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Ti (CxN1−x) nanocomposite powder via high-energy ball milling and subsequent heat treatment
Journal of Alloys and Compounds 334 (2002) 253–260
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Formation of TiB 2 / TiN / Ti (C x N 12x ) nanocomposite powder via high-energy ball milling and subsequent heat treatment Jianlin Li*, Fei Li, Keao Hu, Yong Zhou State Key Laboratory of Metal Matrix Composites, Institute of Composite Materials, Shanghai Jiaotong University, 1954 Huashan Road, Shanghai 200030, PR China Received 7 June 2001; accepted 17 July 2001
Abstract TiB 2 / TiN / Ti (C x N 12x ) nanocomposite powder was fabricated by high-energy ball milling and subsequent heat treatment. The microstructure development of powder mixtures was monitored by X-ray diffraction and transmission electron microscopy. It was found that TiN and TiC formed within 10 h of milling. After 30 h of milling, the resulting powder mixtures were mainly composed of nanocrystalline Ti, TiN, TiC and TiB 2 . The as-milled powder was transformed into Ti (Cx N 12x ), TiN and TiB 2 after subsequent heat treatment at 13008C. During annealing, TiC reacted with TiN, and Ti (C x N 12x ) was thus obtained. 2002 Elsevier Science B.V. All rights reserved. Keywords: Alloys; Nanostructered materials; X-ray diffraction; TEM
1. Introduction The design and manufacture of advanced materials for applications at high stress and high temperatures is one of the most challenging tasks of modern engineering. Refractory materials such as borides, nitrides, carbides, silicides are natural candidates for these demanding applications due to their exceptional hardness and stability at very high temperatures. Boride (TiB 2 , ZrB 2 , HfB 2 ) and nitride (TiN, AlN, BN, ZrN) ceramics have been developed due to their specific properties and potential applications. Although every engineering material has a field of application in its pure form, increasing attention has been devoted to ceramic matrix composites in which pure components are mixed to give new materials with tailored properties [1–3]. Researchers have shown that fine-grained TiB 2 possesses extraordinary resistance to plastic deformation at high temperatures. Furthermore, the fracture toughness of 5 MPa m 1 / 2 is also encouraging. The melting temperature of TiN or TiC, on the other hand, is even higher than that of TiB 2 . However, their elastic modulus and hardness are low and they can deform, resulting in a loss of strength at high temperatures. This combination of extreme resistance to *Corresponding author. E-mail address: [email protected] (J. Li).
plastic deformation of the TiB 2 and the high-temperature plasticity of the TiN and TiC phase suggests that a TiB 2 – Ti (C, N) composite should be attractive as a hightemperature structural ceramic [4,5]. It is believed that this composite approach might yield a unique combination of high-temperature hardness and stability with adequate fracture toughness. In addition, the high electrical and thermal conductivities of both are attractive for functional applications in high performance electrical systems [6]. Porous TiB 2 / TiN composites were obtained by using the conventional self-propagating high temperature synthesis (SHS) method using Ti, B and BN powders as raw materials [7]. In addition, Zhang prepared TiB 2 / Ti (Cx N 12x ) composites by reactive hot pressing at 18508C from TiH 2 , BN and B 4 C in the presence of Ni, which acted as an effective additive [8]. Recently, a modified method of displacement reaction synthesis has been introduced by Olevsky et al. [9], namely, displacement reactions in fully dense, very fine elemental powder blends. Solid-state synthesis of TiB / TiN and TiB 2 / TiN ceramics composites via displacement reactions in fully dense BN–Ti blends has been accomplished in their work. However, the evolution of microstructure in the processing is governed by the diffusion of nitrogen and boron into titanium, and it usually takes a relatively long time to complete the displacement reaction process. Therefore, to prepare bulk materials from powder
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mixture milled for an appropriate time might be a useful production route. With fine submicron / nanometer starting powders, the short diffusion distances allow the synthesis of new phases to be completed over relatively short time periods, and the undesirable coarsening of the microstructure could be prevented. In recent years, high-energy ball milling (mechanical alloying) has been widely used to produce supersaturated crystalline solid solutions, amorphous phases, nanocrystalline solids and compounds through in-situ solid–solid, gas–solid, and liquid–solid reactions. Furthermore, highenergy ball milling can be designed as an intermediate step to promote reactions that otherwise can only be completed at high temperatures. The synthesis of carbides, silicides, nitrides and borides by this processing has been realized by researchers [10–17]. Hence, in view of the specific properties and potential applications of TiB 2 / Ti (C x N 12x ) composites, it is meaningful to study if TiB 2 / Ti (C x N 12x ) composite powder can be obtained by high-energy ball milling process.
2. Experimental procedure The raw materials used in this study were 99.9% pure Ti powder with a sieve size -400 mesh, 99.5% pure BN powder with an average particle size of 4 mm and 98% pure B 4 C powder with an average particle size of 6 mm. Two mixtures A and B were prepared before milling. Mixture A was composed of B 4 C and Ti powders with a molar ratio of 1:3 [Reaction (1)]. Mixture B was composed of BN and Ti powders with a molar ratio of 2:3 [Reaction (2)].
Fig. 1. Diagram of the machine used in this work. The canister can be fixed in the tubular part by screws. This tubular part is jointed on the disc part by a bearing and also jointed to the platform by two cranks. When the motor starts, the disc part begins to rotate around the axle and the tubular part with the canister begins to rotate around the axle and the bearing simultaneously. The canister performs a planetary rotation.
as-milled powders were also investigated by a transmission electron microscopy (TEM, Hitachi H800) with energy dispersive spectrometry (EDS). The operating electron voltage was 200 kV. Differential scanning calorimeter (DSC) analysis was carried out using Rehometric DSC-SP equipment in Ar. The heating rate was 108C / min. Heat treatment was performed in a vacuum at 400, 700 and 13008C, respectively. The heating rate was 108C / min.
3. Results and discussion
3Ti 1 B 4 C → TiC 1 2TiB 2
(1)
3.1. Reaction during ball milling
3Ti 1 2BN → 2TiN 1 TiB 2
(2)
XRD patterns of powder mixtures after different milling times are shown in Fig. 2. After 10 h of milling, BN and B 4 C peaks disappeared completely while TiC and TiN were detected in the mixture by XRD (Fig. 2b). But no TiB 2 was found in the mixture (Fig. 2b). It is suggested that the B 4 C is broken into very small pieces among fine Ti grains. Metallic Ti, therefore, obscures the reflections of B 4 C [18]. On the other hand, BN in the current study has a hexagonal crystal structure (as shown in Fig. 2a). In a previous study, it was found that in ball milling processing, the disappearance of the reflections of BN indicated that BN separated into very small slices [19]. Within 10 h of milling, BN and B 4 C were mixed with Ti on a nanometer scale, and BN and B 4 C began to react with Ti. Thus, TiC and TiN were produced. The lattice parameter of TiN and TiC were found to be 0.421 nm, 0.433 nm, respectively, which did not change during the subsequent milling process. Since no TiB 2 or crystalline B was produced (Fig. 2b), some amorphous B should exist in the mixture. As shown in Fig. 2, no Ti(C, N) was found during
Mixture A and Mixture B with a weight ration of 1:1 were ball-milled in a GN-2 High-energy Ball Milling Machine (Shengyang Science Equipment Factory, China, see Fig. 1). The milling was conducted using hardened steel balls with a diameter of 5 mm. The ball-to-powder weight ratio was 10:1, and the milling speed was 360 rpm. The hardened steel canister was evacuated to 0.2 Pa and flushed with Ar gas. The milling runs were interrupted for 0.5 h to cool the canister whenever the powder was milled for 5 h. After milling for selected times, ball milled powder mixtures were taken out in Ar for analysis. The powders were characterized by X-ray diffraction (XRD, Cu K a radiation: l 50.154 nm). XRD peak broadening was used to determine the crystallite size d. By plotting b cos u ( b the linewidth at half-maximum intensity and u the Bragg angle) again sin u, a straight line was obtained from which the crystal size d was obtained from the intercept. The
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3.2. Reaction mechanism during high-energy ball milling
Fig. 2. XRD patterns of the powder mixtures milled for different times: (a) before milling; (b) 10 h; (c) 20 h; and (d) 30 h.
ball-milling process. Hence, reactions between Ti and BN can be regarded as being independent of the reactions between Ti and B 4 C. With increasing milling time, the diffraction peaks of crystalline Ti become broader and of less intensity, but their positions do not shift. After 20 h of milling, the average diameter of Ti grains is around 20 nm as derived via the Scherrer formula, and TiB 2 peaks appeared (Fig. 2c). With further milling up to 30 h, as shown in Fig. 2d, the decrease in the integral intensity of the Ti peaks is associated with the formation of TiC, TiN and TiB 2 within 30 h of milling. Fig. 3 shows the TEM images of powder milled for 30 h. TiN, TiC, TiB 2 , Ti and amorphous B formed microscale composite particles, and some nanosized particles, composed of TiN, TiC, TiB 2 and a small amount of amorphous B, were wrapped within them (Fig. 3a, b). During the milling process, TiB 2 particles were formed in the TiN and TiC matrix produced previously. Therefore, the heat generated by the formation of TiB 2 possibly led to slight sintering of some nearby nanosized TiN and TiC grains. The existence of a small amount of amorphous B might prompt the sintering (Fig. 3c, d). The corresponding SAD pattern demonstrated that the particles were mainly composed of nanocrystalline TiN, TiC and TiB 2 . The amorphous B shows a thin bright background in the image (Fig. 3c). The SAD pattern demonstrated that the rest part of the particles is free of TiB 2 and is mainly composed of nanocrystalline TiN, TiC, Ti and a small amount of amorphous B.
In a conventional SHS process, the reactant powders are impacted as a pellet for easy propagation of the combustion wave. Although the powders are more loosely dispersed in the vial during ball milling, the mixing of the reactants on a nanometre scale favors mass transfer and the diffusion path length is considerably reduced. In addition, the ignition events must be numerous during the milling process. Therefore, the self-propagating reaction during ball milling may be massive and explosive. It is believed that a high heat of formation serves as the driving force of the combustion reaction during milling process. This is consistent with a conventional SHS reaction. For it to be self-sustaining, the process must be associated with high temperature reactions. Researchers have demonstrated that the value of DH298 / Cp 298 should be above 2000 K in thermally ignited systems [10]. Thus, the value of DH298 / Cp 298 can be used as a rough guide for the existence of combustion in the milling process [14]. Some typical combustion reactions are summarized in Table 1 [20]. Another factor related to self-propagating reactions is the particle size of the reactants. In general, the reaction rate in the powder system is dependent on particle size [10,19]. This is because refinement of particle size increases the reaction interface area and the activities of the reactants. In particular, it has been reported that propagation of the combustion wave in SHS is accelerated by the refinement of particle size, whereas combustion becomes difficult and even disappears as the particle size increases [21]. Previous investigations show that ignition of the combustion reaction requires an initial premilling period (incubation) during which ball milling leads to a change in the factors determining the critical combustion condition. Hence, some researchers have proposed that there exists a critical particle size for ignition of the combustion reaction during the course of high-energy ball milling [14,20]. In this work, the values of DH298 / Cp 298 of Reactions (1) and (2) are 4100 and 4000 K, respectively. Therefore, it is possible that the formation of TiN, TiC and TiB 2 occurs by a self-propagating reaction during ball milling from Ti, B 4 C and BN powders. However, as shown in Fig. 2, the formations of TiN, TiC or TiB 2 occurred by gradual reactions. In a previous study [18], it was found that under the same milling conditions, Reaction (1) took place by a self-propagating reaction when the milling time was 4.5 h, and some TiC particles formed before the combustion reaction. On the other hand, when the mixture of Ti and BN was ball-milled for 10 h, TiN was produced. But no TiB 2 was found in the mixture even after 40 h of milling. Therefore, the mixture of Ti and B 4 C prompted reactions between Ti and BN while the mixture of Ti and BN abated reactions between Ti and B 4 C during the ball-milling processing. In this experiment, the outer shell of some B 4 C particles
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Fig. 3. TEM images of the powder mixtures milled for 30 h (a) and (b), electron diffraction patterns (c) and (d) of Fig. 3b (the areas marked by arrows), showing the existence of nanocrystalline TiC, TiN, TiB 2 and Ti beside a small amount of amorphous B.
would decompose into B and C atoms due to the impact of the milling balls and the reduction of Ti around these B 4 C particles [18]. C and B atoms then diffuse into Ti. The diffusivity of carbon is significantly higher than that of boron, according to the literature data [4], approximately three orders of magnitude higher than that of boron in titanium. Therefore, the formation of TiC was more
favorable than that of TiB 2 . Barsoum et al. [4] reported that when Ti reacted with B 4 C at elevated temperature, carbon diffused rapidly from B 4 C areas into titanium during the initial stage, forming TiC 0.5 and leaving behind areas of high boron activity. TiC 0.5 was one of the intermediate phases. In this experiment, however, TiC, not TiC 0.5 , was detected. Thus, it is assumed that the impact of
J. Li et al. / Journal of Alloys and Compounds 334 (2002) 253 – 260 Table 1 Typical reactions in high-energy ball milling processing Reaction
Heat of formation (kJ mol 21 )
Adiabatic temperature (K)
Mode of reaction
Mo12Si→MoSi 2 Ti12B→TiB 2 Ti1C→TiC 4Al13C→Al 4 C 3 Si1C→SiC W 12Si→WSi 2
2138 2342 2183.8 2215.8 267 –
1900 3190 3210 1200 1800 1500
Combustive Combustive Combustive Gradual Gradual Gradual
the milling balls triggered the reaction between Ti and C near the B 4 C particles where the C content was high enough. But the formation rate of superfine TiC was sluggish and the heat generated by the reaction was too small to ignite a self-propagating reaction due to the dilution of the reactants. The fine TiC grains formed are moved away quickly in the milling processing, so they remain nanosized [16–18] (Fig. 2). A previous study on the displacement reaction of Ti and BN showed that when Ti reacted with BN at elevated temperatures, TiN formed at about 10008C while TiB 2 occurred above 12008C [22]. Namely, the N atoms diffused and reacted with Ti prior to the reaction with B. Therefore, TiN is relatively easier to form in the current work. According to the conventional analysis above, the formation of TiN, TiC and TiB 2 could occur by a selfpropagating reaction, however, there is no extra heat to trigger the reaction during the milling process. Carslaw and Jaeger demonstrated that the majority of heat generated by the impact in ball mill processing is directly proportional to the friction coefficient [21]. In this work, since BN is an excellent lubricant, which can lighten the impact force between balls, the heat generated is small and cannot result in a rapid temperature increment in a given area to ignite the combustion. On the other hand, the heat generated by the formation of TiC or TiN was also too small to trigger a combustion reaction. Due to the sluggish reaction rate, a large amount of heat could not be generated in a short moment. As a result, the heat generated distributed over a relatively large area and finally diffused through the canister to outer space. During 10–30 h of milling, many TiC and TiN particles formed during milling led to an increment of the friction coefficient, and thus reactions between Ti and B 4 C, BN or B might be accomplished by a self-propagating reaction. However, those TiC and TiN particles formed previously acted as diluent and thus a combustion reaction was suppressed. As a result, the formation rate of TiB 2 was remarkably reduced and TiB 2 was slowly formed in the form of superfine particles. In the current work, it was found that when a steel flake was used to quickly clean the powder mixture milled in Ar
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for 10 h and attached to the milling balls, the powder mixture began to combust. This result indicates that the reactants had been mixed on a nanometre scale and that the heat or sparkle, generated by the friction of steel flake and balls, triggered the combustion reaction. However, when a steel flake was used to quickly clean the powder mixture milled in Ar for 20 h, the powder mixtures did not combust. Hence, the reactants were diluted with the TiN, TiC and TiB 2 grains formed during previous milling processes and thus, a combustion reaction was suppressed. As shown in Fig. 2, no intermediate phase, such as TiN x , TiC x or TiB, appeared during the milling process. Therefore, it is assumed that the formation of TiC, TiN or TiB 2 occurred by a local high temperature reaction over a much smaller scale, and the heat generated by the reactions was too small to ignite a self-propagating reaction in the canister. Zhang [8] reported that when a mixture of TiH 2 , B 4 C and BN powders was heated to 18508C and kept at this temperature for 0.5 h, the diffusion of C, N and B atoms into Ti led to the formation of TiB 2 / Ti (C 0.5 N 0.5 ) composites, and no TiN was produced before the formation of Ti (C 0.5 N 0.5 ). Therefore, although the diffusion of C and N atoms played an important role during ballmilling process, no Ti (C x N 12x ) phase was produced during ball-milling processing. This also demonstrated that the TiC or TiN particles formed independently and instantaneously, otherwise some Ti (C x N 12x ) would have been formed. Namely, although the formations of TiC, TiN or TiB 2 were gradual, single grains were formed instantaneously. This result is consistent with the previous conclusion in this work. In summary, the evolution of the powder mixture during milling processing is as follows: During ball milling for 0–10 h, Reactions (3) and (4) take place: Ti 1 B 4 C → TiC 1 B (amorphous)
(3)
Ti 1 BN → TiN 1 B (amorphous)
(4)
During ball milling for 10–30 h, Reaction (5) takes place: Ti 1 B 4 C 1 BN 1 B (amorphous) → TiC 1 TiN 1 TiB 2 (5)
3.3. Effect of subsequent heat treatment on microstructure of powder mixture Fig. 4 shows the XRD patterns of the powder mixtures milled for 30 h followed by heat treatment in a vacuum. In comparison with the powder mixture before heat treatment (Fig. 2d), the amount of TiN and TiC did not increase obviously when the powder mixture was annealed at 400
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Fig. 4. XRD patterns of powder mixtures after 30 h of milling and heat treatment, (a) heated to 13008C and cooled; (b) annealed at 7008C for 1 h; and (c) annealed at 4008C for 1 h.
or 7008C for 1 h, and a trace of Ti was detected in the powder mixture (Fig. 4b, c). The result indicated that almost all TiC, TiN and TiB 2 were produced during the milling process. The XRD patterns demonstrated that the content of Ti or B was low. Namely, Reactions (1) and (2) were nearly accomplished after 30 h of milling. The broader and lower intensity TiN and TiC peaks indicate that the TiN and TiC have nanosized structure (Fig. 4c). When the powder mixture milled for 30 h was annealed at 7008C for 1 h, although the reflections of TiC became less intense, Ti (C x N 12x ) peaks did not appear (Fig. 4b). Namely, TiC and some TiN began to be transformed into superfine Ti (C x N 12x ). Hence, the temperature at which Ti (C x N 12x ) began to form from TiC and TiN should be above 7008C. As shown in Fig. 2, no Ti (C x N 12x ) was produced during the milling process. This indicated that the temperature in the canister did not exceed 7008C, which is consistent with the conclusion that the maximum local milling temperature during ball milling is in the range 300–6008C [23]. When the powder mixture milled for 30 h was heated to 13008C at a heating rate 108C / min, only TiN, Ti (C x N 12x ) and TiB 2 were detected (Fig. 4a). TiC was not detected. The XRD result indicates that the ball-milled mixture has been converted to Ti (C x N 12x ), TiB 2 and TiN after this heat treatment. Namely, Reaction (5) was accomplished up to now, TiC reacted with some TiN, and Ti (C x N 12x ) was produced [Reaction (6)]: TiC 1 TiN → Ti (C x N 12x )
(6)
Although TiC and TiN can be transformed into infinite solid solution, TiN and Ti (C x N 12x ) coexisted in this work due to the low temperature and short holding time in the
Fig. 5. DSC curve, showing no obvious exothermic peak.
heat treatment. Ti (C x N 12x ) was found to be Ti (C 0.7 N 0.3 ) and Ti (C 0.3 N 0.7 ) in this work. In order to study the phase transitions associated with heat treatment, a DSC analysis was performed. The DSC curve is shown in Fig. 5. However, there is no obvious exothermic peak. This result indicates that no obvious exothermic reaction occurred during the heating process. Therefore, the formation of TiC, TiN and TiB 2 should have been nearly completed during ball milling, and the volume fractions of TiC, TiN and TiB 2 formed during the treatment were rather low. This result is consistent with the previous conclusions (Figs. 2–4). The annealed powder shows a yellow color while the powder mixtures before this heat treatment, or those annealed at 400 or 7008C for 1 h are all black, presumably as a result of the very small TiN grain size. The average grain size of Ti (C 0.3 N 0.7 ), TiN and TiB 2 are 25, 80 and 40 nm, respectively. TEM images of the powder mixture milled for 30 h and heated to 13008C are shown in Fig. 6. Fig. 6a shows agglomerates of sintered TiB 2 , TiN and Ti (C x N 12x ) grains, identified by EDS. The grains were found to be 30–60 nm in size, in agreement with the XRD result. In conventional hot pressing processes [8], Ti (Cx N 12x ) and TiB 2 tend to form large grains separately and there are many small pores within the TiB 2 grains. By contrast, the TiB 2 / Ti (C x N 12x ) nanocomposite powder prepared in this work consists of nano-scale TiN / Ti (C x N 12x ) grains incorporated in nanosized TiB 2 particles. If the as-milled powder mixture was impacted and sintered, the TiB 2 / Ti (Cx N 12x ) nanocomposite is expected to form at relatively low temperatures. Further research is being carried out.
4. Conclusion TiB 2 / TiN / Ti (Cx N 12x ) nanocomposite powder has been prepared via high-energy ball milling and subsequent heat treatment. The formations of TiB 2 , TiN and TiC were
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Fig. 6. TEM images of powders milled for 30 h and heated to 13008C (a) and (c), showing nanosized TiN, Ti (C x N 12x ) and TiB 2 grains; (b) and (c) are SAD patterns of (a) and (c), respectively.
gradual, in contrast to the formation by a mechanically induced self-propagating reaction. The formation of TiC and TiN occurs within 10 h of milling, prior to that of TiB 2 during the milling process due to the faster diffusion of carbon and nitrogen atoms compared with boron atoms in
the titanium matrix. No Ti (C x N 12x ) formed during the ball-milling processing. Reactions (1) and (2) were nearly accomplished after 30 h of milling. After heat treatment at 13008C, the final product is composed of nanosized TiB 2 and TiN / Ti (Cx N 12x ) particles.
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References [1] J. Ramberg, C. Woolfe, W.S. Williams, J. Am. Ceram. Soc. 68 (1985) C78–C79. [2] W.S. Williams, J. Appl. Phys. 35 (1964) 1329–1338. [3] F. de Mestral, F. Thevnot, J. Mater. Sci. 26 (20) (1991) 5547–5560. [4] M.W. Barsoum, B. Houng, J. Am. Ceram. Soc. 76 (6) (1993) 1445–1451. [5] D. Brodkin, S.R. Kalidindi, M.W. Barsoum, J. Am. Ceram. Soc. 79 (7) (1996) 1945–1952. [6] K. Nihara, J. Ceram. Soc. Jpn. 99 (10) (1991) 974–986. [7] Z.A. Munir, J. Am. Ceram. Soc. Bull. 67 (2) (1988) 342–349. [8] G.J. Zhang, Ceram. Int. 21 (2) (1995) 182–187. [9] G.L. Caer, E. Bauer-Gross, A. Pianelli, J. Mater. Sci. 29 (1994) 614–631. [10] B.K. Yen, T. Aizawa, J. Kihara, J. Am. Ceram. Soc. 79 (8) (1996) 2221–2223. [11] A. Calka, J.S. Williams, Mater. Sci. Forum 88–90 (1992) 787–794.
[12] L.L. Shaw, Z. Yang, R. Ren, J. Am. Ceram. Soc. 81 (3) (1998) 760–764. [13] L.L. Ye, M.X. Quan, Nanostruct. Mater. 5 (1995) 22–31. [14] N.Q. Wu, Mater. Sci. Tech. 14 (4) (1997) 287–292. [15] A.P. Radlinski, A. Calka, Mater. Sci. Eng. A A134 (1991) 1376– 1379. [16] N.Q. Wu, Mater. Sci. Eng. A257 (1998) 357–360. [17] J. Ding, W.F. Miao, P.G. McCormick, Appl. Phys. Lett. 67 (25) (1995) 3804–3806. [18] J.L. Li, F. Li, K.A. Hu, J. Eur. Ceram. Soc., accepted for publication. [19] J.L. Li, K.A. Hu, Mater. Sci. Eng. A, accepted for publication. [20] C.J. Smithells, Metals Reference Book, 5th Edition, 8-22–8-24, 1983, London, Butterworth. [21] S.C. Deevi, J. Mater. Sci 26 (1991) 3685–3690. [22] G.J. Zhang, J. Am. Ceram. Soc. 67 (2) (1988) 342–349. [23] H.S. Carselaw, J.C. Jaeger, Heat Conduction in Solids, in: Oxford University Press, New York, 1959, p. 255.
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Formation of TiB 2 / TiN / Ti (C x N 12x ) nanocomposite powder via high-energy ball milling and subsequent heat treatment Jianlin Li*, Fei Li, Keao Hu, Yong Zhou State Key Laboratory of Metal Matrix Composites, Institute of Composite Materials, Shanghai Jiaotong University, 1954 Huashan Road, Shanghai 200030, PR China Received 7 June 2001; accepted 17 July 2001
Abstract TiB 2 / TiN / Ti (C x N 12x ) nanocomposite powder was fabricated by high-energy ball milling and subsequent heat treatment. The microstructure development of powder mixtures was monitored by X-ray diffraction and transmission electron microscopy. It was found that TiN and TiC formed within 10 h of milling. After 30 h of milling, the resulting powder mixtures were mainly composed of nanocrystalline Ti, TiN, TiC and TiB 2 . The as-milled powder was transformed into Ti (Cx N 12x ), TiN and TiB 2 after subsequent heat treatment at 13008C. During annealing, TiC reacted with TiN, and Ti (C x N 12x ) was thus obtained. 2002 Elsevier Science B.V. All rights reserved. Keywords: Alloys; Nanostructered materials; X-ray diffraction; TEM
1. Introduction The design and manufacture of advanced materials for applications at high stress and high temperatures is one of the most challenging tasks of modern engineering. Refractory materials such as borides, nitrides, carbides, silicides are natural candidates for these demanding applications due to their exceptional hardness and stability at very high temperatures. Boride (TiB 2 , ZrB 2 , HfB 2 ) and nitride (TiN, AlN, BN, ZrN) ceramics have been developed due to their specific properties and potential applications. Although every engineering material has a field of application in its pure form, increasing attention has been devoted to ceramic matrix composites in which pure components are mixed to give new materials with tailored properties [1–3]. Researchers have shown that fine-grained TiB 2 possesses extraordinary resistance to plastic deformation at high temperatures. Furthermore, the fracture toughness of 5 MPa m 1 / 2 is also encouraging. The melting temperature of TiN or TiC, on the other hand, is even higher than that of TiB 2 . However, their elastic modulus and hardness are low and they can deform, resulting in a loss of strength at high temperatures. This combination of extreme resistance to *Corresponding author. E-mail address: [email protected] (J. Li).
plastic deformation of the TiB 2 and the high-temperature plasticity of the TiN and TiC phase suggests that a TiB 2 – Ti (C, N) composite should be attractive as a hightemperature structural ceramic [4,5]. It is believed that this composite approach might yield a unique combination of high-temperature hardness and stability with adequate fracture toughness. In addition, the high electrical and thermal conductivities of both are attractive for functional applications in high performance electrical systems [6]. Porous TiB 2 / TiN composites were obtained by using the conventional self-propagating high temperature synthesis (SHS) method using Ti, B and BN powders as raw materials [7]. In addition, Zhang prepared TiB 2 / Ti (Cx N 12x ) composites by reactive hot pressing at 18508C from TiH 2 , BN and B 4 C in the presence of Ni, which acted as an effective additive [8]. Recently, a modified method of displacement reaction synthesis has been introduced by Olevsky et al. [9], namely, displacement reactions in fully dense, very fine elemental powder blends. Solid-state synthesis of TiB / TiN and TiB 2 / TiN ceramics composites via displacement reactions in fully dense BN–Ti blends has been accomplished in their work. However, the evolution of microstructure in the processing is governed by the diffusion of nitrogen and boron into titanium, and it usually takes a relatively long time to complete the displacement reaction process. Therefore, to prepare bulk materials from powder
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01769-8
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J. Li et al. / Journal of Alloys and Compounds 334 (2002) 253 – 260
mixture milled for an appropriate time might be a useful production route. With fine submicron / nanometer starting powders, the short diffusion distances allow the synthesis of new phases to be completed over relatively short time periods, and the undesirable coarsening of the microstructure could be prevented. In recent years, high-energy ball milling (mechanical alloying) has been widely used to produce supersaturated crystalline solid solutions, amorphous phases, nanocrystalline solids and compounds through in-situ solid–solid, gas–solid, and liquid–solid reactions. Furthermore, highenergy ball milling can be designed as an intermediate step to promote reactions that otherwise can only be completed at high temperatures. The synthesis of carbides, silicides, nitrides and borides by this processing has been realized by researchers [10–17]. Hence, in view of the specific properties and potential applications of TiB 2 / Ti (C x N 12x ) composites, it is meaningful to study if TiB 2 / Ti (C x N 12x ) composite powder can be obtained by high-energy ball milling process.
2. Experimental procedure The raw materials used in this study were 99.9% pure Ti powder with a sieve size -400 mesh, 99.5% pure BN powder with an average particle size of 4 mm and 98% pure B 4 C powder with an average particle size of 6 mm. Two mixtures A and B were prepared before milling. Mixture A was composed of B 4 C and Ti powders with a molar ratio of 1:3 [Reaction (1)]. Mixture B was composed of BN and Ti powders with a molar ratio of 2:3 [Reaction (2)].
Fig. 1. Diagram of the machine used in this work. The canister can be fixed in the tubular part by screws. This tubular part is jointed on the disc part by a bearing and also jointed to the platform by two cranks. When the motor starts, the disc part begins to rotate around the axle and the tubular part with the canister begins to rotate around the axle and the bearing simultaneously. The canister performs a planetary rotation.
as-milled powders were also investigated by a transmission electron microscopy (TEM, Hitachi H800) with energy dispersive spectrometry (EDS). The operating electron voltage was 200 kV. Differential scanning calorimeter (DSC) analysis was carried out using Rehometric DSC-SP equipment in Ar. The heating rate was 108C / min. Heat treatment was performed in a vacuum at 400, 700 and 13008C, respectively. The heating rate was 108C / min.
3. Results and discussion
3Ti 1 B 4 C → TiC 1 2TiB 2
(1)
3.1. Reaction during ball milling
3Ti 1 2BN → 2TiN 1 TiB 2
(2)
XRD patterns of powder mixtures after different milling times are shown in Fig. 2. After 10 h of milling, BN and B 4 C peaks disappeared completely while TiC and TiN were detected in the mixture by XRD (Fig. 2b). But no TiB 2 was found in the mixture (Fig. 2b). It is suggested that the B 4 C is broken into very small pieces among fine Ti grains. Metallic Ti, therefore, obscures the reflections of B 4 C [18]. On the other hand, BN in the current study has a hexagonal crystal structure (as shown in Fig. 2a). In a previous study, it was found that in ball milling processing, the disappearance of the reflections of BN indicated that BN separated into very small slices [19]. Within 10 h of milling, BN and B 4 C were mixed with Ti on a nanometer scale, and BN and B 4 C began to react with Ti. Thus, TiC and TiN were produced. The lattice parameter of TiN and TiC were found to be 0.421 nm, 0.433 nm, respectively, which did not change during the subsequent milling process. Since no TiB 2 or crystalline B was produced (Fig. 2b), some amorphous B should exist in the mixture. As shown in Fig. 2, no Ti(C, N) was found during
Mixture A and Mixture B with a weight ration of 1:1 were ball-milled in a GN-2 High-energy Ball Milling Machine (Shengyang Science Equipment Factory, China, see Fig. 1). The milling was conducted using hardened steel balls with a diameter of 5 mm. The ball-to-powder weight ratio was 10:1, and the milling speed was 360 rpm. The hardened steel canister was evacuated to 0.2 Pa and flushed with Ar gas. The milling runs were interrupted for 0.5 h to cool the canister whenever the powder was milled for 5 h. After milling for selected times, ball milled powder mixtures were taken out in Ar for analysis. The powders were characterized by X-ray diffraction (XRD, Cu K a radiation: l 50.154 nm). XRD peak broadening was used to determine the crystallite size d. By plotting b cos u ( b the linewidth at half-maximum intensity and u the Bragg angle) again sin u, a straight line was obtained from which the crystal size d was obtained from the intercept. The
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3.2. Reaction mechanism during high-energy ball milling
Fig. 2. XRD patterns of the powder mixtures milled for different times: (a) before milling; (b) 10 h; (c) 20 h; and (d) 30 h.
ball-milling process. Hence, reactions between Ti and BN can be regarded as being independent of the reactions between Ti and B 4 C. With increasing milling time, the diffraction peaks of crystalline Ti become broader and of less intensity, but their positions do not shift. After 20 h of milling, the average diameter of Ti grains is around 20 nm as derived via the Scherrer formula, and TiB 2 peaks appeared (Fig. 2c). With further milling up to 30 h, as shown in Fig. 2d, the decrease in the integral intensity of the Ti peaks is associated with the formation of TiC, TiN and TiB 2 within 30 h of milling. Fig. 3 shows the TEM images of powder milled for 30 h. TiN, TiC, TiB 2 , Ti and amorphous B formed microscale composite particles, and some nanosized particles, composed of TiN, TiC, TiB 2 and a small amount of amorphous B, were wrapped within them (Fig. 3a, b). During the milling process, TiB 2 particles were formed in the TiN and TiC matrix produced previously. Therefore, the heat generated by the formation of TiB 2 possibly led to slight sintering of some nearby nanosized TiN and TiC grains. The existence of a small amount of amorphous B might prompt the sintering (Fig. 3c, d). The corresponding SAD pattern demonstrated that the particles were mainly composed of nanocrystalline TiN, TiC and TiB 2 . The amorphous B shows a thin bright background in the image (Fig. 3c). The SAD pattern demonstrated that the rest part of the particles is free of TiB 2 and is mainly composed of nanocrystalline TiN, TiC, Ti and a small amount of amorphous B.
In a conventional SHS process, the reactant powders are impacted as a pellet for easy propagation of the combustion wave. Although the powders are more loosely dispersed in the vial during ball milling, the mixing of the reactants on a nanometre scale favors mass transfer and the diffusion path length is considerably reduced. In addition, the ignition events must be numerous during the milling process. Therefore, the self-propagating reaction during ball milling may be massive and explosive. It is believed that a high heat of formation serves as the driving force of the combustion reaction during milling process. This is consistent with a conventional SHS reaction. For it to be self-sustaining, the process must be associated with high temperature reactions. Researchers have demonstrated that the value of DH298 / Cp 298 should be above 2000 K in thermally ignited systems [10]. Thus, the value of DH298 / Cp 298 can be used as a rough guide for the existence of combustion in the milling process [14]. Some typical combustion reactions are summarized in Table 1 [20]. Another factor related to self-propagating reactions is the particle size of the reactants. In general, the reaction rate in the powder system is dependent on particle size [10,19]. This is because refinement of particle size increases the reaction interface area and the activities of the reactants. In particular, it has been reported that propagation of the combustion wave in SHS is accelerated by the refinement of particle size, whereas combustion becomes difficult and even disappears as the particle size increases [21]. Previous investigations show that ignition of the combustion reaction requires an initial premilling period (incubation) during which ball milling leads to a change in the factors determining the critical combustion condition. Hence, some researchers have proposed that there exists a critical particle size for ignition of the combustion reaction during the course of high-energy ball milling [14,20]. In this work, the values of DH298 / Cp 298 of Reactions (1) and (2) are 4100 and 4000 K, respectively. Therefore, it is possible that the formation of TiN, TiC and TiB 2 occurs by a self-propagating reaction during ball milling from Ti, B 4 C and BN powders. However, as shown in Fig. 2, the formations of TiN, TiC or TiB 2 occurred by gradual reactions. In a previous study [18], it was found that under the same milling conditions, Reaction (1) took place by a self-propagating reaction when the milling time was 4.5 h, and some TiC particles formed before the combustion reaction. On the other hand, when the mixture of Ti and BN was ball-milled for 10 h, TiN was produced. But no TiB 2 was found in the mixture even after 40 h of milling. Therefore, the mixture of Ti and B 4 C prompted reactions between Ti and BN while the mixture of Ti and BN abated reactions between Ti and B 4 C during the ball-milling processing. In this experiment, the outer shell of some B 4 C particles
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Fig. 3. TEM images of the powder mixtures milled for 30 h (a) and (b), electron diffraction patterns (c) and (d) of Fig. 3b (the areas marked by arrows), showing the existence of nanocrystalline TiC, TiN, TiB 2 and Ti beside a small amount of amorphous B.
would decompose into B and C atoms due to the impact of the milling balls and the reduction of Ti around these B 4 C particles [18]. C and B atoms then diffuse into Ti. The diffusivity of carbon is significantly higher than that of boron, according to the literature data [4], approximately three orders of magnitude higher than that of boron in titanium. Therefore, the formation of TiC was more
favorable than that of TiB 2 . Barsoum et al. [4] reported that when Ti reacted with B 4 C at elevated temperature, carbon diffused rapidly from B 4 C areas into titanium during the initial stage, forming TiC 0.5 and leaving behind areas of high boron activity. TiC 0.5 was one of the intermediate phases. In this experiment, however, TiC, not TiC 0.5 , was detected. Thus, it is assumed that the impact of
J. Li et al. / Journal of Alloys and Compounds 334 (2002) 253 – 260 Table 1 Typical reactions in high-energy ball milling processing Reaction
Heat of formation (kJ mol 21 )
Adiabatic temperature (K)
Mode of reaction
Mo12Si→MoSi 2 Ti12B→TiB 2 Ti1C→TiC 4Al13C→Al 4 C 3 Si1C→SiC W 12Si→WSi 2
2138 2342 2183.8 2215.8 267 –
1900 3190 3210 1200 1800 1500
Combustive Combustive Combustive Gradual Gradual Gradual
the milling balls triggered the reaction between Ti and C near the B 4 C particles where the C content was high enough. But the formation rate of superfine TiC was sluggish and the heat generated by the reaction was too small to ignite a self-propagating reaction due to the dilution of the reactants. The fine TiC grains formed are moved away quickly in the milling processing, so they remain nanosized [16–18] (Fig. 2). A previous study on the displacement reaction of Ti and BN showed that when Ti reacted with BN at elevated temperatures, TiN formed at about 10008C while TiB 2 occurred above 12008C [22]. Namely, the N atoms diffused and reacted with Ti prior to the reaction with B. Therefore, TiN is relatively easier to form in the current work. According to the conventional analysis above, the formation of TiN, TiC and TiB 2 could occur by a selfpropagating reaction, however, there is no extra heat to trigger the reaction during the milling process. Carslaw and Jaeger demonstrated that the majority of heat generated by the impact in ball mill processing is directly proportional to the friction coefficient [21]. In this work, since BN is an excellent lubricant, which can lighten the impact force between balls, the heat generated is small and cannot result in a rapid temperature increment in a given area to ignite the combustion. On the other hand, the heat generated by the formation of TiC or TiN was also too small to trigger a combustion reaction. Due to the sluggish reaction rate, a large amount of heat could not be generated in a short moment. As a result, the heat generated distributed over a relatively large area and finally diffused through the canister to outer space. During 10–30 h of milling, many TiC and TiN particles formed during milling led to an increment of the friction coefficient, and thus reactions between Ti and B 4 C, BN or B might be accomplished by a self-propagating reaction. However, those TiC and TiN particles formed previously acted as diluent and thus a combustion reaction was suppressed. As a result, the formation rate of TiB 2 was remarkably reduced and TiB 2 was slowly formed in the form of superfine particles. In the current work, it was found that when a steel flake was used to quickly clean the powder mixture milled in Ar
257
for 10 h and attached to the milling balls, the powder mixture began to combust. This result indicates that the reactants had been mixed on a nanometre scale and that the heat or sparkle, generated by the friction of steel flake and balls, triggered the combustion reaction. However, when a steel flake was used to quickly clean the powder mixture milled in Ar for 20 h, the powder mixtures did not combust. Hence, the reactants were diluted with the TiN, TiC and TiB 2 grains formed during previous milling processes and thus, a combustion reaction was suppressed. As shown in Fig. 2, no intermediate phase, such as TiN x , TiC x or TiB, appeared during the milling process. Therefore, it is assumed that the formation of TiC, TiN or TiB 2 occurred by a local high temperature reaction over a much smaller scale, and the heat generated by the reactions was too small to ignite a self-propagating reaction in the canister. Zhang [8] reported that when a mixture of TiH 2 , B 4 C and BN powders was heated to 18508C and kept at this temperature for 0.5 h, the diffusion of C, N and B atoms into Ti led to the formation of TiB 2 / Ti (C 0.5 N 0.5 ) composites, and no TiN was produced before the formation of Ti (C 0.5 N 0.5 ). Therefore, although the diffusion of C and N atoms played an important role during ballmilling process, no Ti (C x N 12x ) phase was produced during ball-milling processing. This also demonstrated that the TiC or TiN particles formed independently and instantaneously, otherwise some Ti (C x N 12x ) would have been formed. Namely, although the formations of TiC, TiN or TiB 2 were gradual, single grains were formed instantaneously. This result is consistent with the previous conclusion in this work. In summary, the evolution of the powder mixture during milling processing is as follows: During ball milling for 0–10 h, Reactions (3) and (4) take place: Ti 1 B 4 C → TiC 1 B (amorphous)
(3)
Ti 1 BN → TiN 1 B (amorphous)
(4)
During ball milling for 10–30 h, Reaction (5) takes place: Ti 1 B 4 C 1 BN 1 B (amorphous) → TiC 1 TiN 1 TiB 2 (5)
3.3. Effect of subsequent heat treatment on microstructure of powder mixture Fig. 4 shows the XRD patterns of the powder mixtures milled for 30 h followed by heat treatment in a vacuum. In comparison with the powder mixture before heat treatment (Fig. 2d), the amount of TiN and TiC did not increase obviously when the powder mixture was annealed at 400
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Fig. 4. XRD patterns of powder mixtures after 30 h of milling and heat treatment, (a) heated to 13008C and cooled; (b) annealed at 7008C for 1 h; and (c) annealed at 4008C for 1 h.
or 7008C for 1 h, and a trace of Ti was detected in the powder mixture (Fig. 4b, c). The result indicated that almost all TiC, TiN and TiB 2 were produced during the milling process. The XRD patterns demonstrated that the content of Ti or B was low. Namely, Reactions (1) and (2) were nearly accomplished after 30 h of milling. The broader and lower intensity TiN and TiC peaks indicate that the TiN and TiC have nanosized structure (Fig. 4c). When the powder mixture milled for 30 h was annealed at 7008C for 1 h, although the reflections of TiC became less intense, Ti (C x N 12x ) peaks did not appear (Fig. 4b). Namely, TiC and some TiN began to be transformed into superfine Ti (C x N 12x ). Hence, the temperature at which Ti (C x N 12x ) began to form from TiC and TiN should be above 7008C. As shown in Fig. 2, no Ti (C x N 12x ) was produced during the milling process. This indicated that the temperature in the canister did not exceed 7008C, which is consistent with the conclusion that the maximum local milling temperature during ball milling is in the range 300–6008C [23]. When the powder mixture milled for 30 h was heated to 13008C at a heating rate 108C / min, only TiN, Ti (C x N 12x ) and TiB 2 were detected (Fig. 4a). TiC was not detected. The XRD result indicates that the ball-milled mixture has been converted to Ti (C x N 12x ), TiB 2 and TiN after this heat treatment. Namely, Reaction (5) was accomplished up to now, TiC reacted with some TiN, and Ti (C x N 12x ) was produced [Reaction (6)]: TiC 1 TiN → Ti (C x N 12x )
(6)
Although TiC and TiN can be transformed into infinite solid solution, TiN and Ti (C x N 12x ) coexisted in this work due to the low temperature and short holding time in the
Fig. 5. DSC curve, showing no obvious exothermic peak.
heat treatment. Ti (C x N 12x ) was found to be Ti (C 0.7 N 0.3 ) and Ti (C 0.3 N 0.7 ) in this work. In order to study the phase transitions associated with heat treatment, a DSC analysis was performed. The DSC curve is shown in Fig. 5. However, there is no obvious exothermic peak. This result indicates that no obvious exothermic reaction occurred during the heating process. Therefore, the formation of TiC, TiN and TiB 2 should have been nearly completed during ball milling, and the volume fractions of TiC, TiN and TiB 2 formed during the treatment were rather low. This result is consistent with the previous conclusions (Figs. 2–4). The annealed powder shows a yellow color while the powder mixtures before this heat treatment, or those annealed at 400 or 7008C for 1 h are all black, presumably as a result of the very small TiN grain size. The average grain size of Ti (C 0.3 N 0.7 ), TiN and TiB 2 are 25, 80 and 40 nm, respectively. TEM images of the powder mixture milled for 30 h and heated to 13008C are shown in Fig. 6. Fig. 6a shows agglomerates of sintered TiB 2 , TiN and Ti (C x N 12x ) grains, identified by EDS. The grains were found to be 30–60 nm in size, in agreement with the XRD result. In conventional hot pressing processes [8], Ti (Cx N 12x ) and TiB 2 tend to form large grains separately and there are many small pores within the TiB 2 grains. By contrast, the TiB 2 / Ti (C x N 12x ) nanocomposite powder prepared in this work consists of nano-scale TiN / Ti (C x N 12x ) grains incorporated in nanosized TiB 2 particles. If the as-milled powder mixture was impacted and sintered, the TiB 2 / Ti (Cx N 12x ) nanocomposite is expected to form at relatively low temperatures. Further research is being carried out.
4. Conclusion TiB 2 / TiN / Ti (Cx N 12x ) nanocomposite powder has been prepared via high-energy ball milling and subsequent heat treatment. The formations of TiB 2 , TiN and TiC were
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Fig. 6. TEM images of powders milled for 30 h and heated to 13008C (a) and (c), showing nanosized TiN, Ti (C x N 12x ) and TiB 2 grains; (b) and (c) are SAD patterns of (a) and (c), respectively.
gradual, in contrast to the formation by a mechanically induced self-propagating reaction. The formation of TiC and TiN occurs within 10 h of milling, prior to that of TiB 2 during the milling process due to the faster diffusion of carbon and nitrogen atoms compared with boron atoms in
the titanium matrix. No Ti (C x N 12x ) formed during the ball-milling processing. Reactions (1) and (2) were nearly accomplished after 30 h of milling. After heat treatment at 13008C, the final product is composed of nanosized TiB 2 and TiN / Ti (Cx N 12x ) particles.
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