Ultra-high temperature HfB2-SiC ceramics consolidated by hot-pressing and spark plasma sintering

Two ultra-high temperature HfB2-SiC ceramics were successfully consolidated by hot-pressing (HP) and spark plasma sintering (SPS). The powder mixture HfB2 + 30 vol% SiC was brought to full densification with the addition of 2 vol% TaSi2 as sintering aid, and applying the following conditions: 2100 degrees C for 3 min (SPS), or 1900 degrees C for 35 min (HP). The microstructure consisted of regular micrometric diboride grains and SiC particles homogeneously distributed. The major secondary phases were HfO2, and (Ta, Hf)-mixed or Hf carbides in the materials processed by SPS and HP, respectively. Both SiC and TaSi2 beneficially contributed to boost the sinterability of HfB2 at elevated temperatures. The mechanical properties showed interesting potential. Elastic moduli above 490 GPa were measured. Flexural strengths at room temperature and 1500 degrees C (in air) of the hot-pressed composite were 665 +/- 75 and 480 +/- 30 MPa, respectively. Machining-induced flaws rather than fabrication defects adversely affected the room temperature strength of the spark plasma sintered material, leading to premature failure. The steep cooling up to 1000 degrees C in about 2 min associated to SPS induced large unrelaxed thermal stresses, which enhanced the tendency to micro-cracking during machining. However, such a strained configuration had a beneficial effect on fracture toughness. In the temperature range of 1450-1650 degrees C both the as-fired materials tolerated acceptably the oxidation attack in air. Thermo-gravimetric tests at 1450 degrees C for 20 h had mass gains of 4.10 +/- 0.02 and 3.30 +/- 0.02 mg/cm(2) for the materials processed by HP and SPS, respectively, and decelerating kinetics were recorded, although not conclusively parabolic. (c) 2006 Elsevier B.V. All rights reserved.