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 and 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 specific reference to the level of microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure. Generation of recast layer microcracking is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses which cause microcracks to form. Techniques for the reduction of this thermal shear have been investigated. Optimization of Nd:YAG laser drilling enabled repeatable and high quality processing to be undertaken. However, adaptation of these conventional techniques was required to reduce the level of recast layer microcracking. A method for the numerical characterization of microcracking was developed for this work and was based upon SEM image processing. The use of a high temperature furnace to heat substrates before and after laser processing reduced the level of recast layer microcracking by half, when compared to the ambient temperature process. However, sufficient microcracking was present to cause substrate failure. An unconventional plasma heating technique was developed and applied for the reduction of this cracking. For laser drilling at 1300 °C, the addition of localized plasma heating gave a further 14% mean reduction in recast layer microcracking. This advanced drilling technique also produced a 12% increase in mean hole diameter when compared to the furnace heating method.