Ambient to high-temperature fracture toughness and cyclic fatigue behavior in Al-containing silicon carbide ceramics

A series of in situ toughened, Al, B and C containing, silicon carbide ceramics (ABC-SiC) has been examined with Al contents varying from 3 to 7 wt.%. With increasing Al additions, the grain morphology in the as-processed microstructures varied from elongated to bimodal to equiaxed, with a change in the nature of the grain-boundary film from amorphous to partially crystalline to fully crystalline. Fracture toughness and cyclic fatigue tests on these microstructures revealed that although the 7 wt.% Al containing material (7ABC) was extremely brittle, the 3 and particularly 5 wt.% Al materials (3ABC and 5ABC, respectively) displayed excellent crack-growth resistance at both ambient (25 °C) and elevated (1300 °C) temperatures. Indeed, no evidence of creep damage, in the form of grain-boundary cavitation, was seen at temperatures at 1300 °C or below. The enhanced toughness of the higher Al-containing materials was associated with extensive crack bridging from both interlocking grains (in 3ABC) and uncracked ligaments (in 5ABC); in contrast, the 7ABC SiC showed no such bridging, concomitant with a marked reduction in the volume fraction of elongated grains. Mechanistically, cyclic fatigue-crack growth in 3ABC and 5ABC SiC involved the progressive degradation of such bridging ligaments in the crack wake, with the difference in the degree of elastic vs. frictional bridging affecting the slope, i.e. Paris law exponent, of the crack-growth curve.