**3. Thin film growth techniques**

The thin films growth can be carried out through several techniques where they can be deposited as vapor, liquid, solid phases, or a combination of these phases through plasma enhancement, ion bombardment, self-assembly, chemical treatment, among others [19, 20]. In this chapter, four thin film deposition techniques will be presented: magnetron sputtering, cathodic cage plasma deposition (CCPD), PECVD, and layer-by-layer.

#### **3.1 Magnetron sputtering**

Within the PVD processes, the magnetron sputtering technique is one of the most used in research with nanocomposites [21, 22]. The conventional sputtering technique has very low deposition rates, and an alternative found to increase this rate is the magnetron sputtering technique [23, 24].

The mentioned fact occurs because magnets are positioned in the vicinity of the target to act as electron traps. Due to electron trapping in the region close to the target, the plasma will also be restricted to this region resulting in a higher sputtering rate [24].

**Figure 1** shows the principle of sputtering with a planar magnetron where it is possible to observe the confinement of electrons in the region close to the magnet. During the process, the target is intentionally positioned close to this region to increase the deposition rate.

The advantages of this technique include low plasma impedance and thus high discharge currents from 1 A to 100 A (depending on cathode length) at typical voltages around 500 V; deposition rates in the range from 1 to 10 nm/s; low thermal load to the substrate; dense and well-adherent coatings; large variety of film materials available (nearly all metals and compounds). Some disadvantages of this technique include improvement requirement of target utilization; stabilization of the reactive process in the transition regime [25]. The magnetron sputtering technique can be applied for wear-resistant coatings, low friction coatings, corrosion-resistant coatings, decorative coatings, and coatings with specific optical or electrical properties [21–25]. The **Figure 2** shows schematic diagram of magnetron sputtering process.

**Figure 1.** *The principle of magnetron sputtering [25].*

#### **Figure 2.**

*Schematic diagram of magnetron sputtering process: (1) working chamber, (2) height adjustable substrate holder, (3) quartz window, (4) target to be sputtered, (5) magnetron, (6) molybdenum wire, (7) thermocouple, and (8) substrate [24].*

#### **3.2 Cathodic cage plasma deposition (CCPD)**

This technique is a variation of the reactive sputtering process; it consists of the use of a cage with well-defined geometry where the effect of cylindrical multicathodes is used to obtain coatings and three-dimensional surface treatments. In this

technique, the cage functions as a cathode in which a potential difference that acts only in the cage is applied (for the case of deposition), because the sample remains isolated on an alumina disc [26–28]. **Figure 3** shows a schematic representation of the spatial arrangement of the sample and cage in the sample holder.

During treatment, it is possible to observe the formation of plasma on the surface of the cage as well as the luminous intensification of the cage in each hole of the cage. **Figure 4a** shows the visual aspect of the plasma formed on the cathodic cage.

The process is carried out in a reactor and conducted to very low working pressures (~ 2.5 mbar) with controlled treatment atmosphere (Ar, H2, N2, O, for example). During the sputtering process, the atoms ejected from the surface of the cage can combine with the reactive gas of the plasma atmosphere, forming compounds that deposit on the surface of the sample. In this sense, the cage must be made of the material to be deposited. The technique of CCPD allows both nitriding and deposition of a thin film on a substrate and thus a better adhesion between the film and the substrate [30].

For the case of NTF, an example of arrangement for deposition of thin films would be the use of a graphite cylinder with titanium cover. **Figure 4b** shows the example of the cited arrangement. In this example, the film will present the synergism between the properties of the C from the graphite cylinder representing the soft phase and the Ti due to the cover, representing the hard phase.

The advantages of this technique include treatment of parts with complex geometries; minimizing the edge effect and opening of arcs in relation to the conventional sputtering technique; obtaining uniform layers; allowing to carry out both the nitriding process and the deposition of thin films. Some disadvantages of this technique include requiring a vacuum procedure; the production process of the cages can be complex depending on the type of material. The CCPD technique can be used for oxide and nitride deposition, deposition, desorption, and diffusion. It allows the deposition of thin films of high hardness, high wear, and corrosion resistance [26–30].
