High temperature corrosion of hardmetals

During application of hardmetal high temperatures up to 1000 °C may occur. Under these conditions water vapour is formed by vaporisation of cooling liquids. Degradation of material through high temperature corrosion takes place. Mechanisms and kinetics of these effects are not yet fully understood. It is shown in this article how different hardmetal grades are affected by water vapour and oxygen during heating and isothermal conditions. Thermogravimetry (mass changes) is used for in situ analysis of these reactions. Temperature dependence and kinetics of degradation are discussed. The formed surface layers are characterized by X-Ray Diffraction (phase composition) and FE SEM (morphology). Water vapour can accelerate or retard the mass change rate. The influence of water vapour dep ends on temperature und hardmetal composition. High reaction rates occur at temperatures higher than 600 °C. A comparison of the behaviour of sintered material and hardmetal components (Co, WC) is given.