Micro-hardness and flexural properties of randomly-oriented carbon nanotube composites

The carbon nanotubes possess many unique mechanical and electrical properties, and have been appreciated as new advanced materials for nanocomposite structures, particularly for the development of nanocomposite films. Nanotubes may also be used as nano-reinforcements for matrix system for fibre-reinforced plastic structures in order to improve out-of-plane properties, thus increasing the delamination resistance. However, those properties are highly relied on the structural integrity and homogeneity of the nanotube composites. Unfortunately, only a little works have paid much attention on these issues recently. It has been obviously proved that the atomic architecture on the nanotube's surface may be affected after the nanotubes were chemically reacted with polymer matrix. The weak bonding force among the different layers (bonded by a weak Van Der Waals attractive force) of multiwalled nanotubes may also cause a discontinuous stress transfer from the outer-shell to the inner of the composites. This paper reports the micro-hardness and flexural properties of nanotube composites with different amounts of nanotubes content. Experimental measurements and microscopic observations of the nanotube-epoxy composites before and after the tests are discussed in detail. The results show that the hardness of the nanotube composites varied with different nanotube weight fractions. The flexural strength decreased by 10% for a nanotube composite beam with 2 wt. % of nanotubes. The SEM images also revealed that all nanotubes were completely pulled out after the flexural strength test due to a weak-bonding strength between the nanotube and matrix.