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Effect of (Ni, Mo) and TiN on the microstructure and mechanical properties of TiB2 ceramic tool materials
Materials Science and Engineering A 433 (2006) 39–44
Effect of (Ni, Mo) and TiN on the microstructure and mechanical properties of TiB2 ceramic tool materials Meilin Gu, Chuanzhen Huang ∗ , Bin Zou, Binqiang Liu Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, Jinan 250061, PR China Received 14 April 2006; received in revised form 25 May 2006; accepted 6 July 2006
Abstract TiB2 –TiN composites with (Ni, Mo) as sintering aid have been fabricated by hot-pressing technique, and the microstructure and mechanical properties of the composites were investigated. Grain size and relative density of the composites increase consistently with an increase in the (Ni, Mo). In the case of 10 vol.% (Ni, Mo), the grain size decreases consistently with an increase in the TiN. Ni element is rich in the vicinity around TiN particles. (Ni, Mo) forms a continuous boundary phase around the matrix particles. TiN can improve the bonding strength among the TiB2 grain boundaries and change the fracture pattern from intergranular to transgranular fracture with the increase in TiN. The best mechanical properties of the composites are 1088.20 MPa for three-point flexural strength, 7.54 MPa m1/2 for fracture toughness and 21.53 GPa for Vickers hardness. © 2006 Elsevier B.V. All rights reserved. Keywords: TiB2 ceramic tool material; TiN; (Ni, Mo); Microstructure; Mechanical property
1. Introduction Titanium diboride, which is mainly covalently bonded, has a high melting point about 2980 ◦ C, good thermal conductivity and high Vickers hardness about 32 GPa. Moreover, it has also an excellent electrical conductivity. These properties are especially appropriate for the fabrication of high wear resistant and temperature resistant components such as armour materials and cutting tools. But the extensive application of TiB2 ceramics has been restricted due to a poor fracture toughness and flexural strength. Therefore improvement of the fracture toughness and flexural strength is crucial for the use of TiB2 ceramics as structural materials. Fracture toughness can be increased by several mechanisms, such as phase transformation, microcracks, crack deflection toughening, etc. [1–3]. Crack deflection can occur when a propagating crack tip meets a residual stress field or a reduced grain boundary. The deflection improves the fracture toughness because the crack driving force at the crack tip is reduced. TiB2 has a large crystallographic thermal expansion anisotropy(αa = 6.63 × 10−6 ◦ C−1 , αc = 8.65 × 10−6 ◦ C−1 in
∗
Corresponding author. Tel.: +86 531 88396913; fax: +86 531 88396913. E-mail address: [email protected] (C. Huang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.012
the range of 25–750 ◦ C). This anisotropy produces considerable internal stress during the cooling process and generates microcracks when the grain size is larger than a critical size [4,5]. The microcracks form both in the grain boundaries and in the grain bodies. Microcracking significantly deteriorate the flexural strength [6]. So reducing the grain size is necessary to improve the flexural strength of TiB2 based ceramics. Some investigations [7,8] have demonstrated the dispersion of Al2 O3 and B4 C phase in TiB2 –Al2 O3 and TiB2 –B4 C ceramics, respectively, acts as a grain growth inhibitor and increases the fracture toughness by crack deflection because of a different thermal expansion coefficient and elastic modulus from those of the TiB2 . Moreover, TiB2 is wetted by molten metals such as Iron and Ni etc., and resists molten metals. Recently, research about Ti(C, N) ceramics found [9] that metal bond form a surrounding phase between matrix grains. Appropriate thickness and components of the surrounding phase can improve the mechanical properties of the composite [10]. But surrounding phase in TiB2 ceramics has hardly been reported. In order to develop the new TiB2 composites with higher general mechanical properties, composites TiB2 –TiN was sintered via hot-pressing for 60 min at the condition of 1530 ◦ C and 25 MPa in this study. Microstructure and boundary phase of TiB2 versus TiN and (Ni, Mo) addition are presented. Effect
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M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
of TiN on the mechanical properties of the composites is discussed.
(EDAX) were used to observe the microstructure of the composites.
2. Experimental procedures
3. Experimental results and discussion
Commercially available TiB2 powder, whose average grain size and purity are 0.8 m and 98%, was used as the raw material. A representative chemical analysis of the TiB2 powders is presented in Table 1. The average grain size and purity of TiN are 0.8 m and 99%, respectively. As sintering aids, Ni and Mo powders with an average grain size of 2.3 m and a purity of 98% are added. The compositions of materials are shown in Table 2. The powders were mixed and milled for 48 h in a polyethylene jar by WC balls in ethanol. Then the mixed slurry was dried in vacuum and passed through a 100-mesh sieve. The compacted powders were hot pressed for 60 min at a pressure of 25 MPa and at temperature of 1530 ◦ C in an argon atmosphere. The hot pressed samples were cut into testing specimens by electrical discharge wire cutting method, ground and polished to specimens with a size of 40 mm × 4 mm × 3 mm. Flexural strength was measured at a span of 30 mm and a crosshead speed of 0.5 mm/min by the three-point bending test method on WD-10 electron universal tester. Fracture toughness was measured via the direct indentation method. The indenter is the Vickers DPH type and the applied static load is 196 N for 15 s. Vickers hardness was measured on the polished surfaces using a diamond pyramid indenter under a load of 196 N and loading duration of 15 s. At least 15 specimens were tested for each experimental condition. Bulk density of the sample was measured by Archimedes’ method. Theoretical density was calculated by using a rule of mixture, assuming that only TiB2 , TiN, Ni and Mo were present because other phases were not detected by XRD. Scanning electron microscope (SEM), Transmission electron microscopy (TEM) and energy dispersive X-ray analysis
3.1. Microstructure
Table 1 Chemical analysis of TiB2 powder (wt.%) Element
Amount
Titanium Boron Carbon Oxygen All other metals
68.45 30.84 0.09 0.53 0.09
TEM photos and diffraction patterns of sample no. 3 are shown in Fig. 1. The diffraction pattern reveals TiB2 in the region marked B in Fig. 1(a), G in Fig. 1(b) and I, J in Fig. 1(c). It reveals TiN in the region marked A in Fig. 1(a) and H in Fig. 1(b). It is shown from Fig. 1(a) and (b) that there is an amorphous phase at grain boundaries. Energy dispersive X-ray spectroscopy analysis using a finely focused electron beam (30 nm) in TEM was employed to determine the chemistry of the grain boundary phase. It reveals Ni, Mo and Ti. The Ni and Mo form a liquid phase MoNi at 1309 ◦ C according to the constitutional diagram of alloys. The phase can well wet the grain boundaries and form a continuous boundary phase as shown in Fig. 1(b) and (c), which is identical with what is observed by many other researchers about Ti(C, N) [9,10]. This continuous boundary phase MoNi may enhance mass transfer, so it promotes both densification and grain growth of the composites TiB2 –TiN. Moreover, we found Ni element is rich in the vicinity around TiN particles by contrasting Fig. 1(b) with (c). The content of Ni decreases with apart from the boundary by EDAX as shown in Fig. 2. It can conclude that Ni dissolves in the TiN so as to decrease the liquid phase between matrix particles, which may result in decrease of the grain size and density. Fracture surface morphologies of the composites TiB2 –TiN were observed by SEM as shown in Fig. 3. The black phase is TiB2 , and the gray phase is TiN in Fig. 3. It can be found that the grain of the composites is uniform and equiaxed. The grain size is about 2.3–4.8 m as shown in Table 3, and the grain size decreased distinctly as the amount of TiN addition increased by comparing Fig. 3(a), (d)–(f), which agrees with the result in the previous section. So we can conclude that TiN acts as a grain growth inhibitor. The fracture patterns of the samples no. 0–no. 3 are mostly intergranular with a little transgranular fracture occurring on unconventionally coarse grains, and the fracture surfaces are flat. However, there appears distinct cleavage vein in the samples no. 4 and no. 5, which indicates a lot of transgranular fracture occurring in no. 4 and no. 5. TiB2 has an anisotropic thermal expansion coefficient, which can induce residual stress or microcracks if the Table 3 Grain sizes of composites TiB2 –TiN
Table 2 Compositions of ceramic tool materials TiB2 –TiN (vol.%) Sample no.
TiB2
TiN
Ni
Mo
0 1 2 3 4 5
90 75 73 70 60 50
0 20 20 20 30 40
6.5 3.5 5 6.5 6.5 6.5
3.5 1.5 2 3.5 3.5 3.5
Sample no.
Grain sizea TiB2 (m)
0 1 2 3 4 5
4.8 2.3 2.47 3.14 2.6 2.35 a
Measured by line intercept method.
M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
grain size is large enough during cooling from the densification temperature. In this study, grain size of TiB2 is more less than the critical grain size for microcracks about 15 m [6]. So there is residual tensile stress in the matrix grain boundaries and the little grain size and the little residual stress [11]. On the other hand, the mismatch of thermal expansion coefficient between TiB2 (average α ≈ 8.1 × 10−6 ◦ C−1 ) and TiN (α ≈ 9.4 × 10−6 ◦ C−1 ) can generate compressive stress on the TiB2 grain boundaries, which counteracts the tensile stress arising from thermal anisotropy of the matrix and improves the bonding strength among the TiB2 grain boundaries. So transgranular fracture of the samples no. 4 and no. 5 account for the decrease in grain size and increase in compressive stress on the TiB2 grain boundaries due to the increase in TiN.
41
3.2. Sintering densities It is well known that densification of pure TiB2 ceramics to a reasonably high sintered density is difficult because of its high melting point (2980 ◦ C) and low bulk diffusivity. Densification has been enhanced with a small addition of iron or nickel to form liquid phase using either pressureless or hot-pressing technique [12,13]. In this work, Ni and Mo are used as sintering aids. Fig. 4 shows relative density of the five composites, calculated on the basis of a theoretical density of 4.52, 5.44, 8.9 and 10.2 g/cm3 for TiB2 , TiN, Ni and Mo, respectively. It can be seen that the relative density of the composites increases with an increase of the liquid phase (Ni, Mo). This indicates that the adequate liquid phase can improve the density of the composites
Fig. 1. TEM photos, diffraction pattern and EDAX spectrum of sample no. 3: (a)–(c) bright field image; (d) diffraction pattern of marked B, G, I and J; (e) index of (d); (f) diffraction pattern of marked A and H; (g) index of (f); (h) EDAX spectrum of marked D.
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M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
Fig. 1. (Continued ).
agreeing with the research result of Kwon [14], whose model indicates that the densification rate increases with an increase in the amount of liquid phase. While the relative density of sintered body decreases with an increase of TiN because of the decrease of liquid phase between TiB2 . 3.3. Mechanical properties
Fig. 2. Distribution of Ni element from boundary phase to TiN in sample no. 3.
Table 4 shows the mechanical properties of the composites. The flexural strength, fracture toughness and Vickers hardness
M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
43
Fig. 3. SEM photographs of fracture surfaces: (a) no. 0; (b) no. 1; (c) no. 2; (d) no. 3; (e) no. 4; (f) no. 5.
have a maximum value of 1088.20 MPa, 7.54 MPa m1/2 and 21.53 GPa, respectively. Flexural strength of a polycrystalline ceramic is influenced by the grain size and porosity (or density), etc., small
grain size and high density are necessary for high flexural strength. So the high flexural strength of the composite no. 4 account for the small grain size and high relative density.
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Fig. 4. Relative density of composites TiB2 –TiN. Table 4 Mechanical properties of composites TiB2 –TiN Sample no.
Flexural strength
Fracture toughness
Vickers’
1 2 3 4 5
420.0 670.8 730.5 1088.2 985.8
4.04 6.55 7.12 7.25 7.54
17.00 19.00 21.53 20.47 18.51
Fracture toughness of a polycrystalline ceramic correlates to the fracture energy. The increase of TiN generates obvious cleavage vein in the fracture surface of the samples no. 4 and no. 5, which can consume a lot of fracture energy to improve the fracture toughness. Phase compositions and porosity (or density), etc., affect the Vickers hardness of a polycrystalline ceramic. Abundant horniness phase and high density are useful to Vickers hardness. In composites TiB2 –TiN, the Vickers hardness of TiB2 and TiN phase is 3200 and 2000 GPa, respectively. So the more TiN addition is the lower Vickers hardness is. On the other hand, the more Mo and Ni addition is the higher density is as mentioned in the previous section, this is the reason of higher hardness of no. 3 than that of no. 1 and no. 2. 4. Conclusions (1) Composites TiB2 –TiN can be obtained by hot-pressing for 60 min at 1530 ◦ C, 25 MPa with (Ni, Mo) as sintering aid.
The relative density is up to 99.12% when the addition of (Ni, Mo) is 10 vol.%. (2) The liquid phase of (Ni, Mo) can well wet the grain boundary and form a continuous boundary phase, which enhance mass transfer, so as to promotes both densification and grain growth of the TiB2 –TiN composites. (3) TiN inhibits the grain growth and improves the bonding strength among the TiB2 grain boundaries, which can improve the flexural strength of the composites. The addition of TiN increases to 30 vol.% can generate obvious cleavage vein in the fracture surface of the sample, which can improve the fracture toughness. But the Vickers hardness of the composites decreases with the increase of TiN because of the low Vickers hardness of TiN. Acknowledgements This project was supported by Special Foundation for University Doctor Subject of Ministry of Education of China (No. 20030422012) and Encouragement Foundation for Distinguished Young Scientist of Shandong province, China (No. 03BS153). References [1] R.M. McMeeking, A.G. Evans, J. Am. Ceram. Soc. 65 (5) (1982) 242. [2] A.G. Evans, J. Am. Ceram. Soc. 67 (4) (1984) 255. [3] K.T. Faber, A.G. Evans, Acta Metall. 31 (4) (1983) 565. [4] I.A. Kuszyk, R.C. Bradt, J. Am. Ceram. Soc. 56 (8) (1973) 420. [5] E.C. Case, J.R. Smyth, O. Hunter, J. Mater. Sci. 15 (1980) 149. [6] H.R. Baumgartner, R.A. Steiger, J. Am. Ceram. Soc. 67 (3) (1984) 207. [7] M.L. Gu, C.Z. Huang, J. Wang, Key Eng. Mater. 315/316 (2006) 123. [8] E.S. Kang, C.H. Kim, J. Mater. Sci. 25 (1990) 580. [9] J.K. Yang, H.C. Lee, Mater. Sci. Eng. A 209 (1996) 213. [10] L. He, C.Z. Huang, Y.X. Liu, J. Sun, H.L. Liu, J. Chin. Ceram. Soc. 3 (31) (2003) 324. [11] A.G. Evans, Acta Metall. 26 (1978) 1845. [12] M.A. Einarsrud, E. Hagen, G. Pettersen, T. Grande, J. Am. Ceram. Soc. 80 (12) (1997) 3013. [13] W.M. Wang, Z.Y. Fu, H. Wang, R.Z. Yuan, J. Eur. Ceram. Soc. 22 (2002) 1045. [14] O.H. Kwon, G.L. Messing, Acta Metal. Mater. 36 (9) (1991) 2059.
Effect of (Ni, Mo) and TiN on the microstructure and mechanical properties of TiB2 ceramic tool materials Meilin Gu, Chuanzhen Huang ∗ , Bin Zou, Binqiang Liu Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, Jinan 250061, PR China Received 14 April 2006; received in revised form 25 May 2006; accepted 6 July 2006
Abstract TiB2 –TiN composites with (Ni, Mo) as sintering aid have been fabricated by hot-pressing technique, and the microstructure and mechanical properties of the composites were investigated. Grain size and relative density of the composites increase consistently with an increase in the (Ni, Mo). In the case of 10 vol.% (Ni, Mo), the grain size decreases consistently with an increase in the TiN. Ni element is rich in the vicinity around TiN particles. (Ni, Mo) forms a continuous boundary phase around the matrix particles. TiN can improve the bonding strength among the TiB2 grain boundaries and change the fracture pattern from intergranular to transgranular fracture with the increase in TiN. The best mechanical properties of the composites are 1088.20 MPa for three-point flexural strength, 7.54 MPa m1/2 for fracture toughness and 21.53 GPa for Vickers hardness. © 2006 Elsevier B.V. All rights reserved. Keywords: TiB2 ceramic tool material; TiN; (Ni, Mo); Microstructure; Mechanical property
1. Introduction Titanium diboride, which is mainly covalently bonded, has a high melting point about 2980 ◦ C, good thermal conductivity and high Vickers hardness about 32 GPa. Moreover, it has also an excellent electrical conductivity. These properties are especially appropriate for the fabrication of high wear resistant and temperature resistant components such as armour materials and cutting tools. But the extensive application of TiB2 ceramics has been restricted due to a poor fracture toughness and flexural strength. Therefore improvement of the fracture toughness and flexural strength is crucial for the use of TiB2 ceramics as structural materials. Fracture toughness can be increased by several mechanisms, such as phase transformation, microcracks, crack deflection toughening, etc. [1–3]. Crack deflection can occur when a propagating crack tip meets a residual stress field or a reduced grain boundary. The deflection improves the fracture toughness because the crack driving force at the crack tip is reduced. TiB2 has a large crystallographic thermal expansion anisotropy(αa = 6.63 × 10−6 ◦ C−1 , αc = 8.65 × 10−6 ◦ C−1 in
∗
Corresponding author. Tel.: +86 531 88396913; fax: +86 531 88396913. E-mail address: [email protected] (C. Huang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.012
the range of 25–750 ◦ C). This anisotropy produces considerable internal stress during the cooling process and generates microcracks when the grain size is larger than a critical size [4,5]. The microcracks form both in the grain boundaries and in the grain bodies. Microcracking significantly deteriorate the flexural strength [6]. So reducing the grain size is necessary to improve the flexural strength of TiB2 based ceramics. Some investigations [7,8] have demonstrated the dispersion of Al2 O3 and B4 C phase in TiB2 –Al2 O3 and TiB2 –B4 C ceramics, respectively, acts as a grain growth inhibitor and increases the fracture toughness by crack deflection because of a different thermal expansion coefficient and elastic modulus from those of the TiB2 . Moreover, TiB2 is wetted by molten metals such as Iron and Ni etc., and resists molten metals. Recently, research about Ti(C, N) ceramics found [9] that metal bond form a surrounding phase between matrix grains. Appropriate thickness and components of the surrounding phase can improve the mechanical properties of the composite [10]. But surrounding phase in TiB2 ceramics has hardly been reported. In order to develop the new TiB2 composites with higher general mechanical properties, composites TiB2 –TiN was sintered via hot-pressing for 60 min at the condition of 1530 ◦ C and 25 MPa in this study. Microstructure and boundary phase of TiB2 versus TiN and (Ni, Mo) addition are presented. Effect
40
M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
of TiN on the mechanical properties of the composites is discussed.
(EDAX) were used to observe the microstructure of the composites.
2. Experimental procedures
3. Experimental results and discussion
Commercially available TiB2 powder, whose average grain size and purity are 0.8 m and 98%, was used as the raw material. A representative chemical analysis of the TiB2 powders is presented in Table 1. The average grain size and purity of TiN are 0.8 m and 99%, respectively. As sintering aids, Ni and Mo powders with an average grain size of 2.3 m and a purity of 98% are added. The compositions of materials are shown in Table 2. The powders were mixed and milled for 48 h in a polyethylene jar by WC balls in ethanol. Then the mixed slurry was dried in vacuum and passed through a 100-mesh sieve. The compacted powders were hot pressed for 60 min at a pressure of 25 MPa and at temperature of 1530 ◦ C in an argon atmosphere. The hot pressed samples were cut into testing specimens by electrical discharge wire cutting method, ground and polished to specimens with a size of 40 mm × 4 mm × 3 mm. Flexural strength was measured at a span of 30 mm and a crosshead speed of 0.5 mm/min by the three-point bending test method on WD-10 electron universal tester. Fracture toughness was measured via the direct indentation method. The indenter is the Vickers DPH type and the applied static load is 196 N for 15 s. Vickers hardness was measured on the polished surfaces using a diamond pyramid indenter under a load of 196 N and loading duration of 15 s. At least 15 specimens were tested for each experimental condition. Bulk density of the sample was measured by Archimedes’ method. Theoretical density was calculated by using a rule of mixture, assuming that only TiB2 , TiN, Ni and Mo were present because other phases were not detected by XRD. Scanning electron microscope (SEM), Transmission electron microscopy (TEM) and energy dispersive X-ray analysis
3.1. Microstructure
Table 1 Chemical analysis of TiB2 powder (wt.%) Element
Amount
Titanium Boron Carbon Oxygen All other metals
68.45 30.84 0.09 0.53 0.09
TEM photos and diffraction patterns of sample no. 3 are shown in Fig. 1. The diffraction pattern reveals TiB2 in the region marked B in Fig. 1(a), G in Fig. 1(b) and I, J in Fig. 1(c). It reveals TiN in the region marked A in Fig. 1(a) and H in Fig. 1(b). It is shown from Fig. 1(a) and (b) that there is an amorphous phase at grain boundaries. Energy dispersive X-ray spectroscopy analysis using a finely focused electron beam (30 nm) in TEM was employed to determine the chemistry of the grain boundary phase. It reveals Ni, Mo and Ti. The Ni and Mo form a liquid phase MoNi at 1309 ◦ C according to the constitutional diagram of alloys. The phase can well wet the grain boundaries and form a continuous boundary phase as shown in Fig. 1(b) and (c), which is identical with what is observed by many other researchers about Ti(C, N) [9,10]. This continuous boundary phase MoNi may enhance mass transfer, so it promotes both densification and grain growth of the composites TiB2 –TiN. Moreover, we found Ni element is rich in the vicinity around TiN particles by contrasting Fig. 1(b) with (c). The content of Ni decreases with apart from the boundary by EDAX as shown in Fig. 2. It can conclude that Ni dissolves in the TiN so as to decrease the liquid phase between matrix particles, which may result in decrease of the grain size and density. Fracture surface morphologies of the composites TiB2 –TiN were observed by SEM as shown in Fig. 3. The black phase is TiB2 , and the gray phase is TiN in Fig. 3. It can be found that the grain of the composites is uniform and equiaxed. The grain size is about 2.3–4.8 m as shown in Table 3, and the grain size decreased distinctly as the amount of TiN addition increased by comparing Fig. 3(a), (d)–(f), which agrees with the result in the previous section. So we can conclude that TiN acts as a grain growth inhibitor. The fracture patterns of the samples no. 0–no. 3 are mostly intergranular with a little transgranular fracture occurring on unconventionally coarse grains, and the fracture surfaces are flat. However, there appears distinct cleavage vein in the samples no. 4 and no. 5, which indicates a lot of transgranular fracture occurring in no. 4 and no. 5. TiB2 has an anisotropic thermal expansion coefficient, which can induce residual stress or microcracks if the Table 3 Grain sizes of composites TiB2 –TiN
Table 2 Compositions of ceramic tool materials TiB2 –TiN (vol.%) Sample no.
TiB2
TiN
Ni
Mo
0 1 2 3 4 5
90 75 73 70 60 50
0 20 20 20 30 40
6.5 3.5 5 6.5 6.5 6.5
3.5 1.5 2 3.5 3.5 3.5
Sample no.
Grain sizea TiB2 (m)
0 1 2 3 4 5
4.8 2.3 2.47 3.14 2.6 2.35 a
Measured by line intercept method.
M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
grain size is large enough during cooling from the densification temperature. In this study, grain size of TiB2 is more less than the critical grain size for microcracks about 15 m [6]. So there is residual tensile stress in the matrix grain boundaries and the little grain size and the little residual stress [11]. On the other hand, the mismatch of thermal expansion coefficient between TiB2 (average α ≈ 8.1 × 10−6 ◦ C−1 ) and TiN (α ≈ 9.4 × 10−6 ◦ C−1 ) can generate compressive stress on the TiB2 grain boundaries, which counteracts the tensile stress arising from thermal anisotropy of the matrix and improves the bonding strength among the TiB2 grain boundaries. So transgranular fracture of the samples no. 4 and no. 5 account for the decrease in grain size and increase in compressive stress on the TiB2 grain boundaries due to the increase in TiN.
41
3.2. Sintering densities It is well known that densification of pure TiB2 ceramics to a reasonably high sintered density is difficult because of its high melting point (2980 ◦ C) and low bulk diffusivity. Densification has been enhanced with a small addition of iron or nickel to form liquid phase using either pressureless or hot-pressing technique [12,13]. In this work, Ni and Mo are used as sintering aids. Fig. 4 shows relative density of the five composites, calculated on the basis of a theoretical density of 4.52, 5.44, 8.9 and 10.2 g/cm3 for TiB2 , TiN, Ni and Mo, respectively. It can be seen that the relative density of the composites increases with an increase of the liquid phase (Ni, Mo). This indicates that the adequate liquid phase can improve the density of the composites
Fig. 1. TEM photos, diffraction pattern and EDAX spectrum of sample no. 3: (a)–(c) bright field image; (d) diffraction pattern of marked B, G, I and J; (e) index of (d); (f) diffraction pattern of marked A and H; (g) index of (f); (h) EDAX spectrum of marked D.
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M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
Fig. 1. (Continued ).
agreeing with the research result of Kwon [14], whose model indicates that the densification rate increases with an increase in the amount of liquid phase. While the relative density of sintered body decreases with an increase of TiN because of the decrease of liquid phase between TiB2 . 3.3. Mechanical properties
Fig. 2. Distribution of Ni element from boundary phase to TiN in sample no. 3.
Table 4 shows the mechanical properties of the composites. The flexural strength, fracture toughness and Vickers hardness
M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
43
Fig. 3. SEM photographs of fracture surfaces: (a) no. 0; (b) no. 1; (c) no. 2; (d) no. 3; (e) no. 4; (f) no. 5.
have a maximum value of 1088.20 MPa, 7.54 MPa m1/2 and 21.53 GPa, respectively. Flexural strength of a polycrystalline ceramic is influenced by the grain size and porosity (or density), etc., small
grain size and high density are necessary for high flexural strength. So the high flexural strength of the composite no. 4 account for the small grain size and high relative density.
44
M. Gu et al. / Materials Science and Engineering A 433 (2006) 39–44
Fig. 4. Relative density of composites TiB2 –TiN. Table 4 Mechanical properties of composites TiB2 –TiN Sample no.
Flexural strength
Fracture toughness
Vickers’
1 2 3 4 5
420.0 670.8 730.5 1088.2 985.8
4.04 6.55 7.12 7.25 7.54
17.00 19.00 21.53 20.47 18.51
Fracture toughness of a polycrystalline ceramic correlates to the fracture energy. The increase of TiN generates obvious cleavage vein in the fracture surface of the samples no. 4 and no. 5, which can consume a lot of fracture energy to improve the fracture toughness. Phase compositions and porosity (or density), etc., affect the Vickers hardness of a polycrystalline ceramic. Abundant horniness phase and high density are useful to Vickers hardness. In composites TiB2 –TiN, the Vickers hardness of TiB2 and TiN phase is 3200 and 2000 GPa, respectively. So the more TiN addition is the lower Vickers hardness is. On the other hand, the more Mo and Ni addition is the higher density is as mentioned in the previous section, this is the reason of higher hardness of no. 3 than that of no. 1 and no. 2. 4. Conclusions (1) Composites TiB2 –TiN can be obtained by hot-pressing for 60 min at 1530 ◦ C, 25 MPa with (Ni, Mo) as sintering aid.
The relative density is up to 99.12% when the addition of (Ni, Mo) is 10 vol.%. (2) The liquid phase of (Ni, Mo) can well wet the grain boundary and form a continuous boundary phase, which enhance mass transfer, so as to promotes both densification and grain growth of the TiB2 –TiN composites. (3) TiN inhibits the grain growth and improves the bonding strength among the TiB2 grain boundaries, which can improve the flexural strength of the composites. The addition of TiN increases to 30 vol.% can generate obvious cleavage vein in the fracture surface of the sample, which can improve the fracture toughness. But the Vickers hardness of the composites decreases with the increase of TiN because of the low Vickers hardness of TiN. Acknowledgements This project was supported by Special Foundation for University Doctor Subject of Ministry of Education of China (No. 20030422012) and Encouragement Foundation for Distinguished Young Scientist of Shandong province, China (No. 03BS153). References [1] R.M. McMeeking, A.G. Evans, J. Am. Ceram. Soc. 65 (5) (1982) 242. [2] A.G. Evans, J. Am. Ceram. Soc. 67 (4) (1984) 255. [3] K.T. Faber, A.G. Evans, Acta Metall. 31 (4) (1983) 565. [4] I.A. Kuszyk, R.C. Bradt, J. Am. Ceram. Soc. 56 (8) (1973) 420. [5] E.C. Case, J.R. Smyth, O. Hunter, J. Mater. Sci. 15 (1980) 149. [6] H.R. Baumgartner, R.A. Steiger, J. Am. Ceram. Soc. 67 (3) (1984) 207. [7] M.L. Gu, C.Z. Huang, J. Wang, Key Eng. Mater. 315/316 (2006) 123. [8] E.S. Kang, C.H. Kim, J. Mater. Sci. 25 (1990) 580. [9] J.K. Yang, H.C. Lee, Mater. Sci. Eng. A 209 (1996) 213. [10] L. He, C.Z. Huang, Y.X. Liu, J. Sun, H.L. Liu, J. Chin. Ceram. Soc. 3 (31) (2003) 324. [11] A.G. Evans, Acta Metall. 26 (1978) 1845. [12] M.A. Einarsrud, E. Hagen, G. Pettersen, T. Grande, J. Am. Ceram. Soc. 80 (12) (1997) 3013. [13] W.M. Wang, Z.Y. Fu, H. Wang, R.Z. Yuan, J. Eur. Ceram. Soc. 22 (2002) 1045. [14] O.H. Kwon, G.L. Messing, Acta Metal. Mater. 36 (9) (1991) 2059.