Analysis of the Tribofilm Formation on the Friction Drive Surfaces of a Piezoelectric Motor

Piezoelectric motors typically have a friction drive system to transfer the movement. The miniaturized motor type investigated here has a friction drive system consisting of two drive pads that transfer the oscillating movement of the piezoelectric elements to a linear drive rail. The pads and rail consist of alumina. Tests have shown that the friction coefficient in this type of systems varies between 0.2 and 0.7, depending of wear of the surfaces and the formation of the tribofilm. This study further investigates the friction coefficient and its dependence of the pad oscillation frequency as well as the wear and the structure and of the tribofilm formed. Scanning electron microscope studies of the friction surfaces showed how the tribofilm forms as a function of numbers of cycles and frequency. To further investigate the nature of these tribofilm plateaus, cross section samples were produced with a Focused Ion Beam. Apparently the tribofilm plateaus were formed gradually by sintered/deformed wear debris. The practical implications of these findings for the possibilities to give this type of motors a stable driving force and speed as well as a long lifetime are discussed.