First-principles molecular dynamics study on the surface chemistry and nanotribological properties of MgAl layered double hydroxides

Layered double hydroxides (LDHs) are promising materials for lubrication. However, the underlying mechanism that leads to the low friction of the material is not well-understood. In this study, density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations have been used to study the reduced friction mechanism of MgAl-LDH. Our results indicate that the introduction of trivalent cations has a significant impact on the friction reduction of the LDH. Besides, the lateral force shows a strong correlation with the coverage of the hydroxyl group on the surface. By using AIMD simulation, we show that the water/hydroxide molecules interact with the surface through strong hydrogen bonds that confine the movement and the orientation of the intercalated molecules on the surface. Furthermore, the friction is reduced when the water thickness is increased. The reaction pathways of water with the LDH surface has been investigated using well-tempered metadynamics simulation. We found that the LDH can promote proton transfer, leading to the formation of hydroxide intermediates (OH), which then chemically adsorb on the surface. The chemical adsorption of the hydroxide intermediates can cleave the O-H bonds on the LDH surface.