Commercially available TiO2 (~ 21 nm, XuanchengJingrui New Material Co., Ltd.), ZrO2 (~ 0.6 µm, ChangshaXili Nanometer Lapping Tech. Co., Ltd.), HfO2 (~ 0.3 µm, BeijingFounde Star Sci. & Tech. Co., Ltd.), Nb2O5 (~ 2.0 µm, Beijing Founde Star Sci. & Tech. Co, Ltd.), Ta2O5 (~ 1.0 µm, Beijing Founde Star Sci. & Tech. Co., Ltd.), B4C (~ 1 µm, Mudanjiang Jingangzuan Boron Carbide Co., Ltd.), and graphite (~ 1.5 µm, Shanghai Yifan Graphite Co., Ltd.) powders were used as the initial raw materials in this study. The reactions of using oxides, B4C and C as precursors to prepare high-entropy borides and carbides powders are as follows:
2MO2 + B4C + 3C→2MB2 + 4CO(g) (M = Ti, Zr, Hf) (1)
Me2O5 + B4C + 4C→2MeB2 + 5CO(g) (Me = Nb, Ta) (2)
TiO2 + ZrO2 + HfO2 + 0.5Nb2O5 + 0.5Ta2O5 + 2.5B4C + 8.5C→5(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 + 11CO(g) (3)
MO2 + 2C→MC + 2CO(g) (M = Ti, Zr, Hf) (4)
Me2O5 + 7C→2MeC + 5CO(g) (Me = Nb, Ta) (5)
TiO2 + ZrO2 + HfO2 + 0.5Nb2O5 + 0.5Ta2O5 + 16C→5(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C + 11CO(g) (6)
However, it should be noted that, a large number of researches have shown that when borides are prepared with the stoichiometric amounts of raw powders, according to Reactions (1), (2) and (3), a small amount of residual oxides would invariably remain in the product [10, 16, 18, 23]. This is due to the evaporation of intermediate gas products (i.e., B2O3 and boron-rich oxides), resulting in the loss of B form the initial mixture. In order to obtain two-phase high-entropy ceramics with different high-entropy phase fractions, the amounts of B4C and C were thus adjusted in the present work. For the current study, samples with nominally 100 mol% HEB, 80 mol% HEB + 20 mol% HEC, 60 mol% HEB + 40 mol% HEC, 40 mol% HEB + 60 mol% HEC, 20 mol% HEB + 80 mol% HEC, 100 mol% HEC sintered samples are referred to as HEB, B8C2, B6C4, B4C6, B2C8, and HEC, respectively.
Table 1
Precursor powder molar ratios.
Label | TiO2(mol) | ZrO2(mol) | HfO2(mol) | Nb2O5(mol) | Ta2O5(mol) | B4C(mol) | C(mol) |
HEB | 1 | 1 | 1 | 0.5 | 0.5 | 3.125 | 5.95 |
H8C2 | 1 | 1 | 1 | 0.5 | 0.5 | 2.5 | 7.96 |
H6C4 | 1 | 1 | 1 | 0.5 | 0.5 | 1.875 | 9.97 |
H4C6 | 1 | 1 | 1 | 0.5 | 0.5 | 1.25 | 11.98 |
H2C8 | 1 | 1 | 1 | 0.5 | 0.5 | 0.625 | 13.99 |
HEC | 1 | 1 | 1 | 0.5 | 0.5 | 0 | 16 |
For boro/carbothermal synthesis, the ratio details of these mixes of powders are presented in Table 1. The raw materials were mixed in anhydrous ethanol with Si3N4 milling media for 24 hours, using a roll jar mill, and then dried within a rotary evaporator. The dried mixtures were subsequently passed through a 100-mesh sieve to remove any large agglomerates. The powder mixture was then compacted into 30 mm diameter and ~ 5 mm thickness monoliths, and loaded in a graphite crucible. The preparation of the in-situ synthesized, boro/carbothermal reduction powder is then carried out in a vacuum furnace. The mixtures were heat treated at 1650℃ for 1 h under vacuum (༜10 Pa), with a heating rate of 10 ℃/min, to complete the boro/carbothermal reaction(s) process. The as-prepared high-entropy powders were subsequently ground with an agate mortar and pestle, and then through a 100-mesh sieve. For powder densification, Spark Plasma Sintering (SPS, HPD-10-FL, FCT Systeme GmbH, Germany) consolidation was conducted, using a graphite die with an inner diameter of 30 mm. All samples were sintered at 2000 ℃ for 10 min, with a heating rate of 100 ℃/min, in an Ar atmosphere. A uniaxial pressure of 30 MPa was applied above 1650℃, and then maintained for the remainder of the sintering cycle.
Bulk densities of the sintered compacts were measured using the Archimedes method in distilled water. The Vickers hardness (ISO 14705: 2008, MOD) was measured by the indentation method, with an applied load of 0.2 kg, held for 10 s. The fracture toughness, KIC, was measured by the indentation method with an applied load of 2 kg, held for 10 s [24]. Crystalline phases within the sintered specimens were determined by X-ray diffractometry (XRD; model D8 ADVANCE,Bruker Corp.༌Germany). The Rietveld refinement approach was used to calculated the lattice parameters and phase fraction of the high-entropy ceramics from the recorded XRD pattern, using the Rietveld refinement EXPGUI software. The microstructures of SPS processed samples were examined using scanning electron microscopy (SEM; model SU-8220, Hitachi High-Technologies, Japan), which is equipped with an energy dispersive spectroscopy (EDS) Si-drift detector (X-MaxN50, Oxford, UK) for chemical analysis and mapping.