Effect Of Soot On The Mechanical Properties Of Tribofilm And Its Wear Mechanism Under Boundary Lubrication Regime

Soot,generated from incomplete combustion of fuel,will increase the wear of the cylinder liner / piston ring pair in engine and reduce the service life of it.Especially when the piston reaches top dead center and bottom dead center,this friction pair will run under mixed or boundary lubrication regime,in which case soot will cause more wear.In order to solve this problem,the effect of soot on tribofilm must be ascertained because the tribological property of friction pair,running under mixed or boundary lubrication regime is often determined by the tribofilm.In this thesis,the tribological tests were conducted with liquid paraffin(LP)containing zinc dialkyl dithiophosate(ZDDP)and different contents of soot at different experiment conditions,then the morphology of worn surface and the distribution state of ZDDP tribofilm as well as the mechanical property of worn surface were characterized by modern surface analytical equipment.This provides a new method to study the wear mechanism of soot.The effect of soot content on the mechanical property of tribofilm was studied.The results showed that with the increase of soot,the thickness uniformity of ZDDP tribofilm decreased and its coverage ratio on surface decreased in exponential form.The hardness(H)of worn surface decreased.The elastic modulus(E)of worn surface increased and the average H/E decreased in exponential form.The relative coverage ratio of ZDDP tribofilm and the relative H/E of worn surface had a linear relation.The mechanical property of tribofilm lubricated with or without soot was studied as a variation of friction time.The results showed that after tribological tests without soot and with the increase of friction time,the thickness of ZDDP tribofilm and the hardness(H)of worn surface both increased,and the elastic modulus(E)of worn surface was about 170 GPa.The average H/E increased.After tribological tests with 3% soot and with the increase of friction time,the thickness of ZDDP tribofilm decreased,but the hardness(H)and the elastic modulus(E)of worn surface fluctuated,and these resulted in an irregular variation of average H/E.The relative coverage ratio of tribofilm and the relative H/E both decreased in exponential form.The mechanical property of tribofilm lubricated with or without soot was studied as a variation of contact load.The results showed that after tribological tests without soot and with the increase of contact load,the thickness of ZDDP tribofilm first increased and then decreased,the elastic modulus(E)kept steady,the hardness(H)of worn surface and average H/E both first increased and then decreased.After tribological tests with 3% soot and with the increase of contact load,the thickness of ZDDP tribofilm decreased,the elastic modulus(E)kept steady,the hardness(H)of worn surface and average H/E both first increased and then decreased,the relative coverage ratio of tribofilm decreased in linear form,and the relative H/E first decreased and then increased.The wear mechanism of soot was attributed to that soot will cause abrasive wear.Then the tribofilm on surface will be scraped away,and thus the protection of tribofilm was decreased.Moreover,the mechanical property of friction surface was decreased because of the lack of tribofilm,and the tribological property of friction pair was thus decreased for its deteriorated mechanical property.