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

Abstract

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 (ZrB 2–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 μm), along with controlled thermo-mechanical properties. Average flexure strength and fracture toughness at room temperature (RT) up to about 900 MPa and 4.1 MPa√m, respectively, were measured. Elastic moduli at RT approaching 500 GPa were also measured. At 1500 °C in air, flexure strength decreased to 250 MPa. 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 μm. 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.