Thin films of RF sputtered Aluminium Nitride are columnar in structure and exhibit piezo-electric properties. They can therefore be used as ultrasonic transducers.
This novel kind of transducer has two major advantages over conventional transducers. Firstly they can fit into limited spaces - ideal for a tightly packed transmission system. And secondly, because they are so thin they resonate at very high frequency (in the range 200 - 300 MHz).
In this work a sputtered piezo-coating was used to measure the oil film thickness in a deep groove ball bearing. The photo of the ball bearing shows a coating applied to the outer raceway. The electrical contact was sputtered onto the coating and then wires soldered in place.
This approach is advantageous because since the transducers are of such high frequency and can be activated over a very small area, it is not necessary to focus the sound wave to ensure it all falls within the contact region between the ball and raceway. The results below show the measured film thickness compared against predictions from Downson & Hamrock's EHL solution for a range of bearing loads and speeds.
- Drinkwater B.W, Zhang J1, Kirk K.J , Elgoyhen J, Dwyer-Joyce R.S, (2008) "Measurement of rolling bearing lubrication using piezoelectric thin films." ASME, Journal of Tribology, Volume 131, Issue 1, 011502. DOI:10.1115/1.3002324