Hydrothermal Synthesis And Tribological Properties Of Magnesium Silicate Hydroxide Nanomaterial

To achieve minimum friction and wear,anti-wear additives as one type of the most important lubricant additives has been widely studied by investigators.Meanwhile,developing a new-fashioned "green" additive to replace or partly replace zinc dithiophosphate(ZDDP)and molybdenum dialky dithioearbanate(MoDTC)has been a pressing issue of the moment due to the increasingly severe environmental requirements.Magnesium silicate hydroxide(MSH),as the main component of serpentine-group minerals,is a potential lubricant additive owing to its unique layered structure of alternately stacked silicon-oxygen(Si-O)tetrahedral sheets and magnesium-oxygen/oxyhydrogen(Mg-O/OH)octahedral sheets.Because of the weak bonding force between layers,under the condition of certain pressure and temperature,MSH is easy to occur cleavage and phase transition along the direction of bedding plane,meanwhile releases unsaturated Si-O-Si,O-Si-O,OH-Mg-OH(O),OH-and O-H-O.As a result,a tribofilm forms on the sliding surfaces and reduces wear significantly.However,nano-sized MSH powders are scarcely prepared by high-energy mechanical ball-milling which is not well suited for applications as lubricant additives,and clarification of its tribological mechanisms due to varying morphology and difficulties controlling compositionSynthesizing MSH nanoparticles intended to solve this problem as well as to open path for further performance optimization.In this study,hydrothermal synthesis of MSH nanoparticles was first carried out using silicon dioxide(SiO2)and magnesium oxide(MgO)as raw materials,and NaOH aqueous solution as a medium.The hydrothermal conditions including temperature,pressure,reaction time and stirring speed were controlled by a high-temperature and high-pressure reactor.On the basis of these works,the tribological properties of MSH nanoparticles were investigated.To further improve the anti-wear property of MSH nanoparticles,we combined them with carbon-based materials and synthesized MSH/C composite nanoparticles using graphite as another type of precursor.This simple solution-phase approach also offers a greater possibility in the large-yield,environmental and lattice-perfect produce of graphene.Scanning electron microscope(SEM),transmission electron microscope(TEM),energy dispersive spectroscopy(EDS),X-ray diffractometer(XRD),X-ray photoelectron spectroscopy(XPS),X-ray fluorescence spectrometer(XRF)and Fourier infrared spectroscopy(FTIR)were used to characterize the morphologies,crystal structures and chemical compositions of MSH and MSN/C nanoparticles.A four-ball friction and wear tester and a high-temperature ball-disk friction and wear tester were utilized to test the tribological properties of polyalphaolefin(PAO)suspended synthetic MSH or MSH/C nanoparticles.The experimental conditions included the microstructure and the weight percentage of nanoparticles,applied load,rotational speed and temperature.Morphologies and chemical compositions of the worn surfaces were analyzed by a three-dimensional profile equipment and SEM equipped with EDS.Based on the above experimental results and analyses,we carried out a series of dichromatic interference tests,and revealed the anti-wear mechanisms of synthetic MSH and MSH/C as additives in lubricant oil.In a NaOH solution and under a high-temperature and high-pressure condition,the blend of MgO and SiO2 raw materials can generate ions and agglomerates with a certain concentration.These ions and agglomerates transport continuously to the initial MSH crystal nucleus.And with the growth of the reaction system time,a large amount of crystals will be self-nucleated,crystallized and grow,which contributes to the preparation of MSH nanomaterial.In this process,the addition of the graphite can make the carbon atoms overcome the van der Waals force interaction between the layers,and bond with the active oxygen and/or the hydroxyl groups in MSH.A few-layer graphene,therefore,exfoliates from the graphite and coats on the surfaces of MSH nanoparticles.It connects with MSH via C-OH,C=O(C-O)and C-H bonds.Besides,hydrothermal temperature and time are the main factors affecting the performances of MSH and MSH/C.MSH nanoparticles as additives in oil exhibit excellent anti-wear performance,and even result in infinitesimal wear under a maximum Hertzian contact pressure of 2.71GPa.Their anti-wear mechanisms are various depending on the contact pressure.Under a relative mild pressure,MSH nanoparticles can effectively increase the oil film thickness,thus to achieve a substantial reduction in wear.As the test pressure becomes severer,a tribofilm containing Mg,Si and O would form on the worn surface to provide a secondary anti-wear guarantee.On the other hand,using MSH/C composite nanoparticles as additives in oil can further improve the anti-wear property of MSH.And the worn surfaces had almost no wear even under a maximum Hertzian contact pressure of 3.91 GPa.This can be attribute to the formation of a tribofilm consisting of nanocrystalline graphite and amorphous carbon(a-C)forms on the sliding surfaces no matter how the contact pressure changes.