Abstract: Some ionic liquids of tributylalkyl phosphonium tetrafluoroborate were prepared and characterized by 1H-NMR.Thermal stability was characterized by thermogravimetric analysis(TGA).Tribological properties of these phosphonium ionic liquids as a new kind of promising lubricant for a steel/Cu-Sn alloy contact,were investigated with an Optimal SRV friction and wear tester.As a comparison,phosphazene X-1P and L-P206,were also tested at the same conditions.Elemental composition and chemical nature of the boundary films generated on the worn surface were analyzed with a Kevex energy dispersive X-ray analyzer equipped on a scanning electron microscope,and an X-ray photoelectron spectroscope.As a result,the synthetic ionic liquids showed better friction-reduction and antiwear behavior than the L-P206 did.For applied load ranging from 100 N to 300 N,TPTB exhibited only 1/2 friction coefficient and 1/3 wear volume in comparison with L-P206.