Crack-growth resistance-curve behavior in silicon carbide: Small versus long cracks
- Lawrence Berkeley National Lab., CA (United States). Materials Sciences Div.
Crack-growth resistance-curve (R-curve) behavior for small (<400 {micro}m) surface cracks and long (>3 mm) through-thickness cracks is examined in two silicon carbide (SiC) ceramics that have sharply contrasting fracture properties. The first, an in-situ toughened material designated ABC-SiC fails by intergranular fracture, whereas the second, a commercial SiC (Hexoloy SA), fails by transgranular cleavage. In the former microstructure, hot pressing with aluminum, boron, and carbon additives yields a network of plate-shaped grains, and the presence of an amorphous grain-boundary film that is {approximately}1 nm thick promotes debonding and crack deflection. The resultant grain bridging generates R-curve toughening; in contrast, no evidence of crack-tip shielding is observed in Hexoloy SA. R-curve behavior has been evaluated using two techniques for the different crack-length regimes: a small-crack R-curve has been deconvoluted from indentation-strength data and a long-crack R-curve has been directly measured using fatigue-precracked, disk-shaped compact-tension specimens. Although Hexoloy SA fails catastrophically at <3 MPa{center_dot}m{sup 1/2}, ABC-SiC exhibits much-improved flaw tolerance with significant rising R-curve behavior and a steady-state fracture toughness of {approximately}9 MPa{center_dot}m{sup 1/2} after crack extension of {approximately}600 {micro}m. In ABC-SiC, however, differences in the behavior of long and small cracks exist for crack sizes of less than {approximately}120 {micro}m, with the small-crack measurements demonstrating much-reduced crack-growth resistance; this effect is not observed in Hexoloy SA. Microstructural sources of this behavior are discussed.
- Sponsoring Organization:
- USDOE Office of Energy Research, Washington, DC (United States)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 554014
- Journal Information:
- Journal of the American Ceramic Society, Vol. 80, Issue 9; Other Information: PBD: Sep 1997
- Country of Publication:
- United States
- Language:
- English
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