Slow crack growth in Si3N4 at room temperature

Abstract

Tests of Si3N4 hot pressed with various types and levels of oxide additives show evidence of room temperature slow crack growth in delayed failure tests (using natural flaws), but not in fracture mechanics (e.g. DCB or DT) tests consistent with more limited literature data for these two types of tests. Neither type of test showed slow crack growth in either CVD Si3N4 or RSSN. Further the fracture mode in the latter two materials was essentially all transgranular, while it was predominantly (e.g. 80%) intergranular in the hot-pressed materials. It was thus postulated that (1) the oxide grain boundary phase is responsible for slow crack growth and; (2) varying distribution of the oxide boundary phase and grain boundary character result in sufficient boundaries not susceptible to slow crack growth to pin cracks with macroscopic crack lengths (i.e. as in DCB and DT tests). Both the much smaller crack front lengths and the large number of small (natural, e.g. machining) flaws allows some of these small flaws to grow to critical size, thus leading to delayed failure in the hot-pressed materials.