Preparation,Rheological Behaviors And Tribological Properties Of Surface Modified Nano-SiO₂ Greases

As a kind of inorganic nanomaterial,nano-silica is widely used in national defense transportation,petrochemical industry,daily chemical production,medicine and so on.It has been found to have excellent tribological properties due to its unique structural characteristics,excellent physicochemical properties and environment-friendly characteristics.Because of its strong designability and industrialization,nano-silica gradually become a common component of lubricating materials,especially as the lubricating oil additive which has been widely studied.However,as a grease component,there is relatively little research on nano-silica as a thickener,which restricts the development and application of high-performance nano-silica greases.How to systematically study the rheological behaviors of nano-silica greases and obtain the system’s characteristic relationship and tribological mechanism is the key to solve the above problems.Based on this,the surface modified nano-silica produced by our research group is selected to study the structural role of nano-silica in the system as a grease thickener,as well as the preparation process,synthesis mechanism and performance characteristics of nano-silica greases.The effects of the content of thickener,type of base oil and viscosity of base oil on the rheological behaviors and tribological properties of grease are investigated,and the lubrication mechanism is also discussed.The main research contents and conclusions are as follows:(1)The effect of the addition amount of nano-silica as thickeners on the performance of greasesFive greases with the mass fraction of thickener of 12%,14%,16%,18% and 20% were prepared using nano-silica as thickeners.The influence of nano-silica addition amount on rheological behaviors and tribological properties of grease was investigated.The results showed that the colloidal stability and structural strength of the grease were improved with the increase of nano-silica content,and the viscosity-temperature properties and thixotropy were also changed.The tribological results showed that the anti-friction properties of greases with different thickeners content were correlated with the test temperature.In addition,the low-temperature fluidity test found that the increase of grease flow pressure was basically the same among different penetration grades.(2)The effect of base oil type on rheological behaviors and tribological properties of nano-silica greasesNano-silica grease was prepared with paraffin oil,naphthenic oil,poly-α-olefin(PAO),and polyol ester oil(POE),respectively.The effects of base oil types on the rheological behaviors and tribological properties of grease were investigated.The result showed that the grease could not be successfully prepared with the same experimental materials and preparation process when the polyol ester oil was the base oil.Among the other three successfully prepared greases,the paraffin-based grease had the best colloid stability.The tribological test results showed that the tribological properties of grease were different from the corresponding base oil,and the response of different greases to load was also different,which may be caused by the structure change of the system after preparing into grease,and the participation of nanoparticles in the friction process.The low-temperature fluidity test showed that PAO-based grease had the best pumpability at low temperatures.(3)The effect of base oil viscosity on rheological behaviors and tribological properties of nano-silica greasesFour kinds of nano-silica grease were prepared by four kinds of poly-α-olefin(PAO4,PAO10,PAO40,PAO100)with different viscosities as the base oil.The effect of temperature on rheological behaviors and tribological properties of grease was investigated.The results show that the physicochemical properties of greases change regularly with the increase of viscosity of base oils.Rheological tests show that the structural strength of greases can be improved by increasing the viscosity of base oils,which is mainly related to the stability of the internal skeleton structure.At the same time,it was presented that the thixotropy of greases prepared from low-viscosity base oils was more easily affected by temperature,which was mainly related to the damage to the colloid structure.Besides,the tribological results showed that proper temperature rise was beneficial to improve the anti-wear and anti-friction properties of the grease,and the higher the viscosity of the base oil was,the better the tribological properties of the corresponding grease were.However,the tribological performance of the grease gradually deteriorates with the temperature continued to elevate.In addition,there was a temperature point which made the grease prepared from low-viscosity base oils have better tribological properties.This was mainly related to the dispersion of the aggregate particles and the formation of the deposition film.