TITLE:
A method for producing a thermal shock resistant and high temperature wear resistant
ceramic composite material.
FIELD OF INVENTION:
This invention relates to a method for producing an improved ceramic composite material
with thermal shock resistance and high temperature wear resistance.
BACKGROUND OF THE INVENTION:
The brown fused alumina has the advantages of relatively strong crushing
resistant ability, oxidation resistance, corrosion resistance and good chemical stability,
and is an abrasive having wide use. These properties are exploited in developing various
products using brown fused alumina as the main raw material. The US patent 5139539
describes the invention that sol gel alumina abrasive grain either alone or bonded with
other abrasive grains provide superior performance in bonded abrasives for grinding
cast iron. In another patent US 5143522, it is revealed that an abrasive grain comprising
alumina alongwith about 20-50 wt. % zirconia, 2-10 % titania with total carbon content
<0.5 % is a high performance material for producing coated abrasive products or
bonded abrasive products. Although the levels of impurities are quite low, their influence
on the abrasive’s grinding performance can be most dramatic and significant. US patent 5
494 267 describes an improved castable refractory composition having excellent
resistance to attack from slag. The castable refractory composition includes 36-92 wt. %

silicon carbide, 8-20 wt % colloidal silica binder and balance substantially of other
refractory materials including alumina, microsilica, graphite and a setting agent.
The conveyor systems used in mining industry are subject to considerable wear
from sliding abrasion of the conveyed materials with components of the conveyor
system. US 8485336 reveals a composite liner comprising a base metal having a
ceramic material embedded therein which exhibits improved resistance to wear and
therefore have longer usable life than liners formed of the base metal alone. Refractory
articles must resist thermal, chemical and mechanical attacks. Thermal attacks include
high temperature, often above 1000 0C, and thermal shock caused by quickly changing
the temperature of the article. Frequently, the application in which the article is used
includes or generates damaging chemicals. For example, slags present in steel casting
chemically attacks refractory articles so that articles contact with the slag often include
slag resistant oxides such as zirconia. Similarly, refractory tubes used in aluminum-killed
steels must resist a build-up of alumina that could otherwise clog the tube. Finally the
refractory article must be strong enough to resist mechanical forces such as compressive,
torsional and tensile stresses. Commonly, refractory articles are formed from a
combination of refractory aggregate and a binder. The binder holds the aggregate in

place. Both the aggregator and binder can profoundly affect the properties of the article.
Common aggregates include silica, zirconia, silicon carbide, fused alumina, magnesia,
spinels, calcined dolomite, chrome magnesite, olivine, fosterite, mullite, kyanite,
andalusite, chamotte, carbon, chromite and their combinations.
US patent 8618006 describes a cement free mixture containing brown fused
aluminum oxide, silicon carbide, fumed silica, aluminum metal, an anti-oxidant, reactive
alumina, and techniques to create a carbon bearing material. The mixture can be formed
by conventional techniques to create refractory articles to contain or direct the flow of
liquid metals. The refractory articles formed by the mixture do not require firing to
achieve an initial cure. CA 222 1822 C patent provides a vitreous-bonded abrasive
grinding wheel wherein the abrasive grit portion comprises an alumina abrasive and
wherein the wheel has improved corner or form holding characteristics and mechanical
properties. The invention further includes the bond composition which allows for
improved corner or form holding and mechanical properties with alumina abrasives. The
alumina abrasive comprises of a mixture of white fused alumina and brown fused
alumina. WO2005061411A1 describes a monolithic refractory material comprising of
refractory aggregate and a binder; the binder comprising petroleum pitch, carbon black
and graphite. The refractory aggregate is selected from the group consisting of brown
fused alumina, bauxite, tabular alumina, silicon carbide, calcined bauxitic kaolins,
andalusite, and calcined fireclay and combinations thereof.

Coated abrasive material having blends of abrasive grains of sol gel process
alumina and alumina-zirconia in a volume ratio of at least 15 % sol gel abrasive grain
as described in US patent 5007943 provide improved performance in certain grinding
applications than such products of either abrasive grain alone. The first layer of abrasive
grains is of fused alumina. Similar sol gel based abrasive fused alumina have also been
developed in US patent 5139536. In the US patent US 5147830 and US 5147834, an
improved casting composition utilizing an aqueous finely dispersed silica binder is
reported to provide reduced drying times and reduced cracking and explosion of troughs,
runners and other containment equipment designed for use by the iron and steel industry.
The composition was prepared by mixing 8-14% by weight of the aqueous silica binder
with 55-90% by weight of a refractory base material and up to 35% by weight of silicon
carbide. Preferably, the composition will also include 5-10% by weight of calcined
alumina, 1—10% by weight of microsilica, and 0.02—1% by weight of a setting agent,
Troughs and runners manufactured from the composition also exhibit increased strength
and improved resistance to oxidation, corrosion and erosion.
The above developments confirmed the use of brown fused alumina as an
inexpensive refractory hard material for various applications. However, no report exists
for development of high temperature wear resistant and thermal shock resistant
composite materials as liners for boiler applications. The present study has thus invented
a process for developing a composition using brown fused alumina (BFA) based
material satisfying this requirement.

OBJECTS OF THE INVENTION:
An object of the present invention is to propose a method for producing an improved
ceramic composite material.
Another object of the present invention is to propose a suitable low cost refractory
composite using hard brown fused alumina and calcined alumina materials.
Still another object of the present invention is to propose a method to sinter the material
at high temperature to impart strength and wear resistance.
Further, object of the present invention is to impart thermal shock resistance using the
porous network in the developed composite.
Still further object of the present invention is to study the high temperature wear
resistance of composite in the temperature range of 30-900°C.
BRIEF DESCRIPTION OF THE PRESENT INVENTION:
According to this invention there is provided a method for producing an improved
ceramic a composite material comprising:

Uniform mixing fine alumina based wear resistant composition with suitable amount of
brown fused alumina in presence of a binder in a mixer,
subjecting the mix to the step of fabrication to produce tiles using hydraulic press,
sintering the tiles in a gas operated kiln to incorporate controlled porosity in the structure
to produce a high temperature wear resistant and thermal shock resistant ceramic
composite structure.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 summarizes the effect of impact angle on the erosion volume loss of the
developed material. Similarly figure 2 depicts the effect of temperature on the erosion
volume loss of the material. The FAC based composite displayed least volume loss at
all temperatures tested and at all impact angles. Figure 3 depicts the variation in
modulus of elasticity on the number of thermal shock cycles. Similarly, figure 4
summarizes the effect of erosion volume loss on the number of thermal shock cycles.
Out of all the three short listed composites described in this work, the reduction in
MOE for FAC composite was the least and the most important is the variation of
data for FAC composite was the least as demonstrated by the higher values of R2
coefficient for such composites compared to other two materials. Table 1 summarizes
all the properties of the three composites and as described already, FAC composite
was found to be the best among all the materials studied.

DETAILED DESCRIPTION OF THE INVENTION:
Thermal Stress and High temperature solid particle erosion is a serious issue in
many industries which are subjected to both chemical and mechanical interactions.
Erosion wear or “solid particle erosion” at elevated temperatures is an even more special
issue. The refractory lining materials for circulating fluidized bed (CFB) furnace are
commonly eroded between 8500 C and 11000 C by the coal powders, high-melting
mineral impurity particles and limestone grits used for desulfurizing. The refractory
linings of dry-process cement kiln, cyclone separator and garbage incinerator are also
subject to similar erosion wear at various elevated temperatures by clinker, dust and
garbage. Their services can be severely shortened by such high temperature erosion wear.
Therefore it is essential to fully understand the process and mechanisms of thermal
stresses and high temperature erosion of commonly used refractories, which in turn will
help to develop new generation of refractories with improved erosion wear performance.
Most of the experimental effort have been in testing of ceramic materials for its
thermal stress and high temperature wear resistance. In the present context, interest is
confined to Alumina and Mullite which are suitable for use as high temperature wear
resistant refractory lining. Among the analytical theories for thermal shock behavior,
there exists two fundamental concepts. One relates the parameters that affect crack
initiation by thermal shock and the other relates those parameters responsible for thermal

crack propagation. In the first case, high strength and low modulus of elasticity provides
resistance to thermal shock by preventing crack nucleation. In the second case, low
strength and high modulus of elasticity provides resistance to thermal shock damage by
decreasing stored strain energy at fracture. In the latter case, it is assumed that the
thermal shock environment is too severe for the prevention of crack nucleation, therefore,
the prevention of catastrophic failure by limitation of the damage caused by propagating
cracks is most feasible.
These theories do not immediately suggest a technique for designing materials with
resistance to thermal shock. For most ceramic materials, those microstructural parameters
that affect the modulus of elasticity also affect strength similarly. The alternate approach
is possible, however that does not necessarily depend upon a relationship between
strength and modulus of elasticity. It is necessary to differentiate the effect of thermal
shock upon the mechanical behavior of materials from the more commonly experienced
effect of mechanical loading. In the latter case, the strain of a material is determined by
the appropriate modulus of elasticity. In thermal shock, stress is the materials response to
thermal strain, and it is determined by both modulus of elasticity and thermal expansion
gradients. In this case stress is dependent variable rather than the independent variable.
Therefore, a candidate material must be designed to sustain the required thermal strain
without generating sufficient stress to cause failure.

The elastic-plastic theory cites that the erosion rate of brittle materials at normal
impact is proportional to the particle velocity, diameter and density, and hardness and
inversely proportional to the fracture toughness of the target material. The velocity of the
particles strongly effect the erosion wear rate of the particle. The erosion rate of a
material is usually defined as the ratio of weight loss suffered by the eroding material to
the weight of the erodent particles causing loss. Velocity dependence of erosion rate is
dependent on impact velocity, impact angle, particle size, particle shape etc. The impact
of larger particles produces more erosion than the impact of smaller particles, erosion
being proportional to particle diameter. Sharp, jagged particles are more effective in
causing wear than rounded or spherical particles. The more the hardness of the erodent
particles the more is the material eroded from the target surface. The increase in the
hardness of the target surface shows more resistance to erosion. Impact angle is the angle
at which the particle strikes the target surface. The effect of impact angle is different for
different materials. For ductile materials, the maximum erosion occurs at 20-30 degrees
and for brittle materials maximum erosion occurs at 90 degree impact angle.
Fine alumina has been extensively used as a wear resistant material and many products
including ceramic liners have been commercially exploited. However, the material is
costly and is not good thermal shock resistant. Therefore, the material does not find

application as a good refractory material in severe thermal shock conditions. The
material can however be tailored to act like a refractory material by making suitable
composition with other tough refractory materials like fused alumina. Among the fused
alumina, brown fused alumina is a cheaper source as it is derived from low cost
bauxite compared to white fused alumina which is derived from costly alumina
material.
Brown fused alumina is an excellent material for manufacturing refractory
products because of its abrasive characteristics. The material is produced by the smelting
of calcined bauxite in an electric arc furnace > 2000 0C. The fused alumina is then
allowed to cool and solidify before processing into the desired size fractions. The
material is iron free and extremely tough. It is brown in color as it has a slightly higher
level of impurities than white aluminum oxide. With a Mohs hardness of 9, Brown Fused
Aluminum Oxide is a high strength, wear-resistant material possessing a strong ability to
resist vigorous chemical attacks (such as acid and alkali) at extreme temperatures. It has
properties like high refractoriness, slag resistance, high specific gravity, high thermal
conductivity, creep resistance and abrasion resistance. Generally brown fused alumina is
used in applications that require a superior performance to bauxite but offers a cost
advantage against sintered alumina. The material is used in reusable abrasive, grinding,
lapping and polishing medium, ceramic grinding wheels and grinding media and wear
resistant and refractory products.

The present invention is focused on developing a suitable material which will be
high temperature wear resistant and at the same time thermal shock resistant. The
invention therefore focused on developing suitable composite material with the two
raw materials a fine alumina based wear resistant composition mixed with suitable
amount of brown fused alumina and heat treated at high temperature. There is no report
in the literature for such combination of material especially for application as a liner
in boilers for power plants. The ceramic liners can be fabricated using conventional
pressing method followed by high temperature sintering.
The raw materials used in the powder preparation were alumina, bauxite, brown
fused alumina and mullite in different proportions. All these materials were taken in the
powder form and made into different compositions. The samples for the test were
prepared by mixing various compositions of powders with polyvinyl alcohol as the
binder. The powder was readily mixed and sieved using the mesh of size 45 to check any
lump formation in the powder before pressing. Rectangular bar specimens were pressed
in a 28.5 x 87.5 mm and square tiles in a 120 x 120 mm steel dies by uniaxial pressing at
a pressure of 100 kg/cm2. The samples were dried before firing to remove the moisture
content in the green sample as this moisture present in the sample may have a deleterious
effect by causing pores or cracks when firing subjected to high temperatures. The
Bauxite based samples were sintered in a gas tunnel kiln at a temperature range of 1250-
1280o C and the rest of the samples at 1600 0 C in an industrial batch kiln.

The materials were characterized by different analytical techniques. Most
important of them are high temperature abrasion resistance test following ISO 16349
standard and the thermal shock resistance test of the fired composite. ISO 16349
standard is applicable for the determination of abrasion resistance of refractory materials at
elevated temperature. The ability of the test pieces to resist the surface wear caused by the
mechanical action of moving solids with high speed at elevated temperature is determined. The
test piece with specified dimensions is put into the high temperature test chamber. Its test surface
shall be perpendicular to the sand blast tube. The temperature is raised to the test temperature
after holding for 30 min. The given mass of abrasive is blasted to the test surface of the test piece
by blast gun with compressed air within specified time at that temperature. Then the abrasion loss
of the test piece is measured after cooling the furnace. The abrasive impact angle also can be
varied by positing the test piece at the required angle prior to starting the experiment. Silicon
carbide particles were used as erodent. The velocity of the SiC particles is controlled by line
pressure of air stream which is approximately 4.5 kg/cm2. The abrasive particle velocity was 4
m/s and mass flow rate was 67 g/min. The specimen surface was eroded with silica sand at
different impact angles (i.e. at 30, 45, 60, 90 deg). Amount of wear was determined on
‘mass loss’ basis. It was done by measuring the weight change of the samples. A
precision electronic balance with 0.1 g accuracy was used for weighing for an initial mass
of the flat tile in the range of 500-600 g. Other sand abrasion test following ASTM G65
and sand erosion test following ASTM G 76 standards were also performed using silica
sand as the abrasive material.

Thermal stability of the samples was determined experimentally by water quench
test. The 120 mm x120 mm x8 mm fired tile samples were heated in an electric furnace at
500°C and held for 30 minutes. The samples were then quenched into water and left for 3
minutes and dried before returning to the furnace at 500°C. This procedure was repeated
until failure, the number of quenches to failure is taken as a measure of a thermal shock
resistance. Indirectly, we also measured modulus of elasticity of the sample before and
after every five thermal shock cycles on smaller rectangular bars to understand the
thermal cycling efficiency of these materials.
Although many compositions were experimented, for the clear understanding of
the embodiment of the present invention three composites are chosen for demonstration
and the results are compared to achieve the best material. The base material in all these
cases was a wear resistant commercial alumina composition with alpha alumina as the
major phase and is associated with sintering aid and inorganic binders. This material is
named in this invention as Alumina. The present invention can best be understood with
following examples: The code for example 1 is “MC”, for example 2 is “BC” and for
example 3 is “FAC”.

Table 1 summarizes the results obtained for the selected three composites studied. The
above examples clearly depict the superiority of brown fused alumina based
composition in terms of mechanical, wear and thermal shock properties. Therefore, a
high temperature wear resistance experiment was planned on mullite based composites
and BFA based composite.


Example 1
The commercial wear resistant alumina composition was added with 20 % refractory
grade mullite of 100 mesh and mixed uniformly in a planetary mixer for 30 min. along
with polyvinyl alcohol as the binder. The tiles were fabricated using hydraulic press and
sintered in a gas operated kiln at 1580 0C with a holding time of 1h. The fired sample
resulted a bulk density of 2.95- 3.0 g/cc with apparent porosity in the range of 10-10.5
% and water absorption in the range of 3-3.5 %. The linear shrinkage was in the range of
6 -6.5 %. The Vicker’s hardness at 1Kg. load was in the range of 6-6.2 GPa . The
modulus of elasticity as measured by resonant frequency method was in the range of 105-
115 GPa. The thermal expansion coefficient was in the range of 7.2-7.4 x 10-6 /oC . The
average erosion volume loss as measured on the fired sample following ASTM G 76
standard was in the range of 0.20-0.23 mm3/g. The adjusted volume loss as measured on
fired samples following ASTM G 65 standard yielded a value of 16-19 mm3. The
percentage decrease in modulus of elasticity of samples after 20 thermal shock test was
20 % reduction from the original value.

Example 2
The commercial wear resistant alumina composition was added with 20 %
refractory grade low cost Bauxite of 100 mesh and mixed uniformly in a planetary
mixer for 30 min. along with polyvinyl alcohol as the binder. The tiles were fabricated
using hydraulic press and sintered in a gas operated tunnel kiln at 1250 0C with a
holding time of 1h. The fired sample resulted a bulk density of 2.80- 2.85 g/cc with
apparent porosity in the range of 13-14 % and water absorption in the range of 4.5-5.0 %.
The linear shrinkage was in the range of 8 -8.5 %. The Vicker’s hardness at 1Kg. load
was in the range of 5.8-6.0 GPa . The modulus of elasticity as measured by resonant
frequency method was in the range of 90-95 GPa. The thermal expansion coefficient was
in the range of 7.0-7.2 x 10-6 /oC . The average erosion volume loss as measured on the
fired sample following ASTM G 76 standard was in the range of 0.35-0.40 mm3/g. The
adjusted volume loss as measured on fired samples following ASTM G 65 standard
yielded a value of 40-45 mm3. The percentage decrease in modulus of elasticity of
samples after 20 thermal shock test was 20 % reduction from the original value.

Example 3
The commercial wear resistant alumina composition was added with 20 % refractory
grade brown fused alumina of 100 mesh and mixed uniformly in a planetary mixer for
30 min. along with polyvinyl alcohol as the binder. The tiles were fabricated using
hydraulic press and sintered in a gas operated kiln at 1580 0C with a holding time of 1h.
The fired sample resulted a bulk density of 3.25- 3.35 g/cc with apparent porosity in the
range of 4-5 % and water absorption in the range of 1-1.5 %. The linear shrinkage was in
the range of 6 -6.5 %. The Vicker’s hardness at 1Kg. load was 9-9.5 GPa . The modulus
of elasticity as measured by resonant frequency method was in the range of 220-240 GPa.
The thermal expansion coefficient was in the range of 8.7-8.9 x 10-6 /oC . The average
erosion volume loss as measured on the fired sample following ASTM G 76 standard was
in the range of 0.08-0.10 mm3/g. The adjusted volume loss as measured on fired samples
following ASTM G 65 standard yielded a value of 8-9 mm3. The percentage decrease in
modulus of elasticity of samples after 20 thermal shock test was 15 % reduction from the
original value.

WE CLAIM:
1. A method for producing an improved ceramic composite material comprising:
mixing fine alumina based wear resistant composition with suitable amount of brown
fused alumina in presence of a binder in a mixer,
subjecting the mix to the step of fabrication to produce tiles using hydraulic press,
sintering the tiles in a gas operated kiln to obtain a controlled porosity structure and
Measuring of wear resistance at high temperature and thermal shock resistance.
2. The method as claimed in claim 1, wherein the amount of brown fused alumina of
100 mesh is 20% with particle size in a range of 75 to 80µm.
3. The method as claimed in claim 1, wherein the said binder is preferably polyvinyl
alcohol.
4. The method as claimed in claim 1, wherein the tiles were sintered in a Kiln at a
temperature range of 1260-1600°C with a holding time of 1 hour.
5. The process as claimed in claim 1, wherein the particle size of fine alumina is in the
range of 4-5 µm.

6. The process as claimed in claim 1, wherein the said ceramic composite material
has very high hardness of 9 GPa and high young's modulus of >200 GPa.
7. The process as claimed in claim 1, wherein the said ceramic composite material
has very high abrasion resistance of ~8 mm3 adjusted volume loss and < 0.1 mm3/g
average erosion volume.
8. The process as claimed in claim 1, wherein the said ceramic composite material
has very good high temperature erosion resistance over a wide range of temperature.