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Reactive sintering cBN-Ti-Al composites by spark plasma sintering
Diamond & Related Materials 69 (2016) 138–143
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Reactive sintering cBN-Ti-Al composites by spark plasma sintering Yungang Yuan a, Xiaozhe Cheng a, Rui Chang a, Tianheng Li a, Jianbing Zang a, Yanhui Wang a,⁎, Yiqing Yu b, Jing Lu b, Xipeng Xu b,⁎ a b
State Key Laboratory of Metastable Materials Science & Technology, Yanshan University, Qinhuangdao, Hebei, 066004, P.R. China MOE Engineering Research Center for Brittle Materials Machining, Huaqiao University, Xiamen, 361021, P.R. China
a r t i c l e
i n f o
Article history: Received 3 June 2016 Received in revised form 15 July 2016 Accepted 22 August 2016 Available online 26 August 2016 Keywords: Boron nitride Degradation Spark plasma sintering Microstructure
a b s t r a c t When synthesizing polycrystalline cubic boron nitride (PcBN) at normal pressure, cBN had a trend of hexagonal transformation, which reduces the hardness and strength of PcBN. The cBN-Ti-Al composite was prepared by spark plasma sintering with introducing Ti and Al to absorb hexagonal boron nitride (hBN) transformed from cBN. By the results of X-ray diffraction (XRD), Ti and Al reacted with BN and forming TiN, TiB2, and AlN, which combined cBN as the binder by chemical bonding. The mechanical properties of the prepared composite increased as the increment of sintering temperature. The threshold temperature for preparing composite without hBN phase was at 1400 °C. The composite with optimal mechanical properties was prepared at 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. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Similar to diamond, cubic boron nitride (cBN) possesses many excellent properties, such as high hardness, high thermal conductivity, high chemical stability and chemical inertness [1–3]. However, cBN is more popular than diamond when applied as cutting tools, because it has higher thermal stability and chemical inertness with ferrous alloys than diamond. Furthermore, the cBN tools have higher machining efficiency and better surface processing quality than diamond tools [4–6]. CBN cutting tools have realized the processing of hard alloy steel with high efficiency and cutting without cooling liquid, which makes great contributions to the environment. The big bulk of cBN is often needed, but synthesizing of big single crystalline cBN is very hard. To meet the application, polycrystalline cBN (PcBN) is often synthesized by fine cBN grits [7–9]. Unfortunately, cBN may transform the graphite-like hBN at the high sintering temperature. The appearance of hexagonal boron nitride (hBN) would make the hardness and bending strength of composites declined [9,10]. To avoid the transformation from cBN to hBN, high pressure and high temperature (HPHT) method is mainly used [5,11]. However, it is difficult to synthesize PcBN with large dimensions and complex shape by HPHT method. In addition, the complex device and fabricating process also restrict the application of PcBN.
⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (X. Xu).
http://dx.doi.org/10.1016/j.diamond.2016.08.009 0925-9635/© 2016 Elsevier B.V. All rights reserved.
Spark plasma sintering (SPS) is able to utilize the surface activation effect effectively and self-heating effect among the particles to urge heat-transfer and mass-transfer to be completed instantaneously [12, 13]. And more importantly, SPS can depress the phase transformation of cBN to hBN on account of its high heating rate and short dwell time [8,14]. Besides, in order to make up the condition without high pressure to restrain the phase transformation of cBN to hBN, it is necessary to seek further for a suitable binder absorbing the hBN generated by cBN via SPS. Moreover, the binder plays a significant role in holding cBN grits in composites and endowing the composites with some geometric shape and strength. Aiming at the requirements for the binder in this study, Ti and Al are selected [15,16] based on the following reasons: (i) Ti and Al are able to activate the surface of cBN grits and bond with them effectively. (ii) Al will be melting at high temperature so as to fill the voids among different cBN grits. (iii) Ti and Al are capable of reacting with BN to form high-strength resultants, such as TiN, TiB2, and AlN. However, there are some aluminum boride grains formed during SPS process if there is only Al adding as binder, which is detrimental to the mechanical strength of the composite due to the poor hardness and is easy to cleave [17–19]. The addition of Ti in the composite could absorb the noxious aluminum boride, since that aluminum boride could react with the resultant TiN to form hard-phase AlN and TiB2 [5, 20]. Therefore, Ti as well as Al is used as the binder to prepare cBN-TiAl composite. In this work, the cBN-Ti-Al composite was synthesized by SPS. The mechanical properties, phase transformation of cBN, and the relevant reactions that occurred at the composite were also studied.
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2. Material and methods Commercially available cBN grits (30–40 μm), Ti powders (~ 5 μm), and Al powders (~ 5 μm) were used as raw materials. The three kinds of particles with different proportions were mixed thoroughly, and then loaded into a graphite die with a 20 mm inside diameter. The graphite die was put into the chamber of an SPS device (Model.SPS-3.20MK-IV, Sumitomo Coal Mining Co. Ltd., Japan) and pressed with 50 MPa of uniaxial pressure. The sintering temperatures ranged from 1200 to 1700 °C in an argon atmosphere at a heating rate of 100 °C/min with isothermal hold times of 10 min. The relative density of each composite was measured by an Archimedes method. The bending strength, hardness, and fracture toughness were used to evaluate the mechanical properties of the composite. The bending strength of each composite was tested by a three-point bending method [21], which was carried out on a machine named D2S-II (produced by China Building Material Test & Certification Center). The composite was inserted in phenolic resin, then polishing with diamond grinding paste to a smooth mirror surface. The Vickers hardness (Hv/GPa) and fracture toughness (KIC / MPa·m0.5) were measured on the polished surface at room temperature with a micro-hardness tester (THV-5, China) at a load of 49 N [22,23]. The phase compositions of composites were investigated by X-ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan) and Raman spectroscopy (Renishaw inVia). A field emission scanning electron microscopy (FESEM) (Model S-4800, Japan) was used to observe the morphologies of the polish surface and the fracture surface. 3. Result and discussion Table 1 listed the bending strengths and relative densities of the composite with different material proportions prepared by SPS at 1400 °C for 10 min. It showed that the composite 2, which was consisted of 45% cBN, 35% Ti, and 20% Al in volume (defined as cBN-35Ti-20Al), possessed the maximum bending strength (390.7 ± 4.4 MPa) and relative density (98.9 ± 0.1%). The mechanical properties of the cBN-35Ti-20Al composite fabricated by SPS at different temperatures were shown in Fig. 1. The relative density and bending strength of the composite were shown in Fig. 1(a). It could be seen that both of the relative density and bending strength firstly increased and then fell with the sintering temperature rising, and both of their maximum values occurred at 1400 °C. Meanwhile, the hardness and fracture toughness had the same tendency as the relative density and bending strength, as shown in Fig. 1(b). The results showed that the optimal mechanical properties of the composite occurred at 1400 °C, and the relative density (98.9 ± 0.1%), the bending strength (390.7 ± 4.4 MPa), hardness (14.1 ± 0.5 GPa), and fracture toughness (7.5 ± 0.1 MPa·m0.5 ) reached their maximum. The phase composition of the cBN-35Ti-20Al composite sintered at the temperatures range from 1300 to 1600 °C was identified by XRD,
Fig. 1. (a) Bending strength and relative density of cBN-35Ti-20Al composites sintered at different temperatures. (b) Hardness and fracture toughness of cBN-35Ti-20Al composites sintered at different temperatures.
as shown in Fig. 2. Except the characteristic peaks of cBN at 43.2° and 89.8°, the other peaks corresponding to the phases of TiN, TiB2 and AlN were observed as well. The characteristic peaks of Ti and Al were not found in the composites sintered at 1300–1600 °C, indicating that
Table 1 Bending strengths and relative densities of the composites with different material proportions prepared by SPS. Composite Volume percent (vol.%)
1 2 3 4 5
Weight percent (w.t.%)
cBN Ti
Al
cBN
Ti
Al
40 45 50 55 60
20 20 20 20 20
37.1 42.3 47.7 53.2 59.0
48.4 43.0 37.4 31.6 25.7
14.5 14.7 14.9 15.2 15.3
40 35 30 25 20
Relative density (%)
Bending strength (MPa)
96.6 ± 0.1 98.9 ± 0.1 97.3 ± 0.1 97.6 ± 0.1 93.7 ± 0.1
238.1 ± 7.1 390.7 ± 4.4 261.9 ± 6.1 220.8 ± 8.8 181.3 ± 5.2
Fig. 2. XRD patterns of cBN-35Ti-20Al composites sintered at different temperatures 1300 ° C (a), 1400 °C (b), 1500 °C (c), and 1600 °C (d).
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Ti and Al had been fully reacted with BN. The correlative reactions in the composite were listed below. 3Al þ 2BN ¼ AlB2 þ 2AlN
ð1Þ
3Ti þ 2BN ¼ TiB2 þ 2TiN
ð2Þ
AlB2 þ TiN ¼ AlN þ TiB2
ð3Þ
Because the melting point of Al (660.3 °C) was lower than Ti and cBN, the Al melted firstly during SPS process and filled the gaps between cBN and Ti particles with the aid of pressure. The liquid Al gathered around the cBN grits and reacted with cBN, forming AlB2 and AlN. With the temperature rising, the reaction between Ti and cBN occurred as Eq. (2). The formed TiN could react with the soft-phase AlB2, and hard-phases AlN and TiB2 were obtained (Eq. (3)). It can be explained that no AlB2 was found in the composite. A peak at 26.7° presented in the XRD patterns in Fig. 2(c) and (d) can be attributed to hBN phase. As we know, the transformation from cBN to hBN occurred at a high temperature due to its less thermodynamic stability under normal pressure. The higher the temperature was, the more the transformed hBN was. As the sintered temperature rising (exceeded 1400 °C), the Ti and Al additives could not absorb all transformed hBN effectively, resulting in some residual hBN in the composite. A complete cBN-hBN transformation in composite occurred at 1600 °C, as shown in Fig. 2(d). The morphologies and their corresponding Raman spectra of the cBN-35Ti-20Al composite sintered at 1400, 1500, and 1600 °C were shown in Figs. 3–5, respectively. Fig. 3(a) displayed the microstructure
of the composite sintered at 1400 °C. The Raman spectra of the matrix (I) and (II) were shown in Fig. 3(b) and (c), respectively. Two peaks at 1053.8 cm−1 and 1304.6 cm−1 in Fig. 3(b) were associated with cBN [24]. The Raman spectrum in Fig. 3(c) showed that the matrix was composed by TiB2 (247.3 cm− 1) [25], AlN (309.6 cm− 1) [26], and TiN (560.5 cm−1) [27]. It was found that the hard phases of TiN, TiB2, and AlN were strongly bonded with cBN grains, producing optimal mechanical properties of the composite (as shown in Fig. 1). Fig. 4(a) showed the morphology of the composites sintered at 1500 °C. Except for the bright cBN grits (I) (as shown in Fig. 4(b)) and matrix (II) of TiB2, AlN, and TiN (Fig. 4(c)), some dark grits (III) were found. The corresponding Raman spectrum in Fig. 4(d) showed a peak at 1368.4 cm−1, which was identified as hBN [28]. The presence of the hBN phase caused a decrease in mechanical properties of the composite. When the sintering temperature reached to 1600 °C, as shown in Fig. 5, the complete transformation from cBN to hBN that occurred for the cBN phase disappeared. The Raman spectra in Fig. 5(b) and (c) showed that the composite only consisted hBN grits and the compound matrix (TiB2, AlN, and TiN). Therefore, a conclusion could be drawn that the threshold temperature for preparing the composite without hBN phase was 1400 °C. Fig. 6(a)–(d) presented the polished surface morphologies of the cBN-35Ti-20Al composite sintered at 1300–1600 °C, respectively. Fig. 6(a) presented some voids on the matrix, implying that the sintering temperature of 1300 °C was not high enough for densification of the composite. Comparing with Fig. 6(a), the composite sintered at 1400 ° C was denser, as shown in Fig. 6(b). The cBN grits were tightly bonded with the matrix. The results showed that Al and Ti were forced to infiltrate into the cBN grains at a higher temperature. And the resultants of TiN, TiB2, and AlN were chemically bonded with the cBN grains in composite, which resulted in forming a compact composite with optimal mechanical properties. It could be found that cBN grits in Fig. 6(a) and (b) had sharp edges, while the edges in the grits with dark surfaces
Fig. 3. Microstructure (a) and Raman spectra corresponding to different regions (b, c) of the cBN-35Ti-30Al composite sintered at 1400 °C.
Y. Yuan et al. / Diamond & Related Materials 69 (2016) 138–143
Fig. 4. Microstructure (a) and Raman spectra corresponding to different regions (b, c, d) of the cBN-35Ti-30Al composite sintered at 1500 °C.
Fig. 5. Microstructure (a) and Raman spectra corresponding to different regions (b, c) of the cBN-35Ti-30Al composite sintered at 1600 °C.
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Fig. 6. Polish face morphologies of the of cBN-35Ti-20Al composites sintered at the temperature of 1300 °C (a), 1400 °C (b), 1500 °C (c), and 1600 °C (d).
were not sharp (Fig. 6(c)). Besides, a rough surface was also found in Fig. 6(c), which can ascribe to the transformed hBN from cBN peeled off during polishing. Clearly, the appearance of the hBN phase weakened the strength of the grits and the mechanical properties of the composite decreased, especially the bending strength and the hardness. Nearly all cBN grits transformed into hBN when sintered at 1600 °C. Therefore, the surfaces of BN grits were easily polished due to the lower hardness of the composite compared with others, as shown in Fig. 6(d). Fig. 7 displayed the SEM morphology of the cBN-35Ti-20Al composite sintered at 1400 °C and its corresponding EDS mapping of B, N, Al, and Ti. It was found that the element B and N distributed on the whole composite due to the reaction between cBN and Al and Ti as Eqs. (1) and (2), as shown in Fig. 7(b) and (c). While, the elements Al and Ti exited in the matrix, rather than the BN grits, as shown in Fig. 7(d) and (e). Furthermore, the element Al mainly gathered around the BN grits (Fig. 7(e)). The phenomenon confirmed that compounds produced in the composite (TiN, TiB2, and AlN) were strongly bonded with the cBN grits in composite.
Fig. 8 exhibited the fracture morphologies of the cBN-35Ti-20Al composite sintered at different temperatures. Fig. 8(b) showed that a transcrystalline fracture occurred at the composite sintered at 1400 °C. This phenomenon demonstrated that the matrix of TiN, TiB2, and AlN had an excellent interface adhesive strength with cBN grits [15,16,29, 30], which was even higher than the strength of cBN particle itself. So the composite sintered at 1400 °C showed the maximum bending strength in cBN-35Ti-20Al composites (Fig. 1). Meanwhile, the intercrystalline fracture in both composites sintered at 1300 and 1500 °C was found in Fig. 8(a) and (c), which indicated a poor bond between the matrix and the grits. In Fig. 8(d), the grains surrounded by the matrix were hBN instead of cBN, which led to lower hardness and fracture toughness of the composite. 4. Conclusions The cBN-Ti-Al composites were prepared by the SPS method. The phase transformation of cBN to hBN had been restrained by SPS with
Fig. 7. SEM morphology of cBN-35Ti-20Al composites sintered at 1400 °C (a) and corresponding EDS mapping of Al (b), B (c), Ti (d), and N (e).
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Fig. 8. Fracture morphology of cBN-35Ti-20Al composites sintered at the temperature of 1300 °C (a), 1400 °C (b), 1500 °C (c), and 1600 °C (d).
the addition of Ti and Al. The sintering temperature for cBN-Ti-Al composites without hBN phase was below 1400 °C. The optimal mechanical properties of the cBN-Ti-Al composite were prepared at 1400 °C and the relative density, bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.5 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m0.5, respectively. Moreover, the transcrystalline fracture occurred in the composite sintered at 1400 °C, indicating that the formed matrix of TiN, TiB2, and AlN had an excellent interface adhesive strength with cBN grits. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51272226), and Key Basic Research Program of Hebei Province of China (No. 14961106D). The authors gratefully acknowledge financial and material support from Element Six Co. (No. D09). References [1] R. Wentorf, R.C. De Vries, F. Bundy, Sintered superhard materials, Science 208 (1980) 873–880. [2] L. Vel, G. Demazeau, J. Etourneau, Cubic boron nitride: synthesis, physicochemical properties and applications, Mater. Sci. Eng. B 10 (1991) 149–164. [3] S.N. Monteiro, A.L.D. Skury, M.G. de Azevedo, G.S. Bobrovnitchii, Cubic boron nitride competing with diamond as a superhard engineering material—an overview, J. Mater. Res. Technol. 2 (2013) 68–74. [4] X.-T. Luo, C.-J. Li, G.-J. Yang, Correlations between milling conditions and iron contamination, microstructure and hardness of mechanically alloyed cubic BN particle reinforced NiCrAl matrix composite powders, J. Alloys Compd. 548 (2013) 180–187. [5] X.-Z. Rong, T. Tsurumi, O. Fukunaga, T. Yano, High-pressure sintering of cBN-TiN-Al composite for cutting tool application, Diam. Relat. Mater. 11 (2002) 280–286. [6] C. Brookes, The mechanical properties of cubic boron nitride, Science of hard materials, Proceedings of the International Conference, 1986. [7] M. Hotta, T. Goto, Densification and microstructure of Al 2 O 3 -cBN composites prepared by spark plasma sintering, J. Ceram. Soc. Jpn. 116 (2008) 744–748. [8] B. Yaman, H. Mandal, Spark plasma sintering of Co–WC cubic boron nitride composites, Mater. Lett. 63 (2009) 1041–1043. [9] J. Zhang, R. Tu, T. Goto, Spark plasma sintering of Al2O3–cBN composites facilitated by Ni nanoparticle precipitation on cBN powder by rotary chemical vapor deposition, J. Eur. Ceram. Soc. 31 (2011) 2083–2087. [10] F. Ye, Z. Hou, H. Zhang, L. Liu, Y. Zhou, Spark plasma sintering of cBN/β-SiAlON composites, Mater. Sci. Eng. A 527 (2010) 4723–4726. [11] L. Zhang, Z. Kou, C. Xu, K. Wang, C. Liu, B. Hui, D. He, Sintering behaviors of finegrained cBN–10 wt.% Al3.21Si0.47 system under high pressure, Diam. Relat. Mater. 29 (2012) 84–88.
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Reactive sintering cBN-Ti-Al composites by spark plasma sintering Yungang Yuan a, Xiaozhe Cheng a, Rui Chang a, Tianheng Li a, Jianbing Zang a, Yanhui Wang a,⁎, Yiqing Yu b, Jing Lu b, Xipeng Xu b,⁎ a b
State Key Laboratory of Metastable Materials Science & Technology, Yanshan University, Qinhuangdao, Hebei, 066004, P.R. China MOE Engineering Research Center for Brittle Materials Machining, Huaqiao University, Xiamen, 361021, P.R. China
a r t i c l e
i n f o
Article history: Received 3 June 2016 Received in revised form 15 July 2016 Accepted 22 August 2016 Available online 26 August 2016 Keywords: Boron nitride Degradation Spark plasma sintering Microstructure
a b s t r a c t When synthesizing polycrystalline cubic boron nitride (PcBN) at normal pressure, cBN had a trend of hexagonal transformation, which reduces the hardness and strength of PcBN. The cBN-Ti-Al composite was prepared by spark plasma sintering with introducing Ti and Al to absorb hexagonal boron nitride (hBN) transformed from cBN. By the results of X-ray diffraction (XRD), Ti and Al reacted with BN and forming TiN, TiB2, and AlN, which combined cBN as the binder by chemical bonding. The mechanical properties of the prepared composite increased as the increment of sintering temperature. The threshold temperature for preparing composite without hBN phase was at 1400 °C. The composite with optimal mechanical properties was prepared at 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. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Similar to diamond, cubic boron nitride (cBN) possesses many excellent properties, such as high hardness, high thermal conductivity, high chemical stability and chemical inertness [1–3]. However, cBN is more popular than diamond when applied as cutting tools, because it has higher thermal stability and chemical inertness with ferrous alloys than diamond. Furthermore, the cBN tools have higher machining efficiency and better surface processing quality than diamond tools [4–6]. CBN cutting tools have realized the processing of hard alloy steel with high efficiency and cutting without cooling liquid, which makes great contributions to the environment. The big bulk of cBN is often needed, but synthesizing of big single crystalline cBN is very hard. To meet the application, polycrystalline cBN (PcBN) is often synthesized by fine cBN grits [7–9]. Unfortunately, cBN may transform the graphite-like hBN at the high sintering temperature. The appearance of hexagonal boron nitride (hBN) would make the hardness and bending strength of composites declined [9,10]. To avoid the transformation from cBN to hBN, high pressure and high temperature (HPHT) method is mainly used [5,11]. However, it is difficult to synthesize PcBN with large dimensions and complex shape by HPHT method. In addition, the complex device and fabricating process also restrict the application of PcBN.
⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (X. Xu).
http://dx.doi.org/10.1016/j.diamond.2016.08.009 0925-9635/© 2016 Elsevier B.V. All rights reserved.
Spark plasma sintering (SPS) is able to utilize the surface activation effect effectively and self-heating effect among the particles to urge heat-transfer and mass-transfer to be completed instantaneously [12, 13]. And more importantly, SPS can depress the phase transformation of cBN to hBN on account of its high heating rate and short dwell time [8,14]. Besides, in order to make up the condition without high pressure to restrain the phase transformation of cBN to hBN, it is necessary to seek further for a suitable binder absorbing the hBN generated by cBN via SPS. Moreover, the binder plays a significant role in holding cBN grits in composites and endowing the composites with some geometric shape and strength. Aiming at the requirements for the binder in this study, Ti and Al are selected [15,16] based on the following reasons: (i) Ti and Al are able to activate the surface of cBN grits and bond with them effectively. (ii) Al will be melting at high temperature so as to fill the voids among different cBN grits. (iii) Ti and Al are capable of reacting with BN to form high-strength resultants, such as TiN, TiB2, and AlN. However, there are some aluminum boride grains formed during SPS process if there is only Al adding as binder, which is detrimental to the mechanical strength of the composite due to the poor hardness and is easy to cleave [17–19]. The addition of Ti in the composite could absorb the noxious aluminum boride, since that aluminum boride could react with the resultant TiN to form hard-phase AlN and TiB2 [5, 20]. Therefore, Ti as well as Al is used as the binder to prepare cBN-TiAl composite. In this work, the cBN-Ti-Al composite was synthesized by SPS. The mechanical properties, phase transformation of cBN, and the relevant reactions that occurred at the composite were also studied.
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2. Material and methods Commercially available cBN grits (30–40 μm), Ti powders (~ 5 μm), and Al powders (~ 5 μm) were used as raw materials. The three kinds of particles with different proportions were mixed thoroughly, and then loaded into a graphite die with a 20 mm inside diameter. The graphite die was put into the chamber of an SPS device (Model.SPS-3.20MK-IV, Sumitomo Coal Mining Co. Ltd., Japan) and pressed with 50 MPa of uniaxial pressure. The sintering temperatures ranged from 1200 to 1700 °C in an argon atmosphere at a heating rate of 100 °C/min with isothermal hold times of 10 min. The relative density of each composite was measured by an Archimedes method. The bending strength, hardness, and fracture toughness were used to evaluate the mechanical properties of the composite. The bending strength of each composite was tested by a three-point bending method [21], which was carried out on a machine named D2S-II (produced by China Building Material Test & Certification Center). The composite was inserted in phenolic resin, then polishing with diamond grinding paste to a smooth mirror surface. The Vickers hardness (Hv/GPa) and fracture toughness (KIC / MPa·m0.5) were measured on the polished surface at room temperature with a micro-hardness tester (THV-5, China) at a load of 49 N [22,23]. The phase compositions of composites were investigated by X-ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan) and Raman spectroscopy (Renishaw inVia). A field emission scanning electron microscopy (FESEM) (Model S-4800, Japan) was used to observe the morphologies of the polish surface and the fracture surface. 3. Result and discussion Table 1 listed the bending strengths and relative densities of the composite with different material proportions prepared by SPS at 1400 °C for 10 min. It showed that the composite 2, which was consisted of 45% cBN, 35% Ti, and 20% Al in volume (defined as cBN-35Ti-20Al), possessed the maximum bending strength (390.7 ± 4.4 MPa) and relative density (98.9 ± 0.1%). The mechanical properties of the cBN-35Ti-20Al composite fabricated by SPS at different temperatures were shown in Fig. 1. The relative density and bending strength of the composite were shown in Fig. 1(a). It could be seen that both of the relative density and bending strength firstly increased and then fell with the sintering temperature rising, and both of their maximum values occurred at 1400 °C. Meanwhile, the hardness and fracture toughness had the same tendency as the relative density and bending strength, as shown in Fig. 1(b). The results showed that the optimal mechanical properties of the composite occurred at 1400 °C, and the relative density (98.9 ± 0.1%), the bending strength (390.7 ± 4.4 MPa), hardness (14.1 ± 0.5 GPa), and fracture toughness (7.5 ± 0.1 MPa·m0.5 ) reached their maximum. The phase composition of the cBN-35Ti-20Al composite sintered at the temperatures range from 1300 to 1600 °C was identified by XRD,
Fig. 1. (a) Bending strength and relative density of cBN-35Ti-20Al composites sintered at different temperatures. (b) Hardness and fracture toughness of cBN-35Ti-20Al composites sintered at different temperatures.
as shown in Fig. 2. Except the characteristic peaks of cBN at 43.2° and 89.8°, the other peaks corresponding to the phases of TiN, TiB2 and AlN were observed as well. The characteristic peaks of Ti and Al were not found in the composites sintered at 1300–1600 °C, indicating that
Table 1 Bending strengths and relative densities of the composites with different material proportions prepared by SPS. Composite Volume percent (vol.%)
1 2 3 4 5
Weight percent (w.t.%)
cBN Ti
Al
cBN
Ti
Al
40 45 50 55 60
20 20 20 20 20
37.1 42.3 47.7 53.2 59.0
48.4 43.0 37.4 31.6 25.7
14.5 14.7 14.9 15.2 15.3
40 35 30 25 20
Relative density (%)
Bending strength (MPa)
96.6 ± 0.1 98.9 ± 0.1 97.3 ± 0.1 97.6 ± 0.1 93.7 ± 0.1
238.1 ± 7.1 390.7 ± 4.4 261.9 ± 6.1 220.8 ± 8.8 181.3 ± 5.2
Fig. 2. XRD patterns of cBN-35Ti-20Al composites sintered at different temperatures 1300 ° C (a), 1400 °C (b), 1500 °C (c), and 1600 °C (d).
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Ti and Al had been fully reacted with BN. The correlative reactions in the composite were listed below. 3Al þ 2BN ¼ AlB2 þ 2AlN
ð1Þ
3Ti þ 2BN ¼ TiB2 þ 2TiN
ð2Þ
AlB2 þ TiN ¼ AlN þ TiB2
ð3Þ
Because the melting point of Al (660.3 °C) was lower than Ti and cBN, the Al melted firstly during SPS process and filled the gaps between cBN and Ti particles with the aid of pressure. The liquid Al gathered around the cBN grits and reacted with cBN, forming AlB2 and AlN. With the temperature rising, the reaction between Ti and cBN occurred as Eq. (2). The formed TiN could react with the soft-phase AlB2, and hard-phases AlN and TiB2 were obtained (Eq. (3)). It can be explained that no AlB2 was found in the composite. A peak at 26.7° presented in the XRD patterns in Fig. 2(c) and (d) can be attributed to hBN phase. As we know, the transformation from cBN to hBN occurred at a high temperature due to its less thermodynamic stability under normal pressure. The higher the temperature was, the more the transformed hBN was. As the sintered temperature rising (exceeded 1400 °C), the Ti and Al additives could not absorb all transformed hBN effectively, resulting in some residual hBN in the composite. A complete cBN-hBN transformation in composite occurred at 1600 °C, as shown in Fig. 2(d). The morphologies and their corresponding Raman spectra of the cBN-35Ti-20Al composite sintered at 1400, 1500, and 1600 °C were shown in Figs. 3–5, respectively. Fig. 3(a) displayed the microstructure
of the composite sintered at 1400 °C. The Raman spectra of the matrix (I) and (II) were shown in Fig. 3(b) and (c), respectively. Two peaks at 1053.8 cm−1 and 1304.6 cm−1 in Fig. 3(b) were associated with cBN [24]. The Raman spectrum in Fig. 3(c) showed that the matrix was composed by TiB2 (247.3 cm− 1) [25], AlN (309.6 cm− 1) [26], and TiN (560.5 cm−1) [27]. It was found that the hard phases of TiN, TiB2, and AlN were strongly bonded with cBN grains, producing optimal mechanical properties of the composite (as shown in Fig. 1). Fig. 4(a) showed the morphology of the composites sintered at 1500 °C. Except for the bright cBN grits (I) (as shown in Fig. 4(b)) and matrix (II) of TiB2, AlN, and TiN (Fig. 4(c)), some dark grits (III) were found. The corresponding Raman spectrum in Fig. 4(d) showed a peak at 1368.4 cm−1, which was identified as hBN [28]. The presence of the hBN phase caused a decrease in mechanical properties of the composite. When the sintering temperature reached to 1600 °C, as shown in Fig. 5, the complete transformation from cBN to hBN that occurred for the cBN phase disappeared. The Raman spectra in Fig. 5(b) and (c) showed that the composite only consisted hBN grits and the compound matrix (TiB2, AlN, and TiN). Therefore, a conclusion could be drawn that the threshold temperature for preparing the composite without hBN phase was 1400 °C. Fig. 6(a)–(d) presented the polished surface morphologies of the cBN-35Ti-20Al composite sintered at 1300–1600 °C, respectively. Fig. 6(a) presented some voids on the matrix, implying that the sintering temperature of 1300 °C was not high enough for densification of the composite. Comparing with Fig. 6(a), the composite sintered at 1400 ° C was denser, as shown in Fig. 6(b). The cBN grits were tightly bonded with the matrix. The results showed that Al and Ti were forced to infiltrate into the cBN grains at a higher temperature. And the resultants of TiN, TiB2, and AlN were chemically bonded with the cBN grains in composite, which resulted in forming a compact composite with optimal mechanical properties. It could be found that cBN grits in Fig. 6(a) and (b) had sharp edges, while the edges in the grits with dark surfaces
Fig. 3. Microstructure (a) and Raman spectra corresponding to different regions (b, c) of the cBN-35Ti-30Al composite sintered at 1400 °C.
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Fig. 4. Microstructure (a) and Raman spectra corresponding to different regions (b, c, d) of the cBN-35Ti-30Al composite sintered at 1500 °C.
Fig. 5. Microstructure (a) and Raman spectra corresponding to different regions (b, c) of the cBN-35Ti-30Al composite sintered at 1600 °C.
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Fig. 6. Polish face morphologies of the of cBN-35Ti-20Al composites sintered at the temperature of 1300 °C (a), 1400 °C (b), 1500 °C (c), and 1600 °C (d).
were not sharp (Fig. 6(c)). Besides, a rough surface was also found in Fig. 6(c), which can ascribe to the transformed hBN from cBN peeled off during polishing. Clearly, the appearance of the hBN phase weakened the strength of the grits and the mechanical properties of the composite decreased, especially the bending strength and the hardness. Nearly all cBN grits transformed into hBN when sintered at 1600 °C. Therefore, the surfaces of BN grits were easily polished due to the lower hardness of the composite compared with others, as shown in Fig. 6(d). Fig. 7 displayed the SEM morphology of the cBN-35Ti-20Al composite sintered at 1400 °C and its corresponding EDS mapping of B, N, Al, and Ti. It was found that the element B and N distributed on the whole composite due to the reaction between cBN and Al and Ti as Eqs. (1) and (2), as shown in Fig. 7(b) and (c). While, the elements Al and Ti exited in the matrix, rather than the BN grits, as shown in Fig. 7(d) and (e). Furthermore, the element Al mainly gathered around the BN grits (Fig. 7(e)). The phenomenon confirmed that compounds produced in the composite (TiN, TiB2, and AlN) were strongly bonded with the cBN grits in composite.
Fig. 8 exhibited the fracture morphologies of the cBN-35Ti-20Al composite sintered at different temperatures. Fig. 8(b) showed that a transcrystalline fracture occurred at the composite sintered at 1400 °C. This phenomenon demonstrated that the matrix of TiN, TiB2, and AlN had an excellent interface adhesive strength with cBN grits [15,16,29, 30], which was even higher than the strength of cBN particle itself. So the composite sintered at 1400 °C showed the maximum bending strength in cBN-35Ti-20Al composites (Fig. 1). Meanwhile, the intercrystalline fracture in both composites sintered at 1300 and 1500 °C was found in Fig. 8(a) and (c), which indicated a poor bond between the matrix and the grits. In Fig. 8(d), the grains surrounded by the matrix were hBN instead of cBN, which led to lower hardness and fracture toughness of the composite. 4. Conclusions The cBN-Ti-Al composites were prepared by the SPS method. The phase transformation of cBN to hBN had been restrained by SPS with
Fig. 7. SEM morphology of cBN-35Ti-20Al composites sintered at 1400 °C (a) and corresponding EDS mapping of Al (b), B (c), Ti (d), and N (e).
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Fig. 8. Fracture morphology of cBN-35Ti-20Al composites sintered at the temperature of 1300 °C (a), 1400 °C (b), 1500 °C (c), and 1600 °C (d).
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