Influence of hafnium carbide on vacuum plasma spray processed tantalum carbide microstructures

Five specimens of (TaC)1−x(HfC)x, where x is 0.0, 0.3, 3.0, 16.5, and 19.8 at.%, were fabricated by vacuum plasma spraying. As HfC content increased, the grain size was reduced and the volume fraction of TaC, Ta2C and Ta4C3 changed, with the TaC phase being more dominate. Reduced grain sizes also lead to an increase in Knoop hardness values. The reduction of grain size with increasing HfC content has been explained by the system being driven further into a compositionally lower melting temperature phase field. This increase in liquid fraction caused greater under-cooling and the formation of more nucleation sites that lead to a finer grain size. The changing volume fraction of (TaC)1−x(HfC)x and sub-stoichiometric tantalum carbide phases has been contributed to the loss of carbon intrinsic to vacuum plasma spray processing.