Anisotropic super-hardness of hexagonal WB2-z thin films

Abstract

Physical vapor deposited transition metal borides are an emerging class of materials. Their inherent promising properties range from ultra-low compressibility, highest thermal stability to chemical inertness, allowing an application as protective coating in quite harsh environments. Our recent ab initio calculations suggest an attractive combination of Poisson’s ratio, bulk-, and shear modulus for α-structured WB2-z (space group 191, AlB2-prototype, P6/mmm). This leads to an interesting combination of high hardness while maintaining a sufficient fracture toughness. The stabilization of the α-structure over the intrinsically favored ω-structure (space group 194, W2B5-prototype, P63/mmc) is based on omnipresent growth defects (e.g. various types of 0-dimensional vacancies) throughout the PVD deposition. However, next to the stabilized phases (hence prevalent bonding nature) also the morphology, especially column size and grain boundary interior, has a huge impact on the mechanical response. Here, we focus on the binary WB2 material system in terms of its phase formation using DFT and further illustrate the impact of prevalent lattice orientations on the mechanical properties. We show, that the α-WB2-z structure is preferentially stabilized by B vacancies, exhibiting its energetic minima at sub-stoichiometric compositions of about WB1.5 – being also experimentally underlined. The mechanical properties show that α-WB2-z coatings in 0001 orientation reveal super-hardness (H > 40 GPa) compared to their 101 ̅1 oriented α-WB2-z counterparts (H ~ 30 GPa). This is attributed to differences in the generalized stacking fault energies (GSFE) of basal and pyramidal slip systems in hexagonal diboride crystals. Our results show that the mechanical properties, in particular H, of PVD α -WB2-z coatings can change significantly due to the crystallographic orientation, highlighting the feasibility of tuning mechanical properties by crystallographic orientation relations.