Due to their remarkable properties and unique dimensions, carbon nanotubes (CNTs)
are considered as an exciting nano-reinforcement in a variety of inorganic matrix composites.
However, published data is unable to clearly define the role of CNTs on the properties of these
composites, in particular, the mechanical properties including hardness, stiffness, strength and
fracture toughness. This lack of knowledge is due in part to manufacturing issues, such as the
dispersion of CNTs, densification of composites and microstructural changes during sintering.
Moreover, interest in the electrical and thermal properties of inorganic matrix composites
demands a comprehensive functional property evaluation. The still unexplored technological
properties of these composites, such as thermal shock, ageing, friction and wear resistance, also
deserve particular attention, in order to identify the extent of improvement that can be achieved
due to CNTs. The microstructural characterisation including the nature of CNT distribution and
their embedded morphology in brittle and amorphous matrices is still unclear, together with the
nature of the CNT/matrix interface. Finally, the effect of different CNT aspect ratios on
properties is yet to be investigated in order to choose the most suitable CNT sizes for desired
composite performance.
The present study is, therefore, aimed at developing a model composite system of
uniformly dispersed CNTs of different sizes and loadings in a dense, brittle and amorphous
matrix, and exploring the real effect of CNTs on physical, mechanical, functional and
technological properties of these composites together with their microstructural and interfacial
characterisation.
Indigenously synthesised and functionalised multiwalled carbon nanotubes (MWCNTs)
of four different aspect ratios (~31-65) were used as reinforcement, up to 10wt% (13.2vol%)
loadings, while silica (SiO2) glass was chosen as an inorganic matrix. Heterocoagulation upon
colloidal mixing provided composite powders with homogeneously dispersed MWCNTs while
pressureless sintering produced dense (96-99%) composites. The randomly oriented MWCNTs in the glass matrix showed a mechanical MWCNT/glass interface due to the interlocking of
MWCNTs with the matrix. The indentation fracture toughness was improved, by up to ~100%,
but hardness and stiffness decreased by 21-38% and 20-37%, respectively. The electrical
conductivity increased by >11 orders of magnitude but the thermal conductivity showed limited
improvement, i.e. 41-48%.
The effect of different MWCNT sizes on the mechanical properties, such as hardness,
elastic modulus and indentation fracture toughness, could not be determined due to the
decrease in the densities of the composites containing higher aspect ratio MWCNTs; however,
the functional properties, such as electrical and thermal conductivity, increased in proportion to
the MWCNT size.
The presence of MWCNTs in the thermal shock resistant silica glass matrix did not
produce thermal cracking after a single quench to 20oC from 1200oC or multiple quenches from
1000oC; however, devitrification of the glass was observed. During the thermal ageing of
composites (up to 1000oC for up to 96h), no significant degradation was observed at lower
temperatures (500oC) except limited surface MWCNT oxidation. However, at 750oC,
considerable MWCNT oxidation was noticed, and at 1000oC, cristobalite was also formed
producing surface cracking on cooling. The decarburisation depth due to MWCNT oxidation
increased with time and temperature, and completely porous composites were obtained after
oxidation of all of the embedded MWCNTs. The friction coefficient decreased with increase in
MWCNT content, while the formation of a stable graphitic layer in composites containing 10wt%
MWCNTs reversed the otherwise increasing wear rate.
Finally, the established composite processing route was applied to a commercial
borosilicate glass system containing up to 10wt% (17vol%) MWCNTs. The microstructure along
with the resulting mechanical and functional properties ensured the applicability of the
developed model system, which is believed to serve as a guide in future for preparation of other
technically relevant inorganic matrix composites containing CNTs for improved properties.