Ceramic transition metal diboride superlattices with improved ductility and fracture toughness screened by ab initio calculations.

ABSTRACT

Inherent brittleness, which easily leads to crack formation and propagation during use, is a serious problem for protective ceramic thin-film applications. Superlattice architectures, with alternating nm-thick layers of typically softer/stiffer materials, have been proven powerful method to improve the mechanical performance of, e.g., cubic transition metal nitride ceramics. Using high-throughput first-principles calculations, we propose that superlattice structures hold promise also for enhancing mechanical properties and fracture resistance of transition metal diborides with two competing hexagonal phases, [Formula see text] and [Formula see text]. We study 264 possible combinations of [Formula see text], [Formula see text] or [Formula see text] MB[Formula see text] (where M [Formula see text] Al or group 3-6 transition metal) diboride superlattices. Based on energetic stability considerations, together with restrictions for lattice and shear modulus mismatch ([Formula see text], [Formula see text] GPa), we select 33 superlattice systems for further investigations. The identified systems are analysed in terms of mechanical stability and elastic constants, [Formula see text], where the latter provide indication of in-plane vs. out-of-plane strength ([Formula see text], [Formula see text]) and ductility ([Formula see text], [Formula see text]). The superlattice ability to resist brittle cleavage along interfaces is estimated by Griffith's formula for fracture toughness. The [Formula see text]-type TiB[Formula see text]/MB[Formula see text] (M [Formula see text] Mo, W), HfB[Formula see text]/WB[Formula see text], VB[Formula see text]/MB[Formula see text] (M [Formula see text] Cr, Mo), NbB[Formula see text]/MB[Formula see text] (M [Formula see text] Mo, W), and [Formula see text]-type AlB[Formula see text]/MB[Formula see text] (M [Formula see text] Nb, Ta, Mo, W), are suggested as the most promising candidates providing atomic-scale basis for enhanced toughness and resistance to crack growth.