740 Ph. Colomban N. lapous less drastic. Of course, a sodium aluminosilicate will not be sensitive to corrosion by Na2 SOA(fuel 1. a mixture of tributylborate (TBB, from impurities) or NaCl (marine environment) molten Alfa-Ventron) and of (OBu)2-Al-O-Si(OEt) ester (SiAl, ref. 084, Dynasyl, from formerly In the present paper, we present a sol-gel method Dynamit Nobel, now Huls, France) hereafter for, and discuss the problems encountered in, the called TBB+ SiAl (volume ratio 1: 3 ).This manufacturing of barium(BAS), calcium(CAS)and composition has been designed for sintering sodium (NAs)aluminosilicate-matrix composites temperatures between 1200 and 1400.C. The reinforced with SiC fibres. The of a function precursor is converted by heating into a mullite gradient composite concept, i.e. the association of glass-ceramic with Al2O32SiO2 composition. Its interface and matrix precursors of various mclting temperature is very similar to that of to control the chemical Sian and anorthite between the sic Nicalon NLM202 fibre 2. A mixture of zirconium propor ZrP) Carbon Co. and the alkaline earth or alkali ions of tetraethoxyorthosilicate (TEOS) the matrix. Flexural strengths were obtained by osphate (TBP)(from Alfa- Ventron or from three-point bending tests at room temperature. Fluka)and TBB according to a 1: 3: 1-8: 1. 8 Corrosion mechanisms were studied by X-ray volume ratio(hereafter called ZrSiPB precur diffraction, infrared absorption, Raman micros- sor). This composition has been designed for pectroscopy and scanning electron microscopy. A ntering temperatures between 900 and 1200.C ough comparison of corrosion kinetics is made in The presence of a woven fabric makes sintering order to give a scale for the chemical stability of the between powder grains difficult: (i)the transmission of As, CAs, LaS and nas matrices the applied pressure is inhibited; (ii)the inert geometrically stable nature of the fibre array means that shrinkage necessarily causes crack formation 2 EXPERIMENTAL Furthermore, to deposit an item within the yarns i the fabric, through the voids between the fibres (a fer 2.1 Composite preparation micrometres or less in size) is very difficult. This Our preparation process for two-dimensional woven- possible by using liquid recurs fabric-reinforced composites has been given converted into gel(stage 1 of the process), and then elsewhere. The method consists of the impregnation on firing into (glass) ceramic(stage 3) with a yield of SiC fibres woven along four directions(90, 45)in which is necessarily low(=0.5). In the case of the plane to give a fabric about 1 mm thick(4 dir two-dimensional reinforcements this dilemma is fabric, surface mass 790 g/m2 ) The preparation takes solved by the use of a very reactive matrix precursor lace in three stages (stage 2)in combination with the interface precursor gel which gives rise to a temporary liquid sintering aid 1. Impregnation of the fibre yarns of the fabric (B2 Os rich liquid phase le temperature range with an interface(interphase)precursor: a liqu where matrix densification occurs. The liquid phase alkoxide mixture which slowly hydrolyses and contributes to the densification by mass transport polycondenses into a gel, in situ, by reaction (liquid assisted sintering) but also helps in lubricating with air the matrix-powder/fibre arrangement under pressure 2. Deposition of the fine amorphous and reactive and in maximizing the amount of contact between matrix precursor: a gel powder which has been grains of matrix and the interface precursor, despite heated to about 700C, in air, in order to the presence of the fibre network. The boron-rich remove most of the water and hydroxyl groups ph hase disappears by evaporation and dissolution in the and hence to reduce the shrinkage. This powder mullite matrix. The precursors used are free of is deposited onto the polymerized interface- alkaline earth and alkali ions which are able to react precursor-imprcgnated fabric in the form of strongly with the fibres. When the hot-pressing cycle is suspension in chlorobenzene with the addition over, the interface precursor gives rise to a region between the fibre and the matrix in which the concentration of corrosive ions is lowered. This wi 3. Hot-pressing of three (or five)impregnated and contribute to the acked fabrics in a graphite mould under vacuum(<400oC)and then under N2(1 atm) 2.2 Matrix preparation Two kinds of interface precursors(so-called because The compositions studied were BaAl2Si2 Os (pure the fibre/ matrix interface will result from the reaction celsian);(1-x)BaAl Si2OgrLi2O(=0-05, hereafter between the fibres and the precursor)may be used called BAS5%Li, x=0.1, BAS10%Li); CaAl2Si2O8740 Ph. Colornban, N. Lapous less drastic. Of course, a sodium aluminosilicate will not be sensitive to corrosion by Na$O, (fuel impurities) or NaCl (marine environment) molten salts. In the present paper, we present a sol-gel method for, and discuss the problems encountered in, the manufacturing of barium (BAS), calcium (CAS) and sodium (NAS) aluminosilicate-matrix composites reinforced with Sic fibres. The use of a function gradient composite concept,6 i.e. the association of interface and matrix precursors of various composi￾tions, allows us to control the chemical reaction between the Sic Nicalon NLM202 fibre (Nippon Carbon Co.) and the alkaline earth or alkali ions of the matrix. Flexural strengths were obtained by three-point bending tests at room temperature. Corrosion mechanisms were studied by X-ray diffraction, infrared absorption, Raman micros￾pectroscopy and scanning electron microscopy. A rough comparison of corrosion kinetics is made in order to give a scale for the chemical stability of the BAS, CAS, LAS and NAS matrices. 2 EXPERIMENTAL 2.1 Composite preparation Our preparation process for two-dimensional woven￾fabric-reinforced composites has been given elsewhere.7 The method consists of the impregnation of Sic fibres woven along four directions (90”, 45”) in the plane to give a fabric about 1 mm thick (4 dir fabric, surface mass 790g/m*). The preparation takes place in three stages: 1. 2. 3. Impregnation of the fibre yarns of the fabric with an interface (interphase) precursor: a liquid alkoxide mixture which slowly hydrolyses and polycondenses into a gel, in situ, by reaction with air moisture. Deposition of the fine amorphous and reactive matrix precursor: a gel powder which has been heated to about 7OO”C, in air, in order to remove most of the water and hydroxyl groups and hence to reduce the shrinkage. This powder is deposited onto the polymerized interface￾precursor-impregnated fabric in the form of a suspension in chlorobenzene with the addition of poly(methy1 methacrylate) (PMMA, 2-4% in weight). Hot-pressing of three (or five) impregnated and stacked fabrics in a graphite mould under vacuum ( < 400°C) and then under N2 (1 atm). Two kinds of interface precursors (so-called because the fibre/matrix interface will result from the reaction between the fibres and the precursor) may be used: 1. A mixture of tributylborate (TBB, from Alfa-Ventron) and of (OBu),-Al-0-Si(OEt), ester (SiAl, ref. 084, Dynasyl, from formerly Dynamit Nobel, now Hi.@ France) hereafter called TBB + SiAl (volume ratio 1:3). This composition has been designed for sintering temperatures between 1200 and 1400°C. The precursor is converted by heating into a mullite glass-ceramic with A12032Si02 composition. Its melting temperature is very similar to that of celsian and anorthite. 2. A mixture of zirconium propoxide (ZrP), tetraethoxyorthosilicate (TEOS), tributylph￾osphate (TBP) (from Alfa-Ventron or from Fluka) and TBB according to a 1:3:18:1*8 volume ratio (hereafter called ZrSiPB precur￾sor). This composition has been designed for sintering temperatures between 900 and 1200°C. The presence of a woven fabric makes sintering between powder grains difficult: (i) the transmission of the applied pressure is inhibited; (ii) the ‘inert’ geometrically stable nature of the fibre array means that shrinkage necessarily causes crack formation. Furthermore, to deposit an item within the yarns in the fabric, through the voids between the fibres (a few micrometres or less in size) is very difficult. This is possible by using liquid precursors which are converted into gel (stage 1 of the process), and then on firing into (glass) ceramic (stage 3) with a yield which is necessarily low (5 O-5). In the case of two-dimensional reinforcements this dilemma is solved by the use of a very reactive matrix precursor (stage 2) in combination with the interface precursor gel which gives rise to a temporary liquid sintering aid (B,O,-rich liquid phase) in the temperature range where matrix densification occurs. The liquid phase contributes to the densification by mass transport (liquid assisted sintering) but also helps in lubricating the matrix-powder/fibre arrangement under pressure and in maximizing the amount of contact between grains of matrix and the interface precursor, despite the presence of the fibre network. The boron-rich phase disappears by evaporation and dissolution in the mullite matrix. The precursors used are free of alkaline earth and alkali ions which are able to react strongly with the fibres. When the hot-pressing cycle is over, the interface precursor gives rise to a region between the fibre and the matrix in which the concentration of corrosive ions is lowered. This will contribute to the fibre protection. 2.2 Matrix preparation The compositions studied were BaAl,Si,O, (pure celsian); (1 - x)BaA12Si20sxLi20 (x = 0.05, hereafter called BASS%Li, x = 0.1, BASlO%Li); CaA12Siz0