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)01478-2Materials 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 consid￾erable interest for aerospace applications demanding 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 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 under￾taken 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 Labo￾ratory, National Industrial Research Instiutute of Nagoya (NIRIN), Cooperative Research Center for Advanced Technology, Shimo-Shi￾dami, 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