High temperature creep and slow crack growth properties of HPSN as an example of ceramics containing a glassy phase

Abstract

The high temperature creep kinetics and the slow propagation of cracks in hot-pressed silicon nitride with a glassy phase were investigated. In addition, creep tests under constant load (4-point-bending), compliance measurements and stress relaxation experiments were carried out. Specimens with trapezoidal cross sections were used to determine the ratio of the creep rates in tension and compression. A two phase model, based on the compliance measurements, describes well the kinetics of primary creep. A consideration of the strength criterion for slow crack growth can be neglected for materials undergoing viscous flow or creep. A crack propagates, whenever the free energy of the sample is thereby lowered. Assuming that the voids nucleated in the near crack stress field and in the whole (creeping) specimen contribute to crack propagation, theory and experiments are in agreement.