Differential Influence Law of Rough Lubrication Interfaces on Ultrasonic Oil Film Echo Characteristics and Stability Evaluation of Its Engineering Measurement

Abstract Oil-film thickness is critical for evaluating lubrication and failure risk in bearings, gears and sealing interfaces. Ultrasonic pulse-echo monitoring is suitable for closed metallic contacts, but conventional models usually assume smooth interfaces and therefore neglect roughness-induced changes in local echoes. This study investigates a four-layer lubricated system consisting of 42CrMo steel, ISO VG320 mineral oil, a 42CrMo roughness insert and 42CrMo steel. Surface roughness Ra = 0.2–3.2 and nominal oil-film thickness h = 100–500 µm were examined. A theoretically constrained adaptive-window method was developed to extract local oil-film echoes affected by rough interfaces. The echo packet was identified by locating the Hilbert-envelope peak within a theoretically predicted arrival-time region. Results show that oil-film thickness mainly determines the temporal position of the echo packet, whereas roughness primarily alters its morphology. Envelope peak amplitude and window energy are the most sensitive features under high roughness. Non-monotonic responses at Ra = 0.8–1.6 µm reveal a transition regime in which specular reflection and diffuse scattering coexist. At high roughness, phase linearity is strongly disrupted, reducing the reliability of phase-slope-based thickness inversion. These results provide a laboratory-scale basis for understanding roughness-induced ultrasonic echo distortion and for improving the robustness of future ultrasonic oil-film monitoring methods.