Molecular dynamics (MD) simulations with the ReaxFF reactive force field were carried out to find the atomistic mechanisms for tribochemical 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 atom transfers across the interface occurs during the sliding. In the absence of water molecules, the interfacial mixing is initiated by dehydroxylation followed by the Si-O-Si bond formation bridging two solid surfaces. In the presence of submonolayer thick water, the dissociation of water molecules can provide additions reaction pathways to form the Si-O-Si bridge bonds and atom transfers across the interface. However, when the amount of interfacial water molecules is large enough to form a full monolayer, the degree of atom transfer is substantially reduced since the silicon atoms at the sliding interface are 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 mechanochemical processes during the sliding and provided physical insights into tribochemical wear processes of silicon oxide surfaces observed experimentally.