Effect of base oil polarity on micro and nanofriction behaviour of base oil + ZDDP solutions

Ball on disc tribometer and atomic force microscopy (AFM) were used to analyse the effect of base oil polarity on the friction behaviour of steel–steel contacts lubricated with base oil + zinc dialkyldithiophosphate (ZDDP) solutions. Understanding the lubrication properties of the first chemisorbed layer of additives on work pieces yields important information for the optimisation of lubricant formulation, in particular with regard to the type of additive and amount needed. To characterise the influence of base oil polarity, two reference base oils [hexadecane (non–polar) and diethylenglycol (polar)] were blended with different concentrations of C₄-ZDDP, and the solutions were tested. A monolayer of base oil/additive solution was deposited on an ASI 52100 steel plate and scanned in AFM contact mode under various rubbing times and applied load conditions. An AFM technique was developed to estimate the microscopic values of friction coefficients showing how the oil polarity contributes to the differences in friction behaviour of the solution due to the addition of ZDDP. With different base oils [(hexadecane (non-polar base oil) and diethylenglycol (polar oil)] the authors observed significant different friction behaviours (in micro scale and nano scale) due to the addition of ZDDP compared to the base oil alone. This observation may be attributed to the contribution of base oil to transport the ZDDP additive onto the surface which will be discussed in more details in the paper. These results display the importance of base oil polarity on the friction behaviour of formulated lubricants containing additives.