Research On The Friction And Wear Characteristics And Wear-fatigue Behavior Of Lath Martensitic Steel

Steels,as the most widely used material for key parts and equipments of machineries,are often damaged in the friction condition,which may lead to equipments operation obstruction and resource waste,even cause machines destruction and people deaths.The primary task of controlling the friction and wear loss of steels is to study the friction and wear characteristics systematically and deeply.Especially,the relationship between tribological behavior and plastic deformation,microstructure evolution,properties and composition changes in the wear induced layer.Meanwhile,many steels are served in the condition that combination of the wear and fatigue processes which is called wear-fatigue,such as bucket teeth,brazing tools,flexible wheel and train rail and so on.The wear-fatigue failure behavior is different from the single wear failure or fatigue failure due to friction and wear.It is associated with the loss of surface materials,plastic deformation and structural evolution induced by wear,and there are differences in the performance under different fatigue stress.It is very complicated to predict the wear-fatigue life.Thus,it is important to reveal the mechanism of tribological behavior of steel materials and its impact on wear-fatigue life,and establish a corresponding crack growth model.The rearch results can be usded to evaluate anti-wear ability of steel materials,guide structures design of the surface layer and predict wear-fatigue life.This paper focuses on the dry sliding friction and wear characteristics of low carbon martensite steels.The plastic deformation behavior and the evolution of microstructures in the wear induced layer and their influences on the tribological behavior were studied.The mechanism of martensite structure evolution,oxidation,amorphization and self-lubrication in dry sliding friction was analyzed and corresponding physical models were established for the fist time.Based on the research results of dry friction and wear characteristics,combined with the linear elastic mechanics analysis of stress field and crack growth surface,the relationship between sliding friction and wear-fatigue crack growth was studied,and the wear-fatigue crack growth model was established.The wear-fatigue testing device was designed to study the service behavior under the interaction of large-displacement sliding wear and rotational bending stress,which is different from the contact fatigue?such as the working environment of the rail and wheel?.We got some meaningful conclusions:1.The results of ball?martensite steels?-on-disk?tungsten carbide?dry sliding friction experiments showed that the self-lubrication phenomenon would occur under certain friction conditions.The corresponding wear mechanisms changed from initial abrasive wear,oxidative wear and adhesive wear to delamination and spalling of oxide.The temperature of the surface oxidation was about 200?to 500?which led to the formation of an oxide coating.The generation of a self-lubrication was closely related to the formation of nano-oxide particles?Fe₂O₃ and Fe₃O₄?on the surface,and these nano-oxide particles were the products of structures after oxidation in the plastic deformation layer.2.The results of researches on the plastic deformation behavior in the contact layer showed that when friction was taken into account,the maximum value of the second invariant of the stress bias tensor was located on the contact surface and it was varied with the depth in gradient.The gradient structures formed in the plastic deformation layer under high stress and strain:nano-lamellar structures—bent martensite lath—matrix.Dislocation pile-up and tangling at the boundaries of the lamellar structures?martensite or pearlite?and formed nano-lamellar structures due to the severe plastic deformation.Based on above results,the dynamic evolution model of martensite in the plastic deformation layer was established.3.The lath martensite evolved into nano-lamellar structures and then been oxidized and amorphous due to the severe plastic deformation.The local solid-state amorphization and amorphization by mechanical alloying were the two processes involved in the formation of amorphous structures in dry sliding wear.The amorphous nucleation energy model in the process of friction and wear was established.This model was used to calculate the critical dislocation density value corresponding to the amorphous.It could be determined that whether the amorphous were formed by comparing the actual dislocation multiplication with the calculation results.It indicated that amortization was likely to occur at the structural interfaces with high dislocation density.4.Self-lubrication occurred in the processes of dry friction of martensite and pearlite steels with different element content and carbon content,but the corresponding friction loads and speeds were different.The formation mechanism of self-lubricating layer was revealed on nano-scale:1)the nano-lamellar structures were formed in the severe plastic deformation layer;2)amorphization and oxidation of nano-lamellar structures after the severe plastic deformation were observed;3)nano-lamellar structures in the vicinity of the cracks would be delaminated to form wear debris particles and undergo plastic deformation on the contact surface.4)the wear debris particles with nano-lamellar structures were refined and oxidized after mechanical mixing.Finally,the nano-oxide particles attach tightly to the top surface,forming a self-lubricating layer.5.The wear-fatigue crack growth model under the interaction of wear and bending fatigue can fully reflect the relationship between tribological behavior and wear-fatigue crack growth.It was found that the loss of surface materials,plastic deformation and structural evolution induced by wear had three main effects on the wear-fatigue failure behavior.The effects of tribological behavior on wear-fatigue include:changing crack initiation location,promoting the formation of surface cracks?severe deformation of the contact points?,causing deflection of the crack growth surface?the contact stress field?,changing the crack growth path?plastic flow?and eliminating surface cracks?wear?.6.The wear-fatigue tests of the interaction between wear loads and bending fatigue loads were carried out on a self-designed equipment combined with the wear-fatigue crack propagation model.The wear-fatigue lives under diffirent experimental conditions were obtained.It was found that nanolamellar structures were formed in the wear-induced plastic deformation layer,which forced the crack to propagate parallel to the surface direction,and directly eliminated or blocked the fatigue crack propagating toward the depth direction when the bending stress was set as the fatigue limit.Instead,;When the bending stress was higher than the fatigue limit,increasing the wear load would dramatically reduced the wear-fatigue life;Under the same experimental conditions,higher rotation speed would lead to a significant reduction of the wear-fatigue life.