Journal of the European Cerami so ieis vier scien le limited Printed in Great Britain. All rights reserved PII:s0955-2219(97)00007 0955-221997s1700 Sol-Gel Control of Matrix Net-Shape Sintering in 3D Fibre reinforced Ceramic Matrix Composites Ph Colomban*& m. Wey ONERA, Direction dcs Matcriaux, BP 72, 92322 Chatillon Cedex france (Received 4 October 1996; revised version rcccivcd 26 November 1996 accepted 2 December 1996) Abstract matrix precursor must be incorporated through the voids between fibres (a few microns or less The origin of thermochemical degradation of 3D size). This is only possible by infiltration of liquid reinforced ceramic matrix composites has been or gaseous precursors for which the ceramic yield analysed hy dilatometry, pore-size distribution and is necessarily low, which implies porosity and electron microscopy and correlated to the cracking cracks. The latter method is used for the synthesis arising from the matrix shrinkage in an invariant of covalent bonded ceramics of a rather simple 3D fibre network. The matrix shrinkage of an alu- composition(e.g. C, SiC, BN, .)2 whereas the mina matrix composite has been delayed up to a former is more versatile(for SiC, Si, N4, various higher temperature (+200C) by post-infiltration multicomponent oxides, ...).3.4 The liquid pre with pure alkoxide which was subsequently in situ cursor is a slurry of submicron powder or a hydrolysed, polycondensed and pyrolysed. Mechan- metal-organic reagent(alkoxide, ester, polysil- ical strength of the composites has been increased oxane, polysilazane ng after pyrolysis four times by optimizing alkoxides. The tensile and to oxide, carbide nitride .. In both cases, there flexural mechanical strengths of these alumina is considerable shrinkage after pyrolysis which matrix composites are similar to those obtained produces new voids; however, the presence of the from SiC matrix composites prepared by the CVI woven fabrics can efficiently inhibit the coherent process using the same 3d carbon preform. c 1997 shrinkage of the matrix. In the case of one-or Elsevier Science Limited. two-dimensional reinforcements this dilemma coated fabrics.0 To solve this problem is more 1 Introduction complicated when a three-dimensional (3D)rcin- forcement is used The 3D interconnected array of The great weakness of lithic ceramics fibres segments the matrix, which cracks when trinsic inability to te mechanica sintering takes place. Thus, the methods based without brittle rupture because of their polycrys- on the slow condensation of gaseous precursor talline state and the nature of the chemical bonds Chemical Vapour Infiltration(CVi) is commonly existing in these compounds. The use of long, used. Ihis paper will demonstrate how the post woven ceramic fibres embedded in a refractory infiltration of a specified ceramic precursor--and ceramic matrix can result in a metastable compo- its in situ pyrolysis-between the primary infiI- site material exhibiting greater toughness through trated powder particles by pressure-assisted infil- a specific micromechanism at the fibre-matrix inter- tration can improve the mechanical strength of 3D face: the matrix cracking can be deflected dissociated fibre reinforced ceramic matrix composites and and even arrested when a propagating crack meets that liquid precursors offer an alternative method the reinforcing fibres and thus composite materials of preparing 3D reinforced ceramic matrix com can exhibit a fibrous, pseudo-plastic fracture posites(CMCs) One of the main problems in the preparation of Various sol gel precursors leading to inert ceramic matrix composites is to achieve a low refractory non-reacting interphase(Zr-i-propoxide open porosity in the matrix. The fibres must be as zirconia precursor, Al-butoxide as alumina pre- thoroughly embedded in the matrix and the cursor), reacting interphase (Ti-i- propoxide as rutile precursor, tetraethoxysilane (tEoS)as silica * Corresponding author, also at CNRS, LASIR, 2 rue Henri precursor)or bloating interphase(Al-Si ester, as Dunant. 94320 Thiais. france aluminosilicate precursor) have been considered 1475Journal of the European Ceramic Society 17 (1997) 1475-1483 0 1997 Elsevier Science Limited Printed in Great Britain. All rights reserved PII: SO955-2219(97)00007-l 0955-2219/97/$17.00 Sol-Gel Control of Matrix Net-Shape Sintering in 3D Fibre Rei.nforced Ceramic Matrix Composites Ph. Colomban* & ML Wey ONERA, Direction des MattSriaux, BP 72, 92322 Chatillon Cedex, France (Received 4 October 1996; revised version received 26 November 1996; accepted 2 December 1996) Abstract The origin of thermochemical degradation of 30 reinforced ceramic matri.x composites has been analysed by dilatometry, pore-size distribution and electron microscopy and correlated to the cracking arising from the matrix shrinkage in an invariant 30 jibre network. The matrix shrinkage of an alu￾mina matrix composite has been delayed up to a higher temperature (+2OO”C) by post-injiltration with pure alkoxide which was subsequently in situ hydrolysed, polycondensed and pyrolysed. Mechan￾ical strength of the composites has been increased four times by optimizing alkoxides. The tensile and Jlexural mechanical strengths of these alumina matrix composites are similar to those obtained from Sic matrix composites prepared by the CVI process using the same 30 carbon preform. 0 1997 Elsevier Science Limited. 1 Introduction The great weakness of mo:nolithic ceramics is their intrinsic inability to tolerate mechanical damage without brittle rupture because of their polycrys￾talline state and the naturle of the chemical bonds existing in these compounds. The use of long, woven ceramic fibres embedded in a refractory ceramic matrix can result in a metastable compo￾site material exhibiting greater toughness through a specific micromechanism at the fibre-matrix inter￾face: the matrix cracking can be deflected, dissociated and even arrested when a propagating crack meets the reinforcing fibres and thus composite materials can exhibit a fibrous, pseudo-plastic fracture.’ One of the main problems in the preparation of ceramic matrix composites is to achieve a low open porosity in the matrix. The fibres must be thoroughly embedded in the matrix and the *Corresponding author, also at CNRS, LASIR, 2 rue Hem-i Dunant, 94320 Thiais, France. matrix precursor must be incorporated through the voids between fibres (a few microns or less, in size). This is only possible by infiltration of liquid or gaseous precursors for which the ceramic yield is necessarily low, which implies porosity and cracks. The latter method is used for the synthesis of covalent bonded ceramics of a rather simple composition (e.g. C, Sic, BN, ...)2 whereas the former is more versatile (for Sic, Si,N,, various multicomponent oxides, . . .).3,4 The liquid pre￾cursor is a slurry of submicron powder or a metal-organic reagent (alkoxide, ester, polysil￾oxane, polysilazane, . . .)3,4 leading after pyrolysis to oxide, carbide, nitride, . . . . In both cases, there is considerable shrinkage after pyrolysis which produces new voids; however, the presence of the woven fabrics can efficiently inhibit the coherent shrinkage of the matrix. In the case of one- or two-dimensional reinforcements, this dilemma is solved by the hot pressing of impregnated and coated fabricP To solve this problem is more complicated when a three-dimensional (3D) rein￾forcement is used. The 3D interconnected array of fibres segments the matrix, which cracks when sintering takes place. Thus, the methods based on the slow condensation of gaseous precursor (Chemical Vapour Infiltration (CVI) is commonly used. This paper will demonstrate how the post￾infiltration of a specified ceramic precursor-and its in situ pyrolysis-between the primary infil￾trated powder particles by pressure-assisted infil￾tration can improve the mechanical strength of 3D fibre reinforced ceramic matrix composites and that liquid precursors offer an alternative method of preparing 3D reinforced ceramic matrix com￾posites (CMCs). Various sol-gel precursors leading to inert refractory non-reacting interphase (Zr-i-propoxide as zirconia precursor, Al-butoxide as alumina pre￾cursor), reacting interphase (Ti-i-propoxide as rutile precursor, tetraethoxysilane (TEOS) as silica precursor) or bloating interphase (Al-Si ester, as aluminosilicate precursor) have been considered in 1475