J. Zhang et al Scripta Materialia 52 (2005)381-385 4. conclusions Sic laminates prepared by aqueous tape casting showed an improvement in fracture toughness. The com- position of sintering additives influenced the formation of second phase as well as the final properties of SiC lam inates. The resultant development of microstructure can be well understood and controlled. It is inferred that the presence of residual stress might be the main reason for the improvement in fracture toughness of Sic laminates Acknowledgment 500nm The authors are grateful to the Science and Technol- ogy Committee of Shanghai for providing support under Fig. 5. The crack propagated along the grain boundary and the phase the contract number of 02DJ14065 and 0159nm075 References systems [21]. These microcracks can dissipate the strain energy and shield the main cracks, leading to further in [1 Mistler RE Tape casting, past, present, potential Am Ceram Soc crease in fracture toughness [18, 22]. These thermal resid Bul1998;78(2:82-6 2 Mianxue Wu, Messing GL. Fabrication of oriented Sic-whisker ual stress induced energy-absorbing mechanism is well reinforced mullite matrix composites by tape casting. J Am Ceram documented in literature, i.e. TIC, YAG and TiB2 parti Soc1994:77(10):2586-92 cle reinforced Sic composites [17, 23, 24 3 Kiyoshi Hirao, Masay The thermal expansion coefficient for YAlO3 is even Processing strategy for astrophic silicon nitride. J Am Ceram 687-90 higher than Y3Al5O12. A relatively higher thermal resid ual stress was generated with Y AlO3 as the second phase 14 Hotza D. Greil P. Review: ac ape casting of ceramic owders. Mater Sci Eng A 1995: 202: 206-1 assuming its elastic constant and poissons ratio are the [5] Philippe Boch, Thierry Chartier, Muriel Huttepain. Tape casting same as that for Y3 AlsO1. In this case, more energy is OyzrO2 laminated composites. J Am Ceram Soc dissipated in Sic composites and higher fracture tough 1986:69(8)cl9l-2. ness results. For particulate-reinforced composites, the [6 Xuemin Cui, Shixi Ouyang, Zhiyong Yu, Changan Wang, Yong thermal residual stress was considered a major contrib. casting processing. Mater Lett 2003: 57: 1300-4 utor to the toughing of ceramics [17]. This might be [7 Anirban Das, Giridhar Madras, Niladri Dasgupta, Umarji AM the reason for the difference in fracture toughness for Binder removal studies in ceramic thick shapes made by laminated the two laminated materials, as given in Table 1 bject manufacturing. J Eur Ceram Soc 2003: 23: 1013-7 [8 Yumin Zhang, Xiaodong He, Jiecai Han, Shanyi Du. Ceramic 3. 2.2. Colloidal processing technique green tape extrusion for laminated object manufacturing. Mate Lett1999:40:275-9 Colloidal processes could help to produce very homo-[9] Donald Klosterman, Richard Chartoff, George Graves,Nora geneous green sheets with limited flaws [25]. Rosaura Osborne. Brian Priore. Interfacial characteristics of composites Ham-Su [26 showed that lamination process could fabricated by laminated object manufacturing. Composites A modify faws to a more forgiving morphology and re- (10J Yumin Zhang Jiecai Han, Xinghong Zhiqiang Li, Shanyi Du. Rapid protot d combustion had been observed in laminated AlO3 fabricated by synthesis of TiC: Ni functionally grader Mater Sci Eng non-aqueous tape casting though such improvement A2001;299:218-24. was quite limited [5]. For hot pressed bulk SiC, larger [11 Zhang JX, Jiang DL, Tan SH, Gui LH, Ruan ML. Aqueous defects are inevitably trapped during powder compac rocessing of Sic Green Sheets I. Dispersant. J Mater Res 2002;17(11):2012-8 tion [4]. These defects will reduce the thermal residual 12]Zhang JX, Jiang DL, Tan SH, Gui LH, Ruan ML. Aqueous stress and therefore lower the fracture toughness processing of Sic Green Sheets Il Binder and plasticizer. J Mater [27, 28]. In comparison with laminated SiC samples, Res2002;17(11):2018-25 hot pressed bulk SiC exhibits a low fracture toughness 3] Aksay IA, Pask JA. Stable and metastable In light of these discussions, the formation of YAlO SiO -AlO3. J Am Ceram Soc 1975: 58: 507 is likely to be the main reason for the improvement of [14] Mulla MA, Krstic VD. Low temperature pressureless sintering of silicon carbide with aluminum oxide and yttrium oxide addi- mechanical properties in Sic samples tions. Ceram Bull 1991; 70(3): 439-43.systems [21]. These microcracks can dissipate the strain energy and shield the main cracks, leading to further in￾crease in fracture toughness [18,22]. These thermal resid￾ual stress induced energy-absorbing mechanism is well documented in literature, i.e. TiC, YAG and TiB2 parti￾cle reinforced SiC composites [17,23,24]. The thermal expansion coefficient for YAlO3 is even higher than Y3Al5O12. A relatively higher thermal resid￾ual stress was generated with YAlO3 as the second phase assuming its elastic constant and Poisson
s ratio are the same as that for Y3Al5O12. In this case, more energy is dissipated in SiC composites and higher fracture tough￾ness results. For particulate-reinforced composites, the thermal residual stress was considered a major contrib￾utor to the toughing of ceramics [17]. This might be the reason for the difference in fracture toughness for the two laminated materials, as given in Table 1. 3.2.2. Colloidal processing technique Colloidal processes could help to produce very homo￾geneous green sheets with limited flaws [25]. Rosaura Ham-Su [26] showed that lamination process could modify flaws to a more forgiving morphology and re￾duce the severity of void-tape flaws. This kind of effect had been observed in laminated Al2O3 fabricated by non-aqueous tape casting though such improvement was quite limited [5]. For hot pressed bulk SiC, larger defects are inevitably trapped during powder compac￾tion [4]. These defects will reduce the thermal residual stress and therefore lower the fracture toughness [27,28]. In comparison with laminated SiC samples, hot pressed bulk SiC exhibits a low fracture toughness. In light of these discussions, the formation of YAlO3 is likely to be the main reason for the improvement of mechanical properties in SiC samples. 4. Conclusions SiC laminates prepared by aqueous tape casting showed an improvement in fracture toughness. The com￾position of sintering additives influenced the formation of second phase as well as the final properties of SiC lam￾inates. The resultant development of microstructure can be well understood and controlled. It is inferred that the presence of residual stress might be the main reason for the improvement in fracture toughness of SiC laminates. Acknowledgment The authors are grateful to the Science and Technol￾ogy Committee of Shanghai for providing support under the contract number of 02DJ14065 and 0159nm075. References [1] Mistler RE. Tape casting, past, present, potential. Am Ceram Soc Bull 1998;78(2):82–6. [2] Mianxue Wu, Messing GL. Fabrication of oriented SiC-whisker￾reinforced mullite matrix composites by tape casting. J Am Ceram Soc 1994;77(10):2586–92. [3] Kiyoshi Hirao, Masayoshi Ohashi, Brito ME, Shuzu Kanzaki. Processing strategy for producing highly antistrophic silicon nitride. J Am Ceram Soc 1995;78(6):1687–90. [4] Hotza D, Greil P. Review: aqueous tape casting of ceramic powders. Mater Sci Eng A 1995;202:206–17. [5] Philippe Boch, Thierry Chartier, Muriel Huttepain. Tape casting of Al2O3/ZrO2 laminated composites. J Am Ceram Soc 1986;69(8):c191–2. [6] Xuemin Cui, Shixi Ouyang, Zhiyong Yu, Changan Wang, Yong Huang. A study on green tapes for LOM with water-based tape casting processing. Mater Lett 2003;57:1300–4. [7] Anirban Das, Giridhar Madras, Niladri Dasgupta, Umarji AM. Binder removal studies in ceramic thick shapes made by laminated object manufacturing. J Eur Ceram Soc 2003;23:1013–7. [8] Yumin Zhang, Xiaodong He, Jiecai Han, Shanyi Du. Ceramic green tape extrusion for laminated object manufacturing. Mater Lett 1999;40:275–9. [9] Donald Klosterman, Richard Chartoff, George Graves, Nora Osborne, Brian Priore. Interfacial characteristics of composites fabricated by laminated object manufacturing. Composites A 1998;29:1165–74. [10] Yumin Zhang, Jiecai Han, Xinghong Zhang, Xiaodong He, Zhiqiang Li, Shanyi Du. Rapid prototyping and combustion synthesis of TiC:Ni functionally gradient materials. Mater Sci Eng A 2001;299:218–24. [11] Zhang JX, Jiang DL, Tan SH, Gui LH, Ruan ML. Aqueous processing of SiC Green Sheets I. Dispersant. J Mater Res 2002;17(11):2012–8. [12] Zhang JX, Jiang DL, Tan SH, Gui LH, Ruan ML. Aqueous processing of SiC Green Sheets II. Binder and plasticizer. J Mater Res 2002;17(11):2018–25. [13] Aksay IA, Pask JA. Stable and metastable equilibria in system SiO2–Al2O3. J Am Ceram Soc 1975;58:507. [14] Mulla MA, Krstic VD. Low temperature pressureless sintering of b-silicon carbide with aluminum oxide and yttrium oxide addi￾tions. Ceram Bull 1991;70(3):439–43. Fig. 5. The crack propagated along the grain boundary and the phase boundary. 384 J. Zhang et al. / Scripta Materialia 52 (2005) 381–385