Tribological Properties of Molybdenum Additives on Different Substrates

The effects of sulfur and phosphorus free organic molybdenum (SPFMo) on the tribological properties of lubricating oil (0W-20) on five kinds of different metals (bearing steel (GCr15), aluminium alloy (7A60), titanium alloy (TC4), copper (T2), and gray iron (HT300)) were investigated under different conditions including temperatures, loads and frequencies. 3D measuring laser microscope and scanning electron microscopy were used to measure and analyze the morphology and wear of the five metals. The results showed that SPFMo obviously improved the anti-friction properties of 0W-20 on the five metals. Among them, the friction coefficient of 0W-20 on the bearing steel was reduced by 23%, and the wear rate was maximally dropped by 91%. With the increase of temperature, the friction coefficient of 7A60 first increased and then decreased while the wear rate increased. The wear rate and friction coefficient of other materials increased with the increase of temperature. Additionally, with the increase of temperature, the effect of SPFMo on anti-friction and anti-wear was more obvious. The friction coefficient of four materials except TC4, can be reduced by 18%~23%. As for TC4, SPFMo had better anti-friction effect at lower temperature (≤40 ℃). When the temperature was higher than 40°C, the titanium surface wasa severely worn, and the wear rate increased by 1000 times. Small amount of S, Mo and other elements were detected on the wear tracks. This indicated that the lubrication film containing Mo, S, and O elements was generated in the friction process due to the addition of SPFMo. With the increase of load, the friction coefficient and wear rate of all materials increased. The increase of friction coefficient was most obvious on the T2 and TC4. With the addition of SPFMo, the friction coefficient of 0W20 on TC4 increased dramatically as the load exceeded 100 N. It had a sudden change and an increase of about 10⁴ times as the load was higher than 100 N, which was related to the destruction of titanium oxide layer. With the increase of sliding velocity, the friction coefficient of 7A60 decreased gradually, while the friction coefficient of GCr15 decreased first and then increases, and the friction coefficient of other materials increased gradually. The wear rates of four materials (except T2) increased with the increase of friction velocity. As for TC4, SPFMo had better anti-wear effect at low speed (≤0.8 m/s). The polar groups in SPFMo can form neatly arranged physical adsorption films during the adsorption process on the surface of different materials. The high temperature and pressure environment were generated during the friction process, which enhanced tribochemical reactions between SPFMo with other additives in the lubricating oil and the worn surface of metals. The tribo-chemical reaction film containing MoS₂, MoO₃, FePO₄, FeS was the main reason for the anti-friction and anti-wear of SPFMo. The research results can provide a powerful technical support and theoretical reference for the selection and design of new engine lubricants.