Enhancing superplasticity of engineering ceramics by introducing BN nanotubes

Introducing carbon nanotubes (CNTs) into polymer or ceramic matrices has been a promising approach to obtain ultra-strong, extra-toughened materials as well as multifunctional composites. Most of the previous work on CNT composites has focused on strengthening and toughening of matrix materials at ambient conditions. However, so far there is a lack of information on the mechanical behavior of these composites at elevated temperature. Recently, single-walled CNTs were found to undergo a superplastic deformation with an appealing 280% elongation at a high temperature (Huang et al 2006 Nature 439 281). This discovery implies the high probability for the potential usage of CNTs as reinforcing agents in engineering high-temperature ceramics with improved ductility. Here, for the first time, we demonstrate that a small addition of boron nitride nanotubes (BNNTs) can dramatically enhance the high-temperature superplastic deformation (SPD) of engineering ceramics. More specifically, 0.5 wt% addition of BNNTs leads to an inspiring brittle-to-ductile transition in Al₂O₃ ceramics even at a moderate temperature (1300°C). For Si₃N₄ ceramics, 0.5 wt% addition of BNNTs could also decrease the true stress by 75% under the same deformation conditions. In contrast, addition of micro-sized or nano-sized BN powders has no or a negative effect on the superplasticity of these ceramics. The underlying SPD-enhancement mechanism is discussed in terms of the inhibition of static and dynamic grain growth of the matrix and the energy-absorption mechanism of BNNTs. The unraveled capability of BNNTs to enhance the SPD behavior will make BNNTs promising components in cost-effective complex ceramics with good comprehensive mechanical properties.