*3.2.3 Effect of substrate bias*

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

**Figure 4.** *Film deposition rate as a function of nitrogen flow ratio (RN) in (NbTiAlSiZr)N HEN coatings [41].*

**Figure 5.**

*Surface and cross-section SEM micrographs of (NbTiAlSiZr)N HEN coatings at various RN: (a) RN = 10%, (b) RN = 20%, (c) RN = 30%, (d) RN = 40%, and (e) RN = 50% [41].*

showed FCC solid solution with a similar increasing and decreasing trends of lattice parameters and grain size, respectively. However, in this work, the hardness increased from 22 GPa with no bias to the highest hardness of 42 GPa achieved at −100 V bias. The (AlCrNbSiTiV)N HEN coating showed excellent thermal stability even after annealing at 800°C (5 h) and maintained a hardness of 40 GPa. Such increasing trend in hardness was attributed to changing grain size and residual stress with increasing substrate bias. Similar effect of substrate bias on the coating structure and properties were observed in (TiVCrZrHf)N [45], (AlCrTaTiZr)N [46], (TiHfZrVNb)N [47], and (TiZrHfNbTaY)N [48] HEN coatings.
