The unique structural properties of titanium alloys make the design and development of lubricating materials that applied on titanium alloy surfaces challenging. Ionic liquid (IL) lubricants with high reactivity and controllable performance provide a solution for improving the lubrication performance of titanium alloy interfaces. Herein, two novel ILs (5CP and 5CN) were prepared by using 5-methyl-1H benzotriazole as anion, which was equipped with tetrabutylphosphine and tetrabutylamine as cations, respectively. Then, the traditional IL L-P104 was used as a reference sample, and three ILs were used as lubricants to apply to the steel/titanium alloy interface. The results of physicochemical properties and tribological performance test showed that the synthesized IL lubricants exhibited superior friction-reducing and anti-wear properties on the titanium alloy surface. For IL lubricants, polar adsorption and tribochemical reactions were crucial to the enhancement of the lubrication stability.

In terms of physicochemical properties, the kinematic viscosity of 5CP and 5CN was significantly higher than that of L-P104. Among them, 5CN behaved the largest kinematic viscosity. Their appropriate viscosity value was also conducive to resist internal friction in the shear process to a certain extent. The thermal stability of 5CP/5CN was not as good as that of L-P104, but the thermal decomposition temperature of 5CP and 5CN exceeded 169 °C, implying that the ILs had a certain stability during friction. Furthermore, the adsorption properties of ILs on the titanium chips surface were investigated through the acquisition of adsorption change (∆f) and dissipation (∆D) value. Compared with the control sample L-P104, the ∆f and ∆D of the IL lubricant on the titanium chip were larger, indicating that the highly polar IL could adsorb efficiently on the titanium alloy surface. Besides, the adsorption performance results indicated that 5CP/5CN form a stable flexible adsorption layer at titanium alloy interface. This stable adsorption lays the foundation for its excellent tribological properties.

The tribological test results showed that L-P104 lubricant behaved the poor lubrication, while 5CP and 5CN could effectively reduce friction (the friction coefficient was reduced by 60 %). And the lubrication effect of 5CN was stronger than that of 5CP. The load-bearing performance was obviously improved by the reduction of 69% for wear volume, and the wear surface was enhanced by the detection of surface morphology and element distribution. 5CP and 5CN could effectively reduce the abrasive wear of the titanium alloy interface. And the C element content on the worn surface of titanium alloy after 5CP/5CN lubrication was significantly higher than that of the surface after L-P104 lubrication. The elements of C, N and Ti appeared at the steel ball surface, which indicated that the transfer film may exist in the test ball surface. By means of XPS analysis of the worn surface, it was recommended that the synthesized ILs forms a tribochemical reaction film on the sliding interface of titanium alloy. Besides, the improvement of lubrication performance was attributed to the efficient adsorption of ILs on the surface of titanium alloy and the formation of interfacial lubrication protective film. In addition, the rigid ring structure in the benzotriazole anion structure could complex with the titanium alloy surface, and the flexible alkyl chain on the tetrabutylammonium/phosphorus cations effectively enhanced the stability of tribofilm through van der Waals force.