Phosphazene-based ionic liquids: Synthesis, temperature-dependent viscosity, and effect as additives in water lubrication of silicon nitride ceramics

Abstract

Phosphazene rings with (dimethylamino)ethoxy (1, 2), pyridylmethoxy (3), or (dimethylamino)propoxy (4) chains were synthesized and quaternized at the substitutent nitrogen by treatment with methyl iodide at 35 °C over 3-6 h to give polyiodo salts, 5-8. Subsequent metathesis with LiN(SO2CF 3)2 or NaBF4 gave the respective ionic salts, 9-13. The amide salts, 9-12, were viscous liquids with pour points at 55-100 °C, and the tetrafluoroborate salt, 13, was a solid, mp 168 °C. The compositions of 2 and 5-13 were confirmed by elemental analysis and spectroscopic methods. Compounds 1, 2, and 4 were viscous liquids (d 25 = 1.67 g cm-3; η25 = 0.76-1.56 mPa s-1) with pour points at ∼15 °C. The solid polyquaternary salts, 5-8, melted at 130-194 °C. The ionic liquids, 9-12, had an average density of ∼1.73 g cm-3 at 25 °C, and viscosities (25 °C) ranged between 68.3 and 139.2 mPa s-1. A plot of the viscosities of 9-12 vs temperature revealed an almost linear correlation between 55 and 120 °C. Friction and wear properties of water with 0.25 wt % of 9-12 as boundary lubricant additives were evaluated on Si3N4/Si 3N4 ceramic interfaces. The most significant observation is that they caused a decrease in the running-in period.