Nanometric Cutting of Silicon with an Amorphous-Crystalline Layered Structure: A Molecular Dynamics Study
© The Author(s). 2017
Received: 12 November 2016
Accepted: 2 January 2017
Published: 13 January 2017
Materials with specific nanometric layers are of great value in both theoretical and applied research. The nanometric layer could have a significant influence on the response to the mechanical loading. In this paper, the nanometric cutting on the layered systems of silicon has been studied by molecular dynamics. This kind of composite structure with amorphous layer and crystalline substrate is important for nanomachining. Material deformation, stress status, and chip formation, which are the key issues in nano-cutting, are analyzed. A new chip formation mechanism, i.e., the mixture of extrusion and shear, has been observed. In addition, from the perspective of engineering, some specific composite models show the desired properties due to the low subsurface damage or large material removal rate. The results enrich the cutting theory and provide guidance on nanometric machining.
Materials with a specific layered structure such as film substrates and multilayer systems have many fascinating features. Some of these composite structures that are well designed can provide high strength and high wear resistance and are used as protective coatings . To study the mechanical properties, nanoindentation  and nano-cutting  techniques are always employed. In addition, with the great advances in computer science, numerical simulation has become an essential approach to the understanding of the mechanism of material deformation under these loading conditions, especially on the nanometric scale. For example, Fang et al. studied the nanoindentation and nanoscratch on the Al/Ni multilayer by molecular dynamics method [4, 5]. The contact stiffness increased with more nickel layers in the specimen, and the specific cutting energy decreased under a high scratch speed. The dislocation evolution in the Al/Ni system was analyzed by Cao et al . They found that the semi-coherent boundary hindering the dislocation movement played a key role in mechanical hardening. Sekkal et al. simulated the nanoindentation of TiC/NbC and TiC and discussed the hardness enhancement in the TiC/NbC superlattice . Kizler et al. studied the effects of nanostructures including grain boundary and lattice orientation on the hardness of TiC/NbC system. It indicated that defects in the material would improve the plasticity .
Except for the applications of protective coatings and functional surfaces, the layered composite structure is also very common in the mechanical processes on a nanometric scale. A typical example is the phase transition of covalent crystals (e.g., Si, Ge) during the nanoindentation with fast unloading . Amorphization also occurs during nano-cutting or ultra-precision turning . The amorphous structure is believed to be transformed from a metallic phase (β-Sn for Si and Ge) induced by intensive hydrostatic pressure during loading . Therefore, the crystal being machined in fact has an amorphous-crystalline structure instead of its original lattice. Due to different mechanical properties, the amorphous layer on the top of the crystalline bulk would affect the machining processes significantly such as ultra-precision turning, grinding, and polishing. Besides, an amorphous layer can also be used to facilitate the machining of brittle materials. For example, to control the fracture, surface amorphization, which may be achieved through ion implantation, could be performed before the mechanical processes occur. In this method, the monocrystalline workpiece is exposed to a high-energy ion beam. Large numbers of ions displace the target atoms and trigger a collision cascade . As a result, the initial lattice of the workpiece is destroyed, and an amorphous layer is formed with an abundance of ions. After the implantation, the surface hardness and Young’s modulus decrease, accompanied by an increase in the ductile-brittle transition depth . This method has been applied to silicon, germanium, and silicon carbide, and its validity has been proved [14, 15]. As discussed above, the amorphous-crystalline composite system has great influence on nanomachining but has not been fully researched. Thus, in this work, the nanometric cutting of this layered structure is studied systematically. The effects of various configurations of the amorphous layer on the stress state, material deformation, and chip formation during nano-cutting are analyzed. The aim of our work is also to evaluate which composite structures are preferred for nanomachining.
Numerical simulation is conducted using the molecular dynamics (MD) method. In this section, the cutting model is presented firstly. Then, details of making the layered composite structure are reported.
Nanometric Cutting Model
MD calculations are performed by LAMMPS , assisted by OVITO  for visualization. The nanometric cuttings are simulated in the NVE ensemble with the periodic boundary condition (PBC) in the z dimension. The Verlet algorithm is used to integrate the motion of Newton atoms, and the time step is 2 fs. The interactions between Si-Si and Si-C are described through a Tersoff-type potential . Dynamic relaxation is performed to allow the model to be in an equilibrium state at 293 K before the cutting.
Amorphous and Layered Model Construction
Results and Discussion
The material behaviors of the a-c and c-a-c models are quite different and will be discussed in two separate sections.
For the amorphous-crystalline (c-a) structure, the plastic deformability is enhanced with the increase in the a-Si portion, and stresses are reduced significantly, which has an advantage for the ductile machining of brittle materials. The chip is formed by extrusion independent of the amorphous layer thickness. However, as the a-Si layer extends to the lowest point of the tool, the subsurface damage can be eliminated.
With a thin amorphous layer just beneath the tool edge (c-a-c), the chip formation is realized by extrusion together with shear. Periodicities are observed in both the material removal rate and the formation of the local deformation. The local shear band is formed due to the crack tip-like deformation front. The chip formation in this extrusion-shear mixed manner is sensitive to the location of the thin amorphous layer because there is no shear except for in the model-6.
In this work, both the a-c (model-3) and c-a-c (model-6) models with specific configurations have low subsurface damage, small spring back, and high material removal rate during nanometric cutting. In addition, a distinct decrease in the cutting forces in the c-a-c structure shows a great value for prolonging the life of the diamond tool. Therefore, surface treatments such as ion implantation can be conducted preceding the mechanical processes to modify the material structure at the nanoscale for a better machinability.
This work was supported by the National Natural Science Foundation of China (No: 91423101, 51320105009 and 51375337), the Tianjin Research Program of Application Foundation and Advanced Technology (No. 14JCQNJC05200), and the ‘111’ project by the State Administration of Foreign Experts Affairs and the Ministry of Education of China (Grant No. B07014).
FFZ and ZXD designed and supervised this work; WJS performed the simulation and wrote the manuscript; FFZ and ZXD revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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