October 1997 Fibrous Monolithic ceramics 2483 contribution from cracking of the cell boundaries. T the frac ure energy of the cells, T the interfacial fracture energy, A the cell area, and A, is the interfacial area. The amount of energy that is absorbed because of cracking, therefore, is de pendent on the cell fracture resistance, the interfacial fracture resistance. and the length of the delamination cracks For a given material system, the maximum energy that can be absorbed because of cracking occurs if complete delamina tion of every interphase occurs. Given the measured values of the cell and interfacial fracture energies in the Si3Na-BN sys tem(120 and 30 J/m2, respectively) and the typical specimen size, this corresponds to a maximum of 123 mJ of energy that absorption is occurring because of the creation of interfacial 100010000 crack area, it may seem logical that increasing the interfacial Mean: Crack Spacin fracture resistance would increase the energy absorbed. How ever, recall from the previous section that increasing the inter- facial fracture resistance leads to reduced delamination crack Predicted energy dissipation is plotted versus the distance ugh-thickness cracks in the Si3 N4 cells. Contributions engths because of crack kinking. The optimum value of inter acking(Wc)and frictional sliding(Ws)are shown as well as the facial fracture resistance to maximize energy ab total energy absorption (r) maximum interfacial fracture resistance for which crack de- flection occurs and which does not cause crack kinking (2) Energy Absorption Due to Sliding between Si3N4 rocessed with sinter After failure of the first layer of cells on the tensile surface profoundly in high-temperature pi continued propagation through the thickness of the specimen requires that cells slide relative to one another in a process that analogous to fiber pullout in fiber-reinforced composites fast-fracture tests 47-50 nitests itself at high The amount of energy that is absorbed because of this frictional Hexagonal BN possesses strong covalent bonding within the sliding, Ws, is given by basal plane and weak van der Waals bonding between planes a result, the physical and mechanical properties of hexago- (7) nal BN are highly anisotropic. Furthermore, weak bonding be- tween [0001] basal planes results in a very high out-of-plane where n is the number of cells slipping, 8 the distance slipped, coefficient of thermal expansion, suggesting that the high and Ts the frictional sliding resistance for cracked cell bound temperature mechanical properties in this direction are strongly aries. The maximum distance that can be slipped before the affected upon heating. The presence of glassy phase in the BN layers disengage is the distance between through-thickness interphase also may influence the high-temperature properties cracks in the cells. This distance is maximized if complete of fibrous monolithic ceramics delamination oc However, in flexure, it is usually not pos- sible for sliding to occur until disengagement occurs for every (4 Fast Fracture in Flexure Four-point flexure evaluations were made at room tempera distance between through cracks. Observations suggest that the ture and at I000°,1100°,1200°,1300°,and1400°C. Figure20 average sliding distance, 8, does exceed -2 mm. If crack kink shows representative stress-deflection plots for fibrous mono- ing occurs, 8 is calculated from the average distance between liths as a function of test temperature Specimens remain linear kinks, and the energy absorption capabili lastic up to the peak load for temperatures as high as 1300C further Similar to specimens tested at ambient temperatures, speci mens tested at elevated temperatures exhibit noncatastrophic materials, and results indicate that, for the Si3N-BN system, failure by retaining significant load after the peak load is the sliding resistance .3 MPa. 4 Thus, knowing the size achieved and dissipating a significant amount of energy during and number of cells in a specimen, the energy absorbed be- testing. A slight increase in the work-of-fracture is observed cause of sliding can be calculated using Eq. (7). For a mean as the temperature is increased to 1000C, but the slidin f sliding is -120 mJ for a typical Si3Na-BN fibrous mens tested at 1400 C exhibit nonlinear behavior on I an indication that inelastic deformation is occurring during of the creation of crack area. testing. In Fig. 19, the maximum total energy absorption is plotted Examinations of the side surfaces of the specimens after versus the distance between through-thickness cracks. The ting indicate that cracks are deflected at almost every inter dominant mechanism for dissipating energy depends on the face in the bars tested at 1000C and that delamination dis- distance between through-thickness cracks. when the distance tances are long. The bars tested at 25 C exhibit a similar degree between through-thickness cracks is very small, the contribu- of crack deflection, but delamination distances are shorter than tion from the creation of crack area is greatest. As the distance for the specimens tested at 1000C. A change in fracture m between through-thickness cracks in the cells increases, the phology is observed for specimens tested at 1100-1300.C contribution from frictional sliding becomes increasingly im- Below 1.C, failure in all of the specimens initiates on the portant to energy absorption. For the specimen sizes used in the tensile face of the sample, but, in most of the specimens tested at 1100-1300C, failure initiates in shear at the midplane creation of interfacial crack area and from frictional sliding are Figure 12 shows a load-displacement curve for a representative predicted to be comparable disa ple tested at 1200 C that failed by shear initiation. Note the tinctive change in the specimen compliance after the first () High-Temper load drop. The compliance increases by about a factor of 4 after To understand the failure behavior of fibrous propagation of the shear crack, as expected for a specimen that ceramics at elevated temperatures, the constitutive fails in shear both SiaN, and BN at temperature must be delineat Figure 21(a)is an SEM micrograph of the fracture surface of has been established that a 10-50 A amorphous a BN cell boundary for a specimen tested at 1100.C.Thecontribution from cracking of the cell boundaries, GL the frac￾ture energy of the cells, Gi the interfacial fracture energy, AL the cell area, and Ai is the interfacial area. The amount of energy that is absorbed because of cracking, therefore, is de￾pendent on the cell fracture resistance, the interfacial fracture resistance, and the length of the delamination cracks. For a given material system, the maximum energy that can be absorbed because of cracking occurs if complete delamina￾tion of every interphase occurs. Given the measured values of the cell and interfacial fracture energies in the Si3N4–BN sys￾tem (120 and 30 J/m2 , respectively) and the typical specimen size, this corresponds to a maximum of 123 mJ of energy that can be absorbed by cracking. Because a majority of the energy absorption is occurring because of the creation of interfacial crack area, it may seem logical that increasing the interfacial fracture resistance would increase the energy absorbed. How￾ever, recall from the previous section that increasing the inter￾facial fracture resistance leads to reduced delamination crack lengths because of crack kinking. The optimum value of inter￾facial fracture resistance to maximize energy absorption is the maximum interfacial fracture resistance for which crack de￾flection occurs and which does not cause crack kinking. (2) Energy Absorption Due to Sliding After failure of the first layer of cells on the tensile surface, continued propagation through the thickness of the specimen requires that cells slide relative to one another in a process that is analogous to fiber pullout in fiber-reinforced composites. The amount of energy that is absorbed because of this frictional sliding, WS, is given by WS 4 ndAi tS (7) where n is the number of cells slipping, d the distance slipped, and tS the frictional sliding resistance for cracked cell bound￾aries. The maximum distance that can be slipped before the layers disengage is the distance between through-thickness cracks in the cells. This distance is maximized if complete delamination occurs. However, in flexure, it is usually not pos￾sible for sliding to occur until disengagement occurs for every cell, and the sliding distance usually is much less than the distance between through cracks. Observations suggest that the average sliding distance, d, does exceed ∼2 mm. If crack kink￾ing occurs, d is calculated from the average distance between kinks, and the energy absorption capability is reduced even further. A technique has been developed to measure tS in layered materials, and results indicate that, for the Si3N4–BN system, the sliding resistance is ∼0.3 MPa.44 Thus, knowing the size and number of cells in a specimen, the energy absorbed be￾cause of sliding can be calculated using Eq. (7). For a mean sliding distance of 2 mm, the calculated energy absorbed be￾cause of sliding is ∼120 mJ for a typical Si3N4–BN fibrous monolith, a value comparable to the energy absorbed because of the creation of crack area. In Fig. 19, the maximum total energy absorption is plotted versus the distance between through-thickness cracks. The dominant mechanism for dissipating energy depends on the distance between through-thickness cracks. When the distance between through-thickness cracks is very small, the contribu￾tion from the creation of crack area is greatest. As the distance between through-thickness cracks in the cells increases, the contribution from frictional sliding becomes increasingly im￾portant to energy absorption. For the specimen sizes used in the current study, the contributions to energy dissipation from the creation of interfacial crack area and from frictional sliding are predicted to be comparable. (3) High-Temperature Properties To understand the failure behavior of fibrous monolithic ceramics at elevated temperatures, the constitutive behavior of both Si3N4 and BN at temperature must be delineated. It long has been established that a 10–50 Å amorphous layer exists between Si3N4 grains that are processed with sintering aids that profoundly influence the high-temperature properties of Si3N4. 45,46 This phenomenon is prevalent especially at low strain rates but also manifests itself at high strain rates during fast-fracture tests.47–50 Hexagonal BN possesses strong covalent bonding within the basal plane and weak van der Waals bonding between planes. As a result, the physical and mechanical properties of hexago￾nal BN are highly anisotropic. Furthermore, weak bonding be￾tween [0001] basal planes results in a very high out-of-plane coefficient of thermal expansion, suggesting that the high￾temperature mechanical properties in this direction are strongly affected upon heating. The presence of glassy phase in the BN interphase also may influence the high-temperature properties of fibrous monolithic ceramics. (4) Fast Fracture in Flexure Four-point flexure evaluations were made at room tempera￾ture and at 1000°, 1100°, 1200°, 1300°, and 1400°C. Figure 20 shows representative stress–deflection plots for fibrous mono￾liths as a function of test temperature. Specimens remain linear elastic up to the peak load for temperatures as high as 1300°C. Similar to specimens tested at ambient temperatures, speci￾mens tested at elevated temperatures exhibit noncatastrophic failure by retaining significant load after the peak load is achieved and dissipating a significant amount of energy during testing. A slight increase in the work-of-fracture is observed as the temperature is increased to 1000°C, but the work￾of-fracture decreases again above this temperature. Speci￾mens tested at 1400°C exhibit nonlinear behavior on loading, an indication that inelastic deformation is occurring during testing. Examinations of the side surfaces of the specimens after testing indicate that cracks are deflected at almost every inter￾face in the bars tested at 1000°C and that delamination dis￾tances are long. The bars tested at 25°C exhibit a similar degree of crack deflection, but delamination distances are shorter than for the specimens tested at 1000°C. A change in fracture mor￾phology is observed for specimens tested at 1100°–1300°C. Below 1100°C, failure in all of the specimens initiates on the tensile face of the sample, but, in most of the specimens tested at 1100°–1300°C, failure initiates in shear at the midplane. Figure 12 shows a load–displacement curve for a representative sample tested at 1200°C that failed by shear initiation. Note the distinctive change in the specimen compliance after the first load drop. The compliance increases by about a factor of 4 after propagation of the shear crack, as expected for a specimen that fails in shear. Figure 21(a) is an SEM micrograph of the fracture surface of a BN cell boundary for a specimen tested at 1100°C. The Fig. 19. Predicted energy dissipation is plotted versus the distance between through-thickness cracks in the Si3N4 cells. Contributions from cracking (WC) and frictional sliding (WS) are shown as well as the total energy absorption (WT). October 1997 Fibrous Monolithic Ceramics 2483