Atomistic aspects of ductile responses of cubic silicon carbide during nanometric cutting

Cubic silicon carbide (SiC) is an extremely hard and brittle material having unique blend of material properties which makes it suitable candidate for microelectromechanical systems and nanoelectromechanical systems applications. Although, SiC can be machined in ductile regime at nanoscale through single-point diamond turning process, the root cause of the ductile response of SiC has not been understood yet which impedes significant exploitation of this ceramic material. In this paper, molecular dynamics simulation has been carried out to investigate the atomistic aspects of ductile response of SiC during nanometric cutting process. Simulation results show that cubic SiC undergoes sp3-sp2 order-disorder transition resulting in the formation of SiC-graphene-like substance with a growth rate dependent on the cutting conditions. The disorder transition of SiC causes the ductile response during its nanometric cutting operations. It was further found out that the continuous abrasive action between the diamond tool and SiC causes simultaneous sp3-sp2 order-disorder transition of diamond tool which results in graphitization of diamond and consequent tool wear

An atomistic investigation on the nanometric cutting mechanism of hard, brittle materials

The demand for ultra precision machined devices and components is growing at a rapid pace in various areas such as the aerospace, energy, optical, electronics and bio-medical industries. Because of their outstanding engineering properties such as high refractive index, wide energy bandgap and low mass density, there is a continuing requirement for developments in manufacturing methods for hard, brittle materials. Accordingly, an assessment of the nanometric cutting of the optical materials silicon and silicon carbide (SiC), which are ostensibly hard and brittle, has been undertaken. Using an approach of parallel molecular dynamics simulations with a three-body potential energy function combined with experimental characterization, this thesis provides a quantitative understanding of the ductile-regime machining of silicon and SiC (polytypes: 3C, 4H and 6H SiC), and the mechanism by which a diamond tool wears during the process. The distinctive MD algorithm developed in this work provides a comprehensive analysis of thermal effects, high pressure phase transformation, tool wear (both chemical and abrasive), influence of crystal anisotropy, cutting forces and machining stresses (hydrostatic and von Mises), hitherto not done so far. The calculated stress state in the cutting zone during nanometric cutting of single crystal silicon indicated Herzfeld–Mott transition (metallization) due to high pressure phase transformation (HPPT) of silicon under the influence of deviatoric stress conditions. Consequently, the transformation of pristine silicon to β-silicon (Si-II) was found to be the likely reason for the observed ductility of bulk silicon during its nanoscale cutting. Tribochemical formation of silicon carbide through a solid state single phase reaction between the diamond tool and silicon workpiece in tandem with sp3-sp2 disorder of carbon atoms from the diamond tool up to a cutting temperature of 959 K has been suggested as the most likely mechanism through which a diamond cutting tool wears while cutting silicon. The recently developed dislocation extraction algorithm (DXA) was employed to detect the nucleation of dislocations in the MD simulations of varying cutting orientation and cutting direction. Interestingly, despite of being a compound of silicon and carbon, silicon carbide (SiC) exhibited characteristics more like diamond, e.g. both SiC iii workpiece and diamond cutting tool were found to undergo sp3-sp2 transition during the nanometric cutting of single crystal SiC. Also, cleavage was found to be the dominant mechanism of material removal on the (111) crystal orientation. Based on the overall analysis, it was found that 3C-SiC offers ease of deformation on either (111) , (110) or (100) setups. The simulated orthogonal components of thrust force in 3C-SiC showed a variation of up to 45% while the resultant cutting forces showed a variation of 37% suggesting that 3C-SiC is anisotropic in its ease of deformation. The simulation results for three major polytypes of SiC and for silicon indicated that 4H-SiC would produce the best sub-surface integrity followed by 3C-SiC, silicon and 6H-SiC. While, silicon and SiC were found to undergo HPPT which governs the ductility in these hard, brittle materials, corresponding evidence of HPPT during the SPDT of polycrystalline reaction bonded SiC (RB-SiC) was not observed. It was found that, since the grain orientation changes from one crystal to another in polycrystalline SiC, the cutting tool experiences work material with different crystallographic orientations and directions of cutting. Thus, some of the grain boundaries cause the individual grains to slide along the easy cleavage direction. Consequently, the cutting chips in RB-SiC are not deformed by plastic mechanisms alone, but rather a combination of phase transformation at the grain boundaries and cleavage of the grains both proceed in tandem. Also, the specific-cutting energy required to machine polycrystalline SiC was found to be lower than that required to machine single crystal SiC. Correspondingly, a relatively inferior machined surface finish is expected with a polycrystalline SiC. Based on the simulation model developed, a novel method has been proposed for the quantitative assessment of tool wear from the MD simulations. This model can be utilized for the comparison of tool wear for various simulation studies concerning graphitization of diamond tools. Finally, based on the theoretical simulation results, a novel method of machining is proposed to suppress tool wear and to obtain a better quality of the machined surface during machining of difficult-to-machine materials

From Practice to Policy to Practice

Recent molecular dynamics simulation results have increased conceptual understanding of the grazing and the ploughing friction at elevated temperatures, particularly near the substrate's melting point. In this commentary we address a major constraint concerning its experimental verification

Incipient plasticity in tungsten during nanoindentation: Dependence on surface roughness, probe radius and crystal orientation

The influence of crystallographic orientation, contact size and surface roughness effects on incipient plasticity in tungsten were investigated by nanoindentation with indenters with a range of end radius (150, 350, 720 and 2800 nm) in single crystal samples with the (100) and (111) orientations. Results for the single crystals were compared to those for a reference polycrystalline tungsten sample tested under the same conditions. Surface roughness measurements showed that the Ra surface roughness was around 2, 4, and 6 nm for the (100), (111) and polycrystalline samples respectively. A strong size effect was observed, with the stress for incipient plasticity increasing as the indenter radius decreased. The maximum shear stress approached the theoretical shear strength when W(100) was indented using the tip with the smallest radius. The higher roughness and greater dislocation density on the W(111) and polycrystalline samples contributed to yield occurring at lower stresses

Molecular dynamics simulation investigation on the plastic flow behaviour of silicon during nanometric cutting

Molecular dynamics (MD) simulation was carried out to acquire an in-depth understanding of the flow behaviour of single crystal silicon during nanometric cutting on three principal crystallographic planes and at different cutting temperatures. The key findings were that (i) the substrate material underneath the cutting tool was observed for the first time to experience a rotational flow akin to fluids at all the tested temperatures up to 1200 K. (ii) The degree of flow in terms of vorticity was found higher on the (1 1 1) crystal plane signifying better machinability on this orientation in accord with the current pool of knowledge (iii) an increase in the machining temperature reduces the spring-back effect and thereby the elastic recovery and (iv) the cutting orientation and the cutting temperature showed significant dependence on the location of the stagnation region in the cutting zone of the substrate

Molecular dynamics simulation investigation on the plastic flow behavior of silicon during nanometric cutting

Molecular dynamics (MD) simulation was carried out to acquire an in-depth understanding of the flow behaviour of single crystal silicon during nanometric cutting on three principal crystallographic planes and at different cutting temperatures. The key findings were that (i) the substrate material underneath the cutting tool was observed for the first time to experience a rotational flow akin to fluids at all the tested temperatures up to 1200 K. (ii) The degree of flow in terms of vorticity was found higher on the (1 1 1) crystal plane signifying better machinability on this orientation in accord with the current pool of knowledge (iii) an increase in the machining temperature reduces the spring-back effect and thereby the elastic recovery and (iv) the cutting orientation and the cutting temperature showed significant dependence on the location of the stagnation region in the cutting zone of the substrate

Current trends and future of sequential micro-machining processes on a single machine tool

A sequential micro-machining process chain is described as the machining strategy whereby two or more micro-machining techniques are implemented in sequence on same or different machine tools. This is in contrast to hybrid micro-machining where two standalone machining technologies are integrated together. A recent surge of interest is geared towards building sequential micro-machining capabilities on a single machine tool to avoid realignment and registration errors between processes. One of the major advantages of performing sequential micro-machining on a single machine tool is that it suppresses repositioning errors so enabling much higher levels of accuracy (and thereby tighter tolerances), reduced rejection of machined components, and lower production time; all of these would be otherwise unachievable. Thus, multifunctional micro-machining centres are attracting global interest. Clearly, the necessity of developing reconfigurable, precise and flexible manufacturing is a key driver to this trend. This review aims to provide a critical insight into the recent trends and new classification of sequential micro-machining processes with a special focus on evaluation of such capabilities built on a single machine tool and further potentials. The machining capabilities, advantages and opportunities in the area of sequential micro-machining techniques are evaluated thoroughly and the directions for future work are highlighted

The importance of wavelength for tight temperature control during µ-laser assisted machining

The area of single point diamond turning of brittle materials like semiconductors and ceramics is significantly benefitted by incorporation of laser assistance. In a new developmental technology that is now recognized as micro-laser-assisted machining (μ-LAM), a laser is shone through a diamond tool to soften the high-pressure phase transformed ductile machining phases that in turn allows thermal softening and thereby enables a higher material removal rate during ductile mode machining. One of the lasers currently used in μ-LAM is the neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 100 W (continuous wave) at the wavelength of 1064 nm. Although this configuration has worked to the benefit of the technology, here we report futuristic developments that will significantly enhance temperature control by selecting a laser wavelength according to the material being machined, allowing tunable machining properties. The concept is illustrated with sample calculations for μ-LAM of silicon, and it appears to offer better target temperatures, thus enhancing the performance of the μ-LAM process

Addressing the discrepancy of finding equilibrium melting point of silicon using MD simulations

We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid–liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B 72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon

Molecular dynamics simulation investigation on the plastic flow behaviour of silicon during nanometric cutting

Molecular dynamics (MD) simulation was carried out to acquire an in-depth understanding of the flow behaviour of single crystal silicon during nanometric cutting on three principal crystallographic planes and at different cutting temperatures. The key findings were that (i) the substrate material underneath the cutting tool was observed for the first time to experience a rotational flow akin to fluids at all the tested temperatures up to 1200 K. (ii) The degree of flow in terms of vorticity was found higher on the (1 1 1) crystal plane signifying better machinability on this orientation in accord with the current pool of knowledge (iii) an increase in the machining temperature reduces the spring-back effect and thereby the elastic recovery and (iv) the cutting orientation and the cutting temperature showed significant dependence on the location of the stagnation region in the cutting zone of the substrate