April 1998 Crack Deflection and Propagation in Layered silicon Nitride/Boron Nitride Ceramics %05 50%S Delamination Distance, &(mm) Fig. 7. Spacing between through-thickness cracks in the Si,N4 lay ers. measured for each of the materials the cumulative fraction of the delamination cracks shorter than a given value are shown for each the materials 500um interphase is shown in Fig 8(a). It is clear from this micrograph hat crack deflection occurs within the bn interphase near the g magnification SEM micrograph of the side surface of Si3 N,/BN interface, rather than at the interface between the two ens containing 50 vol% Si3N4 in the interphase materials. As shown in Fig. 8(b), subsequent delamination lection at many of the BN-containing interphases, cracking also occurs within the BN interphase. The crack often t wever, the length of the delamination cracks are limited by cracking meanders within the bn interphase, and no systematic trend with respect to the crack path, could be discerned. The location of the crack within the BN-containing interphase did not seem Is not easy to to change as SiN was added to the interphase vever, a measure (4) Interfacial Fracture Resistance Interfacial fracture resistance was measured using notched etween through-thickness cracks in adjacent SiaN4 layers flexure tests, following the analysis of Charalambides et al, 14 chematic illustrations that show how these distances were from the steady-state load necessary to propagate a delamina- measured are shown in Fig. 6. A cumulative distribution plot of tion crack. One advantage of performing this test on multilayer delamination crack lengths is shown in Fig. 7 for each of tI materials. The delamination distances are longest in the mate specimens rather than on simple bilayer specimens is that re- sidual stresses present due to thermal mismatch between the als that contain 0 vol% and 10 vol% Si,N4 in the interphase. BN and SisN, do not influence the measurement of the inter- Consistent with the micrographs shown in Fig. 4, the delam facial fracture resistance. 15 The applied phase angle (y nation distances decrease markedly as the Si3N4 content is tan- [Ku/kid) was calculated assuming that there was a suffi- cient number of layers so that the elastic properties of a single A higher-magnification SEM micrograph of a through interphase did not influence the overall elastic properties of the hickness crack in a Si3N4 layer that is impinging on a BN specimen. Thus, the measured Young s modulus(E)of the composite Ised to calculate y. For the current experi- ments was cut to approximately the center of the N resulted in a y value of 42 The interfacial fracture resi T was calculated from14 3P2(S-L)(1 Si3N4 H2(1-n)3H3 where P is the applied load at which the delamination crack extends, v the Poisson's ratio of the composite, E the in-plane Youngs modulus of the composite, b the width of the speci- men, H the height of the specimen, and m the distance from the tensile surface of the beam to the delamination crack divided SiaN by the total height of the beam. S and L are the outer span and he inner span in the four-point test fixture, respectively BN Representative load-detlectometer-displacement curves are shown for notched specimens tested in four-point flexure Figs. (aHd) for materials that contain 10, 25, 50, and 80 vol% Si, N, in the interphase. For materials with <50 volSINSA8 the interphase, the crack paths are generally similar. The le increases linearly until a crack is initiated from the notch and propagates into the closest BN-containing interphase, where the distance between the crack is deflected and arrests. Subsequent specimen deflec- througl tion causes the delamination cracks to propagate stably in the interphase to either side of the notch at an almost-constant loadthe length of delamination cracks, because it is not easy to discern the crack tip in the BN interphase. However, a measure of the delamination distances can be obtained from the distance between through-thickness cracks in adjacent Si3N4 layers. Schematic illustrations that show how these distances were measured are shown in Fig. 6. A cumulative distribution plot of delamination crack lengths is shown in Fig. 7 for each of the materials. The delamination distances are longest in the mate￾rials that contain 0 vol% and 10 vol% Si3N4 in the interphase. Consistent with the micrographs shown in Fig. 4, the delami￾nation distances decrease markedly as the Si3N4 content is increased. A higher-magnification SEM micrograph of a through￾thickness crack in a Si3N4 layer that is impinging on a BN interphase is shown in Fig. 8(a). It is clear from this micrograph that crack deflection occurs within the BN interphase near the Si3N4/BN interface, rather than at the interface between the two materials. As shown in Fig. 8(b), subsequent delamination cracking also occurs within the BN interphase. The crack often meanders within the BN interphase, and no systematic trend, with respect to the crack path, could be discerned. The location of the crack within the BN-containing interphase did not seem to change as Si3N4 was added to the interphase. (4) Interfacial Fracture Resistance Interfacial fracture resistance was measured using notched flexure tests, following the analysis of Charalambides et al., 14 from the steady-state load necessary to propagate a delamina￾tion crack. One advantage of performing this test on multilayer specimens rather than on simple bilayer specimens is that re￾sidual stresses present due to thermal mismatch between the BN and Si3N4 do not influence the measurement of the inter￾facial fracture resistance.15 The applied phase angle (C 4 tan−1 [KII/KI ]) was calculated assuming that there was a suffi￾cient number of layers so that the elastic properties of a single interphase did not influence the overall elastic properties of the specimen. Thus, the measured Young’s modulus (E) of the composite was used to calculate C. For the current experi￾ments, the notch was cut to approximately the center of the specimen, which resulted in a C value of 42°. The interfacial fracture resistance, Gi , was calculated from14 Gi = 3P2 ~S − L! 2 ~1 − n2 ! 2Eb2 F 1 H3 ~1 − h! 3 − 1 H3G (1) where P is the applied load at which the delamination crack extends, n the Poisson’s ratio of the composite, E the in-plane Young’s modulus of the composite, b the width of the speci￾men, H the height of the specimen, and h the distance from the tensile surface of the beam to the delamination crack divided by the total height of the beam. S and L are the outer span and the inner span in the four-point test fixture, respectively. Representative load–deflectometer-displacement curves are shown for notched specimens tested in four-point flexure in Figs. 9(a)–(d) for materials that contain 10, 25, 50, and 80 vol% Si3N4 in the interphase. For materials with <50 vol% Si3N4 in the interphase, the crack paths are generally similar. The load increases linearly until a crack is initiated from the notch and propagates into the closest BN-containing interphase, where the crack is deflected and arrests. Subsequent specimen deflec￾tion causes the delamination cracks to propagate stably in the interphase to either side of the notch at an almost-constant load. Fig. 5. Higher-magnification SEM micrograph of the side surface of one of the specimens containing 50 vol% Si3N4 in the interphase, showing crack deflection at many of the BN-containing interphases; however, the length of the delamination cracks are limited by cracking kinking. Fig. 6. Schematic illustration showing how the distance between through-thickness cracks, d, was measured in materials that exhibited (a) delamination cracking and (b) crack kinking. Fig. 7. Spacing between through-thickness cracks in the Si3N4 lay￾ers, measured for each of the materials; the cumulative fraction of the delamination cracks shorter than a given value are shown for each of the materials. April 1998 Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics 1007