Studies on synthesis, densification, and characterization of neodymium hexaboride

Neodymium hexaboride (NdB6) is an important rare-earth hexaboride material that has many structural and functional applications in advanced technologies. This paper presents the results on experimental studies of synthesis, densification, microstructural characterization, and mechanical properties of NdB6. Single-phase NdB6 powder was obtained by the carbothermic reduction of Nd2O3 and B4C at optimized conditions and charge composition. Densification of NdB6 powders was carried out by pressureless sintering and hot pressing. The highest density of 93% theoretical density (TD) was obtained at 1800 degrees C by hot pressing for 2 h in a vacuum, whereas 79% TD was obtained by pressureless sintering at 1900 degrees C. However, 99.9% dense NdB6 was obtained by adding a sintering aid of 5 wt.% B4C using a hot-pressing technique at 1800 degrees C. Microstructural characterization revealed no significant grain growth during hot pressing at 1800 degrees C, and a dominant transgranular mode of fracture was observed in fractography. Hardness values were measured as 23.8 and 28.8 GPa, respectively, by two different techniques micro- and nanohardness testers. Fracture toughness and elastic modulus of hot-pressed NdB6 sample were measured as 3.04 MPa m1/2 and 484 GPa, respectively. The flexural strength of dense NdB6 was obtained as 156 MPa by three-point bend test.