Summary of lubrication mechanism.

Lubrication mechanism	Representative lubricant	Characteristics
Rolling of particles	Nano-borates,57 nano SiO2,63 PS/OMMT66	Spherical particles convert the sliding friction between the friction surfaces into rolling friction, thereby reducing the friction coefficient. In addition, the particles are deposited in the recesses of the friction surface, which can reduce the surface roughness
Adsorption of friction surfaces	Modified vegetable ester,1,30,48 biodiesel,3 polyethylene glycol10	Lubricants are physically adsorbed to the friction surface through hydrogen bonds, electrostatic interactions, intermolecular forces, etc., or chemically adsorbed to the friction surface through metal bonds such as C S bonds. The presence of the adsorption film reduces the mutual contact of the friction surfaces to reduce the coefficient of friction
Sliding of the layered structure	Flake graphite,29 graphene,72 MoS2 (ref. 68)	The lubricant molecules with a layered structure are deposited on the friction surface, so that the direct contact between the friction surfaces is converted into the relative slippage of the lubricant molecular layer, thereby reducing the friction coefficient