Directionally Aligned Ultra‐Long Boron Nitride Nanotube Induced Strengthening of Aluminum‐Based Sandwich Composite

Ultra‐long Boron Nitride Nanotubes (100–200 µm) based layered Al–BNNT–Al composites are fabricated by spark plasma sintering, followed by cold rolling. The BNNT mat is sputter coated with Al to engineer strong metal‐nanotube interface. The BNNTs exhibit perfect alignment along the cold rolling direction. The tensile strength of the composite is found to be 200 MPa, which is 400% greater than the strength of pure Al (≈40 MPa). Young's modulus of this sandwich composite (≈134 GPa) is found to be double the modulus of pure Al (≈70 GPa) (with standard deviations less than 10%). Strengthening is explained by three major mechanisms: superior load transfer for long BNNT reinforcement, improvement in matrix‐nanotube bonding due to trace amount of interfacial product formation, and crack bridging by directionally aligned long nanotubes.