*3.3.2 Effect of substrate temperature*

Braic et al. studied the effect of substrate temperature on the deposition of (CrCuNbTiY)C high-entropy carbide coating with DC co-sputtering of elemental targets with a constant substrate bias and two different CH4 flow ratios. The substrate temperature was increased from 80 to 650°C, and its effect on the structural evolution and mechanical properties were reported. The high-entropy carbide coating with lower carbon concentration showed formation of FCC structures at all the deposition temperatures, while the coating with higher carbon concentration showed poor crystallinity and approaching toward an amorphous phase. The grain size and surface roughness increased with increasing substrate temperature in both coatings. In contrast, the hardness values increased with increasing substrate temperature from 13 GPa (80°C substrate temperature) to 30 GPa (650°C).

#### **3.4 High-entropy oxide coatings**

Most of the research on high-entropy ceramic coatings has been focused on high-entropy nitrides and carbide coatings. Few research works have been reported on high-entropy oxide (HEO) coatings. Initial work on HEO coatings was reported by Chen and Wong, where (AlxCoCrCuFeNi)O (x: 0.5, 1 or 2) HEO coating by RF magnetron sputtering using different oxygen flow ratios (O2/Ar) was developed, and the resulting structural evolution was characterized [59]. The structural evolution was observed to change from FCC, FCC + BCC or BCC (depending on the amount of Al content) to cubic spinal oxides with increasing oxygen flow ratio. The hardness increased from 5–8 GPa to 13–22.6 GPa with increasing O2 flow ratio. In a subsequent work, Lin et al. used strong oxide forming elements to develop (AlCrTaTiZr)O HEO coating using DC magnetron sputtering with increasing O2 flow ratio (RO = O2/(O2 + Ar)) from 0 to 50%. The XRD analysis revealed a metastable amorphous structure irrespective of the O2 flow ratio [33]. The tendency of forming amorphous phases at different O2 flow ratios has been attributed to the large difference in the lattice parameters of oxides from each constituent element of Al, Cr, Ta, Ti, and Zr. The hardness values were reported to be in the range of 8–13 GPa. However, the hardness values increased in the range of 20–22 GPa after annealing at 900°C. The reported hardness values of HEO coatings were relatively higher than most of the reported oxide films, such as Al2O3 (10 GPa) [60], TiO2 (18 GPa) [61], V2O5 (3–7 GPa) [62], and ZrO2 (15 GPa) [63].

#### **3.5 Properties of high-entropy ceramic coatings**

#### *3.5.1 Mechanical properties*

Based on the reported high-entropy nitride (HEN) coatings in literature, it can be seen that the hardness of HEN coating is dependent on the selection of element that are strong nitride formers in multicomponent alloy. In the earlier work on (FeCoNiCrCuAlMn)N and (FeCoNiCrCuAl0.5)N HEN coatings, a maximum hardness of 10.4–11.8 GPa was observed at higher N2 flow ratio [39]. Consequently, HEN coatings containing strong nitride forming elements, such as (AlCrNbSiTiV) N [44], (TiVCrZrHf)N [49], and (TiZrNbAlYCr)N [64], were developed resulting in hardness increase of 40–48 GPa with increasing N2 flow ratio. Furthermore, the mechanical properties have been found to increase with increasing substrate-bias and temperature. Some of the HEN coatings reported in literature with superior mechanical properties have been summarized in **Table 1**.

#### *3.5.2 Tribological properties*

The superior mechanical properties and high temperature stability of highentropy nitride (HEN) and high-entropy carbide coating make them suitable toward tribological applications. Few research works have been reported on tribological studies of HEN and high-entropy carbide coating as a function of N2/

**169**

10<sup>−</sup><sup>6</sup>

mm3

*High-Entropy Ceramics*

decreased from 6.4 × 10<sup>−</sup><sup>6</sup>

**Table 1.**

showed a low wear rate of 2.8 × 10<sup>−</sup><sup>6</sup>

*Mechanical properties of high-entropy nitride coatings.*

and an average wear rate of 2.9 × 10<sup>−</sup><sup>6</sup>

and wear rate of 0.17 and 2.9 × 10<sup>−</sup><sup>7</sup>

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

CH4 gas flow ratio and substrate bias. Lai et al. studied the effect of substrate bias on tribological properties of (AlCrTaTiZr)N HEN coatings against steel counter ball [46]. The resulting wear test showed a high COF of 0.7, while the wear rate

**HEN coating Max. hardness (GPa) Max. Young's modulus (GPa) Ref.** (AlCrTaTiZr)N 32 368 [32] (AlCrMoSiTi)N 35 325 [43] (AlMoNbSiTaTiVZr)N 37 360 [65] (AlCrNbSiTiV)N 42 350 [66] (TiVCrZrHf)N 48 316 [67] (TiZrNbHfTa)N 32.9 — [68] (TiVCrZrHf)N 33 276 [45] (AlCrNbSiTi)N 36.7 425 [69] (TiHfZrVNb)N 44.3 — [47] (AlCrMoTaTi)N 30.6 280 [70] (AlCrMoTaTiSi)N 36 250 [71] (TiVCrZrNbMoHfTa-WAlSi)N 34.8 276.5 [42] (TiZrNbAlYCr)N 47 — [64] (TiZrHfNbTaY)N 40.2 — [48]

mm3

The (AlCrTaTiZr)N HEN coating was found to be stable after the wear test of 70 m sliding distance. In another work, Cheng et al. studied the effect of N2 flow rate on tribological properties of (AlCrMoTaTiZr)N HEN coatings against steel counter ball for a sliding distance of 90 m [72]. The resulting tribological test

mm3

be still high around 0.7. Similarly, Braic et al. studied the tribological behavior of (TiZrNbHfTa)N HEN coating and (TiZrNBHfTa)C high-entropy carbide coating on M2 steel substrate against sapphire counter ball for a sliding distance of 400 m at ambient conditions [68]. The wear test showed an average COF of 0.9

mm3

(TiZrNbHfTa)C high-entropy carbide coating. Following this work, wear test of (TiZrNbHfTa)N HEN coating and (TiZrNBHfTa)C high-entropy carbide coating was carried out in simulated body fluid (SBF) against sapphire counter ball for a sliding distance of 400 m [55]. The resulting wear test showed an average COF

(TiZrNbHfTa)C high-entropy carbide coating, respectively. Furthermore, Braic et al. and Jhong et al. studied the tribological performance of (TiAlCrNbY)C [54], (CuSiTiYZr)C [34], (CrCuNbTiY)C [73], and (CrNbSiTiZr)C [58] high-entropy carbide coatings and showed high-entropy carbide coating possessing superior tribological properties with wear rate and COF values in the range of 0.12–12 ×

Following the initial works on ball-on-disc sliding wear tests on HEN coatings, simulated tests have been performed for cutting tools application. Shen et al. studied the milling performance of (AlCrNbSiTi)N HEN coated WC-Co substrate

mm3

ing, and an average COF of 0.15 and an average wear rate of 8 × 10<sup>−</sup><sup>7</sup>

ing and an average COF and wear rate of 0.12–0.32 and 2–9 × 10<sup>−</sup><sup>7</sup>

/Nm and 0.07–0.4, respectively.

/Nm with increasing substrate bias.

/Nm; however, the COF was found to

/Nm for (TiZrNbHfTa)N HEN coat-

/Nm for (TiZrNbHfTa)N HEN coat-

mm3

mm3

/Nm for

/Nm for

to 3.6 × 10<sup>−</sup><sup>6</sup>


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

#### **Table 1.**

*Engineering Steels and High Entropy-Alloys*

**3.4 High-entropy oxide coatings**

evolution and mechanical properties were reported. The high-entropy carbide coating with lower carbon concentration showed formation of FCC structures at all the deposition temperatures, while the coating with higher carbon concentration showed poor crystallinity and approaching toward an amorphous phase. The grain size and surface roughness increased with increasing substrate temperature in both coatings. In contrast, the hardness values increased with increasing substrate

temperature from 13 GPa (80°C substrate temperature) to 30 GPa (650°C).

(18 GPa) [61], V2O5 (3–7 GPa) [62], and ZrO2 (15 GPa) [63].

mechanical properties have been summarized in **Table 1**.

**3.5 Properties of high-entropy ceramic coatings**

*3.5.1 Mechanical properties*

*3.5.2 Tribological properties*

Most of the research on high-entropy ceramic coatings has been focused on high-entropy nitrides and carbide coatings. Few research works have been reported on high-entropy oxide (HEO) coatings. Initial work on HEO coatings was reported by Chen and Wong, where (AlxCoCrCuFeNi)O (x: 0.5, 1 or 2) HEO coating by RF magnetron sputtering using different oxygen flow ratios (O2/Ar) was developed, and the resulting structural evolution was characterized [59]. The structural evolution was observed to change from FCC, FCC + BCC or BCC (depending on the amount of Al content) to cubic spinal oxides with increasing oxygen flow ratio. The hardness increased from 5–8 GPa to 13–22.6 GPa with increasing O2 flow ratio. In a subsequent work, Lin et al. used strong oxide forming elements to develop (AlCrTaTiZr)O HEO coating using DC magnetron sputtering with increasing O2 flow ratio (RO = O2/(O2 + Ar)) from 0 to 50%. The XRD analysis revealed a metastable amorphous structure irrespective of the O2 flow ratio [33]. The tendency of forming amorphous phases at different O2 flow ratios has been attributed to the large difference in the lattice parameters of oxides from each constituent element of Al, Cr, Ta, Ti, and Zr. The hardness values were reported to be in the range of 8–13 GPa. However, the hardness values increased in the range of 20–22 GPa after annealing at 900°C. The reported hardness values of HEO coatings were relatively higher than most of the reported oxide films, such as Al2O3 (10 GPa) [60], TiO2

Based on the reported high-entropy nitride (HEN) coatings in literature, it can

The superior mechanical properties and high temperature stability of highentropy nitride (HEN) and high-entropy carbide coating make them suitable toward tribological applications. Few research works have been reported on tribological studies of HEN and high-entropy carbide coating as a function of N2/

be seen that the hardness of HEN coating is dependent on the selection of element that are strong nitride formers in multicomponent alloy. In the earlier work on (FeCoNiCrCuAlMn)N and (FeCoNiCrCuAl0.5)N HEN coatings, a maximum hardness of 10.4–11.8 GPa was observed at higher N2 flow ratio [39]. Consequently, HEN coatings containing strong nitride forming elements, such as (AlCrNbSiTiV) N [44], (TiVCrZrHf)N [49], and (TiZrNbAlYCr)N [64], were developed resulting in hardness increase of 40–48 GPa with increasing N2 flow ratio. Furthermore, the mechanical properties have been found to increase with increasing substrate-bias and temperature. Some of the HEN coatings reported in literature with superior

**168**

*Mechanical properties of high-entropy nitride coatings.*

CH4 gas flow ratio and substrate bias. Lai et al. studied the effect of substrate bias on tribological properties of (AlCrTaTiZr)N HEN coatings against steel counter ball [46]. The resulting wear test showed a high COF of 0.7, while the wear rate decreased from 6.4 × 10<sup>−</sup><sup>6</sup> to 3.6 × 10<sup>−</sup><sup>6</sup> mm3 /Nm with increasing substrate bias. The (AlCrTaTiZr)N HEN coating was found to be stable after the wear test of 70 m sliding distance. In another work, Cheng et al. studied the effect of N2 flow rate on tribological properties of (AlCrMoTaTiZr)N HEN coatings against steel counter ball for a sliding distance of 90 m [72]. The resulting tribological test showed a low wear rate of 2.8 × 10<sup>−</sup><sup>6</sup> mm3 /Nm; however, the COF was found to be still high around 0.7. Similarly, Braic et al. studied the tribological behavior of (TiZrNbHfTa)N HEN coating and (TiZrNBHfTa)C high-entropy carbide coating on M2 steel substrate against sapphire counter ball for a sliding distance of 400 m at ambient conditions [68]. The wear test showed an average COF of 0.9 and an average wear rate of 2.9 × 10<sup>−</sup><sup>6</sup> mm3 /Nm for (TiZrNbHfTa)N HEN coating, and an average COF of 0.15 and an average wear rate of 8 × 10<sup>−</sup><sup>7</sup> mm3 /Nm for (TiZrNbHfTa)C high-entropy carbide coating. Following this work, wear test of (TiZrNbHfTa)N HEN coating and (TiZrNBHfTa)C high-entropy carbide coating was carried out in simulated body fluid (SBF) against sapphire counter ball for a sliding distance of 400 m [55]. The resulting wear test showed an average COF and wear rate of 0.17 and 2.9 × 10<sup>−</sup><sup>7</sup> mm3 /Nm for (TiZrNbHfTa)N HEN coating and an average COF and wear rate of 0.12–0.32 and 2–9 × 10<sup>−</sup><sup>7</sup> mm3 /Nm for (TiZrNbHfTa)C high-entropy carbide coating, respectively. Furthermore, Braic et al. and Jhong et al. studied the tribological performance of (TiAlCrNbY)C [54], (CuSiTiYZr)C [34], (CrCuNbTiY)C [73], and (CrNbSiTiZr)C [58] high-entropy carbide coatings and showed high-entropy carbide coating possessing superior tribological properties with wear rate and COF values in the range of 0.12–12 × 10<sup>−</sup><sup>6</sup> mm3 /Nm and 0.07–0.4, respectively.

Following the initial works on ball-on-disc sliding wear tests on HEN coatings, simulated tests have been performed for cutting tools application. Shen et al. studied the milling performance of (AlCrNbSiTi)N HEN coated WC-Co substrate

#### **Figure 6.**

*SEM micrographs of flank wear morphology of cutting inserts coated with: (a) TiN, (b) TiAlN, and (c) AlCrNbSiTi HEN coating [74] (licensed under CC BY 4.0, DOI: 10.3390/coatings5030312).*

against SKD11 steel for a sliding distance of 900 m and compared its performance to commercial TiN and TiAlN coatings [74]. The resulting milling tests showed a lower flank wear of 200 μm/min for (AlCrNbSiTi)N HEN coating as compared to 255 μm/ min in TiN and 270 μm/min in TiAlN, as shown in **Figure 6**. Similarly, machining performance of (TiZrHfVNbTa)N [75], (AlCrNbSiTiV)N [51], and nanolaminate (TiAlCrSiY)N/(TiAlCr)N [76] HEN coatings showed better tribological properties than commercial nitride coatings.

#### *3.5.3 Corrosion properties*

The increase in lattice distortion with high number of principle elements results in formation of amorphous phase in high-entropy alloys, which in return gives better mechanical and electrochemical properties. The corrosion resistance of conventional alloys can be enhanced with amorphous HEA coatings by choosing the appropriate chemical compositions. Lin deposited (TiAlCrSiV)N HEN coatings on a mild steel substrate at different RN flow ratios and studied its electrochemical properties in 3.5 wt.% NaCl solution at room temperature (RT; 22°C). The coating structure changed from amorphous phase at lower RN flow ratio to FCC solid solution at higher RN flow ratio. The highest polarization resistance of 11.36 kΩ/cm2 was observed in the metallic TiAlCrSiV HEA coating, while the polarization resistance slightly decreased to 8.03–8.55 kΩ/cm2 in its nitride coatings. Furthermore, the polarization resistance of HEN coatings was enhanced by developing an interlayer of metallic TiAlCrSiV HEA coating. In another work, Hsueh et al. deposited (AlCrSiTiZr)N HEN coating on 6061 aluminum alloy and mild steel substrates using DC magnetron sputtering at various RN flow ratios, and studied the effect of RN flow ratio and substrate bias during deposition on corrosion properties in 0.1 M H2SO4 aqueous solution at RT [77]. The (AlCrSiTiZr)N HEN coating changed from an amorphous structure to partially crystalline structure at higher RN flow ratios. The resulting corrosion current density (icorr) for 6061 aluminum alloy substrate decreased from 29.1 μA/cm2 (uncoated) to 3.1 μA/cm2 with (AlCrSiTiZr)N HEN coating; while for mild steel substrate, it decreased from 90.4 μA/cm2 (uncoated) to 7.7 μA/cm2 . Similar increase in corrosion/ oxidation resistance with HEN coating on conventional substrate was observed in (TiZrNbHfTa)N and (TiZrNbHfTa)C [55], (AlCrNbSiTi)N [69], nanolaminated AlCrMoNBZr/(AlCrMoNbZr)N [76, 78], and (NbTiAlSiZr)N [41] HEN coating.

### **4. Future possibility and commercialization**

As compared to high-entropy alloys, fewer reports have been published on crystalline high-entropy ceramics (HECs). Due to the high melting point, high hardness, and good thermal and chemical stability, as well as excellent wear and

**171**

*High-Entropy Ceramics*

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

structure and properties of HECs.

transistors (TFETs) [59].

the material design and processing work.

**5. Summary**

oxidation resistance of ceramic materials, most of the bulk HECs such as highentropy borides and carbides are designed to be the new type of ultra-high-temperature ceramics (UHTC) with increased high-temperature stability and superior mechanical properties over conventional ceramics. The current understanding of processing and evolution of properties of bulk HECs was built on and developed from the knowledge of HEAs. For bulk HECs, the preferred ceramic components are group IV, V, and VI metal ceramics (metal boride, carbide, silicide, etc.) due to their closely matched structure and properties. Compared to metallic HEAs system where only elemental materials are used, the combination of HEC composition is greatly limited. More experimental efforts on screening and investigation of HECs with different compositions are required to explore the potential of HECs. Furthermore, most of the reported bulk HECs is based on experimental observations, while a few are systematically discussed in combination with the modeling results. In order to give future researchers a clear guideline of designing HECs, further development on computational methods including corresponded thermodynamic ceramic database is needed to more effectively and accurately predict the

The research on the processing of HECs is progressing, and there have been considerable work performed in the industry on the coatings of HECs toward the next generation of nitride and carbide coatings. High-entropy carbide and nitride coatings have potential applications in biomedical industry, cutting tools, and hard facing die coatings due to their superior mechanical, corrosion, and oxidation resistant properties [55, 74]. HEC coatings of nitride with high thermal stability can be applied as diffusion barrier coatings in integrated circuits to inhibit the diffusion of adjacent materials (e.g., Cu and Si) [79]. Furthermore, high-entropy oxide coatings can be considered as a potential future material for cuprate superconductors, visible-light photocatalysts, and transparent field-effect

Researches of bulk high-entropy ceramic have been reported on metal oxides, refractory carbides, borides, and silicides. HECs with homogenous single-phase structure reveal superior mechanical performance and additional properties like thermal-electrical property of high-entropy oxides. Most scientific effort has been put in exploring various fabrication methods, characterizing the high-entropy structure and the remarkable physical and chemical properties. However, to fully understand HECs that involves a complex multicomponent ceramic system, the problem remains: what are the most significant phase formation rules in a HECs material? How can the potential applications of HECs be realized? A more systematic investigation of the material selection rules that already exists in a metallic high-entropy system is highly demanded in the ceramic system in order to optimize

The reported research in the past 10 years on high-entropy ceramic coatings has shown development of coatings with exceptional mechanical, high temperature, electrical, corrosion, and wear-resistant properties. High-entropy ceramic coatings have great potential in different applications, such as wear and corrosion resistant coatings, thermal barrier coatings, and electrical and biomedical applications. The control on variation of deposition parameters can be advantageous in achieving coating with extremely high strength values and highly densified structures, which can be corrosion resistant and biocompatible. Future work on development of nanocomposite and multilayer high-entropy ceramic coatings

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

*Engineering Steels and High Entropy-Alloys*

than commercial nitride coatings.

*3.5.3 Corrosion properties*

**Figure 6.**

to 8.03–8.55 kΩ/cm2

to 3.1 μA/cm2

decreased from 90.4 μA/cm2

against SKD11 steel for a sliding distance of 900 m and compared its performance to commercial TiN and TiAlN coatings [74]. The resulting milling tests showed a lower flank wear of 200 μm/min for (AlCrNbSiTi)N HEN coating as compared to 255 μm/ min in TiN and 270 μm/min in TiAlN, as shown in **Figure 6**. Similarly, machining performance of (TiZrHfVNbTa)N [75], (AlCrNbSiTiV)N [51], and nanolaminate (TiAlCrSiY)N/(TiAlCr)N [76] HEN coatings showed better tribological properties

*SEM micrographs of flank wear morphology of cutting inserts coated with: (a) TiN, (b) TiAlN, and (c) AlCrNbSiTi HEN coating [74] (licensed under CC BY 4.0, DOI: 10.3390/coatings5030312).*

The increase in lattice distortion with high number of principle elements results in formation of amorphous phase in high-entropy alloys, which in return gives better mechanical and electrochemical properties. The corrosion resistance of conventional alloys can be enhanced with amorphous HEA coatings by choosing the appropriate chemical compositions. Lin deposited (TiAlCrSiV)N HEN coatings on a mild steel substrate at different RN flow ratios and studied its electrochemical properties in 3.5 wt.% NaCl solution at room temperature (RT; 22°C). The coating structure changed from amorphous phase at lower RN flow ratio to FCC solid solution at higher

metallic TiAlCrSiV HEA coating, while the polarization resistance slightly decreased

of HEN coatings was enhanced by developing an interlayer of metallic TiAlCrSiV HEA coating. In another work, Hsueh et al. deposited (AlCrSiTiZr)N HEN coating on 6061 aluminum alloy and mild steel substrates using DC magnetron sputtering at various RN flow ratios, and studied the effect of RN flow ratio and substrate bias during deposition on corrosion properties in 0.1 M H2SO4 aqueous solution at RT [77]. The (AlCrSiTiZr)N HEN coating changed from an amorphous structure to partially crystalline structure at higher RN flow ratios. The resulting corrosion current density

(uncoated) to 7.7 μA/cm2

oxidation resistance with HEN coating on conventional substrate was observed in (TiZrNbHfTa)N and (TiZrNbHfTa)C [55], (AlCrNbSiTi)N [69], nanolaminated AlCrMoNBZr/(AlCrMoNbZr)N [76, 78], and (NbTiAlSiZr)N [41] HEN coating.

As compared to high-entropy alloys, fewer reports have been published on crystalline high-entropy ceramics (HECs). Due to the high melting point, high hardness, and good thermal and chemical stability, as well as excellent wear and

in its nitride coatings. Furthermore, the polarization resistance

with (AlCrSiTiZr)N HEN coating; while for mild steel substrate, it

was observed in the

(uncoated)

. Similar increase in corrosion/

RN flow ratio. The highest polarization resistance of 11.36 kΩ/cm2

(icorr) for 6061 aluminum alloy substrate decreased from 29.1 μA/cm2

**4. Future possibility and commercialization**

**170**

oxidation resistance of ceramic materials, most of the bulk HECs such as highentropy borides and carbides are designed to be the new type of ultra-high-temperature ceramics (UHTC) with increased high-temperature stability and superior mechanical properties over conventional ceramics. The current understanding of processing and evolution of properties of bulk HECs was built on and developed from the knowledge of HEAs. For bulk HECs, the preferred ceramic components are group IV, V, and VI metal ceramics (metal boride, carbide, silicide, etc.) due to their closely matched structure and properties. Compared to metallic HEAs system where only elemental materials are used, the combination of HEC composition is greatly limited. More experimental efforts on screening and investigation of HECs with different compositions are required to explore the potential of HECs. Furthermore, most of the reported bulk HECs is based on experimental observations, while a few are systematically discussed in combination with the modeling results. In order to give future researchers a clear guideline of designing HECs, further development on computational methods including corresponded thermodynamic ceramic database is needed to more effectively and accurately predict the structure and properties of HECs.

The research on the processing of HECs is progressing, and there have been considerable work performed in the industry on the coatings of HECs toward the next generation of nitride and carbide coatings. High-entropy carbide and nitride coatings have potential applications in biomedical industry, cutting tools, and hard facing die coatings due to their superior mechanical, corrosion, and oxidation resistant properties [55, 74]. HEC coatings of nitride with high thermal stability can be applied as diffusion barrier coatings in integrated circuits to inhibit the diffusion of adjacent materials (e.g., Cu and Si) [79]. Furthermore, high-entropy oxide coatings can be considered as a potential future material for cuprate superconductors, visible-light photocatalysts, and transparent field-effect transistors (TFETs) [59].
