Several kinds of titanium complex greases (TCG) were synthesized with the benzoic acid (BA), stearic acid (SA), isopropyl titanate and naphthenic mineral base oil (4016) by varying the molar ratio of BA to SA. The results of FTIR indicated these grease samples were successfully synthesized. The physicochemical properties, rheological behaviors and tribological performances were systematically investigated, and results showed that TCG1, TCG6 and TCG7 could not form grease, while TCG2, TCG3, TCG4 and TCG5 greases were successfully prepared. It demonstrated that the molar ratio of BA and SA had a significant influence on the microstructure and the formation mechanism of the titanium complex grease. The TCG2, TCG3 and TCG4 grease samples performed good physicochemical properties, such as, low penetration value, low oil separation under pressure and high dropping point. However, TCG5 exhibited poor physicochemical properties, such as, large penetration value and low dropping point. The rheological behaviors of titanium complex grease were measured by rotational rheometer, and these rheological parameters, including apparent viscosity and viscoelastic properties were tested. The apparent viscosity of TCG1, TCG6 and TCG7 changed little with the increase of shear rate, which further proved that they behaved more like Newtonian fluids. The apparent viscosity of TCG2, TCG3, TCG4 and TCG5 gradually decreased and finally tended to be stable with the increase of shear rate, which showed obvious shear thinning characteristics. The results of viscoelastic tests of TCG2, TCG3, TCG4 and TCG5 showed that the shear stress at flow point of TCG3 was the highest, followed by TCG2, TCG4 and TCG5, demonstrating the highest structural strength of TCG3. The microstructures of TCG were studied by scanning electron microscope, and results showed that as the SA content increased, the microstructure of thickeners changed from granular pattern with pores to dense fiber pattern, which could be ascribed to that long alkyl chains of SA were entangled with each other to form dense fibrous thickeners. As the SA content continued increasing (for example, TCG1), the grease could not be formed, due to the strong steric hindrance limiting the coordination of titanium atoms and oxygen atoms among different thickeners. Conversely, when the content of SA decreased, the granular structure with many pores would be produced by excessive BA soap, resulting in the decrease in the weak thickening ability. TCG2, TCG3, TCG4 and TCG5 with better rheological behaviors were selected to further explore their tribological properties. Under the experimental conditions of 392 N, 1 200 r /min and 60 min, all their friction coefficients were lower than 0.12, and their wear scar diameters (WSD) were lower than 0.7 mm, indicating their good tribological performances. Among them, TCG4 exhibited the steadiest and lowest friction coefficient curve and smallest WSD value, illustrating that TCG4 gave the best friction-reducing and anti-wear performance. Moreover, TCG4 were also demonstrated to possess the best bearing capacity by obtaining the highest PB value. At the same time, TCG4 was significantly better than the common of complex lithium grease (KK-311) that the average friction coefficient of 0.108, the wear scar diameter of 0.81 mm, the PB value was 98 N. In conclusion, TCG4 not only had excellent friction reducing and anti-wear performance, but also gave the superior bearing capacity.
In summary, these titanium complex grease products with high structural strength and excellent tribological properties were obtained through changing the molar ratio of SA and BA in this work. This method could optimize the performance of grease, provide technical support for the structural regulation and basic performance research of high-performance titanium complex grease, and promote the application in the field of lubrication engineering.
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