《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_j.1151-2916.1997.tb03137.x[1]

ournal Am Ceran. Soc8092421-24(1997) A Strong and Damage-Tolerant Oxide Laminate Dong-Hau Kuo and Waltraud M. Kriven A novel laminar oxide composite was developed. This oxide Yttrium phosphate(YPO,), a xenotime structure is expected laminate, which was fabricated by hot pressing, consisted of to behave as an analog to the rare-earth phosphate, e.g,mona- three types of layers, which were stacked in a repeating zite (LaPO.). which has beer duced as a possible func- sequence of YPOa, yttria-stabilized ZrO2, 30 vol% yttria tional interface for oxide-oxide composites. o Although YPO4 stabilized ZrOx-70 vol% Al2O3, and yttria-stabilized ZrO2. has a tendency to react with alumina(Al2O3)at high tempera- The behavior of the oxide laminate was evaluated by fou tures, it is chemically compatible with yttrium aluminate oint flexural testing and the indentation technique. The (Y3AlsO12) and mullite(3Al2O3, 2SiO2) at least up to flexural strength from one test was 358 MPa, and the load- 1600°C isplacement curve of this test displayed a graceful failure. Zirconia(ZrO,) has been widely applied in the forms of Pronounced interfacial delamination contributed to a high partially stabilized zirconia(PSz)and zirconia-toughened alu- work of fracture and damage tolerance. These properties mina (ZTA). The high strength(2.5 GPa) of the yttria have rarely been observed in oxide composites and are stabilized ZrO, /20 wt %Al2,(Y2 stabilized ZrO, /20 wt% comparable to those of non-oxide composites, such as SiC/ Al, O3)composite has been measured after densification by hot graphite, SiC/BN, and SiaN,/BN laminates. isostatic pressing. 17 High-toughness(15 MParm 2)ZrO2 has been exploited in a Ce-TZP/(Ce-TZP+Al,O laminate. 8Im- proved high-temperature strength(1 GPa at 1000 C)of ZrO2, combination with Al,O3, has been achieved. Strengthening R EINFORCEMENTS(particulate, whisker, and fiber) and phase strategies for ZrOx-containing ceramics at high temperature transformable materials have been incorporated into ce- ave been proposed by Claussen ramics for toughening. Continuous-fiber-reinforced ceramic The purpose of this study was to design and fabricate a composites(CFCCs)are the most-promising materials and strong and tough oxide laminate. Two types of systems have have shown a nonbrittle response. The strong and tough been examined: the first system was a two-layered, YPO lon-oxide CFCC relies upon a carbon or boron nitride(BN) Y2Or-partially stabilized Zro2(YPO,/YPSZ) multilayered ox- film between the fiber and the matrix. The role of the inter- ide laminate, and the second system was a four-layered YPO facial film is to induce debonding and sliding mechanisms Y, O -stabilized ZrO,/30 vol%Y, O -stabilized ZrO-70 vol% ssipation Al2O3/Y2O3-stabilized ZrO2 oxide laminate. The complex con- theless, high-temperature oxidation of carbon or BN results in figuration was chosen mainly to achieve laminates with rea- ittle fracture and has limited the applications of non-oxide sonable strength values and to overcome the deleterious effects CFCS of thermal expansion n mismatch and chemical reactions On the other hand, the development of strong and tough oxides by incorporating oxide fibers in oxide matrices is going. o The oxide materials possess oxidation resistance, low Il. Experimental Procedures thermal conductivity, and high electrical resistivity, as com- ared with non-oxide materials. These oxide-fiber--oxide (1) Materials matrix composites are expected to offer high-temperature oxi- The synthesis of YPOa has been described else- dation resistance. albeit with a mised strength. Limited where. YPSZ (3 mol% .3Y. Tosoh USA, Atlanta, by the availability and properties of fibers, as well as interfacial GA)and Al2O3 powder ( 16-SG. Alcoa Aluminum coatings that behave similar to carbon or BN, oxide CFCCs Pittsburgh, PA)were used pes of tape-cast tapes were have not yet attained the expected properties fabricated YPO4, YPSZ ol% YPSZ-70 vol%Al,O3 Laminated composites with unique and adjustable properties (YZ3A7) tions or by incorporating fibers and whiskers into the lam The procedure for making laminated composites by tape casting was described elsewhere. 21 An 80-layer lam nates. Tough, laminated composites can be obtained by intro- composite was fabricated by stacking these three types of ducing ductile interlayers(e.g, metallic layers, 12)or carbon in the repeating sequence of YPO4, YPSZ, YZ3A7, and Y fiber/epoxy prepregs 3 or by inserting a weak interlayer(e.g, After lamination, organic additives were removed by heating to graphite, s)between ceramic substrates. However, these 800C at a rate of 3.C/h, followed by a holding time of 4 h laminates have problems in high-temperature oxidizing appli- Subsequently, the bulk materials were isostatically cold cations. Strong and tough oxide laminates that are suitable for pressed at-170 MPa for 10 min, Consolidation was performed se at high temperatures in air, also are not yet available by hot pressing at 28 MPa, under an argon atmosphere, at temperatures of 1550C for 2 h. After hot pressing, the lami- nate was annealed at 1000C for 6 h. A YPO//YPSZ laminate without YZ3A7 layers also was fabricated under the same con- F. W. Zok---contributing editor arso (2) Mechanical and Microstructural Evaluation of Laminated Composites Mm版你 May f scientific research through Dr.Alex- The hot-pressed slabs were cut into bars with dimensions of No. AFOSR-F49620-93-1-0562. 25 mm x 2.2 mm x 2.2 mm. a bar was ground and polished to a I um finish with diamond pastes. The edge was chamfered 2421

A Strong and Damage-Tolerant Oxide Laminate Dong-Hau Kuo* and Waltraud M. Kriven* Department of Materials Science and Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801 A novel laminar oxide composite was developed. This oxide laminate, which was fabricated by hot pressing, consisted of three types of layers, which were stacked in a repeating sequence of YPO4, yttria-stabilized ZrO2, 30 vol% yttria￾stabilized ZrO2–70 vol% Al2O3, and yttria-stabilized ZrO2. The behavior of the oxide laminate was evaluated by four￾point flexural testing and the indentation technique. The flexural strength from one test was 358 MPa, and the load– displacement curve of this test displayed a graceful failure. Pronounced interfacial delamination contributed to a high work of fracture and damage tolerance. These properties have rarely been observed in oxide composites and are comparable to those of non-oxide composites, such as SiC/ graphite, SiC/BN, and Si3N4/BN laminates. I. Introduction REINFORCEMENTS (particulate, whisker, and fiber) and phase￾transformable materials have been incorporated into ce￾ramics for toughening. Continuous-fiber-reinforced ceramic composites (CFCCs) are the most-promising materials and have shown a nonbrittle response.1–6 The strong and tough non-oxide CFCC relies upon a carbon or boron nitride (BN) film between the fiber and the matrix.5 The role of the inter￾facial film is to induce debonding and sliding mechanisms, which allow energy dissipation by internal friction.7–9 Never￾theless, high-temperature oxidation of carbon or BN results in brittle fracture and has limited the applications of non-oxide CFCCs. On the other hand, the development of strong and tough oxides by incorporating oxide fibers in oxide matrices is on￾going.10 The oxide materials possess oxidation resistance, low thermal conductivity, and high electrical resistivity, as com￾pared with non-oxide materials. These oxide-fiber–oxide￾matrix composites are expected to offer high-temperature oxi￾dation resistance, albeit with a compromised strength. Limited by the availability and properties of fibers, as well as interfacial coatings that behave similar to carbon or BN, oxide CFCCs have not yet attained the expected properties. Laminated composites with unique and adjustable properties can be achieved by stacking cast tapes of different composi￾tions or by incorporating fibers and whiskers into the lami￾nates. Tough, laminated composites can be obtained by intro￾ducing ductile interlayers (e.g., metallic layers11,12) or carbon fiber/epoxy prepregs13 or by inserting a weak interlayer (e.g., graphite14,15) between ceramic substrates. However, these laminates have problems in high-temperature oxidizing appli￾cations. Strong and tough oxide laminates that are suitable for use at high temperatures in air, also are not yet available. Yttrium phosphate (YPO4), a xenotime structure, is expected to behave as an analog to the rare-earth phosphate, e.g., mona￾zite (LaPO4), which has been introduced as a possible func￾tional interface for oxide–oxide composites.10 Although YPO4 has a tendency to react with alumina (Al2O3) at high tempera￾tures, it is chemically compatible with yttrium aluminate (Y3Al5O12)16 and mullite (3Al2O3z2SiO2) at least up to 1600°C. Zirconia (ZrO2) has been widely applied in the forms of partially stabilized zirconia (PSZ) and zirconia-toughened alu￾mina (ZTA). The high strength (2.5 GPa) of the yttria￾stabilized ZrO2/20 wt % Al2O3 (Y2O3-stabilized ZrO2/20 wt% Al2O3) composite has been measured after densification by hot isostatic pressing.17 High-toughness (ù15 MPazm1/2 ) ZrO2 has been exploited in a Ce-TZP/(Ce-TZP+Al2O3) laminate.18 Im￾proved high-temperature strength (1 GPa at 1000°C) of ZrO2, in combination with Al2O3, has been achieved.19 Strengthening strategies for ZrO2-containing ceramics at high temperature have been proposed by Claussen.20 The purpose of this study was to design and fabricate a strong and tough oxide laminate. Two types of systems have been examined: the first system was a two-layered, YPO4/ Y2O3–partially stabilized ZrO2 (YPO4/YPSZ) multilayered ox￾ide laminate, and the second system was a four-layered, YPO4/ Y2O3-stabilized ZrO2/30 vol% Y2O3-stabilized ZrO2–70 vol% Al2O3/Y2O3-stabilized ZrO2 oxide laminate. The complex con￾figuration was chosen mainly to achieve laminates with rea￾sonable strength values and to overcome the deleterious effects of thermal expansion mismatch and chemical reactions. II. Experimental Procedures (1) Materials The synthesis of YPO4 powders has been described else￾where.21 YPSZ (3 mol% Y2O3) (TZ-3Y, Tosoh USA, Atlanta, GA) and Al2O3 powder (99.8% A16-SG, Alcoa Aluminum, Pittsburgh, PA) were used. Three types of tape-cast tapes were fabricated: YPO4, YPSZ, and 30 vol% YPSZ–70 vol% Al2O3 (YZ3A7). The procedure for making laminated composites by tape casting was described elsewhere.21 An 80-layer laminated composite was fabricated by stacking these three types of tapes in the repeating sequence of YPO4, YPSZ, YZ3A7, and YPSZ. After lamination, organic additives were removed by heating to 800°C at a rate of 3°C/h, followed by a holding time of 4 h. Subsequently, the bulk materials were isostatically cold pressed at ∼170 MPa for 10 min. Consolidation was performed by hot pressing at 28 MPa, under an argon atmosphere, at temperatures of 1550°C for 2 h. After hot pressing, the lami￾nate was annealed at 1000°C for 6 h. A YPO4/YPSZ laminate without YZ3A7 layers also was fabricated under the same con￾ditions for comparison. (2) Mechanical and Microstructural Evaluation of Laminated Composites The hot-pressed slabs were cut into bars with dimensions of 25 mm × 2.2 mm × 2.2 mm. A bar was ground and polished to a 1 mm finish with diamond pastes. The edge was chamfered F. W. Zok—contributing editor Manuscript No. 191784C. Received May 14, 1997; approved May 30, 1997. Supported by the U.S. Air Force Office of Scientific Research through Dr. Alex￾ander Pechenik under Grant No. AFOSR-F49620-93-1-0562. *Member, American Ceramic Society. J. Am. Ceram. Soc., 80 [9] 2421–24 (1997) Journal 2421

2422 Communications of the American Ceramic Sociery Vol. 80. No 9 IYPO4! Y PSZ Fig. 1. SEM micrograph of a YPO/YPSZ/30 vol% YPSZ-70 vol% AL,O,/YPSZ laminate, showing three types of laminar layers YPSZ- YZ3A7 with 15 um diamond paste. Four-point flexural testing was formed using an inner span of 10 mm and an outer span of 20 mm at a crosshead speed of 0.05 mm/min in a screw-driven machine(Model 4502, Instron, Canton, MA). The tensile sur- face was normal to the hot-pressing direction. The apparent work of fracture was obtained by dividing the area under the load-displacement curve by the cross-sectional area of the ample 2 Radial cracks were generated under an indentation oad of 10 kg to study the crack propa teraction with the microstructure. Microstructural character ization was performed by optical microscopy and scanning electron microscopy(SEM)(Model DS-130, International Sci- O5 mm entific Instruments, Santa Clara, CA) IIL. Results he microstructure of the oxide laminate is shown in Fig. 1 Fig 3. (a)Optical and(b)SEM aces of a YPO/YPSZ/30 vol% YPSZ-70 vol%Al,,/YPSZ lami- Interfaces between YPO A and YPSZ and between YPSZ and nate after four-point flexural testing YZ3A7 were straight and distinguishable. No chemical reac- tions were detected by energy-dispersive spectroscopy(EDS) Figure 2 shows a load-displacement se of the ypo nd the fractural behavior was examined using optical micros- YPSZ/YZ3A7/YPSZ hybrid laminate had an ultimate copy and SEM. Figures 3(a)and(b) are micrographs of the four-point strength of 358 MPa, the ste e load decreases four-point fracture response, taken by optical microscopy and beyond the peak stress, are characteristi SEM, respectively. The optical micrograph displays a low- ture. Before the bend bar broke, the flexural test was stopped magnification view of the test bar that was located between the inner loading points. The delaminated interfaces extend later ally upward to the two outer loading points but do not run to the 500 end of the test bar. The SEM micrograph reveals the detailed nature of the fracture. The tensile part of the laminate indicated pronounced interfacial delamination, whereas the compressive part remained intact(Fig. 3(b)). The delaminated interfaces were located only between YPO4 and YPSZ; interfaces be- tween YPSZ and YZ3A7 were strongly bonded without inter facial delamination. The apparent work of fracture was mea- sured to be 8.2 kJ/m2 To examine the interactions between cracks and the lami- nated microstructure, Vickers indentation cracks were intro- duced to this oxide laminate at an orientation of 0 /90(Fig 00 4(a))and 45(Fig. 4(b)), relative to the layer length direction Radial cracks propagated through the YPSZ/YZ3A7 interface which indicated strong bonding at that interface. This behavior was clearly displayed by the 45 indent(Fig. 4(b). In addition to generating radial cracks, the 45 indent produced cracks Crosshead Displacement (mm) along the YPO /YPSZ interface. This observation was consis tent with what had occurred during the flexural test Fig. 2. Load versus d ent curve of a YPO//YPSZ/30 On the other hand, a two-layered YPO,YPSZ laminate sys- YPSZ-70 vol% Alo laminate tested in four-point fl tem was not successfully fabricated. This laminate shattered experimental and( into pieces of broken laminae after hot pressing

with 15 mm diamond paste. Four-point flexural testing was performed using an inner span of 10 mm and an outer span of 20 mm at a crosshead speed of 0.05 mm/min in a screw-driven machine (Model 4502, Instron, Canton, MA). The tensile sur￾face was normal to the hot-pressing direction. The apparent work of fracture was obtained by dividing the area under the load–displacement curve by the cross-sectional area of the sample.22 Radial cracks were generated under an indentation load of 10 kg to study the crack propagation profiles and the interaction with the microstructure. Microstructural character￾ization was performed by optical microscopy and scanning electron microscopy (SEM) (Model DS-130, International Sci￾entific Instruments, Santa Clara, CA). III. Results The microstructure of the oxide laminate is shown in Fig. 1. Interfaces between YPO4 and YPSZ and between YPSZ and YZ3A7 were straight and distinguishable. No chemical reac￾tions were detected by energy-dispersive spectroscopy (EDS) analysis. Figure 2 shows a load–displacement response of the YPO4/ YPSZ/YZ3A7/YPSZ hybrid laminate, which had an ultimate four-point strength of 358 MPa; the stepwise load decreases, beyond the peak stress, are characteristic of the nonlinear frac￾ture. Before the bend bar broke, the flexural test was stopped and the fractural behavior was examined using optical micros￾copy and SEM. Figures 3(a) and (b) are micrographs of the four-point fracture response, taken by optical microscopy and SEM, respectively. The optical micrograph displays a low￾magnification view of the test bar that was located between the inner loading points. The delaminated interfaces extend later￾ally upward to the two outer loading points but do not run to the end of the test bar. The SEM micrograph reveals the detailed nature of the fracture. The tensile part of the laminate indicated pronounced interfacial delamination, whereas the compressive part remained intact (Fig. 3(b)). The delaminated interfaces were located only between YPO4 and YPSZ; interfaces be￾tween YPSZ and YZ3A7 were strongly bonded without inter￾facial delamination. The apparent work of fracture was mea￾sured to be 8.2 kJ/m2 . To examine the interactions between cracks and the lami￾nated microstructure, Vickers indentation cracks were intro￾duced to this oxide laminate at an orientation of 0°/90° (Fig. 4(a)) and 45° (Fig. 4(b)), relative to the layer length direction. Radial cracks propagated through the YPSZ/YZ3A7 interface, which indicated strong bonding at that interface. This behavior was clearly displayed by the 45° indent (Fig. 4(b)). In addition to generating radial cracks, the 45° indent produced cracks along the YPO4/YPSZ interface. This observation was consis￾tent with what had occurred during the flexural test. On the other hand, a two-layered YPO4/YPSZ laminate sys￾tem was not successfully fabricated. This laminate shattered into pieces of broken laminae after hot pressing. Fig. 1. SEM micrograph of a YPO4/YPSZ/30 vol% YPSZ–70 vol% Al2O3/YPSZ laminate, showing three types of laminar layers. Fig. 2. Load versus displacement curve of a YPO4/YPSZ/30 vol% YPSZ–70 vol% Al2O3/YPSZ laminate tested in four-point flexure ((—) experimental and (zzz) fitted). Fig. 3. (a) Optical and (b) SEM micrographs showing the side sur￾faces of a YPO4/YPSZ/30 vol% YPSZ–70 vol% Al2O3/YPSZ lami￾nate after four-point flexural testing. 2422 Communications of the American Ceramic Society Vol. 80, No. 9

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

Communications of the American Ceramic Sociery Vol. 80. No 9 nate system behave comparable to unbroken fibers in fiber M. C. Shaw, D. B. Marshall, M.S. Dadkhah, and A G. Evans, ""Cra reinforced composites. We consider severe interfacial delani echanisms in Ceramic/Metal Multilayers, Acta Metall,41[ nation, as observed in this study, to be an important mechanism 331l-22(1993) that operates in flaw-tolerant materials IZ. Chen and JJ. Mecholsky Jr, "Toughening by Metallic Lamina in ISC. A. Folsom. F. w. Zok. FF. Lange. and D. B. Ma echanical Conclusions Behavior of a Laminar Ceramic/Fiber-Reinforced Epoxy ”JAm Ceram.Soc,75296975(1992) A strong and tough oxide laminate has been developed by 4W.J. Clegg, K. Kendall, N. M. Alford, D. Birchall, and T w. Button, " A tacking three types of tape- cast tapes in the repeating seque Simple Way to Make Tough Ceramics, Nature(London), 347, 455-57(1990) f YPO. Y,Ox-stabilized ZrO,. 30 vol% Y, Ox-stabilized IsW. J. Clegg, ""The Fabrication and Failure of Laminar Ceramic Compos- ites, " Acta Metall, 40 [11] 3085-93(1992) ZrOx-70 vol% A,O3, and Y2O3-stabilized Zro2. This hybrid l6D H. Kuo and W. M. Kriven, "Characterization of Yttrium Phosphate and oxide laminate has a high flexural strength of 358 MPa and an osphate/Yttrium Aluminate Laminate, 'J. Am. Ceram. Soc., apparent work of fracture of -8 2 kJ/m2, this laminate als displays a graceful failure from one four-point flexure test gth and Fracture Toughness Limited debonding and crack deflection at the interfaces of O, and Y,,-Partiall laminates cannot offer ceramics sufficient damage tolerance ZrO,, J. Am. Ceram Soc., 68 [1]C-4-C-5(1985). Pronounced interfacial delamination in this oxide laminate has ered Microcomposites of Ce-ZrO, and Al,O3, ""J. Am. Ceram. Soc., demonstrated an effective route to enhance the damage toler- 14[12]2979-87(1991) ance of the material 19K. Tsukuma, K. Ueda, K. Matsushita, and M. Shimada, High- ure Strength and Fracture Toughness of Y,Oa-Par Acknowledgment: The authors acknowledge Dr David Marshall of the ZrO /AlO3 Composites, J.A. Ceram Soc., 68 [2JC-56-C-58(1985) Rockwell Science Center(Thousand Oaks, CA) for his information regarding a 20N. Claussen, Strategies for ZrO,-Toughened Ceramics at monazite/zirconia system High Temperatures, Mater. Sci. Eng, 71, 23-38(1985) 2D. H. Kuo and W. M. Kriven, ""Chemical Stability, Microstructure, and Mechanical Behavior of LaPO,-Containing Ceramics, Mater. Sci. Eng,A. References A210[-2]123-34(199) K M. Prewo and J High-Strength Silicon Carbide Fiber- 2H. G. Tattersall and G. Tappin, "The Work of Fracture and Its Measure- Mater.sc,ls|2]463-68(1980) ment in Metals, Ceramics and Other Materials, J. Mater. Sci., 1, 296-301 licon Carbide- Yarn- Reinforced Glass- (1966) Matrix Composites, J. 17]1201-206(1982 arbide- Fiber-Reinforced Glas 2D. J. Green, R. H J. Hannink, and M. V. Swain, Transformation Toughen ing of Ceramics. CRC Press, Boca Raton, FL, 1989 2w. D Kingery, H K Bowen, and D R. Uhlmann, Introduction to Ceram- K.M. Prewo, J.J.Brennan, and G K. Layden, "Fiber-Reinforced Glasses ics, 2nd ed. Wiley, New York, 1975 nd Glass Ceramics for High-Performance Applications, Am. Ceram. So Bm,65|2305-13(1986) Cambridge University Press, Cambridge. U. K. facial Characterization of Glass and Glass-Ceramic Ma- 26C. A. Folsom, F. w. Zok, and FF. Lange " Flexural P triN/Nicalon SiC Fiber Composites"; Pp. 549-60 in Tailoring Multiphase and Multilayer Materials: 1, Modeling, J. Am. Ceram. Soc., 77 (3]689-96(1994) Composite Ceramics. Edited by R. T. Tressler, G. L. Messing, C G. Pantano, C. A. Folsom, F. w. Zok, and F. F. Lange, ""Flexural Properties of Brittle and R E Newnham. Plenum Press, New York, 1986 Multilayer Materials: Il, Experiments, 'J. Am. Ceram. Soc., 77 [8]2081-87 6A. G. Evans, ""Perspective on the Development of High-Toughness Ceram- 1994) G. Evans. Effect of Inter- 2D. H Kuo and W. M. Kriven, ""Development of Yttrium Phosphate as ar Meshacaite Pasp ceres mi Pullout n aeom sber- rzi ar L ing of Pacific Rim Ceramic Societies, Cairns, Australia, July 15-17, 1996. 29S. Baskaran, S D. Nunn, D. Popovic, and J. W. Halloran, "Fibrous Mono- latrix Interface in Ceramic Composites, " Am. Ceram. Soc. Bull, 68 242942(1989) Carbide/ Graphite System, "J Anm Ceram. Soc., 76[9]2217-24(1 nics Ih. Fiber-Reintorced Ceramic-matrix Co.mpeosite with hole, "comp asies 25 chanical Properties and Oxidation Behavior of the silicon Carbide/Boron ni- am. Soc 5]124955 E D Morgan and D B. Marshall, "" Ceramic Composites of Monazite 3 H. Liu and S M. Hsu, ""Fracture Behavior of Multilayer Silicon Nitride/ and Alumina, J. Am. Ceram. Soc, 78 6 1553-63(1995 Boron Nitride Ceramics, " J. Am. Ceram Soc., 79 19] 2452-57(1996). O

nate system behave comparable to unbroken fibers in fiber￾reinforced composites. We consider severe interfacial delami￾nation, as observed in this study, to be an important mechanism that operates in flaw-tolerant materials. V. Conclusions A strong and tough oxide laminate has been developed by stacking three types of tape-cast tapes in the repeating sequence of YPO4, Y2O3-stabilized ZrO2, 30 vol% Y2O3-stabilized ZrO2–70 vol% Al2O3, and Y2O3-stabilized ZrO2. This hybrid oxide laminate has a high flexural strength of 358 MPa and an apparent work of fracture of ∼8.2 kJ/m2 ; this laminate also displays a graceful failure from one four-point flexure test. Limited debonding and crack deflection at the interfaces of laminates cannot offer ceramics sufficient damage tolerance. Pronounced interfacial delamination in this oxide laminate has demonstrated an effective route to enhance the damage toler￾ance of the material. Acknowledgment: The authors acknowledge Dr. David Marshall of the Rockwell Science Center (Thousand Oaks, CA) for his information regarding a monazite/zirconia system. References 1 K. M. Prewo and J. J. Brennan, ‘‘High-Strength Silicon Carbide Fiber￾Reinforced Glass-Matrix Composites,’’ J. Mater. Sci., 15 [2] 463–68 (1980). 2 K. M. Prewo and J. J. Brennan, ‘‘Silicon Carbide-Yarn-Reinforced Glass￾Matrix Composites,’’ J. Mater. Sci., 17 [4] 1201–206 (1982). 3 J. J. Brennan and K. M. Prewo, ‘‘Silicon Carbide-Fiber-Reinforced Glass￾Ceramic Matrix Composites Exhibiting High Strength and Toughness,’’ J. Mater. Sci., 17 [8] 2371–83 (1982). 4 K. M. Prewo, J. J. Brennan, and G. K. Layden, ‘‘Fiber-Reinforced Glasses and Glass Ceramics for High-Performance Applications,’’ Am. Ceram. Soc. Bull., 65 [2] 305–13 (1986). 5 J. J. Brennan, ‘‘Interfacial Characterization of Glass and Glass-Ceramic Ma￾trix/Nicalon SiC Fiber Composites’’; pp. 549–60 in Tailoring Multiphase and Composite Ceramics. 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