# Wear Characteristics of Metallic Counterparts under Elliptical-Locus Ultrasonic Vibration
## 3 Results
### Wear Topography  
Wear topography of metallic sliders are provided in Figure 7. Regardless of the wear scars at the sub-millimeter level (upper, \(\times\)100) or micrometer level (lower, \(\times\)1000), differences of wear traces for various metallic sliders in contact with the 2A12 material vibrator in the linear ultrasonic motor are quite obvious. Notably, some hierarchical traces of reticular pattern are shown on the contact surface of the 45 steel slider (Figure 7a). Considering the last working operation in this study before the wear experiment was finish milling, the transverse hardening zones are derived from the final machining processes. After friction and wear, hardening zones are exposed as stripes. The hierarchical and longitudinal grooves along the sliding directions originated from the plowing effect of the asperities of the driving tip and free particles. Result of macroscopic sliding friction between the vibrator contacting the longitudinal distribution of reticular grooves originated from the running rail, and were relatively static. The contact surface was not smooth enough. The hierarchical grooves in the vertical direction appeared on the slider surface caused by sliding friction. The micro level of wear scars is characterized by scaly 5 \(\upmu\)m wear traces along the sliding direction, caused by the cumulative effect of intermittent contact and periodic deformation during the vibrator's operation. Supposing the equivalent radius of asperity of the vibrator tip is \(r\), then both elastic and plastic deformation occurs at the effective surface domain within the slider surface. The excitation frequency of the applied voltage in this experiment was 39.8 kHz, and contact period was about 2.5 \(\times\) 10\({}^{-5}\) s. The speed of the motor was about 200 mm/s under the voltage amplitude of 150 V. The distance between two adjacent scaly pits nearly equal to the sliding distance of the given asperity during dynamic contact between the slider and the vibrator tip. Figure 6: Illustration of: (**a**) Relation of average surface roughness and fractal dimension; and (**b**) comparison of the change rates after wear in the unit sliding distance (1 m). The quenched 45 steel slider showed a high level of hardness (HBS294), and higher contact stiffness which generated slight plastic deformation and few shallower grooves on the surface of the heat-treated slider. Consequently, surface wear of the slider of the quenched 45 steel was fairly mild. Compared to the initial damage of machining, the wear traces and grooves of the sliding friction were relatively few, and not serious. Changes in surface roughness before and after wear further demonstrate this. As shown in the micro-level map, some regular scales appeared on the surface of the metallic slider. The size of scales and their distances apart (3 \(\upmu\)m) were relatively smaller than those of 45 steel slider. Wear traces of slider surface at the extreme location are shown in Figure 7b. As shown, a cylindrical hole with a width of 80 \(\upmu\)m was exposed, along with a few parallel grooves in front of the hole. Few scattered cracks also appear at the non-contact zone near the plastic hole. It is possible to infer that the hole resulted from accumulation between impact deformation and fatigue failure. Wear traces of the 7075 aluminum alloy slider surface are shown in Figure 7c. Thin grooves on the slider surface appeared in the direction of sliding friction, plus cracks caused by adhesive wear and fracture. There were no grooves or ridges in other directions aside from the sliding direction, which diverts from the pattern shown by 45 steel prone to work hardening. On the microscopic level, in addition to the groove-like features of plastic deformation and scalp wear traces, fatigue cracks occurred on the contact surface of the aluminum alloy 7075 slider. The length of crack propagation is clear, and the propagation direction is along the sliding direction of the slider.