VAMAS Round Robin on Fracture Toughness of Silicon Nitride

Abstract

Eight laboratories in Germany, Japan, U.K., and U.S. participated in the VAMAS round robin. The fracture toughness of silicon nitride at room temperature and at 1200deg;C was measured by three methods: the single‐edge V‐notched beam (SEVNB), single‐edge precracked beam (SEPB), and chevron notched beam (CNB). The obtained values show hardly any crosshead speed dependence, irrespective of test temperature and atmosphere. Results may have been influenced by a small amount of slow crack growth, but distinct R‐curve behavior could not be detected within the scope of the tests. The values at 1200deg;C in N2 can be measured by the SEVNB and SEPB methods with small scatters. The oxidation of silicon nitride, caused by heating in air, increases the SEVNB and SEPB values. The CNB values are free from the effects of test temperature and atmosphere, but they show a large scatter between laboratories. However, the chevron V‐notched beam (CVNB) method, which is an improved CNB method, shows values with a small scatter, irrespective of the measurement conditions. The SEVNB and SEPB measurements in N2 and the CVNB measurement under any conditions are recommended for the measurement of high‐temperature fracture toughness.