The use of soft metallic coatings

Wednesday, July 09, 2014

Tags :  coatings

Much of the focus in recent years in terms of coatings used in motorsport powertrains has been on the hard low-friction coatings, such as titanium nitride, chromium nitride and diamond-like carbon. These have served to reduce friction and increase the allowable contact fatigue stress in some critical components.

Metallic coatings of any description have struggled to be newsworthy though, as they are unable to come close to the new engineering coatings in terms of hardness, friction or wear resistance. However, there is a wide range of soft metallic coatings that remain popular and to which the new hard coatings pose no threat. In this article we will look at some uses of these coatings.

Gold plating is something we might associate with cheap jewellery or cutlery that is more for show than use. However, it is used in motor racing, especially in electrical contacts, where its very low strength and stiffness allow it to conform under low loads. Gold-plated components brought into contact with each other quickly deform under light pressure to give large contact areas. This leads to a lower contact resistance and therefore an increase in efficiency. Gold-plated connectors are used for a range of purposes, from electrical power transmission to sensor wiring and earth terminals.

Silver plating, which is again something we might normally associate with jewellery or trinkets, is widely used on fasteners in industry. Such coatings are not the shiny, chemically brightened type used for decorative products but are quite dull, which may or may not disappoint you depending on whether you like your racing powertrains to look like they were made by Harley-Davidson.

Silver-plated fasteners are used widely to prevent seizure, either due to material compatibility problems (stainless bolts installed in stainless tapped holes are a particular problem) or where thread lubricants can’t stand the high temperatures in service. In this latter case, we find silver-plated fasteners used on exhaust manifolds, especially on boosted engines. A number of suppliers offer silver-plated nuts in various styles for this purpose.

The most common use of soft metal plating is on plain bearings. The coatings on bearing shells may consist of one or more extremely soft coatings, commonly based on lead. These very low shear-strength coatings allow the engine to run with little damage during start-up, and they are used in most forms of motorsport for crankshaft main bearings, con rod big-end bearings and sometimes for camshaft bearings. Their combination of coating thickness and softness allows hard particles to become embedded in them, preventing the particles from going any further and causing damage. This ‘embeddability’ can prove crucial in terms of engine durability, preventing critical bearing surfaces from becoming scored.

The cages of rolling element bearings are also often coated with soft metal plating. Copper is widely used for needle roller bearing cages, and silver is also found in this application too. Such bearings are often used to support the gears in cam drives. Silver plating is sometimes found on valve collets as well, helping them to conform to the valve and retainer geometry.

Written by Wayne Ward

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