Long-Term (>30,000-40,000 h) Creep Behavior of an Advanced Si3N4 Ceramic

Long-term creep data are reported for a HIPped Si3N4 ceramic tested at 1250 C in air for 28000 to 38000 h (3.2 to 4.34 years) with no creep failure. Two specimens tested at 150 and 175 MPa each showed extensive primary creep in excess of 10000 h, followed by well-defined steady-state creep. Because the primary creep range was extensively long and reasonably linear, the creep curves appeared to portray two stages of steady-state creep having occurred due to two different creep mechanisms: formation of cavities and growth of cavities. Approximately 57 to 70% of measured apparent creep strain was estimated to be due to the evolution of cavities, while the remaining was attributed to the viscous creep of grain boundary materials. 5 refs.