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, $$\alpha$$ α and $$\omega$$ ω . We study 264 possible combinations of $$\alpha /\alpha$$ α / α , $$\alpha /\omega$$ α / ω or $$\omega /\omega$$ ω / ω MB$$_2$$ 2 (where M $$=$$ = Al or group 3–6 transition metal) diboride superlattices. Based on energetic stability considerations, together with restrictions for lattice and shear modulus mismatch ($$\Delta a<4\%$$ Δ a < 4 % , $$\Delta G>40$$ Δ G > 40 GPa), we select 33 superlattice systems for further investigations. The identified systems are analysed in terms of mechanical stability and elastic constants, $$C_{ij}$$ C ij , where the latter provide indication of in-plane vs. out-of-plane strength ($$C_{11}$$ C 11 , $$C_{33}$$ C 33 ) and ductility ($$C_{13}-C_{44}$$ C 13 - C 44 , $$C_{12}-C_{66}$$ C 12 - C 66 ). The superlattice ability to resist brittle cleavage along interfaces is estimated by Griffith’s formula for fracture toughness. The $$\alpha /\alpha$$ α / α -type TiB$$_2$$ 2 /MB$$_2$$ 2 (M $$=$$ = Mo, W), HfB$$_2$$ 2 /WB$$_2$$ 2 , VB$$_2$$ 2 /MB$$_2$$ 2 (M $$=$$ = Cr, Mo), NbB$$_2$$ 2 /MB$$_2$$ 2 (M $$=$$ = Mo, W), and $$\alpha /\omega$$ α / ω -type AlB$$_2$$ 2 /MB$$_2$$ 2 (M $$=$$ = Nb, Ta, Mo, W), are suggested as the most promising candidates providing atomic-scale basis for enhanced toughness and resistance to crack growth.