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

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. Recast layer microcracking is the Ac hilles heel of laser processing of ceramics. Techniques for the reduction o f the thermal shear caused by laser beam interaction have been investigated . A method for the numerical characterization of microcracking was develope d for this work. and was based upon scanning electron microscopy image proc essing. Optimization of the pulsed Nd:YAG laser drilling and cutting cycles enabled repeatable, high quality processing to be undertaken. Heating of t he partially stabilized tetragonal zirconia (PSTZ) substrates to high tempe ratures before and after laser processing was found to reduce the thermal g radients that cause microcracking. Holes with a mean diameter of 679 mu m w ere percussion drilled through the 8.3 mm thick substrates in 0.75 s, and h ad limited tapering (<150 mu m). Single pass, full depth cutting was achiev ed at a rate of 100 mm min(-1). Laser drilling at 1300 degrees 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 6.7x r eduction in microcracking. The cutting process overall was found to be less damaging to the ceramic substrates by a factor of 12x. A thermal gradient prediction was used to explain this effect, and was based upon the influenc e of thermal diffusivity, thermal conductivity, and the effect of the impin ging assist gas jet. (C) 1999 Laser Institute of America. [S1042-346X(99)00 101-1].