Ultra-high temperature HfB_2-SiC ceramics consolidated by hot-pressing and spark plasma sintering

摘要

Two ultra-high temperature HfB_2-SiC ceramics were successfully consolidated by hot-pressing (HP) and spark plasma sintering (SPS). The powder mixture HfB_2 + 30 vol percent SiC was brought to full densification with the addition of 2 vol percent TaSi_2 as sintering aid, and applying the following conditions: 2100 deg C for 3min (SPS), or 1900 deg C for 35min (HP). The microstructure consisted of regular micrometric diboride grains and SiC particles homogeneously distributed. The major secondary phases were HfO_2, and (Ta, Hf)-mixcd or Hf carbides in the materials processed by SPS and HP, respectively. Both SiC and TaSi_2 beneficially contributed to boost the sinterability of HfB_2 at elevated temperatures. The mechanical properties showed interesting potential. Elastic moduli above 490 GPa were measured. Flexural strengths at room temperature and 1500 deg 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 deg C in about 2min 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 deg C both the as-fired materials tolerated acceptably the oxidation attack in air. Thermo-gravimctrie tests at 1450 deg 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.