In Situ‐Toughened Silicon Carbide

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Abstract
A new processing strategy based on atmospheric pressure sintering is presented for obtaining dense SiC‐based materials with microstructures consisting of (i) uniformly distributed elongate‐shaped α‐SiC grains and (ii) relatively high amounts (20 vol%) of second‐phase yttrium aluminum garnet (YAG). This strategy entails the sintering of β‐SiC powder doped with α‐SiC, Al2O3, and Y2O3. The Al2O3 and Y2O3 aid in the liquid‐phase sintering of SiC and form in situ YAG, which has a significant thermal expansion mismatch with SiC. During a subsequent grain‐growth heat treatment, it is postulated that the α‐SiC “seeds” assist in controlling in situ growth of the elongated α‐SiC grains. The fracture pattern in the in situ‐toughened SiC is intergranular with evidence of copious crack‐wake bridging, akin to toughened Si3N4 ceramics. The elongate nature of the α‐SiC grains, together with the high thermal‐residual stresses in the microstructure, enhance the observed crack‐wake bridging. This bridging accounts for a measured twofold increase in the indentation toughness of this new class of in situ‐toughened SiC relative to a commercial SiC.