Nd:YAG laser cutting and drilling of PSTZ—Influence of substrate heating temperature on recast layer microcracking

Abstract

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. Recast layer microcracking is the Achilles heel of laser processing of ceramics. Techniques for the reduction of the thermal shear caused by laser beam interaction have been investigated. A method for the numerical characterization of microcracking was developed for this work and was based upon scanning electron microscopy image processing. Optimization of the pulsed Nd:YAG laser drilling and cutting cycles enabled repeatable, high quality processing to be undertaken. Heating of the partially stabilized tetragonal zirconia (PSTZ) substrates to high temperatures before and after laser processing was found to reduce the thermal gradients that cause microcracking. Holes with a mean diameter of 679 μm were percussion drilled through the 8.3 mm thick substrates in 0.75 s, and had limited tapering (<150 μm). Single pass, full depth cutting was achieved at a rate of 100 mm min−1. Laser drilling at 1300 °C caused, on average, half the level of microcracking found in the ambient temperature drilled substrates. Laser cutting using the same comparison led to a 6.7× reduction in microcracking. The cutting process overall was found to be less damaging to the ceramic substrates by a factor of 12×. A thermal gradient prediction was used to explain this effect, and was based upon the influence of thermal diffusivity, thermal conductivity, and the effect of the impinging assist gas jet.