Surface Coatings

Metal coatings consist typically in a protective layer of the metallic substrate that is corrosion resistant and protects the surface from environmental conditions. This occurs by changing the properties of the metallic surface itself [2].
There are various techniques to apply metallic coatings to a surface. Some examples of techniques of metallic coating application/use are:

• Hot dip galvanizing consists in a process of coating ferrous materials using a zinc layer by dipping in molten zinc at high temperatures. At the end of the process there is zinc carbonate (ZnCO3) in the surface which protects the steel surfaces from corrosion and scratch due to its strength and stability.
• Electroplating: Coating of a metallic surface by other metal through electrolysis to enhance corrosion resistance of the metal substrate. 
• Anodizing: It is a process of promoting the increase of the thickness of the natural oxide layer of metals using an electrolytic passivation approach. The term “Anodization” comes from the usage of the parts to be treated as “anode” in the electrolytic system cell. 
   

DLC, or, Diamond-like carbon structure are metastable forms of amorphous carbon (in the sp3 form) with high mechanical and chemical resistance, more or less flexibility, as well as optical properties depending of the amorphous carbon used [3]. DLC coatings have been one of the most used carbon coatings due to its widespread range of applications. It can be used as a protective coating in optical windows, magnetic storage disks, car parts, etc.

These coatings are generally produced with physical vapor deposition processes such as sputtering, ion beam, pulsed laser deposition, and cathodic vacuum arc systems. These techniques are mostly based on deposition technologies, where carbon atoms are charged with a constant and known amount of energy and then “bombarded” onto the substrate. The amount of energy per unit ion is different for each deposition technology. Therefore, the DLC coatings prepared by different deposition techniques possess different characteristics.

More recently DLC coatings have been reported as a tribological coating due to its high abrasive/adhesive wear resistance making them excellent materials in high contact pressure conditions. However, in ferrous substrate materials, it’s important to ensure that the coating will not be exposed at high temperatures or the substrate (or the tribological counter-face) could carburize, absorbing the carbon forms of the coating and reducing its hardness. 

Ceramic coatings can vary widely in composition while maintaining their ceramic characteristics. 
These coatings can also be created through physical vapor deposition or chemical vapor deposition processes but are also suitable for application via thermal spray techniques such as high velocity oxy-fuel (HVOF) or cold spray.
Well known examples of ceramic coatings are alumina coatings constituted of Al3O2 and silicon carbide (SiC), but a very wide array of such coatings exist and is ever expanding.

Teflon™ is a registered trademark and a brand name created by Dupont and currently owned by Chemours. The brand Teflon is highly recognized in the coatings industry, not for a specific chemical or single product but for a full range of coatings containing fluoropolymers. This range of coatings delivers the typical properties of fluoropolymer materials. 
“Teflon” coatings are nothing more than polymer coatings that contain fluoropolymers, such as Polytetrafluoroethylene (PTFE), Ethylenefluoroethylene (ETFE), Perfluoroalcoxy (PFA), among others. 

Hybrid ceramic-polymer coatings are produced by synthetic silica or silica-based components in their main structure. Typically, these coatings are produced by Sol-gel techniques. 
Hybrid ceramic-polymer coatings are characterized for having high scratch resistance and hardness as well as chemical and thermal stability, controllable porosity, biological inertness, and high transparency [4].
These coatings can be used in different types of applications due to their capability to be tunable. The Sol-gel process itself is a cost-efficient approach to functionalize and endow surfaces with improved properties.

Polymer coatings are films of polymer (commonly called as plastics) applied as a layer onto surfaces that gives functional, decorative, or protective properties to the coated surface. 
Polymeric materials have a broad range of applications due to their versatile characteristics, cost effectiveness and customization.
Although the polymer synthesis science allows an excellent control of properties of the polymeric material in bulk form, when it comes to understanding the surface interactions and polymer in coating form, there is a large number of variations under study. All the applicability of these materials in coating form, their interactions to substrates and counter-faces (in tribological applications) are driven by the surface science and it can go from the formulation of the polymeric coating “paints” to their manufacturing as well as application.

There is a big variability of polymers (or mixtures of polymers) that can be used as a base for the polymer thin film, as well as fillers (organic or inorganic) that can have an active function on the coating. Examples of these fillers are pigments (typically inorganic) to promote color and opacity to the coating or Polytetrafluoroethylene, PTFE (organic) to reduce the friction in the surface of the coating.

Paint is a general term for mixture/suspension of different ingredients such as pigments, resins/binders/base materials, carrier/solvents and additives in a liquid or paste form.

There are two different types of paints:

• Powder coatings: composed 100% by a mixture of active materials in powder form. 
• Liquid coatings: composed by a mixture of 4 type of components 

1. Fillers provide functional and/or aesthethics properties (which can include pigments).
2. Binders "bind" the fillers together and create the paint film.
3. Solvent/Carrier* are liquids that suspend the ingredients and allow to apply paint on a surface and are completely released during the drying/curing of the coating. The solvents can be organic solvents or water.
4. Additives provide specific paint properties and/or can improve paint film quality.

* - There is a case of binders which does not need solvent/carrier and remains liquid to integrate the rest of additives.

In the category of liquid paints, it is important to refer that the binder can be available in two different forms: 

• Solution-based coatings: where the binder material is initially solubilized to form a liquid resin and then integrate the rest of the components.
• Dispersion-based coatings: where the binder material cannot be solubilized in any industrially available solvent and have to be dispersed into the solvent and then integrate the fillers. It is also mandatory in this step that the binder material have the capability to form a film and create a film. An example is a thermoplastic polymer material, such as Polyetheretherketone (PEEK).

Paints can be applied, usually, with a brush, roller or spray on a surface of an object to form a thin dry film.