Microstructure Evolution And Plastic Failure Behavior In Sliding Friction- Induced Deformation Layer Of 40Cr/GCr15 Tribopairs

One of the most important phenomena during the sliding friction of metallic materials is the formation of worn surface structure(friction-induced deformation layer),which differs from the matrix in microstructures and mechanical properties.Such structure exhibits several remarkable characteristics including severe plastic flow,relatively high strain and strain hardening behavior,which will directly influence the tribological behavior and reliability of materials.Therefore,systematic investigations on the dynamic evolution of friction-induced deformation layer during friction process are quite necessary in revealing the tribological essence and wear mechanism of materials.In this study,lubricated and unlubricated sliding tribological properties of normalized/quenched and tempered 40 Cr steel pin against quenched and tempered GCr15 steel disc were investigated through disc-on-pin contact mode.The microstructural evolution and strain hardening behavior of worn surface/subsurface in 40 Cr pin samples were systematically analyzed using scanning electron microscopy(SEM),transmission electron microscope(TEM)and microhardness tester.The main results are as follows:(1)The tribological behavior and wear mechanism variations of 40Cr/GCr15 tribo-pairs were studied under lubricated and unlubricated conditions.Compared with normalized pin/quenched and tempered disc tribo-pair,the quenched and tempered pin/quenched and tempered disc tribo-pair exhibited lower friction coefficients and wear rates under the identical condition.The wear mechanisms of pins and discs in both tribo-pairs were mainly severe adhesive wear accompanied with oxidation wear,while the mixed mild wear characterized by micro cutting/adhesion and local fatigue was the main wear mechanism under lubricated condition.(2)The microstructural features,microhardness and shear strain gradient evolution in the subsurface of 40 Cr pins were investigated.In the identical friction condition,the quenched and tempered pin sample exhibited lower equivalent strain and deforming depth.Whether normalized or quenched and tempered pin samples,the shear strain and microhardness of upmost subsurface showed the trend which firstly increased and then decreased with the prolongation of dry friction duration,while the deforming depth gradually increased until reaching a stable state.Under the lubricated friction,the deforming depth,equivalent strain and microhardness at the same depth were significantly lower than those under unlubricated condition and all of them also increased with the prolongation of friction duration.(3)The friction-induced deformation layer with different depths beneath the worn surface of normalized pin samples were investigated,the evolution mechanism of subsurface microstructure under the worn surface can be concluded that,as the effect of severe plastic deformation induced by continuous load and friction shear stress,massive proportions of dislocations multiplied and interacted with each other,leading to the refinement or even nano-crystallization of ferrite grains and the fragmentation and dissolution of cementite grains.(4)The formation,development and exfoliation process of subsurface vortex structures under different friction durations were analyzed.Strain localization may result in severe fragmentation during sliding friction contact,these fragmented mesostructures then suffered translation and rotation along sliding direction,resulting in the formation of pronounced vortex structures,which exhibited relatively high microhardness and brittleness.The cyclic process of spalling and regrowing was the main feature of vortex structure evolution during the stable wear stage,while the wear mechanism during this stage manifested itself in the periodic procedure of strain accumulation and plastic failure under the worn surface.