Availableonlineatwww.sciencedirect.com Science Direct CERAMICS INTERNATIONAL ELSEVIER Ceramics International 35(2009)3073-3079 www.elsevier.com/locate/ceramint Sol-gel approach to near-net-shape oxide-oxide composites reinforced with short alumina fibres-The effect of crystallization Milan Kanti Naskar a,, Kunal Basu, Minati Chatterjee a Sol-Gel Division, Central Glass and Ceramic Research Institute(CSIR), Kolkata 700032, india Sagar Institute of Science and Technology, Gandhi Nagar, Bhopal 462034, India Received 24 July 2008: received in revised form 12 March 2009: accepted 14 April 2009 Available online 21 May 2009 Abstract Near-net-shape(NNS) high alumina(alumina: silica=96: 4, in equivalent weight ratio) fibre reinforced ceramic matrix composites(CMCs) were prepared with single and bicomponent sols following sol-gel vacuum infiltration technique. The CMCs were characterized by X-ray diffraction(XRD), three-point bend test and scanning electron microscopy(SEM) Crystallization of tetragonal zirconia(t-ZrO2)in the composite CZY having zirconia-yttria matrix and that of gamma alumina(y-Al2O3)in the composites, CAZ having alumina-zirconia matrix, CAs having alumina-silica matrix and CA having alumina matrix, enhanced the flexural strength values and pseudo-ductile character of CMCs C 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Sol-gel process; B. Fibres; B. Composites; C. Mechanical properties; E. Structural applications 1. Introduction 13, 8-14 Radsick et al. [8 fabricated oxide/oxide matrix composites by slurry infiltration-hot pressing sintering technique The use of monolithic ceramics has some restrictions in (SI-HP) Recently Ruggles-Wrenn et al. [14] have studied the structural applications because of their inherent catastrophic environmental effects on the creep behaviours of oxide/oxide nature. The development of ceramic fibre reinforced ceramic ceramic matrix composites prepared by sol-gel technique. Dey matrix composites(CMCs)has been proved to be promising et al. [ll] have fabricated near-net-shape fibre reinforced ceramic alternative to monolithic ceramics for structural materials matrix composites by sol infiltration technique. Sol-gel vacuum fabrication in engineering field [1, 2]. If CMCs could be made infiltration technique is a promising route for the fabrication of consistently and at reasonable cost, they could be ideal for high oxide-oxide ceramic matrix composite. In the vacuum infiltration temperature corrosive and abrasive environments. One of the technique, a solution containing metal compounds, e. g, metal processes of fabrication of CMCs is to infiltrate a liquid matrix alkoxides, acetates, nitrates or halides are treated chemically to into a reinforcement assemblage of required shape and size form a sol which is then infiltrated into the fibre preform. The sol which is called preform [2]. These preforms may be continuous in the preform is converted to gel form by physical and/or or discontinuous fibres or whiskers or particulates assemblages chemical means, which in turn is subjected to control heating to prepared to the desired volume fraction, shapes and sizes [2,3]. produce the desired products e.g. glass, glass-ceramic or ceramic Several techniques are currently used to fabricate CMCs with matrix. The main advantages of this technique are lower NNS capability which is used to produce NNS components that processing temperature and higher compositional homogeneity do not require machining or working with diamond tools. The with respect to slurry infiltration-hot pressing sintering technique conventional methods of producing CMCs are hot pressing [4], (SI-HP) In this process, preforms provide a uniform distribution melt infiltration [5], chemical vapour infiltration( CVI[6], slurry of fibres and a very high surface area to the matrix gel. However, nfiltration-high pressure sintering(SI-HPS)[3, 7, 8] and sol-gel during drying, shrinkage of matrix results in crack formation in the matrix. It can be minimized by repeated infiltrations with the Corresponding author. Tel. +91 33 2483 8086: fax: +91 33 2473 0957 The properties of CMCs an ellence d by th cteristics of the reinforcing fibres i.e., their strength, aspect ratio 2-8842/$3600C 2009 Elsevier Ltd and Techna Group S.r.L. All rights reserved 10.1016 1-ceramint.2009.04013
Sol–gel approach to near-net-shape oxide–oxide composites reinforced with short alumina fibres—The effect of crystallization Milan Kanti Naskar a, *, Kunal Basu b , Minati Chatterjee a a Sol-Gel Division, Central Glass and Ceramic Research Institute (CSIR), Kolkata 700032, India b Sagar Institute of Science and Technology, Gandhi Nagar, Bhopal 462034, India Received 24 July 2008; received in revised form 12 March 2009; accepted 14 April 2009 Available online 21 May 2009 Abstract Near-net-shape (NNS) high alumina (alumina:silica = 96:4, in equivalent weight ratio) fibre reinforced ceramic matrix composites (CMCs) were prepared with single and bicomponent sols following sol–gel vacuum infiltration technique. The CMCs were characterized by X-ray diffraction (XRD), three-point bend test and scanning electron microscopy (SEM). Crystallization of tetragonal zirconia (t-ZrO2) in the composite, CZY having zirconia–yttria matrix and that of gamma alumina (g-Al2O3) in the composites, CAZ having alumina–zirconia matrix, CAS having alumina–silica matrix and CA having alumina matrix, enhanced the flexural strength values and pseudo-ductile character of CMCs. # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Sol–gel process; B. Fibres; B. Composites; C. Mechanical properties; E. Structural applications 1. Introduction The use of monolithic ceramics has some restrictions in structural applications because of their inherent catastrophic nature. The development of ceramic fibre reinforced ceramic matrix composites (CMCs) has been proved to be promising alternative to monolithic ceramics for structural materials fabrication in engineering field [1,2]. If CMCs could be made consistently and at reasonable cost, they could be ideal for high temperature corrosive and abrasive environments. One of the processes of fabrication of CMCs is to infiltrate a liquid matrix into a reinforcement assemblage of required shape and size which is called preform [2]. These preforms may be continuous or discontinuous fibres or whiskers or particulates assemblages prepared to the desired volume fraction, shapes and sizes [2,3]. Several techniques are currently used to fabricate CMCs with NNS capability which is used to produce NNS components that do not require machining or working with diamond tools. The conventional methods of producing CMCs are hot pressing [4], melt infiltration [5], chemical vapour infiltration (CVI) [6], slurry infiltration-high pressure sintering (SI-HPS) [3,7,8] and sol–gel [3,8–14]. Radsick et al. [8] fabricated oxide/oxide matrix composites by slurry infiltration-hot pressing sintering technique (SI-HP). Recently Ruggles-Wrenn et al. [14] have studied the environmental effects on the creep behaviours of oxide/oxide ceramic matrix composites prepared by sol–gel technique. Dey et al.[11] have fabricated near-net-shape fibre reinforced ceramic matrix composites by sol infiltration technique. Sol–gel vacuum infiltration technique is a promising route for the fabrication of oxide–oxide ceramic matrix composite. In thevacuum infiltration technique, a solution containing metal compounds, e.g., metal alkoxides, acetates, nitrates or halides are treated chemically to form a sol which is then infiltrated into the fibre preform. The sol in the preform is converted to gel form by physical and/or chemical means, which in turn is subjected to control heating to produce the desired products e.g., glass, glass–ceramic or ceramic matrix. The main advantages of this technique are lower processing temperature and higher compositional homogeneity with respect to slurry infiltration-hot pressing sintering technique (SI-HP). In this process, preforms provide a uniform distribution of fibres and a very high surface area to the matrix gel. However, during drying, shrinkage of matrix results in crack formation in the matrix. It can be minimized by repeated infiltrations with the sol matrix and subsequent drying. The properties of CMCs are influenced by the characteristics of the reinforcing fibres i.e., their strength, aspect ratio, www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 35 (2009) 3073–3079 * Corresponding author. Tel.: +91 33 2483 8086; fax: +91 33 2473 0957. E-mail address: milan@cgcri.res.in (M.K. Naskar). 0272-8842/$36.00 # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2009.04.013
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 condition
chemistry 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 tetraethylorthosilicate, 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 customdesigned 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
M.K. Naskar et al. /Ceramics International 35(2009)3073-3079 Manometer Vacuam R MM减 W wh Flar Specimen holder Fig 3. XRD pattern and SEM image (inset)of high alumina fibre pref 3. Results and discussion The XRD of the precursor preforms used in the present investigation indicated the presence of 8-Al2O3 as the only Sol-二-二 crystallized phase(Fig. 3). The microstructure of the fibre preforms shows the presence of inter-fibre pores and voids (inset of Fig. 3). It indicates that the fibres are circular in ag. 2. Schemati of the sol infitration set-up for the fabrication of near-net- diameter with range of 3-7 um. The presence of considerable amount of inter-fibre pores and voids is evident from the microstructure Infiltration of the sols in the preform is expected 2.3. Characterization of the materials to fill these inter-fibre pores and voids, leading to the formation of the continuous phase, i.e., matrix. The infiltrates(sols) were characterized by measuring their In the present investigation, the fibre preforms were pH with a Jencons pH meter(Model 3030, Jencons Scientific infiltrated with the sols of viscosity 10+ 1 mPa s followed Ltd, Bedfordshire, U. K. while the viscosity values were by intermediate heating at 400C. Finally, infiltrations were recorded using a Brookfield viscometer(Model LVTDV Il, carried out with the sols of viscosity 5t I mPa s which helped Brookfield Laboratories, Inc, MA, USA). to minimize matrix cracking and improve mechanical strength Crystallization behaviour of the matrix materials, fibre of CMCs [17]. It is to be noted that most of the interconnected preform and the composites after firing at different tempera- pores and voids of the fibre preform were filled with the sol of tures i. e, 800-1400C was studied by X-ray diffraction(XRD) higher viscosity followed by filling of residual pores and voids (Philips PW-1730 Philips Corporation, Almelo, the Nether- with the sol of lower viscosity lands) with Ni-filtered CuKo radiation. The flexural strength of Crystallization features of the matrix (in the absence of fibres the composites was determined by Instron Universal Testing and fibre preform calcined at different temperatures i.e., 800- Machine, U. K.(Model: 5500 R). The samples of dimensions 1400C are summarized in Tables la and lb respectively. In case 40 mm x 7 mm x 6 mm were cut from the infiltrated preform of MZY (M stands for matrix)matrix in zirconia-yttria system, a for testing. For the measurement of flexural strength, three- mixture of monoclinic(m-)and tetragonal (t-) zirconia(zrO2 point bend test under a crosshead speed of 0.5 mm/min was was found at 800-1000C while they transformed to cubic(c-) performed. The three-point bending stress, o is determined by and t-zrO2 at 1200-1400C. For MAZ (M stands for matrix) the following equation [2] matrix in alumina-zirconia system, delta(8-), gamma(Y-)Al2O3 and t-ZrO, were obtained at 800-1000C while a-AlO3, t-and c-zrO2 were found at 1200-1400C. The appearance of y-Al2O with amorphous silica was revealed for the sample MAS (M stands for matrix) matrix in alumina silica system at 800- where P is the load at break, L is the span length and b and h 1000'C followed by their transformation to orthorhombic are the width and thickness of the test specimen respectively. mullite phase at 1200-1400C. The sample MA (M stands Each strength datum is an average over six samples. Micro- for matrix)matrix in alumina system showed y-Al2O3 at 800oC, structural studies of the fibre preform and the composite y-and 8-Al2O3 at 1000C and a-Al,O3 at 1200-1400C. materials were performed by scanning electron microscopy The fibre preforms(FP) after sintering up to 1200C (SEM)(S 4301, LEO Electronic Microscopy Ltd, Cam- crystallized with 8-and B-Al2O3 along with a trace amount of bridge, U. K) a-Al2O3 and orthorhombic mullite phase (Table 1b). A
2.3. Characterization of the materials The infiltrates (sols) were characterized by measuring their pH with a Jencons pH meter (Model 3030, Jencons Scientific Ltd., Bedfordshire, U.K.) while the viscosity values were recorded using a Brookfield viscometer (Model LVTDV II, Brookfield Laboratories, Inc., MA, USA). Crystallization behaviour of the matrix materials, fibre preform and the composites after firing at different temperatures i.e., 800–1400 8C was studied by X-ray diffraction (XRD) (Philips PW-1730 Philips Corporation, Almelo, the Netherlands) with Ni-filtered CuKa radiation. The flexural strength of the composites was determined by Instron Universal Testing Machine, U.K. (Model: 5500 R). The samples of dimensions 40 mm 7 mm 6 mm were cut from the infiltrated preform for testing. For the measurement of flexural strength, threepoint bend test under a crosshead speed of 0.5 mm/min was performed. The three-point bending stress, s is determined by the following equation [2]: s ¼ 3PL 2bh2 where P is the load at break, L is the span length and b and h are the width and thickness of the test specimen respectively. Each strength datum is an average over six samples. Microstructural studies of the fibre preform and the composite materials were performed by scanning electron microscopy (SEM) (S 430i, LEO Electronic Microscopy Ltd., Cambridge, U.K.). 3. Results and discussion The XRD of the precursor preforms used in the present investigation indicated the presence of d-Al2O3 as the only crystallized phase (Fig. 3). The microstructure of the fibre preforms shows the presence of inter-fibre pores and voids (inset of Fig. 3). It indicates that the fibres are circular in diameter with range of 3–7 mm. The presence of considerable amount of inter-fibre pores and voids is evident from the microstructure. Infiltration of the sols in the preform is expected to fill these inter-fibre pores and voids, leading to the formation of the continuous phase, i.e., matrix. In the present investigation, the fibre preforms were infiltrated with the sols of viscosity 10 1 mPa s followed by intermediate heating at 400 8C. Finally, infiltrations were carried out with the sols of viscosity 5 1 mPa s which helped to minimize matrix cracking and improve mechanical strength of CMCs [17]. It is to be noted that most of the interconnected pores and voids of the fibre preform were filled with the sol of higher viscosity followed by filling of residual pores and voids with the sol of lower viscosity. Crystallization features of the matrix (in the absence of fibres) and fibre preform calcined at different temperatures i.e., 800– 1400 8C are summarized inTables 1a and 1b respectively. In case of MZY (M stands for matrix) matrix in zirconia–yttria system, a mixture of monoclinic (m-) and tetragonal (t-) zirconia (ZrO2) was found at 800–1000 8C while they transformed to cubic (c-) and t-ZrO2 at 1200–1400 8C. For MAZ (M stands for matrix) matrix in alumina–zirconia system, delta (d-), gamma (g-) Al2O3 and t-ZrO2 were obtained at 800–1000 8C while a-Al2O3, t- and c-ZrO2 were found at 1200–1400 8C. The appearance of g-Al2O3 with amorphous silica was revealed for the sample MAS (M stands for matrix) matrix in alumina silica system at 800– 1000 8C followed by their transformation to orthorhombic mullite phase at 1200–1400 8C. The sample MA (M stands for matrix) matrix in alumina system showed g-Al2O3 at 800 8C, g- and d-Al2O3 at 1000 8C and a-Al2O3 at 1200–1400 8C. The fibre preforms (FP) after sintering up to 1200 8C crystallized with d- and u-Al2O3 along with a trace amount of a-Al2O3 and orthorhombic mullite phase (Table 1b). A Fig. 2. Schematic of the sol infiltration set-up for the fabrication of near-netshape ceramic fibre reinforced ceramic matrix composites. Fig. 3. XRD pattern and SEM image (inset) of high alumina fibre preform. M.K. Naskar et al. / Ceramics International 35 (2009) 3073–3079 3075
K. Naskar et al. /Ceramics Intemational 35(2009)3073-3079 Table la zirconia-yttria matrix, showed m-and t-zro2 along with 8-and XRD results of matrix(M)materials calcined I h at different temperatures 8-Al2O3 up to 1200C. A mixture of t-and c-ZrO2 along with a Calcination temperature (C) Crystalline phases trace amount of a-Al2O3 and orthorhombic mullite phases each designation was obtained at 1400C MZY All the composites(CAZ, CAS and CA) prepared with alumina-zirconia, alumina-silica and alumina matrices respec- tively, showed a significant amount of 8-, 0-and Y-Al2O3 at c 800-1000C. The a-Al2O3 which appeared at 1200C became MAZ 8+y-+t prominent at 1400C. At 1200C, along with the other 8+y-+t polymorphs of alumina, t- and c-ZrO2 were found in CAZ and orthorhombic mullite was obtained in cas and CA a 1400C, the a-Al,O3 and mullite were obtained in CAS and MAS Y-+Sio2(amorphous) CA composites while CAz crystallized with a-Al,O3, t-,c- y-+ SiO2(amorphous) ZrO, and trace amount of mullite mmy The load displacement curves of the composites, CZY, CAZ, CAS and Ca sintered at 800-1400C each are shown in Fig. 5 It is evident from the features of the curves that all the samples heat treated at 800-1000C indicate some pseudo-ductile character showing steady falls of load elongation curves afte ote:c-, cubic ZrO2: t, tetragonal ZrO2: y, y-Al2O3 XRD results of fibre preform(FP)calcined I h at different temperatures Calcination designation temperature (C) 0-+0-+a-(trace)+mul(trace) P10 8-+0-+a-(trace)+mul (trace) 8-+0-+a-+mul(trace) FP14 c-(major)+ mul(minor) Note: B, BAl2O3. CAS substantial quantity of a-Al2O3 was found along with a small y入个 amount of mullite phase for 1400C-treated sample Crystallization behaviours of the CMCs(CZY, CAZ, CAs and CA, here C stands for composite) at different temperatures 0.250.35045 are depicted in Fig. 4. XRD of composite material CzY, with CAZ 1400 C+L+a(tr+mu a+t+c+mu(tr) a+mu a(maj)+mu(min 1+C+o+e(tr) 8+8+a+t+c 0+8+mu+ 8+日+a+mu 0050.150250350.45 m+t+8+08+0+tt)8+6+y+a(r)+b+0+y+atr 0.30 0+B+y+t(tr) 8+0+y+a(tr)+ &+e+r+a(tr) Sample ID No 0050.150.250.350.45 Fig. 4. Change in crystalline phases of the composites, CZY, CAZ, CAS and CA at different temperatures. m, monoclinic ZrO2: t, tetragonal ZrO,; c, cubic ZrO2: Fig. 5. Load displacement curves of the composites CZY, CAZ. CAS and CA y, yALO3: a, a-Al2O3: 8, 8-Al2O3: 0, 8-Al2O3; mu, orthorhombic mullin each sintered at(a)800°C,(b)1000°C,(c)1200°Cand(d)14
substantial quantity of a-Al2O3 was found along with a small amount of mullite phase for 1400 8C-treated sample. Crystallization behaviours of the CMCs (CZY, CAZ, CAS and CA, here C stands for composite) at different temperatures are depicted in Fig. 4. XRD of composite material CZY, with zirconia–yttria matrix, showed m- and t-ZrO2 along with d- and u-Al2O3 up to 1200 8C. A mixture of t- and c-ZrO2 along with a trace amount of a-Al2O3 and orthorhombic mullite phases each was obtained at 1400 8C. All the composites (CAZ, CAS and CA) prepared with alumina–zirconia, alumina–silica and alumina matrices respectively, showed a significant amount of d-, u- and g-Al2O3 at 800–1000 8C. The a-Al2O3 which appeared at 1200 8C became prominent at 1400 8C. At 1200 8C, along with the other polymorphs of alumina, t- and c-ZrO2 were found in CAZ and orthorhombic mullite was obtained in CAS and CA. At 1400 8C, the a-Al2O3 and mullite were obtained in CAS and CA composites while CAZ crystallized with a-Al2O3, t-, cZrO2 and trace amount of mullite. The load displacement curves of the composites, CZY, CAZ, CAS and CA sintered at 800–1400 8C each are shown in Fig. 5. It is evident from the features of the curves that all the samples, heat treated at 800–1000 8C indicate some pseudo-ductile character showing steady falls of load elongation curves after Table 1a XRD results of matrix (M) materials calcined 1 h at different temperatures. Sample designation Calcination temperature (8C) Crystalline phases MZY 800 m- + t- 1000 m- + t- 1200 c- + t- 1400 c- + tMAZ 800 d- + g- + t- 1000 d- + g- + t- 1200 a- + t- + c- 1400 a- + t- + cMAS 800 g- + SiO2 (amorphous) 1000 g- + SiO2 (amorphous) 1200 mul 1400 mul MA 800 g- 1000 g- + d- 1200 a- 1400 aNote: c-, cubic ZrO2; t-, tetragonal ZrO2; g-, g-Al2O3; a-, a-Al2O3; d-, d-Al2O3; mul, orthorhombic mullite. Table 1b XRD results of fibre preform (FP) calcined 1 h at different temperatures. Sample designation Calcination temperature (8C) Crystalline phases FP8 800 d- + u- + a- (trace) + mul (trace) FP10 1000 d- + u- + a- (trace) + mul (trace) FP12 1200 d- + u- + a- + mul (trace) FP14 1400 a- (major) + mul (minor) Note: u-, u-Al2O3. Fig. 4. Change in crystalline phases of the composites, CZY, CAZ, CAS and CA at different temperatures. m, monoclinic ZrO2; t, tetragonal ZrO2; c, cubic ZrO2; g, g-Al2O3; a, a-Al2O3; d, d-Al2O3; u, u-Al2O3; mu, orthorhombic mullite. Fig. 5. Load displacement curves of the composites CZY, CAZ, CAS and CA each sintered at (a) 800 8C, (b) 1000 8C, (c) 1200 8C and (d) 1400 8C. 3076 M.K. Naskar et al. / Ceramics International 35 (2009) 3073–3079
M.K. Naskar et al. /Ceramics International 35(2009)3073-307 al strength of the composites calcined at different temperatures. Sample designation Sol viscosity(mPa s) Total no. of infiltration Final sintering temperature (C) Flexural strength(MPa) CZY 10±1(3),5±1(1) 13.67 10±1(3),5±1(1) 10±1(3),5±1(1) 1200 10±1(3),5±1(1) 1400 CAZ 10±1(2),5±1(2) 10±1(2),5±1(2) 444444444 1000 10±1(2),5±1(2) 1200 14.34 10±1(2),5±1(2) 1400 CAS 10±1(1),5±1(3) 10±1(1),5±1(3) 1000 10±1(1),5±1(3) 1200 19.54 10±1(1),5±1(3) 1400 02 10±1(1),5±1(3) 10±1(1),5±1(3) 10±1(1),5±1(3) 1200 14.31 10±1(1),5±1(3) 1400 16.27 ntering temperatu certain loads. Conversely, a brittle ceramic character is noticed observed due to presence of Y2O3 as dopant in ZrO2 host which for the samples sintered at 1200-1400C indicating sudden fall helped to retain t-zrO2(Fig. 4)inhibiting grain growth in the of load elongation curves after certain loads Table 2 shows the fibre-matrix interface. The material sintered at 1200-1400C flexural strength values of the composites sintered at 800- showed ceramic character resulting in higher flexural strength 1400C along with the preceding infiltration steps of the sols values(Fig. 6). The strong interaction at the fibre-matrix e forms. The change in flexural strength with interface developed at 1400C( into the fibre perros for the composites CZY, CAZ, CAS and ceramic in nature. With increasing sintering temperatures, the b). the material becoming sintering temperatur ca is shown in fig. 6. materials became highly dense with the formation of strong The flexural strength of the composite, CZY increased fibre-matrix interface which resulted in higher flexural strength continuously with the increase in sintering temperature from with some sorts of ceramic brittleness 800 to 1400C(Fig. 6). At relatively low sintering It is interesting to point out that the flexural strength of the temperatures i.e.,800-1000C, the flexural strength values composites with an alumina-containing matrix(CAZ, CAS and were quite low but increased sharply after 1000C. The SEM CA)increased slightly with increase in temperature from 800 to fractograph(Fig. 7a) of the sample CZY sintered at 1000C 1000C(Fig. 6). However, it decreased significantly at 1200C after three-point bending test showed pseudo-ductile character followed by slight increase with increase in temperature from with fibre pull-out resulting from the development of weak 1200 to 1400C. The decrease in flexural strength values at fibre-matrix interface. Pseudo-ductility of the material was 1200C is due to the phase transformation of transient Y-Al2O3 to the stable a-Al2O3(Fig 4)accompanying the grain growth of the matrix [18, 19 and fibres at the fibre-matrix interface. The better densification of the materials at 1400C resulted in slight CZY increase of flexural strength values Fig &a and b shows the SEM fractographs of the composites, CAZ and CA respectively sintered at 1000C each after three-point bend test. The fibre pull-out with relatively weak interaction at fibre-matrix interface is revealed from the microstructures. It indicated CAS pseudo-ductility of the composites ga and b shows the fracture surfaces of the composites, CAS and CA respectively sintered at 1400C each. It revealed the formation of strong CAZ bonding between fibres and matrix of the composites indicating brittle ceramic character It is to be noted that for all the composites Czy, CAZ, CAs and CA, pseudo-ductile character was prominent at 1000C showing fibre pull-out of their fracture surfaces. The composites, CAZ, CAS and Ca show the maximum values 出(8= gth at 10oo C,the presence oI y1iue
certain loads. Conversely, a brittle ceramic character is noticed for the samples sintered at 1200–1400 8C indicating sudden fall of load elongation curves after certain loads. Table 2 shows the flexural strength values of the composites sintered at 800– 1400 8C along with the preceding infiltration steps of the sols into the fibre preforms. The change in flexural strength with sintering temperatures for the composites CZY, CAZ, CAS and CA is shown in Fig. 6. The flexural strength of the composite, CZY increased continuously with the increase in sintering temperature from 800 to 1400 8C (Fig. 6). At relatively low sintering temperatures i.e., 800–1000 8C, the flexural strength values were quite low but increased sharply after 1000 8C. The SEM fractograph (Fig. 7a) of the sample CZY sintered at 1000 8C after three-point bending test showed pseudo-ductile character with fibre pull-out resulting from the development of weak fibre–matrix interface. Pseudo-ductility of the material was observed due to presence of Y2O3 as dopant in ZrO2 host which helped to retain t-ZrO2 (Fig. 4) inhibiting grain growth in the fibre–matrix interface. The material sintered at 1200–1400 8C showed ceramic character resulting in higher flexural strength values (Fig. 6). The strong interaction at the fibre–matrix interface developed at 1400 8C (Fig. 7b), the material becoming ceramic in nature. With increasing sintering temperatures, the materials became highly dense with the formation of strong fibre–matrix interface which resulted in higher flexural strength with some sorts of ceramic brittleness. It is interesting to point out that the flexural strength of the composites with an alumina-containing matrix (CAZ, CAS and CA) increased slightly with increase in temperature from 800 to 1000 8C (Fig. 6). However, it decreased significantly at 1200 8C followed by slight increase with increase in temperature from 1200 to 1400 8C. The decrease in flexural strength values at 1200 8C is due to the phase transformation of transient g-Al2O3 to the stable a-Al2O3 (Fig. 4) accompanying the grain growth of the matrix [18,19] and fibres at the fibre–matrix interface. The better densification of the materials at 1400 8C resulted in slight increase of flexural strength values. Fig. 8a and b shows the SEM fractographs of the composites, CAZ and CA respectively sintered at 1000 8C each after three-point bend test. The fibre pull-out with relatively weak interaction at fibre–matrix interface is revealed from the microstructures. It indicated pseudo-ductility of the composites. Fig. 9a and b shows the fracture surfaces of the composites, CAS and CA respectively sintered at 1400 8C each. It revealed the formation of strong bonding between fibres and matrix of the composites indicating brittle ceramic character. It is to be noted that for all the composites CZY, CAZ, CAS and CA, pseudo-ductile character was prominent at 1000 8C showing fibre pull-out of their fracture surfaces. The composites, CAZ, CAS and CA show the maximum values of flexural strength at 1000 8C; the presence of g-Al2O3 in those samples enhanced the flexural strength values. Table 2 Flexural strength of the composites calcined at different temperatures. Sample designation Sol viscosity (mPa s) Total no. of infiltration Final sintering temperature (8C) Flexural strength (MPa) CZY 10 1 (3), 5 1 (1) 4 800 13.67 10 1 (3), 5 1 (1) 4 1000 15.86 10 1 (3), 5 1 (1) 4 1200 21.30 10 1 (3), 5 1 (1) 4 1400 31.08 CAZ 10 1 (2), 5 1 (2) 4 800 18.48 10 1 (2), 5 1 (2) 4 1000 19.90 10 1 (2), 5 1 (2) 4 1200 14.34 10 1 (2), 5 1 (2) 4 1400 14.76 CAS 10 1 (1), 5 1 (3) 4 800 21.71 10 1 (1), 5 1 (3) 4 1000 23.94 10 1 (1), 5 1 (3) 4 1200 19.54 10 1 (1), 5 1 (3) 4 1400 22.02 CA 10 1 (1), 5 1 (3) 4 800 19.98 10 1 (1), 5 1 (3) 4 1000 21.23 10 1 (1), 5 1 (3) 4 1200 14.31 10 1 (1), 5 1 (3) 4 1400 16.27 Note: (i) intermediate sintering temperature = 400 8C/h; (ii) figures in parenthesis indicate the number of infiltration. Fig. 6. Change in flexural strength of the composites, CZY, CAZ, CAS and CA with different temperatures: (&) CZY, (*) = CAZ, (~) = CAS and (^) = CA. M.K. Naskar et al. / Ceramics International 35 (2009) 3073–3079 3077
307 M K. Naskar et al. /Ceramics International 35(2009)3073-3079 Fig. 7. SEM fractographs of CZY sintered at (a) 1000C and(b) 1400C after the three-point bending tes Fig. 8. SEM fractographs of(a)CAZ and (b) CA sintered at 1000'C each after the three-point bending test. Fig 9. SEM fractographs of(a) CAS and(b)CA sintered at 1400C each after the three-point bending test. 4. Conclusions composites proved to be a key factor in optimizing the characteristics of the cmcs Sol-gel vacuum infiltration route was followed for the fabrication of near-net-shape CMCs using high alumina fibre Acknowledgements preform and various sols of single and bicomponent oxide systems as the infiltrates The sintering temperature affected the The authors thank Dr. H.S. Maiti, Director, Central crystallization behaviours of the matrix materials, fibre Glass Ceramic Research Institute(cG CRD, Kolkata preforms and composites as well. The flexural strength of for his constant encouragement and kind permission to the composites was dependent on their state of crystallization. publish this paper. They acknowledge the sincere helps Crystalline phases of the CMCs influenced the fibre-matrix rendered by the colleagues of X-ray Diffraction, SEM interface optimizing the fibre pull out and pseudo-ductility of ESCA and Composite Sections of cg Cri in materials the composites. Therefore, the crystallization behaviours of the characterization
4. Conclusions Sol–gel vacuum infiltration route was followed for the fabrication of near-net-shape CMCs using high alumina fibre preform and various sols of single and bicomponent oxide systems as the infiltrates. The sintering temperature affected the crystallization behaviours of the matrix materials, fibre preforms and composites as well. The flexural strength of the composites was dependent on their state of crystallization. Crystalline phases of the CMCs influenced the fibre–matrix interface optimizing the fibre pull out and pseudo-ductility of the composites. Therefore, the crystallization behaviours of the composites proved to be a key factor in optimizing the characteristics of the CMCs. Acknowledgements The authors thank Dr. H.S. Maiti, Director, Central Glass & Ceramic Research Institute (CG & CRI), Kolkata for his constant encouragement and kind permission to publish this paper. They acknowledge the sincere helps rendered by the colleagues of X-ray Diffraction, SEM & ESCA and Composite Sections of CG & CRI in materials characterization. Fig. 7. SEM fractographs of CZY sintered at (a) 1000 8C and (b) 1400 8C after the three-point bending test. Fig. 8. SEM fractographs of (a) CAZ and (b) CA sintered at 1000 8C each after the three-point bending test. Fig. 9. SEM fractographs of (a) CAS and (b) CA sintered at 1400 8C each after the three-point bending test. 3078 M.K. Naskar et al. / Ceramics International 35 (2009) 3073–3079
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