Fabrication of Ultra High Temperature Ceramics by spark plasma sintering: non-reactive and reactive routes
MENARD T. 1,4, REBILLAT F. 1,4, TÉNÈZE N. 2, LEDAIN O. 3, MAILLÉ L. 1,4
1 Laboratoire des Céramiques Thermostructuraux, Talence, France; 2 CEA/DAM, Le Ripault , Monts, France; 3 CEA/DAM, CESTA, Le Barp, France; 4 Université de Bordeaux, Talence, France
The undergone temperatures by the leading edges during atmospheric re-entry or the hyperveloce flight can exceed 2 000 °C. In order to protect the vehicle, it is necessary to use refractory materials instead of C/C composites, usual candidates. Indeed, Carbon is degraded above 3 000 °C in vacuum, and it is poorly resistant to oxidation above 400 °C with CO (g) and CO2(g) formations. Ultra-High Temperature Ceramics (UHTC) are presented as promising candidates either to replace these C/C composites or to become the material of matrix or to bring a surface protection. UHTC are commonly described as refractory materials with a melting point around 3 000 °C. Among them, the most interesting constituents are carbides (ZrC, HfC) and/or borides (ZrB2, HfB2). These materials form during oxidation an oxide scale, ZrO2 or HfO2, continuous but porous. To increase the efficiency of the protection against diffusion of oxidizing gaseous species, SiC can be added in the system, to form a SiO2 glassy phase in order to fill the porosity of the porous ZrO2 or HfO2 scale and consequently, to increase the oxidation protection. Unfortunately, these UHTC materials are difficult to get dense by natural sintering. Today, spark plasma sintering process (SPS) is often used to obtain high ratios of densification: around 1 900 °C during few minutes.
The goal of this work is to fabricate UHTC by SPS, using reactive sintering routes, in order to lower the process temperature. The chosen system is (Zr,C,Si,B), with a well-known reference material as ZrB2+SiC (20 vol.%). A thermodynamic approach on the solid equilibria helps in the choice to initial mixed elements and/or compounds. In the considered reactions, the most stable compound at the considered sintering temperature is ZrB2, then ZrC and finally SiC. The microstructures of the various systems obtained by reactive and non-reactive routes are compared (SEM, EDS, XRD, …). An oxyacetylene torch is used to test under oxidizing environment the materials. A relation is established between the composition, the sintering route and the oxidation resistance.