Effect of Food-Grade Talc on Tribological Properties of Composite Aluminum-Based Grease

In order to improve the anti-wear performance of as-prepared food-grade grease and enlarge its service life, it is necessary to adopt new additives and investigate their influence the on the tribological properties of the as-prepared food-grade greases. In the study, composite aluminum-based grease was prepared by food-grade white mineral oil and composite aluminum soap thickener. The food-grade talc (F-Talc) was selected as the additive, and its morphology and structure were characterized by scanning electron microscopy and fourier transform infrared spectrometer. The influence of F-Talc mass fraction on thermal stability and tribological properties of the as-prepared composite aluminum-based grease were investigated using Q500 thermogravimetric analyzer and MFT-5000 wear tester with the ball-disc rubbing pair, respectively. The wear scar surfaces of the steel disc were characterized by means of MFP-D white light interferometer, CHI660E electrochemical workstation and 250Xi X-ray photoelectron spectroscopy. Moreover, the safety performances of the composite aluminum-based grease and F-Talc were evaluated using the Listeria monocytogenes method. The results showed that as-prepared composite aluminum-based grease and F-Talc additive-containing grease were verified to be safe by the Listeria monocytogenes method. After F-Talc was added, the dropping point of as-prepared composite aluminum-based grease was increased slightly, but F-Talc had little role on the cone penetration of the as-prepared grease. It was observed that the microscopic morphology of the F-talc powder had a layered structure, implying that F-Talc had a tendency to split into mackerel scales and exhibited the special lubricity. Compared with the as-prepared base grease, the epitaxy initial degradation temperature of as-prepared composite aluminum-based grease containing 1.0% F-Talc increased from 309 ℃ to 320 ℃, corresponding to an increase of about 3.6%. As the final degradation temperature was reached, the residual weight of the as-prepared grease including the F-Talc additive was more than that of the as-prepared base grease. Therefore, F-Talc can improve the thermal stability of the as-prepared composite aluminum-based grease. Due to the use of F-Talc, the rubbing stability of the composite aluminum-based grease was significantly improved. Moreover, the average coefficient of friction of the as-prepared composite aluminum-based grease decreased from 0.107 to 0.080, corresponding to a decrease of around 25%. The depth of the wear scar of the steel disc decreased from 87 μm to 51 μm, corresponding to a decrease of about 41%. The wear volume per unit time of steel disc decreased from 5.89×10⁶ μm³/min to 3.10×10⁶ μm³/min, corresponding to a decrease of about 47%. Moreover, at the 1.0%F-Talc-containing as-prepared grease, its tribological properties was better than that of the commercial grease FM 222. As the mass fraction of F-Talc was 1.0%, the corrosion potential of the disc worn surface increased from −0.462 V to −0.312 V, corresponding to an increase of about 32%, indicating that the corrosion resistance of the disc wear scar surface was the best. Mechanism analysis showed that part of the layered F-Talc can be adsorbed on the surface of the rubbing pair, which was equivalent to the sliding of the "sliding bearing", so as to improve the anti-wear performance of as-prepared composite aluminum-based grease. In addition, the SiO₂ (the major ingredient of F-Talc) can bring about the role of the repairing and micro-polishing during the rubbing process. The presence of silicon and magnesium compounds (such as SiO₂, MgO) and the decrease of Fe concentration on the wear scar of the steel disc confirmed that complex chemical reactions occurred on the surface of the wear scar zone to form the lubricating film, thus reducing the probability of direct contact of the ball-steel disc pair and improving the anti-wear performance of as-prepared composite aluminum-based grease. Hence, the mixed grease with 1.0% F-Talc resulted in optimal performance, exhibiting promise in food processing industry. These results provide valuable insights into the development of novel food-grade lubricants.