Third-Body Dynamics and Tribo-Oxidation Mechanism in Fretting Degradation of an Inconel 718/304 Stainless Steel O-Ring Seal.

Fretting wear at metal O-ring sealing interfaces involves the coupled processes of debris generation, entrapment, and oxidation. Existing accelerated life testing relies on similarity theory, yet it overlooks the dominant role of third-body behavior in governing wear regime transitions, so equivalence between test and service conditions lacks a physical basis. This study investigates reciprocating fretting wear of an Inconel 718/304 stainless steel pair and reinterprets wear evolution using third-body dynamics. Laser scanning confocal microscopy, energy-dispersive spectroscopy, and real-time friction monitoring reveal three distinct stages: debris generation with mild damage, oxide layer formation with steady wear, and oxide layer fracture with material spalling. The friction coefficient passes through running-in, steady-state, and sharp-rise phases, while oxygen content on the wear track rises from 2.1 wt.% to 18.3 wt.%. One-way ANOVA shows significant differences among all stages. Based on Berthier's theory, a state-evolution model is developed that treats third-body oxidation degree and cumulative friction energy dissipation as equivalence criteria. Friction energy and wear mass correlate linearly (R2 = 0.94), giving an effective wear coefficient of 6.3 × 10-7 mg/J and acceleration exponents of m1 = 9.77 for pressure and m2 = -8.71 for frequency. Independent validation shows that the model compresses test duration by 51%, with relative errors below 18% for wear mass, fractal dimension, and fractal roughness. By shifting the focus of accelerated testing from dimensional analysis to mechanism-preserving state tracking, this work provides a failure analysis framework that identifies the root cause of seal degradation as third-body oxidative spalling and offers practical preventive actions for nuclear metal seal reliability assessment.