- Email: [email protected]
Structural evolution of the Ti–Si–C system during mechanical alloying
Journal of Alloys and Compounds 395 (2005) 88–92
Structural evolution of the Ti–Si–C system during mechanical alloying C.J. Lu, Z.Q. Li∗ Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China Received 1 November 2004; received in revised form 22 November 2004; accepted 23 November 2004 Available online 7 January 2005
Abstract Investigation on the structural evolution for the Ti–Si–C system during mechanical alloying (MA) using a planetary ball mill was systematically performed under identical milling conditions. Experimental results on different powder mixtures were examined and compared. Two kinds of mechanically induced chemical reactions were observed in the Ti–Si–C system, which depends on the composition of the ball-milled mixtures. Gradual reaction was observed in both the Ti50 C50 and Si50 C50 mixtures, and TiC and SiC as the resultants were obtained, respectively. The reaction of Ti50 Si50 mixture during MA was also a gradual reaction, but the resultants are crystalline Ti5 Si3 and an amorphous phase. The TiC–SiC composites, however, were synthesized by mechanically alloying the Ti25 Si25 C50 powder mixture through a mechanically induced self-sustaining reaction (MSR) under the same milling conditions as those of Ti50 C50 , Si50 C50 and Ti50 Si50 mixtures. So the MSR process might be applied to synthesize TiC–SiC composites. The structural evolution processes and thermodynamics of the reactions are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Mechanical alloying (MA); Mechanically induced self-sustaining reaction (MSR); TiC–SiC composites
1. Introduction Single-phase ceramics like TiC or SiC are of practical importance due to its high melting point, good thermal stability, and high erosion resistance. The low fracture toughness, however, is a well-known impediment to its applications as structural components. In order to increase the toughness of such ceramics, composites such as TiC–SiC, have been investigated [1–6]. Mechanical alloying (MA) is a powerful technique for synthesizing a variety of materials, such as compounds [7], metastable solid solutions [8], amorphous alloys [9], nanocrystalline materials [10] and so forth. Two kinds of reaction mechanism for MA have been commonly accepted: one is a gradual reaction through elemental diffusion, and the other is a self-sustaining reaction ignited after a certain period of activation time. The mechanically induced self-sustaining reaction (MSR) was usually observed in highly exothermic systems, such as Ti–B [11], Ni–Al [12], Mo–Si [13]. How∗
Corresponding author. Fax: +86 571 87951411. E-mail address: [email protected] (Z.Q. Li).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.11.046
ever, MA process is so complex that it is difficult to predict the precise reaction route and final products from starting materials. Many factors such as milling dynamics and experimental conditions have a strong influence on the final products. For example, TiC may be obtained by self-sustaining reaction using a high energy SPEX 8000 ml mixer mill [14,15], and be also obtained by continuous structural evolution using a planetary ball mill [16]. A self-sustaining reaction was initiated by ball milling binary Sn50 S50 and Zn50 S50 powder mixtures, but the reaction in ternary Sn25 Zn25 S50 system was a gradual reaction under identical milling conditions [17,18]. The present work is mainly concerned with the structural evolution for Ti–Si–C system during MA using a planetary ball mill. The experimental results reveal that the reaction process for the Ti–Si–C system is just contrary to that of the Sn–Zn–S system.
2. Experimental To decrease the oxide layer on silicon powder, fresh silicon obtained by ball milling silicon wafers of 99.9% purity
C.J. Lu, Z.Q. Li / Journal of Alloys and Compounds 395 (2005) 88–92
powder was applied as one of the raw materials for MA. Commercial silicon powder with a purity of 99.9% and an average particle size of 75 m was also used for comparison. Titanium and graphite powders with purities of 99.5 and 99.95%, and average particle sizes of 75 and 100 m, respectively, were used as the starting materials. Four typical compositions of Ti50 C50 , Si50 C50 , Ti50 Si50 , and Ti25 Si25 C50 were ball milled using a QM-SB type planetary ball mill with stainless steel vacuum vials and milling balls at a rotation rate of 300 rpm. The powder-to-ball mass ratio was about 1:20. The vials were filled with argon to protect powders from oxidation during MA. Samples milled for predetermined hours without interruption were characterized by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM) and fourier transform infrared (FTIR) spectroscopy. Some samples were annealed at 950 ◦ C for 1 h in flowing argon. XRD patterns were taken using a Rigaku D/max 2550PC X-ray diffractometer with Cu K␣ radiation, the SEM observation was carried out using a SIRION scanning electron microscope and FTIR spectra were obtained using a Nicolet AVATAR 360 spectroscope.
3. Experimental results and discussion The brittle Si wafers can be easily milled into small particles at the initial stage of milling, and the particle size was about 1 m after 5 h of milling, which means that Si wafers may be used as a raw material for MA. In order to investigate the effect of different Si powders on MA, both the Si powder made from wafers and the commercial Si powder were separately ball milled with graphite powder under identical conditions. Experimental results show that the MA products obtained from different Si powders were nearly the same. Hereafter the Si powder used for sample preparation was all from ball-milled Si wafers. Fig. 1 shows the XRD patterns taken from the as-milled Si50 C50 powders at different milling hours. The intensity of the silicon peaks decreased gradually with increasing milling hours, accompanied by broadening of the full width at half maximums (FWHMs) of the silicon peaks. The intensity of the C peaks, however, decreased obviously with increasing milling hours till they disappear at about 20 h of milling. The silicon carbide was not observed in the XRD pattern. This phenomenon was also observed during MA of C and W. To explore the reason why the C peaks disappeared at the early stage of MA, a similar experiment was carried out by ball milling a mixture of W and MoS2 . Both graphite and MoS2 belong to the hexagonal system with a layered structure, and can be easily cleaved along (0 0 1) planes. Since the atomic scattering factors of Mo and S are much larger than that of C, the (0 0 2) peak of MoS2 is still visible when the mixture of W and MoS2 has been ball milled even for 72 h. The thickness of MoS2 flakes measured from FWHM by a single line method [19] is about 2 nm, which means the MoS2 flakes are composed of several atomic layers. It is suggested from this experimental result
89
Fig. 1. XRD patterns taken from the Si50 C50 mixture milled for different hours. The main diffraction peaks of C, Si and SiC are marked on the diffractograms, and Fe comes from the milling media: (a) 5 h; (b) 72 h; (c) 120 h; (d) 180 h.
that the vanishing of the C peaks in the XRD patterns is mainly due to its low atomic scattering factor and small particle size. Fig. 2 shows the FTIR spectra taken from Si50 C50 samples after different milling hours. Curves 2a and 2b were obtained from the raw Si50 C50 mixture and the 72 h-milled sample, respectively. The small difference between curve 2a and 2b indicates the structural evolution of Si and C before the formation of SiC, which might be related with the gradual reaction between C and Si. After having been ball milled for about 100 h, the SiC phase with considerably broadened peaks was detected in the XRD pattern and the Si peaks were still observed at the same time. When the mixture was ball milled for 180 h, the reaction between C and Si completed and only the SiC peaks
Fig. 2. FTIR spectra taken from Si50 C50 samples milled for different hours: (a) 0 h; (b) 72 h; (c) 180 h.
90
C.J. Lu, Z.Q. Li / Journal of Alloys and Compounds 395 (2005) 88–92
Fig. 3. XRD patterns taken from Ti50 C50 mixture milled for 72 h (a) and 103 h (b). The main diffraction peaks of Ti and TiC are marked on the diffractograms.
with big FWHMs were observed, which means that fine SiC particles were obtained. Curve c in Fig. 2 was obtained from the 180 h-milled sample, and the peak at about 873 cm−1 in curve c is assigned to the Si C bonds, which also confirms the formation of the SiC phase. The experimental results testify that the formation of SiC from ball milling Si and C is a gradual reaction. Fig. 3 shows the XRD patterns taken from mixed Ti50 C50 powders at different milling times. It is very clear from Figs. 1 and 3 that TiC is more easily obtained by ball milling a mixture of Ti and C powders than SiC by ball milling a mixture of Si and C powders under the same milling conditions. A small amount of TiC was produced after 72 h of milling, but SiC was not observed at the same milling time. The amount of TiC increased with increasing milling time, and finally all the Ti and C powders transformed into TiC, which implied that the structural evolution in the Ti50 C50 system is also a continuous reaction. Similar processes have also been observed by other authors with a planetary ball mill [16]. MA of a Ti25 Si25 C50 mixture was also performed under the same milling conditions. The XRD patterns taken from the samples milled for different hours are shown in Fig. 4. A small amount of TiC and a trace of crystalline Ti5 Si3 were observed when the Ti25 Si25 C50 mixture was ball milled for 72 h. The XRD peak widths of Si and Ti increased with increasing milling time till 98 h. The average grain sizes of Si and Ti for the 98 h-milled sample were about 30 and 60 nm, respectively. After having been milled a little longer, see pattern c in Fig. 4, nearly all the Si and Ti peaks disappeared and TiC and SiC peaks appeared with small FWHMs, which means that the reaction completed in a short period during milling was a self-sustaining reaction. Further milling only led to a broadening of the diffraction peaks of TiC and SiC. When the 120 h as-milled sample was annealed at 950 ◦ C for 1 h, the diffraction pattern, including peak positions and
Fig. 4. XRD patterns taken from Ti25 Si25 C50 mixture milled for different hours. The main diffraction peaks of Ti, Si, TiC and SiC are marked in the diagraph. (a) 72 h; (b) 98 h; (c) 103 h; (d) 120 h, and then annealed at 950 ◦ C for 1 h.
FWHMs, had no observable variation. This suggests that the well mixed TiC and SiC particles hampered grain growth, from which practical applications might benefit. Generally, the particle size of an inorganic material decreases with increasing milling time. SEM observation revealed that the particle size of the Ti25 Si25 C50 system had an unusual growth, which was accompanied by the formation of TiC and SiC. Fig. 5a and b show the SEM images of the ball milled Ti25 Si25 C50 mixture before and after the formation of TiC and SiC. The particles of the 98 h-milled sample had an irregular shape and an average size of about 100 nm, but
Fig. 5. SEM micrographs of Ti25 Si25 C50 mixture taken from the samples milled for different hours: (a) 98 h; (b) 103 h.
C.J. Lu, Z.Q. Li / Journal of Alloys and Compounds 395 (2005) 88–92
the particles of the 103 h-milled sample had a rounded shape and an average particle size of about 200 nm. SEM and XRD both reveal that the formation of TiC and SiC during MA of a Ti25 Si25 C50 mixture was a MSR process, and the released heat caused the particles to grow. Munir and Anselmi-Tamburini [20] proposed a H/Cp criterion to decide whether or not a self-sustaining reaction might occur, where H and Cp are the reaction heat and the room temperature heat capacity of the resultant compound. According to his proposal, a self-sustaining reaction can propagate without additional energy from any exterior source when H/Cp > 2000 K. The H/Cp for both the Ti50 C50 and Si50 C50 systems are larger than 2000 K, see Table 1, but no self-sustaining reactions occurred during ball milling in our experiments. However, a combustion reaction of Ti50 C50 powders during milling with a SPEX mill was reported [14,15]. The magnitude of impact energy given to the particles at collision in a planetary ball mill is much lower than that in a SPEX mill. The ignition of MSR process requires hot spots to initiate the MSR reaction. So the impact energy in the planetary ball mill was not high enough to generate plentiful hot spots for the Ti50 C50 system. The heat formation of SiC is −73 kJ/mol, much less than that of TiC, so ignition did not happen either in a planetary ball mill. Thus only a gradual reaction was achieved for the Si50 C50 mixture. However, it is interesting to note that the TiC–SiC composites is obtained abruptly in the planetary ball mill during milling of the Ti25 Si25 C50 mixture. So both the milling dynamics and the reaction between Ti and Si during ball milling should be responsible for the MSR reaction. To explore the reaction between Ti and Si during ball milling, a Ti50 Si50 mixture was ball milled under identical conditions. A trace of Ti5 Si3 was observed in the 48 h-milled sample, see Fig. 6. The heat formation of Ti5 Si3 is −579 kJ/mol, much larger than those of the other phases, see Table 1, which serves as a driving force for the reaction between Ti and Si. An amorphous phase was obtained for the 70 h-milled sample due to its lower Gibbs free energy than that of Ti5 Si3 [21]. The reaction between Ti and Si is highly exothermic, so the Si in the Ti25 Si25 C50 mixture suppressed the formation of TiC. The reaction between Ti and Si was the main reaction at the early stage of ball milling the Ti25 Si25 C50 mixture, and Ti5 Si3 was formed. But Ti and Si powders were surrounded by fine graphite powder in the well-mixed Ti25 Si25 C50 sample; the reaction probability between Ti and Si decreased. The
91
Fig. 6. XRD patterns taken from Ti50 Si50 mixture milled for different hours. The main diffraction peaks of Ti, Si and Ti5 Si3 are marked on the diffractograms. (a) 9 h; (b) 48 h; (c) 70 h.
graphite restrained not only the contact between the Ti and Si particles but also the formation of an amorphous phase. So the reaction between Ti and Si occurred only in a micro-zone and a certain amount of heat was released during the formation of Ti5 Si3 . Besides, it is known from the variation of the FWHMs that the grain size decreased more rapidly in the Ti25 Si25 C50 mixture than that in the Ti50 C50 and Si50 C50 mixtures, so as to ignite the reaction more easily at the interfaces of the Ti25 Si25 C50 mixture. When the grain size reached a critical level, the reaction was ignited locally at the interface of Ti and Si, which served as hot spots. Much more heat should be released while SiC and TiC were formed, and the process escalated into MSR and completed within a short period. It should be noted that the ignition time in our experiment is about 100 h, but the reported ignition time was generally several hours [13,14,22]. The long ignition time in our case could be attributed to the planetary ball mill employed here, whose impact energy was not high enough, and to the relatively low exothermic reaction of the system. Another reason might be that the 500 ml vial is so large for a small amount of specimen that the heat was strongly dissipated. So much longer activation time was needed to ignite the reaction in our case.
4. Conclusions Table 1 The reaction heat and the ratio of heat capacity to the room temperature heat capacity for the investigated systems Composition
Reaction heat, H (kJ/mol)
Reaction heat/RT heat capacity, H/Cp (K)
TiC SiC Ti5 Si3 TiSi TiSi2
−185 −72 −579 −129 −134
5400 2700 3100 – –
Ti50 C50 , Si50 C50 , Ti50 Si50 and Ti25 Si25 C50 composed of elemental powders of Ti, Si and graphite were ball milled with a planetary ball-mill. The structural evolution of the Ti–Si–C system during MA was systematically studied under identical ball milling conditions. Experimental results reveal that the chemical reactions between Ti and C, Si and C are a gradual process, and complete longer than 100 h. Since the reaction heat of TiC is higher than that of SiC, TiC is more easily formed than SiC. Crystalline Ti5 Si3 and an amorphous
92
C.J. Lu, Z.Q. Li / Journal of Alloys and Compounds 395 (2005) 88–92
phase were produced by ball milling a Ti50 Si50 mixture. The TiC–SiC composites, however, were synthesized by mechanically alloying the Ti25 Si25 C50 mixture through a mechanically induced self-sustaining reaction under the same milling conditions as those of Ti50 C50 , Si50 C50 and Ti50 Si50 mixtures. The formation of Ti5 Si3 plays a key role for the abrupt synthesis of TiC and SiC in the Ti–Si–C ternary system. It suggests that the MSR might be used to synthesize TiC–SiC composites.
Acknowledgements This work was supported by the National Nature Science Foundation of China and the Zhejiang Provincial Foundation for Measurement and Test.
References [1] [2] [3] [4]
G.C. Wei, P.F. Becher, J. Am. Ceram. Soc. 67 (1996) 571. H.-G. An, Y.-W. Kim, J.-G. Lee, J. Euro. Ceram. Soc. 21 (2001) 93. L.J. Wang, W. Jiang, L.D. Chen, Mater. Lett. 58 (2004) 1401. L.J. Wang, W. Jiang, L.D. Chen, S.Q. Bai, J. Am. Ceram. Soc. 87 (2004) 1157.
[5] D.L. Jiang, J.H. Wang, Y.L. Li, L.T. Ma, Mater. Sci. Eng. A 109 (1989) 401. [6] Y.-W. Kim, S.-G. Lee, Y.-I. Lee, J. Mater. Sci. 35 (2000) 5569. [7] S.K. Pradhan, S.K. Shee, A. Chanda, P. Bose, M. De, Mater. Chem. Phys. 68 (2001) 166. [8] D.M.J. Wilkes, P.S. Goodwin, C.M. Ward-Close, K. Bagnall, J. Steeds, Mater. Lett. 27 (1996) 47. [9] H. Zhang, E.H. Kisi, J. Alloys Compd. 248 (1997) 201. [10] P. Chatterjee, S.P. Sen Gupta, Appl. Surf. Sci. 182 (2001) 372. [11] Y.H. Park, H. Hashimoto, T. Abe, R. Watanabe, Mater. Sci. Eng. A 181–182 (1994) 1291. [12] M. Atzmon, Phys. Rev. Lett. 64 (1990) 487. [13] L. Takacs, V. Soika, P. Bal´azˇ , Solid State Ionics 141–142 (2001) 641. [14] Z.G. Liu, J.T. Guo, L.L. Ye, G.S. Li, Z.Q. Hu, Appl. Phys. Lett. 65 (21) (1994) 2666. [15] N.J. Calos, J.S. Forrester, G.B. Schaffer, J. Solid State Chem. 158 (2001) 268. [16] L.L. Ye, Z.G. Liu, M.X. Quan, Z.Q. Hu, J. Appl. Phys. 80 (3) (1996) 1910. [17] L. Takacs, M.A. Susol, J. Solid State Chem. 121 (1996) 394. [18] A. Bakhshai, V. Soika, M.A. Susol, L. Takacs, J. Solid State Chem. 153 (2000) 371. [19] Th.H. de Keijser, J.I. Langford, E.J. Mittemeijer, A.B.P. Vogels, J. Appl. Cryst. 15 (1982) 308. [20] Z.A. Munir, U. Anselmi-Tamburini, Mater. Sci. Rep. 3 (1989) 277. [21] J.Y. Yang, J.S. Wu, H. Wei, Physica B 279 (2000) 241. [22] B.K. Yen, J. Alloys Compd. 268 (1998) 266.
Structural evolution of the Ti–Si–C system during mechanical alloying C.J. Lu, Z.Q. Li∗ Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China Received 1 November 2004; received in revised form 22 November 2004; accepted 23 November 2004 Available online 7 January 2005
Abstract Investigation on the structural evolution for the Ti–Si–C system during mechanical alloying (MA) using a planetary ball mill was systematically performed under identical milling conditions. Experimental results on different powder mixtures were examined and compared. Two kinds of mechanically induced chemical reactions were observed in the Ti–Si–C system, which depends on the composition of the ball-milled mixtures. Gradual reaction was observed in both the Ti50 C50 and Si50 C50 mixtures, and TiC and SiC as the resultants were obtained, respectively. The reaction of Ti50 Si50 mixture during MA was also a gradual reaction, but the resultants are crystalline Ti5 Si3 and an amorphous phase. The TiC–SiC composites, however, were synthesized by mechanically alloying the Ti25 Si25 C50 powder mixture through a mechanically induced self-sustaining reaction (MSR) under the same milling conditions as those of Ti50 C50 , Si50 C50 and Ti50 Si50 mixtures. So the MSR process might be applied to synthesize TiC–SiC composites. The structural evolution processes and thermodynamics of the reactions are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Mechanical alloying (MA); Mechanically induced self-sustaining reaction (MSR); TiC–SiC composites
1. Introduction Single-phase ceramics like TiC or SiC are of practical importance due to its high melting point, good thermal stability, and high erosion resistance. The low fracture toughness, however, is a well-known impediment to its applications as structural components. In order to increase the toughness of such ceramics, composites such as TiC–SiC, have been investigated [1–6]. Mechanical alloying (MA) is a powerful technique for synthesizing a variety of materials, such as compounds [7], metastable solid solutions [8], amorphous alloys [9], nanocrystalline materials [10] and so forth. Two kinds of reaction mechanism for MA have been commonly accepted: one is a gradual reaction through elemental diffusion, and the other is a self-sustaining reaction ignited after a certain period of activation time. The mechanically induced self-sustaining reaction (MSR) was usually observed in highly exothermic systems, such as Ti–B [11], Ni–Al [12], Mo–Si [13]. How∗
Corresponding author. Fax: +86 571 87951411. E-mail address: [email protected] (Z.Q. Li).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.11.046
ever, MA process is so complex that it is difficult to predict the precise reaction route and final products from starting materials. Many factors such as milling dynamics and experimental conditions have a strong influence on the final products. For example, TiC may be obtained by self-sustaining reaction using a high energy SPEX 8000 ml mixer mill [14,15], and be also obtained by continuous structural evolution using a planetary ball mill [16]. A self-sustaining reaction was initiated by ball milling binary Sn50 S50 and Zn50 S50 powder mixtures, but the reaction in ternary Sn25 Zn25 S50 system was a gradual reaction under identical milling conditions [17,18]. The present work is mainly concerned with the structural evolution for Ti–Si–C system during MA using a planetary ball mill. The experimental results reveal that the reaction process for the Ti–Si–C system is just contrary to that of the Sn–Zn–S system.
2. Experimental To decrease the oxide layer on silicon powder, fresh silicon obtained by ball milling silicon wafers of 99.9% purity
C.J. Lu, Z.Q. Li / Journal of Alloys and Compounds 395 (2005) 88–92
powder was applied as one of the raw materials for MA. Commercial silicon powder with a purity of 99.9% and an average particle size of 75 m was also used for comparison. Titanium and graphite powders with purities of 99.5 and 99.95%, and average particle sizes of 75 and 100 m, respectively, were used as the starting materials. Four typical compositions of Ti50 C50 , Si50 C50 , Ti50 Si50 , and Ti25 Si25 C50 were ball milled using a QM-SB type planetary ball mill with stainless steel vacuum vials and milling balls at a rotation rate of 300 rpm. The powder-to-ball mass ratio was about 1:20. The vials were filled with argon to protect powders from oxidation during MA. Samples milled for predetermined hours without interruption were characterized by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM) and fourier transform infrared (FTIR) spectroscopy. Some samples were annealed at 950 ◦ C for 1 h in flowing argon. XRD patterns were taken using a Rigaku D/max 2550PC X-ray diffractometer with Cu K␣ radiation, the SEM observation was carried out using a SIRION scanning electron microscope and FTIR spectra were obtained using a Nicolet AVATAR 360 spectroscope.
3. Experimental results and discussion The brittle Si wafers can be easily milled into small particles at the initial stage of milling, and the particle size was about 1 m after 5 h of milling, which means that Si wafers may be used as a raw material for MA. In order to investigate the effect of different Si powders on MA, both the Si powder made from wafers and the commercial Si powder were separately ball milled with graphite powder under identical conditions. Experimental results show that the MA products obtained from different Si powders were nearly the same. Hereafter the Si powder used for sample preparation was all from ball-milled Si wafers. Fig. 1 shows the XRD patterns taken from the as-milled Si50 C50 powders at different milling hours. The intensity of the silicon peaks decreased gradually with increasing milling hours, accompanied by broadening of the full width at half maximums (FWHMs) of the silicon peaks. The intensity of the C peaks, however, decreased obviously with increasing milling hours till they disappear at about 20 h of milling. The silicon carbide was not observed in the XRD pattern. This phenomenon was also observed during MA of C and W. To explore the reason why the C peaks disappeared at the early stage of MA, a similar experiment was carried out by ball milling a mixture of W and MoS2 . Both graphite and MoS2 belong to the hexagonal system with a layered structure, and can be easily cleaved along (0 0 1) planes. Since the atomic scattering factors of Mo and S are much larger than that of C, the (0 0 2) peak of MoS2 is still visible when the mixture of W and MoS2 has been ball milled even for 72 h. The thickness of MoS2 flakes measured from FWHM by a single line method [19] is about 2 nm, which means the MoS2 flakes are composed of several atomic layers. It is suggested from this experimental result
89
Fig. 1. XRD patterns taken from the Si50 C50 mixture milled for different hours. The main diffraction peaks of C, Si and SiC are marked on the diffractograms, and Fe comes from the milling media: (a) 5 h; (b) 72 h; (c) 120 h; (d) 180 h.
that the vanishing of the C peaks in the XRD patterns is mainly due to its low atomic scattering factor and small particle size. Fig. 2 shows the FTIR spectra taken from Si50 C50 samples after different milling hours. Curves 2a and 2b were obtained from the raw Si50 C50 mixture and the 72 h-milled sample, respectively. The small difference between curve 2a and 2b indicates the structural evolution of Si and C before the formation of SiC, which might be related with the gradual reaction between C and Si. After having been ball milled for about 100 h, the SiC phase with considerably broadened peaks was detected in the XRD pattern and the Si peaks were still observed at the same time. When the mixture was ball milled for 180 h, the reaction between C and Si completed and only the SiC peaks
Fig. 2. FTIR spectra taken from Si50 C50 samples milled for different hours: (a) 0 h; (b) 72 h; (c) 180 h.
90
C.J. Lu, Z.Q. Li / Journal of Alloys and Compounds 395 (2005) 88–92
Fig. 3. XRD patterns taken from Ti50 C50 mixture milled for 72 h (a) and 103 h (b). The main diffraction peaks of Ti and TiC are marked on the diffractograms.
with big FWHMs were observed, which means that fine SiC particles were obtained. Curve c in Fig. 2 was obtained from the 180 h-milled sample, and the peak at about 873 cm−1 in curve c is assigned to the Si C bonds, which also confirms the formation of the SiC phase. The experimental results testify that the formation of SiC from ball milling Si and C is a gradual reaction. Fig. 3 shows the XRD patterns taken from mixed Ti50 C50 powders at different milling times. It is very clear from Figs. 1 and 3 that TiC is more easily obtained by ball milling a mixture of Ti and C powders than SiC by ball milling a mixture of Si and C powders under the same milling conditions. A small amount of TiC was produced after 72 h of milling, but SiC was not observed at the same milling time. The amount of TiC increased with increasing milling time, and finally all the Ti and C powders transformed into TiC, which implied that the structural evolution in the Ti50 C50 system is also a continuous reaction. Similar processes have also been observed by other authors with a planetary ball mill [16]. MA of a Ti25 Si25 C50 mixture was also performed under the same milling conditions. The XRD patterns taken from the samples milled for different hours are shown in Fig. 4. A small amount of TiC and a trace of crystalline Ti5 Si3 were observed when the Ti25 Si25 C50 mixture was ball milled for 72 h. The XRD peak widths of Si and Ti increased with increasing milling time till 98 h. The average grain sizes of Si and Ti for the 98 h-milled sample were about 30 and 60 nm, respectively. After having been milled a little longer, see pattern c in Fig. 4, nearly all the Si and Ti peaks disappeared and TiC and SiC peaks appeared with small FWHMs, which means that the reaction completed in a short period during milling was a self-sustaining reaction. Further milling only led to a broadening of the diffraction peaks of TiC and SiC. When the 120 h as-milled sample was annealed at 950 ◦ C for 1 h, the diffraction pattern, including peak positions and
Fig. 4. XRD patterns taken from Ti25 Si25 C50 mixture milled for different hours. The main diffraction peaks of Ti, Si, TiC and SiC are marked in the diagraph. (a) 72 h; (b) 98 h; (c) 103 h; (d) 120 h, and then annealed at 950 ◦ C for 1 h.
FWHMs, had no observable variation. This suggests that the well mixed TiC and SiC particles hampered grain growth, from which practical applications might benefit. Generally, the particle size of an inorganic material decreases with increasing milling time. SEM observation revealed that the particle size of the Ti25 Si25 C50 system had an unusual growth, which was accompanied by the formation of TiC and SiC. Fig. 5a and b show the SEM images of the ball milled Ti25 Si25 C50 mixture before and after the formation of TiC and SiC. The particles of the 98 h-milled sample had an irregular shape and an average size of about 100 nm, but
Fig. 5. SEM micrographs of Ti25 Si25 C50 mixture taken from the samples milled for different hours: (a) 98 h; (b) 103 h.
C.J. Lu, Z.Q. Li / Journal of Alloys and Compounds 395 (2005) 88–92
the particles of the 103 h-milled sample had a rounded shape and an average particle size of about 200 nm. SEM and XRD both reveal that the formation of TiC and SiC during MA of a Ti25 Si25 C50 mixture was a MSR process, and the released heat caused the particles to grow. Munir and Anselmi-Tamburini [20] proposed a H/Cp criterion to decide whether or not a self-sustaining reaction might occur, where H and Cp are the reaction heat and the room temperature heat capacity of the resultant compound. According to his proposal, a self-sustaining reaction can propagate without additional energy from any exterior source when H/Cp > 2000 K. The H/Cp for both the Ti50 C50 and Si50 C50 systems are larger than 2000 K, see Table 1, but no self-sustaining reactions occurred during ball milling in our experiments. However, a combustion reaction of Ti50 C50 powders during milling with a SPEX mill was reported [14,15]. The magnitude of impact energy given to the particles at collision in a planetary ball mill is much lower than that in a SPEX mill. The ignition of MSR process requires hot spots to initiate the MSR reaction. So the impact energy in the planetary ball mill was not high enough to generate plentiful hot spots for the Ti50 C50 system. The heat formation of SiC is −73 kJ/mol, much less than that of TiC, so ignition did not happen either in a planetary ball mill. Thus only a gradual reaction was achieved for the Si50 C50 mixture. However, it is interesting to note that the TiC–SiC composites is obtained abruptly in the planetary ball mill during milling of the Ti25 Si25 C50 mixture. So both the milling dynamics and the reaction between Ti and Si during ball milling should be responsible for the MSR reaction. To explore the reaction between Ti and Si during ball milling, a Ti50 Si50 mixture was ball milled under identical conditions. A trace of Ti5 Si3 was observed in the 48 h-milled sample, see Fig. 6. The heat formation of Ti5 Si3 is −579 kJ/mol, much larger than those of the other phases, see Table 1, which serves as a driving force for the reaction between Ti and Si. An amorphous phase was obtained for the 70 h-milled sample due to its lower Gibbs free energy than that of Ti5 Si3 [21]. The reaction between Ti and Si is highly exothermic, so the Si in the Ti25 Si25 C50 mixture suppressed the formation of TiC. The reaction between Ti and Si was the main reaction at the early stage of ball milling the Ti25 Si25 C50 mixture, and Ti5 Si3 was formed. But Ti and Si powders were surrounded by fine graphite powder in the well-mixed Ti25 Si25 C50 sample; the reaction probability between Ti and Si decreased. The
91
Fig. 6. XRD patterns taken from Ti50 Si50 mixture milled for different hours. The main diffraction peaks of Ti, Si and Ti5 Si3 are marked on the diffractograms. (a) 9 h; (b) 48 h; (c) 70 h.
graphite restrained not only the contact between the Ti and Si particles but also the formation of an amorphous phase. So the reaction between Ti and Si occurred only in a micro-zone and a certain amount of heat was released during the formation of Ti5 Si3 . Besides, it is known from the variation of the FWHMs that the grain size decreased more rapidly in the Ti25 Si25 C50 mixture than that in the Ti50 C50 and Si50 C50 mixtures, so as to ignite the reaction more easily at the interfaces of the Ti25 Si25 C50 mixture. When the grain size reached a critical level, the reaction was ignited locally at the interface of Ti and Si, which served as hot spots. Much more heat should be released while SiC and TiC were formed, and the process escalated into MSR and completed within a short period. It should be noted that the ignition time in our experiment is about 100 h, but the reported ignition time was generally several hours [13,14,22]. The long ignition time in our case could be attributed to the planetary ball mill employed here, whose impact energy was not high enough, and to the relatively low exothermic reaction of the system. Another reason might be that the 500 ml vial is so large for a small amount of specimen that the heat was strongly dissipated. So much longer activation time was needed to ignite the reaction in our case.
4. Conclusions Table 1 The reaction heat and the ratio of heat capacity to the room temperature heat capacity for the investigated systems Composition
Reaction heat, H (kJ/mol)
Reaction heat/RT heat capacity, H/Cp (K)
TiC SiC Ti5 Si3 TiSi TiSi2
−185 −72 −579 −129 −134
5400 2700 3100 – –
Ti50 C50 , Si50 C50 , Ti50 Si50 and Ti25 Si25 C50 composed of elemental powders of Ti, Si and graphite were ball milled with a planetary ball-mill. The structural evolution of the Ti–Si–C system during MA was systematically studied under identical ball milling conditions. Experimental results reveal that the chemical reactions between Ti and C, Si and C are a gradual process, and complete longer than 100 h. Since the reaction heat of TiC is higher than that of SiC, TiC is more easily formed than SiC. Crystalline Ti5 Si3 and an amorphous
92
C.J. Lu, Z.Q. Li / Journal of Alloys and Compounds 395 (2005) 88–92
phase were produced by ball milling a Ti50 Si50 mixture. The TiC–SiC composites, however, were synthesized by mechanically alloying the Ti25 Si25 C50 mixture through a mechanically induced self-sustaining reaction under the same milling conditions as those of Ti50 C50 , Si50 C50 and Ti50 Si50 mixtures. The formation of Ti5 Si3 plays a key role for the abrupt synthesis of TiC and SiC in the Ti–Si–C ternary system. It suggests that the MSR might be used to synthesize TiC–SiC composites.
Acknowledgements This work was supported by the National Nature Science Foundation of China and the Zhejiang Provincial Foundation for Measurement and Test.
References [1] [2] [3] [4]
G.C. Wei, P.F. Becher, J. Am. Ceram. Soc. 67 (1996) 571. H.-G. An, Y.-W. Kim, J.-G. Lee, J. Euro. Ceram. Soc. 21 (2001) 93. L.J. Wang, W. Jiang, L.D. Chen, Mater. Lett. 58 (2004) 1401. L.J. Wang, W. Jiang, L.D. Chen, S.Q. Bai, J. Am. Ceram. Soc. 87 (2004) 1157.
[5] D.L. Jiang, J.H. Wang, Y.L. Li, L.T. Ma, Mater. Sci. Eng. A 109 (1989) 401. [6] Y.-W. Kim, S.-G. Lee, Y.-I. Lee, J. Mater. Sci. 35 (2000) 5569. [7] S.K. Pradhan, S.K. Shee, A. Chanda, P. Bose, M. De, Mater. Chem. Phys. 68 (2001) 166. [8] D.M.J. Wilkes, P.S. Goodwin, C.M. Ward-Close, K. Bagnall, J. Steeds, Mater. Lett. 27 (1996) 47. [9] H. Zhang, E.H. Kisi, J. Alloys Compd. 248 (1997) 201. [10] P. Chatterjee, S.P. Sen Gupta, Appl. Surf. Sci. 182 (2001) 372. [11] Y.H. Park, H. Hashimoto, T. Abe, R. Watanabe, Mater. Sci. Eng. A 181–182 (1994) 1291. [12] M. Atzmon, Phys. Rev. Lett. 64 (1990) 487. [13] L. Takacs, V. Soika, P. Bal´azˇ , Solid State Ionics 141–142 (2001) 641. [14] Z.G. Liu, J.T. Guo, L.L. Ye, G.S. Li, Z.Q. Hu, Appl. Phys. Lett. 65 (21) (1994) 2666. [15] N.J. Calos, J.S. Forrester, G.B. Schaffer, J. Solid State Chem. 158 (2001) 268. [16] L.L. Ye, Z.G. Liu, M.X. Quan, Z.Q. Hu, J. Appl. Phys. 80 (3) (1996) 1910. [17] L. Takacs, M.A. Susol, J. Solid State Chem. 121 (1996) 394. [18] A. Bakhshai, V. Soika, M.A. Susol, L. Takacs, J. Solid State Chem. 153 (2000) 371. [19] Th.H. de Keijser, J.I. Langford, E.J. Mittemeijer, A.B.P. Vogels, J. Appl. Cryst. 15 (1982) 308. [20] Z.A. Munir, U. Anselmi-Tamburini, Mater. Sci. Rep. 3 (1989) 277. [21] J.Y. Yang, J.S. Wu, H. Wei, Physica B 279 (2000) 241. [22] B.K. Yen, J. Alloys Compd. 268 (1998) 266.