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R-curves for two in-situ reinforced silicon nitrides A and B of nominally the same composition are characterized using the Griffith equation and indentation fracture mechanics. These R-curves are calibrated against fine-grained silicon nitrides which have a known chevron-notch (long-crack) toughness and with a nearly flat R-curve behavior. Silicon nitride A, with its coarser microstructure and higher chevron-notch toughness, shows lower resistance to crack growth than silicon nitride B if the crack size is less than -200 pm. These results are consistent with the indentation-strength measurements which show a cmssover of strength between the two materials at an indentation load between 49 and 98 N, and below the crossover A has a lower strength. The toughening behavior is explained using an elastic-bridging model for the short crack, and a pullout model for the long crack. The effects of R-curve properties on design are discussed. [Key words: silicon nitride, R-curve, bridging, indentation, strength.]

The grain size of in situ Si,N, is varied, and its effects on strength-flaw size relations are related to the behavior of a bridging zone behind the crack tip. The bridging-zone properties are calculated from a Dugdale model assuming that the bridging zone has a constant bridging stress (p*) and length (Db) at the moment of the critical fracture. The results show that as grain size increases, p* decreases while D, and the critical bridging zone opening (u*) first increase and then decrease, resulting in a maximum for short-crack fracture toughness at an intermediate grain size. The initial increase of u* and D, with grain size is attributed to an increase in debonding length, while the decrease of p* is attributed to a decrease in strength for bridging grains due to a statistical effect which also causes D, and u* to drop in the large-grain regime. Implications on microstructure design are discussed.

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