CAVITATION EROSION BY SINGLE LASER-PRODUCED BUBBLES

In order to elucidate the mechanism of cavitation erosion, the dynamic s of a single laser-generated cavitation bubble in water and the resul ting surface damage on a flat metal specimen are investigated in detai l. The characteristic effects of bubble dynamics, in particular the fo rmation of a high-speed liquid jet and the emission of shock waves at the moment of collapse are recorded with high-speed photography with f raming rates of up to one million frames/s. Damage is observed when th e bubble is generated at a distance less than twice its maximum radius from a solid boundary (gamma = 2, where gamma = s/R-max, s is the dis tance between the boundary and the bubble centre at the moment of form ation and R-max is the maximum bubble radius). The impact of the jet c ontributes to the damage only at small initial distances (gamma less t han or equal to 0.7). In this region, the impact velocity rises to 83 m s(-1), corresponding to a water hammer pressure of about 0.1 GPa, wh ereas at gamma > 1, the impact velocity is smaller than 25 m s(-1). Th e largest erosive force is caused by the collapse of a bubble in direc t contact with the boundary, where pressures of up to several GPa act on the material surface. Therefore, it is essential for the damaging e ffect that bubbles are accelerated towards the boundary during the col lapse phases due to Bjerknes forces. The bubble touches the boundary a t the moment of second collapse when gamma < 2 and at the moment of fi rst collapse when gamma < 1. Indentations on an aluminium specimen are found at the contact locations of the collapsing bubble. In the range gamma = 1.7 to 2, where the bubble collapses mainly down to a single point, one pit below the bubble centre is observed. At gamma less than or equal to 1.7, the bubble shape has become toroidal, induced by the jet flow through the bubble centre. Corresponding to the decay of thi s bubble torus into multiple tiny bubbles each collapsing separately a long the circumference of the torus, the observed damage is circular a s well. Bubbles in the ranges gamma less than or equal to 0.3 and gamm a = 1.2 to 1.4 caused the greatest damage. The overall diameter of the damaged area is found to scale with the maximum bubble radius. Owing to the possibility of generating thousands of nearly identical bubbles , the cavitation resistance of even hard steel specimens can be tested .