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 investigationmaintained 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 alu￾mina–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 aqu￾eous solution with Al3+ concentration of 1.5 M was prepared by dissolving the aluminium nitrate in deion￾ised water (conductivity of deionised water: 1.4
105 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 visc￾osity 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, sev￾eral 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 mate￾rials [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 evap￾oration. 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 subse￾quently 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 fol￾lowed. To carry out the infiltration experiments, a laboratory made set-up was used. Activated, precursor preforms of dimensions 40 mm
7 mm
6 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 infil￾trated 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 forma￾tion 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 character￾istics 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 cal￾cination 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 dimen￾sions 2 mm
2 mm
1 mm and (iii) wet chemical analysis. M.K. Naskar et al. / Ceramics International 30 (2004) 257–265 259