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Mechanochemical carboaluminothermic reduction of rutile to produce TiC–Al2O3 nanocomposite
Advanced Powder Technology 25 (2014) 423–429
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Mechanochemical carboaluminothermic reduction of rutile to produce TiC–Al2O3 nanocomposite Abdollah Hajalilou a,⇑, Mansor Hashim a, Mahdi Nahavandi b, Ismayadi Ismail a a b
Material Synthesis and Characterization Laboratory, Institute of Advanced Nano Technology (ITMA), Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Faculty of Material Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 10 June 2012 Received in revised form 13 March 2013 Accepted 5 July 2013 Available online 3 August 2013 Keywords: Mechanical alloying (MA) TiC–Al2O3 nanocomposite Mechanochemical process Carboaluminothermic reduction
a b s t r a c t This investigation aims to produce TiC–Al2O3 nanocomposite by reducing rutile with aluminum and graphite powder via a mechanochemical process. The effect of milling time on this process was investigated. The characterization of phase formation was carried out by XRD and SEM. Results showed that after a 10 h milling, the combustion reaction between Al, TiO2 and C was started and promoted by a self-propagation high temperature synthesis. Extending the milling time to 20 h, the reaction was completed. The XRD study illustrated after a 20 h milling, the width of TiC and Al2O3 peaks increased while the crystallite sizes of these phases decreased to less than 28 nm. After annealing at 800 °C for 1 h in a tube furnace, TiC and Al2O3 crystallite sizes remained constant. However, raising the annealing temperature to 1200 °C caused TiC and Al2O3 crystallite size to increase to 49 nm and 63 nm, respectively. No new phase was detected after the heat treatment of the synthesised TiC–Al2O3 nanocomposite. Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Al2O3-based ceramic matrix composites are extensively used in many applications, especially as excellent cutting tools and wear resistance coating [1]. Investigations have shown that the addition of 30–40% of TiC to alumina matrix can improve hardness, fracture toughness and thermal shock resistance at high temperatures [1,2]. Self-propagating high-temperature synthesis (SHS), or combustion synthesis has been used to manufacture ceramic, ceramic/inter metallic, and ceramic/metal composites [3]. Due to the advantage of using inexpensive raw materials in the aluminothermic reaction, the synthesis via aluminothermic reaction has been the focus of interest. TiC–Al2O3, Al2O3–TiB2, WC–Al2O3, Cr3C2–3Al2O3 ceramic composites have been obtained using this technique [4,5]. In most cases, this technique needs expensive furnaces and high temperature which could be higher than the melting point of TiC. Moreover, it is difficult to prepare fine-grained TiC particles [6]. Mechanical alloying (MA) is a powder metallurgy processing technique including cold welding, fracturing and rewelding of powder particles in high-energy ball mill and has recently become an established practical and commercial technique for manufacturing of several advanced materials with unique properties [7], especially for materials that are difficult to be prepared by the ⇑ Corresponding author. Tel.: +60 176015020.
traditional way of liquid metallurgy. Using this method, a variety of advanced engineering materials such as nanocrystalline, high thermally stable metallic glasses and amorphous alloys [8–10], metastable solid solutions and refractory hard materials, involving metal nitrides and silicides [11,12], carbides [13], hydrides [14] are produced at room temperature. Two kinds of reaction mechanism for MA have been proposed [15]: (1) Gradual inter-diffusion of elements and formation of new phases and products by prolonged milling time. (2) Sudden formation of products in a short period of milling time and, therefore, occurrence of mechanically alloyed self-sustaining reaction (MSR). WC formation from W and C raw materials and TiC compound from Ti and C are examples of the first and second mechanism, respectively. There have been some attempts to manufacture TiC–Al2O3 nanocomposite by MA. Jiang and coworkers investigated the in situ synthesis of Al2O3–TiC nanocomposite from a mixture of Ti, graphite and Al2O3 (nano) powders by ball milling [16]. Razavi and Rahimipour, used elemental powders of Ti, Al and carbon black, TiC–Al2O3 was formed during the annealing of milled powder in oxygen atmosphere with some impurities [17]. In this research production of TiC–Al2O3 nanocomposite, using mechanochemical processing of inexpensive raw materials,
E-mail address: [email protected] (A. Hajalilou). 0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2013.07.004
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including TiO2 (rutile), Al and graphite at room temperature, was investigated.
DH0298 ¼ 1071:57 kJ=mol T ad ¼ 2545 K
2. Experimental In order to produced TiC–Al2O3 nanocomposite, titanium dioxide (rutile type), aluminum and graphite powders were used as raw materials. The characteristics of these materials are shown in Table 1. Powders were mixed based on the mass ratio according to the reaction (1). To make TiC–Al2O3 composite there is a vast ratio range for these phases to combine. When the amount of Al2O3 is high in ratio the texture of the composite would be constructed from the continuous phase of Al2O3. In this research, the molar ratio of TiC to Al2O3 was chosen to be equal to 2:3 and the weight ratio was 1.13:1 which made up 44 vol.% TiC composite.
4Al þ 3TiO2 þ 3C ¼ 2Al2 O3 þ 3TiC
ð1Þ
To predict the gradual or instant type of mechanism process, adiabatic temperature of reaction (1) was calculated using thermodynamics data in Table 2 [18]. If the calculated adiabatic temperature were more than 1800 K, the reaction type would be self-propagating combustion [19]. Regarding mechanochemical reaction, if the calculated adiabatic temperature were more than 1300 K, the reaction would be the explosive type [20]. The thermodynamics calculation (see below) suggested the adiabatic temperature was 2545 K; therefore, the mechanochemical reaction was predicted as the explosive type.
Q ¼ ðDH0298 Þ ¼
Z
T ad
Cp
X
products dt
298
DH0298 ¼ ð3DH0298;TiC þ 2DH0298;Al2 O3 Þ ð3DH0298;TiO2 þ 4DH0298;Al þ DH0298;C Þ
DG0298 ¼ 1035 kJ=mol The negative value of DG0298 indicates that reaction (1) is thermodynamically possible at room temperature. DH0298 is also negative showing that this reaction is highly exothermic. Where DH0298 is the heat of reaction of the system at room temperature, Cp(s) and Cp(l) are heat capacity of solid and liquid status of the products, Tm and DHm are the melting point and heat of reaction of the melting products. Ball milling of the powder mixture was carried out in a planetary ball mill at room temperature and under argon atmosphere. Primary mixtures were milled for 8 h, 10 h, 20 h, 30 h and 60 h using steel balls with 10 mm in diameter. The mill had two vials of 45 mm radius each, with the vial and disc rotational speeds being 600 rpm and 400 rpm respectively, giving the ratio 600/ 400 between the two rotational speeds. The ball to powder ratio was 20:1 and the frequency of milling was 600 rpm. In order to investigate the stability of the TiC–Al2O3 nanocomposite after the 20 h milling the mixed powder was compressed under a 1000 kg/ (cm)2 pressure to make some pellets with 18 mm diameter. Then these samples were heat treated in a tube furnace under argon atmosphere at 800 °C and 1200 °C for 1 h. XRD analysis was carried out with Cu Ka (k = 0.15406 nm) radiation with a voltage of 30 kV and current of 25 mA. The morphology of milled powder particles was studied by a Seron technology ALS-2100 scanning electron microscope (SEM). The grain size and lattice strain were calculated via Williamson–Hall method [21] and the lattice parameter was also achieved using High Score software and Bragg’s law [22] through the following equations:
b cos h ¼
DH0298 ¼ 1071:57 kJ=mol DH0298 ¼ 1; 071; 570 Z 1800 Z ¼ ð3Cp ; TiCðsÞ þ 2Cp ; Al2 O3 ðsÞÞdt þ 298
þ 2Cp ; Al2 O3 ðsÞdt þ DHm ; Al2 O3 þ
Z
0:9k þ 2g Sin h Williamson Hall equation d
nk ¼ 2d sin h Bragg’s law 2327
ð3Cp ; TiCðsÞ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 a ¼ d ðk þ h þ l Þ
1800 T ad
ð3Cp ; TiCðsÞ
2327
þ 2Cp ; Al2 O3 dt
where b is full width of peak at half intensity (rad) or the full width at half maximum (FWHM) from High Score software, h is position of peak in the pattern (rad), k the wavelength of X-ray (nm), g strain in the lattice and a the lattice parameter. 3. Results and discussion
Table 1 Properties of raw materials.
3.1. Mechanical alloying
Powder
Average particle size (lm)
Purity (wt.%)
Density (g/cm3)
Titanium dioxide (rutile) Aluminum Graphite
<0.8 <45 <1
99.2 99.4 99.9
4.26 2.7 2.25
Fig. 1 illustrates the X-ray diffraction patterns of milled samples for different time durations. As was observed, XRD patterns after 8 h milling illustrated TiO2 and Al peaks exist and no other kind of reaction occurred among the constituents of the mixture. By
Table 2 Thermo chemical data for TiC, A12O3 and A1 [18]. Element
Temp. range(K)
DH° (kJ/mol)
TiC
298–1800 800–3290
183.69
A12O3
298–1800 1800–2327 2327–3000
1677.44
TiO2
–
944.79
DHm (kJ/mol)
Cp (J/kmol) 49.5 + (3.35 103 T) (14.98 103 T2) 34.2 + (11.58 103 T) + (74.16 105 T2)
111.085
106.61 + (17.78 103 T) + (28.54 105 T2) 128.0 + (5.28 103 T) (80.235 105 T2) 192–464 –
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θ Fig. 1. XRD patterns taken from the Al/TiO2/C powder mixture after; 8 h, 10 h, 20 h, 30 h, 60 h milling; TiC (d), Al2O3 (), TiAl3 (s), TiO2 (|), Al (€), TiAl (4). C (N).
increasing the milling time to 10 h, XRD patterns indicated that Al and TiO2 peaks disappeared and TiC peaks emerged. It can be proposed that TiO2 became either amorphous or completely changed to new phases. Since Al peaks disappeared and Al2O3 and TiC peaks appeared; therefore, TiO2 completely reacted through the SHS process. The formation of titanium carbide and alumina from initial reactants and active energy released from them after a certain milling time, between 8 h and 10 h, proved the SHS reaction to be highly exothermic. By increasing the milling time from 10 h to 20 h, no new compound was detected by XRD and only peaks of interphase compounds (TiAl and Ti3Al) disappeared. This evidence suggested the SHS reaction was completed and the peaks width of Al2O3 and TiC increased, while their intensity decreased. This was due to the fact that grains became smaller and lattice strain increased. During milling from 20 h to 30 h grain sizes of Al2O3 and TiC were almost unchanged and remained constant. This was due to the balances between work hardening and recovery rate. Since the density of dislocation was reduced by hardening sub boundary formation, the rate of recovery reached the rate of hardening [10,23]. By continuing the milling process from 30 h to 60 h, the peak widths of XRD patterns of TiC and Al2O3 were decreased and their intensity increased. This was because the stress was released by grain growth. During the milling period between 30 h and 60 h,
Fig. 2. Williamson–Hall equation calculation in TiO2–C–Al system after 10 h milling.
Table 3 Mean grain size and the strain induced by milling based on Williamson–Hall equation. Milling time (h)
0.9 k/d
dTiC (nm)
g (%)
10 20 60
0.0038 0.0049 0.0033
36 28 41
0.61 0.75 0.50
two processes – mechanical alloying (MA) and combustion reaction-occurred simultaneously. These initially produced finer particles and subsequently TiC Al2O3 production occurred along with increases in temperature and the grain growth. As was observed, since no new phase and compound were detected from 20 h to 60 h periods of milling time and only the crystallite sizes of TiC and Al2O3 changed, it could be concluded that the SHS reaction was completed after the 20 h milling time. To calculate the TiC grain size (d) and lattice strain (g), the XRD measurement and the Williamson–Hall equation were used. Measurement by XRD yields the width of TiC peaks and the diffraction peak angles (2h). Fig. 2 for the 10 h milling sample illustrates the graph of b cos h versus 2 sin h for TiC peaks. According to the
Fig. 3. Gibbs energy of Al2O3, TiC and TiAl3 at different temperatures.
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θ Fig. 4. XRD patterns for 20 h milling time, without heating (a), heating at 800 °C for 1 h (b), heating at 1200 °C for 1 h (c); TiC (d), Al2O3 ().
Williamson–Hall equation, the slope of this graph gives the TiC lattice strain and the intercept between the graph line and vertical axis yields the amount of 0:9k (d is TiC grain size). d By this method the TiC grain size and lattice strain for products of various milling times from 10 h to 60 h are summarized in Table 3. As was observed, the TiC lattice strain increased during periods of time between 10 h and 20 h. That would be due to an increasing number of lattice defects and piling up of dislocations which conforms with the results of a previous study [24]. The samples produced by the MA, continuing from 30 h to 60 h, show the reduction in lattice strain could be due to increasing recovery rate relative to hardening rate. 3.2. Mechanism of synthesis The XRD results showed that the formation of TiC during the mechanochemical reaction was through four main stages. The first stage was local melting of Al. In the second stage the Al liquid went into the pores of TiO2 resulted in reduction by Al and Al2O3 was produced alongside. The third stage was when the produced titanium reacted with Al and interphase compounds were formed. The energy released from this stage caused an increase in temperature of the mixture to reach the ignition point for the initiation of the reaction between Ti and C. In the final stage the produced Ti reacted with graphite and produced Titanium Carbide. Meanwhile, as is shown in Fig. 3, calculation of Gibbs energy of Al2O3, TiAl3, TiC showed that the binding energy between Ti and C was more than that between Ti and Al at the higher temperature. These stages are compatible with the following reactions:
AlðsÞ ! AlðlÞ
ð2Þ
4Al þ 3TiO2 ¼ 2Al2 O3 þ 3Ti
ð3Þ
DH0298 ¼ 164:44 kJ=mol
Ti þ CðsÞ ¼ TiC
DG0298 ¼ 180 kJ=mol
DH0298 ¼ 183:75 kJ=mol During milling at room temperature, due to negative DG0298 (3) and (6) reactions can be completed. However, the reactions (3) and (5) though thermodynamically possible, can be hindered by kinetic barriers at ambient temperatures. It is now well known that mechanochemical processing can supply the means to surmount the kinetics limitations of these reactions [10].
DG0298 ¼ 500:1 kJ=mol; DH0298 ¼ 521:1 kJ=mol TiðsÞ þ AlðlÞ ! TiAlðsÞ ;
ð4Þ
DH0298 ¼ 75:312 kJ=mol TiðsÞ þ 3AlðlÞ ! TiAl3ðsÞ
ð6Þ
ð5Þ Fig. 5. Lattice parameter of TiC as a function of composition.
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Fig. 6. SEM morphology of as-received powder particles.
Fig. 7. SEM morphology of powder particles after; (a)10 h, (b)20 h, (c)30 h, (d)60 h ball milling.
3.3. Heat treatment Nanocrystalline structures are not thermodynamically stable because of the high amount of energy saved in their grain boundaries. In some nanocrystalline materials, the growth of the grain was found to be considerably [8]. Therefore, in order to investigate the stability of TiC–Al2O3 nanocomposite, the 20 h-milled samples were annealed at 800 °C and 1200 °C for an hour under an argon atmosphere. Fig. 4 shows the XRD results for 800 °C in which the width of TiC and
Al2O3 peaks were narrowed slightly about 3% relative to the nonheat treated sample. This indicates there was no significant grain growth and the grain size almost remained constant. But after one hour of heat treatment at 1200 °C it was observed that the peak width of TiC and Al2O3 peaks was reduced about 24% and significant grain growth occurred. The grain sizes of TiC and Al2O3 increased to 49 and 63 nm respectively. The results of heat treating the samples showed that by increasing the temperature the grain sizes increased, but remained in nanometer-scales. No new phases were observed
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Fig. 8. Microstructure photograph of the synthesized Al2O3–TiC nanocomposite after 20 h milling (a) and after 20 h milling with anneal at 1200 °C for 1 h.
after the annealing which indicated that the TiC–Al2O3 nanocomposite produced during mechanical alloying was stable. 3.4. Stoichiometric ratio of titanium carbide The TiC peaks of X-ray diffraction by extension of milling times, shift slightly toward bigger 2h angles. For example, the diffraction
angle of (1 1 1) planes shift from 35.9168 to 35.9552, for 10 h milling and 20 h milling respectively. This may be due to the formation of nonstoichiometric titanium carbide. A solution of some impurities like oxygen and nitrogen in the titanium carbide structure and a change in lattice parameter could lead to nonstoichiometric titanium carbide formation. The titanium carbide crystal structure is cubic similar to that of NaCl and has lattice parameter
Fig. 9. EDX analyses of the synthesized Al2O3–TiC nanocomposite after 20 h milling.
A. Hajalilou et al. / Advanced Powder Technology 25 (2014) 423–429
a = 0.4327 nm [22]. In this research lattice parameter of 10 h milled samples using Brag’s equation was calculated to be equal to a = 0.4326 nm. This slight difference confirms that the lattice parameter changed and also the ratio of C to Ti varied from the stoichiometric ratio by the extension of the milling time. We can consider the titanium carbide formulation as TiC1x. According to Fig. 5, by decreasing the lattice parameter the carbon content falls [25]. And for 10 h milling sample the titanium carbide compound would be suggested as TiC0.73. 3.5. SEM analysis In Fig. 6, the SEM images of as-received raw material powders particles are shown. It can be observed that the morphology of graphite powder particles is angular and flake-like while particles of TiO2 approximately are regular and spherical. The shapes of aluminum powders are spiral-like with different sizes varying between 42 and 47 lm. Fig. 7 shows SEM micrographs of the powder particles after different milling times. SEM observation showed that after 10 h milling, the cluster of titanium carbide burgeoned and the morphology of the powder particles became irregular and their sizes were not homogenous (Fig. 7a). The average powder particles size seems to be within 1–10 lm. By increasing the milling time to 20 h, during MA, powder particles became fragmented frequently and were cold welded together. In fact during MA, between 10 and 20 h, work hardening of powder takes place and fracturing overcame cold welding and the distribution of the powder particles became more homogenous and the morphology of the powder particles became spherical with much smaller size. This can be seen in Fig. 7b. In addition, the plastic deformation among powder particles increases causing the defect densities and internal energy increase. Consequently many fine particles can be observed in the microstructure of the milled powders. Furthermore, the reason for the size decrease of powder particles by increasing milling time could be the production of many dislocations. To decrease their energy, they are formed in sub boundary forms and form a cellular structure. By increasing milling time, gradually sub boundaries increase and later become better grain boundaries [10,24]. With increasing milling time from 20 h to 30 h, the change of powder particle size was not remarkable manifesting that an equal rate of agglomeration and fragmentation of powder particles is obtained, which clearly is shown in Fig. 7c. A SEM image after 60 h milling time is illustrated in Fig. 7d. This image shows that after 60 h milling powder particles adhered together and agglomeration process occurred. This process is due to the much longer extension of milling time and releases heat from the formation of TiC reaction. Fig. 8 shows SEM photographs of combustion products synthesized after 20 h milling followed by pellet making and metallography (a) and after annealing at 1200 °C (b) and spot EDX analysis of an area covering two different phases. Those are shown in Fig. 9. The first point having 86% Ti and no oxygen represents TiC phase. The second point with 40% Al and 42% oxygen shows the Al2O3 phase. Considering the small size of the structure examined under EDX, we believe the results are from TiC and Al2O3. 4. Conclusions 1. Using mechanical alloying (MA), after 10 h milling, the formation of TiC nanocrystalline powder was started and, after 20 h milling, the TiC formation was completed successfully from inexpensive raw materials including rutile, aluminum and graphite.
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2. The XRD results showed that the produced size of TiC and Al2O3 crystallites was on the scale of nanometer and the lattice strain increased up to 20 h milling, then it decreased with increasing milling from 20 h to 60 h it decreased. Furthermore, the TiC lattice parameter deviated slightly from the normal size. 3. During the reaction between Al and TiO2, the formation of interphase compounds such as TiAl3 took place and energy released from this reaction helped to raise the temperature of the mixture to reach the ignition point. 4. Significant grain growth or phase change was not observed after annealing milled TiC–Al2O3 powder at 1200 °C. 5. The fact that the TiC lattice parameter does not match with stoichiometric titanium carbide lattice parameter is due to the formation of TiC0.73. 6. Mechanical alloying as a useful and inexpensive method can be used for the production of the TiC–Al2O3 nanocomposite.
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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Mechanochemical carboaluminothermic reduction of rutile to produce TiC–Al2O3 nanocomposite Abdollah Hajalilou a,⇑, Mansor Hashim a, Mahdi Nahavandi b, Ismayadi Ismail a a b
Material Synthesis and Characterization Laboratory, Institute of Advanced Nano Technology (ITMA), Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Faculty of Material Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 10 June 2012 Received in revised form 13 March 2013 Accepted 5 July 2013 Available online 3 August 2013 Keywords: Mechanical alloying (MA) TiC–Al2O3 nanocomposite Mechanochemical process Carboaluminothermic reduction
a b s t r a c t This investigation aims to produce TiC–Al2O3 nanocomposite by reducing rutile with aluminum and graphite powder via a mechanochemical process. The effect of milling time on this process was investigated. The characterization of phase formation was carried out by XRD and SEM. Results showed that after a 10 h milling, the combustion reaction between Al, TiO2 and C was started and promoted by a self-propagation high temperature synthesis. Extending the milling time to 20 h, the reaction was completed. The XRD study illustrated after a 20 h milling, the width of TiC and Al2O3 peaks increased while the crystallite sizes of these phases decreased to less than 28 nm. After annealing at 800 °C for 1 h in a tube furnace, TiC and Al2O3 crystallite sizes remained constant. However, raising the annealing temperature to 1200 °C caused TiC and Al2O3 crystallite size to increase to 49 nm and 63 nm, respectively. No new phase was detected after the heat treatment of the synthesised TiC–Al2O3 nanocomposite. Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Al2O3-based ceramic matrix composites are extensively used in many applications, especially as excellent cutting tools and wear resistance coating [1]. Investigations have shown that the addition of 30–40% of TiC to alumina matrix can improve hardness, fracture toughness and thermal shock resistance at high temperatures [1,2]. Self-propagating high-temperature synthesis (SHS), or combustion synthesis has been used to manufacture ceramic, ceramic/inter metallic, and ceramic/metal composites [3]. Due to the advantage of using inexpensive raw materials in the aluminothermic reaction, the synthesis via aluminothermic reaction has been the focus of interest. TiC–Al2O3, Al2O3–TiB2, WC–Al2O3, Cr3C2–3Al2O3 ceramic composites have been obtained using this technique [4,5]. In most cases, this technique needs expensive furnaces and high temperature which could be higher than the melting point of TiC. Moreover, it is difficult to prepare fine-grained TiC particles [6]. Mechanical alloying (MA) is a powder metallurgy processing technique including cold welding, fracturing and rewelding of powder particles in high-energy ball mill and has recently become an established practical and commercial technique for manufacturing of several advanced materials with unique properties [7], especially for materials that are difficult to be prepared by the ⇑ Corresponding author. Tel.: +60 176015020.
traditional way of liquid metallurgy. Using this method, a variety of advanced engineering materials such as nanocrystalline, high thermally stable metallic glasses and amorphous alloys [8–10], metastable solid solutions and refractory hard materials, involving metal nitrides and silicides [11,12], carbides [13], hydrides [14] are produced at room temperature. Two kinds of reaction mechanism for MA have been proposed [15]: (1) Gradual inter-diffusion of elements and formation of new phases and products by prolonged milling time. (2) Sudden formation of products in a short period of milling time and, therefore, occurrence of mechanically alloyed self-sustaining reaction (MSR). WC formation from W and C raw materials and TiC compound from Ti and C are examples of the first and second mechanism, respectively. There have been some attempts to manufacture TiC–Al2O3 nanocomposite by MA. Jiang and coworkers investigated the in situ synthesis of Al2O3–TiC nanocomposite from a mixture of Ti, graphite and Al2O3 (nano) powders by ball milling [16]. Razavi and Rahimipour, used elemental powders of Ti, Al and carbon black, TiC–Al2O3 was formed during the annealing of milled powder in oxygen atmosphere with some impurities [17]. In this research production of TiC–Al2O3 nanocomposite, using mechanochemical processing of inexpensive raw materials,
E-mail address: [email protected] (A. Hajalilou). 0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2013.07.004
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including TiO2 (rutile), Al and graphite at room temperature, was investigated.
DH0298 ¼ 1071:57 kJ=mol T ad ¼ 2545 K
2. Experimental In order to produced TiC–Al2O3 nanocomposite, titanium dioxide (rutile type), aluminum and graphite powders were used as raw materials. The characteristics of these materials are shown in Table 1. Powders were mixed based on the mass ratio according to the reaction (1). To make TiC–Al2O3 composite there is a vast ratio range for these phases to combine. When the amount of Al2O3 is high in ratio the texture of the composite would be constructed from the continuous phase of Al2O3. In this research, the molar ratio of TiC to Al2O3 was chosen to be equal to 2:3 and the weight ratio was 1.13:1 which made up 44 vol.% TiC composite.
4Al þ 3TiO2 þ 3C ¼ 2Al2 O3 þ 3TiC
ð1Þ
To predict the gradual or instant type of mechanism process, adiabatic temperature of reaction (1) was calculated using thermodynamics data in Table 2 [18]. If the calculated adiabatic temperature were more than 1800 K, the reaction type would be self-propagating combustion [19]. Regarding mechanochemical reaction, if the calculated adiabatic temperature were more than 1300 K, the reaction would be the explosive type [20]. The thermodynamics calculation (see below) suggested the adiabatic temperature was 2545 K; therefore, the mechanochemical reaction was predicted as the explosive type.
Q ¼ ðDH0298 Þ ¼
Z
T ad
Cp
X
products dt
298
DH0298 ¼ ð3DH0298;TiC þ 2DH0298;Al2 O3 Þ ð3DH0298;TiO2 þ 4DH0298;Al þ DH0298;C Þ
DG0298 ¼ 1035 kJ=mol The negative value of DG0298 indicates that reaction (1) is thermodynamically possible at room temperature. DH0298 is also negative showing that this reaction is highly exothermic. Where DH0298 is the heat of reaction of the system at room temperature, Cp(s) and Cp(l) are heat capacity of solid and liquid status of the products, Tm and DHm are the melting point and heat of reaction of the melting products. Ball milling of the powder mixture was carried out in a planetary ball mill at room temperature and under argon atmosphere. Primary mixtures were milled for 8 h, 10 h, 20 h, 30 h and 60 h using steel balls with 10 mm in diameter. The mill had two vials of 45 mm radius each, with the vial and disc rotational speeds being 600 rpm and 400 rpm respectively, giving the ratio 600/ 400 between the two rotational speeds. The ball to powder ratio was 20:1 and the frequency of milling was 600 rpm. In order to investigate the stability of the TiC–Al2O3 nanocomposite after the 20 h milling the mixed powder was compressed under a 1000 kg/ (cm)2 pressure to make some pellets with 18 mm diameter. Then these samples were heat treated in a tube furnace under argon atmosphere at 800 °C and 1200 °C for 1 h. XRD analysis was carried out with Cu Ka (k = 0.15406 nm) radiation with a voltage of 30 kV and current of 25 mA. The morphology of milled powder particles was studied by a Seron technology ALS-2100 scanning electron microscope (SEM). The grain size and lattice strain were calculated via Williamson–Hall method [21] and the lattice parameter was also achieved using High Score software and Bragg’s law [22] through the following equations:
b cos h ¼
DH0298 ¼ 1071:57 kJ=mol DH0298 ¼ 1; 071; 570 Z 1800 Z ¼ ð3Cp ; TiCðsÞ þ 2Cp ; Al2 O3 ðsÞÞdt þ 298
þ 2Cp ; Al2 O3 ðsÞdt þ DHm ; Al2 O3 þ
Z
0:9k þ 2g Sin h Williamson Hall equation d
nk ¼ 2d sin h Bragg’s law 2327
ð3Cp ; TiCðsÞ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 a ¼ d ðk þ h þ l Þ
1800 T ad
ð3Cp ; TiCðsÞ
2327
þ 2Cp ; Al2 O3 dt
where b is full width of peak at half intensity (rad) or the full width at half maximum (FWHM) from High Score software, h is position of peak in the pattern (rad), k the wavelength of X-ray (nm), g strain in the lattice and a the lattice parameter. 3. Results and discussion
Table 1 Properties of raw materials.
3.1. Mechanical alloying
Powder
Average particle size (lm)
Purity (wt.%)
Density (g/cm3)
Titanium dioxide (rutile) Aluminum Graphite
<0.8 <45 <1
99.2 99.4 99.9
4.26 2.7 2.25
Fig. 1 illustrates the X-ray diffraction patterns of milled samples for different time durations. As was observed, XRD patterns after 8 h milling illustrated TiO2 and Al peaks exist and no other kind of reaction occurred among the constituents of the mixture. By
Table 2 Thermo chemical data for TiC, A12O3 and A1 [18]. Element
Temp. range(K)
DH° (kJ/mol)
TiC
298–1800 800–3290
183.69
A12O3
298–1800 1800–2327 2327–3000
1677.44
TiO2
–
944.79
DHm (kJ/mol)
Cp (J/kmol) 49.5 + (3.35 103 T) (14.98 103 T2) 34.2 + (11.58 103 T) + (74.16 105 T2)
111.085
106.61 + (17.78 103 T) + (28.54 105 T2) 128.0 + (5.28 103 T) (80.235 105 T2) 192–464 –
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θ Fig. 1. XRD patterns taken from the Al/TiO2/C powder mixture after; 8 h, 10 h, 20 h, 30 h, 60 h milling; TiC (d), Al2O3 (), TiAl3 (s), TiO2 (|), Al (€), TiAl (4). C (N).
increasing the milling time to 10 h, XRD patterns indicated that Al and TiO2 peaks disappeared and TiC peaks emerged. It can be proposed that TiO2 became either amorphous or completely changed to new phases. Since Al peaks disappeared and Al2O3 and TiC peaks appeared; therefore, TiO2 completely reacted through the SHS process. The formation of titanium carbide and alumina from initial reactants and active energy released from them after a certain milling time, between 8 h and 10 h, proved the SHS reaction to be highly exothermic. By increasing the milling time from 10 h to 20 h, no new compound was detected by XRD and only peaks of interphase compounds (TiAl and Ti3Al) disappeared. This evidence suggested the SHS reaction was completed and the peaks width of Al2O3 and TiC increased, while their intensity decreased. This was due to the fact that grains became smaller and lattice strain increased. During milling from 20 h to 30 h grain sizes of Al2O3 and TiC were almost unchanged and remained constant. This was due to the balances between work hardening and recovery rate. Since the density of dislocation was reduced by hardening sub boundary formation, the rate of recovery reached the rate of hardening [10,23]. By continuing the milling process from 30 h to 60 h, the peak widths of XRD patterns of TiC and Al2O3 were decreased and their intensity increased. This was because the stress was released by grain growth. During the milling period between 30 h and 60 h,
Fig. 2. Williamson–Hall equation calculation in TiO2–C–Al system after 10 h milling.
Table 3 Mean grain size and the strain induced by milling based on Williamson–Hall equation. Milling time (h)
0.9 k/d
dTiC (nm)
g (%)
10 20 60
0.0038 0.0049 0.0033
36 28 41
0.61 0.75 0.50
two processes – mechanical alloying (MA) and combustion reaction-occurred simultaneously. These initially produced finer particles and subsequently TiC Al2O3 production occurred along with increases in temperature and the grain growth. As was observed, since no new phase and compound were detected from 20 h to 60 h periods of milling time and only the crystallite sizes of TiC and Al2O3 changed, it could be concluded that the SHS reaction was completed after the 20 h milling time. To calculate the TiC grain size (d) and lattice strain (g), the XRD measurement and the Williamson–Hall equation were used. Measurement by XRD yields the width of TiC peaks and the diffraction peak angles (2h). Fig. 2 for the 10 h milling sample illustrates the graph of b cos h versus 2 sin h for TiC peaks. According to the
Fig. 3. Gibbs energy of Al2O3, TiC and TiAl3 at different temperatures.
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θ Fig. 4. XRD patterns for 20 h milling time, without heating (a), heating at 800 °C for 1 h (b), heating at 1200 °C for 1 h (c); TiC (d), Al2O3 ().
Williamson–Hall equation, the slope of this graph gives the TiC lattice strain and the intercept between the graph line and vertical axis yields the amount of 0:9k (d is TiC grain size). d By this method the TiC grain size and lattice strain for products of various milling times from 10 h to 60 h are summarized in Table 3. As was observed, the TiC lattice strain increased during periods of time between 10 h and 20 h. That would be due to an increasing number of lattice defects and piling up of dislocations which conforms with the results of a previous study [24]. The samples produced by the MA, continuing from 30 h to 60 h, show the reduction in lattice strain could be due to increasing recovery rate relative to hardening rate. 3.2. Mechanism of synthesis The XRD results showed that the formation of TiC during the mechanochemical reaction was through four main stages. The first stage was local melting of Al. In the second stage the Al liquid went into the pores of TiO2 resulted in reduction by Al and Al2O3 was produced alongside. The third stage was when the produced titanium reacted with Al and interphase compounds were formed. The energy released from this stage caused an increase in temperature of the mixture to reach the ignition point for the initiation of the reaction between Ti and C. In the final stage the produced Ti reacted with graphite and produced Titanium Carbide. Meanwhile, as is shown in Fig. 3, calculation of Gibbs energy of Al2O3, TiAl3, TiC showed that the binding energy between Ti and C was more than that between Ti and Al at the higher temperature. These stages are compatible with the following reactions:
AlðsÞ ! AlðlÞ
ð2Þ
4Al þ 3TiO2 ¼ 2Al2 O3 þ 3Ti
ð3Þ
DH0298 ¼ 164:44 kJ=mol
Ti þ CðsÞ ¼ TiC
DG0298 ¼ 180 kJ=mol
DH0298 ¼ 183:75 kJ=mol During milling at room temperature, due to negative DG0298 (3) and (6) reactions can be completed. However, the reactions (3) and (5) though thermodynamically possible, can be hindered by kinetic barriers at ambient temperatures. It is now well known that mechanochemical processing can supply the means to surmount the kinetics limitations of these reactions [10].
DG0298 ¼ 500:1 kJ=mol; DH0298 ¼ 521:1 kJ=mol TiðsÞ þ AlðlÞ ! TiAlðsÞ ;
ð4Þ
DH0298 ¼ 75:312 kJ=mol TiðsÞ þ 3AlðlÞ ! TiAl3ðsÞ
ð6Þ
ð5Þ Fig. 5. Lattice parameter of TiC as a function of composition.
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Fig. 6. SEM morphology of as-received powder particles.
Fig. 7. SEM morphology of powder particles after; (a)10 h, (b)20 h, (c)30 h, (d)60 h ball milling.
3.3. Heat treatment Nanocrystalline structures are not thermodynamically stable because of the high amount of energy saved in their grain boundaries. In some nanocrystalline materials, the growth of the grain was found to be considerably [8]. Therefore, in order to investigate the stability of TiC–Al2O3 nanocomposite, the 20 h-milled samples were annealed at 800 °C and 1200 °C for an hour under an argon atmosphere. Fig. 4 shows the XRD results for 800 °C in which the width of TiC and
Al2O3 peaks were narrowed slightly about 3% relative to the nonheat treated sample. This indicates there was no significant grain growth and the grain size almost remained constant. But after one hour of heat treatment at 1200 °C it was observed that the peak width of TiC and Al2O3 peaks was reduced about 24% and significant grain growth occurred. The grain sizes of TiC and Al2O3 increased to 49 and 63 nm respectively. The results of heat treating the samples showed that by increasing the temperature the grain sizes increased, but remained in nanometer-scales. No new phases were observed
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Fig. 8. Microstructure photograph of the synthesized Al2O3–TiC nanocomposite after 20 h milling (a) and after 20 h milling with anneal at 1200 °C for 1 h.
after the annealing which indicated that the TiC–Al2O3 nanocomposite produced during mechanical alloying was stable. 3.4. Stoichiometric ratio of titanium carbide The TiC peaks of X-ray diffraction by extension of milling times, shift slightly toward bigger 2h angles. For example, the diffraction
angle of (1 1 1) planes shift from 35.9168 to 35.9552, for 10 h milling and 20 h milling respectively. This may be due to the formation of nonstoichiometric titanium carbide. A solution of some impurities like oxygen and nitrogen in the titanium carbide structure and a change in lattice parameter could lead to nonstoichiometric titanium carbide formation. The titanium carbide crystal structure is cubic similar to that of NaCl and has lattice parameter
Fig. 9. EDX analyses of the synthesized Al2O3–TiC nanocomposite after 20 h milling.
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a = 0.4327 nm [22]. In this research lattice parameter of 10 h milled samples using Brag’s equation was calculated to be equal to a = 0.4326 nm. This slight difference confirms that the lattice parameter changed and also the ratio of C to Ti varied from the stoichiometric ratio by the extension of the milling time. We can consider the titanium carbide formulation as TiC1x. According to Fig. 5, by decreasing the lattice parameter the carbon content falls [25]. And for 10 h milling sample the titanium carbide compound would be suggested as TiC0.73. 3.5. SEM analysis In Fig. 6, the SEM images of as-received raw material powders particles are shown. It can be observed that the morphology of graphite powder particles is angular and flake-like while particles of TiO2 approximately are regular and spherical. The shapes of aluminum powders are spiral-like with different sizes varying between 42 and 47 lm. Fig. 7 shows SEM micrographs of the powder particles after different milling times. SEM observation showed that after 10 h milling, the cluster of titanium carbide burgeoned and the morphology of the powder particles became irregular and their sizes were not homogenous (Fig. 7a). The average powder particles size seems to be within 1–10 lm. By increasing the milling time to 20 h, during MA, powder particles became fragmented frequently and were cold welded together. In fact during MA, between 10 and 20 h, work hardening of powder takes place and fracturing overcame cold welding and the distribution of the powder particles became more homogenous and the morphology of the powder particles became spherical with much smaller size. This can be seen in Fig. 7b. In addition, the plastic deformation among powder particles increases causing the defect densities and internal energy increase. Consequently many fine particles can be observed in the microstructure of the milled powders. Furthermore, the reason for the size decrease of powder particles by increasing milling time could be the production of many dislocations. To decrease their energy, they are formed in sub boundary forms and form a cellular structure. By increasing milling time, gradually sub boundaries increase and later become better grain boundaries [10,24]. With increasing milling time from 20 h to 30 h, the change of powder particle size was not remarkable manifesting that an equal rate of agglomeration and fragmentation of powder particles is obtained, which clearly is shown in Fig. 7c. A SEM image after 60 h milling time is illustrated in Fig. 7d. This image shows that after 60 h milling powder particles adhered together and agglomeration process occurred. This process is due to the much longer extension of milling time and releases heat from the formation of TiC reaction. Fig. 8 shows SEM photographs of combustion products synthesized after 20 h milling followed by pellet making and metallography (a) and after annealing at 1200 °C (b) and spot EDX analysis of an area covering two different phases. Those are shown in Fig. 9. The first point having 86% Ti and no oxygen represents TiC phase. The second point with 40% Al and 42% oxygen shows the Al2O3 phase. Considering the small size of the structure examined under EDX, we believe the results are from TiC and Al2O3. 4. Conclusions 1. Using mechanical alloying (MA), after 10 h milling, the formation of TiC nanocrystalline powder was started and, after 20 h milling, the TiC formation was completed successfully from inexpensive raw materials including rutile, aluminum and graphite.
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2. The XRD results showed that the produced size of TiC and Al2O3 crystallites was on the scale of nanometer and the lattice strain increased up to 20 h milling, then it decreased with increasing milling from 20 h to 60 h it decreased. Furthermore, the TiC lattice parameter deviated slightly from the normal size. 3. During the reaction between Al and TiO2, the formation of interphase compounds such as TiAl3 took place and energy released from this reaction helped to raise the temperature of the mixture to reach the ignition point. 4. Significant grain growth or phase change was not observed after annealing milled TiC–Al2O3 powder at 1200 °C. 5. The fact that the TiC lattice parameter does not match with stoichiometric titanium carbide lattice parameter is due to the formation of TiC0.73. 6. Mechanical alloying as a useful and inexpensive method can be used for the production of the TiC–Al2O3 nanocomposite.
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