Research On Testing Method Of Plastic Parameters Of Surface Metamorphic Layer Based On Scratch Technique

As the most superficial layer of a component,the surface modification layer directly affects the friction and wear performance and fatigue life of the component,so the characterization of the mechanical properties of the surface modification layer is particularly important.However,the traditional tensile,compression and torsion tests used to extract mechanical properties are not suitable for surface modification layers because the component size is too small or the local gradient of the properties is too large when tested using traditional methods.As a new micro-loss test method,scratch method is more suitable for the characterization of mechanical properties of surface metamorphic layers.In this paper,a convenient and efficient scratch test method for plastic parameters of isotropic/anisotropic materials(surface modification layer)is proposed by combining finite element method and experiment.The main contents are as follows:(1)A new method for determining the plastic parameters such as yield stress,strain hardening index and interfacial friction coefficient of isotropic metallic materials is presented.The yield stress and strain hardening index of the scratch test material can be calculated by scratch width,residual scratch depth and normal force.The interfacial friction coefficient between scratch material and scratch tip can be determined by the ratio of tangential force to normal force.The yield stress and strain hardening index of uncarburized and carburized 18CrNiMo7-6 alloy steels were compared by scratch test and tensile test,and the accuracy of the formulas for calculating plastic parameters was verified.The correlation between the coefficient of interfacial friction calculated from the ratio of tangential load to normal load of diamond flat indentation head and the scratch test results shows that the analytical formula for calculating the coefficient of interfacial friction has good accuracy.(2)A new method for determining the plastic parameters such as yield stress,strain hardening index,yield stress ratio and interfacial friction coefficient of metal-modified surface layer(anisotropic material)is presented.The yield stress and strain hardening index of anisotropic materials can be calculated by using scratch width,residual scratch depth and normal force.The yield stress ratio in two directions perpendicular to each other can be determined by the ratio of the scratch width to the residual scratch depth.According to the yield stress,strain hardening index and the ratio of tangential force to normal force,the interfacial friction coefficient between the test material and the scraper can be determined.The yield stress and strain hardening index of the carburized layer at different depths were determined by the scratch test of the carburized heat treated 18CrNiMo7-6 alloy steel,and the accuracy of the formulas for calculating the plastic parameters based on the anisotropic material was verified.The yield stress ratio obtained by scratch method was compared with that obtained by indentation method,and the accuracy of analytical formula for determining yield stress ratio was further verified.The correlation between the coefficient of interfacial friction calculated based on the ratio of tangential load to normal load of anisotropic material and the scratch test results shows that the analytical formula for calculating the coefficient of interfacial friction has a good accuracy.This work provides a convenient method for simultaneously determining the plastic parameters and interfacial friction coefficient of metal materials,which is of great significance for mechanical analysis,design and safety evaluation of mechanical parts.