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, Os1–xRexB2, and Os1‐xWxB2, 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 Os1–x(TM)xB2 phase varied, which indicated different thermal stabilities, such as OsB2 < Os0.1W0.1B2 < Os0.9Re0.1B2 < Os0.8W0.2B2 < ReB2 < Os0.6W0.4B2 and Os0.5W0.5B2. Among them, Os0.6W0.4B2 and Os0.5W0.5B2 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. [ABSTRACT FROM AUTHOR]