A theoretical assessment of surface defect machining and hot machining of nanocrystalline silicon carbide

In this paper, a newly proposed machining method named “surface defect machining” (SDM) [Wear, 302, 2013 (1124-1135)] was explored for machining of nanocrystalline beta silicon carbide (3C-SiC) at 300K using MD simulation. The results were compared with isothermal high temperature machining at 1200K under the same machining parameters, emulating ductile mode micro laser assisted machining (µ-LAM) and with conventional cutting at 300 K. In the MD simulation, surface defects were generated on the top of the (010) surface of the 3C-SiC work piece prior to cutting, and the workpiece was then cut along the <100> direction using a single point diamond tool at a cutting speed of 10 m/sec. Cutting forces, sub-surface deformation layer depth, temperature in the shear zone, shear plane angle and friction coefficient were used to characterize the response of the workpiece. Simulation results showed that SDM provides a unique advantage of decreased shear plane angle which eases the shearing action. This in turn causes an increased value of average coefficient of friction in contrast to the isothermal cutting (carried at 1200 K) and normal cutting (carried at 300K). The increase of friction coefficient however was found to aid the cutting action of the tool due to an intermittent dropping in the cutting forces, lowering stresses on the cutting tool and reducing operational temperature. Analysis shows that the introduction of surface defects prior to conventional machining can be a viable choice for machining a wide range of ceramics, hard steels and composites compared to hot machining.

Influence of temperature on the anisotropic cutting behaviour of single crystal silicon: A molecular dynamics simulation investigation

Using molecular dynamics (MD) simulation, this paper investigates anisotropic cutting behaviour of single crystal silicon in vacuum under a wide range of substrate temperatures (300 K, 500 K, 750 K, 850 K, 1173 K and 1500 K). Specific cutting energy, force ratio, stress in the cutting zone and cutting temperature were the indicators used to quantify the differences in the cutting behaviour of silicon. A key observation was that the specific cutting energy required to cut the (111) surface of silicon and the von Mises stress to yield the silicon reduces by 25% and 32%, respectively, at 1173 K compared to what is required at 300 K. The room temperature cutting anisotropy in the von Mises stress and the room temperature cutting anisotropy in the specific cutting energy (work done by the tool in removing unit volume of material) were obtained as 12% and 16% respectively. It was observed that this changes to 20% and 40%, respectively, when cutting was performed at 1500 K, signifying a very strong correlation between the anisotropy observed during cutting and the machining temperature. Furthermore, using the atomic strain criterion, the width of primary shear zone was found to vary with the orientation of workpiece surface and temperature i.e. it remains narrower while cutting the (111) surface of silicon or at higher machining temperatures. A major anecdote of the study based on the potential function employed in the study is that, irrespective of the cutting plane or the cutting temperature, the state of the cutting edge of the diamond tool did not show direct diamond to graphitic phase transformation.

Shear instability of nanocrystalline silicon carbide during nanometric cutting

The shear instability of the nanoscrystalline 3C-SiC during nanometric cutting at a cutting speed of 100?m/s has been investigated using molecular dynamics simulation. The deviatoric stress in the cutting zone was found to cause sp3-sp2 disorder resulting in the local formation of SiC-graphene and Herzfeld-Mott transitions of 3C-SiC at much lower transition pressures than that required under pure compression. Besides explaining the ductility of SiC at 1500?K, this is a promising phenomenon in general nanoscale engineering of SiC. It shows that modifying the tetrahedral bonding of 3C-SiC, which would otherwise require sophisticated pressure cells, can be achieved more easily by introducing non-hydrostatic stress conditions.

Temperature and strain rate dependence of syntactic foam under tensile and shear loads

The behaviour of syntactic foam is strongly dependent on temperature and strain rate. This research focuses on the behaviour of syntactic foam made of epoxy and glass microballoons in the glassy, transition and rubbery regions. Both epoxy and epoxy foam are investigated separately under tension and shear loadings in order to study the strain rate and temperature effects. The results indicate that the strength and strain to failure data can be collapsed onto master curves depending on temperature reduced strain rate. The highest strain to failure occurs in the transition zone. The presence of glass microballoons reduces the strain to failure over the entire range considered, an effect that is particularly significant under tensile loading. However, as the microballoons increase the elastic modulus significantly in the rubbery zone but reduce it somewhat in the glassy zone, the effect on the strength is more complicated. Different failure mechanisms are identified over the temperature-frequency range considered. As the temperature reduced strain rate is decreased, the failure mechanism changes from microballoon fracture to matrix fracture and debonding between the matrix and microballoons. © IMechE 2012.