Tribological Properties of Heterocyclic Ionic Liquids as Lubricants for Titanium Alloys

Using 3-(benzothiazol-2-ylthio)-1-propanesulfonate as anion, quaternary phosphonium and quaternary ammonium salts with different alkyl chain lengths and active elements as cations, four kinds of heterocyclic sulfonated ionic liquids (ZILs) were synthesized as lubricants for titanium alloy, where L-F104 as the reference sample. Their physic-chemical and tribological properties between Steel/Ti-6Al-4V alloy friction pairs were assessed. In terms of physical-chemical properties, the viscosity of ZILs was significantly higher than that of L-F104, and the viscosity of quaternary phosphonate ILs was always less than that of ILs with quaternary ammonium cations. The thermal stability data for ZILs was greater than 200 ℃, which meets the basic conditions as ideal lubricants, but slightly lower than that of L-F104. Structurally speaking, for the condition of same anionic chain length, their thermal stability showed the trend: quaternary phosphine salt > quaternary amine cations, which may for the electronegativity of "N" was greater than that of “P”. The lower the electronegativity, the stronger the metallicity, the larger the lattice energy, and the better thermal stability of quaternary phosphine ILs.

For the tribological properties, the friction reducing and anti-wear properties of ZILs were better than the reference sample L-F104 (except N₄₄₄₄ZPS at 25 ℃ and P₈₈₈₈ZPS at 100 ℃). P₄₄₄₄ZPS and N₈₈₈₈ZPS maintain excellent anti-friction and anti-wear effects at 25 ℃ and 100 ℃, mainly attributed to moderate viscosity, increased flexibility of molecular chains and the presence of extreme pressure elements. In the frequency conversion experiment, P₄₄₄₄ZPS exhibits optimal extreme carrying capacity, it may be ascribed to the presence of the extreme pressure element "P" in the cation. Meanwhile, benefiting from the high viscosity, the firmer physical adsorption film and chemical reaction film on the titanium alloy can be obtained. In variable load experiments, N₈₈₈₈ZPS demonstrated lower COF and better extreme pressure carrying capacity, probably originate from the increase in the length of the alkyl chain and the flexibility of ILs. The three-dimensional optical profiler test results of all lubricants had proven to possess excellent wear resistance. The experimental results of SEM and EDS also showed the corresponding conclusion that ZILs demonstrate superior lubricity.

And the lubricating mechanism of ILs on Steel/Titanium friction couples was deeply explored by the contact angle, adsorption capacity, the morphology and element state (XPS analysis) of the wear scar surface X-ray photoelectron spectroscopy (XPS) was used to explore the lubrication mechanism. During friction, ILs with excellent anti-friction and anti-wear effect undergo chemical reactions on the metal surface to form a chemical reaction film, and form a polarity-induced physical adsorption film. Therefore, the physical adsorption film and the chemical reaction film together determine the lubrication performance of the ZILs on the titanium alloy. Combining contact angle (CA) and quartz crystal microbalance with dissipation tests (QCM-D), it was speculated that the polar group SO₃⁻ from ZILs with strong adsorption capacity, followed by van der Waals forces provided by long alkyl chains, increase shear resistance. In addition, under the action of frictional heat and mechanical energy, different active elements (such as N and P and other elements) can chemically react with the metal surface to form a corresponding chemical reaction film.