An atomistic investigation of the effect of strain on frictional properties of suspended graphene

We performed molecular dynamics (MD) simulations of a diamond probe scanned on a suspended graphene to reveal the effect of strain on the fictional properties of suspended graphene. The graphene was subjected to some certain strain along the scanning direction. We compared the friction coefficient obtained from different normal loads and strain. The results show that the friction coefficient can be decreased about one order of magnitude with the increase of the strain. And that can be a result of the decreased asymmetry of the contact region which is caused by strain. The synthetic effect of potential energy and the fluctuation of contact region were found to be the main reason accounting for the fluctuation of the friction force. The strain can reduce the fluctuation of the contact region and improve the stability of friction

Evolution of surface grain structure and mechanical properties in orthogonal cutting of titanium alloy

In this study, a mesoscale dislocation simulation method was developed to study the orthogonal cutting of Titanium alloy. The evolution of surface grain structure and its effects on the surface mechanical properties were studied by using two-dimensional climb assisted dislocation dynamics technology. The motions of edge dislocations such as dislocation nucleation, junction, interaction with obstacles and grain boundaries, and annihilation were tracked. The results indicated that the machined surface has a microstructure composed of refined grains. The fine-grains bring appreciable scale effect and a mass of dislocations are piled up in the grain boundaries and persistent slip bands. In particular, dislocation climb can induce a perfect softening effect, but this effect is significantly weakened when grain size is less than 1.65 μm. In addition, a Hall-Petch type relation was predicted according to the arrangement of grain, the range of grain sizes and the distribution of dislocations

Atomistic investigation of FIB-induced damage in diamond cutting tools under various ion irradiation conditions

Focused Ion Beam (FIB) has been demonstrated as a promising tool to the fabrication of micro- and nanoscale diamond cutting tools. In-depth understanding of the ion-solid interaction in diamond leading to residual damage under different processing parameters are in high demand for the fabrication of nanoscale diamond tools. Molecular dynamics (MD) simulation method has long been regarded as a powerful and effective tool for analysing atomistic interactions with regard to its capacity of tracking each atom dynamically. Developing on the previous research work on single ion collision process in diamond, a novel Gauss random distribution multi-particle collision MD model was developed in this paper to study FIB-induced damage in diamond under various ion irradiation conditions. A multi-timestep algorithm was developed to control the whole collision process. The results show that the proposed model can effectively track the impulse of each single ion leads to atomic displacements in diamond and finally to a U-shape residual damaged layer at the core irradiation area. The multi-timestep algorithm can increase the computing efficiency by 12 times while still holding high simulation accuracy in terms of the thickness of residual damaged layer and the range of incident gallium distribution. The simulation model was further used to study the ion-induced damage layer in diamond under various beam voltages (5 kV, 8 kV, and 16 kV) and incident angles (0˚, 15˚, 30˚, and 45˚). Less damage range were found under the beam energy of 5 kV with the ion incident angle of 45˚, which indicated that a post ion beam polishing process (low beam energy with large incident angle) would be an effective way in practice to remove/minimise the residual damage layer when shaping the diamond cutting tools

Atomistic investigation of the wear of nanoscale diamond cutting tools shaped by focused ion beam

In recent years, micro/nanoscale diamond cutting tools shaped by focused ion beam (FIB) has been developed to the deterministic fabrication of micro/nano-structures owing to its unprecedented merits of high throughput, one-step, and highly flexible precision capabilities. However, the exposure of a diamond tool to FIB will result in the implantation of ion source material and the irradiation damage in cutting edges, and thus affect the tool life. In this work, molecular dynamics (MD) simulation has been carried out to study the effects of FIB induced damage on the wear resistance of nanoscale multi-tip diamond tool under different cutting conditions. A novel nanoscale multi-tip diamond tool model was built with the implanted Ga+ and amorphous damaged layer around tool tips. A damage free tool model with the same tool geometry was built as a reference. The wear resistance of the cutting tool was characterized by the total number of defect atoms formed during nanometric cutting of single crystal copper. The results show that the FIB irradiation induced doping and defects significantly degrade the wear resistance of the diamond tool. For the damage free tool cutting, the sp2 bonded carbon atoms were formed and accumulated on the surface layers. However, the sp2 bonded carbon atoms were found both on the surface and the deep inside of tool when using the tool of predefined defects. The implanted gallium atoms were found to move to the tool surface and left vacuums inside diamond tool tip, which would further degrade the wear resistance of the tool. Moreover, the variation of the sp2-bonded carbon atoms against the depth of cut and the cutting speed has been further analysed. The research findings from this study inform the in-depth understanding of tool wear of FIB shaped multi-tip diamond tool observed in previous nanometric cutting experiment

Review on FIB-induced damage in diamond materials

Background: Although various advanced FIB processing methods for the fabrication of 3D nanostructures have been successfully developed by many researchers, the FIB milling has an unavoidable result in terms of the implantation of ion source materials and the formation of damaged layer at the near surface. Understanding the ion-solid interactions physics provides a unique way to control the FIB produced defects in terms of their shape and location. Methods: We have carefully selected peer-reviewed papers which mainly focusing on the review questions of this paper. A deductive content analysis method was used to analyse the methods, findings and conclusions of these papers. Based on their research methods, we classify their works in different groups. The theory of ion-matter interaction and the previous investigation on ion-induced damage in diamond were reviewed and discussed. Results: The previous research work has provided a systematic analysis of ion-induced damage in diamond. Both experimental and simulation methods have been developed to understand the damage process. The damaged layers created in FIB processing process can significantly degrade/alter the device performance and limit the applications of FIB nanofabrication technique. There are still challenges involved in fabricating large, flat, and uniform TEM samples in undoped non-conductive diamond. Conclusions: The post-facto-observation leaves a gap in understanding the formation process of ioninduced damage, forcing the use of assumptions. In contrast, MD simulations of ion bombardment have shed much light on ion beam mixing for decades. These activities make it an interesting and important task to understand what the fundamental effects of energetic particles on matter are

Influence of cutting parameters on the depth of subsurface deformed layer in nano-cutting process of single crystal copper

Large-scale molecular dynamics simulation is performed to study the nano-cutting process of single crystal copper realized by single-point diamond cutting tool in this paper. The centro-symmetry parameter is adopted to characterize the subsurface deformed layers and the distribution and evolution of the subsurface defect structures. Three-dimensional visualization and measurement technology are used to measure the depth of the subsurface deformed layers. The influence of cutting speed, cutting depth, cutting direction, and crystallographic orientation on the depth of subsurface deformed layers is systematically investigated. The results show that a lot of defect structures are formed in the subsurface of workpiece during nano-cutting process, for instance, stair-rod dislocations, stacking fault tetrahedron, atomic clusters, vacancy defects, point defects. In the process of nano-cutting, the depth of subsurface deformed layers increases with the cutting distance at the beginning, then decreases at stable cutting process, and basically remains unchanged when the cutting distance reaches up to 24 nm. The depth of subsurface deformed layers decreases with the increase in cutting speed between 50 and 300 m/s. The depth of subsurface deformed layer increases with cutting depth, proportionally, and basically remains unchanged when the cutting depth reaches over 6 nm

The influence of cutting parameters on the defect structure of subsurface in orthogonal cutting of titanium alloy

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