Acoustic microscopy for evaluating cracks produced by CO₂ laser shaping of thin alumina plates

The shaping of thin alumina ceramic plates with lasers appears to be an advantageous method for manufacturing. Unfortunately the failure rate is high because of crack initiation during the application of the high power laser. We have begun to address the issue of crack initiation by the use of in-process and post-process analysis. Here we present our results on the evaluation of cracks by optical, scanning laser, scanning electron and scanning acoustic microscopy. We present images of surface and subsurface micro-cracks generated at different power levels of our high power CO₂ laser system. The spatial variation of the Rayleigh wave velocity is measured by the V(z) curve technique. These preliminary data suggest that with improvement the V(z) technique may provide residual stress variation with high spatial resolution. The evaluation described should lead to insight into the fracture mechanism and eventually provide guidance for the choice of laser parameters (e.g., power, focus, scanning rate, emitting duration, or the like).