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Synthesis of tungsten carbide–cobalt composites by the field-activated combustion synthesis method
Journal of Alloys and Compounds 387 (2005) 90–96
Synthesis of tungsten carbide–cobalt composites by the field-activated combustion synthesis method Guojian Jiang∗ , Hanrui Zhuang, Wenlan Li Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 12 April 2004; received in revised form 17 June 2004; accepted 18 June 2004
Abstract The activation of self-propagating combustion reactions in the tungsten carbide–cobalt (WC-Co) system was achieved by using an electric field. Self-sustaining combustion in elemental reactants with composition corresponding to WC-x wt.% Co (0 ≤ x ≤ 10) composites can be activated only when the imposed field is above a threshold value. The nature of combustion products depended on the magnitude of the field. The relative strength of W2 C diffraction peaks in XRD diagram decreased when moving from the reactants region to the final products one or increasing electric field on the reactants. The effect of the relative density of the reactant compacts on the synthesis of WC-Co composites was also investigated. Finally, the reaction mechanism of WC-Co composites was proposed. X-ray and microscopic analysis of the quenched combustion front suggests that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. Molten Co redounds the transformation from W2 C to WC phase. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Chemical synthesis; SEM; X-ray diffraction
1. Introduction Tungsten carbide–cobalt (WC-Co) composites combine the high hardness of WC with the ductility of Co to produce a material of superior wear resistance, yet with sufficient strength for use in practical applications. Examples of such applications are inserts in rock drilling tools, wood cutting teeth and bearings or seals exposed to polluted water. Hard cemented carbides are commonly produced following a powder metallurgy route, which involves product powder synthesis, mixing with metal, granulation, pressing and sintering. Some stages require high temperature and long-time processing, leading to undesirable grain growth (Ostwald ripening) in the material components and the associated benefits of enhanced mechanical properties. A promising and energy-efficient technique known under the acronym of combustion synthesis (CS) or self∗ Corresponding author. Tel.: +86-21-5241-4216; fax: +86-21-5241-3903. E-mail address: [email protected] (G. J. Jiang).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.057
propagating high-temperature synthesis (SHS) for the preparation of advanced materials has been developed. In a typical SHS process, compacts of powder mixtures are ignited at one end to initiate the self-sustaining reaction. However, for the case of some systems, for example, W-C, ignition is impossible without addition activation. Primarily this is due to the thermodynamic limitation, i.e. low reaction enthalpies or the relatively low adiabatic temperatures (Tad ) of these carbides. For example, the adiabatic temperatures of WC and W2 C are 1127 and 673 ◦ C, respectively, which are considerably lower than the empirically established minimum of 1527 ◦ C for SHS reactions. With the addition of Co, the adiabatic temperature will change lower. The effect of the addition of Co on the adiabatic temperature of this system is shown in Fig. 1. Details of calculations of adiabatic temperature have been discussed in some references [1]. For systems that are relatively less exothermic, the common approach has been the preheating of the reactants, which accomplishes the goal of raising Tad . However, this treatment can suffer from the formation of extraneous phases, such as pre-combustion phases produced through diffusion processes.
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Fig. 2. Schematic representation of field-activated combustion synthesis (FACS). Fig. 1. Variation of the adiabatic combustion temperature with the amount (wt.%) of Co for the formation of composites of: WC-x wt.% Co.
Recently, a new method, referred to in the literature as field-activated combustion synthesis (FACS) [2], based on the use of an electric field to activate self-propagating reactions in their relatively low heat of formation or less-exothermic systems, has been developed. Experimental results and modeling studies have led to the conclusion that the effect of the field is to provide Joule heating at a rate of σE2 , with σ being the conductivity and E the field [3–5]. Depending on the electric conductivity of reactants and products, ignition results in the initiation and propagation of a combustion wave in reactant systems, which heretofore could not be synthesized by SHS (without preheating), including SiC, SiC-AlN, B4 C-TiB2 , MoSi2 -SiC, Ti3 Al and others [6–13]. In this paper, we report on the synthesis of WC-Co composites by the FACS method, present the results of the effects of various processing parameters on FACS of WC-Co composites and determine the fast formation mechanism from W, C and Co under electric field. No previous results on FACS of WC-Co composites have been reported. The aim of this study was to provide a better understanding of the role of electric field involved in FACS of WC-Co composites.
2. Experimental procedures Equiatomic mixtures of W and activated carbon with addition of Co powders were used to assess the effect of electric field on SHS reactions. W powder with a reported purity of 99.99% and an average particle size of 0.6 m (Korea Tungsten Co.), was dry-mixed in a mill with 99.9% pure activated carbon powder (an average particle size of 20 m, supplied by Kojondo Chemical Co.) and 99.8% pure Co powder (an average particle size of 30 m, supplied by Aldrich Chemical Co.). Tetragonally shaped pellets with dimensions of 10 mm × 10 mm × 15 mm were cold-pressed under various compaction pressures (100–500 MPa) in a two-plunger steel die. The pellets were then placed between two springloaded copper electrodes across which a voltage was applied. The spring was used to keep two electrodes in intimate contact with the flat top and bottom surface of the sample. Thus,
the distance between electrodes is the thickness of the sample, 10 mm. This experimental geometry provides an electric field perpendicular to the expected direction of wave propagation. A tungsten heating coil, placed near the edge of the sample, was used to initiate the combustion and was turned off immediately after the reaction was initiated. A schematic representation of this set-up is shown in Fig. 2. The activated combustion synthesis experiments were carried out inside a stainless steel pressure chamber in one atmosphere of argon gas. The combustion process evolution was recorded from video camera equipped with a time-code generator. The propagation velocity of combustion wave was determined by timing of the propagation from one end of the sample to the other. The surface temperature profiles of the sample during combustion were measured with a two-color optical pyrometer with a response time of 10 ms focused on the middle portion of the sample. Compositional and microstructural analyses of the products were made through X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.
3. Results and discussion In the absence of a field, the exposure of one end of the sample to an ignition source for several minutes does not result in the initiation of a combustion wave or evidence of any reaction beyond the heat-affected zone. Experiments on WCCo system showed that a self-sustaining combustion wave can not be established unless the field strength exceeds a minimum (threshold) value. The effect of Co amounts in the reactants on the threshold value is shown in Fig. 3. With an increase in the amount of Co, the threshold value increased higher. When the electric field strength is higher than the threshold value, the activation of the ignition source causes the propagation of combustion wave throughout the sample. The figure shows three regions, relative to the strength of the applied field. At low fields, ≤2 V cm−1 , no SHS reaction takes place. On the other hand, when the field is about 25 V cm−1 or higher, no ignition source is required; in other words, the reaction is initiated by the Joule heating of the field itself. The phenomenon observed in Fig. 3 is referred to as simultaneous or volume combustion with the implication
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Fig. 3. The effect of cobalt amounts in the reactants on the threshold value and simultaneous combustion of electric field.
that the reaction is taking place over the entire sample with no wave propagation. The influence of the applied electric field on the propagation wave velocity is shown in Fig. 4 for the synthesis of WC-Co composites from the elements. The velocity of the combustion front increases with the applied field in the interval between 1 and 25 V cm−1 . The effect of the field on the maximum combustion temperature is depicted in Fig. 5. As for WC, the temperature increases from 1660 ◦ C at the lowest applied field of 5 V cm−1 to 1900 ◦ C at the field of 10 V cm−1 . Other reactant systems with different compositions have the same trend. The combustion temperature increased with the
Fig. 4. The dependence of wave propagation velocity on field strength in the synthesis of WC-Co composites (relative density 54.2%).
Fig. 5. The dependence of combustion maximum temperature on field strength in the synthesis of WC-Co composites (relative density 54.2%).
Fig. 6. The effect of the relative density on the measured wave velocity for FACS WC-Co composites at different field strength.
field in the range. During the FACS process, the externally applied electric field provides a Joule heat contribution which together with the heat released by chemical reaction is able to facilitate the initiation and propagation of self-sustained combustion waves. This work also included investigations on the influence of the reactants density on the synthesis process. The dependence of the propagation velocity on sample density is shown in Fig. 6. At the same applied voltage, the velocities begin at low values and then increase with the relative density until maximum value, followed by decreasing with the relative density. The dependence of wave velocity in FACS process on the relative density of reactant compact is consistent with some experiment results of other research systems reported in several investigations [14]. The tentative qualitative interpretation of the observations has focused on the role of thermal conductivity in the FACS process and its relationship with the relative density. Reactant compacts with low relative densities have correspondingly low thermal conductivities leading to a limited heat transfer from the combustion wave to the adjacent layer ahead. Thus, heat is not conducted at a sufficiently high rate to raise the temperature of the layer ahead of the wave to the ignition temperature. On the other hand, if relative density is too high, then heat is conducted far ahead of the wave and dissipated in the entire unreacted sample with the result that the ignition temperature is not reached and thus leads to lower propagation velocity. Moreover, comparing the propagation velocity at the field of 10 V cm−1 with that for 15 V cm−1 , the maximum value moves towards lower relative densities end. For W + C + 10 wt.% Co, the velocity attains the maximum value, 2.31 mm s−1 , at the field of 15 V cm−1 for the sample with the relative density of 47.5%, while at the field of 10 V cm−1 , 1.81 mm s−1 , with the relative density of 54.2%. With the increase of electric field, the function of electric field on the heat transfer becomes evident; thus the maximum propagation velocity will situate at low relative density. Diffraction peaks were obtained for the combustion products under electric fields ranging from 10 to 20 V cm−1 . X-ray diffraction analyses of the synthesized products are shown in Fig. 7. The peak for the unreacted W is not evident in
G. J. Jiang et al. / Journal of Alloys and Compounds 387 (2005) 90–96
Fig. 7. X-ray diffraction patterns of WC-10 wt.% Co composites synthesized at various voltages: (a) 10 V cm−1 , (b) 15 V cm−1 , (c) 20 V cm−1 .
all combustion products. When the synthesis experiment is preformed under progressively higher fields, the combustion reaction becomes violent and the composition of the product changes. With the increase of electric field, the relative strength of W2 C in XRD diagram decreased that is to say more carbon diffuses into W and the carbonization degree increases. Even though the addition metallic Co melts, which is helpful to reactants diffusion and the formation of WC, however, trace W2 C phase can still be detected in the combustion products. As cited in the references [15], the equation of C/W ratio resulted in an incomplete reaction between W and carbon. More carbon was needed to obtain WC single phase. XRD result of the present work from these systems showed the presence of W2 C along with WC, which confirms the above opinion. Moreover, Co3 W3 C phase exists in the combustion product, which is from the reaction among W2 C, metallic W and Co to form mixed compounds of type Wx Cy Coz [16,17] (Co3 W3 C in this system). To understand the mechanism of conversion from the elemental reactants (W + C + 10 wt.% Co) to the final material, experiments were carried out in which the applied voltage (corresponding to E = 10 V cm−1 ) was interrupted after the wave had halfway advanced through the sample. This caused the wave to stop and the combustion process to extinguish immediately. Fig. 8 shows a SEM micrograph of the
Fig. 8. SEM micrograph of the quenched front for the FACS of WC-10 wt.% Co composites.
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quenched front. Three distinct regions (Zones 2–4) can be clearly identified from this figure. Because the solid-phase reaction between W and carbon is relatively slow, two opposite ends of quenched sample were chosen as Zone 1 (reactant zone) and Zone 5 (product zone). A series of SEM micrographs of these regions across the quenched front for the quenched sample and corresponding XRD analysis results of these zones are shown in Figs. 9 and 10, respectively. Zone 1 (Fig. 10a) only contains the reactants. Relatively small W particles are around large carbon particles. No indication of any interaction between the two elements can be observed. In Zone 2 (Fig. 10b), W, carbon along with minor amounts of the combustion product, W2 C and WC, are present. From XRD results, two kinds of carbides are the major phases in Zone 3 (Fig. 10c). The outline of some carbon covered with melting Co is observed. Moreover, Co grains are still clearly discernible, indicating that massive melting of Co has not taken place. In Zone 4 (Fig. 10d), W can not be detected in the combustion product from XRD result. The outline of carbon covered by melting Co was not observed and massive melting of Co is discerned from the micrograph. Finally, only W2 C, WC and Co3 W3 C are observed in Zone 5 (Fig. 10e). Some diffraction peaks of W2 C phase have disappeared as compared with the phase composition of Zone 4, which confirms that with the propagation of combustion wave, the carbonization degree of combustion wave passed zone increased. However, trace W2 C is also detected in the combustion products. As we know, with the decrease of electric field, the combustion temperature and the propagation velocity of combustion wave decrease. In order to confirm whether there exists an intermediate phase or not, the applied voltage was decreased as lower as possible. The quenching experiment at the applied electric field of 2 V cm−1 has been conducted. XRD pattern of the quenched region is shown in Fig. 11. The diffraction peaks of WC were not observed in this diagram, which demonstrates that W2 C is intermediate phase between WC and reactants (W and C). Owing to the low reaction rate between W and C, it is difficult to synthesize WC even with dispersion of Co, as stated above. Thus, long-time exposure of the sample to electric fields is needed, but such treatment caused the sample to change shape. Simultaneous or volume combustion during FACS of carbides (Fig. 3) gives us some information for solving the sample-distortion phenomenon. Therefore, the field-activated and pressure-assisted combustion synthesis (FAPACS) process [18], which is based on the volume combustion synthesis and materials densification, was chosen to synthesize single-phase carbide because it can ensure that no sample-distortion phenomenon takes place and the combustion reaction has enough time to go to completion. The variations of shrinkage displacements and temperatures of the die surface with heating time during FAPACS of W + C + 10 wt.% Co is shown in Fig. 12. From the XRD diagram (Fig. 13), only WC and Co were detected in the combustion product. The detailed work will be discussed in other place further.
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Fig. 9. SEM micrograph of the quenched sample for the FACS of WC-10 wt.% Co composites: (a) Zone 1, (b) Zone 2, (c) Zone 3, (d) Zone 4 and (e) Zone 5.
On the basis of above FACS and FAPACS experimental results, it is possible to postulate the sequence of transformation during FACS WC-Co composites, as summarized in Table 1. Since both adiabatic temperatures of W2 C and WC are less than their melting points (2327–2577 ◦ C for WC and 2785 ◦ C for W2 C, respectively), as well as the melting point of W (3407 ◦ C) and C (3527 ◦ C), a solid–solid mechanism is the only likely process during the combustion synthesis of the carbide phase. Even though W and C have low vapor pressures (according to the equation of vapor pressure and temperature, the vapor pressure of tungsten is about 0.09 atm at about
2000 ◦ C under the electric field of 12.5 V cm−1 ), tungsten carbides can still be obtained through FACS method, which implies they can not be synthesized through gas-solid reaction because the influence of electric field on the gas phase is limited. It is the electric field through current-conducting material (W and C) during synthesis reactions that supplied Joule heating, enhanced mass transport of solid and liquid by electromigration, and markedly influenced on the dynamics of the reaction and the nature of the product. The experimental results suggest that the initial stage consists of the reaction between W and C to form W2 C. This step is followed by the transformation of W2 C to WC, which occurs as
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Fig. 12. The variations of shrinkage displacement and temperature of the die surface with heating time during the processing of W + C + 10 wt.% Co reactants.
Fig. 10. XRD patterns of the corresponding zone of the quenched sample for the FACS WC-10 wt.% Co composites.
Fig. 13. XRD patterns of the sample of the FAPACS WC-10 wt.% Co composites.
Fig. 11. X-ray diffraction patterns of W-C products synthesized at the electric field of 2 V cm−1 .
a consequence of the diffusion of C into the W2 C lattice, and second, by the melting of Co. Because the relative content of Co in the reactants is very small as compared with W and C, thus the reaction mechanism is similar to that for FACS pro-
cess of WC as shown. The main difference is that Co melts and reacts with W and carbon to form Wx Cy Coz , which finally decomposes to Co and WC with the diffusion of carbon. The function of molten Co (the melting point is 1492 ◦ C) increases the interconnection between particles, enhances reactant diffusion and the transformation from the W2 C to WC phase. Cobalt spreads around WC grains to form WC-Co composites.
4. Summary and conclusions Experimental studies were carried out to investigate the effect of electric fields on self-sustaining combustion synthe-
Table 1 Transformation zones and their characteristics during FACS WC-Co composites Zones
Chemical species
Nature of structure transformation
Result of the chemical interaction
Reactants
W, C, Co
No transformation
No interaction
Combustion (1)
W, C, W2 C Co3 W3 C
Diffusion of carbon into W lattice Decomposition of WC to W2 C or W Reaction between W, W2 C and Co
Formation of W2 C Formation of W2 C or W Formation of Co3 W3 C
Combustion (2)
W, C, W2 C, WC Co3 W3 C
Diffusion of carbon into W2 C Transformation of W2 C into WC Reaction between Co3 W3 C and carbon
Formation of WC Formation of WC and Co
WC, Co
Diffusion of carbon into WC
No chemical interaction
Final product
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sis reactions of W, carbon and Co. It was shown that the field plays a major role in these reactions. Self-sustaining combustion waves could be generated in W-C-Co systems only when a threshold field value was added to establish such waves. Higher electric field leads to high combustion temperature. The velocity increase was roughly linear with the applied voltage. Similar studies were also made to investigate the role of the relative density of the reactants. The maximum propagation velocity and combustion temperature occur at a given normalized electric field and relative density. With the decrease in the electric field, the maximum value occurs at lower relative density. In all cases where the wave propagated, the combustion products contained WC and W2 C. However, the relative levels of these combustion reactions, which were reflected in XRD diagram, are different. The relative strength of W2 C diffraction peaks decreased when moving from the reactants region to the final product or with increasing field strength. X-ray analysis on an ‘electrically quenched’ sample revealed the sequence of the field activated reaction between W and C. The process begins with the reaction between W and C to form W2 C. This is followed by the formation of WC.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Acknowledgment
[16]
This work was supported by the National Nature Science Foundation of China (No. 50232020).
[17] [18]
Z.A. Munir, Am. Ceram. Soc. Bull. 67 (1988) 342–349. Z.A. Munir, Mater. Sci. Eng. A 287 (2) (2000) 125–137. I.J. Shon, Z.A. Munir, Mater. Sci. Eng. A 202 (1–2) (1995) 256–261. I.J. Shon, D.H. Rho, H.C. Kim, Z.A. Munir, J. Alloy Compd. 327 (1–2) (2001) 66–72. A. Feng, O.A. Graeve, Z.A. Munir, Comp. Mater. Sci. 12 (2) (1998) 137–155. R. Orr`u, A. Cincotti, G. Cao, Z.A. Munir, Chem. Eng. Sci. 56 (2) (2001) 683–692. F. Maglia, U. Anselmi-Tamburini, C. Milanese, N. Bertolino, Z.A. Munir, J. Alloy. Compd. 319 (1–2) (2001) 108–118. K.S. Yun, J.H. Lee, C.W. Won, Mater. Res. Bull. 35 (10) (2000) 1709–1716. R. Orru, G. Cao, Z.A. Munir, Chem. Eng. Sci. 54 (15–16) (1999) 3349–3355. R. Orr´u, G. Cao, Z.A. Munir, Metall. Mater. Trans. A 30A (4) (1999) 1101–1108. S. Gedevanishvili, Z.A. Munir, Mater. Sci. Eng. A 242 (1–2) (1998) 1–6. H. Xue, Z.A. Munir, J. Eur. Ceram. Soc. 17 (15–16) (1997) 1787–1792. S. Gedevanishvili, Z.A. Munir, Mater. Sci. Eng. A 211 (1–2) (1996) 1–9. E.M. Carrillo-Heian, O.A. Graeve, A. Feng, J.A. Faghih, Z.A. Munir, J. Mater. Res. 14 (5) (1999) 1949–1958. A.P. Hagen, Formation of Bonds to C, Si, Ge, Sn Pb (Part 4), VCH Publishers, New York, 1990. M.E. Vinayo, F. Kassabji, J. Guyonnet, P. Fauchais, J. Vac. Sci. Technol. A3 (6) (1985) 2483–2489. V. Ramnath, N. Jarayaman, Mater. Sci. Technol. 5 (1989) 382–388. Z.A. Munir, Mater. Sci. Eng. A 287 (2) (2000) 125–137.
Synthesis of tungsten carbide–cobalt composites by the field-activated combustion synthesis method Guojian Jiang∗ , Hanrui Zhuang, Wenlan Li Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 12 April 2004; received in revised form 17 June 2004; accepted 18 June 2004
Abstract The activation of self-propagating combustion reactions in the tungsten carbide–cobalt (WC-Co) system was achieved by using an electric field. Self-sustaining combustion in elemental reactants with composition corresponding to WC-x wt.% Co (0 ≤ x ≤ 10) composites can be activated only when the imposed field is above a threshold value. The nature of combustion products depended on the magnitude of the field. The relative strength of W2 C diffraction peaks in XRD diagram decreased when moving from the reactants region to the final products one or increasing electric field on the reactants. The effect of the relative density of the reactant compacts on the synthesis of WC-Co composites was also investigated. Finally, the reaction mechanism of WC-Co composites was proposed. X-ray and microscopic analysis of the quenched combustion front suggests that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. Molten Co redounds the transformation from W2 C to WC phase. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Chemical synthesis; SEM; X-ray diffraction
1. Introduction Tungsten carbide–cobalt (WC-Co) composites combine the high hardness of WC with the ductility of Co to produce a material of superior wear resistance, yet with sufficient strength for use in practical applications. Examples of such applications are inserts in rock drilling tools, wood cutting teeth and bearings or seals exposed to polluted water. Hard cemented carbides are commonly produced following a powder metallurgy route, which involves product powder synthesis, mixing with metal, granulation, pressing and sintering. Some stages require high temperature and long-time processing, leading to undesirable grain growth (Ostwald ripening) in the material components and the associated benefits of enhanced mechanical properties. A promising and energy-efficient technique known under the acronym of combustion synthesis (CS) or self∗ Corresponding author. Tel.: +86-21-5241-4216; fax: +86-21-5241-3903. E-mail address: [email protected] (G. J. Jiang).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.057
propagating high-temperature synthesis (SHS) for the preparation of advanced materials has been developed. In a typical SHS process, compacts of powder mixtures are ignited at one end to initiate the self-sustaining reaction. However, for the case of some systems, for example, W-C, ignition is impossible without addition activation. Primarily this is due to the thermodynamic limitation, i.e. low reaction enthalpies or the relatively low adiabatic temperatures (Tad ) of these carbides. For example, the adiabatic temperatures of WC and W2 C are 1127 and 673 ◦ C, respectively, which are considerably lower than the empirically established minimum of 1527 ◦ C for SHS reactions. With the addition of Co, the adiabatic temperature will change lower. The effect of the addition of Co on the adiabatic temperature of this system is shown in Fig. 1. Details of calculations of adiabatic temperature have been discussed in some references [1]. For systems that are relatively less exothermic, the common approach has been the preheating of the reactants, which accomplishes the goal of raising Tad . However, this treatment can suffer from the formation of extraneous phases, such as pre-combustion phases produced through diffusion processes.
G. J. Jiang et al. / Journal of Alloys and Compounds 387 (2005) 90–96
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Fig. 2. Schematic representation of field-activated combustion synthesis (FACS). Fig. 1. Variation of the adiabatic combustion temperature with the amount (wt.%) of Co for the formation of composites of: WC-x wt.% Co.
Recently, a new method, referred to in the literature as field-activated combustion synthesis (FACS) [2], based on the use of an electric field to activate self-propagating reactions in their relatively low heat of formation or less-exothermic systems, has been developed. Experimental results and modeling studies have led to the conclusion that the effect of the field is to provide Joule heating at a rate of σE2 , with σ being the conductivity and E the field [3–5]. Depending on the electric conductivity of reactants and products, ignition results in the initiation and propagation of a combustion wave in reactant systems, which heretofore could not be synthesized by SHS (without preheating), including SiC, SiC-AlN, B4 C-TiB2 , MoSi2 -SiC, Ti3 Al and others [6–13]. In this paper, we report on the synthesis of WC-Co composites by the FACS method, present the results of the effects of various processing parameters on FACS of WC-Co composites and determine the fast formation mechanism from W, C and Co under electric field. No previous results on FACS of WC-Co composites have been reported. The aim of this study was to provide a better understanding of the role of electric field involved in FACS of WC-Co composites.
2. Experimental procedures Equiatomic mixtures of W and activated carbon with addition of Co powders were used to assess the effect of electric field on SHS reactions. W powder with a reported purity of 99.99% and an average particle size of 0.6 m (Korea Tungsten Co.), was dry-mixed in a mill with 99.9% pure activated carbon powder (an average particle size of 20 m, supplied by Kojondo Chemical Co.) and 99.8% pure Co powder (an average particle size of 30 m, supplied by Aldrich Chemical Co.). Tetragonally shaped pellets with dimensions of 10 mm × 10 mm × 15 mm were cold-pressed under various compaction pressures (100–500 MPa) in a two-plunger steel die. The pellets were then placed between two springloaded copper electrodes across which a voltage was applied. The spring was used to keep two electrodes in intimate contact with the flat top and bottom surface of the sample. Thus,
the distance between electrodes is the thickness of the sample, 10 mm. This experimental geometry provides an electric field perpendicular to the expected direction of wave propagation. A tungsten heating coil, placed near the edge of the sample, was used to initiate the combustion and was turned off immediately after the reaction was initiated. A schematic representation of this set-up is shown in Fig. 2. The activated combustion synthesis experiments were carried out inside a stainless steel pressure chamber in one atmosphere of argon gas. The combustion process evolution was recorded from video camera equipped with a time-code generator. The propagation velocity of combustion wave was determined by timing of the propagation from one end of the sample to the other. The surface temperature profiles of the sample during combustion were measured with a two-color optical pyrometer with a response time of 10 ms focused on the middle portion of the sample. Compositional and microstructural analyses of the products were made through X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.
3. Results and discussion In the absence of a field, the exposure of one end of the sample to an ignition source for several minutes does not result in the initiation of a combustion wave or evidence of any reaction beyond the heat-affected zone. Experiments on WCCo system showed that a self-sustaining combustion wave can not be established unless the field strength exceeds a minimum (threshold) value. The effect of Co amounts in the reactants on the threshold value is shown in Fig. 3. With an increase in the amount of Co, the threshold value increased higher. When the electric field strength is higher than the threshold value, the activation of the ignition source causes the propagation of combustion wave throughout the sample. The figure shows three regions, relative to the strength of the applied field. At low fields, ≤2 V cm−1 , no SHS reaction takes place. On the other hand, when the field is about 25 V cm−1 or higher, no ignition source is required; in other words, the reaction is initiated by the Joule heating of the field itself. The phenomenon observed in Fig. 3 is referred to as simultaneous or volume combustion with the implication
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Fig. 3. The effect of cobalt amounts in the reactants on the threshold value and simultaneous combustion of electric field.
that the reaction is taking place over the entire sample with no wave propagation. The influence of the applied electric field on the propagation wave velocity is shown in Fig. 4 for the synthesis of WC-Co composites from the elements. The velocity of the combustion front increases with the applied field in the interval between 1 and 25 V cm−1 . The effect of the field on the maximum combustion temperature is depicted in Fig. 5. As for WC, the temperature increases from 1660 ◦ C at the lowest applied field of 5 V cm−1 to 1900 ◦ C at the field of 10 V cm−1 . Other reactant systems with different compositions have the same trend. The combustion temperature increased with the
Fig. 4. The dependence of wave propagation velocity on field strength in the synthesis of WC-Co composites (relative density 54.2%).
Fig. 5. The dependence of combustion maximum temperature on field strength in the synthesis of WC-Co composites (relative density 54.2%).
Fig. 6. The effect of the relative density on the measured wave velocity for FACS WC-Co composites at different field strength.
field in the range. During the FACS process, the externally applied electric field provides a Joule heat contribution which together with the heat released by chemical reaction is able to facilitate the initiation and propagation of self-sustained combustion waves. This work also included investigations on the influence of the reactants density on the synthesis process. The dependence of the propagation velocity on sample density is shown in Fig. 6. At the same applied voltage, the velocities begin at low values and then increase with the relative density until maximum value, followed by decreasing with the relative density. The dependence of wave velocity in FACS process on the relative density of reactant compact is consistent with some experiment results of other research systems reported in several investigations [14]. The tentative qualitative interpretation of the observations has focused on the role of thermal conductivity in the FACS process and its relationship with the relative density. Reactant compacts with low relative densities have correspondingly low thermal conductivities leading to a limited heat transfer from the combustion wave to the adjacent layer ahead. Thus, heat is not conducted at a sufficiently high rate to raise the temperature of the layer ahead of the wave to the ignition temperature. On the other hand, if relative density is too high, then heat is conducted far ahead of the wave and dissipated in the entire unreacted sample with the result that the ignition temperature is not reached and thus leads to lower propagation velocity. Moreover, comparing the propagation velocity at the field of 10 V cm−1 with that for 15 V cm−1 , the maximum value moves towards lower relative densities end. For W + C + 10 wt.% Co, the velocity attains the maximum value, 2.31 mm s−1 , at the field of 15 V cm−1 for the sample with the relative density of 47.5%, while at the field of 10 V cm−1 , 1.81 mm s−1 , with the relative density of 54.2%. With the increase of electric field, the function of electric field on the heat transfer becomes evident; thus the maximum propagation velocity will situate at low relative density. Diffraction peaks were obtained for the combustion products under electric fields ranging from 10 to 20 V cm−1 . X-ray diffraction analyses of the synthesized products are shown in Fig. 7. The peak for the unreacted W is not evident in
G. J. Jiang et al. / Journal of Alloys and Compounds 387 (2005) 90–96
Fig. 7. X-ray diffraction patterns of WC-10 wt.% Co composites synthesized at various voltages: (a) 10 V cm−1 , (b) 15 V cm−1 , (c) 20 V cm−1 .
all combustion products. When the synthesis experiment is preformed under progressively higher fields, the combustion reaction becomes violent and the composition of the product changes. With the increase of electric field, the relative strength of W2 C in XRD diagram decreased that is to say more carbon diffuses into W and the carbonization degree increases. Even though the addition metallic Co melts, which is helpful to reactants diffusion and the formation of WC, however, trace W2 C phase can still be detected in the combustion products. As cited in the references [15], the equation of C/W ratio resulted in an incomplete reaction between W and carbon. More carbon was needed to obtain WC single phase. XRD result of the present work from these systems showed the presence of W2 C along with WC, which confirms the above opinion. Moreover, Co3 W3 C phase exists in the combustion product, which is from the reaction among W2 C, metallic W and Co to form mixed compounds of type Wx Cy Coz [16,17] (Co3 W3 C in this system). To understand the mechanism of conversion from the elemental reactants (W + C + 10 wt.% Co) to the final material, experiments were carried out in which the applied voltage (corresponding to E = 10 V cm−1 ) was interrupted after the wave had halfway advanced through the sample. This caused the wave to stop and the combustion process to extinguish immediately. Fig. 8 shows a SEM micrograph of the
Fig. 8. SEM micrograph of the quenched front for the FACS of WC-10 wt.% Co composites.
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quenched front. Three distinct regions (Zones 2–4) can be clearly identified from this figure. Because the solid-phase reaction between W and carbon is relatively slow, two opposite ends of quenched sample were chosen as Zone 1 (reactant zone) and Zone 5 (product zone). A series of SEM micrographs of these regions across the quenched front for the quenched sample and corresponding XRD analysis results of these zones are shown in Figs. 9 and 10, respectively. Zone 1 (Fig. 10a) only contains the reactants. Relatively small W particles are around large carbon particles. No indication of any interaction between the two elements can be observed. In Zone 2 (Fig. 10b), W, carbon along with minor amounts of the combustion product, W2 C and WC, are present. From XRD results, two kinds of carbides are the major phases in Zone 3 (Fig. 10c). The outline of some carbon covered with melting Co is observed. Moreover, Co grains are still clearly discernible, indicating that massive melting of Co has not taken place. In Zone 4 (Fig. 10d), W can not be detected in the combustion product from XRD result. The outline of carbon covered by melting Co was not observed and massive melting of Co is discerned from the micrograph. Finally, only W2 C, WC and Co3 W3 C are observed in Zone 5 (Fig. 10e). Some diffraction peaks of W2 C phase have disappeared as compared with the phase composition of Zone 4, which confirms that with the propagation of combustion wave, the carbonization degree of combustion wave passed zone increased. However, trace W2 C is also detected in the combustion products. As we know, with the decrease of electric field, the combustion temperature and the propagation velocity of combustion wave decrease. In order to confirm whether there exists an intermediate phase or not, the applied voltage was decreased as lower as possible. The quenching experiment at the applied electric field of 2 V cm−1 has been conducted. XRD pattern of the quenched region is shown in Fig. 11. The diffraction peaks of WC were not observed in this diagram, which demonstrates that W2 C is intermediate phase between WC and reactants (W and C). Owing to the low reaction rate between W and C, it is difficult to synthesize WC even with dispersion of Co, as stated above. Thus, long-time exposure of the sample to electric fields is needed, but such treatment caused the sample to change shape. Simultaneous or volume combustion during FACS of carbides (Fig. 3) gives us some information for solving the sample-distortion phenomenon. Therefore, the field-activated and pressure-assisted combustion synthesis (FAPACS) process [18], which is based on the volume combustion synthesis and materials densification, was chosen to synthesize single-phase carbide because it can ensure that no sample-distortion phenomenon takes place and the combustion reaction has enough time to go to completion. The variations of shrinkage displacements and temperatures of the die surface with heating time during FAPACS of W + C + 10 wt.% Co is shown in Fig. 12. From the XRD diagram (Fig. 13), only WC and Co were detected in the combustion product. The detailed work will be discussed in other place further.
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Fig. 9. SEM micrograph of the quenched sample for the FACS of WC-10 wt.% Co composites: (a) Zone 1, (b) Zone 2, (c) Zone 3, (d) Zone 4 and (e) Zone 5.
On the basis of above FACS and FAPACS experimental results, it is possible to postulate the sequence of transformation during FACS WC-Co composites, as summarized in Table 1. Since both adiabatic temperatures of W2 C and WC are less than their melting points (2327–2577 ◦ C for WC and 2785 ◦ C for W2 C, respectively), as well as the melting point of W (3407 ◦ C) and C (3527 ◦ C), a solid–solid mechanism is the only likely process during the combustion synthesis of the carbide phase. Even though W and C have low vapor pressures (according to the equation of vapor pressure and temperature, the vapor pressure of tungsten is about 0.09 atm at about
2000 ◦ C under the electric field of 12.5 V cm−1 ), tungsten carbides can still be obtained through FACS method, which implies they can not be synthesized through gas-solid reaction because the influence of electric field on the gas phase is limited. It is the electric field through current-conducting material (W and C) during synthesis reactions that supplied Joule heating, enhanced mass transport of solid and liquid by electromigration, and markedly influenced on the dynamics of the reaction and the nature of the product. The experimental results suggest that the initial stage consists of the reaction between W and C to form W2 C. This step is followed by the transformation of W2 C to WC, which occurs as
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Fig. 12. The variations of shrinkage displacement and temperature of the die surface with heating time during the processing of W + C + 10 wt.% Co reactants.
Fig. 10. XRD patterns of the corresponding zone of the quenched sample for the FACS WC-10 wt.% Co composites.
Fig. 13. XRD patterns of the sample of the FAPACS WC-10 wt.% Co composites.
Fig. 11. X-ray diffraction patterns of W-C products synthesized at the electric field of 2 V cm−1 .
a consequence of the diffusion of C into the W2 C lattice, and second, by the melting of Co. Because the relative content of Co in the reactants is very small as compared with W and C, thus the reaction mechanism is similar to that for FACS pro-
cess of WC as shown. The main difference is that Co melts and reacts with W and carbon to form Wx Cy Coz , which finally decomposes to Co and WC with the diffusion of carbon. The function of molten Co (the melting point is 1492 ◦ C) increases the interconnection between particles, enhances reactant diffusion and the transformation from the W2 C to WC phase. Cobalt spreads around WC grains to form WC-Co composites.
4. Summary and conclusions Experimental studies were carried out to investigate the effect of electric fields on self-sustaining combustion synthe-
Table 1 Transformation zones and their characteristics during FACS WC-Co composites Zones
Chemical species
Nature of structure transformation
Result of the chemical interaction
Reactants
W, C, Co
No transformation
No interaction
Combustion (1)
W, C, W2 C Co3 W3 C
Diffusion of carbon into W lattice Decomposition of WC to W2 C or W Reaction between W, W2 C and Co
Formation of W2 C Formation of W2 C or W Formation of Co3 W3 C
Combustion (2)
W, C, W2 C, WC Co3 W3 C
Diffusion of carbon into W2 C Transformation of W2 C into WC Reaction between Co3 W3 C and carbon
Formation of WC Formation of WC and Co
WC, Co
Diffusion of carbon into WC
No chemical interaction
Final product
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sis reactions of W, carbon and Co. It was shown that the field plays a major role in these reactions. Self-sustaining combustion waves could be generated in W-C-Co systems only when a threshold field value was added to establish such waves. Higher electric field leads to high combustion temperature. The velocity increase was roughly linear with the applied voltage. Similar studies were also made to investigate the role of the relative density of the reactants. The maximum propagation velocity and combustion temperature occur at a given normalized electric field and relative density. With the decrease in the electric field, the maximum value occurs at lower relative density. In all cases where the wave propagated, the combustion products contained WC and W2 C. However, the relative levels of these combustion reactions, which were reflected in XRD diagram, are different. The relative strength of W2 C diffraction peaks decreased when moving from the reactants region to the final product or with increasing field strength. X-ray analysis on an ‘electrically quenched’ sample revealed the sequence of the field activated reaction between W and C. The process begins with the reaction between W and C to form W2 C. This is followed by the formation of WC.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Acknowledgment
[16]
This work was supported by the National Nature Science Foundation of China (No. 50232020).
[17] [18]
Z.A. Munir, Am. Ceram. Soc. Bull. 67 (1988) 342–349. Z.A. Munir, Mater. Sci. Eng. A 287 (2) (2000) 125–137. I.J. Shon, Z.A. Munir, Mater. Sci. Eng. A 202 (1–2) (1995) 256–261. I.J. Shon, D.H. Rho, H.C. Kim, Z.A. Munir, J. Alloy Compd. 327 (1–2) (2001) 66–72. A. Feng, O.A. Graeve, Z.A. Munir, Comp. Mater. Sci. 12 (2) (1998) 137–155. R. Orr`u, A. Cincotti, G. Cao, Z.A. Munir, Chem. Eng. Sci. 56 (2) (2001) 683–692. F. Maglia, U. Anselmi-Tamburini, C. Milanese, N. Bertolino, Z.A. Munir, J. Alloy. Compd. 319 (1–2) (2001) 108–118. K.S. Yun, J.H. Lee, C.W. Won, Mater. Res. Bull. 35 (10) (2000) 1709–1716. R. Orru, G. Cao, Z.A. Munir, Chem. Eng. Sci. 54 (15–16) (1999) 3349–3355. R. Orr´u, G. Cao, Z.A. Munir, Metall. Mater. Trans. A 30A (4) (1999) 1101–1108. S. Gedevanishvili, Z.A. Munir, Mater. Sci. Eng. A 242 (1–2) (1998) 1–6. H. Xue, Z.A. Munir, J. Eur. Ceram. Soc. 17 (15–16) (1997) 1787–1792. S. Gedevanishvili, Z.A. Munir, Mater. Sci. Eng. A 211 (1–2) (1996) 1–9. E.M. Carrillo-Heian, O.A. Graeve, A. Feng, J.A. Faghih, Z.A. Munir, J. Mater. Res. 14 (5) (1999) 1949–1958. A.P. Hagen, Formation of Bonds to C, Si, Ge, Sn Pb (Part 4), VCH Publishers, New York, 1990. M.E. Vinayo, F. Kassabji, J. Guyonnet, P. Fauchais, J. Vac. Sci. Technol. A3 (6) (1985) 2483–2489. V. Ramnath, N. Jarayaman, Mater. Sci. Technol. 5 (1989) 382–388. Z.A. Munir, Mater. Sci. Eng. A 287 (2) (2000) 125–137.