Development of a reaxff reactive force field for silicon/oxygen/hydrogen/fluoride interactions and applications to hydroxylation and friction

Molecular dynamics (MD) simulations with the ReaxFF reactive force field were carried out to find the atomistic mechanisms for tribo-chemical reactions occurring at the sliding interface of fully-hydroxylated amorphous silica and oxidized silicon as a function of interfacial water amount. The ReaxFF-MD simulations showed a significant amount of mass transfer across the interface occurs during the sliding. In the absence of water molecules, the interfacial mixing was initiated by dehydroxylation followed by the Si-O-Si bond formation bridging two solid surfaces. In the presence of sub-monolayer thick water, the dissociation of water molecules can provide additional reaction pathways to form the Si-O-Si bridge bonds and mass transfers across the interface. However, when the amount of interfacial water molecules was large enough to form full monolayer, the degree of mass transfer was substantially reduced since the silicon atoms at the sliding interface were terminated with hydroxyl groups rather than forming interfacial Si-O-Si bridge bonds. The ReaxFF-MD simulations clearly showed the role of water molecules in atomic scale mechano-chemical processes during the sliding and provided physical insights into tribochemical wear processes of silicon oxide surfaces observed experimentally.;In addition to this, we performed reactive force field molecular dynamics simulation to observe the hydrolysis reactions between water molecules and locally strained SiO2 geometries. We improved the Si/O/H force field from Fogarty et al.1, to more accurately describe the hydroxylation reaction barrier for strained and non-strained Si-O structures, which are about 20 kcal/mol and 30 kcal/mol, respectively. After optimization, energy barrier for the hydroxylation shows a good agreement with DFT data. The observation of silanol formation at the high-strain region of a silica nano-rod also supports the concept that the adsorption of water molecule: hydroxyl formation favors the geometry with higher strain energy. In addition, we found three distinct hydroxylation paths -- H3O+ formation reaction from the adsorbed water, proton donation from H3O+, and the direct dissociation of the adsorbed water molecule. Because water molecules and their hydrogen bond network behave differently with respect to temperature ranges, silanol formation is also affected by temperature. The formation of surface hydroxyl in an amorphous silica double slit displays a similar tendency: SiOH formation prefers high-strain sites. Silanol formation related with H3O + formation and dissociation is observed in hydroxylation of amorphous SiO2, similar with the results from silica nano wire simulation. These results are particularly relevant to the tribological characteristics of surfaces, enabling the prediction of the attachment site of the lubrication film on silica surfaces with a locally strained geometry.