Microstructure and mechanical properties of (TiZrNbTaMo)C high-entropy ceramic

1. Introduction

Ultra-high temperature ceramics are a selected series of transition metallic carbides, nitrides and borides with high melting temperature [1,2]. Due to their high mechanical properties, good phase stability, and extraordinary resistance to erosion and chemical attack, these materials have attracted great attention for extreme environment applications in next-generation hypersonic aircraft, rocket engines and refractory crucibles for active metal smelting, etc. [1,[3], [4], [5], [6], [7], [8]]. Especially, transition metallic carbides are also widely used as cutting tools, abrasives, and wear-resistant parts due to high hardness and abrasion resistance. Nowadays, high-performance ultrahigh temperature materials are urgently needed to meet the requirements for structural applications in Generation IV nuclear energy systems and hypervelocity-finishing cutter [[9], [10], [11]].

Recently, multi-component high-entropy carbide ceramics with equimolar metallic elements and single phase begin to attract considerable attention due to high hardness and strength, low thermal conductivity and excellent creep resistance at elevated temperatures [1,[12], [13], [14], [15], [16], [17], [18], [19], [20]]. Sarker et al. proposed for the first time the entropy-forming-ability (EFA) to evaluate the feasibility of a high-entropy carbide ceramics, and successfully synthesized a series of high-entropy ceramics with a homogeneous compositional distribution, such as (HfNbTaTiZr)C, (HfNbTaTiV)C, (HfNbTaTiW)C, and (HfTaTiWZr)C [13,16]. The phase stability, mechanical and electronic properties of (TaNbHfTiZr)C ceramic are studied by using density functional theory and experiment, simultaneously. The mixing entropy and nanohardness were 0.805R (R is the gas constant) and 40.6 GPa under load of 8 mN [19,21,22]. It should be noted that the measured Vicker’s hardness of (TiZrNbTaHf)C ceramics was highly load-dependent, which was 32.0 GPa, 27.5 GPa, 22.5 GPa and 18.8 GPa at different applied loads of 0.30 N, 0.50 N, 0.98 N and 9.80 N, respectively [13,16,18,19]. Furthermore, the high entropy (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C ceramic exhibits a low thermal conductivity of 6.45 W/(m K) at 29.5 ℃, which is less than that of the constituent binary compounds [20]. A porous high-entropy (TiZrNbTaHf)C ceramic can be further used as light-weight thermal insulation materials at ultrahigh temperatures due to its low thermal conductivity [23]. Several single-phase solid solution carbides have also been explored simultaneously, such as (HfTaZrNb)C and (ZrNbTiV)C [24,25]. The nanohardness of (Zr_0.25Nb_0.25Ti_0.25V_0.25)C reached 30.3 GPa under load of 8 mN [25]. However, current studies on quinary high-entropy carbide ceramics are quite limited, especially on the densification process, phase evolution and the corresponding mechanisms of strengthening and toughening effects.

This work concentrates on a new (TiZrNbTaMo)C high-entropy ceramic due to the following aspects: (1) a large mixed entropy of quinary equimolar (TiZrNbTaMo)C ceramic according to theoretical calculation results of entropy-forming-ability [16]; (2) a strong forming ability of single-phase solid solution determined by the difference in atomic size. The atomic-size effect (δ) in quinary solid solution can be calculated as follows [26]:

$\delta=\sqrt{\sum^{N}_{i=1}X_{i}\left[1-r_{i}/(\sum^{N}_{i=1}X_{i}r_{i})\right]^{2}}$ (1)

where r_i is the nearest diffusion distance of metallic atoms in MC, and X_i is molar content. When M = Zr, Ti, Ta, Nb, r_i equals ai/2, where a_i represents the lattice constant. However, when M = Mo, r_i equals a_i for hexagonal Mo₂C. The calculated δ value of (TiZrNbTaMo)C is 3.42%, which is lower than 4% and favorable for forming a stable single-phase solid solution [27]; (3) promoting the densification process if adding Mo as the strategic main component. Mo₂C has a low melting point of 2410 ℃, which is lower than those of other transition metallic carbides except for Cr₃C₂. Cr atoms from Cr₃C₂ easily segregates at grain boundary during sintering [24]. The addition of Mo effectively reduces the grain size of (Ti,W,Mo)C solid solution ceramics. The grain size of (Ti,W,Mo)C decreases to 1.12 μm as compared with (Ti,W)C without Mo [28]. Therefore, Mo as the strategic main component can be deliberately introduced into the carbide powders to fabricate low-cost high-performance (TiZrNbTaMo)C ceramic.

In the present work, a carbothermal reduction process of equimolar quinary metallic oxides and carbon black was performed to synthesize the carbide solid solution powder. High-entropy (TiZrNbTaMo)C ceramic was then fabricated by using a two-step hot pressing method to investigate the phase evolution, microstructure and mechanical properties.

2. Experimental

The raw powder materials used in this investigation include ZrO₂ (99.9%, 1-3 μm, Shanghai ChaoWei Nanotechnology Co. Ltd., China), TiO₂ (99.5%, 1-2 μm, Ningbo XinFu Titanium Dioxide Co. Ltd., China), Ta₂O₅ (99.9%, 1-2 μm, Beijing XingRongYuan Technology Co. Ltd., China), Nb₂O₅ (99.9%, 1-2 μm, Beijing XingRongYuan Technology Co. Ltd., China), MoO₃ (99.9%, 1-4 μm, Shanghai ZaiBang Chemical Co. Ltd., China) and carbon black (99%, 1-2 μm, Xinjiang YaKeLa Co. Ltd., China). The molar ratio of five raw oxide powders to carbon black was designed as TiO₂:ZrO₂:Nb₂O₅:Ta₂O₅:MoO₃:C of 2:2:1:1:2:33 to finally form a (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C ceramic with equimolar metallic components. The powder mixture was ball-milled for 20 h in a tungsten carbide bottle with tungsten carbide balls as milling media, and the weight ratio of ball-to-powder was 20:1. The ball-milled powder mixture was sieved through a 100-mesh screen, and was then put into a graphite crucible. Afterwards, the carbothermal reduction was carried out at temperatures of 1300 ℃, 1400 ℃ and 1500 ℃, respectively, for 1 h in vacuum. Subsequently, the as-synthesized carbide powder was loaded into a graphite die for two-step hot pressing at 1850 ℃ for 1 h and then 2100 ℃ for 0.5 h under the applied pressure of 30 MPa.

Thermodynamic calculations were performed by using the FactSage software (FactSage 7.1 Thermfact/CRCT, Montreal, Canada) [29]. The partial pressure was assumed to be 50 Pa for calculation, which is nearly equivalent to the experimental condition in vacuum furnace. The phase structures of the as-synthesized carbide powder and ceramic were determined by X-ray diffraction (XRD; D/max-B, Rigaku, Japan), using CuKα radiation, with an operating voltage of 40 kV, a current of 50 mA and a scanning speed of 0.1°/min. Morphologies and microstructure were characterized by a scanning electron microscope (SEM; Quanta 200FEG, USA) and a transmission electron microscope (TEM; Talos F200X, USA) equipped with X-ray energy dispersive spectroscopy (EDS) for qualitative chemical analysis. The average grain size of specimens was determined by using the ImageJ software. The Vicker’s hardness measurements were carried out under different applied loads of 9.8 N, 49 N and 98 N, respectively, for holding 15 s on the polished surfaces. Meanwhile, the nanohardness was measured via nanoindentation under an applied load of 100 mN. The indentation method was used to measure the fracture toughness by using the Vickers’ hardness tester, and the equation for calculation is as follows [30]:

$K_{IC}=P(\pi(\frac{C_{1}+C_{2}}{4}))^{-\frac{3}{2}}(\tan^{-1}\beta)$ (2)

where P is the applied load (N), C1 and C2 are the lengths of diagonal cracks (m), β is a constant of 68°.

3. Results and discussion

3.1. Thermodynamic analysis

In order to avoid oxygen contamination in commercial carbide powders, a carbothermal reduction process of equimolar quinary metallic oxides and carbon black was performed to synthesize the carbide solid solution powder. Five carbothermal reduction reactions associated with (TiZrNbTaMo)C system are shown in Table 1. Although the carbothermal reduction reactions take place in a stepwise manner, the intermediate reactions do not affect the reaction conditions of targeted products from the thermodynamic calculations. The thermodynamic analysis of carbothermal reduction reactions (1)-(5) is studied by using the FactSage 7.1, respectively. The relationship between the difference in Gibbs free energy (ΔG) and reaction temperature are shown in Table 1. When ΔG is lower than zero, the reaction will take place spontaneously, from which the critical reaction temperature can be calculated as illustrated in Table 1. With increasing the reaction temperature, the ΔG of all the five reactions decrease linearly. Clearly, the carbothermal reduction process between zirconia and carbon black needs the highest temperature of about 1135 ℃. Thus, all these reactions occur undoubtedly when temperature is higher than 1200 ℃. As the enthalpies of reactions (1)-(5) are larger than zero, these reactions are clearly endothermic. Thus, an appropriate holding time of 1 h is needed to complete these reactions.

3.2. Carbothermal reduction

The XRD patterns of the ball-milled powder mixture and the as-synthesized carbide powders at different temperatures are shown in Fig. 1(a). The diffraction peaks of milled powder mixture are distinctly broadened in the XRD patterns due to the decrease in particle size by high energy ball milling. The main diffraction peaks of as-synthesized carbide powders at temperatures of 1300-1500 ℃ are assigned as carbide solid solutions. However, some peaks of residual zirconium oxide are still found when the temperature is at 1300 ℃ and 1400 ℃ in Fig. 1(b). Although all carbothermal reduction reactions (1)-(5) take place above 1200 ℃ according to thermodynamic calculations, the kinetics of solid phase reactions determine the reaction rate. From Fig. 1, the reaction temperature is optimized to be 1500 ℃, and the as-synthesized carbide powder has a particle size of only 100 nm without any residual oxides, as shown in Fig. 1(c). The target products after carbothermal reduction are equimolar quinary metallic carbide solutions.

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Fig. 1.   Characterization of the intermediate products powders: (a) XRD patterns of ball-milled powder mixture and as-synthesized powder at different temperatures; (b) enlarged XRD patterns of as-synthesized powder; (c) SEM micrographs of as-synthesized powder at 1500 ℃.

3.3. Phase formation process

The as-synthesized carbide powder was sintered to fabricate high-entropy (TiZrNbTaMo)C ceramic by two-step hot-pressing. The (TiZrNbTaMo)C carbide has a face-centered cubic structure and a lattice parameter of 0.4461 nm by Rietveld Refinement method, as shown in Fig. 2(a) and (b). The as-synthesized carbide powder consists mainly of two main types of face-centered cubic structure, corresponding to TaC- and ZrC-based solid solutions, respectively. No peaks originating from five single componential carbides are identified. After sintering, the diffraction peaks correspond only to a single-phase carbide, which is assumed to be high-entropy (TiZrNbTaMo)C ceramic. In order to further identify the phase structures in the carbide powder and ceramic bulk, the XRD pattern plotted on a logarithmic scale was provided in the supplement materials (see Fig. S1 in supplementary information). Clearly, no other phase is found in Fig. S1, except those marked in Fig. 2.

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Fig. 2.   Characterization of phase formation process: (a) XRD patterns of TaC, ZrC, as-synthesized carbide powder at 1500 ℃ and high-entropy ceramic; (b) Rietveld refinement of (TiZrNbTaMo)C (Fullprof, R_wp = 8.88); (c) schematic illustration showing the formation process of high-entropy ceramic (different colors represent different types of metallic atoms).

The phase evolution occurring during sintering depends mainly on atomic radius and vacancy formation energy of metallic atoms in carbides as follows: (1) metallic atoms in Mo₂C and TiC have small atomic radius and high diffusion coefficient among five single-component carbides, and they can easily diffuse into the lattice of other three carbides [31]; (2) the diffusion behaviors of C and metallic atoms can be treated independently. The self-diffusion coefficient of C is much higher than those of metallic atoms, and the metallic vacancy formation energy in NbC is only 4.1 eV, which is much lower than that in ZrC (9.4 eV) [1]. It deduces that Nb atom can diffuse easily through metal vacancy [32]; (3) the diffusion rate of Zr atom is higher than that of Ta [33]. Namely, the diffusion rate of transition metallic atoms is in the order of Mo > Ti > Nb > Zr > Ta. Therefore, it can be concluded that Ti, Mo and Nb firstly diffuse into the lattice of either TaC or ZrC to form TaC-based or ZrC-based solid solution at early stage of sintering. Subsequently, the interdiffusion of metallic atoms between TaC-based and ZrC-based solid solution leads to the formation of a single-phase high-entropy carbide solid solution at elevated temperature.

In order to further investigate the formation process, two related ceramics of (Ta_0.4Nb_0.2Ti_0.2Mo_0.2)C and (Zr_0.4Nb_0.2Ti_0.2Mo_0.2)C were fabricated under identical sintering condition. The XRD patterns of the as-synthesized powders and ceramics are shown in Fig. 3. The (Zr_0.4Nb_0.2Ti_0.2Mo_0.2)C carbide powder synthesized at 1500 ℃ consists of not only a ZrC-based phase but also a NbC-based phase and a small amount of unassigned carbide solid solution phase. However, the as-synthesized (Ta_0.4Nb_0.2Ti_0.2Mo_0.2)C carbide powder is only composed of a single-phase carbide solution. Therefore, the host carbide materials are only dominated by TaC, NbC and ZrC according to the diffusion rate order of different metallic atoms. Obviously, not all Ti and Mo atoms are able to diffuse into the lattice of NbC or ZrC at 1500 ℃ to generate a single-phase (Zr_0.4Nb_0.2Ti_0.2Mo_0.2)C solid solution powder, however, for comparison Ti, Mo and Nb atoms can diffuse easily into the TaC lattice to form a single-phase (Ta_0.4Nb_0.2Ti_0.2Mo_0.2)C solid solution powder. According to the above-mentioned experimental results, it is concluded that the (TiZrNbTaMo)C carbide powder synthesized in this work consists mainly of ZrC-based and TaC-based carbide solid solutions with face-centered cubic structure. A simple schematic diagram illustrates the formation process of high-entropy (TiZrNbTaMo)C ceramic, as shown in Fig. 2(c).

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Fig. 3.   XRD patterns of carbide powders and ceramics: (a) as-synthesized (Ta_0.4Nb_0.2Ti_0.2Mo_0.2)C powder; (b) (Ta_0.4Nb_0.2Ti_0.2Mo_0.2)C ceramic; (c) as-synthesized (Zr_0.4Nb_0.2Ti_0.2Mo_0.2)C powder; (d) (Zr_0.4Nb_0.2Ti_0.2Mo_0.2)C ceramic.

3.4. Microstructural characterization

SEM micrographs of (TiZrNbTaMo)C ceramic and the corresponding EDS elemental mappings are shown in Fig. 4(a). The relative density of high-entropy (TiZrNbTaMo)C ceramic after two-step sintering is 98.6% in this work, which is clearly higher than those (93.0% and 95.3%) previously reported in (HfZrTaNbTi)C ceramics [19,20]. A very small number of pores are observed on the polished surface of (TiZrNbTaMo)C ceramic, which may originate from incomplete densification or polishing effect. The (TiZrNbTaMo)C ceramic exhibits an average grain size of 8.8 ± 3.0 μm, which is smaller than that (about 16.4 ± 4.5 μm) in previously reported (HfZrTaNbTi)C ceramic [20,34]. The grain refinement in (TiZrNbTaMo)C ceramic is attributed to the influence of Mo as the strategic main component. From EDS elemental mappings, five kinds of metallic components (Ti, Zr, Nb, Ta and Mo) exhibit a homogeneous distribution without any elemental segregation at the microscale level. These five different elements are almost equimolar ratio, and the content of Ta is a little bit higher than other four kinds of metallic elements due to characteristic X-ray overlapping of Ta-M_α and W-M_α, as shown in Fig. 4(b). In this case, a trace amount of W element is introduced from high energy ball milling.

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Fig. 4.   SEM and EDS analysis of (TiZrNbTaMo)C ceramic: (a) SEM image of the polished surface, with the corresponding EDS mapping of Ti, Zr, Nb, Ta and Mo element; (b) elemental component of point as marked in (a); (c) statistical chart of grain size distribution.

To further verify the structure and elemental distribution in the (TiZrNbTaMo)C ceramic, the high-resolution transmission electron microscope (HRTEM) image is shown in Fig. 5(a). The atomic arrangements in the (TiZrNbTaMo)C ceramic exhibits a typical periodic lattice structure. The measured interplanar spacing of (111) plane is about 0.281 nm, and the corresponding lattice parameter is calculated as 0.4867 nm, which is 9.1% higher than that obtained by the Rietveld Refinement method from Fig. 2b). The corresponding selected area electron diffraction (SAED) pattern along zone axis [001] indicates that the (TiZrNbTaMo)C ceramic has a face-centered crystal structure due to the well-arranged typical diffraction spots, as shown in Fig. 5(b). The STEM micrograph and corresponding EDS elemental mappings of (TiZrNbTaMo)C ceramic are also performed. Obviously, the distribution of Ti, Zr, Nb, Ta and Mo elements are very uniform at the nanoscale without any elemental segregation or aggregation in Fig. 5(c). In a word, a dense (TiZrNbTaMo)C ceramic with high compositional uniformity at both microscale and nanoscale levels is obtained based on the above-mentioned characterizations by XRD, SEM and TEM.

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Fig. 5.   TEM analysis of the (TiZrNbTaMo)C ceramic: (a) HRTEM image; (b) SAED pattern; (c) the HADDF image and the corresponding EDS compositional maps.

3.5. Mechanical properties

The hardness and fracture toughness of (TiZrNbTaMo)C ceramic are listed in Table 2. The nanohardness of (TiZrNbTaMo)C ceramic is about 31.3 GPa under an applied load of 100 mN. With increasing the applied load, the hardness exhibits a significant decrement, for example 16.8 GPa under the applied load of 98 N. For a comparative study, the corresponding mechanical properties of high-entropy ceramics are summarized in Table 2. Under identical load of 9.8 N, the hardness value of 25.3 GPa for the (TiZrNbTaMo)C ceramic in this work is comparable to or even higher than those (15 GPa or 18.8 GPa) obtained in other related high-entropy (HfZrTaNbTi)C carbide ceramics [19,20]. It is clearly attributed to high relative density and fine grain size in (TiZrNbTaHf)C ceramic by adding Mo as the strategy main element. However, the fracture toughness of (TiZrNbTaMo)C ceramic is only 3.28 MPa m^1/2, which is not obviously improved due to the lack of effective toughening mechanisms.

4. Conclusion

In summary, a dense high-entropy (TiZrNbTaMo)C ceramic has been successfully fabricated by two-step hot pressing. The carbothermal reduction process of equimolar quinary metallic oxides at 1500 ℃ for 1 h generates a carbide powder mixture, which consists mainly of TaC- and ZrC-based solid solutions. Subsequently, metallic atoms in ZrC-based and TaC-based solid solutions interdiffuse during two-step hot pressing to form a dense single-phase high-entropy (TiZrNbTaMo)C ceramic at elevated temperatures. The elemental distribution in (TiZrNbTaMo)C ceramic is quite uniform from microscale to nanoscale levels. The (TiZrNbTaMo)C ceramic exhibits a relative density of 98.6% and an average grain size of about 8.8 μm. The addition of Mo as the strategic main component promotes the grain refinement and the densification process. The measured hardness values of (TiZrNbTaMo)C ceramic are 25.3 GPa at 9.8 N and 31.3 GPa at 100 mN, respectively, which are clearly higher than those of other available high-entropy carbide ceramics. However, the fracture toughness of (TiZrNbTaMo)C ceramic is only 3.28 MPa m^1/2, which is not obviously improved due to the lack of effective toughening mechanisms.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (Nos. 51602074, 51872061, 51532006 and 51621091), the Natural Science Foundation of Heilongjiang Province (No. E2016026), the China Postdoctoral Science Foundation (No. 2016 M600246) and the Heilongjiang Postdoctoral Foundation (No. LBH-Z16084).