Preparation And Tribological Properties Of Graphene-based Assembled Films

As a typical representative of two-dimensional nanomaterials,graphene has unique physical and chemical properties and excellent mechanical and lubricating properties.It is expected to be used as a nano-lubricating material to reduce adhesion,friction and wear problems of the contact surface of micro-electromechanical systems(MEMS)at the micro-nano scale.In this thesis,a series of self-assembled multilayer films and composite films were prepared on the surface of a single crystal silicon substrate by layer-by-layer self-assembly technology.A variety of surface analysis methods were used to characterize the composition and structure of the film with an emphasis on the tribological properties of the film.The correlation between the film microstructure and its tribological behaviors were analyzed and discussed.The main research contents includes the following aspects:(1)Using graphene oxide(GO)which rich in oxygen-containing groups and amino-terminated 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane(TA)as precursors.The GO-TA-rGO sandwich multilayer film was constructed by a self-assembly technology and thermal reduction process.The sandwich-like films have lower adhesion and excellent load-bearing properties compared with single-layer GO and double-layer GO-TA films.This is because the TA molecules in the film provide a firm and stable interlayer for the sandwich-like film.At the same time,the mechanical and tribological properties of the sandwich-like film are improved by the GO inner and outer layers.(2)The methyl-terminated dodecyltrichlorosilane(DTS)and the trifluoromethyl-terminated 1H,1H,2H,2H-perfluorodecyltrichlorosilane(PFDTS)as the outer layer.Based on the structure of amino-terminated alkylsilane TA,the TA-GO-DTS and TA-GO-PFDTS films were prepared by layer-by-layer assembly.The results showed that TA-GO-PFDTS film containing carbon-fluorinated chains has low surface energy,good hydrophobicity and better friction reduction.However,its anti-wear performance is poorer than that of TA-GO-DTS film containing hydrocarbon chains.This is mainly due to the internal structure of the films.The-CF₃ group with lower surface energy endow the film surface with good hydropho bicity.However,the larger size of-CF₃group and the spiral structure of the fluorocarbon chain in the perfluorinated compound reduce the compactness of the film,and thus affect the tribological behaviors of the film.(3)Using positively charged Poly(diallyldimethylammonium)chloride(PDDA)and negatively charged GO as precursors,the(GO/PDDA)n multilayer film was fabricated by the electrostatic self-assembly method.The composition,intrinsic structure and tribological properties of the films are stud ied,and the influence of the number of layers on the tribological properties is discussed.The results show that(GO/PDDA)₅ film exhibits better anti-wear and anti-friction properties,which is mainly due to the synergistic effect of PDDA and GO.GO has high structural rigidity and good lubrication performance.Polymer PDDA improves the load-bearing capacity of the multilayer film.And it is also used as a binder to connect multi-layer GO films.Therefore,the tribological properties of(GO/PDDA)₅ film are improved under the cooperation of multilayer PDDA and GO.(4)Taking 1-Hexyl-3-methylimidazolium hexafluorophosphate([HMIM][PF ₆])ionic liquid with an anion of PF₆^-and GO nanosheet as precursors.The GO/IL composite films were fabricated by self-assembly method.The influence of the GO concentration on the surface morphology of the film was investigated,and the relationship between the structure and the tribological properties of the composite films was studied.The results showed that the composite film containing 0.1 mg·m L^-1 GO has good friction stability and excellent load-bearing performance.This is due to the excellent tribological properties of GO itself and the good flatness of the composite film.When the content of GO is more than 0.1 mg·m L^-1,the GO nanosheets have serious agglomeration,and the film surface become rough.Therefore,the tribological properties of the composite films reduced with the increased roughness.