Nowadays, parts made from common alloys like steel can be specified with an impressive number of surface coatings and treatments options. As material coating deposition technologies have developed and become more economically viable, more types of materials have become suitable for surface treatments. Advances in physical and chemical vapor deposition processes, and the advent of direct energy techniques like electron and laser beam processes have yielded new possibilities for a wide range of coating specifications. Indeed, surface coating applications span everything from basic coatings such as hot dipped zinc coatings that provide excellent protection of steel from atmospheric corrosion, to substantially more advanced, developmental applications such as the nanocrystalline diamond coatings that are employed in the creation of extremely hard faced cutting tools.
There are many common functional mechanical applications where a surface coating can improve the performance of a part or mechanical subsystem. Profiled below is a cursory look at various mechanical applications that benefit greatly from the applications of additional surface coatings and a few specific examples of economic coatings that can be employed.
Wear Resistance and Friction Performance: For mechanical applications where mating parts move against each other under high loads, such as shaft/bushing pairs, the mating surfaces of each will typically be coated or treated to increase the hardness of the mating surfaces, thereby improving wear resistance. These types of parts can be highly susceptible to surface wear and surface damage. If left unchecked, this can sometimes lead to failure of the material due to cracks that initiate at the surfaces of the mating parts. Thermal spray coating treatments using a chromium nitride (CrN) are commonly employed in these types of applications and more complex surface treatments of alloys like Nickel-Chromium-Boron (NiCrB) can also be applied to a surface where high loads are present and component performance is vital.
Additionally, when running speed is a functional concern, the friction between the parts must be carefully monitored; surface treatments primarily employed to increase wear resistance usually have the added benefit of decreasing the friction between the two mating parts. These coatings (typically an electroless nickel plating technique) create an exceptional finish with very low surface roughness values so that the friction between the parts is as low as possible. This is probably the most common reason to specify a surface coating or treatment like electroless nickel plating.
Corrosion/Oxidation Resistance: Another common reason for specifying a surface treatment on a component has to do with protecting the surface from corrosion or chemical damage when it is used in a chemically hostile environment. The most common surface treatment to protect against corrosion coats the base metal using a hot dipped zinc plating process where the more reactive zinc (Zn) corrodes sacrificially when the barrier is scratched or otherwise breached, protecting the base material. There are many other complete coverage coating methods such as thermal-spray coating processes that can be specified in order to achieve similar corrosion protection.
Other Applications: There are other more specialized applications where a surface treatment would greatly enhance component performance, including applications requiring thermal protection or material diffusion resistance in a saturated environment.
These general applications should provide a few starting principles that can be used to determine if a particular application would benefit from a surface coating treatment. Each specific application’s desired operating environment, designed load capacity and/or operating speed are all central functional design concerns that the engineer should pay close attention to when considering what coating material or process should be specified on critical subcomponents. The chances are that there is a surface treatment that has been developed specifically to help mitigate these concerns.