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Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures
RMHM-04373; No of Pages 6 Int. Journal of Refractory Metals and Hard Materials xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures Sung-Soo Ryu a,⁎, Hae-Ryong Park a,b,1, Young Do Kim b, Hyun Seon Hong c a b c
Korea Institute of Ceramic Engineering and Technology, Icheon 17303, Republic of Korea Dept. of Materials Science & Engineering, Hanyang University, Seoul 04763, Republic of Korea Dept. of Interdisplinary ECO Science, Sungshin University, Seoul 01133, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 July 2016 Received in revised form 28 October 2016 Accepted 27 November 2016 Available online xxxx Keywords: Mechano-chemical process W-Cu Raw materials Solid phase sintering Liquid phase sintering
a b s t r a c t Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO3-CuO powder mixtures was investigated. The mixture of WO3 and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H2 atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W– Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently, W–Cu alloys have attracted a growing interest owing to their superior thermal management properties and high microwave absorption capacities [1–3]. For most of the applications, high-density W– Cu composites exhibiting a homogeneous microstructure are needed to achieve a high performance. Since the W–Cu system exhibits mutual insolubility or negligible solubility [4], W–Cu powder compacts exhibit poor sinterability, even when liquid phase sintering (LPS) is performed above the melting point of the Cu phase [5]. Fine size and well mixing of the components are known to improve the sinterability of the powder, especially in a LPS system, such as the W–Cu system, in which particle rearrangement is known to be the dominant sintering mechanism. The sinterability of W–Cu powder can be increased by employing an activated sintering process [7] with the addition of a small amount of metals such as Co, Ni, or Fe. However, since such activators can have a
⁎ Corresponding author at: Engineering Ceramic Center, Korea Institute of Ceramic Engineering and Technology, 30 Gyeongchung Rd., Sindun, Icheon, Gyeonggi 17303, Republic of Korea. E-mail address: [email protected] (S.-S. Ryu). 1 Present address: Pyeongtaek 451–831, Chang Sung Corporation.
negative influence on the thermal management properties of the W– Cu alloy [8], enhancing the sinterability by activated sintering seems to be one of the least acceptable options. Recently, many efforts towards the fabrication and sintering of W–Cu powders with nano-sized grains have been carried out to achieve excellent densities without adding activators [9,10]. Among various methods used for producing highly dispersed W–Cu nanocomposite powders [11–20], the mechano-chemical process, which involves ball-milling of metal oxide powders and subsequent hydrogen reduction, is a promising approach as W–Cu nanocomposite powders exhibiting a high sinterability can be produced with this method [9,12]. Understanding the microstructure of W–Cu composite powder is essential for elucidating its sintering mechanism [6]. W–Cu nanocomposite powder obtained by performing hydrogen reduction of milled oxide mixtures exhibited similar characteristics as those of micro-homogeneously mixed W–Cu aggregates [21]. A new W–Cu nanocomposite structure, consisting of W nanoparticles coated with a Cu layer, was synthesized previously [22]. The product formation seemed to depend on various factors, including the reduction and milling conditions of the oxide mixtures. In this study, the effect of ball-milling time of WO3CuO powder mixture on the microstructural characteristics and densification behavior of reduced W-Cu composite powder is investigated.
http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: S.-S. Ryu, et al., Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-Cu..., Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012
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S.-S. Ryu et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2016) xxx–xxx
(b)
(a)
Fig. 1. FE-SEM images of the starting materials, (a) WO3 and (b) CuO.
Fig. 2. FE-SEM images of the WO3-CuO powder mixture ball-milled for (a) 1 h and (b) 10 h.
2. Experimental procedure To produce W–20 wt.% Cu (W–20Cu) nanocomposite powder, WO3 powder (Taegu Tec, Korea), with a mean particle size of 3 μm and a purity of 99.9%, and CuO powder (Kojundo, Japan), with a mean particle size of 12 μm and a purity of 99.9%, were used. Ball milling process was performed using a horizontal bead mill (UBM-5, Nanointek, Korea). ZrO2 milling balls and jars were used to avoid contaminants, such as Fe, Cr, and Ni elements, which could act as sintering activators
for W–Cu. More details on the milling experiment are described elsewhere [22]. The WO3–CuO powder mixtures milled for 1 h and 10 h were used as starting materials in this study. The milled WO3-CuO powder mixtures were dried at 80 °C and reduced in a 5 mm powder bed at 200 °C for 1 h. The temperature was then increased to 800 °C for 1 h in dry H2 atmosphere (dew point of −76 °C) with a heating rate of 2 °C/min in order to form the W-20Cu composite powder. Microstructural analysis of the reduced W–Cu powder was conducted using field emission-scanning electron microscopy (FE-SEM, JSM-9701, JEOL, JAPAN) and field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F30 S-Twin, FEI, Netherlands). The reduced W–Cu composite powder was pressed at 100 MPa using the die compaction method, followed by performing cold isostatic pressing at 500 MPa in order to minimize the shrinkage anisotropy. The green density of the powder compact was found to be 45 ± 1% of the theoretically obtained value. Sintering was also conducted in the temperature range of 1050–1250 °C for 1 h, with a heating rate of 5 °C/min in H2 atmosphere. Density of the sintered sample was measured using the Archimedes' displacement method, and the sintered microstructure was examined using FE-SEM (JSM-9701, JEOL, JAPAN). 3. Results and discussion
Fig. 3. XRD patterns of W-Cu powder reduced from WO3-CuO powder mixture ball-milled for (a) 1 h and (b) 10 h in the bead mill.
Fig. 1 shows the FE-SEM images indicating the morphologies of the WO3 and CuO powders, which were used as starting materials in this study. As shown in the figure, both the powders consist of micronsized particles. WO3 consists of agglomerates of needle shaped primary particle with ultra-fine size. The CuO powder consists of uniform micron-sized particles. Fig. 2 shows the FE-SEM images of the WO3-CuO powder mixture ball-milled for 1 h and 10 h in the bead mill. As shown in Fig. 2(a), the
Please cite this article as: S.-S. Ryu, et al., Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-Cu..., Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012
S.-S. Ryu et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2016) xxx–xxx
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Fig. 4. FE-SEM images of the W-Cu composite powder produced from (a) 1 h-milled and (b) 10 h-milled WO3-CuO powder mixtures.
WO3 particle size is decreased to obtain nanoparticles when the milling was performed for 1 h, and the shape of the CuO particles changed from polyhedral shape to micron-sized plate-like structure. WO3 particles could be easily milled as compared with CuO particles. When the milling was performed for 10 h, nanoparticles of WO3 and CuO were obtained (Fig. 2(b)). Fig. 3 shows the XRD patterns of reduced W-Cu powder obtained after heat-treating the milled WO3-CuO powder mixtures in H2 atmosphere. After reducing the high-energy ball-milled WO3-CuO mixture at 800 °C, pure phases of W and Cu are observed in the XRD pattern of the products irrespective of the milling time, indicating that all the oxides are transformed to form the W–Cu composite powder. Fig. 4 shows the FE-SEM images of the W–Cu composite powder reduced at 800 °C. W and Cu particles can be easily distinguished from the FE-SEM images in the case of 1 h-milled sample. White regions in the image (Fig. 4(a)) correspond to nanoparticles of W and the grey regions correspond to Cu microparticles. Some of the W nanoparticles are attached on the surfaces of Cu microparticles. The characteristics of this sample could be related to those of a milled oxide mixture containing WO3 nanoparticles and CuO microparticles. The surface of Cu microparticles (previously H2-reduced) might act as a nucleation site for the W nanoparticles owing to the reduction including the chemical vapor transport process of W oxide in H2 atmosphere as suggested by Kim et al. [23]. As a result, the W-Cu powder consists of W nanoparticles attached on the surface of Cu microparticles. Lee et al. [24] reported that W-Cu composite powder consisting of W-coated Cu particles could be produced by mixing (ball milling) the powders containing WO3 nanoparticles and CuO microparticles.
The W-Cu powder obtained with 10 h milling consists of nanocomposite powder containing uniform shaped particles exhibiting a mean particle size of about 50 nm (Fig. 4(b)). Owing to the resolution limit in FE-SEM, it is difficult to investigate the degree of homogeneity of the constituents present in the reduced W–Cu nanocomposite powder. Hence, STEM-EDS mapping was employed (Fig. 5). Surface analysis of the reduced W–Cu nanocomposite powder was also carried out with a combined use of STEM and EDS analyses, and the results are described in detail elsewhere [22]. According to these analyses, the reduced W– Cu nanocomposite powder is found to consist of ~ 50 nm W particles surrounded by a Cu nanolayer. Schematic diagram illustrating the microstructure of W–Cu powders produced from the constituent oxide mixtures using different milling times (1 h and 10 h) is shown in Fig. 6. W nanoparticle-attached Cu microparticles and W nanoparticles with the Cu layer could be obtained with 1 h-milled and 10-h milled oxide mixtures, respectively. Fig. 7 gives the sintered densities of the W-20Cu samples as a function of the sintering temperature after performing isothermal sintering for 1 h in H2 atmosphere. A high densification with a relative density of 92.8% of theoretical density (TD) is obtained on performing solid phase sintering (SPS) at 1050 °C with the 10 h-milled specimen. In addition, the full densification of Cu-coated W nanoparticles is achieved on sintering at 1100 °C. This excellent sinterability could be attributed to the maximization of the rearrangement-controlled densification, which reduces the number of W-W contacts before the melting of Cu [22]. Cu phase spreading, i.e., spreading-assisted particle rearrangement process [22,24] plays an important role in the enhanced densification occurring in the SPS stage.
Fig. 5. (a) STEM image and EDS elemental mapping images corresponding to (b and c) Cu and (d and e) W [22].
Please cite this article as: S.-S. Ryu, et al., Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-Cu..., Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012
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Sintered density (%)
100 80 60 40 o
Sintering temp.: 1050 C 20 0
Fig. 6. Schematic diagram of W-Cu powders prepared from (a) 1 h-milled and (b) 10 hmilled WO3-CuO powder mixtures.
The 1 h-milled W-Cu specimen exhibited a poor sinterability in the SPS stage with 65% TD obtained at 1050 °C. However, its densification is significantly enhanced in the temperature range of LPS above the Cu melting temperature of 1083 °C. High densities of 97.0% TD and 98.9% TD could be obtained on sintering the sample at 1100 °C and 1200 °C, respectively. This significant increase in densification could be related to the presence of W nanoparticles in the sample. Johnson et al. [25] reported on the basis of dilatometry results for W-20Cu powder compacts that even on sintering a W-20Cu mixture containing W nanoparticles (0.11 μm) and Cu microparticles (5.71 μm), a significant increase in shrinkage occurs when Cu melts, thereby achieving a nearly full density at 1200 °C. The effect of sintering time on the densification behavior of two WCu powders occurring in the SPS stage is analyzed for the samples sintered at 1050 °C. Fig. 8 shows the sintered density of the W-20Cu specimens sintered at 1050 °C for 1 h and 5 h in H2 atmosphere. In the case of the 10 h-milled W-Cu specimen, a high densification occurs even in the SPS stage. The sintered density increases up to 96.0% TD after performing sintering for 5 h, whereas the 1 h-milled W-Cu powder containing W nanoparticles and Cu microparticles exhibits a relatively lower sintered density. After 5 h-sintering, the density is not greatly enhanced (increases to 75.9% TD from 65.0% TD). The densification occurring in the SPS stage seems to depend strongly on the characteristics of the Cu phase. In case of Cu-coated W powder, the Cu phase spreading, i.e., spreading-assisted particle rearrangement process [24], could take place even in the SPS stage, contributing remarkably to densification. However, in the case of 1 h-milled W-Cu powder, the Cu diffusion rate during SPS might be slow owing to the presence of a large number of Cu micro particles and their inhomogeneous
Sintered density (%)
100 90 80 70 10 h-milled 1 h-milled
60 50 1000
1100
1200
o
1300
Sintering temperature ( C) Fig. 7. The relative sintered density of W-Cu powder compacts sintered at different temperatures for 1 h in H2 atmosphere with a heating rate of 5 °C/min.
10 h-milled 1 h-milled 0
1
2
3
4
5
6
Sintering time (h) Fig. 8. The relative density of W-Cu powder compacts sintered for 1 h and 5 h at 1050 °C in H2 atmosphere with a heating rate of 5 °C/min.
distribution as compared with those of Cu-coated W nanopowder, which limits sintering by Cu phase spreading. Fig. 9 shows the FE-SEM micrographs of the W-20Cu specimens sintered at 1050 °C for 1 h and 5 h in H2 atmosphere. In the case of the 1 h-milled W-Cu powder shown in Fig. 9(a–b), the microstructure analysis indicates the presence of many pores in the samples sintered for 1 h and 5 h and that the sintering locally takes place by Cu spreading. In the case of 10 h-milled W-Cu powder shown in Fig. 9(c–d), a highly densified microstructure could be observed owing to fast Cu spreading. After performing sintering for 5 h, the sintered sample exhibits similar features as those of a typical fully densified W–Cu alloy formed via LPS. The FE-SEM images of W-20Cu specimens sintered isothermally in a LPS temperature range of 1100–1200 °C are shown in Fig. 10. All specimens exhibit similar sintered microstructures even though a large difference in the microstructural characteristics of the 1 h-milled W-Cu powder and the 10 h-milled W-Cu powder. On sintering, a typical fully densified W–Cu alloy is produced with the LPS stage, thereby achieving a well-dispersed W–Cu sintered specimen containing ultrafine particles. Each W grain is completely surrounded with liquid Cu, and homogeneously distributed W particles are obtained by the rearrangement of W grains in the Cu matrix. The W grain size is slightly increased with an increase in the temperature. 4. Conclusions The microstructure of the reduced W-Cu powder and its densification behavior significantly depend on the milling time of WO3 and CuO powder mixtures. Two types of W-Cu powders exhibiting different microstructural characteristics were produced from oxide mixtures by ball-milling the mixture for 1 h and 10 h. The W–Cu powder, which consisted of W nanoparticle-attached Cu microparticles, was obtained when the mixture was milled for 1 h, whereas Cu-coated W nanocomposite powder containing W nanoparticles coated by a Cu layer was produced with the 10 h milled mixture. The relationship between the microstructure of the reduced W-Cu powder and its densification behavior was examined. The microstructure of the reduced W-Cu powder influences its densification behavior. Sintering results indicate that the densification occurring in the SPS stage, which was mainly proceeded by Cu spreading, strongly depended on the microstructure of the reduced W-Cu, especially on the characteristics of the Cu phase. The densification occurring in the LPS stage is greatly influenced by the size of the W particles since most of the shrinkage occurring in the LPS stage could be attributed to the W particle rearrangement taking place in the melted Cu phase. Thus, for both the W-Cu powders containing nano-sized particles, a nearly full density could be achieved with the LPS process.
Please cite this article as: S.-S. Ryu, et al., Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-Cu..., Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012
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Fig. 9. FE-SEM fractographs of the W-Cu specimen sintered at 1050 °C in H2 atmosphere with a heating rate of 5 °C/min; 1 h-milled W-Cu specimen sintered for (a) 1 h and (b) 5 h; 10 hmilled W-Cu specimen sintered for (c) 1 h and (d) 5 h.
Fig. 10. (a) FE-SEM fractographs of 1 h-milled W-Cu specimen sintered at (a) 1100 °C, (b) 1150 °C, and (c) 1200 °C as wells as of 10 h-milled W-Cu specimen sintered at (d) 1100 °C, (e) 1150 °C, and (f) 1200 °C. Sintering was carried out for 1 h in H2 atmosphere with a heating rate of 5 °C/min.
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Please cite this article as: S.-S. Ryu, et al., Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-Cu..., Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures Sung-Soo Ryu a,⁎, Hae-Ryong Park a,b,1, Young Do Kim b, Hyun Seon Hong c a b c
Korea Institute of Ceramic Engineering and Technology, Icheon 17303, Republic of Korea Dept. of Materials Science & Engineering, Hanyang University, Seoul 04763, Republic of Korea Dept. of Interdisplinary ECO Science, Sungshin University, Seoul 01133, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 July 2016 Received in revised form 28 October 2016 Accepted 27 November 2016 Available online xxxx Keywords: Mechano-chemical process W-Cu Raw materials Solid phase sintering Liquid phase sintering
a b s t r a c t Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO3-CuO powder mixtures was investigated. The mixture of WO3 and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H2 atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W– Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently, W–Cu alloys have attracted a growing interest owing to their superior thermal management properties and high microwave absorption capacities [1–3]. For most of the applications, high-density W– Cu composites exhibiting a homogeneous microstructure are needed to achieve a high performance. Since the W–Cu system exhibits mutual insolubility or negligible solubility [4], W–Cu powder compacts exhibit poor sinterability, even when liquid phase sintering (LPS) is performed above the melting point of the Cu phase [5]. Fine size and well mixing of the components are known to improve the sinterability of the powder, especially in a LPS system, such as the W–Cu system, in which particle rearrangement is known to be the dominant sintering mechanism. The sinterability of W–Cu powder can be increased by employing an activated sintering process [7] with the addition of a small amount of metals such as Co, Ni, or Fe. However, since such activators can have a
⁎ Corresponding author at: Engineering Ceramic Center, Korea Institute of Ceramic Engineering and Technology, 30 Gyeongchung Rd., Sindun, Icheon, Gyeonggi 17303, Republic of Korea. E-mail address: [email protected] (S.-S. Ryu). 1 Present address: Pyeongtaek 451–831, Chang Sung Corporation.
negative influence on the thermal management properties of the W– Cu alloy [8], enhancing the sinterability by activated sintering seems to be one of the least acceptable options. Recently, many efforts towards the fabrication and sintering of W–Cu powders with nano-sized grains have been carried out to achieve excellent densities without adding activators [9,10]. Among various methods used for producing highly dispersed W–Cu nanocomposite powders [11–20], the mechano-chemical process, which involves ball-milling of metal oxide powders and subsequent hydrogen reduction, is a promising approach as W–Cu nanocomposite powders exhibiting a high sinterability can be produced with this method [9,12]. Understanding the microstructure of W–Cu composite powder is essential for elucidating its sintering mechanism [6]. W–Cu nanocomposite powder obtained by performing hydrogen reduction of milled oxide mixtures exhibited similar characteristics as those of micro-homogeneously mixed W–Cu aggregates [21]. A new W–Cu nanocomposite structure, consisting of W nanoparticles coated with a Cu layer, was synthesized previously [22]. The product formation seemed to depend on various factors, including the reduction and milling conditions of the oxide mixtures. In this study, the effect of ball-milling time of WO3CuO powder mixture on the microstructural characteristics and densification behavior of reduced W-Cu composite powder is investigated.
http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: S.-S. Ryu, et al., Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-Cu..., Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012
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S.-S. Ryu et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2016) xxx–xxx
(b)
(a)
Fig. 1. FE-SEM images of the starting materials, (a) WO3 and (b) CuO.
Fig. 2. FE-SEM images of the WO3-CuO powder mixture ball-milled for (a) 1 h and (b) 10 h.
2. Experimental procedure To produce W–20 wt.% Cu (W–20Cu) nanocomposite powder, WO3 powder (Taegu Tec, Korea), with a mean particle size of 3 μm and a purity of 99.9%, and CuO powder (Kojundo, Japan), with a mean particle size of 12 μm and a purity of 99.9%, were used. Ball milling process was performed using a horizontal bead mill (UBM-5, Nanointek, Korea). ZrO2 milling balls and jars were used to avoid contaminants, such as Fe, Cr, and Ni elements, which could act as sintering activators
for W–Cu. More details on the milling experiment are described elsewhere [22]. The WO3–CuO powder mixtures milled for 1 h and 10 h were used as starting materials in this study. The milled WO3-CuO powder mixtures were dried at 80 °C and reduced in a 5 mm powder bed at 200 °C for 1 h. The temperature was then increased to 800 °C for 1 h in dry H2 atmosphere (dew point of −76 °C) with a heating rate of 2 °C/min in order to form the W-20Cu composite powder. Microstructural analysis of the reduced W–Cu powder was conducted using field emission-scanning electron microscopy (FE-SEM, JSM-9701, JEOL, JAPAN) and field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F30 S-Twin, FEI, Netherlands). The reduced W–Cu composite powder was pressed at 100 MPa using the die compaction method, followed by performing cold isostatic pressing at 500 MPa in order to minimize the shrinkage anisotropy. The green density of the powder compact was found to be 45 ± 1% of the theoretically obtained value. Sintering was also conducted in the temperature range of 1050–1250 °C for 1 h, with a heating rate of 5 °C/min in H2 atmosphere. Density of the sintered sample was measured using the Archimedes' displacement method, and the sintered microstructure was examined using FE-SEM (JSM-9701, JEOL, JAPAN). 3. Results and discussion
Fig. 3. XRD patterns of W-Cu powder reduced from WO3-CuO powder mixture ball-milled for (a) 1 h and (b) 10 h in the bead mill.
Fig. 1 shows the FE-SEM images indicating the morphologies of the WO3 and CuO powders, which were used as starting materials in this study. As shown in the figure, both the powders consist of micronsized particles. WO3 consists of agglomerates of needle shaped primary particle with ultra-fine size. The CuO powder consists of uniform micron-sized particles. Fig. 2 shows the FE-SEM images of the WO3-CuO powder mixture ball-milled for 1 h and 10 h in the bead mill. As shown in Fig. 2(a), the
Please cite this article as: S.-S. Ryu, et al., Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-Cu..., Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012
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Fig. 4. FE-SEM images of the W-Cu composite powder produced from (a) 1 h-milled and (b) 10 h-milled WO3-CuO powder mixtures.
WO3 particle size is decreased to obtain nanoparticles when the milling was performed for 1 h, and the shape of the CuO particles changed from polyhedral shape to micron-sized plate-like structure. WO3 particles could be easily milled as compared with CuO particles. When the milling was performed for 10 h, nanoparticles of WO3 and CuO were obtained (Fig. 2(b)). Fig. 3 shows the XRD patterns of reduced W-Cu powder obtained after heat-treating the milled WO3-CuO powder mixtures in H2 atmosphere. After reducing the high-energy ball-milled WO3-CuO mixture at 800 °C, pure phases of W and Cu are observed in the XRD pattern of the products irrespective of the milling time, indicating that all the oxides are transformed to form the W–Cu composite powder. Fig. 4 shows the FE-SEM images of the W–Cu composite powder reduced at 800 °C. W and Cu particles can be easily distinguished from the FE-SEM images in the case of 1 h-milled sample. White regions in the image (Fig. 4(a)) correspond to nanoparticles of W and the grey regions correspond to Cu microparticles. Some of the W nanoparticles are attached on the surfaces of Cu microparticles. The characteristics of this sample could be related to those of a milled oxide mixture containing WO3 nanoparticles and CuO microparticles. The surface of Cu microparticles (previously H2-reduced) might act as a nucleation site for the W nanoparticles owing to the reduction including the chemical vapor transport process of W oxide in H2 atmosphere as suggested by Kim et al. [23]. As a result, the W-Cu powder consists of W nanoparticles attached on the surface of Cu microparticles. Lee et al. [24] reported that W-Cu composite powder consisting of W-coated Cu particles could be produced by mixing (ball milling) the powders containing WO3 nanoparticles and CuO microparticles.
The W-Cu powder obtained with 10 h milling consists of nanocomposite powder containing uniform shaped particles exhibiting a mean particle size of about 50 nm (Fig. 4(b)). Owing to the resolution limit in FE-SEM, it is difficult to investigate the degree of homogeneity of the constituents present in the reduced W–Cu nanocomposite powder. Hence, STEM-EDS mapping was employed (Fig. 5). Surface analysis of the reduced W–Cu nanocomposite powder was also carried out with a combined use of STEM and EDS analyses, and the results are described in detail elsewhere [22]. According to these analyses, the reduced W– Cu nanocomposite powder is found to consist of ~ 50 nm W particles surrounded by a Cu nanolayer. Schematic diagram illustrating the microstructure of W–Cu powders produced from the constituent oxide mixtures using different milling times (1 h and 10 h) is shown in Fig. 6. W nanoparticle-attached Cu microparticles and W nanoparticles with the Cu layer could be obtained with 1 h-milled and 10-h milled oxide mixtures, respectively. Fig. 7 gives the sintered densities of the W-20Cu samples as a function of the sintering temperature after performing isothermal sintering for 1 h in H2 atmosphere. A high densification with a relative density of 92.8% of theoretical density (TD) is obtained on performing solid phase sintering (SPS) at 1050 °C with the 10 h-milled specimen. In addition, the full densification of Cu-coated W nanoparticles is achieved on sintering at 1100 °C. This excellent sinterability could be attributed to the maximization of the rearrangement-controlled densification, which reduces the number of W-W contacts before the melting of Cu [22]. Cu phase spreading, i.e., spreading-assisted particle rearrangement process [22,24] plays an important role in the enhanced densification occurring in the SPS stage.
Fig. 5. (a) STEM image and EDS elemental mapping images corresponding to (b and c) Cu and (d and e) W [22].
Please cite this article as: S.-S. Ryu, et al., Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-Cu..., Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.11.012
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Sintered density (%)
100 80 60 40 o
Sintering temp.: 1050 C 20 0
Fig. 6. Schematic diagram of W-Cu powders prepared from (a) 1 h-milled and (b) 10 hmilled WO3-CuO powder mixtures.
The 1 h-milled W-Cu specimen exhibited a poor sinterability in the SPS stage with 65% TD obtained at 1050 °C. However, its densification is significantly enhanced in the temperature range of LPS above the Cu melting temperature of 1083 °C. High densities of 97.0% TD and 98.9% TD could be obtained on sintering the sample at 1100 °C and 1200 °C, respectively. This significant increase in densification could be related to the presence of W nanoparticles in the sample. Johnson et al. [25] reported on the basis of dilatometry results for W-20Cu powder compacts that even on sintering a W-20Cu mixture containing W nanoparticles (0.11 μm) and Cu microparticles (5.71 μm), a significant increase in shrinkage occurs when Cu melts, thereby achieving a nearly full density at 1200 °C. The effect of sintering time on the densification behavior of two WCu powders occurring in the SPS stage is analyzed for the samples sintered at 1050 °C. Fig. 8 shows the sintered density of the W-20Cu specimens sintered at 1050 °C for 1 h and 5 h in H2 atmosphere. In the case of the 10 h-milled W-Cu specimen, a high densification occurs even in the SPS stage. The sintered density increases up to 96.0% TD after performing sintering for 5 h, whereas the 1 h-milled W-Cu powder containing W nanoparticles and Cu microparticles exhibits a relatively lower sintered density. After 5 h-sintering, the density is not greatly enhanced (increases to 75.9% TD from 65.0% TD). The densification occurring in the SPS stage seems to depend strongly on the characteristics of the Cu phase. In case of Cu-coated W powder, the Cu phase spreading, i.e., spreading-assisted particle rearrangement process [24], could take place even in the SPS stage, contributing remarkably to densification. However, in the case of 1 h-milled W-Cu powder, the Cu diffusion rate during SPS might be slow owing to the presence of a large number of Cu micro particles and their inhomogeneous
Sintered density (%)
100 90 80 70 10 h-milled 1 h-milled
60 50 1000
1100
1200
o
1300
Sintering temperature ( C) Fig. 7. The relative sintered density of W-Cu powder compacts sintered at different temperatures for 1 h in H2 atmosphere with a heating rate of 5 °C/min.
10 h-milled 1 h-milled 0
1
2
3
4
5
6
Sintering time (h) Fig. 8. The relative density of W-Cu powder compacts sintered for 1 h and 5 h at 1050 °C in H2 atmosphere with a heating rate of 5 °C/min.
distribution as compared with those of Cu-coated W nanopowder, which limits sintering by Cu phase spreading. Fig. 9 shows the FE-SEM micrographs of the W-20Cu specimens sintered at 1050 °C for 1 h and 5 h in H2 atmosphere. In the case of the 1 h-milled W-Cu powder shown in Fig. 9(a–b), the microstructure analysis indicates the presence of many pores in the samples sintered for 1 h and 5 h and that the sintering locally takes place by Cu spreading. In the case of 10 h-milled W-Cu powder shown in Fig. 9(c–d), a highly densified microstructure could be observed owing to fast Cu spreading. After performing sintering for 5 h, the sintered sample exhibits similar features as those of a typical fully densified W–Cu alloy formed via LPS. The FE-SEM images of W-20Cu specimens sintered isothermally in a LPS temperature range of 1100–1200 °C are shown in Fig. 10. All specimens exhibit similar sintered microstructures even though a large difference in the microstructural characteristics of the 1 h-milled W-Cu powder and the 10 h-milled W-Cu powder. On sintering, a typical fully densified W–Cu alloy is produced with the LPS stage, thereby achieving a well-dispersed W–Cu sintered specimen containing ultrafine particles. Each W grain is completely surrounded with liquid Cu, and homogeneously distributed W particles are obtained by the rearrangement of W grains in the Cu matrix. The W grain size is slightly increased with an increase in the temperature. 4. Conclusions The microstructure of the reduced W-Cu powder and its densification behavior significantly depend on the milling time of WO3 and CuO powder mixtures. Two types of W-Cu powders exhibiting different microstructural characteristics were produced from oxide mixtures by ball-milling the mixture for 1 h and 10 h. The W–Cu powder, which consisted of W nanoparticle-attached Cu microparticles, was obtained when the mixture was milled for 1 h, whereas Cu-coated W nanocomposite powder containing W nanoparticles coated by a Cu layer was produced with the 10 h milled mixture. The relationship between the microstructure of the reduced W-Cu powder and its densification behavior was examined. The microstructure of the reduced W-Cu powder influences its densification behavior. Sintering results indicate that the densification occurring in the SPS stage, which was mainly proceeded by Cu spreading, strongly depended on the microstructure of the reduced W-Cu, especially on the characteristics of the Cu phase. The densification occurring in the LPS stage is greatly influenced by the size of the W particles since most of the shrinkage occurring in the LPS stage could be attributed to the W particle rearrangement taking place in the melted Cu phase. Thus, for both the W-Cu powders containing nano-sized particles, a nearly full density could be achieved with the LPS process.
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Fig. 9. FE-SEM fractographs of the W-Cu specimen sintered at 1050 °C in H2 atmosphere with a heating rate of 5 °C/min; 1 h-milled W-Cu specimen sintered for (a) 1 h and (b) 5 h; 10 hmilled W-Cu specimen sintered for (c) 1 h and (d) 5 h.
Fig. 10. (a) FE-SEM fractographs of 1 h-milled W-Cu specimen sintered at (a) 1100 °C, (b) 1150 °C, and (c) 1200 °C as wells as of 10 h-milled W-Cu specimen sintered at (d) 1100 °C, (e) 1150 °C, and (f) 1200 °C. Sintering was carried out for 1 h in H2 atmosphere with a heating rate of 5 °C/min.
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