High-temperature fracture mechanism of low-Ca-doped silicon nitride
- Osaka Univ., Ibaraki (Japan). Inst. for Scientific and Industrial Research
- Lawrence Berkeley Lab., CA (United States)
High-purity Si{sub 3}N{sub 4} (with 2.5 wt% glassy SiO{sub 2}) 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 1,400 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 1,400 C was divided into three regions: ``brittle``, SCG, and creep failure as a function of both external strain rate and Ca concentration, C{sub ca}. From the investigation of the C{sub 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{sub 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 Si{sub 3}N{sub 4} polycrystalline materials containing intergranular glassy-SiO{sub 2} film.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 79780
- Journal Information:
- Journal of the American Ceramic Society, Journal Name: Journal of the American Ceramic Society Journal Issue: 3 Vol. 78; ISSN 0002-7820; ISSN JACTAW
- Country of Publication:
- United States
- Language:
- English
Similar Records
High-temperature strength of fluorine-doped silicon nitride
Fracture toughness and time-dependent strength behavior of low-doped silicon nitrides for applications at 1400 C