Chemical Reactivity: In Search of Better Processing of HfB2/SiC UHTC Composites

MB2/SiC and MB 2 /MC/SiC composites (M = Hf or Zr) are materials of choice for ultrahigh-temperature applications, primarily in aerospace designs. Conventionally, they are processed by hot pressing at very high temperature (1900 to 2300°C). This study continues the evaluation of "low-temperature" reactions (i.e., below 1500°C) between metallic Hf and various B- and C-containing reagents leading to the formation of hafnium boride and carbide phases. Such reactions may allow the formation of structures and coatings made of HfB 2 /SiC and HfB 2 /HfC/SiC to be processed under pressureless or low-pressure conditions at convenient processing temperatures. In an earlier study, large Hf grains were found to disappear during reactions between loosely mixed (i.e., not compacted by pressing) reactant powders, leading to the assumption that Hf melts during the exothermic reaction. Such local melting would accelerate the remaining reaction by increasing the contact interface area between the Hf and the B and C donors. Reactions between loosely spread powders of one type on top of substrates made of a material capable of forming either HfB 2 or HfC phases have been evaluated after heating at 1500°C in a conventional tube furnace under argon at atmospheric pressure. The study focused on forming a molten phase, although all the reactants studied have melting points above 2200°. The microstructure and phase evolution in the reactions between B, B 4 C, BN, and C powders with Hf thin plates, and inversely, between Hf metal powder and B, BN, B 4 C, and C surfaces at 1500°C, are evaluated and discussed.