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cBN–Al–HfC composites: Sintering behaviors and mechanical properties under high pressure
Int. Journal of Refractory Metals and Hard Materials 50 (2015) 221–226
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
cBN–Al–HfC composites: Sintering behaviors and mechanical properties under high pressure Lili Zhang a,b,c,⁎, Feng Lin a,b,c, Zhi Lv a,b,c, Chao Xu d, Xulin He a,c, Wenlong Wang a,c, Liwei Li a,c, Changlong Zhang a,b,c, Chao Chen a,b,c, Luojun Xia a,c a
Guangxi Key Laboratory of Superhard Materials, Guilin 541004, China National Engineering Research Center for Special Mineral Materials of China, Guilin 541004, China China Nonferrous Metal (Guilin) Geology and Mining Co., Ltd, Guilin 541004, China d College of Science, Wuhan University of Science and Technology, Wuhan 430081, China b c
a r t i c l e
i n f o
Article history: Received 26 August 2014 Received in revised form 23 January 2015 Accepted 24 January 2015 Available online 26 January 2015 Keywords: PcBN Sintering behaviors Mechanical properties High pressure and high temperature
a b s t r a c t Cubic boron nitride (cBN) composites, using Al and HfC as the additives, were sintered under static high pressure of 5.0 GPa and at temperatures of 700 °C–1500 °C for 80 s. By analyzing the phase components of the sintered samples through the X-ray diffraction (XRD) analysis, we found that when the temperature increased from 700 °C to 900 °C, cBN reacted with Al and HfC, and produced AlN, AlB2, HfB2 and B2C5N2. Above 1000 °C, AlB2 was not stable. It decomposed and finally generated AlB12. At the meantime, the content of AlN, AlB12, HfB2 and B2C5N2 increased with Al disappearing. The Vickers hardness of the sintered samples increased with increasing the cBN content. The SEM and the abrasion ratio tests revealed that the well-sintered samples with homogeneous microstructure and the best wear resistance could be obtained at 5.0 GPa, 1300 °C for 80 s with the cBN content of 80 wt.%. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Cubic boron nitride (cBN) has high hardness and high thermal conductivity second only to diamond. With regard to chemical and thermal stabilities, cBN is superior to diamond. Because of those unique properties, cBN is widely used as cutting tools for cutting hardened steel, cast iron, ferrous powder metal and heat resisting alloy [1–6]. For polycrystalline cubic boron nitride (PcBN) composites, they can be divided into two categories by checking whether they have cemented carbide substrates or not. The former type with cemented carbide substrates needs to be welded to the cutter body for cutting, however, there exists a risk that the cutter head may fall off owing to the failure of the welding point. In comparison, the solid PcBN composites without any cemented carbide substrates have the better reliability and economy. For the absence of the cemented carbide substrate, the thermal conductivity as well as the strength of the cutting tools could be significantly enhanced, and therefore it is able to mount on the standard carbide tool shank with safety. In addition, owing to omit the welding process, it avoids the influence of
⁎ Corresponding author at: National Engineering Research Center for Special Mineral Materials of China, Guilin 541004, China. E-mail address: [email protected] (L. Zhang).
http://dx.doi.org/10.1016/j.ijrmhm.2015.01.015 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
welding face on the cutting performance, thus greatly improving the cutting performance and machining quality. What's more, it provides a plurality of cutting edges with double side and has a longer service life, reducing the blade cost of the apportioned to each parts production. The blade reaching the blade tool life can even be processed into the lower level specifications for reutilizations. Therefore, it has the very important practical significance for the study of the solid PCBN inserts. For the sintering of PcBN composites, appropriate binders are often added. Conventionally, metals of the groups IVB, VB, and VIB of the periodic table or their compounds are chosen as additives. Besides, materials like aluminum, cobalt, nickel, titanium, aluminum nitride and beryllium oxide can also be used as additives in the sintering process. In this study, HfC and Al are selected as the binders. For HfC, it has good properties like high hardness (up to 33 GPa) [7], high oxidation resistance, high electrical and thermal conductivity, excellent chemical stability, excellent wear resistance and good resistance to corrosion [8–10]. For Al, it often used as the binder in the sintering of PcBN products because of its low melting point, and the reaction between Al and cBN which can occur under a wide temperature range. Though both HfC and Al are considered as the good binders for the PcBN sintering, the information of the cBN–Al–HfC system has rarely been reported. Therefore, the purpose of this study is to introduce the sintering behaviors of cBN–Al–HfC system and the mechanical properties of the sintered bodies.
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The reaction mechanisms of cBN–Al–HfC system as well as the mechanical properties of the sintered PcBN samples were studied. The phase composition of the sintered products under various P–T conditions was analyzed through X-ray diffraction (XRD). Microstructures of the sintered samples were checked through the scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). For the mechanical property characterizations, Vickers indentation hardness and abrasion ratio tests were conducted. 2. Experimental procedures 2.1. Sample preparation In the present work, high purity cBN powder (b10 μm), Al powder (b 5 μm) and HfC powder (b2 μm) were used as the initial materials. The cBN content of the initial materials ranged from 70 wt.% to 90 wt.%, with the mass ratio of 2:1 for the Al and HfC additives. Before the sintering process, the initial powders were firstly mixed manually in ethyl alcohol using an agate mortar and pestled for 4 h, and then dried in vacuum atmosphere to get rid of the adsorption such as vapors, oxygen and ethanol on the powder surface, with the pressure of 3.0 × 10−3 Pa and temperature of 500 °C for 1 h. After that, about 1.25 g of the dried powders were compacted into the graphite capsule and set into the sample assembly as shown in Fig. 1 for the high pressure and high temperature (HPHT) sintering. The experiments were carried out using a DS6 × 8MN cubic press (Guilin Hengxin Technology Co., Ltd, China), with the pressures up to 5.0 GPa, and temperatures at 700 °C–1500 °C with intervals of 100 °C and heating durations of 80 s. The cell temperature was directly measured with Pt6%Rh–Pt30%Rh thermocouples, and the pressure was calibrated with a method of aluminum melting point [11]. In this study, the samples were first compressed to the required high pressure, and then heated to the desired temperature. After keeping the high pressure and high temperature conditions for 80 s, the samples were quenched to room temperature and then decompressed to ambient pressure. In order to remove the graphite on their surface, the well sintered samples were grinded into a wafer of about 10 mm in diameter and 3.3 mm in thickness by a diamond wheel. Afterwards, the samples were polished to a smooth mirror surface using a polishing machine with 10 μm and 3.5 μm diamond pastes.
2.2. Sample characterization XRD analysis (D/max-2500v/pc, Rigaku, Japan) was conducted to investigate the phase composition of the samples sintered under various conditions. The microstructure was observed by means of SEM (S-4800, Hitachi, Japan) equipped with EDS (IE250, Oxford, England). The mechanical properties of the sintered samples like Vickers hardness and abrasion ratio were also tested. Vickers hardness of the polished samples was tested by a Vickers hardness tester (VH-5, Everone, China) with 29.4 N of applied load and 20 s dwelling time. In order to assess the grinding performance, the sintered samples were firstly polished into the shape with 0.2 mm × 20° chamfer, and then were used for the abrasion ratio tests. The grinding performance of the sintered samples was tested using an abrasion ratio tester (DHM-2, Zhengzhou Dahua Mechanical and Electrical Technology Co., Ltd, China) through the grinding between the 80 # ceramic bond green SiC grinding wheel and the sintered samples according to “JB/T 3235–1999 Method for determination of the abrasion ratio of the artificial diamond sintered body”. The abrasion ratio was defined as the ratio between the weight loss of the grinding wheel wear and the abrasion loss of the samples. In our experiments, the weight loss of the grinding wheel and the samples was measured with a common scale and the one over ten thousand scale, respectively. During our tests, each sample was tested for 3 min, with the linear velocity of the grinding wheel of 25 m/s and the frequency of the pendulum of the workbench of 40/min.
3. Results and discussion 3.1. X-ray diffraction analysis and reaction mechanism Fig. 2 shows the XRD patterns of the samples (cBN content: 80 wt.%) sintered at 5.0 GPa with different temperatures. Clearly, the phases of the sintered samples varied with increasing sintering temperatures. According to Fig. 2, the major phases detected by XRD were cBN, HfC, Al, AlN, AlB2, HfB2 and B2C5N2 at 700 °C [Fig. 2(a)]. When the temperature increased from 700 °C to 900 °C, the phases of the sintered samples did not change except for their content. As the sintering temperatures increase, the amount of AlN, HfB2 and B2C5N2 increased, while that of HfC, AlB2 and Al reduced [Fig. 2 (a–c)]. When the temperature
15 2
4 27
Intensity(a.u.)
2 4
4 7 6
6 3
5 6 1 24 2 27 4 24 2 41 2 7 3 4 3
1.BN 2.HfB2 3.AlB2 4.AlN 5.B2C5N2 6.Al 7.HfC
e d c b a
20
Fig. 1. Cell assembly for HPHT sintering experiments. 1—steel ring, 2—titanium sheet, 3—dolomite, 4—sample, 5—thermocouple, 6—joint, 7—graphite furnace, 8—pyrophyllite.
30
40
50 60 2(degree)
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Fig. 2. X-ray diffraction patterns of the sintered samples from the cBN–Al–HfC (cBN content: 80 wt.%) mixture at: (a) 700 °C, (b) 800 °C, (c) 900 °C, (d) 1000 °C, (e) 1500 °C under 5.0 GPa for 80 s.
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continuously increased from 1000 °C to 1500 °C, Al and AlB2 disappeared, and the products only contained the phases of cBN, HfC, AlN, HfB2 and B2C5N2 [Fig. 2 (d–e)]. According to the phase evolutions of the samples analyzed above, we may speculate the possible reaction mechanism among cBN, Al and HfC. As shown in Fig. 2, the major phases were cBN, HfC, Al AlN, AlB2, HfB2 and B2C5N2 at 700 °C, 800 °C and 900 °C, indicating cBN reacted with both HfC and Al to create AlN, AlB2, HfB2 and B2C5N2. The possible chemical reaction equation was shown as:
temperatures were higher than 1000 °C under high pressure, AlB2 was not stable and easy to decompose, generating Al and AlB12, as shown in Eq. (2). H.S.L. Sithebe et al. indicated that Al can easily react with cBN, and generate AlN and AlB12 under high pressure and high temperature above 1000 °C [14]. Therefore, the diffraction peaks of Al and AlB2 disappeared when the temperature was above 1000 °C at 5.0 GPa in our experiments, indicating that Al reacted completely, and the AlB2 also completely decomposed. In all, the above reaction processes may describe as:
ð2x þ 12ÞBN þ ð3x þ 10ÞAl þ 5HfC ¼ ð2x þ 10ÞAlN þ x AlB2 þ 5HfB2 þ B2 C5 N2 :
ð12y þ 12ÞBN þ ð13y þ 10ÞAl þ 5HfC
ð1Þ
When temperature was increased to 1000 °C, the major phases changed to be cBN, HfC, AlN, HfB2 and B2C5N2 as the peaks of Al and AlB2 disappeared. During this reaction process, the system may undergo the following equations: ð2x þ 12ÞBN þ ð3x þ 10ÞAl þ 5HfC
ð4Þ
¼ ð12y þ 10ÞAlN þ yAlB12 þ 5HfB2 þ B2 C5 N2 :
ð1Þ
From the Eq. (4), it can be seen clearly that the molar ratio of AlN and AlB12 was (12y + 10):y. The quantity of AlB12 was obviously much less than that of AlN. As a result, the diffraction peaks of AlB12 in the XRD were rarely detected. The similar phenomenon has been reported in the precious reports [15,16].
¼ ð2x þ 10ÞAlN þ x AlB2 þ 5HfB2 þ B2 C5 N2 3.2. Microstructure analysis 6AlB2 ¼ 5Al þ AlB12
ð2Þ
13Al þ 12BN ¼ 12AlN þ AlB12
ð3Þ
At these P–T conditions, the reaction of Eq. (1) still occurred, in the meantime, another two reactions may also happen. According to the phase diagram of AlB2 and the previous literature [12,13], when the
Fig. 3 shows the back scanning electron microscopy (BSEM) image and EDS microanalysis of the polished sample (80 wt.% of cBN) sintered at 5.0 GPa, 1300 °C for 80 s. The EDS analysis displayed that the dark areas were rich in the elements of boron and nitrogen, thus it corresponded to the cBN phase. The gray areas were rich in the elements of boron, aluminum, carbon and nitrogen, corresponding to the
Fig. 3. Microstructure of the sample sintered at 1300 °C under 5.0 GPa for 80 s.
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accordance with the results shown in Fig. 5, thus the micrographs were not shown.
3.3. Vickers hardness test Fig. 6 shows the Vickers hardness of cBN–Al–HfC samples versus sintering temperatures and the cBN content. In our experiments, five indentations were conducted at different places on the polished surface for each sample, and the hardness was calculated by using the average diagonal value of those indentations. According to Fig. 6, the Vickers hardness of the sintered samples depended on the sintering temperature and the content of the cBN. For a sample with certain content of cBN, the Vickers hardness increased with increasing sintering temperature. This may attribute to that the plastic deformation was easy to occur for the cBN and the binders above 1000 °C. Therefore, it was more easily to be in combination with each other among the particles, making the microstructure to be denser and getting the higher hardness. However, the Vickers hardness of the sintered samples strongly depended on the content of the cBN. As can be seen in Fig. 6, the more cBN contained in the sample, the higher the Vickers hardness was. When the cBN content was 70 wt.%, the hardness of the sample sintered at 1500 °C was about 26.8 GPa. As the content of the cBN increased to 80 wt.%, the hardness was greatly enhanced to about 35 GPa or higher. This may be due to the fact that the hardness of the sintered samples came from the comprehensive effect of cBN and the bonding phases. As we know, the hardness of the cBN was much higher than that of the bonding phases. As we can see in Fig. 4, when the cBN content was 90 wt.%, the samples had more hard phase (cBN) based on the XRD and EDS analysis, resulting the higher Vickers hardness. Therefore, the increasing amount of cBN would remarkably improve the hardness of the samples.
3.4. Abrasion ratio test
Fig. 4. Microstructure of the samples with different cBN content sintered under the same P–T conditions: (a) 70 wt.%, (b) 80 wt.%,(c) 90 wt.%.
reaction products of AlN, B2C5N2 and AlB12. The white areas were rich in the elements of hafnium, boron, oxygen, aluminum and carbon, corresponding to the reaction products of HfB2, and the HfC, not reacting completely, and Al2O3 oxidized by the starting materials as well. The XRD analysis did not detect the existence of the Al2O3 because of its too small content. The BSEM image revealed that the microstructure was homogeneous and the cBN grains were surrounded by the binder phases. And the cBN bonded very well without the micro cracks observed. Fig. 4 is the BSEM images of the polished samples with different cBN content sintered at 1300 °C under 5.0 GPa. Comparing Fig. 4a,b and c, the dark portions became more, and the white and the gray portions became less with the cBN content increased. This indicated that the bonding phases decreased with increasing the cBN content. Fig. 5 shows the microstructure of the fracture surface of the samples (80 wt.% of cBN) sintered at different temperatures. From Fig. 5a and b, we can see that the cBN grains combined closely without micro cracks observed. However, micro cracks and gaps were founded in Fig. 5c and d. This phenomenon may be attributed to that the temperatures got too high, so the relatively large residual stress formed during the cooling process, which leads to the generation of the micro cracks. The SEM results of samples with the cBN content of 70 wt.% and 90 wt.% were in
Abrasion ratio tests of the sintered samples were conducted to investigate the grinding performance. In order to ensure the accuracy of the measurement, each sample was cleaned by the ultrasonic cleaning machine to remove the dust on the sample surface before and after the tests, and dried to the constant weight, then to weigh. Each PCBN sample was measured 3 times, and the final abrasion ratio value was taken according to their average. The dependence of the sample abrasion ratio on the cBN content and the synthesis temperature was plotted in Fig. 7. It was shown that at a fixed cBN content, the abrasion ratio of the sintered samples increased first and achieved the highest abrasion ratio at about 1.3 × 104 when the temperature grew to 1300 °C, and finally decreased with the continuous increase in temperature. We hypothesized that this phenomenon was associated with micro cracks and gaps existed in the sintered samples. From Fig. 5 we can see that micro cracks and gaps were observed when the temperature was above 1300 °C. The cBN grains would fell off along the micro cracks and gaps during the abrasion ratio tests, which caused the wear resistance of the sintered samples to sharply decline. At the meantime, the abrasion ratio of the sintered samples increased when the cBN content increased from 70 wt.% to 80 wt.%, but it decreased when the cBN content continued to increase to 90 wt.% instead. As it was illustrated in Fig. 4, we can see that the bonding phases decreased while the cBN content increased. We speculated that when the cBN content was 90 wt.%, the sintered samples contained less adhesives, which was not enough to hold the cBN particles, and the cBN grains easily pulled out as a whole and fell off during the abrasion ratio tests. Therefore, when the cBN content was 90 wt.%, the abrasion ratio was decreased. Thus according to the abrasion ratio tests, we suggest that to get the sintered samples with the best wear resistance, it needs the sintering conditions of 5.0 GPa, 1300 °C for 80 s with the cBN content of 80 wt.%.
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Fig. 5. Microstructure of the sample sintered at: (a) 1200 °C, (b) 1300 °C, (c) 1400 °C, (d) 1500 °C under 5.0 GPa for 80 s.
4. Conclusion In this work, the possible reactions of the cBN–Al–HfC system in the HPHT sintering process were studied. We found that when the temperature increased from 700 °C to 900 °C, cBN reacted with Al and HfC, and produced AlN, AlB2, HfB2 and B2C5N2. When the temperature was above 1000 °C, AlB2 was not stable. It decomposed and finally generated AlB12. At the meantime, the content of AlN, AlB12, HfB2 and B2C5N2 increased with Al disappearing. The Vickers hardness of the sintered samples increased with increasing the cBN content. The SEM image and the abrasion ratio tests revealed that the well-sintered
55
Acknowledgments This work was funded by the National Engineering Research Center for Special Mineral Materials of China and partially supported by the special funding of the Guangxi Key Laboratory of Super hard Materials (Grant No. 14-045-16).
16000
70% 80% 90%
50
70% 80% 90%
14000
45
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40 Abrasion ratio
Vickers Hardness(GPa)
samples with homogeneous microstructure and the best wear resistance could be obtained at 5.0 GPa, 1300 °C for 80 s with the cBN content of 80 wt.%.
35 30 25
10000 8000 6000
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o
Temperature( C) Fig. 6. Vickers hardness of cBN–Al–HfC samples versus sintering temperatures and cBN content for 80 s under 5.0 GPa.
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o
Temperature( C) Fig. 7. Abrasion ratio of cBN–Al–HfC samples versus sintering temperatures and cBN content for 80 s under 5.0 GPa.
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References [1] Y. Sahin, Comparison of tool life between ceramic and cubic boron nitride (cBN) cutting tools when machining hardened steels, J. Mater. Process. Technol. 209 (2009) 3478–3489. [2] J. Kopac, P. Krajnik, High-performance grinding—a review, J. Mater. Process. Technol. 175 (2006) 278–284. [3] A. Onodera, K. Inoue, H. Yoshihara, H. Nakae, T. Matsuda, T. Hirari, Synthesis of cubic boron nitride from rhombohedral form under high static pressure, J. Mater. Sci. 25 (1990) 4279–4284. [4] K.S. Neo, M. Rahman, X.P. Li, H.H. Khoo, M. Sawa, Y. Maeda, Performance evaluation of pure cBN tools for machining of steel, J. Mater. Process. Technol. 140 (2003) 326–331. [5] A. Badzian, T. Badzian, Recent developments in hard materials, Int. J. Refract. Met. Hard Mater. 15 (1997) 3–12. [6] H. Sumiya, S. Uesaka, S. Satoh, Mechanical properties of high purity polycrystalline cBN synthesized by direct conversion sintering method, J. Mater. Sci. 35 (2000) 1181–1186. [7] S. Guicciardi, L. Silvestroni, G. Pezzotti, D. Sciti, Depth-sensing indentation hardness characterization of HfC-based composites, Adv. Eng. Mater. 9 (2007) 389–392. [8] A. Sayir, Carbon fiber reinforced hafnium carbide composite, J. Mater. Sci. 39 (2004) 5995–6003.
[9] A. Krajewski, L. D'Alessio, G. De Maria, Physico-chemical and thermophysical properties of cubic binary carbides, Cryst. Res. Technol. 33 (1998) 341–374. [10] G.Q. Li, G.Y. Li, Microstructure and mechanical properties of hafnium carbide coatings synthesized by reactive magnetron sputtering, J. Coat. Technol. Res. 7 (2010) 403–407. [11] S.M. Wang, D.W. He, W.D. Wang, L. Lei, Pressure calibration for the cubic press by differential thermal analysis and the high-pressure fusion curve of aluminum, High Pressure Res. 29 (2009) 806–814. [12] X.M. Wang, The formation of AlB2 in an Al–B master alloy, J. Alloys Compd. 403 (2005) 283–287. [13] C. Rizzoli, P.S. Salamakha, O.L. Sologub, G. Bocelli, X-ray investigation of the Al–B–N ternary system: isothermal section at 1500 °C: crystal structure of the Al0.185B6CN0.256 compound, J. Alloys Compd. 343 (2002) 135–141. [14] H.S.L. Sithebe, D. McLachlan, I. Sigalas, M. Herrmann, Ceram. Int. 34 (2008) 1367–1371. [15] X.Z. Rong, T. Yano, TEM investigation of high-pressure reaction-sintered cBN–Al composites, J. Mater. Sci. 39 (2004) 4705–4710. [16] L.L. Zhang, Z.L. Kou, C. Xu, K.X. Wang, C.L. Liu, B. Hui, et al., Sintering behavior of finegrained cBN-10 wt.% Al3.21Si0.47 system under high pressure, Diamond Relat. Mater. 29 (2012) 84–88.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
cBN–Al–HfC composites: Sintering behaviors and mechanical properties under high pressure Lili Zhang a,b,c,⁎, Feng Lin a,b,c, Zhi Lv a,b,c, Chao Xu d, Xulin He a,c, Wenlong Wang a,c, Liwei Li a,c, Changlong Zhang a,b,c, Chao Chen a,b,c, Luojun Xia a,c a
Guangxi Key Laboratory of Superhard Materials, Guilin 541004, China National Engineering Research Center for Special Mineral Materials of China, Guilin 541004, China China Nonferrous Metal (Guilin) Geology and Mining Co., Ltd, Guilin 541004, China d College of Science, Wuhan University of Science and Technology, Wuhan 430081, China b c
a r t i c l e
i n f o
Article history: Received 26 August 2014 Received in revised form 23 January 2015 Accepted 24 January 2015 Available online 26 January 2015 Keywords: PcBN Sintering behaviors Mechanical properties High pressure and high temperature
a b s t r a c t Cubic boron nitride (cBN) composites, using Al and HfC as the additives, were sintered under static high pressure of 5.0 GPa and at temperatures of 700 °C–1500 °C for 80 s. By analyzing the phase components of the sintered samples through the X-ray diffraction (XRD) analysis, we found that when the temperature increased from 700 °C to 900 °C, cBN reacted with Al and HfC, and produced AlN, AlB2, HfB2 and B2C5N2. Above 1000 °C, AlB2 was not stable. It decomposed and finally generated AlB12. At the meantime, the content of AlN, AlB12, HfB2 and B2C5N2 increased with Al disappearing. The Vickers hardness of the sintered samples increased with increasing the cBN content. The SEM and the abrasion ratio tests revealed that the well-sintered samples with homogeneous microstructure and the best wear resistance could be obtained at 5.0 GPa, 1300 °C for 80 s with the cBN content of 80 wt.%. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Cubic boron nitride (cBN) has high hardness and high thermal conductivity second only to diamond. With regard to chemical and thermal stabilities, cBN is superior to diamond. Because of those unique properties, cBN is widely used as cutting tools for cutting hardened steel, cast iron, ferrous powder metal and heat resisting alloy [1–6]. For polycrystalline cubic boron nitride (PcBN) composites, they can be divided into two categories by checking whether they have cemented carbide substrates or not. The former type with cemented carbide substrates needs to be welded to the cutter body for cutting, however, there exists a risk that the cutter head may fall off owing to the failure of the welding point. In comparison, the solid PcBN composites without any cemented carbide substrates have the better reliability and economy. For the absence of the cemented carbide substrate, the thermal conductivity as well as the strength of the cutting tools could be significantly enhanced, and therefore it is able to mount on the standard carbide tool shank with safety. In addition, owing to omit the welding process, it avoids the influence of
⁎ Corresponding author at: National Engineering Research Center for Special Mineral Materials of China, Guilin 541004, China. E-mail address: [email protected] (L. Zhang).
http://dx.doi.org/10.1016/j.ijrmhm.2015.01.015 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
welding face on the cutting performance, thus greatly improving the cutting performance and machining quality. What's more, it provides a plurality of cutting edges with double side and has a longer service life, reducing the blade cost of the apportioned to each parts production. The blade reaching the blade tool life can even be processed into the lower level specifications for reutilizations. Therefore, it has the very important practical significance for the study of the solid PCBN inserts. For the sintering of PcBN composites, appropriate binders are often added. Conventionally, metals of the groups IVB, VB, and VIB of the periodic table or their compounds are chosen as additives. Besides, materials like aluminum, cobalt, nickel, titanium, aluminum nitride and beryllium oxide can also be used as additives in the sintering process. In this study, HfC and Al are selected as the binders. For HfC, it has good properties like high hardness (up to 33 GPa) [7], high oxidation resistance, high electrical and thermal conductivity, excellent chemical stability, excellent wear resistance and good resistance to corrosion [8–10]. For Al, it often used as the binder in the sintering of PcBN products because of its low melting point, and the reaction between Al and cBN which can occur under a wide temperature range. Though both HfC and Al are considered as the good binders for the PcBN sintering, the information of the cBN–Al–HfC system has rarely been reported. Therefore, the purpose of this study is to introduce the sintering behaviors of cBN–Al–HfC system and the mechanical properties of the sintered bodies.
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The reaction mechanisms of cBN–Al–HfC system as well as the mechanical properties of the sintered PcBN samples were studied. The phase composition of the sintered products under various P–T conditions was analyzed through X-ray diffraction (XRD). Microstructures of the sintered samples were checked through the scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). For the mechanical property characterizations, Vickers indentation hardness and abrasion ratio tests were conducted. 2. Experimental procedures 2.1. Sample preparation In the present work, high purity cBN powder (b10 μm), Al powder (b 5 μm) and HfC powder (b2 μm) were used as the initial materials. The cBN content of the initial materials ranged from 70 wt.% to 90 wt.%, with the mass ratio of 2:1 for the Al and HfC additives. Before the sintering process, the initial powders were firstly mixed manually in ethyl alcohol using an agate mortar and pestled for 4 h, and then dried in vacuum atmosphere to get rid of the adsorption such as vapors, oxygen and ethanol on the powder surface, with the pressure of 3.0 × 10−3 Pa and temperature of 500 °C for 1 h. After that, about 1.25 g of the dried powders were compacted into the graphite capsule and set into the sample assembly as shown in Fig. 1 for the high pressure and high temperature (HPHT) sintering. The experiments were carried out using a DS6 × 8MN cubic press (Guilin Hengxin Technology Co., Ltd, China), with the pressures up to 5.0 GPa, and temperatures at 700 °C–1500 °C with intervals of 100 °C and heating durations of 80 s. The cell temperature was directly measured with Pt6%Rh–Pt30%Rh thermocouples, and the pressure was calibrated with a method of aluminum melting point [11]. In this study, the samples were first compressed to the required high pressure, and then heated to the desired temperature. After keeping the high pressure and high temperature conditions for 80 s, the samples were quenched to room temperature and then decompressed to ambient pressure. In order to remove the graphite on their surface, the well sintered samples were grinded into a wafer of about 10 mm in diameter and 3.3 mm in thickness by a diamond wheel. Afterwards, the samples were polished to a smooth mirror surface using a polishing machine with 10 μm and 3.5 μm diamond pastes.
2.2. Sample characterization XRD analysis (D/max-2500v/pc, Rigaku, Japan) was conducted to investigate the phase composition of the samples sintered under various conditions. The microstructure was observed by means of SEM (S-4800, Hitachi, Japan) equipped with EDS (IE250, Oxford, England). The mechanical properties of the sintered samples like Vickers hardness and abrasion ratio were also tested. Vickers hardness of the polished samples was tested by a Vickers hardness tester (VH-5, Everone, China) with 29.4 N of applied load and 20 s dwelling time. In order to assess the grinding performance, the sintered samples were firstly polished into the shape with 0.2 mm × 20° chamfer, and then were used for the abrasion ratio tests. The grinding performance of the sintered samples was tested using an abrasion ratio tester (DHM-2, Zhengzhou Dahua Mechanical and Electrical Technology Co., Ltd, China) through the grinding between the 80 # ceramic bond green SiC grinding wheel and the sintered samples according to “JB/T 3235–1999 Method for determination of the abrasion ratio of the artificial diamond sintered body”. The abrasion ratio was defined as the ratio between the weight loss of the grinding wheel wear and the abrasion loss of the samples. In our experiments, the weight loss of the grinding wheel and the samples was measured with a common scale and the one over ten thousand scale, respectively. During our tests, each sample was tested for 3 min, with the linear velocity of the grinding wheel of 25 m/s and the frequency of the pendulum of the workbench of 40/min.
3. Results and discussion 3.1. X-ray diffraction analysis and reaction mechanism Fig. 2 shows the XRD patterns of the samples (cBN content: 80 wt.%) sintered at 5.0 GPa with different temperatures. Clearly, the phases of the sintered samples varied with increasing sintering temperatures. According to Fig. 2, the major phases detected by XRD were cBN, HfC, Al, AlN, AlB2, HfB2 and B2C5N2 at 700 °C [Fig. 2(a)]. When the temperature increased from 700 °C to 900 °C, the phases of the sintered samples did not change except for their content. As the sintering temperatures increase, the amount of AlN, HfB2 and B2C5N2 increased, while that of HfC, AlB2 and Al reduced [Fig. 2 (a–c)]. When the temperature
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Fig. 1. Cell assembly for HPHT sintering experiments. 1—steel ring, 2—titanium sheet, 3—dolomite, 4—sample, 5—thermocouple, 6—joint, 7—graphite furnace, 8—pyrophyllite.
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Fig. 2. X-ray diffraction patterns of the sintered samples from the cBN–Al–HfC (cBN content: 80 wt.%) mixture at: (a) 700 °C, (b) 800 °C, (c) 900 °C, (d) 1000 °C, (e) 1500 °C under 5.0 GPa for 80 s.
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continuously increased from 1000 °C to 1500 °C, Al and AlB2 disappeared, and the products only contained the phases of cBN, HfC, AlN, HfB2 and B2C5N2 [Fig. 2 (d–e)]. According to the phase evolutions of the samples analyzed above, we may speculate the possible reaction mechanism among cBN, Al and HfC. As shown in Fig. 2, the major phases were cBN, HfC, Al AlN, AlB2, HfB2 and B2C5N2 at 700 °C, 800 °C and 900 °C, indicating cBN reacted with both HfC and Al to create AlN, AlB2, HfB2 and B2C5N2. The possible chemical reaction equation was shown as:
temperatures were higher than 1000 °C under high pressure, AlB2 was not stable and easy to decompose, generating Al and AlB12, as shown in Eq. (2). H.S.L. Sithebe et al. indicated that Al can easily react with cBN, and generate AlN and AlB12 under high pressure and high temperature above 1000 °C [14]. Therefore, the diffraction peaks of Al and AlB2 disappeared when the temperature was above 1000 °C at 5.0 GPa in our experiments, indicating that Al reacted completely, and the AlB2 also completely decomposed. In all, the above reaction processes may describe as:
ð2x þ 12ÞBN þ ð3x þ 10ÞAl þ 5HfC ¼ ð2x þ 10ÞAlN þ x AlB2 þ 5HfB2 þ B2 C5 N2 :
ð12y þ 12ÞBN þ ð13y þ 10ÞAl þ 5HfC
ð1Þ
When temperature was increased to 1000 °C, the major phases changed to be cBN, HfC, AlN, HfB2 and B2C5N2 as the peaks of Al and AlB2 disappeared. During this reaction process, the system may undergo the following equations: ð2x þ 12ÞBN þ ð3x þ 10ÞAl þ 5HfC
ð4Þ
¼ ð12y þ 10ÞAlN þ yAlB12 þ 5HfB2 þ B2 C5 N2 :
ð1Þ
From the Eq. (4), it can be seen clearly that the molar ratio of AlN and AlB12 was (12y + 10):y. The quantity of AlB12 was obviously much less than that of AlN. As a result, the diffraction peaks of AlB12 in the XRD were rarely detected. The similar phenomenon has been reported in the precious reports [15,16].
¼ ð2x þ 10ÞAlN þ x AlB2 þ 5HfB2 þ B2 C5 N2 3.2. Microstructure analysis 6AlB2 ¼ 5Al þ AlB12
ð2Þ
13Al þ 12BN ¼ 12AlN þ AlB12
ð3Þ
At these P–T conditions, the reaction of Eq. (1) still occurred, in the meantime, another two reactions may also happen. According to the phase diagram of AlB2 and the previous literature [12,13], when the
Fig. 3 shows the back scanning electron microscopy (BSEM) image and EDS microanalysis of the polished sample (80 wt.% of cBN) sintered at 5.0 GPa, 1300 °C for 80 s. The EDS analysis displayed that the dark areas were rich in the elements of boron and nitrogen, thus it corresponded to the cBN phase. The gray areas were rich in the elements of boron, aluminum, carbon and nitrogen, corresponding to the
Fig. 3. Microstructure of the sample sintered at 1300 °C under 5.0 GPa for 80 s.
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accordance with the results shown in Fig. 5, thus the micrographs were not shown.
3.3. Vickers hardness test Fig. 6 shows the Vickers hardness of cBN–Al–HfC samples versus sintering temperatures and the cBN content. In our experiments, five indentations were conducted at different places on the polished surface for each sample, and the hardness was calculated by using the average diagonal value of those indentations. According to Fig. 6, the Vickers hardness of the sintered samples depended on the sintering temperature and the content of the cBN. For a sample with certain content of cBN, the Vickers hardness increased with increasing sintering temperature. This may attribute to that the plastic deformation was easy to occur for the cBN and the binders above 1000 °C. Therefore, it was more easily to be in combination with each other among the particles, making the microstructure to be denser and getting the higher hardness. However, the Vickers hardness of the sintered samples strongly depended on the content of the cBN. As can be seen in Fig. 6, the more cBN contained in the sample, the higher the Vickers hardness was. When the cBN content was 70 wt.%, the hardness of the sample sintered at 1500 °C was about 26.8 GPa. As the content of the cBN increased to 80 wt.%, the hardness was greatly enhanced to about 35 GPa or higher. This may be due to the fact that the hardness of the sintered samples came from the comprehensive effect of cBN and the bonding phases. As we know, the hardness of the cBN was much higher than that of the bonding phases. As we can see in Fig. 4, when the cBN content was 90 wt.%, the samples had more hard phase (cBN) based on the XRD and EDS analysis, resulting the higher Vickers hardness. Therefore, the increasing amount of cBN would remarkably improve the hardness of the samples.
3.4. Abrasion ratio test
Fig. 4. Microstructure of the samples with different cBN content sintered under the same P–T conditions: (a) 70 wt.%, (b) 80 wt.%,(c) 90 wt.%.
reaction products of AlN, B2C5N2 and AlB12. The white areas were rich in the elements of hafnium, boron, oxygen, aluminum and carbon, corresponding to the reaction products of HfB2, and the HfC, not reacting completely, and Al2O3 oxidized by the starting materials as well. The XRD analysis did not detect the existence of the Al2O3 because of its too small content. The BSEM image revealed that the microstructure was homogeneous and the cBN grains were surrounded by the binder phases. And the cBN bonded very well without the micro cracks observed. Fig. 4 is the BSEM images of the polished samples with different cBN content sintered at 1300 °C under 5.0 GPa. Comparing Fig. 4a,b and c, the dark portions became more, and the white and the gray portions became less with the cBN content increased. This indicated that the bonding phases decreased with increasing the cBN content. Fig. 5 shows the microstructure of the fracture surface of the samples (80 wt.% of cBN) sintered at different temperatures. From Fig. 5a and b, we can see that the cBN grains combined closely without micro cracks observed. However, micro cracks and gaps were founded in Fig. 5c and d. This phenomenon may be attributed to that the temperatures got too high, so the relatively large residual stress formed during the cooling process, which leads to the generation of the micro cracks. The SEM results of samples with the cBN content of 70 wt.% and 90 wt.% were in
Abrasion ratio tests of the sintered samples were conducted to investigate the grinding performance. In order to ensure the accuracy of the measurement, each sample was cleaned by the ultrasonic cleaning machine to remove the dust on the sample surface before and after the tests, and dried to the constant weight, then to weigh. Each PCBN sample was measured 3 times, and the final abrasion ratio value was taken according to their average. The dependence of the sample abrasion ratio on the cBN content and the synthesis temperature was plotted in Fig. 7. It was shown that at a fixed cBN content, the abrasion ratio of the sintered samples increased first and achieved the highest abrasion ratio at about 1.3 × 104 when the temperature grew to 1300 °C, and finally decreased with the continuous increase in temperature. We hypothesized that this phenomenon was associated with micro cracks and gaps existed in the sintered samples. From Fig. 5 we can see that micro cracks and gaps were observed when the temperature was above 1300 °C. The cBN grains would fell off along the micro cracks and gaps during the abrasion ratio tests, which caused the wear resistance of the sintered samples to sharply decline. At the meantime, the abrasion ratio of the sintered samples increased when the cBN content increased from 70 wt.% to 80 wt.%, but it decreased when the cBN content continued to increase to 90 wt.% instead. As it was illustrated in Fig. 4, we can see that the bonding phases decreased while the cBN content increased. We speculated that when the cBN content was 90 wt.%, the sintered samples contained less adhesives, which was not enough to hold the cBN particles, and the cBN grains easily pulled out as a whole and fell off during the abrasion ratio tests. Therefore, when the cBN content was 90 wt.%, the abrasion ratio was decreased. Thus according to the abrasion ratio tests, we suggest that to get the sintered samples with the best wear resistance, it needs the sintering conditions of 5.0 GPa, 1300 °C for 80 s with the cBN content of 80 wt.%.
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Fig. 5. Microstructure of the sample sintered at: (a) 1200 °C, (b) 1300 °C, (c) 1400 °C, (d) 1500 °C under 5.0 GPa for 80 s.
4. Conclusion In this work, the possible reactions of the cBN–Al–HfC system in the HPHT sintering process were studied. We found that when the temperature increased from 700 °C to 900 °C, cBN reacted with Al and HfC, and produced AlN, AlB2, HfB2 and B2C5N2. When the temperature was above 1000 °C, AlB2 was not stable. It decomposed and finally generated AlB12. At the meantime, the content of AlN, AlB12, HfB2 and B2C5N2 increased with Al disappearing. The Vickers hardness of the sintered samples increased with increasing the cBN content. The SEM image and the abrasion ratio tests revealed that the well-sintered
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Acknowledgments This work was funded by the National Engineering Research Center for Special Mineral Materials of China and partially supported by the special funding of the Guangxi Key Laboratory of Super hard Materials (Grant No. 14-045-16).
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