The chief o bjective is to develop silver-free electrical contacts for low voltage circuit breakers. In order to find new materials, the first step is to acquire a better knowledge of the mechanisms giving rise to contact material deterioration (oxidation, sticking, abrasion from arcing) to identify the physicochemical properties of the contact material involved in the phenomena, and finally to evaluate the contribution of the microstructure of the contact material to their properties and performance in operation. The second step is to define, from the previous results, the guidelines for new materials. The first part of the study is carried out on Cu-Ni/Graphite composites used as model materials.
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.
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 PROGRAMME CONCERNS :
- THE STUDY OF THE MECHANISMS GIVING RISE TO CONTACT MATERIAL DETERIORATION (OXIDATION, STICKING, ABRASION FROM ARCING OR MOTION).
- THE IDENTIFICATION OF THE PHYSICO-CHEMICAL PROPERTIES INVOLVED IN THE PHENOMENA.
- THE DETERMINATION OF THE RELATIONSHIP BETWEEN MICROSTRUCTURE, PROPERTIES AND MATERIAL PERFORMANCE IN OPERATION.
THE NOVEL ASPECTS OF THE PROGRAMME MAINLY RELATE TO :
- THE STUDY OF THE INFLUENCE OF THE MICROSTRUCTURE AND THE INTERFACE CHARACTERISTICS OF THE MATERIALS ON THE PERFORMANCE IN-SERVICE.
- THE CLOSE COLLABORATION OF THREE SPECIALIZED GROUPS OF SCIENTISTS INCLUDING METALLURGISTS, PHYSICO-CHEMISTS AND ELECTRICAL ENGINEERS.
CONTACT MATERIALS GENERALLY CONSIST OF AG- BASED COMPOSITES, TYPICALLY WITH AG/NI, OR AG/C WITH AG PLATED CU. SUCH MATERIALS COMBINE THE PROPERTIES OF HIGH ELECTRICAL CONDUCTIVITY OF THE SILVER MATRIX WITH THE ERROSION AND ARCING RESISTANCE OF HIGH MELTING OR REFRACTORY ELEMENTS (NI, C, W, ...) OR COMPOUNDS (CARBIDES, OXIDES). THE SUBSTITUTION OF SILVER, BY CHEAPER ELEMENTS WOULD RESULT IN SIGNIFICANT FINANCIAL BENEFITS. FURTHERMORE, THE LIMITED WORLDWIDE RESSOURCES OF AG MUST FURTHER RESTRICT THE FUTURE USE OF CONTACT MATERIALS CONTAINING AG. THESE TWO ECONOMIC AND STRATEGIC ASPECTS CONSTITUTE A STRONG INCENTIVE TO PROMOTE THE RESEARCH FOR NEW AG-FREE MATERIALS FOR MERLIN-GERIN, 3RD EUROPEAN PRODUCER OF LOW VOLTAGE CIRCUIT BREAKERS IN THE WORLD. FROM A PRELIMINARY STUDY CARRIED OUT BY MERLIN-GERIN AND LTPCM, A CU-BASED COMPOSITE SEEMS TO HAVE SATISFACTORY CONTACT RESISTANCE AND NON-WELDING PROPERTIES. CONSEQUENTLY, THE COMPLETE STUDY OF THIS MATERIAL, WHICH CONTAINS GRAPHITE PARTICLES (5%) DISPERSED IN A CU-NI (5-10%) MATRIX, IS OF INTEREST IN VIEW OF FURTHER INDUSTRIAL DEVELOPMENT.