Toward Stronger Al-BN Nanotube Composite Materials: Insights into Bonding at the Al/BN Interface from First-Principles Calculations

Lightweight mechanically strong composite Al/BN nanotube materials may find applications in the automotive and space industries because addition of a few percent of the nanotubes considerably improves the mechanical properties of the Al matrix. At the same time, experiments indicate that bonding at the interface between Al and the nanotubes is rather weak, which limits the performance of the composites. To get precise microscopic knowledge of the atomic structure and bonding at the interface between the Al matrix and BN nanotubes and to suggest ways to improve the adhesion, we employ density functional theory with van der Waals exchangecorrelation functionals and carry out first-principles calculations of the interface between the Al(111) surface and hexagonal BN sheets, mimicking BN nanotubes with large diameters. We estimate the bonding energy and the interfacial critical shear stress and compare the theoretical results to the experimental data. We further assess how point defects, such as atomic vacancies and substitutional impurities, affect the interface bonding. We show that some point defects (e.g., single B vacancies) dramatically change the atomic structure of the interface, giving rise to stronger bonding and higher critical shear stress values. Larger vacancies and substitutional C impurities in the BN sheets have a stronger effect on the adhesion. Finally, we demonstrate that even though defects deteriorate the mechanical properties of BN nanotubes, this effect is rather weak at defect concentrations of a few percent, so that an overall improvement of the characteristics of Al/BN nanotube composites after introduction of defects at the interface can be expected. We suggest possible routes for the development of nanostructured BNAl materials with improved mechanical characteristics.