Characterization of thermophysical and mechanical properties of hafnium carbonitride fabricated by hot pressing sintering

Dense HfCxN1−x carbonitride ceramics are very promising as potential ultra-high temperature ceramics (UHTCs) for application under extremely harsh environments. However, the thermophysical and mechanical properties of the HfCxN1−x carbonitrides have not been investigated clearly. The present work prepared HfCxN1−x (x = 0.3, 0.4, 0.5, 0.6, 0.7) ceramics at 1950 °C under 30 MPa in flowing Ar atmosphere by using hot pressing sintering method. The relative densities of the samples obtained reached above 96%. Thermal conductivity of the as-prepared HfCxN1−x carbonitrides ranged from 19 to 24 W m−1 K−1 at room temperature. The increased role of electrons in thermal conduction caused by both increasing nitrogen content and increasing temperature, resulted in improved thermal conductivity, varying from 32 to 39 W m−1 K−1. With increasing nitrogen content, the electrical conductivity also increased, ranging from 149 to 213 × 104 Ω−1 m−1. With the increase of nitrogen content, Hf-C covalent bonds are gradually replaced by Hf-N covalent bonds with lower bond strength, resulting in HfC0.7N0.3 exhibiting the highest room-temperature flexural strength and hardness, HfC0.3N0.7 exhibiting the highest fracture toughness. Their mechanical properties are greatly improved over the binary HfC and HfN. The high-temperature flexural strength of the HfC0.7N0.3 decreased from 324 MPa at 1000 °C, to 139 MPa at 1600 °C and 100 MPa at 2000 °C. Meanwhile, it was revealed that the high-temperature flexural strength decreased with increasing nitrogen content for the as-prepared HfCxN1−x carbonitrides, similar to the changing trend of room-temperature flexural strength. The HfC0.3N0.7 possessed high-temperature plasticity at 2000 °C, attributed to the ability of the coarser grain to produce numerous layer dislocations.