Calibration and validation of ultrasonic reflection methods for thin-film measurement in tribology

Interfacial thin liquid films between solid bodies have been measured using ultrasonic reflective techniques in a range of tribological applications such as those present in hydrodynamic bearings, piston rings and mechanical seals (Dwyer-Joyce et al (2004) Tribol. Lett. 17 337-48, Dwyer-Joyce et al (2006) Proc. Instn Mech. Eng. A 220 619-28, Reddyhoff et al (2006) Tribol. Trans. 51 140-49, Harper et al (2005) Tribol. Interface Eng. Ser. 48 305-12). There are two main ultrasonic methods used, the spring model and film resonance techniques. For very thin films (<20 mu m) a simple quasi-static spring model has been used to show that the proportion of the wave reflected by the liquid layer depends on the stiffness of the layer. This stiffness can be related to the layer thickness and its material properties. In the film resonance technique the incident ultrasonic wave is used to resonate the liquid layer. The frequency of resonance can then be related to the layer thickness. This paper collates four experiments where oil film thickness is measured by both ultrasonic reflection and an independent method. In the first three experiments the film thickness is measured ultrasonically and this measurement is compared to the thickness inferred from the geometry of the surfaces constraining the liquid film. In the final experiment the ultrasonic results are compared to measurements taken using capacitive and verified laser interferometer techniques. Excellent correlation was observed between the measurement methods in all of the experiments. In particular the film resonance technique showed repeatable and consistent results across a wide range of film thicknesses. The spring model showed a degree of agreement for films above a few microns but this relationship diverged as the film thickness approached the magnitude of the surface roughness.