1. Project Concept: An Investigation on the mechanical properties of a graphene
reinforced alumina ceramic matrix composite (CMC).
Brief Literature Background:
Since the report by Iijma et al in 1991 on CNT reinforcement there has been a number of
studies aimed at mechanical property improvement through CNT reinforced composites
(Yamamoto et al., 2008; Iijma et al., 1991). The understanding is that CNTs possess high
tensile strength values (20-100 GPa) and high aspect ratio (>5000) making them suitable for
reinforcement applications (Iijma et al., 1991). They play a critical role in absorbing most of
the strain related energy due to their high elasticity. The use of CNTs to reinforce ceramics
has been motivated by the fibre bridging effect resulting from debonding and sliding
resistance which strengthens the composites and improve fracture toughness (Yamamoto et
al. 2008). The commonly accepted mechanisms contributing to improved strength and
toughness in CNT reinforced nanoceramics include; crack deflection at the CNT/ceramic
interface; crack bridging by the CNT and pullout of CNTs on the crack plane (Yamamoto et
al., 2008; Sahetat et al., 1999).
In light of the above, a number of CNT reinforced-composites have been studied so far, and
in a number of cases they show some improvement in mechanical properties especially
strength and for fracture toughness. Yamamoto et al (2008) reported a 25% increase in
fracture toughness for a 5vol% CNT addition (in the form of multi-walled carbon nanotubes
(MWCNTs)) to alumina (Yamamoto et al., 2008; Baughman et al., 2002). Zhan et al (2003)
obtained a fracture toughness of 9.7 MPa.m1/2
with a 10vol%-SWCN (Single Wall Carbon
Nanotubes) reinforced alumina composite. This was enabled by the lower sintering
temperatures and shorter times required to obtain fully dense materials in SPS compared to
conventional sintering methods. Furthermore, a remarkably higher fracture toughness of 13.5
MPa.m1/2
was obtained for an alumina-SWCNT system in which Nb was added as a ductile
phase (Kuntz et al., 2002; Kuntz et al., 2004). The higher fracture toughness is attributed to a
more ductile Nb metallic phase. In another study the fracture toughness of zirconia toughened
alumina (ZTA) was improved by a margin of 44% by incorporating CNTs (Bocanegra &
Echeberria, 2011).

2. The development of CNT-reinforced composites is still limited owing to the difficulty in
obtaining homogeneous mixtures and the high cost of CNTs plane (Yamamoto et al. 2008;
Dusza et al., 2012). Furthermore, CNTs have a tendency to agglomerate and form ‘ropes’and
‘bundles ‘which makes it difficult to homogenously disperse CNTs in the matrix to form
adequate interfacial connectivity between the two phases (Yamamoto et al. 2011; Gat et al.
2004). This explains the bulk of the disappointing results that have been reported so far.
Yamamoto et al (2011) through electron microscopy studies observed some form of
debonding of MWCNTs from the matrix and recorded a fracture toughness of 4.74 MPa.m0.5
and a binding strength of 543.8 MPa with 0.9 vol% of MWCNTs in an alumina matrix. This
result is not far off from the one obtained for the monolithic alumina synthesized under
similar conditions (4.37 MPa.m0.5
and 502.3 MPa respectively) (Yamamoto et al., 2011).
Recently there has been wide interest in the fabrication of graphene reinforced composites as
an alternative to CNTs. Graphene is a 2-dimensional counterpart of CNTs consisting of a
one-atom thick layer of C-atoms in a honeycomb (the parallel stacked layers found in
graphite) (Liu, Yan & Jiang 2013).The motivation in their use is the higher fracture strength
in comparison to CNTs for similar type of defects and the easiness of homogeneous
dispersion in ceramic matrices (Hirata et al. 2004; Yu et al. 2000). Graphene is normally
referred in literature as graphene platelets (GPLs), graphene nanosheets (GNS), multi-layer
graphene nanosheets (MGN) or graphene nanoplatelets (GNPs) and consists of several layers
of graphene with a thickness of 100 nm (Basu & Balani, 2008). They are normally made by
micromechanical exfoliation of expanded graphite, chemical processing or subjecting
graphite to shear stresses which induce some slipping of the stacked layers such as in high
energy ball milling (Potts et al., 2011).
Several attempts have been made to sinter GPL-reinforced composites in the SPS. Wang et al
reported a 53% increase in fracture toughness with Al2O3 containing 2 wt% GPLs. In a
separate study Liu et al also presented a 40% increase in fracture with 0.81vol%GPL in
Al2O3. In a recent study a 27.20% improvement in fracture and 30.75% increase in flexural
strength have been reported by Liu et al (2013) for an alumina-GPL composite. Walker et al
(2011) prepared Si3N4-1.5wt%GPL and obtained a fracture toughness of 6.6 MPa.m0.5
which
was 136% higher than the monolithic binderless Si3N4 obtained using similar conditions.
There is a clear indication based on the above studies, there is significant improvement in
fracture toughness and strength for the CNT, GPL-reinforced composites however there is

3. still some inconsistences in the results obtained. The potential of obtaining even better results
lies in the ability to process the raw powders more effectively (homogeneous mixing) and
minimizing the level of impurities in the composites. It should be noted however that the
major challenge in obtaining graphene reinforced CMCs is limited due to the low thermal
stability of graphene which decomposes at temperatures of 6000
C (Walker et al., 2011).
However the use of the SPS technique has been found to be successful in obtaining well
sintered products with improved properties due to the short sintering times and well
controlled sintering parameters (Walker et al., 2011).
Aim:
To develop an alumina-graphene reinforced ceramic matrix composite with improved
mechanical properties through a novel sol-gel synthesis technique and SPS sintering.
Specific Objectives:
1) To optimise synthesis parameters for the production of a homogeneous composite.
2) To evaluate the powder characteristics.
3) To oprimise sintering parameters.
4) To evaluate sintered CMC properties.
Methodology:
1) Synthesis: synthesis of alumina-graphene composites
2) with different vol% using sol gel synthesis method.
3) Characterisation of powders.
4) Optimistion of sintering parameters and SPS sintering.
5) Evaluation of mechanical properties (hardness,fracture toughness, strength, wear
properties, microstructural analysis, phase evolution)

4. Experimental Procedure:
Synthesis of alumina-graphene composites:
The sol–gel method for RGO/Al2O3 nanocomposite synthesis utilizes metal organic
aluminum compound (triethylaluminum) as Al2O3 NPs precursor. The innovative GO
reduction process is realized in situ during the two-stage reaction process in which
oxygen is transferred from GO to Al2O3 precursor (alumoxane) and subsequently,
organics are completely removed as a result of thermal decomposition process.
A 500 ml of dry hexane will be introduced to the 1000 ml reactor equipped with a stirrer,
0.055 g GO flakes will be added and stirred for 60 minutes. After, 1.5mL of
triethylaluminium will be introduced. Thus prepared, the reaction mixture will be stirred until
a spontaneous evaporation of the solvent, a light brown solid of a precursor will be obtained.
After a thermal decomposition of the precursor at 280ºC for 3 hours, the dark brown product
of the Al2O3 composite will be obtained.
Powder preparation:
Alumina powder with the purity of about 99.85% will be used as a metal matrix material, an
average size of 150nm and a surface area of 10 m²/g will be used in this project. The GPL
will be produced via rapid thermal expansion of graphite that has been intercalated by using
sulphuric acid. Appropriate quantities of GPLs will first be dispersed in DMF and sonicated
for 1 hour. AL2O3 powder will be added and then the mixture will further be sonicated for a
predetermined time. This will be followed by a ball milling procedure. The milled slurry
mixture will be dried then sieved.
Spark plasma sintering (SPS):
Sintering is the process of heating powder in a furnace below its melting point so that
bonding takes place by atomic diffusion, leading to individual powder particles adhering to
each other in a dense compact. SPS is a new technique which only takes a few minutes to
complete as compared to the conventional way of sintering.
A known quantity of the dried and milled alumina powder will be taken in a cylindrical die
lined with graphite sheet which will facilitate easy removal of the sintered sample. Switch on
the compressor and the machine, the die containing the alumina powder sample will then be
placed inside the SPS chamber. The sintering process will be conducted under 5 Pa vacuum

5. conditions. A uniaxial pressure of 50 MPa will be applied throughout the sintering process.
The sintering temperature will be measured and controlled using a thermocouple for
temperatures below 1000ºC and an optical pyrometer for temperatures above 1000ºC.
Shrinkage, displacements, heating current and voltage will also be recorded during the
sintering process. After sintering the samples will be grinded using a SiC paper and polished
using a 0.5µm diamond suspension followed. Hardness will be tested using a Vickers
hardness tester. Fracture toughness of the ceramic composites will also be measured using a
v-notched beam method under ambient conditions. The test specimens will be f sizes 3mm
witdth x 4 mm thickness x 36mm length. A notch in the centre of the test specimen will be
cut using a diamond wheel and the fracture surface will be examined using a scanning
electron microscope (SEM). The wear tests will be carried out in a reciprocating wear tester
with an applied load of 20 N and a sliding speed of o.o6 ms-1 for predetermined sliding
distance.

6. Workplan:
Work plan for 2017
Research Task Jan Feb Mar Apr May Jun Jul Aug Sep Oct
No
v Dec
Research methodology course
and compilation of research
proposal.
Equipment training.
Powder metallurgy and
sintering course.
Gaining access and permission
to work in a particular area,
have access to data.
Literature review
Designing a methodology to
obtain improved mechanical
properties of ceramic matrix
composite
Proposal submission
Proposal presentation
Performing Experiment
Work plan for 2018
Literature review
Progress report
Raw tabulations/draft analysis
of qualitative data
Data analysis
Submission of first draft
Submission of final report
Revising and proofing of final
report
Research presentation

7. Project Scheduling:
 January-April: Research Methodology course & Compilation of research proposal.
 April-June: Equipment training and attending training courses in powder metallurgy
and sintering fundamentals.
Budget:
 Alumina price (2kg): R7.89 – 14.07/kg
 Graphene Oxide (Small Flakes): 1 g: R3500
 SiC papers (grit: 320, 500, 800, 1200): R8000
 0.5µm diamond paste (500 ml): R1200
 SEM:
Total = 12728.00

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