Creep testing of SNW-1000 sintered silicon nitride

There is a need to predict the lifetime of ceramic components that are subjected to creep at elevated temperatures. Tensile creep data are reported for a sintered silicon nitride that is currently being investigated as a candidate material for advanced heat engine applications. Specimens were tested in pure uniaxial tensile creep at 1100 and 1200°C. Creep strain was monitored with the help of a noncontact laser extensometer. Results indicate that the steady state creep rate is a function of both the temperature and the applied stress, temperature being a more important factor than the stress. Even at lower stresses, steady state creep rates were higher for specimens tested at 1200°C than at 1100°C. The effect of creep on microstructure was analyzed by examining representative transverse and longitudinal sections from the gage section using optical and scanning electron microscopies. Results indicated that after creep, the edges of pores in the material had softened and fissures extended beyond the pores, indicating the possibility for the linkage of pores during creep. For specimens tested at 1200°C, an effective threshold in applied stress was observed, above which the steady state creep rate increased drastically. EDS analyses indicated that there was a difference in chemical compositions in the pore areas, especially in the yttrium and silicon contents in the specimens subjected to creep as compared to specimens not subjected to creep. The creep rupture behavior of this material can be approximated by a straight line on a logarithmic plot of steady state creep rate versus creep rupture time, regardless of temperature or applied stress.