1. Materials and Manufacturing Processes, 21: 652–657, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6914 print/1532-2475 online
DOI: 10.1080/10426910600611581
Synthesis of Alumina–Zirconia Nanocomposites by Solgel Process
S. K. Malhotra1
, Paramanand Singh1
, and A. Thirunavukkarasu2
1
Composites Technology Centre, Indian Institute of Technology, Madras, Chennai, India
2
Department of Metallurgical & Materials Engineering, Indian Institute of Technology, Madras, Chennai, India
Al2O3–ZrO2 nanocomposites were developed starting with the solgel process. Composite alumina–zirconia nanopowders were synthesized from
metallorganic precursors (Aluminium secondary butoxide and zirconium Iso propoxide) using the solgel process. The parameters affecting the
synthesis—solvent, concentration of precursor, R/H ratio (i.e., dilution of water in solvent)—were varied as also the temperature and pH. BET and
TEM were used to measure nanosize. Diffuse reflectance spectroscopy and also qualitative optical absorption led to identical particle size estimate.
The variation of process parameters was used to study the effect and interdependence of process parameters. Artificial Neural Networks was used
to rigorously analyze the process. Although this led to confirmation of interdependence of parameters, the presence of a single overwhelming
solvent variable was also established. Then the optimal process was used to synthesize more nanopowder. To produce bulk nanocomposite the
nanopowders were sintered by varying the temperature and time period. The sintered lithoids were probed with a vickers hardness tester to measure
elastic modulus, hardness, and fracture toughness. The results showed high elastic modulus, modest hardness, and very high fracture toughness.
Keywords Alumina; ANN; Ceramic; Characterization; Composite; Indentation; Nanocomposite; Nanopowder; Optimization; Process;
Properties; Sintering; Solgel; Ultrafine; Zirconia.
A material having two or more distinct constituent
materials or phases such that the integrated material
has properties noticeably different from constituents is a
composite material [1]. Nanocomposites have at least one
of the phases with dimensions in the nanometer range [2].
They can be classified based on composition as
metal-based, ceramic-based and polymer-based. Based
on microstructure, Niihara classified nanocomposites
as intragranular, intergranular, hybrid, and nano/nano
composites. Intragranular has a nanosized phase inside the
grain of the larger phase. In intergranular the nanophase is
in the grain boundary of the larger size phase. In hybrid
the nanophase occurs both inside the grain and in the
grain boundary. Nano/Nano composites have both phases
of nanosize dimensions.
A similar microstructure classification based on
connectivity concept (by Newnham) describes nanocompo-
sites as 0–3, 1–3, 2–2 etc., where the first digit denotes
dimensionality of second phase and the latter of the
matrix (e.g., 2–2 means an interpenetrating two-dimensional
microstructure) [3].
Making nanocomposites involves the control of diffusion
through process variables such that nuclei form but their
coarsening and growth are suppressed [3].
Nanocomposites offer a remarkable potential for new
properties and technologies. This is due to their novel
properties. Nano/Nano ceramics are ductile as opposed to
brittle microscopic ceramics. They have higher toughness
and very high elastic modulus. However, they can be made
with even finer nanosize phases. In turn, this may further
increase the properties. This also includes their verified and
Received November 9, 2005; Accepted December 14, 2005
Address correspondence to S. K. Malhotra, Composites Technology
Centre, Indian Institute of Technology, Madras, Chennai 600 036, India;
E-mail: malhotra@iitm.ac.in
potential high strength and wear resistance. Nanosize phases
is a new way for highly tough ceramics.
While preliminary studies have been carried out, further
studies are necessary both to confirm the initial studies and
also to develop an understanding of these materials.
A number of studies to develop nanomaterials for
their mechanical, physical, and functional properties have
been carried out. But the verification of their suitability
is not yet complete. The present work is aimed at
developing alumina–zirconia nanocomposite for cutting tool
application. Previous attempts to fabricate such a cutting
tool had a minimum phase size of 30 nanometers (nm). If
the phase size can be decreased further, it would lead to a
better tool. It would mean higher fracture toughness. In the
two-phase material the nano zirconia grains would provide
insulation as well as high toughness. The alumina phase
would provide toughness but would be more conducting to
pass the tool friction heat to the interior. In addition, a two
phase composite structure would vastly increase the ability
to restrict the size of both phases to very fine nanosize.
A number of methods are in use to synthesize ceramic
nanocomposites. But they also have shortcomings. Some
lead to material with impurities. Others use high temperature
and lead to coarse nanosize greater than 10nm. A
few of them are restricted in the amount of material
volumes they produce. However the solgel process with the
organometallics as precursor offers an effective alternative
to all these processes. Solgel would be used to synthesize
nanopowder. They can be consolidated to nanocomposite.
There are few published reports on mechanical properties
of nano/nano composites. But a few reports that have been
made predict superior/very good mechanical properties.
In addition, oxide/oxide system of Alumina–Zirconia is
very suitables as nano/nano composite for cutting tool
application.
A number of ceramic nanocomposites have been shown
to possess favorable properties for structural and functional

2. SYNTHESIS OF ALUMINA–ZIRCONIA 653
applications. The present work is aimed to do the same for
a cutting tool material; however, it also attempts to make
a realistic target of applying it and close the gap between
properties and actual application.
Alcohol-based solgel processes can have homogeneity on
a molecular scale (when using organometallic precursors).
In the case of inorganic salt precursor this solgel process
requires ability to coprecipitate to maintain homogeneity on
an atomic scale. For Alumina–Zirconia system the inorganic
salts do not coprecipitate.
Solgel process, being a popular as well as versatile
process, has been used in multiple studies and material
systems. It has been used to make ceramics at low
temperature—say, to coat on polymer or maintain single
phase without phase separation or to prevent phase
coarsening. It has also been used to produce nearly full
density at low temperature due to high surface area
of solgel product. Some have used solgel to vary and
control composition. It can be a good process to obtain
uniform dispersion of a second molecular species/second
phase. The solgel process can enable the tailoring of
particle/coating size. Other studies have used solgel to
overcome agglomeration and enable scaling-up. Another
study has used solgel to synthesize sol stable for long
periods of time. The present study aims to incorporate most
of these features in a single process.
Solgel being the low temperature process, it offers
the greatest scope for the smallest nanosize material.
Also, organometallics are expected to produce narrow size
distribution. Organometallic precursors are also of very high
purity. This is the first time nanomaterial is produced from
organometallics. These precursors would lead to lowest
nanosize.
Composite nanopowder from organometallics would be a
major breakthrough for a number of reasons. The problem of
agglomerations of two different phases would be overcome.
A molecular level mixing would lead to complete mixing
of two phases. Because they are mixed at molecular
level coarsening of both phases during sintering would be
avoided. This would enable a nano/nano composite of very
fine phase size. Also a room temperature process that can
be scaled up would be possible. The actual reaction would
be a few hours and other steps such as calcination can be
split into separate operations of a few hours each.
Hence, the material development began with the synthesis
of composite nanopowder.
Synthesis of nanopowder
The composite alumina zirconia nanopowders were
synthesized by the solgel process using the organometallic
precursors aluminium secondary butoxide and zirconium
n-propoxide.
Initial Solgel Synthesis
Aluminium secondary butoxide is dissolved in a solvent
containing acetyl acetone. The solution is raised to
appropriate temperature and pH additives incorporated.
This is hydrolyzed by addition of a distilled water and
anhydrous solvent mixture and stirred for one hour in
ambient environment. Subsequently, zirconium n-propoxide
is added and stirred for two hours. Finally, a water and
solvent mixture containing water is added in a continuous
stream under vigorous stirring. This leads to the formation
of a clear yellowish sol [4].
The sol obtained was dried in an oven at 110 C for four
or more hours to cause gellation and drying of the gel.
The dried gel was calcined at 500 C for two hours to
produce powders. The organic residues decompose and
burn at 500 C, hence, the choice of this temperature for
calcination.
The calcined powder was then milled in a planetary mill
to break the lumps of powder. The milling was carried out
for five hours at 100rpm.
Extended Studies on Solgel
It was realized that a number of variables influence the
solgel synthesis. These include temperature, concentration
[5], pH [6], and solvent type [5]. Concentration is an
important factor. It comprises of concentration of precursor
and concentration of water. Extended studies were carried
out to qualitatively study the nature of the solgel process.
These factors were varied in a number of experiments
and the result on the nanopowder—its particle size
was measured. However, the concentration was varied
systematically. The objective was to develop a qualitative
understanding of the nature of the solgel process in terms
of the influence of the variables.
Characterization of composite
alumina–zirconia nanopowders
Various studies were carried out to characterize the
properties of the composite nanopowders. Apart from the
powder particle size, the nature of the processing method
and an insight into how it affects the powder was also
obtained.
Brunauer, Emmett, and Teller (BET) Surface Area
The powders were studied by the BET method to find
the surface area per unit mass. The method involves the
adsorption of a monolayer of gas atoms on the powder.
The BET procedure was carried out using the equipment
sorptomatic, carlo Erba strumentazinone.
The BET surface area was them used to calculate the
average particle size. The relation between surface area and
particle size is [7]:
d = Ks/ S (1)
where d = particle size, meter, Ks = shape factor = 6, S =
surface area (in m2
/kg , = density, kg/m3
Transmission Electron Microscopy
Transmission Electron Microscopy (TEM) was used to
characterize the solgel nanopowder. The particle size of
the nanopowder was analyzed. A phillips CM12 STEM
120kV instrument was used. The magnifications ranged
from 45,000X to 1,00,000X. Different methods were used
for sample preparation. The first method used a solder metal

3. 654 S. K. MALHOTRA ET AL.
Table 1.—BET surface area and corresponding particle size.
Sample Solvent Concentration (M) R/H Temperature (C) pH H2O:MOR ratio BET surface aream2
/gm D(nm)
Old T0 E 0 381 2 45 7 1 170 8 45
Old T2 40E + 60A 0 76 5 37 7 1 204 7 05
Old T3 70E+30A 0 381 10 45 9 1 212 6 79
Old T4p 25E+75A 1 523 5 37 9 1.5 214 6 74
Old T6 90E+10A 0 381 2 30 7 1 183 7 88
T19 10E+19A 0 76 5 30 2 1 150 9 57
Debsi But 1 52 2 30 7 1 243 5 89
Sysc But 0 76 2 30 7 1 209 6 89
SysRH But 1 523 5 30 7 1 151 9 50
E = ethanol, A = acetone, But = butanol, R/H = ratio of volume of alcohol, R-OH to the volume of water (H-OH), MOR = Metal
alkoxide.
to incorporate powder, then hot pressing and thinning by
electropolishing.
The second method ultrasonicated the powder in a
Schoeller & Co. ultrasonic agitator in acetone medium. The
frequency of the instrument was 30kHz. A drop of the
suspension was placed on a carbon-coated grid and allowed
to dry. This was observed in the TEM.
Diffuse Reflectance Spectroscopy (DRS)
Diffuse Reflectance Spectroscopy was carried out to
analyze the particle size of the nanopowder [8]. The
instrument used was Varian Cary Model No. 5E UV-NIR
spectrophotometer. The two sources used were a tungsten
lamp for the range 2500 to 400nm and a deuterium lamp
for the region 400 to 200nm.
The powder was mixed with silica (SiO2) and pressed into
a pellet by application of medium pressure. The spectrum
was then obtained for the UV to near IR region. This,
on comparison with data reported in literature, led to an
estimate of particle size.
Qualitative Optical Absorption
Optical absorption spectroscopy is a method used to
characterize materials by measuring the absorption of light
as the wavelength of the incident monochromatic beam is
varied. The instrument used in the present study is model
No. U3400 of Hitachi Ltd., Japan. It uses a double beam to
measure the incident and transmitted beam intensities. The
source of light is a tungsten filament lamp for the visible and
near infrared ranges (350nm to 2.5 m). The detector is a
photomultiplier in the visible range and a photoconductor in
the near infrared region. A hydrogen (deuterium) lamp is the
source for the near ultraviolet wavelengths (350–200nm).
The old T0 powder was impregnated in araldite resin to
produce a polymer film that was transparent. A blank film of
the same material was also made to check for the absorption
by the matrix.
Particle size of synthesized nanopowders
The results of the particle size analysis of the nano-
powders by various methods were encouraging. They
demonstrated the accuracy and suitability of different and
interdisciplinary methods of particle size analysis. The
widely divergent methods and modifications in view of the
present requirements were proved to be correct by the results
(Table 1).
Old T0 powder had a particle size of 8.45nm. All the
synthesis experiments led to very high surface area and
very fine nanosize less than 10nm. Hence, the successful
synthesis of nanopowders, of very fine nanosize has been
demonstrated (Fig. 1).
TEM showed particle size of 8nm (Fig. 2).
The DRS showed no feature up to 1000nm. The above
is the spectrum for 800nm to 200nm.
The diffuse reflectance spectra (DRS) of old T0 powder
shows a shoulder at 340nm. K. G. Kanade and colleagues
[9] have reported an absorption edge at 351nm for Zirconia
powder of 20nm particle size. This absorption is due to
surface defect abundance of high surface area nanopowders.
Hence the data is relevant to crystalline as well as
amorphous nanopowders. The powder in present study has
an absorption shoulder at 340nm. This is clearly less than
351nm. The particle size in probably less then 10nm.
Figure 1.—TEM photograph of old T0.

4. SYNTHESIS OF ALUMINA–ZIRCONIA 655
Figure 2.—Diffuse reflectance spectrum of old T0 in the band 200 to 800nm.
Qualitative Optical Absorption
The optical absorption led to qualitative information on
the nanopowder particle size. According to literature [10]
alumina film of 7nm has an absorption edge at 280nm. The
powder from solgel synthesis old T0 showed no absorption
upto 300nm. This shows that the powder is nanosize and
that of size less than 10nm.
Thus all the characterisation methods led to an identical
particle size around 8.5nm.
Analysis of solgel process by artificial
neural networks (ANN)
ANN was used to analyse the nature of the solgel process.
This included the variables that affect the process and the
extent of influence of different variables. The procedure was
carried out in Matlab 6.5 using the ANN tool Box.
The results showed that hydrophobic nature of the solvent
was the only important factor when it was present and,
by itself, it led to very high surface area i.e., very small
particle size. In the absence of hydrophobic solvent, basic
pH, high water to alkoxide ratio, and low hydrogen bonding
increased surface area. In presence of hydrophobic solvent,
other factors were not important. They could not improve
the results of hydrophobic solvent.
Fabrication and characterization of bulk
nano/nano composite
The nano/nano composites were fabricated from
synthesised debsiM nanopowder having a particle size of
5.9nm. The powders were cold pressed with 1.73 tonnes
load corresponding to a pressure of 150MPa. This pressure
was chosen based on a previous study [11]. Three green
Table 2.—Size of nanophases of bulk nano/nano alumina–zirconia composite.
-Al2O3 dimension m-ZrO2 dimension
Sample name Temperature ( C) Time (minutes) (024) (104) (211) (¯1 02) (¯1 11) (022)
CTC 4402 800 C 120 5.5 23 29 5.5 23 29
METTHIRU 800 C 30 3.5 20 27 3.5 3nm & 20nm 27
9902ATHIRUN 750 C 30 2.25 17 24 2.25 17 24
compacts were made, one weighing 1gm and two weighing
0.75gm each.
Time–temperature studies were carried out to fabricate
nano/nano composite. The first sample from 1gm powders,
CTC 4402, was sintered/crystallized at 800 C for two hours.
Each of the other two compacts had been made from 0.75gm
powder each. The first one, METTHIRU, was sintered at
800 C for 30 minutes. The second, 9902 ATHIRUN, was
sintered at 750 C for 30 minutes.
All the three samples were characterized using XRD. The
phase size was determined from peak broadening using the
relation [12]:
d = 0 9 / B cos (2)
where d = phase size, = wavelength of X-ray = 1.542Å,
B = full width at half maximum in radians, and =
position of the X-ray peak.
The XRD was carried out on a SHIMADZU XD-D1
X-ray diffractometer.
Because the sample was a regular circular disk the
dimensions were measured to obtain its volume, then, from
the weight of the piece, the apparent density was calculated.
This on comparison with the reference density gave the
porosity.
Nanosintering results
The XRD results of bulk sintered nano/nano composites
(Table 2).
Nanosintering involved on one hand, the separation of
Alumina and Zirconia phases from the molecular mixture
of them in the solgel nanopowder [13]. In another way,
the separated Alumina and Zirconia phases crystallize.
High temperature with long sintering time as well as low
temperature with less hold time are inadequate. We should
enable diffusion with high temperature, but we should not
allow too much time since the phases will be of coarse sizes
(due to more diffusion [14]). Thus the sintering process
must be optimized.
The density measurement showed 60% porosity. The
green compact was 100% dense, but at high temperature of
sintering they loose molecular water so much that it leads
to 60% porosity. The solgel nanoparticle have Aluminium
(Al) and Zirconium (Zr) along on the surface of the particle
forming Al(OH)3 and Zr(OH)4. On sintering at 800 C the
hydroxyl groups are volatilized and the hydroxides of Al
and Zr convert to respective oxides loosing mass. This leads
to 60% porosity.
Thus the time–temperature studies led to an in-depth
understanding of the nature of the phenomena involved in

5. 656 S. K. MALHOTRA ET AL.
sintering of nanopowders. The sintering was also optimized
through these studies.
The indentation method for measuring
mechanical properties
The sample METTHIRUN processing was optimal
leading to best nano-nano composite phase sizes. Hence it
was studied to obtain the mechanical properties. This was
implemented by the indentation method using the hardness
tester Wolpert Vickers Hardness Tester-Amsler Diamond
Tester 2RC. A load of 5kg was used. The vickers hardness
was calculated [4]:
The elastic modulus was calculated using the relation
[15]:
e = K5N cot 1
he
h
(3)
where K5 = constant depending on shape and material of
indenter = 0.35, N = constant calculated from elastic and
plastic penetration, and K5 = 0 76, 1 = angle of Vickers
diamond indenter = 68 , he = h ⇒ he/h = 1 .
Elastic modulus,
E = −1 08 1 − 1 − 2 2
1 ×
HV
e
(4)
where 1 =Poisson’s ratio = 0 2 (for ceramic), HV =
Vickers Hardness.
The fracture toughness was calculated from the lengths
of the two diagnols of vickers diamond indentation and
average crack length at each vertex of the indentation. A
number of relations were used for obtaining the fracture
toughness.
Mechanical properties of bulk nano/nano
composites
The data from the indentation are:
Load diagonal diagonal Average crack length
d1 d2 at each corner
5kg 0.4mm 0.4mm 125µm
The hardness is 58VHN (=0 142GPa). The elastic modulus
was calculated as 419.66GPa.
The fracture toughness from the various relations are:
Relation I [19] II [20] III [21] IV [22] V [23] VI [23] VII [23]
KIC
(MPa
√
m) 26.65 29.42 30.90 20.08 34.52 34.95 26.89
Thus the fracture toughness KIC is about 30MPa
√
m.
Conclusions
Composite nanopowder and bulk nano/nano composites
were developed by controlling the diffusion processes.
Nanopowders were successfully synthesised from high
purity organometallic precursors.
Following conclusions can be drawn from the above
work:
1. The presence of hydrophobic solvent was found to be an
important factor leading to very fine nanosize.
2. Different methods of particle size estimation led to
consistent estimate of particle size (6–10nm range).
3. The yield was 40–50% and bulk nano/nano composites
produced by cold pressing and sintering had high elastic
modulus and very high fracture toughness.
4. Artificial Neural Network analysis showed that, in
the presence of hydrophobic solvent, it was the only
important factor. However, in its absence other factors
led to very fine nanosize.
The high hardness was acceptable in view of very high
porosity (nano materials have inverse Hall–Petch relation
[16]). Reference 24 says that ZrO2 of nanosize has inverse
Hall-Petch relation. Other authors have said ceramics have
inverse Hall-Petch relation and metals have traditional Hall–
Petch relation. The high elastic modulus is an indication of
high strength. The processing by hot pressing may improve
all properties to unprecedented high levels.
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