Microstructure evolution of ultra-fine grained HfC ceramics with high hardness sintered under ultrahigh pressure by plastic deformation densification mechanism

HfC ceramics has potential for serving in extreme environment but it is impossible to achieve full densification with fine grains. Currently, ultra-high pressure sintering is an effective method to promote densification of ultra-high temperature ceramics and suppress grain growth. In this study, fully dense HfC ceramics with high hardness and ultrafine grains was fabricated under 15 GPa at relative low temperature. The nanocrystalline HfC ceramic with grain size of 130 nm, high Vickers hardness of 23.5 GPa and fracture toughness of 2.54 MPa & sdot;m1/2 was sintered at 1500 degrees C. The microstructure characterization identified that HfC ceramics sintered at 1500 degrees C formed low angle subgrains, twin boundaries and stacking faults, which enhanced the hardness and refined grains. However, the low angle subgrain boundaries transformed to high angle grain boundaries and grains grow up with temperature increasing while the dislocation density decreased. These experimental phenomenon and calculation model demonstrate that plastic deformation was the dominating densification mechanism. The nonequilibrium grain boundary and lattice strain field results proved that the plastic deformation densification mechanism exhibited positive influence on the mechanical properties.