An atomistic investigation of nanometric cutting process using a multi-tip single crystal diamond tool

In recent years great efforts are being made for the design and fabrication of periodic nanostructures used in emerging nano-products, such as plasmonic lens, nano-grating and high density hard disk etc. In our previous research work, a deterministic fabrication approach to cost-effectively manufacturing nano gratings over large area has been developed through single point diamond turning by using a multi-tip nano-scale single crystal diamond tool fabricated by FIB (Focus Ion Beam). However, the machining mechanism and technical limits of this approach i.e. the minimum dimension of nanostructures that can be obtained has not known yet. Due to the limitation of real-time detect equipment as well as the high research cost, it is difficult to obtain a quick answer through experimental work. On the other hand molecular dynamics (MD) simulation provides a cost-effective solution for this problem. Based on the merit offered by the large-scale molecular dynamics simulation method and new progresses made in high performance computing (HPC) technique, this paper proposes a new MD model for nanometric cutting process using a multi-tip single crystal diamond (SCD) tools to machine single crystal copper workpieces. By using centrosymmetry parameter (CSP) method and combining it with the dislocation nucleation and propagation theory, the machining mechanism and generation of nanostructures are studied through MD simulation. In order to reveal the dependence of the depth of cut on the integrality of generated nanostructures, a number of MD simulations have been carried out under depth of cut varying from 0.5, 1.0, 1.5, 2.0, and 3.0nm. The simulation results show that the depth of cut has significant influence on the integrality of the machined nanostructured surfaces and cutting force. A concept of maximum depth of cut to obtain high precision nanostructured surfaces in a single cutting pass is proposed based on analysis of the dimensional accuracy of the integrality machined nanostructures. In all simulations the cutting forces fluctuate around a constant value after chip formation

Smooth particle hydrodynamics study of surface defect machining for diamond turning of silicon

Acknowledgments The authors would like to thank EPSRC (EP/K018345/1) and Royal Society-NSFC International Exchange Scheme for providing financial support to this research.Peer reviewedPublisher PD

Prediction of residual stress in precision milling of AISI H13 steel

Surface integrity describes the attributes of a surface and it influences the functional performance of a work piece significantly. Residual stress is one of the major characterization parameters of surface integrity. Non-favorable residual stresses on a machined surface can reduce the fatigue life and performance of the machined part. It therefore requires a prediction model for residual stress in order to establish machining strategy to obtain favorable residual stress for prolonged fatigue life. Hardened tool steels have been widely used to make molds and dies by precision milling in aerospace and automotive industries. Knowledge of the relationship between residual stress on the machined molds and machining conditions is very important for process control. In this work, a prediction model for residual stress was developed by using a model-based approach on an Artificial Neural Network. This model is expected to predict the residual stress based on cutting parameters such as cutting speed, feed rate, depth of cut and tool lead angle. Several precision milling trials were carried out using a central composite design method. The networks have been trained and validated by experimental results. The performance of a feed forward neural network model with backpropagation was assessed and compared with a radial basis function network model by criterion of least mean squared error. Furthermore, the neural network prediction model was supported by the finite element simulation of the milling process to understand the formation mechanism of the residual stress in the machined surface. It was found, that the predicted values by the neural network model matched well with the experimental results. The radial basis function network showed better results than the feed forward network and was therefore chosen to take forward in the analysis. The feed rate was in this case the most influential factor, because it contributes significantly to heat and deformation on the work piece. The model could be used to optimize machining processes to obtain machining strategy for generating favorable residual stress and increasing fatigue life performance of the machined parts

Measurement of spindle error motions by an improved multi-probe method

This paper proposes an improved multi-probe method for measurement of spindle error motions. Four degree of freedoms (DOFs) of error motions of a spindle are measured in a dedicated setup using capacitive sensors. Three sets of probe angle set are carefully selected in order to overcome the harmonic suppression problems commonly encountered in the multi-probe measurement approach. The error contribution of each set of angles is analysed and then the measurement results are modified in frequency domain so as to minimise the effect of harmonic suppression. The evaluation of measurement results shows that this method is effective and possesses good agreement with repeated measurements

Molecular dynamics simulation study of deformation mechanisms in 3C-SiC during nanometric cutting at elevated temperatures

Molecular dynamics (MD) simulation was employed in this study to elucidate the dislocation/amorphization-based plasticity mechanisms in single crystal 3C–SiC during nanometric cutting on different crystallographic orientations across a range of cutting temperatures, 300 K to 3000 K, using two sorts of interatomic potentials namely analytical bond order potential (ABOP) and Tersoff potential. Of particular interesting finding while cutting the (110) was the formation and subsequent annihilation of stacking fault-couple and Lomer–Cottrell (L–C) lock at high temperatures, i.e. 2000 K and 3000 K, and generation of the cross-junctions between pairs of counter stacking faults meditated by the gliding of Shockley partials at 3000 K. Another point of interest was the directional dependency of the mode of nanoscale plasticity, i.e. while dislocation nucleation and stacking fault formation were observed to be dominant during cutting the (110), low defect activity was witnessed for the (010) and (111) crystal setups. Nonetheless, the initial response of 3C–SiC substrate was found to be solid-state amorphization for all the studied cases. Further analysis through virtual X-ray diffraction (XRD) and radial distribution function (RDF) showed the crystal quality and structural changes of the substrate during nanometric cutting. A key observation was that the von Mises stress to cause yielding was reduced by 49% on the (110) crystal plane at 3000 K compared to what it took to cut at 300 K. The simulation results were supplemented by additional calculations of mechanical properties, generalized stacking faults energy (GSFE) surfaces and ideal shear stresses for the two main slip systems of 3C–SiC given by the employed interatomic potentials

Hybrid micro-machining processes : a review

Micro-machining has attracted great attention as micro-components/products such as micro-displays, micro-sensors, micro-batteries, etc. are becoming established in all major areas of our daily life and can already been found across the broad spectrum of application areas especially in sectors such as automotive, aerospace, photonics, renewable energy and medical instruments. These micro-components/products are usually made of multi-materials (may include hard-to-machine materials) and possess complex shaped micro-structures but demand sub-micron machining accuracy. A number of micro-machining processes is therefore, needed to deliver such components/products. The paper reviews recent development of hybrid micro-machining processes which involve integration of various micro-machining processes with the purpose of improving machinability, geometrical accuracy, tool life, surface integrity, machining rate and reducing the process forces. Hybrid micro-machining processes are classified in two major categories namely, assisted and combined hybrid micro-machining techniques. The machining capability, advantages and disadvantages of the state-of-the-art hybrid micro-machining processes are characterized and assessed. Some case studies on integration of hybrid micro-machining with other micro-machining and assisted techniques are also introduced. Possible future efforts and developments in the field of hybrid micro-machining processes are also discussed

Molecular dynamics simulation investigation of hot nanometric cutting of single crystal silicon

In this study, molecular dynamics (MD) simulation is employed to investigate mechanisms occurring during nanometric cutting process of single crystal silicon on different crystallographic planes under a wide range of workpiece temperatures (300-1500 K) by comparing the results obtained from two types of interatomic potential functions i.e. an analytical bond order potential (ABOP) and a modified version of Tersoff potential. It was found that resultant forces decrease up to 25% at workpiece temperature of 1173 K. A steep decrease of tool temperature at 1500 K was noticed on the (010) and (110) crystal planes when modified Tersoff potential function was used, attributable to the decrease of the tool-chip contact length at 1500 K. Another point of interest was the decrease of magnitude of von Mises stresses on the cutting edge with the increase of the workpiece temperature for the different crystallographic planes. The variation of the local potential energy in the primary deformation zone was also monitored so as to obtain a superior appreciation of the elastic and plastic deformation processes

An atomistic simulation investigation on chip related phenomena in nanometric cutting of single crystal silicon at elevated temperatures

Nanometric cutting of single crystal silicon on different crystal orientations and at a wide range of temperatures (300-1500 K) was studied through molecular dynamics (MD) simulations using two sorts of interatomic potentials, an analytical bond order potential (ABOP) and a modified version of Tersoff potential, so as to explore the cutting chip characteristics and chip formation mechanisms. Smaller released thermal energy and larger values of chip ratio (ratio of the uncut chip thickness to the cut chip thickness) as well as shear plane angle were obtained when cutting was performed at higher temperatures or on the (1 1 1) crystal plane, implying an enhancement in machinability of silicon. Nonetheless, the subsurface deformation depth was observed to become deeper under the aforementioned conditions. Further analysis revealed a higher number of atoms in the chip when cutting was implemented on the (1 1 0) crystal plane, attributable to the lower position of the stagnation region which triggered less ploughing action of the tool on the silicon substrate. Regardless of temperature of the substrate the minimum chip velocity angle was found while cutting the (1 1 1) crystal plane of silicon substrate whereas the maximum chip velocity angle appeared on the (1 1 0) surface. A discrepancy between the two potential functions in predicting the chip velocity angle was observed at high temperature of 1500 K, resulting from the overestimated phase instability and entirely molten temperatures of silicon by the ABOP function. Another key observation was that the resultant force exerted by the rake face of the tool on the chip was found to decrease by 24% when cutting the (1 1 1) surface at 1173 K compared to that at room temperature. Besides, smaller resultant force, friction coefficient at the tool/chip interface and chip temperature was witnessed on the (1 1 1) crystal plane, as opposed to the other orientations

Investigation of a scale-up manufacturing approach for nanostructures by using a nanoscale multi-tip diamond tool

Increasing interest in commercializing functional nanostructured devices heightens the need for cost-effective manufacturing approaches for nanostructures. This paper presents an investigation of a scale-up manufacturing approach for nanostructures through diamond turning using a nanoscale multi-tip diamond tool (four tip tool with tip width of 150 nm) fabricated by focused ion beam (FIB). The manufacturing capacity of this new technique is evaluated through a series of cutting trials on copper substrates under different cutting conditions (depth of cut 100–500 nm, spindle speed 12–120 rpm). The machined surface roughness and nanostructure patterns are measured by using a white light interferometer and a scanning electron microscope, respectively. Results show that the form accuracy and integrity of the machined nanostructures were degraded with the increase of the depth of cut and the cutting speed. The burr and the structure damage are two major machining defects. High precision nano-grooves (form error of bottom width < 6.7 %) was achieved when a small depth of cut of 100 nm was used (spindle speed = 12 rpm). Initial tool wear was found at both the clearance cutting edge and the side edges of tool tips after a cutting distance of 2.5 km. Moreover, the nanometric cutting process was emulated by molecular dynamic (MD) simulations. The research findings obtained from MD simulation reveal the underlying mechanism for machining defects and the initialization of tool wear observed in experiments

Atomic-scale characterization of occurring phenomena during hot nanometric cutting of single crystal 3C-SiC

Nanometric cutting of single crystal 3C-SiC on the three principal crystal orientations at various cutting temperatures spanning from 300 K to 3000 K was investigated by the use of molecular dynamics (MD) simulation. The dominance of the (111) cleavage was observed for all the tested temperatures. An observation of particular interest was the shift to the (110) cleavage at cutting temperatures higher than 2000 K. Another key finding was the increase of anisotropy in specific cutting energy from ~30% at 300 K to ~44% at 1400 K, followed by a drop to ~36% and ~24% at 1700 K and 2000 K, respectively. The obtained results also indicated that the specific cutting energies required for cutting surfaces of different orientations decrease by 33%-43% at 2000 K compared to what are required at 300 K. Moreover, the position of stagnation region was observed to vary with changes in temperature and crystallographic orientation. Further analysis revealed that the subsurface deformation was maximum on the (111) surface whereas it was minimum on the (110) plane. This is attributable to the occurrence of cleavage and the location of the stagnation region. In addition, the amount of subsurface damage scaled linearly with the increase of cutting temperature. A vortex flow of atoms beneath the cutting tool was also observed, which is qualitatively analogous to the plastic flow of silicon. The simulations also predicted that the atom-by-atom attrition wear and plastic deformation of the diamond cutting tool could be alleviated while cutting at high temperatures. Nevertheless, chemical wear i.e. dissolution-diffusion and adhesion wear is plausible to be accelerated at high temperatures