Investigation On CuW/CuCr Integrated Electrical Contact Materials For Ultrahigh Voltage Interrupters

With the development of high voltage interrupters to ultrahigh voltage and larger capability, the higher demands are put forward to the arc electrical contacts which acted as the key part in interrupters. Therefore, it is essential that novel CuW/CuCr integral electrical contact materials with excellent properties should be developed to guarantee operating reliability under more severe service conditions. Based on the common failure mode of electrical contacts, the interfacial bonding strength of CuW/CuCr integral materials and electric arc erosion resistance were investigated systematically in this paper.Firstly, the effects of alloying elements on the wettability of Cu/W system and its interfacial characteristics were studied. The results show that the wetting angle between Cu and W substrates decreases with increasing the single addition of Ni, Cr and Fe elments into Cu, and decreases further with raising the wetting temperatures. The interfacial charateristics between different sessile droplets and W substrate were characterized by SEM, EPMA and X-ray diffraction. The results reveal that mutual dissolution and reaction occurs at the Cu/W interfacial regions on some extent owing to the addition of Ni, Cr, Fe elments. The liquid-solid interfacial energy is decreased due to the formation of an alloying transition layer at the interface between Cu and W, and thus the wettability is improved. The wettability of Cu/W system can be improved by applying an electric field. The mutual diffusion and dissolution among Cu, Fe and W elements occurred at the interfacial region are accelerated, the extent and depth of interfacial alloying layer are improved by applying an external electric field, so the wetting angles between Cu-Fe alloys and W substrate are decreased further by applying an electric field.An ideal model for calculating interfacial bond strength of CuW/CuCr dissimilar materials was established. According to the theoretical model, the interfacial strength of CuW/CuCr integral materials containing different W contents can be predicated when the corresponding microstructures of Cu-W alloys were assessed. And the bond strength of CuW/CuCr materials depends on the distribution and area fractions of Cu, W phase at the interface, and on the strength of CuCr alloy side. A series of CuW/CuCr integral materials with alloying interlayers containing different Fe contents were prepared by the sintering-infiltration technique, the interfacial tensile strength of these integrated materials were tested, and microstructures adjacent to interfacial regions were analysed. The results indicated that the amount of Fe element diffused from the alloying layer to the CuW side is larger than to the CuCr alloy side. As for Cu/W interphase and CuW/CuCr interface, the metallurgical bond are achieved owing to a small amount of Fe addition. The CuW/CuCr integrated material with Cu-5%Fe alloy interlayer exhibits good interfacial bond strength. The W skeletons close to CuW/CuCr interface can be dissolved and eroded, and the amount of eutectic phase in the microstructure of CuCr alloy side can be increased if more Fe element is added into the interfacial alloying interlayers, and thus resulting in a poor interfacial bond strength for CuW/CuCr integrated materials. The temperature fields of CuW/CuCr electrical contacts under the action of high voltage electric arc were simulated by utilizing the finite element software ANSYS, and the temperature vs time curves at the interface of CuW/CuCr integrated materials with different W contents were obtained by numerical simulation. The results show that the interfacial peak temperature of Cu70W/CrCu material is 480℃under the action of electric arc for 11 seconds. The simulated results can provide the experimental basis for the subsequent investigation on the bond strength of CuW/CuCr integrated materials under thermal cycling introduced by electric arc during service.According to the numerical simulation results, the service process of CuW/CuCr integrated materials were simulated by thermal cycling tests, the changes of the bond strength of CuW/CuCr integrated materials and the microstructural evaluation of CuCr alloy were studied. The results show that the interfacial bond strength increases with increasing thermal cycles, and the fracture location leans to the CuW alloy side when the thermal cycling temperature is 450℃.After different thermal cycles at 500℃and 550℃. the fracture locations occur at the interface, and the fracture location are inclined to CuCr alloy part with increasing the temperature of thermal cycling. After thermal cycling at 600℃, the fracture of CuW/CuCr integrated material occurs at the CuCr alloy side, and the interfacial bond strength decreases gradually with increasing thermal cycling times. The bond strength of CuW/CuCr integrated materials is closely related to microstructures and properties of CuCr alloy after undergoing different thermal cycling conditions. Microstructures of Cu-Cr alloy were analyzed by OM, TEM and HRTEM. It reveals that the recrystallization does not appeared, finer and dispersed Cr precipitated particles with the fcc crystal lattice structure are fully coherent with the Cu matrix when the thermal cycling temperature is 500"C. However, when the temperature was reached up to 600℃, the recrystallization and grain growth appear, the Cr precipitated particles with bec structure are coarsened obviously and lost perfect coherency with Cu matrix.The vacuum electrical breakdown characteristics and arc erosion behavior of Cu-W alloys were investigated. For the Cu-W alloy, the electric breakdown occurs in the region of rich Cu phase, the splash of liquid copper was serious, and larger cathode craters were formed. As for Cu-W alloys added WC, TiC, CeO2 and Y2O3 particles separately into W skeletons, the breakdown take place in these additive phases with lower work function, the electric arc spread around the suface of these Cu-W alloys due to the dispersed distribution of these additive particles in the W skeletons. So the splash of liquid copper was slight, lesser and dispersive cathode craters were formed. At the same time, the breakdown strength of Cu-W alloys is enhanced slightly, the chopping current is decreased, electric arc life-span is prolonged. In order to strengthen the weak breakdown Cu phase, CuFeW material was prepared by the infiltration method in which CuFe alloy was infiltrated into W skeletons. Nano-scale precipitated Fe particles are dispersed in the zone of rich Cu phase, so the electrical breakdown occures discontinuously and selectively in the copper matrix phases, rather than continuously and concentratively in the rich Cu zone and Cu/W interphase. The lesser and dispersive cathode spots are formed, and the splash of liquid copper is slight.