Nano machining process of silicon mono crystals often causes significant structural changes such as phase transformation and oxidation and thereby changes the mechanical properties of the material. Although a number of theoretical and experimental papers have discussed the phase transformation process, the oxidation of silicon during the machining process has not been studied in detail. In their experimental study on the sliding process, Zhang et al.  reported that oxygen penetrates into the amorphous layer, changes the atomic bonding of silicon and thereby increases the wear rate. However, the mechanism of this chemical process is not yet known. This work aims to explore the effect of O2 on the indentation of silicon using molecular dynamics simulation.
Here we model the indentation of Si(100) surface in the presence of oxygen molecules using an inert hemisphere carbon indentor. Interactions between Si atoms are calculated using Tersoff potential and all other interactions are calculated using Morse potential with appropriate parameters. Initial velocities of the Si and O atoms follow the Maxwell distribution.
Our results show that on indentation, the O2 molecule in the appropriate position and orientation dissociates into oxygen atoms, penetrates into the surface and forms chemical bonds with silicon atoms. These oxygen atoms are three/four-fold co-ordinated by silicon atoms. Throughout the loading phase, the coordination of the oxygen atoms did not change significantly. On unloading, some of the previously formed Si-O bonds are broken and some new Si-O bonds are formed. After the indentation, the oxygen atoms are found in the 6th - 7th layer region. A comparison of the number of neighbours around oxygen / dummy oxygen atoms during the indentation process, with and without O2, shows that the oxygen atoms have more Si neighbours. This indicates that the penetration of oxygen atoms would increase the wear rate of silicon.
 Liangchi Zhang, Irena Zarudi, Wear 225-229 (1999) 669-677.