HIGH-SPEED PHOTOGRAPHIC STUDIES OF LASER DRILLING OF CERAMICS AND CERAMIC COMPOSITES

High-speed photographic techniques were used to study plumes generated above a material during drilling of ceramics and ceramic composites with a carbon dioxide laser. The prinicipal objectives were to identify the mechanism of material removal (spattering, particulate and fiber debris, liquid droplets) and plume phenomena (plume shapes and sizes) for ceramics and ceramic composites. High-speed photographic (1000 frames per second) visualization of laser drilling was undertaken for two monolithic ceramics, (sintered alpha-silicon carbide (alpha-SiC) and hot-pressed silicon nitride (Si3N4)) and two continuous fiber-ceramic matrix composites, (carbon fibers in a silicon carbide matrix (C-SIC) and silicon carbide fibers in a silicon carbide matrix (SiC-SiC)). The results of this study indicate that each of these ceramic materials decomposes differently during laser processing and that the material removal mechanism for a given material is possibly different at different stages of the process. In all cases, material removal is initially through high-temperature decomposition and/or vaporization. As the hole depth increases, the walls of the hole become steeper resulting in a substantial reduction of the laser flux incident on the surface; this in turn appears to modify the mechanism of material removal depending upon the composition and porosity of the material. The photographic evidence presented here clearly shows ablative material removal for sintered alpha-silicon carbide (alpha-SiC), periodic ejection of liquid silicon during laser drilling of silicon nitride and ejection of particulate/fiber debris during laser processing of the composite materials SiC-SIC and C-SiC.