The Evaluation of Silver/Diboride Composites for use in Electrical Contact Applications

Silver/refractory metal composites are widely used in protective switchgear. These materials, as typified by silver/tungsten, are plagued by the formation of a high resistivity surface layer caused by the propensity of tungsten to oxidize under arcing conditions. One approach to decreasing the interfacial resistance of silver/refractory composites is to replace the tungsten by the more oxidation-resistant Group IVB transition metal diborldes, ZrB2, HfB2, and TiB2. To evaluate the suitability of silver/diboride composites for use as electrical contacts, several electrical properties of the silver/diborides have been measured and compared with those of silver/tungsten. In tests simulating a switching breaker application, the silver/diborides show a significant decrease in millivolt drop and temperature rise compared to silver/tungsten. The erosion rate of the silver/diborides is, however, higher than that of silver/tungsten. The improvement in the interfacial properties of the silver/diborides is related to the surface chemistry of the material. The silver/diborides exhibit a significant amount of free silver on the surface, an oxide layer no more than a few nanometers thick, and a distinct phase segregation at the surface. In contrast, the silver/tungsten material displays an oxygen permeated reaction zone extending 75 m into the bulk. The concentration of silver on the surface of the silver/tungsten samples is lower than that found in the silver/diborides. Evidence also suggests that, in the case of silver/tungsten, an appreciable amount of the surface silver is bound into insulating compounds of silver, tungsten, and oxygen and therefore is not available for participation in the conduction process.