Studies on the phase formation of cobalt contacted with zinc vapour

Abstract

In cemented carbides cobalt serves as a binding agent between tungsten carbide grains. The zinc process exhibits an important technique to recycle these materials. The decomposition takes place at temperatures of 900–1000 °C and the role of gaseous zinc in this process is poorly investigated. A specific experimental set-up was used to ensure that only gaseous zinc reacts with solid cobalt. By varying the temperatures, times and Zn:Co ratios, it was possible to ensure the formation of intermetallic phases. According to the binary CoZn phase diagram, phases of different composition are formed, depending on temperature and pressure. It was found that not all of the indicated phases occur simultaneously, but several do. With the support of the findings from the layer evolution between two solid as well as solid and liquid substances, it is explained which layers may form in the CoZn system. The multiple phase formation depends on diffusitivity and other factors such as the different melting points, the atomic radii and the occurrence of cracks. Of these, the occurrence of cracks across or between two layers represents the most likely reason.