Hafnium reactivity with boron and carbon sources under non-self-propagating high-temperature synthesis conditions

The homogeneous and heterogeneous reactions of hafnium (Hf) with boron (B) or carbon (C) powders were studied under non-self-propagating high-temperature synthesis (SHS) conditions in searching for chemically aided processes of ultra-high-temperature ceramics at mild and practical conditions. The threshold interactions of the Hf/B and Hf/C powder mixtures consisting of relatively large Hf particles occur at 700 degrees and 800 degrees C, respectively, with no observation of particle melting as previously postulated for igniting SHS reactions. A microscale melting phenomenon at the Hf surface was observed by scanning electron microscopy analysis in heterogeneous reactions between Hf metal strips and B or C powders at the temperature range of 1100 degrees-1200 degrees C, more than 1000 degrees C below the incipient melting point of the metal. The non-SHS conditions allow halting the reactions at intermediate stages for analytical purposes. Kinetic studies of the homogeneous and heterogeneous reactions, heat transfer model analyses, and their association with the investigation of microstructural phenomena were used to postulate the mechanism involved in the observed reactions.