Effect of structural anisotropy on the dislocation nucleation and evolution in 6HSiC under nanoindentation

Single crystalline 6HSiC possesses complex microstructure and its deformation is strongly anisotropic. With the aid of molecular dynamics analysis, this paper investigated the dislocation nucleation and evolution in 6HSiC under nanoindentation on three major planes, i.e., $\left(0001\right)$, $\left(01\overline{1}0\right)$ and $\left(2\overline{1}\overline{1}0\right)$. It was found that the half loops of prismatic dislocations could form during the nanoindentation on the $\left(0001\right)$plane, while the prismatic dislocation loops emerged on the $\left(01\overline{1}0\right)$ and $\left(2\overline{1}\overline{1}0\right)$planes. Further analysis revealed that the half loops were generated via the interaction of the nucleated dislocations in the basal plane and the first prismatic planes $\left\{01\overline{1}0\right\}$; while the formation of the prismatic loops can be attributed to either the “lasso”-like mechanism or the combination of dislocation interaction and “lasso”-like mechanism. Such strong effect of structural anisotropy was clarified through the generalised stacking fault (GSF) energy surface and stress distribution.     

Molecular dynamics simulation for orientation dependence of deformations in monocrystalline AlN during nanoindentation

Molecular dynamics simulations were performed for the nanoindentations using a virtual cylindrical indenter on monocrystalline aluminum nitride (AlN) with the indentation surface orientations of [0001], $[10\overline{1}0]$, $[\overline{1}2\overline{1}0]$ and $[\overline{1}01\overline{2}]$, respectively, to investigate the orientation dependence of the material. Vashishta potential was used to model the interactions between Al-Al, N-N, and Al-N atoms in the specimens. Simulation results indicated that the deformation mechanism varies with surface orientations at the initial inelastic stage. In the specimens with the surface orientations of [0001] and $[10\overline{1}0]$ phase transformation plays a predominant part, in the case of $[\overline{1}2\overline{1}0]$ dislocation slip dominates the inelastic deformation, whereas in the case of $[\overline{1}01\overline{2}]$ both phase transformation and dislocation slip act a leading role. However, the phase transformation and dislocation slip occur in all the samples during the further indentation. We found two paths of B4 to B1 phase transitions. Path I includes two steps: Al and N atoms move along the [0001] axis by an anti-parallel vertical motion, followed by horizontal relative movement of the two types of atoms. Path II is similar to Path I, but the sequence of relative movement of Al and N atoms is different. Path I occurs in the cases of [0001] and $[\overline{1}01\overline{2}]$ orientations, while Path II in the cases of the other two orientations.     

Material removal mechanism and crack propagation in single scratch and double scratch tests of single-crystal silicon carbide by abrasives on wire saw

Single-crystal silicon carbide is a promising semiconductor material widely applied in micro-electronics. However, its high hardness and brittleness make it difficult to be sliced into wafers by wire sawing technology. In this paper, both single scratch and double scratch tests are carried out on the (0001) surface of 4H-SiC by the abrasives on wire saw instead of using a standard indenter to obtain an in-depth understanding of the material removal mechanism and crack propagation in wire sawing. The scratching groove in single scratch tests is found to be composed of several small grooves because of the actual irregular shape of the abrasive. Stress analysis shows that this phenomenon has relatively small effects on the radial cracks propagating near the $\{1\overline{1}00\}$ primary cleavage plane. Radial cracks play an important role in the process of material removal by means of the interaction between radial cracks and the interaction between radial cracks and lateral cracks in double scratch tests. Based on fracture mechanics, the variation of stress intensity factors (SIFs) at the tip of radial cracks with the separation distance between the two grooves is calculated. Results show that the values of SIFs increase with the decrease of the separation distance, which means that radial cracks are more easily to propagate when the separation distance decreases. The theoretical analysis agrees with the experimental observation.