Study On The Effect Of Friction Reducing And Anti-wear Additives On The Rheological And Tribological Properties Of Polyurea Grease

According to the basic performance of grease,urea base is selected as thickener of grease.Additive is an important means to improve the friction and anti-wear performance of grease.In contrast to polyurea grease without additives,and according to the friction and anti-wear performance of additives,four kinds of friction and antiwear additives,zinc dialkyl dithiophosphate(ZDDP),potassium borate(PB),benzotriazole octadecylamine(BTA),molybdenum dialkyl dithiophosphate(Mo DDP)and ZDDP/PB,ZDDP/Mo DDP,BTA/PB and PB/Mo DDP,are selected as additives for polyurea grease.four additive packages as additives for polyurethanes.The tribological performance of the grease is further improved by the synergistic effect between the additives.The effects of single and compound additives on the rheological properties and microstructure of grease were verified by rheometer and SEM.The additives cause changes in the microstructure of the grease thickener and also cause an increase in the apparent viscosity,shear stress,structural strength,energy storage modulus and loss modulus of polyurea grease.The stronger the polarity of the additive,the more obvious this phenomenon is,and the greater the degree of entanglement of the thickener fibers and the platform modulus of the polyurea grease.When the friction reducing and antiwear compound additives act simultaneously,the thickener structure and rheological properties change significantly,and the structural strength of the grease increases significantly.The tribological properties of the grease at room temperature and under different loads were investigated by a friction and wear testing machine(SRV).At room temperature(30°C),PB/Mo DDP grease showed better friction reduction,anti-wear performance and extreme pressure performance under variable load than other greases,and its average friction coefficient was reduced by 31.5-40.1% and wear rate was reduced by 45.2-62.1% under different loads.The anti-wear performance of BTA grease and ZDDP/Mo DDP grease at room temperature was excellent,and their wear rates were reduced by 11.3%-79.2% and 35.5%-77.3% under different loads.At room temperature,abrasive wear,fatigue wear and adhesive wear mainly occur on the wear surface of steel balls.The greases containing friction reducing and anti-wear additives all have excellent anti-wear performance and reduce the abrasive wear phenomenon on the wear surface.Next,whether the synergistic effect of additives remained effective at high temperatures(130°C)and the tribological performance of the additive combinations under various contact loads were compared.Compared with room temperature,the friction reduction effect of different greases at high temperature decreased and the flaking of wear surface increased.mo DDP grease and PB/BTA grease showed better friction reduction performance at high temperature,and their average friction coefficients were reduced by 9.1%-28.1% and 16.1%-30.3%,respectively.Under high temperature and low load(130℃,1.68GPa),PB/MODDP grease has excellent friction reduction performance,anti-wear performance and extreme pressure performance under variable load,and its average friction coefficient is reduced by 43.2% and wear rate is reduced by 43.1%;BTA grease,PB/BTA grease and ZDDP/BTA grease have better antiwear performance,and their wear rates are reduced by 66.8%,72.1% and 50.1 %respectively,72.1% and 50.1% respectively.Under high temperature and high load,Mo DDP grease and PB/BTA grease had excellent friction reduction performance,and their average friction coefficients were reduced by 9.1%-28.1% and 16.1%-30.3%,respectively.Under normal temperature and high temperature and low load,PB/Mo DDP can meet the requirements of friction reduction and anti-wear of polyurea grease at the same time.However,at high temperature and very high load(>1.68 GPa),both the friction reduction and anti-wear additives provided in this thesis cannot meet the requirements of friction reduction and anti-wear performance of polyurea grease at the same time.The elemental content and generated products on the wear surface were analyzed by scanning electron microscopy(SEM),energy spectrometry(EDS)and X-ray photoelectron spectroscopy(XPS)to determine the synergistic mechanism of PB/Mo DDP additives with excellent tribological properties.The results indicate that physical adsorption and complex tribochemical reactions are involved in the friction process.The excellent tribological properties are attributed to the complex tribological film produced during the friction process,mainly composed of molybdenum disulfide,molybdenum oxide,sulfide,phosphate and other metal oxides.At high temperatures,the content of metal phosphates and molybdenum disulfide decreases,the content of metal oxides rises,the adsorption capacity of additives decreases,and the instability of the friction film rises,making the anti-wear and friction-reducing capacity of the grease decrease and the wear of the metal surface increase.