Preparation And Tribological Behavior Of Oil-Miscible Tungsten Disulfide And Its Nanocomposites Over Wide Temperature Range

With the rapid development of mechanical equipment industries in recent years,the lubrication of mechanical systems under severe conditions involving high temperature and high load is increasingly arousing concerns and requiring lubricant additives to work over a wide temperature range so as to achieve desired fuel economy in association with efficient environmental protection.Conventional antiwear and friction-reducing additives,unfortunately,are often less competitive in this respect.WS2 nanoparticles have good thermal stability,mechanical stability and chemical stability.As antiwear and friction-reducing additives of lubricating oil,they also exhibit excellent tribological properties,which adds to their value of research and application in lubrication field.This research uses high temperature liquid phase thermal decomposition method,mild condition liquid phase method and spray drying-solid phase thermal decomposition method to prepare a series of oil-miscible tungsten disulfide nanosheets,oil-miscible tungsten disulfide/nickel oxide nanocomposites and oil-miscible reduced graphene oxide/tungsten disulfide nanocomposites.It also deals with the tribological behavior and lubrication mechanism of oil-miscible tungsten disulfide and its nanocomposites as lubricant additives over a wide temperature range.The main contents and conclusions are as follows:1)Oil-miscible WS2 nanosheets were prepared by in-situ surface modification in association with high temperature liquid phase thermal decomposition route.The oil-miscible WS2 nanosheets were obtained through the liquid phase thermal decomposition of ammonium dithiotungstate precursor at 350? in the presence of surface modifier oleylamine(OA).This preparation method avoids the emission of highly toxic H2S gas in the traditional preparation of oil-miscible WS2 nanoparticles.The thermal stability of oil-miscible WS2 nanosheets and their dispersion stability in lubricating oil were investigated.The tribological properties of the additives in poly-alpha olefin(PA06)base oil from room temperature to 200? were evaluated.Findings show that OA-modified tungsten disulfide nanosheets as the lubricant additive can significantly improve the antiwear and friction-reducing properties of PA06 from room temperature to 200?.The reason lies in that OA-modified WS2 nanosheets as the lubricant additive can form physical adsorption lubrication film and tribochemical reaction film on the rubbed surfaces of the frictional pairs.Therefore,OA-modified WS2 nanosheets as the lubricant additive of PA06 are favorable for providing continuous lubrication over a wide temperature range especially under elevated temperature.2)Oil-miscible WS2 nanosheets and oil-miscible NiO/WS2 nanocomposites were prepared by thermal decomposition of the mixture of ammonium dithiotungstate and nickel formate at 350?,with the hope to produce lubrication film on rubbed surfaces of frictional pairs so as to achieve continuous lubrication under oil-lack working condition owing to the failure of lubrication system.The tribological properties of the as-prepared lubricant additives and zinc dialkyldithiophosphates(ZDDP)under oil lubrication and oil-lack lubrication conditions were evaluated in a ball-on-disk contact mode.The mechanism of the sustained lubrication of oil-miscible WS2 nanosheets and oil-miscible NiO/WS2 nanocomposites under severe oil-lack conditions was analyzed.Findings demonstrate that,different from ZDDP,the as-prepared oil-miscible WS2 nanosheets and NiO/WS2 nanocomposites can maintain good antiwear and friction-reducing properties under oil-lack lubrication condition;and the NiO/WS2 nanocomposites exhibit better tribological properties than WS2 nanosheets.This could be because the NiO-encapsulated WS2 nanosheets are able to form a dense and cohesive lubrication film on the rubbed surfaces of the frictional pairs.As a result,the direct contact of the frictional pairs under severe oil-lack condition is effectively avoided,and the antiwear and friction-reducing abilities are highly improved.The as-prepared NiO/WS2 nanocomposites as the lubricant additive could be of significance for maintaining the stable and smooth operation of machines and equipments facing lubrication system failure under severe working condition.3)With a view to the disadvantage of the oil-miscible WS2 nanosheets prepared by high temperature liquid phase thermal decomposition method,a mild liquid phase method(under relatively lowered temperature of 200?)was adopted to fabricate oil-miscible WS2 nanosheets with tungsten hexachloride and thioacetamide as the raw materials.The tribological properties of the product as lubricant additive from room temperature to 300? were evaluated,and its lubrication mechanism was analyzed.Four-ball friction and wear test results show that OA-modified WS2 nanosheets obtained by the mild liquid phase method can considerably enhance the friction-reducing and antiwear abilities of PA06 oil from 75? to 200?.The sliding tests of piston ring-cylinder liner demonstrate that the as-prepared lubricant additive exhibits better antiwear ability from room temperature to 300? than ZDDP.This is because OA-modified WS2 nanosheets as the lubricant additive in PA06 can form a protective and lubricious layer via adsorption on worn steel surface while they can also participate in tribochemical reaction to afford tribochemical reaction film composed of WO3,FeSO4,FeS,and Fe3O4 thereon.The as-prepared OA-modified WS2 nanosheets as the potential substitute for ZDDP could find promising application in the development of high-performance and high-temperature engine oil.4)With the assistance of the good dispersibility of crumpled graphene,dispersible reduced graphene oxide(RGO)/WS2 nanocomposites were prepared by spray drying-solid phase thermal decomposition so as to increase the thermal stability of oil-miscible WS2 nanosheets.The tribological properties of the as-prepared RGO/WS2 nanocomposites over a wide temperature range(from room temperature to 300?)were evaluated in a piston ring-steel plate contact mode.Results indicate that the dispersible RGO/WS2 nanocomposites have better thermal stability than WS2 nanoparticles prepared by in-situ surface-modification method,exhibiting a weight loss of 2.2%at 800?.In the meantime,the dispersible RGO/WS2 nanocomposites exhibit good antiwear and friction-reducing properties from room temperature to elevated temperature of 300?.Particularly,they exhibit better antiwear and friction-reducing properties than RGO under elevated temperatures,and they also have good synergistic action with the antiwear and friction-reducing additives of engine oils.This is because dispersible RGO/WS2 nanocomposites in PA06 can form physical adsorption film with RGO and WS2 as the main components and tribochemical reaction film composed of WO3,FeSO4 and iron oxides on the rubbed surfaces of the frictional pairs.The synergistic action of the physical adsorption film and tribochemical reaction film accounts for the excellent tribological properties of the dispersible RGO/WS2 nanocomposites as the lubricant additive of PA06 over a wide temperature range.