R ‐Curve Behavior and Strength for In‐Situ Reinforced Silicon Nitrides with Different Microstructures

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 resitance to crack growth than silicon nitride B if the crack size is less than ∼200 μm. These results are consistent with the indentation–Strength measurements which show a crossover 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.