MATERIALS SCIENCE AND CHEMISTRY

Shear stress can cause plastic deformation in crystalline materials. In soft materials such as metals this can occur through the motion of dislocations. In contrast, deformation of covalent materials such as silicon and diamond can cause the formation of thin, amorphous layers at the sliding interface. The amorphization of silicon, for example, is responsible for the nanoscale wear of silicon tips in atomic force microscopes and affects the surface quality of silicon wafers. The amorphization of diamond plays a central role in polishing of jewelry and industrial diamonds.

The elementary processes that govern friction and wear usually occur on small length scales which makes scientific studies very challenging. In experiments it is usually not possible to directly observe the processes at the sliding interface. This is often referred to as the “buried interface” problem. Therefore, research on these processes relies on simulations. In the chfr09 project presented here, simulations on atomic length scales were performed on GCS supercomputers to study the behavior of silicon and diamond under combined shear and compression. As shown in Fig. 1, the simulations show the formation of an amorphous layer with fluid-like properties. In case of silicon, the amorphous material has a lower density than the crystal. As a result, the formation of the amorphous layer is favored by pressure applied to the system from the outside. Increasing the pressure between the surfaces leads to faster formation of the amorphous layer. In case of diamond, however, the amorphous layer has a lower density than the crystal. Therefore, an increased pressure slows down amorphization.

For silicon, shear-induced recrystallization counteracts amorphization resulting in a constant thickness of the amorphous zone of only a few atomic layers for low pressures. The elastic response of silicon crystals to shear is anisotropic and therefore the pairing of differently oriented crystals can lead to the migration of the amorphous zone. As shown in Fig. 2 this results in the “triboepitaxial” growth of one crystal at the expense of the other.