Utilizing the full mechanical capabilities of individual nanotubes is a primary research goal in nanotube reinforced nanocomposite materials. Practical use of these nanomaterials requires creating stable and strong linkages between nanotubes without sacrificing their mechanical advantage. Cross-linking between shells via electron beam irradiation and application of large compressive forces have been studied and offer a viable approach to improve tube-to-tube load transfer and hence, mechanical properties. However, these approaches result in unwanted mechanical degradation and have limitations in scale-up for their applications to macroscopic nanocomposite materials.
Carbon nanotubes (CNTs) have received significant attention due to their outstanding combination of mechanical, electrical, and thermal properties. However, applications requiring greater thermal and chemical stability (above 300-400 °C in air) have led to the exploration of alternative compositions that provide similar structural performance. Among those compositions are boron nitride nanotubes (BNNTs), which offer mechanical and thermal properties comparable to CNTs. In addition, the chemical and thermal stability of BNNTs make them attractive for aerospace applications.
This work focuses on advantages arising from the use of amorphous carbon (a-C) to physically adhere individual BNNTs to produce macroscopic nanostructural materials. Multiple in situ tensile, compressive, and lap shear tests of a-C bonded BNNT hybrid nanostructures were performed. Both fractured and completely broken specimens were subsequently repaired multiple times between tests by deposition of additional a-C using electron beam irradiation, and then retested with the repaired area positioned within the gauge length.
While it would clearly be preferable to achieve pristine BNNT properties in a bulk material, the weak load transfer between concentric shells in multiwalled tubes and between adjacent tubes in bundles prevents their effective use in mechanical applications, especially under tensile loading. Utilization of these nanomaterials in a bulk structural component will, therefore, require some means of creating strong physical or chemical bonds between them.
This work demonstrates the viability of using a-C to form stable connections between the tubes using electron beam irradiation. Specifically, in situ transmission electron microscope (TEM)-atomic force microscope (AFM) techniques were used to precisely position BNNT specimens, and electron beam radiation was used to deposit a-C to modify and join BNNTs. Mechanical properties, including tensile, compressive, and lap shear strength, were measured for both freshly prepared specimens and, repeatedly, for the same samples after electron beam-induced deposition (EBID) repair of the fractures. The resulting properties were substantially reduced from those of pristine BNNTs, but comparable with those of currently available structural fibers such as CNT yarn, carbon fibers, and carbon fiber reinforced composites. Therefore, the current approach of a-C welding represents one possible approach for transferring load between the tubes for future structural material designs.