The Preparation And Tribological Properties Of Magnesium Silicate Hydroxide/C Composite Nanoparticles

Friction reduction and anti-wear additives are one of the research hotspots in the field of tribology.Zinc dialkyl dithiophosphate(ZDDP),as the traditional anti-wear additive,has been developed for more than 70 years and was seen as the most successful additives.However,with the rapid development of technology,the higher demand for additive in more severe working condition push people to find the specific additive with higher tribological performance,and meanwhile should be environment friendly.Magnesium silicate hydroxide(MSH)as the main component of natural serpentine mineral has been attracting more attentions for its outstanding tribological properties.In this paper,we try to control the morphology of MSH/C by adding organic acid in its synthetic process.MSH/C composite powders were prepared by the decomposition of organic acid in subcritical hydrothermal conditions.By the characterization of composite powders,it was found that the addition of oleic acid(OA)in the synthetic process could not only prevent the occurrence of Mg(OH)₂ impurity but also promote the flake morphology transferring to the granular morphology as compared with pure MSH powders.Besides,the decomposition products of OA in subcritical hydrothermal conditions could lead to the occurrence of a coating layer composed of amorphous carbon and carbon cluster.This coating with a thickness of about 3-5nm can be coated on the particles with granular morphology,which is also the reason why it can inhibit the growth of particles with tubular morphology.The different addition ratio of OA in the synthetic process responds to different carbon-related content in the final prepared powders.By the characterization of FTIR,Raman,TG-DSC analysis,and RMD simulation,it is believed the C-C bond at allylic positions and the C-C bond adjacent to the carboxyl group could cleavage for its lower bond dissociation energy,which caused the presence of large amounts of hydrocarbons with short-chain.Some of these initial cracking products could go through the further cracking to form smaller molecules.Some of these short-chain hydrocarbons polymerized to form graphite ring structure and finally formed carbon clusters,and the other part was disorderly distributed,finally forming an amorphous carbon structure.For friction-reducing and antiwear properties of MSH/C as the lubricant additive,we discussed the influence of the morphology,particle size,the amount of oleic acid added,and the friction working condition.The four-ball experiments show that under mixed lubrication region,the tribological performance of the powder mainly depends on two relationships:the relationship between the particle size and the thickness of the oil film,and the relationship between the friction work and the tribochemical reaction.Compared with pure MSH powders,the worn surface lubricated with oil containing MSH/C composite powders shows higher organic carbon content,as XPS results indicated,which participate in the building of tribofilm and lead to the better tribological performance.Besides,the composite powders with different morphologies and sizes under different synthetic conditions show an obvious difference in tribological performance.To compare the possibility of particles with different morphologies entering the contact area,equivalent diameter(De)was introduced to transform morphology difference into the particle size difference,which can be used to compare the possibility of entering the friction contact area.By comparing the four-ball friction test under two working condition with a large difference in oil film thickness,it is believed that when the De of the powder is close to or less than the center oil film thickness(h_c),the powder is more likely to enter the friction contact area,and then undergo a rolling-deformation-destroy-decomposition process.We also tested the composite with a certain size under different working conditions.The results show that the added composite powder exhibits better anti-wear and anti-wear performance than the base oil under different working conditions.In terms of wear scar diameter(WSD),the anti-wear rate can be up to 55%.For the composite powder,when the load is constant,the friction coefficient shows a significant decrease trend with the speed increases.Moreover,when the load reaches400N,the wear scar diameter shows a steadily decreasing trend with increasing speed,which is opposed to the variation trend of PAO base oil.We believe that this is not only due to the increase in the oil film thickness caused by the increase in rotational speed,which is conducive to the tribological performance of the composite powder,but also because of the influence of the frictional heat caused by the changes in the working conditions.It can be confirmed by the changing trend of the wear reduction rate:as the speed increases,the turning point of the anti-wear rate from falling to rising is moving toward the small load direction.It is known that MSH/C composite powder exerts anti-wear and friction reduction through physical functions including adsorption,spreading,filling and deposition,and tribochemical reaction between the powder and the contact area.The protective film formed by physical action will be destroyed due to the gradual increase of the load or speed,resulting in a decrease in the protective ability,which reflects a decrease in the anti-wear ability,but on the other hand,the harshness of this working condition is beneficial for the tribochemistry.The friction contact area will generate more friction energy in the form of friction heat due to friction,which will destroy the structure of the MSH/C composite powder and release a large amount of Mg-O,Si-O active groups and the surface-coated carbon layer,which will cause the increasing trend after reaching the inflection point on the curve of the wear reduction rate.The multilayer films on the substrate surface were prepared through the Burnish process,which includes the bottom Sb₂O₃ layer and the top MSH/C composite powder layer.The tribological performance of this solid film was tested by a ball-on-disk tester.Through the characterization of the worn surface,it was found that the Sb₂O₃ layer can effectively isolate oxygen and prevent the oxidation of the substrate under high-temperature conditions.In addition,the rough surface brought by the Burnish process allows it to accommodate more composite powders in the initial running-in phase.Under high temperature,this Sb₂O₃ layer becomes ordered with the effect of frictional stress,which makes the friction coefficient of the contact area drop rapidly(could be reduced to below 0.1).At 300°C,with the support of Sb₂O₃ layer,the top MSH/C composite layer decomposes,releasing active groups and carbon.Analysis of the films further supported the findings of the study suggesting that the superlibric nature of the coating is a synergistic effect of the antimony oxide adhesion layer,tribocatalytically-active MSH powders,and tribocatalytically-generated surface carbon film.