Atomistic insight into the minimum wear depth of Cu(111) surface
© Li et al.; licensee Springer. 2013
Received: 13 October 2013
Accepted: 27 November 2013
Published: 5 December 2013
In the present work, we investigate the minimum wear depth of single crystalline Cu(111) under single asperity friction by means of molecular dynamics simulations. The atomistic mechanisms governing the incipient plasticity are elucidated by characterizing specific defect structures and are correlated to the observed mechanical and frictional responses of the material. Furthermore, the effect of probe radius on the friction process is studied. Our simulations indicate that the formation of wear impression is closely associated with defect nucleation and the minimum wear depth is equivalent to the critical penetration depth at which plasticity initiates. It is found that the probe radius has a strong influence on the formation of defect structures and the observed mechanical responses.
With the feature size of miniaturized mechanical components shrinking down to the nanometer regime, friction and wear, as the major causes of mechanical failures and dissipative energy losses, play pronounced and even dominant role in determining the functionality of nanoelectromechanical system (NEMS) devices [1–3]. Therefore, reducing the friction and wear between contacting surfaces of components is of significant importance for the application of NEMS devices. Specifically, a fundamental understanding of the atomic scale origin of the friction-induced wear is essentially required for the rational design of the components that possess good wear resistance.
During the course of friction, wear phenomena are closely accompanied with permanent deformation and even removal of the materials under applied mechanical loads. Thus, identifying and characterizing the initiation of plasticity of the materials under friction are central to the understanding of the atomic scale origin of wear phenomena. In the past few decades, both experimental investigations and atomistic simulations have been conducted to investigate the incipient plasticity of metallic and semiconductor materials under nanoindentation [4–8]. Recently, Paul et al. performed nanoindentation experiments to study the minimum threshold of the incipient plasticity of a gold single crystal. They found that the indentation-induced elastic deformation and plastic deformation can be well identified by features observed in the force-displacement curves, and the first pop-in phenomenon reflects the onset of plasticity . However, a rather limited effort has been taken to study the incipient plasticity which occurs under friction. Compared to the localized uniaxial stress state of nanoindentation, the multi-axial states of localized stress induced by friction action may lead to more complex mechanical responses at the onset of plasticity. On the other hand, it is crucial to correlate microstructure evolution that occurs within the materials with the observed features in force-displacement curves, which is of great challenge for the experimental investigations because of the involvement of nanometer length scale. As a complement to experiments, molecular dynamics (MD) simulation has been demonstrated to be one powerful tool to investigate the atomic scale phenomena of friction and wear [10–20]. Although previous MD simulations have provided valuable insights into the nanoscale friction and wear processes, our knowledge about the incipient plasticity under friction process, particularly the relationship between specific defect structures and observed wear phenomena, is still scarce.
In the present work, we perform MD simulations to investigate the incipient plasticity of single crystalline copper under single asperity friction with a spherical probe. The deformation mechanisms of the material are analyzed in detail, and the specific defect structures are particularly characterized and are correlated to the mechanical and frictional responses. Our simulations demonstrate that the minimum wear depth is determined by the formation of permanent defects such as dislocations and vacancies and is strongly probe radius-dependent. This paper is outlined as follows. In ‘Methods’ Section, we describe the simulation method. The simulation results are presented and discussed in Section ‘Results and discussion’. Finally, we summarize the results in ‘Conclusions’ Section.
The atoms in the as-created substrate first undergo global energy minimization at 0 K, and then the substrate is relaxed to its equilibrium configuration at 30 K and 0 bar through dynamic NPT relaxation for 50 ps. After relaxation, the substrate is subjected to friction by placing the probe above the free surface of the substrate with a distance of 0.2 nm. The friction process is composed of two stages of first penetration and following scratching, as illustrated in Figure 1. In the penetration stage, the probe moves along negative Y direction with constant velocity of 20 m/s to penetrate into the substrate until a pre-determined penetration depth is reached. In the following scratching stage, the probe scratches at 12.2 nm along negative X direction with constant velocity of 20 m/s. Both the penetration and scratching velocities of 20 m/s are a few orders of magnitude higher than the typical velocities utilized in nanoscratching experiments due to the intrinsic requirement of integration timesteps to be of the order of 1 fs. All the MD simulations are completed using the IMD code with a time step of 1 fs . The detailed description about the friction procedure can also be found elsewhere . To identify the defects generated within the substrate, a modified bond angle distribution (BAD) method is utilized . In the present work, the perfect face-centered cubic (FCC) atoms are not shown for better viewing of the defect structures, and the coloring scheme for various defects is as follows: red stands for surface atoms, blue indicates hexagonal close-packed (HCP) atoms, and the remaining atoms are categorized into defects including dislocation cores and vacancies. The software AtomEye is employed to visualize MD data and generate MD snapshots .
Results and discussion
Determination of minimum wear depth
Effect of probe radius on minimum wear depth
Mechanical responses of the substrate under friction with different probe radiuses
Critical penetration force (nN)
Critical penetration depth (nm)
Critical contact pressure (GPa)
Average friction coefficient
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.
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. Christoph Begau in ICAMS, Ruhr-University Bochum for helpful discussion and providing advanced analyzing technique of lattice defects.
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