Photoacoustic Signal from Subsurface Defects in Ceramics

Abstract

The laser scanning photoacoustic (PA) technique was used to detect subsurface slots in Si3N4, SiC and AlN ceramics. The PA signal amplitude peak width across the slot decreased with increasing chopping frequency, up to 480 Hz. At low frequencies, the peak width for SiC and AlN was considerably larger than that for Si3N4. This is ascribed to the difference in thermal diffusivity among these ceramics. The amplitude peak height vs subsurface slot depth relationships for Si3N4 are in good agreement with the calculation based on the Rosencwaig-Gersho theory. However, the relation-ships for SiC and AlN disagree with the calculation. This suggests that at low frequencies the PA signal on the slot is affected by the thermal diffusion outside the slot.