In-situ ultrasonic shear wave sensing of thin metallic coatings on journal bearing shells

Ultrasonic testing using shear polarised waves is widely applied in medical and engineering fields, commonly employed for hardness or stress measurement. The advantage of ultrasonic shear waves for wear measurement lies in their lower wave propagation speed and their sensitivity in measuring the wear scars formed through the shear motion. An in-situ wear measurement method of thin metallic coatings using ultrasonic shear waves is proposed in this paper. A finite element analysis was used to investigate the interaction between an ultrasonic shear wave and various geometry wear scars. Compared with longitudinal waves, the result shows that 10 MHz shear waves and 22 MHz longitudinal waves has a similar performance in the measurement of undamaged metallic coatings. But for discontinuously distributed scars, 10 MHz shear wave shows an amplitude decrease, where the energy has been reflected to both sides. Then, the in-situ tests were conducted, and the shear wave measurements of wear compared with microscope results. For 350 μm aluminium-alloy coated samples, the maximum deviation between shear wave results and microscope results was 5.13 μm, with a relative error of 1.5%. For 250 μm bronze-lead coatings, the maximum deviation was 5.54 μm, with a relative error of 2.51%. The practicality of using shear waves to determine continuous wear progression of a bearing is briefly discussed and their potential to monitor the health of bearing coatings in service.