effects of processing variability on thermo- mechanical...
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International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
336 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
Effects of Processing Variability on Thermo-
Mechanical Properties of Graded Epoxy-Graphite
Composites Fabricated by Gravity Method
Tirumali Manoj1, K Balasubramanian
1*, A Kumaraswamy
2
Abstract- This experimental research article presents the
investigation carried out with graded concentration (0.5-1.0 wt%)
of graphite powder (50µm) in epoxy and the effects of the varying
parameters on thermo-mechanical properties such as the
concentration and gel / solidification time of each layer to form
the bulk epoxy-graphite graded composites. The samples of 3, 4
and 7 Layers were experimented. The graded materials have been
characterised by XRD, FESEM, TGA, Flexural & Tensile testing
and Nano-indentation. The XRD images exhibit amorphous peak
with the graphite peak indicating a shift to lower intensity. The
investigation show the effect of variability of either the
concentration, number of layers or the gel time on the 2θ value
vis-à-vis that of pure epoxy indicating a variation of 7 to 9% and
being highest for higher filler materials. The FE SEM examined
the surface morphology of fractured specimens across the cross-
section. The images indicate continuous gradation with graphite
particles being well dispersed in epoxy. The TGA results of
graded composites depict a volatile decomposition with enhanced
char yield weight loss as compared to pure epoxy. The graded
composites especially, 3 layers with respect to pure epoxy has
shown enhancement in flexural and tensile properties. The nano-
indentation analysis indicates especially for 2 and 3 layers an
increase in hardness and modulus values at the interface across the
cross-section. The experiment indicated that process variability
has an effect on thermo-mechanical properties and may be tailored
for effective end use applications especially for aerospace
requirements.
Index Terms: Fabrication; graded polymer; thermo-mechanical;
gel time; nano-indentation; gravity method
Nomenclature
σf Flexural stress, MPa
P Applied load, N
LG Span length, mm
D Maximum deflection of the center of the beam, mm
b Width of the specimen tested, mm
d Depth of the specimen tested, mm
εf Flexural strain, mm/mm
Eb Modulus of elasticity in bending, MPa
m Slope of the tangent to the initial straight-line portion of
the load-deflection curve, N/mm
Vm Volume concentration of matrix
Vp Volume concentration of particle
Ec Modulus of Elasticity of composite, MPa
Em Modulus of Elasticity of matrix, MPa
Ep Modulus of Elasticity of particle, MPa
1. INTRODUCTION
The roadmap of exploration on polymer graded materials
for their processing techniques, characterisation and bulk
production began sometime in 1990s. This evolved
soonafter the progress on Functionally Graded Materials
(FGMs) especially the work related to metals and ceramics
took successful predominance over conventional
composites especially for high temperature applications.
The basis of study on graded materials began with the
theoretical work in 1970s by Bever and Duwez [1] and
Shen et al. [2]. This was recognized that it was later
followed up for its implementation by Klingshirn et al. [3].
Later, Niino et al. [4] in 1980s described the work on
graded materials of Metals and Ceramics emphasizing its
utilization for high temperature space applications. The use
of graded polymers for high temperature applications has
been on the rise. The use of Phthalonitrile-graphite polymer
has been researched for high temperature applications for
missiles and rocket applications [5]. The other resins like
the oligomers, PMR-15, the Bismaleimides (BMI) etc have
also been explored with fillers of carbon allotropes of micro
/ nano sizes [6]. This further led to many programs as
conducted by Germany as well as Japan and other European
and Western Nations for professionalising the fabrication
technology and characterization of graded materials for cost
effective and bulk production [7]-[17]. The processing and
bulk production of metals and ceramic gradation materials
has been more or less established that the polymer graded
materials became the point of research. The polymers of
thermoplastic as well as thermosets along with the graded
reinforcements (fillers, fibres, and hybrids) became the
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
337 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
subjects of study. The gradation of microstructure or the
composition along one dimension are being researched with
types of polymer-reinforcement combinations for obtaining
tailored properties useful for varied science and engineering
applications. Jyongsik Yang et al. [10] have demonstrated
on the fabrication and the mechanical properties of glass
and carbon fibre polypropylene functionally graded
material which has been developed by varying the filler
concentration (Carbon fibre or Glass fibre) spatially.
Nowadays, there are several literature reviews and research
work on issues concerning studies on various aspects of
thermo-mechanical and functional properties such as
erosive wear resistant, heat transfer solutions, stress
analysis and fracture toughness, electrical conductivity &
resistivity in a material, electromagnetic shielding and UV
absorption etc. that could establish the importance of a
graded material [15], [18]-[25]. In particular, polymer
graded materials are being explored for deriving both their
primary stiffness to weight and strength to weight
advantages as well as the added contra properties within a
material due to tailored gradation of polymer matrix-
reinforcement mix.
The polymer gradation fabrication techniques are broadly
classified under either the casting or the pressing
techniques. The casting which is by gravitational method
has been seen quite economical and effective [12]-[13],
[26]-[31]. The present article is bringing out the research
work carried out in the fabrication of epoxy/ graphite
powder (Micrometer size, 50µm) composite using the
gravity method. The graphite filler concentration has been
in the order of 0.5 to 1.5wt% in steps of 0.5wt%. The
literature reviews have shown studies carried out on graded
epoxy-graphite polymers with varying graphite or other
carbon allotropes of filler concentration of micro or nano
sizes varying from 2.5wt% to 80wt% [1]-[2], [20]-[21],
[29]-[30], [31]-[34]. The materials, epoxy and graphite
have been chosen, as their source is in abundance and also
have vast applications in Science and Engineering,
especially in the aerospace applications [13]-[15]. The other
essential advantage gathered from Zurale et al. [35] has
been the cost effectiveness obtained by adding fillers such
as graphite in epoxy. The low viscosity epoxy resin with
amine curing agent that enables good crosslinking at room
temperature [36]-[38] has been taken alongwith the fine
powder graphite which has the affinity to epoxy in forming
a good bonding between each other at room temperature for
stronger crosslinking and networking during curing. The
article discusses the feasibility of fabricating a graded
epoxy-graphite composite and also the effect of variability
w.r.t. the concentration change, gel time, layer-wise
solidification time and the number of layers forming the
bulk on the thermo-mechanical properties. This research
work is a prelude to further study under progress to tailor
the resin-graded reinforcement combination for required
functional properties as applicable for aerospace
applications. The characterisation include mechanical
testing for flexural and tensile properties which is supported
by the non-destructive technique i.e. nano-indentation
technique, Field Emission Scanning Electron Microscope
(FE SEM) to study the surface morphology and dispersion
characteristics of the graded filler in the matrix. Thermo
Gravimetric Analysis (TGA) for analysing the thermal
stability status with variation of filler content or the gel
time of each layer or the number of layers and finally with
XRD technique to understand the structure of the graded
composite. The analysis has shown the preliminary
feasibility of fabricating by the gravity technique.
2. EXPERIMENTAL DETAILS
Materials
The materials used in the process for fabrication of graded
polymer are given in Table 1.
Table 1. Materials for experimentation
The materials are of analytical grade, which are used
directly without any treatment. Material key data as per
Original Equipment Manufacturer (OEM) data sheet is
mentioned in Table 2.
Table 2. Material key data
Experimental
The gradation is based on change from 100% pure polymer
(Epoxy + Hardner) to graded polymer (Epoxy + Hardener +
(0.5 / 1.0 / 1.5) wt% Gr) layerwise along the thickness
direction with the net weight equal to 100wt%. The process
by simple gravity method has been carried out mainly for
samples of 2, 3, 4 and 7 layers of graded polymer-filler
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
338 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
mix. The specifications of different layered gradation
samples are given in Table 3. Table 3. Specimen specification
The standard technique of uniform dispersion of graphite in
a solvent and then mixing the epoxy followed by adding the
hardner for polymerization has been carried out for each
layer of concentration of filler-polymer mix [38-39]. The
fabrication design of the open mold made of aluminum
material for tensile test specimens and flexural test
specimens has been as per the specification given in ASTM
D 638-08 (Type I) and ASTM D 790-07 (Procedure A).
The span length is taken as greater than 16:1 ratio for
ASTM D 790-07 specimens. The molds are shown in
Fig 1 (A) and (B). The flexural properties are based on the
relations as per the beams supported at large support spans.
The equations used to determine the flexural properties are
given below.
𝜎𝑓 = (3PLG)/2bd2) [1 + 6 (D/LG))
2 - 4 (d/LG)) (D/LG)] (1)
εf = 6Dd/LG2 (2)
Eb=LG3m/4bd
3 (3)
Fig 1. Aluminum molds for mechanical testing (A) Tensile
(B) Flexural
The pure epoxy composite layers (epoxy + hardner in the
ratio of 100:34) are processed as per the schematic shown
in Fig 2 (A). The fabrication of graded polymer composite
is carried out by placing layers of filler-polymer mix as per
design when each is at semi-solid state after pouring and
maintaining to a specified gel / solidification time for
partial crosslinking. The bulk which is produced after
pouring of all layers as per thickness of the specimen will
be cured at first at room temperature for 24hours and later
post cured @ 3630K for 8h as per OEM data sheet
recommendations. The overall weight of each layer of
polymer-filler mix is 100g where the concentration of filler
and Epoxy-Hardner is proportionately weighed and mixed.
The process followed in the fabrication of
Epoxy-Graphite graded polymer has taken into reference
the work carried out by Dilini et al. in the fabrication of
Graphene oxide-Epoxy Nanocomposite, [11]. The
schematic of the graded polymer process is as shown in
Fig 2 (B).
Fig 2. Schematics of graded layers process (A) Epoxy (B) Epoxy-Graphite
The bulk specimens both of tensile test and flexural test are
shown in Fig 3 (A) and (B) respectively.
Kieback et. al., [12]. and Stabik et. al. [29] have elucidated
the gradation process, which mainly consists of three
processes namely the constitutive, homogenization and
segregation. The present experiment has followed same
procedures in forming the bulk with continuous gradation.
The process of development of graded polymer as indicated
in the schematic involves several stages, first stage
involves, preparation of thermoset polymer solution by
dispersion of weighted fine graphite powder in 20ml
acetone solvent. This mixture is further subjected to
ultrasonic bath (Indian make, 20kHz pulse at 20W power)
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
339 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
for 10mins for uniform dispersion of graphite particles
followed by another dispersion process for 10mins in the
probe sonicator (Sonics India, 25khz, 50% amplitude). The
finely dispersed particles are then mixed with proportionate
weighted epoxy to make the total 100g as per the gradation
design of each layer. This mixture is then kept at room
temperature for 10-15mins for thorough mixing on a
magnetic stirrer which is rotated @ 220 rpm. Subsequently,
the solvent is removed by evaporation by heating the mix
@ 328-3430K for 12h on the magnetic stirrer. Soonafter
complete evaporation of solvent (acetone), the mixing of
hardner in the OEM recommended ratio of 100:34, 36
(i.e.
100 parts of epoxy: 34 parts of hardner) with the epoxy-
graphite mixture is undertaken. The polymer mix is then
de-aerated or moisture is removed in a vacuum desiccator.
The bulk mix is then weighed for pouring into each die in
the mold as per the thickness of each layer depending on
the number of layers the graded polymers is made of. The
samples of specimens with different layers prepared with
varying gel time maintained at 30mins / 1h/ 1.5h / 2h/ 4h or
a combination of these for especially 4 and 7 layer bulk
composites. Stabik et. al, 30
, has reported fabrication of
graded composite with a gel time of 33mins. The varying
gel time leads to the interfacial bonding due to
sedimentation of particles at the interface as each layer will
be partially crosslinked before the next is poured.
Fig 3. Specimens as per ASTMs (A) Tensile (D 638-08)
(B) Flexural (D 790-07)
In the current investigation, the work carried out by Stabik
et al., [26]-[30] and Ehsan et al., [31] in the development of
graded polymer was perused. The novelty is the
development of 3, 4, and 7 layers graded polymer
composite in an open mold by conventional gravity
method. The experiment demonstrates the development and
its effect on thermo-mechanical properties. The preliminary
preparation of the samples of 0.5 and 1.0wt% forming 3, 2
and 4 layers of uniform thickness and 7 layers of varying
thickness have been processed. The schematic of these
specimens are shown at Fig 4. Of these specimens, samples
of 3 layers with 0.5 and 1wt% layers as sandwich between
pure epoxy and the specimens with varying gel times were
characterised for mechanical testing, XRD, FE SEM, and
TGA, whereas, 2, 4, and 7 layers were characterised only
through FE SEM, XRD and TGA. The present process is
restricted to the laboratory work and may be extracted for
industry production in future.
Fig 4. Schematic of Graded Layers (2, 3, 4 and 7 layers)
3. CHARACTERISATION
X-ray diffraction (XRD)
XRD was used to verify the structure of the graded
composite. The XRD was performed on a Bruker D8
Advance Diffractometer with Cu Ka radiation (λ=1.541 Å)
having the slide width of 6mm operating at 40 kV and 40
mA. The scanning range was 10 - 60 with a scanning speed
of 0.1step/sec.
Flexural and tensile testing
Flexural tests were carried out on an Instron 2T capacity
Universal testing machine. The 3 point bending fixture was
mounted on this machine for the flexural tests. The
crosshead speed was maintained at 1mm/min. The tensile
tests were performed on a Tinius Olsen H25KS Mechanical
Testing Machine at a crosshead rate of 5.0 mm/min. The
flexural and tensile tests were performed as per ASTM D
790-07 and ASTM D 638-08 respectively. Minimum of 5
tests of 3 layers (0.5 and 1.0Wt%) and 3layers of 1wt% of
varying gel time (1/ 1.5 / 2 / 4h) were tested for 3 point
bending while 3layers of 1Wt% was tested for tensile
testing. The limited selection was only to ascertain the
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
340 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
feasibility of fabricating the graded structure and later
optimize the process for further development based on the
effects of wt% and gel time variation on properties seen in
the selected samples tested.
Microscopy
The cross-sectional surface cut with a diamond cutting tool
(ISOMET model, low speed saw, Buleher Make) or the
fractured surfaces obtained either from mechanical testing
or cryogenic by using liquid nitrogen were examined using
a Zigma Field Emission Scanning Electron Microscope
(FE SEM). The 3 and 7 layers were examined for their
surface morphology as a sample to establish the feasibility
of graded development. The diffusion of graphite particles
between layers and the interfacial bonding were also
analysed. The graded specimens for FE SEM were
prepared by gold sputtering in vacuum for 3min to avoid
charging. The FE SEM was carried out in the Secondary
Electron Mode with accelerating voltage at 5kV. Images
were captured for analysis at 49-60X and further zoomed
for 500X for distinct analysis of the surface morphology of
each layer.
Thermo gravimetric analysis (TGA)
The graded composite was examined for their thermal
stability vis-à-vis that of pure epoxy or graphite. The
investigation was carried out using M/S Perkin Elmer
Model No. FTA 6000 instrument. The TGA recording was
carried out at 20 0C/min under constant nitrogen flow of
100 ml/min from room temperature to 10730K. At least five
tests were carried out for each type of samples namely 2, 3,
4, 7 layers and also of 3 layers 0.5wt% and 3 layers 1.0wt%
of 4h gel time.
Nano indentation test
Nano-indentation tests were carried out on samples of 2, 3,
4, 7 layers and also of 3layers 0.5wt% and 3layers 1.0wt%
of 4h gel time. The tests were carried out on M/s Agilent
Nano-indenter, Model No. G 300 at a strain rate of
5mm/min. The samples were examined on their both cross-
section as well as on the top surface. The Young‘s
Modulus, hardness and strain hardening exponent have
been determined.
4. RESULTS AND DISCUSSION
XRD Measurements
XRD scans of pure epoxy and the graded materials are
shown in Fig 5 and of pure graphite powder in Fig 6. The
images of graded composites exhibit amorphous peak with
2θ ranging at 12.8060-13.163
0 corresponding to the
interlayer distance ranging from 6.92 - 7.34 Å. The graded
composite peaks exhibited are seen to be around that of
epoxy phase at 12.260 corresponding to 7.21 Å while the
pure graphite powder phase is observed at 27.60. The
graphite powder phase is observed to be close to the
reported value of 26.50 as mentioned in literature, [31],
[39]. Man Wai et al.and others, [41]-[42] have reported the
phase of epoxy resin to be of amorphous nature with
diffraction angle close to 200. The XRD scan of graphite /
epoxy graded composite is analysed taking into similarity
of the investigation made by Dilani et al., [11] and
Ding et al., [42] As per Dilani et al., [11], the diffraction
peak of graphite did not occur in the diffraction
spectrogram of graphene oxide indicating complete
oxidation of graphite to graphene oxide. The interlayer
distance is attributed to the stretching of crystal lattice
length along the axis c. Also, from Ding et al.,[42] the
observed broad peaks of amorphous nature has been
reasoned as to ZnO being encapsulated or covered by epoxy
resin and thereby weakening the ZnO peaks. This
phenomenon is seen while the ZnO is being synthesised by
in-situ method. The observation has also been associated to
the little weight concentration that has been used for the
synthesis. In the present synthesis of graded graphite-epoxy
composite, the process has been in-situ method with
graphite of little concentration mixed with epoxy. The
spectrogram of graded polymer indicates no graphitic peak
which is analogous to the scan investigated in
Dilani et al., [11]. The deduction from this analogy will be
that the graphite has undergone oxidation during the 12h
mixing process with epoxy and thereby the graphite peak
has not occurred. Also from the analogy of Ding et al.,[42]
it has been deduced that the low concentration of graphite
may have been encapsulated by epoxy matrix that the
graphite peak seems to have shifted downwards to lower
angles (12.800-13.16
0). This work also makes a reference to
the GO/epoxy composites XRD scan of Hae Kyung et al.,
[43] in which GO peaks occurs at 12.840, (001) plane. In
the present work, the investigation of XRD scan show the
effect of variability of either the concentration, number of
layers or the gel time on the 2θ value vis-à-vis that of the
pure epoxy indicating a variation of 7.47 to 9.48% and that
the intensity of higher filler material (GP7) is seen to be
highest.
Fig 5. XRD of epoxy graded composite
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
341 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
Fig 6. XRD of graphite powder
Surface morphology
The FE SEM examined the surface morphology of
fractured as well as cut specimens across the cross-section.
The intent was to study the surface morphology of graded
specimens and establish the continuous gradation by the
fabrication process adopted. The images shown in Fig 7 and
Fig 8 distinctly indicate the continuous gradation. The
graphite particles have dispersed well and there are less
signs of agglomeration. Nevertheless, the images need
further clarity as there could be ingress of moisture while
partial solidification. The continuous grading seen is a good
confidence measure on the process adopted to make graded
materials by gravity method. The 1.0GP31 graded polymer
composite has demonstrated a continuous gradation with
good thermo-mechanical properties. The 1.0GP34
composite though indicates a comparable property, the
gradation layers are seen to be step wise and do not show
the smooth and continuous gradation as the crosslinking
time is more and less diffusion of particles occur during this
period.
Fig 7. FE SEM of 3 graded layers of E-E+Gr-E layers
Fig 8. FE SEM of 7 layers Graded Materials of Epoxy-Graphite
Effect of filler concentrations on mechanical
properties
The flexural and tensile test results are plotted for different
filler concentrations for 3 layers, 4h gel time (0.5 and
1.0wt%) and varying gel time for 1.0wt% 3 layers graded
specimen. The results demonstrates enhancement in
flexural strength with that of pure epoxy and with increase
in filler concentration for 3 layer, 4h graded material,
as shown in Fig 9. The flexural rigidity which represents
bending stiffness is also high with increase in graphite
concentration in comparison to the pure epoxy value, as
shown in Fig 10. The flexural strain is proportionately
varying as per the modulus and strength, shown in Fig 11.
The plots of peak load over area Vs the varying wt% and
gel time, as shown in Fig 12, indicates that 1.0 wt% and 1h
gel time providing improved property. The tensile results
also indicate improved properties with gradation vis-à-vis
pure epoxy values, as shown in Fig 13 and Fig 14. This is
in close agreement with the literature reports on how
gradation has shown to improve mechanical properties with
increase in filler concentration. Subita et al.[32], Kaushik et
al. [20] and Suhermana et al., [33] have reported in their
work on the enhancement of mechanical properties with
increase in filler concentration. The elasticity of modulus
values for graded polymeric materials lies between the two
extremes which is the iso-strain based on Voigt model and
the iso-stress based on the Reuss Model, [44]-[45]. The
extreme modulus of elasticity is given below at Eq 4 and 5.
In case of any positive deviation from the extremes values
it will indicate the matrix constraints.
Ec = Vm Em + Vp Ep ---- (4)
Ec= (Em Ep) / (VmEp+VpEm) ---- (5)
The 1.0GP34 graded material has demonstrated a good
gradation and thermo-mechanical properties over that of
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
342 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
pure epoxy. The improvement in properties mainly
depends on the dispersion and good interfacial bonding.
The dispersion in turn depends on several factors such as
the vibration frequency for cavitation, and the timing set
which will be related to the temperature exposed to the
solution and the resulting degradation. Birgit et. al. in their
work has shown the effects of these factors on dispersion
and the corresponding effects on the mechanical properties
of the composite. The results with respect to change in gel
time seem to indicate that the 1h gel time is providing
better flexural properties than the rest as shown in Fig 15.
There seems to be an increase of 0.61% flexural strength,
26.27% flexural modulus and 23.1 % peak load /sample
area vis-à-vis that of pure epoxy. The young‘s modulus
values for the specimens are further corroborated by the
nano-indentation results which show that the average
Young‘s modulus is around 3.3 GPa and the average
hardness value is about 0.14 GPa. There seems to be no
variation in these values for any changes in terms of
concentration of fillers or number of layers or gel time.
These values measured at nano-scale level may have certain
aberrations particularly for a graded material which are
having some inhomogeneity at the interfaces across the
cross section. The results of nano-indentation have been
discussed in the subsequent section. The correlation of
mechanical testing results with that of nano-indentation
needs further analysis.
Fig 9. Av Flexural Strength Vs Wt%
Fig 10. Av. Flexural Modulus Vs Wt%
Fig 11. Av Flexural Strain Vs wt%
Fig 12. Peak load/sample area Vs Gel Time
Fig 13. Av Tensile Stress Vs Wt%
Fig 14. Av Tensile Modulus Vs wt%
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
343 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
Fig 15. Flexural Strength and Modulus Vs Gel Time of E-Gr Graded
Material
Thermo Gravimetric Analysis (TGA)
The TGA scans for varying concentration, number of layers
and gel time have been analysed. The scans obtained for
various specimens of epoxy-graphite graded composites as
well as of pure epoxy and graphite is shown in Fig 16. The
figure indicates the decomposition in multi-stages
especially for epoxy and 7 layers composite. The scan of
graphite powder indicates a good thermal stability after the
initial decomposition due to moisture indicated by initial
wt% loss at 5130K. The char yield is 94% at 1067
0K. The
independent scan of epoxy at Fig 17 indicates multi- phase
decomposition with the initial volatile decomposition
commencing at 5720K with a weight loss of 0.94%. The
char yield of epoxy indicates 1.21% (Fig 16 & 17) at
10700K. Hui et al., [46] and Asma et al., [39] have reported
an onset of epoxy decomposition at 6330K which is
established in the test as shown in Fig 17. The epoxy-
graphite graded composites of varying concentration based
on number of layers and filler concentration have shown
volatile decomposition commencing from a minimum of
4930K (GP2) to a maximum of 630
0K (GP3). However the
char yield weight loss of all graded composites except GP7
has shown enhanced values compared to epoxy. The
percentage of char yield varies from 0.145% (GP7) to
5.28% (GP3) and the temperature at this point is varying
from 10050K (GP7) to 1072
0K (GP3 & GP4). The increase
in char yield indicates the resistive path the matrix will
have with filler diffused in-homogenously at the interface
layers. The free movement of the matrix chain is restricted
giving rise to enhanced mechanical and thermal properties.
The extent of dispersion of particles, particle size and
saturation limits of the matrix-filler element bonding results
to the increase or decrease of thermal stability. The increase
in concentration with number of layers has seen a trend of
enhanced thermal stability up to some layers beyond which
the thermal stability reduces as noted in the case of 7 layers.
Fig 16. TGA of E-Gr graded layer
Fig 17. TGA of pure epoxy
Nano-indentation Measurement
Nakamura et al., [44] and Carmine et al., [47] have reported
micro indentation modeling work on gradation polymers
using inverse analysis method and Finite Element Method
for studying the tensile properties of a graded composite.
This paper reports the properties through nano-indentation
of graded polymers and of pure epoxy. The average
hardness and modulus of elasticity values of flat and cross
sectional surface of specimens tested are given in Table 4
and Table 5 respectively. The analysis indicates especially
for 2 and 3 layers composite an increase in hardness and
modulus values at the interface across the cross-section.
This is attributed to the effects of interface diffusion and
strengthening of bonds. The test on the cross-section is
schematically represented in Fig 18. The small triangles on
the cross section depict the location of indentation. In the
computation, Poisson‘s Ratio assumed for Epoxy was 0.30
(approx.). However, it is seen that by increasing the number
of layers of epoxy-graphite, hardness and modulus values
of the composite is getting decreased as can be seen from
the indentation test of 4 (GP4) and 7 layer (GP7)
specimens. Pop-in observed in the unloading curves shown
in Fig 19 could be due to surface cracking. Also specimen
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
344 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
Table 4 Nano-indentation on flat surface
Table 5 Nano-indentation on Cross-section
of epoxy and 4 layers (GP4) were sample tested with
different methods mentioned below to check whether the
results of hardness or modulus are same or not. The
methods M1 and M2 mentioned below have given the same
results for hardness as well as modulus of elasticity.
a) G-Series CSM Standard Hardness, Modulus,
and Tip Cal (M1): Continuous Stiffness
Measurement (CSM) option to return hardness (H)
and elastic modulus (E) as a continuous function
of penetration into the test surface. With the CSM
option, every indentation test returns complete
depth profiles of Young‘s modulus and hardness.
b) G-Series Basic Hardness, Modulus, Tip Cal,
and Load Control (M2): Returns hardness (H)
and elastic modulus (E) vs. penetration depth
using multiple load/unload cycles at each test site.
Constant loading rate force application. Here we
set load time, max load, and number of cycles.
Fig 18. Indentation on Cross-sectional area (W x T)
Fig 19. Load Vs Displacement of graded layers and epoxy
5. CONCLUSION
The mechanical / material characterisation results have
demonstrated the feasibility of continuous gradation. The
study of various graded specimens demonstrates 1.0GP31
(1wt%, 3 layers, 1hr gel time) composite to be preferable
comparatively due to better thermo-mechanical properties.
The nano-indentation analysis of graded polymer
composites of 2 & 3 layers indicate an increase in hardness
and modulus values at the interface across the cross-
section. The experimentation is seen to be encouraging for
tailoring functionally graded polymers for effective
aerospace applications.
Acknowledgment
The authors are grateful to the Dr Prahlada, Vice
Chancellor, DIAT (DU) and Director, MILIT, Pune for
encouragement and support. The authors extend gratitude to
R & DE (Engr), Dighy, Pune, CMTI, Bangalore, UoP,
Pune, HEMRL, Pune for support in providing raw
materials, and testing facilities. The authors are grateful to
International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015
345 ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR
―DIAT—NANO project EPIPR/ER/1003883/M/01/908/
2012/D (R&D)/1416‖ for support. The authors like to
acknowledge the technical staff of Material Eng., Physics
and Workshop for their technical support.
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Tirumali Manoj is a research scholar in the Department of Materials Engineering in Defence Institute of Advanced Technology (Deemed
University)., Pune, India. He received master‘s degree in mechanical
engineering (Air Armament) from University of Pune, India. He has authored a chapter in Structural Nanocomposites; Perspective for future applications,
published by Springers in 2013. He is presently in Armed Forces serving in
Indian Air Force. His active areas of research interest include the development of functionally graded and hybrid composites.
Dr K Balasubramanian is an Associate Professor and Head of Materials Engineering Department at Defence Institute of Advanced Technology
(Deemed University) at Pune, India. He has years of industrial experience at
UK, an eminent scholar and well acknowledged academician. Dr. K Balasubramanian obtained various prestigious fellowships including
award for Technical Excellence at UK Materials Research Institute, Selected
for ‗Hind Rattan award‘ for outstanding contribution in the area of science and technology, India and a Professional Fellow of Institute of Technology,
UK. He has published over 200 articles, patents and conference proceedings
in the field of materials science and technology. He authored a chapter in
Structural Nanocomposites; Perspective for future applications, published by
Springers in 2013. (*Corresponding author. [email protected])
Dr A Kumaraswamy is an Associate Professor in Mechanical Engineering Department at Defence Institute of Advanced Technology (Deemed
University), Pune, India. He has been a reputed academician and a visiting professor to well known engineering colleges and Institutes in India. He has
been listed in Who‘s Who in the World (29th edition), Marquis, New Jersey,
2012. Dr. A Kumaraswamy obtained various prestigious fellowships and awards including Sir Issac Newton scientific award for excellence in 2012.
He has published over 80 articles, and conference proceedings in the field of
solid mechanics, contact mechanics, FEA, Nanoindentation, Composites
and metal forming and cutting. ([email protected])