Study On Structure Evolution Of Friction Transfer Film For Several Typical Solid Lubricating Materials

Solid lubrication refers to the use of solid powders,thin films,or other integral materials to reduce friction and wear between two surfaces in contact having relative motion.Due to its wide range of application,it can play an important role in lubrication in the case where liquid lubrication is not valid.Therefore,solid lubrication materials represented by DLC,graphite and Mo S2 have been widely used.However,the influence of structural changes of transfer film on friction in the friction process is still controversial.In order to study the mechanism of action between transfer film and solid lubrication material,three typical solid lubricating materials,a-C:H film,graphite and Mo S2-based composite film were selected.Scanning electron microscopy(SEM),energy dispersion spectroscopy(EDS),Raman spectroscopy(Raman),X-ray diffraction(XRD)and other analytical methods were used to characterize the morphology,composition and structure of the materials.The friction test was carried out by using the ball-on-disk friction tester,and the wear morphology of the film was observed by non-contact surface three-dimensional profiler.The formation of transfer film and the effect of structure evolution on friction and wear of different materials were observed.The main results are as follows:1.a-C:H thin films were prepared by multi-target intermediate frequency magnetron sputtering,and friction tests were carried out in air and dry nitrogen atmosphere with Al2O3 and Si3N4 balls as counterparts.The experimental results show that when the counterpart is Al2O3,the friction coefficient of a-C:H film in dry nitrogen is lower and the wear life is longer than that in air.This is the result that the carbon transfer film with typical DLC characteristic will be formed on Al2O3 ball in nitrogen atmosphere and it can exist stably.In addition,in dry nitrogen,the structure of the wear track and the transfer film tend to be"graphitized".Therefore,it is the combined action of the above two that results in lower friction and wear of a-C:H film in N2 atmosphere than in air.When the counterpart is Si3N4,a transfer film is formed on the surface of the Si3N4 ball under 10 N in air,and the transfer film becomes more and more with the increase of time.The transfer film can protect the friction interface,which can further reduce the friction coefficient and wear rate.Friction in nitrogen,due to the strong adhesion of Si3N4 to carbon,the larger shear force under high load will quickly peel off the surface of the film and the peeling film will act as an abrasive between the friction interfaces,aggravating friction and wear.However,under low load,due to the low shear force,the hanging bond is partially passivated during friction,and a transfer film is formed on the surface of the Si3N4 ball to reduce wear.When the irradiation dose is low,the friction and wear of Ni~+ion implanted Si3N4 ball is similar to that of unimplanted.With the increase of irradiation dose,the peak concentration of Ni~+on the surface of the counterpart ball increases,the adhesion of Si3N4 ball to carbon is reduced,and the generation of a-C:H debris between friction interfaces is reduced,which promotes the generation of transfer film,thus reducing the wear rate.2.The friction tests were carried out between Al2O3 counterpart balls and three kinds of graphite(amorphous graphite,pyrolytic graphite and highly oriented graphite)with different degree of crystallization and ordering,respectively,to study the influence of substrate ordering on friction.The results show that the friction coefficient increases after friction with three kinds of graphite materials,and the higher the degree of ordering of the substrate,the longer the graphite keeps low friction.The friction of graphite is also affected by its own mechanical properties,the HOPG with lower hardness has the largest plastic deformation in the friction,so the wear rate is the largest.Comparing with the original graphite surface,the degree of defects in all wear tracks and transfer film structure is higher.The defect is related to the transition from two-dimensional original graphite to 3D lattice disorder structure in all wear tracks.Disorder is caused by graphite debris produced by the fracture of layered graphite bonds.3.The Mo S2-Ti composite films were prepared by R.F.magnetron sputtering and were annealed at different temperatures(200,300,400,500,600℃)in vacuum.Friction tests were carried out with different counterpart balls(Al2O3,Si3N4,Si C and GCr15)in vacuum to study the effect of transfer film formation on friction.The results show that 500℃is the best recrystallization temperature for Mo S2-Ti composite films,and the films have the highest crystallization degree and the best mechanical properties.After friction,the transfer film is easier to form on the softer GCr15 dual surface,but the existence of the transfer film can not guarantee low wear.In order to achieve low wear,it is necessary to ensure that the hardness of the counterpart ball is higher than that of the film itself,so as to avoid the occurrence of abrasive wear.Friction can induce the ordering transition of amorphous Mo S2-Ti composite films,and the more ordering the film itself is,the more ordering the transfer film is,and the lower the friction coefficient is.It is speculated that the wear rate of the film is dominated by the mechanical properties of the film,followed by the crystallinity of the film.