Tribological Investigation Of Zinc Sulfide And Its Derivative As Lithium Grease Additives

With the development of modern industry,traditional lubricants can not meet the requirements under harsh working conditions.Solid lubricants can enhance the friction-reducing,anti-wear and extreme pressure properties,to make up for the defects of lubricating grease in harsh conditions.Metal sulfides are an important solid lubricant additives,among which molybdenum disulfide has been widely used in various lubricant products due to its effective friction and wear reducing properties.However,molybdenum element is rare metal element on the earth.With the sustainable development of global resources,it needs to find abundant elements to prepare excellent solid lubricant additives for supplementing and enriching lubricant products.ZnS is a widely used metal sulfide materials.Now ZnS usually be used as an additive in oils or making coatings,little research has been reported in lubricating greases.In this paper,layered ZnS·?en?_0.5?ethylenediamine,abbreviated en?,micron-sized ZnS and micro-sphere ZnS were synthesized to meet the requirements as grease-additives.A four-ball tester and Optimol SRV-V oscillating reciprocating tester were employed to investigate the tribological properties.The worn surfaces were analyzed by SEM,EDS,3D non-contact surface mapping profilerand XPS.The main contents and conclusions are as follows:1.Layered ZnS·?en?_0.5(ZnS·?en?_0.5-sheet)with a particle size of 3⁵?m was prepared in the ZnCl₂-S-ethylenediamine system.Micron-sized ZnS?ZnS-sheet?with the same particle size and morphology as ZnS·?en?_0.5-sheet was prepared by annealing ZnS·?en?_0.5-sheet.Micro-sphere ZnS?ZnS-ball?with a particle size of 3⁵?m?stacked from 100 nm to 300 nm nanosheets?was prepared in the Zn?NO₃?₂-CH₄N₂S-C₄H₁₃N₃-H₂O system.2.The tribological properties of ZnS·?en?_0.5-sheet and ZnS-sheet were studied by a four-ball tester.The results show that ZnS·?en?_0.5.5 and ZnS can improve the maximum non-seizure load?P_B?and sintered load?P_D?of base grease.The P_B value and P_D value for 5.0 wt.%ZnS·?en?_0.5 are 882 N and 1568N,respectively.The P_B value and P_D value for 5.0 wt.%ZnS are 1372 N and1961 N,respectively.ZnS is superior to ZnS·?en?_0.5.5 in improving extreme pressure property.Under 392 N-300 rpm-120 min test condition,the WSD values for ZnS·?en?_0.5 grease and ZnS grease is 0.53 mm and 0.78 mm,respectively.Under 490 N-1450 rpm-120 min test condition,the WSD values for ZnS·?en?_0.5 grease and ZnS grease is 0.88 mm and 1.03 mm,respectively.ZnS·?en?_0.5 is significantly better than that of ZnS in anti-wear performance.3.From the SEM and 3D morphology of the wear surface of the steel ball,the wear surface lubricated by ZnS·?en?_0.5.5 grease has less and shallower furrow than that of ZnS grease.From XPS analysis results,both ZnS·?en?_0.5 and ZnS have complex frictional chemical reactions on the surface of steel ball and form tribological chemical reaction film.According to the crystal structure,when the crystal is subjected to extrusion,layered ZnS·?en?_0.5 with interlaminar pillared ethylenediamine has a certain elastic space between the layer,and make it easily shear,so ZnS·?en?_0.5 exhibit superior antiwear performance.For ZnS crystal,the Zn atoms and S atoms on each layer were closely packed and strongly bonded to each other,the packed interlayer spacing make it difficult to slip between interlayer,but the crystal strong interplanar bond forces can provide ZnS higher carrying load.Therefore,the difference of the crystal structure of ZnS·?en?_0.5and ZnS makes differences in the lubricating properties.4.The Optimol SRV-V oscillating reciprocating tester was employed to study the tribological properties of ZnS-ball and ZnS-sheet.Under the frequency of 50 Hz and load ranged from 300⁶00 N,the wear volume of ZnS-ball grease and ZnS-sheet grease is?5.72¹3.83?±0.57×10^-4 mm³,?21.46¹82.90?±2.15×10^-4 mm³,respectively.Under the applied load of 300 N and frequency ranged from 20⁵0 Hz,the wear volume of ZnS-ball grease and ZnS-sheet grease is?4.99⁷.58?±0.50×10^-4 mm³,?4.57²2.05?±0.46×10^-4 mm³,respectively.With the increase of load and frequency,ZnS-ball still keep the stable and lowest wear volume,and wear resistance properties is always better than that of ZnS-sheet.5.From the SEM and 3D morphology analysis of the wear scar of the lower disk,the wear scar for ZnS-ball grease is less and smoother,while the surface furrow and scratches of the ZnS-sheet grease is more and deeper.This may be due to the fact that ZnS-ball particles like balls can reciprocate roll to the surface of the friction pair and rapidly fill the gap between the ball and disk to prevent direct contact between friction pairs,especially in high-frequency motion.In high-load motion,part of the microsphere ZnS stacked with nanosheets will be extruded into nanosheets,which can be filled into the concave and convex to in time to protect the friction pair.For micro-sheet ZnS particles,they cannot quickly slip into contact surface,and only a little particles stay in the friction pair surface,causing the wear volume to increase as the load and frequency increase during high-frequency reciprocating motion.