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The Utility of R-Curves for Understanding Fracture Toughness-Strength Relations in Bridging Ceramics

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The mechanical behavior of four rare earth (RE)-Mg-doped Si3N4 ceramics (RE=La, Lu, Y, Yb) with varying grain-boundary adhesion has been examined with emphasis on materials containing La and Lu (which represent the extremes of RE ionic radius). Fracture-resistance curves (R-curves) for all ceramics rose very steeply initially, giving them exceptional strength and relative insensitivity to flaw size. The highest strength was seen in the Lu-doped material, which may be explained by its steeper initial R-curve; the highest “apparent” toughness (for fracture from millimeter-scale micronotches) was seen in the lowest strength La-doped material, which may be explained by its slowly rising R-curve at longer crack lengths. Excellent agreement was found between the predicted strengths from R-curves and the actual strengths for failures originating from natural flaws, a result attributed to careful estimation of the early part of the R-curve by deducing the intrinsic toughness, K0, and the fact that this portion of the R-curve is relatively insensitive to sample geometry. Finally, it was found that RE elements with relatively large ionic radius (e.g., La) tended to result in lower grain-boundary adhesion. This implies that there is a small window of optimal grain-boundary adhesion which can lead to the fastest rising R-curves (for short cracks) and the highest strengths. The importance of this work is that it reinforces the notion that factors which contribute to the early part of the R-curve are critical for the design of ceramic microstructures with both high strength and high toughness.
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Document Type: Research Article

Affiliations: 1: Materials Science, School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, Oregon 97331 2: Institute for Ceramics in Mechanical Engineering, University of Karlsruhe, D-76131 Karlsruhe, Germany 3: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720

Publication date: 2008-06-01

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