The effect of the addition of phosphate esters to paraffinic base oils on their lubricating performance under sliding conditions

Using a modified Timken machine, the effect of the addition of different phosphate esters to paraffinic base oils on their lubricating performance was investigated. The aryl phosphate ester tricresyl phosphate (TCP) has a greater resistance to scuffing than the alkyl phosphate ester trioctyl phosphate (TOP) when used as a lubricant. In contrast, base oils containing TOP are superior to those containing TCP, although phosphate esters are effective only in base oils of higher viscosity than the esters.The lubricating performance of high viscosity base oils containing TOP is improved by coupling the ability of TOP to form a beneficial surface film with the ability of the base oils to build up thick oil films. The addition of TCP, which has a higher adsorption ability but a lower reactivity than TOP, seems, however, to prevent the formation of the oxide film which is formed with base oils alone owing to the marked ability of TCP to adhere to a metal surface. In addition, because of the low reactivity of TCP, iron phosphate is hardly formed. Consequently, the addition of TCP increases the scuffing load of the base oils but has the disadvantage of increasing the coefficient of friction.