Compatibility of Refractory Metal Boride/Oxide Composites at Ultrahigh Temperatures

At 2073 K, the compatibility couple of (symbolized as O/Si/B/O) shows that the Hf‐Si‐O system is complicated by the existence of a two‐liquid region in the pseudoquaternary phase diagram. The diffusion of the oxide ion through the silicate liquid phase appears to control the development of product phases between the and phases at 2073 K. In addition, the reaction product consists of two liquid phases, silicate and silicide, between the and phases. In the formation of the two liquids, the hafnium ions partition into the silicide phase leaving the silicate anions to polymerize and form complicated silicate rings which establishes the free oxide ion activity. Hafnium boride was determined to be relatively inert to either the oxide and silicide phase when examined with an optical microscope, scanning electron microscope, and electron microprobe. At 1873 K, the compatibility couple of O/Si/B/O showed no measurable movement of the oxide/silicide interface and suggests that a stable solid‐solid immiscibility of and exists. The immobility of the interface also suggests that a liquid is necessary for the interfacial reaction to occur and dissolve hafnia.