Multi-pass scratch test on pearlitic steel: Phase identification and crystallographic orientation analysis of the sub-surface layers

Abstract

In this study, the abrasive wear of pearlitic steel was investigated, including the corresponding microstructural changes at the sub-surface. Multi-pass scratch tests were conducted under different conditions of normal load (4 and 18 N) and number of sliding cycles (from 1 to 12). Scanning electron microscopy (SEM) was employed to characterize the worn surface following the scratch tests. The microstructure after the tests was analyzed via focused ion beam/SEM, and electron backscattered diffraction (EBSD). A geometrically necessary dislocation (GND) analysis was utilized to evaluate the dislocation density from the EBSD data. The results facilitated the establishment of a relationship between the coefficient of friction (COF) and the test parameters. It was found that a decrease in COF was associated with an increase in the number of indenter passes while the COF was independent (statistically) of the normal load. During the multi-pass tests, the work-hardening behavior of the material controls the evolution of the microstructures beneath the scratch. The density of dislocations did not show any significant difference across the test conditions. The nano-hardness analysis showed the presence of a multi-layer with some microstructural transformation. The values of hardness obtained suggested the formation of nanocrystalline martensite in the topmost layer known as the white etching layer (WEL). The microstructure of the layer below the hard WEL is compatible with nanocrystalline martensite and saturated ferrite (based on the high hardness values). The third layer is characterized by nanocrystalline ferrite and partially dissolved cementite. No correlation was found between the crystallographic orientations prior to the tests and work hardening. The plastically affected region did not show any predominant crystallographic orientation.