Low Temperature, Low Pressure Fabrication of Ultra High Temperature Ceramics (UHTCs)

摘要

The US Air Force is interested in developing fiber-reinforced ceramic composites that perform at ultra-high temperatures (greater or more than 1500 degrees C) under oxidative conditions, especially for hypersonic vehicles. Two potential approaches are: (a) Utilizing existing carbon-fiber- reinforced carbon-matrix composites (C/C) or carbon-fiber-reinforced silicon carbide-matrix composites (C/SiC) coated by thick (>100 um) ultra-high temperature ceramic (UHTC) coatings, or (b) Replacing the C and SiC matrices of such composites with an ultra-high temperature matrix, processed by conventional composite techniques. The most investigated UHTCs are ZrB2/SiC and HfB2/SiC particulate composites (70:30 to 80:20 volume ratio). Wet processing via slurries is potentially a practical method for making such thick coatings and matrices. The project focused on developing slurry processing for thick ZrB2/SiC coatings on SiC and, to a limited extent, C/SiC composite substrates using preceramic and precarbon polymers combined with inert fillers and/or reactive metals. The evolved coatings were tested for their oxidation resistance under various conditions. A limited effort to assess the capability of bulk compositions made of slurries suitable for processing matrices for fiber-reinforced composites was also performed. Out of two distinctly different approaches and various compositions and preceramic polymers, the most promising stepwise approach was determined to be (1) forming ZrB2/C porous coatings ('preforms') processed from phenolic-based slurries, then (2) reacting the preform coatings with molten Si to form SiC and (3) converting residual Si to SiC. This technique resulted in highly dense, well adhering composite coatings that were 100 um thick and over. Thick coatings made by this approach provided much better characteristics and performance than other formulations and processes.