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Platinum Metals Rev., 1970, 14, (2), 63

Light Duty Electrical Contacts

The Selection of Materials

  • L. B. H.

It is not so very many years ago that an electrical con –tact operating of course in a fairly simple circuit – could be more or less adequately described as either “good” or “bad”. More recently, and in particular during the last fifteen years, the study of electrical contact phenomena has been greatly stimulated by the rapid developments in electronics and in automation and control, so that a much more sophisticated approach has been necessary to the proper understanding and selection of contact materials.

In the light duty field – which may be broadly defined as involving currents of not more than 1.0 A at voltages of the order of 250, down to very small radio and audiofrequency currents – the primary consideration is the use of surfaces free from tarnish films in order to ensure minimum contact resistance. For this basic reason the platinum metals and gold, with a number of their alloys, comprise the materials from which a final selection has to be made to suit individual conditions. A useful and comprehensive survey of this subject has just been published in the series of Metallurgical Reviews, sponsored by the Institute of Metals, prepared by H. C. Angus of International Nickel (Met. Rev., 1970, 15, 13). This covers the production, properties, and behaviour of contact materials with a reasonable minimum of the mathematics involved in modern contact theory. The literature on contact physics has become extensive in recent years – particularly from the researches of Ragnar Holm and Professor Llewellyn-Jones – and it is valuable to have a full and up-to-date review.

The factors involved in the selection of contact materials are outlined – their density, electrical and thermal conductivities, hardness, melting and boiling points – and these are discussed in relation to the range of gold and the platinum metals and their alloys. While gold has the highest conductivities, it is soft and mechanically weak and has the lowest boiling point (a factor of more importance than might readily be supposed in relation to the very high local temperatures attained at contact surfaces), rhodium, ruthenium, and iridium are difficult to fabricate so that little or no advantage has yet been taken of their high boiling points combined with reasonably good conductivities. Thus platinum and palladium provide the great bulk of light duty contacts, but improved methods of fabricating the more refractory platinum metals should lead to their more extensive use as relay contacts.

A marked feature of contact technology in the last few years has been the great increase in the use of electrodeposited surfaces, particularly with alloyed golds, but improved electrolytic processes for platinum and palladium have extended the scope of these metals as contacts, while more recently methods for ruthenium deposition have enabled greater advantages to be taken of its useful properties.