Friction And Wear Properties Of Rare Earth Modified Cnts Composite Films On Silion Substrate

Micro/Nano-electromechanical-systems?MEMS/NEMS?have been widely used in biomedical,aerospace,military,machine industries,and agricultural engineering,due to their small size,light weight,low energy consumption,high degree of integration and intelligent control.Due to the micro-and nano-dimensions in MEMS/NEMS,friction and adhesion between the contact surfaces are the major failure mode affecting the performance and reliability of MEMS/NEMS.Research on surface treatment of MEMS/NEMS mainly focuses on lubrication films with strong adhesion and good friction properties.Self-assembled mono-layer?SAM?films can effectively reduce the friction of MEMS/NEMS components due to their stable performance,low friction coefficient,thin thickness,and strong binding energy.However,the applications of SAM films in MEMS/NEMS have been limited due to their low bearing capacity and poor wear resistance.There is an urgent need to develop new types of self-assembled lubricating films to meet the requirements of MEMS/NEMS under severe operating conditions.Based on the special physical and chemical properties of rare-earth?RE?elements and carbon nanotubes?CNTs?,RE modified CNTs?CNTs/RE?composite film has been prepared in this research by using self-assembly techniques.The composite film has shown strong interfacial bonding strength to the silicon substrate,and can effectively improve the tribological properties of the silicon substrates.Firstly,the surface of CNTs was modified by RE by a chemical route to achieve CNTs/RE composite material.The silicon substrates is pre-treated by grafting a phosphorylated 3-amino?propyl triethoxy silane??APTES?monolayer,and then deposited with the CNTs/RE composite film to form a self-assembled film?SAM?via chemical bonding between RE and the oxygen-containing functional group of phosphorylated APTES.The influences of self-assembly temperature,self-assembly time,RE modified CNTs concentration in dispersion solution,and content of the RE modification agent on film formation were studied.Surface morphology and chemical properties of the composite film were characterized by X ray photoelectron spectroscopy?XPS?,scanning electron microscopy?SEM?,etc.The formation mechanism of the composite film was elucidated.Secondly,the self-assembling mechanism of the RE in CNTs and the film forming capability of the CNTs/RE composite film were studied in the perspective of thermodynamics.From thermodynamic calculation of the chemical reaction and the surface free energy equation,it is found that the surface free energy of RE elements in the film reduces after chemical reaction between La³⁺and oxygen-containing functional group of phosphorylated APTES-SAM,indicating that RE can be spontaneously adsorbed on the substrate through chemical reaction.With the aid of Materials Studio,the absorption process of the film was simulated using molecular dynamics simulation.The energy variation through the whole process also proved that the chemical bonding between RE and the functional groups was spontaneous.Thirdly,macroscopic friction and wear properties of CNTs/RE composite film were systematically valuated by the UMT-2 multi-functional friction and wear testing machine.The influences of RE modifier concentration,self-assemble time,normal load and sliding speed on the friction coefficient and friction lifetime of the composite film were investigated.Based on the measured data and theoretical calculation,a model of molecular springs under macro-friction conditions was established,which showed the composite film has excellent friction and wear resistance properties owing to the chemical bonding between RE elements and oxygen-containing functional groups on the silicon substrates surface.CNTs assembled on the surface of phosphorylated APTES-SAM enhance the bearing capacity and wear resistance of the film,and improve friction and wear resistance properties of Si substrates.Fourthly,nano-friction and wear properties of CNTs/RE composite film were studied by atomic force microscopy?AFM?,and the influences of load,sliding speed,air humidity and temperature on the adhesion and friction of the composite film were investigated.Effects of normal load,friction coefficient,sliding speed and moving distance on wear depth of the composite film were analysed,and the model of rolling-logs under nanowear conditions was established.The results showed that the composite film has lower friction and adhesion erosion,and superior wear resistance than bare silicon substrate,therefore,can effectively improve the nanofriction properties of the Si substrates.In summary,CNTs/RE composite film with excellent anti-friction and wear resistance performance was deposited and chemically bonded to the Si substrate surface.Optimal preparation parameters of the film have been defined,the tribological properties of the composite film were investigated and the mechanism was clarified.This study provides a new approach to investigate lubrication in MEMS/NEMS under nano-scale and promotes application of RE and CNTs in surface engineering field.Moreover,this study provides theoretical and experimental basis for the application of CNTs nano coatings in the field of MEMS/NEMS.