Effect of mono- and bi-layer of Al on physical properties of predicted MAB phases Zr2AlB2 and ZrAlB for industrial applications – DFT investigations

We conducted first-principles calculations to study the impact of mono- and bi-layers of Al on the physical characteristics of hypothesized MAB phases Zr₂AlB₂ and ZrAlB. First the stabilities of the predicted MAB phases are confirmed. The studied materials exhibit metallic behaviour and anisotropic elastic properties. Analysis of Mulliken atomic population and charge density verifies the existence of covalent and ionic bonds. Bond length statistics highlight the relatively stronger nature of B–B bond. The elastic moduli of the phase with monolayer of Al are larger than those of bi-layer of Al, suggesting greater hardness and anisotropic behavior. Single Al layered phase has brittle properties, whereas the double Al layered phase shows ductility. Zr₂AlB₂ exhibits higher fracture toughness than ZrAlB which is suitable for heavy-duty industrial applications. Again, ZrAlB possesses higher damage tolerance than Zr₂AlB₂. The phase with bi-layer of Al exhibits very good lubricant behaviour compared to the other phase which can reduce the frictional force and improve the performance behaviour. Both compounds possess high melting temperature and low thermal expansion coefficient. Minimum thermal conductivities of Zr₂AlB₂ and ZrAlB are 1.15 and 0.98 W/mK, respectively, which are smaller than the reference value (1.25 W/mK). Hence both borides may be suitable candidates for thermal barrier coating materials. The phases are optically anisotropic, and exhibit promises for optical display systems (LED) due to their large refractive indices. Further the phases can serve as effective absorbers of visible and ultraviolet radiation and thus have the potential to mitigate solar heating across various energy regions.