Study On The Regulation Of Molybdenum Disulfide Crystal Form By Freeze-drying And Its Friction And Wear Properties

New and technologies with friction-reducing and life-extending effects are the focus and hotspot of current industrial development.Nano lubrication technology is an effective method to reduce wear and extend the life of friction pairs.The traditional lubricating oil can not severe the high load,high speed and the harsh service conditions of sliding.Nano-MoS₂has good chemical and electronic properties and excellent tribological properties due to their nano-size effect and surface effects.Previous studies have shown that nano-MoS₂can effectively reduce friction,wear and energy loss as lubricant additive.However,nano-MoS₂has a large surface energy,resulting to agglomerating during the preparation process,and nano-MoS₂has various forms,such as spherical and flake,less literature reported on the crystal forms effect for the friction properties,and the lubrication mechanism is still unclear.It is of great significance for the life extension and maintenance of key parts to study the anti-friction and anti-wear properties of different crystal nano-MoS₂as lubricating oil additives for the mechanical components.In this thesis,high-performance nano-MoS₂was prepared by freeze-drying method to regulate the crystalline structure of nanoparticles under different reaction conditions and process parameters.Ammonium molybdate and sodium molybdate was used as molybdenum sources,and thiourea and thioacetamide as sulfur sources,the nano-MoS₂precursor solution was prepared,and flake and spherical crystal nano-MoS₂was obtained by freeze-drying method.The composition and micromorphological structure were characterized by XRD and SEM,and the effects of reaction conditions and process parameters were studied,containing reaction time,reaction temperature,p H and freezing rate on the micromorphological structure were studied.The results show that the reaction time of optimal process for flaky nano-MoS₂is 24h for reaction time,220°C for reaction temperature,and suitable freezing method is liquid nitrogen freezing;the optimal process of spherical nano-MoS₂is the reaction time of 12h,the reaction temperature of 220℃,the p H of the reaction system is 3,and the liquid nitrogen is used for freezing.The tribological properties and dispersion stability of freeze-dried nano-MoS₂was investigated in the lubricating oil system.Its lubricating mechanism was studied,and the results show that the dispersion stability of the freeze-dried nano-MoS₂oil sample is more stable than that of the commercial nano-MoS₂oil sample.The freeze-dried nano-MoS₂oil sample has better extreme pressure resistance grinding performance than tht of the base oil.When the optimal concentrations of flaky and spherical nano-MoS₂are 3.0wt%and 2.5wt%,respectively.The average friction coefficients of flaky and spherical nano-MoS₂are 29.3%and 37.9%,lower than that of paraffin oil,respectively.The average friction coefficient of spherical nano-MoS₂decreased by 17.7%and 33.6%,respectively.And the average friction coefficient of spherical nano-MoS₂was 12.2%lower than that of flake nano-MoS₂.Spherical nano-MoS₂shows better lubricating effect than flake nano-MoS₂.In addition,freeze-dried nano-MoS₂can shorten the running-in period of lubricating oil.Under the same condition,the anti-frictional performance of nano-MoS₂prepared by liquid nitrogen freezing is better,and the friction coefficient of nano-MoS₂prepared by freezing at-20°C is reduced by 15.4%.The main reason is that the freezing rate of liquid nitrogen is faster when the size of nano-MoS₂is more uniform.It is found that both flake and spherical nano-MoS₂have better lubricating properties under heavy loads and high speeds through investigating the friction properties of nano-MoS₂oil samples under different loads and sliding speeds.Spherical and flaky nano-MoS₂nanofluid models were established by molecular dynamics theory,and the tribological behavior between Fe-Fe friction pairs was simulated,moreover,the lubrication mechanism of nano-MoS₂was explained.The simulation results show that the flaky nano-MoS₂has translational motion between the friction pairs,which can be deposited on the surface of the Fe plate to form friction films.The spherical nano-MoS₂has both translational and rotational motions between the sliding tribo-pairs,which verifies the"ball effect",further showing better anti-friction and anti-wear performance.In addition,nano-MoS₂is probable to exert interlayer flow and ball effect under larger load and translation speed,which is consistent with the tribological results of the as-prepared nano-MoS₂particles.