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Near-net-shape fibre-reinforced ceramic matrix composites by the sol infiltration technique
Materials Letters 57 (2003) 2919 – 2926 www.elsevier.com/locate/matlet
Near-net-shape fibre-reinforced ceramic matrix composites by the sol infiltration technique A. Dey a, M. Chatterjee a,*, M.K. Naskar a, K. Basu b a
Sol – Gel Division, Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India b Regional Research Laboratory, Bhopal 462 026, India Received 7 October 2002; accepted 15 October 2002
Abstract The infiltration of a ceramic fibre preform by a ceramic sol is a cost-effective method for the manufacture of ceramic matrix composites (CMCs). The ceramic sol after calcination forms the matrix and the discontinuous fibres act as the reinforcement agents. In this paper, a technique has been presented in which discontinuous mullite preform with 15 vol.% of fibre content was infiltrated with the stabilized zirconia – yttria (ZrO210 wt.% Y2O3) sol to fabricate near-net-shape CMCs. The effects of sol viscosity, number of infiltration, in situ deposition of carbon in the CMC samples, and calcination (in air and nitrogen atmosphere) temperature on physico-mechanical properties of fabricated CMCs were examined. The characterization of the preform and the developed CMCs were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD indicated the presence of both cubic (c) and tetragonal (t) zirconia in the CMCs calcined even at 1400 jC. The flexural strength of the CMCs (calcined in air at 1400 jC) when determined by the three-point bend test was found to be almost 14 MPa under a given set of experimental conditions, while calcination of the same materials at 1400 jC in nitrogen atmosphere (carbon-containing CMCs) exhibited a modulus value of almost 51 GPa. SEM indicated multiple fractures of the matrix, which gave rise to pseudo-ductility. This is also evident from the load – elongation curve of the three-point bend test. The reinforced fibres acted as crack arresters, which prevented the propagation of the cracks. SEM studies also indicated fibre pull-out in the fracture surface of the CMCs. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Sol – gel preparation; Composite materials; Vacuum infiltration; Preforms
1. Introduction Recently, the development of ceramic fibre-reinforced ceramic matrix composites (CMCs) has attracted attention as a promising alternative to monolithic ceramics for obtaining structural materials suitable for engineering applications. The incorporation * Corresponding author. Tel.: +91-33-483-8086; tel./fax: +9133-473-0957. E-mail address: [email protected] (M. Chatterjee).
of reinforcing inorganic fibres into the ceramic matrix develops CMCs that exhibit pseudo-ductility, preventing catastrophic crack growth by such mechanisms as crack bridging, fibre debonding, and fibre pull-out [1– 7]. The characteristics of the CMCs will depend upon the shape, the size and dispersion of the reinforcement, the microstructure of the ceramic matrix, and the physico-chemical nature of the interface between the fibres and the matrix. The interaction between the fibre and the matrix is expected to take place, and a judicious control of processing parame-
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01397-6
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ters is necessary to establish the optimum degree of interfacial bonding [1,7]. There are several methods of producing CMCs (e.g., hot pressing, melt infiltration, polymer pyrolysis, and sol –gel) [4,5,8,9]. The sol – gel method of fabricating CMCs has several advantages, such as better homogeneity, low processing and sintering temperatures, and near-net-shape fabrication. 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]. Keeping in mind the aforementioned points, an attempt has been made in the present investigation towards: (i) the development of a technique for the fabrication of near-net-shape CMCs using discontinuous mullite fibre preforms and stabilized zirconia – yttria (ZrO210 wt.% Y2O3) sol as the infiltrant; (ii) the optimization of the process parameters for the fabrication of CMCs; and (iii) the characterization of the developed CMCs by different analytical techniques.
2. Experimental
oxychloride octahydrate (ZrOCl28H2O) and hydrous yttrium nitrate (both from M/s Indian Rare Earths, Mumbai), each with a purity of about 99.9%, were used as the starting materials [10,11]. Precipitates of hydrated zirconia were obtained by adding 1:1 vol/vol aqueous ammonia solution (25 wt.%, G.R., E. Merck, India) to a solution of zirconium oxychloride octahydrate in deionized water [10,11]. The washed (almost electrolyte free) precipitate was peptized with glacial acetic acid (99.8%; AnalaR, BDH, India) under stirring at 65 F 1 jC. The sol thus obtained had a Zr4 + concentration of 1.2 M. To a known volume of the zirconium acetate sol, a required amount of yttrium nitrate (10 wt.% equivalent Y2O3) was mixed under stirring. The pH and the viscosity of the resulting yttrium containing zirconia sol (parent sol) were found to be 2.53 F 0.02 and 2.8 F 1 mPa s, respectively. No organics were added to the sol as the viscosity controlling agent. From the parent sol, several sols of viscosities ranging from 3 F 1 to 90 F 1 mPa s (Table 1) were prepared by solvent evaporation. The pH of the sols was measured with a Jencons pH meter (Model 3030) while the viscosity values were recorded using a Brookfield viscometer (Model LVTDV-II).
2.1. Preparation of precursor fibre preforms Samples of dimensions 40 7 6 mm were cut from as-received 15% volume fraction discontinuous mullite fibre preforms of 100 mm diameter and 10 mm height (from M/s Orient Cerlane, 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 jC (at the heating rate of 1 jC/min) with a dwell time of 1 h to remove most of volatiles, if any, absorbed on the fibre surface and the intrafibre 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 infiltrants (liquid matrix precursor) for the fabrication of CMCs Zirconia (ZrO2) sols, stabilized with 10 wt.% Y2O3, were used as the infiltrant (i.e., the liquid matrix precursor). For the preparation of this sol, zirconium
2.3. Preparation of CMCs by the vacuum infiltration technique (VIT) In the present investigation, the vacuum infiltration technique, using the zirconia –yttria sol (Section 2.2) of various viscosities (Table 1) as the infiltrant, was followed [1,8,12]. To carry out the infiltration experiments, a laboratory-made set-up was used. Activated, precursor preforms of dimensions 40 7 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 infiltrated preform samples were placed in air at ambient temperature to convert the sol penetrating into the preform to the corresponding gel. The samples were further dried in an air-circulating oven at 100 F 2 jC for 4 h and subsequently calcined at 500, 800, and 1000 jC in air under static condition to remove the volatiles and decomposable materials. Calcining led to the formation of voids and cracking of matrix due to shrinkage. The above infiltration
A. Dey et al. / Materials Letters 57 (2003) 2919–2926
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Table 1 Characteristics of the products obtained under different experimental conditions Run
Sol viscosity (mPa s)
Number of infiltration
1 2 3
30 F 1 60 F 1 85 F 1
2 3 2
100 100 100
1000 1000 1000
0.97 0.93 1.15
4
80 F 1
3
1000
1400
5.59
5
60 F 1
3
1000
1400
5.64
6
60 F 1
5
800
1400
11.49
7
60 F 1(1)a + 40 F 1(2)a
3
800
1400
5.21
8
60 F 1(1)a + 40 F 1(4)a
5
800
1000
6.25
9
60 F 1(1)a + 40 F 1(4)a
5
800
1200
7.31
10
60 F 1(1)a + 40 F 1(4)a
5
800
1400
13.95
11
60 F 1(1)a + 40 F 1(2)a
3
500
1000b
6.32
12
60 F 1(1)a + 40 F 1(2)a
3
500
1400b
6.21
a b
Intermediate sintering temperature (jC)
Final sintering temperature (jC)
Flexural strength (MPa)
Characteristics of the products Fragile Fragile Fragile, formation of surface coating Formation of surface coating Brittle, good surface (ceramic character) Brittle, good surface (ceramic character) Good surface (pseudo-ductility) Brittle, good surface (ceramic character) Brittle, good surface (ceramic character) Brittle, good surface (ceramic character) Good surface (pseudo-ductility) Good surface (pseudo-ductility)
Figures in the parentheses indicate the number of infiltration. N2 atmosphere.
process was repeated to examine the effect of the number of infiltrations on the characteristics of the CMCs. The final sintering of the infiltrated preforms was performed at 1000, 1200, and 1400 jC in air under static condition. All the CMCs were white in colour. For the preparation of carbon-containing CMCs, the intermediate heating of the infiltrated preforms was performed at 500 jC with a dwell time of 1 h in static air followed by the final calcination at 1000 and 1400 jC, each for 1 h in nitrogen (N2) atmosphere with a flow rate of N2 at 1 l/min (runs 11 and 12 of Table 1). The CMCs obtained in N2 atmosphere were black in colour.
(SEM; Model Leo 400c) on samples of dimensions 2 2 1 mm; and (iii) wet chemical analysis. (b) The infiltrated materials (CMCs) were characterized by (i) XRD as described in Section 2.4 (a)(i) above; (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)(ii) above; and (iii) the flexural strength and modulus measurement on samples of dimensions 40 7 6 mm using a three-point bend test with the help of the Instron Universal Testing Machine (Model 5500 R) under a cross-head speed of 0.5 mm/min. Each strength datum is an average over six samples.
2.4. Characterization of the materials 3. Results and discussion (a) The as-received fibre preforms of tensile strength of about 2.5 GPa and modulus of about 100 GPa were characterised by: (i) X-ray diffraction (XRD; Model Philips PW 1730) using Ni-filtered CuKa radiation; (ii) scanning electron microscopy
3.1. Characteristics of the precursor preforms The XRD of the precursor preforms used in the present investigation indicated the presence of mullite
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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 Am. The shot content in the fibre preform was found to be negligible. The presence of considerable amounts of interfibre pores and voids is evident from the microstructure. The infiltration of the sol in the preform is expected to fill these interfibre 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 infiltrant (liquid matrix precursor) As alumina and alumino-silicate/ZrO2 composites are known to exhibit good mechanical properties [13 – 16], in the present study, yttria-stabilised zirconia sol was used to act as the matrix material for mullite fibre preforms. The Y2O3 additive in the sol helped to retain the t-ZrO2 polymorph even at 1400 jC by inhibiting grain growth. The sols (infiltrant) of various viscosities used for the infiltration of the samples of the preforms are given in Table 1. The characteristics of the infiltrant should be such that its decomposition takes place at a temperature below that at which sintering occurs, thus preventing interference with the densification process [17]. The homogenous mixing of the multicomponent oxides, higher purity, and
Fig. 1. SEM of the preform showing interfibre voids and pores.
low processing temperature are the main advantages of using a sol as the liquid infiltrant [1,3,4,18]. Moreover, in the infiltration process, the sol wets and encapsulates the fibres [19,20]. Keeping in mind the aforementioned points, CMC preparation using bicomponent sols as the infiltrant has been taken into consideration in the present investigation. The higher the sol viscosity, the more difficult it is to achieve good wetting of the fibres by the sol as well as the infiltration of the sol into all the interconnected pores and voids. On the other hand, too low a viscosity requires long processing time for achieving high density. Therefore, to examine the effect of sol viscosity on the characteristics of the CMCs, several sols of viscosities ranging from 3 F 1 to 90 F 1 mPa s were used. Sols with high viscosity (80 F 1 mPa s and above) were difficult to infiltrate due to the formation of (i) considerable amounts of air bubbles during operation and (ii) fragmented coatings on the surface of the preform after air drying, which prevented further penetration of sols inside the preform during repeated infiltration [10]. On the other hand, sols of low viscosity (30 F 1 mPa s) were also found to be unsuitable because in such cases, even after nine cycles of infiltrations, the samples exhibited considerably brittle character. Based on the above results, two different sols of viscosities 40 F 1 and 60 F 1 mPa s were used as the infiltrants in the present investigation. 3.3. Characteristics of the CMC samples prepared by the VIT Table 1 presents the characteristics of sample CMCs obtained in the present investigation. VIT has been followed for the preparation of CMCs in the present study as VIT aids in the removal of entrapped air in the fibre preform, thereby facilitating the infiltration process [1,7]. Further, the application of vacuum causes an increase in capillary pressure difference across the curved fibre surfaces [7], thus leading to enhanced infiltration of the liquid medium into the interfibre regions, thereby causing an increase in flexural strength of the CMCs (Table 1). To overcome a large shrinkage of matrix materials during drying, leading to extensive matrix cracking and residual fine scale porosity [7,8], multiple infiltration cycles followed by intermediate heat treatment was adopted in the present study [1,4,8]. Multiple infiltrations in the green state help to improve
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Fig. 2. Load – displacement curve of the product of run 7 of Table 1.
the green strength and green machinability and handling. Unless the decomposable materials present in the green body (even after multiple infiltration) are removed during intermediate sintering steps, a 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 jC had confirmed the removal of almost all the decomposable and carbonaceous materials at 800 jC [21] with the formation of whitecoloured fibres. On the other hand, the fibres calcined at 500 jC were black in colour due to the deposition of carbon via the decomposition of acetate groups present in the fibres. The acetate groups in the fibres originated from the corresponding zirconium acetate precursor sols. Based on this result, the intermediate sintering temperatures of 500, 800, and 1000 jC were selected for multiple infiltration of CMC samples in the present investigation. The final sintering of the samples was performed at temperatures of 1000, 1200, and 1400 jC. Table 1 indicates that the initial infiltration with a sol of high viscosity (60 F 1 mPa s), followed by intermediate sintering at 800 jC and further infiltration with a sol of low viscosity (40 F 1 mPa s), improves the flexural strength of the samples. The infiltrated sol, in the interfibre region of the sample preforms, after gel formation followed by calcination may form agglomerates of particles at the intermediate sintering temperature of 800 jC, which undergoes densification after further sintering
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at 1400 jC. 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 too high a temperature, the material becomes brittle because of the high interfacial reaction, while at too low a temperature, the matrix does not sinter adequately [1,3]. This is supported by the strength values of the CMCs sintered at different temperatures (Table 1). Thus, an optimum sintering temperature is needed. Table 1 further shows that the number of infiltration cycles also affects the characteristics of the CMCs. Comparing the results of runs 7 and 10, it reveals that with the decrease in the number of cycles of infiltration, keeping other parameters unaltered, the strength of the infiltrated samples decreases (run 7), but it exhibits pseudo-ductility. This may be due the development of weak interaction between the fibre and the matrix materials. Further, the reinforced fibres acted as crack arresters, which prevented the propagation of cracks. This is revealed from the load –elongation curve of the three-point bend test presented in Fig. 2. Fig. 3 represents the SEM of the fracture surface of the sample of run 7 after a three-point bend test, which indicates fibre pull-out in the sample. Multiple fractures of the matrix, which gave rise to pseudo-ductility, are also evident from the SEM of Fig. 4. XRD of the samples sintered at the 1400 jC confirmed the presence of both cubic (c) and tetragonal (t) ZrO2 along with the mullite, without any formation of monoclinic polymorphs. Obviously,
Fig. 3. SEM of the fracture surface of the CMC after a three-point bend test (product of run 7 of Table 1) showing fibre pull-out from the matrix.
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Fig. 4. SEM of the top surface of the CMC (product of run 7 of Table 1) showing multiple fractures of the matrix.
the Y2O3 additive in the matrix materials helped to retain t-ZrO2 at 1400 jC by inhibiting grain growth [10]. In contrast to run 7, the sample of run 10 of Table 1 fails to exhibit pseudo-ductility. The development of strong interaction between the fibre and the matrix material may be the reason of such failure. This is reflected from the load – elongation curve of the threepoint bend test presented in Fig. 5. Fig. 6 represents the SEM of the fracture surface after the three-point bend test of the product of run 10 of Table 1. Filling up of interfibre voids and pores by the infiltration of sol is clearly discernible from the figure. It is to be noted that all the CMC samples corresponding to runs 1 – 10 were white in colour (almost free from carbon) after heat treatment in air and exhibited modulus values in the range 1 –3 GPa.
Fig. 5. Load – displacement curve of the product of run 10 of Table 1.
Fig. 6. SEM of the fracture surface of the CMC after a three-point bend test (product of run 10 of Table 1) showing filling up of the interfibre voids and pores by the infiltration of sol.
Intermediate heat treatment of the products of runs 11 and 12 of Table 1 at 500 jC for 1 h in air resulted in the formation of black-coloured materials due to the in situ deposition of carbon from the decomposable acetate groups present in the infiltrated preforms [21]. The black colour of the above materials is retained after final calcination at 1000 and 1400 jC in N2 atmosphere. It is to be noted that the presence of carbon in the developed products corresponding to runs 11 and 12 of Table 1 caused an increase in their flexural strengths in comparison with that obtained for run 7 (free from carbon). Further, comparing the results of the products of run 7 (modulus value, 3 GPa) and run 12 (modulus value 51 GPa), it is observed that the presence of carbon in the fibre/ matrix composite material significantly increased the modulus values, thereby increasing pseudo-ductility
Fig. 7. Load – displacement curve of the product of run 12 of Table 1.
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for the development of CMCs with pseudo-ductile characteristics. 3. XRD indicates the presence of both c-ZrO2 and tZrO2 along with mullite in the CMCs calcined at 1400 jC. 4. The pseudo-ductile character developed in the CMCs is evident from the load – elongation curve of the three-point bend test. 5. SEM indicates fibre pull-out in the fracture surface of the CMCs. Acknowledgements Fig. 8. SEM of the fracture surface of the carbon-containing CMC after a three-point bend test (product of run 12 of Table 1) showing fibre pull-out from the matrix.
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 [22 – 25]. This is discernible from the load – displacement curve of the three-point bend test in Fig. 7 and the fibre pullout from the SEM of Fig. 8 of the same sample. Therefore, the bonding at the fibre matrix interface can be tailored by changing the characteristics of the infiltrant, number of infiltration, intermediate and final sintering temperatures, and finally via in situ deposition of carbon in the fibre/matrix composite materials by a proper choice of carbon-generating decomposable groups present in the corresponding precursor sols.
4. Conclusions 1. A sol – gel vacuum infiltration technique has been developed for the fabrication of near-net-shape CMCs using discontinuous mullite fibre preform with 15 vol.% fibre content and ZrO210 wt.% Y2O3 sol as the infiltrant. 2. Effects of sol viscosity, number of infiltration, in situ deposition of carbon in the composite materials, and calcination temperature on physico-mechanical properties of CMCs are examined. Multiple infiltration using sols of viscosities 40 F 1 and 60 F 1 mPa s has been found to be effective
The authors thank Dr. H.S. Maiti, Director, Central Glass and Ceramic Research Institute (CG and CRI) and Dr. N. Ramakrishnan, Director, Regional Research Laboratory, Bhopal, for their kind permission to publish this paper. The authors sincerely thank Dr. K.K. Phani, Head, Composite Division, for providing valuable suggestions throughout this work. They also thank colleagues of the X-ray, SEM, Composite and Analytical Chemistry Sections for their kind help in materials characterization. The financial assistance provided by the Aeronautics Research and Development Board (AR and DB), Ministry of Defence, Government of India, is also thankfully acknowledged. References [1] R.S. Russell-Floyd, B. Harris, R.G. Cooke, J. Laurie, F.W. Hammett, R.W. Jones, T. Wang, J. Am. Ceram. Soc. 76 (1993) 2635. [2] H.-K. Liu, W.-S. Kuo, B.-H. Lin, J. Mater. Sci. 33 (1998) 2095. [3] K.K. Chawla, Ceramic Matrix Composites, Chapman & Hall, London, 1993. [4] J.R. Strife, J.J. Brennan, K.M. Prewo, Ceram. Eng. Sci. Proc. 11 (1990) 871. [5] Anon., Am. Ceram. Soc. Bull. 65 (1986) 297. [6] J. Wu, F.R. Jones, P.F. James, Advances in Ceramic Matrix Composite II, p. 177. [7] X. Gu, P.A. Trusty, E.G. Butler, C.B. Ponton, J. Eur. Ceram. Soc. 20 (2000) 675. [8] S.-M. Sim, R.J. Kerans, Ceram. Eng. Sci. Proc. 13 (1992) 632. [9] W.B. Hilling, J. Am. Ceram. Soc. 71 (1988) C-96. [10] P.K. Chakrabarty, M. Chatterjee, M.K. Naskar, B. Siladitya, D. Ganguli, J. Eur. Ceram. Soc. 21 (2001) 355. [11] M.K. Naskar, D. Ganguli, J. Mater. Sci. 31 (1996) 6263. [12] E.H. Moore, T. Mah, K.A. Keller, Ceram. Eng. Sci. Proc. 15 (1994) 113.
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[13] E.A. Puger, P.E.D. Morgan, J. Am. Ceram. Soc. 69 (1986) C-120. [14] K. Ishida, K. Hirota, O. Yamaguchi, J. Am. Ceram. Soc. 77 (1994) 1391. [15] S.C.H. Koh, K.K. Aik, R. McPherson, in: S. Somiya, N. Yamamoto, H. Yanagida (Eds.), Advances in Ceramics, vol. 24A, The American Ceramic Society, Westerville, OH, 1986, p. 293. [16] P. White, H.K. Bowen, B. Fegley Jr., in: S. Somiya, N. Yamamoto, H. Yanagida (Eds.), Advances in Ceramics, vol. 24A, The American Ceramic Society, Westerville, OH, 1986, p. 301. [17] B.R. Marple, D.J. Green, J. Am. Ceram. Soc. 71 (1988) C-471. [18] H.-K. Liu, J. Mater. Sci. 31 (1996) 5093.
[19] B. Ben-Nissan, D. Martin, J. Sol-Gel Sci. Technol. 6 (1996) 187. [20] M.K. Cinibulk, Ceram. Eng. Sci. Proc. 17 (1996) 241. [21] M. Chatterjee, M.K. Naskar, D. Ganguli, Trans. Indian Ceram. Soc. 52 (1993) 51. [22] C.A. Doughan, R.L. Lehman, V.A. Greenhut, Ceram. Eng. Sci. Proc. 10 (1989) 912. [23] R.W. Rice, J.R. Spann, D. Lewis, W. Coblenz, Ceram. Eng. Sci. Proc. 5 (1984) 614. [24] K.A. Keller, T. Mah, T.A. Parthasarathy, C.M. Cooke, J. Am. Ceram. Soc. 83 (2002) 329. [25] H.-K. Liu, B.-H. Lin, Mater. Lett. 48 (2001) 230.
Near-net-shape fibre-reinforced ceramic matrix composites by the sol infiltration technique A. Dey a, M. Chatterjee a,*, M.K. Naskar a, K. Basu b a
Sol – Gel Division, Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India b Regional Research Laboratory, Bhopal 462 026, India Received 7 October 2002; accepted 15 October 2002
Abstract The infiltration of a ceramic fibre preform by a ceramic sol is a cost-effective method for the manufacture of ceramic matrix composites (CMCs). The ceramic sol after calcination forms the matrix and the discontinuous fibres act as the reinforcement agents. In this paper, a technique has been presented in which discontinuous mullite preform with 15 vol.% of fibre content was infiltrated with the stabilized zirconia – yttria (ZrO210 wt.% Y2O3) sol to fabricate near-net-shape CMCs. The effects of sol viscosity, number of infiltration, in situ deposition of carbon in the CMC samples, and calcination (in air and nitrogen atmosphere) temperature on physico-mechanical properties of fabricated CMCs were examined. The characterization of the preform and the developed CMCs were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD indicated the presence of both cubic (c) and tetragonal (t) zirconia in the CMCs calcined even at 1400 jC. The flexural strength of the CMCs (calcined in air at 1400 jC) when determined by the three-point bend test was found to be almost 14 MPa under a given set of experimental conditions, while calcination of the same materials at 1400 jC in nitrogen atmosphere (carbon-containing CMCs) exhibited a modulus value of almost 51 GPa. SEM indicated multiple fractures of the matrix, which gave rise to pseudo-ductility. This is also evident from the load – elongation curve of the three-point bend test. The reinforced fibres acted as crack arresters, which prevented the propagation of the cracks. SEM studies also indicated fibre pull-out in the fracture surface of the CMCs. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Sol – gel preparation; Composite materials; Vacuum infiltration; Preforms
1. Introduction Recently, the development of ceramic fibre-reinforced ceramic matrix composites (CMCs) has attracted attention as a promising alternative to monolithic ceramics for obtaining structural materials suitable for engineering applications. The incorporation * Corresponding author. Tel.: +91-33-483-8086; tel./fax: +9133-473-0957. E-mail address: [email protected] (M. Chatterjee).
of reinforcing inorganic fibres into the ceramic matrix develops CMCs that exhibit pseudo-ductility, preventing catastrophic crack growth by such mechanisms as crack bridging, fibre debonding, and fibre pull-out [1– 7]. The characteristics of the CMCs will depend upon the shape, the size and dispersion of the reinforcement, the microstructure of the ceramic matrix, and the physico-chemical nature of the interface between the fibres and the matrix. The interaction between the fibre and the matrix is expected to take place, and a judicious control of processing parame-
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01397-6
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A. Dey et al. / Materials Letters 57 (2003) 2919–2926
ters is necessary to establish the optimum degree of interfacial bonding [1,7]. There are several methods of producing CMCs (e.g., hot pressing, melt infiltration, polymer pyrolysis, and sol –gel) [4,5,8,9]. The sol – gel method of fabricating CMCs has several advantages, such as better homogeneity, low processing and sintering temperatures, and near-net-shape fabrication. 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]. Keeping in mind the aforementioned points, an attempt has been made in the present investigation towards: (i) the development of a technique for the fabrication of near-net-shape CMCs using discontinuous mullite fibre preforms and stabilized zirconia – yttria (ZrO210 wt.% Y2O3) sol as the infiltrant; (ii) the optimization of the process parameters for the fabrication of CMCs; and (iii) the characterization of the developed CMCs by different analytical techniques.
2. Experimental
oxychloride octahydrate (ZrOCl28H2O) and hydrous yttrium nitrate (both from M/s Indian Rare Earths, Mumbai), each with a purity of about 99.9%, were used as the starting materials [10,11]. Precipitates of hydrated zirconia were obtained by adding 1:1 vol/vol aqueous ammonia solution (25 wt.%, G.R., E. Merck, India) to a solution of zirconium oxychloride octahydrate in deionized water [10,11]. The washed (almost electrolyte free) precipitate was peptized with glacial acetic acid (99.8%; AnalaR, BDH, India) under stirring at 65 F 1 jC. The sol thus obtained had a Zr4 + concentration of 1.2 M. To a known volume of the zirconium acetate sol, a required amount of yttrium nitrate (10 wt.% equivalent Y2O3) was mixed under stirring. The pH and the viscosity of the resulting yttrium containing zirconia sol (parent sol) were found to be 2.53 F 0.02 and 2.8 F 1 mPa s, respectively. No organics were added to the sol as the viscosity controlling agent. From the parent sol, several sols of viscosities ranging from 3 F 1 to 90 F 1 mPa s (Table 1) were prepared by solvent evaporation. The pH of the sols was measured with a Jencons pH meter (Model 3030) while the viscosity values were recorded using a Brookfield viscometer (Model LVTDV-II).
2.1. Preparation of precursor fibre preforms Samples of dimensions 40 7 6 mm were cut from as-received 15% volume fraction discontinuous mullite fibre preforms of 100 mm diameter and 10 mm height (from M/s Orient Cerlane, 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 jC (at the heating rate of 1 jC/min) with a dwell time of 1 h to remove most of volatiles, if any, absorbed on the fibre surface and the intrafibre 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 infiltrants (liquid matrix precursor) for the fabrication of CMCs Zirconia (ZrO2) sols, stabilized with 10 wt.% Y2O3, were used as the infiltrant (i.e., the liquid matrix precursor). For the preparation of this sol, zirconium
2.3. Preparation of CMCs by the vacuum infiltration technique (VIT) In the present investigation, the vacuum infiltration technique, using the zirconia –yttria sol (Section 2.2) of various viscosities (Table 1) as the infiltrant, was followed [1,8,12]. To carry out the infiltration experiments, a laboratory-made set-up was used. Activated, precursor preforms of dimensions 40 7 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 infiltrated preform samples were placed in air at ambient temperature to convert the sol penetrating into the preform to the corresponding gel. The samples were further dried in an air-circulating oven at 100 F 2 jC for 4 h and subsequently calcined at 500, 800, and 1000 jC in air under static condition to remove the volatiles and decomposable materials. Calcining led to the formation of voids and cracking of matrix due to shrinkage. The above infiltration
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Table 1 Characteristics of the products obtained under different experimental conditions Run
Sol viscosity (mPa s)
Number of infiltration
1 2 3
30 F 1 60 F 1 85 F 1
2 3 2
100 100 100
1000 1000 1000
0.97 0.93 1.15
4
80 F 1
3
1000
1400
5.59
5
60 F 1
3
1000
1400
5.64
6
60 F 1
5
800
1400
11.49
7
60 F 1(1)a + 40 F 1(2)a
3
800
1400
5.21
8
60 F 1(1)a + 40 F 1(4)a
5
800
1000
6.25
9
60 F 1(1)a + 40 F 1(4)a
5
800
1200
7.31
10
60 F 1(1)a + 40 F 1(4)a
5
800
1400
13.95
11
60 F 1(1)a + 40 F 1(2)a
3
500
1000b
6.32
12
60 F 1(1)a + 40 F 1(2)a
3
500
1400b
6.21
a b
Intermediate sintering temperature (jC)
Final sintering temperature (jC)
Flexural strength (MPa)
Characteristics of the products Fragile Fragile Fragile, formation of surface coating Formation of surface coating Brittle, good surface (ceramic character) Brittle, good surface (ceramic character) Good surface (pseudo-ductility) Brittle, good surface (ceramic character) Brittle, good surface (ceramic character) Brittle, good surface (ceramic character) Good surface (pseudo-ductility) Good surface (pseudo-ductility)
Figures in the parentheses indicate the number of infiltration. N2 atmosphere.
process was repeated to examine the effect of the number of infiltrations on the characteristics of the CMCs. The final sintering of the infiltrated preforms was performed at 1000, 1200, and 1400 jC in air under static condition. All the CMCs were white in colour. For the preparation of carbon-containing CMCs, the intermediate heating of the infiltrated preforms was performed at 500 jC with a dwell time of 1 h in static air followed by the final calcination at 1000 and 1400 jC, each for 1 h in nitrogen (N2) atmosphere with a flow rate of N2 at 1 l/min (runs 11 and 12 of Table 1). The CMCs obtained in N2 atmosphere were black in colour.
(SEM; Model Leo 400c) on samples of dimensions 2 2 1 mm; and (iii) wet chemical analysis. (b) The infiltrated materials (CMCs) were characterized by (i) XRD as described in Section 2.4 (a)(i) above; (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)(ii) above; and (iii) the flexural strength and modulus measurement on samples of dimensions 40 7 6 mm using a three-point bend test with the help of the Instron Universal Testing Machine (Model 5500 R) under a cross-head speed of 0.5 mm/min. Each strength datum is an average over six samples.
2.4. Characterization of the materials 3. Results and discussion (a) The as-received fibre preforms of tensile strength of about 2.5 GPa and modulus of about 100 GPa were characterised by: (i) X-ray diffraction (XRD; Model Philips PW 1730) using Ni-filtered CuKa radiation; (ii) scanning electron microscopy
3.1. Characteristics of the precursor preforms The XRD of the precursor preforms used in the present investigation indicated the presence of mullite
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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 Am. The shot content in the fibre preform was found to be negligible. The presence of considerable amounts of interfibre pores and voids is evident from the microstructure. The infiltration of the sol in the preform is expected to fill these interfibre 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 infiltrant (liquid matrix precursor) As alumina and alumino-silicate/ZrO2 composites are known to exhibit good mechanical properties [13 – 16], in the present study, yttria-stabilised zirconia sol was used to act as the matrix material for mullite fibre preforms. The Y2O3 additive in the sol helped to retain the t-ZrO2 polymorph even at 1400 jC by inhibiting grain growth. The sols (infiltrant) of various viscosities used for the infiltration of the samples of the preforms are given in Table 1. The characteristics of the infiltrant should be such that its decomposition takes place at a temperature below that at which sintering occurs, thus preventing interference with the densification process [17]. The homogenous mixing of the multicomponent oxides, higher purity, and
Fig. 1. SEM of the preform showing interfibre voids and pores.
low processing temperature are the main advantages of using a sol as the liquid infiltrant [1,3,4,18]. Moreover, in the infiltration process, the sol wets and encapsulates the fibres [19,20]. Keeping in mind the aforementioned points, CMC preparation using bicomponent sols as the infiltrant has been taken into consideration in the present investigation. The higher the sol viscosity, the more difficult it is to achieve good wetting of the fibres by the sol as well as the infiltration of the sol into all the interconnected pores and voids. On the other hand, too low a viscosity requires long processing time for achieving high density. Therefore, to examine the effect of sol viscosity on the characteristics of the CMCs, several sols of viscosities ranging from 3 F 1 to 90 F 1 mPa s were used. Sols with high viscosity (80 F 1 mPa s and above) were difficult to infiltrate due to the formation of (i) considerable amounts of air bubbles during operation and (ii) fragmented coatings on the surface of the preform after air drying, which prevented further penetration of sols inside the preform during repeated infiltration [10]. On the other hand, sols of low viscosity (30 F 1 mPa s) were also found to be unsuitable because in such cases, even after nine cycles of infiltrations, the samples exhibited considerably brittle character. Based on the above results, two different sols of viscosities 40 F 1 and 60 F 1 mPa s were used as the infiltrants in the present investigation. 3.3. Characteristics of the CMC samples prepared by the VIT Table 1 presents the characteristics of sample CMCs obtained in the present investigation. VIT has been followed for the preparation of CMCs in the present study as VIT aids in the removal of entrapped air in the fibre preform, thereby facilitating the infiltration process [1,7]. Further, the application of vacuum causes an increase in capillary pressure difference across the curved fibre surfaces [7], thus leading to enhanced infiltration of the liquid medium into the interfibre regions, thereby causing an increase in flexural strength of the CMCs (Table 1). To overcome a large shrinkage of matrix materials during drying, leading to extensive matrix cracking and residual fine scale porosity [7,8], multiple infiltration cycles followed by intermediate heat treatment was adopted in the present study [1,4,8]. Multiple infiltrations in the green state help to improve
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Fig. 2. Load – displacement curve of the product of run 7 of Table 1.
the green strength and green machinability and handling. Unless the decomposable materials present in the green body (even after multiple infiltration) are removed during intermediate sintering steps, a 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 jC had confirmed the removal of almost all the decomposable and carbonaceous materials at 800 jC [21] with the formation of whitecoloured fibres. On the other hand, the fibres calcined at 500 jC were black in colour due to the deposition of carbon via the decomposition of acetate groups present in the fibres. The acetate groups in the fibres originated from the corresponding zirconium acetate precursor sols. Based on this result, the intermediate sintering temperatures of 500, 800, and 1000 jC were selected for multiple infiltration of CMC samples in the present investigation. The final sintering of the samples was performed at temperatures of 1000, 1200, and 1400 jC. Table 1 indicates that the initial infiltration with a sol of high viscosity (60 F 1 mPa s), followed by intermediate sintering at 800 jC and further infiltration with a sol of low viscosity (40 F 1 mPa s), improves the flexural strength of the samples. The infiltrated sol, in the interfibre region of the sample preforms, after gel formation followed by calcination may form agglomerates of particles at the intermediate sintering temperature of 800 jC, which undergoes densification after further sintering
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at 1400 jC. 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 too high a temperature, the material becomes brittle because of the high interfacial reaction, while at too low a temperature, the matrix does not sinter adequately [1,3]. This is supported by the strength values of the CMCs sintered at different temperatures (Table 1). Thus, an optimum sintering temperature is needed. Table 1 further shows that the number of infiltration cycles also affects the characteristics of the CMCs. Comparing the results of runs 7 and 10, it reveals that with the decrease in the number of cycles of infiltration, keeping other parameters unaltered, the strength of the infiltrated samples decreases (run 7), but it exhibits pseudo-ductility. This may be due the development of weak interaction between the fibre and the matrix materials. Further, the reinforced fibres acted as crack arresters, which prevented the propagation of cracks. This is revealed from the load –elongation curve of the three-point bend test presented in Fig. 2. Fig. 3 represents the SEM of the fracture surface of the sample of run 7 after a three-point bend test, which indicates fibre pull-out in the sample. Multiple fractures of the matrix, which gave rise to pseudo-ductility, are also evident from the SEM of Fig. 4. XRD of the samples sintered at the 1400 jC confirmed the presence of both cubic (c) and tetragonal (t) ZrO2 along with the mullite, without any formation of monoclinic polymorphs. Obviously,
Fig. 3. SEM of the fracture surface of the CMC after a three-point bend test (product of run 7 of Table 1) showing fibre pull-out from the matrix.
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Fig. 4. SEM of the top surface of the CMC (product of run 7 of Table 1) showing multiple fractures of the matrix.
the Y2O3 additive in the matrix materials helped to retain t-ZrO2 at 1400 jC by inhibiting grain growth [10]. In contrast to run 7, the sample of run 10 of Table 1 fails to exhibit pseudo-ductility. The development of strong interaction between the fibre and the matrix material may be the reason of such failure. This is reflected from the load – elongation curve of the threepoint bend test presented in Fig. 5. Fig. 6 represents the SEM of the fracture surface after the three-point bend test of the product of run 10 of Table 1. Filling up of interfibre voids and pores by the infiltration of sol is clearly discernible from the figure. It is to be noted that all the CMC samples corresponding to runs 1 – 10 were white in colour (almost free from carbon) after heat treatment in air and exhibited modulus values in the range 1 –3 GPa.
Fig. 5. Load – displacement curve of the product of run 10 of Table 1.
Fig. 6. SEM of the fracture surface of the CMC after a three-point bend test (product of run 10 of Table 1) showing filling up of the interfibre voids and pores by the infiltration of sol.
Intermediate heat treatment of the products of runs 11 and 12 of Table 1 at 500 jC for 1 h in air resulted in the formation of black-coloured materials due to the in situ deposition of carbon from the decomposable acetate groups present in the infiltrated preforms [21]. The black colour of the above materials is retained after final calcination at 1000 and 1400 jC in N2 atmosphere. It is to be noted that the presence of carbon in the developed products corresponding to runs 11 and 12 of Table 1 caused an increase in their flexural strengths in comparison with that obtained for run 7 (free from carbon). Further, comparing the results of the products of run 7 (modulus value, 3 GPa) and run 12 (modulus value 51 GPa), it is observed that the presence of carbon in the fibre/ matrix composite material significantly increased the modulus values, thereby increasing pseudo-ductility
Fig. 7. Load – displacement curve of the product of run 12 of Table 1.
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for the development of CMCs with pseudo-ductile characteristics. 3. XRD indicates the presence of both c-ZrO2 and tZrO2 along with mullite in the CMCs calcined at 1400 jC. 4. The pseudo-ductile character developed in the CMCs is evident from the load – elongation curve of the three-point bend test. 5. SEM indicates fibre pull-out in the fracture surface of the CMCs. Acknowledgements Fig. 8. SEM of the fracture surface of the carbon-containing CMC after a three-point bend test (product of run 12 of Table 1) showing fibre pull-out from the matrix.
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 [22 – 25]. This is discernible from the load – displacement curve of the three-point bend test in Fig. 7 and the fibre pullout from the SEM of Fig. 8 of the same sample. Therefore, the bonding at the fibre matrix interface can be tailored by changing the characteristics of the infiltrant, number of infiltration, intermediate and final sintering temperatures, and finally via in situ deposition of carbon in the fibre/matrix composite materials by a proper choice of carbon-generating decomposable groups present in the corresponding precursor sols.
4. Conclusions 1. A sol – gel vacuum infiltration technique has been developed for the fabrication of near-net-shape CMCs using discontinuous mullite fibre preform with 15 vol.% fibre content and ZrO210 wt.% Y2O3 sol as the infiltrant. 2. Effects of sol viscosity, number of infiltration, in situ deposition of carbon in the composite materials, and calcination temperature on physico-mechanical properties of CMCs are examined. Multiple infiltration using sols of viscosities 40 F 1 and 60 F 1 mPa s has been found to be effective
The authors thank Dr. H.S. Maiti, Director, Central Glass and Ceramic Research Institute (CG and CRI) and Dr. N. Ramakrishnan, Director, Regional Research Laboratory, Bhopal, for their kind permission to publish this paper. The authors sincerely thank Dr. K.K. Phani, Head, Composite Division, for providing valuable suggestions throughout this work. They also thank colleagues of the X-ray, SEM, Composite and Analytical Chemistry Sections for their kind help in materials characterization. The financial assistance provided by the Aeronautics Research and Development Board (AR and DB), Ministry of Defence, Government of India, is also thankfully acknowledged. References [1] R.S. Russell-Floyd, B. Harris, R.G. Cooke, J. Laurie, F.W. Hammett, R.W. Jones, T. Wang, J. Am. Ceram. Soc. 76 (1993) 2635. [2] H.-K. Liu, W.-S. Kuo, B.-H. Lin, J. Mater. Sci. 33 (1998) 2095. [3] K.K. Chawla, Ceramic Matrix Composites, Chapman & Hall, London, 1993. [4] J.R. Strife, J.J. Brennan, K.M. Prewo, Ceram. Eng. Sci. Proc. 11 (1990) 871. [5] Anon., Am. Ceram. Soc. Bull. 65 (1986) 297. [6] J. Wu, F.R. Jones, P.F. James, Advances in Ceramic Matrix Composite II, p. 177. [7] X. Gu, P.A. Trusty, E.G. Butler, C.B. Ponton, J. Eur. Ceram. Soc. 20 (2000) 675. [8] S.-M. Sim, R.J. Kerans, Ceram. Eng. Sci. Proc. 13 (1992) 632. [9] W.B. Hilling, J. Am. Ceram. Soc. 71 (1988) C-96. [10] P.K. Chakrabarty, M. Chatterjee, M.K. Naskar, B. Siladitya, D. Ganguli, J. Eur. Ceram. Soc. 21 (2001) 355. [11] M.K. Naskar, D. Ganguli, J. Mater. Sci. 31 (1996) 6263. [12] E.H. Moore, T. Mah, K.A. Keller, Ceram. Eng. Sci. Proc. 15 (1994) 113.
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