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Synthesis and characterization of novel Ti3SiC2–cBN composites
Diamond & Related Materials 43 (2014) 29–33
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Synthesis and characterization of novel Ti3SiC2–cBN composites Zhengyang Li a, Aiguo Zhou a,⁎, Liang Li a,1, Libo Wang a, Meihua Hu a, Shangsheng Li a, Surojit Gupta b a Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China b Department of Mechanical Engineering, University of North Dakota, Grand Forks, ND 58201, USA
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Article history: Received 1 November 2013 Received in revised form 12 January 2014 Accepted 13 January 2014 Available online 22 January 2014 Keywords: Ti3SiC2 Cubic boron nitride Composites High pressure high temperature
a b s t r a c t In this paper, synthesis of novel super hard and high performance composites of titanium silicon carbide–cubic boron nitride (Ti3SiC2–cBN) was evaluated at three different conditions: (a) high pressure synthesis at ~4.5 GPa, (b) hot pressing at ~35 MPa, and (c) sintering under ambient pressure (0.1 MPa) in a tube furnace. From the analysis of experimental results, the authors report that the novel Ti3SiC2–cBN composites can be successfully fabricated at 1050 °C under a pressure of ~4.5 GPa from the mixture of Ti3SiC2 powders and cBN powders. The subsequent analysis of the microstructure and hardness studies indicates that these composites are promising candidates for super hard materials. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Super hard composites consist of super hard particles and a binder. Common examples of super hard particles are diamond or cubic boron nitride (cBN). Usually, there are two kinds of binders: (a) metal [1–4] and (b) ceramics (vitrified) [4–7]. Metal binder, for example Co or Al [2–4], possesses excellent room-temperature strength, but is easily softened and oxidized at high temperatures. Ceramic binders, for example TiC or TiN [4,7], have excellent high-temperature properties and oxidation resistance, however, its toughness is low, and the composites with ceramic binder are easily damaged at room-temperature. If a material combines the excellent properties of both ceramics and metals, then it can be used as a binder for fabricating remarkable super hard materials. It is well known that Ti3SiC2 is a ternary carbide that combines the properties of ceramics and metals [8–10]. Thus, it can be a promising candidate material as a binder for fabricating super hard composites with high performance. Jaworska et al. [11] fabricated Ti3SiC2–diamond composites under a pressure of ~ 8 GPa at 1800 °C. The starting materials were diamond particles and Ti3SiC2 powders. Benko et al. [11] made cBN–Ti3SiC2 composites under ~7 GPa at 1750 °C. The starting materials were cBN particles and Ti3SiC2 powders. The high pressure and high temperature (HPHT) in those processes is a typical condition for fabricating super hard materials. The high pressure (7 or 8 GPa) is generally used to avoid the phase transformation of cBN or diamond at higher ⁎ Corresponding author. Tel.: +86 391 3986936; fax: +86 391 3986908. E-mail address: [email protected] (A. Zhou). 1 Present address: Department of Materials Science and Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan. 0925-9635/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.diamond.2014.01.008
temperatures. However, the two papers did not show convincing evidence about the existence of Ti3SiC2 in those composites. For example, in the XRD pattern of HPHT Ti3SiC2–diamond composite (Fig. 5 of Jaworska's paper) [12], Ti3SiC2 peaks were very weak as compared to other phases. Similarly, Benko et al. [11] did not report any XRD pattern of HPHT Ti3SiC2–cBN composite. The TEM micrographs (Fig. 1 of Benko's paper) confirmed the presence of cBN, TiC, SiC and TiB2 by electron diffraction but no Ti3SiC2 was detected. Recently, Qin et al. [13] reported that Ti3SiC2 decomposes under high pressure at high temperature; for example, Ti3SiC2 starts to decompose at temperatures higher than 800 °C under ~5 GPa. The decomposition temperature also decreases with an increase of pressure. Thus, the existence of Ti3SiC2 in the above mentioned composites is debatable. Therefore, it is possible that most of the Ti3SiC2 in those composites [11,12] transformed to other materials at high temperature and high pressure. The critical question is, if the high pressure up to 5 GPa is unfavorable for the fabrication of Ti3SiC2 bonded super hard composites, whether low pressure is an advantage for the fabrication of the composites. Recently, Mu et al. [14] fabricated Ti3SiC2–diamond composites by spark plasma sintering at 1400 °C and at a low pressure of ~ 20 MPa. However, in the XRD pattern of those composites (Fig. 2 of Mu's paper), the peaks of diamond were very weak and almost cannot be detected. Most probably, a significant percentage of diamonds have transformed to graphite, and subsequently, reacted to form Ti3SiC2. Rampai et al. [14] also fabricated Ti3SiC2–cBN composites by a similar method at 1400 °C and ~20 MPa. From the XRD pattern of that sample (Fig. 13 of Rampai's paper), it can be concluded that significant amount of cBN transformed to hBN, and subsequently reacted with Ti3SiC2 to form TiN and TiB2, etc.
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Z. Li et al. / Diamond & Related Materials 43 (2014) 29–33
From the material processing perspective, there is an interesting dilemma, whether to use high pressures to fabricate the composites, because high pressure is required to resist the phase transformation of diamond or cBN, however, the high pressure also induces the decomposition of Ti3SiC2. Ti3SiC2 is stable at room temperature at pressures up to 61 GPa [15]. Ti3SiC2 is also stable up to ~ 200 MPa between 1400 and 1600 °C, since pressure is used to synthesize highly pure Ti3SiC2 by hot pressing (~40 MPa) or hot isostatic stressing (200 MPa) [10]. Therefore, it is fundamentally important to investigate and understand the synthesis of the composites at different pressures. Earlier, the authors had successfully fabricated Ti3SiC2 [16], Ti3SiC2– diamond composites [16] and Ti3AlC2–cBN composites [17] under 4.5 GPa by HPHT (Note: Ti3AlC2 has crystal structures and properties similar with those of Ti3SiC2 [18]). In this paper, we fabricated Ti3SiC2– cBN composites over a range of pressures by using different fabrication methods. The main aim of this paper is to determine the optimum pressure–temperature range and precursor materials for fabricating Ti3SiC2–cBN composites, and the role of pressure on the fabricating process is clarified.
X-ray diffraction (XRD, Bruker AXS Co., Germany) was used to determine the phase compositions of fabricated composites. Collected samples were examined by scanning electron microscopy (SEM, JSM-6390LV, JEOL, Japan) equipped with energy dispersive spectroscopy (EDS, INCA-ENERAGY 250, Oxford, UK) to reveal the microstructure. SEM was also used to observe the worn surfaces of some samples after friction test. Friction tests were carried out on a room-temperature tribometer with a pin-on-disk configuration (CFT-I Material Surface Performance Comprehensive Test Instrument, Zhongke KaiHua Instrument Equipment Co., China). The specimens were fixed in a square disk made of WC-based cermet. The pin was Si3 N 4 ball with the size of Φ ~ 4 mm. Hardness test measurement was done by a motor-driven Rockwell hardness tester (500MRA, Wolpert Wilson Instruments). HRA hardness of the sample was tested at a load of 60 kgf (588 N), and the loading time was 10 s.
3. Results and discussion 3.1. HPHT process
2. Experimental Starting materials used during this study were Ti (325 mesh, 99.3 wt.% pure, Aladdin Reagent Co., China), Si (200 mesh, 99 wt.% pure, Tianjin Weichen Chemical Reagents Co., China), SiC (325 mesh, 98 wt.% pure, Aladdin Reagent Co., China), TiC (200 mesh, 99.8 wt.% pure, Aladdin Reagent Co., China), lab-made Ti3SiC2 powders by a tube furnace [19], cBN particles (20–30 μm, Xingyang New Source Chemical Co., China) and hBN (200 mesh, 99.9 wt.% pure, Aladdin reagent Co., China). The compositions used during the fabrication of Ti3SiC2–cBN composites are listed in Table 1. All the powders were weighed and mixed by an airport star mixing machine for 4 h. High pressure and high temperature (HPHT) process was performed in a cubic anvil high-pressure apparatus (XKY-6×1200MN, Xianyang Superhard Materials Plant, Shaanxi, Chian) under 4.5 GPa at 1050 °C. The mixtures of starting materials were compacted in a steel mold into compacts with a diameter and height of 12 mm and 6 mm, respectively. The compacts were then put in the high-pressure apparatus. The desired temperature and pressure were achieved in ~3 min. All samples were then processed at the desired temperature and pressure for 10 min. Thereafter, the heating electrical power was shut off, and the pressure was released in ~15 min. Hot pressing (HP) processes were carried out in a vacuum hot press sintering furnace (ZTY-40-20, Shanghai Chenhua Electric Furnace Co., China) under ~35 MPa at 1450 °C for 60 min. The starting powder mixtures were put in a graphite mold protected by graphite foil with inner diameter of 45 mm. The mold was placed in the HP furnace. The HP furnace was heated at 10 °C/min to 1450 °C. The furnace was turn off after sintering at 1450 °C for 60 min, and the sample was cooled in the furnace. Pressureless sintering (PLS) was performed in a tube furnace with flowing Ar atmosphere (SJG-16B, Luoyang Shenjia Kiln Co., China). The starting powders were compacted in a steel mold. The compacts were sintered at 1500 °C for 240 min on an alumina boat. The heating rate was 10 °C/min during the PLS process.
Fig. 1 shows the XRD patterns of compositions 1, 2, 3, 4 and 5 after HPHT processing. Composition 1 was mainly composed of Ti3SiC2 and cBN with a little amount of TiC (Fig. 1). The mass ratio of TiC:Ti3SiC2 was estimated to be 15:85 according to a previous reported method [20]. Therefore, Ti3SiC2–cBN compacts can be sintered to fully dense bulk solid without the phase transformation of cBN, and a little amount of decomposition of Ti3SiC2 into TiC under reaction conditions of ~4.5 GPa and 1050 °C by HPHT. XRD pattern of compositions 2 and 3 shows the presence of Ti3SiC2. Thus, Ti3SiC2 can also be synthesized from the mixture of Ti–Si–C or Ti–SiC–C with the co-existence of cBN by HPHT. However, TiNx was also detected as impurities. Therefore, at ~4.5 GPa and 1050 °C, cBN reacts with Ti (compositions 2 and 3). Because Ti is indispensable as the starting materials for fabricating Ti3SiC2, thus it is not a favorable choice to in situ synthesize Ti3SiC2 during the fabrication of Ti3SiC2–cBN composites. For both compositions 2 and 3, an obvious peak at 26.5° was observed in the XRD patterns. This peak can be attributed to either unreacted graphite or hBN formed from the phase transformation of cBN at high temperatures. This peak was also observed in the XRD pattern of Ti3AlC2–cBN made by HPHT [17]. Similar to the earlier work [17], we think that the observed peak should belong to hBN. If C (graphite) exists in the starting materials, then as it is very active, it will react with Ti at high temperatures. As the reaction between C and Ti is exothermal, a lot of heat is generated in a short time. Local flash
Table 1 Recipes to make Ti3SiC2–cBN composites. No.
Starting materials
Mole ratio
Mass ratio
1) 2) 3) 4) 5) 6) 7)
Ti3SiC2:cBN Ti:Si:C:cBN Ti:SiC:C:cBN Ti:Si:TiC:cBN Ti:SiC:TiC:cBN Ti:Si:C:hBN Ti:SiC:C:hBN
1:1 3:1:2:1 3:1:1:1 1:1:2:1 2:1:1:1 3:1:2:1 3:1:1:1
89:11 65:13:11:11 65:18:5:11 22:13:54:11 43:18:27:11 65:13:11:11 65:18:5:11
Fig. 1. XRD patterns of recipes 1, 2, 3, 4, and 5 after HPHT processing.
Z. Li et al. / Diamond & Related Materials 43 (2014) 29–33
temperatures can induce the phase transformation of cBN to form hBN. Some hBN may react with Ti to form TiNx, whereas a small amount may exist in the sample also. Therefore, it is reasonable to attribute the 26.5° peak in Fig. 1 to hBN though we cannot rule out the possibility that it may belong to graphite. TiC and TiSix were detected from HPHT of the mixtures of Ti–Si–TiC (composition 4) and Ti–TiC–SiC (composition 5). Clearly, these mixtures are not suitable to fabricate Ti3SiC2 by HPHT because no Ti3SiC2 was synthesized by this process. Interestingly, another super hard material C3N4 was synthesized (composition 5). Most probably, the reactions between SiC, TiC and cBN formed the new phase, C3N4. It is well know that cBN powder production is a costly process as they are prepared from hBN by HPHT with the help of metal catalysts [21]. During HPHT process, if hBN can transform to cBN in the presence of Ti and Si, then cheap hBN powders can also be used to fabricate Ti3SiC2–cBN composites. We tried recipes 6 and 7 by the HPHT process. The XRD results are shown in Fig. 2. No Ti3SiC2 phase was synthesized from the mixture of Ti–SiC–C–hBN by HPHT. For the sample from Ti–Si–C–hBN, Ti3SiC2 peak at 39° and cBN peak at 43.5° were detected. It is possible to in situ synthesize Ti3SiC2 and transform hBN to cBN by HPHT from Ti–Si–C–hBN mixture. However, many impurities such as hBN, TiN, and TiC exist in the composites. Therefore, in future, it may be possible to fabricate Ti3SiC2–cBN composites from hBN, but more studies are needed to optimize and understand the manufacturing process. The hardness of the Ti3SiC2–cBN composition was 86.2 ± 1 HRA. It is equivalent to ~10.55 GPa. The composite was relatively soft as a super hard material. In this composite, the molar ratio of Ti3SiC2:cBN was 1:1, and the mass ratio was only 89:11. The composite with 11 wt.% cBN was easy to make but not very hard as a super hard material. For most super hard composites, the content percentage of cBN is much higher than this value. Therefore, Ti3SiC2–cBN composites with more cBN will have higher hardness. We are currently fabricating such composites and their results will be reported in the future. Fig. 3 shows the SEM micrographs of HPHT samples fabricated from Ti3SiC2–cBN. Fig. 3a shows the fractured surface of HPHT Ti3SiC2–cBN composites. In general, dark particles are evenly distributed in gray matrix. According to EDS analysis (not shown), the dark particles mainly consist of nitrogen element. Because only nitrogen element cannot form solids and boron element cannot be detected by EDS, combined with the results of XRD, it is reasonable that those dark particles are cBN. The main composition of the gray matrix is Ti3SiC2. Fig. 3b is a high magnification graph to show the interface. The left yellow rectangle indicates the layered microstructure of Ti3SiC2. The middle red rectangle indicates a clear interface and excellent bonding between Ti3SiC2 and cBN. However, a transition zone also exists as shown in the left blue rectangle. Therefore, there are two kinds of interface in the composite: clear interface without any inter diffusion and transition zone
Fig. 2. XRD patterns of recipes 6 and 7 after HPHT processing.
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Fig. 3. SEM graphs of HPHT sample from Ti3SiC2–cBN (a) fracture surface in low magnification, the dark particles are cBN and the gray matrix is Ti3SiC2 (b) bonding interface between cBN particle and Ti3SiC2 matrix (c) microstructure of worn surface after friction test.
with some inter diffusion between cBN and the matrix. Fig. 3c is the micrograph of HPHT samples after friction test. It can be seen that some areas of the worn surface show slight signs of scuffing and is extraordinarily smooth as shown in the left red rectangle. This indicates that the samples exhibit excellent bonding between Ti3SiC2 and cBN. Some areas are rough and show signs of severe scuffing as shown in the right blue cycle. This indicates the spalling of Ti3SiC2 particles from matrix. Fig. 4 shows the SEM results of HPHT samples from other starting materials. Fig. 4a is for the sample from Ti–Si–C–cBN powders, and Fig. 4b is for the sample from Ti–SiC–C–cBN powders. Both figures show similar morphology that dark cBN particles are distributed in gray matrix with complex structure. The matrix is mainly composed of Ti3SiC2, TiC, TiNx, TiBx, etc. Due to multitude of phases in the matrix, it is difficult to sinter and densify these compositions. Fig. 4c shows the SEM results of sample from Ti–Si–C–hBN mixture. Unlike other SEM
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Fig. 5. XRD patterns of HPed samples from Ti–Si–C–cBN, Ti–SiC–C–cBN, Ti–Si–TiC–cBN and Ti–SiC–TiC–cBN powders.
of Ti3SiC2. The authors believe that at the temperatures above 1300 K, cBN powders react with Ti to form TiB2 and TiNx [25]. Unlike the TiC formed from diamond and Ti, the TiB2 and TiNx cannot react with Si to form Ti3SiC2. Thus, the existence of cBN inhibits the synthesis of Ti3SiC2. 3.3. PLS process Fig. 6 shows the XRD patterns of PLS samples and the comparison of samples made at different pressures. No Ti3SiC2 peaks appear in the XRD patterns for samples made by PLS under 0.1 MPa. Ti3SiC2 cannot be
Fig. 4. SEM micrograph of HPHT sample from (a) Ti–Si–C–cBN powders, (b) Ti–SiC–C–cBN powders, and (c) Ti–Si–C–hBN powders.
micrographs, there are no any dark cBN particles in the matrix. This observation agrees with the XRD result of Fig. 2 that cBN was not obtained from hBN by HPHT. 3.2. HP process Fig. 5 shows the XRD patterns of HPed samples. The peaks of cBN are present in all the four analyzed samples. The sample with the composition Ti–Si–TiC–cBN has weak peaks of Ti3SiC2. This is an interesting result. It is well known that nearly pure Ti3SiC2 can be fabricated by hot pressing under similar conditions from the starting materials without cBN additions [22,23]. cBN is unstable at the temperature and pressure used during HP. Therefore, initially we assumed that Ti3SiC2 can be synthesized and almost no cBN is left by this process. During a previous work, Mu et al. [24] fabricated Ti3SiC2–diamond composites at 1400 °C under 20 MPa. Strong peaks of Ti3SiC2 in the XRD pattern of assynthesized sample were observed, and almost no diamond peaks were present (Fig. 2 of Mu's paper). Interestingly, unlike the addition of diamond, cBN addition in the starting materials inhibits the synthesis
Fig. 6. XRD patterns of samples made by different methods under different pressures from (a) Ti3SiC2–cBN, and (b) Ti–Si–C–cBN.
Z. Li et al. / Diamond & Related Materials 43 (2014) 29–33
synthesized in the presence of cBN even though PLS process is a common method to synthesize Ti3SiC2 [26,27]. It is well known that Ti3SiC2 is thermally stable in Ar atmosphere up to at least 1800 °C [28], the loss of Ti3SiC2 is not due to thermal decomposition at high temperature. It is due to the existence of cBN. Therefore, an interesting conclusion can be obtained from Fig. 6a — that the high pressure up to 4.5 GPa at HPHT keeps the thermal stability of Ti3SiC2 rather than cBN. By decreasing the pressure to a normal pressure of ~ 0.1 MPa can transform some cBN to hBN. However, Ti3SiC2 completely reacts with cBN or hBN to form TiB2 or TiN, thus no Ti3SiC2 phase was detected. Similar conclusion can be obtained from Fig. 6b. It is possible that a small amount of Ti3SiC2 was synthesized under 4.5 GPa with the existence of cBN though the high pressure is unfavorable for the synthesis of pure Ti3SiC2. If pressure was further decreased to 35 MPa, then only negligible amount of Ti3SiC2 was synthesized, however, cBN still existed. If pressure was further decreased to ~ 0.1 MPa, then neither Ti3SiC2, nor cBN was detected. 4. Conclusions Ti3SiC2–cBN composites can be successfully prepared by HPHT at 1050 °C and 4.5 GPa from the mixture of Ti3SiC2–cBN. From the analysis of composition, microstructure and primary mechanical test results, it can be concluded that these composites are promising candidates as super hard materials. Ti3SiC2 can be synthesized by HPHT from Ti– SiC–C–cBN or Ti–Si–C–cBN mixture with TiNx, TiSix, etc. as impurities. If C is replaced by TiC in both the mixtures as starting materials, then Ti3SiC2 cannot be synthesized. If the pressure during sintering is decreased to 35 MPa (as in HP) or 0.1 MPa (as in PLS), then some cBN can transform to hBN, and can react with Ti. Due to this reason almost no Ti3SiC2 was detected in the mixture. Therefore, high pressure up to 4.5 GPa is necessary to fabricate Ti3SiC2–cBN composites. Prime novelty statement In this article, Ti3SiC2–cBN composites were “really” made. The only temperature–pressure range and starting materials combination that can make these composites were obtained; the effect of pressure on the synthesis of these composites was understood. Acknowledgments This work was supported by the National Nature Science Foundation of China (51002045, 51205111), Plan for Scientific Innovation Talent of Henan Province (134100510008), Program for Innovative Research Team of Henan Polytechnic University (T2013-4), Fundamental and Advanced Research Project of Henan Province (102300410235), Doctoral Fund from Henan Polytechnic University (B2009-95), and the State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University (KF201313).
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Synthesis and characterization of novel Ti3SiC2–cBN composites Zhengyang Li a, Aiguo Zhou a,⁎, Liang Li a,1, Libo Wang a, Meihua Hu a, Shangsheng Li a, Surojit Gupta b a Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China b Department of Mechanical Engineering, University of North Dakota, Grand Forks, ND 58201, USA
a r t i c l e
i n f o
Article history: Received 1 November 2013 Received in revised form 12 January 2014 Accepted 13 January 2014 Available online 22 January 2014 Keywords: Ti3SiC2 Cubic boron nitride Composites High pressure high temperature
a b s t r a c t In this paper, synthesis of novel super hard and high performance composites of titanium silicon carbide–cubic boron nitride (Ti3SiC2–cBN) was evaluated at three different conditions: (a) high pressure synthesis at ~4.5 GPa, (b) hot pressing at ~35 MPa, and (c) sintering under ambient pressure (0.1 MPa) in a tube furnace. From the analysis of experimental results, the authors report that the novel Ti3SiC2–cBN composites can be successfully fabricated at 1050 °C under a pressure of ~4.5 GPa from the mixture of Ti3SiC2 powders and cBN powders. The subsequent analysis of the microstructure and hardness studies indicates that these composites are promising candidates for super hard materials. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Super hard composites consist of super hard particles and a binder. Common examples of super hard particles are diamond or cubic boron nitride (cBN). Usually, there are two kinds of binders: (a) metal [1–4] and (b) ceramics (vitrified) [4–7]. Metal binder, for example Co or Al [2–4], possesses excellent room-temperature strength, but is easily softened and oxidized at high temperatures. Ceramic binders, for example TiC or TiN [4,7], have excellent high-temperature properties and oxidation resistance, however, its toughness is low, and the composites with ceramic binder are easily damaged at room-temperature. If a material combines the excellent properties of both ceramics and metals, then it can be used as a binder for fabricating remarkable super hard materials. It is well known that Ti3SiC2 is a ternary carbide that combines the properties of ceramics and metals [8–10]. Thus, it can be a promising candidate material as a binder for fabricating super hard composites with high performance. Jaworska et al. [11] fabricated Ti3SiC2–diamond composites under a pressure of ~ 8 GPa at 1800 °C. The starting materials were diamond particles and Ti3SiC2 powders. Benko et al. [11] made cBN–Ti3SiC2 composites under ~7 GPa at 1750 °C. The starting materials were cBN particles and Ti3SiC2 powders. The high pressure and high temperature (HPHT) in those processes is a typical condition for fabricating super hard materials. The high pressure (7 or 8 GPa) is generally used to avoid the phase transformation of cBN or diamond at higher ⁎ Corresponding author. Tel.: +86 391 3986936; fax: +86 391 3986908. E-mail address: [email protected] (A. Zhou). 1 Present address: Department of Materials Science and Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan. 0925-9635/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.diamond.2014.01.008
temperatures. However, the two papers did not show convincing evidence about the existence of Ti3SiC2 in those composites. For example, in the XRD pattern of HPHT Ti3SiC2–diamond composite (Fig. 5 of Jaworska's paper) [12], Ti3SiC2 peaks were very weak as compared to other phases. Similarly, Benko et al. [11] did not report any XRD pattern of HPHT Ti3SiC2–cBN composite. The TEM micrographs (Fig. 1 of Benko's paper) confirmed the presence of cBN, TiC, SiC and TiB2 by electron diffraction but no Ti3SiC2 was detected. Recently, Qin et al. [13] reported that Ti3SiC2 decomposes under high pressure at high temperature; for example, Ti3SiC2 starts to decompose at temperatures higher than 800 °C under ~5 GPa. The decomposition temperature also decreases with an increase of pressure. Thus, the existence of Ti3SiC2 in the above mentioned composites is debatable. Therefore, it is possible that most of the Ti3SiC2 in those composites [11,12] transformed to other materials at high temperature and high pressure. The critical question is, if the high pressure up to 5 GPa is unfavorable for the fabrication of Ti3SiC2 bonded super hard composites, whether low pressure is an advantage for the fabrication of the composites. Recently, Mu et al. [14] fabricated Ti3SiC2–diamond composites by spark plasma sintering at 1400 °C and at a low pressure of ~ 20 MPa. However, in the XRD pattern of those composites (Fig. 2 of Mu's paper), the peaks of diamond were very weak and almost cannot be detected. Most probably, a significant percentage of diamonds have transformed to graphite, and subsequently, reacted to form Ti3SiC2. Rampai et al. [14] also fabricated Ti3SiC2–cBN composites by a similar method at 1400 °C and ~20 MPa. From the XRD pattern of that sample (Fig. 13 of Rampai's paper), it can be concluded that significant amount of cBN transformed to hBN, and subsequently reacted with Ti3SiC2 to form TiN and TiB2, etc.
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From the material processing perspective, there is an interesting dilemma, whether to use high pressures to fabricate the composites, because high pressure is required to resist the phase transformation of diamond or cBN, however, the high pressure also induces the decomposition of Ti3SiC2. Ti3SiC2 is stable at room temperature at pressures up to 61 GPa [15]. Ti3SiC2 is also stable up to ~ 200 MPa between 1400 and 1600 °C, since pressure is used to synthesize highly pure Ti3SiC2 by hot pressing (~40 MPa) or hot isostatic stressing (200 MPa) [10]. Therefore, it is fundamentally important to investigate and understand the synthesis of the composites at different pressures. Earlier, the authors had successfully fabricated Ti3SiC2 [16], Ti3SiC2– diamond composites [16] and Ti3AlC2–cBN composites [17] under 4.5 GPa by HPHT (Note: Ti3AlC2 has crystal structures and properties similar with those of Ti3SiC2 [18]). In this paper, we fabricated Ti3SiC2– cBN composites over a range of pressures by using different fabrication methods. The main aim of this paper is to determine the optimum pressure–temperature range and precursor materials for fabricating Ti3SiC2–cBN composites, and the role of pressure on the fabricating process is clarified.
X-ray diffraction (XRD, Bruker AXS Co., Germany) was used to determine the phase compositions of fabricated composites. Collected samples were examined by scanning electron microscopy (SEM, JSM-6390LV, JEOL, Japan) equipped with energy dispersive spectroscopy (EDS, INCA-ENERAGY 250, Oxford, UK) to reveal the microstructure. SEM was also used to observe the worn surfaces of some samples after friction test. Friction tests were carried out on a room-temperature tribometer with a pin-on-disk configuration (CFT-I Material Surface Performance Comprehensive Test Instrument, Zhongke KaiHua Instrument Equipment Co., China). The specimens were fixed in a square disk made of WC-based cermet. The pin was Si3 N 4 ball with the size of Φ ~ 4 mm. Hardness test measurement was done by a motor-driven Rockwell hardness tester (500MRA, Wolpert Wilson Instruments). HRA hardness of the sample was tested at a load of 60 kgf (588 N), and the loading time was 10 s.
3. Results and discussion 3.1. HPHT process
2. Experimental Starting materials used during this study were Ti (325 mesh, 99.3 wt.% pure, Aladdin Reagent Co., China), Si (200 mesh, 99 wt.% pure, Tianjin Weichen Chemical Reagents Co., China), SiC (325 mesh, 98 wt.% pure, Aladdin Reagent Co., China), TiC (200 mesh, 99.8 wt.% pure, Aladdin Reagent Co., China), lab-made Ti3SiC2 powders by a tube furnace [19], cBN particles (20–30 μm, Xingyang New Source Chemical Co., China) and hBN (200 mesh, 99.9 wt.% pure, Aladdin reagent Co., China). The compositions used during the fabrication of Ti3SiC2–cBN composites are listed in Table 1. All the powders were weighed and mixed by an airport star mixing machine for 4 h. High pressure and high temperature (HPHT) process was performed in a cubic anvil high-pressure apparatus (XKY-6×1200MN, Xianyang Superhard Materials Plant, Shaanxi, Chian) under 4.5 GPa at 1050 °C. The mixtures of starting materials were compacted in a steel mold into compacts with a diameter and height of 12 mm and 6 mm, respectively. The compacts were then put in the high-pressure apparatus. The desired temperature and pressure were achieved in ~3 min. All samples were then processed at the desired temperature and pressure for 10 min. Thereafter, the heating electrical power was shut off, and the pressure was released in ~15 min. Hot pressing (HP) processes were carried out in a vacuum hot press sintering furnace (ZTY-40-20, Shanghai Chenhua Electric Furnace Co., China) under ~35 MPa at 1450 °C for 60 min. The starting powder mixtures were put in a graphite mold protected by graphite foil with inner diameter of 45 mm. The mold was placed in the HP furnace. The HP furnace was heated at 10 °C/min to 1450 °C. The furnace was turn off after sintering at 1450 °C for 60 min, and the sample was cooled in the furnace. Pressureless sintering (PLS) was performed in a tube furnace with flowing Ar atmosphere (SJG-16B, Luoyang Shenjia Kiln Co., China). The starting powders were compacted in a steel mold. The compacts were sintered at 1500 °C for 240 min on an alumina boat. The heating rate was 10 °C/min during the PLS process.
Fig. 1 shows the XRD patterns of compositions 1, 2, 3, 4 and 5 after HPHT processing. Composition 1 was mainly composed of Ti3SiC2 and cBN with a little amount of TiC (Fig. 1). The mass ratio of TiC:Ti3SiC2 was estimated to be 15:85 according to a previous reported method [20]. Therefore, Ti3SiC2–cBN compacts can be sintered to fully dense bulk solid without the phase transformation of cBN, and a little amount of decomposition of Ti3SiC2 into TiC under reaction conditions of ~4.5 GPa and 1050 °C by HPHT. XRD pattern of compositions 2 and 3 shows the presence of Ti3SiC2. Thus, Ti3SiC2 can also be synthesized from the mixture of Ti–Si–C or Ti–SiC–C with the co-existence of cBN by HPHT. However, TiNx was also detected as impurities. Therefore, at ~4.5 GPa and 1050 °C, cBN reacts with Ti (compositions 2 and 3). Because Ti is indispensable as the starting materials for fabricating Ti3SiC2, thus it is not a favorable choice to in situ synthesize Ti3SiC2 during the fabrication of Ti3SiC2–cBN composites. For both compositions 2 and 3, an obvious peak at 26.5° was observed in the XRD patterns. This peak can be attributed to either unreacted graphite or hBN formed from the phase transformation of cBN at high temperatures. This peak was also observed in the XRD pattern of Ti3AlC2–cBN made by HPHT [17]. Similar to the earlier work [17], we think that the observed peak should belong to hBN. If C (graphite) exists in the starting materials, then as it is very active, it will react with Ti at high temperatures. As the reaction between C and Ti is exothermal, a lot of heat is generated in a short time. Local flash
Table 1 Recipes to make Ti3SiC2–cBN composites. No.
Starting materials
Mole ratio
Mass ratio
1) 2) 3) 4) 5) 6) 7)
Ti3SiC2:cBN Ti:Si:C:cBN Ti:SiC:C:cBN Ti:Si:TiC:cBN Ti:SiC:TiC:cBN Ti:Si:C:hBN Ti:SiC:C:hBN
1:1 3:1:2:1 3:1:1:1 1:1:2:1 2:1:1:1 3:1:2:1 3:1:1:1
89:11 65:13:11:11 65:18:5:11 22:13:54:11 43:18:27:11 65:13:11:11 65:18:5:11
Fig. 1. XRD patterns of recipes 1, 2, 3, 4, and 5 after HPHT processing.
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temperatures can induce the phase transformation of cBN to form hBN. Some hBN may react with Ti to form TiNx, whereas a small amount may exist in the sample also. Therefore, it is reasonable to attribute the 26.5° peak in Fig. 1 to hBN though we cannot rule out the possibility that it may belong to graphite. TiC and TiSix were detected from HPHT of the mixtures of Ti–Si–TiC (composition 4) and Ti–TiC–SiC (composition 5). Clearly, these mixtures are not suitable to fabricate Ti3SiC2 by HPHT because no Ti3SiC2 was synthesized by this process. Interestingly, another super hard material C3N4 was synthesized (composition 5). Most probably, the reactions between SiC, TiC and cBN formed the new phase, C3N4. It is well know that cBN powder production is a costly process as they are prepared from hBN by HPHT with the help of metal catalysts [21]. During HPHT process, if hBN can transform to cBN in the presence of Ti and Si, then cheap hBN powders can also be used to fabricate Ti3SiC2–cBN composites. We tried recipes 6 and 7 by the HPHT process. The XRD results are shown in Fig. 2. No Ti3SiC2 phase was synthesized from the mixture of Ti–SiC–C–hBN by HPHT. For the sample from Ti–Si–C–hBN, Ti3SiC2 peak at 39° and cBN peak at 43.5° were detected. It is possible to in situ synthesize Ti3SiC2 and transform hBN to cBN by HPHT from Ti–Si–C–hBN mixture. However, many impurities such as hBN, TiN, and TiC exist in the composites. Therefore, in future, it may be possible to fabricate Ti3SiC2–cBN composites from hBN, but more studies are needed to optimize and understand the manufacturing process. The hardness of the Ti3SiC2–cBN composition was 86.2 ± 1 HRA. It is equivalent to ~10.55 GPa. The composite was relatively soft as a super hard material. In this composite, the molar ratio of Ti3SiC2:cBN was 1:1, and the mass ratio was only 89:11. The composite with 11 wt.% cBN was easy to make but not very hard as a super hard material. For most super hard composites, the content percentage of cBN is much higher than this value. Therefore, Ti3SiC2–cBN composites with more cBN will have higher hardness. We are currently fabricating such composites and their results will be reported in the future. Fig. 3 shows the SEM micrographs of HPHT samples fabricated from Ti3SiC2–cBN. Fig. 3a shows the fractured surface of HPHT Ti3SiC2–cBN composites. In general, dark particles are evenly distributed in gray matrix. According to EDS analysis (not shown), the dark particles mainly consist of nitrogen element. Because only nitrogen element cannot form solids and boron element cannot be detected by EDS, combined with the results of XRD, it is reasonable that those dark particles are cBN. The main composition of the gray matrix is Ti3SiC2. Fig. 3b is a high magnification graph to show the interface. The left yellow rectangle indicates the layered microstructure of Ti3SiC2. The middle red rectangle indicates a clear interface and excellent bonding between Ti3SiC2 and cBN. However, a transition zone also exists as shown in the left blue rectangle. Therefore, there are two kinds of interface in the composite: clear interface without any inter diffusion and transition zone
Fig. 2. XRD patterns of recipes 6 and 7 after HPHT processing.
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Fig. 3. SEM graphs of HPHT sample from Ti3SiC2–cBN (a) fracture surface in low magnification, the dark particles are cBN and the gray matrix is Ti3SiC2 (b) bonding interface between cBN particle and Ti3SiC2 matrix (c) microstructure of worn surface after friction test.
with some inter diffusion between cBN and the matrix. Fig. 3c is the micrograph of HPHT samples after friction test. It can be seen that some areas of the worn surface show slight signs of scuffing and is extraordinarily smooth as shown in the left red rectangle. This indicates that the samples exhibit excellent bonding between Ti3SiC2 and cBN. Some areas are rough and show signs of severe scuffing as shown in the right blue cycle. This indicates the spalling of Ti3SiC2 particles from matrix. Fig. 4 shows the SEM results of HPHT samples from other starting materials. Fig. 4a is for the sample from Ti–Si–C–cBN powders, and Fig. 4b is for the sample from Ti–SiC–C–cBN powders. Both figures show similar morphology that dark cBN particles are distributed in gray matrix with complex structure. The matrix is mainly composed of Ti3SiC2, TiC, TiNx, TiBx, etc. Due to multitude of phases in the matrix, it is difficult to sinter and densify these compositions. Fig. 4c shows the SEM results of sample from Ti–Si–C–hBN mixture. Unlike other SEM
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Fig. 5. XRD patterns of HPed samples from Ti–Si–C–cBN, Ti–SiC–C–cBN, Ti–Si–TiC–cBN and Ti–SiC–TiC–cBN powders.
of Ti3SiC2. The authors believe that at the temperatures above 1300 K, cBN powders react with Ti to form TiB2 and TiNx [25]. Unlike the TiC formed from diamond and Ti, the TiB2 and TiNx cannot react with Si to form Ti3SiC2. Thus, the existence of cBN inhibits the synthesis of Ti3SiC2. 3.3. PLS process Fig. 6 shows the XRD patterns of PLS samples and the comparison of samples made at different pressures. No Ti3SiC2 peaks appear in the XRD patterns for samples made by PLS under 0.1 MPa. Ti3SiC2 cannot be
Fig. 4. SEM micrograph of HPHT sample from (a) Ti–Si–C–cBN powders, (b) Ti–SiC–C–cBN powders, and (c) Ti–Si–C–hBN powders.
micrographs, there are no any dark cBN particles in the matrix. This observation agrees with the XRD result of Fig. 2 that cBN was not obtained from hBN by HPHT. 3.2. HP process Fig. 5 shows the XRD patterns of HPed samples. The peaks of cBN are present in all the four analyzed samples. The sample with the composition Ti–Si–TiC–cBN has weak peaks of Ti3SiC2. This is an interesting result. It is well known that nearly pure Ti3SiC2 can be fabricated by hot pressing under similar conditions from the starting materials without cBN additions [22,23]. cBN is unstable at the temperature and pressure used during HP. Therefore, initially we assumed that Ti3SiC2 can be synthesized and almost no cBN is left by this process. During a previous work, Mu et al. [24] fabricated Ti3SiC2–diamond composites at 1400 °C under 20 MPa. Strong peaks of Ti3SiC2 in the XRD pattern of assynthesized sample were observed, and almost no diamond peaks were present (Fig. 2 of Mu's paper). Interestingly, unlike the addition of diamond, cBN addition in the starting materials inhibits the synthesis
Fig. 6. XRD patterns of samples made by different methods under different pressures from (a) Ti3SiC2–cBN, and (b) Ti–Si–C–cBN.
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synthesized in the presence of cBN even though PLS process is a common method to synthesize Ti3SiC2 [26,27]. It is well known that Ti3SiC2 is thermally stable in Ar atmosphere up to at least 1800 °C [28], the loss of Ti3SiC2 is not due to thermal decomposition at high temperature. It is due to the existence of cBN. Therefore, an interesting conclusion can be obtained from Fig. 6a — that the high pressure up to 4.5 GPa at HPHT keeps the thermal stability of Ti3SiC2 rather than cBN. By decreasing the pressure to a normal pressure of ~ 0.1 MPa can transform some cBN to hBN. However, Ti3SiC2 completely reacts with cBN or hBN to form TiB2 or TiN, thus no Ti3SiC2 phase was detected. Similar conclusion can be obtained from Fig. 6b. It is possible that a small amount of Ti3SiC2 was synthesized under 4.5 GPa with the existence of cBN though the high pressure is unfavorable for the synthesis of pure Ti3SiC2. If pressure was further decreased to 35 MPa, then only negligible amount of Ti3SiC2 was synthesized, however, cBN still existed. If pressure was further decreased to ~ 0.1 MPa, then neither Ti3SiC2, nor cBN was detected. 4. Conclusions Ti3SiC2–cBN composites can be successfully prepared by HPHT at 1050 °C and 4.5 GPa from the mixture of Ti3SiC2–cBN. From the analysis of composition, microstructure and primary mechanical test results, it can be concluded that these composites are promising candidates as super hard materials. Ti3SiC2 can be synthesized by HPHT from Ti– SiC–C–cBN or Ti–Si–C–cBN mixture with TiNx, TiSix, etc. as impurities. If C is replaced by TiC in both the mixtures as starting materials, then Ti3SiC2 cannot be synthesized. If the pressure during sintering is decreased to 35 MPa (as in HP) or 0.1 MPa (as in PLS), then some cBN can transform to hBN, and can react with Ti. Due to this reason almost no Ti3SiC2 was detected in the mixture. Therefore, high pressure up to 4.5 GPa is necessary to fabricate Ti3SiC2–cBN composites. Prime novelty statement In this article, Ti3SiC2–cBN composites were “really” made. The only temperature–pressure range and starting materials combination that can make these composites were obtained; the effect of pressure on the synthesis of these composites was understood. Acknowledgments This work was supported by the National Nature Science Foundation of China (51002045, 51205111), Plan for Scientific Innovation Talent of Henan Province (134100510008), Program for Innovative Research Team of Henan Polytechnic University (T2013-4), Fundamental and Advanced Research Project of Henan Province (102300410235), Doctoral Fund from Henan Polytechnic University (B2009-95), and the State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University (KF201313).
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