CERAMICS INTERNATIONAL ELSEVIER Ceramics International 30(2004)257-265 www.elsevier.com/locate/ceramint Effects of processing parameters on the fabrication of near-net-shape fibre reinforced oxide ceramic matrix composites via sol-gel route M.K. Naskar, M. Chatterjee, * A. Deya, K. Basub Sok-Gel d Central Glass Ceramic Research Institute, Kolkata 700 032. India Regional Research laboratory, Bhopal 462026, India Received 5 March 2003: received in revised form 12 March 2003: accepted 2 May 2003 Abstract sol infiltration technique was found to be very effective for the fabrication of near-net-shape mullite fibre reinforced ceramic composites(CMCs). The infiltrated sol. in single- and bi-component oxide systems with equivalent molar compositions of (A), 60 Al,O3: 40 SiO,(AS), 87 Al2O3: 13 ZrO,(AZ), and 94 Zro2: 06Y2O3(ZY), after drying and calcination, formed the matrix. The discontinuous mullite fibres (as preforms with 15 vol. fibre content) acted as the reinforcement agents. The charac- teristics of the CMCs were found to be strongly dependent on the type of the sols (infiltrates)and their viscosity, presence of non- reactive fillers in the sol, number of infiltrations, intermediate and final sintering temperatures and in-situ deposition of carbon in the fabricated materials. The CMCs were characterised by X-ray diffraction(XRD), scanning electron microscopy (SEM) and the three point bend test. SEM indicated fibre pull-out in the fracture surface of the CMCs. The pseudo-ductile character, developed in the CMCs, was evident from the load-elongation curve of the three-point bend test. The carbon-containing CMCs exhibited a modulus value of almost 51 gpa C 2003 Elsevier Ltd and Techna S r.L. All rights reserved Keywords: A: Soh-gel process; B: Composites; C: Mechanical properties; D: Mullite: E: Structural applications 1. Introduction ceramic matrix and the physicochemical nature of the interface between the fibres and the matrix. The inter- Ceramic fibre reinforced ceramic matrix composites action between the fibre and the matrix is expected to (CMCs) are considered as promising candidates for use take place and judicious control of processing para in high temperature structural application due to their meters is necessary to establish the optimum degree of high strength, high modulus and toughness. The fibre interfacial bonding [1, 71 reinforced ceramic materials have been successful in Although numerous processing methods are already eliminating the catastrophic behaviour of monolithic known for producing CMCs, e.g. polymer impregnation ceramics [1, 2]. Incorporation of reinforcing inorganic and pyrolysis, melt infiltration, chemical vapour infil- fibres into the ceramic matrix develops CMCs which tration(CvI), hot pressing and sol-gel [4, 5, 8, 9], the sol- exhibit pseudo ductility, preventing catastrophic crack gel infiltration has proved to be a viable technique for growth by such mechanisms as crack bridging, fibre fabrication of CMCs using fibrous preforms as the pre- debonding, fibre pull-out [1-7. The characteristics of cursor materials, as the method involves several advan- the CMCs will depend upon the shape, size and disper- tages, such as better homogeneity, low processing sion of the reinforcement, the microstructure of the temperatures, near-net-shape fabrication [9]. However, the technique requires multiple infiltration with inter Corresponding author. Tel. +91-33-2483-8086: fax: +91-33- mediate heat-treatment to overcome matrix cracking 2473-0957. due to excessive shrinkage of matrix during drying and E-mailaddress:minati33(@hotmail.com(M.Chatterjee) sintering [1, 4, 8, 9]. Further, well dispersed fillers, i 0272-8842/03/$30.00@ 2003 Elsevier Ltd and Techna S.r.l. All rights reserved. doi:10.1016/S0272-8842(03)007X
Effects of processing parameters on the fabrication of near-net-shape fibre reinforced oxide ceramic matrix composites via sol–gel route M.K. Naskara , M. Chatterjeea,*, A. Deya , K. Basub a Sol–Gel Division, Central Glass & Ceramic Research Institute, Kolkata 700 032, India bRegional Research Laboratory, Bhopal 462 026, India Received 5 March 2003; received in revised form 12 March 2003; accepted 2 May 2003 Abstract The sol infiltration technique was found to be very effective for the fabrication of near-net-shape mullite fibre reinforced ceramic matrix composites (CMCs). The infiltrated sol, in single- and bi-component oxide systems with equivalent molar compositions of Al2O3 (A), 60 Al2O3:40 SiO2 (AS), 87 Al2O3: 13 ZrO2 (AZ), and 94 ZrO2:06 Y2O3 (ZY), after drying and calcination, formed the matrix. The discontinuous mullite fibres (as preforms with 15 vol.% fibre content) acted as the reinforcement agents. The characteristics of the CMCs were found to be strongly dependent on the type of the sols (infiltrates) and their viscosity, presence of nonreactive fillers in the sol, number of infiltrations, intermediate and final sintering temperatures and in-situ deposition of carbon in the fabricated materials. The CMCs were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and the three point bend test. SEM indicated fibre pull-out in the fracture surface of the CMCs. The pseudo-ductile character, developed in the CMCs, was evident from the load–elongation curve of the three-point bend test. The carbon-containing CMCs exhibited a modulus value of almost 51 GPa. # 2003 Elsevier Ltd and Techna S.r.l. All rights reserved. Keywords: A: Sol–gel process; B: Composites; C: Mechanical properties; D: Mullite; E: Structural applications 1. Introduction Ceramic fibre reinforced ceramic matrix composites (CMCs) are considered as promising candidates for use in high temperature structural application due to their high strength, high modulus and toughness. The fibre reinforced ceramic materials have been successful in eliminating the catastrophic behaviour of monolithic ceramics [1,2]. Incorporation of reinforcing inorganic fibres into the ceramic matrix develops CMCs which exhibit pseudo ductility, preventing catastrophic crack growth by such mechanisms as crack bridging, fibre debonding, fibre pull-out [1–7]. The characteristics of the CMCs will depend upon the shape, size and dispersion of the reinforcement, the microstructure of the ceramic matrix and the physicochemical nature of the interface between the fibres and the matrix. The interaction between the fibre and the matrix is expected to take place and judicious control of processing parameters is necessary to establish the optimum degree of interfacial bonding [1,7]. Although numerous processing methods are already known for producing CMCs, e.g. polymer impregnation and pyrolysis, melt infiltration, chemical vapour infiltration (CVI), hot pressing and sol–gel [4,5,8,9], the sol– gel infiltration has proved to be a viable technique for fabrication of CMCs using fibrous preforms as the precursor materials, as the method involves several advantages, such as better homogeneity, low processing temperatures, near-net-shape fabrication [9]. However, the technique requires multiple infiltration with intermediate heat-treatment to overcome matrix cracking due to excessive shrinkage of matrix during drying and sintering [1,4,8,9], Further, well dispersed fillers, i.e. 0272-8842/03/$30.00 # 2003 Elsevier Ltd and Techna S.r.l. All rights reserved. doi:10.1016/S0272-8842(03)00097-X Ceramics International 30 (2004) 257–265 www.elsevier.com/locate/ceramint * Corresponding author. Tel.: +91-33-2483-8086; fax: +91-33- 2473-0957. E-mail address: minati33@hotmail.com (M. Chatterjee)
M.K. Naskar et al. /Ceramics International 30(2004)257-265 Table I infiltration technique for the fabrication of near-net-shape Characteristics of the parent sols(infiltrates) CMCS. The samples were activated at 200C(at the Sol Composition H ofl°C/min) with a dwe f I h designation mo1%) (mPa s) remove most of volatiles, if any, absorbed on the fibre surface and the intra-fibre regions of the sample pre ZrO2:Y2O3=94062.53±0.013±1 4.91±0.01 form. The activated precursor fibre preform samples were preserved in a desiccator maintained at a relative :133.21±0.01 10±1 humidity of 25%. These samples will function as the reinforcing material submicron particles in the sol may also counter matrix 2. 2. Preparation of single- and bi-component infiltrates shrinkage [8, 10-12], thereby enhancing characteristics (liquid matrix precursor) for the fabrication of CMCs of CMCs. Keeping the afore-mentioned points in view, an attempt has been made in the present investigation: 2.2.1. Preparation of alumina(single-component)sol to examine the effects of processing parameters on For the preparation of a parent aque g.r., Merck, eous alumina sol continuous mullite fibre preforms(15 vol. fibre content) India with purity >99% was used as the starting mate- as the reinforcement agent and single- and bi-compo- rial [13]. Boelunite particles were precipitated from this nent oxides in systems with molar compositions Al2O aluminium nitrate solution at 80-90oC. with ammonia 60 Al,O3: 40 Sio,, 87 AlO3: 13 ZrO, and 94 ZrO2: 06 solution (25 wt %GR, Merck, India). The washed Y2O3 as the matrix materials, based on the vacuum sol precipitate was peptized with nitric acid (69 wt % G R infiltration technique, (ii) to characterize the developed Merck, India) to obtain a colloidal sol. The pH and CMCs by different analytical techniques and (ii) to viscosity of the present sol(designated as'A")were examine the characteristics of the fibre /matrix interface 4.91+0.01 and 4+l mPa s respectively (Table 1). From by in-situ deposition of carbon in the matrix material. this parent sol, several sols of different viscosities were prepared by solvent evaporation (Table 2). The pH of the sols was measured with a Jencon pH meter(model: 2. Experimental procedure 3030 while the viscosity values were recorded using Brookfield viscometer(model: LVTDV-ID) 2.1. Preparation of precursor fibre preforms 2. 2. Preparation of alumina-silica(bi-component ) sol Samples of dimensions 40 mmx7 mmx6 mm were cut A calculated quantity of tetraethylorthosilicate from as-received 15% volume fraction discontinuous TEOS, (purity 98%; Fluka Chemie AG, Switzerland) mullite fibre preforms of 100 mm diameter and 10mm was slowly added to the parent alumina sol prepared as thickness(from M/s Orient Cerlane Limited, Gujrat)for in Section 2.2. 1 under stirring. The molar ratio of alu- investigating and establishing the parameters of sol mina to silica in the bi-component colloidal sol was Table 2 Characteristics of some typical CMCs obtained under different experimental conditions Sample Sol viscosity No of Intermediate Final Flexural Characteristics of infiltration sintering ntering ngth the products temperature(C) temperature(C) (MPa) ZYI 1(1)+40±1(2)3 800 1400 Good surface(pseudo ducti ±1(1)+40±1(4)5 Brittle, good surface(ceramic character) ZY3 60±1(1)+40±1(4)5 1400 Brittle, good surface(ceramic character 40±1( ZY560±1(1)+40±1(2)3 10004 Good surface (pseudo ductility) 60±1(1)+40±1(2)3 14004 Good surface(pseudo ductility) 40±1(1)+20±1(4)5 400 1400 Brittle, good surface(ceramic character) 30±1(5) 1400 Good surface(pseudo ductility) 60±1(1)+14±1(4)5 1400 Brittle, good surface(ceramic character) 40±1(1)+14±1(4)5 1400 ood surface(pseudo ductility) AZI 60±1(3)+40±1(1)4 1400 Good surface(pseudo ductility) ±1(3)+40±1(2)5 400 1400 Brittle, good surface(ceramic character) Figures in parentheses indicate the number of infiltration
submicron particles in the sol may also counter matrix shrinkage [8,10–12], thereby enhancing characteristics of CMCs. Keeping the afore-mentioned points in view, an attempt has been made in the present investigation: (i) to examine the effects of processing parameters on the fabrication of near-net-shape CMCs using discontinuous mullite fibre preforms (15 vol.% fibre content) as the reinforcement agent and single- and bi-component oxides in systems with molar compositions Al2O3, 60 Al2O3:40 SiO2, 87 Al2O3:13 ZrO2 and 94 ZrO2:06 Y2O3 as the matrix materials, based on the vacuum sol infiltration technique, (ii) to characterize the developed CMCs by different analytical techniques and (iii) to examine the characteristics of the fibre/matrix interface by in-situ deposition of carbon in the matrix material. 2. Experimental procedure 2.1. Preparation of precursor fibre preforms Samples of dimensions 40 mm7 mm6 mm were cut from as-received 15% volume fraction discontinuous mullite fibre preforms of 100 mm diameter and 10mm thickness (from M/s Orient Cerlane Limited, Gujrat) for investigating and establishing the parameters of sol infiltration technique for the fabrication of near-net-shape CMCs. The samples were activated at 200 C(at the heating rate of 1 C/min) with a dwell time of 1 h to remove most of volatiles, if any, absorbed on the fibre surface and the intra-fibre regions of the sample preform. The activated precursor fibre preform samples were preserved in a desiccator maintained at a relative humidity of 25%. These samples will function as the reinforcing material. 2.2. Preparation of single- and bi-component infiltrates (liquid matrix precursor) for the fabrication of CMCs 2.2.1. Preparation of alumina (single-component) sol For the preparation of a parent aqueous alumina sol, Al(NO3)39H2O (Guaranteed Reagent (G.R.), Merck, India with purity >99% was used as the starting material [13]. Boelunite particles were precipitated from this aluminium nitrate solution at 80–90 C, with ammonia solution (25 wt.% G.R., Merck, India). The washed precipitate was peptized with nitric acid (69 wt.%, G.R. Merck, India) to obtain a colloidal sol. The pH and viscosity of the present sol (designated as ‘A’) were 4.910.01 and 41 mPa s respectively (Table 1). From this parent sol, several sols of different viscosities were prepared by solvent evaporation (Table 2). The pH of the sols was measured with a Jencon pH meter (model: 3030) while the viscosity values were recorded using a Brookfield viscometer (model: LVTDV-II). 2.2.2. Preparation of alumina-silica (bi-component) sol A calculated quantity of tetraethylorthosilicate, TEOS, (purity 98%; Fluka Chemie AG, Switzerland) was slowly added to the parent alumina sol prepared as in Section 2.2.1 under stirring. The molar ratio of alumina to silica in the bi-component colloidal sol was Table 1 Characteristics of the parent sols (infiltrates) Sol designation Composition (mol%) pH Viscosity (mPa s) ZY ZrO2:Y2O3=94:06 2.530.01 31 A Al2O3=100 4.910.01 41 AS Al2O3:SiO2=60:40 3.960.01 61 AZ Al2O3:ZrO2=87:13 3.210.01 101 Table 2 Characteristics of some typical CMCs obtained under different experimental conditions Sample no. Sol viscosity (mPa s) No. of infiltration Intermediate sintering temperature (C) Final sintering temperature (C) Flexural strength (MPa) Characteristics of the products ZY1 601(1)+401(2) 3 800 1400 5.2 Good surface (pseudo ductility) ZY2 601(1)+401(4) 5 800 1200 7.3 Brittle, good surface (ceramic character) ZY3 601(1)+401(4) 5 800 1400 13.9 Brittle, good surface (ceramic character) ZY4 401 (5) 5 800 1400 6.8 Good surface (pseudo ductility) ZY5 601(1)+401(2) 3 500 1000a 6.3 Good surface (pseudo ductility) ZY6 601(1)+401(2) 3 500 1400a 6.2 Good surface (pseudo ductility) A1 401(1)+201(4) 5 400 1400 2.8 Brittle, good surface (ceramic character) A2 301(5) 5 400 1400 2.4 Good surface (pseudo ductility) AS1 601(1)+141(4) 5 400 1400 4.8 Brittle, good surface (ceramic character) AS2 401(1)+141(4) 5 400 1400 4.2 Good surface (pseudo ductility) AZ1 601(3)+401(1) 4 400 1400 3.0 Good surface (pseudo ductility) AZ2 601(3)+401(2) 5 400 1400 3.8 Brittle, good surface (ceramic character) Figures in parentheses indicate the number of infiltration. a N2 atmosphere. 258 M.K. Naskar et al. / Ceramics International 30 (2004) 257–265
M.K. Naskar et al. /Ceramics International 30(2004)257-265 maintained at 60: 40(mullite composition). The pH and 2. 2.5. Preparation of stabilised zirconia (with the molar viscosity of the parent bicomponent sol(designated as composition ZrO2: Y203 as 94: 6) powder (filler) AS)was3.96±0.0land6± I mPa s respectively a known amount of the parent ZY sol (Section 2. 2. 4) (Table 1). From this parent sol, several sols of different was evaporated to dryness at about 80oC and the viscosities were prepared by solvent evaporation resulting gel powder was subjected to calcination at Table 2)as described in Section 2.2.1 800oC with I h dwell time in air under static condition followed by grinding. The calcined powder was used as 2. 2.3. Preparation of alumina-zirconia(bi-component) the filler and dispersed in the ZY sol which was subse- quently used as the infiltrate for the fabrication of The precursor materials for the preparation of alu- CMCS mina-zirconia sols with the AlO3: ZrO, molar ratios of 87: 13 were aluminium nitrate nonahydrate, Al 2.3. Preparation of CMCs by vacuum infiltration NO3)3.9H,O(GR, Merck, India, purity >99%)and technique(vIT zirconium oxychioride octahydrate, ZrOCl2.8H2O, (Indian Rare Earths Limited, purity >99%). An aqu In the present investigation, the vacuum infiltration eous solution with Al+ concentration of 1.5 technique [9, 16 using four different sols of various prepared by dissolving the aluminium nitrate in deion viscosities(Tables 1 and 2)as the infiltrates was fol- ised water(conductivity of deionised water: 1. 4x10 lowed. To carry out the infiltration experiments, a mho). The calculated quantity of zirconium oxychlonde laboratory made set-up was used. Activated, precurso solution with Zr+ concentration of 1.5 M in deionised preforms of dimensions 40 mmx7 mmx6 mm were water was added to the aluminium nitrate solution. The immersed in the sol for 10 min on the bed of a specially resulting solution was then subjected to sol formation at designed infiltration unit. The sol was then removed 80+1C by the addition of concentrated ammonia slowly (3-5 ml/min) by a rotary vacuum pump(model solution(25 wt % G.R., Merck, India). The final visc- TSRP/100)attached to the infiltration unit. The infil- osity of the transparent bi-component sols(designated trated preform samples were placed in air at ambient as AZ)was determined to be 10+I mPa s. The ph of temperature for converting the sol penetrated into the le sol at this stage was found to be 3.21+0.01 preform to the corresponding gel. The samples were Table 1). From the bi-component sol further dried in an air circulating oven at 100+2 C for 87Al2O3 13ZrO2 in equivalent oxide mole content, sev- 4 h and subsequently calcined(intermediate heating)at eral sols of viscosities ranging from 40 to 60 mPa s were 400, 500 and 800C(according to the necessity) in air prepared by solvent evaporation(Table 2) under static condition to remove the volatiles and decomposable materials. Calcining lead to the forma 2.2.4. Preparation of zirconic-ytiria(bi-component)sol tion of voids and cracking of matrix due to shrinkage L For the preparation of zirconia( ZrO2) sols, stabilised The above infiltration process was repeated to examine ith 6 mol%Y,O3, zirconium oxychionde octahydrate the effect of number of infiltrations on the character (ZrOCl,8H,O)and hydrous yttrium nitrate(both from istics of the CMCs. Final sintering of the infiltrated M/s Indian Rare Earths Limited, Mumbai), each with a preforms were performed at 1000, 1200 and 1400C in purity of about 99.9%, were used as the starting mate air under static condition all the cmcs were white in rials [14, 15]. Precipitates of hydrated zirconia were colour. For the preparation of carbon containing obtained by adding aqueous ammonia solution (25 CMCs, the intermediate heating of the infiltrated ZY wt%, G.R., Merck, India) to a solution of zirconium containing preforms were performed at 500C with a oxychloride octahydrate in deionized water. The washed dwell time of 1 h in static air followed by the final cal precipitate was peptised with glacial acetic acid (99.8%0, ination at 1000 and 1400C, each for I h in nitrogen Analar, BDH, India) at 65+lC. The sol thus (N2) atmosphere with a flow rate of N2 as 1 I / min obtained had a Zr4+ concentration of 1.2M (Sample nos. ZY5 and ZY6 of Table 2). The CMCs To a known volume of the zirconium acetate sol, a obtained in N2 atmosphere were black in colour required amount of yttrium nitrate(6 mol% equivalent Y203) was mixed under stirring. The pH and viscosity 2. 4. Characterisation of the materials of the resulting yttrium containing zirconia sol(parent sol), designated as ZY, was found to be 2.53+0.01 and (a)The as-received fibres of tensile strength of 3+l mPa s respectively (table 1). No organics were about 2.5 gPa and modulus of about 100 gpa dded to the sol as viscosity controlling agent From the were characterised by:(1)XRD:(model: Philips parent sol, several sols of viscosities ranging from 40+1 PW 1730) using Ni-filtered CuKg radiation (ii) to 60+I mPa s(Table 2)were prepared by solvent evap- (SEM: model: Leo 400c) on samples of dimen oration Table 1 summarises the characteristics of sols sions 2 mmx2 mmx I mm and (iii) wet chemical used in the present investigation
maintained at 60:40 (mullite composition). The pH and viscosity of the parent bicomponent sol (designated as ‘AS’) was 3.960.01 and 61 mPa s respectively (Table 1). From this parent sol, several sols of different viscosities were prepared by solvent evaporation (Table 2) as described in Section 2.2.1. 2.2.3. Preparation of alumina–zirconia (bi-component) sol The precursor materials for the preparation of alumina–zirconia sols with the Al2O3:ZrO2 molar ratios of 87:13 were aluminium nitrate nonahydrate, Al (NO3)39H2O (G.R, Merck, India, purity >99%) and zirconium oxychioride octahydrate, ZrOCl28H2O, (Indian Rare Earths Limited, purity >99%). An aqueous solution with Al3+ concentration of 1.5 M was prepared by dissolving the aluminium nitrate in deionised water (conductivity of deionised water: 1.4105 mho). The calculated quantity of zirconium oxychlonde solution with Zr4+ concentration of 1.5 M in deionised water was added to the aluminium nitrate solution. The resulting solution was then subjected to sol formation at 801 Cby the addition of concentrated ammonia solution (25 wt.%, G.R., Merck, India). The final viscosity of the transparent bi-component sols (designated as AZ) was determined to be 101 mPa s. The pH of the sol at this stage was found to be 3.210.01 (Table 1). From the bi-component sol, i.e. 87Al2O313ZrO2 in equivalent oxide mole content, several sols of viscosities ranging from 40 to 60 mPa s were prepared by solvent evaporation (Table 2). 2.2.4. Preparation of zirconia–ytiria (bi-component) sol For the preparation of zirconia (ZrO2) sols, stabilised with 6 mol% Y2O3, zirconium oxychionde octahydrate (ZrOCl28H2O) and hydrous yttrium nitrate (both from M/s Indian Rare Earths Limited, Mumbai), each with a purity of about 99.9%, were used as the starting materials [14,15]. Precipitates of hydrated zirconia were obtained by adding aqueous ammonia solution (25 wt.%, G.R., Merck, India) to a solution of zirconium oxychloride octahydrate in deionized water. The washed precipitate was peptised with glacial acetic acid (99.8%, AnalaR, BDH, India) at 65l C. The sol thus obtained had a Zr4+ concentration of 1.2M. To a known volume of the zirconium acetate sol, a required amount of yttrium nitrate (6 mol% equivalent Y2O3) was mixed under stirring. The pH and viscosity of the resulting yttrium containing zirconia sol (parent sol), designated as ZY, was found to be 2.530.01 and 31 mPa s respectively (Table 1). No organics were added to the sol as viscosity controlling agent. From the parent sol, several sols of viscosities ranging from 401 to 601 mPa s (Table 2) were prepared by solvent evaporation. Table 1 summarises the characteristics of sols used in the present investigation. 2.2.5. Preparation of stabilised zirconia (with the molar composition ZrO2:Y2O3 as 94:6) powder (filler) A known amount of the parent ZY sol (Section 2.2.4) was evaporated to dryness at about 80 Cand the resulting gel powder was subjected to calcination at 800 Cwith 1 h dwell time in air under static condition followed by grinding. The calcined powder was used as the filler and dispersed in the ZY sol which was subsequently used as the infiltrate for the fabrication of CMCs. 2.3. Preparation of CMCs by vacuum infiltration technique (VIT) In the present investigation, the vacuum infiltration technique [9,16] using four different sols of various viscosities (Tables 1 and 2) as the infiltrates was followed. To carry out the infiltration experiments, a laboratory made set-up was used. Activated, precursor preforms of dimensions 40 mm7 mm6 mm were immersed in the sol for 10 min on the bed of a specially designed infiltration unit. The sol was then removed slowly (3–5 ml/min) by a rotary vacuum pump (model: TSRP/100) attached to the infiltration unit. The infiltrated preform samples were placed in air at ambient temperature for converting the sol penetrated into the preform to the corresponding gel. The samples were further dried in an air circulating oven at 1002 Cfor 4 h and subsequently calcined (intermediate heating) at 400, 500 and 800 C(according to the necessity) in air under static condition to remove the volatiles and decomposable materials. Calcining lead to the formation of voids and cracking of matrix due to shrinkage. The above infiltration process was repeated to examine the effect of number of infiltrations on the characteristics of the CMCs. Final sintering of the infiltrated preforms were performed at 1000, 1200 and 1400 Cin air under static condition. All the CMCs were white in colour. For the preparation of carbon containing CMCs, the intermediate heating of the infiltrated ZY containing preforms were performed at 500 Cwith a dwell time of 1 h in static air followed by the final calcination at 1000 and 1400 C, each for 1 h in nitrogen (N2) atmosphere with a flow rate of N2 as 1 l/min (Sample nos. ZY5 and ZY6 of Table 2). The CMCs obtained in N2 atmosphere were black in colour. 2.4. Characterisation of the materials (a) The as-received fibres of tensile strength of about 2.5 GPa and modulus of about 100 GPa were characterised by: (1) XRD: (model: Philips PW 1730) using Ni-filtered CuKa radiation (ii) (SEM: model: Leo 400c) on samples of dimensions 2 mm2 mm1 mm and (iii) wet chemical analysis. M.K. Naskar et al. / Ceramics International 30 (2004) 257–265 259
f.K. Naskar et al. Ceramics international 30(2004)257-265 b) The infiltrated materials(CMCs) were charac AlO3 as the major constituents with trace impurities of terised by(i) XRD as described previously in Fe2O3, TiO2, K2O, NayO and LOI Section 2.4(a),(ii) SEM in which the fracture urfaces and the top surfaces of samples of the 3. 2. Characteristics of the infiltrates(sols) CMCs of same dimensions were examined as mentioned in Section 2. 4 (a),(iii)thermo- Table 2 summarises the characteristics of cmcs fab- gravimetry analysis(TGA)(Model: Netzsch STa ricated using sols of different viscosities in single-and 409c)from 30 to 1000C with a heating rate of bi-component oxide systems following VIT. In the pres- 10C/mm in argon atmosphere and (iv) the ent investigation, four different types of sols(Table 1) flexural strength and modulus measurement on were considered. It is to be noted that for each type of samples of dimensions 40 mmx7 mmx6 mm the sol, the higher the viscosity, the more difficult it was using three point bend test (Instron Universal to achieve good wetting of the fibres by the sol and Testing Machine, model: 5500 R)under a cross- infiltrate the sol into the interconnected pores and void head speed of 0.5 mm/min. Each strength datum [9, 10, 16]. Further, infiltration of the high viscosity sols an average over six samples gave rise to the formation of considerable amounts of (c) The stabilised zirconia powder (with the molar air bubbles during operation and deposition of frag composition of ZrO2: Y2O3 as 94: 06) was char- mented coatings on the surface of the preform after acterised by: ()XRD and (i) SEM as described drying which prevented further penetration of sols earlier and (iii) particle size analyser(Model: inside the preform during repeated infiltration. On the Autosizer IIC, Malvern Instruments) other hand, sols of low viscosity were also found to be unsuitable because in such cases even after nine cycles of infiltration, the sample exhibited considerably brittle 3. Results and discussion character. Based on the above results, in the present Inves igation, the viscosity of each type of sol was kept 3. 1. Characteristics of the precursor preforms fixed in certain optimum ranges(Table 2) which in turn were found to depend on their polymerisation beha The XRD of the precursor preforms used in the pres- viour and preparative principles. pH of the sol is also an ent investigation indicated the presence of mullite as the important point to be considered, as it affects both the only phase. The microstructural feature of the preform viscosity and stability of a sol. with the increase in pH in Fig. I indicates that the fibres are circular in diameter viscosity of a sol increased and finally caused gel for with the diameter distribution in the range 3-8 um. The mation. Thus, an optimum value of both the pH and shot-content in the fibre preform was found to be neg- viscosity of different sols are necessary for carrying out ligible. Presence of considerable amounts of inter-fibre the infiltration experiments [ll pores and voids is evident from the microstructure Infiltration of the sol in the preform is expected to fill 3.3. Characteristics of the CMCs obtained by the sol these inter-fibre pores and voids, leading to the forma- infiltration technique tion of the continuous phase, i.e. the matrix which is the primary aim of this investigation. The preforms Characteristics of the CMCs fabricated unde were found to contain 54.88 wt. SiO2 and 43.05 wt. ent experimental conditions have been presented in Table 2. as described in table 2. characteristics of the CMCs have been found to be affected by the following 3.3.1. Cracking in the matrix of the CMCs Although the sol infiltration technique of fabricating CMCS has several advantages, such as, homogeneous mixing of the multicomponent oxides, higher purity, low processing temperature, the method suffers from the serious problem of excessive shrinkage during drying due to removal of considerable amount of volatiles giving rise to extensive matrix cracking and residual fine scale porosity in composites followed by degradation of mechanical properties. In the present investigation, this 25 problem was tried to be minimised by(i) multiple infiltra tion of the infiltrated preform followed by intermediate Fig. l. SEM of the preform showing inter-fibre voids and porosities heat-treatment at the lowest possible temperature for
(b) The infiltrated materials (CMCs) were characterised by (i) XRD as described previously in Section 2.4 (a), (ii) SEM in which the fracture surfaces and the top surfaces of samples of the CMCs of same dimensions were examined as mentioned in Section 2.4 (a), (iii) thermogravimetry analysis (TGA) (Model: Netzsch STA 409c) from 30 to 1000 Cwith a heating rate of 10 C/mm in argon atmosphere and (iv) the flexural strength and modulus measurement on samples of dimensions 40 mm7 mm6 mm using three point bend test (Instron Universal Testing Machine, model: 5500 R) under a crosshead speed of 0.5 mm/min. Each strength datum is an average over six samples. (c) The stabilised zirconia powder (with the molar composition of ZrO2:Y2O3 as 94:06) was characterised by: (i) XRD and (ii) SEM as described earlier and (iii) particle size analyser (Model: Autosizer IIC, Malvern Instruments). 3. Results and discussion 3.1. Characteristics of the precursor preforms The XRD of the precursor preforms used in the present investigation indicated the presence of mullite as the only phase. The microstructural feature of the preform in Fig. 1 indicates that the fibres are circular in diameter with the diameter distribution in the range 3–8 mm. The shot-content in the fibre preform was found to be negligible. Presence of considerable amounts of inter-fibre pores and voids is evident from the microstructure. Infiltration of the sol in the preform is expected to fill these inter-fibre pores and voids, leading to the formation of the continuous phase, i.e. the matrix, which is the primary aim of this investigation. The preforms were found to contain 54.88 wt.% SiO2 and 43.05 wt.% Al2O3 as the major constituents with trace impurities of Fe2O3, TiO2, K2O, Na2O and LOI. 3.2. Characteristics of the infiltrates (sols) Table 2 summarises the characteristics of CMCs fabricated using sols of different viscosities in single- and bi-component oxide systems following VIT. In the present investigation, four different types of sols (Table 1) were considered. It is to be noted that for each type of the sol, the higher the viscosity, the more difficult it was to achieve good wetting of the fibres by the sol and infiltrate the sol into the interconnected pores and voids [9,10,16]. Further, infiltration of the high viscosity sols gave rise to the formation of considerable amounts of air bubbles during operation and deposition of fragmented coatings on the surface of the preform after drying which prevented further penetration of sols inside the preform during repeated infiltration. On the other hand, sols of low viscosity were also found to be unsuitable because in such cases even after nine cycles of infiltration, the sample exhibited considerably brittle character. Based on the above results, in the present investigation, the viscosity of each type of sol was kept fixed in certain optimum ranges (Table 2) which in turn were found to depend on their polymerisation behaviour and preparative principles. pH of the sol is also an important point to be considered, as it affects both the viscosity and stability of a sol. With the increase in pH, viscosity of a sol increased and finally caused gel formation. Thus, an optimum value of both the pH and viscosity of different sols are necessary for carrying out the infiltration experiments [1]. 3.3. Characteristics of the CMCs obtained by the sol infiltration technique Characteristics of the CMCs fabricated under different experimental conditions have been presented in Table 2. As described in Table 2, characteristics of the CMCs have been found to be affected by the following factors. 3.3.1. Cracking in the matrix of the CMCs Although the sol infiltration technique of fabricating CMCs has several advantages, such as, homogeneous mixing of the multicomponent oxides, higher purity, low processing temperature, the method suffers from the serious problem of excessive shrinkage during drying due to removal of considerable amount of volatiles, giving rise to extensive matrix cracking and residual fine scale porosity in composites followed by degradation of mechanical properties. In the present investigation, this problem was tried to be minimised by (i) multiple infiltration of the infiltrated preform followed by intermediate Fig. 1. SEM of the preform showing inter-fibre voids and porosities. heat-treatment at the lowest possible temperature for 260 M.K. Naskar et al. / Ceramics International 30 (2004) 257–265
f.K. Naskar et al. Ceramics international 30(2004)257-265 avoiding fibre/matrix reaction [1, 8, 9 and(ii) adding ing number of infiltrations dramatically changed the non-reactive fillers to the sol to counter this shrinkage fibre/matrix characteristics, leading to the formation of [10-12. Multiple infiltration in the green state helps to ceramic materials with monolithic character. Therefore improve the green strength and green machinability and an optimum number of infiltration is necessary for handling. Table 2 indicates that initial infiltration with a obtaining CMC with desired mechanical properties sol of high viscosity, followed by intermediate sintering It has been reported [10-12] that the addition of and further infiltration with a sol of low viscosity, non-reactive solid particles(fillers) to the sol increases minimises matrix cracking and improves the flexural infiltration efficiency and reduces matrix cracking by strength of the CMCs(Sample nos. ZYI and AS2 of minimizing shrinkage. Comparing the results of Sample Table 2). Fig. 2 represents the SEM of the fracture sur- nos. ZYI and ZY4 of Table 2, it is observed that the face of the Sample no. ZYl of Table 2 after three-point addition of 10 wt. of sol-gel derived ZY powder bend test which indicates fibre pull-out in the sample ( Section 2.2.5), with the tetragonal(t-) and cubic (c-) while the load-elongation curve in Fig. 3 reflects the ZrO, and mean size of about 0.5 um to the Zy sol (as development of pseudoductile character in the same the infiltrate) helped to modify the fibre/matrix interface sample. In contrast to Sample no. ZYl, the Sample no characteristics of the developed CMCs. The dispersed ZY3 of Table 2 fails to exhibit pseudo ductility Devel- particles in the sol produced unaggregated colloidal opment of strong interaction at the fibre/matrix inter- particles. Both the particle size and loading of the fillers face may be the reason of such failure. This is reflected in the sol are the dominant factors for controlling the rom the load-elongation curve of the three point bend characteristics of the CMCs, thereby tailoring the fibre/ test presented in Fig. 4. Fig. 5 represents the SEM of matrix interaction. As the addition of fillers caused an the fracture surface after three point bend test of the increase in the viscosity of the sol [1], a maximum load- Sample no. ZY3 of Table 2. Filling up inter-fibre voids ing up to about 10 wt was found to be the optimum ind pores by infiltration of excess sol through increas- in the present case. Figs. 6 and 7 represent the particle 350 250 200 81 0000040080.190.160900.24 Displacement(mm) 25μm Fig 4. Load-displacement curve of sample no. zY3 of Table 2. Fig. 2. SEM of the fracture of CMC after three- point bend test (Sample no. ZYI of Table 2)showing fibre pull-out from the matrix. 19 6 0.000040080.190.160.20024 25μm Displacement (mm) Fig. 5. SEM of the fracture surface of CMC after three-point bend Fig 3. Load-displacement curve of sample no ZYI of Table 2. test(sample no. ZY3 of Table 2
avoiding fibre/matrix reaction [1,8,9] and (ii) adding non-reactive fillers to the sol to counter this shrinkage [10–12]. Multiple infiltration in the green state helps to improve the green strength and green machinability and handling. Table 2 indicates that initial infiltration with a sol of high viscosity, followed by intermediate sintering and further infiltration with a sol of low viscosity, minimises matrix cracking and improves the flexural strength of the CMCs (Sample nos. ZY1 and AS2 of Table 2). Fig. 2 represents the SEM of the fracture surface of the Sample no. ZY1 of Table 2 after three-point bend test which indicates fibre pull-out in the sample while the load–elongation curve in Fig. 3 reflects the development of pseudoductile character in the same sample. In contrast to Sample no. ZY1, the Sample no. ZY3 of Table 2 fails to exhibit pseudo ductility. Development of strong interaction at the fibre/matrix interface may be the reason of such failure. This is reflected from the load-elongation curve of the three point bend test presented in Fig. 4. Fig. 5 represents the SEM of the fracture surface after three point bend test of the Sample no. ZY3 of Table 2. Filling up inter-fibre voids and pores by infiltration of excess sol through increasing number of infiltrations dramatically changed the fibre/matrix characteristics, leading to the formation of ceramic materials with monolithic character. Therefore an optimum number of infiltration is necessary for obtaining CMC with desired mechanical properties. It has been reported [10–12] that the addition of non-reactive solid particles (fillers) to the sol increases infiltration efficiency and reduces matrix cracking by minimizing shrinkage. Comparing the results of Sample nos. ZY1 and ZY4 of Table 2, it is observed that the addition of 10 wt.% of sol–gel derived ZY powder (Section 2.2.5), with the tetragonal (t-) and cubic (c-) ZrO2 and mean size of about 0.5 mm to the ZY sol (as the infiltrate) helped to modify the fibre/matrix interface characteristics of the developed CMCs. The dispersed particles in the sol produced unaggregated colloidal particles. Both the particle size and loading of the fillers in the sol are the dominant factors for controlling the characteristics of the CMCs, thereby tailoring the fibre/ matrix interaction. As the addition of fillers caused an increase in the viscosity of the sol [1], a maximum loading up to about 10 wt.% was found to be the optimum in the present case. Figs. 6 and 7 represent the particle Fig. 2. SEM of the fracture of CMC after three-point bend test (Sample no. ZY1 of Table 2) showing fibre pull-out from the matrix. Fig. 3. Load–displacement curve of sample no. ZY1 of Table 2. Fig. 4. Load–displacement curve of sample no. ZY3 of Table 2. Fig. 5. SEM of the fracture surface of CMC after three-point bend test (sample no. ZY3 of Table 2). M.K. Naskar et al. / Ceramics International 30 (2004) 257–265 261
M.K. Naskar et al. /Ceramics International 30(2004)257-265 the decomposable and carbonaceous materials at 800C [17] with the formation of white coloured fibres. Based on this result, the intermediate sintering temperature of 6 800C was selected for multiple infiltration of ZY sol for fabrication of CMCs in the present investigation For the other sols. ie. A. aZ and as. based on their thermogravimetry (TG) results, an intermediate sinter ing temperature of 400C corresponding to the removal 251090501002005009000 of maximum amount of volatiles and decomposable Particle size(nm) materials was selected. Sintering temperatures above 800C proved to be ineffective probably due to the Fig. 6. Particle size distribution of the sol-gel derived ZY particles fibre/matrix interaction. The infiltrated sol, in the inter-fibre region of the sample preforms, after gel formation followed by calci nation may form agglomerates of particles at the inter mediate sintering temperature of 400, 500 or 800 oC which undergoes densification after further sintering at higher temperatures, e. g 1400C. Hence the strength of the CMCs has been found to depend to a great extent on its final sintering temperature which affects the degree of interaction between fibres and matrix. At a because of interfacial reaction. while at a too low tem- perature, the matrix does not sinter adequately This is supported by the strength values of the CMCs sintered at different temperatures (Table 2). Thus, an optimum sintering temperature is needed. It is to be 10 um noted that, in the present study, since the CMCs fabri cated at the final sintering temperature of 1000 oC Fig.7.SEM of the sol-gel derived ZY particles(fillers)showing the exhibited very low flexural strength, i.e. below I MPa, ormation of submicrometre sized particle those values were not presented in Table 2. ze distribution and SEM, respectively of the ZY par- 3.3.3. In-situ deposition of carbon in CMCs ticles used as fillers in the present study. Both the figures Intermediate heat treatment of the Sample nos. ZY5 indicate the formation of submicrometre sized particles. and ZY6 of Table 2 at 500C for I h in air resulted in It is to be noted that although attempts were made to the formation of black coloured materials due to the in increase the flexural strength of CMCs by optimising situ deposition of carbon from the decomposable ace- different process parameters, however, insignificant tate groups present in the infiltrated preforms [17]. The improvements in the results were obtained in these pre- black colour of the above materials is retained after liminary experiments. Further work in this area to final calcination at 1000 and 1400C in N2 atmosphere improve the strength values is under study It is to be noted that the presence of carbon in the developed products corresponding to Sample nos. ZY5 3.3.2. Intermediate and final sintering temperature and zY6 of Table 2 caused an increase in their flexural It has already been mentioned in Section 3.3. 1 that strengths in comparison with that obtained for Sample the multiple infiltration followed by intermediate sin no. ZYl(free from carbon). Further, comparing the tering minimise matrix cracking, thereby enhancing the results of the Sample no. ZYl(modulus value 3 GPa) mechanical properties. Unless the decomposable mate- and ZY5(modulus value 51 GPa), it is observed that the rials present in the green body, after each infiltration, presence of carbon in the fibre/matrix composite mate- are properly removed during intermediate sintering rial significantly increased the modulus values and steps, high strength of the CMCs is difficult to attain. pseudo ductility in the materials. This may be explained The choice of this intermediate sintering temperature, to be due to the fact that the in situ deposition of carbon however, depends on the system under consideration. in the composite material presumably protected the Fourier transform infrared(FTIR) spectra of the sol- fibre/matrix interface from strong interaction and acted gel CaO-doped ZrO, fibres prepared from the zirconium as the crack arrester [18-20]. This is discernible from the acetate sols and calcined at different temperatures from load-displacement curve of the three-point bend test in 30 to 1000C had confirmed the removal of almost all Fig 8 and the fibre pull-out from the SEM of Fig 9 of
size distribution and SEM, respectively of the ZY particles used as fillers in the present study. Both the figures indicate the formation of submicrometre sized particles. It is to be noted that although attempts were made to increase the flexural strength of CMCs by optimising different process parameters, however, insignificant improvements in the results were obtained in these preliminary experiments. Further work in this area to improve the strength values is under study. 3.3.2. Intermediate and final sintering temperature It has already been mentioned in Section 3.3.1 that the multiple infiltration followed by intermediate sintering minimise matrix cracking, thereby enhancing the mechanical properties. Unless the decomposable materials present in the green body, after each infiltration, are properly removed during intermediate sintering steps, high strength of the CMCs is difficult to attain. The choice of this intermediate sintering temperature, however, depends on the system under consideration. Fourier transform infrared (FTIR) spectra of the sol– gel CaO-doped ZrO2 fibres prepared from the zirconium acetate sols and calcined at different temperatures from 30 to 1000 Chad confirmed the removal of almost all the decomposable and carbonaceous materials at 800 C [17] with the formation of white coloured fibres. Based on this result, the intermediate sintering temperature of 800 Cwas selected for multiple infiltration of ZY sol for fabrication of CMCs in the present investigation. For the other sols, i.e. A, AZ and AS, based on their thermogravimetry (TG) results, an intermediate sintering temperature of 400 Ccorresponding to the removal of maximum amount of volatiles and decomposable materials was selected. Sintering temperatures above 800 Cproved to be ineffective probably due to the fibre/matrix interaction. The infiltrated sol, in the inter-fibre region of the sample preforms, after gel formation followed by calcination may form agglomerates of particles at the intermediate sintering temperature of 400, 500 or 800 C which undergoes densification after further sintering at higher temperatures, e.g. 1400 C. Hence the strength of the CMCs has been found to depend to a great extent on its final sintering temperature which affects the degree of interaction between fibres and matrix. At a too high temperature, the material becomes brittle because of interfacial reaction, while at a too low temperature, the matrix does not sinter adequately [1,3]. This is supported by the strength values of the CMCs sintered at different temperatures (Table 2). Thus, an optimum sintering temperature is needed. It is to be noted that, in the present study, since the CMCs fabricated at the final sintering temperature of 1000 C exhibited very low flexural strength, i.e. below 1 MPa, those values were not presented in Table 2. 3.3.3. In-situ deposition of carbon in CMCs Intermediate heat treatment of the Sample nos. ZY5 and ZY6 of Table 2 at 500 Cfor 1 h in air resulted in the formation of black coloured materials due to the insitu deposition of carbon from the decomposable acetate groups present in the infiltrated preforms [17]. The black colour of the above materials is retained after final calcination at 1000 and 1400 Cin N2 atmosphere. It is to be noted that the presence of carbon in the developed products corresponding to Sample nos. ZY5 and ZY6 of Table 2 caused an increase in their flexural strengths in comparison with that obtained for Sample no. ZY1 (free from carbon). Further, comparing the results of the Sample no. ZY1 (modulus value 3 GPa) and ZY5 (modulus value 51 GPa), it is observed that the presence of carbon in the fibre/matrix composite material significantly increased the modulus values and pseudo ductility in the materials. This may be explained to be due to the fact that the in situ deposition of carbon in the composite material presumably protected the fibre/matrix interface from strong interaction and acted as the crack arrester [18–20]. This is discernible from the load–displacement curve of the three-point bend test in Fig. 8 and the fibre pull-out from the SEM of Fig. 9 of Fig. 7. SEM of the sol-gel derived ZY particles (fillers) showing the formation of submicrometre sized particles. Fig. 6. Particle size distribution of the sol-gel derived ZY particles (fillers). 262 M.K. Naskar et al. / Ceramics International 30 (2004) 257–265
n.K. Naskar et al. Ceramics International 30(2004)257-265 Table XRD results of CMCs calcined at different temperatures Sol Calcination Duration Crystalline designation temperature (h) hases c+t-ZrO, 0.00004 0.08 0.12 Fig 8. Load-displacement curve of sample no. ZY6 of Table mullite (orthorhombic) D-Al,O3+t-ZrO 25μm Fig 9. SEM of the fracture surface of the carbon containing CMc after three-point bend test(sample no. ZY6 of Table 2) the same sample. Therefore, instead of using carbon 15m deposited carbon in the bulk matrix may also modify Fig. 10. SEM of the fracture surface of the CMC(sample no. A2 of the fibre/matrix interface and act as the crack arrester 3.3.4. Crystallisation behaviour of the CMCs CMCs fabricated after calcining at 1000, 1200 and transformation of the transient phases, e.g. y-and 8-Al20 1400C, each with a dwell time of I h, were examined to the stable phase a-AlO3 during heat-treatment up to by XRD and the identified crystalline phases are pre- 1400C followed by grain growth with temperature sented in Table 3. It is to be noted that the phases other and thereby degrading the mechanical properties than mullite(fibre preform) crystallised fferent temperatures from the matrix part of the CMCs have Addition of 13 mol% ZrO, to the AlO3 matrix, been listed in Table 3 however, exhibited an increase in the flexural strength at Considering the flexural strength values in Table 2 the same temperature. Crystallisation of t-ZrO2 in the and XRD phases in Table 3, it is to be noted that the alumina matrix is believed to inhibit the growth of the MCs fabricated from the alumina sol exhibited the a-AlO3, resulting in the increase in strength lowest flexural strength compared with those developed [23, 24]. Similarly, incorporation of 40 mol% SiO, in the from the other three sols, irrespective of the final sin- alumina matrix caused crystallisation of orthorhombic tering temperature. The fracture surface of the Sample mullite at 1200-1400C and thereby helped to increase no. A2 of Table 2 as a typical case, under SEM showed the flexural strength of the fabricated CMCs(Table 2) the presence of considerable amounts of cracks in the [25]. Further, XRD of the CMCs infiltrated with the Z matrix part of the matenals, as indicated in Fig. 10 The sol followed by sintering at the 1400oC confirmed the above phenomenon may be due to the reconstructive presence of both c-and t-ZrO2, along with the mullite
the same sample. Therefore, instead of using carboncoated fibres as the reinforcement agents, the in situ deposited carbon in the bulk matrix may also modify the fibre/matrix interface and act as the crack arrester. 3.3.4. Crystallisation behaviour of the CMCs CMCs fabricated after calcining at 1000, 1200 and 1400 C, each with a dwell time of 1 h, were examined by XRD and the identified crystalline phases are presented in Table 3. It is to be noted that the phases other than mullite (fibre preform) crystallised at different temperatures from the matrix part of the CMCs have been listed in Table 3. Considering the flexural strength values in Table 2 and XRD phases in Table 3, it is to be noted that the CMCs fabricated from the alumina sol exhibited the lowest flexural strength compared with those developed from the other three sols, irrespective of the final sintering temperature. The fracture surface of the Sample no. A2 of Table 2 as a typical case, under SEM showed the presence of considerable amounts of cracks in the matrix part of the matenals, as indicated in Fig. 10 The above phenomenon may be due to the reconstructive transformation of the transient phases, e.g. g- and d-Al2O3 to the stable phase a-Al2O3 during heat-treatment up to 1400 Cfollowed by grain growth with temperature, and thereby degrading the mechanical properties [21,22]. Addition of 13 mol% ZrO2 to the Al2O3 matrix, however, exhibited an increase in the flexural strength at the same temperature. Crystallisation of t-ZrO2 in the alumina matrix is believed to inhibit the growth of the a-Al2O3, resulting in the increase in strength values [23,24]. Similarly, incorporation of 40 mol% SiO2 in the alumina matrix caused crystallisation of orthorhombic mullite at 1200–1400 Cand thereby helped to increase the flexural strength of the fabricated CMCs (Table 2) [25]. Further, XRD of the CMCs infiltrated with the ZY sol followed by sintering at the 1400 Cconfirmed the presence of both c- and t- ZrO2, along with the mullite Fig. 8. Load–displacement curve of sample no. ZY6 of Table 2. Fig. 9. SEM of the fracture surface of the carbon containing CMC after three-point bend test (sample no. ZY6 of Table 2). Table 3 XRD results of CMCs calcined at different temperatures Sol designation Calcination temperature ( C) Duration (h) Crystalline phases ZY 1000 1 c-+t-ZrO2 1200 1 c-+t-ZrO2 1400 1 c-+t-ZrO2 A 1000 1 g+d-Al2O3 1200 1 a-Al2O3 1400 1 a-Al2O3 AS 1000 1 g-Al2O3+SiO2 (amorphous) 1200 1 Mullite (orthorhombic) 1400 1 Mullite (orthorhombic) AZ 1000 1 g-Al2O3+t-ZrO2 1200 1 a-Al2O3+t-ZrO2 1400 1 a-Al2O3+t-ZrO2 Fig. 10. SEM of the fracture surface of the CMC (sample no. A2 of Table 2) showing multiple fracture of the matrix. M.K. Naskar et al. / Ceramics International 30 (2004) 257–265 263
M.K. Naskar et al. /Ceramics International 30(2004)257-265 preform), without any formation of monoclinic poly- materials characterization. Financial assistance pro- orphs. Obviously, the Y,O3 additive in the matrix vided by the Aeronautics Research and Development materials helped to retain t-ZrO2 at 1400C by inhibit- Board(AR&DB), ministry of Defence, Govt. of India, ing grain growth [14], thus increasing the flexural is also thankfully acknowledged strength of the CMCs and developing pseudo ductile character in the materials From the foregoing discussion, it may be stated that References the flexural strength of the CMCs fabricated from the four different sols decreased in the order. []R.S. Russel-Floyd, B. Harris, R.G. Cooke, J. Laurie, ZY>AS>AZ>A F.W. Hammett, R.w. Jones, et al., Application of sol-gel for the manuf amIcs,J.Am. Ceran.Soc.76(10)(1993)26352643 Therefore, bonding at the fibre/matrix interface can be 22KK. 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The sol-gel vacuum infiltration technique is very [7 J. Brandt, K. Rundgren, R. Pompe, H. Swan, C. O'Meara, effective for the fabrication of near-net-shape R. Lundberg, et al., SiC continuous fibre-reinforced Si3N4 by CMCs using discontinuous mullite fibre preform infiltration and reaction bonding, Ceram. Eng. Sci. Proc. having 15 vol. of fibre content and various sols (9-10)(1992)622-631 (in single- and bi-component oxide systems)as [8S.M. Sim, R.l. Kerans, Slurry infiltration of 3-D woven compo- sites, Ceram. Eng. Sci. Proc. 13(9-10)(1992)632-641 9A. Dey, M. Chatterjee, M. K. Naskar, K. Basu, Near-net-shap 2. Effects of sol viscosity, number of infiltration, in fibre reinforced ceramic matrix composites by the sol infiltration situ deposition of carbon in the composite technique, Mater. Lett. 57(2003)2919-2126 materials, characteristics of the infiltrates and [ X. Gu, P.A. Trusty, E.G. Butler, C.B. Ponton, Deposition of calcination temperature on physicomechanical Ceran.Soc.20(6)(2000)675-684 properties of CMCs are examined. Multiple infil [1 H.K. Liu, B.H. Lin, The effect of sol/particle reaction on prop. trations and presence of fillers in the infiltrates has been found to be effective for the development of Lett.48(34)(2001)230-241 CMCs with pseudo ductile characteristics [2] E.H. Moore, 3-D composite fabrication through matrix slurry ssure infiltration, Ceram. Eng. Sci. Proc. 15(4)(1994)113-120 3. The pseudo ductile character developed in the [13 M. Chatteijee, D. Enkhtuvshin, B. Siladitya, D. Ganguli, Hollow CMCs is evident from the load-elongation curve alumina microspheres from boehmite sols, J. Mater. Sci. 33(20) of the three-point bend test. (1998)4937-4942 4. SEM indicates fibre pull-out in the fracture [14] P.K. Chakrabarty, M. Chatterjee, M.K. Naskar, B. Siladitya, urface of the Cmcs D. Ganguli, Zirconia fibre mats prepared by a sol-gel spinning technique, J. Eur. Ceram Soc. 21(3)(2001)355-3 [15 M K. Naskar, D. Ganguli, Rare-earth doped zirconia fibres by gel processing, J Mater. Sci. 31(23)(1996)6263-6267 Acknowledgements [16 A. Dey, M. Chatterjee, M K. Naskar, S. Dalui, K. Basu, A novel technique for fabrication of near-net-shape CMCs, Bull. Mater. The authors thank Dr. H.s. Maiti, Director, Central Sci.25(6)(2002)493-495 Glass& Ceramic Research Institute(CG&Cri) and Dr [7 M. Chatterjee, M.K. Naskar, D. Ganguli, Synthesis of poly ZrOr-Cao fibres by sol-gel N. Ramakrishnan, Director, Regional Research ind. Ceram.Soc.5202)(1993)51-55 Laboratory, Bhopal for their kind permission to publish [18] C.A. Doughan, R L. Lehman, V.A. Greenhut, Interfacial prop- this paper. The authors sincerely thank Dr K.K. Phani erties of C-coated alumina fiber/glass matrix fiber composites Head, Composite Division, for providing valuable sug- Ceram.Eng.Sci.Proc.10(78)(1989)912-924 gestions throughout this work. The [19RW. Rice, J.R. Spann, D. Lewis, W. Coblenz, The effect of ev aIso the room temperature mechanical gues of the X-ray, sEM, Composite, Refractories and behaviour of ceramic-fiber composites, Ceram. Eng. Sci. Proc. 5 Analytical Chemistry Sections for their kind help in (4)(1984)614624
(preform), without any formation of monocliic polymorphs. Obviously, the Y2O3 additive in the matrix materials helped to retain t-ZrO2 at 1400 Cby inhibiting grain growth [14], thus increasing the flexural strength of the CMCs and developing pseudo ductile character in the materials. From the foregoing discussion, it may be stated that the flexural strength of the CMCs fabricated from the four different sols decreased in the order: ZY > AS > AZ > A Therefore, bonding at the fibre/matrix interface can be tailored by changing the characteristics of the infiltrate, number of infiltration, intermediate and final sintering temperatures, addition of non-reactive fillers, and finally via in situ deposition of carbon in the fibre/matrix composite materials by proper choice of carbon generating decomposable groups present in the corresponding precursor sols. 4. Conclusions 1. The sol–gel vacuum infiltration technique is very effective for the fabrication of near-net-shape CMCs using discontinuous mullite fibre preform having 15 vol.% of fibre content and various sols (in single- and bi-component oxide systems) as the infiltrate. 2. Effects of sol viscosity, number of infiltration, in situ deposition of carbon in the composite materials, characteristics of the infiltrates and calcination temperature on physicomechanical properties of CMCs are examined. Multiple infiltrations and presence of fillers in the infiltrates has been found to be effective for the development of CMCs with pseudo ductile characteristics. 3. The pseudo ductile character developed in the CMCs is evident from the load–elongation curve of the three-point bend test. 4. SEM indicates fibre pull-out in the fracture surface of the CMCs. Acknowledgements The authors thank Dr. H.S. Maiti, Director, Central Glass & Ceramic Research Institute (CG&CRI) and Dr. N. 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