RELATIONSHIP BETWEEN HARDNESS AND STRENGTH CHARACTERISTICS OF Cu-Cr MICROLAYER COMPOSITE MATERIAL AT ELEVATED TEMPERATURES

摘要

Copper and chrome composites are widely used as the most efficient electric contact materials for arcing contacts of arc-extinguishing chambers in vacuum circuit breakers. In addition to the conventional methods of powder metallurgy, these materials are produced by high rate electron-beam evaporation/condensation of copper and chrome from separate water-cooled crucibles and layer-by-layer condensation on a metal substrate (a rotating steel disc). The electron-beam physical vapor deposition (EBPVD) technology makes it possible to produce, within one production cycle, sheet copper-chrome composite materials with a given microlayer structure and chemical composition, and condensates with the content of gaseous impurities no higher than that in the initial material, even in the case of evaporation of active metals such as chrome. One more peculiar feature of this method is that it allows the creation of combined contacts with a working layer of an arc-extinguishing composite material obtained by physical vapor deposition, which is cohesive with the copper substrate that provides heat removal from the working layer. The optimum content of chrome in these composites that provides the most favorable combination of electrical, mechanical and chemical characteristics of the material is from 30 to 40 mass%, the tensile strength of composites being 400 to 550 MPa, hardness from 1600 to 1800 MPa, and the resistivity not exceeding 2.5 .10~-8 Ω.m. A high-temperature annealing of the condensed materials Cu-(30 to 40 mass%) Cr is responsible for the decrease in their strength and resistivity down to 30%, but, in fact, does not influence the hardness. Owing to the specific structure of these materials, their unique physico-mechanical and operating characteristics are formed. At present, condensed Cu-Cr microlayer composite materials have been produced as pilot lots by the "Gekont" Research and Production Enterprise (Vinnytsya, Ukraine) [1-7]. In the process of operation, materials of contact pairs for high-current vacuum circuit breakers are subjected not only to intense corrosion but also to mechanical loads at elevated temperatures. In this connection, the investigation of their mechanical characteristics at operating temperatures is of scientific and practical interest.The paper presents the results of the experimental investigations into the structure, strength, hardness and plasticity of condensed Cu-Cr-base materials for electric contacts in the temperature range from 290 to 1070 K.