Abstract
D.Phil.
The dispersion of multi-walled carbon nanotubes (MWCNTs) in NiAl matrix was successfully carried out through the combination of two steps planetary ball millings. Low energy ball milling (LEBM) was employed for preliminary milling of the starting powders at 150 rpm. Subsequently, the milled powders were transferred to the high energy ball milling (HEBM) for auxiliary milling, and to ensure proper dispersion. This was performed at different milling hours (1 h, 2 h and 3 h) to further de-bundle the MWCNTs. The milled powders were consolidated via spark plasma sintering (SPS) using the sintering temperature of 1000 oC, sintering pressure of 50 MPa, holding time of 10 min and heating rates from 50-150 oC/min. The morphology of the milled powders and the consolidated compacts were observed using the scanning electron microscopy (SEM) and the structural and phase identifications were examined through the X-ray diffractometer (XRD). Raman spectroscopy was used to determine the structural integrity of MWCNTs in the metal matrix and Transmission electron microscopy (TEM) revealed the level of damages or defects done to the MWCNTs. The SEM micrograph of the consolidated NiAl-1wt% MWCNTs composite (HEBM, 1 hour) revealed a relative dispersion of MWCNTs with a few agglomerations and this was corroborated by the TEM images. Furthermore, the Vickers microhardness and relative density of the consolidated samples was observed to decrease as the heating rate was increased and further increase in heating rate led to the increase in the density of the consolidated NiAl. Conversely, the Vickers microhardness and relative density of the consolidated NiAl-1wt% MWCNTs composites decreased as the heating rate was increased from 50-150 oC/min. The nanoindentation plot indicates that the sintered samples with higher penetration depths at the indentation load of 75 mN had lower nanohardness values. MWCNTs was observed to have decreased the coefficient of friction due to the formation of lubricating tribolayer at the contact...