CATERTALIA Pergamon Acta mater.48(200046094618 www.elsevier.com/locate/actamat THE CONCEPT OF A STRONG INTERFACE APPLIED TO SiC/SIC COMPOSITES WITH A BN INTERPHASE F. REBILLAT, J. LAMONT and A. GUETTE Laboratoire des Composites Thermostructuraux, UMR 5801, CNRS-SNECMA-CEA-UB1, 3, allee de la Boetie 33600 Pessac. france natrix composites(CMC). The concept of a strong interface has been established in SiC/SiC composit with pyrocarbon(Py C)or multilayered(PyC/SiC) fiber coatings(also referred to as interphases). The present reports an attempt directe applying the concept of a strong interface to SiC/SiC composites with BN coating(referred to as SiC/BN/SiC). Fiber bonding and frictional sliding were investigated by means of push-out tests performed on 2D-composites as well as on microcomposite samples, and tensile tests perfor- d on microcomposites. The stress-strain behavior of the SiC/BN/SiC composites and microcomposites is discussed with respect to interface characteristics and location of debonding either in the coating or in the fiber/coating interface. 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AlI rights reserved. 1 INTRODUCTION expansion mismatch. Fiber/matrix interfaces in the most advanced ceramic matrix composites consist of of composites because load transfers from the matrix a thin coating layer(less than l um thick )of one or to the fiber and vice versa must occur through the several materials deposited on the fiber (interphase) interface. Therefore, it exerts a profound infuence Recently, SiC/SiC composites with strong interfaces upon the mechanical behavior and the lifetime. Thus, have been developed. The coating/fiber bond was sig- as a function of end use applications through optimiz. treated 3-5). Features of the mechanical behavior of SiC/SiC composites with strong fiber/coating inter- 6-1 In fiber-reinforced ceramic composites, most Experiments as well as models have demonstrated increase fracture toughness. The major contribution that a strong interface is beneficial to the strength, the to toughness is attributed to crack bridging and fiber toughness, the lifetime and the creep resistance 14, 6 composite strength. A high strength requires efficient mental trast, weak interfaces are shown to be det- he once ept of strong interfaces has been estab- load transfers which are obtained with strong inter- lished on Sic/C/SiC composites with PyC and multi- faces. This implies short debond cracks and/or sig- layered(Py C/SiC)fiber coatings. In the present paper, nificant sliding friction. These latter requirements, to be met for strong composites, are therefore incompat- it is applied to SiC/BN/SiC composites with boron ible with the former ones for tough composites, if nitride fiber coatings. BN is foreseen to be an alterna- toughening is based solely upon the above mentioned tive fiber coating to improve the oxidation resistance weak interface-based mechanisms of ceramic matrix composites at high temperature Fiber/matrix bonding results from diffusion or chemical reactions(chemical bonding)or from fiber 2. FEATURES OF STRONG INTERFACES VS WEAK clamping by residual stresses induced by thermal INTERFACES n prc uce of a hom all correspondence should be addressed. Tel: strong interface, we recall first the basic features of 844-703;fax:+33-556-841-225 interface phenomena in CMCs subject to an essen- ddress: admin@lcts. u-bordeaux fr(. Lamon) tially tensile load. These phenomena influence the 1359-6454100/520.00@ 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved PI:S1359-6454(00)00247-0Acta mater. 48 (2000) 4609–4618 www.elsevier.com/locate/actamat THE CONCEPT OF A STRONG INTERFACE APPLIED TO SiC/SiC COMPOSITES WITH A BN INTERPHASE F. REBILLAT, J. LAMON† and A. GUETTE Laboratoire des Composites Thermostructuraux, UMR 5801, CNRS-SNECMA-CEA-UB1, 3, alle´e de la Boe´tie, 33600 Pessac, France Abstract—Strong interfaces have been shown to allow improvement of the mechanical properties of ceramic matrix composites (CMC). The concept of a strong interface has been established in SiC/SiC composites with pyrocarbon (PyC) or multilayered (PyC/SiC) fiber coatings (also referred to as interphases). The present paper reports an attempt directed at applying the concept of a strong interface to SiC/SiC composites with a BN coating (referred to as SiC/BN/SiC). Fiber bonding and frictional sliding were investigated by means of push-out tests performed on 2D-composites as well as on microcomposite samples, and tensile tests perfor￾med on microcomposites. The stress–strain behavior of the SiC/BN/SiC composites and microcomposites is discussed with respect to interface characteristics and location of debonding either in the coating or in the fiber/coating interface.  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Interphase; Interface; Composites 1. INTRODUCTION The fiber–matrix interfacial domain is a critical part of composites because load transfers from the matrix to the fiber and vice versa must occur through the interface. Therefore, it exerts a profound influence upon the mechanical behavior and the lifetime. Thus, it may be expected that composites could be tailored as a function of end use applications through optimiz￾ation of interfaces. In fiber-reinforced ceramic composites, most authors promote the concept of weak interfaces to increase fracture toughness. The major contribution to toughness is attributed to crack bridging and fiber pull-out [1, 2]. Weak interfaces are detrimental to composite strength. A high strength requires efficient load transfers which are obtained with strong inter￾faces. This implies short debond cracks and/or sig￾nificant sliding friction. These latter requirements, to be met for strong composites, are therefore incompat￾ible with the former ones for tough composites, if toughening is based solely upon the above mentioned weak interface-based mechanisms. Fiber/matrix bonding results from diffusion or chemical reactions (chemical bonding) or from fiber clamping by residual stresses induced by thermal † To whom all correspondence should be addressed. Tel.: 133-556-844-703; fax: 133-556-841-225. E-mail address: admin@lcts.u-bordeaux.fr (J. Lamon) 1359-6454/00/$20.00  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S13 59-6454(00)00247-0 expansion mismatch. Fiber/matrix interfaces in the most advanced ceramic matrix composites consist of a thin coating layer (less than 1 µm thick) of one or several materials deposited on the fiber (interphase). Recently, SiC/SiC composites with strong interfaces have been developed. The coating/fiber bond was sig￾nificantly stronger when fibers had been previously treated [3–5]. Features of the mechanical behavior of SiC/SiC composites with strong fiber/coating inter￾faces has been examined in several papers [4, 6–10]. Experiments as well as models have demonstrated that a strong interface is beneficial to the strength, the toughness, the lifetime and the creep resistance [4, 6– 11]. By contrast, weak interfaces are shown to be det￾rimental. The concept of strong interfaces has been estab￾lished on SiC/C/SiC composites with PyC and multi￾layered (PyC/SiC) fiber coatings. In the present paper, it is applied to SiC/BN/SiC composites with boron nitride fiber coatings. BN is foreseen to be an alterna￾tive fiber coating to improve the oxidation resistance of ceramic matrix composites at high temperature. 2. FEATURES OF STRONG INTERFACES VS WEAK INTERFACES In order to properly introduce the concept of a strong interface, we recall first the basic features of interface phenomena in CMCs subject to an essen￾tially tensile load. These phenomena influence the