Analytical Surface Energy Model of Fine Copper-Graphite Core-Shell Particles in Oil Lubricant

Surface energy is a key quantity that controls many physical, mechanical, and tribological properties of materials.However, determining their values at various scales remains challenging to evaluate how they affect dispersion behavior and frictional performance.Analytical surface energy modelling for lubricating additives in die lubricant oil is made to evaluate the surface energy as a function of tribological characteristics with complex compositional dispersion in two conditions: (i) separately adding graphite and copper particles at the same time to the oil and (ii) adding the particles to the oil in the form of copper-graphite core-shell composite structure.We have introduced the surface-induced excess energy and discussed this value before and after friction according to the particle diameter and powder content wt.% in the lubricant oil.Results revealed that the adding of graphite and copper particles as a core-shell composite structure can play an excellent role as compared to separately adding the graphite and copper particles to the oil.The ratio of total surface energy after friction process to that of before friction in separately introduced the components, and in the form of core-shell composite was equal to 7 and 5, respectively.Meanwhile, the ratio of the total surface energy of the core-shell composite structure to that of separately introduced the components before and after the friction process is equal to 2 and 1.4, respectively.However, the composite with (2.5-3) wt.% content and a 10 µm diameter of particles indicates 3 and 3.4 values of the total surface energy ratio in cases of separate and core-shell structure conditions, respectively.The results indicate that the total surface energy for the core-shell composite is more effective than separate powders of graphite and copper added in oil.