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. 18. Example of the evolution of the friction coefficient from tests with the cylindrical part taken from the gears

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SP

Fig. 19. Marks on the lower cylinder after 340,000 cycles.

5. Discussion The strong surface misorientations of the SP tooth grains before the test confirm the shot peening effect with plastic deformations probably accompanied by work hardening. The loss of crystallinity of the surface layer after the test indicates that this work hardened layer becomes amorphous due to normal and tangential stresses during contact with the opposing tooth. This structure is certainly less resistant to wear compared to a tooth without shot peening, which explains the reasons why more wear is observed on the shot peened tooth. Friction test on the tribometer has successfully reproduced micropitting with the hard turned tooth at the top of the grooves, confirming the role of friction force in micropitting generation. However, under the same test conditions, not as much micropitting is found on the shot peened tooth surface. The specific roughness of the SP surface most likely favors boundary lubrication compared to the turned surface in tribometer tests, which can greatly reduce the risk of micropitting formation. The rolling-sliding motion in the contact of the two gear teeth may also promote micropitting formation compared to pure sliding conditions in the tribometer test. 6. Conclusion This study provides an understanding of the complex role of shot peening in the micropitting behavior of case hardened gears. While shot peening significantly enhances surface properties by introducing compressive residual