Phase transformation in tungsten carbide–cobalt composite during high temperature treatment in microwave hydrogen plasma

To study phase formation up to 2300 K, tungsten carbide–cobalt (WC–Co) samples were exposed to concentrated solar radiation in hydrogen plasma. The reducing atmosphere was a non-equilibrium hydrogen plasma created in a microwave cavity at the nominal power of 1000 W. The dissociation fraction of hydrogen molecules in plasma was of the order of 10% assuring for almost optimal reduction of any oxide that could be formed on the surface of samples due to the residual atmosphere. Micro-structural characterization was performed by XRD and AES depth profiling. The results showed the appearance of Co₆W₆C phase at the temperature of 1050±50 K. This phase almost vanished at the temperature of 1300±90 K and was replaced by the Co₃W₃C phase. This phase vanished at 1690±150 K where only WC and Co peaks were detected by XRD. The AES depth profiles showed enrichment of the surface film with Co at elevated temperatures. At extremely high temperature of 2300 K, the Co vanished from the surface layer but remained in crystalline form in the bulk material. SEM imaging showed an evolution of the material crystallinity up to perfectly recognizable crystals of the dimension of approx. 1 μm at the maximum temperature. The results are explained by mobility of Co atoms, surface segregation and sublimation at very high temperature.