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Preparation of AlON–TiC composites via reaction-bonding
Materials Research Bulletin 40 (2005) 447–451 www.elsevier.com/locate/matresbu
Preparation of AlON–TiC composites via reaction-bonding K.F. Caia,b,c,*, D.S. McLachlanb a
Functional Materials Research Laboratory, Tongji University, 67 Chifeng Road, Shanghai 200092, PR China b School of Physics and Materials Physics Research Institute, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa c Institute of Materials Research, German Aerospace Center, D-51147, Cologne, Germany Received 15 April 2003; accepted 3 December 2004
Abstract AlON–TiC composites were fabricated via a reaction-bonding technique, using Al, Al2O3 and TiC powders as the starting materials. A composite sample sintered at 1850 8C after nitriding is highly densified and the Vickers hardness and fracture toughness of the sample are about 1751.1 kg/mm2 and 5.3 MPa m1/2, respectively. The composition and microstructure of the sample are characterized by means of XRD and SEM/EDX. # 2004 Elsevier Ltd. All rights reserved. Keywords: A: Composites; B: Chemical synthesis; D: Mechanical properties; D: Microstructure
Reaction bonding technology, which exhibits several advantages, including inexpensive raw materials, high stability, high purity and near net shape formation capability, has been used successfully for the production of Si3N4 (RBSN) and SiC (RBSC) [1,2]. Since 1989, when Claussen et al. [3] introduced reaction-bonding aluminium oxide (RBAO), many ceramists have studied RBAO [4–7]. A few years ago, Cai et al. [8,9] introduced another reaction-bonding technique, i.e. reaction-bonding aluminium nitride (RBAN). More recently, Cai et al. [10] have successfully fabricated Al2O3–AlON and Al2O3–AlN composites, and Cai and McLachlan [11], AlN ceramics, via RBAN. The reaction bonded AlN ceramics show high hardness, high electrical resistivity, moderate dielectric properties [11], and high thermal conductivity after annealing [12]. In this letter, we report on preparation of AlON–TiC composites via RBAN. AlON–TiC composites have, to our knowledge, not yet been previously studied. * Corresponding author. Tel.: +86 21 65980255; fax: +86 21 65980255. E-mail address: [email protected] (K.F. Cai). 0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2004.12.001
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K.F. Cai, D.S. McLachlan / Materials Research Bulletin 40 (2005) 447–451
23.5 wt.% of Al (5 mm, Cerac Incorporated), 33.8 wt.% of Al2O3 (0.3 mm, AKP-50, Sumitomo), and 42.7 wt.% of TiC (average grain size 1.5 mm) were homogenized by ball milling (the weight ratio of powder to WC based cement balls is 0.1) for 15 h in hexane and then dried in a vacuum. The homogenized powders were uniaxially pressed at 100 MPa in a steel die to obtain 20 mm diameter and 3 mm high pellets. The pellets were nitrided inside an Al2O3 tube in N2–4% H2 flowing at 0.5 liter/ min. The heating rate was 2 and 1 8C/min below and above 660 8C, respectively. The pellets remained at 1200 8C for 12 min. The pellets were then cooled at 5 8C/min to room temperature and removed from the furnace for further sintering. The samples were pressurelessly sintered at 1750, 1800 and 1850 8C for 1 h in Ar, with a heating and cooling rate of 20 8C/min. The density of the sintered samples was determined by a displacement method in water. Vickers hardness of the sintered samples was evaluated at a load of 1 kg. Fracture toughness was determined from radial cracks produced by Vickers indentation at a load of 1 kg [13]. Samples evaluated for indentation were polished with a 3-mm diamond paste. Five separate indentations were made on each sample and the results averaged. The phase composition of the nitrided and sintered samples was determined by X-ray diffraction (XRD) and the microstructure of the sintered samples was characterized by means of scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The density of the samples sintered at 1750, 1800 and 1850 8C is about 3.1511, 3.4211 and 3.7182 g/ cm3, respectively, and the apparent porosity is about 15.25, 9.04 and 1.75%, respectively. In the XRD spectrum for the nitrided samples (Fig. 1 a), there are no peaks corresponding to Al, while there are weak peaks corresponding to hexagonal AlN. There are also very strong peaks for TiC and Al2O3. These results indicate that all the Al has been nitrided into hexagonal AlN. Because the newly formed AlN grains are nanosized and not well crystallized [9], the peaks for AlN are broad and weak. Fig. 1b shows the XRD spectrum of the samples sintered at 1850 8C (the spectrum of the samples sintered at 1750 or 1800 8C is very similar to that of the samples sintered at 1850 8C), the Al2O3 peaks have disappeared, the AlN peaks have become still weaker, and so that now, besides the strong TiC peaks, strong peaks for a newly formed phase of Al2.7 + 0.33xO4 xNx (0.22 x 0.50) have also appeared [10]. At first thought, the intensity of the AlN peaks in Fig. 1b should have become stronger than that in Fig. 1a, as the crystallization and the size of the AlN grains should have increased after sintering. However, during the sintering process, the AlN reacted with the Al2O3 to form Al2.7 + 0.33xO4 xNx according to the following equation [12]: AlN þ Al2 O3 ! Al2:7 O4x Nx ;
(1)
with 0.22 x 0.50. It can be seen from the Eq. (1) that in order to form certain amount of Al2.7 + 0.33xO4 xNx (written as AlON below), a higher molar content of Al2O3 than that of AlN is
Fig. 1. XRD spectra of the nitrided (a) and the sample sintered at 1850 8C (b).
K.F. Cai, D.S. McLachlan / Materials Research Bulletin 40 (2005) 447–451
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needed. The composition of the starting materials was chosen such that the molar content of AlN, assuming all the Al was nitrided into AlN, is higher than Al2O3. Therefore, in the XRD pattern of the sintered samples (Fig. 1b) there are still very weak AlN peaks but no Al2O3 peaks, which is in good agreement with Gogotsi et al.’s [13] work. Gogotsi et al. [13] have hot pressed AlON–TiN composites at 1900 8C under 30 MPa for 30 min, using AlN, Al2O3, and TiN as the starting materials. They also found very little AlN left in the sample as the starting materials consisted of 29 wt.% AlN, 29 wt.% Al2O3 and 42 wt.% TiC, which is similar to the composition of the present work (which after nitriding is AlN:Al2O3:TiC = 31.56:30.64:37.8 wt.%, assuming the Al was fully converted into AlN). An AlN-free AlON–TiC composite could be fabricated by adjusting the starting composition of Al and Al2O3. Because both the samples sintered at 1750 and also 1800 8C have higher porosities, further information of these two samples are not given here. Fig. 2a and b are typical secondary electron (SE) and back-scattered electron (BSE) micrograph of fracture surface of the samples sintered at 1850 8C, respectively. It can be seen from Fig. 2a that the samples are reasonably dense (very few
Fig. 2. Typical secondary electron micrograph (a) and back-scattered electron microscopy (b) of the fracture surface of sintered sample.
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Fig. 3. Typical SEM micrograph of the polished section of the samples sintered at 1850 8C with the indentation-induced crack.
observable micropores). In the BSE micrograph (Fig. 2b), the large white phase areas are the TiC grains. Note that the TiC grains are randomly dispersed in the AlON matrix. Tungsten carbide was identified as main impurity in the composites, using EDAX. The Vickers hardness of the sample sintered at 1850 8C is about 1751.1 kg/mm2, and the fracture toughness is 5.3 MPa m1/2. The values of Vickers hardness and fracture toughness are comparable to the hot-pressed AlON–TiN composites with a near-theoretical density [13]. Compared with the Vickers hardness and fracture toughness of pure AlON (being about 1200 kg/mm2 and 2.0–2.9 MPa m1/2, respectively [13]), the AlON–TiC composites sintered at 1850 8C have a higher hardness and fracture toughness. This is because TiC is harder than AlON, and the effects of crack deflection and grain bridging by TiC grains as clearly shown in Fig. 3. Crack deflection is due to the intrinsic stresses produced by thermal mismatch between the AlON and TiC phases during cooling (the thermal expansion coefficient of AlON and TiC being 5.37–7.6 106 and 7.4 106/K, respectively [13,14]). In conclusion, AlON–TiC composites have been successfully prepared using reaction bonding. The composites sintered at 1850 8C have good mechanical properties. This work suggests that a series of AlON based composites, such as AlON–TiN/TiB2 (using TiN or TiB2 instead of TiC), and AlN–AlON– TiC/TiN/TiB2 or Al2O3–AlON–TiC/TiN/TiB2 (by adjusting the starting composition of Al and Al2O3 and using TiC, or TiN or TiB2 as the second phase), could be fabricated by using the RBAN technique. Acknowledgment One of the authors (K.F. Cai) would like to thank the Alexander von Humboldt Foundation of Germany for a research fellowship. References [1] A.J. Moulson, J. Mater. Sci. 28 (1993) 3709. [2] C.W. Forrest, P. Kennedy, J.V. Sherman, Ceramics for High Performance Applications, Brook, Hill, Chestnut Hill, MA, 1975, p. 99.
K.F. Cai, D.S. McLachlan / Materials Research Bulletin 40 (2005) 447–451 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
451
N. Claussen, T. Le, S. Wu, J. Eur. Ceram. Soc. 5 (1989) 29. S.H. Yokota, L.C. De Jonghe, M.N. Rahaman, J. Mater. Sci. 9 (1994) 4177. J.H. Park, Z.S. Ahn, J. Mater. Sci. Lett. 13 (1994) 650. J. Luten, J. Cooymans, P. Diels, J. Sleurs, in: P. Duran, J.F. Fernandez (Eds.), Third Euro-Ceramics, vol. 1, 1993, p. 657. E. Suvaci, G.L. Messing, J. Mater. Sci. 34 (1999) 3249. K.F. Cai, C.W. Nan, R.Z. Yuan, Chin. Sci. Bull. 40 (1995) 879. K.F. Cai, C.W. Nan, R.Z. Yuan, J. Chin. Ceram. Soc. 24 (1996) 216. K.F. Cai, D.S. Mclachlan, I. Sigalas, J. Mater. Sci. Lett. 20 (2001) 193. K.F. Cai, D.S. Mclachlan, Mater. Res. Bull. 37 (2002) 575. K.F. Cai, D.S. McLachlan, G. Sauti, E. Mueller, Solid State Sci., in press. Y.G. Gogotsi, F. Porz, V.P. Yaroshenko, J. Am. Ceram. Soc. 75 (8) (1992) 2251. K.F. Cai, D.S. Mclachlan, N. Axen, R. Manyatsal, Ceram. Int. 28 (2002) 217.
Preparation of AlON–TiC composites via reaction-bonding K.F. Caia,b,c,*, D.S. McLachlanb a
Functional Materials Research Laboratory, Tongji University, 67 Chifeng Road, Shanghai 200092, PR China b School of Physics and Materials Physics Research Institute, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa c Institute of Materials Research, German Aerospace Center, D-51147, Cologne, Germany Received 15 April 2003; accepted 3 December 2004
Abstract AlON–TiC composites were fabricated via a reaction-bonding technique, using Al, Al2O3 and TiC powders as the starting materials. A composite sample sintered at 1850 8C after nitriding is highly densified and the Vickers hardness and fracture toughness of the sample are about 1751.1 kg/mm2 and 5.3 MPa m1/2, respectively. The composition and microstructure of the sample are characterized by means of XRD and SEM/EDX. # 2004 Elsevier Ltd. All rights reserved. Keywords: A: Composites; B: Chemical synthesis; D: Mechanical properties; D: Microstructure
Reaction bonding technology, which exhibits several advantages, including inexpensive raw materials, high stability, high purity and near net shape formation capability, has been used successfully for the production of Si3N4 (RBSN) and SiC (RBSC) [1,2]. Since 1989, when Claussen et al. [3] introduced reaction-bonding aluminium oxide (RBAO), many ceramists have studied RBAO [4–7]. A few years ago, Cai et al. [8,9] introduced another reaction-bonding technique, i.e. reaction-bonding aluminium nitride (RBAN). More recently, Cai et al. [10] have successfully fabricated Al2O3–AlON and Al2O3–AlN composites, and Cai and McLachlan [11], AlN ceramics, via RBAN. The reaction bonded AlN ceramics show high hardness, high electrical resistivity, moderate dielectric properties [11], and high thermal conductivity after annealing [12]. In this letter, we report on preparation of AlON–TiC composites via RBAN. AlON–TiC composites have, to our knowledge, not yet been previously studied. * Corresponding author. Tel.: +86 21 65980255; fax: +86 21 65980255. E-mail address: [email protected] (K.F. Cai). 0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2004.12.001
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K.F. Cai, D.S. McLachlan / Materials Research Bulletin 40 (2005) 447–451
23.5 wt.% of Al (5 mm, Cerac Incorporated), 33.8 wt.% of Al2O3 (0.3 mm, AKP-50, Sumitomo), and 42.7 wt.% of TiC (average grain size 1.5 mm) were homogenized by ball milling (the weight ratio of powder to WC based cement balls is 0.1) for 15 h in hexane and then dried in a vacuum. The homogenized powders were uniaxially pressed at 100 MPa in a steel die to obtain 20 mm diameter and 3 mm high pellets. The pellets were nitrided inside an Al2O3 tube in N2–4% H2 flowing at 0.5 liter/ min. The heating rate was 2 and 1 8C/min below and above 660 8C, respectively. The pellets remained at 1200 8C for 12 min. The pellets were then cooled at 5 8C/min to room temperature and removed from the furnace for further sintering. The samples were pressurelessly sintered at 1750, 1800 and 1850 8C for 1 h in Ar, with a heating and cooling rate of 20 8C/min. The density of the sintered samples was determined by a displacement method in water. Vickers hardness of the sintered samples was evaluated at a load of 1 kg. Fracture toughness was determined from radial cracks produced by Vickers indentation at a load of 1 kg [13]. Samples evaluated for indentation were polished with a 3-mm diamond paste. Five separate indentations were made on each sample and the results averaged. The phase composition of the nitrided and sintered samples was determined by X-ray diffraction (XRD) and the microstructure of the sintered samples was characterized by means of scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The density of the samples sintered at 1750, 1800 and 1850 8C is about 3.1511, 3.4211 and 3.7182 g/ cm3, respectively, and the apparent porosity is about 15.25, 9.04 and 1.75%, respectively. In the XRD spectrum for the nitrided samples (Fig. 1 a), there are no peaks corresponding to Al, while there are weak peaks corresponding to hexagonal AlN. There are also very strong peaks for TiC and Al2O3. These results indicate that all the Al has been nitrided into hexagonal AlN. Because the newly formed AlN grains are nanosized and not well crystallized [9], the peaks for AlN are broad and weak. Fig. 1b shows the XRD spectrum of the samples sintered at 1850 8C (the spectrum of the samples sintered at 1750 or 1800 8C is very similar to that of the samples sintered at 1850 8C), the Al2O3 peaks have disappeared, the AlN peaks have become still weaker, and so that now, besides the strong TiC peaks, strong peaks for a newly formed phase of Al2.7 + 0.33xO4 xNx (0.22 x 0.50) have also appeared [10]. At first thought, the intensity of the AlN peaks in Fig. 1b should have become stronger than that in Fig. 1a, as the crystallization and the size of the AlN grains should have increased after sintering. However, during the sintering process, the AlN reacted with the Al2O3 to form Al2.7 + 0.33xO4 xNx according to the following equation [12]: AlN þ Al2 O3 ! Al2:7 O4x Nx ;
(1)
with 0.22 x 0.50. It can be seen from the Eq. (1) that in order to form certain amount of Al2.7 + 0.33xO4 xNx (written as AlON below), a higher molar content of Al2O3 than that of AlN is
Fig. 1. XRD spectra of the nitrided (a) and the sample sintered at 1850 8C (b).
K.F. Cai, D.S. McLachlan / Materials Research Bulletin 40 (2005) 447–451
449
needed. The composition of the starting materials was chosen such that the molar content of AlN, assuming all the Al was nitrided into AlN, is higher than Al2O3. Therefore, in the XRD pattern of the sintered samples (Fig. 1b) there are still very weak AlN peaks but no Al2O3 peaks, which is in good agreement with Gogotsi et al.’s [13] work. Gogotsi et al. [13] have hot pressed AlON–TiN composites at 1900 8C under 30 MPa for 30 min, using AlN, Al2O3, and TiN as the starting materials. They also found very little AlN left in the sample as the starting materials consisted of 29 wt.% AlN, 29 wt.% Al2O3 and 42 wt.% TiC, which is similar to the composition of the present work (which after nitriding is AlN:Al2O3:TiC = 31.56:30.64:37.8 wt.%, assuming the Al was fully converted into AlN). An AlN-free AlON–TiC composite could be fabricated by adjusting the starting composition of Al and Al2O3. Because both the samples sintered at 1750 and also 1800 8C have higher porosities, further information of these two samples are not given here. Fig. 2a and b are typical secondary electron (SE) and back-scattered electron (BSE) micrograph of fracture surface of the samples sintered at 1850 8C, respectively. It can be seen from Fig. 2a that the samples are reasonably dense (very few
Fig. 2. Typical secondary electron micrograph (a) and back-scattered electron microscopy (b) of the fracture surface of sintered sample.
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Fig. 3. Typical SEM micrograph of the polished section of the samples sintered at 1850 8C with the indentation-induced crack.
observable micropores). In the BSE micrograph (Fig. 2b), the large white phase areas are the TiC grains. Note that the TiC grains are randomly dispersed in the AlON matrix. Tungsten carbide was identified as main impurity in the composites, using EDAX. The Vickers hardness of the sample sintered at 1850 8C is about 1751.1 kg/mm2, and the fracture toughness is 5.3 MPa m1/2. The values of Vickers hardness and fracture toughness are comparable to the hot-pressed AlON–TiN composites with a near-theoretical density [13]. Compared with the Vickers hardness and fracture toughness of pure AlON (being about 1200 kg/mm2 and 2.0–2.9 MPa m1/2, respectively [13]), the AlON–TiC composites sintered at 1850 8C have a higher hardness and fracture toughness. This is because TiC is harder than AlON, and the effects of crack deflection and grain bridging by TiC grains as clearly shown in Fig. 3. Crack deflection is due to the intrinsic stresses produced by thermal mismatch between the AlON and TiC phases during cooling (the thermal expansion coefficient of AlON and TiC being 5.37–7.6 106 and 7.4 106/K, respectively [13,14]). In conclusion, AlON–TiC composites have been successfully prepared using reaction bonding. The composites sintered at 1850 8C have good mechanical properties. This work suggests that a series of AlON based composites, such as AlON–TiN/TiB2 (using TiN or TiB2 instead of TiC), and AlN–AlON– TiC/TiN/TiB2 or Al2O3–AlON–TiC/TiN/TiB2 (by adjusting the starting composition of Al and Al2O3 and using TiC, or TiN or TiB2 as the second phase), could be fabricated by using the RBAN technique. Acknowledgment One of the authors (K.F. Cai) would like to thank the Alexander von Humboldt Foundation of Germany for a research fellowship. References [1] A.J. Moulson, J. Mater. Sci. 28 (1993) 3709. [2] C.W. Forrest, P. Kennedy, J.V. Sherman, Ceramics for High Performance Applications, Brook, Hill, Chestnut Hill, MA, 1975, p. 99.
K.F. Cai, D.S. McLachlan / Materials Research Bulletin 40 (2005) 447–451 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
451
N. Claussen, T. Le, S. Wu, J. Eur. Ceram. Soc. 5 (1989) 29. S.H. Yokota, L.C. De Jonghe, M.N. Rahaman, J. Mater. Sci. 9 (1994) 4177. J.H. Park, Z.S. Ahn, J. Mater. Sci. Lett. 13 (1994) 650. J. Luten, J. Cooymans, P. Diels, J. Sleurs, in: P. Duran, J.F. Fernandez (Eds.), Third Euro-Ceramics, vol. 1, 1993, p. 657. E. Suvaci, G.L. Messing, J. Mater. Sci. 34 (1999) 3249. K.F. Cai, C.W. Nan, R.Z. Yuan, Chin. Sci. Bull. 40 (1995) 879. K.F. Cai, C.W. Nan, R.Z. Yuan, J. Chin. Ceram. Soc. 24 (1996) 216. K.F. Cai, D.S. Mclachlan, I. Sigalas, J. Mater. Sci. Lett. 20 (2001) 193. K.F. Cai, D.S. Mclachlan, Mater. Res. Bull. 37 (2002) 575. K.F. Cai, D.S. McLachlan, G. Sauti, E. Mueller, Solid State Sci., in press. Y.G. Gogotsi, F. Porz, V.P. Yaroshenko, J. Am. Ceram. Soc. 75 (8) (1992) 2251. K.F. Cai, D.S. Mclachlan, N. Axen, R. Manyatsal, Ceram. Int. 28 (2002) 217.