• (1) Control friction and wear,
  • (2) Improve corrosion resistance,
  • (3) Change physical properties, such as conductivity, resistivity and reflectivity,
  • (4) Modify the size,
  • (5) Change appearance, such as color and roughness,
  • (6) Reduce costs.

General surface treatments can be divided into two main types: treatments that cover the surface and treatments that change the surface.

1. Cover the surface

The treatment of covering the surface includes organic coating and inorganic coating.

Inorganic coatings include electroplating, conversion coating, thermal spraying, thermal dipping, melting in a furnace, or coating the surface of materials with films, glass, or ceramics.

Electroplating is an electrochemical process in which an electric current is passed through an electroplating bath to deposit metal on the substrate.

There is usually an anode (positive electrode), which is the source of the material to be precipitated; electrochemical reaction is an intermediate process in which metal ions are exchanged and migrated to the substrate to be covered; and a cathode (negative electrode), which is the substrate to be covered.

Plating is carried out in an electroplating bath which is usually a non-metallic container (generally plastic). The container is filled with electrolyte containing the metal to be plated in an ionic state.

The anode is connected to the anode of the power supply. The anode is usually the metal to be plated (it is assumed that the metal can corrode in the electrolyte). For easy handling, the metal is in the form of solid small pieces and placed in an inert metal basket made of corrosion-resistant metal (such as titanium or stainless steel).

The cathode is the workpiece, the substrate to be plated, connected to the negative pole of the power supply. The power supply is well adjusted to minimize fluctuations and provide a stable and predictable current under load changes (as seen in the plating container).

Once the current is applied, the positive metal ions from the solution are attracted to the negatively charged cathode and precipitate on it. As a supplement to these precipitated ions, the metal from the anode is dissolved and enters the solution to balance the ion potential.

Thermal spraying process: Thermal spraying metal coating is a metal deposition layer that is projected onto the substrate immediately after the metal is melted. The metal used and the application system can vary, but most applications are to coat a thin layer on a surface that requires improved corrosion or wear resistance.

Thermal spraying is a general term used for a large class of related processes. The molten droplets sprayed on the surface to produce a coating can be metal, ceramic, glass and/or polymer, forming an independent approximate pure shape or producing unique properties Design materials.

In general, any material with a stable melting state can be thermally sprayed, and a wide range of pure and synthetic materials can generally be sprayed for research and industrial purposes. The deposition rate is very high compared to alternative coating technologies.

The thickness of the precipitation is generally 0.1 to 1mm, and for some materials, the thickness of the precipitation can reach more than 1cm.

The application process of spraying metal is relatively simple and consists of the following stages:

  • (1) Melting metal in the spray gun.
  • (2) Spray liquid metal on the prepared substrate by compressed air.
  • (3) The molten particles are projected on the cleaned substrate.

There are two main types of wire applications available, namely arc spray and gas spray.

Arc spraying—When a pair of wires are connected together by a hand-held spray gun, they are energized across their ends to ignite the arc. The compressed air is blown through the arc to atomize and drive the automatic feeding of metal wire particles onto the prepared workpiece.

Gas spraying—The continuously moving metal wire passes through the hand-held spray gun in the combustion flame spray and is melted by the conical nozzle of the combustion gas. The tip of the molten metal wire enters the cone to atomize and drive it to the substrate.

Thin film coating: Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are the two most common types of thin film coating methods.

Physical evaporation precipitation coating involves the precipitation of various materials on a solid substrate in a vacuum device where atoms are close to atoms, molecules close to molecules or ions.

Thermal evaporation utilizes the mist of particles formed by the evaporation of the coated metal in a vacuum environment to cover all surfaces in the visible range between the substrate and the target. It is often used when generating thinner (0.5μm), decorative, glossy coatings on plastic parts.

However, this thin coating is fragile and not suitable for wear applications. The thermal evaporation process can also cover the jet engine parts with very thick (1mm) coatings of heat-resistant materials, such as MCrAIY—a metal, chromium, aluminum, and yttrium alloy.

The “reactive sputtering method” applies high-tech coatings such as ceramics, metal alloys, organic and inorganic compounds by connecting workpieces and materials with specific compositions to high-voltage direct current in an argon vacuum device.

A plasma zone is formed between the substrate (workpiece) and the target (raw material) and transfers the sputtered target atoms to the surface of the substrate.

"If the substrate is not conductive, such as polymer, radio frequency (RF) sputtering is used instead. Reactive sputtering can produce thinner (less than 3μm (120μin)), hard film coatings, like titanium nitride (TIN), which is harder than the hardest metals.

Now the reactive sputtering method has been widely used in cutting tools, molding tools, injection molds, and general-purpose appliances such as punches and dies to enhance their wear resistance and service life.

Chemical evaporation precipitation can produce thicker, dense, extensible and adhesive coatings on metals and non-metals like glass and plastics. Compared with physical evaporation precipitation in the "visible range", chemical evaporation precipitation can cover all surfaces of the substrate.

The conventional chemical evaporation precipitation coating process requires a metal compound that is easy to volatilize at a relatively low temperature and decomposes into pure metal when it contacts the substrate at a higher temperature.

The most well-known example of chemical evaporation precipitation is the 2.5mm (0.1in.) thick nickel carbonyl (NiCO4) coating on glass windows and containers to make them resistant to bursting or breaking.

In order to increase the surface hardness of cutting tools, a diamond chemical evaporation precipitation coating process is introduced. However, this process can only be achieved at temperatures higher than 700°C (1300°F), which will soften most tool steels.

Therefore, the application of diamond chemical evaporation precipitation is limited by the material, and the material is required to not soften at this temperature, such as cemented carbide.

The plasma-assisted chemical evaporation deposition coating process can operate at a lower temperature than the diamond chemical evaporation deposition coating. This chemical evaporation precipitation is used to cover the diamond coating or silicon carbide isolation coating on plastic films and semiconductors (including the case of artificial 0.25 μm semiconductors).

2. Change the surface

The treatment to change the surface includes quenching treatment, high energy machining and special treatment.

High-energy processing is a relatively new surface treatment method. They can change the surface properties without changing the surface size.

Electron beam treatment: The electron beam treatment changes the surface properties by rapidly heating with electron beams and rapidly cooling at 106°C/sec in the very shallow (100μm) area near the surface. This technique is also used for surface hardening to produce "surface alloys".

Ion implantation: Ion implantation uses electron beam or plasma to accelerate through a magnetic coil in a vacuum chamber to impact gas atoms into ions with sufficient energy, and to embed these ions in the atomic lattice of the matrix. The mismatch between the ion implantation and the metal surface produces atomic defects on the hardened surface.

Laser beam processing: Similar to electron beam processing, laser beam processing changes the surface properties by rapidly heating and rapidly cooling a very shallow area close to the surface. It can also be used for surface hardening to produce "surface alloys".

But the preliminary results seem to be promising. High-energy processing requires further development, especially implant dosage and processing methods.