Evolution Of Gradient Nanostructures Of Martensitic High Strength Steel Under Sliding

Sliding cause large plastic deformation and form a gradient nanostructure on the contact surface.This microstructural change directly caused by sliding usually affects the tribological properties of metal materials to a great extent.Numerous studies have been carried out on pure metals and low-strength alloys.However,there is still a lack of research on the formation of gradient nanostructures in martensite high-strength steel under sliding,and the tribological properties of the gradient nano-structural martensite high-strength steel also need to be discussed.Grain coarsening occurs in the nano-laminated structure of martensite high-strength steel under lubricated sliding.Grain coarsening caused by sliding is mainly concentrated on pure metals and dual-alloys with nanostructures,which has not been reported in the sliding research of commonly used tribological martensite materials.Therefore,this paper firstly carries out dry sliding on martensitic high strength steel to generate gradient nanostructures,and discusses the formation mechanism of gradient nanostructures and changes in tribological properties.Then lubricated sliding is carried out,and the changes of grain coarsening and microstructure under different sliding times are continuously studied,with the emphasis on the coarsening mechanism of nano-laminated structure.The main conclusions are as follows:(1)Plastic deformation induced by dry sliding causes different changes in the microstructure of worn subsurface.This is due to different worn mechanisms under different loads and worn times.The adhesive wear mechanism of the former is much more capable of accumulating shear strain than the abrasive wear caused by the latter.Therefore,strain gradients with excessive differences in depths lead to great discrepancy between deformed tissues,forming gradient structures.Shear strain and strain gradient increase with the increase of load and sliding time,but the change trend is almost the same.(2)The worn surface is composed of nano laminated structure,where the shear strain is the largest.High strain means high dislocation density in the tissue.Dislocations plug up along the sliding direction at the martensite lath boundaries,leading to formation of new dislocation boundaries: geometrically necessary boundary(GNB)and incidental dislocation boundary(IDB).The formation of nano laminated structure is a process in which dislocation cell blocks and elongated grains are fragmented by continuously generated GNBs and IDBs.This process is not just a mechanical process.Due to the temperature rise of friction heating,it is affected by thermal effect.This study gives the upper and lower limits of temperature rise caused by friction heating.Partial recrystallization was confirmed by some clear grains in TEM micrographs.(3)The degree of grain coarsening is highly dependent on the initial grain size.After lubricated sliding,the nano laminated structure is coarsened into equiaxed grains and subgrains,or still remains lamellar.Grain grows preferentially in the direction of lamellar spacing.Continuous grain coarsening is mechanically driven and does not involve thermal effects.The nano laminated structure consists of triple junctions with high density,which is beneficial to the growth and merge of nano laminated grains.(4)Under the condition of high tribological stress,the wear resistance of martensitic highstrength steel with gradient nanostructures does improve,but the coefficient of friction does not decrease,indicating that the wear resistance increases because the nanostructures improve the hardness.However,grain coarsening does not change the hardness of the nanostructure and will not affect the tribological properties.