Preparation And Tribological Properties Of Nano Magnesium Silicate Hydroxide Based Lubricating Oil Additives

All mechanical moving parts will involve friction,wear and lubrication,and friction consumes 1/3 to 1/2 of the world’s primary energy.About 60%of mechanical parts fail or fail due to wear or poor lubrication caused by friction.Lubrication can reduce unnecessary energy consumption and improve the efficiency of industrial production and machine operation.As the main way to improve mechanical lubrication,lubricating oil has always been a research hotspot in the fields of energy,machinery,materials and so on.Lubricating oil is generally composed of base oil and additives.Anti-friction and anti-wear additives are used by most lubricating oils because they can reduce the friction and wear of friction pairs,reduce the direct contact of wear surfaces,save energy consumption and eliminate vibration noise.As a natural and green antifriction and anti-wear additive,serpentine is mainly composed of Magnesium Silicate Hydroxide（MSH）,which is a silicate mineral composed of Si-O tetrahedron and Mg-O octahedron.The mineral has a layered crystal structure,which is easy to slide between layers under the action of external force,which makes serpentine have good antifriction and wear resistance.However,the engineering application of serpentine as lubricating oil additive is limited by the following disadvantages:regional differences,uneven size and various impurities.Moreover,its complex composition also masks the real anti-wear mechanism of serpentine.Based on the above engineering and scientific problems,this study adopts the artificial method to synthesize MSH,and on this basis,it is compounded with molybdenum disulfide（Mo S₂）to give the lubrication performance of MSH,so as to improve the anti-friction performance of MSH as a whole.In addition,the hydrothermal synthesis of MSH-Mo S₂ also provides a new idea and way for the controllable preparation of Mo S₂ materials.The specific research contents are as follows:（1）Using Mg Cl₂·6H₂O and Na Si O₃·9H₂O as precursors and Na OH as p H regulating medium,MSH nanoparticles with uniform particle size and about 20 nm diameter were obtained by setting hydrothermal temperature of 200℃and hydrothermal time of 24 h.When MSH nanoparticles are used as lubricating additives,the friction coefficient and wear rate of MSH oil samples with the addition amount of 1 wt.%are the lowest.In the friction process,MSH nanoparticles play the role of rolling bearing and are physically adsorbed on the wear surface.During the friction process,MSH nanoparticles decompose and tribo-chemical reactions occur to form a friction film mainly composed of MSH,Fe₂O₃,Fe₃O₄ and graphite.（2）MSH-Mo S₂ nanoparticles were synthesized by step-by-step hydrothermal method.The microstructure of the particles is layered Mo S₂ surrounded by MSH nanotubes.The results of friction and wear test show that PAO-MSH-Mo S₂ oil sample has the best anti-friction and wear resistance compared with PAO,PAO-MSH and PAO-Mo S₂.Compared with PAO,the anti-friction and anti-wear properties of GCr15 ball disk lubricated by composite powder oil sample are improved by 27.7%and 37.4%respectively.In addition,the results of long-term test and variable load test show that the anti-friction and anti-wear characteristics of PAO-MSH-Mo S₂ are suitable for all working conditions in this test.The excellent tribological properties of MSH-Mo S₂ are attributed to the formation of a special friction film on the wear surface,which is mainly composed of Mo S₂,Mo O₃,MSH,Fe₂O₃,Fe₂（SO₄）₃ and graphite.Compared with the wear mark surface lubricated by GCr15 disc and other oil samples,the friction film has the highest hardness,higher elastic modulus and maximum plastic deformation resistance.（3）MSH-Mo S₂ nanoparticles have good self-repairing performance.After five hours of friction test,PAO-MSH-Mo S₂ oil sample repaired 30%of the wear volume of GCr15 disc.According to the self-repairing test results and the analysis of the morphology and composition of the worn surface,it is speculated that the self-repairing mechanism of MSH-Mo S₂ is that the nanoparticles dispersed in Poly Alpha Olefin（PAO）are captured by the pits generated by the original wear,and form a protective film with the diffusion and chemical reaction with other elements,so as to reduce the friction coefficient,improve the wear resistance and repair the surface damage.（4）By adjusting the reaction temperature and the thermal decomposition characteristics of chitosan,nanoparticles with different morphology and structure were obtained.The nano particles（MM2）synthesized at a hydrothermal temperature of 220℃gain their bonding strength due to the pinning effect at the edge of the incoherent interface between MSH and Mo S₂.In addition,due to the abundant active sites on the surface of MM2,it shows good dispersion stability in oil samples.（5）The composite particles with different morphology and structure were added into Pao as lubricating oil additives,and the anti-friction and anti-wear properties of the particles were tested.Compared with PAO,the friction coefficient of GCr15 steel ball under PAO-MM2 lubrication decreased by 44.8%,the average wear spot diameter decreased by 41.2%,and the maximum non seizure load increased by 116.6%.In addition,due to the good cooperation between MSH and Mo S2,a unique double-layer friction film is formed on the wear surface.The upper layer of the friction film is mainly composed of Mo S₂,and the lower layer mainly includes MSH,Mo O_x,Fe₂O₃,Fe S and Fe₂（SO₄）₃.The friction film with this special structure has good friction reduction and anti-wear effect and protects the friction pair well in the friction process.