Friction Behavior and Structural Evolution of Hexagonal Boron Nitride: A Relation to Environmental Molecules Containing −OH Functional Group

Sliding contact experiments and first-principles calculations were performed to elucidate the roles of environmental molecules containing −OH functional groups on the friction behavior and structural evolution of hexagonal boron nitride ( h -BN). A significant decrease in the friction coefficient (COF) is established by the physisorption and dissociative adsorption of molecules containing −OH functional groups on h -BN, compared with that in a H 2 or N 2 atmosphere. A key finding is the existence of two friction mechanisms to reconstruct the sliding interface for h -BN crystallites in humid air and carbon contaminant (CH 3 OH and C 2 H 5 OH) atmospheres, which is verified by the friction behavior and morphologies of the wear track. There is a running-in period in the friction process to induce the formation of defects in h -BN in humid air, which facilitates dissociative adsorption of water molecules on h -BN. The formation of nanostructured water at defect sites will promote lamellar slip of h -BN crystal materials for friction reduction. In carbon contaminant environments, both molecules exhibit strong adsorption on the h -BN surface regardless of the presence of defects, thereby weakening the structural damage rate and enhancing the bearing capacity. C 2 H 5 OH molecules are more likely to dissociate and bind onto defect sites, endowing h -BN with high in-plane stress to form a coiled structure. h -BN in the annular or tubular form would exhibit a self-protective effect, facilitate incommensurate contact, and reduce the contact area to enhance lubrication. Our work may establish the fundamental basis for future applications of h -BN in new energy vehicles.