High Fracture Toughness of Al2O3-TiN0.3 Composites

High Fracture Toughness of Al2O3-TiN0.3 Composites High fracture toughness of Al2O3-TiN0.3 composites prepared via spark plasma sintering Lina Qiaoa, b, Yucheng Zhaoa, Mingzhi Wanga, à ¯Ã‚ Ã¢â‚¬ ºÃƒ ¯Ã¢â€š ¬Ã‚ ªÃƒ ¯Ã‚ Ã‚ , Yana Yea, Junxing Zhanga, Qin Zoua, Qian Yanga, Hua Dengc, Ying Xingc Abstract Al2O3–TiN0.3 composites with different TiN0.3 contents were spark plasma sintered at 1300–1600  °C for 10 min. Phase identification was characterized through X-ray diffraction. Microstructures were observed using a scanning electron microscope. The fracture toughness of the composite with 30 vol% TiN0.3 sintered at 1500  °C reaches to the highest value of 6.91 MPa m1/2. Based on the first-principles density functional theory, the density of states for TiN and TiNx was calculated. The covalent bonding is weakened and the metallic bonding is strengthened as the nitrogen concentration is reduced in nonstoichiometric TiNx. The active slip systems determined by covalent bonding for the nitrides are possibly increased by adding nonstoichiometric TiN0.3, which improves the fracture toughness of Al2O3-based composites. Keywords: Al2O3–TiN0.3 composites;Fracture toughness; Slip system; Bond calculation 1. Introduction Alumina (Al2O3) ceramics are essential structural materials, but the inherent brittleness has inhibited their applications [1, 2]. The fracture toughness can be improved substantially by adding a secondary reinforcing phase into the matrix. The effects of TiN particles on the mechanical properties of Al2O3-based composites have been widely studied [3–9]. Shen et al. [9] reported that the fracture toughness of Al2O3–TiN composites prepared via spark plasma sintering (SPS) at 1500  °C reaches to a maximum value of 5.7 MPa m1/2. Li et al. [1] studied the mechanical properties of TiN–Al2O3 nanocomposites prepared by hot pressing at 1550  °C, and pointed out that the highest fracture toughness is 5.27 MPa m1/2. However, there have been few reports about the effects of nonstoichiometric TiN0.3 on the fracture toughness of Al2O3-based composites. In this study, nonstoichiometricTiN0.3 was added into Al2O3 matrix, and the effects of TiN0.3 on the mechanical properties (especially fracture toughness) of the composites were discussed. Nonstoichiometric TiN0.3 synthesized via mechanical alloying (MA) possesses fine grain size and TiN-type structure with numerous N vacancies [10, 11], which are conducive to improving sinterability [11–14]. Furthermore, weakening covalent bond and strengthening metallic bond in TiN0.3 structure [15, 16] may have an important influence on the fracture toughness. This study aims to verify whether or not adding nonstoichiometric materials can increase the fracture toughness of Al2O3-based composites. 2. Experimental Raw materials used include TiN0.3 synthesized through MA [10, 11] and commercial powders Al2O3 (analytically pure, an average particle size of 1 ÃŽ ¼m). Powder mixtures were milled for 2 h in absolute ethanol using WC milling media on a Pulverisette 4 Vario-Planetary Mill (FRTSCH German) at 300 rpm. SPS (3.20 MK-IV, Sumitomo Coal Mining Co., Ltd.) was performed in vacuum (6Ãâ€"10−3 Pa) at different heat treatment temperatures (1300–1600  °C) for 10min at 30 MPa. The heating rate was 100  °C/min. The temperature was determined using an optical pyrometer focused on the non-through hole located on the surface of the graphite die. Phase identification was performed through X-ray diffraction (XRD) with Cu KÃŽ ± radiation by using a D/MAX-2500PC diffractometer (Rigaku). Microstructures of the specimen’s polished surface and fracture cross-sections were observed using an S-3400N (Hitachis) scanning electron microscope (SEM) equipped with electron back-scattered diffraction (Edax-Tsl, Ametek). The bending strength was measured with Instron-5848 MicroTester (America) using the three point bending test with a span length of 13 mm and crosshead speed of 0.5 mm/min. Fracture toughness was determined through the Vickers indentation method proposed by Anstis et. al [17]. Measurements of the hardness and fracture toughness were conducted using an FM-700 Vickers hardness tester (Future-Tech, Japan) by indentation using a pyramidal indenter and applying a 10 kg load for 10 s. 3. Results 3.1 XRD identification and morphology observation Fig. 1 shows the XRD patterns of Al2O3–30 vol% TiN0.3 composite sintered via SPS at different temperatures in vacuum (6Ãâ€"103 Pa) for 10 min. Only TiN0.3 and ÃŽ ±-Al2O3 phases are detected in the XRD patterns. It suggests that no chemical reaction occurs between the second phase and the matrix. Fig. 2 shows the back-scattered SEM micrograph of the polished surface of Al2O3–30 vol% TiN0.3 composite sintered via SPS at 1400  °C in vacuum (6Ãâ€"103 Pa) for 10 min. The gray grains are Al2O3, while the white ones are TiN0.3. TiN0.3 grains are uniformly dispersed in Al2O3 matrix. Fig. 3 shows the microstructure of the fracture cross-sections of Al2O3–30 vol% TiN0.3 composite sintered via SPS at different temperatures in vacuum (6Ãâ€"103 Pa) for 10 min. When the sintering temperature is raised to 1400  °C, the grain size of the composite is fine and approximately 2 ÃŽ ¼m for Al2O3; the fracture mode is mainly intergranular (Fig. 3 b). Then the gains grew obviously with further raising the sintering temperature, here ~3-4 ÃŽ ¼m at 1500  °C and ~4-5 ÃŽ ¼m at 1600  °C for Al2O3; the fracture modes are intergranular and transgranular (Fig. 3 c and d). In addition, Al2O3–30 vol% TiN0.3 composite has not reached full density at 1300  °C, as indicated both by the SEM observations (Fig. 3) and measured hardness values (Fig. 5). Fig. 4 shows the microstructure of the fracture cross-sections of Al2O3–TiN0.3 composites with different TiN0.3 contents sintered via SPS at 1400  °C in vacuum (6Ãâ€"103 Pa) for 10 min. The grain size of Al2O3 existed in all samples does not change significantly. It is not agreement with the previous study that the addition of TiN effectively inhibits the grain growth of Al2O3 [9]. This phenomenon may be attributed to the fact that Al2O3–TiN0.3 composites have good sinterability. In addition, the fracture morphology is influenced by TiN0.3 content in these samples. The fracture mode of Al2O3–TiN0.3 composites with TiN0.3 contents from 10 vol% to 30 vol% (Fig. 4 a–c) is mainly intergranular. But, the fracture modes of Al2O3–TiN0.3 composite with 40 vol% TiN0.3 (Fig. 4 d) are intergranular and transgranular. The explanation for the fracture mode change is that the grain boundaries in Al2O3–TiN0.3 composites are strengthened, inhibiting inter granular crack propagation. 3.2 Mechanical properties Fig. 5 a shows the Vickers hardness of Al2O3–30 vol% TiN0.3 composite sintered at different temperatures. The Vickers hardness of Al2O3–30 vol% TiN0.3 composite sintered at 1400  °C reaches to the highest value of 18.75 GPa, then slightly reduces with raising the sintering temperature, which is due to grain growth [9, 18, 19] (Fig. 3 b-d). Fig. 5 b shows the Vickers hardness of Al2O3–TiN0.3 composites sintered at 1400  °C versus TiN0.3 content. The Vickers hardness of Al2O3–TiN0.3 composites with different TiN0.3 contents from 10 vol% to 40 vol% reaches to a range of 17–19 GPa, which is no significant difference from that of pure Al2O3 and close to that of Al2O3–TiN nanocomposites [1]. Fig. 6 shows the bending strength of Al2O3–TiN0.3 composites sintered at 1400  °C versus TiN0.3 content. The bending strength of Al2O3–TiN0.3 composites sintered at 1400  °C increases with increasing TiN0.3 contents from 10 vol% to 40 vol%, and is higher than that of Al2O3 ceramics. As adding TiN0.3 into Al2O3 matrix, the microstructure is improved and the grain boundaries are strengthened, which lead to an increase in the bending strength of Al2O3–TiN0.3 composites. The fracture toughness of the composite with 30 vol% TiN0.3 sintered at 1500  °C reaches to the highest value of 6.91 MPa m1/2, as shown in Fig. 5 a, which is much higher than that of nano- or micron-sized Al2O3–TiN composites [1, 4, 5, 9, 20]. And the fracture toughness of the composites sintered at 1400  °C increases with the addition of TiN0.3, and presents a maximum value of 6.60 MPa m1/2 at 30 vol% TiN0.3, then decreases with further increasing the amount of TiN0.3, as shown in Fig. 5 b. These results are in agreement with previous studies [1, 4, 5, 9, 20]. For particulate reinforced composites, many toughening mechanisms such as crack pinning, microcrack toughening, crack deflection, residual stress toughening and crack bridging have been proposed. For TiN–Al2O3 composites, Li et al. [1] reported that possible toughening mechanisms are crack deflections and/ or crack pinning; Shen et al. [9] pointed out that the predominating toughening mechanism is related to crack tilting and twisting caused by thermal expansion and/ or elastic modulus mismatch stresses. It is difficult to indicate a prevailing toughening mechanism. In this research, maybe some of these toughening mechanisms are active at the same time. Nonetheless, due to structure defect, TiN0.3 may have an important influence on the fracture toughness. It will be discussed subsequently in more detail. 4. Discussion The above experimental results suggest that adding a nonstoichiometric TiN0.3 phase is more effective for improving the fracture toughness of Al2O3-based composites. To explain the phenomenon, based on the first-principles density functional theory [15, 16, 21], the density of states (DOS) for TiN and TiNx was calculated, as shown in Fig 7. Close to the Fermi level, the DOS for TiN consists of hybridized Ti-3d and N-2p states, as shown in Fig. 7. It can be seen that the DOS for TiN at the Fermi level is not at the minimum and mainly dominated by Ti-3d states. This is an evidence that the cohesion in TiN is a complex mixture of covalent, ionic (a little) and metallic types. The new structures in the DOS for TiNx near the Fermi level can clearly be seen in Fig. 7, which are called ‘vacancy state associated structures’ [15, 16]. It can be explained by symmetry changes resulting from the vacancy sites in the lattice. Titanium atoms are completely equivalent in a perfect stoichiometric rocksalt structure. But, in a nonstoichiometric structure, both Ti neighboring levels of symmetry interact together through a vacancy (symbolized by à ¢- ¡) to establish a Ti–à ¢- ¡Ã¢â‚¬â€œTi bond which is absent in the stoichiometric titanium compound. In other words, the covalent bonding is weakened and the metallic bonding is strengthened as the nitrogen concentration is reduced in nonstoichiometric TiNx, which are indicated by the peaks observed near the Fermi level on the DOS curves in Fig. 7 and in accordance with Refs. [15, 16]. Al2O3 is a kind of brittle material due to the lack of active slip system essentially. The active slip systems determined by covalent bonding for the nitrides can be increased by adding a nonstoichiometric material. Rowcliffe et al. [22] had reported that TiC has the {111} 0> slip system at high temperature and the {110} 0> slip system at room temperature. The root cause of the change of the slip systems is that the cohesion in TiC is a complex mixture of covalent, ionic (a little) and metallic types. At low temperature, the bonding is covalent with strong, directional bonding between neighboring carbon and metal atoms; as the temperature is raised, electrons are transferred from these bonds into less localized metallic states. Such a transfer has the effect of reducing both the directionality and strength of the bonds. They also pointed out that the covalent contribution to bonding becomes less as the carbon concentration in nonstoichiometric TiCx decreases [20]. TiC and TiN crystals belong to the same space group (FM-3M, cubic system) and the atomic radii of C and N are closed. It is inferred that TiN (or TiNx) has the similar slip system. Same as previous analysis, the nitrogen concentration in TiN0.3 is very low, which leads to weakening covalent bond and strengthening metallic bond. Maybe the {111} 0> slip system, or some of it, is active at room temperature. In other words, there may be more active slip systems at room temperature in Al2O3–TiN0.3 composites. This is a major reason for the improvement of the fracture toughness of Al2O3–TiN0.3 composites. 4. Conclusions This paper introduces a new and effective method to improve the fracture toughness of Al2O3-based composites by adding a nonstoichiometric material. Al2O3–TiN0.3 composites were prepared via SPS at a relatively low temperature. The fracture toughness and bending strength of the composites have been greatly improved and the hardness is almost identical to that of Al2O3 matrix. Based on the first-principles density functional theory, the DOS for TiN and TiNx was calculated. The covalent bonding is weakened and the metallic bonding is strengthened as the nitrogen concentration is reduced in nonstoichiometric TiNx. The active slip systems determined by covalent bonding for the nitrides are possibly increased by adding nonstoichiometric TiN0.3, which improves the fracture toughness of Al2O3-based composites. Acknowledgments The authors gratefully acknowledge financial support from Key Laboratory of Metastable Materials Science and Technology, the Science and Technology Foundation of Hebei (E2012203116), the Key Item of Education Department of Hebei (ZH2012003), Synergy Innovation Plan Project of College of Mechanical Engineering (JX2014-3), and Heavy Machinery Synergy Innovation Plan Project (ZX01-20140100-01). References [1] Jingguo Li, Lian Gao, Jingkun Guo. Mechanical properties and electrical conductivity of TiN–Al2O3 nanocomposites. J. Eur. Ceram. Soc. 2003; 23: 74-6. [2] Songlin Ran, Lian Gao. Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering. Ceram. Int. 2012; 38: 4928-6. [3] Bellosi A., Guicciardi S., Tampieri A.. 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Soc. 1976; 59 (7-8): 372-2. [20] Egawa T., Ichikizaki T., Tsukamoto H., Tsunoda H., Shimoyama T.. Material characteristics and cutting performance of TiN–Al2O3 ceramic tool. Int. J. Jpn. Soc. Precis. Eng. 1995; 29 (3): 228-7. [21] Y. Yang, H. Lu, C. Yu, J.M. Chen. First-principles calculations of mechanical properties of TiC and TiN. J. Alloys Compd. 2009; 485: 547-6. [22] R. H. J. Hannink, D. L. Kohlstedt, M. J. Murray. Slip system determination in cubic carbides by hardness anisotropy. ProcRoy Soc. 1972; 326 A (1566): 420-12. Figure captions Fig. 1 X-ray diffraction patterns of Al2O3–30 vol% TiN0.3 composite sintered via SPS at different temperatures in vacuum (6Ãâ€"103 Pa) for 10 min. Fig. 2 Back-scattered SEM micrograph of polished surface of Al2O3–30 vol% TiN0.3 composite sintered via SPS at 1400  °C in vacuum (6Ãâ€"103 Pa) for 10 min. Fig. 3 SEM micrographs of fracture cross-sections of Al2O3–30 vol.% TiN0.3 composite sintered via SPS at different temperatures in vacuum (6Ãâ€"103 Pa) for 10 min: (a) 1300  °C; (b) 1400  °C; (c) 1500  °C; (d) 1600  °C. Fig. 4 SEM micrographs of fracture cross-sections of the composites sintered via SPS at 1400  °C in vacuum (6Ãâ€"103 Pa) for 10 min: (a) Al2O3–10 vol% TiN0.3; (b) Al2O3–20 vol% TiN0.3; (c) Al2O3–30 vol% TiN0.3; (d) Al2O3–40 vol% TiN0.3. Fig. 5 Vickers hardness and fracture toughness of (a) Al2O3–30 vol% TiN0.3 composite versus sintering temperature; (b) Al2O3–TiN0.3 composites sintered at 1400  °C versus TiN0.3 content. Fig. 6 Bending strength of Al2O3–TiN0.3 composites sintered at 1400  °C versus TiN0.3 content. Fig. 7 Density of states for TiN and TiNx. à ¯Ã‚ Ã¢â‚¬ ºÃƒ ¯Ã¢â€š ¬Ã‚ ªÃƒ ¯Ã‚ Ã‚  Corresponding author. Tel (fax): 086-0335-8061671; E-mail: [emailprotected] Supported by the Hebei Province Scientific Committee of China (Nos. E2012203116, ZH2012003, JX2014-3 and ZX01-20140100-01).