World Journal of Nano Science and Engineering, 2011, 1, 7-14
doi:10.4236/wjnse.2011.11002 Published Online March 2011 (http://www.scirp.org/journal/wjnse)
7
The Development and Characterization of Zirconia-Silica
Sand Nanoparticles Composites
Tahir Ahmad, Othman Mamat
Mechanical Engineering Department, Universiti Teknologi Petronas, Tronoh, Malaysia
Received February 7, 2011; revised March 14, 2011; accepted March 21, 2011
Abstract
The present study aims to develop zirconia-Silica sand nanoparticles composites through powder processing
route and to study the physical properties, mechanical properties and microstructure of the composites. Zir-
conia based silica sand nanoparticles composite with 5, 10, 15 and 20 wt.% were developed through powder
processing technique and sintered at 1500℃ for two hours. A decreasing trend of green density however an
improvement in sintered density was observed. Also the addition of silica sand nanoparticles with 20 wt.%
increased the hardness up to 12.45 GPa and microstructures indicated the diffusion mechanism of silica sand
nanoparticles into pore sites of the composites.
Keywords: Zirconia-Silica Sand Nanoparticles Composites, Physical Properties, Mechanical Properties, Mi-
crostructures
1. Introduction
A great progress in dental restoration technique has been
established by the use of ceramic materials since the 70’s.
Ceramics show some advantages like being the material
that best mimic the bone tissue, although present low
toughness when compared with the metallic materials.
Alumina-zirconia composites are one of the relatively
good and promising candidates for biomaterials appli-
cation, due to biocompatibility and their mechanical pro-
perties that combine high flexural strength with high
toughness. [1]
Hirvonen et al. [2] prepared nanocomposites by pres-
sureless sintering method (PSM). The properties of the
yttrium stabilized zirconia (Y-TZP)/zircon (ZrSiO4) com-
posites obtained by PSM are discussed in terms of its
structure and fabrication condition that include the con-
tent of cordierite in initial mixture of cordierites and Y-
TZP nanopowders and sintering temperature. The inter-
esting finding of this study concerns the coefficient of
thermal conductivity of Y-TZP based composites with
zircon, fabricated from the powder with more than 15 vol.%
content of cordierites, which appears to be lower than
that of pure zirconia, despite the mixed materials exhibit
high thermal conductivity. The DTA-TG examination
confirmed excellent stability of the compo-site at ele-
vated temperatures and proved the lack of the nanocom-
posite oxidation.
Wen-Cheng et al. [3] studied ZrO2/mullite composites
with homogeneously dispersed ZrO2 grains from co-
lloidal or sol-gel processes of the precursors, which were
a mixture of colloidal pseudo-boehmite (γ- AlOOH),
zirconia and silicic acid gel, or prepared from dissociated
zircon with alumina powder. Experimental results in-
dicated that heat treatment from 1300 to 1600 in℃-
fluence the growth of mullite and fine ZrO2 grains in
ZMCs, especially for the composite prepared from sol-
gel methods in which the ZrO2 grew from tens of nano-
meters to micrometer size. The growth of fine ZrO2
grains in a mullite matrix belongs to a mechanism of
coalescence.
Pyda et al. [4] studied the reaction between carbon and
titanium originated from zirconia-titania solid solution to
produce TiC inclusions in situ in zirconia powders and
dense materials. The co-precipitated powder composed
of 1.5 mol% Y2O3, 18 mol% TiO2 and 80.5 mol% ZrO2
was mixed with phenol-formaldehyde resin in an amount
giving 200% excess of carbon in terms of stoichiometry
of the carbothermal reduction of TiO2. The mixture was
heated for 2 h at 1500℃ in argon. The sintered bodies
were composed mainly of tetragonal and monoclinic
phase and contained nano-crystalline TiC inclusions
which showed two different morphologies: oval and
elongated. The role of oxygen vacancies in the in situ

T. AHMAD ET AL. 8
formation of elongated TiC inclusions is discussed. Im-
provement in the characteristics of the zirconia materials
was obtained.
Anne et al. [5] prepared Yttria-stabilised zirconia (Y-
TZP) based composites with carbide (WC) content up to
50 vol.% from nanopowders by means of conventional
hot pressing. The mechanical properties were investi-
gated as a function of the WC content. The hardness in-
creased from 12.3 GPa for pure Y-TZP up to 16.4 GPa
for the composites with 50 vol.% WC, whereas the bend-
ing strength reached a maximum of 1551 MPa for 20 vol.%
WC composites. The toughness of the composites could
be optimised by judicious adjustment of overall yttria
content by mixing monoclinic and 3 mol% Y2O3 co-
precipitated ZrO2 starting powders. An optimum fracture
toughness of 9 MPa m1/2 was obtained for a 40 vol.%
WC composites with an overall yttria content of 2 mol%.
The hardness, strength as well as fracture toughness of
the ultrafine grained composites with a nanosized WC
source was significantly higher than with micron-sized
WC.
Liang et al. [6,7] prepared zirconia-mullite nano-
composite ceramics by in-situ controlled crystallization
of Si-Al-Zr-O amorphous bulk, which were first treated
at 900-1000℃ for nucleation, then treated at higher tem-
perature for crystallization to obtain ultra-fine zirconia-
mullite composite ceramics. The effects of treating tem-
perature and ZrO2 addition on mechanical properties and
microstructure were analyzed. A unique structure in
which there are a lot of near equated t-ZrO2 grains and
fine yield-cracks has been developed in the samples with
15% zirconia addition treated at 1150℃. This specific
microstructure is much more effective in toughening
ceramics matrix and results in the best mechanical prop-
erties. The flexural strength and fracture toughness are
520 MPa and 5.13 MPa.m1/2.
Zhan et al. [8] prepared the Y-TZP ceramic containing
up to 30 vol.% TiC particles. A physical mixture of 3 mol%
Y2O3-ZrO2 powder with the TiC one was used. The
cold isostatically pressed compacts were hot-pressed
for 40 min at 1700℃ under 30 MPa in argon. Adding
TiC particles to Y-TZP did not changed practically the
bending strength and it improved the fracture toughness
and hardness. With 20 vol.% TiC particles the bending
strength, fracture toughness and Vickers hardness of
composites reached~1.1 GPa, 14.6 MPa.m0.5 and 16 GPa
respectively.
Haberko et al. [9] prepared a TiC-TZP composite
starting from a solid solution powder of composition
3 mol % Y2O3, 18 mol % TiO2 and 79 mol% ZrO2. The
material, hot-pressed at 1500℃ under 25 MPa in argon,
showed a density of + 98%, a hardness of 17 GPa, a
Young’s modulus of 221 GPa and a fracture toughness of
4.1 MPa.m0.5 in the presence of 8.3 vol.% TiC inclusions.
From above cited literature, no researcher still had used
silica sand nanoparticles as reinforcement in zirconia
based composites, therefore the present study’s aim to
develop zirconia based silica sand nanoparticles compos-
ites through powder processing technique and to investi-
gate the physical, mechanical properties, microstructure
and XRD analysis of the composites.
2. Experimental Work
The silica sand was originated naturally from Tronoh,
Perak, Malaysia was washed with water to remove mud
particles and dried in oven at 120℃ for 3 hours. The
dried silica sand was meshed about 600 μm using sieve
analysis. The 600 μm size of natural silica was ground to
nanoparticles by using dry ball mill with zirconium ball
(beads) as grinding media. This high energy milling was
known as one of the “top-down” nanoparticles type ap-
proach which generally relies on physical methods for
their production.[10] In this work it was used to reduce
the macro and micro-scale of silica sand to a nanoscale
particle size (which is thereafter referred to “silica (SiO2)
nanoparticles”). The high-energy milling processes in-
volve the comminution of bulk materials. The principle
of comminution is centred on applying physical forces to
bulk material so as to affect breakage into smaller sizes.
The forces required to effect breakage are usually a com-
bination of either impact or shear. Material is introduced
into a milling chamber in which grinding (milling) media
are contained. Milling occurs when the media is made to
move either by stirring (using a rotor) or by shak-
ing/vibrating the chamber. These results in the milling
media making contact the bulk material thus imparting,
depending on the milling parameters, either impact or
shear forces on it. Breakage can occur through a variety
of mechanisms and are generally described as attrition,
abrasion, fragmentation or chipping, and occur both at
the macro and microscopic level. [10] The nanoparticles
obtained were verified by ZetaSizer, Nano ZS (ZEN
3600) (MALVERN) analyser. The ZetaSizer analyser is
based on Photon Correlation Spectroscopy (PCS) or Dy-
namic Light Scattering (DLS) which is a process used to
determine particle size by examining the diffusion rates
(i.e. Brownian motion) of suspended particles. The tech-
nique utilizes a light source to illuminate the samples and
scattered light is collected at a detector typically posi-
tioned at a fixed scattering angle. The latest generation of
PCS/DLS instruments, such as the Zetasizer Nano, utilize a
backscatter angle of 173 degrees. This optical configuration
accommodates both high and low sample concentration,
low sample volumes, and reduces the probability of in-
tensity biasing, a phenomenon that occurs in 90 degree

T. AHMAD ET AL. 9
Figure 1. Particle size distribution of the silica sand nanoparticles (by intensity) produce d fr om
Zetasizer Nano Analyzer. The nanoparticles average size is 54.18 nm.
Figure 2. FESEM image of the ball-milled silica sand nanoparticles.
systems where the magnitude of light scattered by a large
particle overshadows the light collected from the smaller
particles. The nanoparticles size produced was then veri-
fied microscopically through Field Emission Scanning
Electron Microscope (FESEM). The chemical composi-
tion of silica sand nanoparticles was analyzed using
XRF technique. Silica sand nanoparticles were mixed
with ZrO2 (Merck KGaA* 64271 Darmstadt Germany
(zirconium (IV) oxide special grade TechnipurTM (M =
123.22 g/mol, average particles size 400 nm and mono-
clinic crystalline phase) by using ball milling for 1
hour. Paraffin wax had been used as binding agent. The
autopellet machine (capacity 80 kN) at 142 MPa force by
using metallic mould of diameter of 13 mm was used to
make the composites. The following compositions were
developed: pure ZrO2, ZrO2-5wt.% silica sand, ZrO2-
10wt.% silica sand, ZrO2-15wt.% silica sand and ZrO2-
20wt.% silica sand nanoparticles. After the compaction,
the green density of the samples was measured by the
Archimedes method. The samples were sintered at 1500℃
for 2 hour in argon atmosphere respectively. The heating
and cooling rates of sintered process were 5℃/min and
10℃/min respectively. After sintering process, the sin-
tered densities were measured by Archimedes method.
Microstructures of composites were analyzed by FESEM.
XRD-Bruker AXS D8 Advance was used for identifica-
tion of phases present in composites. The hardness was
measured in accordance with ASTM C 1327-9925. The
indentation load was 10 kgf (98.1 N) during fifteen sec-
onds. After the diagonal length measurement, the values
of the Vickers hardness (GPa) were calculated, by the
equation: [1]
Hv = 0.0018544

2
P
d (1)
where:
Hv = Vickers hardness (GPa)
P = applied (N)
d = arithmetic mean of the two diagonal length (mm).
54.18 nm
0.1
In
te
n
s
i
ty(%)
Intensity(%)

T. AHMAD ET AL. 10
3. Results and Discussion
3.1. Morphology, Size and Distribution Analysis
of Silica Sand Nanoparticles
In this work silica sand particles, the macro and micro-
scale were reduced to a nanoscale particle size (which
then called as “silica (SiO2) sand nanoparticles”). The
high-energy milling processes involve the comminution
of bulk materials. The principle of comminution is centred
on applying physical forces to bulk material so as to af-
fect breakage into smaller sizes. The forces required to
effect breakage are usually a combination of either im-
pact or shear. Material is introduced into a milling cham-
ber, which contained grinding (milling) media are. Mill-
ing occurs when the media is made to move either by
stirring (using a rotor) or by shaking/vibrating the cham-
ber and contacts the bulk material thus imparting, de-
pending on the milling parameters, either impact or shear
forces on it. Breakage can occur through a variety of
mechanisms and are generally described as attrition,
abrasion, fragmentation or chipping and occur both at the
macro and microscopic level. The silica (SiO2) sand par-
ticle was quantitatively analysed by using the ZetaSizer,
Nano ZS (ZEN 3600) (MALVERN) and its result is
shown in (Figure 1). The average size for the silica par-
ticles was found to be 54.18 nm, which qualified the
product to be considered as SiO2 nanoparticles.
Morphology, size and distribution of the SiO2 sand
nanoparticles are shown in Figure 2. It can be seen that
some of SiO2 particulates which have fused together re-
sulted from squeezing action of the ball-milling process.
Thereafter ball-milled SiO2 nanoparticles formed wide
range of shape from irregular or rod-like aggregates and
agglomerates to rounded nanoparticles. The high surface
energy of nanoparticles SiO2 is responsible for the ag-
gregation. The conglomeration of submicron SiO2 parti-
cles is a spontaneous process because the Gibbs energy
decreases during this process.
The FESEM image shown in Figure 2 also illustrates
various structures of the SiO2 particles including spherical
particles and irregular shape particles as well as agglom-
erates. This strong tendency for agglomeration which
exists in the particles was induced by the Van-der-Waal
forces acting between the individual particles. The mor-
phology of such agglomerates may vary from chain-like
(1-D) to heavily aggregated (3-D). This alone would not
constitute a major problem since soft agglomerates held
together by Van-der-Waal forces only are easily broken
down in the next step of “dry” pressing that was incorpo-
rated in the powder processing. Several primary particles
seem to cluster or fuse at their faces, which is possibly
due to growth and “sintering” of individual crystals dur-
ing the ball-milling process. EDS analysis on the silica
sand rich phase shown in Figure 3, revealed the presence
of high Si with little amount of Al.
3.2. Chemical Composition of Tronoh Silica
Sand Nanoparticles
The chemical composition of the silica sand analyzed by
XRF analysis gave the results shown in Table 1 .
3.3. Characterization of the Zirconia-Silica Sand
Nanoparticles Composites
3.3.1. Zirconia Matrix Composites with Silica Sand
Nanoparticles Property: Density
A decrease in green density is shown in Figure 4, as
increasing amount of silica sand nanoparticles, which
indicates the presence of porosity in the green samples.
Contrary to that, the rein-forcing of silica sand nanopar-
ticles shows an improvement in densities after sintering.
Also, after sintering, the porosity of the samples de-
creased and the presence of silica sand nanoparticles in
zirconia probably reduce the mobility of zirconia grain
boundaries during sintering and shows full densification.
It also shows a better adhesion between zirconia matrix
and silica sand nanoparticles which improved the sin-
tered densities. Hirvonen [2] also observed such results
of decreasing green densities and improvement in sin-
tered densities in case of corderite/ZrO2 composites.
3.3.2. FESEM Analysis of the ZrO2-SiO2
Nanoparticles Composites
Good interfacial integrity has been observed between the
matrix and the reinforcement and homogeneous distri-
bution of reinforcement can be seen form microstructures
regardless of the size difference between matrix and the
reinforcement. This is due to diffusion of silica sand na-
noparticles in the pores sites of the composites as shown
in Figure 5. In case of 15 and 20 wt.% of silica sand
zirconia based high resolution microstructures reveal the
shrinkage of big pores into shallow inter-connected pores,
with evidence of diffusion mechanism. Formation of
bigger grains, necking and bridging structures was due to
pre-liquid phase sintering.
3.3.3. EDS Analysis of the ZrO2-SiO2 Nanoparticles
Composites
Figure 6 (a, b, c and d), shows the EDS analysis that
reveals the presence of silica sand nanoparticles in 5, 10,
15 and 20 wt.% zirconia based composites. Increasing
trend of silica sand nanoparticles are verified by the EDS
analysis, as the curve of 20 wt.% silica sand is more pro-
nounced as compared to 5 wt.% silica sand nano-

T. AHMAD ET AL. 11
Table 1. Chemical composition of Tronoh silica sand nanoparticles.
Compound Al2O3 SiO2 P
2O5 K 2O CaO TiO2 Fe2O3
wt.% 2.99 95.22 0.77 0.095 0.139 0.16 0.121
Figure 3. EDS analysis of the silica sand nanoparticles.
0
1
2
3
4
5
6
0510 1520 25
Amount of Silica Sa nd Nanoparticles in Zirconia (w t.%)
Green an d S intered Densi ties of
Com po sites (gcc)
Green Densi ty of c om posit esS i ntered Den si ty of c om posit es
Figure 4. Green and Sintered densities of ZrO2-SiO2 composites.
Figure 5. FESEM analysis of ZrO2-SiO2 nanoparticles composites with (a) 5wt.%
SiO2, (b) 10wt.% SiO2, (c) 15wt.% SiO2, (d) 20wt.% SiO2.
Elements Weight %Atomic %
O K 64.95 76.47
Al K 0.35 0.37
Si K 34.52 23.15
Total 100 100
Si
O
Al
0 1 2 3 4 5 6 7 8 9
Full Scale 9016 cts Cursor: 0.000 keV
(a)
(c) (d)
(b)

T. AHMAD ET AL. 12


Figure 6. EDS analysis of ZrO2-SiO2 nanoparticles composites with (a) 5wt.% SiO2, (b) 10wt.% SiO2, (c) 15wt.% SiO2, (d)
20wt.% SiO2.
particles in composites. It also verifies the homogeneity
of the composites because all of EDS analysis of com-
posites shows the peaks of silicon, oxygen and zirconium.
The EDS analysis verified the presence of silicon, oxy-
gen and zirconium, which may lead to the formation of
ZrSiO4, Hirvonen [2] found such observation in case of
corderite/ZrO2 composites, which revealed the existence
of ZrSiO4. It is also frequently indicated that ZrSiO4 is a
proper candidate for high temperature ceramics, and as
such, it matches the requirements concerning compo-
nents of this “new composites”.
3.3.4. Hardness measurements of ZrO2 -SiO2
Nanoparticles Composites
An increasing trend of hardness was observed with in-
creasing trend of silica sand nanoparticles in zirconia-
based composites as shown in Figure 7. The increase in
average hardness can be attributed primarily due to the
presence of nano-size reinforcement particulates to higher
tion. Both the matrix and reinforcement are ceramic ma-
terials, therefore this assist in obstructing localized plas-
constrain to localized matrix deformation during indenta-
tic deformation of the matrix during hardness test. Fur-
(c)
500μm Electron Image 1
(b-1)
0 1 2 3 4 5 6 7 8 9 10
Full Scale 4624 cts Cursor: 0.000 keV
Si
Zr
Zr
O
Spectrum 1
(b)
500μm Electron Image 1
(d)
500μm Electron Image 1
(d-1)
0 1 2 3 4 5 6 7 8 9 10
Full Scale 4624 cts Cursor: 0.000 keV
Si
Zr
Zr
O
Spectrum 1
(c-1)
0 1 2 3 4 5 6 7 8 9 10
Full Scale 4624 cts Cursor: 0.000 keV
Si
Zr
Zr
O
Spectrum 1
(a-1)
0 1 2 3 4 5 6 7 8 9 10
Full Scale 4624 cts Cursor: 0.000 keV
Zr
Zr
Si
O
Spectrum 1
500μm Electron Image 1
(a)

T. AHMAD ET AL. 13
0
2
4
6
8
10
12
14
0510 1520 25
Amount of Silica Sand Nano
p
articles in Zirconia
(
wt.%
)
Hard ne ss Hv (G P a) o f composi tes
Figure 7. Hardness of ZrO2-SiO2 nanoparticles composites
with (a) 5wt.% SiO2, (b) 10wt.%SiO2, (c) 15wt.% SiO2, (d)
20wt.% SiO2.
thermore, the difference in coefficient of thermal expan-
sion between the matrix and reinforcement particulates
leads to higher dislocation density in the matrix and this
resulted in the hardening of the matrix. [11,12] Higher
hardness values imply good wear and scratch resistance.
However, machining can be very hard to be done. The
indentation load is critical in the measurement of hard-
ness and fracture toughness using the indentation method.
The amount of load could affect the indentation of the
sample dimension and the microstructure of the material.
[1] Solid loading plays an important role in determining
the porosity and mechanical strength characteristics of
the sintered samples. The strength of porous ceramic is
also affected not only by the porosity, but also by forma-
Figure 8. XRD analysis of ZrO2-SiO2 nanoparticles composites.

T. AHMAD ET AL. 14
tion of sintering neck on the wall as well as the smaller
pore size. The loading of the solid materials and sintering
temperature significantly contributes to change of ce-
ramic struts and porosity. [13]
3.3.5. XRD Analysis of ZrO2-SiO2 Nanoparticles
Composites
XRD phase analysis of composites is shown in Figure 8.
The peaks have been identified as belonging to phases of
zirconia and silica. It was noted that as the weight per-
centage of reinforcement increases, the intensity peaks
becomes stronger. XRD results also reveals that almost
all the composites show similar peaks which result
proper mixing of the composites constituents. Originally
the zirconia powder was monoclinic and when it is sin-
tered at 1500℃ only a little bit phase trans-formation
occur from monoclinic to tetragonal at this temperature.
This quantity decreased with increasing temperature.
This decrease could be due to grain growth of the ZrO2,
lack of phase stabilizer, such as CaO or MgO and the
thermal stress induced transformation and such behav-
iour has also been discussed by Wen-Cheng et al. [3].
4. Conclusions
The outcomes of this research show that the addition of
silica sand nanoparticles to zirconia decreased the green
densities. However, an improvement in sintered densities
is observed. The incorporation of silica sand nanoparti-
cles enhanced the hardness of the composites up to
12.45 GPa. From FESEM and EDS analysis, it was ob-
served that the silica sand nanoparticles diffused into the
porous sites of composites causing an improvement in
mechanical properties. The XRD results show almost
similar peaks for all composites. These observations in-
dicate that silica sand nanoparticles may be used to de-
velop new zirconia based composites better properties.
5. Acknowledgements
The authors would like to thank the Universiti Teknologi
PETRONAS for providing necessary support in complet-
ing this research.
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