Controlled Surface Flaws in Hot‐Pressed Si ₃ N ₄

Abstract

Surface flaws of controlled size and shape were produced in high‐strength hot‐pressed Si 3 N 4 with a Knoop microhardness indenter. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a 4‐point‐bend specimen, with attendant reduction in the measured magnitude and scatter of the fracture strength. The stress required to propagate the controlled flaw was used to calculate the critical stress‐intensity factor, K IC , from standard fracture‐mechanics formulas for semielliptical surface flaws in bending. After the bend specimen had been annealed, the room‐temperature K IC values for HS‐130 Si 3 N 4 increased to a level consistent with values obtained from conventional fracture‐mechanics tests. It was postulated that annealing reduces the residual stresses produced by the microhardness indentation. The presence of residual stresses may account for the low K IC , values. Elevated‐temperature K IC values for HS‐130 Si 3 N 4 were consistent with double‐torsion data. Controlled flaws in HS‐130 Si 3 N 4 exhibited slow crack growth at high temperatures.