Grain Growth of Zirconium Diboride

Hot-pressed zirconium diboride compacts which had four kinds of relative densities were heated in graphite at temperatures ranging from 1400°C to 2500°C. The surfaces of the heated compacts and their textures were examined by X-ray diffraction and optical microscope. Weight and relative density as well as average grain size of each compact were then measured.Through heating at the lower temperatures ZrO2 appeared on the surface of the compacts and then changed to ZrC at the temperatures between 1700°C and 1800°C. Lattice constants of ZrC changed to the normal values from the smaller ones with increasing temperatures. From this fact, it was supposed that ZrN was also formed simultaneously in this temperature range and made solid-solution with ZrC. While oriented graphite layers covered the surfaces of the compacts heated at temperatures above 2400°C, thin films of graphite were observed over the molten surfaces of the compacts heated at 2500°C.Relative density of each compact reached about 0.97 when it was heated at 2500°C for 100 min. Data on grain growth were processed under the application of the following equation:D2-D02=ktm, the value of m being either 0.66-0.78 or about 1 for heating temperatures of 2100°-2300°C or 2400°C. When the compact was heated at 2300°C, grain growth was rapid at the surface layer compared with the inner part and the m reached about 1 at this layer.Grain growth rate constant, k in the above equation increased with the increase in the initial densities of the compacts. But the same temperature dependence was obtained for all the compacts heated at the temperatures between 2100°C and 2300°C. The activation energy in the process of grain growth was calculated to be about 60kcal/mol.