In this work, as a new type of oil-based additive, a phosphate mixture of (Sr_0.9Ca_0.1)₃(PO₄)₂ and Sr₃(PO₄)₂ with flower-like structure (SrP) was synthesized. When lubricating titanium alloy, compared with pure PAO8, using 20 wt.% SrP for lubrication can reduce the coefficient of friction (COF) by 69.89% and the wear rate (WR) by 99.86%. It was demonstrated that the extraordinary tribological performance was attributed to the deposition layer of SrP on the surface of titanium alloy. On the one hand, the deposition layer formed by SrP can avoid direct contact between friction pairs, protect the surface of titanium alloy, and prevent adhesion wear of titanium alloy. On the other hand, the low-shear interlayer sliding of SrP nanosheets inside the deposition layer was beneficial for friction reduction. The XPS confirmed that after the frictional sliding, the active group phosphate in SrP was activated, and other metals were oxidized to produce a series of oxides. Besides, the phosphate can form the P-O-Ti bonds with titanium at the interface, which was the key to SrP deposition and adsorption on the surface of titanium alloy. The SrP additive not only exhibited excellent performance in lubricating titanium alloy discs, but also stainless steel 304, 42CrMo, and tin bronze. After lubrication with 20 wt.% SrP additive, the wear tracks of stainless steel 304 and 42CrMo can not be detected, and the WR of tin bronze decreased by 92%. The interface lubrication mechanism has been concluded and proposed that may be beneficial for the design and application of new lubricating materials.

In this work, the advantage of Coulomb repulsion in the intermolecular forces experienced by molecules on the solid–liquid nanosized contact interface is taken, and the superior friction-reducing property of Cu₃(PO₄)₂·3H₂O (CuP) oil-based additives has been confirmed for titanium alloy. Three-dimensional (3D) CuP nanoflowers (CuP-Fs) with a strong capillary absorption effect are prepared to achieve the homogeneous mixing of solid CuP and lubricating oil. Lubrication by CuP-Fs additives for titanium alloy, friction coefficient (COF) can be reduced by 73.68%, and wear rate (WR) reduced by 99.69%. It is demonstrated that the extraordinary friction-reducing property is due to the repulsive solid–liquid interface with low viscous shear force originating from Coulomb repulsion between polar water molecules in CuP and non-polar oil molecules. However, any steric hindrance or connection between this repulsive solid–liquid interface will trigger the adhesion and increase the viscous shear force, for example, dispersant, hydrogen bondings, and shaky adsorbed water molecules. Besides, the lamellar thickness of CuP and the molecular size of lubricant both have a great influence on tribological properties. Here the lubrication mechanism based on interface Coulomb repulsion is proposed that may help broaden the scope of the exploration in low-friction nanomaterial design and new lubricant systems.