Abstract:
Along with other oxide ceramics, alumina is an important and widely used industrial
material. Its applications include prostheses and dental implants used as bio-medical
replacements, wear- resistant components and speedy cutting tools, thermal and
electrical insulations and coatings for high temperature use. The effectiveness of
alumina for such uses is credited to its excellent corrosion resistance, high hardness,
good electrical and thermal insulating properties and high compressive strength.
However, regardless of its excellent potentials and properties, its use for structural
applications has significantly been limited due to its low-fracture toughness and low-
fracture strength. The potential use of carbon nanotubes reinforced ceramic
nanocomposites for various engineering applications has unlocked an interesting area
of research.
In the current work, two kinds of sintering routes, namely pressureless and spark
plasma sintering are used for the synthesis of multiwalled carbon nanotube reinforced
alumina matrix nanocomposites. The characterization of the resulted nanocomposites
is carried out and their comparison with the sintering behavior of monolithic alumina
is presented. Two types of composites were prepared by using pressureless and spark
plasma sintering techniques both contained 1, 2 and 3 wt% of as-received and
functionalized carbon nanotubes. The mixing and dispersion of carbon nanotubes in
alumina was done by a novel technique of gas purging sonication. Varying
percentages of carbon nanotubes in the composites were compacted using a uniaxial
press followed by pressureless sintering at 1600°C in flowing argon with a dwell time
of 15 minutes and spark plasma sintering at 1400°C under a pressure of 60 MPa for a
holding time of 10 minutes.
Pressureless sintered nanocomposites with 1 wt% carbon nanotubes gave 98.5%
relative density with no degradation of carbon nanotubes. Moreover, it also resulted in
an increase in fracture toughness from 8.1% and 9.4% and Young’s modulus by 5%
and 7% when compared to as-received and functionalized carbon nanotube
nanocomposites respectively with respect to pure alumina. This investigation has
shown that the densification can be achieved without degradation of carbon nanotubes
at elevated temperatures in the carbon nanotube-alumina nanocomposites sintered by
conventional route.
vWell-dispersed carbon nanotube-reinforced alumina nanocomposites have been
synthesized successfully having a high density by spark plasma sintering. At 1 wt%
functionalized carbon nanotubes addition in alumina, a near full density is achieved
that contributes to the improvement in mechanical properties of the nanocomposites.
On addition of 1wt% CNTs, fracture toughness values increased by approximately
18.6% and 14% for functionalized and as-received CNT-alumina nanocomposites
respectively. However further addition of CNTs up to 3 wt % slightly decreased the
hardness and the fracture toughness. Young’s modulus was improved by 6.5% for
functionalized and 4% for as-received CNT-alumina nanocomposites over monolithic
alumina. Average grain size of monolithic alumina is observed as 2.0 ± 0.5 μm while
that of 1wt% CNT-alumina nanocomposite was less than 1 μm. The well-dispersed
carbon nanotubes within the alumina matrix enhanced the pullout resistance, bridged
the gaps between cracks and held up the crack propagation by using elasticity that
lead to improved fracture toughness.