# Wear Characteristics of Metallic Counterparts under Elliptical-Locus Ultrasonic Vibration
## 1 Introduction  
Elliptical-locus ultrasonic vibration has been favored in recent years for its capability to cut non-conductive, brittle workpiece materials [1]. The reason is that the frictional force between the tool rake and the chip is effectively reduced by reversing the frictional direction, and the reversed frictional force assists the chip to flow out [2]. More importantly, the development of ultrasonic vibration has brought about new trends in the miniaturization and multifunctionality of special sensors, actuators and/or motors. Compared with conventional electromagnetic motors, remarkable advantages of the ultrasonic motor [3,4] make itself an attractive technology for applications in civilian industries, automation production, biological medicine, precision engineering, the aerospace industry, and others. Because of such promising applications in various industries, research into improving the output power, efficiency and longevity are quite valuable. Currently, several inevitable problems yet exist to be solved before moving the ultrasonic motor from laboratories to factories. As is well-known, ultrasonic vibration in different existing forms play specific roles in components. Unilateral vibration either in sliding direction or normal to sliding direction would reduce the coefficient of friction [5], thus the in-plane ultrasonic vibration is hardly devoted to the power transmission by using ultrasonic vibration [6]. Fundamentally, ultrasonic motors are driven through ultrasonic vibration and friction action [7]. Inherent friction and wear between the driving tip and the driven slider become crucial considerations for the output performance and life-span of ultrasonic motors. Therefore, high coefficient of friction and low wear rate, specifically, are the primary goal of ultrasonic motor design [8] and parameterization. Besides, dynamic contact of the driving tip and the slider, especially for standing-wave motors, is intermittent and impacting [9,10], resulting in severe wear and material loss over traveling wave motors [11]. In addition, special requirementsfor the drive signals of different vibrators are necessary for sufficient impedance matching [12], and have not yet been standardized; this problem would be enhanced by the dynamic contact process, with a consequence of the erratic output performance of ultrasonic motor induced by the variable friction force. Last but not least, ultrasonic motors make a concession to the service life of the contact interface, thereafter the output mechanical power is small and their working efficiency needs improve further. Consequently, friction and wear of the contact surfaces between the stator and vibrator are critical for both output characteristics and service life-span of ultrasonic motors. This issue is related to kinematics of driving tip, dynamic contact mechanics of contact interfaces, friction materials of counterparts, wear mechanisms and/or modes, and so on. Given the contact interface of a driving tip and a slider of standing-wave ultrasonic motor turn into a critical and urgent problem for engineers and technologists, design of friction materials and comprehension of wear mechanisms for the interface of driving and driven parts would play an effective role. Relatively little research currently exists regarding the optimization of surface characteristics or illustration of wear mechanisms for standing-wave ultrasonic motors. [PERSON] et al. [13] reported that stainless steel drive shafts and sliders exhibit favorable dynamic behaviors in linear ultrasonic motors. Tribological performances of the friction pair of a middle carbon steel driving tip and alumina slider were investigated [14; 15], where results showed that friction increases with the increase of preload applied to the contact surface, higher temperature, and decrease of sliding speed and humidity. [PERSON] et al. [16] compared two kinds of alumina ceramic surfaces with different roughness, presenting to the relatively smooth surface with relatively low specific wear amount.