Processing and properties of ultra-high temperature ceramics for space applications

The processing and the properties of two ultra-high temperature ceramics (UHTCs) designed for the manufacturing of aerospace sharp-shaped hot-structures are presented, along with the results obtained in the electrical discharge machining (EDM) of these UHTCs into sharp hot-structure components.The powder mixtures in the (ZrB2-SiC)-based systems were brought to full density by hot-pressing. The hot-pressed bodies were characterized by fine and uniform microstructures (typical grain size 5 micron), along with controlled thermo-mechanical properties. Average flexure strength and fracture toughness at room temperature (RT) up to about 900MPa and 4.1MPavm, respectively, were measured. Elastic moduli at RT approaching 500 GPa were also measured. At 1500°C in air, flexure strength decreased to 250MPa.Beyond basic mechanical properties, the machinability of UHTC blocks into a more complex shape by means of the EDM was also assessed. Sharp-shaped hot-structures in the form of a nose-cone were produced from 12 cm×10 cm hot-pressed cylindrical blocks. The machined surfaces showed limited roughness Ra 1 micron.With the exception of the typical size of critical flaws induced by EDM seeming slightly larger than those of specimens machined with conventional diamond-loaded tools, the EDM technique proved to be as effective in machining UHTC sintered pieces into more complex components.