Wear behavior of filled polymers for ultrasonic motor in vacuum environments

Friction materials with favorable mechanical and tribological characteristics are critical to the drivability and reliability of ultrasonic motors. Preliminary screening at atmospheric conditions suggests that filled polymer composites such as poly-tetrafluoroethylene (PTFE), phenol formaldehyde resins (PF), and poly-p-hydroxybenzoic acid (Ekonol®) are potential candidates for use as friction materials ultrasonic motors. The wear behavior of these polymers in contact with duralumin (2A12) stators was examined and compared under various environmental conditions: room temperature (RT) and atmospheric pressure, RT and low vacuum (5 Pa), RT and high vacuum (5×10−3 Pa), and both low temperature and low vacuum. The wear loss of these various polymers was characterized by tracking changes in their surface roughness parameters (Ra and Rv). Wear tracks on the surfaces of different frictional materials were inspected using scanning electron microscopy. Results show that the wear resistance of Ekonol and PF composites is poorer in vacuum than in ambient conditions, whereas that of PTFE composite is not readily susceptible to ambient conditions. Lastly, wear modes and mechanisms at the various conditions are discussed.