*3.1.1 Sputtering*

Sputtering is a physical vapor deposition (PVD) technique where a solid surface (target) is bombarded with accelerated charged ions (Ar+ ) resulting into backward 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

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 nitride, carbide, and oxide ceramic coatings.
