| Unlike conventional liquid lubricants for electrical contact,ionic liquids are believed to have combined property of good lubrication as well as good electrical conductivity.Up to now,current-loading tribological performance of typical electrical contact in the presence of ionic liquid is still far from fully understood.Ionic liquids as lubricants for electrical contact are expected to open a new window for the lubrication of wire/slip ring system employed in spacecraft based on the merits in property(e.g.low vapor pressure,wide electrochemical window,high thermal stability,and many others).In this connection,this thesis focuses on understanding the tribological mechanism involved in metal/metal contact.By ensuring good conductivity at the tribo-interface,the aim of reducing adhesion and work-hardening of prow is established by using an ionic liquid lubricant of LP108(1-methyl-3-octyl imidazolium hexafluorophosphate).A pin-on-disk reciprocating tribo-meter is used to investigate the tribological performance of electroless silver film in the presence of LP108.Tribo-physics(interfacial adhesion,formation of silver transfer film and tribo-layer,and genesis of wear debris),tribo-chemistry(composition and microstructure of boundary lubrication film)under low sliding speed and heavy-load conditions are separately investigated under non current-loading condition and current-loading condition.The electrochemical and tribochemical mechanisms and their effects on tribological properties are discussed.Some basic data for engineering applications are intentionally obtained in this thesis.The main results and conclusions are as follows:1.LP108 is lubricious and provides anti-adhesion to Cu on electroless Ag film contact.Under a contact stress of 0.77GPa(10 N in load)and a sliding rate of 0.78mm/s,the copper(pin)/electroless silver film contact(2 mm in film thickness,simplified as Cu/Ag contact)suffers severe adhesive wear(a thick silver transfer layer on the surface of the copper pin)with friction coefficient as high as 1.0-1.4.LP108 effectively reduces interfacial adhesion with a friction coefficient as low as 0.35.A thin silver transfer film can be found on the worn surface of the copper pin.The wear debris is mainly generated during the run-in period.Mild wear for both pin and disk occurs during the steady-state stage.The wear lifetime is greater than 6 hours at a contact stress of 1 GPa(29 N in load).2.The current-loading tribological properties of the Cu/Ag contact in presence of LP108 depend on the polarity of the pin and disk.(1)For MSR mode(mechanical separation and recontact mode),the tribological property is strongly influenced by electric arc.For the pin(+)/disk(-),friction coefficient as low as 0.3,mild wear and stable contact resistance are obtained.In addition,rainbow-like film on Ag film by electric arc is observed.While,for negative Cu pin against positive Ag film disk,removal of Ag film is observed and this produces dark ionic liquid-Ag suspension.In both cases,electric arc does not degrade LP108.(2)Electric arc and electric arc-induced damage on Ag film can be eliminated by ensuring good contacting surface conformity.3.The silver tribo-layer and the good boundary lubrication property of LP108 render Cu/Ag contact excellent current-loading tribological property.In the presence of flooded LP108,the formation of silver@pin/silver@disk interface is observed.In specific,an Ag transfer film on the worn surface of pin and a tribo-layer of Ag on the worn surface of disk are found.Because of the good boundary lubrication properties by anion(PF6-)in LP108,a boundary lubrication film rich in element F(CF3)of several nanometers in thickness on top of the transfer film and tribo-layer is identified.Elements of P and N are too low to detect on the worn surface.It is revealed that no obvious difference in the chemical composition and microstructure of the tribochemical film between non current-loading condition and current-loading condition.4.Ionic liquid lubricated wire/slip ring system shows potential application.In this thesis,we presents two kinds of results.The first is the design of the frictional interface at macro-level and the second is the storage stability of the LP108 film @electroless silver film.Three kinds of friction interfaces,i.e.(a)tribo-interface I,bare copper pin against Ag film on disk,(b)tribo-interface II,Ag film on copper pin against bare copper disk,(c)tribo-interface III,Ag film on copper pin against Ag film on disk.Different tribological properties(genesis of wear debris and formation of tribo-layer)are observed for the three tribo-interface.A large number of flaky wear debris are found for tribo-interface I and III;while,wear debris for tribo-interface II can hardly be collected.Thin film LP108@electroless silver film has good storage stability and shows considerable resistance to hot air and thermal oxidation in air. |
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