Availableonlineatwww.sciencedirect.com SCIENCE DIRECT E噩≈S ELSEVIER Journal of the European Ceramic Society 26 www.elsevier.com/locate/jeurceramsoc Fabrication and superplasticity of Al2O3/3Y-TZP laminated composite Kaifeng Zhang, Guofeng Wang, Zhenjie Wang, Changwen Wang, Wenbo Han School of Material Science and Technology, Harbin Institute of Technology, Harbin, China Received I June 2004; received in revised form 14 October 2004; accepted 6 November 2004 Available online 12 January 2005 Abstract Tape casting and hot-press sintering are used to fabricate an Al2O3/3Y-TZP laminated composite. The as-prepared material is deep drawn significant residual porosity was detected from SEM observations at the interfaces between the two types of layers. The superplastic forming experiment shows that, when the strain rate is constant, temperature has a great influence on the superplasticity of the Al2O3 /3Y-TZP laminated composite. A hat-like part with the greatest deform height is obtained at 1500C. The processing will be less effective at higher or lower C 2005 Elsevier Ltd. All rights reserved. Keywords: Tape casting: Hot pressing, Laminated composite; AlO3: TZP 1. Introduction Flacher and Blandin found superplastic compressiv properties in an Al2O3/ZrO2 laminated composite at 1470C Since 1990, ceramic/ceramic laminated composites have Manuel and Clauss examined the high temperature plasti emerged as promising candidates to overcome the inherent deformation of an Al2O3/Al2O3-YTZP laminated compos- brittleness of ceramics for use in structural application ite in uniaxial compression testing. The composite exhibited Al2O3/TZP laminated composites have been receiving grow creep properties at low strain rate. The fine grain sizes and ing attention, mainly due to their oxidation stability at high the good inter facial adhesion of the layers impart creep resis- temperature and to the drastic increases in strength and espe ance and ductility simultaneously to the laminates. How- ially in fracture toughness at room temperature because of ever, these studies are limited to compressive deformation the various crack-shielding phenomena related to the pres- In the present work, superplastic deep drawing of Al2 O3/Y- ence of the layers. .,6 TZP laminated composites is investigated in consideration of Tomaszewski'fabricated Y-TZP/Al2O3 laminated com- the importance of plastic deformation under tensile stress for posites and researched the effect of residual stress on the char- engineering applications acter of crack propagation. Chartier and rouxel made a sig nificant improvement of both the fracture resistance and the toughness, from 380 to 560 MPa and from 3.7 to 8 MPam/2 respectively, between the pressed alumina monolith and the 2. Experimental tape cast Al2O3/Zro2 laminated composites Because of the e great a dvantage of laminated composites An a-Al2O3 powder and a 3Y-TZP powder, with particle the plastic forming technology becomes very important izes of 50-200 nm and 15-40 nm. were used. TEM mor phologies of the powders are shown in Fig. 1. It can be seen that the powders have no hard aggregate Corresponding author pes of AlO3 and 3Y-TZP were E-mail address: kezhang a hit. edu. cn(K. Zhang) casting, the details of which can be seen in the paper of Cui 0955-22197S-see front matter 2005 Elsevier Ltd. All rights reserved doi: 10.1016/j-jeurceramsoc 2004.1 1.008
Journal of the European Ceramic Society 26 (2006) 253–257 Fabrication and superplasticity of Al2O3/3Y-TZP laminated composite Kaifeng Zhang∗, Guofeng Wang, Zhenjie Wang, Changwen Wang, Wenbo Han School of Material Science and Technology, Harbin Institute of Technology, Harbin, China Received 1 June 2004; received in revised form 14 October 2004; accepted 6 November 2004 Available online 12 January 2005 Abstract Tape casting and hot-press sintering are used to fabricate an Al2O3/3Y-TZP laminated composite. The as-prepared material is deep drawn at high temperature to research its superplastic formability. It is found that the microstructure of the material sintered at 1550 ◦C is fine and no significant residual porosity was detected from SEM observations at the interfaces between the two types of layers. The superplastic forming experiment shows that, when the strain rate is constant, temperature has a great influence on the superplasticity of the Al2O3/3Y-TZP laminated composite. A hat-like part with the greatest deform height is obtained at 1500 ◦C. The processing will be less effective at higher or lower temperature. © 2005 Elsevier Ltd. All rights reserved. Keywords: Tape casting; Hot pressing; Laminated composite; Al2O3; TZP 1. Introduction Since 1990, ceramic/ceramic laminated composites have emerged as promising candidates to overcome the inherent brittleness of ceramics for use in structural applications.1–4 Al2O3/TZP laminated composites have been receiving growing attention, mainly due to their oxidation stability at high temperature and to the drastic increases in strength and especially in fracture toughness at room temperature because of the various crack-shielding phenomena related to the presence of the layers.5,6 Tomaszewski7 fabricated Y-TZP/Al2O3 laminated composites and researched the effect of residual stress on the character of crack propagation. Chartier and Rouxel8 made a significant improvement of both the fracture resistance and the toughness, from 380 to 560 MPa and from 3.7 to 8 MPa m1/2, respectively, between the pressed alumina monolith and the tape cast Al2O3/ZrO2 laminated composites. Because of the great advantage of laminated composites, the plastic forming technology becomes very important. ∗ Corresponding author. E-mail address: kfzhang@hit.edu.cn (K. Zhang). Flacher and Blandin9 found superplastic compressive properties in an Al2O3/ZrO2 laminated composite at 1470 ◦C. Manuel and Clauss10 examined the high temperature plastic deformation of an Al2O3/Al2O3-YTZP laminated composite in uniaxial compression testing. The composite exhibited creep properties at low strain rate. The fine grain sizes and the good interfacial adhesion of the layers impart creep resistance and ductility simultaneously to the laminates.11 However, these studies are limited to compressive deformation. In the present work, superplastic deep drawing of Al2O3/3YTZP laminated composites is investigated in consideration of the importance of plastic deformation under tensile stress for engineering applications. 2. Experimental An -Al2O3 powder and a 3Y-TZP powder, with particle sizes of 50–200 nm and 15–40 nm, were used. TEM morphologies of the powders are shown in Fig. 1. It can be seen that the powders have no hard aggregate. The tapes of Al2O3 and 3Y-TZP were prepared by tape casting, the details of which can be seen in the paper of Cui 0955-2219/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2004.11.008
K. Zhang et al. /Journal of the European Ceramic Society 26 (2006)253-257 她蟹 2.0 200nm ig. 1. TEM morphologies of the powders(a)3Y-TZP; (b)Al2O3 Fig. 2. Simultaneous TG-DSC analysis on green tape, up to 700C et al. The dried tape was punched into o 30 mm discs. The Al2 O3 circles and 3Y-tzp discs were stacked in an alterna- tion sequence to a total of 41 layers, with both of the outer two types of layers are well defined with interfaces and no surface layers being Al2O3 800 C for 2 h for binder burnout. Cooling was controlle& significant residual porosity was detected from SEM obser The discs were placed in a low-temperature furnace vations at the interfaces between the two types of layers. The thickness of each layer is about 90 um. However, it must be mentioned that the interfaces are not so straight because the The laminates were uniaxial pressure sintered in a cylin- material was pressed during the sintering. This will advantage drical graphite die with diameter of 30 mm. The heating the strength of the interfaces rate was 20C/min and the pressure was 25 MPa. The lam In order to research the influence of temperature on the inates were heated to the required temperature and held for microstructure of laminated composite, different sintering 90 min. The thickness of all discs was 2 mm prior to the temperatures, 1450, 1550 and 1650C, w forming tests the samples were polished on one side(dia- shows the cross section of the laminated composite sintered mond paste 0.3 um). The discs were placed on a cylindri at different temperatures cal graphite ring with an inner diameter of 20 mm. A hemi- The grain size of the laminated composite sintered at spherical graphite punch with radius of 8 mm was pushed 1450C is very fine Mean grain sizes were determined to on the non-polished side of the laminate using a velocity of be 150 and 400 nm, respectively for the 3Y-TZP and Al2O3 0.2 mm/min while the force was monitored. The tests were performed in vacuum at 1400, 1500 and 1600C. The axial layers. However, there are many cavities in the Al2O3 layer The composite sintered at 1550 C has a larger grain size displacement was measured externally during all tests. MI- Mean grain sizes are 200 and 800nm, respectively for the crostructure examination of samples after hot pressing and 3Y-TZP and Al2O3 layers. The relative density has great im- perplastic forming was carried out using a scanning electron provement. There is little porosity existing in the Al2O3 layer microscope(SEM, Philips XL 20)equipped with an image When the temperature reaches 1650oC, grains grow rapidl They are l and 10 um, respectively for the 3Y-TZP and Al2O3 (Fig. 4) 3. Results and discussion 3.1. Hot-pressing sintering During the preparation procedure of the tapes, a lot of organic addition was used. This must be burned out com- pletely before sintering. Otherwise, it will do harm to the properties of the laminated composite. The tapes were an- alyzed with TG-DSC and the results are shown in Fig. 2 It can be seen that the pyrolytic decomposition appened mainly between 200 and 400C and almost all of the or- ganic addition was burned out when the temperature reached Fig 3 shows the cross section of the as-prepared laminated Fig 3. Morphologies of the cross section of laminated composite after sin- composite. The dark zones correspond to Al2O3 layers. The
254 K. Zhang et al. / Journal of the European Ceramic Society 26 (2006) 253–257 Fig. 1. TEM morphologies of the powders (a) 3Y-TZP; (b) Al2O3. et al.12 The dried tape was punched into ø 30 mm discs. The Al2O3 circles and 3Y-TZP discs were stacked in an alternation sequence to a total of 41 layers, with both of the outer surface layers being Al2O3. The discs were placed in a low-temperature furnace at 800 ◦C for 2 h for binder burnout. Cooling was controlled at 3 ◦C/min. The laminates were uniaxial pressure sintered in a cylindrical graphite die with diameter of 30 mm. The heating rate was 20 ◦C/min and the pressure was 25 MPa. The laminates were heated to the required temperature and held for 90 min. The thickness of all discs was 2 mm. Prior to the forming tests the samples were polished on one side (diamond paste 0.3 m). The discs were placed on a cylindrical graphite ring with an inner diameter of 20 mm. A hemispherical graphite punch with radius of 8 mm was pushed on the non-polished side of the laminate using a velocity of 0.2 mm/min while the force was monitored. The tests were performed in vacuum at 1400, 1500 and 1600 ◦C. The axial displacement was measured externally during all tests. Microstructure examination of samples after hot pressing and superplastic forming was carried out using a scanning electron microscope (SEM, Philips XL 20) equipped with an image analyzer. 3. Results and discussion 3.1. Hot-pressing sintering During the preparation procedure of the tapes, a lot of organic addition was used. This must be burned out completely before sintering. Otherwise, it will do harm to the properties of the laminated composite. The tapes were analyzed with TG-DSC and the results are shown in Fig. 2. It can be seen that the pyrolytic decomposition happened mainly between 200 and 400 ◦C and almost all of the organic addition was burned out when the temperature reached 700 ◦C. Fig. 3 shows the cross section of the as-prepared laminated composite. The dark zones correspond to Al2O3 layers. The Fig. 2. Simultaneous TG-DSC analysis on green tape, up to 700 ◦C. two types of layers are well defined with interfaces and no significant residual porosity was detected from SEM observations at the interfaces between the two types of layers. The thickness of each layer is about 90 m. However, it must be mentioned that the interfaces are not so straight because the material was pressed during the sintering. This will advantage the strength of the interfaces. In order to research the influence of temperature on the microstructure of laminated composite, different sintering temperatures, 1450, 1550 and 1650 ◦C, were chosen. Fig. 4 shows the cross section of the laminated composite sintered at different temperatures. The grain size of the laminated composite sintered at 1450 ◦C is very fine. Mean grain sizes were determined to be 150 and 400 nm, respectively for the 3Y-TZP and Al2O3 layers. However, there are many cavities in the Al2O3 layer. The composite sintered at 1550 ◦C has a larger grain size. Mean grain sizes are 200 and 800 nm, respectively for the 3Y-TZP and Al2O3 layers. The relative density has great improvement. There is little porosity existing in the Al2O3 layer. When the temperature reaches 1650 ◦C, grains grow rapidly. They are 1 and 10 m, respectively for the 3Y-TZP and Al2O3 layers (Fig. 4). Fig. 3. Morphologies of the cross section of laminated composite after sintering.
K. Zhang et al. / Joumal of the European Ceramic Society 26(2006)253-257 255 2750 2000 0-1400C 1500C 口-1600 5um 3Y-TZ Fig. 5. Displacement vs applied force during deep drawing at different de- formation temperatures At 1500C the laminated composite can be stretched to a height of ll l mm without fracture. From Fig. 5 it is also clear that higher and lower forming temperature require more force for identical displacements This influence of deformation temperature on deformation can be explained in terms of the change of microstructures, which are shown in Fig. 6. A slightly grain growth occurs when the specimen was deep drawn at 1400C. Mean grain sizes are 3 um and 500 nm for Al2O3 and 3Y-TZP, respec AlO tively, as shown in Fig 6a. However, many cavities appear in the al2O3 layer because of its bad deformability at lower tem- perature. It is widely accepted that cavity nucleation arises from stress concentrations generated at second phase parti cles or triple points as a result of grain boundary sliding. At a certain concentration and/or size the cavities are interlinking which is associated with the onset of tensile failure. Because of the stronger cavity growth at lower forming temperature higher stresses are present in the ceramics. This explains the higher forces necessary for the same displacement at lower ing te Grains grow rapidly when deep drawing at 1600C Mean 15u grain sizes are 8 and I um for Al2O3 and 3Y-TZP, respec The grain-size effect on superplasticity has been often ob- Fig 4. SEMmorphe served in the literature and can be explained by the fact that sintering at different temperatures:(a)1450°C,(b)1550°C;c)1650° grain boundary sliding is the dominant deformation mode High-temperature deformation behavior in fine-grained 3. 2. Superplastic deep drawing ramics is usually described by Q In Fig. 5 the displacement of the punch and the resulting dp exp(rt force on the sample are given 1500 and 1600C using the laminated composite sintered at where A is a numerical constant, o the flow stress and n its 1550C Here the influence of deep-drawing temperature is exponent, d the grain size with exponent P, Q the apparent clearly visible. If a higher or lower forming temperature is activation energy and T is the temperature used, the chance of fracturing during deep drawing increases Generally, superplastic deformation occurs in ceramics (fracturing is indicated in the figure by an arrow downwards). due to grain boundary sliding. Accompanying the grain
K. Zhang et al. / Journal of the European Ceramic Society 26 (2006) 253–257 255 Fig. 4. SEM morphologies of the cross section of laminated composites after sintering at different temperatures: (a) 1450 ◦C; (b) 1550 ◦C; (c) 1650 ◦C. 3.2. Superplastic deep drawing In Fig. 5 the displacement of the punch and the resulting force on the sample are given during deep drawing at 1400, 1500 and 1600 ◦C using the laminated composite sintered at 1550 ◦C. Here the influence of deep-drawing temperature is clearly visible. If a higher or lower forming temperature is used, the chance of fracturing during deep drawing increases (fracturing is indicated in the figure by an arrow downwards). Fig. 5. Displacement vs. applied force during deep drawing at different deformation temperatures. At 1500 ◦C the laminated composite can be stretched to a height of 11.1 mm without fracture. From Fig. 5 it is also clear that higher and lower forming temperature require more force for identical displacements. This influence of deformation temperature on deformation can be explained in terms of the change of microstructures, which are shown in Fig. 6. A slightly grain growth occurs when the specimen was deep drawn at 1400 ◦C. Mean grain sizes are 3m and 500 nm for Al2O3 and 3Y-TZP, respectively, as shown in Fig. 6a. However, many cavities appear in the Al2O3 layer because of its bad deformability at lower temperature. It is widely accepted that cavity nucleation arises from stress concentrations generated at second phase particles or triple points as a result of grain boundary sliding. At a certain concentration and/or size the cavities are interlinking which is associated with the onset of tensile failure. Because of the stronger cavity growth at lower forming temperature, higher stresses are present in the ceramics. This explains the higher forces necessary for the same displacement at lower forming temperature. Grains grow rapidly when deep drawing at 1600 ◦C. Mean grain sizes are 8 and 1 m for Al2O3 and 3Y-TZP, respectively, as shown in Fig. 6c. The grain-size effect on superplasticity has been often observed in the literature and can be explained by the fact that grain boundary sliding is the dominant deformation mode. High-temperature deformation behavior in fine-grained ceramics is usually described by:13 ε˙ = Aσn dp exp − Q RT (1) where A is a numerical constant, σ the flow stress and n its exponent, d the grain size with exponent p, Q the apparent activation energy and T is the temperature. Generally, superplastic deformation occurs in ceramics due to grain boundary sliding. Accompanying the grain
K Zhang ef al. / Joumal of the European Ceramic Society 26 (2006)253-257 Fig. 7. Al2O3/3Y-TZP laminated composite part formed by superplastic deep drawing: (a) the part, (b)the cross section. It is also shown that the 3 Y-TZP layers, which ha superplastic forming ability, help the Al2O3 layers, which have poor superplastic forming ability, when the laminated composite is deep drawn at a suitable temperature. However if we want to improve further the superplastic forming abil ity of the Al2O3/3Y-TZP laminated composite, grain growth of Al2O3 must be controlled. Park et al. +reported that the Fig. 6. SEM morphologies of cross section of laminated composites after addition of sio or ZrO2 can effectively inhibited the growth superplastic forming at different temperatures: (a)1400"; (b)1500"C;(c) of Al2O3. This will be researched in the future 1600°C boundary sliding the general understanding is that accom- 4. Concluding remarks modation involving diffusion and/or dislocation movement is required to maintain grain boundary cohesion. If accom- It is shown that tape casting and hot pressing can be applied modation is incomplete or slower than the grain displacement to the fabrication of an Al2 O3/3Y-TZP laminated composite rate then cavities will occur. The effect of grain growth is to with fine grain size and crack-free microstructure at 1550C make the accommodation process more difficult by increas- The as-prepared laminated composite has good superplastic the unit path length for the operative mechani ty. At 1500C it can be deformed to form a hemisphere Grain sizes are fine and few cavities appear after deep without fracture. However, the superplastic forming ability drawing at 1500C. So, the best superplastic deep-drawing will decreases rapidly at higher or lower temperature. If we temperature for the Al2O3/3Y-TZP laminated composite want to improve further the superplastic forming ability of an Al2O3/3Y-TZP laminated composite, grain growth of Al2O3 An Al2O3/3Y-TZP laminated composite part has been must be controlled formed by superplastic deep drawing at 1500C, the par and its cross section are shown in Fig. 7. The diameter of the flange reduces from 30 to 25 mm and the mean thickness Acknowledgements reduces from 2 to 1.7 mm. This shows that the shape of hemi- sphere is achieved by the shrinkage of the diameter and the This work was supported by the Scientific Research Foun reduction of thickness dation of Harbin Institute of Technology under grant number
256 K. Zhang et al. / Journal of the European Ceramic Society 26 (2006) 253–257 Fig. 6. SEM morphologies of cross section of laminated composites after superplastic forming at different temperatures: (a) 1400 ◦C; (b) 1500 ◦C; (c) 1600 ◦C. boundary sliding the general understanding is that accommodation involving diffusion and/or dislocation movement is required to maintain grain boundary cohesion. If accommodation is incomplete or slower than the grain displacement rate then cavities will occur. The effect of grain growth is to make the accommodation process more difficult by increasing the unit path length for the operative mechanism. Grain sizes are fine and few cavities appear after deep drawing at 1500 ◦C. So, the best superplastic deep-drawing temperature for the Al2O3/3Y-TZP laminated composite is 1500 ◦C. An Al2O3/3Y-TZP laminated composite part has been formed by superplastic deep drawing at 1500 ◦C; the part and its cross section are shown in Fig. 7. The diameter of the flange reduces from 30 to 25 mm and the mean thickness reduces from 2 to 1.7 mm. This shows that the shape of hemisphere is achieved by the shrinkage of the diameter and the reduction of thickness. Fig. 7. Al2O3/3Y-TZP laminated composite part formed by superplastic deep drawing: (a) the part; (b) the cross section. It is also shown that the 3Y-TZP layers, which have good superplastic forming ability, help the Al2O3 layers, which have poor superplastic forming ability, when the laminated composite is deep drawn at a suitable temperature. However, if we want to improve further the superplastic forming ability of the Al2O3/3Y-TZP laminated composite, grain growth of Al2O3 must be controlled. Park et al.14 reported that the addition of SiO2 or ZrO2 can effectively inhibited the growth of Al2O3. This will be researched in the future. 4. Concluding remarks It is shown that tape casting and hot pressing can be applied to the fabrication of an Al2O3/3Y-TZP laminated composite with fine grain size and crack-free microstructure at 1550 ◦C. The as-prepared laminated composite has good superplasticity. At 1500 ◦C it can be deformed to form a hemisphere without fracture. However, the superplastic forming ability will decreases rapidly at higher or lower temperature. If we want to improve further the superplastic forming ability of an Al2O3/3Y-TZP laminated composite, grain growth of Al2O3 must be controlled. Acknowledgements This work was supported by the Scientific Research Foundation of Harbin Institute of Technology under grant number
K. Zhang et aL Joumal of the European Ceramic Sociery 26(2006)253-257 257 HIT.2002.36 and the National Natural Science Foundation of 7. Tomaszewski, H, Residual stresses in layered ceramic China under grant number 50375037 Eur Ceram 1999,19(6-7),1329-1331 8. Chartier, T. and Rouxel, T, Tape-cast alumina-zirconia sing and mechanical properties. J. Eur. Ceram. Soc., 1997, 17 References 9. Flacher, O. and Blandin, JJ, Microstructural aspects of superplastic deformation of Al2 O3/ZrOz laminate composites. Mater. Sci. Eng. A 2 w.J. et al, A simple way to toughen ceramic Nature, 1990, 1996,219,148-155 455-461 10. Manuel, J. M. and Clauss, C, Microstructure and high-temperatu L. and Krstic, V. D, High toughness silicon carbide/graphite mechanical behavior of alumina/alumina-yttria-stab tetragonal laminar composite by slip casting. Theor. Appl. Fract. Mech., 1995. zirconia multilayer composites. J. Am. Ceram. Soc., 1997, 80(8) 4(1),13-19 2126-2130 3. Wang, C, Huang, Y and Zan, Q. F, Biomimetic structure design- 11. Manuel, J M. and Gutierrez Mora, F, Effect of layer interfaces on the ssible approach to cha ne brittleness of ceramics in nature high-temperature mechanical ies of alumina/zirconia laminate Mater. Sci Eng, 2000, 11(1),9-12 mposites. Acta Mater, 2000, 48, 4715-4720 Ohji, T, Shigegaki, Y, Kondo, N. et al, Fracture toughness of mul 12. Cui, X.M., Ouyang, s.x. et al, A study on green tapes for LOM tilayer silicon nitride with crack deflection. Mater. Let 1999, 40(6) th water based tape casting processing. Mater Lett., 2003, 57 280-284 1300-1304. 5. She. J. and Inoue. T. Damage resistance and R-curve behavior 13. Langdon, T. G, Superplastic forming of structural alloys. Metall. Soc of multilayer Al]O3/SiC ceramics. Ceram. Int, 2000, 26(8),801- AME,1982,27-40 805 14. Park, S. Y, Saruhan, B and Schneider, H, Mullite/zirconia laminate 6. Chen, I. W. and Xue, L. A, Development of superplastic structural composites for high temperature application. J. Eur. Ceram. Soc. ceramics. J. Am. Ceram. Soc. 1990. 73. 2585-2609 2000,20,2463-2468
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