*2.2.4 Other high-entropy ceramics—silicide, borocarbide, sulfide, etc.*

To date, the work of bulk high-entropy ceramics is mostly focused on oxide, boride, and carbide, and several other classes of high-entropy materials like silicide, sulfide, and borocarbide have been reported.

**163**

*High-Entropy Ceramics*

at 773 K was obtained.

ultrahigh hardness of 35 GPa.

**3.1 Preparation methods**

*3.1.1 Sputtering*

**3. High-entropy ceramic coatings**

(target) is bombarded with accelerated charged ions (Ar<sup>+</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.89527*

The synthesis of high-entropy silicide was reported by Gild et al. [26] and Qin

By adding B4C into the four-component HEC (HfMoTaTi)C, Zhang et al. [29] studied the capability of a high-entropy carbide system on accommodating one more nonmetal element, boron. The quaternary (HfMoTaTi)C contains FCC structures, while the addition of B4C induced the formation of a small fraction of hexagonal phase. Similar with the diffusion process reported by Castle et al. [19], TaC is suggested to act as the host lattice in the borocarbide system. The effect of employing different particle sizes for host carbide TaC and other constituent carbides on the phase formation and mechanical properties of the HEC composites was discussed. A high-entropy B4(HfMo2TaTi)C ceramic exhibiting hexagonal structure, with alternating metal and nonmetal layers in the lattice, was found when SiC whiskers are introduced to the borocarbide system [30]. The hexagonal HEC solid solution shows great improvement of the mechanical property with an

Sputtering is a physical vapor deposition (PVD) technique where a solid surface

scattering of surface atoms of solid. Sputtering technique is used to deposit thin coatings. There are several sputtering systems employed for deposition of thin coatings, such as direct current (DC) magnetron sputtering and radio frequency (rf) sputtering being the widely used techniques. DC-magnetron sputtering is a low pressure sputtering system for metal deposition and electrically conductive target coating materials. The magnetic field from magnetron lowers the sputtering gas pressure and increases the deposition rate of sputtered coatings. On the other hand, when metallic target is replaced by an insulator target, the plasma discharge cannot be maintained due to buildup of surface charge of positive ions on the target. Thus, an rf power supply is used instead of dc voltage power supply to sustain the glow discharge on an insulator target [31]. Another advantage of using sputtering for thin coating deposition is the use of reactive gases, substrate bias, and substrate temperature to vary the composition and properties of deposited coatings. In reactive sputtering, different reactive gases, such as oxygen, nitrogen, and CH4 can be used to deposit ceramic coatings of oxides, nitrides, and carbides, respectively [32–34]. Furthermore, deposition of carbonitride and oxynitride coatings can be

) resulting into backward

et al. [27]. Gild utilized SPS and sintered high-entropy silicide (Mo0.2Nb0.2 Ta0.2Ti0.2W0.2)Si2 at 1923 K, resulting in a single hexagonal C40 crystal structure (space group P6222). Hardness enhancement over the constituent silicide and low thermal conductivity similar to high-entropy carbide was reported. On the other hand, Qin produced (Ti0.2Zr0.2Nb0.2Mo0.2W0.2) Si2 from compositional elemental powders, leading to the formation of a hexagonal structure highentropy silicide, with the same space group P6222. Zhang et al. [28] developed a data-driven model to design high-entropy sulfides for thermoelectric applications. Compositions Cu5SnMgGeZnS9 and Cu3SnMgInZnS7 were picked to be synthesized via high energy ball milling and SPS. Single-phase high-entropy sulfide with homogeneous distribution of the elements is reported. By increasing the Sn content in the Cu5SnMgGeZnS9 system, a figure of merit zT value of 0.58

#### *High-Entropy Ceramics DOI: http://dx.doi.org/10.5772/intechopen.89527*

*Engineering Steels and High Entropy-Alloys*

**162**

solid solution via SPS.

**Figure 3.**

*(a) The energy distribution spectra of different configurations and the corresponding EFA value of the nine five-metal carbides. (b) The X-ray patterns of the sintered carbides with the same compositions, by Sarker* 

crystal structures was commonly understood to impede the formation of the highentropy phase because of the mismatch of lattice structure. It was observed that the addition of group VI elements (Cr, Mo, and W) is likely to reduce the chance of forming a single phase, as group VI metal monocarbides are generally demonstrated as non-cubic structure at room temperature. However, the MoNbTaVWC5 composition that simultaneously contains tungsten carbide (W2C) and molybdenum carbide (Mo2C), which exhibit orthorhombic and hexagonal structure, respectively, shows the highest EFA value among the 56 analyzed compositions and demonstrates single-phase FCC structure after SPS. The mechanical properties measured by experiments showed that the Vickers hardness and elastic modulus have significant enhancement compared to the predicted value from the rule of mixture, contributed by mass disorder in the structure and solid solution hardening. Following the work of the pioneers, a systematic study focused on the phase stability and mechanical properties of high-entropy carbides synthesized from group IV, V, and VI metal carbides [25] was carried out. It was verified that carbide system that was stated to have EFA value lower than 45 cannot form single-phase

*et al. [23] (licensed under CC BY 4.0, https://doi.org/10.1038/s41467-018-07160-7).*

*2.2.4 Other high-entropy ceramics—silicide, borocarbide, sulfide, etc.*

sulfide, and borocarbide have been reported.

To date, the work of bulk high-entropy ceramics is mostly focused on oxide, boride, and carbide, and several other classes of high-entropy materials like silicide,

The synthesis of high-entropy silicide was reported by Gild et al. [26] and Qin et al. [27]. Gild utilized SPS and sintered high-entropy silicide (Mo0.2Nb0.2 Ta0.2Ti0.2W0.2)Si2 at 1923 K, resulting in a single hexagonal C40 crystal structure (space group P6222). Hardness enhancement over the constituent silicide and low thermal conductivity similar to high-entropy carbide was reported. On the other hand, Qin produced (Ti0.2Zr0.2Nb0.2Mo0.2W0.2) Si2 from compositional elemental powders, leading to the formation of a hexagonal structure highentropy silicide, with the same space group P6222. Zhang et al. [28] developed a data-driven model to design high-entropy sulfides for thermoelectric applications. Compositions Cu5SnMgGeZnS9 and Cu3SnMgInZnS7 were picked to be synthesized via high energy ball milling and SPS. Single-phase high-entropy sulfide with homogeneous distribution of the elements is reported. By increasing the Sn content in the Cu5SnMgGeZnS9 system, a figure of merit zT value of 0.58 at 773 K was obtained.

By adding B4C into the four-component HEC (HfMoTaTi)C, Zhang et al. [29] studied the capability of a high-entropy carbide system on accommodating one more nonmetal element, boron. The quaternary (HfMoTaTi)C contains FCC structures, while the addition of B4C induced the formation of a small fraction of hexagonal phase. Similar with the diffusion process reported by Castle et al. [19], TaC is suggested to act as the host lattice in the borocarbide system. The effect of employing different particle sizes for host carbide TaC and other constituent carbides on the phase formation and mechanical properties of the HEC composites was discussed. A high-entropy B4(HfMo2TaTi)C ceramic exhibiting hexagonal structure, with alternating metal and nonmetal layers in the lattice, was found when SiC whiskers are introduced to the borocarbide system [30]. The hexagonal HEC solid solution shows great improvement of the mechanical property with an ultrahigh hardness of 35 GPa.
