The ability of a lubricating oil to reduce friction in mechanical surfaces which are in relative sliding motion depends on the base oil behaviour. Previous studies have demonstrated that temperature has a significant influence on the friction behaviour of mineral and synthetic base oils by using a laboratory based friction testing machine. However, the effect of a constantly changing load under different temperature conditions has not been explored fully. In this study, the effect of an increasing load on the friction behaviour of four six different mineral base oils and a polyalphaolefin (PAO) base oil were studied using the SRV4® tribometer. The sliding surfaces were AISI 52100 steel ball and disc. The average loads (range: 50 – 250 N), temperatures (range: 40 – 120 oC), relative humidity of 20 % and a sliding speed of 0.2 m.s-1 were selected as the test conditions. The seven base oils were selected from four API base oil groups. Stribeck curves were used as a tool to characterize the friction behaviour of the base oils. The results show that for all the base oils, the coefficient of friction and the Stribeck parameter decrease gradually with the increase in applied normal load under constant temperature conditions. The increase in temperature increased the coefficient of friction and decreased the Stribeck parameter at each load stage. The external friction mechanisms dominated the friction behaviour under all test conditions. Viscosity showed a strong influence on the film forming characteristics of the seven base oils only at 40 and 60 oC. Between 80 and 120 oC, the oil-surface interactions were predominant. The results further demonstrated that effect of an increasing temperature on the coefficient of friction was bigger between 80 and 100 oC for all Group III base oils and was consistent between 40 and 120 oC for the Group III+ and PAO base oil. The highly saturated (PAO and Group III+) base oils have demonstrated good thermal stability and less reactivity compared to the less saturated base oils (GI and GIII) under all test conditions. The friction behaviour of the PAO base oil was the most affected by the presence of dissolved water. The presence of water proved to increase the friction at the sliding steel interfaces. / Dissertation (MEng)--University of Pretoria, 2018. / Chemical Engineering / MEng / Unrestricted