Tribological Properties of Combination of Surface Texture and Ionic Liquids

Titanium alloys have been widely used in engineering fields due to their high corrosion resistance, good toughness and specific strength and many other excellent properties. However, titanium alloys are especially sensitive to adhesive wear with poor wear resistance under higher loads. In past several decades, many researchers have found that the textured surface can improve tribological performance, which is suitable for severe working environments. The influence of surface texture on the tribological properties mainly depends on various factors such as texture shape, depth and size parameters and processing accuracy. In past twenty years, ionic liquids have been extensively and intensively studied as lubricant oils and lubricant additives. In our work, laser processing technology was used to construct circular dimple texture patterns with different diameters on titanium alloy (Ti-6Al-4V) surface. Two kinds of ionic liquids （1-butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)-imide and Tributyltetradecyl phosphonium Bis(2-ethylhexyl)-phosphate) were synthesized and characterized. MS-T3001 tribometer was used to test friction coefficients of Ti-6Al-4V surfaces under different conditions, which included surfaces with and without circular dimple texture, with and without ionic liquids, and the lubricant combination of textured surfaces and ionic liquids. Before friction process, the texture of circular pits was observed on a metallurgical microscope. A scanning electron microscope was used to analyze the morphology of the textured surface before and after rubbing. ANSYS Fluent software was used to simulate and analyze the influence mechanism of surface texture parameters and ionic liquid’s viscosity on tribological performance. In experimental process, both textured surface and ionic liquid exhibited good friction-reduction behavior. Wear scar analysis were also consistent with the decreasing trend of friction coefficient. Lower friction coefficients from textured surfaces could be attributed to dimple’s collecting and storing worn debris. Tribological performances of ionic liquids were excellent due to their special physical and chemical properties. In the lubricant combination system, size parameters of textured surface and species of ionic liquid were chosen to optimize tribological properties. Under the combination conditions, it was found that friction coefficients decreased with the increase of diameters of the circular dimple texture and the ionic liquid with higher viscosity also provided better friction-reduction behavior. Two-dimensional simplified simulation of friction process was carried out in ANSYS Fluent to simulate the lifting force in the lubricant combination. As the lubricant fluid, ionic liquid produced a wedge effect in the convergence interval, which generated a lifting force on the upper friction pair. The simulation results showed that in the lubricant combination, the friction coefficients decreased with the increase of diameter of the circular dimple on textured surfaces, simultaneously ionic liquid with higher viscosity producing a higher lifting force on the upper friction pair. The simulated results about the lubricant combination of surface texture and ionic liquids were consistent with the experimental results, which confirmed to optimize the tribological performance by tuning the size of texture surface and choosing the species of ionic liquids.