NEW CU NI GRAPHITE COMPOSITES FOR LOW VOLTAGE ELECTRICAL CONTACTS

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Copper-nickel/graphite composites are potential substitutes for silver based materials used in low voltage circuit breakers. The microstructure versus properties relationships in composties containing a dispersion of graphite particles were thoroughly investigated. New composites have also been prepared using graphite fibres. The electrical conductivity is found to be significantly affect by the graphite volume fraction and by the anisotropy factors of the microstructure while the electrical contact characteristics mainly depend on the graphite fraction adn are stable and sufficient in the graphite range 8 to 12%. The mechanical properties are decreased by the graphite content and the poor interface adhesion between the copper-nickel matrix and graphite. In order to promote interface adhesion composties were prepared using carbide forming elements (tungsten, silicon, chromium). The chromium content and processing conditions should be tuned in order to promote adhesion without inducing the extensive formation of a brittle phase along the interfaces. Copper nickel graphite composites when associated with silver carbide yields excellent result for contact resistance and wear.

The research covered the development of silver free electrical contacts for low voltage circuit breakers and minimisation of production costs of electrical contacts by increasing the effectiveness of composites which could replace silver in low voltage circuit breakers. There are, in theory, 6 alternative ways of obtaining a satisfactory composite by combining 2 alternative ways of presenting graphite (particles or fibres) and 3 alternative consolidation methods (pressureless sintering, hot unaxial pressing and a third, alternative method developed by the researchers). In practice, only 3 out of the 6 combinations were practicable. The first step in carrying out the project used copper-nickel/graphite composites after preliminary microstructural studies indicated their promising possibilities to replace silver in electrical contacts for low voltage circuit breakers. Although partners expected to extend the results to composites of similar characteristics, information obtained was hardly extendable to other composites. As a consequence, the second step of the research focussed on the study of only copper-nickel/graphite composites; good performances were obtained for low voltage breakers used in consumer terminal distributions. For such devices, the breaking and life demands are not too heavy. For higher voltage industrial breakers, however, further work is necessary and, moreover, their electrical performance is not yet satisfactory.

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