Applying finishes to metals can radically adjust their properties, providing manufacturers with an enormous palette of manufacturing solutions and improved performance characteristics.
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Possible surface treatments include painting, plating, heat-treating, anodising and powder coating. Choosing the right treatment can impart qualities such as conductivity or insulation, heat resistance, corrosion resistance, lubrication, wear resistance or improved appearance. The purpose of some treatments is to prepare a surface to successfully accept a subsequent one. For example, anodising can allow metal surfaces to absorb dyes.
Here is a quick description of some of the best options.
Fettling, linishing, blasting and peening
These are metal-working processes rather than substantially different surfaces. Nevertheless, subsequent treatments often depend on them. Cast components usually need deburring and fine adjustment by the fettler or grinder, and they sometimes shot blasting or lighter peening to provide a good clean key for the following process.
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Heat treatments
Heating and rapid cooling is a well-known method of hardening surfaces. It can also be used to eliminate internal stresses created by shaping processes.
Powder coating and painting
Aesthetics and durability are the two main reasons for applying wet paint or dry powder coatings. Anti-corrosion paint provides protection from humid and salty environments. Some coatings work by forming a barrier, while others depend on a chemical inhibitor. These coatings are often far more resilient if they are preceded by anodising, especially on aluminium.
Dry film lubricants
These can be used for several purposes, but the main one is wear resistance. Liquid oils are often unsuitable where there is electricity or heat. Dry coatings can perform a similar job, and they often do it better. Examples include fluoropolymers like PTFE, graphite, molybdenum and tungsten disulphides.
Anodising
Anodising is an electro-chemical method of creating a columnar oxide layer on the metal surface. Aluminium is the most common substrate, but titanium anodising is also common. Varying the chemistry, current, temperature and duration can provide radically different properties. A good description of these processes can be found at https://www.poeton.co.uk/standard-treatments/.
Anodised surfaces have better durability in their own right but also readily absorb a range of subsequent treatments, imparting desirable qualities that include artistic finishes and the extreme heat tolerances often required in aerospace. Look for specialists accredited by the National Aerospace and Defense Contractors Accreditation Program (NADCAP).
Conversion coatings
Hexavalent chrome has often been used to protect aluminium but is no longer recommended under hazardous chemical regulations like Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH). New alternatives include Surtec 650, Iridite NCP and Lanthane.
Thermal sprays
These methods use high-energy equipment such as plasma arcs to deposit a very firm surface layer. They are often programmed and applied robotically.
Plating
Metal surface layers can be deposited over other metals (or non-metals) electrochemically or auto-catalytically. Corrosion and wear resistance are the usual benefits, but they can also enhance appearance (such as with gold and silver jewellery) or conductivity. Other suitable metals include chrome, nickel, zinc phosphate, cadmium and copper.
