TENSION COMPRESSION CREEP ASYMMETRY IN SI3N4

We have deformed a commercial HIP-ed silicon nitride in both tension and compression at 1430 degrees C at stresses between 40 and 300 MPa. The material creeps up to 100 times faster in tension than in compression. The stress dependence of the tensile strain rate is not characterized by a single stress exponent rising from about two at low stresses to about five at higher stresses. When stressed in tension the material responds by cavitating, with the cavitation contributing significantly to the tensile strain. In compression, however, the material exhibits a stress exponent of unity, and cavitation is almost completely suppressed. The asymmetry in creep behavior between tension and compression is due to the dilatation of the material.