Wide-area film thickness measurement method of water-lubricated polymer bearing based on ultrasonic reflection coefficient phase

Accurate identification of wide-area film thickness is crucial for detecting lubrication failure in water-lubricated bearings. Ultrasonics is an excellent non-destructive method for measuring thickness. However, due to the heterogeneity and acoustic attenuation of polymer materials, obtaining water film information with highfrequency ultrasonic sensors is challenging. There is a blind area in measurement between the resonance model and the phase matching model under low-frequency probes. Additionally, the significant acoustic impedance difference between friction pairs renders the traditional spring model inapplicable. To address these issues, the first zero-crossing method covering the blind area and the spring model-phase method for the thin films was proposed. The proposed method based on the reflection coefficient phase, in combination with existing methods, can achieve wide-area measurement of film thickness. A calibration platform using a polyetheretherketone and stainless-steel is constructed to validate the accuracy of the method. Film thickness measurement tests are conducted, and the sensitivity of various ultrasonic models is analyzed. The results demonstrate that the mean absolute error is less than 5 mu m in the water film ranging from 10 mu m to 360 mu m. When the film thickness exceeds 30 mu m, the mean relative error is less than 5 %, conversely, that is 16.61 %.