# Study on the Tribological Properties of F-T DS/ZnFe-LDH Composite Lubricating Material
## 1 Introduction  
With the development of the social economy, Chinese car ownership has increased substantially, and the demand for oil and gas resources has further increased [1]. However, Chinese energy resources distribution characteristics are rich in coal, poor in oil, and minimal in gas, which means Chinese dependence on imported oil and gas resources remain in high proportion. As a result, using Chinese rich coal resources to solve the problem of shortage of oil and gas resources is feasible [2]. The production of diesel from coal liquefaction is not only national energy security and strategic reserve technology but also an important means of solving the Chinese energy crisis to balance fossil fuel consumption and energy structure. Chinese coal-to-liquid technologies mainly include indirect coal liquefaction, direct coal liquefaction, coal-oil co-process, and coal-to-methanol technologies. Table 1 is a comparison table of the physical and chemical properties of coal-to-diesel (Fischer-Tropsch synthesis diesel, F-T diesel) and commercial 0\({}^{6}\) diesel. As shown in Table 1, in comparison with 0\({}^{6}\) diesel, F-T diesel has a higher closed flash point, cetaneindex, density, polycyclic aromatic hydrocarbon content and freezing point, lower kinematic viscosity, oxidation stability sediment, acidity, and ash content. At the same time, other physical and chemical properties are similar to 0\({}^{\#}\) diesel. With the development of coal-to-oil technology, coal-to-oil has gradually become an ideal substitute for engine fuel. Regardless of the fuel, it inevitably produces soot due to incomplete combustion during engine operation. After the soot generated by the engine enters the lubrication system, it causes abrasive wear, adhesion wear, and poor-oil lubrication, which will shorten the service life of the machine [3]. Nonetheless, soot is not useless and completely harmful. In addition, fuel soot, carbon nanotubes, graphene, fullerene, etc., are all carbon materials. When a small amount of soot is used as an additive to lubrication oil and applied to other machinery, it can have a bright anti-friction and anti-wear effect to some degree [4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. Hydrotalcite, a layered nanomaterial, has good tribological properties. As a lubricant additive, it can improve the lubricating performance of the base oil and reduce friction and wear [14; 15; 16; 17; 18; 19; 20; 21]. As lubricating oil additives, soot and hydrotalcite are free of heavy metals, are non-toxic and environmentally friendly, conform to the development trend of green and environmental protection, and have good anti-friction and anti-wear properties. Therefore, in this study, we synthesized a composite (F-T DS/ZnFe-LDH, FS/ZnFe-LDH) of F-T diesel soot (F-T DS, FS) and zinc-iron hydrotalcite compound (ZnFe-LDH) via hydrothermal synthesis. The physicochemical properties and tribological properties of FS, ZnFe-LDH, and FS/ZnFe-LDH were investigated by various analytical methods. The purpose was to explore a new type of lubricating material based on coal-to-diesel soot to improve the tribological properties of lubricating oil and provide ideas for the recycling of soot from alternative fuels for engines.