Microstructure/Properties Relations of Advanced Materials ournal . Am Ceram. So, 80 [7] 1677-83 (1997) Debonding in Multilayered Composites of Zirconia and Lapo4 David B. Marshall, Peter E D Morgan, and Robert M. Housley Rockwell Science Center, Thousand Oaks, Califomia 91360 Multilayered composites consisting of LaPO. (La-monazite) I. Experimental Procedure layers alternating with various Zro,based materials were fabricated to investigate whether Lapo, provides a weakly Laminar composites consisting of alternating layers of bonded interface suitable for promoting toughening, as pre- LaPO, and zirconia were fabricated by colloidal techniques viously observed in the system LaPO./Al,O,. The following Four types of composites containing different compositions in Zro,based materials were assessed: Y-zrO2, Y-Zro the"zirconia"layers were fabricated: one with 3 mol%Y,O AL,O,, Ce-ZrO2, and Ce-ZrO, / AL,O3. Debonding was addition (Tosoh 3Y); one with 12 mol%CeO2 addition(Tosoh observed in all cases. The composites containing Yzro, 12Ce); one with a mixture(50% by volume)of Al2 0, (Sumi and Y-Zro2 /Al,O, were stable, with no reactions, at tem. tomo AKP30)and Y-ZrO2; and one with a mixture of Al2O3 (50% by volume)and Ce-zrO2. The layer thicknesses were in peratures up to at least 1600C. However, in the composites the range 5 to 50 um for the LaPO layers and 20 to 200 um for containing Ce-ZrO2, interdiffusion of Ce and La occurred, resulting in formation of a pyrochlore like phase and, in the Zro2 layers The composites were consolidated by sequential centrifuging the case of the Ce-rO, AL,O, composite, a(Ce, LaAl Ois or vacuum slip casting of colloidal suspensions of the various se was made of a colloidal technique developed by Velamakanni et d velamakannia in which an I. Introduction aqueous electrolyte (NH.NO,) was used (after dispersing the wders at pH 2) to produce short-range repulsive hydration SF EVERAL mechanisms have been used to achieve toughen forces and reduce the magnitudes of the longer range electro- ing in multilayered ceramic composites. One involves static forces between the suspended particles, Such conditions enhanced transformation toughening in systems containing zir weakly attractive network of particles, which pre conia with strongly bonded interfaces, While large increases vents mass segregation during centrifugation and slip casting in fracture toughness een achieved in such composites but because of the lubricating action of the short-range repul failure in tensile loading involves growth of a single crack: sive forces, allows particles to pack to high green density the composites do not exhibit a large nonlinear behavior of The prevention of mass segregation is especially important for distributed damage. Other mechanisms involve deflection of forming uniform two-phase layers such as Al,O, Zro, from cracks or secondary cracking caused by the presence of weak suspensions containing mixtures of the two types of powder ral defects. -If the degree of crack deflection is sufficient to 350 MPa, then sintered in air (in the case of Y-containing cause splitting between layers, a damage-tolerant response and composites)or oxygen( Ce-containing composites )at 1600oC distributed damage can be achieved in flexural loading, as dem- for 2 h. Several specimens containing Y-ZrO2 were packed in onstrated initially in systems containing layers of carbon alumina powder after the cold-pressing step and hot-pressed in BN -However, the usefulness of systems containing carbon graphite dies at 1400'C for 1 h. Sections were cut, polished and bn is limited by their sensitivity to oxidation. and thermally etched at 1400'C for microstructural analysis Attempts have been made recently to avoid this limitation by Microstructural characteristics of the multilayered compos- developing analogous oxide-based systems, using either layers ites were assessed using X-ray diffraction, electron microprobe SEM, secondary electrons, backscattered electrons, and cathod (La-monazite)and YPO, (xenotime)to cause debonding. In the luminescence were used for imaging, and elements were identi ystem LaPO.,O3, the interfacial toughness is sufficiently fied by energy-dispersive spectroscopy (EDS)of fluorescence X-rays. The response of the cathodoluminescence detector was mally incident crack to deflect along the interface rather than strongly peaked in the blue wavelengths. The electron micro- cross it.This system is stable for long periods in air at temper- probe measurements were taken at an operating voltage of 15kV atures at least as high as 1600oC. 3. I4 In this paper, we preser preliminary study of the stability and debonding characteristics cerium oxide (Electron microprobe measurements were done at logy Department at Caltech, using a JEOL JXA-733 of achieving toughening from both interlaminar cracking and instrument. In the various imaging modes the following phase transformations is also explored ndary electrons, alumina was darker than zirconia and mona- Ite. while zirconia in cathodoluminescence, ( La, Ce)PO, was bright, while Al David J. Green-contributing editor and zro, did not luminesce Debonding at the lapo layers was tested using two methods One involved forming cracks in a polished surface(normal to the ayers by indenting with a vickers indenter. the other involved loading rectangular beams in four-point bending and monitoring crack growth in situ using an optical microscope. The beams Numbers were oriented with the layers normal to the loading direction, so Member, American Ceramic Soc that initial crack growth was normal to the layers. Some beams 1677f.:' .. ':,:.,);,,?,yii&.is ., 1,:... ._ Microstructure/Properties Relations of Advanced Materials J ~m c.mnr soc, M) 171 1677-83 (im Debonding in Multilayered Composites of Zirconia and LaPO, David B. Marshall,. Peter E. D. Morgan,' and Robert M. Housley. Rockwell Science Center, Thousand Oaks, California 91360 Multilayered composites consisting of LaPO, (La-monazite) layers alternating with various Zr0,-based materials were fabricated to investigate whether LaPO, provides a weakly bonded interface suitable for promoting toughening, as pre￾viously observed in the system LaPO, /Al,O,. The following Zr0,-based materials were assessed: Y-ZrO,, Y-ZrO,/ A1203, Ce-ZrO,, and Ce-ZrO,/Al,O,. Debonding was observed in all cases. The composites containing Y-ZrO, and Y-ZrO,/Al,O, were stable, with no reactions, at tem￾peratures up to at least 1600°C. However, in the composites containing Ce-ZrO,, interdiffusion of Ce and La occurred, resulting in formation of a pyrochlore-like phase and, in the case of the Ce-ZrO,/Al,O, composite, a (Ce,La)Al,,O,, magnetoplumbite phase. I. Introduction EVERAL mechanisms have been used to achieve toughen- S ing in multilayered ceramic composites.' One involves enhanced transformation toughening in systems containing zir￾conia with strongly bonded interfaces." While large increases in fracture toughness have been achieved in such composites, failure in tensile loading involves growth of a single crack: the composites do not exhibit a large nonlinear behavior or distributed damage. Other mechanisms involve deflection of cracks or secondary cracking caused by the presence of weak layers, weak interfaces, residual stresses, or other microstruc￾tural If the degree of crack deflection is sufficient to cause splitting between layers, a damage-tolerant response and distributed damage can be achieved in flexural loading, as dem￾onstrated initially in systems containing layers of carbon or BN."' However, the usefulness of systems containing carbon and BN is limited by their sensitivity to oxidation. Attempts have been made recently to avoid this limitation by developing analogous oxide-based systems, using either layers of magnetoplumbite/P-alumina cornpo~nds,'-''-'~ porous oxide layers,I2 or rare-earth orthophosphates'3-16 such as LaPO, (La-monazite) and YPO, (xenotime) to cause debonding. In the system LaPo,-Al,O,, the interfacial toughness is sufficiently low to satisfy the criterion of He and Hutchinson" for a nor￾mally incident crack to deflect along the interface rather than cross it.I3 This system is stable for long periods in air at temper￾atures at least as high as 1600°C.13*'4 Inthis paper, we present a preliminary study of the stability and debonding characteristics of LaPO, with several layered zirconia systems. The possibility of achieving toughening from both interlaminar cracking and transformations is also explored. David J. Green-contributing editor Manuscript No. 191602.,Received August 15,1996; approved February 10,1997. Resented at a Symposium on Microstruc~Ro rty Relations of Advanced Materials, a symposium in honor of Professor Arthur R"uefs 60th birthday, held at Max-Planck-Institut fllr Metallforschun , Stuttgart. Germany. April 29-30,1996. Suppomd by the Air Force Office ot@Scientific Research under Coneact Numbers F49620-92-C-0028 and F49620-96-C-0026 monitored by Dr. A. Pechenik. 'Member. American Ceramic Society. II. Experimental Procedure Laminar composites consisting of alternating layers of LaPO, and zirconia were fabricated by colloidal techniques. Four types of composites containing different compositions in the "zirconia" layers were fabricated: one with 3 mol% Y203 addition (Tosoh 3Y); one with 12 mol% CeO, addition (Tosoh 12Ce); one with a mixture (50% by volume) of A1203 (Sumi￾tom0 AKP30) and Y-Zro,; and one with a mixture of A1,0, (50% by volume) and Ce-ZrO,. The layer thicknesses were in the range 5 to 50 p,m for the LaPO, layers and 20 to 200 pm for the Zro, layers. The composites were consolidated by sequential centrifuging or vacuum slip casting of colloidal suspensions of the various layers. Use was made of a colloidal technique developed by Velamakanni er al." and Chang and VelamakanniZ2 in which an aqueous electrolyte (NH,NO,) was used (after dispersing the powders at pH 2) to produce short-range repulsive hydration forces and reduce the magnitudes of the longer range electro￾static forces between the suspended particles. Such conditions produce a weakly attractive network of particles, which pre￾vents mass segregation during centrifugation and slip casting, but because of the lubricating action of the short-range repul￾sive forces, allows particles to pack to high green density. The prevention of mass segregation is especially important for forming uniform two-phase layers such as Al,03-Zro, from suspensions containing mixtures of the two types of powder. The consolidated bodies were cold isostatically pressed at 350 MPa, then sintered in air (in the case of Y-containing composites) or oxygen (Ce-containing composites) at 1600OC for 2 h. Several specimens containing Y-ZrO, were packed in alumina powder after the cold-pressing step and hot-pressed in graphite dies at 1400°C for 1 h. Sections were cut, polished, and thermally etched at 1400OC for microstructural analysis. Microstructural characteristics of the multilayered compos￾ites were assessed using X-ray diffraction, electron microprobe analysis, and scanning electron microscopy (SEM). In the SEM, secondary electrons, backscattered electrons, and cathodo￾luminescence were used for imaging, and elements were identi￾fied by energy-dispersive spectroscopy (EDS) of fluorescence X-rays. The response of the cathodoluminescence detector was strongly peaked in the blue wavelengths. The electron micro￾probe measurements were taken at an operating voltage of 15 kV using standards of lanthanum phosphate, zirconium silicate, and cerium oxide. (Electron microprobe measurements were done at the Geology Department at Caltech, using a JEOL JXA-733 instrument.) In the various imaging modes the following phase distinctions could be readily made: with backscattered and sec￾ondary electrons, alumina was darker than zirconia and mona￾zite, while zirconia and monazite were almost indistinguishable; in cathodoluminescence, (La,Ce)PO, was bright, while A1203 and ZrO, did not luminesce in the blue and were indis￾tinguishable. Debonding at the LaPO, layers was tested using two methods. One involved forming cracks 1z1 a polished surface (normal to the layers) by indenting with a Vickers indenter. The other involved loading rectangular beams in four-point bending and monitoring crack growth in sifu using an optical microscope.. The beams were oriented with the layers normal to the loading direction, so that initial crack growth was normal to the layers. Some beams 1677