Pathways for Engineering Boron Nitride Nanotube Based High-Strength Metal Matrix Composites

Boron Nitride Nanotube (BNNT) displays excellent elastic modulus (> 1 TPa) and tensile strength (> 60 GPa), and can survive at elevated temperatures (as high as 1000°C) without oxidizing or degrading. These brilliant thermo-mechanical properties of BNNT are exploited to develop high-strength and lightweight metal matrix composites based on Aluminum and Titanium. Very long (100-200 µm) and fine (5-10 nm) nanotubes are used as reinforcing nanofiller for superior strengthening. Three major processing routes are adopted: spark plasma sintering (powder metallurgy), metal solidification (casting) and plasma spray (additive manufacturing). BNNTs are found to survive the extreme temperature and pressure conditions involved in these processes. Interfacial chemical reactions, and dispersion and integration of BNNTs in the metal matrix are examined by electron microscopy, X-ray diffraction and energy dispersive spectroscopy. BNNT addition is found to enhance the elastic modulus, hardness and tensile strength of the composite, indicating effective stress transfer from the matrix to the nanotube. Post failure microscopy shows nanotube pull out and crack-bridging as prominent load-bearing mechanisms. These findings evidence the suitability of BNNT for developing advanced high-performance metal matrix composites for aerospace applications.