Cyclic fatigue and resistance-curve behavior of an in situ toughened silicon carbide with Al-B-C additions

• Published in: Acta materialia 1996-08, Vol.44 (8), p.3199-3214
• Main Authors: GILBERT, C. J, CAO, J. J, MOBERLYCHAN, W. J, DEJONGHE, L. C, RITCHIE, R. O
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Fatigue, brittleness, fracture, and cracks; Mechanical and acoustical properties of condensed matter; Mechanical properties of solids; Physics
• ISSN: 1359-6454
• DOI: 10.1016/1359-6454(95)00409-2

The room-temperature crack-growth properties of an in situ toughened, monolithic silicon carbide are reported. Hot pressing was performed at 1900 deg C with 3 wt.% Al, 2 wt.% C and 0.6 wt.% B additions. Compared to a commercial SiC (Hexoloy SA), significant improvements in both the fracture toughness and cyclic fatigue-crack propagation resistance have been achieved through control of the beta to alpha transformation. Using fatigue-precracked, disk-shaped compact-tension speicmens, marked rising resistance-curve behavior was measured over the first approx600 mu m of crack extension, leading to a "plateau" fracture toughness of K sub c approx9.1 MParoot m; this represents more than a threefold increase over the toughness of Hexology, where a K sub c value of 2.5 MPa sub SRm was measured with no evidence of a resistance curve. 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 resistance was found to be far superior to that of Hexology. Whereas cracking in the commercial SiC became unstable when the maximum stress intensity K sub max exceeded approx2 MParoot m, thresholds for fatigue-crack growth in the in situ toughened material exceeded a K sub max of 7 MParoot m. Such dramatic improvements in the crack-growth resistance of SiC are attributed to a microstructure consisting of a network of interlocking, plate-like predominantly alpha -phase grains, which combine to both bridge and deflect the crack. Under cyclic loads, fatigue-crack growth is promoted by the cycle-dependent decay in such crack-tip shielding due to frictional-wear degradation of the zone of grain bridging ligaments in the crack wake. These results represent the first reported evidence of cyclic fatigue behavior in a monolithic silicon carbide and the first direct measurement of the resistance curve properties in this ceramic.