There are many different methods by which a metallic coating can be applied as a surface treatment to increase the mechanical, electrochemical, and thermal performance of a material. The technique used largely depends on material application as well as the desired material coating or depth of the surface treatment. Although the subject of metallic and non-metallic coating techniques is a big subject possessing many technologically advanced methods, there are a few basic principles and techniques that, when understood, can be the key to a broader understanding of surface coating processes.
At the risk of over generalizing, the two most commonly specified families of surface coating techniques can be categorized as either a plating processes or a vapor deposition processes. However, within these two distinctions, there are further sub-distinctions as well; common examples of each process are provided and profiled below.
- Hot-Dipping: This process is typically a continuous or batch immersion process where the base alloy (typically steel) is fully submerged into a molten zinc bath. The hot-dipped zinc coatings form a coherent coating due to metallurgical bonds that form between the base metal and the zinc at the high bonding temperature. This technique is widely used across applications that demand a high level of atmospheric protection from corrosion of the base steel. Although most commonly used on steel alloys, variations galvanic hot dipping process are also feasible for coating other metals, such as aluminum.
- Electroplating: This is an electro-chemical process where metallic ions are deposited onto the surface of the bulk material via a cathodic polarization reaction. Chromium is a metal that is frequently electroplated onto materials to add wear resistance or even simply a decorative finish to the material, like what is commonly seen in a large number of “chrome-plated” automotive parts. Thicker electrodeposited chromium coatings provide excellent abrasion and wear resistance.
- Cladding: This process is literally the joining of two dissimilar metals by mechanical means such as rolling or extruding the materials together under high temperatures which results in a pressure-welded joint of two dissimilar metallic alloys. This method is advantageous because the “composite” material retains all the beneficial properties of both of the requisite materials.
Vapor Deposition Processes:
- Physical Vapor Deposition (PVD): This is the category of processes where the material surface coating is applied through the condensation of the vaporized alloy coating onto the material surface, in a physical process involving the movement of particles across physical material phase boundaries (from gas to solid). To put it simply, the surface coating alloy is first vaporized and then sprayed onto the surface of the base metal (typically within a vacuum). There are other variations and subsets of PVD, but this is the basic operative principle behind the concept. PVD is commonly used to apply thin coatings of titanium nitride (TiN) in order to harden metal cutting tools and greatly increase wear resistance of the tool.
- Chemical Vapor Deposition (CVD): In contrast with PVD, in CVD there is an actual chemical interaction between a mixture of gases and the bulk surface of the material which causes chemical decomposition of some of the specific gas constituents, forming a solid coating on the surface of the base material. CVD is employed in a wide range of industry applications, such as the deposition of refractory materials (non-metallic materials that can withstand extremely high temperatures) on turbine blades in order to greatly increase the wear resistance and thermal shock resistances of the blades.
There are other types of surface treatment processes that can be employed to improve a material’s mechanical, thermal, and/or electrical characteristics such as diffusion, ion implementation, or conversions coating techniques, but these processes are not strictly material coating processes where one dissimilar bulk material is overlaid on top of another. Nevertheless, this brief survey of material coating technologies should provide design engineers with a better understanding of the processes behind surface coatings, enabling them to better specify both the desired coating and coating process for each desired application.