The Research On The Preparation And Tribological Properties Of 2D BN Nano-Lubricant Materials For Lubricant Applications

Friction and wear can be seen everywhere in daily life and various industrial applications.Wear caused by friction always lead to various problems including surface damage,degradation of equipment performance and reliability.In this case,lubrication is the most effective way to control friction and wear.More importantly,the introduction of lubricant additives was employed as a resultful method to enhance lubricant performance.Although the proportion of additives is small,the roles of additives are non-negligible in bring new features and making up for the shortcomings of lubricants.Hexagonal boron nitride(h-BN)in nano-lubricating materials has an ultra-flat surface,and the low strength of its atomic shear plane,which provide low friction during friction.In addition,the excellent mechanical strength,high chemical inertness and thermal stability make h-BN as a new type of nano-additive with great likelihood.This paper focused on the preparation and tribological properties of 2D BN nano-lubricant materials for lubricant applications.Main researches of the thesis involves the following aspects:(1)Preparation of BNNSs nanosheets by Na+/K+hydrothermal method and oxalic acid thermal expansion stripping process.A series of characterizations was utilized to determine the size of nanosheets obtained by hydrothermal method and corresponding thickness of nanosheets was 200-500 nm at 1-3 layers.Nanosheets fabricated by the hydrothermal method showed better stability of dispersion in base oil compared with h-BN.The size of the nanosheets obtained by the thermal expansion method of oxalic acid was 100-200 nm and the thickness was 3-5 nm with the yield of about 13.6%.Compared with powdered h-BN,nanosheets prepared by the thermal expansion method had better dispersion and stability in ethanol and base oil.According to the friction performance tests of the nanosheets obtained by these two methods,BNNSs obtained by the hydrothermal method and the thermal expansion of oxalic acid displayed better tribological properties.The average friction coefficient and average wear scar diameter of the BNNS-base oil were reduced by about 35.7%and 35.2%,45%and 51.3%,respectively.(2)We proposed a novel approach to massively exfoliate BNNSs via microwave-assisted liquid nitrogen exfoliation.Under the combined action of microwave liquid nitrogen,we obtained the nanosheets with a thickness of 2-6 nm(3-12 layers)and an average size of 200 nm.In addition,the nanosheets could achieve a stable dispersion of 0.565 mg/ml in ethanol solution,and the yield of the combined stripping process was about 7.91%.According to the friction performance tests of obtained nanosheets,BNNSs(0.08 wt.%)showed better friction performance than the base grease.The average friction coefficient and average wear volume were decreased by 20.10%and 55.8%compared with the base grease.(3)We successfully prepared the 2D nanostructured h-BN/WS2 heterojunction by the hydrothermal-assisted exfoliation of bulk h-BN and WS2.The morphology and structure of the as-prepared heterojunction were determined by a series of characterizations.The forming mechanism of the as-prepared heterojunction was proposed.The friction coefficient and wear rate of the as-prepared h-BN/WS2 heterojunction-grease were decreased by 33.9%and 45.9%compared to the grease.In addition,the friction coefficient was close to 0.01 in certain intervals of operation during tribological tests,which was expected to achieve super lubrication.(4)Scale-up experiments on innovative preparation methods were carried out.The feasibility of the 2D BN preparation process was analyzed based on the comparison of the important indicators such as the shape,quality and yield of the nanosheets between scale-up tests and small-scale tests.