3.1. Microstructures of HEC powders
After high-energy ball-milling, all of seven system powders became Face-centered cubic (Fcc) solid solution as seen in Fig. 2, and the related studies on the microstructure about HEC-1, HEC-2 and HEC-3 powders have been introduced in our previous reports . As far as we know, the formation of solid solution is closely related to size factor and atomic electronegativity difference. Because the radius ratios of B, C, N to metal atoms (rx/rM) (X = B, C, N; M = Ta, Nb, Hf, Zr, Ti, …) are all less than 0.59, it is possible for B, C, N atoms to form interstitial compound (MX) with transition metals. Moreover, the interstitial compounds with the same structure are promoted to form infinite solid solution, because the radius differences of metal atoms are less than 15%. Specially, the solid solution process of atoms still needs further study, which is one of the key points to our next work.
According to the mixing rule and Bragg’s law, HEC-6 system has the minimum average metal atom size, the minimum interplanar distance (the specific diffraction angle can be seen in Table 3) and the largest crystal plane diffraction angle, which is consistent with the XRD results in Fig. 2. The diffraction angle of (111) crystal plane decreases in the wake of the average metal atom size increasing, which can be seen from the line chart intuitively in Fig. 3. Obviously, after high-energy ball-milling, the mixed powders agglomerated, and all kinds of powders are similar to sphere as seen in Fig. 4. We have reported the similar microstructure of HEC-1 ~ 3 in the Ref. .
3.2. High temperature thermal stability of HEC powders
Obviously, weight loss mainly occurs in 100–200 oC and 1000–1100 oC from the TG curve of HEC-1 ~ 7 powders from RT to 1400 oC in Fig. 5. The H2O adsorbed on the powder surface volatilized, causing a small amount of weight losses in 100 oC. When the temperature was up to 1000 oC, powders lost weight quickly, which was probably caused by the volatilization of B2O3 generated by oxidation of B element. Compared with the values of weight loss from HEC-1 to HEC-7, HEC-2 has the lest weight loss and the values are shown in Table 4. Therefore, we choose HEC-2 system as a specimen to study the subsequent sintering process. The specific sintering process has been introduced in Sect. 2.2.
3.3. Microstructures of HEC bulk ceramics
After hot-pressing sintering at 1900 oC, the HEC-2 mainly has a Fcc phase, followed by a small amount of oxide impurities and BN(C) phase (Fig. 6 (c)). XRD pattern shows that the crystallinity of Fcc phase increases with sintering temperature increasing, at the same time the FWHM of XRD diffraction peak decreases and the diffraction intensity increases (Fig. 6 (a), (b) and (c)). What’s more, the amorphous BCN phase crystallized forming BN/C phase when heating to1900 oC. In Ref. , it was found that oxides formed when the powders heated at a lower temperature (1100 oC) in argon atmosphere, because oxygen impurities were inevitably brought into the powder preparation process by adopting high-energy ball-milling. Besides, the use of high-activity metal elemental as raw material powders also made it easier to introduce oxygen impurities. With the increase of sintering temperature (> 1700 oC), the oxides have a phase transformation from monoclinic phase to high-temperature stable tetragonal phase which is consistent with the existing researches [36–37]. In addition, when the sintering temperature reached 1900 oC, a small amount of BN/C precipitated.
The microcracks and holes on the ceramic surfaces indicate that the ceramics are not completely dense (Fig. 7). It can be seen from the fracture morphology of the ceramics that when the sintering temperature is 1700 oC, the ceramics are composed of nano-sized small particles, and no obvious cracks or holes appear on the fracture section and particles are intact on section in Fig. 8. When the sintering temperature is up to 1800 oC, small particles connect and there are clearly cracks on section. When sintering temperature is 1900 oC, there is obvious lamellar phase in ceramics. According to XRD analysis, the lamellar substance is BN/C phase, and the fracture surface has obvious traces of lamellar tearing and large particles pulling out with sintering temperature increasing.
In order to further analyze the microstructures characteristics, HEC-2119 ceramic after hot-pressing sintering was tested by TEM (Fig. 9). Based on the electron diffraction spots, the position 1, 2, 3 and 4 are the Fcc phase, the oxide phase, the mixture of amorphous phase and nanocrystalline and the BN/C phase, respectively. It can be seen from the EDS patterns that all elements in the Fcc high-entropy phase are uniformly distributed, but the solid solubility of N in the high-entropy phase is far less than others, whose atomic size is the smallest and which mainly concentrates in BN phase. One of the reasons for that situation is probably that besides atomic size, electronegativities of atoms also have a great influence on solubility. Compared with C and B, the electronegativity difference between N and transition metal is the largest, causing the smallest solubility.
We speculate that oxides are solid solutions composed of hafnium, niobium, zirconium and oxide based on the EDS results, which can be expressed as (Hf, Nbx, Zry)Oz. By compared the content of metal atoms between oxide phase and high-entropy phase, it can be found that the hafnium and zirconium contents of former are higher than latter, while it is opposite for Nb elements. By studying the oxidation sequence of elements in subgroup Ⅳ and Ⅴ under the condition of high entropy, Backman L et al  found that the predatory ability to oxygen of metal element is Hf > Zr > Ti > Ta > Nb, so Hf and Zr atoms are oxidized first. However, Nb atoms with the least oxidation tendency are oxidized, which may be due to the great solubility of oxygen. Due to the limitation of oxygen content, Nb atoms are not oxidized into the most stable Nb2O5, but exist in the form of NbO2. According to the principle of similar compatibility, HfO2, ZrO2 and NbO2 are dissolved into single solid-solution oxides because of the same crystal structure. It is worth noting that ZrO2 has a phase transformation from monoclinic-phase to tetragonal-phase when temperature is higher than 1170 oC , and changes to monoclinic-phase back when the temperature decreases. But that phenomenon is not observed during the sintering process of HEC-21 powders, we speculate that the formation of oxide phase is based on HfO2 structure. It proves once again that HEC-21 is a multiphase ceramic mixed with high-entropy phase, oxides and BN/C phase sintered at 1900 oC, 60 MPa in argon atmosphere.
3.4. Effects of nonmetal content on phase and microstructure
The addition amount of BCN has a certain influence on phase and microstructure. Besides high-entropy phase and oxides phase, hexagonal phase MiB2 was precipitated with the decrease of BCN, while the diffraction peak of BN/C phase disappeared (Fig. 6 (c), (d) and (e)). It can be seen from the fracture morphology that the lamellar BN/C gradually decreases with the decrease of BCN, and there is basically no lamellar phase in HEC-23, which is consistent with the results of XRD patterns (Fig. 6 (e)). Although there is a small amount of layered phase in HEC-22, it is not shown in Fig. 6 (d) because of its little content.
From the fracture section (Fig. 10), the fracture morphology of HEC-21 ceramic is composed of a large number of randomly oriented lamellar BN/C and a small number of intact particles covered by lamellar. The fracture morphology of HEC-22 ceramic is mainly composed of intact particles and a small number of lamellar BN/C. The fracture morphology of HEC-23 ceramic with the least BCN content is composed of a large number of intact grains and a small number of grains with tearing characteristic, and there is substantially no lamellar phase. Compared with the grain size, it is found that with the decrease of BCN content, the grain size increases gradually. The main reason is that BN/C distribution among particles can effectively hinder grain growth.
3.5. Mechanical properties of HEC-2 bulk ceramics
With the sintering temperature increasing, the density and flexural strength increase gradually, and the surface porosity decreases. The specific values are in Table 5. After sintering at 1900 oC, the flexural strength of HEC-2119 is as high as 495.82 MPa. In addition, the Vickers hardness increases first and then decreases with the increase of sintering temperature (Fig. 11). The Vickers hardness of HEC-2118 ceramic is 7.57 GPa, and the hardness of HEC-2119 is 5.56 GPa. The reason for this variation tendency is that the ceramic form lamellar BN/C at 1900 oC that is easy to slip between lamellae, leading to the decrease of Vickers hardness. With the decrease of BCN content, the density, hardness and bending strength increase, but the surface porosity decreases. To a certain extent, the decrease of porosity indicates the increase of density, which is due to the high BCN content in HEC-21 ceramic, which effectively hinders the grain growth. The decrease of BCN content in HEC-22 ceramic makes the grain size larger than the former, and the increased grain squeezes lamellar to increase its density. There is no lamellar BN/C phase in HEC-23 ceramic to hinder the grain growth, which makes the grain size of this system the largest. The Vickers hardness, bending strength and open porosity of HEC-2319 are 24.54 GPa, 522.00 MPa and 0.05%, respectively. When BCN content decreases, the precipitation amount of lamellar BN/C phase decreases, and the proportion of high-entropy phase increases, which effectively improves its Vickers hardness and bulk density, and the specific change of hardness is shown in Fig. 12.