High-Temperature Fracture Mechanism of Low-Ca-Doped Silicon Nitride |
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Authors: | Isao Tanaka Ken'ichiro Igashira Taira Okamoto Koichi Niihara Rowland M. Cannon |
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Affiliation: | Institute for Scientific and Industrial Research, Osaka University, Ibaraki, 567 Osaka, Japan;Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 |
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Abstract: | High-purity Si3N4 (with 2.5 wt% glassy SiO2) doped with 0 to 450 at.ppm of Ca was prepared as a model system to investigate the effects of grain-boundary segregants on fracture phenomenology at 1400°C. Subcritical crack-growth (SCG) resistance as well as creep resistance was degraded significantly by the presence of a small amount of Ca. The internal friction of the doped materials exhibited the superposition of a grain-boundary relaxation peak and a high-temperature background, and the apparent viscosity of the grain-boundary film was determined from the peak. Based on these experimental data, the fracture mechanism at 1400°C was divided into three regions: "brittle," SCG, and creep failure as a function of both external strain rate and Ca concentration, C Ca. From the investigation of the C Ca dependence of the critical strain rate for the transition from "brittle" to SCG fractures, the SCG phenomenon is suggested to be triggered by small-scale, grain-boundary sliding. The C Ca dependence of "steady-state" creep rate was far from the theoretical dependence of diffusional creep via a solution-precipitation mechanism. The discrepancy was interpreted to be due to the presence of an impurity-insensitive creep component. This component may correspond to the lowest limit of the tensile creep rate in Si3N4 polycrystalline materials containing intergranular glassy-SiO2 film. |
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