Fluorescence‑Based Calibration Model for In‑Situ Measurement of Micro‑scaled Lubricant Thickness Distribution at Indentation Interface

This study proposes a model for the measurement of microscale liquid film thickness distribution using fluorescence signals.The interfacial conditions between the tool and the workpiece in mechanical machining are important for understanding these phenomena and mechanisms.In this study,indentation tests with transparent tools were used to observe interfaces;however,it was challenging to obtain the signal from a thin fluorescent liquid film on smooth and steeply inclined surfaces.Therefore,fluorescence-based measurement,such as laser-induced fluorescence,was employed.To measure the absolute thickness of the thin fluorescent film,calibration of the measurement system is necessary.Therefore,a theoretical model was proposed considering the multiple reflections of excitation light and fluorescence at the inclined surface between the indenter and workpiece.By measuring the profile of the surface topography of the indented workpiece and comparing the results with those measured by a surface profiler,the validity of the proposed calibration method and the performance of this measurement system were demonstrated.The measured surface profiles,including scratches of 2–4μm,were in good agreement,demonstrating the validity of the proposed method.