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

Abstract

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.]

Effect of sintering atmosphere on the mechanical properties of liquid-phase-sintered SiC

“…The same microstructural ingredients also enhance the efficacy of the toughening mechanism crack-wake bridging, making those ceramics tougher. 2,26 This is borne out by the fact that Ar-LPS SiC has a higher toughness (K IC =3.3 AE 0.1 MPa m 1/2 ), compared with N 2 -LPS SiC (K IC =2.2 AE 0.1 MPa m 1/2 ) with equiaxed SiC grains and presumably stronger SiC-YAG interfaces. In a previous study it was demonstrated that N 2 -atmopshere sintering of LPS SiC results in the incorporation of nitrogen in the intergranular phase of N 2 -LPS SiC.…”

Determining the Toughness of Ceramics from Vickers Indentations Using the Crack‐Opening Displacements: An Experimental Study

“…2-5. Additionally, for bridging ceramics such a grain-elongated Si 3 N 4 and ABC-SiC, [5][6][7] this method, if successful, has the potential to allow for the determination of the intrinsic toughness, or beginning point of the R-curve, by deconvoluting the displacements due to the residual and contact stresses (Eq. 2-5) and those due to the bridging stresses.…”

A Residual Stress Intensity Factor Solution for Knoop Indentation Cracks

“…In silicon nitrides toughening is achieved by crack face bridging by elongated grains (see e.g. Li et al 1992). For an indentation crack, crack opening displacements near the indenter impression are larger than for a through crack of the same length which renders grain bridging at this location less effective.…”