MIATERIAL TENGE ENGMEERIM ELSEVIER Materials Science and Engineering A241(1998)241-250 fracture of multilayer oxide composites Dong- Hau Kuo Waltraud M. Kriven Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 3 February 1997; received in revised form 9 June 1997 Abstract Ceramics with high strength and damage tolerance have been realized in the well-recognized non- oxide systems, e.g. silicon arbide/graphite and silicon carbide/boron nitride laminates. All-oxide systems without compliant materials functioning as do graphite and boron nitride are still facing the problem of catastrophic failure. In this study, ten kinds of multilayered, all-oxide laminates fabricated by a low-cost tape casting technique were evaluated These materials were yttrium phosphate (YPO4)-and lanthanum phosphate (LaPO4)-containing zirconia (ZrO2) laminates, and aluminum phosphate(AlPO4)-containing alumina (Al,O3)laminates. A YPOa- containing ZrO, laminate demonstrated excellent strength and improved damage tolerance. A LaPOa-containing ZrO, laminate also displayed a satisfactory result. An AlPO4/Al,O3 laminate was weak. The AlPO,/Al,O minate was strengthened by reinforcing AlPO4 layers with Al,O3, while retaining non-brittle fracture. Different flexural behaviors in different oxide laminates were discussed. @1998 Elsevier Science s.a Keywords: Y ttrium phosphate: Lanthanum phosphate: Aluminum phosphate; Zirconia; Alumina; Laminate; Mechanical property 1. Introduction between fiber and matrix, to weaken the fiber/matrix interface. Without a weak fiber/matrix interface, the Ceramics have many excellent properties that make fiber-reinforced composites demonstrate catastrophic their use as structural materials very attractive. These failure. However, there are still two remaining prob- properties include high strength, high hardness, wear lems: one is the high temperature oxidation of the resistance, high melting temperature, excellent thermal interlayer and the polymer-derived fibers, and the other and chemical stability, low density, and a unique set of is the high fabrication cost electrical, thermal, and other properties. However, their All-oxide(oxide fiber/oxide matrix) composites might brittleness has prevented their use in structural applica- be the ultimate materials as far as the effect of oxidizing tions to date environment is concerned. Nevertheless, the develop- The most promising candidates for improving ce- ment of oxide fibers and oxide interlayers behaving like amic brittleness have been continuous fiber ceramic carbon and boron nitride as weak interlayers in non-ox composites(CFCCs) which have shown high strength ide systems is nt re- and toughness [1]. The development of these advanced search effort. a potential oxide interphase is lanthanum structural materials for high temperature applications phosphate(LaPO4), having a monazite structure, which nportant technological goal. Progress in was proposed by Morgan et al. [2-4]. Yt phos- many areas of technology, such as gas turbines, heat phate (YPO4, with a xenotime structure, expected to exchangers, space re-entry vehicle design and the like, is behave as an analogue to monazite (LaPO4), has also dependent on the development of these high tempera- been evaluated ture structural materials. These basic materials include Tape-casting has been used to fabricate versatile silicon carbide (Sic) fiber-reinforced ceramic com- laminate composites by stacking tapes of different com- posites with a carbon(C) or boron nitride (BN) layer positions, as well as to incorporate fibers and whiskers into the laminates. Material properties can be g author trolled by adjusting the tape compositions, reinforce- address: Department of Materials Science and Engineer. ment orientation, and stacking sequence. Clegg and Dong Hwa University, Hualien, Taiwan, R.O.C coworkers [9, 10 produced a laminar fabric of Sic 0921-5093/98/S19.00 0 1998 Elsevier Science S.A. All rights reserved PIS09215093097100498·X
Materials Science and Engineering A241 (1998) 241–250 Fracture of multilayer oxide composites Dong-Hau Kuo 1 , Waltraud M. Kriven * Department of Materials Science and Engineering, Uni6ersity of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Received 3 February 1997; received in revised form 9 June 1997 Abstract Ceramics with high strength and damage tolerance have been realized in the well-recognized non-oxide systems, e.g. silicon carbide/graphite and silicon carbide/boron nitride laminates. All-oxide systems without compliant materials functioning as do graphite and boron nitride are still facing the problem of catastrophic failure. In this study, ten kinds of multilayered, all-oxide laminates fabricated by a low-cost tape casting technique were evaluated. These materials were yttrium phosphate (YPO4)- and lanthanum phosphate (LaPO4)-containing zirconia (ZrO2) laminates, and aluminum phosphate (AlPO4)-containing alumina (Al2O3) laminates. A YPO4-containing ZrO2 laminate demonstrated excellent strength and improved damage tolerance. A LaPO4-containing ZrO2 laminate also displayed a satisfactory result. An AlPO4/Al2O3 laminate was weak. The AlPO4/Al2O3 laminate was strengthened by reinforcing AlPO4 layers with Al2O3, while retaining non-brittle fracture. Different flexural behaviors in different oxide laminates were discussed. © 1998 Elsevier Science S.A. Keywords: Yttrium phosphate; Lanthanum phosphate; Aluminum phosphate; Zirconia; Alumina; Laminate; Mechanical property 1. Introduction Ceramics have many excellent properties that make their use as structural materials very attractive. These properties include high strength, high hardness, wear resistance, high melting temperature, excellent thermal and chemical stability, low density, and a unique set of electrical, thermal, and other properties. However, their brittleness has prevented their use in structural applications to date. The most promising candidates for improving ceramic brittleness have been continuous fiber ceramic composites (CFCCs) which have shown high strength and toughness [1]. The development of these advanced structural materials for high temperature applications remains an important technological goal. Progress in many areas of technology, such as gas turbines, heat exchangers, space re-entry vehicle design and the like, is dependent on the development of these high temperature structural materials. These basic materials include silicon carbide (SiC) fiber-reinforced ceramic composites with a carbon (C) or boron nitride (BN) layer between fiber and matrix, to weaken the fiber/matrix interface. Without a weak fiber/matrix interface, the fiber-reinforced composites demonstrate catastrophic failure. However, there are still two remaining problems: one is the high temperature oxidation of the interlayer and the polymer-derived fibers, and the other is the high fabrication cost. All-oxide (oxide fiber/oxide matrix) composites might be the ultimate materials as far as the effect of oxidizing environment is concerned. Nevertheless, the development of oxide fibers and oxide interlayers behaving like carbon and boron nitride as weak interlayers in non-oxide systems is ongoing and requires a significant research effort. A potential oxide interphase is lanthanum phosphate (LaPO4), having a monazite structure, which was proposed by Morgan et al. [2–4]. Yttrium phosphate (YPO4), with a xenotime structure, expected to behave as an analogue to monazite (LaPO4), has also been evaluated [5–8]. Tape-casting has been used to fabricate versatile laminate composites by stacking tapes of different compositions, as well as to incorporate fibers and whiskers into the laminates. Material properties can be controlled by adjusting the tape compositions, reinforcement orientation, and stacking sequence. Clegg and coworkers [9,10] produced a laminar fabric of SiC * Corresponding author. 1 Present address: Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan, R.O.C. 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(97)00 49 8- X
D -H. Kuo, W.M. Kricen/ Materials Science and Engineering 4241(1998)241-250 interleaved graphite films. Folsom et al. [ll] demon- 2.3. Laminate fabrication strated a laminar ceramic/carbon fiber-reinforced epoxy composite. Baskaran and coworkers [12-14 The procedure for making laminated composites by died fibrous monolithic ceramics of a SiC/C sys- tape casting was similar to those described elsewhere tem [13] and a SiC/BN system [14]. Shaw et al. [15] [19]. The slurry formulation contained 20 vol% oxide and Chen and Mecholsky Jr. [16] incorporated powders, 60 vol% solvent consisting of a mixture of metallic layers with ceramics to increase ceram trichloroethylene and ethanol, as well as a dispersant, a toughness. Liu and Hsu [17 fabricated multilayer sil- binder and plasticizers. Slurries were tape cast to yield icon nitride(Si,N4/BN ceramics. All of these materi- laminae of 100-200 um thickness with a doctor blade als have problems in high temperature oxidizing opening of 250-350 um. 80-Layer laminated com- environments posites were fabricated by periodically stacking two or 9 In this paper, all-oxide ceramics were fabricated by three kinds of oxide laminae having dimensions of 25 low-cost, tape casting technique without incorpo mm x 51 mm. Thermocompression was done by hold rating expensive fibers. The materials were YPO- ing for I h at 50-80 C under a 10 MPa pressure. The and LaPOa-containing zirconia (ZrO2) oxide lami organic additives were removed by heating to 650C at nates, and aluminum phosphate(AlPO4)-containing a rate of 3C/h, followed by a 3-h holding time. Subse alumina(Al,O )laminates. Fractural behaviors of quently the bulk materials were isostatically cold nese laminates were evaluated by 4-point flexural pressed at. -170 MPa for 10 min, then loaded in a testing, indentation and microstructural examination nates or Al,O3 powders for AlPO4 laminates respec- tively surrounding the pressed laminates Consolidation was performed by hot pressing, under an Experimental procedures argon atmosphere at 28 MPa, at a temperature of 1550C for YPO a and LaPO4 laminates and 1600C for AlPO4 laminates, both for 2 h. After hot pressing, the 2.I. Materials aminate was annealed at 1000%C for 6 h YPO4, LaPO4, and a 50/50 vol%(AlPO4+ Al,O3) owders were prepared by the Pechini method asTable was used for LaPO4 earlier [18]: 3 mol% yttria-par Symbol and mechanical response of YPO4, LaPO", and AlPO-con- ially stabilized(TZ-3Y or Y-ZrO2)and un-stabilized taining oxide laminates zirconia(TZ-0 or ZrO2) powders from Tosoh, At- System Symbol of stacking periodStrength/damage lanta. GA. were used for the ZrO, source: 99.8% tolerance Al6-SG(Alcoa, Pittsburgh, PA)alumina powder for the Al,O3 source; mullite (3Al2O3. 2SiO2) from Kyor- YPO -containing Zro2 laminates itsu, Nagoya, Japan; AlPO4 from Aldrich, Milwau Y(a) YPO/Y-Zr02YZ3-A7/Y- Goo kee, Wi as one of the AlPO, sources: and 99.9% Zro cerium(Iv) oxide(Aldrich),99.9% strontium oxide Y(b) YP7-YZ3/(YCeSr)Z7-A3 Medium/ bad Y(c) YP7-YZ3/( CeSr) (Aldrich), and 99.9% yttrium oxide(Molycorp, White Plains, NY) for the additives Z7-A3/A/YP7-YZ3 Y(d) YPO/Y-ZrO Shattered after hot 2. 2. Chemical compatibility and microstructural LaPOa-containing ZrO. laminates aPO/Y-Zro Shattered during speci- (b) LaPO//(YCeSr)Z7-A3 Medium/medium patibility were carried out on pressed pellets composed of YPO4, LaPO4,or Z7-Mu3/A/(Y CeSr)Z7-Mu3 AlPO4 powders as one component and Y-ZrO L(d) LaPO4/Y-Zr02/Z3./Y Shattered after hot lets were cold isostatically pressed at 86 MPa for 5 AlPO-containing ZrO2 laminates pressing AL,O, as the other. After uniaxial pressing, the pel- min. These materials were fired at 1500%C. 1550C or 600oC for 3 h and the phases were identified using A(b)(AlPO4+AL,O3/AL,O Medium/medium X-ray diffractometry(XRD, Model D-Max, Rigaku Danvers, MA). Microstructural characterization was () Mu: mullite(3A103 2SiO2);(i)(YCeSr)Z7-A3 and (YCeSrz7. performed by optical microscopy and Mu3 were doped with ceria and strontium oxide in addition to yttria; (i)YZ3-A7 and Z3A7 represent the ZrO/,O, composites with tron microscopy(SEM, Model DS-130, International nd without stabilized yttria, respectively; (iv)(AlPO4+Al,O3 ) 50 Scientific Instruments, Santa Clara, CA) vol AlPO,+50 vol% AlO
242 D.-H. Kuo, W.M. Kri6en / Materials Science and Engineering A241 (1998) 241–250 interleaved graphite films. Folsom et al. [11] demonstrated a laminar ceramic/carbon fiber-reinforced epoxy composite. Baskaran and coworkers [12–14] studied fibrous monolithic ceramics of a SiC/C system [13] and a SiC/BN system [14]. Shaw et al. [15] and Chen and Mecholsky Jr. [16] incorporated metallic layers with ceramics to increase ceramic toughness. Liu and Hsu [17] fabricated multilayer silicon nitride (Si3N4)/BN ceramics. All of these materials have problems in high temperature oxidizing environments. In this paper, all-oxide ceramics were fabricated by a low-cost, tape casting technique without incorporating expensive fibers. The materials were YPO4- and LaPO4-containing zirconia (ZrO2) oxide laminates, and aluminum phosphate (AlPO4)-containing alumina (Al2O3) laminates. Fractural behaviors of these laminates were evaluated by 4-point flexural testing, indentation and microstructural examination. 2. Experimental procedures 2.1. Materials YPO4, LaPO4, and a 50/50 vol% (AlPO4+Al2O3) powders were prepared by the Pechini method as was used for LaPO4 earlier [18]; 3 mol% yttria-partially stabilized (TZ-3Y or Y-ZrO2) and un-stabilized zirconia (TZ-0 or ZrO2) powders from Tosoh, Atlanta, GA, were used for the ZrO2 source; 99.8% A16-SG (Alcoa, Pittsburgh, PA) alumina powder for the Al2O3 source; mullite (3Al2O3 · 2SiO2) from Kyoritsu, Nagoya, Japan; AlPO4 from Aldrich, Milwaukee, WI as one of the AlPO4 sources; and 99.9% cerium (IV) oxide (Aldrich), 99.9% strontium oxide (Aldrich), and 99.9% yttrium oxide (Molycorp, White Plains, NY) for the additives. 2.2. Chemical compatibility and microstructural characterization Studies of chemical compatibility were carried out on pressed pellets composed of YPO4, LaPO4, or AlPO4 powders as one component and Y-ZrO2 or Al2O3 as the other. After uniaxial pressing, the pellets were cold isostatically pressed at 86 MPa for 5 min. These materials were fired at 1500°C, 1550°C or 1600°C for 3 h and the phases were identified using X-ray diffractometry (XRD, Model D-Max, Rigaku, Danvers, MA). Microstructural characterization was performed by optical microscopy and scanning electron microscopy (SEM, Model DS-130, International Scientific Instruments, Santa Clara, CA). 2.3. Laminate fabrication The procedure for making laminated composites by tape casting was similar to those described elsewhere [19]. The slurry formulation contained 20 vol% oxide powders, 60 vol% solvent consisting of a mixture of trichloroethylene and ethanol, as well as a dispersant, a binder and plasticizers. Slurries were tape cast to yield laminae of 100–200 mm thickness with a doctor blade opening of 250–350 mm. 80-Layer laminated composites were fabricated by periodically stacking two or three kinds of oxide laminae having dimensions of 25 mm×51 mm. Thermocompression was done by holding for 1 h at 50–80°C under a 10 MPa pressure. The organic additives were removed by heating to 650°C at a rate of 3°C/h, followed by a 3-h holding time. Subsequently the bulk materials were isostatically cold pressed at 170 MPa for 10 min, then loaded in a graphite die with Y-ZrO2 for YPO4 and LaPO4 laminates or Al2O3 powders for AlPO4 laminates respectively, surrounding the pressed laminates. Consolidation was performed by hot pressing, under an argon atmosphere at 28 MPa, at a temperature of 1550°C for YPO4 and LaPO4 laminates and 1600°C for AlPO4 laminates, both for 2 h. After hot pressing, the laminate was annealed at 1000°C for 6 h. Table 1 Symbol and mechanical response of YPO4-, LaPO4-, and AlPO4-containing oxide laminates System Symbol of stacking period Strength/damage tolerance YPO4-containing ZrO2 laminates Y(a) YPO4/Y-ZrO2/YZ3-A7/Y- Good/good ZrO2 Y(b) YP7-YZ3/(YCeSr)Z7-A3 Medium/bad Y(c) Medium YP7-YZ3/(YCeSr) /bad Z7-A3/A/YP7-YZ3 Y(d) Shattered after hot YPO4/Y-ZrO2 pressing LaPO4-containing ZrO2 laminates LaPO4 L(a) /Y-ZrO2 Shattered during specimen cutting LaPO4 L(b) /(YCeSr)Z7-A3 Medium/medium LaPO4 L(c) Low /(YCeSr) /low Z7-Mu3/A/(YCeSr)Z7-Mu3 LaPO4/Y-ZrO2 L(d) Shattered after hot /Z3-A7/YZrO2 pressing AlPO4-containing ZrO2 laminates A(a) AlPO4/Al2O3 Low/medium A(b) (AlPO4+Al2O3)/Al2O3 Medium/medium (i) Mu: mullite (3Al2O3 · 2SiO2); (ii) (YCeSr)Z7-A3 and (YCeSr)Z7- Mu3 were doped with ceria and strontium oxide in addition to yttria; (iii) YZ3-A7 and Z3-A7 represent the ZrO2/Al2O3 composites with and without stabilized yttria, respectively; (iv) (AlPO4+Al2O3): 50 vol% AlPO4+50 vol% Al2O3.
D -H. Kuo, W.M. Kricen/ Materials Science and Engineering 4241(1998)241-250 Ten kinds of YPO4, LaPO4, and AlPOa laminat (a) ere fabricated, as listed in Table 1. For example, three aLaP4o￥2 kinds of tape-cast tapes were fabricated: YPO4, 3 mol% Y-ZrO2, and 30 vol% Y-ZrO2-70 vol% Al,O3(YZ3 A7)tapes. An 80-layer laminated composite was formed by stacking these three kinds of tapes in the ·Y1o4oYZO2 repeating sequence of YPO4→Y-ZrO2→YZ3-A7→Y Zro2(labelled as the Y(a) system) The (Y CeSr)Z7-A3 and (YCeSr)Z7-Mu3 layers were omposed of 70 vol% ZrO2 and 30 vol% Al,, or mullite with 2 mol%Y,O3 and 4 mol% CeO, based on O Y-Zr( O2 ZrO, and 9. 1 mol% Sro based on Al,O3(molar ratio of hese additives were used due to concerns of the strength and the humidity sensitivity of he composites (4Y, 4Ce)-ZrO2/Al,O3)fabri cated by using wet chemical methods have shown high resistance to the tetragonal-to-monoclinic(t-m) phase 045505560 transformation during low temperature agi Simultaneous additions of SrO and Al, to ZrOz, as Fig. 1. XRD of(a)LaPO/Y-ZrO2(b)YPO,/Y-ZrO2 and(c)Y-Zro2 proposed by Cutler et al. [21] can lead to the in situ ellets fired at 1550C for 3 h formation of strontium aluminate platelets. This type of zirconia can have high strength and hardness without reaction compound. The Y-ZrO2 phase remains tetrag- loss of toughness Chemical stability between AlPO4 and Al,O3 was 2. 4. Mechanical evaluation of laminated composites studied by XRD of a powder compact(35 vol% AlPO4 and 65 vol% Al,O3) which was fired at 1600C for 3 h The hot pressed slabs were cut into bars with dimen Fig. 2 shows the XRD result which indicates the chem- point flexural tests having an outer span of 20 mm and AlPO, was in the a-cristobalite form nd ions of25mm×2.0-2.5mm×2.0-2.5mm.Four- ical compatibility between AlPO Al,O3. The an inner span of 10 mm were performed at room emperature. The tensile surface was parallel to the 3.2. Mechanical responses and microstructure of laminate and tested in a screw-driven machine (model laminated composites 4502, Instron, Canton, MA)with a crosshead speed of 0.05 mm/min. Apparent work-of-fracture was obtained Fig 3 shows two load-displacement responses of the y dividing the area under the load-displacement curve YPO4 /Y-ZrO2,/YZ3-A7/Y-ZrO2 laminate(the Y(a)sys- by the cross-sectional area of the sample Radial cracks tem)tested in two different specimens. These responses were generated under a 10-kg indentation load on the Y(a) system and a 5-kg load on the A(a) and A(b) laminates in order to study crack propagation profiles and interaction with the microstructure 口APO 3. Results 口 Studies of chemical reactions between YPO4 and 1500C/3h Y-ZrO, and between LaPO, and Y-zrO, were carried out by firing pellets at 1500°C,1550°C,andl600°Cfo 3 h. It was found that the xrd results were the same for the different firing temperatures. XRD results of Y-ZrO2, YPOA/Y-ZrO2(50/50 vol) and LaPO4/Y-ZrO (50/50 vol) pellets fired at 1550oC for 3 h are shown in Fig. 1. These XRD results indicate that there is chemi- cal compatibility between YPOA and Y-ZrO2 and be Fig. 2. XRD of an AIPO ALO, pellet fired at 1600C for 3 h, tween Lapo, and Y-ZrO, without the formation of a pared with that of an Al,O, pellet fired at 1500oC for 3h
D.-H. Kuo, W.M. Kri6en / Materials Science and Engineering A241 (1998) 241–250 243 Ten kinds of YPO4, LaPO4, and AlPO4 laminates were fabricated, as listed in Table 1. For example, three kinds of tape-cast tapes were fabricated: YPO4, 3 mol% Y-ZrO2, and 30 vol% Y-ZrO2 –70 vol% Al2O3 (YZ3- A7) tapes. An 80-layer laminated composite was formed by stacking these three kinds of tapes in the repeating sequence of YPO4Y-ZrO2YZ3-A7YZrO2 (labelled as the Y(a) system). The (YCeSr)Z7-A3 and (YCeSr)Z7-Mu3 layers were composed of 70 vol% ZrO2 and 30 vol% Al2O3 or mullite with 2 mol% Y2O3 and 4 mol% CeO2 based on ZrO2 and 9.1 mol% SrO based on Al2O3 (molar ratio of SrO/Al2O30.1). These additives were used due to concerns of the strength and the humidity sensitivity of ZrO2. The composites ((4Y,4Ce)-ZrO2/Al2O3) fabricated by using wet chemical methods have shown high resistance to the tetragonal-to-monoclinic (tm) phase transformation during low temperature aging [20]. Simultaneous additions of SrO and Al2O3 to ZrO2, as proposed by Cutler et al. [21] can lead to the in situ formation of strontium aluminate platelets. This type of zirconia can have high strength and hardness without loss of toughness. 2.4. Mechanical e6aluation of laminated composites The hot pressed slabs were cut into bars with dimensions of 25 mm×2.0–2.5 mm×2.0–2.5 mm. Fourpoint flexural tests having an outer span of 20 mm and an inner span of 10 mm were performed at room temperature. The tensile surface was parallel to the laminate and tested in a screw-driven machine (Model 4502, Instron, Canton, MA) with a crosshead speed of 0.05 mm/min. Apparent work-of-fracture was obtained by dividing the area under the load-displacement curve by the cross-sectional area of the sample. Radial cracks were generated under a 10-kg indentation load on the Y(a) system and a 5-kg load on the A(a) and A(b) laminates in order to study crack propagation profiles and interaction with the microstructure. 3. Results 3.1. Chemical compatibility Studies of chemical reactions between YPO4 and Y-ZrO2 and between LaPO4 and Y-ZrO2 were carried out by firing pellets at 1500°C, 1550°C, and 1600°C for 3 h. It was found that the XRD results were the same for the different firing temperatures. XRD results of Y-ZrO2, YPO4/Y-ZrO2 (50/50 vol) and LaPO4/Y-ZrO2 (50/50 vol) pellets fired at 1550°C for 3 h are shown in Fig. 1. These XRD results indicate that there is chemical compatibility between YPO4 and Y-ZrO2 and between LaPO4 and Y-ZrO2 without the formation of a Fig. 1. XRD of (a) LaPO4/Y-ZrO2, (b) YPO4/Y-ZrO2 and (c) Y-ZrO2 pellets fired at 1550°C for 3 h. reaction compound. The Y-ZrO2 phase remains tetragonal. Chemical stability between AlPO4 and Al2O3 was studied by XRD of a powder compact (35 vol% AlPO4 and 65 vol% Al2O3) which was fired at 1600°C for 3 h. Fig. 2 shows the XRD result which indicates the chemical compatibility between AlPO4 and Al2O3. The AlPO4 was in the a-cristobalite form. 3.2. Mechanical responses and microstructure of laminated composites Fig. 3 shows two load-displacement responses of the YPO4/Y-ZrO2/YZ3-A7/Y-ZrO2 laminate (the Y(a) system) tested in two different specimens. These responses Fig. 2. XRD of an AlPO4/Al2O3 pellet fired at 1600°C for 3 h, as compared with that of an Al2O3 pellet fired at 1500°C for 3 h.
D -H. Kuo, W.M. Kricen/ Materials Science and Engineering 4241(1998)241-250 The other laminated system which showed moderate Max, g= 392 MPa strength and damage tolerance was the LaPO4/ wOF= 10 kJ/m (YCeSr)Z7-A3 laminate(the L(b) system). Fig. 6 shows WOF =8,2k] two load-displacement responses of this laminate under 4-point flexural tests. The ultimate strengths were 276 and 254 MPa with corresponding work-of-fracture val ues of 3.3 and 2.9 kJ/m. This laminate had the zirconia layers modified by alumina which reduced the mis- match between LaPO4 and doped ZrO2 in the coeffi cient of thermal expansion. The l(b) laminate with less thermal expansion mismatch was fabricated success- 00050.10.150 25030.350 fully, but the LaPO/Y-ZrO2 laminate(the l(a) system) Crosshead Displ nt(mm) disintegrated during bend bar cutting. Fig. 7 is the side-view micrograph of a 4-point fractured specimen Fig. 3. Load versus displacement curves for two YPO /Y-Zro, /30 as seen by SEM. Interfacial delamination occurred to vol% Y-ZrO2 -70 vol% AL,O,/Y-ZrO2 laminated specimens tested in prevent catastrophic fracture. 4-point flexure Not all the YPO4 and LaPO4 laminates displayed high strength and damage tolerance. The LaPo4/ had ultimate 4-point strengths of 358 and 392 MPa, (YCeSr)Z7-Mu3/A/(YCeSr)Z7-Mu3 laminate(the L(c) respectively. The step-wise load drops, beyond the peak system)had strengths, after two measurements, of 124 stress, were characteristic of the non-catastrophic frac nd 91 MPa. with work-of-fracture values of 1. 2 and ture. Before the bend bar broke the flexural test of the laminate with flexural strength of 358 MPa was stopped, and the specimen was examined under optical nd scanning electron microscopes. Fig. 4(a) and Fig. 4(b)are micrographs of this specimen as seen by optical microscopy and SEM, respectively. The optical mi- crograph shows a low-magnification view of the test bar located between the inner loading points. The delani- nated interfaces extended laterally up to the two outer loading points, but did not run to the end of the test bar. The SEM micrograph revealed the detailed nature of the fracture. The tensile(bottom) part of the lami nate displayed extensive interfacial delamination, while le compressive(top) part stayed intact(Fig. 4(b). The 1 mm delaminated interfaces were only located between YPOa and Y-Zo2. The YPO/Y-ZrO2 interface located close to the mid-plane was severely damaged. Interfaces be- b tween Y-ZO2 and Yz3-A7 were strongly bonded with out interfacial delamination. It was observed that a delaminated interface showed up periodically after each four-layer configuration. Apparent work-of-fracture values were measured to be 8.2 and 10 kJ/m, respec- tively This high strength and damage tolerant oxide lam nate was qualitatively examined by the indentation technique. Vickers indentation cracks were introduced to this oxide laminate at an orientation of 45(Fig. 5) relative to the layer length direction. The radial cracks were generated from the Vickers indent. In addition to the indentation-induced radial cracks, a preferred prop- agation path along the YPO4/Y-ZrO2 interface was observed. This observation was consistent with what Fig. 4.(a) Optical and (b) SEM micrographs showing the side surfaces of a YPO/Y-ZrO,/30 vol% Y-ZrO2-70 vol% AL,./Y-ZrO had occurred during the flexural test laminate after 4-point flexural testing
244 D.-H. Kuo, W.M. Kri6en / Materials Science and Engineering A241 (1998) 241–250 Fig. 3. Load versus displacement curves for two YPO4/Y-ZrO2/30 vol% Y-ZrO2 –70 vol% Al2O3/Y-ZrO2 laminated specimens tested in 4-point flexure. The other laminated system which showed moderate strength and damage tolerance was the LaPO4/ (YCeSr)Z7-A3 laminate (the L(b) system). Fig. 6 shows two load-displacement responses of this laminate under 4-point flexural tests. The ultimate strengths were 276 and 254 MPa with corresponding work-of-fracture values of 3.3 and 2.9 kJ/m2 . This laminate had the zirconia layers modified by alumina which reduced the mismatch between LaPO4 and doped ZrO2 in the coeffi- cient of thermal expansion. The L(b) laminate with less thermal expansion mismatch was fabricated successfully, but the LaPO4/Y-ZrO2 laminate (the L(a) system) disintegrated during bend bar cutting. Fig. 7 is the side-view micrograph of a 4-point fractured specimen as seen by SEM. Interfacial delamination occurred to prevent catastrophic fracture. Not all the YPO4 and LaPO4 laminates displayed high strength and damage tolerance. The LaPO4/ (YCeSr)Z7-Mu3/A/(YCeSr)Z7-Mu3 laminate (the L(c) system) had strengths, after two measurements, of 124 and 91 MPa, with work-of-fracture values of 1.2 and had ultimate 4-point strengths of 358 and 392 MPa, respectively. The step-wise load drops, beyond the peak stress, were characteristic of the non-catastrophic fracture. Before the bend bar broke, the flexural test of the laminate with flexural strength of 358 MPa was stopped, and the specimen was examined under optical and scanning electron microscopes. Fig. 4(a) and Fig. 4(b) are micrographs of this specimen as seen by optical microscopy and SEM, respectively. The optical micrograph shows a low-magnification view of the test bar located between the inner loading points. The delaminated interfaces extended laterally up to the two outer loading points, but did not run to the end of the test bar. The SEM micrograph revealed the detailed nature of the fracture. The tensile (bottom) part of the laminate displayed extensive interfacial delamination, while the compressive (top) part stayed intact (Fig. 4(b)). The delaminated interfaces were only located between YPO4 and Y-ZO2. The YPO4/Y-ZrO2 interface located close to the mid-plane was severely damaged. Interfaces between Y-ZO2 and YZ3-A7 were strongly bonded without interfacial delamination. It was observed that a delaminated interface showed up periodically after each four-layer configuration. Apparent work-of-fracture values were measured to be 8.2 and 10 kJ/m2 , respectively. This high strength and damage tolerant oxide laminate was qualitatively examined by the indentation technique. Vickers indentation cracks were introduced to this oxide laminate at an orientation of 45° (Fig. 5) relative to the layer length direction. The radial cracks were generated from the Vickers indent. In addition to the indentation-induced radial cracks, a preferred propagation path along the YPO4/Y-ZrO2 interface was observed. This observation was consistent with what had occurred during the flexural test. Fig. 4. (a) Optical and (b) SEM micrographs showing the side surfaces of a YPO4/Y-ZrO2/30 vol% Y-ZrO2 –70 vol% Al2O3/Y-ZrO2 laminate after 4-point flexural testing
D -H. Kuo, W.M. Kricen/ Materials Science and Engineering 4241(1998)241-250 Y73-A7 Y-ZrO2 w LaPO4 CYCeSr)Z7-A3 50m Fig. 5. SEM micrograph of an indentation crack pattern in a YPO/ Fig. 7. SEM micrograph showing the side surface of a LaPO/70 YZr( 02 30 vol% Y-Zr02-70 vol% AL,O3/Y-LrO, laminate Indents vol%(YCeSr)Zro2 -30 vol% AlO, laminate after 4-point flexural were oriented at an orientation of 45 relative to the layer direction Cesr)Z7-A3 laminate(the Y(b) system) had flexural inate. A flexural strength of 47 MPa was measured for strengths measured from two specimens as 187 and 163 the AlPO/AL,O, laminate and of 225 MPa for the MPa and fractured with a brittle nature. The YP7-YZ3/(AlPO4+ AlO3)/Al,O3 laminate, with a non- (YCeSr)Z7-A3/A/(YCeSr)Z7-A3 laminate(the Y(c)sys- catastrophic fracture from one test for each tem) had a flexural strength of 217 MPa with a view of the non-brittle A(b) bar was examined under catastrophic failure from one test. SEM micrographs SEM and is shown in Fig. 10(b). Cracks propagating showing the side surfaces of a Y(b)laminate and a Y(c) laterally were responsible for this non-brittle fracture laminate after 4-point flexural testing are displayed in behavior. Fig. 9(a) and Fig. 9(b) Vickers indentation cracks were also introduced to Some laminates did not fabricate successfully. YPO./ the A(a) and A(b) laminates, at an orientation of 45 Y-ZrO2(the Y(d)system)and LaPO/Y-ZrO,/Z3-A7/(Fig. 11) relative to the layer length direction. On Y-ZrO2(the L(d) system)shattered after removal from propagating from the(AlPO4+Al2 O3)layers towards the hot press die. The previously mentioned LaPo4/Y- the Al2O3 layers, the radial cracks preferentially ZrO, laminate stayed intact after removal from the die deflected along the(AlPO4+ AlO3)/AlO Interrace and annealing, but shattered during bend bar cutting ... Max o= 276 MPa WOF= 2,9kJ/ 6SA1203xa (YCeSr)Z7-Mu3 mmm 00050.10150.20.250.30.350.4 Crosshead Displacement(mm) Fig. 8. SEM micrograph showing the side surface of a LaPo/70 Fig. 6. Load of a LaPo/70 vol vol%(YCeSr)ZrO, -30 vol% mullite/Al,,/70 vol%(Y CeSr)ZrO2-30 (YCeSrZrO2 -30 vol% Al,O, laminate tested in 4-point flexure. vol% mullite laminate after 4-point flexural testin
D.-H. Kuo, W.M. Kri6en / Materials Science and Engineering A241 (1998) 241–250 245 Fig. 5. SEM micrograph of an indentation crack pattern in a YPO4/ Y-ZrO2/30 vol% Y-ZrO2 –70 vol% Al2O3/Y-ZrO2 laminate. Indents were oriented at an orientation of 45° relative to the layer direction. Fig. 7. SEM micrograph showing the side surface of a LaPO4/70 vol% (YCeSr)ZrO2 –30 vol% Al2O3 laminate after 4-point flexural testing. 1.0 kJ/m2 , respectively. The flexural strengths and the work-of-fracture values were low. Fig. 8 is its SEM micrograph after a 4-point fracture test. The YP7-YZ3/ (YCeSr)Z7-A3 laminate (the Y(b) system) had flexural strengths measured from two specimens as 187 and 163 MPa and fractured with a brittle nature. The YP7-YZ3/ (YCeSr)Z7-A3/A/(YCeSr)Z7-A3 laminate (the Y(c) system) had a flexural strength of 217 MPa with a catastrophic failure from one test. SEM micrographs showing the side surfaces of a Y(b) laminate and a Y(c) laminate after 4-point flexural testing are displayed in Fig. 9(a) and Fig. 9(b). Some laminates did not fabricate successfully. YPO4/ Y-ZrO2 (the Y(d) system) and LaPO4/Y-ZrO2/Z3-A7/ Y-ZrO2 (the L(d) system) shattered after removal from the hot press die. The previously mentioned LaPO4/YZrO2 laminate stayed intact after removal from the die and annealing, but shattered during bend bar cutting. Load-displacement responses of 4-pt flexural tests are presented in Fig. 10(a) for an AlPO4/Al2O3 (A(a)) laminate and for an (AlPO4+Al2O3)/Al2O3 (A(b)) laminate. A flexural strength of 47 MPa was measured for the AlPO4/Al2O3 laminate and of 225 MPa for the (AlPO4+Al2O3)/Al2O3 laminate, with a noncatastrophic fracture from one test for each. A side view of the non-brittle A(b) bar was examined under SEM and is shown in Fig. 10(b). Cracks propagating laterally were responsible for this non-brittle fracture behavior. Vickers indentation cracks were also introduced to the A(a) and A(b) laminates, at an orientation of 45° (Fig. 11) relative to the layer length direction. On propagating from the (AlPO4+Al2O3) layers towards the Al2O3 layers, the radial cracks preferentially deflected along the (AlPO4+Al2O3)/Al2O3 interface. Fig. 8. SEM micrograph showing the side surface of a LaPO4/70 vol% (YCeSr)ZrO2 –30 vol% mullite/Al2O3/70 vol% (YCeSr)ZrO2 –30 vol% mullite laminate after 4-point flexural testing. Fig. 6. Load versus displacement curves of a LaPO4/70 vol% (YCeSr)ZrO2 –30 vol% Al2O3 laminate tested in 4-point flexure