《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_sIc

JOURNAL OF MATERIALS SCIENCE 38(2003)4735-4738 Fabrication and characterization of laminated SiC ceramics with self-sealed ring structure Z.B. YUV.D KRSTIC Centre for Manufacturing of Advanced Ceramics and Nanomaterials, Department of Mechanical Engineering, Queens University, Nico/ Hall, 60 Union Street, Kingston, Ontario, Canada, K7L 3N6 One of the most creative achievements of researchers Sic slurry was prepared by mixing 88 wt% of Sic. in the structural ceramics field is ceramic-matrix com- 8 wt% of Al O3 and 4 wt% of Y,O3 with water with posite materials. Like so many materials science solid to liquid ratio of 30/70 by volume. The concen- complishments, uni-directionally aligned continuous tration of carbon in the water based graphite slurry fibre reinforced ceramic composites [1-3]or laminated was between 2.5 and 5%by volume. SiC/graphite lam plate-form ceramic composites [4-12] actually par- inates were slip-cast with alternate SiC and graphite tially imitate the structure of a natural material, in this layers in a plaster of Paris mould with a casting cham- case wood, either in one or two dimensions. The ad- ber 10 mm in diameter and 50 mm in length. all the antage of layered structures is that they perform the laminates were fabricated in such a way that the out function of fibre-reinforced ceramic composites, but ermost layer and core were SiC. The thickness of the are much easier to fabricate Plate-form laminated ce- SiC layer was varied from 50 to 600 um and that of ramic composites with a weak interface such as the the graphite layer from 5-20 um by adjusting the vis- SiC/graphite system, usually show a high work of frac- cosity of the slurry and casting time. The number of ture and fracture toughness as high as 14 MPam/ Sic layers in the structure was varied from 20 to 25 [4, 5]. However, the major problem associated with After slip casting, the green bodies were slowly dried the plate-form laminate is that it possesses two delam- in air for 48 h and sintered at temperatures ranging ination directions, which prevents the laminates from from 1, 800 to 1, 850C for I h in an Ar atmosphere enjoying widespread applications as structural compo- at I atm pressure. Bulk densities were measured by nents. Therefore other structures are needed, either at the Archimedes method. The theoretical density of the the micro- or meso-/macro-level, to address this prob- laminates was calculated on the basis of the rule of lem. From the meso-/macro-structure point of view, one mixtures possible solution to the intrinsic delamination problem Three-point bending tests on specimens with a span in plate-form ceramic laminates is to once again look to of 25. 4 mm were conducted using an Instron machine a natural material and imitate the ring structure of wood(8502)at room temperature. The crosshead speed was in three-dimensions. This strategy reduces the potential 0.06 mm/min. Since ceramic rods with circular cross- delamination directions in a laminated ceramic mate- sections were used for the mechanical property tests, rial from two to zero when the structure of a laminate the work of fracture/failure work was used to character changes from plate form to a self-sealed ring, i. e, a ize the fracture resistance of the silicon carbide/carbon highly anisotropic structure laminates. The total work per unit volume during a The objective of the present work is to create a ce- bending test can be written as: amic laminate by imitating the concentric cylinder tree trunk structure and to examine the delamination resis- Imax tance and fracture behaviour of the structure. A simple F(y)dymax shaping technique, a modified slip casting method, has been used to achieve a self-sealed ring structure for a variety of ceramic laminates. In our experiment, the where ymax is the roller displacement, F(y)is the load, silicon carbide/carbon system was chosen as an exam- and r and L are the radius of the specimen and loading ple to demonstrate this structure. This system has two span of three-point bending test, respectively. As the characteristics:(1)the graphite not only happens to be load F(y) cannot be expressed as a single and simple weak interface [4, 5] and(2)our previous research [4] the calculation of the work of fracture for the lami showed that the carbon layers can be converted into nated composites was done by measuring the area under porous silicon carbide layers. Hence a single-phase Sic the load-displacement curve. To achieve higher accu ceramic with a better oxidation resistance can be ob- racy, the area under the load-displacement curve was tained while at the same time retaining a suitably weak divided into tiny rectangular strips each with a width of about 100 um and multiplied by the corresponding 0022-2461 2003 Kluwer Academic Publishers 4735

JOURNAL OF MATERIALS SCIENCE 3 8 (2 003) 4735 – 4738 Fabrication and characterization of laminated SiC ceramics with self-sealed ring structure Z. B. YU, V. D. KRSTIC Centre for Manufacturing of Advanced Ceramics and Nanomaterials, Department of Mechanical Engineering, Queen’s University, Nicol Hall, 60 Union Street, Kingston, Ontario, Canada, K7L 3N6 One of the most creative achievements of researchers in the structural ceramics field is ceramic-matrix com￾posite materials. Like so many materials science ac￾complishments, uni-directionally aligned continuous fibre reinforced ceramic composites [1–3] or laminated plate-form ceramic composites [4–12] actually par￾tially imitate the structure of a natural material, in this case wood, either in one or two dimensions. The ad￾vantage of layered structures is that they perform the function of fibre-reinforced ceramic composites, but are much easier to fabricate. Plate-form laminated ce￾ramic composites with a weak interface such as the SiC/graphite system, usually show a high work of frac￾ture and fracture toughness as high as 14 MPam1/2 [4, 5]. However, the major problem associated with the plate-form laminate is that it possesses two delam￾ination directions, which prevents the laminates from enjoying widespread applications as structural compo￾nents. Therefore other structures are needed, either at the micro- or meso-/macro-level, to address this prob￾lem. From the meso-/macro-structure point of view, one possible solution to the intrinsic delamination problem in plate-form ceramic laminates is to once again look to a natural material and imitate the ring structure of wood in three-dimensions. This strategy reduces the potential delamination directions in a laminated ceramic mate￾rial from two to zero when the structure of a laminate changes from plate form to a self-sealed ring, i.e., a highly anisotropic structure. The objective of the present work is to create a ce￾ramic laminate by imitating the concentric cylinder tree trunk structure and to examine the delamination resis￾tance and fracture behaviour of the structure. A simple shaping technique, a modified slip casting method, has been used to achieve a self-sealed ring structure for a variety of ceramic laminates. In our experiment, the silicon carbide/carbon system was chosen as an exam￾ple to demonstrate this structure. This system has two characteristics: (1) the graphite not only happens to be a successful sintering aid for SiC but also provides a weak interface [4, 5] and (2) our previous research [4] showed that the carbon layers can be converted into porous silicon carbide layers. Hence a single-phase SiC ceramic with a better oxidation resistance can be ob￾tained while at the same time retaining a suitably weak interface. SiC slurry was prepared by mixing 88 wt% of SiC, 8 wt% of Al2O3 and 4 wt% of Y2O3 with water with solid to liquid ratio of 30/70 by volume. The concen￾tration of carbon in the water based graphite slurry was between 2.5 and 5% by volume. SiC/graphite lam￾inates were slip-cast with alternate SiC and graphite layers in a plaster of Paris mould with a casting cham￾ber 10 mm in diameter and 50 mm in length. All the laminates were fabricated in such a way that the out￾ermost layer and core were SiC. The thickness of the SiC layer was varied from 50 to 600 µm and that of the graphite layer from 5–20 µm by adjusting the vis￾cosity of the slurry and casting time. The number of SiC layers in the structure was varied from 20 to 25. After slip casting, the green bodies were slowly dried in air for 48 h and sintered at temperatures ranging from 1,800 to 1,850◦C for 1 h in an Ar atmosphere at 1 atm pressure. Bulk densities were measured by the Archimedes method. The theoretical density of the laminates was calculated on the basis of the rule of mixtures. Three-point bending tests on specimens with a span of 25.4 mm were conducted using an Instron machine (8502) at room temperature. The crosshead speed was 0.06 mm/min. Since ceramic rods with circular cross￾sections were used for the mechanical property tests, the work of fracture/failure work was used to character￾ize the fracture resistance of the silicon carbide/carbon laminates. The total work per unit volume during a bending test can be written as: W =
y max 0 F(y)dymax πr 2L (1) where ymax is the roller displacement, F(y) is the load, and r and L are the radius of the specimen and loading span of three-point bending test, respectively. As the load F(y) cannot be expressed as a single and simple function of y in the entire roller displacement range, the calculation of the work of fracture for the lami￾nated composites was done by measuring the area under the load-displacement curve. To achieve higher accu￾racy, the area under the load-displacement curve was divided into tiny rectangular strips each with a width of about 100 µm and multiplied by the corresponding 0022–2461 C 2003 Kluwer Academic Publishers 4735

1111/1/ 800um igure I Reflected light micrograph of a SiC/C laminate sintered at 1,850.C for 1 h Grey layers are dense SiC layers and black layers are porous load according to the Equation: (∑y=F(y)△y) 2s o 5s TrL o20s口40s The bending strength was calculated from the Equation 爸 g= 3FL/Ir3 E17 16 The cross-sectional structure of a sintered laminate with the sic/C thickness ratio of about 20: 1 is shown Except for the first few outer layers, the inner layers Layer number are homogeneous, and the interface uniformity between Sic and graphite layers is in the microme ter ran Figure 2 Relationships between: (a) Layer thickness and casting time With this modified slip casting method, the layered and(b) Shrinkage and layer thickn ramics can be tailored to have structures with dif- ferent thicknesses of SiC and graphite layers. As ex- The apparent densities of the laminates sintered at pected, the longer the slip casting time, the thicker the different temperatures are approximately 95% of theo layer produced, as shown in Fig 2a. The largest thick- retical density (TD), which is lower than the 97%TD ness of SiC layer produced was approximately 600 um obtained for monolithic SiC ceramic. Examination and the smallest was approximately 50 um. In addition of porosities in the laminates revealed that the open to the slip casting time, the viscosity and the solid-to- porosity decreased with increasing sintering tempera- liquid ratio of the slurry also play an important role ture. The monolithic SiC ceramic specimen showed a in determining the layer thickness. During the casting lower(<0.5%)open porosity than the laminated ones of thicker layers, the thickness of previous layers be-(0.5-1. 8%); and laminates with thicker SiC layers had a comes the dominating factor controlling the thickness lower open porosity than those with thinner SiC layers of new layers. As the layer thickness increases, the re- because of a relatively high fraction of pores in the lat- moval of water through the wall of the green body be This also explain the shrinkage of Sic/C comes more difficult, and for a given slip casting time, laminates as presented in Fig. 2b the layer thickness becomes smaller. This phenomenon It is worth noting that the higher shrinkage did not explains the non-linear relationship between the layer result in higher density of the laminates because of thickness and the casting time. It was noted that struc- the creation of highly porous SiC layers. This is why tural features such as layer thickness and layer thick- laminates with thicker SiC layers experienced higher ness ratio of Sic/C etc have a strong effect on the final shrinkage properties of the laminates as well as on the sintering The fracture behavior of the laminates and mono- lithic Sic ceramics was evaluated in a three-point bend 4736

Figure 1 Reflected light micrograph of a SiC/C laminate sintered at 1,850◦C for 1 h. Grey layers are dense SiC layers and black layers are porous SiC layers. load according to the Equation: W = y max y=0 F(y)y  πr 2L (2) The bending strength was calculated from the Equation: σ = 3FL/πr 3 (3) The cross-sectional structure of a sintered laminate with the SiC/C thickness ratio of about 20:1 is shown in Fig. 1. Except for the first few outer layers, the inner layers are homogeneous, and the interface uniformity between SiC and graphite layers is in the micrometer range. With this modified slip casting method, the layered ceramics can be tailored to have structures with dif￾ferent thicknesses of SiC and graphite layers. As ex￾pected, the longer the slip casting time, the thicker the layer produced, as shown in Fig. 2a. The largest thick￾ness of SiC layer produced was approximately 600 µm and the smallest was approximately 50 µm. In addition to the slip casting time, the viscosity and the solid-to￾liquid ratio of the slurry also play an important role in determining the layer thickness. During the casting of thicker layers, the thickness of previous layers be￾comes the dominating factor controlling the thickness of new layers. As the layer thickness increases, the re￾moval of water through the wall of the green body be￾comes more difficult, and for a given slip casting time, the layer thickness becomes smaller. This phenomenon explains the non-linear relationship between the layer thickness and the casting time. It was noted that struc￾tural features such as layer thickness and layer thick￾ness ratio of SiC/C etc. have a strong effect on the final properties of the laminates as well as on the sintering behavior. Figure 2 Relationships between: (a) Layer thickness and casting time and (b) Shrinkage and layer thickness. The apparent densities of the laminates sintered at different temperatures are approximately 95% of theo￾retical density (TD), which is lower than the ∼97%TD obtained for monolithic SiC ceramic. Examination of porosities in the laminates revealed that the open porosity decreased with increasing sintering tempera￾ture. The monolithic SiC ceramic specimen showed a lower (<0.5%) open porosity than the laminated ones (0.5–1.8%); and laminates with thicker SiC layers had a lower open porosity than those with thinner SiC layers because of a relatively high fraction of pores in the lat￾ter. This effect can also explain the shrinkage of SiC/C laminates as presented in Fig. 2b. It is worth noting that the higher shrinkage did not result in higher density of the laminates because of the creation of highly porous SiC layers. This is why laminates with thicker SiC layers experienced higher shrinkage. The fracture behavior of the laminates and mono￾lithic SiC ceramics was evaluated in a three-point bend 4736

160 g:0 00 Displacement (mm) 8821K Inn ND29 50Na15 Figure 3 Fracture behaviour of SiC ceramics with an average thickness ratio of SiC/C of 30: I sintered at 1, 850.C for I h(a) Load-displacement urve, notably similar to those obtained for continuous fibre-reinforced ceramic composites. (b)Fractured cross-sectional surface (c) Deflected crack between layers. The role of the interface and the unique structure in deflecting cracks and eliminating undesired delamination can be clearly seen. 4737

(a) (b) (c) Figure 3 Fracture behaviour of SiC ceramics with an average thickness ratio of SiC/C of 30:1 sintered at 1,850◦C for 1 h. (a) Load-displacement curve, notably similar to those obtained for continuous fibre-reinforced ceramic composites. (b) Fractured cross-sectional surface. (c) Deflected crack between layers. The role of the interface and the unique structure in deflecting cracks and eliminating undesired delamination can be clearly seen. 4737

test and a representative load-displacement curve presented in Fig. 3 The curve indicates that the fracture behavior of lam- inated SiC is entirely different from that of monolithic SiC. The former showed non-catastrophic failure with lar displacement and the latter always shows brittle fracture with a minimal deformation Af- ter the bending tests were complete, the laminated spec imens held together in one piece rather than break- ing into two pieces as in the case of the monolithic Sic samples. The laminates showed a non-linear load displacement behavior with some peaks occurring ei s101213182330 ther before or after the maximum load, depending on KC thickness ratio their structural features, but all exhibited gradual, non catastrophic failure. These peaks correspond to the fail ure of either a single layer or multiple layers indicating R strong crack deflection caused by the weak/porous in- terfaces(Fig. 3b and c). It was also noticed that even when the maximum failure load of the solid sic core had been reached, the laminates with thicker Sic layers could still resist a relatively high load. The thick Sic layered composites also showed a large displacement over a high load range, which can contribute signifi cantly to work of fracture s101131售82330 Our initial results(Fig. 4) show that the monolithic Scc thickness rato Sic has a strength of one and a half times that of the However, even though the laminates showed a lower strength, their work of fracture was 2-4 times higher than the monolithic sic. it seems that there exists a threshold value of the thickness ratio between the Sic References layer and the carbon layer. Higher ratios(>20) re- 1.R I.R. A. J SAMBELL, D. H. BOWEN and D. C. PHILLIPS, sult in materials with higher fracture toughness. The J Mater. Sci. 7(1972)663 2. K. M. PREWO and J. J. BRENNAN, ibid. 15(1980)463 high work of fracture of the laminates is attributed 3. z. B. YU and D. P. THOMPSON. J. Eur: Ceram Soc. 22(2002) to the self-sealed ring structure which eliminates the delamination completely. Clearly, the newly formed 4 L. ZHANG and v. D. KRSTIC, Theor Appl. Fract. Mech. 24 porous SiC layers serve to deflect the propagating 5.w. J. CLEGG,K.KENDALL, N. MCN. ALFORD, crack leading to a remarkable improvement in fracture T. W. BUTTON and J. D. BIRCHALL, Nature 374(1990) In summary, this work demonstrated for the first 6. w. J. CLEGG, Acta Metal. Mater. 40(1992)308 time that, by imitating the concentric circle tree-trunk 7 H. LIU and S. M. HSU, J. Amer. Ceram Soc. 79(1996)2452 structure in three-dimensions, the self-sealed layered 8. D. B. MARSHALL, J. J. RATTO and F. F. LANGE, ibid structure can be created which effectively eliminates 9. J. REQUENA, R. MORENO andJ. S. MAYA, ibid. 72(1989) undesired delamination problem associated with plate form laminates. The self-sealed layered SiC ceramics 10 E. LUCCHINI and O. SBAIZERO, JEur CeramSoc.I demonstrate graceful failure behaviour almost identical (1995)975 to that of oriented continuous fibre composites. Cur 11M. P. RAO A. J. SANCHEZ-HERENCIA rently, the experiments are under way to achieve the TZ, R. M. MCMEEKING and F. F. LANGE, Science self-sealed structure in silicon nitride/carbon system 12. G. J. ZHANG. X. M. YuE andT. WATANABE. J.Amer which may open the avenue for the design and mar Ceram Soc. 82(1999)3 ufacture of a large number of ceramics with properties 4738

test and a representative load-displacement curve is presented in Fig. 3. The curve indicates that the fracture behavior of lam￾inated SiC is entirely different from that of monolithic SiC. The former showed non-catastrophic failure with a large crosshead displacement and the latter always shows brittle fracture with a minimal deformation. Af￾ter the bending tests were complete, the laminated spec￾imens held together in one piece rather than break￾ing into two pieces as in the case of the monolithic SiC samples. The laminates showed a non-linear load￾displacement behavior with some peaks occurring ei￾ther before or after the maximum load, depending on their structural features, but all exhibited gradual, non￾catastrophic failure. These peaks correspond to the fail￾ure of either a single layer or multiple layers indicating strong crack deflection caused by the weak/porous in￾terfaces (Fig. 3b and c). It was also noticed that even when the maximum failure load of the solid SiC core had been reached, the laminates with thicker SiC layers could still resist a relatively high load. The thick SiC layered composites also showed a large displacement over a high load range, which can contribute signifi- cantly to work of fracture. Our initial results (Fig. 4) show that the monolithic SiC has a strength of one and a half times that of the laminates. However, even though the laminates showed a lower strength, their work of fracture was 2–4 times higher than the monolithic SiC. It seems that there exists a threshold value of the thickness ratio between the SiC layer and the carbon layer. Higher ratios (>20) re￾sult in materials with higher fracture toughness. The high work of fracture of the laminates is attributed to the self-sealed ring structure, which eliminates the delamination completely. Clearly, the newly formed porous SiC layers serve to deflect the propagating crack leading to a remarkable improvement in fracture resistance. In summary, this work demonstrated for the first time that, by imitating the concentric circle tree-trunk structure in three-dimensions, the self-sealed layered structure can be created which effectively eliminates undesired delamination problem associated with plate￾form laminates. The self-sealed layered SiC ceramics demonstrate graceful failure behaviour almost identical to that of oriented continuous fibre composites. Cur￾rently, the experiments are under way to achieve the self-sealed structure in silicon nitride/carbon system which may open the avenue for the design and man￾ufacture of a large number of ceramics with properties of fibre composites without the difficulty and expense of incorporating fibres. Figure 4 Mechanical properties of laminates: (a) Strength versus composition. (b) Work of fracture versus composition. References 1. R. A. J. SAMBELL, D. H. BOWEN and D. C. PHILLIPS , J. Mater. Sci. 7 (1972) 663. 2. K. M. PREWO and J. J. BRENNAN, ibid. 15 (1980) 463. 3. Z. B. Y U and D. P . THOMPSON, J. Eur. Ceram. Soc. 22 (2002) 225. 4. L. ZHANG and V. D. KRSTIC, Theor. Appl. Fract. Mech. 24 (1995) 13. 5. W. J. CLEGG, K. KENDALL, N. MCN. ALFORD, T. W. BUTTON and J. D. BIRCHALL, Nature 374 (1990) 455. 6. W. J. CLEGG, Acta Metal. Mater. 40 (1992) 3085. 7. H. LIU and S . M. HSU, J. Amer. Ceram. Soc. 79 (1996) 2452. 8. D. B. MARSHALL, J. J. RATTO and F . F . LANGE, ibid. 74 (1991) 2979. 9. J. REQUENA, R. MORENO and J. S . MAYA, ibid. 72 (1989) 1511. 10. E. LUCCHINI and O. SBAIZERO, J. Eur. Ceram. Soc. 15 (1995) 975. 11. M. P . RAO, A. J. SANCHEZ-HERENCIA, G. E. BELTZ, R. M. MCMEEKING and F . F . LANGE, Science 286 (1999) 102. 12. G. J. ZHANG, X. M. YUE and T. WATANABE, J. Amer. Ceram. Soc. 82 (1999) 3257. Received 22 January and accepted 31 July 2003 4738

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