Investigation Of Cu/B Doping On The Structure,mechanical And Tribological Properties Of MoS₂ Films

In recent years,with the rise of modern industry and the rapid economic growth,the energy problem and environmental crisis are getting worse.In the modern industrial system,friction and wear have led to the huge global energy consumption,which has become an urgent problem to be solved.Therefore,lots of kinds of lubricants emerge as the times require to reduce friction.With the rapid development of the industry,the moving parts of high-end equipment will serve in a more complex and harsh environment,such as the extremely high pressure environment,extreme temperature environment and ultra-high vacuum environment.Traditional liquid lubricating materials are no longer be applied perfectly in actual working conditions.Nowadays,the development of new solid lubricating materials with excellent lubricity,good oxidation resistance,and high chemical stability has become the focus of research in the field of lubrication.Molybdenum disulfide(MoS₂),typically transition metal dichalcogenides(TMDs),enhibiting excellent lubrication performance in vacuum,has been successfully used as solid lubricating materials in the aerospace industry,becoming a research hotspot.The highly anisotropic layered structure of MoS₂endows it with a very weak interlayer shear force,in which the Mo atom is covalently bonded to two S atoms,while the adjacent lamellae are held together through the very weak van der Waals forces.However,the exposed dangling chemical bonds at the edge of the plane and the film defects will easily react with oxygen and water vapor and promote the oxidation of MoS₂,leading to the formation of Mo O₃and the destruction of easy-shearing lamellar structure,resulting in a high friction and a short lifetime.In addition,the extremely low hardness and low load-bearing capacity of pure MoS₂film severely limit the application of pure MoS₂in atmosphere.In order to solve the above problems,in our work,we decide to improve the mechanical and tribological properties of MoS₂film in stages and steps through the strategies of component regulation and microstructure regulation,using magnetron sputtering technology.We systematically and meticulously study the effects of different modification methods on the structure,mechanical properties and tribological properties of MoS₂films.We finally combine the advantages of the two modification strategies to obtain the films with excellent mechanical and tribological properties in the atmosphere.The main research contents and conclusions are as follows:1.Effect of boron doping on the structure,mechanical and tribological properties of sputtered Mo-S-B films.We first investigate the effect of film composition on the properties of MoS₂films.In this work,Mo-S-B films with different B content are deposited by magnetron sputtering.With the doping of B atoms,the film surface morphology changes from porous and fibrous to dense and smooth,corresponding to a transition from columnar structure to dense feature,along with a phase transition from hexagonal-MoS₂to amorphous phase.The hardness lineally increases from 0.75±0.26GPa for MoS₂to 8.38±0.70 GPa for the Mo-S-B film.The tribological test results indicate that the increased B content led to the enhancement of wear resistance for Mo-S-B films,the wear rate of Mo-S-B films is reduced to a minimum of 5.5×10^-7mm³/Nm,which is two orders of magnitude lower than pure MoS₂.The friction coefficient of Mo-S-B films are slightly larger than that of pure MoS₂(0.14±0.01),keeping it in the stable low value range of 0.15-0.24,which can be ascribed to the formation of MoS₂tribofilm.2.Effect of group IB metals Cu doping on the structure,mechanical and tribological properties of sputtered Mo-S-Cu films.In the first part of the work,although the doping of B greatly improves the hardness of Mo-S-B film,the friction coefficient of the film increases slightly,meaning the tribological properties are not significantly improved.In this work,we modify MoS₂film by doping the catalytic metal Cu to induce the film to form curved structure.We deposit Mo-S-Cu films with turbulence-like MoS₂(TL-MoS₂)using magnetron sputtering technology,which exhibit higher hardness and lower friction coefficient in the atmosphere than pure MoS₂.The hardness increases from 0.31±0.06GPa for pure MoS₂to 3.14±0.09 GPa for Cu/MoS₂,which is one orders of magnitude higher.The improvement of hardness for Mo-S-Cu film can be attributed to the denser structure and TL-MoS₂.TL-MoS₂contributes to the improvement of the hardness by extending the strength of the planar network into a three-dimension space network.Moreover,the curvature for TL-MoS₂composed of the curved S-Mo-S planes reduces the number of exposed dangling bonds at the edges of the plane,which prevents oxidation and promotes the formation of MoS₂tribofilm during sliding.The friction coefficient of the Mo-S-Cu film is reduced from 0.18±0.02 to 0.07±0.01,which is 2.6 times lower than that of pure MoS₂film.3.Effect of Cu and B co-doping on the structure,mechanical and tribological properties of sputtered Mo-S-Cu-B films.Considering the results of the first two experiments,in this work,we combine two strategies of component regulation and microstructure regulation to modify MoS₂film.The Cu atoms with catalytic properties and light element B atoms are co-doped into MoS₂to deposited Mo-S-Cu-B films with turbulence-like MoS₂(TL-MoS₂)using magnetron sputtering technology,showing much higher hardness and lower friction coefficient in atmosphere than pure MoS₂.On the one hand,Cu atoms induce the formation of TL-MoS₂consists of curved S-Mo-S layers,which reduces the number of exposed dangling bonds at the edges of the plane to buffer the oxidation of MoS₂.Meanwhile,the film surface becomes smooth,corresponding to a transition from the columnar growth to dense structure.On the other hand,the doping B atoms are in direct contact with the sulfur-deficient MoS₂phase and occupy vacant sulfur sites,which improves the structural stabilization and reduces the number of growth-related defects,helping in inhibiting the oxidation and forming MoS₂tribofilm.The friction coefficient of Mo-S-Cu-B film is significantly reduced to 0.09±0.01.The hardness of Mo-S-Cu-B film is 6.65±0.11 GPa,which is nearly 21 times higher than that of pure MoS₂film.Our strategy provides guidance for preparing MoS₂-based films with excellent mechanical and tribological properties for application in the atmosphere.Moreover,good structural stabilization and oxidation resistance help Mo-S-Cu-B film to maintain extremely low friction coefficient and wear rate after annealing at 300?and400?,which provides potential possibility for the industrial application of Mo-S-Cu-B films in high temperature fields.