Thermal stability of ReB2-type transition metal diborides

The thermal stability of metastable ReB₂-type transition metal diborides (TMB₂), which are considered as new type of superhard material, is of vital importance to obtain bulk samples. In the present work, thermal stability of four kinds of ReB₂-type TMB₂ powders, ReB₂, OsB₂, Os_1–xRe_xB₂, and Os_1-xW_xB₂, were synthesized with varied transition metal (TM)-to-B molar ratio by mechanochemical methods and the subsequent annealing was compared. The as-synthesized powders were then consolidated using a pressureless sintering technique. The results showed that the B content required to obtain the pure hexagonal ReB₂-type Os_1–x(TM)_xB₂ phase varied, which indicated different thermal stabilities, such as OsB₂ < Os_0.1W_0.1B₂ < Os_0.9Re_0.1B₂ < Os_0.8W_0.2B₂ < ReB₂ < Os_0.6W_0.4B₂ and Os_0.5W_0.5B₂. Among them, Os_0.6W_0.4B₂ and Os_0.5W_0.5B₂ were found to be relatively thermally stable and could be synthesized with a stoichiometric molar ratio of (Os + W):B = 1:2. It was also found that the thermal stability of TMB₂ with a hexagonal ReB₂ structure could be mainly governed by the length of lattice constant c. The results have guiding significance for the design and preparation of new type of TM borides. In addition, the hardness of TMB₂ can be increased by tailoring the B content in the raw materials more precisely.