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Microstructure and mechanical properties of NbC matrix cermets using Ni containing metal binder
Metal Powder Report Volume 00, Number 00 May 2016
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Microstructure and mechanical properties of NbC matrix cermets using Ni containing metal binder S.G. Huang1, J. Vleugels1, H. Mohrbacher2 and M. Woydt3 1
Department of Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium NiobelCon bvba, B-2970 Schilde, Belgium 3 BAM Federal Institute for Materials Research and Testing, Division 6.3 Tribology & Wear Protection, Unter den Eichen 44-46, D-12203 Berlin, Germany 2
The current study reports on the effect of the addition of Al metal and secondary refractory carbides on the microstructure and mechanical properties of Ni bonded NbC matrix cermets. Powder mixtures were pressurelessly sintered for 1 h at 14208C in vacuum. Microstructural and elemental mapping of the sintered cermets were performed by electron probe microanalysis (EPMA) to reveal the effect of the additions on the NbC grain morphology, grain growth and binder composition. Results indicated that NbC grain growth was suppressed and a homogeneous NbC grain size distribution was obtained in the cermets with the addition of Al and secondary carbides, i.e. Mo2C + VC + WC, as compared to the pure Ni binder. The liquid phase sintered NbC-12 vol% Ni cermet had a Vickers hardness (HV10) of 1130 22 kg/mm2 and indentation toughness of 11.8 0.4 MPa m1/2. With the addition of 4 vol% WC-4 vol% VC-4 vol% Mo2C, the hardness increased to 1490 15 kg/mm2, whereas the corresponding toughness decreased to 9.2 0.4 MPa m1/2. Addition of 4 vol% Mo2C into WC-13.8 vol% Ni-52.3 vol% NbC mixture further increased the hardness to 1620 19 kg/mm2 in combination with a moderate fracture toughness of 7.7 0.2 MPa m1/2. Introduction WC-Co cemented carbides are the most widely used hard materials for wear applications, combining an excellent hardness, toughness and strength. Pure niobium carbide (NbC) has a high hardness (1960 kg/mm2), low density (7.79 g/cm3) and high melting temperature (36008C) [1]. Moreover, the hardness of cubic structure NbC can be tailored by the Nb/C ratio [1]. However, NbC on the other hand has been hardly explored as a major hard phase for cemented carbides and cermets. Very recently, NbC-Co and NbC-Fe3Al cermets prepared by solid state sintering were however qualified as a competitive or even superior alternative in terms of wear resistance to WC- and Cr3C2-based cemented carbides and ceramics [2]. The NbC cermets clearly demonstrate the potential for tribological applications [2,3]. Earlier work revealed a remarkable NbC grain growth in Co bonded NbC cermets when pressureless liquid-phase E-mail address: [email protected].
sintered [4]. NbC grain growth however was largely suppressed when consolidating NbC-Co cermets at lower temperatures [3] or with the addition of secondary carbides [4,5]. To further improve the hardness of NbC-Co cermets, novel NbC-Co cermet grades containing a significant amount of nanosized WC grains were developed, together with the addition of VC and Cr3C2 as grain growth inhibitors [6,7]. Aluminides, a potential binder phase for NbC, are attractive for high-temperature structural material applications [8]. Based on these studies, it is concluded that the hardness of the NbC cermets can be tailored to different levels. However, the fracture toughness remained to be rather low when compared to the widely used WC-Co materials. In the present study, the effect of Al and secondary carbides on the microstructure and mechanical properties of Ni bonded NbC cermets were investigated. Ni is chosen as the binder for NbC due to its good wetting behavior with cubic carbides, such as TiN and TiC. The NbC based cermets were prepared by conventional liquid
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Metal Powder Report Volume 00, Number 00 May 2016
SPECIAL FEATURE FIGURE 1
Morphology of the NbC (a) and Ni powder (b). FIGURE 2
phase sintering. The goal was to develop novel NbC-Ni cermets with tailored mechanical properties, especially an improved fracture toughness, through the variation of the chemical composition of the binder phase.
Experimental procedure Powders, NbC (Seco, 0.3 mm, Sweden), WC (CRC-015, 0.5 mm, ¨ tten GmbH, Austria), VC (Treibacher, Wolfram Bergbau und Hu FSSS = 1.15 mm, Austria) and Mo2C (Chempur, 2 mm) as well as metal powders, Ni (AEE, Ni-110, 4–8 mm) and Al (AEE, Al-100, 1–5 mm) were used to prepare NbC-based cermets. The morphology of the NbC and Ni is shown in Fig. 1. The agglomerate size of the NbC and Ni powders is below 10 mm. The agglomerates consist of sub-mm size particles. The NbC powder has a total carbon content of 10.75 wt% and oxygen content of 0.17 wt%.
Calculated phase diagrams of the Nb-C-12 vol% Ni system (a) and phase evolution with temperature of the studied NbC-12 vol% Ni cermet (b).
The chemical composition of the investigated cermets is listed in Table 1. All the cermets contain NbC as hard phase and Ni or NiAl as metal binder. The Ni-Al binder is a powder mixture of Ni and Al powders. Different amounts of refractory carbide, such as WC, VC and Mo2C, were added into the NbC-Ni powder mixtures. The weighed powders were then mixed on a multi-directional mixer (Turbula, WAB, Switzerland) in ethanol for 24 h using WC-6 wt% Co milling balls (Ceratizit grade H20C, Ø10 mm). The suspension was dried in a rotating evaporator at 658C. Cold isostatically pressed (300 MPa) powder compacts (Ø20 mm 10 mm) were densified by conventional pressureless liquid phase sintering for 1 h at 14208C in a dynamic vacuum (7 Pa) with a heating and cooling rate of 208C/min.
TABLE 1
Chemical composition of the investigated NbC cermets. Cermet
Metal binder (vol%)
Composition
T.D. (g/cm3)
NbC12Ni NbC12Ni30Al NbC12Ni4Mo2C4VC4WC WC14Ni52NbC4Mo2C
12 Ni 12 (Ni-30 mol% Al) 12 Ni 13.8 Ni
NbC-12 vol% Ni NbC-12 vol% (Ni-30 mol% Al) NbC-12Ni-4Mo2C-4VC-4WC (vol%) WC-13.8Ni-52.3NbC-4.1Mo2C (vol%)
7.92 7.76 8.21 10.30
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The bulk density of the sintered cermets was measured in ethanol. The microstructure of polished surfaces was examined by electron probe microanalysis (EPMA, JXA-8530F, JEOL Ltd.), equipped with five WDS and one EDS-type detector. The Vickers hardness (HV10) was measured (Model FV-700, Future-Tech Corp., Tokyo) with an indentation load of 98.1 N. The fracture toughness, KIC, was measured from the length of the radial cracks originating from the corners of these indentations and calculated according to the formula of Shetty [9]. The reported values are the mean and standard deviation of five indentations.
Results and discussion Thermodynamics of the NbC-Ni system A thermodynamic evaluation of the NbC-Ni system was carried out using Thermo-Calc [10] and the database TCFE7. The influence of the total carbon content and temperature on the phase evolution of the Nb-C-12 vol% Ni system is shown in Fig. 2a. At a low carbon content of 9.25 wt%, the equilibrium phases at 1500 and 11008C are NbC + liquid and NbC + NbNi3 respectively. With increasing carbon content of 9.75 wt%, the corresponding phases
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change to NbC + Fcc at 11008C and this structure can be retained at room temperature. When graphite is present, the eutectic temperature of the NbC-12 vol% Ni-C system is estimated at 12658C. The liquid-phase forming temperature is around 12708C at a carbon content between 9.3 and 10.0 wt%. It is apparent that a sintering temperature of 14208C is in the liquidphase region of the Ni alloy binder. The influence of temperature on the phase evolution of the NbC-12 vol% Ni system is shown in Fig. 2b. The NbC-12 vol% Ni powder mixture has a total carbon content of 9.27 wt% based on the carbon analysis of the NbC starting powder. When the powder mixture is heated to 12808C, the solid Ni binder converts to a liquid phase. Formation of NbNi3 occurs at 10938C. With decreasing temperature, more and more NbNi3 phase is formed.
Microstructural analysis Backscattered electron micrographs (BSE) of the liquid phase sintered NbC cermets with 12 vol% metal binders, i.e., Ni and Ni-30 mol% Al, are shown in Fig. 3a and b. For the pure Ni binder, the bright and dark contrast phases correspond to the NbC and Ni
FIGURE 3
BSE micrographs of NbC cermets vacuum sintered at 14208C. NbC-12 vol% Ni (a), NbC-12 vol% (Ni-30Al) (b), NbC-12Ni-4Mo2C-4WC-4VC (vol%) (c) and WC13.8Ni-52.3NbC-4.1Mo2C (vol%) (d). 3 Please cite this article in press as: S.G. Huang, et al., Met. Powder Rep. (2016), http://dx.doi.org/10.1016/j.mprp.2016.05.009
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based binder respectively. The shape of the large NbC grains is well-faceted with a slight rounding of the edges, while the small grains are almost spherical. The NbC grain size is in the 5–20 mm range. Although coarse Ni and NbC agglomerates were used as starting powders, the Ni binder is evenly distributed in the NbC matrix, indicating a good wetting between NbC and Ni binder. The most obvious effect of the 30 mol% Al addition in the Ni binder is a reduced sintered NbC grain size to less than 10 mm, as shown in Fig. 3b. An enhanced angular NbC grain morphology was observed in the cermet. Besides the NbC and metal binder grains, dark contrast Al2O3 grains were also found in the cermets. The Ni-Al binder is evenly distributed and the NbC grains have a narrow grain size distribution in the 5–10 mm range. It was reported that the grain growth of NbC in Co, Ni and Fe binders is realized through dissolution and re-precipitation [11]. During liquid phase sintering, the small NbC grains dissolved in the Ni and Ni-Al binder and re-precipitated on larger NbC grains. The reduction of surface energy of the solid particles is the major driving force for small grains to dissolve and large grains to grow.
Metal Powder Report Volume 00, Number 00 May 2016
When Al is dissolved in the binder, the surface energy of the liquid binder and the NbC solubility in the binder are changed, lowering the driving force for NbC dissolution-reprecipitation. The X-ray elemental map of Nb, Ni, Al, C and O of the liquid phase sintered 12 vol% (Ni-30 mol% Al) cermet is shown in Fig. 4. The mapping clearly demonstrates that only Nb and C are present in the carbide grains and both Ni and Al, as well as Nb and C are present in the binder phase. The Nb/C ratio in the NbC phase seems to have shifted to a lower carbon content due to the presence of C in the Ni binder. The presence of sub-mm sized dark Al2O3 grains is mainly due to the oxidation of Al metal powder during the powder mixture preparation procedure. Some of the oxide can be also from the fine Al starting powder. The microstructure of the 4 Mo2C-4 WC-4 VC (vol%) added NbC-12 vol% Ni cermet, vacuum sintered for 1 h at 14208C, is shown in Fig. 3c. Although the cermet has 12 vol% secondary carbides, only a bright-gray NbC phase and a dark-gray Ni based binder were observed. The NbC-12 vol% Ni cermet (Fig. 3a) has relatively large (<20 mm) facet NbC grains with a slight
FIGURE 4
WDS elemental mapping of the NbC-12 vol% (Ni-30 mol% Al) cermet. 4 Please cite this article in press as: S.G. Huang, et al., Met. Powder Rep. (2016), http://dx.doi.org/10.1016/j.mprp.2016.05.009
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rounding of the edges and mostly separated by the Ni binder, whereas the sintered NbC grain size is significantly reduced to 0.5–2 mm upon addition of the secondary carbides. There is only a limited NbC grain growth compared to the size of the 0.3 mm NbC starting powder. These carbides are known to have a very high solubility (WC > Mo2C > VC) in Ni at the sintering temperature [12]. The surface energy of the liquid Ni binder was therefore changed during sintering and dissolution of the secondary carbides, especially WC and VC into NbC also influenced the surface energy of NbC, therefore affecting the NbC grain growth behavior. A backscattered electron image of the WC-13.8Ni-52.3NbC4Mo2C (vol%) cermet is shown in Fig. 3d. Bright WC, gray NbC and dark Ni binder phases could be differentiated. The gray NbC grains form a 52 vol% skeleton and are actually (Nb,W)C solid solution grains. The Ni binder is mainly present among the bright WC grains and not at the NbC grain boundaries, due to the better wettability in the WC-Ni than in the NbC-Ni system [13]. Free Mo2C grains could not be observed in the microstructure due to its low concentration and homogeneous distribution in the binder and carbide phases. Considering the final WC and NbC grain size,
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it is apparent that there is a limited growth of the WC grains (<0.6 mm) in the cermet, whereas the NbC grains are in the range of 5–10 mm. The present observation is similar to the Co bonded WC-NbC cemented carbides [7], where a small amount of VC and Cr3C2 significantly limit the WC grain growth. Elemental mappings of the mixed carbide cermet are shown in Fig. 5. According to the Nb and W distribution, the W is present both in the WC and NbC grains. The dissolution of WC into the NbC grains forms (Nb,W)C solid solution carbide grains. The Ni binder is mostly located around the WC grains, confirming that Ni has a better wettability with WC than NbC.
Mechanical properties The Vickers hardness (HV10) and fracture toughness (KIC) of the investigated cermets are summarized in Table 2 and compared in Fig. 6. The theoretical density (TD) was calculated based on the mixture rule. According to the observed microstructure, all cermets were sintered to full density, although the NbC-12 vol% (Ni-30 mol% Al) has a relative density of only 96.7% TD. The underestimated density level of the NbC-12 vol% (Ni-30 mol% Al) cermet can be due to the inaccurate theoretical density value,
FIGURE 5
WDS elemental mapping of the WC-13.8Ni-52.3NbC-4.1Mo2C (vol%) cermet. 5 Please cite this article in press as: S.G. Huang, et al., Met. Powder Rep. (2016), http://dx.doi.org/10.1016/j.mprp.2016.05.009
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TABLE 2
Hardness and fracture toughness of the investigated cermets. Cermet
Density (g/cm3)
R.D. (%)
HV10 (kg/mm2)
KIC (MPa m1/2)
Crack resistance
NbC-12 vol% Ni NbC-12 vol% (Ni-30 mol% Al) NbC-12Ni-4Mo2C-4WC-4VC (vol%) WC-13.8Ni-52.3NbC-4.1Mo2C (vol%)
7.90 7.50 8.19 10.29
100 96.7 99.8 99.9
1130 22 1360 29 1490 15 1620 19
11.8 0.4 6.4 0.2 9.2 0.4 7.7 0.2
1.63 0.35 0.28 0.01 0.73 0.07 0.48 0.03
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due to the formation of Ni-Al alloy and Al2O3. It is known that the hardness of WC-Co cemented carbides is mainly dependent on the WC grain size (Hall–Petch relationship) and Co binder content. The finer the WC grain size, the higher the hardness for a given Co volume fraction. In this study, the 12 vol% Ni bonded NbC cermet has the lowest hardness, followed by the NbC-12 vol% (Ni-30 mol% Al), the NbC-12 Ni-4Mo2C-4VC-4WC (vol%) and the WC-13.8Ni-52.3NbC-4.1Mo2C (vol%) cermet, which has the highest hardness. Since the NbC matrix cermets have the same binder content, the difference in hardness of the NbC matrix cermets can be directly linked to the properties of the binder and the NbC grain size. According to the observed microstructures, NbC-12 vol% Ni has the largest NbC grain size, followed by the Ni-30 mol% Al cermet and the NbC-12 Ni-4Mo2C-4VC-4WC (vol%) cermet. Although the NbC grains are much coarser in the NbC substituted WC-Ni cermet, the extremely fine WC grains should contribute to the high hardness of the WC-NbC-Ni-Mo2C cermet. Furthermore, the increased hardness of this cermet might also be partially due to the solid solution hardening of the NbC phase. The higher hardness of the (Nb,W,Mo)C phase however remains to be proven. According to Fig. 6, there is an antagonistic correlation between the hardness and fracture toughness of NbC matrix cermets, except for the Ni-Al binder cermet. Usually, the metal binder plays a crucial role in shielding the stress field in front of a crack tip to improve the toughness [14]. Ductile failure of the Co, Ni and Fe binder is usually found on fractured NbC-Co/Ni/Fe surfaces. Since the Ni-Al binder has an intrinsically low room temperature ductility, the Al doped cermet exhibits a lower fracture toughness of 6.4 MPa m1/2. The 12 vol% Ni bonded NbC cermet has the highest toughness of 11.8 MPa m1/2. With increased hardness to
1490 kg/mm2, the NbC-12 Ni-4Mo2C-4VC-4WC (vol%) cermet has a toughness of 9.2 MPa m1/2, whereas the WC-13.8Ni-52.3NbC4.1Mo2C (vol%) cermet has a toughness of 7.7 MPa m1/2 and hardness of 1620 kg/mm2. The crack resistance, defined as P/4L (P is the indentation load (98.1 N) and L is the crack length (mm)) is also summarized in Table 2. It is apparent that the pure Ni binder (NbC-12 vol% Ni) has the highest crack resistance, i.e. shortest crack length, followed by the NbC-12Ni-4Mo2C-4WC-4VC (vol%), WC-13.8Ni-52.3NbC4.1Mo2C (vol%) and NbC-12 vol% (Ni-30 mol% Al) cermets. Two representative crack propagation patterns, induced by Vickers
FIGURE 7 FIGURE 6
Vickers hardness and fracture toughness of the investigated NbC cermets.
Crack propagation mode of the cermet NbC-12 Ni-4 Mo2C-4 WC-4 VC (vol%) (a) and WC-13.8 Ni-52.3 NbC-4.1 Mo2C (vol%) (b).
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indentations are shown in Fig. 7. The NbC-12Ni-4Mo2C-4WC-4VC (vol%) cermet mainly intergranularly fractures at the NbC/Ni interface, whereas the WC-13.8Ni-52.3NbC-4.1Mo2C (vol%) cermet exhibits both inter-granular fracture at the NbC/NbC and NbC/Ni interface, as well as transgranular fracture at the WC/Ni interface.
Acknowledgements
Conclusions
References
Full densification of Ni bonded NbC based cermets was achieved by pressureless liquid phase sintering in vacuum for 1 h at 14208C. The addition of Al to the Ni binder or the addition of multiple carbides (WC, Mo2C, VC) in NbC-Ni cermets influenced not only the morphology of the NbC grains, but also the NbC grain size as well as the Ni binder distribution. The hardness and fracture toughness of the NbC matrix cermets can be mainly tailored by the secondary carbide additions. An optimal combination of hardness and toughness was found for NbC-Ni cermets with multiple carbide addition The NbC-12 Ni-4 Mo2C-4 WC-4 VC (vol%) cermet exhibits a hardness of 1490 15 kg/mm2 and fracture toughness of 9.2 0.4 MPa m1/2.
[1] H.O. Pierson, Handbook of Refractory Carbides and Nitrides, Noyes, New Jersey, USA, 1996. [2] M. Woydt, H. Mohrbacher, Wear 321 (2014) 1–7. [3] S.G. Huang, et al. Mater. Lett. 61 (2) (2007) 574–577. [4] S.G. Huang, et al. J. Alloys Compd. 430 (1–2) (2007) 158–164. [5] S.G. Huang, et al. Int. J. Refract. Met. Hard Mater. 48 (2015) 418–426. [6] S.G. Huang, et al. Int. J. Refract. Met. Hard Mater. 26 (5) (2008) 389–395. [7] S.G. Huang, et al. J. Alloys Compd. 464 (1–2) (2008) 205–211. [8] R. Subramanian, J.H. Schneibel, Mater. Sci. Eng.: A 244 (1) (1998) 103–112. [9] D.K. Shetty, et al. J. Mater. Res. 20 (1985) 1873–1882. [10] B. Sundman, B. Jansson, J.O. Andersson, Calphad 9 (1985) 153–190. [11] R. Warren, J. Less Common Met. 17 (1969) 65–72. [12] H.E. Exner, Int. Mater. Rev. 24 (1) (1979) 149–173. [13] G.A. Yasinskaya, Sov. Powder Metall. Met. Ceram. 5 (7) (1966) 557–559. [14] M. Kotoul, Acta Mater. 45 (8) (1997) 3363–3376.
7 Please cite this article in press as: S.G. Huang, et al., Met. Powder Rep. (2016), http://dx.doi.org/10.1016/j.mprp.2016.05.009
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We gratefully acknowledge the financial support of the Companhia Brasileira de Metalurgia e Minerac¸a˜o (CBMM), Sa˜o Paulo, Brazil. The authors thank the Hercules Foundation (project ZW09-09) and the Fund for Scientific Research Flanders (FWO) under project G.0772.16N.
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Microstructure and mechanical properties of NbC matrix cermets using Ni containing metal binder S.G. Huang1, J. Vleugels1, H. Mohrbacher2 and M. Woydt3 1
Department of Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium NiobelCon bvba, B-2970 Schilde, Belgium 3 BAM Federal Institute for Materials Research and Testing, Division 6.3 Tribology & Wear Protection, Unter den Eichen 44-46, D-12203 Berlin, Germany 2
The current study reports on the effect of the addition of Al metal and secondary refractory carbides on the microstructure and mechanical properties of Ni bonded NbC matrix cermets. Powder mixtures were pressurelessly sintered for 1 h at 14208C in vacuum. Microstructural and elemental mapping of the sintered cermets were performed by electron probe microanalysis (EPMA) to reveal the effect of the additions on the NbC grain morphology, grain growth and binder composition. Results indicated that NbC grain growth was suppressed and a homogeneous NbC grain size distribution was obtained in the cermets with the addition of Al and secondary carbides, i.e. Mo2C + VC + WC, as compared to the pure Ni binder. The liquid phase sintered NbC-12 vol% Ni cermet had a Vickers hardness (HV10) of 1130 22 kg/mm2 and indentation toughness of 11.8 0.4 MPa m1/2. With the addition of 4 vol% WC-4 vol% VC-4 vol% Mo2C, the hardness increased to 1490 15 kg/mm2, whereas the corresponding toughness decreased to 9.2 0.4 MPa m1/2. Addition of 4 vol% Mo2C into WC-13.8 vol% Ni-52.3 vol% NbC mixture further increased the hardness to 1620 19 kg/mm2 in combination with a moderate fracture toughness of 7.7 0.2 MPa m1/2. Introduction WC-Co cemented carbides are the most widely used hard materials for wear applications, combining an excellent hardness, toughness and strength. Pure niobium carbide (NbC) has a high hardness (1960 kg/mm2), low density (7.79 g/cm3) and high melting temperature (36008C) [1]. Moreover, the hardness of cubic structure NbC can be tailored by the Nb/C ratio [1]. However, NbC on the other hand has been hardly explored as a major hard phase for cemented carbides and cermets. Very recently, NbC-Co and NbC-Fe3Al cermets prepared by solid state sintering were however qualified as a competitive or even superior alternative in terms of wear resistance to WC- and Cr3C2-based cemented carbides and ceramics [2]. The NbC cermets clearly demonstrate the potential for tribological applications [2,3]. Earlier work revealed a remarkable NbC grain growth in Co bonded NbC cermets when pressureless liquid-phase E-mail address: [email protected].
sintered [4]. NbC grain growth however was largely suppressed when consolidating NbC-Co cermets at lower temperatures [3] or with the addition of secondary carbides [4,5]. To further improve the hardness of NbC-Co cermets, novel NbC-Co cermet grades containing a significant amount of nanosized WC grains were developed, together with the addition of VC and Cr3C2 as grain growth inhibitors [6,7]. Aluminides, a potential binder phase for NbC, are attractive for high-temperature structural material applications [8]. Based on these studies, it is concluded that the hardness of the NbC cermets can be tailored to different levels. However, the fracture toughness remained to be rather low when compared to the widely used WC-Co materials. In the present study, the effect of Al and secondary carbides on the microstructure and mechanical properties of Ni bonded NbC cermets were investigated. Ni is chosen as the binder for NbC due to its good wetting behavior with cubic carbides, such as TiN and TiC. The NbC based cermets were prepared by conventional liquid
0026-0657/ß 2016 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mprp.2016.05.009
1 Please cite this article in press as: S.G. Huang, et al., Met. Powder Rep. (2016), http://dx.doi.org/10.1016/j.mprp.2016.05.009
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Metal Powder Report Volume 00, Number 00 May 2016
SPECIAL FEATURE FIGURE 1
Morphology of the NbC (a) and Ni powder (b). FIGURE 2
phase sintering. The goal was to develop novel NbC-Ni cermets with tailored mechanical properties, especially an improved fracture toughness, through the variation of the chemical composition of the binder phase.
Experimental procedure Powders, NbC (Seco, 0.3 mm, Sweden), WC (CRC-015, 0.5 mm, ¨ tten GmbH, Austria), VC (Treibacher, Wolfram Bergbau und Hu FSSS = 1.15 mm, Austria) and Mo2C (Chempur, 2 mm) as well as metal powders, Ni (AEE, Ni-110, 4–8 mm) and Al (AEE, Al-100, 1–5 mm) were used to prepare NbC-based cermets. The morphology of the NbC and Ni is shown in Fig. 1. The agglomerate size of the NbC and Ni powders is below 10 mm. The agglomerates consist of sub-mm size particles. The NbC powder has a total carbon content of 10.75 wt% and oxygen content of 0.17 wt%.
Calculated phase diagrams of the Nb-C-12 vol% Ni system (a) and phase evolution with temperature of the studied NbC-12 vol% Ni cermet (b).
The chemical composition of the investigated cermets is listed in Table 1. All the cermets contain NbC as hard phase and Ni or NiAl as metal binder. The Ni-Al binder is a powder mixture of Ni and Al powders. Different amounts of refractory carbide, such as WC, VC and Mo2C, were added into the NbC-Ni powder mixtures. The weighed powders were then mixed on a multi-directional mixer (Turbula, WAB, Switzerland) in ethanol for 24 h using WC-6 wt% Co milling balls (Ceratizit grade H20C, Ø10 mm). The suspension was dried in a rotating evaporator at 658C. Cold isostatically pressed (300 MPa) powder compacts (Ø20 mm 10 mm) were densified by conventional pressureless liquid phase sintering for 1 h at 14208C in a dynamic vacuum (7 Pa) with a heating and cooling rate of 208C/min.
TABLE 1
Chemical composition of the investigated NbC cermets. Cermet
Metal binder (vol%)
Composition
T.D. (g/cm3)
NbC12Ni NbC12Ni30Al NbC12Ni4Mo2C4VC4WC WC14Ni52NbC4Mo2C
12 Ni 12 (Ni-30 mol% Al) 12 Ni 13.8 Ni
NbC-12 vol% Ni NbC-12 vol% (Ni-30 mol% Al) NbC-12Ni-4Mo2C-4VC-4WC (vol%) WC-13.8Ni-52.3NbC-4.1Mo2C (vol%)
7.92 7.76 8.21 10.30
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The bulk density of the sintered cermets was measured in ethanol. The microstructure of polished surfaces was examined by electron probe microanalysis (EPMA, JXA-8530F, JEOL Ltd.), equipped with five WDS and one EDS-type detector. The Vickers hardness (HV10) was measured (Model FV-700, Future-Tech Corp., Tokyo) with an indentation load of 98.1 N. The fracture toughness, KIC, was measured from the length of the radial cracks originating from the corners of these indentations and calculated according to the formula of Shetty [9]. The reported values are the mean and standard deviation of five indentations.
Results and discussion Thermodynamics of the NbC-Ni system A thermodynamic evaluation of the NbC-Ni system was carried out using Thermo-Calc [10] and the database TCFE7. The influence of the total carbon content and temperature on the phase evolution of the Nb-C-12 vol% Ni system is shown in Fig. 2a. At a low carbon content of 9.25 wt%, the equilibrium phases at 1500 and 11008C are NbC + liquid and NbC + NbNi3 respectively. With increasing carbon content of 9.75 wt%, the corresponding phases
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change to NbC + Fcc at 11008C and this structure can be retained at room temperature. When graphite is present, the eutectic temperature of the NbC-12 vol% Ni-C system is estimated at 12658C. The liquid-phase forming temperature is around 12708C at a carbon content between 9.3 and 10.0 wt%. It is apparent that a sintering temperature of 14208C is in the liquidphase region of the Ni alloy binder. The influence of temperature on the phase evolution of the NbC-12 vol% Ni system is shown in Fig. 2b. The NbC-12 vol% Ni powder mixture has a total carbon content of 9.27 wt% based on the carbon analysis of the NbC starting powder. When the powder mixture is heated to 12808C, the solid Ni binder converts to a liquid phase. Formation of NbNi3 occurs at 10938C. With decreasing temperature, more and more NbNi3 phase is formed.
Microstructural analysis Backscattered electron micrographs (BSE) of the liquid phase sintered NbC cermets with 12 vol% metal binders, i.e., Ni and Ni-30 mol% Al, are shown in Fig. 3a and b. For the pure Ni binder, the bright and dark contrast phases correspond to the NbC and Ni
FIGURE 3
BSE micrographs of NbC cermets vacuum sintered at 14208C. NbC-12 vol% Ni (a), NbC-12 vol% (Ni-30Al) (b), NbC-12Ni-4Mo2C-4WC-4VC (vol%) (c) and WC13.8Ni-52.3NbC-4.1Mo2C (vol%) (d). 3 Please cite this article in press as: S.G. Huang, et al., Met. Powder Rep. (2016), http://dx.doi.org/10.1016/j.mprp.2016.05.009
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based binder respectively. The shape of the large NbC grains is well-faceted with a slight rounding of the edges, while the small grains are almost spherical. The NbC grain size is in the 5–20 mm range. Although coarse Ni and NbC agglomerates were used as starting powders, the Ni binder is evenly distributed in the NbC matrix, indicating a good wetting between NbC and Ni binder. The most obvious effect of the 30 mol% Al addition in the Ni binder is a reduced sintered NbC grain size to less than 10 mm, as shown in Fig. 3b. An enhanced angular NbC grain morphology was observed in the cermet. Besides the NbC and metal binder grains, dark contrast Al2O3 grains were also found in the cermets. The Ni-Al binder is evenly distributed and the NbC grains have a narrow grain size distribution in the 5–10 mm range. It was reported that the grain growth of NbC in Co, Ni and Fe binders is realized through dissolution and re-precipitation [11]. During liquid phase sintering, the small NbC grains dissolved in the Ni and Ni-Al binder and re-precipitated on larger NbC grains. The reduction of surface energy of the solid particles is the major driving force for small grains to dissolve and large grains to grow.
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When Al is dissolved in the binder, the surface energy of the liquid binder and the NbC solubility in the binder are changed, lowering the driving force for NbC dissolution-reprecipitation. The X-ray elemental map of Nb, Ni, Al, C and O of the liquid phase sintered 12 vol% (Ni-30 mol% Al) cermet is shown in Fig. 4. The mapping clearly demonstrates that only Nb and C are present in the carbide grains and both Ni and Al, as well as Nb and C are present in the binder phase. The Nb/C ratio in the NbC phase seems to have shifted to a lower carbon content due to the presence of C in the Ni binder. The presence of sub-mm sized dark Al2O3 grains is mainly due to the oxidation of Al metal powder during the powder mixture preparation procedure. Some of the oxide can be also from the fine Al starting powder. The microstructure of the 4 Mo2C-4 WC-4 VC (vol%) added NbC-12 vol% Ni cermet, vacuum sintered for 1 h at 14208C, is shown in Fig. 3c. Although the cermet has 12 vol% secondary carbides, only a bright-gray NbC phase and a dark-gray Ni based binder were observed. The NbC-12 vol% Ni cermet (Fig. 3a) has relatively large (<20 mm) facet NbC grains with a slight
FIGURE 4
WDS elemental mapping of the NbC-12 vol% (Ni-30 mol% Al) cermet. 4 Please cite this article in press as: S.G. Huang, et al., Met. Powder Rep. (2016), http://dx.doi.org/10.1016/j.mprp.2016.05.009
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rounding of the edges and mostly separated by the Ni binder, whereas the sintered NbC grain size is significantly reduced to 0.5–2 mm upon addition of the secondary carbides. There is only a limited NbC grain growth compared to the size of the 0.3 mm NbC starting powder. These carbides are known to have a very high solubility (WC > Mo2C > VC) in Ni at the sintering temperature [12]. The surface energy of the liquid Ni binder was therefore changed during sintering and dissolution of the secondary carbides, especially WC and VC into NbC also influenced the surface energy of NbC, therefore affecting the NbC grain growth behavior. A backscattered electron image of the WC-13.8Ni-52.3NbC4Mo2C (vol%) cermet is shown in Fig. 3d. Bright WC, gray NbC and dark Ni binder phases could be differentiated. The gray NbC grains form a 52 vol% skeleton and are actually (Nb,W)C solid solution grains. The Ni binder is mainly present among the bright WC grains and not at the NbC grain boundaries, due to the better wettability in the WC-Ni than in the NbC-Ni system [13]. Free Mo2C grains could not be observed in the microstructure due to its low concentration and homogeneous distribution in the binder and carbide phases. Considering the final WC and NbC grain size,
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it is apparent that there is a limited growth of the WC grains (<0.6 mm) in the cermet, whereas the NbC grains are in the range of 5–10 mm. The present observation is similar to the Co bonded WC-NbC cemented carbides [7], where a small amount of VC and Cr3C2 significantly limit the WC grain growth. Elemental mappings of the mixed carbide cermet are shown in Fig. 5. According to the Nb and W distribution, the W is present both in the WC and NbC grains. The dissolution of WC into the NbC grains forms (Nb,W)C solid solution carbide grains. The Ni binder is mostly located around the WC grains, confirming that Ni has a better wettability with WC than NbC.
Mechanical properties The Vickers hardness (HV10) and fracture toughness (KIC) of the investigated cermets are summarized in Table 2 and compared in Fig. 6. The theoretical density (TD) was calculated based on the mixture rule. According to the observed microstructure, all cermets were sintered to full density, although the NbC-12 vol% (Ni-30 mol% Al) has a relative density of only 96.7% TD. The underestimated density level of the NbC-12 vol% (Ni-30 mol% Al) cermet can be due to the inaccurate theoretical density value,
FIGURE 5
WDS elemental mapping of the WC-13.8Ni-52.3NbC-4.1Mo2C (vol%) cermet. 5 Please cite this article in press as: S.G. Huang, et al., Met. Powder Rep. (2016), http://dx.doi.org/10.1016/j.mprp.2016.05.009
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TABLE 2
Hardness and fracture toughness of the investigated cermets. Cermet
Density (g/cm3)
R.D. (%)
HV10 (kg/mm2)
KIC (MPa m1/2)
Crack resistance
NbC-12 vol% Ni NbC-12 vol% (Ni-30 mol% Al) NbC-12Ni-4Mo2C-4WC-4VC (vol%) WC-13.8Ni-52.3NbC-4.1Mo2C (vol%)
7.90 7.50 8.19 10.29
100 96.7 99.8 99.9
1130 22 1360 29 1490 15 1620 19
11.8 0.4 6.4 0.2 9.2 0.4 7.7 0.2
1.63 0.35 0.28 0.01 0.73 0.07 0.48 0.03
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due to the formation of Ni-Al alloy and Al2O3. It is known that the hardness of WC-Co cemented carbides is mainly dependent on the WC grain size (Hall–Petch relationship) and Co binder content. The finer the WC grain size, the higher the hardness for a given Co volume fraction. In this study, the 12 vol% Ni bonded NbC cermet has the lowest hardness, followed by the NbC-12 vol% (Ni-30 mol% Al), the NbC-12 Ni-4Mo2C-4VC-4WC (vol%) and the WC-13.8Ni-52.3NbC-4.1Mo2C (vol%) cermet, which has the highest hardness. Since the NbC matrix cermets have the same binder content, the difference in hardness of the NbC matrix cermets can be directly linked to the properties of the binder and the NbC grain size. According to the observed microstructures, NbC-12 vol% Ni has the largest NbC grain size, followed by the Ni-30 mol% Al cermet and the NbC-12 Ni-4Mo2C-4VC-4WC (vol%) cermet. Although the NbC grains are much coarser in the NbC substituted WC-Ni cermet, the extremely fine WC grains should contribute to the high hardness of the WC-NbC-Ni-Mo2C cermet. Furthermore, the increased hardness of this cermet might also be partially due to the solid solution hardening of the NbC phase. The higher hardness of the (Nb,W,Mo)C phase however remains to be proven. According to Fig. 6, there is an antagonistic correlation between the hardness and fracture toughness of NbC matrix cermets, except for the Ni-Al binder cermet. Usually, the metal binder plays a crucial role in shielding the stress field in front of a crack tip to improve the toughness [14]. Ductile failure of the Co, Ni and Fe binder is usually found on fractured NbC-Co/Ni/Fe surfaces. Since the Ni-Al binder has an intrinsically low room temperature ductility, the Al doped cermet exhibits a lower fracture toughness of 6.4 MPa m1/2. The 12 vol% Ni bonded NbC cermet has the highest toughness of 11.8 MPa m1/2. With increased hardness to
1490 kg/mm2, the NbC-12 Ni-4Mo2C-4VC-4WC (vol%) cermet has a toughness of 9.2 MPa m1/2, whereas the WC-13.8Ni-52.3NbC4.1Mo2C (vol%) cermet has a toughness of 7.7 MPa m1/2 and hardness of 1620 kg/mm2. The crack resistance, defined as P/4L (P is the indentation load (98.1 N) and L is the crack length (mm)) is also summarized in Table 2. It is apparent that the pure Ni binder (NbC-12 vol% Ni) has the highest crack resistance, i.e. shortest crack length, followed by the NbC-12Ni-4Mo2C-4WC-4VC (vol%), WC-13.8Ni-52.3NbC4.1Mo2C (vol%) and NbC-12 vol% (Ni-30 mol% Al) cermets. Two representative crack propagation patterns, induced by Vickers
FIGURE 7 FIGURE 6
Vickers hardness and fracture toughness of the investigated NbC cermets.
Crack propagation mode of the cermet NbC-12 Ni-4 Mo2C-4 WC-4 VC (vol%) (a) and WC-13.8 Ni-52.3 NbC-4.1 Mo2C (vol%) (b).
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indentations are shown in Fig. 7. The NbC-12Ni-4Mo2C-4WC-4VC (vol%) cermet mainly intergranularly fractures at the NbC/Ni interface, whereas the WC-13.8Ni-52.3NbC-4.1Mo2C (vol%) cermet exhibits both inter-granular fracture at the NbC/NbC and NbC/Ni interface, as well as transgranular fracture at the WC/Ni interface.
Acknowledgements
Conclusions
References
Full densification of Ni bonded NbC based cermets was achieved by pressureless liquid phase sintering in vacuum for 1 h at 14208C. The addition of Al to the Ni binder or the addition of multiple carbides (WC, Mo2C, VC) in NbC-Ni cermets influenced not only the morphology of the NbC grains, but also the NbC grain size as well as the Ni binder distribution. The hardness and fracture toughness of the NbC matrix cermets can be mainly tailored by the secondary carbide additions. An optimal combination of hardness and toughness was found for NbC-Ni cermets with multiple carbide addition The NbC-12 Ni-4 Mo2C-4 WC-4 VC (vol%) cermet exhibits a hardness of 1490 15 kg/mm2 and fracture toughness of 9.2 0.4 MPa m1/2.
[1] H.O. Pierson, Handbook of Refractory Carbides and Nitrides, Noyes, New Jersey, USA, 1996. [2] M. Woydt, H. Mohrbacher, Wear 321 (2014) 1–7. [3] S.G. Huang, et al. Mater. Lett. 61 (2) (2007) 574–577. [4] S.G. Huang, et al. J. Alloys Compd. 430 (1–2) (2007) 158–164. [5] S.G. Huang, et al. Int. J. Refract. Met. Hard Mater. 48 (2015) 418–426. [6] S.G. Huang, et al. Int. J. Refract. Met. Hard Mater. 26 (5) (2008) 389–395. [7] S.G. Huang, et al. J. Alloys Compd. 464 (1–2) (2008) 205–211. [8] R. Subramanian, J.H. Schneibel, Mater. Sci. Eng.: A 244 (1) (1998) 103–112. [9] D.K. Shetty, et al. J. Mater. Res. 20 (1985) 1873–1882. [10] B. Sundman, B. Jansson, J.O. Andersson, Calphad 9 (1985) 153–190. [11] R. Warren, J. Less Common Met. 17 (1969) 65–72. [12] H.E. Exner, Int. Mater. Rev. 24 (1) (1979) 149–173. [13] G.A. Yasinskaya, Sov. Powder Metall. Met. Ceram. 5 (7) (1966) 557–559. [14] M. Kotoul, Acta Mater. 45 (8) (1997) 3363–3376.
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We gratefully acknowledge the financial support of the Companhia Brasileira de Metalurgia e Minerac¸a˜o (CBMM), Sa˜o Paulo, Brazil. The authors thank the Hercules Foundation (project ZW09-09) and the Fund for Scientific Research Flanders (FWO) under project G.0772.16N.