Creep of a Silicon Nitride Under Various Specimen/Loading Configurations

Extensive creep testing of a hot-pressed silicon nitride (NC132) was performed at 1300°C in air using five different specimen/loading configurations, including pure tension, pure compression, four-point uniaxial flexure, ball-on-ring biaxial flexure, and ring-on-ring biaxial flexure. Nominal creep strain and its rote for a given nominal applied stress were greatest in tension, least in compression, and intermediate in uniaxial and biaxial flexure. Except for the case of compressive loading, nominal creep strain generally decreased with time, resulting in less-defined steady-state condition. Of the four different creep formulations - power-law, hyperholic sine, step, and redistribution models - the conventional power-law model still provides the most convenient and reasonable means to estimate simple, quantitative creep parameters of the material. Predictions of creep deformation for the ease of multiaxial stress state Cniaxial flexure) were made based on pure tension and compression creep data by using the design code CARESCreep.