September 1997 Communications of the American Ceramic Sociery 2423 YPO4 Dosed of YPOA, YPSZ, an YZ3A7 laminae between delaminated interfaces. Only two of the composite layers were broken. It is assumed that, based on YPSZ of the layers, startin YZ3A7 and experimental work of Folsom et al.7 Cortical analysis nates, the load-displacement response following cracking in Fig. 2 can be fitted by the predicted nominal-stress-nominal- where 8 is deflection and g. and 8 are the nominal strain and deflection at the onset of cracking. the fitting in Fig. 2 is rformed by choosing 8, to be 0. 124 mm at a loading of 339 N. This result confirms the idea that the nonlinear behavior is due to tensile cracks and the associated (b) YPSZ Different hybrid oxide laminates have previously been in- restigated in this laboratory 16,28 After four flexural test YZ3A7 ing, these laminates displayed catastrophic fracture, although here was limited interfacial delamination or severe crack de- flection. Comparing the fracture behavior of hese examples with that of the YPO, /YPSZ/YZ3A7/YPSZ laminate in this YPO nounced interfacial delamination could make laminates toler d coworkers 4, Is produced a laminar fabric of sili- con carbide(SiC)that was interleaved with graphite films, which had flexural stress-strain behavior that was comparable 50 mm to fiber-reinforced composites. The three-point flexural strength was as high as 633 MPa with a work of fracture. which was calculated by the same method that was used in this study in th e ranse of 4.6--6.7 kJ/m2 Folsom et al. 13 demonstrated a laminar ceramic-carbon-fiber-reinforced-epoxy composite tha had a four-point flexural of 400 MPa and failure. Baskaran and coworkers29, 30 studied fibrous monolithic oriented(a)0/90 and (b)45 relative to the layer length direction ceramics, which exhibited a four-point flexural strength of Delaminating interfacial cracks are marked by arrows -250 MPa for a SiC/graphite system and 300-375 MPa for a SiC/BN system, with graceful fracture. The apparent values for the work of fracture(using the same definition that has been IV. Discussion used in this study for the fibrous monolithic ceramics were 1.3 and 2.4 kJ/m for SiC/graphite and SiC/BN syste A YPO /YPSZ/YZ3A7/YPSZ oxide laminate has been suc- tively. Liu and Hsu l fabricated multilayer silicon nitride/boron cessfully fabricated. On the other hand the laminate composed nitride (Si, n,/BN) ceramics with four-point flexural stren only of YPOA and YPSZ was shattered, because of a large of 437 and 196 MPa and apparent work-of-fracture values of thermal expansion mismatch between YPO4(coefficient of 6.5 and 5.5 kJ/m2, respectively. In comparison, the YPO4/ thermal expansion, a, of-86x 10-6/C) 6 and YPSZ (o= 10.6 YPSZ/YZ3A7/YPSZ oxide laminate of this work has demon- 10/oC).23The incorporated YZ3A7 layers had an estimated strated a four-point flexural strength of 358 MPa and an ap a value of.3 x 10-6/C, which was based on the a value of parent work of fracture of.2 kJ/m2 from one test. Thus, this 8.8 x 10-6/C for Al2O3. 24 Although the YZ3A7 layers did oxide laminate had comparable mechanical properties with not participate in providing a weak interface, the function of those of non-oxide composites and far-improved properties this laye as to increase the stiffness and modify the residual comparison with those of oxide composites stresses in the oxide laminate which enabled fabrication of the The YPO,YPSZ/YZ3A7/YPSZ laminate s stre hybrid oxide laminate. From the comparison with the YPO,/ ZrO2-containing laminae(YPSZ and YZ3A7)and" weak YPSZ laminate. there were two reasons for the successful ox- YPO/YPSZ interfaces. Ceramics that have been toughened ide laminate with pronounced interfacial delamination in Fig. by PSZ have shown high strength. 7 Further enhancemer 3:(i) the nature of the YPO/YPSZ interface and(ii) residual of the damage tolerance in this laminate is achieved by the stress-assisted delamination. In essence, this YPO//YPSZ YPO /YPSZ interface delamination. During the cracking/ terface was weakened by the assistance of residual stresses delamination events, ZrO2 has an important role. The strong Furthermore, the symmetrical stacking also had the effect of YPSZ and YZ3A7 layers in the discrete composite layers can preventing shape distortion of this laminate and local micro- support the applied load after the cracking and delamination structural damage and failure. 25 In symmetrical laminates, the events occur, which keeps the oxide laminate from fracturing oupled forces, because of property differences in each layer, catastrophically. This fracture behavior is similar to that which largely cancel out and hold the laminate without distortion IS OC in fiber-reinforced ceramic composites that have a There is a close relationship between load decreases and weak interface: interfacial debonding and delamination are fol- cracking/delamination events. As shown in Fig 3, eight de- lowed by load redistribution among the unfractured part and laminated interfaces have been displayed, which have left dis- he unbroken fibers. The discrete composite la our lami-IV. Discussion A YPO4/YPSZ/YZ3A7/YPSZ oxide laminate has been suc￾cessfully fabricated. On the other hand, the laminate composed only of YPO4 and YPSZ was shattered, because of a large thermal expansion mismatch between YPO4 (coefficient of thermal expansion, a, of ∼8.6 × 10−6/°C)16 and YPSZ (a ≈ 10.6 × 10−6/°C).23 The incorporated YZ3A7 layers had an estimated a value of ∼9.3 × 10−6/°C, which was based on the a value of ∼8.8 × 10−6/°C for Al2O3. 24 Although the YZ3A7 layers did not participate in providing a weak interface, the function of this layer was to increase the stiffness and modify the residual stresses in the oxide laminate, which enabled fabrication of the hybrid oxide laminate. From the comparison with the YPO4/ YPSZ laminate, there were two reasons for the successful ox￾ide laminate with pronounced interfacial delamination in Fig. 3: (i) the nature of the YPO4/YPSZ interface and (ii) residual stress-assisted delamination. In essence, this YPO4/YPSZ in￾terface was weakened by the assistance of residual stresses. Furthermore, the symmetrical stacking also had the effect of preventing shape distortion of this laminate and local micro￾structural damage and failure.25 In symmetrical laminates, the coupled forces, because of property differences in each layer, largely cancel out and hold the laminate without distortion. There is a close relationship between load decreases and cracking/delamination events. As shown in Fig. 3, eight de￾laminated interfaces have been displayed, which have left dis￾crete composite layers that are composed of YPO4, YPSZ, and YZ3A7 laminae between delaminated interfaces. Only two of the composite layers were broken. It is assumed that, based on the micrographs in Fig. 3, fracture proceeds by tensile cracking of the layers, followed by delamination of the layers, starting from the tensile cracks and propagating along the specimen length to the outer loading pins. From the theoretical analysis and experimental work of Folsom et al.26,27 on the glass lami￾nates, the load–displacement response following cracking in Fig. 2 can be fitted by the predicted nominal-stress–nominal￾strain response: s so = S « «o D −2 = S d do D −2 where d is deflection and «o and do are the nominal strain and deflection at the onset of cracking. The fitting in Fig. 2 is performed by choosing do to be 0.124 mm at a loading of 339 N. This result confirms the idea that the nonlinear behavior is due to tensile cracks and the associated stress redistribution. Different hybrid oxide laminates have previously been in￾vestigated in this laboratory.16,28 After four-point flexural test￾ing, these laminates displayed catastrophic fracture, although there was limited interfacial delamination or severe crack de￾flection. Comparing the fracture behavior of these examples with that of the YPO4/YPSZ/YZ3A7/YPSZ laminate in this study, we understood that (i) limited delamination, especially located at the end of crack propagation, and severely crack deflection did not benefit damage tolerance, and (ii) only pro￾nounced interfacial delamination could make laminates tolerant to damage (Fig. 3). Clegg and coworkers14,15 produced a laminar fabric of sili￾con carbide (SiC) that was interleaved with graphite films, which had flexural stress–strain behavior that was comparable to fiber-reinforced composites. The three-point flexural strength was as high as 633 MPa with a work of fracture, which was calculated by the same method that was used in this study, in the range of 4.6–6.7 kJ/m2 . Folsom et al.13 demonstrated a laminar ceramic–carbon-fiber-reinforced-epoxy composite that had a four-point flexural strength of 400 MPa and a graceful failure. Baskaran and coworkers29,30 studied fibrous monolithic ceramics, which exhibited a four-point flexural strength of ∼250 MPa for a SiC/graphite system and 300–375 MPa for a SiC/BN system, with graceful fracture. The apparent values for the work of fracture (using the same definition that has been used in this study) for the fibrous monolithic ceramics were 1.3 and 2.4 kJ/m2 for SiC/graphite and SiC/BN systems, respec￾tively. Liu and Hsu31 fabricated multilayer silicon nitride/boron nitride (Si3N4/BN) ceramics with four-point flexural strengths of 437 and 196 MPa and apparent work-of-fracture values of 6.5 and 5.5 kJ/m2 , respectively. In comparison, the YPO4/ YPSZ/YZ3A7/YPSZ oxide laminate of this work has demon￾strated a four-point flexural strength of 358 MPa and an ap￾parent work of fracture of ∼8.2 kJ/m2 from one test. Thus, this oxide laminate had comparable mechanical properties with those of non-oxide composites and far-improved properties in comparison with those of oxide composites. The YPO4/YPSZ/YZ3A7/YPSZ laminate possesses strong ZrO2-containing laminae (YPSZ and YZ3A7) and ‘‘weak’’ YPO4/YPSZ interfaces. Ceramics that have been toughened by PSZ have shown high strength.17 Further enhancement of the damage tolerance in this laminate is achieved by the YPO4/YPSZ interface delamination. During the cracking/ delamination events, ZrO2 has an important role. The strong YPSZ and YZ3A7 layers in the discrete composite layers can support the applied load after the cracking and delamination events occur, which keeps the oxide laminate from fracturing catastrophically. This fracture behavior is similar to that which is occurring in fiber-reinforced ceramic composites that have a weak interface: interfacial debonding and delamination are fol￾lowed by load redistribution among the unfractured part and the unbroken fibers. The discrete composite layers in our lami￾Fig. 4. SEM micrographs of indentation crack patterns in a YPO4/ YPSZ/30 vol% YPSZ–70 vol% Al2O3/YPSZ laminate; indents were oriented (a) 0°/90° and (b) 45° relative to the layer length direction. Delaminating interfacial cracks are marked by arrows. September 1997 Communications of the American Ceramic Society 2423