M.K. Naskar et al. /Ceramics International 35(2009)3073-3079 chemistry and thermal stability [2, 15]. The use of sintering aid 8-9. The precipitate was washed with deionized water to free also influenced the mechanical strength of the CMC [16]. For foreign ions e.g., Cl, NH4 etc. followed by peptization with the development of a high quality CMC, matrix properties are glacial acetic acid(99 8%0, AnalaR, bdh, mumbai, India)at lso to be taken into account. Important characteristics of the 65+1C. a required amount of Y(NO3)3(6 mol %o equivalent matrix include thermal compatibility with the fibres and fibre- Y203)solution was added to the above peptized zirconia sol matrix interface reaction. The strength and toughness of CMc under stirring for 15 min at rt (30C)to prepare ZY sol are primarily governed by the interfacial bonding at the fibre/(Fig. 1) matrix interface [1, 15]. Interfacial strength can be influenced For the preparation of Az sol of composition oy a number of factors, most important being degree of bonding 87Al2O3 13ZrO2(in equivalent mole), required amount of between the fibre and the matrix and their difference in thermal Al(NO3)3 solution(1.5 M)was mixed with required amount of expansion behaviour. ZrOCl2 solution(1. 5 M)under stirring for 15 min An aqueous Crystallization behaviour i. e, the development of crystalline ammonia solution(25 wt %, G.R. Merck, Mumbai, India)was phases in the composites with sintering temperatures has an added to the above mixed solution containing Al and zr important role for the characteristics of the CMCs. In the under stirring at 80+1C in a covered container(Fig. 1). In present investigation, a sol-gel vacuum infiltration route was this case the adjustment of pH at around 3 is the crucial step for followed for the preparation of NNS ceramic fibre reinforced stabilizing of AZ sol ceramic matrix composites using various sols as the infiltrates. For the preparation of alumina sol (A), ammonia solution The effect of crystallization in the composites towards their (25 wt. G.R. Merck, Mumbai, India) was slowly added to characteristics i.e., mechanical strength, pseudo-ductility and 1.5 M of Al(NO3)3 solution at 80+ lC under stirring in a hence the nature of fibre-matrix interface has been studied in covered container. The pH of the sol(A)was then adjusted up to his work about 3 by control addition of NH,OH in warm condition 2. Experimental procedure A calculated quantity TEOS was slowly added under stirring to the required amount of alumina sol(A)as prepared above 2.1. Preparation of precursor sols under stirring to obtain AS sol of composition 60Al20340SiO (in equivalent mole). The stirring was continued for 15 min for Zirconium oxychloride octahydrate, ZrOCI2-8H2O(Indian homogenization(Fig. 1) tare Earths Ltd, Mumbai, India, purity >99%), hydrated The viscosity values of the above sols(zY, Az, A and as) R yttrium nitrate (Indian Rare Earths Ltd, Mumbai, India, were adjusted up to either 10 or 5 mPa s by solvent evaporation. purity>99%), aluminum nitrate, Al(NO3)39H20(GR. Two series of sols were prepared with different viscosities: one Merck, Mumbai, India, purity >99%)and tetraethylorthosi- with a viscosity of 10 mPa s for the very first infiltration steps and licate,TEOS(Fluka Chemie AG, Switzerland, purity >98%) the other with a viscosity of 5 mPa s for the last infiltration step(s) were used as the starting materials for ZrO2, Y2O3, Al2O3 and SiO2 respectively. Fig. I shows schematically the preparative 2.2. Infiltration of the sols in the high alumina fibre steps of zirconia-yttria(ZY), alumina-zirconia(AZ), alumina preforms (A)and alumina-silica(AS) sols For the preparation of ZY sol with the composition In the present investigation, the high alumina fibre preform 94ZrO2- 6Y2O3(in equivalent mole), ZrOCI28H2O(1.5M) (M/s Thermal Ceramics, USA)(fibre content=30 vol %, aqueous ammonia solution(25 wt %, G.R. Merck, Mumbai, 125 mm( diameter)X 25 mm(thickness)was infiltrated with India)under stirring, maintaining the pH of the solution up to the above prepared sols, ZY, AZ, AS and A using a custom- designed set-up(Fig. 2). In this technique, the fibre preform was ZroCl solution(1.5M) (NO3) solution placed on a filter bed(sintered disc)of the infiltration unit with Zroc12(1.5M) the help of a specimen holder. The preform was immersed with the prepared sol of viscosity 10 mPa s by sucking it up through pH89) Stirring at80°C the filter bed with the help of a rotary vacuum pump attached to stimng the infiltration unit. The samples were kept for 5 min in the sol Hydrated zirconia precipitate Solution of Al"andZr"Alumina(A)sol under immersed condition followed by releasing pressure. The TEOS Peptization at80°C sol infiltrated preform was dried at 100C to convert the penetrated sol into the corresponding wet gel. This process was Zirconia sol lumina-zirconia(AZ)sol Alumina-silica(AS) continued thrice with the same sol of viscosity 10 mPas. The (NO3) wet infiltrated preform was then heated at 400C in air under solution, stirring static condition to remove the volatiles and decomposable ZrOz-Y2O3(ZY)sol materials. Finally, the 400C-treated sample was infiltrated following the above procedure with the sol of viscosity 5 mPa s Fig 1. Schematic for the preparation of zirconia-yttria(ZY), alumina-zirconia followed by heating at different temperatures, i.e,800- (AZ). alumina(A)and alumina-silica(AS) sols 1400C in air under static conditionchemistry and thermal stability [2,15]. The use of sintering aid also influenced the mechanical strength of the CMC [16]. For the development of a high quality CMC, matrix properties are also to be taken into account. Important characteristics of the matrix include thermal compatibility with the fibres and fibre– matrix interface reaction. The strength and toughness of CMC are primarily governed by the interfacial bonding at the fibre/ matrix interface [1,15]. Interfacial strength can be influenced by a number of factors, most important being degree of bonding between the fibre and the matrix and their difference in thermal expansion behaviour. Crystallization behaviour i.e., the development of crystalline phases in the composites with sintering temperatures has an important role for the characteristics of the CMCs. In the present investigation, a sol–gel vacuum infiltration route was followed for the preparation of NNS ceramic fibre reinforced ceramic matrix composites using various sols as the infiltrates. The effect of crystallization in the composites towards their characteristics i.e., mechanical strength, pseudo-ductility and hence the nature of fibre–matrix interface has been studied in this work. 2. Experimental procedure 2.1. Preparation of precursor sols Zirconium oxychloride octahydrate, ZrOCl28H2O (Indian Rare Earths Ltd., Mumbai, India, purity > 99%), hydrated yttrium nitrate (Indian Rare Earths Ltd., Mumbai, India, purity > 99%), aluminum nitrate, Al(NO3)39H2O (G.R. Merck, Mumbai, India, purity > 99%) and tetraethylorthosi￾licate, TEOS (Fluka Chemie AG, Switzerland, purity > 98%) were used as the starting materials for ZrO2, Y2O3, Al2O3 and SiO2 respectively. Fig. 1 shows schematically the preparative steps of zirconia–yttria (ZY), alumina–zirconia (AZ), alumina (A) and alumina–silica (AS) sols. For the preparation of ZY sol with the composition 94ZrO26Y2O3 (in equivalent mole), ZrOCl28H2O (1.5 M) was precipitated as hydrated zirconia by the addition of aqueous ammonia solution (25 wt.%, G.R. Merck, Mumbai, India) under stirring, maintaining the pH of the solution up to 8–9. The precipitate was washed with deionized water to free foreign ions e.g., Cl, NH4 + etc. followed by peptization with glacial acetic acid (99.8%, AnalaR, BDH, Mumbai, India) at 65  1 8C. A required amount of Y(NO3)3 (6 mol% equivalent Y2O3) solution was added to the above peptized zirconia sol under stirring for 15 min at r.t (30 8C) to prepare ZY sol (Fig. 1). For the preparation of AZ sol of composition 87Al2O313ZrO2 (in equivalent mole), required amount of Al(NO3)3 solution (1.5 M) was mixed with required amount of ZrOCl2 solution (1.5 M) under stirring for 15 min. An aqueous ammonia solution (25 wt.%, G.R. Merck, Mumbai, India) was added to the above mixed solution containing Al3+ and Zr4+ under stirring at 80  1 8C in a covered container (Fig. 1). In this case the adjustment of pH at around 3 is the crucial step for stabilizing of AZ sol. For the preparation of alumina sol (A), ammonia solution (25 wt.% G.R. Merck, Mumbai, India) was slowly added to 1.5 M of Al(NO3)3 solution at 80  1 8C under stirring in a covered container. The pH of the sol (A) was then adjusted up to about 3 by control addition of NH4OH in warm condition (Fig. 1). A calculated quantity TEOS was slowly added under stirring to the required amount of alumina sol (A) as prepared above under stirring to obtain AS sol of composition 60Al2O340SiO2 (in equivalent mole). The stirring was continued for 15 min for homogenization (Fig. 1). The viscosity values of the above sols (ZY, AZ, A and AS) were adjusted up to either 10 or 5 mPa s by solvent evaporation. Two series of sols were prepared with different viscosities: one with a viscosity of 10 mPa s for the very first infiltration steps and the other with a viscosity of 5 mPa s for the last infiltration step(s). 2.2. Infiltration of the sols in the high alumina fibre preforms In the present investigation, the high alumina fibre preform (M/s Thermal Ceramics, USA) (fibre content = 30 vol.%, length = 200–250 mm, diameter = 3–7 mm) of dimension 125 mm (diameter)  25 mm (thickness) was infiltrated with the above prepared sols, ZY, AZ, AS and A using a custom￾designed set-up (Fig. 2). In this technique, the fibre preform was placed on a filter bed (sintered disc) of the infiltration unit with the help of a specimen holder. The preform was immersed with the prepared sol of viscosity 10 mPa s by sucking it up through the filter bed with the help of a rotary vacuum pump attached to the infiltration unit. The samples were kept for 5 min in the sol under immersed condition followed by releasing pressure. The sol infiltrated preform was dried at 100 8C to convert the penetrated sol into the corresponding wet gel. This process was continued thrice with the same sol of viscosity 10 mPa s. The wet infiltrated preform was then heated at 400 8C in air under static condition to remove the volatiles and decomposable materials. Finally, the 400 8C-treated sample was infiltrated following the above procedure with the sol of viscosity 5 mPa s followed by heating at different temperatures, i.e., 800– 1400 8C in air under static condition. Fig. 1. Schematic for the preparation of zirconia–yttria (ZY), alumina–zirconia (AZ), alumina (A) and alumina–silica (AS) sols. 3074 M.K. Naskar et al. / Ceramics International 35 (2009) 3073–3079