Contribution to the thermodynamic study and oxidation behavior of HfB2-SiC Ultra-High Temperature Ceramics

International audienceUltra-High Temperature Ceramics (UHTCs), more specifically diborides of the IVb group, are potential candidates for a large variety of applications, in particular as structural materials for thermal protection of hypersonic vehicles exposed to high temperatures (> 1800°C) and to oxidation1. The aim of this work consists in elaborating ultra-high temperature HfB2-SiC ceramic composites used as structural materials for several applications, such as aeronautic and aerospace areas and to understand oxidation mechanisms involved during their entry into the atmosphere. The sintering of these materials at different compositions has been carried out by Spark Plasma Sintering (SPS). This process is used to obtain fully dense materials (> 98% relative density) with fine grains, at lower temperatures and shorter dwell times than conventional techniques (natural sintering, hot pressing)2. In order to favor densification and to obtain a homogeneous distribution of both phases in the final material, a study on the raw powders granulometry has been necessary. An optimization of sintering conditions (applied load, temperature and its hold time, heating rate) has been subsequently carried out in order to elaborate fully dense samples by using parameters adapted to their environment. Obtained bulk bodies have been oxidized at several temperatures with a solar furnace to approach conditions to which hypersonic vehicles are subjected. Oxidation mechanisms have been highlighted through the study of the oxidized layers by combining X-ray diffraction analysis (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectrometry (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). This work was helped by the obtaining of kinetic data during oxidation tests. In order to supplement this study, a first thermodynamic approach of the Hf-B-Si-C system has been made through the modeling of ternary diagrams, the aim being to constitute a coherent data base of this quaternary system.References1 F. Monteverde and A. Belosi, “The resistance to oxidation of an HfB2-SiC composite”, J. Eur. Ceram. Soc., 25, pp.1025-1031, 2005.2 D. Sciti, and al. “Spark Plasma Sintering of HfB2 with low additions of silicides of molybdenum and tantalum”, J. Eur. Ceram. Soc., 30, pp.3253-3258, 2010