As shown in Table 1, the hardness and fracture toughness of HEC-1 are similar with other high entropy ceramics owning finer grains which were prepared by powder metallurgy. This is further confirmed by the fact that the structure of HEC-1 was relatively dense and homogeneous, which was benefit from high entropy effect and solid solution strengthening mechanism. For HEC-2, as an IVB group element, hafnium possesses excellent plasticity and it is beneficial to refine grain. Additionally, the volume fraction of grain boundaries will increase with a decrease in grain size. As a result, an increase of residual stress in the structure inhibits the propagation of cracks, leading to an improvement of fracture toughness.
Table 1. Values of microhardness and fracture toughness of HEC-1 and HEC-2 for a comparation.
| Samples | Microhardness (GPa) | Fracture toughness (KIC, MPa/m1/2) | References |
|---|---|---|---|
| (Zr0.25Nb0.25Ti0.25Vo0.25)C | 20.8(9.8 N) | 4.7 | [15] |
| (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)C | 18.8 (9.8 N) | 3.0 | [16] |
| (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C | 25.3 (9.8 N) | 3.28 | [23] |
| (V0.2Nb0.2Mo0.2Ta0.2W0.2)C | 23.8 (4.9 N) | 3.34 | [37] |
| (Hf0.2Ta0.2Ti0.2Nb0.2Mo0.2)C | 18.4 (9.8 N) | N/A | [47] |
| (Ti-Zr-Hf-Nb-Ta-Mo)C | 23.2 (9.8 N) | 3.7 | [31] |
| HEC-1 | 18.75 (9.8 N) | 3.2 | Present work |
| HEC-2 | 17.83 (9.8 N) | 12.9 | Present work |
To investigate their toughness mechanism furtherly, representative images of Vickers indentations under the load of 4.9 N are demonstrated in Fig. 6. The behaviors of crack propagation in both HEC-1 and HEC-2 would be explored based on the microcrack deflection toughening mechanism [15]. It can be seen clearly from Fig. 6(a) that the radial cracks propagated along the direction labeled by the red arrow on well-polished surface, indicating the stresses concentrate at the indentation corner. As usual, the radical crack from the indentation propagate along the flat path in conventional single-phase ceramics [48,49]. In contrast, the radical crack in Fig. 6(a) exhibits a zigzag propagation path (noted by the red circle). The deflection in the propagation process absorbed the energy of crack propagation, which gave rise to an increase of the resistance of microcrack, causing HEC-1 to be toughened. Besides, crack branching is also an operative toughening mechanism [50]. With the crack split into branches, both of the stress intensity and crack driving force decrease which promotes the improvement of fracture toughness. Nevertheless, it can be clearly found that an indentation with good morphology is not available for HEC-1, suggesting the fragility of high-entropy carbide ceramic. Similar phenomenon is not observed in HEC-2, for there is no obvious spread of long microcrack from Fig. 6(b), indicating a better plasticity compared with HEC-1. The indentation depth of HEC-2 shown in Fig. 6(d) is deeper than that of HEC-1, suggesting a lower hardness. In sum, it can be concluded that the refining grain size and the doping of element Hf improved the toughness according to the research of HEC-1 and HEC-2. But a deeper understanding of the mechanism in toughness enhancement of HECs requires further study. Based on these properties, the newly developed HECs exhibit a great potential as candidates used in extreme conditions, such as aeronautic and automotive applications.
Fig. 6
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