Preparation And Tribological Performances Of Nanoparticles Based On Magnesium Silicate Hydroxide

With the development of high-speed and over-loading operating conditions of machinery and the increasing awareness of environmental protection,research of nanoparticles as lubricant additive in the field of tribology has been widely paid attention by researchers all over the world.The magnesium silicate hydroxide（MSH）is the main chemical composition of natural serpentine minerals with layered silicate structure.The artificial synthesis of MSH and its related materials as lubricant additives are not only conducive to overcoming the disadvantages of uncertainty impurity of natural minerals,but also promising to solve the environmental pollution problem of anti-wear additive zinc dialkyldithiophosphate（ZDDP）existing in commercial lubricant.Diamond like carbon（DLC）is a kind of amorphous carbon（a-C）material with excellent mechanical and tribological properties.Therefore,the combination of MSH and a-C is of great significance to improve its tribological properties as lubricant additive.In this paper,the core-shell nanocomposite particles MSH@C:F were prepared on the basis of synthetic MSH.Polytetrafluoroethylene（PTFE）can be decomposed into fluorine-doped amorphous carbon（a-C:F）coated on MSH surface under subcritical hydrothermal conditions.The a-C:F can significantly inhibit the formation of MSH with tubular morphology by influencing the crystallization process of MSH,and the increase of temperature,p H value and reaction time are conducive to the defluoridation of PTFE.Ab-initio molecular dynamics（AIMD）and classical molecular dynamics（CMD）were used to simulate the defluoridation of PTFE.The results show that the C-F bond in PTFE is attacked by Na OH,which results in the introduction of C=O group to the carbon site bound with removed F atom,and the core structure MSH can promote the above-mentioned defluoridation process through surface"dissolution".The tribological performances and mechanism of MSH@C:F as lubricant additive were studied in this paper.The MSH@C:F shows excellent anti-wear performance and friction stability under 3.91 GPa,and is superior to fully formulated commercial lubricant 5W30 containing with ZDDP and to pure MSH nanoparticle.The results show that the external cladding a-C:F of MSH@C:F can form a protective tribofilm on the worn surface with high graphitic structural order,which results from friction-induced rehybridization（dehydrogenation and defluorination）and adaptively sliding oriented rearrangement.Moreover,the degree of friction-induced rehybridization（FRD）positively correlated with anti-wear rate（AR）.In addition,the F C and O C ratios of a-C:F affect its lipophilicity and the adsorption stability to worn surface,and there exists an equilibrium point,which provides reference for the further regulation of the type and content of chemical groups in a-C:F.The thermal stability of MSH with different morphologies and the morphology,phase composition,chemical group information and specific surface area of the annealed products were studied.Compared with MSH200,MSH300 has higher thermal stability,and the thermal decomposition of MSH consists of the first endothermic valley caused by loss of adsorpted water and interlaminar water,the second endothermic valley caused by dehydroxylation reaction,and the exothermic region caused by solid phase transition.The results show that the 400℃annealing product MSH300-AHT2 can maintain the layered structure and possess weakened interlaminar hydrogen bond interaction and interlaminar shear strength induced by partial removal of interlaminar water.In addition,the BET specific surface area of the annealing product is lower than that of original sample,which is attributed to the partial melting and sintering of nanoparticle induced by the annealing treatment.The tribological performances and mechanism of annealed MSH products as lubricant additives were studied in this paper.The appropriate annealing products MSH300-AHT2 as lubricant additive has the best friction reduction and anti-wear performances,which is attributed to the decrease of interlaminar shear strength and the"micro-bearing effect"of tubular nanoparticle.Excessive annealing heat treatment deteriorates the friction reduction and anti-wear performances of MSH nanoparticle,which is due to the severe three-body abrasive wear caused by the hard phase transformation products.The optimal ratio of MSH300-AHT2 to OA is about 1:6,which is attributed to the"competitive"relationship between them.Compared with 200 rpm condition,the increase of oil film thickness at 400 and 600 rpm conditions is beneficial for MSH300-AHT2 to enter the friction clearance smaller than its axial size in a favorable attitude,thus making it exhibit excellent tribological performances.The results of dichromatic optical interferometric tests show that the addition of MSH300-AHT2 could increase the central film thicknessh_c in the contact area.The results of CMD simulation show that the"flash temperature"induced by friction can not only cause the dehydroxylation reaction of MSH,but also release the Mg²⁺by decomposing the Mg-O octahedron.The results of AIMD simulation show that the active components(Mg²⁺and-OH)released under the action of"flash temperature"can cause the dehydrogenation reaction of PAO10 base oil,which enables MSH to convert the PAO10 into a-C.After appropriate annealing treatment,MSH300-AHT2 with lower interlaminar shear strength is easy to undergo interlaminar cleavage under the action of friction,and then expose more active components to convert PAO10 base oil into a-C protective tribofilm with higher graphitic structural order.