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Effect of sintering temperature on synthesis of PCBN in cBN-Ti-Al-W system
Diamond & Related Materials 103 (2020) 107714
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Effect of sintering temperature on synthesis of PCBN in cBN-Ti-Al-W system☆
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Mo Peicheng , Chen Chao, Chen Jiarong, Jia Guang, Xie Delong, Xiao Leyin, Pan Xiaoyi, Lin Feng China Nonferrous Metal (Guilin) Geology and Mining Co, Ltd, Guilin 541004, Guangxi, China Guangxi Key Laboratory of Superhard Material, Guilin 541004, Guangxi, China National Engineering Research Center for Special Mineral Material, Guilin 541004, Guangxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: PCBN High-temperature ultrahigh pressure Sintering temperature Bonding agent
Polycrystalline cubic boron nitride (PCBN) was prepared under high-temperature and ultra high-pressure. The influence of sintering temperature on composition, micro-structure, relative density, porosity, micro-hardness and flexural strength of the PCBN was investigated by means of X-ray diffraction (XRD), field scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the phase components of PCBN have no significant difference, which are composed of BN, TiN, TiB2, W, AlN, W2B and Al3Ti when the sintering temperature at 1500 °C–1600 °C. The cBN crystals connect with each other by the reaction product. With the increase of sintering temperature, the porosity of PCBN decreased from 1.05% to 0.53%, and the relative density increased from 94.5% to 99%. When the sintering temperature increasing to 1600 °C, the PCBN exhibit optimal comprehensive mechanical properties with a micro-hardness of 34.64GPa and a flexural strength of 1005.23 MPa.
1. Introduction Cubic boron nitride (cBN) has an excellent thermal stability, chemically inert properties, and great hardness. It is widely used as tool material for cutting hardened steel, cast iron or other refractory materials [1–7]. It has become the most promising tool material after diamond, which fully compensates for the defects that diamond is unsuitable for cutting iron materials. Since cBN is composed of a highstrength covalent bonds, it is difficult to obtain a pure cBN sintered body even using high temperature and high pressure (HTHP) technology. Therefore, a synthesis method of polycrystalline cubic boron nitride (PCBN) is proposed by researchers, which combines cBN with an appropriate amount of binder at high temperature and high pressure (HTHP) conditions. The binder plays an important role in the synthesis of polycrystalline cubic boron nitride materials at high-temperature (above 1400 °C) and ultra high-pressure (> 4.5 GPa). The addition of an appropriate amount (5 wt%–45 wt%) of binder not only can reduce the sintering temperature and pressure, but also improves the properties of the sintered body. Conventionally, metal of the groups IVB, VB, and VIB of the periodic table or their compounds are commonly chosen as activated sintering agents, such as Al, Ti, W, Hf, et al. [8–13]. Aluminum is often
used as the binder in the sintering of PCBN products because of its low melting point. When the temperature is > 660 °C, Al melts into liquid phase to fill the gap between cBN crystals, and Al reacts with BN to produce ceramic phase AlN and AlB2, which will inhibit the transition of cBN to hBN phase, beneficial to the preparation of PCBN [14]. Ran Lv et al. [15] used 15 wt% Al and 20 wt% AlN as additives to sinter PCBN at 5.0 GPa and 1300–1700 °C. The results of the study indicate that a liquid phase sintering and reaction process were observed in the cBN Al system. The hardness decreases with increase of sintering temperature and reaches the highest Vickers hardness of 32.1 GPa at 1350 °C. Titanium is very reactive and easily reacts with cBN to form TiN and TiB2, the productions have high hardness and high-temperature stability, which can enhance the thermal stability, red hardness and fracture toughness of PCBN [16,17]. Yungang Yuan et al. [18] used the cBN-TiAl composite was prepared by spark plasma sintering. The composite with optimal mechanical properties was prepared for 1400 °C, and the relative density, the bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.4 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m0.5, respectively. Tungsten is a high melting point metal with a melting point of 3410 °C. It has good compatibility with other elements, and its thermal expansion coefficient is close to cBN. The expansion coefficient of W is 4.5*10−6 and the expansion
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Supported by Innovation-driven Development Special Fund Project of Guangxi Province, China (AA17204098). Corresponding author at: China Nonferrous Metal (Guilin) Geology and Mining Co, Ltd, Guilin 541004, Guangxi, China. E-mail address: [email protected] (M. Peicheng).
https://doi.org/10.1016/j.diamond.2020.107714 Received 16 August 2019; Received in revised form 13 January 2020; Accepted 15 January 2020 Available online 16 January 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved.
Diamond & Related Materials 103 (2020) 107714
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coefficient of cBN is 4.7*10−6. There is no direct reaction to W and cBN. Only when B* in cBN is replaced by Al, W will combine with B*, so the addition of W will not cause the decrease of cBN content. W is a high melting point metal that does not cause the PCBN tool to soften during the cutting process. The addition of tungsten can significantly reduce the expansion coefficient of the binder, reduce thermal stress, and reduce the occurrence of cracks in the sample. In this paper, Ti-Al-W was added as a binder, and the binder melted into a liquid state under high-temperature and high-pressure, and slipped and flowed into the gap between the cBN crystals, and reacted with cBN to form a dense sintered body. Reaction mechanisms of cBNTi-Al-W system as well as the morphology, fracture mode, chemical composition and mechanical properties of the sintered PCBN samples were studied.
conditions. The relative density and porosity of PCBN samples were determined by the Archimedes drainage method. The microstructure, grain morphology and cross section of PCBN were characterized by field emission scanning electron micros-cope (FSEM) (S-4800, Hitachi High-Technologies Corpora- tion/Oxford Instruments, Japan/England), and the composition was analyzed by energy-dispersive spectrometry (EDS). The microhardness was measured by microhardness tester (MH6), with 3 kg of extrinsic load and a 15 s dwell time, and five positions on surfaces were respectively tested to determine the microhardness average value. The three-point bending strength was measured by universal material testing machine (model AG-1 50KN), the sample size was φ13 mm*6 mm, the span was 10 mm, and the loading speed was 0.5 mm/min. 3. Results and discussion
2. Experimental
3.1. X-ray diffraction analysis
2.1. Sample preparation
Fig. 2 shows the typical XRD patterns of PCBN samples at different temperatures (sintering time of 10 min and sintering pressure of 5.5 GPa) by cBN-Al-Ti-W system. According to Fig. 2, the major phases detected by XRD were BN、TiN, TiB2, W, W2B5, AlN and Al3Ti at different sintering temperatures. When the temperature increased to 1500 °C, the W2B new phase was detected. W2B has a high melting point, good metal conductivity and high hardness. It is often used for wear-resistant coating of wear parts, so the formation of W2B will not have a negative impact on the performance of PCBN. Except for BN and W, the rest of the materials are produced during the sintering process. According to the phase evolution of the samples analyzed above, we may speculate the possible reaction mechanism among cBN, Al, Ti and W. And all of Ti and Al are completely reacted and W partly reacts. The possible chemical reactions are as follows [19–22]:
In this experiment, cBN powders (4–8 μm, > 99.9 wt% purity), Ti powders (3–6 μm, > 99.5 wt% purity), and Al powder (3–6 μm, > 99.8 wt% purity), W powder (3–6 μm, > 99.9 wt% purity) were used as the initial materials. The mass percentages of the raw materials were 60 wt% cBN, 25 wt% Ti, 9 wt% Al, and 6 wt% W, respectively. In a typical experiment, the powders were mixed and ground in a N-heptane medium for 2 h using a stainless steel tank. Then, the mixtures were dried in an oven for 6 h at 120 °C. After cooling to room temperature, the mixtures were screened through a 100-mesh sieve. Next, the mixture was packed into the cylindrical shell made of molybdenum cup and then cold pressed. And then dried in high-temperature vacuum atmosphere to get rid of the adsorption such as vapors, oxygen and n-heptane on the powder surface, with the pressure of the 1.0× 10−3 Pa and temperature of 960 °C for 1 h. The vacuum treated samples were chilled into blocks and assembled according to Fig. 1. The sintering parameters were set as follows: the sintering pressure (5.5 GPa), the sintering temperature (1350 °C–1450 °C–1500 °C–1550 °C–1600 °C), and the sintering holding time (600 s). 2.2. Sample polishing and characterisation
3Al + Ti = Al3 Ti
(1)
Al + 4Ti + 2BN = AlTi2 N + TiB2 + TiN
(2)
Al + BN = AlN + B∗
(3)
3Ti + 2BN = 2TiN + TiB2
(4)
2W + B ∗ = W2 B
(5)
2W + 5B ∗ = W2 B5
(6)
Table 1 shows the diffraction peak intensities and Diffraction full width at half maximum (FWHM) of cBN, TiB2, W and W2B in PCBN at different sintering temperatures. It can be seen from the table that as the sintering temperature increases, the intensity of the TiB2 (101) crystal plane and the W2B (211) crystal plane increase continuously, indicating that the amount of TiB2 and W2B is increasing. The intensity of the diffraction peaks of the BN (111) crystal plane and the W (200) crystal plane gradually decreased, indicating that the amounts of BN and W were decreasing. The diffraction peak half height width of the TiB2 (101) crystal plane and the W (200) crystal plane is continuously reduced. The smaller half height width and the sharper the peak, the better the crystal crystallinity. The diffraction peak half height width of the BN (111) crystal plane and W2B (211) crystal planes increases continuously.
The well-sintered PCBN samples were polished into a mirror surface on a diamond automatic polishing machine, with a 0.5–5 μm diamond pastes. Using XRD analysis (X'Pert PRO, PANalytical, Netherlands) to investigate the phase composition of the samples sintered under various
3.2. Porosity and relative density analysis Fig. 3 shows the porosity and relative density of the PCBN sintered body at different sintering temperatures. We can know from the Fig. 3 that the relative density of the sample increases with the increase of temperature, and the porosity decreases with the increase of temperature. It can be seen from the figure that when the sintering temperature is 1600 °C, the porosity of PCBN is only 0.534%, and the relative density reaches 99%. Because of the low sintering temperature, the
Fig. 1. The cell assembly for HPHT sintering experiments. 1-steel ring, 2-pyrophyllitic, 3-titanium flake, 4-graphite furnace, 5-mixed powders of NaCl and C, 6-magnesium capsule, 7-molybdenum capsule, 8-thermocouple, 9-sample. 2
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Fig. 2. XRD patterns of the PCBN prepared at different sintering temperatures.
pressure, with the solid titanium and Al became liquid phase, the liquid titanium and Al underwent flow mass transfer in the system, and are filled between the cBN and cBN crystals, accelerated the chemical reaction of titanium and BN, and generated high strength and high hardness TiN, TiB2 and AlN, which firmly bond cBN crystals to reduce the generation of pores. When the temperature increases, the shrinkage of the sintered body increases, and the bond between the crystals inside the sample becomes better and better, so that the formed PCBN phase distribution is more uniform and the structure is more compact. However, if the sintering temperature is too high, the diffusion coefficient of the mass transfer atoms will become large, which will cause the rapid movement of the crystals boundaries and cause the crystal to grow abnormally, which is not conducive to the sintering and densification of the PCBN.
Table 1 Intensity of diffraction peaks and full width at half maximum (FWHM) values of cBN, TiB2, W and W2B in PcBN prepared at different sintering temperatures. T(°C)
1350 1450 1500 1550 1600
Diffraction intensity
FWHM
cBN
TiB2
W
W2 B
cBN
TiB2
W
W2B
1718 1517 1545 1574 1423
249 318 399 525 873
1569 1167 1110 1025 516
– – 83 75 137
0.256 0.271 0.268 0.283 0.283
0.292 0.284 0.272 0.264 0.259
0.387 0.345 0.332 0.273 0.229
– – 0.069 0.139 0.214
1.1
1.00
Porosity% Relative density%
1.0
3.3. Microstructure analysis
Porosity,%
0.98
0.8 0.7 0.96 0.6
Relative density,%
0.9
Fig. 4 shows the BSEM images of polished PCBN samples at different sintering temperatures (e) 1350 °C (f) 1500 °C. From the Fig. 4, cBN particles are distributed homogeneously indicating good bonding with the binder. More pores can be observed (see Fig. 4e) in samples of low sintering temperatures. At low temperatures, the TiB2 rod-shaped crystals produced are long and easily support each other to form voids, which is advantageous for the generation of pores at relatively low temperatures. When the temperature rises to 1500 °C, the aspect ratio of TiB2 rod-shaped crystals decreases, the surface porosity of the sample decreases, and the relative density increases. Fig. 5 shows the results of PCBN microstructure and the corresponding EDS analysis of cBN-Ti-Al-W system synthesized at different sintering temperatures. Fig. 5(a), (b), and (c) are typical fracture surface images of PCBN synthesized at 1350 °C, 1500 °C and 1600 °C corroded by HF. The intermetallic (Ti3Al) and glass phase are usually easily corroded in HF. The cBN, W, TiB2, AlN, TiN, W2B5 and W2B are hardly corroded. As can be seen from Fig. 5, the angle of cBN becomes much round, as indicated by the elliptical dotted line in the figure, indicating that a significant chemical reaction occurs with the binder on the surface of the cBN crystals. From the Fig. 5, there are some rodshaped crystals and fibrous crystals with long diameters are observed,
0.5 1350
1400
1450
1500
1550
1600
0.94
Sintering temperature,ć Fig. 3. Relative density and porosity of the PCBN prepared at different sintering temperatures.
binder may not permeate into the cBN crystals or fully react. When the sintering temperature is between 1500 °C and 1600 °C, the variation of porosity and relative density is small, and PCBN with good compactness can be obtained in this temperature range. It shows that the increase of sintering temperature has a promoting effect on the compactness of PCBN. Under the condition of high-temperature and ultra-high 3
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Fig. 4. BSEM images of polished PCBN samples at different sintering temperatures (e) 1350 °C (f) 1500 °C.
atoms increases, W begins to combine with B to form W2B, which consumes part of B atoms. Therefore, the amount of Ti-Al-B in the system can still be maintained in a state in which rod crystals can be formed, but the amount and the aspect ratio of the rod crystals are significantly lower than the amount and the aspect ratio of the rod crystals at 1350 °C. When the temperature rises, the rod-shaped crystal aspect ratio decreases, the liquid phase flow accelerates, and the reaction between the binder and cBN is accelerated, and the product is filled in the pores to facilitate densification.
which all extend from the boundary portion of cBN, indicating that the formation of these crystals is related to cBN. When the temperature is raised to 1500 °C and 1600 °C, the aspect ratio of the rod-shaped crystals and fibrous crystals formed is significantly < 1350 °C. Moreover, fibrous crystals gradually disappeared and transformed into rodshaped crystals, and the appearance of equiaxed crystals was also found in the scanning images at 1500 °C and 1600 °C. To elucidate the compositions of rod-shaped crystals (A in Fig. 5.c) and equiaxed crystals (B in Fig. 5.c), EDS data was researched carefully. The results are shown in Fig. 5.e. Combined with XRD analysis, it was found that the rodshaped crystals and equiaxed crystals were TiB2 and TiN, respectively. According to the Ti-Al-B ternary alloy phase diagram [23], TieB rod crystal is formed in a liquid phase in which the content of B is very low and the contents of Al and Ti are much higher. The B atom is displaced by the reaction of Al with cBN, and a hexagonal phase of AlN [24] is formed. Therefore, at 1350 °C, due to the sintering temperature is low, the chemical reaction between Al and BN is not completely performed, and the content of B atoms in the material system is low, so that many rod-shaped crystals and fibrous crystals are formed at this time. The rod-shaped crystals formed at a low temperature have a long length and support each other to form pores, which is favorable for the generation of pores. As the sintering temperature further increased, the reaction of Al, Ti and cBN accelerated, the content of B atoms began to increase, and the content of Al and Ti began to decrease. As the content of B
3.4. Mechanical performance analysis The micro-hardness and flexural strength of the PCBN samples sintered at different temperature are shown in Fig. 6. The micro-hardness of the PCBN increased with the increase in temperature. We can know from the Fig. 6 that when the temperature increases from 1350 °C to 1600 °C, the hardness of the sample from 28.35 GPa to 34.64 GPa. Hardness of the PCBN sintered body is related to the phase component content and degree of crystallization [25]. The higher the degree of crystallization, the greater the hardness of the material. The atomic spacing is reduced by the combination of temperature and pressure during the sintering process. The glass phase has good fluidity under high temperature conditions and promotes the flow of the binder in the system, and the temperature rise is favorable for the crystallization of
Fig. 5. The fracture surface images of the PCBN prepared at different sintering temperatures (a) 1350 °C, (b) 1500 °C, (c) 1600 °C and the correspond EDS results (e) under ultrahigh pressure of 5.5 GPa. 4
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36
Eγ σf = A ⎛ ⎞ 1/2 ⎝ c ⎠
34
where: γ represents the fracture energy; σf represents the fracture stress; c represents the defect size; E represents the elastic modulus; A is a constant, meaningless, depending on the geometry of the sample and the defect. It can be seen from the formula that the fracture strength of the material is greatly affected by the defect size. When the elastic modulus and fracture energy are constant, the larger the defect size, the smaller the fracture strength. When the sintering temperature is 1350 °C, sample strength is only 673.54 MPa, which is due to the existence of a large number of pores inside the PCBN sample during low temperature sintering, poor compactness and loose microstructure. Although there are fibrous and rod-shaped crystals in the system, the amount of rod-shaped crystals is small, and the fibrous crystals are fine, and the load of the entire sample cannot be carried under a large stress, and cracking easily occurs. When the sintering temperature is further increased, the liquid phase formed in the system is filled with the internal pores, the internal structure tends to be dense, the defect size is reduced, and the bending strength is greatly improved. Fig. 7 is a graph of the uncorroded fracture of the sample at 1450 °C and 1600 °C. It can be seen from the Fig. 7 that the shape of the cBN crystals is obvious, the cBN reacts with the binder uniformly distributed around it, and the product acts as a binder phase to firmly bond the cBN crystals together to form a densified structure. The binder was completely melted and not dispersed binder particles were seen. The interface between the cBN particles and the molten binder was clearly visible in the figure. Obvious crystals boundaries and smooth crystal faces can be observed in the figure, indicating that there is an intergranular fracture in each sample. There is also the rod crystals pull-out in the fracture process (the dotted line in the graphs m and n). At the fracture of the 1600 °C sample, obvious jagged crystals (the active line in the graphs m and n) were also observed, indicating that the interface bonding strength between the cBN and the binder was strong. The occurrence of transgranular fracture and intergranular fracture phenomenon is accompanied that the fracture strength of PCBN is significantly improved. There is large amount of rod-like crystals in the binder, and when the material propagates to the rod-like crystal during the fracture, the rod-like crystal has high elastic modulus and strength without broken. When the rod crystal is pulled out from the matrix or the crack propagates around the rod crystal, the crack propagation path is increased. This greatly consumes the energy of crack propagation, thereby increasing the toughness of the material. In this work, since the rod crystal acts like fiber and whisker toughening, it can enhance PCBN by rod pull-out, bridging and crack deformation.
Flexural strength,MPa Hardnss,GPa
900 32 800 30
Hardness,GPa
Flexural strength,MPa
1000
700 28 1350
1400
1450
1500
1550
1600
Sintering temperature,ć Fig. 6. The mechanical properties of the PCBN prepared at different sintering temperatures under ultrahigh pressure of 5.5 GPa.
the glass phase. Since the same formulation is sintered at different temperatures, there is no significant difference in the phase composition of PCBN at different sintering temperatures, which were proved by the XRD results. However, due to the large difference in hardness between the phases of the sintered body, the micro-hardness of the ceramic material, TiB2, was 36 GPa [26,27], which is barely less than that of the cBN in the PCBN. So content and crystallinity of TiB2 are a major factor affecting the micro-hardness of the PCBN. According to the results of XRD analysis, the TiB2 diffraction peak becomes more and more sharply as the sintering temperature increases, indicating that the content of TiB2 increased and the degree of crystallization becomes better. Therefore, the hardness of the sintered body of PCBN gradually increases, and the micro-hardness was 34.64 GPa at 1600 °C. It can be seen from Fig. 5 that the flexural strength of PCBN increased from 673.54 MPa to 1005.23 MPa with the increase of temperature. Research shows that the flexural strength of ceramic materials is related to these factors such as porosity, maximum defect size and grain diameter [28]. The porosity itself as a defect may become the most dangerous crack inside. From the results of the porosity test, it is known that when the sintering temperature is 1350 °C. There are many pores inside the PCBN. The presence of pores hinders the connection of the reinforcing phase to the matrix, reduces the effective cross-sectional area of the applied load, reduces the stress that the material can withstand, and cracks are easily generated at the pores, causing the material to fracture under lower stresses [29,30]. According to the Griffith fracture theory, the fracture strength of a material has the following relationship with the defect size [31]:
Fig. 7. Uncorroded fracture profile of the sample at 1450 °C and 1600 °C. 5
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4. Conclusion
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In this paper, the effect of sintering temperature on the synthesis of PCBN in cBN-Ti-Al-W system was studied. The composition, morphology, fracture mode, mechanical properties etc., were investigated and analyzed in detail. As the sintering temperature increases, the porosity of PCBN decreases significantly and the relative density increases. When the temperature increased to 1600 °C, the composition of PCBN consisted of BN, TiN, TiB2, W, W2B5, AlN, Al3Ti and W2B, which were distributed homogeneously. The mechanical properties of PCBN are best at 1600 °C, the microhardness and the flexural strength are 34.64 GPa and 1005.23 MPa, respectively. The fracture of PCBN is the result of the interaction of crystal fracture and transgranular fracture. CRediT authorship contribution statement Mo Peicheng:Conceptualization, Methodology, Data curation, Writing - original draft, Writing - review & editing, Visualization.Chen Chao:Supervision, Project administration, Funding acquisition.Chen Jiarong:Validation, Formal analysis, Investigation.Jia Guang:Formal analysis, Investigation.Xie Delong:Formal analysis, Investigation.Xiao Leyin:Formal analysis, Investigation.Pan Xiaoyi:Formal analysis, Investigation.Lin Feng:Supervision, Project administration, Funding acquisition. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgments This study was financially supported by the Innovation-driven Development Special Fund Project of Guangxi Province, China (AA17204098). References [1] Y.B. Zhao, X.H. Peng, T. Fu, C. Huang, C. Feng, D.E. Yin, Z.C. Wang, Molecular dynamics simulation of nano-indentation of (111) cubic boron nitride with optimized tersoff potential [J], Appl. Surf. Sci. 382 (2016) (2016) 309–315. [2] X. Yang, Q.L. Ye, Synthesis of high-quality octahedral cBN crystals with large size using lithium metal as a catalyst [J], J. Alloys Compd. 580 (12) (2013) 1–4. [3] Y.B. Li, H.X. Jiang, G.Z. Yuan, A.L. Chen, X. Wang, T.G. Dai, H.S. Yang, Electronic structure and impurity states of S-doped cBN: a first-principle study [J], J. Alloys Compd. 531 (2012) 82–85. [4] G.Z. Wang, Summarization about the peculiarities of cBN [J], Superhard Mater. Eng. 17 (5) (2005) 41–45. [5] J. Angseryd, M. Elfwing, E. Olsson, H.O. Andren, Detailed microstructure of a cBN based cutting tool material [J], Int. J. Refract. Met. Hard Mater. 27 (2) (2009) 249–255. [6] Y.F. Wang, High Toughness Polycrystalline Cubic Boron Nitride (PcBN) Compound Preparation Technology and Performance Study [D], Henan university of technology, Henan, 2012.
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Effect of sintering temperature on synthesis of PCBN in cBN-Ti-Al-W system☆
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Mo Peicheng , Chen Chao, Chen Jiarong, Jia Guang, Xie Delong, Xiao Leyin, Pan Xiaoyi, Lin Feng China Nonferrous Metal (Guilin) Geology and Mining Co, Ltd, Guilin 541004, Guangxi, China Guangxi Key Laboratory of Superhard Material, Guilin 541004, Guangxi, China National Engineering Research Center for Special Mineral Material, Guilin 541004, Guangxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: PCBN High-temperature ultrahigh pressure Sintering temperature Bonding agent
Polycrystalline cubic boron nitride (PCBN) was prepared under high-temperature and ultra high-pressure. The influence of sintering temperature on composition, micro-structure, relative density, porosity, micro-hardness and flexural strength of the PCBN was investigated by means of X-ray diffraction (XRD), field scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the phase components of PCBN have no significant difference, which are composed of BN, TiN, TiB2, W, AlN, W2B and Al3Ti when the sintering temperature at 1500 °C–1600 °C. The cBN crystals connect with each other by the reaction product. With the increase of sintering temperature, the porosity of PCBN decreased from 1.05% to 0.53%, and the relative density increased from 94.5% to 99%. When the sintering temperature increasing to 1600 °C, the PCBN exhibit optimal comprehensive mechanical properties with a micro-hardness of 34.64GPa and a flexural strength of 1005.23 MPa.
1. Introduction Cubic boron nitride (cBN) has an excellent thermal stability, chemically inert properties, and great hardness. It is widely used as tool material for cutting hardened steel, cast iron or other refractory materials [1–7]. It has become the most promising tool material after diamond, which fully compensates for the defects that diamond is unsuitable for cutting iron materials. Since cBN is composed of a highstrength covalent bonds, it is difficult to obtain a pure cBN sintered body even using high temperature and high pressure (HTHP) technology. Therefore, a synthesis method of polycrystalline cubic boron nitride (PCBN) is proposed by researchers, which combines cBN with an appropriate amount of binder at high temperature and high pressure (HTHP) conditions. The binder plays an important role in the synthesis of polycrystalline cubic boron nitride materials at high-temperature (above 1400 °C) and ultra high-pressure (> 4.5 GPa). The addition of an appropriate amount (5 wt%–45 wt%) of binder not only can reduce the sintering temperature and pressure, but also improves the properties of the sintered body. Conventionally, metal of the groups IVB, VB, and VIB of the periodic table or their compounds are commonly chosen as activated sintering agents, such as Al, Ti, W, Hf, et al. [8–13]. Aluminum is often
used as the binder in the sintering of PCBN products because of its low melting point. When the temperature is > 660 °C, Al melts into liquid phase to fill the gap between cBN crystals, and Al reacts with BN to produce ceramic phase AlN and AlB2, which will inhibit the transition of cBN to hBN phase, beneficial to the preparation of PCBN [14]. Ran Lv et al. [15] used 15 wt% Al and 20 wt% AlN as additives to sinter PCBN at 5.0 GPa and 1300–1700 °C. The results of the study indicate that a liquid phase sintering and reaction process were observed in the cBN Al system. The hardness decreases with increase of sintering temperature and reaches the highest Vickers hardness of 32.1 GPa at 1350 °C. Titanium is very reactive and easily reacts with cBN to form TiN and TiB2, the productions have high hardness and high-temperature stability, which can enhance the thermal stability, red hardness and fracture toughness of PCBN [16,17]. Yungang Yuan et al. [18] used the cBN-TiAl composite was prepared by spark plasma sintering. The composite with optimal mechanical properties was prepared for 1400 °C, and the relative density, the bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.4 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m0.5, respectively. Tungsten is a high melting point metal with a melting point of 3410 °C. It has good compatibility with other elements, and its thermal expansion coefficient is close to cBN. The expansion coefficient of W is 4.5*10−6 and the expansion
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Supported by Innovation-driven Development Special Fund Project of Guangxi Province, China (AA17204098). Corresponding author at: China Nonferrous Metal (Guilin) Geology and Mining Co, Ltd, Guilin 541004, Guangxi, China. E-mail address: [email protected] (M. Peicheng).
https://doi.org/10.1016/j.diamond.2020.107714 Received 16 August 2019; Received in revised form 13 January 2020; Accepted 15 January 2020 Available online 16 January 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved.
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coefficient of cBN is 4.7*10−6. There is no direct reaction to W and cBN. Only when B* in cBN is replaced by Al, W will combine with B*, so the addition of W will not cause the decrease of cBN content. W is a high melting point metal that does not cause the PCBN tool to soften during the cutting process. The addition of tungsten can significantly reduce the expansion coefficient of the binder, reduce thermal stress, and reduce the occurrence of cracks in the sample. In this paper, Ti-Al-W was added as a binder, and the binder melted into a liquid state under high-temperature and high-pressure, and slipped and flowed into the gap between the cBN crystals, and reacted with cBN to form a dense sintered body. Reaction mechanisms of cBNTi-Al-W system as well as the morphology, fracture mode, chemical composition and mechanical properties of the sintered PCBN samples were studied.
conditions. The relative density and porosity of PCBN samples were determined by the Archimedes drainage method. The microstructure, grain morphology and cross section of PCBN were characterized by field emission scanning electron micros-cope (FSEM) (S-4800, Hitachi High-Technologies Corpora- tion/Oxford Instruments, Japan/England), and the composition was analyzed by energy-dispersive spectrometry (EDS). The microhardness was measured by microhardness tester (MH6), with 3 kg of extrinsic load and a 15 s dwell time, and five positions on surfaces were respectively tested to determine the microhardness average value. The three-point bending strength was measured by universal material testing machine (model AG-1 50KN), the sample size was φ13 mm*6 mm, the span was 10 mm, and the loading speed was 0.5 mm/min. 3. Results and discussion
2. Experimental
3.1. X-ray diffraction analysis
2.1. Sample preparation
Fig. 2 shows the typical XRD patterns of PCBN samples at different temperatures (sintering time of 10 min and sintering pressure of 5.5 GPa) by cBN-Al-Ti-W system. According to Fig. 2, the major phases detected by XRD were BN、TiN, TiB2, W, W2B5, AlN and Al3Ti at different sintering temperatures. When the temperature increased to 1500 °C, the W2B new phase was detected. W2B has a high melting point, good metal conductivity and high hardness. It is often used for wear-resistant coating of wear parts, so the formation of W2B will not have a negative impact on the performance of PCBN. Except for BN and W, the rest of the materials are produced during the sintering process. According to the phase evolution of the samples analyzed above, we may speculate the possible reaction mechanism among cBN, Al, Ti and W. And all of Ti and Al are completely reacted and W partly reacts. The possible chemical reactions are as follows [19–22]:
In this experiment, cBN powders (4–8 μm, > 99.9 wt% purity), Ti powders (3–6 μm, > 99.5 wt% purity), and Al powder (3–6 μm, > 99.8 wt% purity), W powder (3–6 μm, > 99.9 wt% purity) were used as the initial materials. The mass percentages of the raw materials were 60 wt% cBN, 25 wt% Ti, 9 wt% Al, and 6 wt% W, respectively. In a typical experiment, the powders were mixed and ground in a N-heptane medium for 2 h using a stainless steel tank. Then, the mixtures were dried in an oven for 6 h at 120 °C. After cooling to room temperature, the mixtures were screened through a 100-mesh sieve. Next, the mixture was packed into the cylindrical shell made of molybdenum cup and then cold pressed. And then dried in high-temperature vacuum atmosphere to get rid of the adsorption such as vapors, oxygen and n-heptane on the powder surface, with the pressure of the 1.0× 10−3 Pa and temperature of 960 °C for 1 h. The vacuum treated samples were chilled into blocks and assembled according to Fig. 1. The sintering parameters were set as follows: the sintering pressure (5.5 GPa), the sintering temperature (1350 °C–1450 °C–1500 °C–1550 °C–1600 °C), and the sintering holding time (600 s). 2.2. Sample polishing and characterisation
3Al + Ti = Al3 Ti
(1)
Al + 4Ti + 2BN = AlTi2 N + TiB2 + TiN
(2)
Al + BN = AlN + B∗
(3)
3Ti + 2BN = 2TiN + TiB2
(4)
2W + B ∗ = W2 B
(5)
2W + 5B ∗ = W2 B5
(6)
Table 1 shows the diffraction peak intensities and Diffraction full width at half maximum (FWHM) of cBN, TiB2, W and W2B in PCBN at different sintering temperatures. It can be seen from the table that as the sintering temperature increases, the intensity of the TiB2 (101) crystal plane and the W2B (211) crystal plane increase continuously, indicating that the amount of TiB2 and W2B is increasing. The intensity of the diffraction peaks of the BN (111) crystal plane and the W (200) crystal plane gradually decreased, indicating that the amounts of BN and W were decreasing. The diffraction peak half height width of the TiB2 (101) crystal plane and the W (200) crystal plane is continuously reduced. The smaller half height width and the sharper the peak, the better the crystal crystallinity. The diffraction peak half height width of the BN (111) crystal plane and W2B (211) crystal planes increases continuously.
The well-sintered PCBN samples were polished into a mirror surface on a diamond automatic polishing machine, with a 0.5–5 μm diamond pastes. Using XRD analysis (X'Pert PRO, PANalytical, Netherlands) to investigate the phase composition of the samples sintered under various
3.2. Porosity and relative density analysis Fig. 3 shows the porosity and relative density of the PCBN sintered body at different sintering temperatures. We can know from the Fig. 3 that the relative density of the sample increases with the increase of temperature, and the porosity decreases with the increase of temperature. It can be seen from the figure that when the sintering temperature is 1600 °C, the porosity of PCBN is only 0.534%, and the relative density reaches 99%. Because of the low sintering temperature, the
Fig. 1. The cell assembly for HPHT sintering experiments. 1-steel ring, 2-pyrophyllitic, 3-titanium flake, 4-graphite furnace, 5-mixed powders of NaCl and C, 6-magnesium capsule, 7-molybdenum capsule, 8-thermocouple, 9-sample. 2
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Fig. 2. XRD patterns of the PCBN prepared at different sintering temperatures.
pressure, with the solid titanium and Al became liquid phase, the liquid titanium and Al underwent flow mass transfer in the system, and are filled between the cBN and cBN crystals, accelerated the chemical reaction of titanium and BN, and generated high strength and high hardness TiN, TiB2 and AlN, which firmly bond cBN crystals to reduce the generation of pores. When the temperature increases, the shrinkage of the sintered body increases, and the bond between the crystals inside the sample becomes better and better, so that the formed PCBN phase distribution is more uniform and the structure is more compact. However, if the sintering temperature is too high, the diffusion coefficient of the mass transfer atoms will become large, which will cause the rapid movement of the crystals boundaries and cause the crystal to grow abnormally, which is not conducive to the sintering and densification of the PCBN.
Table 1 Intensity of diffraction peaks and full width at half maximum (FWHM) values of cBN, TiB2, W and W2B in PcBN prepared at different sintering temperatures. T(°C)
1350 1450 1500 1550 1600
Diffraction intensity
FWHM
cBN
TiB2
W
W2 B
cBN
TiB2
W
W2B
1718 1517 1545 1574 1423
249 318 399 525 873
1569 1167 1110 1025 516
– – 83 75 137
0.256 0.271 0.268 0.283 0.283
0.292 0.284 0.272 0.264 0.259
0.387 0.345 0.332 0.273 0.229
– – 0.069 0.139 0.214
1.1
1.00
Porosity% Relative density%
1.0
3.3. Microstructure analysis
Porosity,%
0.98
0.8 0.7 0.96 0.6
Relative density,%
0.9
Fig. 4 shows the BSEM images of polished PCBN samples at different sintering temperatures (e) 1350 °C (f) 1500 °C. From the Fig. 4, cBN particles are distributed homogeneously indicating good bonding with the binder. More pores can be observed (see Fig. 4e) in samples of low sintering temperatures. At low temperatures, the TiB2 rod-shaped crystals produced are long and easily support each other to form voids, which is advantageous for the generation of pores at relatively low temperatures. When the temperature rises to 1500 °C, the aspect ratio of TiB2 rod-shaped crystals decreases, the surface porosity of the sample decreases, and the relative density increases. Fig. 5 shows the results of PCBN microstructure and the corresponding EDS analysis of cBN-Ti-Al-W system synthesized at different sintering temperatures. Fig. 5(a), (b), and (c) are typical fracture surface images of PCBN synthesized at 1350 °C, 1500 °C and 1600 °C corroded by HF. The intermetallic (Ti3Al) and glass phase are usually easily corroded in HF. The cBN, W, TiB2, AlN, TiN, W2B5 and W2B are hardly corroded. As can be seen from Fig. 5, the angle of cBN becomes much round, as indicated by the elliptical dotted line in the figure, indicating that a significant chemical reaction occurs with the binder on the surface of the cBN crystals. From the Fig. 5, there are some rodshaped crystals and fibrous crystals with long diameters are observed,
0.5 1350
1400
1450
1500
1550
1600
0.94
Sintering temperature,ć Fig. 3. Relative density and porosity of the PCBN prepared at different sintering temperatures.
binder may not permeate into the cBN crystals or fully react. When the sintering temperature is between 1500 °C and 1600 °C, the variation of porosity and relative density is small, and PCBN with good compactness can be obtained in this temperature range. It shows that the increase of sintering temperature has a promoting effect on the compactness of PCBN. Under the condition of high-temperature and ultra-high 3
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Fig. 4. BSEM images of polished PCBN samples at different sintering temperatures (e) 1350 °C (f) 1500 °C.
atoms increases, W begins to combine with B to form W2B, which consumes part of B atoms. Therefore, the amount of Ti-Al-B in the system can still be maintained in a state in which rod crystals can be formed, but the amount and the aspect ratio of the rod crystals are significantly lower than the amount and the aspect ratio of the rod crystals at 1350 °C. When the temperature rises, the rod-shaped crystal aspect ratio decreases, the liquid phase flow accelerates, and the reaction between the binder and cBN is accelerated, and the product is filled in the pores to facilitate densification.
which all extend from the boundary portion of cBN, indicating that the formation of these crystals is related to cBN. When the temperature is raised to 1500 °C and 1600 °C, the aspect ratio of the rod-shaped crystals and fibrous crystals formed is significantly < 1350 °C. Moreover, fibrous crystals gradually disappeared and transformed into rodshaped crystals, and the appearance of equiaxed crystals was also found in the scanning images at 1500 °C and 1600 °C. To elucidate the compositions of rod-shaped crystals (A in Fig. 5.c) and equiaxed crystals (B in Fig. 5.c), EDS data was researched carefully. The results are shown in Fig. 5.e. Combined with XRD analysis, it was found that the rodshaped crystals and equiaxed crystals were TiB2 and TiN, respectively. According to the Ti-Al-B ternary alloy phase diagram [23], TieB rod crystal is formed in a liquid phase in which the content of B is very low and the contents of Al and Ti are much higher. The B atom is displaced by the reaction of Al with cBN, and a hexagonal phase of AlN [24] is formed. Therefore, at 1350 °C, due to the sintering temperature is low, the chemical reaction between Al and BN is not completely performed, and the content of B atoms in the material system is low, so that many rod-shaped crystals and fibrous crystals are formed at this time. The rod-shaped crystals formed at a low temperature have a long length and support each other to form pores, which is favorable for the generation of pores. As the sintering temperature further increased, the reaction of Al, Ti and cBN accelerated, the content of B atoms began to increase, and the content of Al and Ti began to decrease. As the content of B
3.4. Mechanical performance analysis The micro-hardness and flexural strength of the PCBN samples sintered at different temperature are shown in Fig. 6. The micro-hardness of the PCBN increased with the increase in temperature. We can know from the Fig. 6 that when the temperature increases from 1350 °C to 1600 °C, the hardness of the sample from 28.35 GPa to 34.64 GPa. Hardness of the PCBN sintered body is related to the phase component content and degree of crystallization [25]. The higher the degree of crystallization, the greater the hardness of the material. The atomic spacing is reduced by the combination of temperature and pressure during the sintering process. The glass phase has good fluidity under high temperature conditions and promotes the flow of the binder in the system, and the temperature rise is favorable for the crystallization of
Fig. 5. The fracture surface images of the PCBN prepared at different sintering temperatures (a) 1350 °C, (b) 1500 °C, (c) 1600 °C and the correspond EDS results (e) under ultrahigh pressure of 5.5 GPa. 4
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36
Eγ σf = A ⎛ ⎞ 1/2 ⎝ c ⎠
34
where: γ represents the fracture energy; σf represents the fracture stress; c represents the defect size; E represents the elastic modulus; A is a constant, meaningless, depending on the geometry of the sample and the defect. It can be seen from the formula that the fracture strength of the material is greatly affected by the defect size. When the elastic modulus and fracture energy are constant, the larger the defect size, the smaller the fracture strength. When the sintering temperature is 1350 °C, sample strength is only 673.54 MPa, which is due to the existence of a large number of pores inside the PCBN sample during low temperature sintering, poor compactness and loose microstructure. Although there are fibrous and rod-shaped crystals in the system, the amount of rod-shaped crystals is small, and the fibrous crystals are fine, and the load of the entire sample cannot be carried under a large stress, and cracking easily occurs. When the sintering temperature is further increased, the liquid phase formed in the system is filled with the internal pores, the internal structure tends to be dense, the defect size is reduced, and the bending strength is greatly improved. Fig. 7 is a graph of the uncorroded fracture of the sample at 1450 °C and 1600 °C. It can be seen from the Fig. 7 that the shape of the cBN crystals is obvious, the cBN reacts with the binder uniformly distributed around it, and the product acts as a binder phase to firmly bond the cBN crystals together to form a densified structure. The binder was completely melted and not dispersed binder particles were seen. The interface between the cBN particles and the molten binder was clearly visible in the figure. Obvious crystals boundaries and smooth crystal faces can be observed in the figure, indicating that there is an intergranular fracture in each sample. There is also the rod crystals pull-out in the fracture process (the dotted line in the graphs m and n). At the fracture of the 1600 °C sample, obvious jagged crystals (the active line in the graphs m and n) were also observed, indicating that the interface bonding strength between the cBN and the binder was strong. The occurrence of transgranular fracture and intergranular fracture phenomenon is accompanied that the fracture strength of PCBN is significantly improved. There is large amount of rod-like crystals in the binder, and when the material propagates to the rod-like crystal during the fracture, the rod-like crystal has high elastic modulus and strength without broken. When the rod crystal is pulled out from the matrix or the crack propagates around the rod crystal, the crack propagation path is increased. This greatly consumes the energy of crack propagation, thereby increasing the toughness of the material. In this work, since the rod crystal acts like fiber and whisker toughening, it can enhance PCBN by rod pull-out, bridging and crack deformation.
Flexural strength,MPa Hardnss,GPa
900 32 800 30
Hardness,GPa
Flexural strength,MPa
1000
700 28 1350
1400
1450
1500
1550
1600
Sintering temperature,ć Fig. 6. The mechanical properties of the PCBN prepared at different sintering temperatures under ultrahigh pressure of 5.5 GPa.
the glass phase. Since the same formulation is sintered at different temperatures, there is no significant difference in the phase composition of PCBN at different sintering temperatures, which were proved by the XRD results. However, due to the large difference in hardness between the phases of the sintered body, the micro-hardness of the ceramic material, TiB2, was 36 GPa [26,27], which is barely less than that of the cBN in the PCBN. So content and crystallinity of TiB2 are a major factor affecting the micro-hardness of the PCBN. According to the results of XRD analysis, the TiB2 diffraction peak becomes more and more sharply as the sintering temperature increases, indicating that the content of TiB2 increased and the degree of crystallization becomes better. Therefore, the hardness of the sintered body of PCBN gradually increases, and the micro-hardness was 34.64 GPa at 1600 °C. It can be seen from Fig. 5 that the flexural strength of PCBN increased from 673.54 MPa to 1005.23 MPa with the increase of temperature. Research shows that the flexural strength of ceramic materials is related to these factors such as porosity, maximum defect size and grain diameter [28]. The porosity itself as a defect may become the most dangerous crack inside. From the results of the porosity test, it is known that when the sintering temperature is 1350 °C. There are many pores inside the PCBN. The presence of pores hinders the connection of the reinforcing phase to the matrix, reduces the effective cross-sectional area of the applied load, reduces the stress that the material can withstand, and cracks are easily generated at the pores, causing the material to fracture under lower stresses [29,30]. According to the Griffith fracture theory, the fracture strength of a material has the following relationship with the defect size [31]:
Fig. 7. Uncorroded fracture profile of the sample at 1450 °C and 1600 °C. 5
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4. Conclusion
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In this paper, the effect of sintering temperature on the synthesis of PCBN in cBN-Ti-Al-W system was studied. The composition, morphology, fracture mode, mechanical properties etc., were investigated and analyzed in detail. As the sintering temperature increases, the porosity of PCBN decreases significantly and the relative density increases. When the temperature increased to 1600 °C, the composition of PCBN consisted of BN, TiN, TiB2, W, W2B5, AlN, Al3Ti and W2B, which were distributed homogeneously. The mechanical properties of PCBN are best at 1600 °C, the microhardness and the flexural strength are 34.64 GPa and 1005.23 MPa, respectively. The fracture of PCBN is the result of the interaction of crystal fracture and transgranular fracture. CRediT authorship contribution statement Mo Peicheng:Conceptualization, Methodology, Data curation, Writing - original draft, Writing - review & editing, Visualization.Chen Chao:Supervision, Project administration, Funding acquisition.Chen Jiarong:Validation, Formal analysis, Investigation.Jia Guang:Formal analysis, Investigation.Xie Delong:Formal analysis, Investigation.Xiao Leyin:Formal analysis, Investigation.Pan Xiaoyi:Formal analysis, Investigation.Lin Feng:Supervision, Project administration, Funding acquisition. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgments This study was financially supported by the Innovation-driven Development Special Fund Project of Guangxi Province, China (AA17204098). References [1] Y.B. Zhao, X.H. Peng, T. Fu, C. Huang, C. Feng, D.E. Yin, Z.C. Wang, Molecular dynamics simulation of nano-indentation of (111) cubic boron nitride with optimized tersoff potential [J], Appl. Surf. Sci. 382 (2016) (2016) 309–315. [2] X. Yang, Q.L. Ye, Synthesis of high-quality octahedral cBN crystals with large size using lithium metal as a catalyst [J], J. Alloys Compd. 580 (12) (2013) 1–4. [3] Y.B. Li, H.X. Jiang, G.Z. Yuan, A.L. Chen, X. Wang, T.G. Dai, H.S. Yang, Electronic structure and impurity states of S-doped cBN: a first-principle study [J], J. Alloys Compd. 531 (2012) 82–85. [4] G.Z. Wang, Summarization about the peculiarities of cBN [J], Superhard Mater. Eng. 17 (5) (2005) 41–45. [5] J. Angseryd, M. Elfwing, E. Olsson, H.O. Andren, Detailed microstructure of a cBN based cutting tool material [J], Int. J. Refract. Met. Hard Mater. 27 (2) (2009) 249–255. [6] Y.F. Wang, High Toughness Polycrystalline Cubic Boron Nitride (PcBN) Compound Preparation Technology and Performance Study [D], Henan university of technology, Henan, 2012.
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