Fracture of hot-pressed silicon nitride at elevated temperatures
MgO-doped and Al/sub 2/O/sub 3/-Y/sub 2/O/sub 3/-doped hot-pressed silicon nitrides were fractured between 1100 and 1350/sup 0/C in four-point bend at three displacement rates. Fracture stress and critical stress intensity factor were plotted vs temperature for each displacement rate. Fracture stress and critical stress intensity factor showed a strain rate dependence above 1200/sup 0/C. The MgO-doped Si/sub 3/N/sub 4/ exhibited a peak in fracture stress before the values decreased at high temperatures. This peak corresponds to the onset of subcritical crack growth prior to catastrophic failure. Subcritical cracking is believed to occur by the nucleation, growth and coalescence of cavities in the secondary, amorphous grain boundary phase. The coalescence of cavities along with grain boundary separation results in the formation of microcracks in the vicinity of the main crack. The main crack then advances slowly by joining up with these nearby microcracks. Cavities and regions of grain boundary deformation were observed with SEM and TEM. A map was drawn of displacement rate versus temperture showing regions where subcritical cracking was and was not observed as well as a transition region. Both silicon nitrides exhibited a peak in effective critical stress intensity factor prior to the decrease. A decrease in K/sub 1//sub C/ results as the material weakens at high temperatures. The Y/sub 2/O/sub 3/-Al/sub 2/O/sub 3/-doped silicon nitride sustained its strength and K/sub 1//sub C/ to higher temperatures due to its more refractory Y/sub 2/O/sub 3/-Al/sub 2/O/sub 3/-SiO/sub 2/ crystalline compound grain boundary phase than the amorphous magnesium-silicate phase present in the MgO-doped silicon nitride.
- Research Organization:
- Illinois Univ., Urbana (USA). Dept. of Ceramic Engineering
- DOE Contract Number:
- AC02-76ER01198
- OSTI ID:
- 6596903
- Report Number(s):
- DOE/ER/01198-1395; ON: DE83005236
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
360203* -- Ceramics
Cermets
& Refractories-- Mechanical Properties
ALUMINIUM COMPOUNDS
ALUMINIUM OXIDES
AUGER ELECTRON SPECTROSCOPY
BENDING
CAVITIES
CHALCOGENIDES
CRACK PROPAGATION
CRYSTAL STRUCTURE
DOPED MATERIALS
ELECTRON MICROSCOPY
ELECTRON SPECTROSCOPY
FABRICATION
FRACTURE PROPERTIES
GRAIN BOUNDARIES
HOT PRESSING
MATERIALS
MATERIALS WORKING
MECHANICAL PROPERTIES
MICROSCOPY
MICROSTRUCTURE
NITRIDES
NITROGEN COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PNICTIDES
PRESSING
SILICON COMPOUNDS
SILICON NITRIDES
SPECTROSCOPY
STRAIN RATE
STRESS INTENSITY FACTORS
TEMPERATURE DEPENDENCE
TRANSITION ELEMENT COMPOUNDS
VERY HIGH TEMPERATURE
YTTRIUM COMPOUNDS
YTTRIUM OXIDES