The lubricating ability of one halogen-free and one halogen-containing phosphonium-based ionic liquids are investigated as neat lubricants, lubricant additives and thin lubricant layers in steel-steel contact. The use of the ionic liquids in any of the three lubricating methods reduced friction and wear compared to a base mineral oil. The halogen-free ionic liquid outperformed the halogen-containing ionic liquid in the three methods of lubrication. The highest friction and wear reduction were obtained when ionic liquids were used as neat lubricants. Under this condition, friction reductions of 37.21% and 25.73 %, and wear reduction of 47.12% and 41.18% compared to the based mineral oil were obtained for the halogen-free and halogen-containing ionic liquids respectively. The wear mechanisms and surface interactions are discussed in terms of ionic liquid-metal surface interactions from optical and SEM images and EDS analysis.

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