Aluminum as well as its alloys has been extensively used in automotive and aerospace applications owing to its lightweight and selected materials attributes. Therefore, aluminum with sufficient strength have been fabricated only that their load bearing capacity is still low for critical structural components applications. For this reason steel and other expensive titanium alloys have been used consequently resulting in more weight, higher fuel cost, and increased operation cost.
To address this issue more researchers have focused on developing high strength and lightweight aluminum composites. Carbon nanotube reinforced aluminum composites been considered and researched widely. However, integration of carbon nanotubes in aluminum matrix is faced with major challenges of poor dispersion and oxidation of the carbon nanotubes at temperatures exceeding 400°C. For this reason, high temperature processing methods such as hot rolling, forging, and casting cannot be employed easily without damaging carbon nanotubes.
Boron Nitride nanotube as an analogue of carbon nanotube with alternate nitrogen and boron atoms has unique mechanical properties. Boron nitride is resistant to oxidation at temperatures around 1000°C. Aluminum melting point is approximately 660°C; therefore, boron nitride opens up the opportunity of manufacturing boron nitride nanotube aluminum reinforced composites through casting.
In a recent paper published in Acta Materialia researchers led by Professor Arvind Agarwal at Florida international University prepared boron nitride nanotubes integrated with molten aluminum to make a composite using the equilibrium solidification method. They were able to investigate the interfacial phenomena as well as the reactions between the nanotubes and the molten aluminum.
The authors obtained long and fine nanotubes in the form of boron nitride fibril balls. These nanotube fibers were then mixed with aluminum pellets in a crucible and then heated up to 700 °C in a muffle furnace. The molten mixture was then stirred to obtain a homogeneous mix. Thereafter, the mixture was soaked and allowed to cool to ambient temperature.
The authors observed that the boron nitride nanotubes survived the high temperature as well as reactive conditions that were involved in aluminum melting. Limited interfacial reaction occurred which led to traces of aluminum nitride and aluminum boride compounds formed. However, aluminum nitride was observed as the principle product, and this led to enhanced interfacial wetting. Computations based on the surface energies indicated improved work of the interfacial adhesion owing to the formation of aluminum nitride.
Boron nitride nanotubes were seen to be well integrated in the molten aluminum matrix. This signified that the resulting aluminum nitride led to excellent interfacial wetting. The authors also reported capillary induced high temperature filling of the boron nitride nanotubes by the aluminum. This filling was enhanced by aluminum nitride formation. The filling attribute was however dependent on nanotube wetting which further improved when aluminum-boron nitride nanotube reaction progressed to form aluminum nitride.
The results of their study indicate controlled reaction induced appreciable wettability of the boron nitride nanotubes by molten aluminum. This supports the development of high strength boron nanotube reinforced aluminum composites by the casting method.
Figure credit of Acta Materialia
Pranjal Nautiyal, Ankur Gupta, Sudipta Seal, Benjamin Boesl, Arvind Agarwal. Reactive wetting and filling of boron nitride nanotubes by molten aluminum during equilibrium solidification. Acta Materialia 126 (2017) 124-131.Go To Acta Materialia