The Effect Of The Grain Size Of 316 Stainless Steel On The Growth Of Tribofilm Of ZDDP

Zinc dialkyl dithiophosphate(ZDDP)is a kind of widely used automotive lubricant additive,which is commonly used in engine piston rings.ZDDP will decompose at the interface during the friction process,and the formation of the tribofilm can reduce contact of the steels or other metal materials on the surface of the grinding metal to protect the surface.Due to the use of ZDDP will make the friction loss increased,and production of compounds containing Zn,P,S,and phosphorus precipitates especially zinc phosphate will make the purifying agent of the catalytic converters poisoning,reducing the efficiency and life of catalytic converter.However,it is not yet well understood that the growth mechanism of the tribofilm,which limits the further development of an alternative and more excellent additive.In order to study the growth mechanism of ZDDP tribofilm,the nano layer and different grain size samples were prepared by ultrasonic cold forging technology(UCFT)and vacuum annealing.The friction and wear behaviors of samples with different grain sizes were investigated by using CSM friction and wear tester in PAO4 and 1.0 wt% ZDDP.Using scanning electron microscopy,optical microscopy,NanoMap-D 3D surface profilometer,X ray diffraction and micro hardness tester to analyse the microtopography,microstructure and microhardness of samples.Using atomic force microscope(AFM)to monitor the growth and properties of the tribofilms in situ in well-defined single-asperity sliding nano contacts.The results are as follows:(1)The average grain size of the top surface layer of 316 L stainless steel was about 12.7 nm,and the surface hardness was increased from 190 HV to 341 HV.After vacuum annealing,the grain size becomes larger.UCFT sample has the highest hardness and the lowest friction coefficient and wear rate.Grain refinement is helpful to reduce friction and wear.(2)In the single asperity sliding test,with the increase of the sliding cycles,the tribofilm of ZDDP grows.The nucleation position is randomly distributed,and the sliding of the probe promotes the growth of the tribofilm.The tribofilm has the characteristics of rough,patchy and plate shape.The film formation rate of UCFT sample is faster than that of Untreated sample.The height of tribofilm on UCFT sample is higher than Untreated sample when the same sliding cycles.This may be because there has more grain boundary of surface after UCFT processing,and the grain size of surface is smaller,grain refinement and atom density of grain boundary is lower,easy to produce more bond breaking,so as to improve the surface activity,promote the formation of surface adsorption film or tribofilm.(3)The Zn,P,S regional characteristics of elements were shown on the inside tribofilm of the single asperity sliding test sample,not shown on the outside tribofilm,which demonstrates that the decomposition of ZDDP has occurred in the process of friction and wear.The phosphate tribofilm is produced,which plays an antiwear role.