Effect of the MgO/SiO2Ratio on the Strength of Hot-Pressed Si3N4. Interrelation between Creep and Slow Crack Growth. Crack Extension and Arrest:Theory and Experiments for Contact Stress Fields.

摘要

The strength of hot-pressed Si3N4 containing different MgO/SiO2 molar ratios was studied. Significant increases in strength at 1400 C were observed as the MgO/SiO2 ratio increased to 3-4. Smaller decreases in strength at both 25 C and 1400 C were observed when MgO/SiO2 > 4. These observations are briefly discussed with regard to possible chemistry changes of the grain boundary phase believed to be present in hot-pressed Si3N4. Phenomenological interrelations between creep strain and slow crack growth are derived based on the fact that the total creep strain at fracture is determined by the growth kinetics of a pre-existing crack. It is shown that measurements of the creep strain in a region remote from the growing crack can be used to predict failure periods for materials that concurrently exhibit creep and slow crack growth. Sudden crack extension and arrest is observed when indenters are pressed into the surface of brittle materials. The energitics of this system is examined to determine the criteria for crack extension and arrest. Experimental observations are presented to confirm the first criterion of crack extension and its relation to material properties. The experiments were performed on hot-pressed Si3N4 and SiC.