GB402894A - Improvements in electrical make-and-break contacts and method of making same - Google Patents

Abstract

In the manufacture of metallic make-and-break contacts made of tungsten or molybdenum and adapted for ignition systems in motor vehicles, the material is treated so as to increase the size of the surface granules or to reduce the amount of amorphous material between the granules, such treatment consisting in heating the tungsten &c., with a compound of silicon, such as sodium silicate, which is reduced to oxide of silicon by the heat. The metal is then mechanically worked to form a body having an interlocked grain structure. In a preferred process a mixture of oxide of tungsten with 0.5 per cent of sodium silicate is fired in a silica crucible for about an hour at a temperature of 1100 DEG C. and then re-heated for a longer period in a reducing atmosphere of hydrogen at a temperature increasing from about 400 DEG C. to 900 DEG C. The mixture is removed from the reduction furnace and, after sieving, pressed into slugs under considerable pressure; the slugs are then baked in a non-oxidizing atmosphere for 15 minutes at about 1250 DEG C. and finally heated electrically in an atmosphere of hydrogen or equivalent to a temperature of 3000-3200 DEG C., care being exercised to avoid equiaxing of the granules but to maintain the interlocked structure, while at the same time a considerable proportion of the silicon is volatilized. After rapid cooling, the slugs are hot-swaged into rods, and the process of electrical heating to a temperature of about 2400 DEG C. in hydrogen repeated.