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

Abstract

Ultra-long Boron Nitride Nanotubes (100-200 mm) 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.

Quantitative Characterization of Structural and Mechanical Properties of Boron Nitride Nanotubes in High Temperature Environments

“…For example, BNNTs can survive at up to 800 °C in air, while CNTs start to oxidize at 400 °C 16 , 18 . The unique light and strong characteristics together with their extraordinary chemical and thermal stability enable BNNTs to excel in high temperature applications, such as reinforcing additives for metal and ceramic nanocomposites that are typically involved with extreme thermal processing and/or working conditions 19 , 20 . Another unique physical characteristic of BNNTs as reinforcing fillers in metal and ceramic nanocomposites is their insulating property, as B-N bonds have a large bandgap of 5–6 eV 21 .…”

Strengthening in Boron Nitride Nanotube Reinforced Aluminum Composites Prepared by Roll Bonding

“…Using Equation , the critical length was calculated to be ≈5.3 μm. The reported BNNT length of up to ≈200 μm is well above the critical length, indicating that the shear lag model is a valid explanation for tensile strengthening. In Figure b, the nanotubes can be seen distinctly embedded in the Al matrix, suggesting that there is good interfacial adhesion between the BNNTs and Al.…”

“…This provides BNNT an edge over CNTs for the processing of metal matrix composites. Already, researchers have proven BNNTs as feasible structural reinforcements of Al‐matrix composites by physical vapor deposition, molten fabrication, severe plastic deformation techniques, hot working, and powder metallurgy techniques . Al‐BNNT composites manufactured by melt spinning, magnetron sputtering, and high‐pressure torsion techniques have exhibited remarkable improvements in tensile stress, two to nine times greater than unreinforced aluminum.…”

“…Nonetheless, the study used “bamboo‐shaped” BNNTs which have structural disadvantages for mechanical properties . Nautiyal et al examined the mechanical performance of a sandwich Al‐BNNT composite prepared by spark plasma sintering and cold rolling. The study reported a notable 400% increase in tensile strength and near 100% increase in elastic modulus.…”

“…These enhancements were ascribed to an excellent load transfer, enhanced matrix‐nanotube interface due to trace amounts of interfacial product formation, and substantial crack bridging by ultra‐long nanotubes. These are the only two studies on Al‐BNNT composites that utilize cold working techniques . Cold working as a manufacturing process can be advantageous due to short production time, minimal or no heating, and improved strength as a result of strain hardening.…”