A

PROJECT REPORT

ON

PREPARATION OF CERAMIC-CERAMIC COMPOSITES AND STUDY THE

CRACK HEALING BEHAVIOUR OF THESE COMPOSITES

Submitted in the partial fulfillment of the

Requirement for the award of degree

Of

MASTER IN TECHNOLOGY

IN

METALLURGICAL AND MATERIALS ENGINEERING

(INDUSTRIAL METALURGY)

Submitted by

ANUP TIGGA

Under the guidance of

Dr. ANJAN SIL

Associate Professor

DEPARTMENT OF METALLURGICAL AND MATERIALS ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY, ROORKEE

ROORKEE-247667

JUNE 2009

1. INTRODUCTION

The word ceramic, derives its name from the Greek keramos, meaning "pottery", which

in turn is derived from an older Sanskrit root, meaning "to burn". The Greeks used the term to

mean "burnt stuff" or "burned earth". Thus the word was used to refer to a product obtained

through the action of fire upon earthy materials. A ceramic is an inorganic, non-metallic solid

prepared by the powdered materials (i.e. Al2O3, SiC, TiO2 etc) by the process of compaction

followed by sintering. They are fabricated in the product through the application of heat or heat

as well as pressure. Ceramic materials may have a crystalline or partly crystalline structure, or

may be amorphous, i.e., a glass. As most common ceramics are crystalline, the definition of

ceramic is often restricted to inorganic crystalline materials, as opposed to the non-crystalline

glasses.

Depending on their method of formation ceramic can be dense and lightweight and show

various characteristic properties i.e. high hardness, high temperature resistance, high strength,

low thermal conduction and high brittleness. Thus due to their low conductivity they are

extremely used in the electrical appliances as insulator. They are best suited as a refractory

material due to their high hardness and high thermal resistance. Ceramics are also used in pottery

products, and sanitary ware.

Ceramic bonds are mixed, ionic and covalent, with a proportion that depends on the

particular ceramics. The ionic character is given by the difference of electro negativity between

the cations (+) and anions (-). Covalent bonds involve sharing of valence electrons. Very ionic

crystals usually involve cations which are alkalis or alkaline-earths (first two columns of the

periodic table) and oxygen or halogens as anions.

The building criteria for the crystal structure are two:

maintain electrical neutrality

charge balance dictates chemical formula

achieve closest packing

The condition for minimum energy implies maximum attraction and minimum repulsion.

This leads to contact, configurations where anions have the highest number of cation neighbors

and vice versa.

1.1. CERAMIC CLASSIFICATION

Ceramic products are usually divided into four sectors according to their application and

character.

1.1.1. CERAMIC CLASSIFICALTION BASED ON PROCUCTS

1.1.1.1. Structural ceramics

Structural ceramics include bricks, pipes, floor, roof tiles, etc.

1.1.1.2. Refractory ceramics

Such as kiln linings, gas fire radiant’s, steel and glass making crucibles.

1.1.1.3. White wares

Includes tableware, wall tiles, pottery products, and sanitary ware, etc

1.1.1.4. Technical ceramic

Technical, is also known as Engineering, Advanced, Special, and in Japan, Fine

Ceramics. Such items include tiles used in the Space Shuttle program, gas burner nozzles,

ballistic protection, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine

blades, and missile nose cones. Frequently the raw materials do not include clay. Technical

ceramics are of three types:

1.1.1.1.1. Oxides : Alumina, zirconia

1.1.1.1.2. Non-oxides : Carbides, borides, nitrides, silicates

1.1.1.1.3. Composites: Particulate reinforced combinations of oxides and non-oxides.

1.1.2. CERAMIC CLASSIFICALTION BASED ON MATERIALS

A ceramic material is often understood as restricted to inorganic crystalline oxide

material. It is solid and inert. Ceramic materials are brittle, hard, and strong in compression,

weak in shearing and tension. They withstand chemical errosion that occurs in an acidic or

caustic environment. Ceramics generally can withstand very high temperatures such as

temperatures that range from 1000°C to 1600°C. Exceptions include inorganic materials that do

not include oxygen such as silicon carbide or silicon nitride. A glass is often not understood as a

ceramic because of its amorphous (non-crystalline) character. However, glass making involves

several steps of the ceramic process and its mechanical properties are similar to ceramic

materials.

Traditional ceramic raw materials include clay minerals such as kaolinite, whereas more

recent materials include aluminium oxide, more commonly known as alumina. The modern

ceramic materials, which are classified as advanced ceramics, include silicon carbide and

tungsten carbide. Both are valued for their abrasion resistance, and hence find use in applications

such as the wear plates of crushing equipment in mining operations. Advanced ceramics are also

used in the medicine, electrical and electronics industries

1.1.2.1. Crystalline ceramics

Crystalline ceramic materials are not amenable to a great range of processing. Methods

for dealing with them tend to fall into one of two categories - either makes the ceramic in the

desired shape, by reaction in situ, or by "forming" powders into the desired shape, and then

sintering to form a solid body. Ceramic forming techniques include shaping by hand (sometimes

including a rotation process called "throwing"), slip casting, tape casting (used for making very

thin ceramic capacitors, etc.), injection molding, dry pressing, and other variations.

1.1.2.2. Non-crystalline ceramics

Non-crystalline ceramics, being glasses, tend to be formed from melts. The glass is

shaped when either fully molten, by casting, or when in a state of toffee-like viscosity, by

methods such as blowing to a mold. If later heat-treatments cause this glass to become partly

crystalline, the resulting material is known as a glass-ceramic.

1.2. COMPOSITE MATERIALS

A composite is commonly defined as a combination of two or more distinct materials,

each of which retains its own distinctive properties. Composite material may have two or more

than two phases.

Some common classifications of composites are:

Reinforced plastics

Metal-matrix composites

Ceramic matrix composites

Concrete

1.2.1. CERAMIC MATRIX COMPOSITES(CMC)

Ceramic matrix composites (CMCs) combine reinforcing ceramic phases with a ceramic

matrix to create materials with new and superior properties. In ceramic matrix composites, the

primary goal of the ceramic reinforcement is to provide toughness to an otherwise brittle ceramic

matrix. Fillers can also be added to the ceramic matrix during processing to enhance

characteristics such as electrical conductivity, thermal conductivity, thermal expansion, and

hardness.

The desirable characteristics of CMCs include high-temperature stability, high thermal

shock resistance, high hardness, high corrosion resistance, light weight, nonmagnetic and

nonconductive properties, and versatility in providing unique engineering solutions. The

combination of these characteristics makes ceramic matrix composites attractive alternatives to

traditional processing industrial materials such as high alloy steels and refractory metals.

Ceramic matrices can be categorized as either oxides or non-oxides and in some cases

may contain residual metal after processing. Some of the more common oxide matrices include

alumina, silica, mullite, barium alumino-silicate, lithium alumino-silicate and calcium

aluminosilicate. Of these, alumina and mullite have been the most widely used because of their

in-service thermal and chemical stability and their compatibility with common reinforcement.

1.2.2. TYPES OF CMC

1.2.2.1. Particulate-reinforced ceramics

In particulate reinforced ceramic composite, the particles of ceramic material are added to

the matrix material. The particle material doesn’t react with the matrix material and fracture

toughness increases by crack deflection and hence the crack healing ability of the material. If

particles are of irregular in shape or much longer in grain size than matrix, some bridging can be

occur and if the particles are significantly different in thermal expansion coefficient than the

matrix, some toughening by micro cracking formation can occur, which is very important in

crack healing ability of ceramics.

1.2.2.2. Whiskers-reinforced ceramics

In whiskers reinforced usually whiskers form of the material is used. Whiskers range in

size from 0.5 to 10 μm in diameter and 5 to 20 mm in length. Whiskers are added widely in

effort to achieve increased toughness and bending strength.

1.2.2.3. Fiber-reinforced ceramics

Reinforcing ceramics with long fibers can increase the distance over which a toughening

mechanism acts and lead to enough strain to failure that fracture is no longer catastrophic, this

approach can be used in the crack healing ability of the ceramics.

Ceramic fibers are generally available as continuous strands wound onto a spool and has

similar to textile fibers. Fibers are generally amorphous or polycrystalline with very small grain

size (nanometers). Glasses fibers, carbon fiber, silicon carbide fibers are few examples of fibers.

1.3. CRACK GROWTH AS STABILITY PROBLEM

Crack generally starts in ceramics materials due to the thermal stress at higher

temperature. Static and cyclic load cause the initiation of the crack and once the crack develop

in the ceramic material it vigorously propagate and fail in the brittle manner. It can propagate in

inter-granular or trans-granular way. This is the disadvantage of the ceramic material that they

are very brittle and have low fracture toughness due to this, their structural integrity is severely

affected.

Once the crack initiate in the ceramic material it is very difficult to stop that and failure

occurs is catastrophic in nature. Energy in the crack tip is maximum, which further creates new

surfaces for the propagation of crack. Micro flaw which are there in the material acts as the crack

initiator which lead to the failure of the material. Knowing it then also occurrence of flaws is not

completely avoidable in the processing, fabrication, or service of a material component. Flaws

may appear as cracks, voids, metallurgical inclusions in the material.

The toughness of ceramic is very low as compared to metals. At the tip of the crack, the

stress intensity is high and microscopic plasticity is limited and when the strain energy released

is equal to or greater than the energy required to grow the crack surface(s a crack will grow

spontaneously). The stability condition can be written as:

Elastic energy released = surface energy created

So as to achieve crack growth stability the toughness of ceramics should be increased

and it has founded the Composites exhibiting. The highest level of fracture toughness is

typically made of a pure alumina or some silica-alumina (SiO2 /Al2O3) matrix with tiny

inclusions of zirconia (ZrO2) dispersed as uniformly as possible within the solid matrix. And

cracked healed zone shows higher fracture toughness than the unhealed sample.

So if the crack healing ability of the ceramic can be increased then along with it fracture

toughness and crack stability will be automatically get increased.

1.4. DESSERTATION OUTLINE

Structural ceramics have excellent heat resistance, corrosion resistance, and wear

resistance and may be indispensible material of new millennium. However, but due to their high

brittleness and low fracture toughness its applicability is limited. Once crack develops it fails in

catastrophic manner. So, it is important to increase its reliability to widen its application area.

So there is two ways to increase its reliability of ceramic:

(1) First to stop the crack propagation by healing the crack.

(2) Secondly, increase the fracture toughness.

As alumina is a very popular ceramics, exhibits excellent mechanical properties

and excellent oxidation resistance than metals at high temperature. Thus it is expected to

apply in the structural component operating at high temperature in various fields.

However due to its low fracture toughness, low strength and low crack resistance it

applicability is hampered. As it has discovered the SiC carbide shows excellent crack

healing ability and as well as it has high fracture toughness.

So, in my dissertation work i have taken matrix of alumina dispersed with SiC

with varying the composition of SiC to study the crack healing behavior on the composite

of alumina / SiC. To investigate the maxima crack which is healed and the optimal crack

healing conditions.

2. LITERATURE REVIEW

2.1. CRACK HEALING ABILITY

Structural ceramics are brittle and sensitive to flaws. As a result, the structural integrity of

ceramic component may be seriously affected. The following can be the excellent methodology

to overcome these problems:

(a) Toughen the ceramic by matrix reinforcement, etc.

(b) Activate the crack-healing ability.

If a crack healing ability is used on structural components for engineering use, considerable

advantages can be anticipated. With this motivation, it was discovered that mullite, alumina, SiC,

show very strong crack healing abilities.

Systematic studies were made on the above subjects by the authors. As a result, in the case of

most ceramics above, the crack-healed zones exhibited excellent mechanical properties almost

up to the heat-proof temperature for the strength of the base material, if the ceramics were healed

at the optimized conditions. The temperature where bending strength starts to decrease rapidly

with increasing testing temperature is defined as heat proof temperature. These test results

suggest that the crack healing ability can be used as a method to guarantee the structural integrity

of a ceramic component. However, oxygen is necessary for the crack healing process. Thus,

embedded flaws and micro structural flaws such as abnormally large grains cannot be healed.

This fact was confirmed many times examining the crack initiation sites using SEM, these facts

suggests the importance of a proof test to ensure higher reliability.

There is much of research is done on the crack healing behavior of the structural ceramics such

as alumina, mullite, SiC. Recently, the interesting tests results were obtain by the authors:

(1) Si3N4 and mullite showed excellent crack healing ability even under the constant and

cyclic stress at temperatures from 800 to 1200°C and from 1000°C to 1200°C,

respectively;

(2) The healed sample exhibits almost the same mechanical properties as the base material at

the temperature of healing. Namely, it can be said that both ceramics have excellent crack

healing ability.

However, if the crack can be healed during the service, it would be very desirable for structural

integrity.

2.2. EFFECTS ON CRACK HEALING BEHAVIOR

2.2.1. Effect of environment on the crack healing behavior

The standard surface crack used for evaluating the basic crack-healing behavior. The

specimens used were made according to JIS standard. The details of the above information of

mullite/SiC1436 used mainly were as follows:

The mullite powder used has a mean particle size of 0.2 µm and an alumina content of 71.8%.

The SiC powder has a mean particle size of 0.27 µm. The quantity of SiC powder added is 15

mass%, in contrast to mullite powder. The mixture is hot-pressed at 923 K, 4 h and 35MPa in

nitrogen. The sintered material has an average grain size of 0.46 µm and SiC particles are located

in grain boundary and distributed uniformly.

The semi-elliptical surface crack was induced by an indentation technique using a

Vickers indenter. A surface crack length 2C ≈ 100 µm was induced by controlling the Vickers

indentation load, and the crack depth was about 45 µm This crack was defined as the standard

crack. Fig. 2.1 shows the effect of environment on the crack-healing behavior of mullite/SiC and

Al2O3/SiC.The contrast between bending strength (σB) of smooth and cracked samples was

shown by the left-most column of Fig. 2.1.

In these tests, the following three types of fracture were observed: (1) crack initiation

from a pre-crack. This type of fracture usually occurred when crack-healing was incomplete.

(2) Crack initiation from the base material, and the crack-healed zone did not fracture, and no

special flaw can be seen on the site. (3) The sample fractured into many pieces and the crack

initiation site could not be found. In the case of high bending strength (σB) most samples showed

this type of fracture. The symbol (*) indicates that samples fractured outside the crack-healed

zone. All samples of both ceramics healed in air recovered σB completely, and showed that the

cracks were healed completely. Especially, healed smooth specimen and crack-healed sample

exhibited higher bending strength than that of smooth sample, because either small cracks on

smooth sample and standard crack (2C = 100 µm were healed completely.

Samples of both ceramics healed in vacuum, Ar gas and N2 gas indicated that the

strength recovery was insufficient, and all samples fractured from the crack-healed zone as

shown in Fig. 2.1.

Fig.2.1. showing bending strength and crack healing environments for different specimen ???

These test results showed that a crack in Mullite / SiC and Al2O3/SiC can be healed

completely only in an air environment similar to experience with silicon nitride.7 This test result

clearly shows that crack-healing needs oxygen in the air, thus an embedded crack cannot be

healed

2.2.2. Effect of temperature and time on crack healing behavior

Crack-healing behavior depends both on healing temperature (TH ??) and time (tH ??).

To find this relationship, 14 kinds of healing conditions were tested, using mullite/SiC.32 ?? The

test results are shown in Fig. 2.2.

Fig.2.2. shows bending strength verses time and temperature of healing

The bending strength σB of smooth and cracked specimens are compared in the left-most

column. The symbol (*) indicates that fracture occurred from outside the crack-healed zone. The

symbol (_) indicates the _B obtained by healing time tH = 1 h at each healing temperatures.

Note that σB does not recover below TH = 1223 K, but it recovers considerably at TH = 1373

and 1473 K. However, when considering that many fractures occurred from a pre-crack, the

strength recovery is not sufficient. On the other hand, at TH = 1573 K, the average σB of the

healed specimen is higher than that of the smooth specimen. In conclusion, the lowest crack-

healable temperature for tHM = 1 h is THL = 1573 K.In the same way, the lowest crack-healable

temperature conditions for tHM = 10 h (_) and tHM = 100 h (_) are THL = 1473K and THL =

1373 K, respectively.

From the σB versus healing temperature curve, the lowest temperatures (THL) were

selected, where the average σB of the crack-healed sample exceeded the average σB of smooth

specimens, for each healing time (tHM = 1, 10, 100 h), and the THL were plotted in the

Arrhenius graph In short, THL = 1573K is for tHM1 h, THL = 1473K is for tHM = 10 h, and TH

= 1373K is for tHM = 100 h. Where, the minimum healing time for complete strength recovery

is denoted by (tHM), since this expression is convenient for the Arrhenius graph. The Arrhenius

plots of four kinds of ceramics. Symbol (_) indicates the result on mullite/SiC23 used mainly for

this study, symbols (_), (_) and (_) show the results for monolithic alumina, Al2O3/SiC22 and

SNC-Y8,25 respectively. The crack sizes of these three specimens are 2C _ 100 _m, and they are

healed in an air environment. Symbol (↑) shows that the crack can be healed within this time

period. With respect to these four kinds of ceramics, both (THL −1) and (1/tHM) are in closely

proportional relation. Therefore, the crack-healing behavior of these ceramics follows the Eq.

(1).

where AH is a proportionality constant (h−1), QaH is the activation energy of crack-

healing (kJ/mol), R is the gas constant (kJ/mol K) and THL is the lowest absolute temperature of

the healing (K).. A crack-healable condition of the standard crack can be evaluated easily as a

function of temperature and time.

2.2.3. Activation energy for the crack healing

The crack healing behavior depends on both the healing temperature and the healing

time, as mention in section above.

It has been reported that crack healing is caused by the oxidation reaction as follows:

Therefore, the crack-healing rate obeys the Arrhenius law. For each healing time (tH =1,

10 and 100 h), the TH was determined and the relationship tH and TH was expressed as

Arrhenius plots, as shown in Fig. 2.3.

Fig.2.3. showing Arrhenius plots between time and temperature of healing

The crack-healing behavior follows Eq.

Where tH is the minimum healing time for complete strength recovery (h), AH is

proportionality constant (h−1), QH is the activation energy (kJ/mol), and R is a gas constant

(kJ/mol K). The activation energy (QH) for the crack-healing of mullite/SiC/Y2O3 is determined

to be ≈566 kJ/mol. In earlier studies, the activation energies for the crack healing of several

ceramics were examined. The evaluated of mullite/SiC, Al2O3/SiC and Si3N4/SiC are 413

kJ/mol, 334 kJ/mol and 227 kJ/mol, respectively. The activation energy for the crack-healing of

mullite/SiC/Y2O3 is similar to that of mullite/SiC.

2.2.4. Effect of pre-crack size on the crack healing behavior

A surface crack (2C = 100−250_m, aspect ratio = 0.9) was introduced on the mullite/SiC

sample, and healed under the standard condition of mullite/SiC (TH = 1573 K, tH = 1 h, air

environment). Subsequently the bending test was carried out at room temperature. The

relationship between the crack length (2C) and bending strength (σB) is shown in Fig.2.4.

Fig.2.4. shows relationship between the crack length (2C) and bending strength

The symbols (_) and (_) shows the σB of the heat-treated smooth and crack-healed

sample, respectively. The σB of crack-healed sample regains the same level σB as the heat-

treated smooth specimens, when 2C is smaller than 200 _m. But the σB decreases suddenly,

when 2C is over 200_m. Therefore, the maximum crack size to be healed completely is 2C = 200

_m. The critical parameter for the complete crack-healing is not crack-length but crack-depth. If

a crack is deep enough, oxygen cannot supplied well, thus the crack cannot be healed

completely. SNC-Y8, SNC-Y5A3, Al2O3/SiC and SiC were also tested in the same way. Table 1

lists the maximum crack size to be healed completely for six kinds of structural ceramics. Only

SiC could heal a large crack of 2C = 450 _m by itself. The other materials can heal a crack up to

2C = 200 _m, completely.

2.2.5. Bending strength of the crack-healed samples at elevated temperature

For the practical use of the crack-healing technology, the bending strength (σB) of the

crack-healed sample at elevated temperature is very important. The temperature dependence of

the σB in six crack-healed ceramics is shown in Fig. 2.5.

Fig.2.6. shows relationship between the bending strength and test temperature.

Monolithic Al2O3 was healed at 1723 K, 1 h in air. For this case, crack-healing is a re-

sintering mechanism, and the heated sample showed the same value of σB as that of the base

material up to 1573K and numerous samples fracture outside the crack-healed zone.35

Mullite/SiC36 and Al2O3/SiC10 were healed at 1573 K, after 1 h in air.

Crack-healed mullite/SiC and Al2O3/SiC showed high heat resistance up to 1473 and

1573 K, respectively and most samples fractured outside the crack-healed zone up to 1573 K.

The SiC was healed at 1773 K, after 1 h in air. The base material showed a high σB up to 1673

K, however, the heat-proof temperature of the crack-healed sample was about 873K and

considerably lower than that of base material.34 Recently, SiC having a heat-proof temperature

of 1473K of the crack-healed zone has been developed.12 The crack-healed zone of SNC-Y5A3

is a glassy phase, so its heat-proof temperature is moderate, being about 1273 K, however, the

crack-healed zone of SNC-Y8 healed at 1573 K, after 1 h in air is crystalline SiO2, thus the

healed zone showed a higher heat-proof temperature of 1673 K.

2.3. RESERCHERS CONTRIBUTION

Hae Sook Kima, Mi Kyung Kimb, Sun Bae Kangc, Seok Hwan Ahnd, Ki Woo Namd

sintered the Al2O3/SiC composite ceramics and Al2O3 monolithic ceramics and subjected to

three-point bending tests. Especially, Al2O3/SiC composite ceramics were investigated in effect

of the additive powder Y2O3. The crack-healing behavior and the room temperature bending

strength of the crack-healed specimens were investigated after heat treatment at 1373–1723 K, 1

h in air. Al2O3/SiC composite ceramics could completely heal semicircular cracks with

diameters 100 μm. The bending strength of Al2O3/SiC composite ceramics significantly

increased after heat treatment at 1573K for 1 h in air. The crackhealing ability of Al2O3/SiC

composite ceramics was the best in additive powder 3 wt.% Y2O3.

Sang-Kee Lee a, Wataru Ishida a, Seung-Yun Lee b, Ki-Woo Namc, Kotoji Ando,

investigated systematically the basic crack-healing behavior of the commercial SiC which

showed the best crack-healing behavior among three tested previously was, as a function of

crack-healing temperature, time, crack size and temperature dependence of the resultant bending

strength. The main conclusions they obtained were as follows:

(1) The optimized crack-healing condition was 1773 K, 1 h in air.

(2) The maximum semi-elliptical crack size (2cmax) in diameter that could be healed completely

was dependent on crack-healing conditions; At healing conditions of 1773 K, 1 h in air: 2cmax

≈450_m in diameter. At healing conditions of 1573 K, 6 h in air: 2cmax ≈50_m in diameter.

(3) The limiting temperature for bending strength for the bending strength of smooth specimen of

SiC is over 1573 K. However, that of crack-healed specimen was about 873 K, and crack-healing

conditions have no effect on the limiting temperature for bending strength.

Wataru Nakao, Shuntaro Mori, Jun Nakamura, Koji takahashi, and Kotoji Ando, hot

pressed the mullite/SiC particle/ SiC whisker multi- composites . They investigated crack-

healing abilities and mechanical properties of these sintered composites. From the obtained

results, the usefulness of the mullite composites as materials for springs was discussed.

MS15W10P can be completely heal the pre -crack at a lower temperature than MS25W

despite having the same SiC content. Admixing above 20 vol% SiC whiskers alone with mullite

yielded enough crack healing ability that the cracks healed had higher room temperature strength

than the base materials. However, heat-resistance limits temperatures of 1073 and 1273 K,

respectively. In contrast, the cracks healed in MS15W10P retained high reliability over the

measured temperature range.

Fracture toughness increased as SiC content were increased over the whole SiC range

that was prepared in the present study. The value of γmax content of 20%, because shows a

maximum at an SiC content of 20 vol%, because the young modulus increases with an increase

in SiC content, but the bending strengths are almost constant above as SiC content of 20 vol %.

Therefore, it is confirmed that MS15W10P has the best potential to be a material for

ceramic springs used at high temperatures, because MS15W10P has an adequate crack-healing

ability as well as a shear deformation ability that is almost two times greater than that of

monolithic mullite.

Kotoji Andoa,∗, Kotokaze Furusawa b, Koji Takahashi a, Shigemi Sato c was proposed

the new methodology to guarantee the structural integrity of ceramic components which may be

called “[crack-healing + proof test + in situ crack-healing]” and the flow chart was shown.

During machining, many surface cracks may be induced in ceramic components. By the crack-

healing under the optimized condition, the surface cracks can be healed completely and strength

recovered completely. However, oxygen is necessary for the crack-healing, thus embedded

cracks cannot be healed at all. Proof test is very useful to reject the member that has

unacceptable flaws. Thus, the structural integrity of a ceramics component before service can be

guaranteed by [crack-healing + proof test]. However, if a crack initiates during service, the

reliability of the component will decrease considerably depending on the crack size. If the

materials used have excellent crack-healing ability during service (namely; in situ crack-healing

ability), this problem will be overcome easily. Then a new concept [crack-healing + proof test +

in situ crack-healing] is a very useful technology to guarantee the structural integrity of a

ceramic component over all its lifetime, if the material used has large crack-healing ability

Kotoji Andoa, Min-Cheol Chua,*, Kiichi Tsujib, Toshikazu Hirasawac, Yasuyoshi

Kobayashid, Shigemi Satod investigated the crack-healing behavior and high-temperature

strength characteristics systematically using hot-pressed mullite/SiC composite ceramics. This

material exhibits very interesting crack-healing behaviour. The main results of this experiment

are described below.

1. The strength of the pre-cracked specimen was recovered by healing the pre-crack using pre-

oxidation at an elevated temperature in air.

2. The optimum crack healing condition in this experiment range was 1300 _C for 1 h in air. A

semi-elliptical crack 200 mm in diameter could be healed perfectly under this condition.

3. The crack-healed zone had sufficient bending strength compared with that of the matrix up to

about 1200 _C.

Sang-Kee Lee, Masato Ono, Wataru Nakao, Koji Takahashi, Kotoji Ando studied

the Mullite/SiC/Y2O3 composite and following results they obtain.

(1) Mullite/SiC/Y2O3 composite ceramics have the ability to heal after cracking from 1423K to

1673K in air.

(2) The heat-resistance limit temperature for strength of the crack-healed specimen is ∼= 1473

K, and (68% of the specimens fractured from outside the crack-healed zone at the tested-

temperature range 300–1573 K.

(3) Due to the crack-healing treatment, the strength of the machined specimen increased by 40–

200%. Local fracture strength was assumed to recover completely, and the cracks formed by

machining were healed completely.

(4) A large self-crack-healing ability is desirable for achieving a higher structural integrity in

ceramic components.

Marc-Oliver Nandy,a Siegfried Schmauder,a* Byung-Nam Kim,b Makoto

Watanabeb and Teruo Kishi experimentally observed the fracture toughness of SiC could be

increased through the addition of 20 vol% spherical alumina particles by a factor of 2. Crack

paths in this composite material are similar to those obtained by simulation: Residual stress®

fields in the matrix due to the mismatch of thermal expansion coefficients are forcing the crack

to detect around particles. Residual tensile stresses in the radial direction of the particles are

acting as a crack opening mode, residual compressive stresses in tangential direction contribute

to close the crack. In graded particle distributions, the stress ®fields of the particles in densely

populated areas superpose. When clustering occurs, the stress ®eld around the cluster forces the

crack to propagate around the cluster. Superposed residual stresses can result in local Kca values

higher than Kcm. In order to come to more realistic results as regards fracture toughness, it is

supposed to consider 3D effects, micro cracking and crack wake bridging in future

considerations.

Toshio Osada a,∗ , Wataru Nakaoa, Koji Takahashi a, Kotoji Andoa, Shinji Saito

A nano-composite Al2O3/SiCwas sintered and small bending specimens (22mm×4mm×3 mm)

were made. A semicircular groove was machined hardly using a diamond ball-drill at the center

of the specimen’s surface. The as-machined specimens were crack-healed under various

conditions. The fracture stresses of these specimens after crack-healing were evaluated

systematically at RT and high temperatures. The main conclusions are as follows:

(1) Hard machining introduced many cracks and chips in the specimens. All as-machined

specimens were fractured by the cracks introduced by machining. The cracking reduced the local

fracture stress at RT from approximately 845 to 250MPa.

(2) The cracks introduced by machining were healed almost completely, and the local fracture

stress of the machined specimen recovered from approximately 250 to 773MPa by healing at

1400 ◦C for 1 h. However, the chips were not healed and affected the strength of the specimen;

thus most machined specimens healed (10 of 12) fractured from the chip. As a result, the average

local fracture stress (773MPa) exhibited a slightly lower value than that (845MPa) of the healed

smooth specimen.

(3) The machined specimen healed exhibited a little higher or the same level strength to the

healed smooth specimen up to 1300 ◦C.

(4) These results demonstrated crack-healing to be a very effective technique to reduce

machining costs and to improve the structural integrity of machined alumina.

3. EXPERIMENT OVERVIEW

As discussed in earlier chapter the prime objective of the dissertation is to improve the

crack healing ability of oxide with the help of the dispersion of particles. Researchers already

contributed to improve the crack healing ability in oxide ceramics.

Alumina was selected for the study of the crack healing ability with the dispersion of SiC

on it. Alumina was taken as matrix composite because not much of study was carried out for the

alumina based composite. This chapter gives a brief outline of the various steps adopted in the

experiment. All details of these were discussed in the next chapter.

The sample were made of alumina in which SiC were dispersed on that by pressing with

the help of uniaxial press. Then the samples were sintered at 1500°C for 12 hours to the study the

crack healing behavior of the samples. Then the samples were polished with the 3μm diamond

paste for inducing indentation (Vickers indentation). Then the microstructure is studied and

crack lengths were measured. Also FESEM, EDAX and XRD are done to the sintered samples to

check the various properties, after that the samples were fired at different temperature for 1 hours

in the presence of air. Final crack length was measure finally.

The dependence of crack healing ability on various factors such as percentage of SiC

dispersion, firing temperature, firing time, and change in crack length were studied in details.

SEM and FESEM techniques for micro structural investigations analyzed the samples. For phase

analysis the sample were studied with XRD

The flow diagram shows the various steps performed to carry out during the experiment.

Powder mixing (alumina-SIC)

Uniaxial pressing

Sintering at 1500°C for 12 hours

SEM and FESEM analysis

Initial polished with by rubbing samples on each

other

Final polished with diamond paste

Inducing crack by Vickers indentation

Measurement of the initial crack length

Firing the samples at different temperatures for 1

hours

Measurement of the final crack length

Plotting graph between temperature and crack healed

length

Comparing initial and final crack length

Result and discussion

4. EXPERIMENT PROCEDURE

As discussed earlier alumina is very important structural ceramics which is used in the

various applications. But due to its low fracture toughness and high brittleness its applications is

restricted. If any how if this two drawbacks can be overcome then it structural integrity will be

increased and it may be the new material for the millennium.

It has been seen that the SiC show very excellent crack healing ability as well as it has

good fracture toughness. So in my dissertation work I have taken the matrix of alumina in which

SiC has been dispersed with varying composition to study the crack healing ability of the

alumina/SiC composite

4.1. MATERIAL SELECTION

Alumina powder was taken of average particle size = 0.4μm.

Silicon carbide powder of average particle size= 220 mesh.

Silicon carbide was dispersed to the alumina with varying volume percentage. the volume

percentage of silicon carbide were taken are:

1. 7.5% Silicon carbide

2. 12.5% Silicon carbide

3. 15% Silicon carbide

Table 1. Property data for Alumina (99.9% purity)

Names Aluminum oxide, corundum

Molecular formula Al2O3

Color White

Density 3.97 gm/cm3

Meting point 2072 °C

Boiling point 2980 °C

Solubility in water insoluble

Refractive index nω=1.768 - 1.772 nε=1.760 - 1.763,

Birefringence 0.008

Coordination geometry octahedral

Flexural Strength 330 MPa (lb/in2x10

3)

Elastic Modulus 300 GPa (lb/in2x10

6)

Shear Modulus 124 GPa (lb/in2x10

6)

Bulk Modulus 165 GPa (lb/in2x10

6)

Poisson’s Ratio 0.21

Compressive Strength 2100 MPa (lb/in2x10

3)

Hardness 1175 Kg/mm2

Fracture Toughness KIC 3.5 MPa•m1/2

Maximum Use Temperature

(no load)

1700 °C

Table 1. Property data for Silicon carbide

Names Silicon oxide

Molecular formula SiC

Color Black

Density 3.22 g/cm3

Meting point 2730°C

Boiling point

Solubility in water nsoluble

Refractive index ~2.6 (all forms)

Coordination geometry

Flexural Strength 550 MPa (lb/in2x10

3)

Elastic Modulus 410 GPa (lb/in2x10

6)

Shear Modulus -

Bulk Modulus -

Poisson’s Ratio 0.14

Compressive Strength 3900 MPa (lb/in2x10

3)

Hardness 2800 Kg/mm2

Fracture Toughness KIC 4.6 MPa•m1/2

Maximum Use Temperature

(no load)

1650°C

4.2. POWDER PROCESSING

Proper powder selection, sizing and pre-consolidation processing are vital to achieving

the final desired shape and properties. Contamination and non-uniformity during powder

processing will be carried throughout the rest of the processing and will usually lead to a

deficient or undesirable component. So powder processing is the initial process and it should be

done adequately. Before proceeding further lets know the accessories and precaution which are

needed during processing.

4.2.1. Accessories required

i. Butter papers

ii. Tissue papers

iii. Spatula

iv. Motor and passel

v. Die punch(20mm diameter)

vi. Acetone

vii. Binder(poly vinyl alcohol)

4.2.2. Processing

Firstly for preparing 5 samples, 10.91 grams of alumina powder and 7.5% vol. of silicon

powder (which come to be 1.26 grams) were taken in the motor.

Acetone was added to it and it was thoroughly mixed by passel until the powers again get

dried.

For providing enough strength in the green body to permit handling. Binder as poly vinyl

alcohol was used. PVA (3-4%weight) was mixed with the warm water and then it is

added to the alumina/SiC mixture and it was again thoroughly mixed by passel until the

powder gets dried.

The same sequence is repeated for the 12.5% and 15% SiC.

Now the powder is ready for the pellet making by compaction.

4.2.3. Precaution

Alumina and silicon carbide powder should be kept in airtight bottle to avoid

contamination

Use of separate spatula for alumina and silicon carbide powder.

Use of butter paper during the measurements of powder weights.

Cleaning of die punch with acetone and tissue paper in successive injection.

Use of spatula to remove the powders from the corners of the motor.

4.3. POWDER COMPACTION

The powder mixtures are now ready for forming into the required shapes. Pressing is

accomplished by placing the powder (premixed with the suitable binders and lubricants and pre-

consolidated so that it is free flowing).

So for compaction, uniaxial press is used for the compaction of the powder into a rigid

die by applying pressure along the single direction through a rigid punch, the figure of uniaxial

press given below and it has the maximum capacity of 25 tons.

4.3.1. Processing

Firstly powers were weighed in the electronic weighing machine. Approximately average

weights of 2.28 -2.39 grams power were taken for making pellet of 2 mm diameter.

The punches preposition in the die body to form a cavity predetermined (based on the

compaction ratio of the power) to contain the correct volume to achieve the required

green dimensions after compaction.

Now the pressures of 12.5 and 17.5 ton is applied on the top punch and simultaneously

upper and lower punches compress the powder

Now the compacted powder is ejected and all samples were weighed and its dimensions

were measured to study the linear, volume metric shrinkage and density measurement.

4.3.2. Uniaxial pressing problem

The following are some of the problems that can be encountered with uniaxial pressing:

Improper density or size

Die wear

Cracking

Density variation

The first two are easy to detect by simple measurements on the green compact

immediately after pressing. Improper density or size are often associated with off-specification

powder batches and are therefore relatively easy and resolve. Die wear shows up as progressive

change in dimensions. The source of cracking may be more difficult to locate. It may be due to

improper die design, air-entrapment, rebound during ejection from the die, die wear or other

cause and perhaps most important problem is non uniform density in uniaxial press. One source

of density variation is the friction between the powder and the die wall and between powder

particles.

Use of suitable binders and lubricants can reduce both wall and particle-particle friction

and thus reduce density variation in the compact. Applying pressures from the both ends of the

die also helps.

4.4. SINTERING

Sintering is essentially a removal of the pores between the starting particles

(accompanied by shrinkage of the component), combined with growth together and strong

bonding between adjacent particles. The following criteria must be met before sintering can

occur:

The mechanism of material transport must be present

Source of energy to activate and sustain this material transport must be present.

The primary mechanisms for transport are diffusion and material transport. Heat is the primary

source of energy, in conjunction with energy gradients due to particle-particle contact and

surface tension.

Sintering can occur by a variety of mechanism as summarized below in the table. Each

mechanism can work alone or in the combination with other mechanisms to achieve

densification.

Table showing sintering mechanisms:

Type of sintering Material transport

mechanism

Driving energy

Vapor phase Evaporation-condensation Difference in vapor pressure

Solid state Diffusion Difference in free energy or

chemical potential

Liquid phase Viscous flow, diffusion Capillary pressure, surface

tension

Reactive liquid Viscous flow, solution-

precipitation

Capillary pressure, surface

tension

4.4.1. Processing

Solid state sintering is carried out in the chamber furnace (furnace specification is given

below) at 1500°C for 12 hours for all the compositions at the rate of 10°C/min of

temperature gradient. The graph shows the heating cycle for the sintering at 1500°C for

12 hours.

After sintering was over the samples weights and dimension were measured to study the

linear, volume metric shrinkage and density measurement.

Fig.4.1. SEM PHOTOGRAPH SHOWING MICROSTRUCTURE

4.5. INDUCING THE CRACK AND MEASURMENT

As thesis deals with the study of crack healing behavior. So to study crack healing

behavior it is necessary that material should have crack on it and for that crack is developed to it

by the Vickers indentation.

So prior to crack inducement samples were firstly initial polished by rubbing samples

through each other of same composition and then it is finally polished by using diamond

paste on the diamond polishing machine.

After polishing the samples microstructures were taken and EDAX and FESEM analysis

were done.

Now the cracks were induced to the samples at the load of 20 kg.

The figures were taken of the crack for further analysis which is shown below.

After inducing the crack on the samples its crack length was measured.

Fig.4.2. showing the crack indentation by Vickers indentor

4.6. FIRING THE SAMPLE

It has been earlier determined that for crack healing behavior heating in the presence of

air is required. Firing allows forming of various phases in the samples in the presence of air.

Formation of phases depends upon the temperature and time given for the firing.

Firstly, samples were kept in the boat and firing is done in the chamber furnace at various

temperatures such as 1100°C, 1200°C, and 1300°C for 1 hours.

4.7. MEASUREMENT OF UNHEALED CRACK LENGTH

To investigate the crack healing behavior on the samples after firing, final crack length

was measured and it was correlated with the initial crack length. Finally graph is plotted between

the temperature and healed crack length for the various compositions of the samples.

Fig.4.3. showing the crack indentation after crack healing

5. RESULTS

Tables below shows that the weight, diameter, density of the green compact and sintered samples

showing density and volume shrinkage after sintering.

Table 5.1. Showing table for 7.5% silicon carbide, load applied=17.5 ton, theorical thickness=

2.5 mm and theorical density=3.913.

Table 2. Showing table for 12.5% silicon carbide, load applied=17.5 ton, theorical thickness= 2.5

mm and theorical density=3.875

Green compact Sintered samples

Weight (grams)

Diameter (mm)

Thickness (mm)

Density (gms/mm3 )

Weight (grams)

Diameter (mm)

Thickness (mm)

Density (mm)

1 2.392 20.04 3.01 2.520 2.357 18.47 2.88 3.056

2 2.387 20.04 2.99 2.531 2.345 18.50 2.80 3.119

3 2.397 19.97 3.01 2.542 2.360 18.48 2.85 3.087

4 2.384 20.03 3.04 2.488 2.343 18.47 2.81 3.112

5 2.391 20.02 3.02 2.515 2.356 18.45 2.88 3.059

Green compact Sintered samples

Weight (grams)

Diameter (mm)

Thickness (mm)

Density (gms/mm3 )

Weight (grams)

Diameter (mm)

Thickness (mm)

Density (mm)

1 2.284 20.09 2.86 2.510 2.218 18.14 2.70 3.178

2 2.280 20.08 2.82 2.550 2.222 18.15 2.62 3.277

3 2.286 20.09 2.83 2.551 2.228 18.15 2.61 3.299

4 2.273 20.09 2.81 2.551 2.220 18.16 2.65 3.234

5 2.271 20.08 2.77 2.558 2.218 18.15 2.60 2.297

Table 3. Showing table for 15% silicon carbide, load applied=17.5 ton, theorical thickness= 2.5

mm and theorical density=3.857.

Green compact Sintered samples

Weight (grams)

Diameter (mm)

Thickness (mm)

Density (gms/mm3 )

Weight (grams)

Diameter (mm)

Thickness (mm)

Density (mm)

1 2.373 20.00 2.91 2.595 2.337 18.61 2.81 3.057

2 2.364 20.03 2.90 2.587 2.332 18.58 2.76 3.116

3 2.363 20.03 2.93 2.559 2.328 18.59 2.79 3.074

4 2.358 20.02 2.90 2.583 2.322 18.59 2.78 3.077

5 2.363 20.08 2.88 2.590 2.326 18.65 2.76 3.085

Table 5.2. showing various sample fired at different temperature and initial crack length,

final crack length, healed crack length and percentage of healing.

1. Samples healed at 1300°C for 1 hour.

Serial

no.

Serial no.

of crack

indentation

Crack

number

Initial

crack

length

(μm)

Final

crack

length

(μm)

Healed

crack

length

(μm)

Average of

crack

length of

one

indentation

Percentage

of healing

%

7.5%

SiC

1 1 1 139 104 35

2 150 49 101

3 214 100 114

4 191 84 107 357/694 51.21

2 1 0 0 0

2 0 0 0

3 229 110 119

4 190 78 112 231/419 55.13

3 1 173 67 106

2 0 0 0

3 101 45 56

4 209 89 120 282/483 58.38

4 1 187 92 95

2 232 122 110

3 302 185 117

4 0 0 0 332/721 46.04

5 1 160 42 118

2 184 66 118

3 144 40 104

4 215 98 117 457/703 65.00

12.5%

SiC

2 1 1 139 46 93

2 188 67 121

3 0 0 0

4 285 162 123 226/612 36.92

2 1 125 51 74

2 0 0 0

3 217 94 123

4 268 140 128 325/610 53.27

3 1 0 0 0

2 0 0 0

Serial

no.

Serial no.

of crack

indentation

Crack

number

Initial

crack

length

(μm)

Final

crack

length

(μm)

Healed

crack

length

(μm)

Average of

crack

length of

one

indentation

Percentage

of healing

12.5%

SiC

3 121 20 101

4 0 0 0 101/121 83.47

4 1 170 34 136

2 154 33 121

3 145 27 118

4 88 22 66 441/557 79.17

5 1 242 116 126

2 233 100 133

3 0 0 0

4 206 85 121 380/681 55.80

15%

SiC

3 1 1 160 48 113

2 190 88 102

3 198 57 141

4 159 59 100 456/707 64.49

2 1 186 37 150

2 181 42 139

3 0 0 0

4 167 28 139 428/534 80.01

3 1 267 94 173

2 244 85 159

3 277 189 80

4 209 53 156 568/997 56.97

4 1 183 94 89

2 0 0 0

3 313 147 166

4 255 102 153 408/751 54.32

5 1 330 190 140

2 265 135 130

3 363 198 165

4 204 105 99 534/1162 45.95

6 1 251 72 179

2 178 36 142

3 0 0 0

4 200 45 155 476/629 75.67

2. Samples healed at 1200°C for 1 hour.

Serial

no.

Serial no.

of crack

indentation

Crack

number

Initial

crack

length

(μm)

Final

crack

length

(μm)

Healed

crack

length

(μm)

Average of

crack

length of

one

indentation

Percentage

of healing

7.5%

SiC

4 1 1 143 49 84

2 147 74 73

3 0 0 0

4 151 91 66 223/441 50.56

2 1 251 176 75

2 255 183 72

3 281 230 51

4 228 146 82 280/1015 27.58

3 1 270 134 36

2 202 93 69

3 303 230 73

4 0 0 0 115/775 14.83

4 1 287 236 51

2 189 158 31

3 330 301 29

4 95 81 14 125/901 13.87

5 1 207 180 27

2 0 0 0

3 115 92 23

4 173 165 8 58/495 11.71

6 1 302 302 30

2 0 0 0

3 86 27 59

4 64 29 35 124/452 37.43

12.5%

SiC

5 1 1 123 80 49

2 0 0 0

3 222 121 101

4 173 98 75 252/518 48.64

2 1 230 203 27

2 162 88 74

3 113 74 15

4 90 84 10 126/595 21.17

Serial

no.

Serial no.

of crack

indentation

Crack

number

Initial

crack

length

(μm)

Final

crack

length

(μm)

Healed

crack

length

(μm)

Average of

crack

length of

one

indentation

Percentage

of healing

3 1 245 207 38

2 275 251 24

3 130 123 7

4 0 0 0 69/650 10.61

4 1 306 209 97

2 106 108 52

3 213 161 52

4 0 0 0 201/625 32.16

5 1 138 78 60

2 228 156 72

3 95 80 15

4 0 0 0 147/461 31.88

6 1 198 162 36

2 115 66 49

3 156 105 51

4 0 0 0 136/469 28.99

15%

SiC

6 1 1 125 37 88

2 0 0 0

3 229 118 111

4 126 89 37 236/480 49.16

2 1 95 31 64

2 144 35 109

3 265 160 105

4 160 79 89 268/664 40.36

3 1 0 0 0

2 0 0 0

3 182 60 120

4 146 57 89 209/328 63.71

4 1 159 76 83

2 230 86 144

3 112 17 95

4 255 47 128 450/756 59.52

5 1 143 72 77

2 141 35 106

3 106 45 61

4 150 62 82 326/540 60.37

3. Samples healed at 1100°C for 1 hour.

Serial

no.

Serial no.

of crack

indentation

Crack

number

Initial

crack

length

(μm)

Final

crack

length

(μm)

Healed

crack

length

(μm)

Average of

crack

length of

one

indentation

Percentage

of healing

7.5%

SiC

7 1 1 132 79 53

2 175 123 52

3 139 81 58

4 0 0 0 163/346 47.10

2 1 158 125 33

2 229 197 32

3 179 95 44

4 112 68 44 153/678 22.56

3 1 172 127 45

2 0 0 0

3 202 159 51

4 0 0 0 96/374 25.66

4 1 0 0 0

2 160 100 60

3 145 89 56

4 165 112 53 169/470 35.95

12.5%

SiC

8 1 1 145 106 39

2 234 163 71

3 226 168 58

4 155 84 71 239/760 31.44

2 1 153 93 60

2 0 0 0

3 144 69 75

4 117 59 58 193/414 46.61

3 1 211 135 76

2 159 82 77

3 201 146 55

4 0 0 0 208/571 36.42

4 1 188 110 78

2 159 61 78

3 0 0 0

4 106 34 72 228/453 50.33

Serial

no.

Serial no.

of crack

indentation

Crack

number

Initial

crack

length

(μm)

Final

crack

length

(μm)

Healed

crack

length

(μm)

Average of

crack

length of

one

indentation

Percentage

of healing

15%

SiC

9 1 1 208 141 67

2 164 104 60

3 228 139 89

4 125 52 72 288/725 39.72

2 1 0 0 0

2 203 106 97

3 198 92 106

4 167 76 91 294/568 51.17

3 1 185 86 99

2 143 78 65

3 266 175 51

4 158 94 64 279/752 37.10

4 1 233 151 81

2 0 0 0

3 256 154 102

4 136 34 104 287/725 39.58

5 1 190 97 93

2 201 98 103

3 195 106 89

4 0 0 0 285/586 48.63

Various crack length before healing and after healing is measured and healed crack length were

calculated. Finally graph is plotted between the maximum crack healed length and temperature

for various composition (7.5%, 12.5% and 15% Silicon carbide). Graph have been shown below

Vertical axis showing maximum crack healed in μm.

7.5% S iC

0

20

40

60

80

100

120

140

1100°C 1200°C 1300°C

7.5%S iC

Temperature (°C)

Fig.5.1. graph showing max crack healed length for 7.5% SiC

12.5% S iC

0

20

40

60

80

100

120

140

160

1100°C 1200°C 1300°C

12.5%S iC

Fig.5.2. graph showing max crack healed length for12.5% SiC

Temperature (°C)

Fig.5.3. graph showing max crack healed length for 15% SiC

Fig.5.4. graph showing max crack healed length for different composition of SiC

FESEM AND EDAX OF THE SAMPLE SINTERED AT 1500°C FOR 12 HOURS

1. For 7.5 % SiC showing elemental percentage and microstructure at 3000X.

Fig.5.5. showing elemental percentage and microstructure of 7.5% SiC

Element Wt% At%

CK 03.46 06.08

OK 34.84 45.89

AlK 56.81 44.37

SiK 04.89 03.67

Matrix Correction ZAF

KV 20.0 MAG 3000 TILT 0.0 MICRONSPERPIXY 0.083

2. For 12.5 % SiC showing elemental percentage and microstructure at 3000X.

Fig.5.6. showing elemental percentage and microstructure of 12.5% SiC

Element Wt% At%

CK 04.32 07.35

OK 39.33 50.23

AlK 47.56 36.02

SiK 08.79 06.39

Matrix Correction ZAF

KV 20.0 MAG 3000 TILT 0.0 MICRONSPERPIXY 0.083

3. For 15 % SiC showing elemental percentage and microstructure at 3000X.

Fig.5.7. showing elemental percentage and microstructure of 15% SiC

CK 05.29 09.20

OK 33.57 43.81

AlK 50.56 39.12

SiK 10.57 07.86

Matrix Correction ZAF

KV 20.0 MAG 3000 TILT 0.0 MICRONSPERPIXY 0.083

XRD analysis of the powder and sintered sample.

Fig.5.8. XRD taken for the alumina powder

Fig.5.9. XRD taken for the silicon carbide powder

Fig.5.10. Mullite phase is present after sintering at 1500C for 12 hours

Figure showing before and after cracks healing of the

sample

Fig.5.11. showing before and after crack healing of 7.5% SiC dispersed in alumina matrix

Fig.5.12. showing before and after crack healing of 12.5% SiC dispersed in alumina matrix

Fig.5.13. showing before and after crack healing of 15% SiC dispersed in alumina matrix

6. DISCUSSION

Results showed that in Al2O3/SiC composite crack was healed after firing the sample at

various temperatures. This means than Al2O3/SiC show good crack healing ability but it should

be investigated that actually what are the factor which are causing crack healing behavior and

what are the cause for crack healing. So, from the work I have investigated some factors which

are responsible for the crack healing behavior. The following factors which are very helpful in

crack healing behavior are;

6.1. Effect of environment on crack healing behavior

From the earlier experiment it was concluded that when samples were crack healed at

different environment (hydrogen, nitrogen, argon and air) it was showed that maximum crack

healing was achieved by fired in the presence of air. I decided to continue my project in the

presence of air. How the crack gets healed in presence of air? Actually what happen is that when

crack is developed in the Al2O3/SiC composite new sites gets open which contain SiC due to the

opening of crack. This SiC which is present at the free surface which reacts with the air forming

a new phase material which gets deposited on the crack bridging the crack and thus healing the

crack. The reaction which takes place is give below:

Thus for crack healing air is necessary.

6.2. Effect of pre crack shape on crack healing behavior

As the crack was developed on the surface the crack shape was broader near the indentation and

throughout it length broadening goes on deceasing or crack shape becomes finer and finer. When

the broadening is more at that condition crack was not heal that means that the void area is much

more than that of free open surface. This also depends on the particle to particle distance it the

particle to particle distance will be low the crack will be finer. This merely depend on the density

of the material as the density will be higher the material will be more dense and particle to

particle will closely attached. Thus shape of the crack is also responsible for the crack healing of

the ceramics. That means that when density is lower, crack healing is also lower this may

happen due to in lower density crack broadening is more and SiC which is present on the surface

of crack is insufficient to fill the void of larger volume. As particle to particle distance is as much

as closer crack healing ability is more.

6.3. Effect of temperature and time on crack healing behavior

Crack healing depends on the crack healing temperature and time. To verify this crack healing is

done on the different temperatures at 1100°C, 1200°C, and 1300°C for one hour. And the result

showed that as the temperature is increased crack healing is also more. Graph is shown below of

maximum crack healed at different temperatures.

Fig.6.1. Graph showing maximum crack healed at different temperatures

As, the crack-healing behavior of these ceramics follows the Eq.

where AH is a proportionality constant (h−1), QaH is the activation energy of crack-healing

(kJ/mol), R is the gas constant (kJ/mol K) and THL is the lowest absolute temperature of the

healing (K).

So, for maximum healing of the crack

TH (temperature of healing) = should be high

QH (activation energy) =should be low

R (gas constant) = should be high and the equation shows that time of healing is inversely

proposnal to temperature of healing. After calculating with some values I got that little variance

in time cause high variance in healing time. So, 1 hour’s time selected and crack was healed at

different temperatures to get the amount of crack healing.

6.4. Effect of SiC percentage on crack healing behavior

To check that how the percentage of SiC in the alumina matrix affect the crack healing

behavior. Three different composition of SiC was dispersed in the matrix of the alumina. The

different composition of SiC which were taken are 7.5%, 12.5% and 15% by volume in the

matrix of alumina. Result showed that as the percentage of SiC is increased in the matrix of

alumina the crack healing ability of SiC/Al2O3 composite increases the graph shown below

clarifies it,

Fig.6.2.Graph showed that max healing of crack at different percentage of SiC

6.5. Crack healing after firing.

Considering all the FESEM-EDAX data and taking into the XRD results, when the green

compact is sintered formation of some mullite phase take place in the matrix of alumina/SiC

composite and rest is alumia/SiC matrix. The mullite phase transformation is shown below by

the reaction:

3Al2O3+2SiO2 = 3Al2O3.2SiO2(mullite)

When the crack is induced in the sample by the Vickers indentation. The crack gets opens

opening of the site for un-reacted silicon carbide which are inside the surface. Again when the

thermal energy is supplied to it, the un-reacted silicon carbide which was open to the surface

reacts with air to form a new phase compound which gets deposited into the crack bridging the

two side of the crack and thus healing the crack. The new phase formation is shown be below by

the reaction.

Experiment was done taking three different temperatures and three different compositions

(which are discussed earlier) which showed that crack healing depends on the temperature of

healing and percentage of SiC. It has been already proved that once a crack is healed, fracture

toughness of the healed part gets increased. So, crack healing in the structural ceramics not only

important in the aspect of its durability but also by the crack healing method fracture toughness

of the structural ceramic can be increased.

7. CONCLUSIONS

The crack healing behavior in the Al2O3/SiC was studied with varying the composition

of SiC in the matrix of alumina and temperature of healing for one hour and conclusions came

out to be;

1. The maximum crack healed in the 7.5% SiC at 1100°C was 60μm, at 1200°C was 84μm

and at 1300°C was 120.

2. The maximum crack healed in the 12.5% SiC at 1100°C was 78μm, at 1200°C was

101μm and at 1300°C was 136μm.

3. The maximum crack healed in the 15% SiC at 1100°C was 106μm, at 1200°C was144μm

and at 1300°C was 179μm.

4. So, optimal crack healing condition was 1300°C for 15% SiC in which maximum crack

length healed was 179μm.

Results showed that a large self crack healing ability is introduced in Al2O3/SiC

composites. Since ceramic have good heat resistance, corrosion resistance, wear resistance

but due to their brittleness and low fracture toughness is application is limited. As self crack

healing ability in the structural ceramics showed a wide approach toward its structural

integrity. Self crack healing behavior not only overcome crack recovery but also is increases

the fracture toughness of the material. Instead of using single material if composite are

prepared it will show higher fracture toughness. This is the new area of research and lots of

research’s are going on developing maximum crack healing ability in structural ceramics and

increasing the fracture toughness. Thus, if above two drawbacks can be overcome the

application of structural ceramic can be widen as well as it structural integrity will be

increased.

8. RECOMMENDATION

Structural ceramics have excellent heat resistance, corrosion resistance, and wear

resistance but due to their high brittleness and low fracture toughness its applicability is limited.

Once crack develops it fails in catastrophic manner. So, it is important to increase its reliability

to widen it application area.

So, to study crack healing behavior composite of Al2O3/SiC was taken and experiment

was done. Although Al2O3/SiC composite showed good crack healing behavior but by doing

further some improvement in the processing the crack healing ability can be further increased. If

the following processing is modified can its crack healing ability as well as fracture toughness

can be well improved.

In my experiment I had used uniaxail press instead of it if iso-static press being used then

it crack healing May further increased. As use of it may increase the density and the

particle to particle distance will be very closer. The crack boarding will be low, thus free

surface volume will be much lower than free surface area.

Instead of sintering the sample in the air atmosphere if it is sintered in the other

atmosphere such as nitrogen, argon etc then during firing it may show higher crack

healing behavior as earlier when it is sintered in the presence of the air the earlier only

some SiC gets oxidized. Thus the amount of unreacted SiC left after sintering might be

less.

Dispersion of some fibrous compound inside the matrix may increase further the fracture

toughness. As it may interlock the matrix thus reducing the crack propagation resistance.

Although lot of research are going the field of structural ceramics to increase its

crack healing ability as well as fracture toughness this two are the main drawbacks of

structural ceramics if these are recovered in the structural ceramics the its structural

integrity will be increased wide its applications.

9. REFERENCES

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