CRACK-GROWTH RESISTANCE-CURVE BEHAVIOR IN SILICON-CARBIDE - SMALL VERSUS LONG CRACKS

Crack-growth resistance-curve (R-curve) behavior for small (<400 mu m) surface cracks and long (>3 mm) through-thickness cracks is examined in two silicon carbide (SIG) ceramics that have sharply contrasting fr acture properties, The first, an in-situ toughened material designated ABC-SiC, fails by intergranular fracture, whereas the second, a comme rcial SIC (Hexoloy SA), fails by transgranular cleavage, In the former microstructure, hot pressing with aluminum, boron, and carbon additiv es yields a network of plate-shaped grains, and the presence of an amo rphous grain-boundary film that is similar to 1 nm thick promotes debo nding and crack deflection, The resultant grain bridging generates R-c urve toughening; in contrast, no evidence of crack-tip shielding is ob served in Hexoloy SA, R-curve behavior has been evaluated using two te chniques 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-sha ped compact-tension specimens, Although Hexoloy SA fails catastrophica lly at <3 MPa.m(1/2), ABC-SiC exhibits much-improved flaw tolerance wi th significant rising R-curve behavior and a steady-state fracture tou ghness of similar to 9 MPa.m(1/2) after crack extension of similar to 600 mu m. In ABC-SiC, however, differences in the behavior of long and small cracks exist for crack sizes of less than similar to 120 mu m, with the small-crack measurements demonstrating much-reduced crack-gro wth resistance; this effect is not observed in Hexoloy SA, Microstruct ural sources of this behavior are discussed.