Relation between Strength, Microstructure, and Grain‐Bridging Characteristics in In Situ Reinforced Silicon Nitride

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Abstract
The grain size of in situ Si3N4 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 Db 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 Db 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 Db and u* to drop in the large‐grain regime. Implications on microstructure design are discussed.

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