Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Journal of the European Ceramic Society 28(2008)447-453 www.elsevier.comlocate/jeurceramsoc Interface engineering in mullite fiber/mullite matrix composites KK. Chawla k Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL 35294-4461, USA Available online 6 April 2007 Abstract Mullite fiber/mullite matrix composites are attractive because of their inherent oxidation resistance at high temperatures. Mullite has better creep resistance than alumina. However. chemical int between oxides are often very severe; with the result no gain is made over monolithic mullite in terms of toughness. Even in the absence of chemical bonding, a strong mechanical bond component may be present. This originates from radial compressive stress due to thermal expansion mismatch and/or the surface roughness of interface. Thus, the microstructure and behavior of the interface region are the key factors in obtaining an effective control of damage in composites and enhancement of toughness. This body of work on mullite/mullite composites shows the feasibility of producing fully dense tough oxide/oxide composites by interface engineering. Coatings such as Bn alone or Sic/BN double coating function effectively for mullite fiber/mullite matrix composites in that they provide a nonbrittle fracture and increased work of fracture at room temperature. It would appear that for use at high temperatures in air, one needs to identify structural analogs of bN among oxides o 2007 Elsevier Ltd. All rights reserved. Keywords:Fibers:Mullite;Composites:Fracture:Toughness and toughening Introduction nber fallure Mullite fiber/mullite matrix composites form an impor Mullite-based composites are attractive because of their inher It has been amply suggested that reinforcement with con- tinuous fibers such as carbon. alumina. silicon carbide. and ent oxidation resistance at high temperatures and their favorable mullite fibers in brittle matrices can result in toughening 3 thermal shock and damage tolerance properties. In fiber rein- forced ceramic matrix composites, the local response of the It is not necessary for all these failure mechanisms to oper- ate simultaneously for a given fiber/matrix system and often fiber/matrix interface during fracture is of great importance in many composite systems only one or two of these tough- the key factors in obtaining an effective control of damage Interface engineering approach involves incorporation of fiber in composites and enhancement of toughness. When a crack coatings that can bring into play energy absorbing processes noves through a matrix containing unidirectional fibers, a such as crack deflection and fiber pullout, and thus make the variety of failure mechanisms may come into play as shown in Fig. 1 composite damage-tolerant. It would appear that the debond- ing of the fiber/matrix interface is a prerequisite for phenomena such as crack deflection, crack bridging by fibers, and fiber ● matrix fracture pullo interfacial debonding at the crack tip, followed by crack deflection. interfacial debonding in the crack wake, followed by crack 2. Criteria for interfacial debonding deflection: frictional sliding between the fiber and matrix There are two main criteria for interfacial bonding both of hem are difficult to use in practice. We indicate a third one that is relatively simple and involves only radial stress (i.e, normal Tel:+12059759725;fax:+12059348485 to the fiber/matrix interface)component. A brief description of E-mail address. chawla(uab.edu these criteria follows. 0955-2219/S-see front matter o 2007 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2007.03.008Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 447–453 Interface engineering in mullite fiber/mullite matrix composites K.K. Chawla ∗ Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL 35294-4461, USA Available online 6 April 2007 Abstract Mullite fiber/mullite matrix composites are attractive because of their inherent oxidation resistance at high temperatures. Mullite has better creep resistance than alumina. However, chemical interactions between oxides are often very severe; with the result no gain is made over monolithic mullite in terms of toughness. Even in the absence of chemical bonding, a strong mechanical bond component may be present. This originates from radial compressive stress due to thermal expansion mismatch and/or the surface roughness of interface. Thus, the microstructure and behavior of the interface region are the key factors in obtaining an effective control of damage in composites and enhancement of toughness. This body of work on mullite/mullite composites shows the feasibility of producing fully dense, tough oxide/oxide composites by interface engineering. Coatings such as BN alone or SiC/BN double coating function effectively for mullite fiber/mullite matrix composites in that they provide a nonbrittle fracture and increased work of fracture at room temperature. It would appear that for use at high temperatures in air, one needs to identify structural analogs of BN among oxides. © 2007 Elsevier Ltd. All rights reserved. Keywords: Fibers; Mullite; Composites; Fracture; Toughness and toughening 1. Introduction Mullite fiber/mullite matrix composites form an impor￾tant subpart of the oxide fiber/oxide matrix compsites.1–3 Mullite-based composites are attractive because of their inher￾ent oxidation resistance at high temperatures and their favorable thermal shock and damage tolerance properties. In fiber rein￾forced ceramic matrix composites, the local response of the fiber/matrix interface during fracture is of great importance. The microstructure and behavior of the interface region are the key factors in obtaining an effective control of damage in composites and enhancement of toughness. When a crack moves through a matrix containing unidirectional fibers, a variety of failure mechanisms may come into play as shown in Fig. 1: • matrix fracture; • interfacial debonding at the crack tip, followed by crack deflection; • interfacial debonding in the crack wake, followed by crack deflection; • frictional sliding between the fiber and matrix; ∗ Tel.: +1 205 975 9725; fax: +1 205 934 8485. E-mail address: kchawla@uab.edu. • fiber failure; • fiber pullout. It has been amply suggested that reinforcement with con￾tinuous fibers such as carbon, alumina, silicon carbide, and mullite fibers in brittle matrices can result in toughening.3 It is not necessary for all these failure mechanisms to oper￾ate simultaneously for a given fiber/matrix system and often in many composite systems only one or two of these tough￾ness contributions will dominate the total fracture toughness. Interface engineering approach involves incorporation of fiber coatings that can bring into play energy absorbing processes such as crack deflection and fiber pullout, and thus make the composite damage-tolerant. It would appear that the debond￾ing of the fiber/matrix interface is a prerequisite for phenomena such as crack deflection, crack bridging by fibers, and fiber pullout. 2. Criteria for interfacial debonding There are two main criteria for interfacial bonding. Both of them are difficult to use in practice. We indicate a third one that is relatively simple and involves only radial stress (i.e., normal to the fiber/matrix interface) component. A brief description of these criteria follows: 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.03.008