Low-Temperature Reactivities of Ultra-High Temperature Ceramics (Hf-X System)

摘要

A growing interest in ultra high temperature ceramics and their aerospace and turbine applications has led to a renewal of activities to fabricate MB2/SiC composites as the materials of choice, because of their high thermal and oxidation resistance stability. Unfortunately, these composites are currently formed by an expensive, size and shape limited hot-press operation at a temperature range of 1900 to 2200 C. The project investigated chemical reactions at and below 1500 C that can lead to structures and coatings consisting of monolithic HfB2/SiC composite microstructures (and also HfC, ZrB2, and ZrC composites) under pressureless conditions. These can be employed in reactive and shape-forming processes in which ceramic precursors or reactive powders are employed. Special attention was paid to exothermic reactions and formers of in situ liquid phases, especially reactions involving elemental Hf and Si. Substantial reactions between loosely mixed powders or between powders and reactive surfaces were detected by X-ray diffraction analyses, microstructural studies, and energy-dispersive X-ray spectroscopy. These findings confirmed significant reactivities starting at 700 C between Hf metal or its ceramic compounds. Bulk melting of Hf at external temperatures of 1100 C and higher (over 1000 C) below its adiabatic melting point was observed in many of the reactions.