Research On PVDF-Based Friction Materials Of Ultrasonic Motors

An ultrasonic motor utilizes the converse piezoelectric effect of piezoelectric materials to vibratethe elastic body （stator） in the ultrasonic frequency band. The mechanical movement and torque areobtained by means of the frictional contact force between the stator and the rotor. Friction material isone of the key elements of ultrasonic motors. It is known as "the heart of the ultrasonic motor".Theefficiency of energy transform and serving life of ultrasonic motors are obviously determined byproperties of friction materials.PVDF has excellent properties such as weather ability, abrasion resistance, impact resistance, andhigh mechanical strength, etc., but it has poor heat resistance. When the inorganic particles, carbonfibers and friction properties adjusting material are added into PVDF, it is wear resistance can befurther improved. So it has the potential of friction materials for ultrasonic motor. But little attentionhas been paid to potential application in ultrasonic motor tribological field.In order to develop high performance friction materials for ultrasonic motor, carbon fibers, POBand nano-Al₂O₃were chosen to modify the properties of PVDF. The effects of composition, process,temperature stability, mechanical and tribological properties on PVDF-based friction materials werestudied. Worn surface morphology was analysed, and friction and wear mechanism was discussed.Relative conclusions were summarized as follows:1. PVDF-based friction materials of ultrasonic motor filled with carbon fiber （CF） were prepared.The effects of CF content on the properties of PVDF-based friction materials were investigated andthe optimal content of CF was confirmed. The results show that CF not only increases the mechanicalproperties and the tribological characteristics of PVDF composites, but also improves the temperaturestability of PVDF-based composites. On the whole, PVDF-based friction materials filled with15wt.%CF is the preferable friction materials for ultrasonic motors.2. PVDF-based friction materials of ultrasonic motors filled with poly-p-oxybenzoyl （POB） wereprepared. The effects of POB content on the properties of PVDF-based friction materials wereinvestigated and the optimal content of POB was confirmed. The results show that POB increases thehardness and elastic modulus of PVDF composites, and improves the temperature stability and thethermal conductivity of PVDF composites at the same time. Under the experimental conditions，PVDF composite has the lowest friction coefficient and wear mass loss when the weight content of POB is5wt.%．The USM has higher no-load speed and stall torque with this composite as frictionmaterial．3. The effects of nano-Al₂O₃which was processed by silane coupling agent KH550on theproperties of PVDF-based friction materials were studied. The results show that lower content ofnano-Al₂O₃can induce the crystal form transformation of PVDF from α to β, and improve thetribological of the materials. The wear mechanism transforms from adherence wear to abrasion wearwith the increase of the content of nano-Al₂O₃. Considering the improvement of the no-load speedand the efficiency of the motor output, PVDF-based friction materials filled with2wt.%nano-Al₂O₃is the preferable friction materials of ultrasonic motors.