E≈S Journal of the European Ceramic Society 20(2000)1929-1938 Sic/SiC minicomposites with structure-graded BN interphases S Jacques, A. Lopez-Marure, C. Vincent, H. Vincent, J. Bouix Laboratoire des multimateriaux et Interfaces- UMR 5615 CNRS/University of Lyon I 43. boulevard du // Novembre 1918. F-69622 Villeurbanne Cedex france Received 28 October 1999: received in revised form 10 February 2000: accepted 14 February 2000 Abstract BN interphases in SiC/Sic minicomposites were produced by infiltration of fibre tows from BF3-NH3-H2 gaseous system dur- ing interphase one-step processing, the tow travels through a reactor containing a succession of different hot areas. By tem char- acterization, the Bn interphases were found to be made of a structural gradient from isotropic to highly anisotropic. The very first coating is poorly organised and allows to protect the fibre from a further chemical attack by the reactant mixture. The mini- composites were tensile tested at room temperature with unloading-reloading cycles. The Bn interphases act as mechanical the fibre/matrix bonding intensity ranges from weak to rather strong depending on the tow travelling rate during interphase infill tration. The specimen lifetimes at 700C under a constant tensile loading were measured in dry and moist air. Compared to a pyr ocarbon reference interphase, the bn interphases significantly improve the oxidation resistance of the Sic/Sic minicomposit C 2000 Elsevier Science Ltd. All rights reserved Keywords: BN; Composites: Electron microscopy; Interphase: Oxidation resistance; SiC/SiC 1. Introduction 2. Experimental procedure Sic/SiC type ceramic matrix composites, a good 2.1. Samples toughness can be achieved by adding between the fibre ind the brittle matrix a thin film of a compliant material The samples used in this study were SiC/SiC mini- called"interphase. If highly anisotropic pyrolytic(ex- composites. A minicomposite is a ID model composite BF3)boron nitride can replace advantageously the poor It consists of a tow in which the Bn or PyC interphase oxidation resistance pyrocarbon(PyC), its chemical and then the SiC matrix are infiltrated by CVI. The tow vapour infiltration(CVI)processing requires to protect is composed of around 500 Hi-Nicalon fibres(mono the fibre from a chemical attack with an isotropic Bn filaments)(from Nippon Carbon, Japan)(Fig. 1) pre-coating obtained with less aggressive conditions The geometry of the minicomposite is simpler than Thus, in the case of a classical static isothermal CVI, the that of the actual composite. It can properly simulate its N interphase must be infiltrated by following several behaviour and specially enables to determine the inter separate steps (i.e. changing the boron precursor gas phase properties and CVI parameters during the experiment.) In our case, the fibre volume fraction Vr of the mini- The aim of the present work was to prepare within ID composites was about 20%(measured by weighing) minicomposites bn interphases with structural gra- dients by using a one-step continuous dynamic CVI 2.2. Interphase processing process and to characterize these minicomposites The process used for the interphase preparatic new process called gradient-chemical vapour infiltration ( TG-CVi) le reactor was a hot wal furnace: a graphite 35 cm) placed inside a silica tube(inner diameter: 24 mm) is heated by dm.univ-lyonlfr(S Jacques). induction. The fibre tow was unwound from a first spool 0955-2219/00/S-see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00064-9SiC/SiC minicomposites with structure-graded BN interphases S. Jacques *, A. Lopez-Marure, C. Vincent, H. Vincent, J. Bouix Laboratoire des MultimateÂriaux et Interfaces Ð UMR 5615 CNRS/University of Lyon 1, 43, boulevard du 11 Novembre 1918, F±69622 Villeurbanne Cedex, France Received 28 October 1999; received in revised form 10 February 2000; accepted 14 February 2000 Abstract BN interphases in SiC/SiC minicomposites were produced by in®ltration of ®bre tows from BF3±NH3±H2 gaseous system. Dur￾ing interphase one-step processing, the tow travels through a reactor containing a succession of di€erent hot areas. By TEM char￾acterization, the BN interphases were found to be made of a structural gradient: from isotropic to highly anisotropic. The very ®rst coating is poorly organised and allows to protect the ®bre from a further chemical attack by the reactant mixture. The mini￾composites were tensile tested at room temperature with unloading-reloading cycles. The BN interphases act as mechanical fuses; the ®bre/matrix bonding intensity ranges from weak to rather strong depending on the tow travelling rate during interphase in®l￾tration. The specimen lifetimes at 700C under a constant tensile loading were measured in dry and moist air. Compared to a pyr￾ocarbon reference interphase, the BN interphases signi®cantly improve the oxidation resistance of the SiC/SiC minicomposites. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: BN; Composites; Electron microscopy; Interphase; Oxidation resistance; SiC/SiC 1. Introduction In SiC/SiC type ceramic matrix composites, a good toughness can be achieved by adding between the ®bre and the brittle matrix a thin ®lm of a compliant material called ``interphase''.1 If highly anisotropic pyrolytic (ex￾BF3) boron nitride can replace advantageously the poor oxidation resistance pyrocarbon (PyC), its chemical vapour in®ltration (CVI) processing requires to protect the ®bre from a chemical attack with an isotropic BN pre-coating obtained with less aggressive conditions.2,3 Thus, in the case of a classical static isothermal CVI, the BN interphase must be in®ltrated by following several separate steps (i.e. changing the boron precursor gas and CVI parameters during the experiment...). The aim of the present work was to prepare within 1D minicomposites BN interphases with structural gra￾dients by using a one-step continuous dynamic CVI process and to characterize these minicomposites. 2. Experimental procedure 2.1. Samples The samples used in this study were SiC/SiC mini￾composites. A minicomposite is a 1D model composite. It consists of a tow in which the BN or PyC interphase and then the SiC matrix are in®ltrated by CVI. The tow is composed of around 500 Hi-Nicalon ®bres (mono- ®laments) (from Nippon Carbon, Japan) (Fig. 1). The geometry of the minicomposite is simpler than that of the actual composite. It can properly simulate its behaviour and specially enables to determine the inter￾phase properties. In our case, the ®bre volume fraction Vf of the mini￾composites was about 20% (measured by weighing). 2.2. Interphase processing The process used for the interphase preparation was a new process called thermal gradient-chemical vapour in®ltration (TG-CVI).4,5 The reactor was a hot wall furnace: a graphite susceptor (length: 35 cm) placed inside a silica tube (inner diameter: 24 mm) is heated by induction. The ®bre tow was unwound from a ®rst spool 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00064-9 Journal of the European Ceramic Society 20 (2000) 1929±1938 * Corresponding author. E-mail address: sylvain.jacques@adm.univ-lyon1.fr (S. Jacques)