Metallic plating

Thursday, July 04, 2013

Tags :  coatings

The applications of plating in a racing powertrain aren’t as widespread as we might find in an automotive engine. Engines in general are reasonably benign environments for metallic components: oil is an excellent anti-corrosion fluid and most of the stressed components in an engine or transmission come into contact with oil during use, so many of the applications of metallic plating in an engine are for corrosion resistance. Racing engines are also stripped and inspected much more regularly than their road-going counterparts, which means that corrosion can be much more easily monitored.

Cylinder bores were once commonly chromium plated, although this has been largely supplanted by a composite electroless nickel/ceramic coating or by one of a number of thermal spray coatings. There are though some specialist processes used for chromium plating of cylinder bores in motorsport.

Fasteners are one component that we can often find with metallic plating. This is commonly for corrosion resistance, but we can often find fasteners that have been plated to modify the friction coefficient or provide some lubricity in high-temperature applications where a grease will simply not withstand the heat applied. Cadmium and silver are commonly applied to high-temperature nuts for this reason.

Running surfaces on shafts are commonly plated with hard chromium, which can subsequently be ground to the correct size. This means that a thick, hard layer can be applied to the surface of a shaft that is not necessarily as hard as the seal or bearing requires. This is also often used as a repair scheme for damaged shafts that have been worn by seals.

Case-hardened components are often subject to a copper-plating process where areas need to be masked to prevent them becoming hardened. Such areas might require further machining, have features that require some toughness, or thin areas that might otherwise be ‘through-carburised’, rendering them brittle and prone to overload failure.

With the advent of hybrid systems and their growing use in motorsport, metallic platings have found another area of use in the racing powertrain. Again they play a number of different roles here in electrical and electronic applications. Soft metallic platings, commonly silver and gold, are used in order to provide a layer that conforms to provide good electrical contact, even under modest loads. For a typical metallic joint, the two counterparts will only reach 100% apparent contact when the yield strength of the weaker surface has been reached. This can be difficult to achieve over anything more than a very small area. However, if we use a very low-strength coating on the surface of a metallic part requiring good electrical contact, we can get much better contact than with unplated parts. This leads to lower contact resistance and thus lower electrical losses through ohmic heating.

Where contacts in electrical connections require a high-strength surface, especially where some sliding is involved in their engagement, a hard metallic plating may be applied to an otherwise low-strength material such as copper. The copper is selected for its electrical conductivity, but this may not prove durable enough for repeated connection/disconnection.

Written by Wayne Ward

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