In all cases an intermediate nickel coating is used for protection in order to prevent diffusion of the material.
Hard gold has a passive surface, so it is an ideal electrical contact partner with very good chemical resistance. The special gold alloy, with a micro hardness of up to 400 HV, is much harder than pure gold, but there are limits with regard to wear behaviour. The abrasion behaviour can be influenced positively if the conditions of use are optimised, for example by the avoidance of radial movements.
Rhodium is one of the platinum metals. A very high level of wear resistance is achieved because of the very high degree of hardness of up to 1000 HV. However, because of the stored oxygen, a higher level of layer brittleness must be expected. This problem can be reduced by means of a special layer build-up and reduced layer thicknesses. However, rhodium plating is unsuitable when heavy impact loads are involved. If the brittle and very thin rhodium layer is damaged, this may have a negative effect on the initially good electrical contact characteristics.
Nickel is distinguished by even layer deposition with excellent contour accuracy, which is especially advantageous for the function of pointed tips and sharp-edged tip styles. This nickel-plating has a micro hardness of approx. 600 HV, which can, as an option, be increased to 1000 HV and more by means of subsequent heat treatment. This is accompanied by very good wear resistance. Maximum corrosion resistance is achieved by the intercalation of phosphorus. However, the resulting inactive surface does not result in such good contacts as those of gold or rhodium.
Silver provides the best electrical conductivity among metals, which is why it is used, for example, for high-current applications. In this case, the spring of the test probe used is silver-coated in order to increase conductivity. In addition, some of the probe tips in the high-current range are fitted with caps made of a silver alloy, because this prevents restrictions to the conductivity caused by contact pitting.