《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_porous matrix-1.pdf_大学文库

MIATERIALS IENGE& ENGMEERN A ELSEVIER Materials Science and Engineering A325(2002)19-24 www.elsevier.com/locate/msea Effect of cyclic infiltrations on microstructure and mechanical behavior of porous mullite/mullite composites Jihong she * Peter Mechnich. Hartmut Schneider. Martin Schmucker, Bernd Kanka Institute of Materials Research, German Aerospace Center(DLR), 51147 Koln, Germany Received 14 August 2000 received in revised form 19 March 2001 Abstract a porous mullite fiber/mullite matrix composite was cyclically infiltrated in an AlCl, solution to introduce alumina into the pore pace. It is shown that the matrix porosity decreases with the number of infiltration cycles. Moreover, the microstructural observations indicate that the distribution of residual pores within the infiltrated composites is heterogeneous, with a lower porosity in the surface region relative to the interior region Beyond 4 cycles of infiltration, surface embrittlement occurs due to a significant enhancement of the interparticle bonds and the fiber/matrix interfaces in the surface region, leading to a notable reduction in fracture energy. C 2002 Elsevier Science B.v. All rights reserved Keywords: Mullite: Composites; Infiltration; Microstructure; Mechanical behavior 1. Introduction taken to assess the effects of the matrix microstructure on the mechanical performance. For this purpose,an Mullite fiber /mullite matrix composites are of consid infiltration process was used to incorporate alumina crable interest for aerospace applications demanding into the porous mullite matrix long-term and high-temperature stability in an oxidiz- ing atmosphere. However, the main problem of this system is that the sintering temperature for matrix 2. Experimental procedure densification is usually high enough to cause som physical or chemical interactions at the fiber/matrix Unidirectional mullite- fiber/mullite-matrix preforms interfaces, giving rise to brittle failure [1, 2]. In order to were prepared by a slurry impregnation method using achieve a 'graceful fracture behavior, it is necessary to uncoated mullite fibers(Nextel 720, 3M Corp, MN) coat the fibers with a suitable interphase, such as bn and ultra-fine mullite powders (Siral Il, Condea 34], SIC/BN [56], ZrO2 [7, 8] or monazite [9]. This Chemie, Hamburg, Germany). In order to strengthen limits the applications of mullite/mullite composites due the matrix so that the preforms could withstand the to the complexity of the fiber-coating processes subsequent processing steps, a light sintering treatment e Recently, Kanka and Schneider [lO] have successfully was performed at 1300oC for I h. Infiltration was ricated a damage-t tolerant mullite/ mullite composite carried out by compl letely immersing the rectangular through the use of a porous matrix for crack deflection sof~1.8×4×40mm3ina5 M AICI without the requirement of a fiber/matrix interphase. In solution, which was prepared by dissolving 120.7 g of the present work, a further investigation was under- aluminum-chloride-hexahydrate [AlCl3 6H,O](Fluka Chemie, Germany) in 100 ml of distilled water Prior to Corresponding author. Present address: Synergy Ceramics Labo- infiltration, air entrapped within the specimens was not ratory, National Industrial Research Instiutute of Nagoya ( NIRIN Cooperative Research Center for Advanced Technology, Shimo-Sh evacuated. To ensure that the infiltration process had dami, Moriyama-ku, Nagoya 463-8687, Japan. Fax: +81-52-739. reached completion, the specimens were immersed in the AlCl, solution for a period of 20 h. Then, the E-imail address: jhshe(@nirin go jp (J. She) infiltrated specimens were soaked in a 32% ammonia 0921-5093/02/S- see front matter c 2002 Elsevier Science B V. All rights reserved. PI:S0921-5093(01)01478-2

Materials Science and Engineering A325 (2002) 19–24 Effect of cyclic infiltrations on microstructure and mechanical behavior of porous mullite/mullite composites Jihong She *, Peter Mechnich, Hartmut Schneider, Martin Schmu¨cker, Bernd Kanka Institute of Materials Research, German Aerospace Center (DLR), 51147 Koln, Germany Received 14 August 2000; received in revised form 19 March 2001 Abstract A porous mullite fiber/mullite matrix composite was cyclically infiltrated in an AlCl3 solution to introduce alumina into the pore space. It is shown that the matrix porosity decreases with the number of infiltration cycles. Moreover, the microstructural observations indicate that the distribution of residual pores within the infiltrated composites is heterogeneous, with a lower porosity in the surface region relative to the interior region. Beyond 4 cycles of infiltration, surface embrittlement occurs due to a significant enhancement of the interparticle bonds and the fiber/matrix interfaces in the surface region, leading to a notable reduction in fracture energy. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Mullite; Composites; Infiltration; Microstructure; Mechanical behavior www.elsevier.com/locate/msea 1. Introduction Mullite fiber/mullite matrix composites are of considerable interest for aerospace applications demanding long-term and high-temperature stability in an oxidizing atmosphere. However, the main problem of this system is that the sintering temperature for matrix densification is usually high enough to cause some physical or chemical interactions at the fiber/matrix interfaces, giving rise to brittle failure [1,2]. In order to achieve a ‘graceful’ fracture behavior, it is necessary to coat the fibers with a suitable interphase, such as BN [3,4], SiC/BN [5,6], ZrO2 [7,8] or monazite [9]. This limits the applications of mullite/mullite composites due to the complexity of the fiber-coating processes. Recently, Kanka and Schneider [10] have successfully fabricated a damage-tolerant mullite/mullite composite through the use of a porous matrix for crack deflection, without the requirement of a fiber/matrix interphase. In the present work, a further investigation was undertaken to assess the effects of the matrix microstructure on the mechanical performance. For this purpose, an infiltration process was used to incorporate alumina into the porous mullite matrix. 2. Experimental procedure Unidirectional mullite-fiber/mullite-matrix preforms were prepared by a slurry impregnation method using uncoated mullite fibers (Nextel 720, 3M Corp., MN) and ultra-fine mullite powders (Siral II, Condea Chemie, Hamburg, Germany). In order to strengthen the matrix so that the preforms could withstand the subsequent processing steps, a light sintering treatment was performed at 1300 o C for 1 h. Infiltration was carried out by completely immersing the rectangular specimens of 1.8×4×40 mm3 in a 5 M AlCl3 solution, which was prepared by dissolving 120.7 g of aluminum-chloride-hexahydrate [AlCl3·6H2O] (Fluka Chemie, Germany) in 100 ml of distilled water. Prior to infiltration, air entrapped within the specimens was not evacuated. To ensure that the infiltration process had reached completion, the specimens were immersed in the AlCl3 solution for a period of 20 h. Then, the infiltrated specimens were soaked in a 32% ammonia * Corresponding author. Present address: Synergy Ceramics Laboratory, National Industrial Research Instiutute of Nagoya (NIRIN), Cooperative Research Center for Advanced Technology, Shimo-Shidami, Moriyama-ku, Nagoya 463-8687, Japan. Fax: +81-52-739- 0136. E-mail address: jhshe@nirin.go.jp (J. She). 0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 4 7 8 - 2

J. She et al. Materials Science and Engineering 4325(2002)19-24 solution(Merck, Germany) for 2 h to achieve alu- 3. Results and discussion ninum-hydroxide [Al(OH)3] gelation. After drying on a hot plate, the specimens were further heated at A350 Fig. 1 shows the change of weight increase and oC for 15 min to decompose Al(OH)3 into Al,O3. The matrix porosity with cyclic infiltrations for porous mul- processes of infiltration, gelation and decomposition lite/mullite composites. As expected, the weight gain repeated ten times. To stabilize amorphous Al, O, increases and the matrix porosity decreases as the num n equilibrium o-structure, a final heat-treatment ber of infiltration cycles increases. After ten infiltra- was given at 1200C for 30 min. The weight of the tions, the matrix porosity decreased from 65 to 52% specimens before infiltration and after heat-treatment due to the introduction of c 12 wt% alumina into the vas measured to calculate the amount of Alo that void space within the composite. Based on a theoretical was incorporated into the mullite matrix. Density and analysis [ll], the matrix porosity, Pm, is related to the porosity were determined by Archimedes' method number of infiltration cycles, N, through the following Flexural strength was measured by a three-point bend distance of 20 Pm=P(1-Y2), cross-head speed of 0.5 mm min. Tests were inter rupted when the load dropped below 5N where Pm is the initial content of pores in the matrix. cross-head displacement exceeded 0.4 mm. Load-dis and Ye is the ceramic yield by volume of the infiltrant placement responses were recorded with a computerized It can be seen in Fig. 1(b) that the fitted curve with data-acquisition system. Fracture energy was evaluated Pm=64.6% and Y=2.28% is in good agreement with from the area under the load-displacement curve in the the experimental data. If an infiltrant with a higher Yc non-elastic region. Polished sections and fracture sur value is used, the infiltration process would be more faces were observed in a scanning electron microscope effective in reducing the porosity (SEM) It has been demonstrated in the earlier work [12] that there is a non-uniform distribution of the residual pores within the infiltrated composites. As shown in Fig. 2, the matrix porosity is lower in the surface region, and tends to increase towards the interior. This is thought to result from two events which may occur during infiltration and gelation. As reported elsewhere [12], the intruded solution is filtered' as it passes through the pore channels due to a good adsorbability of amor- phous alumina that was introduced into the pore space 6 by previous cycles. On the other hand, the gelling process of the intruded solution in the central region is prevented due to the formation of the Al(OH)3 gelation in the surface region. Since the former may produce a decreasing concentration of the intruded solution from 0 the surface inward and the latter may cause an incom- plete gelation of the intruded solution in the interior region, any or both of them could give rise to less alumina and thus more pores in the interior region relative to the surface region. This phenomenon is less 62 noticeable for the composites within four infiltrations, while it becomes substantially more pronounced for five to ten infiltrations. Moreover, it was found that the thickness of the 'dense surface region increased with the infiltrated composites, some closer inspections were further performed on the polished sections. Fig. 3 illus trates a typical microstructure of the composite after 0 4 8 10 ten infiltration cycles, where mullite appears gray and Number of Infiltration Cycles alumina is in bright contrast. Evidently, the alumina phase is always located within the interstices between Fig. 1. Weight increase and matrix porosity as a function of the matrix particles or between matrix and fibers. Since number of infiltration cycles for porous mullite/mullite composites amorphous alumina sinters readily above 800C [13]

20 J. She et al. / Materials Science and Engineering A325 (2002) 19–24 solution (Merck, Germany) for 2 h to achieve aluminum-hydroxide [Al(OH)3] gelation. After drying on a hot plate, the specimens were further heated at 350 o C for 15 min to decompose Al(OH)3 into Al2O3. The processes of infiltration, gelation and decomposition were repeated ten times. To stabilize amorphous Al2O3 to an equilibrium -structure, a final heat-treatment was given at 1200 o C for 30 min. The weight of the specimens before infiltration and after heat-treatment was measured to calculate the amount of Al2O3 that was incorporated into the mullite matrix. Density and porosity were determined by Archimedes’ method. Flexural strength was measured by a three-point bending test with a support distance of 20 mm and a cross-head speed of 0.5 mm min−1 . Tests were interrupted when the load dropped below 5 N or the cross-head displacement exceeded 0.4 mm. Load–displacement responses were recorded with a computerized data-acquisition system. Fracture energy was evaluated from the area under the load–displacement curve in the non-elastic region. Polished sections and fracture surfaces were observed in a scanning electron microscope (SEM). 3. Results and discussion Fig. 1 shows the change of weight increase and matrix porosity with cyclic infiltrations for porous mullite/mullite composites. As expected, the weight gain increases and the matrix porosity decreases as the number of infiltration cycles increases. After ten infiltrations, the matrix porosity decreased from 65 to 52% due to the introduction of 12 wt.% alumina into the void space within the composite. Based on a theoretical analysis [11], the matrix porosity, Pm, is related to the number of infiltration cycles, N, through the following expression: Pm=P m 0 (1−Yc) N, (1) where P m 0 is the initial content of pores in the matrix, and Yc is the ceramic yield by volume of the infiltrant. It can be seen in Fig. 1(b) that the fitted curve with P m 0 =64.6% and Yc=2.28% is in good agreement with the experimental data. If an infiltrant with a higher Yc value is used, the infiltration process would be more effective in reducing the porosity. It has been demonstrated in the earlier work [12] that there is a non-uniform distribution of the residual pores within the infiltrated composites. As shown in Fig. 2, the matrix porosity is lower in the surface region, and tends to increase towards the interior. This is thought to result from two events which may occur during infiltration and gelation. As reported elsewhere [12], the intruded solution is ‘filtered’ as it passes through the pore channels due to a good adsorbability of amorphous alumina that was introduced into the pore space by previous cycles. On the other hand, the gelling process of the intruded solution in the central region is prevented due to the formation of the Al(OH)3 gelation in the surface region. Since the former may produce a decreasing concentration of the intruded solution from the surface inward and the latter may cause an incomplete gelation of the intruded solution in the interior region, any or both of them could give rise to less alumina and thus more pores in the interior region relative to the surface region. This phenomenon is less noticeable for the composites within four infiltrations, while it becomes substantially more pronounced for five to ten infiltrations. Moreover, it was found that the thickness of the ‘dense’ surface region increased with infiltration cycles. In order to highlight the microstructural features of the infiltrated composites, some closer inspections were further performed on the polished sections. Fig. 3 illustrates a typical microstructure of the composite after ten infiltration cycles, where mullite appears gray and alumina is in bright contrast. Evidently, the alumina phase is always located within the interstices between matrix particles or between matrix and fibers. Since amorphous alumina sinters readily above 800 o C [13], Fig. 1. Weight increase and matrix porosity as a function of the number of infiltration cycles for porous mullite/mullite composites

J. She et al. Materials Science and Engineering 4325 (2002)19-24 (b) 10 um um ig. 5. SEM micrographs of the fracture surface of a porous mullite/mullite composite after 3 cycles of infiltration, showing(a) extensive fiber pullout and (b) a broken fiber with matrix adhered to it. bserved over the whole fracture surface. This is proba- occur in a spatially random fashion, as indicated in Fig bly due to the fact that porous matrix between the 6(b) by the broken fibers with different crack planes. In fibers disintegrated into smaller pieces during the frac- fact, the fibers fracture sequentially, according to the ture process, as evidenced in Fig. 5(b) by a noticeable probabilistic distribution of their strengths. This pro- amount of the matrix debris attached to the fibers cess results in the gradual load decreases in Fig 4 With five to seven infiltrations, the composites exhib- When the composites were infiltrated for 8-10 cycles, ited a non-catastrophic failure, but the load dropped he initial cracking event in the surface region leads to suddenly to a certain level after the maximum load. The an abrupt drop in the nominal stress to a relatively low sudden load drop is believed to be associated with level, followed immediately by a load decrease. This is surface embrittlement. As observed in Fig. 2(b), the attributed primarily to the reduction in the thickness of matrix porosity is considerably decreased in the surface the porous interior region, where the fiber pullout may region. Especially, the bonds between the fibers and the occur during fracture matrix are significantly promoted by the alumina Furthermore. it was observed that all the infiltrated bridges. In this case, the matrix crack, which initiates composites remained intact after testing, but the degree on the tensile surface at the peak stress, would propa- of load retention decreased with increasing number of gate through the fibers rather than deflect between the infiltrations. As presented in Fig 4, the composites with fibers. As can be seen in Fig. 6(a), the fracture topogra- three, six and eight infiltrations have the ability to phy of the surface region is nearly planar, with almost withstand a flexural stress of 30, 22 and 10 MPa at a no fiber pullout. This cracking event causes the sharp cross-head displacement of 0.4 mm, while the com- load drops in Fig. 4. However, such a crack is arrested posite with ten infiltrations has almost no load-bearing ind deflected by the porous matrix in the interior capability even at a cross-head displacement of 0. 2 mm region. This allows the measured load to rise again with On the other hand, it is worth to note in the stress ncreasing cross-head displacement, until some cracks displacement curves of Fig. 4 that the slope of the initiate within the fibers. Subsequently, fiber failures initial linear region increases slightly with the number

22 J. She et al. / Materials Science and Engineering A325 (2002) 19–24 Fig. 5. SEM micrographs of the fracture surface of a porous mullite/mullite composite after 3 cycles of infiltration, showing (a) extensive fiber pullout and (b) a broken fiber with matrix adhered to it. observed over the whole fracture surface. This is probably due to the fact that porous matrix between the fibers disintegrated into smaller pieces during the fracture process, as evidenced in Fig. 5(b) by a noticeable amount of the matrix debris attached to the fibers. With five to seven infiltrations, the composites exhibited a non-catastrophic failure, but the load dropped suddenly to a certain level after the maximum load. The sudden load drop is believed to be associated with surface embrittlement. As observed in Fig. 2(b), the matrix porosity is considerably decreased in the surface region. Especially, the bonds between the fibers and the matrix are significantly promoted by the alumina ‘bridges’. In this case, the matrix crack, which initiates on the tensile surface at the peak stress, would propagate through the fibers rather than deflect between the fibers. As can be seen in Fig. 6(a), the fracture topography of the surface region is nearly planar, with almost no fiber pullout. This cracking event causes the sharp load drops in Fig. 4. However, such a crack is arrested and deflected by the porous matrix in the interior region. This allows the measured load to rise again with increasing cross-head displacement, until some cracks initiate within the fibers. Subsequently, fiber failures occur in a spatially random fashion, as indicated in Fig. 6(b) by the broken fibers with different crack planes. In fact, the fibers fracture sequentially, according to the probabilistic distribution of their strengths. This process results in the gradual load decreases in Fig. 4. When the composites were infiltrated for 8–10 cycles, the initial cracking event in the surface region leads to an abrupt drop in the nominal stress to a relatively low level, followed immediately by a load decrease. This is attributed primarily to the reduction in the thickness of the porous interior region, where the fiber pullout may occur during fracture. Furthermore, it was observed that all the infiltrated composites remained intact after testing, but the degree of load retention decreased with increasing number of infiltrations. As presented in Fig. 4, the composites with three, six and eight infiltrations have the ability to withstand a flexural stress of 30, 22 and 10 MPa at a cross-head displacement of 0.4 mm, while the composite with ten infiltrations has almost no load-bearing capability even at a cross-head displacement of 0.2 mm. On the other hand, it is worth to note in the stress– displacement curves of Fig. 4 that the slope of the initial linear region increases slightly with the number

J. She et al. Materials Science and Engineering 4325 (2002)19-24 2000 4. Conclusions 1600 An infiltration process was used to incorporate alu mina into a porous mullite fiber/mullite matrix com posite. The matrix porosity was found to decrease with 81200 the cycles of infiltration. SEM examinations revealed an inhomogeneous distribution of residual pores within the infiltrated composites, with an increasing porosity from the surface to the interior. Beyond four infiltration cycles, surface embrittlement was observed to occur due E400 to a significant enhancement of the interparticle bonds and the fiber/matrix interfaces by the alumina 0 at the contact points between matrix particles tween matrix and fibers. As a result. the desi provements in mechanical properties were not achieved Number of Infiltration Cycles However, the results of this work indicate the possibil- ity of the infiltration approach to modify the thermal Fig8. Variation in fracture energy with the number of infiltration conductivity of porous mullite/mullite composites, les for porous mullite/mullite composites. without a noticeable loss of the damage-tolerant behav lor fracture energy would contain the elastic strain energy if calculations were made from the total area under the load-displacement curve. As noted earlier, the Young Acknowledgements modulus of the infiltrated composites increases slightly with the number of infiltrations. whereas the flexural Jihong She wishes to acknowledge the alexander strength remains almost constant. Therefore, a slight Humboldt(AvH) Foundation for a financial support during his stay at German Aerospace Center (DLR) multiple infiltrations. This is in good agreement with the observation that the area under the load-displace ment curve in the linear region decreases with increas- References ing number of infiltrations. To investigate the effects of cyclic infiltrations on the 'true' fracture er []R.N. Singh, M. K. Brun, Ceram. Eng. Sci. Proc. 8(1987)636 process, calculations were conducted on the area under 2O. Yeheskel, M. L. Balmer, D.C. Cranmer, Ceram. Eng. Sci the load-displacement curve in the non-elastic region Proc.9(1988)687 As can be seen in Fig. 8, the fracture energy of the B]JS. Ha, K.K. Mater. Sci. Eng. A161 (1993)303 infiltrated composites with N-4 is in excess of 1250 J 4J.S. Ha, K.K. [J.S. Ha, KK c Mater. Sci. Eng. A203(1995)171. m due to extensive fiber pullout during the fracture J. Mater. Sci. Lett. 12(1993)84. 6K. K. Chawla, Z.R. Xu, J.S. Ha, J. Eur. Ceram. Soc. 16(1996) process. It has been reported [14 that the energy ab- sorption comes predominantly from the disintegration [7] E. Mouchon, P Colomban, Composites 26(1995)175 of the porous matrix during fiber pullout. This is con 8 P Colomban, E. Bruneton, J. L. Lagrange, E. Mouchon, J. Eur. sistent with the phenomenon in Fig. 5(b) that a notable Ceram Soc. 16(1996)30 mount of matrix remains on the surface of the broken 9 P.E. D. Morgan, D B. Marshall, Mater. Sci. Eng. A162(1993) fibers. Beyond four infiltration cycles, the extent of fiber (10 B.Kanka, H.Schneider,J. Eur. Ceram Soc. 20(2000)619 pullout decreases considerably with further infiltration [11]Y. Hirata, T. Matsura, K. Hayata, J. Am. Ceram Soc. 83(2000) due to surface embrittlement. Since the thickness of infiltration cycles, ah gion increases with increasing [12]JH.She, P. Mechnich, H Schneider,B Kanka, M. Schmucker, embrittled surfac he fracture energy is decrease J. Mater. Sci. Lett. 20(2001)51 [13] O. Sudre, FF. Lange, J. Am. Ceram Soc. 75(1992) markedly to 300 J m-2 for the composite with ten (14)JJ.Haslam,KE.Berroth, FF.Lange,J.Eur.CeramSoc

24 J. She et al. / Materials Science and Engineering A325 (2002) 19–24 Fig. 8. Variation in fracture energy with the number of infiltration cycles for porous mullite/mullite composites. 4. Conclusions An infiltration process was used to incorporate alumina into a porous mullite fiber/mullite matrix composite. The matrix porosity was found to decrease with the cycles of infiltration. SEM examinations revealed an inhomogeneous distribution of residual pores within the infiltrated composites, with an increasing porosity from the surface to the interior. Beyond four infiltration cycles, surface embrittlement was observed to occur due to a significant enhancement of the interparticle bonds and the fiber/matrix interfaces by the alumina ‘bridges’ at the contact points between matrix particles or between matrix and fibers. As a result, the desired improvements in mechanical properties were not achieved. However, the results of this work indicate the possibility of the infiltration approach to modify the thermal conductivity of porous mullite/mullite composites, without a noticeable loss of the damage-tolerant behavior. Acknowledgements Jihong She wishes to acknowledge the Alexander von Humboldt (AvH) Foundation for a financial support during his stay at German Aerospace Center (DLR). References [1] R.N. Singh, M.K. Brun, Ceram. Eng. Sci. Proc. 8 (1987) 636. [2] O. Yeheskel, M.L. Balmer, D.C. Cranmer, Ceram. Eng. Sci. Proc. 9 (1988) 687. [3] J.S. Ha, K.K. Chawla, Mater. Sci. Eng. A161 (1993) 303. [4] J.S. Ha, K.K. Chawla, Mater. Sci. Eng. A203 (1995) 171. [5] J.S. Ha, K.K. Chawla, J. Mater. Sci. Lett. 12 (1993) 84. [6] K.K. Chawla, Z.R. Xu, J.S. Ha, J. Eur. Ceram. Soc. 16 (1996) 293. [7] E. Mouchon, P. Colomban, Composites 26 (1995) 175. [8] P. Colomban, E. Bruneton, J.L. Lagrange, E. Mouchon, J. Eur. Ceram. Soc. 16 (1996) 301. [9] P.E.D. Morgan, D.B. Marshall, Mater. Sci. Eng. A162 (1993) 15. [10] B. Kanka, H. Schneider, J. Eur. Ceram. Soc. 20 (2000) 619. [11] Y. Hirata, T. Matsura, K. Hayata, J. Am. Ceram. Soc. 83 (2000) 1044. [12] J.H. She, P. Mechnich, H. Schneider, B. Kanka, M. Schmu¨cker, J. Mater. Sci. Lett. 20 (2001) 51. [13] O. Sudre, F.F. Lange, J. Am. Ceram. Soc. 75 (1992) 519. [14] J.J. Haslam, K.E. Berroth, F.F. Lange, J. Eur. Ceram. Soc. 20 (2000) 607. fracture energy would contain the elastic strain energy if calculations were made from the total area under the load–displacement curve. As noted earlier, the Young modulus of the infiltrated composites increases slightly with the number of infiltrations, whereas the flexural strength remains almost constant. Therefore, a slight decrease in the elastic strain energy would be caused by multiple infiltrations. This is in good agreement with the observation that the area under the load–displacement curve in the linear region decreases with increasing number of infiltrations. To investigate the effects of cyclic infiltrations on the ‘true’ fracture energy in the process, calculations were conducted on the area under the load–displacement curve in the non-elastic region. As can be seen in Fig. 8, the fracture energy of the infiltrated composites with N-4 is in excess of 1250 J m−2 due to extensive fiber pullout during the fracture process. It has been reported [14] that the energy absorption comes predominantly from the disintegration of the porous matrix during fiber pullout. This is consistent with the phenomenon in Fig. 5(b) that a notable amount of matrix remains on the surface of the broken fibers. Beyond four infiltration cycles, the extent of fiber pullout decreases considerably with further infiltration due to surface embrittlement. Since the thickness of the embrittled surface region increases with increasing infiltration cycles, the fracture energy is decreased markedly to 300 J m−2 for the composite with ten infiltrations

《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_porous matrix-1

《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_porous matrix-1