The nano-machining of silicon mono crystals often causes significant structural changes such as phase transformation and oxidation and thereby changes the mechanical properties of the material. The oxidation of silicon during a machining process in a non-vacuum environment has not been studied in detail; in an experimental study on sliding wear, Zhang and Zarudi reported that the oxygen penetrates into the amorphous layer, changes the atomic bonding of silicon and accelerates wear.
Nano indentation is the simplest mechanical process that can generate stress field more or less similar to that in the nano-machining process. The present study investigates the penetration of oxygen in silicon during nano-indentation by atomistic simulation method. Oxygen molecules are placed in between the silicon (100) substrate and the indentor at different positions and orientations. The indentation is done with an inert hemisphere carbon indentor at room temperature. A three-body Tersoff potential is used for the interaction between silicon atoms and a two-body Morse potential is used for all other interactions as used successfully in a large number of machining and indentation process of various materials.
Our results show the oxygen molecule in the appropriate position and orientation dissociates into atoms, penetrates into the substrate and forms three/four coordination with Si atoms throughout loading and unloading. At the end of the indentation process, the oxygen atoms are found about 8Å below the surface in the amorphous phase. A comparison of the number of silicon atoms around oxygen/dummy oxygen atom throughout the indentation process with and without oxygen shows there are more silicon atoms around oxygen. This clearly indicates that the oxygen penetration causes damage to the silicon substrate. Thus in nano-tribological sliding and precision surfacing where the many moving surface asperities are abrasive grits, which can be viewed as moving indentors, would increase the chance of oxidation.