Chemical reactivities of hafnium and its derived boride, carbide and nitride compounds at relatively mild temperature

MB 2 /SiC composites are materials of choice for ultra-high-temperature structural applications, primarily in the aerospace arena. These composites are processed in a hot-press operation at a temperature range of 1900 to 2200°C. This article assesses potential mild-temperature (below 1500°C) chemical reactions that may lead to structures and coatings made of HfB 2 /SiC under pressureless or mild-pressure conditions. The reactions are anticipated to be involved in reactive and shape-forming processes, where ceramic precursors and/or reactive powders are incorporated. This article pays special attention to exothermic reactions as well as to formers of a liquid phase; both can aid the desired phase formation, microstructure development, and sintering of the composite under milder conditions than currently practiced. Reactions between loosely mixed powders with melting points significantly above 1500°C were detected by X-ray diffraction (XRD) analyses. Significant solid-phase reactions of the loose powder mixtures were observed at this mild temperature in powder form. Preliminary microstructural studies using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray Spectroscopy (EDX) techniques have confirmed the presence of unique reaction mechanisms between the loosely connected particles. Good examples are the reactions between Hf powder and powders of BN or B 4 C, all having melting points above 2200°C, which form at 1500°C, or below HfB 2 /HfN and HfB 2 /HfC crystalline domains, respectively. These reactions are less intuitive than the reaction with B 2 O 3 , which forms HfB 2 /HfO 2 , potentially via molten or gaseous phases of boron oxide.