Minimization of elastic thermal stresses during laser machining of ceramics using a dual-beam arrangement

Lasers are emerging as a valuable tool for shaping and cutting hard and brittle ceramics. Unfortunately, the large, concentrated heat flux rates that allow the laser to efficiently cut and shape the ceramic also result in large localized thermal stresses in a small heat-affected zone. These notable thermal stresses can lead to micro-cracks, a decrease in strength and fatigue life, and possibly catastrophic failure. In order to assess where, when, and what stresses occur during laser scribing, an elastic stress model has been incorporated into a two-dimensional drilling as well as a three-dimensional scribing and cutting code. The results of the analysis for a single Gaussian laser source show that substantial tensile stresses develop over a thick layer below and parallel to the surface, which may be the cause of experimentally observed subsurface cracks. In this work the effect of splitting the beam into two parts is discussed: a highly-focussed beam that does the actual drilling or scribing, and a partially defocussed beam, used to heat the material ahead of, around, or behind the laser beam. Different secondary beam scenarios are investigated, and their effect on reducing the damaging tensile stresses is evaluated.