Tribological Properties and Mechanism of Diisooctyl Dithiodibenzoate as a Lubricant for Copper and Aluminum Alloys

The ester lubricant (Phe-S-C_i8) was synthesized by using dithiodibenzoic acid and isooctanol as raw materials. The physical and chemical properties of the sample were evaluated using a variety of instruments. The lubrication performance of the sample on different friction pairs was evaluated and compared with using trioctyl trimellitate (Phe-3C_i8) and dioctyl sebacate (1088). The friction mechanism was tentatively studied by using the data of scanning electron microscopy and surface element analysis.

The results showed that Phe-S-C_i8 had high viscosity both at 40 ℃ and 100 ℃, which enabled it to form stable lubricating film on the surface of the friction pair, but the viscosity index of Phe-S-C_i8 was too small so that its viscosity changed significantly with temperature. This may be because there were two aromatic rings in Phe-S-C_i8, which increased the viscosity and reduced the viscosity index of Phe-S-C_i8. Phe-S-C_i8 was found to have higher pour point, flash point and oxidation stability than the control samples. The reason for the higher thermal stability and pour point of Phe-S-C_i8 was that Phe-S-C_i8 had larger molecular weight and more aromatic rings than the comparing samples, which make the molecular structure of Phe-S-C_i8 more stable and increased the pour point of Phe-S-C_i8. In addition, a p-π-conjugated system may be formed between the thioether structure and the aromatic rings, this system allowed increasing the thermal stability and pour point of Phe-S-C_i8. Phe-S-C_i8 had significantly high oxidation stability. The reason for the good oxidation stability of Phe-S-C_i8 was that there was a thioether structure in the Phe-S-C_i8 molecule, which was a natural antioxidant structure. it formed the SRV test results showed that the synthetic Phe-S-C_i8 had stable friction coefficient and lower wear volume as lubricant for steel/copper and steel/aluminum friction pairs compared with the reference samples. The SEM results showed that abrasion on the copper surface was mainly abrasive wear, while abrasion on the aluminum surface was mainly adhesive wear. The extreme pressure test results of Phe-S-C_i8 showed that Phe-S-C_i8 also exhibited significantly better extreme pressure bearing capacity than the reference samples on the above two friction pairs.

Corrosion experiments of Phe-3C_i8, 1088, and Phe-S-C_i8 were performed. The experimental results showed that the corrosion of Phe-3C_i8, 1088, and Phe-S-C_i8 on copper sheet increased in turn. The strong corrosiveness of Phe-S-C_i8 may be due to the introduction of active sulfur into the molecular, which was highly corrosive to non-ferrous metals, so Phe-S-C_i8 showed slightly stronger corrosiveness.

By analyzing the surface elements of the wear scar, it could be concluded that the friction reducing and anti-wear performances of Phe-S-C_i8 on copper friction pair were mainly due to the physical adsorption of the lubricant molecules on the sliding surface, while their performances on aluminum friction pair were mainly due to the chemical reaction between Phe-S-C_i8 and the friction interface.