An investigation on the mechanics of nanometric cutting for hard-brittle materials at elevated temperatures

Due to their exceptional physical and chemical properties such as high strength, high thermal conductivity, high stability at high temperature, high resistance to shocks, low thermal expansion and low density, silicon and silicon carbide (SiC) have become consummate candidates for optoelectronics, semiconductor and tribological applications. In particular, 3C-SiC, as a zinc blende structured SiC, possesses the highest fracture toughness, hardness, electron mobility and electron saturation velocity amongst the SiC polytypes. Thus, it has drawn substantial attention as a candidate substrate material for nano-devices which require high performance in extreme environments.Nanometric cutting is a promising ultra precision manufacturing process for manufacturing of 3D silicon and SiC based components which require submicron form accuracy and nanometric smooth finish. However, silicon and 3C-SiC have poor machinability at room temperature due to their relatively low fracture toughness and high hardness. A common understanding is that the yield strength and hardness of silicon and 3C-SiC will reduce under high temperature. As such, their fracture toughness increase which will ease plastic deformation and improve their machinabilities primarily as a result of thermally-generated intrinsic defects and thermal softening processes. However, the extent has never been reported although this knowhow could be vital in implementing the hot machining of silicon and SiC with the assistance of laser processing.This dissertation aims to gain an in-depth understanding of nanoscale mechanisms involved in nanometric cutting of hard-brittle materials such as silicon and 3C-SiC at elevated temperatures through molecular dynamics (MD) simulation and experimental trials. To this end, three-dimensional MD models of nanometric cutting were developed and different types of interatomic potential functions i.e. Tersoff, modified Tersoff, ABOP and SW were adopted to describe the interactions between atoms. In order to obtain reliable results, the equilibrium lattice constants were calculated at different temperatures for the employed potential functions. To perform the MD simulations, LAMMPS software was employed on a HPC service which was coupled with OVITO to visualise and post-process the atomistic data. Material flow behaviour, cutting chip characteristics, cutting forces and specific cutting energy, yielding stresses, stress and temperature on the cutting edge of the diamond tool, tool wear, defect-mediated plasticity and amorphization processes were calculated and analysed to quantify the differences in the cutting behaviour at different temperatures. Furthermore, In-situ high temperature nanoscratching (~500°C) of silicon wafer under reduced oxygen condition through an overpressure of pure Argon was carried out using a Berkovich tip with a ramp load at low and high scratching speeds. Ex-situ Raman spectroscopy and AFM analysis were performed to characterize high pressure phase transformation, nanoscratch topography, nanoscratch hardness and condition of the nanoindenter tip in nanoscratching at room and elevated temperatures.MD simulation results showed that the workpiece atoms underneath the cutting tool experienced a rotational flow akin to fluids. Moreover, the degree of flow in terms of vorticity was found higher on the (111) crystal plane, signifying better machinability on this orientation. Furthermore, it was observed that the degree of turbulence in the machining zone increases linearly with machining operation temperature. The cutting temperature showed significant dependence on the location and position of the stagnation region in the cutting zone of the substrate. In general, when cutting was performed on the (111) plane, the stagnation region (irrespective of the cutting temperature) was observed to locate at an upper position than that for the (010) and (110) planes. Also, at high temperatures, the stagnation region was observed to shift downwards than what was observed at room temperature. Another point of interest was the increase of subsurface deformation depth of the workpiece while cutting the (111) crystal plane at elevated temperatures.;Dislocation nucleation and formation of stacking faults were identified in conjunction with amorphization of silicon as the meditators of crystal plasticity in single crystal silicon during nanometric cutting process on different crystallographic planes at various temperatures. MD simulations revealed strong anisotropic dependence behaviour of dislocation activation and stacking fault formation. Likewise, while cutting 3C-SiC on the (110), formation and subsequent annihilation of stacking fault-couple 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 were observed. An observation of particular interest, while cutting 3C-SiC, was the shift to the (110) cleavage at cutting temperatures higher than 2000 K. The initial response of both the silicon and 3C-SiC substrates 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. No symptom of any atom-by-atom attrition wear and plastic deformation of the diamond cutting tool was observed during nanometric cutting of silicon irrespective of the cutting plane or the cutting temperature under vacuum condition. However, while cutting 3C-SiC, cutting tool showed severe wear and plastic deformation. It was found that the atom-by-atom attrition wear and plastic deformation of the diamond cutting tool could be alleviated while cutting 3C-SiC at high temperatures. Nevertheless, chemical wear i.e. dissolution-diffusion and adhesion wear is plausible to be accelerated at high temperatures.Raman spectroscopy was successfully used to identify the formation of metastable silicon phases during nanoscratching experiments at room and high temperatures. The probability of forming high pressure phases of Si-III and Si-XII was found to increase above the threshold load of 5 mN during room temperature nanoscratching experiment at low scratching speed. At high scratching speed, small remnants of Si-XII and Si-III phases were detected when the scratching load was greater than a threshold value i.e. ~9.5 mN. When high temperature nanoscratching was carried out at low and high speeds, no remnants of polymorph phases were observed along the nanoscratch residual track, suggesting the transition of metastable silicon phases (Si-III and Si-XII) into thermodynamic stable Si-I. Further analysis using AFM showed that the residual scratch morphologies and nanoscratch hardness were profoundly influenced by the temperature and scratching speed.Due to their exceptional physical and chemical properties such as high strength, high thermal conductivity, high stability at high temperature, high resistance to shocks, low thermal expansion and low density, silicon and silicon carbide (SiC) have become consummate candidates for optoelectronics, semiconductor and tribological applications. In particular, 3C-SiC, as a zinc blende structured SiC, possesses the highest fracture toughness, hardness, electron mobility and electron saturation velocity amongst the SiC polytypes. Thus, it has drawn substantial attention as a candidate substrate material for nano-devices which require high performance in extreme environments.Nanometric cutting is a promising ultra precision manufacturing process for manufacturing of 3D silicon and SiC based components which require submicron form accuracy and nanometric smooth finish. However, silicon and 3C-SiC have poor machinability at room temperature due to their relatively low fracture toughness and high hardness. A common understanding is that the yield strength and hardness of silicon and 3C-SiC will reduce under high temperature. As such, their fracture toughness increase which will ease plastic deformation and improve their machinabilities primarily as a result of thermally-generated intrinsic defects and thermal softening processes. However, the extent has never been reported although this knowhow could be vital in implementing the hot machining of silicon and SiC with the assistance of laser processing.This dissertation aims to gain an in-depth understanding of nanoscale mechanisms involved in nanometric cutting of hard-brittle materials such as silicon and 3C-SiC at elevated temperatures through molecular dynamics (MD) simulation and experimental trials. To this end, three-dimensional MD models of nanometric cutting were developed and different types of interatomic potential functions i.e. Tersoff, modified Tersoff, ABOP and SW were adopted to describe the interactions between atoms. In order to obtain reliable results, the equilibrium lattice constants were calculated at different temperatures for the employed potential functions. To perform the MD simulations, LAMMPS software was employed on a HPC service which was coupled with OVITO to visualise and post-process the atomistic data. Material flow behaviour, cutting chip characteristics, cutting forces and specific cutting energy, yielding stresses, stress and temperature on the cutting edge of the diamond tool, tool wear, defect-mediated plasticity and amorphization processes were calculated and analysed to quantify the differences in the cutting behaviour at different temperatures. Furthermore, In-situ high temperature nanoscratching (~500°C) of silicon wafer under reduced oxygen condition through an overpressure of pure Argon was carried out using a Berkovich tip with a ramp load at low and high scratching speeds. Ex-situ Raman spectroscopy and AFM analysis were performed to characterize high pressure phase transformation, nanoscratch topography, nanoscratch hardness and condition of the nanoindenter tip in nanoscratching at room and elevated temperatures.MD simulation results showed that the workpiece atoms underneath the cutting tool experienced a rotational flow akin to fluids. Moreover, the degree of flow in terms of vorticity was found higher on the (111) crystal plane, signifying better machinability on this orientation. Furthermore, it was observed that the degree of turbulence in the machining zone increases linearly with machining operation temperature. The cutting temperature showed significant dependence on the location and position of the stagnation region in the cutting zone of the substrate. In general, when cutting was performed on the (111) plane, the stagnation region (irrespective of the cutting temperature) was observed to locate at an upper position than that for the (010) and (110) planes. Also, at high temperatures, the stagnation region was observed to shift downwards than what was observed at room temperature. Another point of interest was the increase of subsurface deformation depth of the workpiece while cutting the (111) crystal plane at elevated temperatures.;Dislocation nucleation and formation of stacking faults were identified in conjunction with amorphization of silicon as the meditators of crystal plasticity in single crystal silicon during nanometric cutting process on different crystallographic planes at various temperatures. MD simulations revealed strong anisotropic dependence behaviour of dislocation activation and stacking fault formation. Likewise, while cutting 3C-SiC on the (110), formation and subsequent annihilation of stacking fault-couple 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 were observed. An observation of particular interest, while cutting 3C-SiC, was the shift to the (110) cleavage at cutting temperatures higher than 2000 K. The initial response of both the silicon and 3C-SiC substrates 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. No symptom of any atom-by-atom attrition wear and plastic deformation of the diamond cutting tool was observed during nanometric cutting of silicon irrespective of the cutting plane or the cutting temperature under vacuum condition. However, while cutting 3C-SiC, cutting tool showed severe wear and plastic deformation. It was found that the atom-by-atom attrition wear and plastic deformation of the diamond cutting tool could be alleviated while cutting 3C-SiC at high temperatures. Nevertheless, chemical wear i.e. dissolution-diffusion and adhesion wear is plausible to be accelerated at high temperatures.Raman spectroscopy was successfully used to identify the formation of metastable silicon phases during nanoscratching experiments at room and high temperatures. The probability of forming high pressure phases of Si-III and Si-XII was found to increase above the threshold load of 5 mN during room temperature nanoscratching experiment at low scratching speed. At high scratching speed, small remnants of Si-XII and Si-III phases were detected when the scratching load was greater than a threshold value i.e. ~9.5 mN. When high temperature nanoscratching was carried out at low and high speeds, no remnants of polymorph phases were observed along the nanoscratch residual track, suggesting the transition of metastable silicon phases (Si-III and Si-XII) into thermodynamic stable Si-I. Further analysis using AFM showed that the residual scratch morphologies and nanoscratch hardness were profoundly influenced by the temperature and scratching speed

Investigating Interaction Partners Of Yeast UBPS and Human USP7 Deubiquitinases

Ubiquitin-specific proteases (USPs; Ubps) are the most abundant family of deubiquitinating enzymes. Their involvement in various cellular processes, which are implicated in development and diseases such as cancer, have made them an important subject of investigation. Protein function correlates with its three-dimensional fold; therefore, structural studies can help identify evolutionary conserved features, catalytic mechanisms and provide clues to specificity and interactions. We examined the expression of soluble Saccharomyces cerevisiae deubiquitinating enzymes in E. coli for structural studies. Following extensive optimizations Ubp1, the Ubp6 catalytic domain and the Ubp12 N-terminal domain crystallized. The Ubp6 crystals led to the determination of its three-dimensional structure and revealed that the catalytic triad residues are arranged in an active conformation. We showed that the Ubp6 catalytic domain superimposes well with that of USP14, its human homologue. This suggests a similar regulatory mechanism for yeast Ubp6 as compared to human USP14. Further, we investigated the mode of protein interaction of yeast Ubp15. Both S. cerevisiae Ubp15 and its human homologue, USP7, harbor a highly conserved DWGF motif in their N-terminal domains. The DWGF motif in USP7-NTD efficiently binds substrates or interacting proteins containing (E/P/A)XXS sequences. We showed that similar to USP7-NTD, the DWGF motif in Ubp15-NTD mediates interaction with (E/P/A)XXS motifs. To establish latency, human herpesviruses (HHVs) have evolved various mechanisms for host immune evasion. Proteins expressed by some members of the HHV family interfere with the USP7-p53-Hdm2 pathway. They competitively bind to USP7, block USP7-substrate interaction and suppress p53 mediated apoptosis. We identified an EGPS motif in both vIRF1 protein (expressed by Kaposi's sarcoma-associated herpesvirus; KSHV) and pp71 protein (expressed by human cytomegalovirus; HCMV). This motif is identical to the sequence reported in EBNA1 (of Epstein-Barr virus) and ORF45 (of KSHV) responsible for mediating their interaction with USP7-NTD. We demonstrated that both vIRF1 and pp71 interact with USP7-NTD in vitro and characterized the interaction using functional studies. The crystal structures of the USP7-NTD:vIRF1 and USP7-NTD:pp71 peptide complexes revealed identical modes of binding as that of the EBNA1 EGPS peptide to USP7-NTD. Our results support new roles for vIRF1 and pp71 through deregulation of the deubiquitinating enzyme USP7, which destabilize p53 and inhibit cellular antiviral responses

Destructive Attacks Detection and Response System for Physical Devices in Cyber-Physical Systems

Nowadays, physical health of equipment controlled by Cyber-Physical Systems (CPS) is a significant concern. This paper reports a work, in which, a hardware is placed between Programmable Logic Controller (PLC) and the actuator as a solution. The proposed hardware operates in two conditions, i.e. passive and active. Operation of the proposed solution is based on the repetitive operational profile of the actuators. The normal operational profile of the actuator is fed to the protective hardware and is considered as the normal operating condition. In the normal operating condition, the middleware operates in its passive mode and simply monitors electronic signals passing between PLC and Actuator. In case of any malicious operation, the proposed hardware operates in its active mode and both slowly stops the actuator and sends an alert to SCADA server initiating execution of the actuator’s emergency profile. Thus, the proposed hardware gains control over the actuator and prevents any physical damage on the operating devices. Two sample experiments are reported in which, results of implementing the proposed solution are reported and assessed. Results show that once the PLC sends incorrect data to actuator, the proposed hardware detects it as an anomaly. Therefore, it does not allow the PLC to send incorrect and unauthorized data pattern to its actuator. Significance of the paper is in introducing a solution to prevent destruction of physical devices apart from source or purpose of the encountered anomaly and apart from CPS functionality or PLC model and operation

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

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

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

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