Crack-free processing of hot-pressed silicon nitride ceramics using pulsed YAG laser. (2nd report. Analysis on transient thermal stress and crack formation mechanism in laser-processing of silicon nitride ceramics).

Abstract

Thermal-stress-induced cracks are generated during the laser-processing of ceramics. This paper describes the temperature profile and thermal stress propagation for silicon nitride ceramics heated by Gaussian heat flux. The thermal stress distribution is calculated with consideration of the temperature dependence on material properties, the influence of laser pulse duration and its repetition rate using FEM. The tensile stress in circumferential, radial and axial directions induces the radial lateral and median cracks, respectively. The use of short laser pulses below 20μs with a repetition frequency under 10 kHz is experimentally recommended to reduce thermal stress and to avoid crack formation.