Controlled Surface Flaws in Hot‐Pressed Si3N4

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Abstract
Surface flaws of controlled size and shape were produced in high‐strength hot‐pressed Si3N4with 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,KIC, from standard fracture‐mechanics formulas for semielliptical surface flaws in bending. After the bend specimen had been annealed, the room‐temperatureKICvalues for HS‐130 Si3N4increased 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 lowKIC, values. Elevated‐temperatureKICvalues for HS‐130 Si3N4were consistent with double‐torsion data. Controlled flaws in HS‐130 Si3N4exhibited slow crack growth at high temperatures.

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