Hafnium carbide (HfC) is a potential candidate of ultrahightemperature ceramics (UHTCs) and has attracted significantly widespread interest in recent years. Here, we have synthesized high-purity HfC samples with NaCl-type structure by using a high-pressure solid−solid reaction. The structural stability, equation of state, plastic deformation, yield strength, and bonding properties under high pressure are investigated by a series of in situ high-pressure synchrotron-radiation angle-dispersive X-ray diffraction experiments combined with first-principles calculations. The yield strength of HfC (∼18 GPa) is obtained from analyzing the plastic deformation behavior under high pressure. In addition, we have successfully prepared bulk HfC ceramics with high density using a high-pressure and high-temperature method. The synthesized sample possesses a desirable Vickers hardness of 24.2 GPa and an excellent fracture toughness of 5.0 MPa•m 1/2 . The present results offer insights into the achievable application of HfC ceramics under extreme conditions and provide a powerful guide for the further design and synthesis of other high-performance UHTCs.

Hafnium diboride (HfB2) is one of the most promising hard-brittle ceramic materials with unique physical properties. Here, we have synthesized the well-crystallized HfB2 by a high-pressure solid-state reaction and employ in situ high-pressure synchrotron radiation angle-dispersive X-ray diffraction to investigate the size-effect of HfB2. An abnormal physical behavior of HfB2 under high pressure is observed. The microsized HfB2 shows slight anisotropy along a and c axes; however, the nanosized HfB2 reveals a different compression behavior with pressure-induced shell structural transition from a disordered symmetric amorphous shell state to an asymmetric amorphous shell state. In addition, the results indicate that the pressure calibrations are sensitive to the grain size. The present findings offer insights into the physical behaviors of different sized HfB2, which may also provide valuable information for other transition metal borides under high pressure.

Polycrystalline cubic boron nitride (PcBN) has been synthesized at 14 GPa and high temperatures of 1300–2000 °C in a two-stage multi-anvil cell. Sintered PcBN synthesized at 1700–1800 °C and 14 GPa with a grain size of ∼200 nm is optically transparent with a transmittance of ∼70% at wavelengths of 400–1500 nm and has the Vickers hardness of ∼63–69 GPa. Analyses with scanning and transmission electron microscopy reveal that PcBN can be strengthened by introducing nanometer-scale grains and microscopic defects at high pressure and temperature. The optical transparency of the bulk PcBN synthesized at high pressure and temperature can be explained by the very thin intergranular films between grains. The present sintered PcBN is the second-hardest transparent material after diamond and can be used for windows in extreme conditions.

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