Effect of ultrasonic nanocrystalline surface modification process on fretting wear behavior of laser surface textured 20CrMoH steel

Laser surface texturing (LST) followed by ultrasonic surface rolling (USR) was applied to 20CrMoH steel to improve the friction and wear behavior. In this paper, the fretting friction and wear mechanisms of the modified (USR, LST + USR) 20CrMoH steel under different loads and lubrication conditions were systematically investigated. Results indicated that the regular coupling distribution of micro-texture and gradient nanostructure (GNS) was formed on the surface layer of the combined treated material. Simultaneously, the depth of the GNS was up to 22 mu m and the surface microhardness was increased from 800 HV0.2 to 860 HV0.2. Besides, the surface roughness Ra value was decreased from 0.24 mu m to 0.01 mu m and the maximum residual compressive stress (RCS) value was -610 MPa. Under dry friction and high load conditions, compared with the untreated specimen, the average friction coefficient and worn losses of the combined treated specimen were reduced by about 10% and 30%, respectively. In comparison with the untreated specimen, the average friction coefficient of the combined treated specimen was decreased by 26% under the conditions of low load and oil lubrication. Especially, the fretting wear resistance was obviously improved and its worn loss was significantly reduced by 92.05% compared with the untreated material. The predominant wear mechanisms of the combined treated material under dry friction and oil lubrication were oxidation wear and abrasive wear, respectively.