Tantalum Boride and Its Ultra-High Hardness

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tantalum boride (TaB) and its oxide (TaB2O3 ) are compounds of tantalum and boron most remarkable for their extreme hardness (30 GPa). They are stable to oxidation below 700 degC, and to acid corrosion. These materials are also capable of forming crystals or films in the following space groups: TaB (orthorhombic, Thallium(I) iodide-type, Cmcm), Ta5B6 (Cmmm), Ta3B4 (Immm), and Ta2 (hexagonal, aluminum diboride-type, P6/mmm).

Superhard and high thermal stability of boron-rich tantalum borides is an important research topic for both the theoretical and experimental studies. These ultra-hard borides can be used as protective coatings for machining tools, heavy-duty friction couplings, fuel cells, and aerospace vehicles. However, the hardness of these materials is still not well understood because their complex chemical behavior has not been systematically investigated.

Microwave plasma chemical vapor deposition (MPCVD) is an efficient method for the synthesis of tantalum boride films using plasma initiated from a feedgas mixture containing hydrogen and diborane. MPCVD results are dependent on substrate temperature and substrate bias. Increased substrate temperature leads to increased TaB formation and solid solution hardening, while application of negative substrate bias did not further increase the hardness.

m-TaB4 is the hardest of all boron-rich tantalum borodes, which can be attributed to its strong covalent bonding and the presence of a planar boron six-membered ring unit. Moreover, the DOS of ELF shows that m-TaB4 is metallic because its partial density of states (DOS) originates mostly from d electrons of tantalum and p electrons of boron, which are essential for the hardness of transition metal borides.