Silver coating is primarily a lubricant for high temperature applications on corrosion resistant fasteners in steel and other structures where galvanic corrosion is not a problem.
It is also used because it does not sublimate. The specification is usually AMS2410. Other coatings such as Electrofilm are really lubricants and provide minimal corrosion protection. Soft metals like Lead (Pb) can be sheared easily, so that in certain circumstances they can also be used as solid lubricants. However, they cannot support heavy loads without deforming, so they are used as thin coatings on stronger metal surfaces. Soft metal films are mainly used as lubricants only in a vacuum.
Fastener qualification data has been generated on the use of Ion Vapour Deposited (IVD) aluminium (IVADISE) as a replacement for cadmium in general engineering use (Reference 14.2).
The conclusions can be summarised as follows:
- a. IVD aluminium does not produce any detrimental effects on the mechanical properties.
- b. The coefficient of friction of aluminium is higher than cadmium, therefore higher installation forces are necessary. These higher values, however, are within the working ranges presently used for cadmium in most cases. A closer attention to the type of lubricants used can be necessary for interference fit fasteners.
With a silver-plated A286 nut, an IVD coated titanium bolt, and using EVERLUBE 812 as a lubricant, a consistently low coefficient of friction can be obtained for multiple installations. Kalgard 2240 (an aluminium rich paint) can be also used instead of the IVADIZE coating (Reference 14.3).
Refer to Reference 14.4 for processing requirements for platings and coatings. Since plating process bears the risk of hydrogen embrittlement sustained tensile load tests can be needed (see references 14.5, 14.6 and 14.7).
Note that the use of bolts with coatings based on chemically deposited lubricants is preferred w.r.t. the implementation of a lubricant during the installation of the fastener. The deposited layer guarantee a more controlled low coefficient of friction also for multiple installation. Typical examples are deposited MOS2 and Dicronite.
Other plating and coatings are cadmium, zinc, nickel, phosphate, chromium, etc. for details see Reference 14.8.