PSI - Issue 75

D. Jbily et al. / Procedia Structural Integrity 75 (2025) 158–175 Author name / Structural Integrity Procedia (2025)

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Fig. 4. Comparison of the micropitting safety factor according to ISO 6336-22 between the REF and SP gears.

The objective of this study is to conduct complementary analyses to better understand the behaviors involved specifically, why shot-peened gears, which exhibit better residual stress, lower surface roughness, and higher theoretical safety against micropitting, tend to damage more quickly by micropitting.

3. Materials and Methods Gear specimens, referred to as REF (reference) and SP (shot-peened), were manufactured from 18CrNiMo7-6 steel. The SP treatment was applied after grinding. Residual stress profiles and surface roughness were measured before and after fatigue testing. Surface analyses were conducted using optical microscopy and 3D interferometric profilometry on both original surfaces and replicas. EBSD analyses were performed in worn and unworn areas of both REF and SP specimens to characterize grain orientation, grain size, local misorientation (KAM), and band contrast, thereby revealing subsurface microstructural changes induced by fatigue. Tribological tests were carried out on a TE-77 tribometer in a crossed-cylinder configuration, using counterbody samples extracted from REF and SP gears, in order to measure the coefficient of friction. 4. Results 4.1. Surface Condition and Micropitting An optical microscope equipped with various objective lenses is utilized at CETIM's Metallurgy Laboratory to conduct detailed inspections of gear tooth flanks. This setup enables precise visualization of micropitting initiation and progression by providing high resolution images of the affected areas. To facilitate these observations, a replication medium precisely adapted to the tooth flank geometry is applied to the gear surface at different stages of the testing cycles. Prior to replication, the gear surface is thoroughly cleaned with a solvent and wiped to remove oils and contaminants. The extracted replicas are then analyzed under the microscope to assess micropitting development over time. In addition to visual inspection, digital imaging and interferometric profiling techniques are used to analyze the gear surfaces. These methods provide both qualitative and quantitative assessments of surface degradation. Optical microscopy reveals surface features and micropit patterns, while interferometric profiling offers detailed measurements of surface topography. Surface analyses are conducted on both worn and unworn areas of two gear types (REF and SP), before and after testing. The results (see Fig. 5) show that micropitting can develop as early as the end of the running in period in the