Controlled surface-flaw-initiated fracture in reaction-bonded Si3N4

Abstract

Reproducible controlled surface flaws were introduced in reaction-bonded Si3N4 by Knoop microhardness indentation. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a four-point bend specimen, and the stress required to propagate the controlled flaw catastrophically was used to calculate the critical stress intensity factor KIC from standard fracture-mechanics formulae for semi-elliptical surface flaws in bending. KIC was unchanged up to 1200° C, but values above this temperature together with those at room temperature after vacuum annealing were reduced approximately 25%. This reduction is thought to be due to reaction with oxygen concentrations present in the vacuum “atmosphere” rather than to a change in material properties. Although flaw “healing” was observed for annealing in air, it was accompanied by a decrease in fracture stress. It is suggested that this is due to volume mismatch between Si3N4 and the oxidation product, cristobalite, which introduces tensile stresses in the matrix material.