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 and omega. We study 264 possible combinations of alpha /alpha, alpha /omega or omega /omega MB_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%, Delta 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}, where the latter provide indication of in-plane vs. out-of-plane strength (C_{11}, C_{33}) and ductility (C_{13}-C_{44}, 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-type TiB_2/MB_2 (M = Mo, W), HfB_2/WB_2, VB_2/MB_2 (M = Cr, Mo), NbB_2/MB_2 (M = Mo, W), and alpha /omega-type AlB_2/MB_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.