Preparation And Tribological Properties Of The Bearing Steel With Gradient Nanostructured Surface Layers

Bearings are the vital component of major mechanical equipment in the industrial field,wherein bearing often works under harsh conditions（high temperature,heavy loading,etc.）,unavoidably leading to wear failure of the chosen bearing steels.Thus it is urgently required to elevate the tribological properties of bearing steels in an effort to prolong the service lifetime of the bearings.Common design strategies for developing wear-resistant bearing steels include alloying and surface carburizing,etc.In recent years,it has been establised that the architecture of gradient nano-grained structure（GNG）on the metallic surface can promise the materials possess superior strength-ductility combination and strain delocalization capability without changing the chemical compositions,thereby significantly mitigating the friction and wear of metallic materials.This novel design guideline is greatly anticipated to improve the friction and wear properties and service life of bearing steels.The third generation CSS-42L bearing steel and traditional AISI 316L stainless steel were processed by surface mechanical rolling treatment（SMRT）to prepare gradient nanostructures and the SMRT processing parameters were adjusted to obtain GNG samples with optimized friction and wear properties.The microstructure evolution of surface and subsurface layer of coarse-grained（CG）,nanocrystalline（NG）and GNG bearing steels under high temperature or heavy load friction were systematically investigated to delineate the friction and wear mechanisms of GNG bearing steels.The conclusions are drawn as follows:（1）Gradient nanostructures were fabricated on the surface of CG CSS-42L bearing steel by SMRT and processed for 1,3 and 5 passes,respectively.The thickness of the nanocrystalline layer on the topmost layer is 2μm and the hardness of the topmost layer of the sample increases from 494 HV of the CG sample to 635 HV of the SMRT-5P sample,and the SMRT-5P sample possesses the lowest friction coefficient and wear rate.Strikingly,the GNG CSS-42L bearing steel（5 passes）displays ultra-low friction coefficient（0.180,0.188）and wear rate(3.8×10^-6mm³/Nm,4.0×10^-6mm³/Nm)at room temperature and 200℃,markedly lower than the friction coefficient（0.64,0.57）and wear rate(8.47×10^-6mm³/N,1.04×10^-5mm³/Nm)of CG samples.At 350℃,the friction coefficient of the GNG sample is 0.43 and the wear rate is 1.96×10^-5mm³/Nm,still lower than the friction coefficient（0.5）and wear rate(3.66×10^-5mm³/Nm)of the CG samples.The friction coefficient and wear rate of the CG and GNG samples are similar at 500℃,but the wear rate of the GNG samples is lowered by about 3 times than that at 350℃.（2）The wear mechanism of GNG CSS-42L is abrasive wear at both room temperature and 200℃.At 350℃,slight oxidation occurs on the surface and the wear mechanism is mainly abrasive wear and oxidative wear as well.At 500℃,oxidative wear is dominant.The subsurface microstructure of GNG CSS-42L remains almost unchanged after 1800 cycles at room temperature,and an equiaxed nanocrystalline layer is formed at 200℃.While for the sample at 350℃,lamellar gradient nanostructure and an oxide layer with thickness of about 300 nm are observedin the subsurface layer.AT 500℃,an oxide layer of 3μm thick can be observed and there are obvious cracks in the oxide layer,which accounts for the wear of the material.The excellent wear resistance of GNG samples from room temperature to 350℃can be attributed to the high hardness and good oxidation resistance of the surface gradient nanostructured layer,and the stable gradient nanostructured surface enabling supressionof strain localization and consequent surface roughening.（3）The friction and wear of the GNG 316L stainless steel can be greatly reduced in the single scratch process.After scratching,the surface roughness of the CG and NG samples increases significantly and a large number of microcracks are formed.The microstructure of CG and NG samples are inhomogenous at the front of the scratch scar,unavoidably resulting in stress concentration and crack initiation under high tribological stress.Under tensile stress generated by the indenter passing through the crack zone,the crack propagates and intensifies the material wear.The stable gradient nanostructure of GNG samples at the front and right below the scratch can effectively inhibit the strain localization,surface roughening and the generation of delaminating tribolayer,thereby contributing to the elevation of the scratch resistance.