Unraveling the superlattice effect for hexagonal transition metal diboride coatings

Superlattice structures enable the simultaneous enhancement in hardness (H) and fracture toughness (KIC) of ceramic-like coatings. While a deeper understanding of this effect has been gained for fcc-structured transition metal nitrides (TMN), hardly any knowledge is available for hexagonal diborides (TMB2). Here we show that superlattices can—similarly to nitrides—increase the hardness and toughness of diboride films. For this purpose, we deposited TiB2/WB2 and TiB2/ZrB2 superlattices with different bilayer periods (Λ) by non-reactive sputtering. Nanoindentation and in-situ microcantilever bending tests yield a distinct H peak for the TiB2/WB2 system (45.5 ± 1.3 GPa for Λ = 6 nm) but no increase in KIC related to a difference in shear moduli (112 GPa). Contrary, the TiB2/ZrB2 system shows no peak in H, but for KIC with 3.70 ± 0.26 MPa∙m1/2 at Λ = 4 nm originating from differences in lattice spacing (0.14 Å), hence causing coherent stresses retarding crack growth.