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VC and Cr3C2 doped WCoB-TiC ceramic composites prepared by hot-pressing
International Journal of Refractory Metals & Hard Materials 68 (2017) 24–28
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Short communication
VC and Cr3C2 doped WCoB-TiC ceramic composites prepared by hotpressing
MARK
Deqing Ke, Yingjun Pan⁎, Yuanyuan Xu, Pan Wang, Run Wu The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: WCoB-TiC ceramic composites Hot-pressing Grain growth inhibitors Microstructure Mechanical properties
The grain growth inhibitors (GGIs) VC and Cr3C2 doped WCoB-TiC ceramic composites were fabricated by hotpressing. The microstructure, hardness, transverse rupture strength (TRS), fracture toughness (KIC) and wearresistance of WCoB-TiC ceramic composites were investigated. The results reveal that the grains can obviously become refined and the densification temperature of WCoB-TiC ceramic composites will be increased due to the VC and Cr3C2. The typical microstructure of WCoB-TiC ceramic composites mainly consist of bright W2CoB2 grains, gray TiC particles, dark TiB2 and pores. WCoB-TiC ceramic composites doped with 0.3 wt% VC and 0.3 wt% Cr3C2 hot-pressing at 1420 °C show the optimum mechanical properties (hardness, TRS and KIC are 92.6 HRA, 1976 MPa and 14.8 MPa m1/2, respectively) and the best dry sliding wear-resistance.
1. Introduction As a typical kind of ternary boride based cermets, WCoB shows tremendous development potential in engineering materials with extremely high-hardness, high-melting point, excellent wear-resistance and exidation-resistance, such as cutting tools, drawing dies, bearing surfaces, etc. [1]. Nevertheless, the inherent brittleness severely limits its application as structural materials. In recent years, research has been focused on the synthesis of WCoB-TiC ceramic composites in order to obtain high performance. A. Saez et al. [2] fabricated WCoB-TiC ceramic composites for the WC-Co-TiB2 raw material powder system and studied the influence of initial Co contents (varying between 10–47 wt%) on the microstructure and mechanical properties of WCoBTiC ceramic composites, and claimed that WCoB-TiC ceramic composites with no Co binder show enormous application prospect in oxidizing environments. In addition, it has been found that refractory carbides, such as VC, Cr3C2, NbC, TaC and Mo2C, can control the microstructure evolution of ceramic phases during sintering and change the mechanical properties [3–10]. With high solubility and mobility in the cobalt phase, VC and Cr3C2 are the most effective GGIs in WC-Co cemented carbides [4,11–14]. Our previous research revealed that typical refractory carbides Cr3C2 can effectively suppress the grain growth, but the densification response of the WCoB-TiC ceramic composites prepared by vacuum sintering was not good [15]. It is widely known that hot-pressing method has been proposed to achieve the densification of ceramic ⁎
Corresponding author. E-mail address: [email protected] (Y. Pan).
http://dx.doi.org/10.1016/j.ijrmhm.2017.06.005 Received 1 May 2017; Received in revised form 15 June 2017; Accepted 18 June 2017 Available online 19 June 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
materials [16–21]. However, it has been rarely reported that WCoB-TiC ceramic composites were made by hot pressing method. So, it is worthy of further studying the hot-pressed WCoB-TiC ceramic composites with grain growth inhibitor addition. This work is aimed to investigate the ability of GGIs to influence the microstructure and mechanical properties of WCoB-TiC ceramic composites by hot-pressing at different sintering temperatures. The densification behavior, microstructure evolution and mechanical properties of hot-pressed specimens were investigated. 2. Experimental procedures Characteristics of the raw powders are listed in Table 1. The ratios of the mixed powders and hot-pressing temperatures are listed in Table 2. The powders WC-19.6 wt%Co-21.2 wt%TiB2 (WT) and WC19.6 wt%Co-21.2 wt%TiB2-0.3 wt%VC-0.3 wt%Cr3C2 (WT-GGI) were premixed and ball milled with a QM-1SP4 planetary ball milling machine under argon gas atmosphere for 45 h, respectively. The ratio of ball to powder weight was 10:1 and the rotation speed of the mill was 350 rpm. Both the vial and milling balls (10 mm in diameter) were made of cemented carbide materials. After debindering, the milled composite powders were hot-pressed in a graphite die using a vacuum hot-pressing furnace at the temperatures of 1400, 1420 or 1440 °C under the pressure of 39.6 MPa in a vacuum (about1.3 × 10− 1 Pa) atmosphere for 30 min. The densities of the sintered specimens were measured by the density measuring instrument (METTLER TOLEDO AB-104N). The
International Journal of Refractory Metals & Hard Materials 68 (2017) 24–28
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phase identification of the specimens was studied by an X-radial Diffractometer (XRD, XPert PRO MPD) equipped with a Cu Kα radiation (λ = 0.15418 nm) at 400 kV and 200 mA. The 2θ range was 10°–90° with a step of 0.05° and the measure time was 10 s per step. The field emission scanning electron microscopy (FE-SEM, FIB Nova 400 Nano) was used to analyze the microstructure of the sintered specimens. Their chemical composition was examined by energy dispersive spectroscopy (EDS). The hardness of the hot-pressed specimens was measured with a standard Rockwell hardness measuring device (HRA-150) with the load of 30 kg and the indentation time of 10 s. At least ten indentations were formed to calculate an average value. Before TRS test, the four long face of the hot-pressed specimens were processed by a diamond wheel with enough coolant, and the surface roughness Ra ≤ 0.4 μm and the angle of chamfer was 45°. The TRS at room temperature was measured by a three-point bend test with a span length of 30 mm and a cross-head speed of 0.5 mm/min. The fracture toughness was calculated based on the radial crack length produced by Vickers (HV30) indentation according to the formula proposed by Shetty et al. [22]. Wear tests were carried out under dry sliding conditions with a tribometer pin-on-disc (ball-on-disc configuration) manufactured by MICROTEST MT2/60/ SCM/T, according to ASTM wear testing standard G99-05 [23].
Table 1 Characteristics of the raw powders. Powder
Mean particle size (μm)
Adulterant (wt%)
WC Co TiB2 VC Cr3C2
~4.0 ~2.0 ~2.0 ~2.0 ~2.0
< 0.1 < 0.1 < 0.1 < 0.1 < 0.1
Table 2 Ratios of the mixed powders and hot-pressing temperature. Specimens
A B C D E F
Powder ratios/wt%
Hot-pressing temperature/°C
WC
Co
TiB2
VC
Cr3C2
59.2 59.2 59.2 58.6 58.6 58.6
19.6 19.6 19.6 19.6 19.6 19.6
21.2 21.2 21.2 21.2 21.2 21.2
0 0 0 0.3 0.3 0.3
0 0 0 0.3 0.3 0.3
1400 1420 1440 1400 1420 1440
3. Results and discussion 3.1. Densification The density of specimens hot-pressed at different temperatures are shown in Fig. 1. As shown in Fig. 1, when hot-pressing temperature increased, the densities of the WT ceramic composites did not decrease significantly. While the densities of the WT-GGI ceramic composites reached a peak at 1420 hot-pressing temperature and presented a sharp decline. Furthermore, the densities of the WT-GGI ceramic composites are higher than WT ceramic composites at the same hot-pressing temperature. This indicates that the densification temperature of WCoB-TiC ceramic composites increased with the addition of 0.3 wt% VC and 0.3 wt% Cr3C2. 3.2. Microstructure
Fig. 1. The density of WT and WT-GGI ceramic composites hot-pressed at different
The typical XRD patterns of WT and WT-GGI ceramic composites are shown in Fig. 2. It was found that the phases of WT and WT-GGI ceramic composites were WCoB, W2CoB2, TiB2, TiC and Co2B. The appearance of the W2CoB2 and TiC phase is mainly attributed to the following reaction [24]: 2WC + 5Co + 2TiB2 → W2CoB2 + 2TiC + 2Co2B
(1)
The microstructure of WT and WT-GGI ceramic composites is shown in Fig. 3. As shown in Fig. 3, the typical microstructure of WCoB-TiC ceramic composites is composed of bright W2CoB2 grains, gray TiC particles, dark TiB2 and pores. It could be seen that, WT ceramic composites have a grain size about 1.0–2.2 μm (Fig. 3a, b, c), while the grains of WT-GGI are refined to 0.6–1.5 μm (Fig. 3d, e, f). This indicates that the microstructure (coarse/fine) of the ceramic composites can be tailored by altering hot-pressing temperature and/or adding the GGIs (VC and Cr3C2). The WT ceramic composites that were hot pressed at 1400 °C were selected to characterize the composition of different color regions by EDS, and the results are shown in Fig. 4. It could be seen that, the white region contains W, Co and Ti with the ratio of main element W up to
Fig. 2. XRD patterns of WT and WT-GGI ceramic composites.
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Fig. 3. Microstructure of WT and WT-GGI composites hotpressed at different temperature: (a) WT 1400 °C; (b) WT 1420 °C; (c)WT 1440 °C; (d) WT-GGI 1400 °C; (e) WT-GGI 1420 °C; (f) WT-GGI 1440 °C.
highest hardness, TRS and fracture toughness of 92.6 HRA, 1976 MPa and 14.8 MPa m1/2, respectively. This indicates that the optimum hotpressing temperature of WT and WT-GGI composites are 1400 °C and 1420 °C, respectively. Fig. 6 shows the relationship between wear time and mass loss. It can be seen that mass loss increased linearly with the increase of wear time. The wear-resistance of WT-GGI ceramic composites is better than WT ceramic composites. The reason maybe that the specimen has a relatively smooth surface and more densified structure with the GGIs addition. Fig. 6 also shows that WT-GGI ceramic composites hot-pressed at 1420 °C showed the best dry sliding wear-resistance compared with other specimens. As we know, Hardness is usually accepted as the best indicator of wear-resistance. Materials with higher hardness usually have better wear-resistance. This result is identical to our previous research. Meanwhile, the TiC particle size, TiC content and the bonding strength of the TiC particle with the WCoB matrix may exert influence
80.01 wt%, while the dark region mainly contains Ti element, and the ratio of Ti is up to 90.81 wt%. 3.3. Mechanical properties The hardness, TRS and fracture toughness of hot-pressed specimens was measured at room temperature, and the results were summarized in Fig. 5. It was found that the hardness, TRS and fracture toughness of WT-GGI ceramic composites are higher than those of WT ceramic composites at the same hot-pressing temperature. This can be attributed to the fact that GGIs can fine WCoB-grains and TiC-particle, resulting in a higher hardness, TRS and fracture toughness. When the hot-pressing temperature is raised, the hardness, TRS and fracture toughness of WT ceramic composites almost decreased linearly. However, the hardness, TRS and fracture toughness of WT-GGI ceramic composites increased first and then decreased with the hot-pressing temperature increased. In addition, WT-GGI ceramic composites hot-pressed at 1420 °C have the
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Fig. 6. The relationship between wear time and mass loss of WT and WT-GGI ceramic composites.
4. Conclusions A detailed investigation was carried out on the microstructure, hardness, transverse rupture strength, fracture toughness and wear-resistance of WCoB-TiC ceramic composites with/without 0.3 wt% VC and 0.3 wt% Cr3C2 prepared by hot-pressing. It was found that the results reveal that the grains can obviously become refined and the densification temperature of WCoB-TiC ceramic composites will be increased due to the VC and Cr3C2. The typical microstructure of WCoBTiC ceramic composites mainly consist of bright W2CoB2 grains, gray TiC particles, dark TiB2 and pores. WCoB-TiC ceramic composite doped with 0.3 wt% VC and 0.3 wt% Cr3C2 by hot-pressing at 1420 °C show the optimum mechanical properties (hardness, TRS and KIC are 92.6 HRA, 1976 MPa and 14.8 MPa m1/2, respectively) and the best dry sliding wear-resistance.
Fig. 4. EDS analysis of different color regions in WT ceramic composites hot-pressed at 1400 °C.
Acknowledgement on the wear-resistance of WT and WT-GGI ceramic composites. The optimal conditions used in the hot-pressing technique are the main reason for small grain sizes, improvement in hardness and high contiguity of the TiC network. Therefore, the wear-resistance of WCoB-TiC hot-pressed ceramic composites is governed by these properties.
We would like to appreciate the financial support provided by the Youth Fund of The State Key Laboratory of Refractories and Metallurgy under project No. 2016QN18.
Fig. 5. Hardness, TRS and KIC of WT and WT-GGI composites hotpressed at different temperature.
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References [13] [1] Sue A J, Fang Z, White A C. Boronized wear-resistant materials and methods thereof: US, US 6478887 B1[P]. 2002. [2] A. Saez, F. Arenas, E. Vidal, Microstructure development of WCoB-TiC based hard materials, Int. J. Refract. Met. Hard Mater. 21 (2003) 13–18. [3] V. Bonache, M.D. Salvador, V.G. Rocha, et al., Microstructural control of ultrafine and nanocrystalline WC-12Co-VC/Cr3C2 mixture by spark plasma sintering, Ceram. Int. 37 (2011) 1139–1142. [4] C. Ouyang, S. Zhu, H. Qu, VC and Cr3C2 doped WC-MgO compacts prepared by hotpressing sintering[J], Mater. Des. 40 (2012) 550–555. [5] Y. Li, D. Zheng, X. Li, et al., Cr3C2 and VC doped WC-Si3N4 composites prepared by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 41 (2013) 540–546. [6] W. Dong, S. Zhu, Y. Wang, et al., Influence of VC and Cr3C2 as grain growth inhibitors on WC-Al2O3 composites prepared by hot press sintering, Int. J. Refract. Met. Hard Mater. 45 (2014) 223–229. [7] W. Zhang, Y. Du, Y. Peng, Effect of TaC and NbC addition on the microstructure and hardness in graded cemented carbides: simulations and experiments, Ceram. Int. 42 (2015) 428–435. [8] W. Wan, J. Xiong, M. Liang, Effects of secondary carbides on the microstructure, mechanical properties and erosive wear of Ti (C,N)-based cermets, Ceram. Int. 43 (2017) 944–952. [9] X.G. Wang, J.X. Liu, Y.M. Kan, et al., Effect of solid solution formation on densification of hot-pressed ZrC ceramics with MC (M = V, Nb, and Ta) additions, J. Eur. Ceram. Soc. 32 (2012) 1795–1802. [10] M. Mahmoodan, H. Aliakbarzadeh, R. Gholamipour, Sintering of WC-10%Co nano powders containing TaC and VC grain growth inhibitors, Trans. Nonferrous Metals Soc. China 21 (2011) 1080–1084. [11] H. Wang, T. Webb, J.W. Bitler, Different effects of Cr3C2 and VC on the sintering behavior of WC-Co materials, Int. J. Refract. Met. Hard Mater. 53 (2015) 117–122. [12] Y. Jin, B. Huang, C. Liu, et al., Phase evolution in the synthesis of WC-Co-Cr3C2-VC
[14] [15]
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[19]
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nanocomposite powders from precursors, Int. J. Refract. Met. Hard Mater. 41 (2013) 169–173. L. Sun, T. Yang, C. Jia, et al., VC, Cr3C2 doped ultrafine WC-Co cemented carbides prepared by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 29 (2011) 147–152. S. Luyckx, M.Z. Alli, Comparison between V8C7 and Cr3C2 as grain refiners for WCCo, Mater. Des. 22 (2001) 507–510. K. Deqing, P. Yingjun, L. Xufeng, et al., Influence and effectivity of Sm2O3 and Cr3C2 grain growth inhibitors on sintering of WCoB-TiC based cermets, Ceram. Int. 41 (2015) 15235–15240. H. Zhao, W. Wang, Z. Fu, et al., Thermal conductivity and dielectric property of hotpressing sintered AlN-BN ceramic composites, Ceram. Int. 35 (2009) 105–109. X. Zhang, G.E. Hilmas, W.G. Fahrenholtz, et al., Hot pressing of tantalum carbide with and without sintering additives, J. Am. Ceram. Soc. 90 (2010) 393–401. L. Jia, X. Zhang, W. Zhi, et al., Microstructure and mechanical properties of hotpressed ZrB2 -SiC-ZrO2f ceramics with different sintering temperatures, Mater. Des. 34 (2012) 853–856. B. Zou, H. Zhou, K. Xu, et al., Study of a hot-pressed sintering preparation of Ti (C7N3)-based composite cermets materials and their performance as cutting tools, J. Alloys Compd. 611 (2014) 363–371. J.M. Lonergan, W.G. Fahrenholtz, G.E. Hilmas, Sintering mechanisms and kinetics for reaction hot-pressed ZrB2, J. Am. Ceram. Soc. 98 (2015) 2344–2351. J.X. Xue, H.T. Liu, Y. Tang, et al., Effect of sintering atmosphere on the densification behavior of hot pressed TiN ceramics, Ceram. Int. 39 (2013) 8531–8535. D.K. Shetty, I.G. Wright, P.N. Mincer, et al., Indentation fracture of WC-Co cermets, J. Mater. Sci. 20 (1985) 1873–1882. ASTM G99-05, Standard Test Method for Wear Testing with a Pin-on-disc Apparatus, West Conshohocken, PA, (2005). J. Song, C. Huang, B. Zou, et al., Effects of sintering additives on microstructure and mechanical properties of TiB2-WC ceramic-metal composite tool materials, Int. J. Refract. Met. Hard Mater. 30 (2012) 91–95.
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Short communication
VC and Cr3C2 doped WCoB-TiC ceramic composites prepared by hotpressing
MARK
Deqing Ke, Yingjun Pan⁎, Yuanyuan Xu, Pan Wang, Run Wu The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: WCoB-TiC ceramic composites Hot-pressing Grain growth inhibitors Microstructure Mechanical properties
The grain growth inhibitors (GGIs) VC and Cr3C2 doped WCoB-TiC ceramic composites were fabricated by hotpressing. The microstructure, hardness, transverse rupture strength (TRS), fracture toughness (KIC) and wearresistance of WCoB-TiC ceramic composites were investigated. The results reveal that the grains can obviously become refined and the densification temperature of WCoB-TiC ceramic composites will be increased due to the VC and Cr3C2. The typical microstructure of WCoB-TiC ceramic composites mainly consist of bright W2CoB2 grains, gray TiC particles, dark TiB2 and pores. WCoB-TiC ceramic composites doped with 0.3 wt% VC and 0.3 wt% Cr3C2 hot-pressing at 1420 °C show the optimum mechanical properties (hardness, TRS and KIC are 92.6 HRA, 1976 MPa and 14.8 MPa m1/2, respectively) and the best dry sliding wear-resistance.
1. Introduction As a typical kind of ternary boride based cermets, WCoB shows tremendous development potential in engineering materials with extremely high-hardness, high-melting point, excellent wear-resistance and exidation-resistance, such as cutting tools, drawing dies, bearing surfaces, etc. [1]. Nevertheless, the inherent brittleness severely limits its application as structural materials. In recent years, research has been focused on the synthesis of WCoB-TiC ceramic composites in order to obtain high performance. A. Saez et al. [2] fabricated WCoB-TiC ceramic composites for the WC-Co-TiB2 raw material powder system and studied the influence of initial Co contents (varying between 10–47 wt%) on the microstructure and mechanical properties of WCoBTiC ceramic composites, and claimed that WCoB-TiC ceramic composites with no Co binder show enormous application prospect in oxidizing environments. In addition, it has been found that refractory carbides, such as VC, Cr3C2, NbC, TaC and Mo2C, can control the microstructure evolution of ceramic phases during sintering and change the mechanical properties [3–10]. With high solubility and mobility in the cobalt phase, VC and Cr3C2 are the most effective GGIs in WC-Co cemented carbides [4,11–14]. Our previous research revealed that typical refractory carbides Cr3C2 can effectively suppress the grain growth, but the densification response of the WCoB-TiC ceramic composites prepared by vacuum sintering was not good [15]. It is widely known that hot-pressing method has been proposed to achieve the densification of ceramic ⁎
Corresponding author. E-mail address: [email protected] (Y. Pan).
http://dx.doi.org/10.1016/j.ijrmhm.2017.06.005 Received 1 May 2017; Received in revised form 15 June 2017; Accepted 18 June 2017 Available online 19 June 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
materials [16–21]. However, it has been rarely reported that WCoB-TiC ceramic composites were made by hot pressing method. So, it is worthy of further studying the hot-pressed WCoB-TiC ceramic composites with grain growth inhibitor addition. This work is aimed to investigate the ability of GGIs to influence the microstructure and mechanical properties of WCoB-TiC ceramic composites by hot-pressing at different sintering temperatures. The densification behavior, microstructure evolution and mechanical properties of hot-pressed specimens were investigated. 2. Experimental procedures Characteristics of the raw powders are listed in Table 1. The ratios of the mixed powders and hot-pressing temperatures are listed in Table 2. The powders WC-19.6 wt%Co-21.2 wt%TiB2 (WT) and WC19.6 wt%Co-21.2 wt%TiB2-0.3 wt%VC-0.3 wt%Cr3C2 (WT-GGI) were premixed and ball milled with a QM-1SP4 planetary ball milling machine under argon gas atmosphere for 45 h, respectively. The ratio of ball to powder weight was 10:1 and the rotation speed of the mill was 350 rpm. Both the vial and milling balls (10 mm in diameter) were made of cemented carbide materials. After debindering, the milled composite powders were hot-pressed in a graphite die using a vacuum hot-pressing furnace at the temperatures of 1400, 1420 or 1440 °C under the pressure of 39.6 MPa in a vacuum (about1.3 × 10− 1 Pa) atmosphere for 30 min. The densities of the sintered specimens were measured by the density measuring instrument (METTLER TOLEDO AB-104N). The
International Journal of Refractory Metals & Hard Materials 68 (2017) 24–28
D. Ke et al.
phase identification of the specimens was studied by an X-radial Diffractometer (XRD, XPert PRO MPD) equipped with a Cu Kα radiation (λ = 0.15418 nm) at 400 kV and 200 mA. The 2θ range was 10°–90° with a step of 0.05° and the measure time was 10 s per step. The field emission scanning electron microscopy (FE-SEM, FIB Nova 400 Nano) was used to analyze the microstructure of the sintered specimens. Their chemical composition was examined by energy dispersive spectroscopy (EDS). The hardness of the hot-pressed specimens was measured with a standard Rockwell hardness measuring device (HRA-150) with the load of 30 kg and the indentation time of 10 s. At least ten indentations were formed to calculate an average value. Before TRS test, the four long face of the hot-pressed specimens were processed by a diamond wheel with enough coolant, and the surface roughness Ra ≤ 0.4 μm and the angle of chamfer was 45°. The TRS at room temperature was measured by a three-point bend test with a span length of 30 mm and a cross-head speed of 0.5 mm/min. The fracture toughness was calculated based on the radial crack length produced by Vickers (HV30) indentation according to the formula proposed by Shetty et al. [22]. Wear tests were carried out under dry sliding conditions with a tribometer pin-on-disc (ball-on-disc configuration) manufactured by MICROTEST MT2/60/ SCM/T, according to ASTM wear testing standard G99-05 [23].
Table 1 Characteristics of the raw powders. Powder
Mean particle size (μm)
Adulterant (wt%)
WC Co TiB2 VC Cr3C2
~4.0 ~2.0 ~2.0 ~2.0 ~2.0
< 0.1 < 0.1 < 0.1 < 0.1 < 0.1
Table 2 Ratios of the mixed powders and hot-pressing temperature. Specimens
A B C D E F
Powder ratios/wt%
Hot-pressing temperature/°C
WC
Co
TiB2
VC
Cr3C2
59.2 59.2 59.2 58.6 58.6 58.6
19.6 19.6 19.6 19.6 19.6 19.6
21.2 21.2 21.2 21.2 21.2 21.2
0 0 0 0.3 0.3 0.3
0 0 0 0.3 0.3 0.3
1400 1420 1440 1400 1420 1440
3. Results and discussion 3.1. Densification The density of specimens hot-pressed at different temperatures are shown in Fig. 1. As shown in Fig. 1, when hot-pressing temperature increased, the densities of the WT ceramic composites did not decrease significantly. While the densities of the WT-GGI ceramic composites reached a peak at 1420 hot-pressing temperature and presented a sharp decline. Furthermore, the densities of the WT-GGI ceramic composites are higher than WT ceramic composites at the same hot-pressing temperature. This indicates that the densification temperature of WCoB-TiC ceramic composites increased with the addition of 0.3 wt% VC and 0.3 wt% Cr3C2. 3.2. Microstructure
Fig. 1. The density of WT and WT-GGI ceramic composites hot-pressed at different
The typical XRD patterns of WT and WT-GGI ceramic composites are shown in Fig. 2. It was found that the phases of WT and WT-GGI ceramic composites were WCoB, W2CoB2, TiB2, TiC and Co2B. The appearance of the W2CoB2 and TiC phase is mainly attributed to the following reaction [24]: 2WC + 5Co + 2TiB2 → W2CoB2 + 2TiC + 2Co2B
(1)
The microstructure of WT and WT-GGI ceramic composites is shown in Fig. 3. As shown in Fig. 3, the typical microstructure of WCoB-TiC ceramic composites is composed of bright W2CoB2 grains, gray TiC particles, dark TiB2 and pores. It could be seen that, WT ceramic composites have a grain size about 1.0–2.2 μm (Fig. 3a, b, c), while the grains of WT-GGI are refined to 0.6–1.5 μm (Fig. 3d, e, f). This indicates that the microstructure (coarse/fine) of the ceramic composites can be tailored by altering hot-pressing temperature and/or adding the GGIs (VC and Cr3C2). The WT ceramic composites that were hot pressed at 1400 °C were selected to characterize the composition of different color regions by EDS, and the results are shown in Fig. 4. It could be seen that, the white region contains W, Co and Ti with the ratio of main element W up to
Fig. 2. XRD patterns of WT and WT-GGI ceramic composites.
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Fig. 3. Microstructure of WT and WT-GGI composites hotpressed at different temperature: (a) WT 1400 °C; (b) WT 1420 °C; (c)WT 1440 °C; (d) WT-GGI 1400 °C; (e) WT-GGI 1420 °C; (f) WT-GGI 1440 °C.
highest hardness, TRS and fracture toughness of 92.6 HRA, 1976 MPa and 14.8 MPa m1/2, respectively. This indicates that the optimum hotpressing temperature of WT and WT-GGI composites are 1400 °C and 1420 °C, respectively. Fig. 6 shows the relationship between wear time and mass loss. It can be seen that mass loss increased linearly with the increase of wear time. The wear-resistance of WT-GGI ceramic composites is better than WT ceramic composites. The reason maybe that the specimen has a relatively smooth surface and more densified structure with the GGIs addition. Fig. 6 also shows that WT-GGI ceramic composites hot-pressed at 1420 °C showed the best dry sliding wear-resistance compared with other specimens. As we know, Hardness is usually accepted as the best indicator of wear-resistance. Materials with higher hardness usually have better wear-resistance. This result is identical to our previous research. Meanwhile, the TiC particle size, TiC content and the bonding strength of the TiC particle with the WCoB matrix may exert influence
80.01 wt%, while the dark region mainly contains Ti element, and the ratio of Ti is up to 90.81 wt%. 3.3. Mechanical properties The hardness, TRS and fracture toughness of hot-pressed specimens was measured at room temperature, and the results were summarized in Fig. 5. It was found that the hardness, TRS and fracture toughness of WT-GGI ceramic composites are higher than those of WT ceramic composites at the same hot-pressing temperature. This can be attributed to the fact that GGIs can fine WCoB-grains and TiC-particle, resulting in a higher hardness, TRS and fracture toughness. When the hot-pressing temperature is raised, the hardness, TRS and fracture toughness of WT ceramic composites almost decreased linearly. However, the hardness, TRS and fracture toughness of WT-GGI ceramic composites increased first and then decreased with the hot-pressing temperature increased. In addition, WT-GGI ceramic composites hot-pressed at 1420 °C have the
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Fig. 6. The relationship between wear time and mass loss of WT and WT-GGI ceramic composites.
4. Conclusions A detailed investigation was carried out on the microstructure, hardness, transverse rupture strength, fracture toughness and wear-resistance of WCoB-TiC ceramic composites with/without 0.3 wt% VC and 0.3 wt% Cr3C2 prepared by hot-pressing. It was found that the results reveal that the grains can obviously become refined and the densification temperature of WCoB-TiC ceramic composites will be increased due to the VC and Cr3C2. The typical microstructure of WCoBTiC ceramic composites mainly consist of bright W2CoB2 grains, gray TiC particles, dark TiB2 and pores. WCoB-TiC ceramic composite doped with 0.3 wt% VC and 0.3 wt% Cr3C2 by hot-pressing at 1420 °C show the optimum mechanical properties (hardness, TRS and KIC are 92.6 HRA, 1976 MPa and 14.8 MPa m1/2, respectively) and the best dry sliding wear-resistance.
Fig. 4. EDS analysis of different color regions in WT ceramic composites hot-pressed at 1400 °C.
Acknowledgement on the wear-resistance of WT and WT-GGI ceramic composites. The optimal conditions used in the hot-pressing technique are the main reason for small grain sizes, improvement in hardness and high contiguity of the TiC network. Therefore, the wear-resistance of WCoB-TiC hot-pressed ceramic composites is governed by these properties.
We would like to appreciate the financial support provided by the Youth Fund of The State Key Laboratory of Refractories and Metallurgy under project No. 2016QN18.
Fig. 5. Hardness, TRS and KIC of WT and WT-GGI composites hotpressed at different temperature.
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D. Ke et al.
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