Hexagonal OsB2: Sintering, microstructure and mechanical properties

• ReB2-type hexagonal OsB2 powder has been densified by spark plasma sintering.
• The sintered OsB2 contains ∼80 wt.% hexagonal and ∼20 wt.% orthorhombic phases.
• The average grain size of the sintered OsB2 sample was 0.56 ± 0.26 μm.
• H = 31 ± 9 GPa and E = 574 ± 112 GPa measured by nanoindentation.

The metastable high pressure ReB2-type hexagonal OsB2 bulk ceramics was produced by spark plasma sintering. The phase composition, microstructure, and mechanical behavior of the sintered OsB2 were studied by X-ray diffraction, optical microscopy, TEM, SEM, EDS, and nanoindentation. The produced ceramics was rather porous and contained a mixture of hexagonal (∼80 wt.%) and orthorhombic (∼20 wt.%) phases as identified by X-ray diffraction and EBSD analysis. Two boron-rich phases, which do not contain Os, were also identified by TEM and SEM/EDS analysis. Nanoindentation measurements yielded a hardness of 31 ± 9 GPa and Young’s modulus of 574 ± 112 GPa, indicating that the material is rather hard and very stiff; however, it is very prone to crack formation and propagation, which is indicative of a very brittle nature of this material. Improvements in the sintering regime are required in order to produce dense, homogeneous and single phase hexagonal OsB2 bulk ceramics.

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