Cobalt capping: Why is sintered hardmetal sometimes covered with binder?

Some sintered hardmetal grades come out of the furnace covered with a thin layer of binder at the surface, while others do not. The layer can be beneficial for subsequent brazing, but it must be removed by grinding or etching if the hardmetal is to be coated. It would be beneficial to be able to control this “cobalt capping” during sintering by applying suitable sintering conditions. For this purpose, the mechanism of cobalt capping was investigated. The capping occurs on the cooling ramp, at the eutectic temperature of the binder. If the binder solidifies first in the interior, the contraction caused by the phase transition squeezes melt out of the exterior zones. The contraction of the specimen was verified with dilatometry. Conditions leading to cobalt capping are: higher carbon content in the surface than in the bulk (decrease of melting point), high binder content, especially in the surface zone, and a low cooling rate.