Comparison of damage mechanisms: Acoustic cavitation versus series of single laser-induced bubbles

Cavitation is the formation and collapse of bubbles due to pressure changes in fluids. In the vicinity of a solid surface, shock waves, an impinging water jet, and other effects of collapsing bubbles may cause severe damage. Cavitation erosion is extensively studied using techniques generating clouds of bubbles, e.g. flow channels or ultrasonic oscillations. Single bubbles can be generated in a highly controlled manner by evaporating fluid by a short laser pulse. This technique is typically used to study bubble dynamics and the damage from one single bubble on very soft materials. In the present study, two austenitic steels and a NiAl-bronze are exposed to standard acoustic cavitation and repeated laser-induced single bubbles. The evolution of surface damage and the underlying mechanisms are investigated. Surface changes are not observed before 200 single bubbles. After 50,000 bubbles the three alloys are still within the incubation phase. Comparable damage mechanisms act on the materials under both testing techniques. Since the surface area affected by repeated single bubbles is relatively small (approximate to 500 mu m diameter), the weight loss could not be measured and correlations are based on surface roughening and the mechanisms of damage specific for each material.