High‑throughput nanoindentation for the mechanical characterization of bearing steel subjected to fretting mechanisms

This work proposes an innovative approach to characterize the impact of false brinelling on the microstructure and mechanical properties of a 100Cr6 bearing steel. The originality lies in the combination of large-scale nanoindentation tests (around 1,000 indents) and low load to quantify local hardness changes induced by tribological tests. Plastic deformation of metallic material generally leads to local increase in hardness but, in the case of tempered 100Cr6, the work hardening mechanism has never been sufficiently highlighted nor quantified. The hardness increase related to mechanical deformation in such material is inherently difficult to achieve (the microstructure is already optimized in terms of martensite fraction, carbide distribution, and dislocation density) and detecting and quantifying a shallow gradient requires a high-resolution and statistically robust technique. The complexity of the material, composed of a martensitic matrix with cementite-type carbide precipitates, results in a highly heterogeneous mechanical response at the microscale. A specific methodology was developed: (1) isolation of the mechanical contribution of martensite and cementite, and (2) comparison of hardness distributions between the deformed zone, the intact surface, and the bulk material. This approach was made possible by recent advances in indentation techniques, which combine high-resolution testing capabilities with significantly optimised processing times. The results reveal a shift toward higher hardness values in the area affected by false brinelling, limited to the first 50 micrometres, while the bulk retains its initial properties. This statistical analysis, coupled with microstructural observations from EBSD analyses in a previous study, provides insight into local damage mechanisms. • Nanoindentation enables the characterization of martensite and cementite in 100Cr6 steel. • High-density nanoindentation mapping resolves subtle hardness variations induced by tribological loading, revealing a measurable increase in hardness within the near‑surface region. • The affected subsurface zone is confined to the first ~50 µm, beyond which the hardness returns to bulk values. • This approach demonstrates that fast‑mapping nanoindentation can quantify small mechanical modifications in heterogeneous microstructures with high spatial resolution.