In a nanoscale friction test,wear of an atomic force microscope(AFM)tip is inevitable.The shape of the worn tip influences the friction force measured.In order to eliminate the influence,nanowear pretreatment should be conducted for the AFM tip.In our study,pretreatment of three kinds of tips,i.e.Si3N4 tip,Si tip and silica colloidal tip,was performed using AFM.The results show that the shape of the tips changes with the increase of sliding distance,which leads to the variety of friction force.Whereas,when the tip gets blunt,the shape of tip tends to become stable and the friction force becomes stable correspondingly.To a certain degree,it reveals that the pretreated tips can be applied to study the friction force of samples.
In this work,a super-low friction coefficient of 0.003 was found between a silicon nitride ball and a sapphire plate lubricated by phosphoric acid solution.The wear mainly occurred in the running-in period and disappeared after superlubricity was achieved.The friction coefficient was effectively reduced from 0.3 to 0.003 at a constant speed of 0.076 m/s,accompanied by a 12-nm-thickness film.The lubrication regime was indicated to change from boundary lubrication in the running-in period to elastohydrodynamic lubrication in the superlubricity period,which is also supported by the results of the friction coefficient versus sliding speed.In addition,the experimental results showed good agreement with theoretical calculations based on the elastohydrodynamic lubrication theory,suggesting a significant hydrodynamic effect of phosphoric acid on superlubricity.
Mingming DENGChenhui ZHANGJinjin LILiran MAJianbin LUO
In this study,the tribological behaviors of graphene as a lubricant additive for steel/copper and steel/steel friction pairs were compared.For the steel/copper friction pair,the graphene sheets remarkably decreased the coefficient of friction and wear scar depth under low loads,but these slightly increased under high loads.The steel/steel friction pair showed excellent tribological properties even under high loads.Severe plastic deformation on the copper surface reduced the stability of the graphene tribofilm because of a rough copper transfer film on the steel during the running-in period.The results provide a better understanding of the mechanism of graphene as a lubricant additive.
Junyuan MAOGuangyan CHENJun ZHAOYongyong HEJianbin LUO
Graphene is a promising material as a lubricant additive for reducing friction and wear.Here,a dispersing method which combines chemical modification of graphene by octadecylamine and dicyclohexylcarbodiimide with a kind of effective dispersant has been successfully developed to achieve the remarkable dispersion stability of graphene in base oil.The stable dispersion time of modified graphene(0.5 wt%)with dispersant(1 wt%)in PAO-6 could be up to about 120 days,which was the longest time reported so far.At the same time,the lubricant exhibits a significant improvement of tribological performance for a steel ball to plate tribo-system with a normal load of 2 N.The coefficient of friction between sliding surfaces was~0.10 and the depth of wear track on plate was~21 nm,which decreased by about 44%and 90%when compared to pure PAO-6,respectively.Furthermore,the analysis of the lubricating mechanisms in regard to the sliding-induced formation of nanostructured tribo-film has been contacted by using Raman spectra and TEM.
Pu WUXinchun CHENChenhui ZHANGJiping ZHANGJianbin LUOJiyang ZHANG