Development of carbon nanotubes reinforced titanium metal matrix composites

This study has explored the potential of carbon nanotubes (CNTs) as a reinforcement material in titanium metal matrix composites (TMCs). CNTs exhibit elastic modulus in terapascal (TPa) range, tensile strength of 150-200 GPa, and thermal conductivity in the range of 1000-3000 W/m.K. These extraordinary mechanical and thermal properties of CNTs advocate their use as a reinforcement material in TMCs. These unique properties of CNTs are attributed to their seamless cylindrical morphology which is comprised of an intact sp2 carbon-carbon (C-C) bonds. However, in the production of CNTs reinforced TMCs, a major hindrance is the high reactive nature of Ti at higher temperatures during the high temperature consolidation stages which cause excessive damages to CNTs and formation of unwanted brittle phases, e.g., carbides, at the interface of CNTs and Ti matrix. The formation of interfacial products, e.g., titanium carbide (TiC) during the processing of CNTs reinforced TMCs has the potential to either improve or deteriorate the mechanical properties of TMCs. To date there is still a substantial gap in existing literature, whether to promote or avoid the formation of TiC during the processing of CNTs reinforced TMCs. Previous research on the fabrication of CNTs reinforced TMCs has been carried out on using empirically selected parameters during high energy ball milling (HEBM) to improve the dispersion of CNTs in the Ti matrix. Also, there is no consensus the relationships between the process parameters and resulting properties of the fabricated TMCs. The selection of dispersion methods such as HEBM, sonication, solution ball milling, the processing parameters, and the influence of impact energies provided to CNTs during the dispersion process have not been adequately studied. Such non-standardised fabrication approaches can potentially lead to ambiguous results, making it difficult to understand the strengthening mechanisms and to compare different data sets.<br><br>The aims of this study were to investigate the relationship of various processing parameters of dispersion and consolidation on the evolution of CNTs, microstructure, and resultant mechanical properties of the TMCs. For the first time, impact energies transferred to CNTs-Ti powders during dispersion processing, were reported which control the in situ formation of TiC nanorods in the powder mixtures. Homogenous formation of an interfacial layer of TiC around CNTs during HEBM protected them from undergoing further deterioration and interfacial interactions with surrounding Ti matrix at high temperature consolidation processing. Further, investigations were made to improve the dispersion and de-bundling of CNTs in Ti matrix without compromising their structural integrity and formation of any interfacial products which significantly enhanced the mechanical and tribological properties of the TMCs. In addition, strengthening efficiencies of CNTs in TMCs were quantified and processing conditions were proposed to improve them by preserving the characteristic graphitic structure of CNTs. In conclusions, these studies demonstrate that in order to manipulate the reinforcing effect of CNTs in composites, the selection of processing parameters during dispersion and consolidation stages are critical and can play a significant role in resultant mechanical properties of fabricated composites. It also highlights that there is a need for a development of standardised dispersion methods to obtain uniform dispersion of CNTs in the hosting matrices with minimum damage to their structural integrity. The results of this research will contribute to lay the foundations for the development of high strength CNTs reinforced TMCs that can be used in various advanced engineering applications.