Phosphazene-Based Ionic Liquids: Synthesis, Temperature-Dependent Viscosity, and Effect as Additives in Water Lubrication of Silicon Nitride Ceramics

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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(SO2CF3)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 (d25 = 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/Si3N4 ceramic interfaces. The most significant observation is that they caused a decrease in the running-in period.