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.m1/2, ABC‐SiC exhibits much‐improved flaw tolerance with significant rising R‐curve behavior and a steady‐state fracture toughness of ∼9 MPa.m1/2 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.