Influence from humidity on the alumina friction drive system of an ultrasonic motor

Miniaturized ultrasonic motors, based on piezoelectric movement, are rapidly developing and the number of commercial applications is steadily growing. The details of the friction drive system of these motors still need systematic studies to fully utilize the potential of the technique. The friction drive system transfers the high frequency oscillatory movement from the stator to a drive rail. The friction force should be maximized and stabilized to achieve the best motor performance, while maintaining a low wear rate. The mating parts of a friction drive system typically consist of alumina which is selected due to its relatively low wear rate and relatively high coefficient of friction. With increasing relative humidity, the friction coefficients of alumina ceramics generally decrease. This work examines how water and humidity affect a friction drive system with respect to coefficient of friction and wear of the mating surfaces. Ball-on-disc experiments were used to evaluate the tribological properties. The worn surfaces were studied by scanning electron microscopy. The coefficient of friction was found to be relatively low but stable with water on the surface but higher and more fluctuating in dryer conditions. The character of the worn surfaces did not clearly correlate to the friction behaviour.