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.CeramSoc24 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 alu￾mina into a porous mullite fiber/mullite matrix com￾posite. 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 be￾tween matrix and fibers. As a result, the desired im￾provements in mechanical properties were not achieved. However, the results of this work indicate the possibil￾ity of the infiltration approach to modify the thermal conductivity of porous mullite/mullite composites, without a noticeable loss of the damage-tolerant behav￾ior. 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–displace￾ment curve in the linear region decreases with increas￾ing 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 ab￾sorption comes predominantly from the disintegration of the porous matrix during fiber pullout. This is con￾sistent 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