop observed

in the force-depth curve. The specific permanent defects governing the wear phenomena are composed of stair-rod dislocations and prismatic dislocation loops as well as vacancies. While the contact pressure for the nucleation of initial dislocation is independent on probe radius, the minimum wear depth increases with probe radius. Further analysis of the shear strain distribution implies that a larger probe results in more compliant deformation of the material, which leads to larger volume of wear debris and wider extent of defect structures. Acknowledgements The authors greatly acknowledge financial supports from the NSFC (51005059 and 51222504), China Postdoctoral Science Foundation (20100471047 and 2012 M511463), and Heilongjiang Postdoctoral Foundation of China (LBH-Z11143). JZ also greatly acknowledges Dr. Alexander Hartmaier and Dr.