CYCLIC FATIGUE AND RESISTANCE-CURVE BEHAVIOR OF AN IN-SITU TOUGHENED SILICON-CARBIDE WITH AL-B-C ADDITIONS

The room-temperature crack-growth properties of an in situ toughened, monolithic silicon carbide are reported. Hot pressing was performed at 1900 degrees C with 3 wt.% Al, 2 wt.% C and 0.6 wt.% B additions. Com pared to a commercial SiC (Hexoloy SA), significant improvements in bo th the fracture toughness and cyclic fatigue-crack propagation resista nce have been achieved through control of the beta to alpha transforma tion. Using fatigue-precracked, disk-shaped compact-tension specimens, marked rising resistance-curve behavior was measured over the first s imilar to 600 mu m of crack extension, leading to a ''plateau'' fractu re toughness of K-c similar to 9.1 MPa root m; this represents more th an a threefold increase over the toughness of Hexoloy, where a K-c val ue of 2.5 MPa root m was measured with no evidence of a resistance cur ve. Cyclic fatigue-crack growth rates in the toughened SiC were found to be faster than those under sustained loads (static fatigue) at the same stress-intensity level. The cyclic fatigue-crack growth resistanc e was found to be far superior to that of Hexoloy. Whereas cracking in the commercial SiC became unstable when the maximum stress intensity K-max exceeded similar to 2 MPa root m, thresholds for fatigue-crack g rowth in the in situ toughened material exceeded a K-max of 7 MPa root m. Such dramatic improvements in the crack-growth resistance of SiC a re attributed to a microstructure consisting of a network of interlock ing, plate-like predominantly ct-phase grains, which combine to both b ridge and deflect the crack. Under cyclic loads, fatigue-crack growth is promoted by the cycle-dependent decay in such crack-tip shielding d ue to frictional-wear degradation of the zone of grain bridging ligame nts in the crack wake. These results represent the first reported evid ence of cyclic fatigue behavior in a monolithic silicon carbide and th e first direct measurement of the resistance curve properties in this ceramic.