Investigating the effect of overbased sulfonates on calcium sulfonate complex grease: enhancements in physicochemical, rheological, and tribological properties

Overbased sulfonate plays a crucial role in calcium sulfonate complex grease, significantly impacting the grease's performance characteristics. Herein, the calcium sulfonate complex grease was formulated using overbased calcium sulfonate (T106D) and overbased magnesium sulfonate (T107) in ratios of 1 : 2, 1 : 1, and 2 : 1, labeled as CMSCG (1 : 2), CMSCG (1 : 1), and CMSCG (2 : 1), respectively. This study examined the effects of overbased sulfonates on the physicochemical, anti-corrosion, rheological, and tribological properties of the grease. Results showed that CMSCG (1 : 2) exhibited superior physicochemical properties, with the highest dropping point (354 °C), the lowest penetration (161 (0.1 mm)), and the least oil separation (1.25%). Exposure to a salt spray environment significantly altered the grease's rheological properties. The combination of T106D and T107 enhanced the corrosion resistance of the grease, attributed to the formation of a corrosion inhibition layer. Incorporating T107 increased both the yield stress and hysteresis area of the calcium sulfonate complex grease. The CMSCG (1 : 2), CMSCG (1 : 1), and MSCG showed the high thixotropic ring area, indicating the poor thixotropy. The calcium sulfonate complex grease formulated with T107 showed the highest yield stress (558 Pa). The friction mechanism revealed that MSCG showed the optimal friction reduction properties, and CMSCG (1 : 1) demonstrated the optimal wear resistance, which are attributed to the synergistic effects of tribo-chemical films composed of CaO, CaCO₃, FeSO₄, MgCO₃, and iron oxide.