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Title: An investigation on the mechanics of nanometric cutting for hard-brittle materials at elevated temperatures
Author: Chavoshi, Saeed Zare
ISNI:       0000 0004 6499 4876
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2016
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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) < 001̅ >, 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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral