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Chromium carbide coating of diamond particles using low temperature molten salt mixture
Journal of Alloys and Compounds 805 (2019) 648e653
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Chromium carbide coating of diamond particles using low temperature molten salt mixture Hwa-Jung Kim, Hee-Lack Choi, Yong-Sik Ahn* Dept. of Materials Sci. & Eng., Pukyong National University, 45 Yongso-Ro, Nam-Gu, Busan, 48513, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 March 2019 Received in revised form 25 June 2019 Accepted 26 June 2019 Available online 4 July 2019
For diamond/metal composite fabrication, coating diamond particles with metal carbide provides improved wettability between the diamond particles and the matrix. In this study, diamond particles were coated with chromium (Cr) using a molten salt method. Diamond and Cr powders were heated at 600e950 C in molten salts of lithium chloride (LiCl), potassium chloride (KCl), and sodium chloride (NaCl) to create a chromium carbide (here Cr7C3) coating on the diamond particles. Carbide formation was influenced by the diffusion rate and eutectic point of the molten salt mixture. The resulting Cr7C3 coating surface and microstructure were characterized by X-ray diffraction and scanning electron microscopy, respectively. Coating layer thickness was determined using particle size analysis and confirmed by energy dispersive spectroscopy results. A uniform Cr7C3 layer formed on the surface of the diamond particles at a relatively low temperature, at which the graphitization of diamond is avoided. Compared with a mixture of LiCl, KCl, and calcium chloride (CaCl2), which had been used in our previous study, the LiCleKCleNaCl mixture increased the reaction rate and initiated coating at a much lower temperature. The differences between the two molten salt mixtures for coating diamond are discussed in terms of the diffusion parameters. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ceramics Coating materials Diffusion Microstructure Scanning electron microscopy (SEM) X-ray diffraction
1. Introduction The performance and reliability of electronic components depend on effective thermal management. Key developments and advances in the electronic industry have relied on the ability to find materials with both a high thermal conductivity and a low thermal expansion coefficient. Carbon (C) materials, in the form of diamond, graphite, carbon fiber, carbon nanotubes, and graphene, have proven to be promising candidates, as they can be used directly for thermal management applications or as part of a composite with other materials, such as metals [1,2]. Diamond/copper (Cu) composites have a high thermal conductivity, ranging from 350 to 780 W/mK, but the poor wettability of the diamond/metal interface and possible harmful interfacial chemical reactions during sintering deteriorate the thermal properties of the composites [2e5]. Several methods are currently available for fabricating diamond/ Cu composites with high thermal conductivity, including hot pressing, infiltration of a melted metal into a matrix, and spark
* Corresponding author. E-mail addresses: [email protected] (H.-L. Choi), [email protected] (Y.-S. Ahn).
(H.-J.
https://doi.org/10.1016/j.jallcom.2019.06.333 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Kim),
[email protected]
plasma sintering [2]. However, these methods require high fabrication temperatures of 800e1000 C, which can lead to graphitization of diamond [4]. A diamond surface undergoes graphitization in the temperature range of 697e1397 C in vacuum [5,6]. When a diamond particle is coated with strong carbide-forming elements, e.g., titanium, chromium (Cr), molybdenum (Mo), or tungsten (W), the particle surface can be protected from graphitization, even at relatively high temperatures; additionally, the carbide coating improves interfacial bonding between the diamond and metal without degrading the thermal conductivity [2,7,8]. Carbide coatings can be formed on diamond particles using the molten salt method [7], vacuum vapor deposition [3,9], sol-gel processes [8], diffusion methods [4,10], and plating techniques [11]. However, vacuum and chemical vapor deposition and the sol-gel process tend to be expensive, due to the materials and equipment required, and plating methods with carbide-forming elements tend to be relatively complex [11]. Molten salt synthesis is arguably one of the simplest methods for forming coatings on the surface of diamond. During the coating reaction, the molten salt liquid environment acts as a reaction medium for rapid carbide formation at relatively low temperatures [12e15]. The salt mixture is required to be stable, readily available,
H.-J. Kim et al. / Journal of Alloys and Compounds 805 (2019) 648e653
inexpensive, and easily washed away with water [14]. A typical molten salt synthesis procedure uses a mixture of chlorides or sulfates to form the coatings. In many cases, sodium chloride (NaCl)-potassium chloride (KCl) eutectic salt mixtures have been used to lower the liquid formation temperature [7,16]. Kang et al. coated diamond particles with Cr carbide by heating diamond and metallic Cr in a molten mixture of NaCl and KCl at 900 C [16]. In this study, diamond particles were coated with chromium carbide (here Cr7C3) using a molten salt mixture of lithium chloride (LiCl), KCl, and NaCl. Cr metal was chosen for the diamond surface coating because it easily coats the surface, is less expensive, and is more suitable for industrial production than W or Mo [3,5]. In our previous study, we used another molten salt mixture of LiCl, KCl, and calcium chloride (CaCl2); the coating temperature for this mixture was 800 C [17]. The eutectic point of the LiCleKCleNaCl mixture is 346 C, which is lower than that (400 C) of LiCleKCleCaCl2 [18]. However, to date the LiCleKCleNaCl composition has not been used as a molten mixture for coating diamond. Therefore, we investigated the use of a eutectic mixture of LiCleKCleNaCl as a molten salt for coating diamond particles with Cr at a low temperatures and compared the results with those obtained using the LiCleKCleCaCl2 salt mixture. 2. Experimental procedures CRJ1002-type diamond particles (particle size: 120/140 mesh) were used in this study. Cr powder (>99% purity, Sigma-Aldrich Corp., St. Louis, MO, USA) was used as the coating material. The molten salt mixture consisted of reagent-grade LiCl, KCl, and NaCl (99% purity; Junsei Chemical Company, Ltd., Tokyo, Japan). The LiCl:KCl:NaCl molar ratio in the mixture was 55:33:12, which corresponds to that at the eutectic point (346 C) of the LiCleKCleNaCl three-phase system. The LiCl:KCl:CaCl2 was set to 50.5:44.2:5.3,
649
corresponding to the eutectic composition of the LiCleKCleCaCl2 three-phase system [18]. Diamond particles and Cr powders were mixed at a molar ratio of 10:1 [16], and the molten salt mixture and diamond/Cr powders were mixed with a 3:2 wt ratio [19]. The molten salt mixture was placed in an alumina crucible, heat-treated at 600e900 C for 15e60 min in a tube furnace under a high-purity argon atmosphere, and then furnace-cooled to room temperature. After cooling, the salts in the mixture were removed by boiling in distilled water. The remaining diamond powder was then dried. The surface and microstructure of the coated diamond particles were characterized by X-ray diffraction (XRD; Ultima IV, Rigaku) and scanning electron microscopy (SEM; S-2700, Hitachi Ltd., Tokyo, Japan). The average thickness of the coating layer was determined using particle size analysis (PSA). 3. Result Fig. 1 presents a schematic diagram of the Cr coating process on diamond particles in molten salts. First, a large amount of Cr dissolves in the molten salt and becomes dissociated into mobile cations and delocalized electrons [20]. Cr powder of 3.2 g was put into molten salt mixture of 52.8 g, of which 0.9 g of Cr powder was remaining after the coating. This indicate that 2,3 g of Cr was dissolved in the molten salt. The dissolution rate of Cr is considerably higher than that of the diamond particles; thus, product layers form on the diamond surface to create the Cr carbide coating [14,15]. The molten salt mixture is thus believed to facilitate the dissolution and transport of Cr cations to the surface of the diamond particle, to react subsequently to form the carbide layer. Previous research has shown that the molten salt mixture promotes the reaction between the Cr powder and diamond particles [17]. Cr can be dissolves into the melt, and the molten salt experiences a metalesalt reaction by Eq. (1) [21]:
Fig. 1. Schematic diagram of the chromium (Cr) coating process on diamond particles in molten salts.
Fig. 2. Scanning electron microscopy (SEM) images of (a) raw diamond particles and (b,c) Cr-coated diamond particles processed at 700 C for 60 min in lithium chloride (LiCl)potassium chloride (KCl)-sodium chloride (NaCl) molten salts.
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Fig. 3. X-ray diffraction patterns of Cr-coated diamond particles processed at 600e900 C for 60 min in LiCleKCleNaCl molten salts.
2Cr3þ þ Cr / 3Cr2þ
(1)
and this reaction equilibrium is completely shifted to the right. Because the equilibrium of reaction in Eq. (1) is practically shifted completely to the right, chromium in the melt is present only in the form of chromium chlorides. These chlorides diffuse to the diamond and, if chromium dissolved into melt salt, the following reaction [Eq. (2)] might happen [21]: 21CrCl2 þ 3C (diamond) ¼ Cr7C3 þ 14CrCl3
Fig. 5. Particle size analysis of raw diamond and Cr-coated diamond particles processed at 700e900 C for 60 min in LiCleKCleNaCl molten salts.
(2)
Fig. 2 presents the morphology of raw diamond particles and Crcoated diamond particles processed at 700 C for 60 min in LiCleKCleNaCl molten salts. The coated diamond particles in Fig. 2b and c have an even appearance, which would result from a uniform coating reaction on the diamond surface. The coated materials were analyzed by XRD. Fig. 3 presents the XRD patterns of diamond particles with Crpowder heated at 600e900 C for 60 min in LiCleKCleNaCl molten salts. The sample heated at 600 C shows only the diamond peak, which indicates that the diamond particles had yet to be
coated. At 700 C, both Cr7C3 and diamond peaks were identified, which indicates that the coating reaction of diamond/Cr-powder had already occurred; additionally, there was no evidence of diamond graphitization at this temperature. With an increase in temperature of up to 900 C, the Cr7C3 peak increased. Diamond particles were also coated with a different chemical composition of chromium carbide using a second molten salt mixture of LiCleKCleCaCl2 at temperatures >800 C, as tested in a previous study [17]. Fig. 4 presents diamond particles coated at 800 and 900 C for 30 or 60 min in LiCleKCleNaCl and LiCleKCleCaCl2 molten salt mixtures. The resulting coated diamond particles using the LiCleKCleNaCl molten salt mixture had more uniform and evenly coated surfaces compared with those processed with the LiCleKCleCaCl2 mixture. These differences are attributable to the diffusion rates of the two mixtures. Fig. 5 presents the particle size distributions of raw diamond and diamonds coated at various temperatures in LiCleKCleNaCl molten salt. The coated diamond particles had a relatively uniform particle size distribution, with an average size larger than that of
Fig. 4. SEM micrograph of Cr-coated diamond particles processed with LiCleKCleNaCl and LiCleKCl-calcium chloride (CaCl2) molten salt mixtures at 800 C and 900 C and for reaction times of 30 and 60 min.
H.-J. Kim et al. / Journal of Alloys and Compounds 805 (2019) 648e653 Table 1 Average particle size and coated thickness of Cr-coated diamond in LiCleKCleNaCl. Temperature ( C) Raw diamond 900
800
700
Time (min)
Particle size (mm)
Coating thickness (mm)
60 30 15 60 30 15 60
132.9 142.8 139.5 137.7 138.8 135.9 133.7 134.8
9.9 6.6 4.8 5.9 3 0.8 1.9
651
Table 2 Single salt melt constant and activation energy for viscous motion in Eq. (3) for viscosity calculations [23].
LiCl NaCl KCl CaCl2
A (103 )
Eh (J/mol)
Temperature range (K)
33.06 18.60 49.84 10.88
7007 9308 6586 11997
910e1080 1090e1240 1060e1200 1060e1240
Table 3 Viscosity for a single salt at various temperatures, calculated using Eq. (3). Temperature ( C)
LiCl
NaCl
KCl
CaCl₂
700 800 900
7.86 7.25 6.78
5.88 5.28 4.83
11.25 10.43 9.79
4.79 4.18 3.72
Fig. 6. Coating thickness of diamond particles coated with Cr at 700e900 C in LiCleKCleNaCl and LiCleKCleCaCl2 molten salt mixtures.
raw diamond; the size increased with the coating temperature. The coated thickness was calculated from the size difference between the raw and coated diamond. Table 1 lists particle sizes and coating thicknesses with respect to the temperature and processing time. The thickness of the coated layer ranged from 1 to 10 mm. Fig. 6 presents the relationship between the thickness of the coated layer and coating temperature of the two molten salt mixtures of LiCleKCleNaCl and LiCleKCleCaCl2. The average thickness of the coated layer was calculated using PSA results. The coating thickness increased with the coating temperature. In the LiCleKCleNaCl mixture, the coating was much thicker than that using the LiCleKCleCaCl2 mixture, regardless of temperature. This indicates that the coating reaction or diffusion rate in the LiCleKCleNaCl molten salt mixture was much faster than that in the LiCleKCleCaCl2 mixture.
4. Discussion The diffusion rate of an element or an atom in a melt can be expressed using the WilkeeChang expression [22], in which the diffusion coefficient is inversely proportional to the viscosity of the melt. The viscosity for a certain substance h can be described as a function of temperature, as follows [23]:
h ¼ A exp
Eh ðcpÞ RT
(3)
where cp is the viscosity, A is a constant of the single salt melts, R is the gas constant [ ¼ 8.314 J/(molㆍK)], T is the absolute temperature, and Eh is the activation energy of the melt. Table 2 lists the
Fig. 7. Viscosities of two molten salt mixtures over the temperature range of 700e900 C.
single salt melt constant and activation energy of LiCl, NaCl, KCl, and CaCl2 for viscous motion over the temperature range of 910e1240 K. Table 3 lists the calculated viscosity for single salt melts at 700, 800, and 900 C using Eq. (3). Fig. 7 presents the calculated viscosity of LiCleKCleNaCl and LiCleKCleCaCl2 molten salt mixtures as a function of temperature. The viscosity of the molten salt mixture decreased as the temperature increased. The LiCleKCleNaCl molten salt mixture had a much lower viscosity than the LiCleKCleCaCl2 mixture. Thus, the LiCleKCleNaCl mixture is expected to have a higher diffusion rate, so this salt mixture should provide a more uniform coating of the diamond surface. The LiCleKCleNaCl mixture enabled coating at 700 C, a temperature lower than that of the LiCleKCleCaCl2 mixture: the coating reaction in the LiCleKCleCaCl2 mixture occurred only at temperatures higher than 800 C [17]. Notably, the eutectic point of the LiCleKCleNaCl mixture is 346 C, whereas that of LiCleKCleCaCl2 is 400 C [18]. Fig. 8 presents an SEM image of the focused ion beam crosssection of diamond particles coated at 700 C for 60 min in LiCleKCleNaCl molten salt. Fig. 8c shows the concentration changes of C and Cr elements via energy dispersive X-ray spectroscopy (EDS) line-scanning analysis, in which Cr was observed at a distance of 1.9 mm from the surface to the center. Thus, the
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Fig. 8. SEM images of Cr-coated diamond particles processed at 700 C for 60 min in LiCleKCleNaCl molten salts: (a,b) focused ion beam cross-section and (c) energy dispersive spectroscopy line-scanning analysis.
Fig. 9. Electron probe microanalysis (EPMA) image of Cr-coated diamond particles processed at 800 C for 60 min in LiCleKCleNaCl molten salts: (a) polished samples and (b) EPMA analysis surface.
thickness of the interfacial Cr7C3 layer was approximately 1.9 mm, which coincides exactly with the measured coating thickness results, as given in Table 1. This carbide was already identified as Cr7C3 in XRD analyses (Fig. 3). Fig. 9 presents an electron probe microanalysis image of diamond particles coated at 800 C for 60 min in LiCleKCleNaCl molten salts; element mapping of the polished sample revealed high Cr content at the diamond particle surface using EDS, which indicated a uniform Cr7C3 coating. The average thickness of the interfacial Cr7C3 layer was about 5 mm. These results are in good agreement with the coating layer thickness determined by SEM analysis (Table 1).
5. Conclusion Diamond particles were coated with a carbide layer by heating diamond and Cr-particles in a molten mixture of LiCleKCleNaCl over the temperature range of 700e900 C. The coated layer was identified as Cr7C3. The thickness of the coating layer increased linearly with the coating temperature and time. The temperature required for coating was 700 C for the LiCleKCleNaCl mixture, which was much lower than that of the LiCleKCleCaCl2 mixture (800 C), as determined in a previous study. The coating reaction rate with the LiCleKCleNaCl mixture was much faster, so the
coated layer was much thicker than that using the LiCleKCleCaCl2 mixture. These results are attributable to the diffusion rate and the viscosity. The viscosity of the LiCleKCleNaCl molten salt mixture was much lower, resulting in a higher diffusion coefficient. The coated thickness measured by PSA coincided exactly with that obtained with EDS.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Chromium carbide coating of diamond particles using low temperature molten salt mixture Hwa-Jung Kim, Hee-Lack Choi, Yong-Sik Ahn* Dept. of Materials Sci. & Eng., Pukyong National University, 45 Yongso-Ro, Nam-Gu, Busan, 48513, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 March 2019 Received in revised form 25 June 2019 Accepted 26 June 2019 Available online 4 July 2019
For diamond/metal composite fabrication, coating diamond particles with metal carbide provides improved wettability between the diamond particles and the matrix. In this study, diamond particles were coated with chromium (Cr) using a molten salt method. Diamond and Cr powders were heated at 600e950 C in molten salts of lithium chloride (LiCl), potassium chloride (KCl), and sodium chloride (NaCl) to create a chromium carbide (here Cr7C3) coating on the diamond particles. Carbide formation was influenced by the diffusion rate and eutectic point of the molten salt mixture. The resulting Cr7C3 coating surface and microstructure were characterized by X-ray diffraction and scanning electron microscopy, respectively. Coating layer thickness was determined using particle size analysis and confirmed by energy dispersive spectroscopy results. A uniform Cr7C3 layer formed on the surface of the diamond particles at a relatively low temperature, at which the graphitization of diamond is avoided. Compared with a mixture of LiCl, KCl, and calcium chloride (CaCl2), which had been used in our previous study, the LiCleKCleNaCl mixture increased the reaction rate and initiated coating at a much lower temperature. The differences between the two molten salt mixtures for coating diamond are discussed in terms of the diffusion parameters. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ceramics Coating materials Diffusion Microstructure Scanning electron microscopy (SEM) X-ray diffraction
1. Introduction The performance and reliability of electronic components depend on effective thermal management. Key developments and advances in the electronic industry have relied on the ability to find materials with both a high thermal conductivity and a low thermal expansion coefficient. Carbon (C) materials, in the form of diamond, graphite, carbon fiber, carbon nanotubes, and graphene, have proven to be promising candidates, as they can be used directly for thermal management applications or as part of a composite with other materials, such as metals [1,2]. Diamond/copper (Cu) composites have a high thermal conductivity, ranging from 350 to 780 W/mK, but the poor wettability of the diamond/metal interface and possible harmful interfacial chemical reactions during sintering deteriorate the thermal properties of the composites [2e5]. Several methods are currently available for fabricating diamond/ Cu composites with high thermal conductivity, including hot pressing, infiltration of a melted metal into a matrix, and spark
* Corresponding author. E-mail addresses: [email protected] (H.-L. Choi), [email protected] (Y.-S. Ahn).
(H.-J.
https://doi.org/10.1016/j.jallcom.2019.06.333 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Kim),
[email protected]
plasma sintering [2]. However, these methods require high fabrication temperatures of 800e1000 C, which can lead to graphitization of diamond [4]. A diamond surface undergoes graphitization in the temperature range of 697e1397 C in vacuum [5,6]. When a diamond particle is coated with strong carbide-forming elements, e.g., titanium, chromium (Cr), molybdenum (Mo), or tungsten (W), the particle surface can be protected from graphitization, even at relatively high temperatures; additionally, the carbide coating improves interfacial bonding between the diamond and metal without degrading the thermal conductivity [2,7,8]. Carbide coatings can be formed on diamond particles using the molten salt method [7], vacuum vapor deposition [3,9], sol-gel processes [8], diffusion methods [4,10], and plating techniques [11]. However, vacuum and chemical vapor deposition and the sol-gel process tend to be expensive, due to the materials and equipment required, and plating methods with carbide-forming elements tend to be relatively complex [11]. Molten salt synthesis is arguably one of the simplest methods for forming coatings on the surface of diamond. During the coating reaction, the molten salt liquid environment acts as a reaction medium for rapid carbide formation at relatively low temperatures [12e15]. The salt mixture is required to be stable, readily available,
H.-J. Kim et al. / Journal of Alloys and Compounds 805 (2019) 648e653
inexpensive, and easily washed away with water [14]. A typical molten salt synthesis procedure uses a mixture of chlorides or sulfates to form the coatings. In many cases, sodium chloride (NaCl)-potassium chloride (KCl) eutectic salt mixtures have been used to lower the liquid formation temperature [7,16]. Kang et al. coated diamond particles with Cr carbide by heating diamond and metallic Cr in a molten mixture of NaCl and KCl at 900 C [16]. In this study, diamond particles were coated with chromium carbide (here Cr7C3) using a molten salt mixture of lithium chloride (LiCl), KCl, and NaCl. Cr metal was chosen for the diamond surface coating because it easily coats the surface, is less expensive, and is more suitable for industrial production than W or Mo [3,5]. In our previous study, we used another molten salt mixture of LiCl, KCl, and calcium chloride (CaCl2); the coating temperature for this mixture was 800 C [17]. The eutectic point of the LiCleKCleNaCl mixture is 346 C, which is lower than that (400 C) of LiCleKCleCaCl2 [18]. However, to date the LiCleKCleNaCl composition has not been used as a molten mixture for coating diamond. Therefore, we investigated the use of a eutectic mixture of LiCleKCleNaCl as a molten salt for coating diamond particles with Cr at a low temperatures and compared the results with those obtained using the LiCleKCleCaCl2 salt mixture. 2. Experimental procedures CRJ1002-type diamond particles (particle size: 120/140 mesh) were used in this study. Cr powder (>99% purity, Sigma-Aldrich Corp., St. Louis, MO, USA) was used as the coating material. The molten salt mixture consisted of reagent-grade LiCl, KCl, and NaCl (99% purity; Junsei Chemical Company, Ltd., Tokyo, Japan). The LiCl:KCl:NaCl molar ratio in the mixture was 55:33:12, which corresponds to that at the eutectic point (346 C) of the LiCleKCleNaCl three-phase system. The LiCl:KCl:CaCl2 was set to 50.5:44.2:5.3,
649
corresponding to the eutectic composition of the LiCleKCleCaCl2 three-phase system [18]. Diamond particles and Cr powders were mixed at a molar ratio of 10:1 [16], and the molten salt mixture and diamond/Cr powders were mixed with a 3:2 wt ratio [19]. The molten salt mixture was placed in an alumina crucible, heat-treated at 600e900 C for 15e60 min in a tube furnace under a high-purity argon atmosphere, and then furnace-cooled to room temperature. After cooling, the salts in the mixture were removed by boiling in distilled water. The remaining diamond powder was then dried. The surface and microstructure of the coated diamond particles were characterized by X-ray diffraction (XRD; Ultima IV, Rigaku) and scanning electron microscopy (SEM; S-2700, Hitachi Ltd., Tokyo, Japan). The average thickness of the coating layer was determined using particle size analysis (PSA). 3. Result Fig. 1 presents a schematic diagram of the Cr coating process on diamond particles in molten salts. First, a large amount of Cr dissolves in the molten salt and becomes dissociated into mobile cations and delocalized electrons [20]. Cr powder of 3.2 g was put into molten salt mixture of 52.8 g, of which 0.9 g of Cr powder was remaining after the coating. This indicate that 2,3 g of Cr was dissolved in the molten salt. The dissolution rate of Cr is considerably higher than that of the diamond particles; thus, product layers form on the diamond surface to create the Cr carbide coating [14,15]. The molten salt mixture is thus believed to facilitate the dissolution and transport of Cr cations to the surface of the diamond particle, to react subsequently to form the carbide layer. Previous research has shown that the molten salt mixture promotes the reaction between the Cr powder and diamond particles [17]. Cr can be dissolves into the melt, and the molten salt experiences a metalesalt reaction by Eq. (1) [21]:
Fig. 1. Schematic diagram of the chromium (Cr) coating process on diamond particles in molten salts.
Fig. 2. Scanning electron microscopy (SEM) images of (a) raw diamond particles and (b,c) Cr-coated diamond particles processed at 700 C for 60 min in lithium chloride (LiCl)potassium chloride (KCl)-sodium chloride (NaCl) molten salts.
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Fig. 3. X-ray diffraction patterns of Cr-coated diamond particles processed at 600e900 C for 60 min in LiCleKCleNaCl molten salts.
2Cr3þ þ Cr / 3Cr2þ
(1)
and this reaction equilibrium is completely shifted to the right. Because the equilibrium of reaction in Eq. (1) is practically shifted completely to the right, chromium in the melt is present only in the form of chromium chlorides. These chlorides diffuse to the diamond and, if chromium dissolved into melt salt, the following reaction [Eq. (2)] might happen [21]: 21CrCl2 þ 3C (diamond) ¼ Cr7C3 þ 14CrCl3
Fig. 5. Particle size analysis of raw diamond and Cr-coated diamond particles processed at 700e900 C for 60 min in LiCleKCleNaCl molten salts.
(2)
Fig. 2 presents the morphology of raw diamond particles and Crcoated diamond particles processed at 700 C for 60 min in LiCleKCleNaCl molten salts. The coated diamond particles in Fig. 2b and c have an even appearance, which would result from a uniform coating reaction on the diamond surface. The coated materials were analyzed by XRD. Fig. 3 presents the XRD patterns of diamond particles with Crpowder heated at 600e900 C for 60 min in LiCleKCleNaCl molten salts. The sample heated at 600 C shows only the diamond peak, which indicates that the diamond particles had yet to be
coated. At 700 C, both Cr7C3 and diamond peaks were identified, which indicates that the coating reaction of diamond/Cr-powder had already occurred; additionally, there was no evidence of diamond graphitization at this temperature. With an increase in temperature of up to 900 C, the Cr7C3 peak increased. Diamond particles were also coated with a different chemical composition of chromium carbide using a second molten salt mixture of LiCleKCleCaCl2 at temperatures >800 C, as tested in a previous study [17]. Fig. 4 presents diamond particles coated at 800 and 900 C for 30 or 60 min in LiCleKCleNaCl and LiCleKCleCaCl2 molten salt mixtures. The resulting coated diamond particles using the LiCleKCleNaCl molten salt mixture had more uniform and evenly coated surfaces compared with those processed with the LiCleKCleCaCl2 mixture. These differences are attributable to the diffusion rates of the two mixtures. Fig. 5 presents the particle size distributions of raw diamond and diamonds coated at various temperatures in LiCleKCleNaCl molten salt. The coated diamond particles had a relatively uniform particle size distribution, with an average size larger than that of
Fig. 4. SEM micrograph of Cr-coated diamond particles processed with LiCleKCleNaCl and LiCleKCl-calcium chloride (CaCl2) molten salt mixtures at 800 C and 900 C and for reaction times of 30 and 60 min.
H.-J. Kim et al. / Journal of Alloys and Compounds 805 (2019) 648e653 Table 1 Average particle size and coated thickness of Cr-coated diamond in LiCleKCleNaCl. Temperature ( C) Raw diamond 900
800
700
Time (min)
Particle size (mm)
Coating thickness (mm)
60 30 15 60 30 15 60
132.9 142.8 139.5 137.7 138.8 135.9 133.7 134.8
9.9 6.6 4.8 5.9 3 0.8 1.9
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Table 2 Single salt melt constant and activation energy for viscous motion in Eq. (3) for viscosity calculations [23].
LiCl NaCl KCl CaCl2
A (103 )
Eh (J/mol)
Temperature range (K)
33.06 18.60 49.84 10.88
7007 9308 6586 11997
910e1080 1090e1240 1060e1200 1060e1240
Table 3 Viscosity for a single salt at various temperatures, calculated using Eq. (3). Temperature ( C)
LiCl
NaCl
KCl
CaCl₂
700 800 900
7.86 7.25 6.78
5.88 5.28 4.83
11.25 10.43 9.79
4.79 4.18 3.72
Fig. 6. Coating thickness of diamond particles coated with Cr at 700e900 C in LiCleKCleNaCl and LiCleKCleCaCl2 molten salt mixtures.
raw diamond; the size increased with the coating temperature. The coated thickness was calculated from the size difference between the raw and coated diamond. Table 1 lists particle sizes and coating thicknesses with respect to the temperature and processing time. The thickness of the coated layer ranged from 1 to 10 mm. Fig. 6 presents the relationship between the thickness of the coated layer and coating temperature of the two molten salt mixtures of LiCleKCleNaCl and LiCleKCleCaCl2. The average thickness of the coated layer was calculated using PSA results. The coating thickness increased with the coating temperature. In the LiCleKCleNaCl mixture, the coating was much thicker than that using the LiCleKCleCaCl2 mixture, regardless of temperature. This indicates that the coating reaction or diffusion rate in the LiCleKCleNaCl molten salt mixture was much faster than that in the LiCleKCleCaCl2 mixture.
4. Discussion The diffusion rate of an element or an atom in a melt can be expressed using the WilkeeChang expression [22], in which the diffusion coefficient is inversely proportional to the viscosity of the melt. The viscosity for a certain substance h can be described as a function of temperature, as follows [23]:
h ¼ A exp
Eh ðcpÞ RT
(3)
where cp is the viscosity, A is a constant of the single salt melts, R is the gas constant [ ¼ 8.314 J/(molㆍK)], T is the absolute temperature, and Eh is the activation energy of the melt. Table 2 lists the
Fig. 7. Viscosities of two molten salt mixtures over the temperature range of 700e900 C.
single salt melt constant and activation energy of LiCl, NaCl, KCl, and CaCl2 for viscous motion over the temperature range of 910e1240 K. Table 3 lists the calculated viscosity for single salt melts at 700, 800, and 900 C using Eq. (3). Fig. 7 presents the calculated viscosity of LiCleKCleNaCl and LiCleKCleCaCl2 molten salt mixtures as a function of temperature. The viscosity of the molten salt mixture decreased as the temperature increased. The LiCleKCleNaCl molten salt mixture had a much lower viscosity than the LiCleKCleCaCl2 mixture. Thus, the LiCleKCleNaCl mixture is expected to have a higher diffusion rate, so this salt mixture should provide a more uniform coating of the diamond surface. The LiCleKCleNaCl mixture enabled coating at 700 C, a temperature lower than that of the LiCleKCleCaCl2 mixture: the coating reaction in the LiCleKCleCaCl2 mixture occurred only at temperatures higher than 800 C [17]. Notably, the eutectic point of the LiCleKCleNaCl mixture is 346 C, whereas that of LiCleKCleCaCl2 is 400 C [18]. Fig. 8 presents an SEM image of the focused ion beam crosssection of diamond particles coated at 700 C for 60 min in LiCleKCleNaCl molten salt. Fig. 8c shows the concentration changes of C and Cr elements via energy dispersive X-ray spectroscopy (EDS) line-scanning analysis, in which Cr was observed at a distance of 1.9 mm from the surface to the center. Thus, the
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Fig. 8. SEM images of Cr-coated diamond particles processed at 700 C for 60 min in LiCleKCleNaCl molten salts: (a,b) focused ion beam cross-section and (c) energy dispersive spectroscopy line-scanning analysis.
Fig. 9. Electron probe microanalysis (EPMA) image of Cr-coated diamond particles processed at 800 C for 60 min in LiCleKCleNaCl molten salts: (a) polished samples and (b) EPMA analysis surface.
thickness of the interfacial Cr7C3 layer was approximately 1.9 mm, which coincides exactly with the measured coating thickness results, as given in Table 1. This carbide was already identified as Cr7C3 in XRD analyses (Fig. 3). Fig. 9 presents an electron probe microanalysis image of diamond particles coated at 800 C for 60 min in LiCleKCleNaCl molten salts; element mapping of the polished sample revealed high Cr content at the diamond particle surface using EDS, which indicated a uniform Cr7C3 coating. The average thickness of the interfacial Cr7C3 layer was about 5 mm. These results are in good agreement with the coating layer thickness determined by SEM analysis (Table 1).
5. Conclusion Diamond particles were coated with a carbide layer by heating diamond and Cr-particles in a molten mixture of LiCleKCleNaCl over the temperature range of 700e900 C. The coated layer was identified as Cr7C3. The thickness of the coating layer increased linearly with the coating temperature and time. The temperature required for coating was 700 C for the LiCleKCleNaCl mixture, which was much lower than that of the LiCleKCleCaCl2 mixture (800 C), as determined in a previous study. The coating reaction rate with the LiCleKCleNaCl mixture was much faster, so the
coated layer was much thicker than that using the LiCleKCleCaCl2 mixture. These results are attributable to the diffusion rate and the viscosity. The viscosity of the LiCleKCleNaCl molten salt mixture was much lower, resulting in a higher diffusion coefficient. The coated thickness measured by PSA coincided exactly with that obtained with EDS.
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