Verification of the stresses developed in silicon nitride by repeated thermal shocks

A new testing method was used to test the resistance of silicon nitride to repeated thermal shocks. Specimens with cracks initiated by Vicker's indentor were cyclically heated to 1100 °C and cooled to 500 °C. Temperature and stress progress was computed in two points. One point was located on the surface of the specimen and the other one in a region that is most dangerous for crack growth. The temperature progress of the surface point was verified using a thermocouple. First thermal shock caused the highest temperature peak and amplitude. The temperature stabilised after different number of cycles in the two analyzed points. Stress oscilation had a different character. We defined an increasing, a stabilised and a decreasing stage. Calculated critical stress, needed for unstable crack growth (132.2 MPa) was assigned to the 13th stress peak. During practical experiments, unstable crack growth occured after the 4th cycle (114.9 MPa). The difference between this value and the value of calculated critical stress was 15%.