High Temperature Crack Growth in Silicon Nitride with Two Different Grain Sizes Under Static and Cyclic Loads

Abstract

The crack growth behaviour at 1300°C of two silicon nitrides with different grain sizes was studied and compared with the behaviour at room temperature. It was found that at 1300°C, unlike at room temperature, both materials exhibited a beneficial effect of cyclic loading on the crack growth resistance. From observations of crack paths and fracture surfaces of specimens with cracks grown at elevated temperature but fractured at room temperature it was found that the formation of bridges was promoted by high stress intensities and static loading. In contrast to at room temperature the bridges consisted not only of single grains but also multiple grains. The room temperature fracture toughnesses of testpieces with cracks grown under static and cyclic loads at 1300°C confirmed that more bridges were formed during static loading and at high K-levels. The lower crack growth rates observed under cyclic loading as compared to static loading was suggested to be the result of a higher crack opening displacement rate which increased the load transferred to the bridges even if the amount of bridges were larger under static loading. With increasing K-levels both the crack growth rate and the crack opening displacement rate increased and consequently the effect of bridging increased very rapidly in the case of static loading. For high K-levels the crack growth rates under static loading became similar to those under cyclic loading. The crack growth resistance was higher for the large grain size material both for cyclic and static loading.