Influence of sizes on tribological properties and tribofilm formation of lanthanum borate nanospheres in soybean oil

Abstract

Vegetable oils exhibit excellent lubrication properties owing to their polar functional groups, which are liable to adsorb on the metal surfaces to form adsorption film and tribofilm. Additionally, nanoparticles play significant roles in enhancing the tribological performances of base oils by means of forming a tribofilm between friction parts. However, little work has been done on clarifying the interaction of nanoparticles and vegetable oils in the film-forming process. In this paper, two varieties of lanthanum borate nanospheres with average diameters of 50 nm (named LBN-1) and 105 nm (named LBN-2) were synthesized. The morphologies, size distribution, and chemical compositions of the nanospheres were characterized using a scanning electron microscopy, dynamic light scattering, Fourier transform infrared, and X-ray diffraction. The tribological characteristics of soybean oils with lanthanum borate nanospheres were evaluated by a four-ball tribo-tester. The tribological performances of soybean oils were obviously improved by lanthanum borate nanospheres. The focused-ion beam/transmission electron microscope analyses results revealed a uniform tribofilm containing LBN-1 was formed, contributing to excellent friction-reducing and anti-wear performances. Whereas the tribofilm of soybean oil/LBN-2 was uneven in thickness and contained more wear debris. The outcome of this work provides significant insights into the tribofilm formation for metal surfaces lubricated with vegetable oils containing different sizes of lanthanum borate nanoparticles.