Printed in Northern Ireland. All righ S0266·3538(96)00015·2 NEW SOL-GEL MATRICES OF CHEMICALLY STABLE COMPOSITES OF BAS. NAS AND CAS Ph Colomban N. lapous ONERA, Direction des materiaux, BP 72, 92322 Chatillon; and CNRS, LaSIR, 2 rue Henri Dunant, 94320 Thiais, france Received 22 November 1994; revised 31 March 1995; accepted 16 June 1995) Abstract (and Mg" )ion leaching due to the fast diffusion and The preparation of ceramic-matrix composites made ion exchange of small-sized cations in loosely packed with celsian(BAS), anorthite(CAS), and amorphous frameworks. Constriction or expansion, up to 10% of albite(NAS) matrices and reinforced with Nicalon@ the unit cell parameters, may result from ion NLM202 SiC woven fibers is reported. The method exchange, by smaller or larger ions, respectively. used to make composites is a three stage sol-gel Whereas the constriction resulting from the exchange process:(i) in situ gelation of a mixture of alkoxides in does not cause deterioration of the composite, the a ceramic fibre fabric; (ii) deposit of a matrix expansion of the matrix by a few percent leads to a precursor onto the impregnated fabrics; and (iii) total crumbling of the material. 4,In both cases, the hot-Pressing. Emphasis is given to the preparation of a composite becomes very sensitive to thermal cycling single-step.firing matrix powder leading to dense Hitherto, mechanical criteria have imposed the choice composites. Three-point flexural strengths have been of low-expansion matrices in order to keep the matrix determined. The matrix and fibre corrosion by proton compressed by the Sic fibre inside the composite and or sodium ions is discussed. The BAs matrix presen hence to prevent matrix microcracking. Unfortun od compromise between chemical, mechanical ately, low-expansion structures belong generally to a and manufacturing criteria. 1996 Elsevier Science loosely packed frameworks which also promote fast ion diffusion and ion exchange. The easy of molecular Keywords: ceramic-matrix composites, sol-gel, Nicalon SiO4, AlO4, AlO6, etc. )arising from the low fibre, aluminosilicates, corrosion compactness of the structure allows compensation of the thermal expansion of the chemical bond up to a phase transition. A large temperature domain in 1 INTRODUCTION which the material exhibits a low expansion coefficient can thus be present. Loosely packed frameworks consequently involve rather low mechanical strength At present, the compositions of matrices designed for and Youngs modulus, and hence composites made composites working at medium temperatures are with such matrices exhibit a low limit to their elastic lithium and magnesium aluminosilicates(LAS and behaviour. A large amount of microcracking MAS). The corrosion of these matrices by Na ions result and promote the corrosion On the other hand and protons'-3is rapid and these matrices cannot be dense frameworks (e. g. Al2O3)and, to a lesser degree used for long periods of time. The preparation of parts compositions free of mobile ions(e.g. mullite) exhibit for aircraft engines or electrical plant turbines requires larger thermal expansion coefficients, higher melting materials exhibiting lower corrosion rates. Aluminos tcmperaturcs and higher mechanical strengths but are licates exhibit many advantages, either from the not sensitive to ion leaching. Selection criteria may (compatibility with SiC fibre) and mechanical (low well as mechanical aspect. graphic and chemical as hermal expansion coefficient) points of view. Low Generally, the diffusion of alkali and alkaline earth n rate aluminosilicate matrices are thus highly ions of larger size and/or larger electrical charge is desirable. We report here a comparative study of the hindered. Furthermore, the substitution of Li corrosion of various aluminosilicate-matrix composites inforced with Nicalon NLM202 refractoriness and hence lowers the diffusion rate at a The main mechanism of corrosion for B-spodumene given temperature. Finally, the mechanical degrada (LAS)and cordierite(MAS)matrices arises from Li tion which results from ion exchange by smaller ions
Composites Science and Technology 56 (1996) 739-746 0 1996 Elsevier Science Limited ELSEVIER SO266-3538(96)00015-Z Printed in Northern Ireland. All rights reserved 0266-3538/96/$15.00 NEW SOL-GEL MATRICES OF COMPOSITES OF BAS, CHEMICALLY STABLE NAS AND CAS Ph. Colomban & N. Lapous ONERA, Direction des Materiaux, BP 72, 92322 Chatillon; and CNRS, LASIR, 2 rue Henri Dunant, 94320 Thiais, France (Received 22 November 1994; revised 31 March 1995; accepted 16 June 1995) Abstract The preparation of ceramic-matrix composites made with celsian @AS), anorthite (CAS), and amorphous albite (NAS) matrices and reinforced with Nicalon@ NLM.202 Sic woven fibers is reported. The method used to make composites is a three stage sol-gel process.* (i) in situ gelation of a mixture of alkoxides in a ceramic jibre fabric; (ii} deposit of a matrix precursor onto the impregnated fabrics; and (iii) hot-pressing. Emphasis is given to the preparation of a single-step-jiring matrix powder leading to dense composites. Three-point flexural strengths have been determined. The matrix and fibre corrosion by proton or sodium ions is discussed. The BAS matrix presents a good compromise between chemical, mechanical and manufacturing criteria. @ 1996 Elsevier Science Limited Keywords: ceramic-matrix composites, sol-gel, Nicalon fibre, aluminosilicates, corrosion 1 INTRODUCTION At present, the compositions of matrices designed for composites working at medium temperatures are lithium and magnesium aluminosilicates (LAS and MAS). The corrosion of these matrices by Na+ ions and protons’-3 is rapid and these matrices cannot be used for long periods of time. The preparation of parts for aircraft engines or electrical plant turbines requires materials exhibiting lower corrosion rates. Aluminosilicates exhibit many advantages, either from the manufacturing point of view or from the chemical (compatibility with Sic fibre) and mechanical (low thermal expansion coefficient) points of view. Low corrosion rate aluminosilicate matrices are thus highly desirable. We report here a comparative study of the corrosion of various aluminosilicate-matrix composites reinforced with Nicalon@ NLM202. The main mechanism of corrosion for /3-spodumene (LAS) and cordierite (MAS) matrices arises from Lit 739 (and Mg’+) ion leaching due to the fast diffusion and ion exchange of small-sized cations in loosely packed frameworks. Constriction or expansion, up to 10% of the unit cell parameters, may result from ion exchange, by smaller or larger ions, respectively. Whereas the constriction resulting from the exchange does not cause deterioration of the composite, the expansion of the matrix by a few percent leads to a total crumbling of the material.4*s In both cases, the composite becomes very sensitive to thermal cycling. Hitherto, mechanical criteria have imposed the choice of low-expansion matrices in order to keep the matrix compressed by the Sic fibre inside the composite and hence to prevent matrix microcracking. Unfortunately, low-expansion structures belong generally to a class of loosely packed frameworks which also promote fast ion diffusion and ion exchange. The easy rotation of molecular bricks of the framework (e.g. Si04, A104, A106, etc.) arising from the low compactness of the structure allows compensation of the thermal expansion of the chemical bond up to a phase transition. A large temperature domain in which the material exhibits a low expansion coefficient can thus be present. Loosely packed frameworks consequently involve rather low mechanical strength and Young’s modulus, and hence composites made with such matrices exhibit a low limit to their elastic behaviour. A large amount of microcracking will result and promote the corrosion. On the other hand, dense frameworks (e.g. A1203) and, to a lesser degree, compositions free of mobile ions (e.g. mullite) exhibit larger thermal expansion coefficients, higher melting temperatures and higher mechanical strengths but are not sensitive to ion leaching. Selection criteria may take into account crystallographic and chemical as well as mechanical aspects. Generally, the diffusion of alkali and alkaline earth ions of larger size and/or larger electrical charge is hindered. Furthermore, the substitution of Li+ and Mg2+ ions by Ba2+ or Ca2+ ions increases the refractoriness and hence lowers the diffusion rate at a given temperature. Finally, the mechanical degradation which results from ion exchange by smaller ions is
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); CaAl2Si2O8
740 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 compositions, 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 microspectroscopy 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 wovenfabric-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 interfaceprecursor-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), tributylphosphate (TBP) (from Alfa-Ventron or from Fluka) and TBB according to a 1:3:18:1*8 volume ratio (hereafter called ZrSiPB precursor). 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
Sol-gel matrices of BAs, NAS and CAs 741 (anorthite, CAS); NaAISi3O(albite, NAS); and a (350C)and NaCl (950C) for various times between mixed CNAS composition (7CAS+ 3NAS). Gel 6 h powders were prepared by instant hydrolysis of a mixture of alkoxides diluted in 2-propanol(volume 3 RESULTS AND DISCUSSION ratio 1: 20) by an aqueous solution and dispersion of alkali and alkaline earth reagents. Silicon and 3.1 Densification and mechanical properties aluminium were introduced by using TEOS and Figure 1 compares the plots of shrinkage versus aluminium-s-butoxide, respectively. Sodium is intro- temperatur of the different matrices: an initial duced by using NaOH solution (and lithium doping shrinkage occurs below 300C and is associated with from LiNO, solution) whereas barium and calcium orous glass (or xerogel) were introduced by using a milky dispersion of Cao transformation. The main shrinkage takes place and Bao powder in CO2-free water. A large excess of the dehydroxylation-(nucleation)-densification water (volume ratio of water:alkoxide= 20: 1) was tion at about 650C (NAS),800C(CNAS), used, and there was vigorous mixing during and after(BAS5 %Li and CAS)and 900C(BAS). A strong the hydrolysis. Alternatively, the use of alkaline earth viscous flow is observed for the low refractory NAs nitrates or salicylates dissolved in the alkoxide- and CNAs compositions. The lowering of the propanol mixture leads to gels exhibiting shrinkage densification temperature arising from the use of behaviour(e. g. expansion after a first shrinkage step, sol-gel precursor is thus not significant for NAS and related to the dehydroxylation-nucleation reaction) CNAS compositions. On the other hand, CAS and which are not compatible with a good densification. BAS fired compositions which are geometrically stable The addition of small amounts of lithium is however up to about 1500C seem to be, a priori, the most possible by the use of a three-liquid mixture(alcoholic suitable matrices for the preparation of low solution, base aqueous solution, lithium nitrate temperature-sinterable refractory composites aqueous solution) The highest mechanical strength was measured for leads to a gel dispersion in water. The BAS-matrix composites. The ultimate three-point water and alcohol were removed by drying under Ir flexural strength reaches 250 MPa for composites bulbs. We obtained a fine white, submicronic, exhibiting open porosity close to 5%(Table 1). This amorphous gel powder which was fired for 2 h at value is higher than the mechanical strength of 700C in air. The resulting powder remained amorphous and mesoporous. By heating above hexagonal celsian(BAS), monoclinic celsian (BASS %Li, BAS10%Li), anorthite (CAS, CNAS)and glass(NAS)were obtained, respectively 2.3 Techniques The shrinkage measurements were made by using an BAS Adamel Lhormergy DI24 apparatus(Instrument SA) with alumina rod and support. Curves were drawn in BAS coLi air at heating rate of 5C/min. Fracture surfaces and sliced or polished sections of the composites were observed in a 200 kV Cambridge scanning electron microscope. Flexural strengths were measured by CAS three-point bending tests on bar specimens(35 mm in length)at a cross-head speed of 0.1 mm/min at room temperature. Typically, three samples were broken for each composite and the mean value is given. X-ray infraction powder patterns were recorded on CNAS powdered samples. Micro-Raman spectra wer orded at the 514. 5 nm exciting wavelength of an Ar laser with an XY Dilor multichannel microprobe NAS equipped with a liquid nitrogen cooled Wright CCD (1 300)array measured by the Archimedean water impregnation thod 1. Linear of BaAl2Si2O8 (BAS), Chemical attacks were performed on 0.,o. 0-05 S5%Li), CaAl Si2O8(CAS) sections of composites immersed in boiling (CNAS) and NaAlSi,O (NAS) gels versus are(at room temperature, gels trated sulphuric acid (340C) or in molten contain about 40 wt% water)
Sol-gel matrices of BAS, NAS and CAS 741 (anorthite, CAS); NaAlS&Oa (albite, NAS); and a mixed CNAS composition (7CAS + 3NAS). Gel powders were prepared by instant hydrolysis of a mixture of alkoxides diluted in 2-propanol (volume ratio 1:20) by an aqueous solution and dispersion of alkali and alkaline earth reagents. Silicon and aluminium were introduced by using TEOS and aluminium-s-butoxide, respectively. Sodium is introduced by using NaOH solution (and lithium doping from LiN03 solution) whereas barium and calcium were introduced by using a milky dispersion of CaO and BaO powder in COz-free water. A large excess of water (volume ratio of water:alkoxide = 2O:l) was used, and there was vigorous mixing during and after the hydrolysis. Alternatively, the use of alkaline earth nitrates or salicylates dissolved in the alkoxidepropanol mixture leads to gels exhibiting shrinkage behaviour (e.g. expansion after a first shrinkage step, related to the dehydroxylation-nucleation reaction) which are not compatible with a good densification. The addition of small amounts of lithium is however possible by the use of a three-liquid mixture (alcoholic solution, base aqueous solution, lithium nitrate aqueous solution). The process leads to a gel dispersion in water. The water and alcohol were removed by drying under IR bulbs. We obtained a fine white, submicronic, amorphous gel powder which was fired for 2 h at 700°C in air. The resulting powder remained amorphous and mesoporous. By heating above lOOO”C, in air, hexagonal celsian (BAS), monoclinic celsian (BASS%Li, BASlO%Li), anorthite (CAS, CNAS) and glass (NAS) were obtained, respectively. 2.3 Techniques The shrinkage measurements were made by using an Adamel Lhormergy D124 apparatus (Instrument SA) with alumina rod and support. Curves were drawn in air at heating rate of S”C/min. Fracture surfaces and sliced or polished sections of the composites were observed in a 200 kV Cambridge scanning electron microscope. Flexural strengths were measured by three-point bending tests on bar specimens (35 mm in length) at a cross-head speed of O-1 mm/min at room temperature. Typically, three samples were broken for each composite and the mean value is given. X-ray diffraction powder patterns were recorded on powdered samples. Micro-Raman spectra were recorded at the 514.5 nm exciting wavelength of an Ar+ laser with an XY Dilor multichannel microprobe equipped with a liquid nitrogen cooled Wright CCD (1200-300) array detector. The open porosity was measured by the Archimedean water impregnation method. Chemical attacks were performed on polished sections of composites immersed in boiling concentrated sulphuric acid (340°C) or in molten NaNO, (350°C) and NaCl (950°C) for various times between 15 min and 6 h. 3 RESULTS AND DISCUSSION 3.1 Densikation and mechanical properties Figure 1 compares the plots of shrinkage versus temperature of the different matrices: an initial shrinkage occurs below 300°C and is associated with the (aqua) gel to mesoporous glass (or xerogel) transformation. The main shrinkage takes place with the dehydroxylation-(nucleation)-densification reaction at about 650°C (NAS), 800°C (CNAS), 850°C (BASS%Li and CAS) and 900°C (BAS). A strong viscous flow is observed for the low refractory NAS and CNAS compositions. The lowering of the densification temperature arising from the use of sol-gel precursor is thus not significant for NAS and CNAS compositions. On the other hand, CAS and BAS fired compositions which are geometrically stable up to about 1500°C seem to be, a priori, the most suitable matrices for the preparation of lowtemperature-sinterable refractory composites. The highest mechanical strength was measured for BAS-matrix composites. The ultimate three-point flexural strength reaches 250MPa for composites exhibiting open porosity close to 5% (Table 1). This value is higher than the mechanical strength of I I I I I%* 1. Linear shrinkage of BaA1,Si,OB (BAS), 0.95BaAl,Si,O, O.OSLi,O (BASS%Li), CaAl&O, (CAS), i 0.7CaAl,OSi,O, 0*3NaAlSi,O, (CNAS) and NaAlSi,O, (NAS) gels versus temperature (at room temperature, gels contain about 40 wt% water)
742 Ph Colomban, N. lapous Table 1. Main parameters for the preparation of BAS, CAS, NAS and CNAS matrix composites and resulting physical properties Matrix nterface Open porosity Fibre volume Three-Dos x fraction flexual streng Temp.C°C)Dwel(h) (MPa) CAS TBB SiAl 1250 035 ZrSiPB 030 ZrSiPB 7CAS-3NAS ZrSiPB 1050 030 BAS TBB+ SiAl 0·30 l250 8 030 BASS %L TBB+SiAl 85 033 TBB+ SiA 3 033 BAS10%Li TBB SiAl 124 03 BAS10%LI ZrSiPB 1200 035 250 Symbols explained in the text. Hot-pressing at 20 MPa Not measured corresponding monolithic BAS ceramics. At this level chosen in preference. An electron microscope study of of densification, the observed mechanical properties the fibre matrix interface and of its ageing will be are higher than those of LAs glass-ceramic- required to determine the most suitable interface matrix/SiC-fibre composites (<180 MPa) prepared precursor. However, the evaporation of boron during with the same fabric but by the usual molten glass the hot-pressing cycle occurs more readily than that of process. The mechanical properties of CAS and NAs phosphorus and hence the use of the ZrSiPB interface matrix composites are lower, despite a rather similar precursor may lower the refractoriness of the open porosity(Table 1). composites and consequently reduce their use above he results are not very sensitive to the interface precursor used (TBB+ SiAl or ZrSiPB). However, Figure 2 shows the good impregnation of the fabric previous studies have shown that the use of ZrSiPb voids by the ceramic. Evidence for the dissipative precursor promotes the formation of a thick behaviour of the fracture is shown in Fig 3. The use the Sic Nicalon NLM202 fibre by alkali or alkaline of zirconia which recursors leads to precipitatio carbon rich interphase as a result of the attack of of ZrSiPB interface appears white on electron earth ions. The presence of a carbon-rich interphase photomicrographs(Fig. 2, composites 2 and 3). The may promote oxidation and the sial precursor may be precipitation results from the reaction between Ba matrix), 2(CAS matrix)and 3(NAS matrix)(mean fibre diameter=11 um). See Table 1 for preparation details, AS Fig. 2. Sliced sections of Sic composites of Nicalon Sic fibre in aluminosilicate Sic Nicalon matrices; composites 5(B
742 Ph. Colomban, N. Lapous Table 1. Main parameters for the preparation of BAS, CAS, NAS and CNAS matrix composites and resulting physical properties Composite number Matrix” Interface precursor Sinteringb Open porosity Fibre volume Three-point (%) fraction Temp. (“C) Dwell (h) flexual strenght (MPa) 1 CAS TBB + SiAl 12.50 1 6 0.35 _c 2 CAS ZrSiPB 1200 0.5 7 o-30 -100 3 NAS ZrSiPB 1050 0.5 8 0.30 -70 4 7CAS-3NAS ZrSiPB 1050 1 6 0.30 _= 2 BAS TBB ZrSiPB + SiAl 1250175 1 5.5 8 0.30 200 0.30 120 7 BASS%Li TBB + SiAl 1150 1 8.5 0.33 180 8 BASS%Li TBB + SiAl 1250 1 3 0.33 250 9 BASlO%Li TBB + SiAl 1250 1 4 0.35 _= 10 BASlO%Li ZrSiPB 1200 1 6 0.35 250 a Symbols explained in the text, b Hot-pressing at 20 MPa. ’ Not measured. corresponding monolithic BAS ceramics. At this level of densification, the observed mechanical properties are higher than those of LAS glass-ceramicmatrix/Sic-fibre composites ( < 180 MPa) prepared with the same fabric but by the usual molten glass process. The mechanical properties of CAS and NAS matrix composites are lower, despite a rather similar open porosity (Table 1). The results are not very sensitive to the interface precursor used (TBB + SiAl or ZrSiPB). However, previous studies have shown that the use of ZrSiPB precursor promotes the formation of a thick carbon-rich interphases,9 as a result of the attack of the Sic Nicalon NLM202 fibre by alkali or alkaline earth ions. The presence of a carbon-rich interphase may promote oxidation and the SiAl precursor may be chosen in preference. An electron microscope study of the fibre matrix interface and of its ageing will be required to determine the most suitable interface precursor. However, the evaporation of boron during the hot-pressing cycle occurs more readily than that of phosphorus and hence the use of the ZrSiPB interface precursor may lower the refractoriness of the composites and consequently reduce their use above 700°C. Figure 2 shows the good impregnation of the fabric voids by the ceramic. Evidence for the dissipative behaviour of the fracture is shown in Fig. 3. The use of ZrSiPB interface precursors leads to precipitation of zirconia which appears white on electron photomicrographs (Fig. 2, composites 2 and 3). The precipitation results from the reaction between Ba2+ Fig. 2. Sliced sections of Sic composites c )f Nicalon SIC fibre in aluminosihcate Sic Nicalon matrices; composites 1 aatrix), 2 (CAS matrix) and 3 (NAS matrix) (mean fibre diameter = 11 pm). See Table 1 for preparation detail (BAS
ol-gel matrices of BAS, NAs and CAS 743 250 MPa l=3.33 JLLL LLLLL Strain (arbitrary units Fig 3. Stress/strain plots for composites(a)5(BAS matrix),(b )10(BAS10%Li matrix)and (c)interslab polished section of composite 6(BAS matrix). See Table 1 for details (or Ca2+) ions of the matrix with the zircono- the CAs and NAS matrix composi able 1) silicophosphate glass issued from the pyrolysis of the although the preparation cycle (temperature cycle ZrSiPB precursor. We therefore have a multilevel dwell, relative volume of fibre and of interface and reinforced composite(fibre- and particle-reinforced matrix precursors, etc. )was not optimized. However previous studies have shown that alkaline earth and Composite 5 was made with a BAS matrix free of phosphorus ions react with the Sic fibre. This reaction lithium addition but exhibiting a large amount of promotes the formation of a carbon-rich fibre/matrix celsian with monoclinic symmetry. Usually the interphase which leads to a sliding interface, but too monoclinic celsian is obtained by lithium addition or large an attack lowers the safe fibre cross-section and after long annealing at high tcmpcraturc. 5. 10 Thc hence the mechanical properties. Optimization of the achievement of the monoclinic phase in this composite relative proportion of each precursor is required. The not due to the reducing atmosphere related to the SiAl interface precursor seems to preserve the presence of the graphite felts and resistor in the oven fibre 4, I>although a higher temperature is needed for and to contact with the Sic fibre but may be related to sintering the low sintering temperature (matrix of the BAs composite 6 hot-pressed at 1250C is hexagonal celsian) or to the use of the TBB+ SiAl interphase 3.2 Acid and alkaline corrosion precursor Table 1). The section polished parallel to the fabric (interlab 3.2.1 Celsian(BAS) matrix polished section of composite 6, Fig 3(c)shows a Figure 4 compares the microstructures of polished regular array of cracks. This phenomenon may be sections of composites after attack by boiling sulphuric related to the expansion mismatch between the Sic acid or molten sodium nitrate. Cracks at the grain fabric (--3 5X10/c) and hexacelsian (8 boundary and a darkening of the grain limit are 10"/C)expansion coefficients, and can be compared observed for BAS matrices. The origin of this to the value of the monoclinic phase expansion phenomenon can be found in the fast attack of the coefficient(2. 3 x 10-7/ C). 12 The rather large expan- grain boundary second phase or in the grain sion coefficient of anorthite(6X10/'C)seems to constriction associated with ion exchange. Comparison be compatible with that of SiC fibre since no cracks of Raman spectra recorded on various grains by the are observed. On the other hand, a crack array was Raman microprobe (analysed area -3 um X 3 um) found in the NAS-matrix composite(3). Note that the clearly shows the band-broadening characteristic of a thermal expansion coefficients of pyrolysed SiAl and loss of crystallinity and a strong reduction in the ZrSiPB interface precursors are very similar to that of intensity of the 110 cm band assigned to the iC fibre. 11 The mismatch between the thermal translational motion of Ba2+ ions in monoclinic celsian expansion coefficients of SiC fibre and NAS and CAs ( Fig. 5). This indicates leaching of Ba ions. A atrix may explain the lower mechanical properties of longer duration of the chemical attack leads to a more
Sol-gel matrices of BAS, NAS and CAS 743 A 200MPa a) BO - 2 ,111 111111111) Strain (a rbitrary unita) & Fig. 3. Stress/strain plots for composites (a) 5 (BAS matrix), (b) 10 (BASlO%Li matrix) and (c) interslab polished section of composite 6 (BAS matrix). See Table 1 for details. A io- 250 MPa IO - (or Ca*+) ions of the matrix with the zirconosilicophosphate glass issued from the pyrolysis of the ZrSiPB precursor. We therefore have a multilevel reinforced composite (fibre- and particle-reinforced composite). Composite 5 was made with a BAS matrix free of lithium addition but exhibiting a large amount of celsian with monoclinic symmetry. Usually the monoclinic celsian is obtained by lithium addition or after long annealing at high temperature.5,10 The achievement of the monoclinic phase in this composite is not due to the reducing atmosphere related to the presence of the graphite felts and resistor in the oven and to contact with the Sic fibre but may be related to the low sintering temperature (matrix of the BAS composite 6 hot-pressed at 1250°C is hexagonal celsian) or to the use of the TBB + SiAl interphase precursor (Table 1). The section polished parallel to the fabric (interlab polished section of composite 6, Fig. 3(c)) shows a regular array of cracks. This phenomenon may be related to the expansion mismatch between the SIC fabric ( - 35 X 10-7/“C)‘1 and hexacelsian (8 X lo-‘/“C) expansion coefficients, and can be compared to the value of the monoclinic phase expansion coefficient (2.3 X 10-7/“C).12 The rather large expansion coefficient of anorthite (6 X 10-7/“C)13 seems to be compatible with that of Sic fibre since no cracks are observed. On the other hand, a crack array was found in the NAS-matrix composite (3). Note that the thermal expansion coefficients of pyrolysed SiAl and ZrSiPB interface precursors are very similar to that of Sic fibre.” The mismatch between the thermal expansion coefficients of SIC fibre and NAS and CAS matrix may explain the lower mechanical properties of the CAS and NAS matrix composites (Table l), although the preparation cycle (temperature cycle, dwell, relative volume of fibre and of interface and matrix precursors, etc.) was not optimized. However, previous studie@’ have shown that alkaline earth and phosphorus ions react with the Sic fibre. This reaction promotes the formation of a carbon-rich fibre/matrix interphase which leads to a sliding interface,’ but too large an attack lowers the safe fibre cross-section and hence the mechanical properties.’ Optimization of the relative proportion of each precursor is required. The SiAl interface precursor seems to preserve the fibre14,15 although a higher temperature is needed for sintering.ll 3.2 Acid and alkaline corrosion 3.2.1 Celsian (BAS) matrix Figure 4 compares the microstructures of polished sections of composites after attack by boiling sulphuric acid or molten sodium nitrate. Cracks at the grain boundary and a darkening of the grain limit are observed for BAS matrices. The origin of this phenomenon can be found in the fast attack of the grain boundary second phase or in the grain constriction associated with ion exchange. Comparison of Raman spectra recorded on various grains by the Raman microprobe (analysed area -3 pm X 3 pm) clearly shows the band-broadening characteristic of a loss of crystallinity and a strong reduction in the intensity of the llOcm-’ band assigned to the translational motion of Ba” ions in monoclinic celsian (Fig. 5).5 This indicates leaching of Ba” ions. A longer duration of the chemical attack leads to a more
744 Ph Colomban, N. lapous uniform habit and the detachment of some grains is corrosion is straightforward and the platelet habit is observed. The corrosion rate is faster for hexag clearly visible at the limit between the pyrolysed celsian and especially for lithium-containing matrices ZrSiPB interface precursor and the matrix (monoclinic celsian, see Fig. 4(a)). The anisotropic Typical microstructure char ges induce Fig. 4. Polished sections of BAS-matrix composites after attack for: (a)15 min(5);(a )30 min(5);(a)30 min(10)by boiling sulphuric acid. (c) Polished sections of CAS-matrix composite after attack by sulphuric acid for 90 min site 2. interslab section). The corrosion after chemical attack by a NaNO, melt is shown in(b) composite 5, BAs matrix NAS matrix, 6 h. See Table 1 for preparation details
744 Ph. Colomban, N. Lapous unifo am habit and the detachment of some grains is obser -ved. The corrosion rate is faster for hexagonal celsia m and especially for lithium-containing matrices (mon loclinic celsian, see Fig. 4(a”)). The anisotropic corrosion is straightforward and the platelet habit is clearly visible at the limit between the pyrolysed ZrSiPB interface precursor and the matrix. Typical microstructure changes induced by the Fig. 4. Polished sections of BAS-matrix composites after attack for: (a) 1.5 min (5); (a’) 30 min (5); (a”) 30 min (10) by boiling sulphuric acid. (c) Polished sections of CAS-matrix composite after attack by sulphuric acid for 90 min (composite 2, interslab section). The corrosion after chemical attack by a NaNO, melt is shown in (b) composite 5, BAS matrix, 6 h; (d) composite 3, NAS matrix, 6 h. See Table 1 for preparation details
Sol-gel matrices of Bas, NAS and CAS 745 and in the resulting low diffusion rate of Ba ions from the matrix to the siC fibre related to the use of barium-free interface precursors as diffusion barriers The stability of the BAS matrix relative to protons and sodium ions is rather god heing similar to that of amorphous l As but CAs and NAS matrices exhibit a better chemical stability. This behaviour may be related to the presence of an amorphous phase in these last matrices The Sic Nicalon NLM202 fibre is more rapidly b) corroded by sodium ions than the matrices studied in this work The bas matrix seems to present a good compromise expansion coefficient, high mechanical strength and 00100 Youngs modulus), the chemical criteria (low corro sion rate by proton and sodium ions, high melting Fig. 5. Raman spectrum recorded on a monoclinic temperature)and the manufacture criteria(low celsian-matrix grain (composite 5)(a) before and(b) after sintering temperature, easy handling of the sulphuric acid attack for 15 min precursors) sodium nitrate attack are shown in Fig. 4(b). In REFERENCES cases a strong chemical attack of the SiC Nicalon NLM202 fibre is observed. whatever the matrix. The phenomenon is more drastic when using an NaCl ceramics by sodium sulphate at 1000'C. J. Mater. Sci. melt. It can be assumed that there is a diffusion of 2. Kim, H. E.& Moorhead, A. J, Effect of hydrogen alkali ions into the fibre during the hot-pressing, as ater atmospheres on corrosion and flexural strength of previously observed at high temperature with sintered a-silicon carbide J. Am. Ceram. Soc., 73 (1990 sodium-rich NASICON matrix and that this diffusion 94-699 promotes the chemical attack by sodium ions of the 3. Scanu, T.& Colomban Corrosion of ceran nitrate melt. However, the attack is more important matrix composites. J. Phys. IV, C7(1993)1927-1930. 4. Lenfant, P, Plas, D, Ruffo, M, boilot, P. in the inner region of the fibre between the periphery Colomban, Ph Ceramiqucs d'aluminc p ct de ferrite B and the core than at the fibre surface(Fig. 4(d)).A pour sonde a protons, Mater. Res. Bull., 15(1980) small attack also seems to occur in the fired interface 1817-1827 precursor near the fibre periphery. 5. Scanu,T, Guglielmi, J& Colomban, Ph, lon exchange and hot corrosion of ceramic composite matrix:A vibrational and microstructural study. Solid State Ionics 3.2.2 Anorthite(CAS) and amorphous albite(NAS) 70-71(1994)109-120 No chemical attack was found with these matrices 6. Colomban, Ph, Composites ceramique multiniveaux ou after 15 min in boiling sulphuric acid small cracks are Tinteret des methodes sol-gel. Compte-Rendus des only observed in the pyrolysed ZrSiPB precursor in 8emes Journees Nationales sur les Composites (NC-8 CAS-matrix composites, as observed for LAS-matrix ed. o. Allix, J. P. Favre P. Ladeveze. AMAC. Paris composites. Cracks are evident after 90 min in boilingColomban,Ph,Menet,M,Mouchon,E,Cour sulphuric acid for the CAS matrix, which may be temanchc C.& Parlier, M, Composites ceramique proof of a partial Ca/H ion exchange and ique multicouches elabores en utilisant un associated constriction seur d interface et un precurseur de matrice ONERA Fr2672383 EP92002355, 07/830904,199 4 CONCLUSION 8. Mouchon, E, Lagrange, J. L. &Colomban,Ph Composites SiC Nicalon/matrice Nasicon: Illustration Almost fully dense aluminosilicate-matrix composites u role des ions alcalins sur la formation d'une interface have been prepared by a sol-gel route by using lissant. Compte-Rendus des &emes Journees Nati tailored polymeric interface precursors compatible nales sur les Composites (NC-8), ed. O. Allix, J.P. Favre P Ladeveze. AMAC, Paris, 1992, pp 85-96 with SiC Nicalon fibres. Mechanical properties are 9. Mouchon, E. Colomban, Ph, Origin of the carbon ery similar to those of composites prepared by the rich sliding interface in alkali containing matrix-SiC injection of a LAs melt. The highest mechanical n nbrc composite.购,,C70(9 strength was obtained by using a monoclinic celsian matrix. The origin of this phenomenon can be found 10. Chen, M, Lee, W.E.& James, P. F, Synthesis of monoclinic celsian glass-ceramic from alkoxides n the rather dense structure of monoclinic celsian Non-Crysi. Solids,147-148(1992)532-536
Sol-gel matrices of BAS. NAS and CAS 745 800 700 600 500 400 300 200 100 Wavenumbers (cm-l) Fig. 5. Raman spectrum recorded on a monoclinic celsian-matrix grain (composite 5) (a) before and (b) after sulphuric acid attack for 15 min. sodium nitrate attack are shown in Fig. 4(b). In all cases a strong chemical attack of the Sic Nicalon NLM202 fibre is observed, whatever the matrix. The phenomenon is more drastic when using an NaCl melt. It can be assumed that there is a diffusion of alkali ions into the fibre during the hot-pressing, as previously observed at high temperature with sodium-rich NASICON matrix’ and that this diffusion promotes the chemical attack by sodium ions of the nitrate melt.16 However, the attack is more important in the inner region of the fibre between the periphery and the core than at the fibre surface (Fig. 4(d)). A small attack also seems to occur in the fired interface precursor near the fibre periphery. 3.2.2 Anorthite (CAS) and amorphous albite (NAS) No chemical attack was found with these matrices after 15 min in boiling sulphuric acid: small cracks are only observed in the pyrolysed ZrSiPB precursor in CAS-matrix composites, as observed for LAS-matrix composites.5 Cracks are evident after 90 min in boiling sulphuric acid for the CAS matrix, which may be proof of a partial Ca’+/H+ ion exchange and associated constriction. 4 CONCLUSION Almost fully dense aluminosilicate-matrix composites have been prepared by a sol-gel route by using tailored polymeric interface precursors compatible with Sic Nicalon fibres. Mechanical properties are very similar to those of composites prepared by the injection of a LAS melt. The highest mechanical strength was obtained by using a monoclinic celsian matrix. The origin of this phenomenon can be found in the rather dense structure of monoclinic celsian, and in the resulting low diffusion rate of Ba2+ ions from the matrix to the Sic fibre related to the use of barium-free interface precursors as diffusion barriers. The stability of the BAS matrix relative to protons and sodium ions is rather good, the corrosion rate being similar to that of amorphous LAS’ but CAS and NAS matrices exhibit a better chemical stability. This behaviour may be related to the presence of an amorphous phase in these last matrices. The Sic Nicalon NLM202 fibre is more rapidly corroded by sodium ions than the matrices studied in this work. The BAS matrix seems to present a good compromise between the mechanical criteria (low expansion coefficient, high mechanical strength and Young’s modulus), the chemical criteria (low corrosion rate by proton and sodium ions, high melting temperature) and the manufacture criteria (low sintering temperature, easy handling of the precursors). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Bianco, R. & Jacobson, N., Corrosion of cordierite ceramics by sodium sulphate at 1000°C. J. Muter. Sk., 24 (1989) 2903-2910. Kim, H. E. & Moorhead, A. J., Effect of hydrogenwater atmospheres on corrosion and flexural strength of sintered a-silicon carbide. J. Am. Ceram. Sot., 73 (1990) 694-699. Scanu, T. & Colomban, Ph., Corrosion of ceramic matrix composites. J. Phys. IV, C7 (1993) 1927-1930. Lenfant, P., Plas, D., Ruffo, M., Boilot, J. P. & Colomban, Ph., Ceramiques d’alumine 0 et de ferrite p pour sonde a protons. Mater. Rex Bull., 15 (1980) 1817-1827. Scanu, T., Guglielmi, J. & Colomban, Ph., Ion exchange and hot corrosion of ceramic composite matrix: A vibrational and microstructural study. Solid State Zonics, 70-71(1994) 109-120. Colomban, Ph., Composites ceramiques multiniveaux ou l’intCr&t des methodes sol-gel. Compte-Rendus des 8smes JournCes Nationales sur les Composites (JNC-8), ed. 0. Allix, J. P. Favre & P. Ladeveze. AMAC, Paris, 1992, pp. 73-84. Colomban, Ph., Menet, M., Mouchon, E., Courtemanche C. & Parlier, M., Composites ceramiqueceramique multicouches Clabores en utilisant un precurseur d’interface et un precurseur de matrice. Brevets ONERA Fr2672383, EP9200235.5, USO7/830904,1991. Mouchon, E., Lagrange, J. L. & Colomban, Ph., Composites Sic Nicalon/matrice Nasicon: Illustration du role des ions alcalins sur la formation d’une interface glissante. Compte-Rendus des 8smes Journe’es Nationales sur les Composites (JNC-8), ed. 0. Allix, J. P. Favre & P. Ladeveze. AMAC, Paris, 1992, pp. 85-96. Mouchon, E. & Colomban, Ph., Origin of the carbon rich sliding interface in alkali containing matrix-sic Nicalon fibre composites. J. Phys. N, C7 (1993) 1941-1944. Chen, M., Lee, W. E. & James, P. F., Synthesis of monoclinic celsian glass-ceramic from alkoxides. J. Non-Cryst. Solids, 147-148 (1992) 532-536
746 Ph. Colomban, N. Lapous 11. Mouchon, E, Composites ceramique a matrice oxyde through a sol-gel ature cerar renforcee par des fibres longues tissees, a proprietes matrix composites. Proc. HTCM ACM ed R. thermiques et electromagnetiques specifiques: Interet Lamon D. Doumeingts. Woodhead d'une voie sol-gel. Thesis, Universite Paris VI, 1993 Publishers, Cambridge, 1993, Pp. 159-166 12. Moya Corral, J. S& Garcia Verduch, A, Silica-celsian 15. Bruneton, E, Culomban, Ph. Michel, D, Carbonl-free solid solution. Trans. J. Br. Ceram. Soc., 77(1978) sliding interface in sol-gel processed Sic Nicalon fiber 40-44. refractory oxide matrix composites. J. Phys. IV, C7 13. Reade, R. F, Anorthite glass-ceramics, US Patent (1993)1937-1940 4187.115,5 February1980 16. Colomban, Ph, Ec 14. Colomban, Ph. Mouchon, E, Processing and effects SiC-matrice silicate et corrosion haute temperature des of zirconia precipitates and(Ge)zirco mullite matrix-SiC woven fabric composites prepared Industries. ceramique et vitroceramique. Silicates
746 Ph. Colomban, N. Lapous 11. Mouchon, E., Composites dramiques 21 matrice oxyde renforde par des fibres longues tissees, a proprietes thermiques et ClectromagnCtiques specifiques: Inter&t d’une voie sol-gel. Thesis, Universitd Paris VI, 1993. 12. Moya Corral, J. S. & Garcia Verduch, A., Silica-celsian solid solution. Trans. J. Br. Ceram. Sot., 77 (1978) 40-44. 13. Reade, R. F., Anorthite glass-ceramics, US Patent 4.187.115, 5 February 1980. 14. Colomban, Ph. & Mouchon, E., Processing and effects of zirconia precipitates and (Ge) zirconia interphase in mullite matrix-sic woven fabric composites prepared through a sol-gel route in high temperature ceramic matrix composites. Proc. HTCMCI, 6th EACM, ed. R. Naslain, J. Lamon & D. Doumeingts. Woodhead Publishers, Cambridge, 1993, pp. 159-166. 15. Bruneton, E., Colomban, Ph. & Michel, D., Carbon-free sliding interface in sol-gel processed Sic Nicalon fiber refractory oxide matrix composites. J. Phys. IV, C7 (1993) 1937-1940. 16. Colomban, Ph., Echanges ioniques, reactions fibre Sic-matrice silicatee et corrosion haute temperature des composites ceramique et vitrodramique. Silicates Industriels (in press)