*3.2.2 Effect of N2 flow rate*

The high-entropy nitride (HEN) coatings developed by DC-magnetron sputtering were reported by research group of Yeh et al. using targets of FeCoNiCrCuAlMn, FeCoNiCrCuAl, AlxCoCrCuFeNi (x: 0.5 and 2), and AlCrNiSiTi [39]. In all the cases, the resulting HEN coatings showed formation of FCC solid solution at low N2 flow rate (RN) and became amorphous at higher N2 flow rate (RN) due to severe lattice distortion and presence of weak nitride forming elements, such as Al and Si, where RN = (N2/(Ar + N2)). Following this work, Lai et al. developed (AlCrTaTiZr)N HEN coating, where the coating structure changed from amorphous phase metallic coating to FCC solid solution HEN with increasing N2 flow rate [32]. The sputtering rate decreased from 35 nm/min (RN = 0%) to 15 nm/min (RN = 60%) with increasing N2 flow ratio (RN). The decreasing sputtering rate was attributed to a lower sputtering yield due to nitrogen absorption, nitridation of the target, and/or decreasing sputtering efficiency of reactive gases with increasing RN as compared to argon ions. The increasing RN ratio resulted in increasing the hardness from 9 GPa (0% RN) to 32 GPa (15% RN). In another study, Tsai et al. developed octonary principal element (AlMoNbSiTaTiVZr)N HEN coatings using magnetron sputtering with nitrogen flow ratio from 0 to 70% RN [40]. The deposition rate decreased from 31 nm/min (RN = 0%) to 8.3 nm/ min (RN = 67%) with increasing nitrogen flow ratio. In contrast, the hardness values increased from 13.5 GPa (0% RN) to 37 GPa (50% RN). Such high increase in hardness has been attributed to the stronger bonding between N and target elements. The coating morphology of (AlMoNbSiTaTiVZr)N HEN changed from coarser grain-like morphology (RN = 0%; grain size: 30–100 nm) to reduced grain morphology (RN = 11%; grain size: 10 nm) and then to rougher morphology (RN = 33%). Similarly, the cross-section of the coating changed from a glass-like

**165**

**Figure 4.**

*High-Entropy Ceramics*

respectively.

(0% RN) to 34.8 GPa (50% RN).

*3.2.3 Effect of substrate bias*

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

featureless morphology to fine columnar structure with increasing nitrogen flow ratio from RN = 0–11% to RN = 33%, respectively. Similarly, Xing et al. developed (NbTiAlSiZr)N HEN coating using RF sputtering with increasing nitrogen flow rate from 10 to 50% [41]. The coating thickness and deposition rate were found to

The cross-section of (NbTiAlSiZr)N HEN coatings is shown in **Figure 5**. The coating thickness was found to decrease from 298.8 to 200 nm with increasing nitrogen flow rate. Furthermore, the hardness was found to increase from 9.5 to 12 GPa with the increasing nitrogen flow rate of RN = 10% to RN = 50%,

In another work, Chang developed a duodenary (TiVCrZrNbMoHfTaWAlSi)N HEN coatings with RN from 0 to 50% using DC magnetron sputtering and similar structural evolution from amorphous to FCC solid solution was observed with increasing RN [42]. The hardness after reactive sputtering increased from 13 GPa

The changing substrate bias during coating deposition effects the chemical composition, microstructure, and mechanical properties of coatings. Chang et al. studied the effect of substrate bias from 0 to −200 V on (AlCrMoSiTi)N HEN coatings developed by DC magnetron sputtering at 50% RN [43]. The coating showed FCC solid solution structure even though it contained immiscible nitrides, such as AlN, TiN, and Si3N4. However, the lattice parameter increased from 4.15 to 4.25 Å, and the grain size decreased from 16.8 to 3.3 nm with increasing substrate bias. This change in the lattice parameter was attributed to increase in adatom mobility, and the decreasing grain was due to increase in nucleation rate at ion-induced surface defects with changing substrate bias. The increasing substrate bias had a small effect on the hardness properties from 25 GPa with no bias to highest hardness of 32 GPa observed at −100 V bias. Following these findings, Huang et al. studied the effect of increasing substrate bias from 0 to −160 V on (AlCrNbSiTiV)N HEN coating using RF magnetron sputtering with RN and substrate temperature kept constant at 28% and 300°C, respectively [44]. The XRD analysis of HEN coatings

*Film deposition rate as a function of nitrogen flow ratio (RN) in (NbTiAlSiZr)N HEN coatings [41].*

decrease with increasing nitrogen flow rate, as shown in **Figure 4**.

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

*Engineering Steels and High Entropy-Alloys*

**3.2 Structural evolution**

*3.2.2 Effect of N2 flow rate*

*3.2.1 High-entropy nitride coatings*

achieved by using a mixture of CH4 + N2 and O2 + N2, respectively [35, 36]. In reactive sputtering, a mixture of Ar and reactive gas (5–70%) is used for sputtering. The increasing bias between the target and substrate can affect the chemical composite, microstructure, and mechanical properties of the coating by giving compact coating structures. Lastly, substrate temperature can be varied in reactive sputtering to enhance the ion mobility and the interaction between the deposited ions, which can affect the microstructure, composition, and the properties of the coating. The following section will be used to highlight the use of reactive sputtering to deposit

High-entropy alloys based on multi-principle elements make it possible to design various nitride coatings. The use of nitride forming multi-principle elements can enhance the physical and mechanical properties that are not achievable in the conventional binary- and ternary-nitride coatings [37]. The high-entropy nitride coatings form amorphous and/or solid solution with face-centered cubic (FCC) structure. The severe lattice distortion and solid solution hardening help in developing high strength nitride coatings for various applications that require wear resistance, corrosion resistance, diffusion barrier, electrical resistivity, biocompatibility or light reflectivity [38]. The versatile sputtering deposition can be used to vary the N2 flow, substrate temperature, and substrate bias to obtain nitride coatings with

high-entropy nitride, carbide, and oxide ceramic coatings.

good physical, chemical, and mechanical properties, respectively.

The high-entropy nitride (HEN) coatings developed by DC-magnetron sputtering were reported by research group of Yeh et al. using targets of FeCoNiCrCuAlMn, FeCoNiCrCuAl, AlxCoCrCuFeNi (x: 0.5 and 2), and

AlCrNiSiTi [39]. In all the cases, the resulting HEN coatings showed formation of FCC solid solution at low N2 flow rate (RN) and became amorphous at higher N2 flow rate (RN) due to severe lattice distortion and presence of weak nitride forming elements, such as Al and Si, where RN = (N2/(Ar + N2)). Following this work, Lai et al. developed (AlCrTaTiZr)N HEN coating, where the coating structure changed from amorphous phase metallic coating to FCC solid solution HEN with increasing N2 flow rate [32]. The sputtering rate decreased from 35 nm/min (RN = 0%) to 15 nm/min (RN = 60%) with increasing N2 flow ratio (RN). The decreasing sputtering rate was attributed to a lower sputtering yield due to nitrogen absorption, nitridation of the target, and/or decreasing sputtering efficiency of reactive gases with increasing RN as compared to argon ions. The increasing RN ratio resulted in increasing the hardness from 9 GPa (0% RN) to 32 GPa (15% RN). In another study, Tsai et al. developed octonary principal element (AlMoNbSiTaTiVZr)N HEN coatings using magnetron sputtering with nitrogen flow ratio from 0 to 70% RN [40]. The deposition rate decreased from 31 nm/min (RN = 0%) to 8.3 nm/ min (RN = 67%) with increasing nitrogen flow ratio. In contrast, the hardness values increased from 13.5 GPa (0% RN) to 37 GPa (50% RN). Such high increase in hardness has been attributed to the stronger bonding between N and target elements. The coating morphology of (AlMoNbSiTaTiVZr)N HEN changed from coarser grain-like morphology (RN = 0%; grain size: 30–100 nm) to reduced grain morphology (RN = 11%; grain size: 10 nm) and then to rougher morphology (RN = 33%). Similarly, the cross-section of the coating changed from a glass-like

**164**

featureless morphology to fine columnar structure with increasing nitrogen flow ratio from RN = 0–11% to RN = 33%, respectively. Similarly, Xing et al. developed (NbTiAlSiZr)N HEN coating using RF sputtering with increasing nitrogen flow rate from 10 to 50% [41]. The coating thickness and deposition rate were found to decrease with increasing nitrogen flow rate, as shown in **Figure 4**.

The cross-section of (NbTiAlSiZr)N HEN coatings is shown in **Figure 5**. The coating thickness was found to decrease from 298.8 to 200 nm with increasing nitrogen flow rate. Furthermore, the hardness was found to increase from 9.5 to 12 GPa with the increasing nitrogen flow rate of RN = 10% to RN = 50%, respectively.

In another work, Chang developed a duodenary (TiVCrZrNbMoHfTaWAlSi)N HEN coatings with RN from 0 to 50% using DC magnetron sputtering and similar structural evolution from amorphous to FCC solid solution was observed with increasing RN [42]. The hardness after reactive sputtering increased from 13 GPa (0% RN) to 34.8 GPa (50% RN).
