Transient elastic and viscoelastic thermal stresses during laser drilling of ceramics

Abstract

Lasers appear to be particularly well suited to drill and shape hard and brittle ceramics, which are almost impossible to netshape to tight tolerances, and can presently only be machined by diamond grinding. Unfortunately, the large, focussed heat flux rates that allow the ready melting and ablation of material, also result in large localized thermal stresses within the narrow heat-affected zone, which can lead to micro-cracks, significant decrease in bending strength, and even catastrophic failure. In order to assess the where, when and what stresses occur during laser drilling, that are responsible for cracks and decrease in strength, elastic and viscoelastic stress models have been incorporated into our two-dimensional drilling code. The code is able to predict temporal temperature fields as well as the receding solid surface during CW or pulsed laser drilling. Using the resulting drill geometry and temperature field, elastic stresses as well as viscoelastic stresses are calculated as they develop and decay during the drilling process. The viscosity of the ceramic is treated as temperature-dependent, limiting viscoelastic effects to a thin layer near the ablation front where the ceramic has softened.Lasers appear to be particularly well suited to drill and shape hard and brittle ceramics, which are almost impossible to netshape to tight tolerances, and can presently only be machined by diamond grinding. Unfortunately, the large, focussed heat flux rates that allow the ready melting and ablation of material, also result in large localized thermal stresses within the narrow heat-affected zone, which can lead to micro-cracks, significant decrease in bending strength, and even catastrophic failure. In order to assess the where, when and what stresses occur during laser drilling, that are responsible for cracks and decrease in strength, elastic and viscoelastic stress models have been incorporated into our two-dimensional drilling code. The code is able to predict temporal temperature fields as well as the receding solid surface during CW or pulsed laser drilling. Using the resulting drill geometry and temperature field, elastic stresses as well as viscoelastic stresses are calculated as they develop and ...