In order to elucidate the mechanism of cavitation erosion, the
dynamics of a single
laser-generated cavitation bubble in water and the resulting surface damage
on a
flat metal specimen are investigated in detail. The characteristic effects
of bubble
dynamics, in particular the formation of a high-speed liquid jet and the
emission of
shock waves at the moment of collapse are recorded with high-speed photography
with framing rates of up to one million frames/s. Damage is observed
when the
bubble is generated at a distance less than twice its maximum radius from
a solid
boundary (γ=2, where γ=s/Rmax,
s is the distance between the boundary and
the bubble centre at the moment of formation and Rmax
is the maximum bubble
radius). The impact of the jet contributes to the damage only at small
initial distances
(γ[les ]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, whereas at γ>1,
the impact velocity is smaller than 25 m s−1. The largest
erosive
force is caused by the collapse of a bubble
in direct contact with the boundary, where pressures of up to several GPa
act on
the material surface. Therefore, it is essential for the damaging effect
that bubbles are
accelerated towards the boundary during the collapse phases due to Bjerknes
forces.
The bubble touches the boundary at the moment of second collapse when γ<2
and
at the moment of first collapse when γ<1. Indentations on an aluminium
specimen
are found at the contact locations of the collapsing bubble. In the range
γ=1.7 to 2,
where the bubble collapses mainly down to a single point, one pit below
the bubble
centre is observed. At γ[les ]1.7, the bubble shape has become toroidal,
induced by
the jet flow through the bubble centre. Corresponding to the decay of this
bubble
torus into multiple tiny bubbles each collapsing separately along the circumference
of the torus, the observed damage is circular as well. Bubbles in the ranges
γ[les ]0.3
and γ=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.
A Cavitation Erosion Model for Ductile Materials