Wear-Mechanism-Dependent Relationship Between the Coefficient of Friction and Acoustic Emission Characteristics in Sliding Contacts

Acoustic emission (AE) is sensitive to interfacial damage, but its relationship with the coefficient of friction (COF) changes as wear evolves, limiting mechanism-based condition monitoring. This study investigated the wear-mechanism-dependent relationship between COF and AE characteristics during reciprocating sliding of SUS304 against SUJ2 and Si3N4 balls under a load of 20 N for 500 cycles. AE waveforms and statistical features were analyzed with surface morphology and elemental distribution. Under the SUJ2 ball condition, persistent adhesive deformation produced high-amplitude burst-type AE signals, while centroid frequency remained nearly constant at 240 kHz. Accordingly, AE signal energy showed a positive linear trend with frictional work. Under the Si3N4 ball condition, the surface evolved from grooving and plastic deformation to increased oxygen coverage, followed by cracking and spallation within an oxygen-rich tribolayer. This evolution produced a three-stage nonlinear frictional work–AE relationship, with fracture-related spectral components near 580–600 kHz. Although both conditions showed comparable COF values of 0.78–0.8 at the later stage, AE energy under the SUJ2 ball condition was over one order of magnitude higher. These results indicate that a combined analysis of multiple acoustic emission features provides a more reliable basis for monitoring evolving friction and wear processes.