Wear

Ultrasonic wear falls into three main categories —

• Cavitation erosion occurs when a material is exposed to a cavitating fluid. The collapsing cavitation bubbles cause intense shock waves and microjets which, in turn, cause highly localized surface stresses. Repetition of this loading due to repeated bubble collapses causes local surface fatigue failure and the subsequent detachment or flaking off of pieces of material. (Brennen, section 3.6, p. 91)
• Abrasion occurs when a hard rough surface slides across a softer surface — for example, when a horn abrades as it vibrates against a glass-filled plastic. The rate of abrasion depends on the relative hardnesses between the contacting materials.
• Fretting occurs when two contacting surfaces experience cyclic motion (oscillatory tangential displacement) of small amplitude. Fretting is a problem at joints and metal welding tooling surfaces.