On the formation and in situ observation of thin surface layers of cobalt on sintered cemented carbides

The formation of a surface layer of cobalt binder during sintering of cemented carbides is widely known and has been thoroughly discussed in literature. The phenomenon occurs upon cooling, however the details of the mechanism are still uncertain. To further investigate the mechanism, different cooling experiments and atmospheres were combined with in situ observations of sample surfaces during cooling, acquiring new data about the timescale and temperature range in which the formation of the layer takes place. Based on these results, the authors propose a new mechanism for the cobalt layer formation.After an initial transport of liquid cobalt towards the surface, caused by the slightly decarburizing conditions during vacuum sintering, the high surface tension of liquid cobalt governs the formation of the layer. If the formation of a cobalt surface layer is provoked, this implies limits for the achievable layer thickness and also confirms known measures to prevent the cobalt layer formation. Fast cooling or cooling under carburizing conditions can prevent the cobalt layer formation.

► In situ observation of the cobalt layer formation above 1300 °C.
► Observation of the timespan and temperature interval the cobalt layer formation takes place.
► Study of the effects of different cooling rates and different sintering atmospheres on cobalt layer formation.
► Proposal of a new mechanism for the cobalt layer formation.