This study aims at the characterisation of physicochemical properties and tribological behaviour of quaternary phosphonium based ionic liquids (QPILs) P(44414)Sac and P(44414)Ace at ambient temperature and elevated temperature. The kinematic viscosity, viscosity index (VI), thermal stability, corrosivity and tribological properties of the ionic liquids (ILs) were systematically investigated. The copper strip corrosion test results revealed that the two green ILs are non-corrosive because of their halogen-free characteristic. Tribological data demonstrated that the present ILs exhibited extremely superior friction-reducing and anti-wear properties to conventional IL lubricant 1-Butyl-3-methylimidazolium hexafluorophosphate (L-P104), 1-n-Octyl-3-methyl imidazolium hexafluorophosphate (L-P108) and commercially available synthetic oil PAO10. Electrical contact resistance (ECR) and surface analysis of the worn-out steel disc surfaces revealed that the synergistic effect of a polarity induced physical adsorption layer and a tribo-chemical reaction film formed a boundary film to protect steel-steel contact between rubbing surfaces.