Rising Fracture Resistance of Whisker-Reinforced Alumina and Silicon Nitride
Fracture resistance behaviour of two different whisker reinforced ceramics (Al2O3 + SiC-whiskers and Si3N4 + Si3N4-whiskers) was studied by means of indentation technique. The measurements were performed using the controlled flaw / strength (CFS) technique. The microstructure and fracture properties were studied on the polished specimen surfaces using light and scanning electron microscopy. The toughening mechanisms acting in the wake zone behind the crack tip were studied both, on fracture surfaces and indentation fracture lines.
The silicon nitride based material showed a considerably higher fracture toughness. Both materials exhibited a rising R-curve behaviour as a function of crack extension. The R-curves according to power-law prediction were constructed. As main mechanisms causing the rising R-curve effect crack deflection and crack bridging by whiskers were indentified. Using a simple model of the bridging forces in the wake region of the crack tip, the crack closure stress profiles were evaluated. Thus, effectiveness of the whisker reinforcing in the studied materials was showed.
KeywordsCrack Length Silicon Nitride Indentation Load Crack Growth Resistance Closure Stress
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