Journal of the American Ceramic Sociery-Kovar et al. Vol. 80. No. 10 crack deflection behavior, the composition of the BN-con- tensile surface. In contrast, the sample with 40 vol% Si3N aining cell boundary was varied. Previously, it has been ob dded to the cell boundary has very few extensive delamination erved that the addition of a strong phase to a weak interphase cracks, although cracks deflect a short distance at each cell increases the interfacial fracture resistance. II As BN in the cell boundary. Similar results have been observed in Sic-graphite oundary is replaced with Si3N4, the fracture resistance of the ous mono 7 cell boundary increases, and the tendency for crack deflection Kovar et al. 35 have conducted a more detailed study decreases significantly. SEM micrographs showing the tensile conventional, two-dimensional, layered ceramics. In this work, and side ces of two specimens after flexural testing are the interphase composition was again modified by adding shown in Fig. 13. The specimen with 10 vol% Si3N4 added to Si3Na to the Bn interphase. The interfacial fracture resistanc the cell boundary, which had the lower interfacial energy or these materials was measured directly on the Si3Na-BN shows extensive delamination on the side surface and on the laminates that revealed that the interfacial fracture resistance increased from -30 to 90 J/m as the SigNa content in the interphase was increased from 0 to 50 vol%, as shown in Fig 14. The fracture resistance of the monolithic Sis Na layers is 120 J/m2 The results of the interfacial fracture resistance measurements together with the predictions of He and Hutch- inson33 are shown in Fig. 15. This plot shows that crack de flection occurs at values of the interfacial fracture resistance hat are significantly higher than the predicted values There are several factors that may contribute to the d ncy between the observations of crack deflection and heory. For example, bn is highly anisotropic in its elastic properties as well as other mechanical properties, 5 which is ot accounted for in the theory Furthermore this theory does ot account for residual stresses that may develop because of differences in thermal expansion between Si3N4 and BN. How- ever. the microcracks in the bN. which have been observed using tEM, should relieve most of the residual stress that develops because of thermal mismatch. These microcracks also should make it easier for cracks to deflect at the BN layers. 36, 37 ≡HH crack deflection at an interface, but we observe that crack deflection and crack propagation al ways occur within the BN cell boundary, rather than at the interface between Si3N4 and BN. Figure 16 shows a crack propagating within the Bn cell The cracks wander within the bn cell boundary, bu never at the interface. Cracks seem to grow by link-up of preexisting microcracks, although it is difficult to image the near-tip region of the cracks (2) Delamination Cracking versus Crack Kinking Although crack deflection is an essential mechanism for dis- sipating energy in layered materials,38,39 crack deflection by itself does not ensure that a laminate will absorb significant amounts of energy during fracture. For example, materials with up to 50% Si3 N4 in the bn interphase have observable crack deflection but relatively little work-of-fracture, because the ex- tent of the delamination cracking decreases significantly as the dissipation depends upon the extent of delamination ci Delamination has little effect on the energy dissipation of a laminate if the delamination crack kinks and reenters the Si3N4 cell after propagating only a short distance Bulk I for Si, N4 mn Fig. 13. Side and tensile surfaces of a specimen with(a)10 vol% and tcrphasc(") vith(b)40 vol% Si, Na in the cell boundary are shown after testing Delamination cracking is much more extensive in the specimen w Fig. 14. Plot of interfacial fracture resistance as a function of Si3 Na less Si, N, in the cell boundary content in the BN-containing interphasecrack deflection behavior, the composition of the BN-con￾taining cell boundary was varied. Previously, it has been ob￾served that the addition of a strong phase to a weak interphase increases the interfacial fracture resistance.11 As BN in the cell boundary is replaced with Si3N4, the fracture resistance of the cell boundary increases, and the tendency for crack deflection decreases significantly. SEM micrographs showing the tensile and side surfaces of two specimens after flexural testing are shown in Fig. 13. The specimen with 10 vol% Si3N4 added to the cell boundary, which had the lower interfacial energy, shows extensive delamination on the side surface and on the tensile surface. In contrast, the sample with 40 vol% Si3N4 added to the cell boundary has very few extensive delamination cracks, although cracks deflect a short distance at each cell boundary. Similar results have been observed in SiC–graphite fibrous monoliths.7 Kovar et al.35 have conducted a more detailed study using conventional, two-dimensional, layered ceramics. In this work, the interphase composition was again modified by adding Si3N4 to the BN interphase. The interfacial fracture resistance for these materials was measured directly on the Si3N4–BN laminates that revealed that the interfacial fracture resistance increased from ∼30 to 90 J/m2 as the Si3N4 content in the interphase was increased from 0 to 50 vol%, as shown in Fig. 14. The fracture resistance of the monolithic Si3N4 layers is ∼120 J/m2 . The results of the interfacial fracture resistance measurements together with the predictions of He and Hutch￾inson33 are shown in Fig. 15. This plot shows that crack de￾flection occurs at values of the interfacial fracture resistance that are significantly higher than the predicted values. There are several factors that may contribute to the discrep￾ancy between the observations of crack deflection and the theory. For example, BN is highly anisotropic in its elastic properties as well as other mechanical properties,25 which is not accounted for in the theory. Furthermore, this theory does not account for residual stresses that may develop because of differences in thermal expansion between Si3N4 and BN. How￾ever, the microcracks in the BN, which have been observed using TEM, should relieve most of the residual stress that develops because of thermal mismatch. These microcracks also should make it easier for cracks to deflect at the BN layers.36,37 Finally, the He and Hutchinson33 theory defines conditions for crack deflection at an interface, but we observe that crack deflection and crack propagation always occur within the BN cell boundary, rather than at the interface between Si3N4 and BN. Figure 16 shows a crack propagating within the BN cell boundary. The cracks wander within the BN cell boundary, but never at the interface. Cracks seem to grow by link-up of preexisting microcracks, although it is difficult to image the near-tip region of the cracks. (2) Delamination Cracking versus Crack Kinking Although crack deflection is an essential mechanism for dis￾sipating energy in layered materials,38,39 crack deflection by itself does not ensure that a laminate will absorb significant amounts of energy during fracture. For example, materials with up to 50% Si3N4 in the BN interphase have observable crack deflection but relatively little work-of-fracture, because the ex￾tent of the delamination cracking decreases significantly as the Si3N4 content in the interphase is increased. Clearly, energy dissipation depends upon the extent of delamination cracking. Delamination has little effect on the energy dissipation capacity of a laminate if the delamination crack kinks and reenters the Si3N4 cell after propagating only a short distance. Fig. 13. Side and tensile surfaces of a specimen with (a) 10 vol% and with (b) 40 vol% Si3N4 in the cell boundary are shown after testing. Delamination cracking is much more extensive in the specimen with less Si3N4 in the cell boundary. Fig. 14. Plot of interfacial fracture resistance as a function of Si3N4 content in the BN-containing interphase. 2480 Journal of the American Ceramic Society—Kovar et al. Vol. 80, No. 10