Controlled Surface Flaws in Hot-Pressed Si3N4 |
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Authors: | J J PETROVIC L A JACOBSON P K TALTY A K VASUDEVAN |
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Affiliation: | Metallurgy and Ceramics Research Laboratory, Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433 |
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Abstract: | Surface flaws of controlled size and shape were produced in high-strength hot-pressed Si3N4 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 Si3N4 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 KIC values for HS-130 Si3N4 were consistent with double-torsion data. Controlled flaws in HS-130 Si3N4 exhibited slow crack growth at high temperatures. |
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