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 2421A 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