Densification mechanism and enhanced properties of fine‐grain hafnia ceramics prepared by high‐pressure sintering

Hafnia has become a promising thermal protective material due to its low thermal conductivity and high mechanical strength. However, it is difficult to balance the densification and grain growth of the material with the traditional sintering technology. In this study, HfO 2 ceramics with high‐density (97.4%), ultra‐fine grain size (149 nm), enhanced mechanical properties of hardness (12.23 GPa), and fracture toughness (3.00 MPa m 1/2 ), low thermal conductivity (3.75 W m −1 K −1 ) and excellent ablation‐resistant properties were prepared under a high pressure (200 MPa) and a low temperature (1350°C) by spark plasma sintering (SPS) from nanocrystalline raw power (77.4 nm). According to calculation, the relative density could reach up to78.9% after the contribution of plastic deformation under high pressure of 200 MPa at 1350°C without thermal diffusion. The high pressure induced high‐density lattice distortions such as lamination faults and twins in HfO 2 ceramics. These lattice distortions contributed to high mechanical strength and low thermal conductivity.