Silver Refractory Materials
| The silver refractory contact materials are made by powder metallurgy. This makes it possible to combine silver with such refractory materials as tungsten and tungsten carbide. These materials offer the contact user the high conductivity of silver and the high melting temperature of the refractory material. Use of these materials is in the high current ranges and heavy duty devices. Due to the high melting temperature of the refractory material, there is less chance of welding and greater resistance to arc erosion. Selection of the optimum material is a compromise between conductivity and non-welding properties. Higher silver content will produce less temperature rise whereas the higher refractory will give less erosion.
Contacts made of these materials are necessarily of composite construction where they are brazed to base metal backings such as screws or rivets. DMC-280 (35% silver, 65% tungsten) is normally used in air circuit breaker in the fifty to hundred ampere range. It contains enough silver to be a good conductor yet resists erosion and welding. DMC-87 (50% silver, 50% tungsten), with a higher silver content, is a better current carrying material and is used in relays where other silver alloys would not withstand arcing. DMC-316 (40% silver, 60% tungsten carbide) is a material with very high hardness and provides maximum arc erosion characteristics. Since the tungsten carbide does not oxidize as readily as tungsten, this material will give less surface resistance than a silver tungsten of comparable composition. DMC-316 is used mainly in high current circuit breakers. DMC-281 (50% silver, 50% tungsten carbide) and DMC-86 (65% silver, 35% tungsten carbide), both containing more silver than DMC-316 are capable of carrying high loads without undue temperature rise. These materials are widely used in industrial contactors. Properties of Silver Refractory Materials
Note: Hardness values are for comparative purposes only and should not be used in design specifications. |