Superlattice effect on the mechanical properties of transition metal diboride coatings

Abstract

PVD deposited superlattice structures enable the simultaneous enhancement of the hardness (H) and the fracture toughness (KIC) of thin ceramic coatings. While a deeper understanding of this effect has been gained for transition metal nitrides (TMN), hardly any knowledge is yet available for hexagonal diborides (TMB2). Here we show that superlattices can—similarly to nitrides—increase both the hardness and toughness of diboride coatings. For this purpose, we non-reactively deposited the systems TiB2/WB2 and TiB2/ZrB2, a high difference in shear modulus characterizes the former, while the latter features a high lattice mismatch of the participating layer materials. Applying nanoindentation and in-situ microcantilever bending tests, we proved a distinct increase in hardness for TiB2/WB2 (up to 45.5 ± 1.3 GPa), but no increase in KIC. Vice versa, TiB2/ZrB2 shows no increase in H, while KIC increases by ~ 20% up to 3.70 ± 0.26 MPa∙m1/2. These results are discussed using X-ray diffraction analysis (XRD) complemented by an extensive literature review.