Crack-Growth Resistance-Curve Behavior in Silicon Carbide : Small versus Long Cracks

Crack-growth resistance-curve (R-curve) behavior for small (<400 μm) 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 ∼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·m^1/2, ABC-SiC exhibits much-improved flaw tolerance with significant rising R-curve behavior and a steady-state fracture toughness of ∼9 MPa·m^1/2 after crack extension of ∼600 μm. In ABC-SiC, however, differences in the behavior of long and small cracks exist for crack sizes of less than ∼120 μm, 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.