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 infiltrationsubmicron 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 dis￾continuous mullite fibre preforms (15 vol.% fibre content) as the reinforcement agent and single- and bi-compo￾nent 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 mm
7 mm
6 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 pre￾form. 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 mate￾rial [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 alu￾mina 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