**3. Preparation method of SPE coating**

The preparation method of the coating is physical vapor deposition (PVD) based on plasma technology. Physical vapor deposition is a common technology for preparing hard coatings. Its principles include: (1) gasification of coating materials, that is, providing energy to coating materials through collision, sputtering and other methods to ionize the coating materials and become a source of plasma gasification; (2) After sputtering and collision, the atoms or ions in the gasified coating materials are effectively separated from the plasma coating materials by the migration of atoms, molecules or ions in the coating materials, and the atoms or molecules in the coating materials are controlled to migrate to the substrate surface by magnetic field and other means; and (3) Coating atoms, molecules or ions are deposited on the body, that is, the separated atoms, molecules or ions are deposited or bombarded on the target surface by voltage or other methods. Physical vapor deposition (PVD) has become a promising coating preparation technology due to its high bonding strength, low deposition temperature, rich materials and multilayer coating.

The commonly used PVD technologies are mainly magnetron sputtering ion plating and arc ion plating. Magnetron sputtering ion plating is not suitable for preparing thick coatings because its ionization rate is much lower than that of arc ion plating due to its collision miss mechanism; At the same time, the toughness of the coating prepared by magnetron sputtering is far less than that of arc ion plating, and its erosion resistance is poor, so it is not suitable for the preparation of erosion resistant coatings. The other commonly used PVD technology, arc ion plating, is widely used for the preparation of hard coatings. Because the traditional cathodic arc ion plating is a thermal field emission miss target mechanism, the deposition particle energy is high, the flux is large, and the ionization rate is high (about 80%). At the same time, the technology can install different targets to achieve the deposition of multiple coatings, but the target will partially melt and eject micron sized molten metal particles due to arc discharge (as shown in **Figure 4**). These protrusions formed by micro-particles are easy to be eroded by sand and gravel, and will form pits after the micro-particles on the surface are washed away. These pits are easy to become stress concentration points of fatigue or erosion damage, thus leading to rapid failure of the coating.

#### **Figure 4.**

*Schematic diagram of the surface morphology of the arc ion plating coating and the influence of large particles on the erosion.*

Therefore, how to eliminate or reduce the metal droplets in the anti-erosion coatings prepared by arc ion plating is one of the current research focuses.
