Nd : YAG laser drilling of 8.3 mm thick partially stabilized tetragonal zirconia-control of recast layer microcracking using localized heating techniques

The machining of ceramic components using conventional techniques is slow a nd expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. However, the performance of ceramic substrates is dependent on the quality of the processed region, with specif ic reference to the level of microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure, Generation o f recast layer microcracking is the Achilles heel of laser processing of ce ramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer an d bulk substrate. This in turn leads to the stresses which cause microcrack s to form. Techniques for the reduction of this thermal shear have been inv estigated, Optimization of Nd:YAG laser drilling enabled repeatable and hig h quality processing to be undertaken. However, adaptation of these convent ional techniques was required to reduce the level of recast layer microcrac king. A method for the numerical characterization of microcracking was deve loped for this work and was based upon SEM image processing. The use of a h igh temperature furnace to heat substrates before and after laser processin g reduced the level of recast layer microcracking by half, when compared to the ambient temperature process. However, sufficient microcracking was pre sent to cause substrate failure. An unconventional plasma heating technique was developed and applied for the reduction of this cracking. For laser dr illing at 1300 degrees C, the addition of localized plasma heating gave a f urther 14% mean reduction in recast layer microcracking. This advanced dril ling technique also produced a 12% increase in mean hole diameter when comp ared to the furnace heating method. (C) 1999 Laser Institute of America. [S 1042-346X(99)00404-0].