Ultra-high Speed Imaging of Laser-Induced Spallation

In this work real-time images of the initiation and arrest of a laser-induced substrate spallation event have been captured over its ~40 ns duration using ultra-high speed photography. A modified version of the laser spallation technique is presented, introducing additional optical components allowing for control of the laser loading fluence (energy/unit area) without changing the emitted loading pulse energy, spot size, and profile. Spallation experiments were conducted on (100) silicon wafer substrates, 500 μm thick, coated with a 100 nm thick Al thin film at three fluence levels, 0.124, 0.174 and 0.336 J/mm². The capabilities of the modified laser spallation technique capture spallation area evolution and spallation front velocity results over time are reported for the fluence levels examined. Following the spallation event, stress wave reflections and residual thin film stresses drive additional film delamination and substrate failure. Comparisons are made between the real-time spallation images and postmortem static images highlighting the effects of stress wave reflections.