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shell-type microparticles consisting of cBN cores aluminum coating via composite method
Accepted Manuscript Preparation core/shell-type microparticles consisting of cBN cores aluminum coating via composite method Zhufeng Jiang, Jilin Wang, Peng Wang, Shanshan Yang, Zhengguang Zou, Yi Wu PII:
S0925-8388(18)33357-7
DOI:
10.1016/j.jallcom.2018.09.117
Reference:
JALCOM 47536
To appear in:
Journal of Alloys and Compounds
Received Date: 9 April 2018 Revised Date:
4 September 2018
Accepted Date: 11 September 2018
Please cite this article as: Z. Jiang, J. Wang, P. Wang, S. Yang, Z. Zou, Y. Wu, Preparation core/shelltype microparticles consisting of cBN cores aluminum coating via composite method, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.09.117. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Preparation Core/Shell-Type Microparticles Consisting of cBN Cores Aluminum Coating via Composite Method Zhufeng
Jianga,b,Jilin
Wanga,
Peng
Wanga,Shanshan
Yanga,
a
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Zhengguang Zoua , Yi Wua* School of Materials Science and Engineering, Key Laboratory of Nonferrous Materials and New
Processing Technology of Ministry of Education, Guilin University of Technology, Guilin 541004, China
College of Chemistry, Chemical Engineering and Materials Science, Soochow University,
SC
b
Soochow 215000, China
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Keywords: Brownian motion, Self-assembly, Cubic boron nitride, Core-shell structure, Ceramics, Metallic composites
Abstract. The mechanically mixed aluminum and cubic boron nitride (cBN) powder are important raw materials for preparing super-hard metal cutting tools. So aluminum shell-coated
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cBN may have significant economic potential. A Brownian motion based electrostatic self-assembly route in liquid salt of high temperature was developed using silica nanolayer bridge
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joint for the aluminum coating of cBN microparticles. In this approach, piranha solution method was first used to activate the surface of cBN cores, the silica nanolayer via sol-gel method served
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as the substrate for an outer aluminum coating of cBN cores. Coating of aluminum on cBN cores was accomplished by a self-assembly process from a liquid state solution of molten salt and aluminum powders. To distinguish small amounts of non-reacted aluminum powders which acted as aluminum source with the core - shell structure product, bromoform (CHBr3) was used to separate both of them due to the density differences. Samples were characterized by SEM, XRD, EDX and DSC. The contribution of our work lies in the creation of a novel strategy to fabricate a light metal element coating on the inert material particles in view of the versatility of sol-gel 1
ACCEPTED MANUSCRIPT process and the controllability of the molten salt process. Introduction Cubic boron nitride (cBN) is one of the hardest known materials with excellent thermal
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stability and chemical inertness.[1-4] The polycrystalline cubic boron nitride (PCBN) materials, a composite of cBN, that is sintered at high temperature and high pressure with fine micrometer cBN powders and other binder powders has excellent high speed cutting performance as
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superhard material for hardened steel parts.[5, 6] Among these binder material, aluminum is of
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excellent performance.[7] Since the aluminum powder and the boron nitride powder as raw materials are difficult to totally thoroughly mix mechanically, and the chemical wettability between aluminum and boron nitride is poor , resulting in the clustering of binder and the aggregation of cBN particles during the sintering period.[8,
9]
One idea to increase the
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homogeneous is coating every individual cBN particles with binder material.[10] This would decrease the contact between cBN particles and improve the interaction between the cBN and the binder material.
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But because the electrode potential of aluminum is too low, it is not suitable for
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electroplating. Moreover, aluminum is easily oxidized, so cubic boron nitride and metal aluminum have never been made into a core-shell structure. However, the boron nitride-aluminum core-shell structure has significant potential commercial value, and it can be used to produce a boron nitride-aluminum sintered body under high temperature and high pressure, which is an expensive superhard material for metal cutting. Many ideas regarding core-shell microparticles were reported in academia, providing some valuable process to the hard alloy industry. Among them the electrostatic self-assembly is a good 2
ACCEPTED MANUSCRIPT option.[11-13] Jiao et al reported that chemically grafted quantum dots can be assembled on nanotube structures via self-assembly, and diamond particles after surface modification can be coated by graphene via wet chemistry method, inspiring us that after some chemical process cBN
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particles that are also inert material may be coated with aluminum shell via electrostatic self-assembly after some surface modification as well.[14,15]Previously it's reported that silica can improve the wettability to aluminum.[16] And the silica coating can be prepared with a mature
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Stober method using tetraethyl orthosilicate (TEOS), though it cannot be used for nonpolar
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particles.[17] This inspired us that with the silica nanolayer the surface modification cBN core can be connected with metal coating via a composite route. It is previously reported that a TiN-TiB2 coating on cBN powders was prepared using a molten salts technique due to the chemical reaction between Ti and boron nitride. Nevertheless, it has a disadvantage that small amounts of
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non-reacted titanium metal particles, acting as titanium source for the disproportionation reaction in molten salts, are contained within the treated cBN powders.[18,19] In this study, we proposed to polarize the cBN via piranha solution treatment, and then
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modify the surface by silica nanolayer, to improve the wettability and avoid the difficult reaction
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condition between boron nitride and aluminum. The surface-modified cBN particles were coated with aluminum metal shell by molten salt process. The density difference between boron nitride and metallic aluminum is used to separate small amounts of non-reacted aluminum metal particles, solving all the problems before, successfully achieving core/shell-type microparticles consisting of cBN cores with aluminum coating via silica nanolayer bridging intermediary. In view of many kinds of superhard material in the industry has always suffered from that the internal phase structure is not uniform enough. And our investigation may improve this. 3
ACCEPTED MANUSCRIPT Experimental Preparation of Aluminum Coated cBN Composite Particles cBN powder (Funik Ultrahard Material Co., LTD) with average powder size of 4~5um
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and TEOS (Xilong Scientific Co., LTD) were used as starting materials for the preparation of surface silica modified cBN powder. The cBN powders were subjected to Piranha solution which is a mixture of 3 parts of concentrated sulfuric acid (H2SO4) and 1 part of 30% hydrogen peroxide
SC
(H2O2) solution. The Piranha solution is a strong oxidizer which can activate the surface of most
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materials.[20] After being washed, the cBN powders were dried in air at 100℃ for 12 hrs. Surface treated cBN was then suspended in 90% ethanol solution in a beaker to which aqua ammonia had been added for a concentration of 0.5 M, and the solution was ultrasonically dispersed for 30 min. The amount of cBN with respect to the suspension was adjusted to 4 wt% of cBN in suspension.
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The suspension was vigorously stirred with a paddle stirrer at about 500 rpm. And then TEOS had been added for a concentration of 0.004 M. The resulting mixture was heated at 50℃ for 24 hrs. The resulting coated particles were removed from suspension by filtration and drying process.
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Sodium chloride and potassium chloride were mixed in a mole ratio of 1:1. This is the mixed salt
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of which the melting point is about 657℃. And then the mixed salt, powdered aluminum and as-prepared silica-cBN nanocomposite powders were mixed in a weight ratio of 42:7:9. The mixture were calcined at 700 ℃ for 2 hrs. After cooling the molten salt mixture, the insoluble solids were removed from the lump of salts. The insoluble solids consisting of treated cBN particles and unreacted aluminum powder were washed well with distilled water and dried in a vacuum at 100℃ for 24 hrs. Final purification of the treated cBN was done by density centrifugation procedures with the heavy liquid of bromoform. Since the specific gravity of 4
ACCEPTED MANUSCRIPT bromoform is 2.89 which is a little higher than 2.7 the aluminum and much lower than 3.5 the cBN. Sample Characterization
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Powder samples were observed by SEM (S-4800, Hitachi) with electron diffraction and energy dispersive X-ray analysis (EDS). The phase compositions of the composite powders were identified using X-ray diffractometry (X’Pert PRO, PANalytical) the treated cBN
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powders were subjected to DSC (STA6000, PerkinElmer) analysis at a heating rate of
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5°C/min up to 800°C in the N2 atmosphere.
Results and Discussion
Characterization of cBN@ aluminum Core/Shell-type microparticles
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Figure 1 shows sample photograph after bromoform flotation purification in (a) and the XRD patterns of the cBN powders before and after Al coating preparation in (b). As shown in XRD curve (a), in the case of the as-prepared silica coated powders via sol–gel, peaks due to cBN only were observed, thus indicating that the silica coating is an amorphous phase. As shown in curve (b),
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after treated, aluminum and aluminum oxide peaks were seen to coexist with the cBN phase.
observed.
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Besides the evidence for the existence of small amounts of aluminum oxide silicate were
5
ACCEPTED MANUSCRIPT Fig. 1. XRD patterns of the treated cBN powders taken after (a) sol–gel reaction, (b) molten salt heat treatment
The SEM images in Figure 2 show the morphology of the treated cBN particles in various stages of their preparation. There are sharp edges and a cleavage fracture surface resulting from the
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crushing of cBN grit on the untreated cBN, shown in Figure 2(a), this is the typical morphology of the cBN powders of size of 3~5um. The cBN particles are covered with a silica nanolayer via sol-gel process, as seen in Figure 2(b), the morphology has little change. It's believed that is
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because the silica nanolayer is too thin. However, the morphology of the cBN change a lot after
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molten salt heat treatment, as can be seen in Figure 2(c), the cBN was covered by obvious shell. Energy dispersive X-ray spectroscopy revealed the presence of Al along with N, B, Si and O, suggesting that these small particles are coated with aluminum via silica bridging. (B)
(C)
(D)
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(A)
Fig. 2. SEM images showing the morphology of the coated layer on cBN surface. (a) uncoated cBN particle, (b) sol–gel coating silica nanolayer on cBN surface,(c) Aluminum coated cBN particles via silica bridging and (d) EDS result of the coated cBN particles.
6
ACCEPTED MANUSCRIPT DSC and TG analyses were performed on the aluminum coated cBN composite powder in the temperature range from 25 to 800°C with a heating rate of 10°C /min in N2 atmosphere for the as-prepared coated cBN particles separated by Bromoform centrifugation and washed by Ethanol.
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In Figure 3 the first stage of slight mass loss can be ascribed to the evaporation of water and Ethanol until 300°C. No further mass loss can be observed in the next stage from 300 to 800°C. An endothermic peak at 267°C can be observed. It is reported that this may be due to Ethanol
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desorption. [21]
Figure 3. DSC/TGA curves of the cBN@ aluminum Core/Shell-type powder heated in N2.
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Possible Reaction Mechanism
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Figure 4 shows the effect of different heat treatments on the formation of metallic aluminum coating. As can be seen in Figure 4(a), under the ordinary heat treatment conditions described in the preceding, there was relatively uniform coating on the cBN surface. But if the silica modified cBN and aluminum were in direct contact without molten salt, as can be seen in Figure 4(b), the coating will not be formed, suggesting the molten salt plays a key role in this experiment.
7
ACCEPTED MANUSCRIPT (B)
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(A)
Fig. 4. SEM images of the surface silica modified cBN subject to heat treatment (a) with molten salt and (b) without molten salt
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We believe that the mechanism is based on the Brownian motion and electrostatic
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self-assembly. Brownian motion is the random motion of particles suspended in a fluid resulting from their collision with the fast-moving atoms or molecules in the gas or liquid.[22] The co-melting point of KCl-NaCl at 1: 1 molar ratio is 660°C. When the temperature rises to 700°C or higher, the Salt melted to form a conductive liquid environment. At the same time aluminum
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powder also melted, dispersed into liquid nano droplets and diffused via the Brownian motion. The cBN particles undergo random motions in the liquid environment of molten salt due to Brownian motion. Since the liquid molten salt is an electrolyte, it's conductive and the metallic
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aluminum is also conductive. Aluminum powder after melting, of which a small part of electron
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will be thermally activated into free electrons. The liquid molten salt is conductive, so some free electrons in the molten salt and dispersed liquid metal aluminum get free movement and become shared electronic cloud for both conductive molten salt and aluminum. As a result, due to a slight loss of electrons, small aluminum droplets will be charged with a positive charge. So small aluminum droplets will be charged with a positive charge. Further more，Weyl et al studied the solid surface structure, suggesting that the surface of solid is semi-saturated.[23] As for silica, the oxygen anion was pushed to the outside to form a negatively charged layer, which will adsorb 8
ACCEPTED MANUSCRIPT positively charged aluminum. Under the Brownian motion, both modified cBN and Aluminum was dispersed in a conductive liquid environment and kept in thermal motion. The surface silica modified cBN, its surface with a negative charge, adsorbing positively charged aluminum.
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The schematic diagram in Figures 5 shows the morphological variations of the coated layer during the preparation of Aluminum coated cBN particles via silica bridging by the
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composite process.
Fig. 5. Schematic diagram showing various stages in the preparation of Aluminum coated cBN powders via silica bridging.
Conclusion
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cBN powder was successfully coated with aluminum shell by two-step process. The first step was preparation of electronegative nano-silica bridging layer via sol-gel method. The second step was
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electropositive molten aluminum's self-assembly in fuse salt. It's believed that the mechanism is based on the Brownian motion and electrostatic self-assembly.
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Acknowledgement
This research was supported by grant (Code No. AA17204098) from Guangxi Innovation Driven Development Special Fund Project and grant (Code No.2016GXNSFBA380155) from Guangxi Natural Science Foundation References [1] M. Bindal, R. Nayar, S. Singhal , et al. High-pressure sintering of cubic boron nitride[J], 4347-4351. https://doi.org/10.1007/BF01106554
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J. Mater. Sci. 21 (1986)
ACCEPTED MANUSCRIPT [2] F. Bundy, R. Wentorf Jr, Direct transformation of hexagonal boron nitride to denser forms[J], J. Chem. Phys. 38 (1963) 1144-1149. https://doi.org/10.1063/1.1733815 [3] R. Wentorf, R.C. DeVries, F. Bundy, Sintered superhard materials[J], Science.208 (1980) 873-880. https://doi.org/10.1126/science.208.4446.873
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[4] G.Z. Wang, Summarization about the peculiarities of cBN[J], Superhard Mater Eng.17(2005)41-45. https://doi.org/10.3969/j.issn.1673-1433.2005.05.011
[5] J. Barry, G. Byrne, Cutting tool wear in the machining of hardened steels: Part I: Alumina/TiC cutting tool wear[J], Wear. 247 (2001) 139-151. https://doi.org/10.1016/S0043-1648(00)00531-7
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[6] S. Takatsu, H. Shimoda, K. Otani, Effects of CBN content on the cutting performance of polycrystalline CBN tools[J], Int. J. Refract. Hard Met.2 (1983) 175-178.
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[7] X.L. Zhao, Study on the Development of New Polycrystalline Cubic Boron Nitride (PCBN) Tool and Its Cutting Mechanism(in Chinese), [D].Shandong University, 2011. https://doi.org/10.7666/d.y1936853 [8] J. Angseryd, E. Coronel, M. Elfwing, et al. The microstructure of the affected zone of a worn PCBN cutting tool characterised with SEM and TEM[J], Wear, 267 (2009) 1031-1040. https://doi.org/10.1016/j.wear.2008.12.075 [9] J. Angseryd, M. Elfwing, E. Olsson, et al. Detailed microstructure of a cBN based cutting tool material[J], Int. J.
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Refract. Met. Hard Mater. 27 (2009) 249-255. https://doi.org/10.1016/j.ijrmhm.2008.09.008 [10] A. Zhang, Z. Li, Z. Li, et al. Preparation and characterization of SiO2-Al2O3-Na2O glass coated cBN abrasive particles via sol–gel route[J], J. Sol-Gel Sci. Technol. 49 (2008) 6-11. https://doi.org/10.1007/s10971-008-1839-4
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[11]X.Huang ,T.Jiao,Q.Liu, et al. Hierarchical electrospun nanofibers treated by solvent vapor annealing as air filtration mat for high-efficiency PM2. 5 capture[J]. Science China Materials, 2018: 1-14.
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https://doi.org/10.1007/s40843-018-9320-4
[12]Y.Liu, C.Hou, T.Jiao, et al. Self-assembled AgNP-containing nanocomposites constructed by electrospinning as efficient dye photocatalyst materials for wastewater treatment[J]. Nanomaterials, 2018, 8(1): 35. https://doi.org/10.3390/nano8010035 [13]Y.Meng, L.Zhang, R.Xing, et al. Facile preparation and electrochemical characterization of self-assembled core-shell diamond-polypyrrole nanocomposites[J]. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018. https://doi.org/10.1016/j.colsurfa.2018.08.007 [14]S.Huo, P.Duan, T.Jiao, et al. Self-Assembled Luminescent Quantum Dots To Generate Full‐Color and White Circularly Polarized Light[J]. Angewandte Chemie International Edition, 2017, 56(40): 12174-12178. https://doi.org/10.1002/ange.201706308
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ACCEPTED MANUSCRIPT [15]X.Zhao, K.Ma, T.Jiao, et al. Fabrication of hierarchical layer-by-layer assembled diamond-based core-shell nanocomposites as highly efficient dye absorbents for wastewater treatment[J]. Scientific reports, 2017, 7: 44076. https://doi.org/10.1038/srep44076 [16] Y.K. Hao, D.T. Huang, X. Zhao, Effect of Silicon Oxide Coating and Silicon on Compatibility between Silicon
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Carbide Fiber and Aluminum(in Chinese)[J], Journal of National University of Defense Technology. 7 (1986) 82-92. [17] T. Aubert, F. Grasset, S. Mornet, et al. Functional silica nanoparticles synthesized by water-in-oil microemulsion processes[J], J. Colloid Interface Sci. 341 (2010) 201-208. https://doi.org/10.1016/j.jcis.2009.09.064
[18] H. Yoshida, S. Kume, Very high pressure sintering of cBN fine particles coated with TiN–TiB 2 layer formed by
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disproportionation reaction in molten salts[J], J. Mater. Res. 12 (1997) 585-588. https://doi.org/10.1557/jmr.1997.0084
[19] D. Xiang, M.S. Li, K.G. Liu, et al. Study on Salt-bath Plating Ti Layers on Diamond Surface[J], Hot.
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Working. Technol, 38 (2009) 74-77. https://doi:10.3969/j.issn.1001-3814.2009.04.023
[20] L. Jian, Preparation of SrTiO3 Functional Ceramic Thin Film on Glass Wafers Treated byPiranha Solution with Liquid Phase Deposition Method[J], Chin. J. Inorg. Chem. 28 (2012) 1065-1069.
[21] W. Cai, H. Li, Y. Zhang, Preparation of nanofibrous γ-Al2O3 particles with large pore volume from H2O2 and sodium aluminate solutions[J], J. Chem. Ind. Eng. 1155 (2004) 1976-1981. https://doi.org/10.3321/j.issn:0438-1157.2004.12.008
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[22] A. Einstein, Investigations on the Theory of the Brownian Movement, first ed., Courier Corporation[M], New York, 1956. https://doi.org/10.2307/3609228
[23] Y. Gu, C.S. Zhao, Materials Science and Engineering Foundation, second ed(in Chinese)[M]., Chemical
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Industry Press, Beijing, 2011.
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Highlights The easily oxidized light metal Aluminum shell coated on the inert material microparticles in liquid salt of high temperature. Develop new Aluminum- cubic boron nitride core-shell composite materials. Wet chemical sol-gel method combined with the heat treatment of metals. Significant potentials in design novel functional light metal coatings.
S0925-8388(18)33357-7
DOI:
10.1016/j.jallcom.2018.09.117
Reference:
JALCOM 47536
To appear in:
Journal of Alloys and Compounds
Received Date: 9 April 2018 Revised Date:
4 September 2018
Accepted Date: 11 September 2018
Please cite this article as: Z. Jiang, J. Wang, P. Wang, S. Yang, Z. Zou, Y. Wu, Preparation core/shelltype microparticles consisting of cBN cores aluminum coating via composite method, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.09.117. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Preparation Core/Shell-Type Microparticles Consisting of cBN Cores Aluminum Coating via Composite Method Zhufeng
Jianga,b,Jilin
Wanga,
Peng
Wanga,Shanshan
Yanga,
a
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Zhengguang Zoua , Yi Wua* School of Materials Science and Engineering, Key Laboratory of Nonferrous Materials and New
Processing Technology of Ministry of Education, Guilin University of Technology, Guilin 541004, China
College of Chemistry, Chemical Engineering and Materials Science, Soochow University,
SC
b
Soochow 215000, China
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Keywords: Brownian motion, Self-assembly, Cubic boron nitride, Core-shell structure, Ceramics, Metallic composites
Abstract. The mechanically mixed aluminum and cubic boron nitride (cBN) powder are important raw materials for preparing super-hard metal cutting tools. So aluminum shell-coated
TE D
cBN may have significant economic potential. A Brownian motion based electrostatic self-assembly route in liquid salt of high temperature was developed using silica nanolayer bridge
EP
joint for the aluminum coating of cBN microparticles. In this approach, piranha solution method was first used to activate the surface of cBN cores, the silica nanolayer via sol-gel method served
AC C
as the substrate for an outer aluminum coating of cBN cores. Coating of aluminum on cBN cores was accomplished by a self-assembly process from a liquid state solution of molten salt and aluminum powders. To distinguish small amounts of non-reacted aluminum powders which acted as aluminum source with the core - shell structure product, bromoform (CHBr3) was used to separate both of them due to the density differences. Samples were characterized by SEM, XRD, EDX and DSC. The contribution of our work lies in the creation of a novel strategy to fabricate a light metal element coating on the inert material particles in view of the versatility of sol-gel 1
ACCEPTED MANUSCRIPT process and the controllability of the molten salt process. Introduction Cubic boron nitride (cBN) is one of the hardest known materials with excellent thermal
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stability and chemical inertness.[1-4] The polycrystalline cubic boron nitride (PCBN) materials, a composite of cBN, that is sintered at high temperature and high pressure with fine micrometer cBN powders and other binder powders has excellent high speed cutting performance as
SC
superhard material for hardened steel parts.[5, 6] Among these binder material, aluminum is of
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excellent performance.[7] Since the aluminum powder and the boron nitride powder as raw materials are difficult to totally thoroughly mix mechanically, and the chemical wettability between aluminum and boron nitride is poor , resulting in the clustering of binder and the aggregation of cBN particles during the sintering period.[8,
9]
One idea to increase the
TE D
homogeneous is coating every individual cBN particles with binder material.[10] This would decrease the contact between cBN particles and improve the interaction between the cBN and the binder material.
EP
But because the electrode potential of aluminum is too low, it is not suitable for
AC C
electroplating. Moreover, aluminum is easily oxidized, so cubic boron nitride and metal aluminum have never been made into a core-shell structure. However, the boron nitride-aluminum core-shell structure has significant potential commercial value, and it can be used to produce a boron nitride-aluminum sintered body under high temperature and high pressure, which is an expensive superhard material for metal cutting. Many ideas regarding core-shell microparticles were reported in academia, providing some valuable process to the hard alloy industry. Among them the electrostatic self-assembly is a good 2
ACCEPTED MANUSCRIPT option.[11-13] Jiao et al reported that chemically grafted quantum dots can be assembled on nanotube structures via self-assembly, and diamond particles after surface modification can be coated by graphene via wet chemistry method, inspiring us that after some chemical process cBN
RI PT
particles that are also inert material may be coated with aluminum shell via electrostatic self-assembly after some surface modification as well.[14,15]Previously it's reported that silica can improve the wettability to aluminum.[16] And the silica coating can be prepared with a mature
SC
Stober method using tetraethyl orthosilicate (TEOS), though it cannot be used for nonpolar
M AN U
particles.[17] This inspired us that with the silica nanolayer the surface modification cBN core can be connected with metal coating via a composite route. It is previously reported that a TiN-TiB2 coating on cBN powders was prepared using a molten salts technique due to the chemical reaction between Ti and boron nitride. Nevertheless, it has a disadvantage that small amounts of
TE D
non-reacted titanium metal particles, acting as titanium source for the disproportionation reaction in molten salts, are contained within the treated cBN powders.[18,19] In this study, we proposed to polarize the cBN via piranha solution treatment, and then
EP
modify the surface by silica nanolayer, to improve the wettability and avoid the difficult reaction
AC C
condition between boron nitride and aluminum. The surface-modified cBN particles were coated with aluminum metal shell by molten salt process. The density difference between boron nitride and metallic aluminum is used to separate small amounts of non-reacted aluminum metal particles, solving all the problems before, successfully achieving core/shell-type microparticles consisting of cBN cores with aluminum coating via silica nanolayer bridging intermediary. In view of many kinds of superhard material in the industry has always suffered from that the internal phase structure is not uniform enough. And our investigation may improve this. 3
ACCEPTED MANUSCRIPT Experimental Preparation of Aluminum Coated cBN Composite Particles cBN powder (Funik Ultrahard Material Co., LTD) with average powder size of 4~5um
RI PT
and TEOS (Xilong Scientific Co., LTD) were used as starting materials for the preparation of surface silica modified cBN powder. The cBN powders were subjected to Piranha solution which is a mixture of 3 parts of concentrated sulfuric acid (H2SO4) and 1 part of 30% hydrogen peroxide
SC
(H2O2) solution. The Piranha solution is a strong oxidizer which can activate the surface of most
M AN U
materials.[20] After being washed, the cBN powders were dried in air at 100℃ for 12 hrs. Surface treated cBN was then suspended in 90% ethanol solution in a beaker to which aqua ammonia had been added for a concentration of 0.5 M, and the solution was ultrasonically dispersed for 30 min. The amount of cBN with respect to the suspension was adjusted to 4 wt% of cBN in suspension.
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The suspension was vigorously stirred with a paddle stirrer at about 500 rpm. And then TEOS had been added for a concentration of 0.004 M. The resulting mixture was heated at 50℃ for 24 hrs. The resulting coated particles were removed from suspension by filtration and drying process.
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Sodium chloride and potassium chloride were mixed in a mole ratio of 1:1. This is the mixed salt
AC C
of which the melting point is about 657℃. And then the mixed salt, powdered aluminum and as-prepared silica-cBN nanocomposite powders were mixed in a weight ratio of 42:7:9. The mixture were calcined at 700 ℃ for 2 hrs. After cooling the molten salt mixture, the insoluble solids were removed from the lump of salts. The insoluble solids consisting of treated cBN particles and unreacted aluminum powder were washed well with distilled water and dried in a vacuum at 100℃ for 24 hrs. Final purification of the treated cBN was done by density centrifugation procedures with the heavy liquid of bromoform. Since the specific gravity of 4
ACCEPTED MANUSCRIPT bromoform is 2.89 which is a little higher than 2.7 the aluminum and much lower than 3.5 the cBN. Sample Characterization
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Powder samples were observed by SEM (S-4800, Hitachi) with electron diffraction and energy dispersive X-ray analysis (EDS). The phase compositions of the composite powders were identified using X-ray diffractometry (X’Pert PRO, PANalytical) the treated cBN
SC
powders were subjected to DSC (STA6000, PerkinElmer) analysis at a heating rate of
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5°C/min up to 800°C in the N2 atmosphere.
Results and Discussion
Characterization of cBN@ aluminum Core/Shell-type microparticles
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Figure 1 shows sample photograph after bromoform flotation purification in (a) and the XRD patterns of the cBN powders before and after Al coating preparation in (b). As shown in XRD curve (a), in the case of the as-prepared silica coated powders via sol–gel, peaks due to cBN only were observed, thus indicating that the silica coating is an amorphous phase. As shown in curve (b),
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after treated, aluminum and aluminum oxide peaks were seen to coexist with the cBN phase.
observed.
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Besides the evidence for the existence of small amounts of aluminum oxide silicate were
5
ACCEPTED MANUSCRIPT Fig. 1. XRD patterns of the treated cBN powders taken after (a) sol–gel reaction, (b) molten salt heat treatment
The SEM images in Figure 2 show the morphology of the treated cBN particles in various stages of their preparation. There are sharp edges and a cleavage fracture surface resulting from the
RI PT
crushing of cBN grit on the untreated cBN, shown in Figure 2(a), this is the typical morphology of the cBN powders of size of 3~5um. The cBN particles are covered with a silica nanolayer via sol-gel process, as seen in Figure 2(b), the morphology has little change. It's believed that is
SC
because the silica nanolayer is too thin. However, the morphology of the cBN change a lot after
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molten salt heat treatment, as can be seen in Figure 2(c), the cBN was covered by obvious shell. Energy dispersive X-ray spectroscopy revealed the presence of Al along with N, B, Si and O, suggesting that these small particles are coated with aluminum via silica bridging. (B)
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Fig. 2. SEM images showing the morphology of the coated layer on cBN surface. (a) uncoated cBN particle, (b) sol–gel coating silica nanolayer on cBN surface,(c) Aluminum coated cBN particles via silica bridging and (d) EDS result of the coated cBN particles.
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ACCEPTED MANUSCRIPT DSC and TG analyses were performed on the aluminum coated cBN composite powder in the temperature range from 25 to 800°C with a heating rate of 10°C /min in N2 atmosphere for the as-prepared coated cBN particles separated by Bromoform centrifugation and washed by Ethanol.
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In Figure 3 the first stage of slight mass loss can be ascribed to the evaporation of water and Ethanol until 300°C. No further mass loss can be observed in the next stage from 300 to 800°C. An endothermic peak at 267°C can be observed. It is reported that this may be due to Ethanol
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desorption. [21]
Figure 3. DSC/TGA curves of the cBN@ aluminum Core/Shell-type powder heated in N2.
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Possible Reaction Mechanism
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Figure 4 shows the effect of different heat treatments on the formation of metallic aluminum coating. As can be seen in Figure 4(a), under the ordinary heat treatment conditions described in the preceding, there was relatively uniform coating on the cBN surface. But if the silica modified cBN and aluminum were in direct contact without molten salt, as can be seen in Figure 4(b), the coating will not be formed, suggesting the molten salt plays a key role in this experiment.
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Fig. 4. SEM images of the surface silica modified cBN subject to heat treatment (a) with molten salt and (b) without molten salt
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We believe that the mechanism is based on the Brownian motion and electrostatic
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self-assembly. Brownian motion is the random motion of particles suspended in a fluid resulting from their collision with the fast-moving atoms or molecules in the gas or liquid.[22] The co-melting point of KCl-NaCl at 1: 1 molar ratio is 660°C. When the temperature rises to 700°C or higher, the Salt melted to form a conductive liquid environment. At the same time aluminum
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powder also melted, dispersed into liquid nano droplets and diffused via the Brownian motion. The cBN particles undergo random motions in the liquid environment of molten salt due to Brownian motion. Since the liquid molten salt is an electrolyte, it's conductive and the metallic
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aluminum is also conductive. Aluminum powder after melting, of which a small part of electron
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will be thermally activated into free electrons. The liquid molten salt is conductive, so some free electrons in the molten salt and dispersed liquid metal aluminum get free movement and become shared electronic cloud for both conductive molten salt and aluminum. As a result, due to a slight loss of electrons, small aluminum droplets will be charged with a positive charge. So small aluminum droplets will be charged with a positive charge. Further more，Weyl et al studied the solid surface structure, suggesting that the surface of solid is semi-saturated.[23] As for silica, the oxygen anion was pushed to the outside to form a negatively charged layer, which will adsorb 8
ACCEPTED MANUSCRIPT positively charged aluminum. Under the Brownian motion, both modified cBN and Aluminum was dispersed in a conductive liquid environment and kept in thermal motion. The surface silica modified cBN, its surface with a negative charge, adsorbing positively charged aluminum.
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The schematic diagram in Figures 5 shows the morphological variations of the coated layer during the preparation of Aluminum coated cBN particles via silica bridging by the
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composite process.
Fig. 5. Schematic diagram showing various stages in the preparation of Aluminum coated cBN powders via silica bridging.
Conclusion
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cBN powder was successfully coated with aluminum shell by two-step process. The first step was preparation of electronegative nano-silica bridging layer via sol-gel method. The second step was
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electropositive molten aluminum's self-assembly in fuse salt. It's believed that the mechanism is based on the Brownian motion and electrostatic self-assembly.
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Acknowledgement
This research was supported by grant (Code No. AA17204098) from Guangxi Innovation Driven Development Special Fund Project and grant (Code No.2016GXNSFBA380155) from Guangxi Natural Science Foundation References [1] M. Bindal, R. Nayar, S. Singhal , et al. High-pressure sintering of cubic boron nitride[J], 4347-4351. https://doi.org/10.1007/BF01106554
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Highlights The easily oxidized light metal Aluminum shell coated on the inert material microparticles in liquid salt of high temperature. Develop new Aluminum- cubic boron nitride core-shell composite materials. Wet chemical sol-gel method combined with the heat treatment of metals. Significant potentials in design novel functional light metal coatings.