This paper describes the measurement of lubricant-film thickness in a rolling element bearing using a piezoelectric thin film transducer to excite and receive ultrasonic signals. High frequency (200 MHz) ultrasound is generated using a piezoelectric aluminum nitride film deposited in the form of a very thin layer onto the outer bearing raceway. This creates a transducer and electrode combination of total thickness of less than 10 μm. In this way the bearing is instrumented with minimal disruption to the housing geometry and the oil-film can be measured noninvasively. The high frequency transducer generates a fine columnar beam of ultrasound that has dimensions less than the typical lubricated contact ellipse. The reflection coefficient from the lubricant-layer is then measured from within the lubricated contact and the oil-film thickness extracted via a quasistatic spring model. The results are described on a deep groove 6016 ball bearing supporting an 80 mm shaft under normal operating conditions. Good agreement is shown over a range of loads and speeds with lubricant-film thickness extracted from elastohydrodynamic lubrication theory.

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Thin film ultrasonic transducers have been designed which operate over an important frequency range, 300 kHz to 10 MHz. The transducers were made using piezoelectric aluminium nitride films a few microns thick. The films would have a fundamental thickness mode resonance at 1-3 GHz if fabricated as an unsupported film, however operation at much lower frequencies has been demonstrated when the transducers are fabricated on bulk substrates. This would enables them to be used in ultrasonic non-destructive testing in circumstances where the film can be deposited directly onto the object under test. We have found that the major factors influencing the belowresonance operation of the thin film transducers are the device impedance, the spectrum of the excitation pulse, and any mechanical (mass) loading applied to the back face of the transducer. Results are presented showing that the evolution of device impedance as a function of device area could be predicted using a PSpice model of the thin film transducer. The ability of the transducer to generate longitudinal mode pulses rather than shear wave pulses was found to depend on increasing the mechanical loading at the back face of the transducer. This mechanism for pulse generation was confirmed by Finite Element Modelling using PZFlex.