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 ionsComposites 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. Aluminosi￾licates 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. Unfortun￾ately, 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 degrada￾tion which results from ion exchange by smaller ions is