CHAPTER V

MULLITE - SILICON CARBIDE NANOCOMPOSITES

FROM SOL-GEL COATED PRECURSORS

5.1 INTRODUCTION

Ceramic-Matrix composites have been receiving increasing attention

largely due to their significantly enhanced mechanical properties, high

temperature stability and suitability to specific applications [1,2].

Nanocomposites consist of very small particles of guest material (having a

diameter less than lOOnrn or 1000" A) in a host matrix. Only those materials in

which a binder (matrix) material completely surrounds its reinforcing material

(fibre or particle) are considered as true composites. These two phases act

together to produce characteristics not attainable by either constituents acting

alone. A composite is designed to exhibit the best properties of its constituents

or some properties possessed by neither of the constituents [3]. Thus composites

are made (a) to increase strength, stifkess, toughness (b) to reduce permeability

of gases and liquids (c) to modify electrical properties (d) to reduce cost (e) to '\

reduce thermal expansion and (0 to increase chemical and corrosion resistance.

5.1.1 Ceramic Nanocorn~osites

The term and concept "nanocomposite" was formally adopted for

ceramic materials by Roy, Komarneni and colleagues a decade ago [4,5]. They

developed hybrid ceramic metal nanocomposite material synthesised by sol-gel

process. The concept of "structural nanocomposite" was proposed by Niihara in

1991 and can be seen as an adoption of the nanocomposite approach for the

nanostructural tailoring of structural ceramic composites [6] . A general

summary of several classes of synthetic nanocomposites together with some

example are listed in Table 5.1 .

Table 5.1 A general summary of several classes of synthetic nanocomposites

together with some example.

Type of Nanocomposite Example

1. Low temperature sol-gel Mullite1SiO2, Al2O31SiO2 , SiOz/MgO, derived nanocomposite A1203 I Ti02, MullitelZr02, Mullite~TiO~

2. Structural ceramics Al2O31SiC, MullitelSiC, Si3N41SiC nanocomposites MgOISiC

3. Glass Ceramics, glasslmetal nanocomposite Photosensitive glasses

4. Electro ceramic nanocomposites ColCr

5. Nanocomposite films nanocomposite Lead zirconate titanate(PZC)lnickel

6. Entrapment type Zeolitelorganic complexes

7. Layered nanocomposite Pillared clays (montmorilloniteloxide sol particles)

8. Organo ceramic nanocomposite Polymeric matrix/PbTi03

9. Metallceramic nanocomposite Fe-Cr/Alz03, Ni/A1203

Another way of classification of composite is based on the reinforcement

forms present. Thus there are particulate reinforced composite, fibre reinforced

composite and linear composites.

(a) Particulate reinforcement composite: A reinforcement is considered

to be a particle and all of its dimensions are roughly equal. This include those

reinforced by spheres, rods, flakes and many other shapes of roughly equal axes.

The particles either metallic or non-metallic do not chemically combine with the

matrix material . The size, shape, spacing of particles, their volume fraction and

their distribution all contribute to the properties of the material.

(b) Fibre reinforcement composite: Fibre reinforced composites contain

reinforcements having lengths much greater than their cross sectional

dimensions. Fibre reinforcement composites are further divided into

discontinuous fibre composite. Discontinuous fibre composites are those in

which the properties of composite vary with fibre length. The composite is

considered to be a continuous fibre reinforced composite if the change in length

of fibre does not affect the properties.

(c) Laminar composite: Laminar composites are those composed of two or

more layers with two of their dimensions being much larger than their third

dimension. In this type of composite layers two different solid materials are

bonded together.

111 tile ii~ict.oco~~~posites, iliicro-size second phases such as particulate,

platelet, whisker and fibre are dispersed at the grain boundaries of the matrix.

The main purpose of these composites is to improve the fracture toughness. The

nanocomposite can be grouped into three types, intragranular and intergranular

composites and nanoinano composites. This can be schematically represented as

Intra type Inter type

Intra / inter type Nanofnano type

Fig. 5.1 The classification of ceramic nanocomposites

In the intra and inter granular nanocomposites, the nanosize particles are

dispersed mainly within the matrix grains or the grain boundaries of the matrix

respectively. h e high temperature mechanical properties such as hardness,

strength and creep and fatigue fracture resistance are very much improved in

these nanocomposites. In nanolnano composites the dispersoids and the matrix

gains are in the nanometer size. The development of such nanocomposite added

the functions such as machinability and superplasticity like metals to ceramics

16,71.

5.1.2 Svnthesis of Ceramic Nanocomuosites

Chemical vapour deposition method was first reported for the preparation

of ceramic nanocomposites by Niihara and Hirai in 1986. The CVD process is a

very preferable method to disperse the nano-size second phases into the matrix

grains or at the grain boundaries. However, the main disadvantage of this

method is to fabricate the large and complex shaped component for the mass

production and also the high cost. Niihara proposed that for the oxide-based

material such as AI2O3/SiC, MulliteISiC the solid state sintering process was

applied to prepare the nanocomposites, while the non-oxide ones such as

Si3N41SiC, B4CISiC through the liquid phase sintering.

The greatest disadvantage of the ceramics is considered to be its

brittleness. The fracture strength of brittle materials can be improved by an

increase in fracture toughness. The fracture toughness can be increased

reasonably by incorporating various energy-dissipating components into the

ceramic microstructure. The components can be inclusions such as whiskers,

platelets or particles. The reinforcement, serve to deflect the crack or to provide

bridging elements hindering further opening of the crack. Another concept is to

incorporate metallic ligaments into the ceramic matrix to form crack bridging

elements thar absorb energy by plastic deformation [8,9].

Sol-gel method is found to be a potential method in the fabrication of

composites [10,11]. This involves the formation of a homogeneous sol of raw

material and subsequent gelation of the sol to form a porous amorphous oxide.

Upon firing. densification \%,ill proceed to give a glass or a polycrystalline

ceramic. Many factors influence the formation of a gel such as amount of water

added, the nature and concentration of catalyst, the solvent and sequence of

mixing. During drying and firing of gels substantial shrinkage and stresses result

in the samples which frequently lead to the cracking of the amorphous porous

oxide. Some of the advantages of sol-gel method, as far as ceramic composites

are concerned, are (a) molecular level homogeneity can be achieved through

mixing since the starting materials are liquids (b) the raw materials used are

synthesised to ensure a much higher purity, (c) heat treatment to form

polycrystalline ceramic is usually achieved without resorting to as high a

temperature as in conventional process, (d) since the pores in the gel are

continuous, they can be infiltrated with gases and liquids and also the pore size

distribution can be controlled. However the major disadvantage is the

mechanical weakness of the wet gel which is a problem during fabrication of

large monoliths. This is probably solved through hyper critical drying.

The various pathways for processing sol-gel derived ceramic matrix

composite can be divided into the following categories.

1) Mixing of two or more sols to form a homogeneous mixture. The

differtnt components can be tailored so that they do not react with each

other lo form new compounds. This method gives a good uniformity of

the composite.

2) Dispersion of a solid phase such as fme powders of fibres in a sol before

gelation. The composite formed this way will have good homogeneity

and intimate contact between particles and matrix.

3) Infiltration or coating of fibres, laminates or three dimension fibre fabric

by a low viscosity sol. Infiltration can be reported to achieve dense

bodies.

5.1.3 Mullite-Matrix Composites

As described in Chapter 1 (Introduction), mullite is a candidate material

for high temperature engineering applications due to its good creep resistance,

chemical stability, mechanical strength at high temperature and low thermal

expansion. However. its fracture toughness is relatively low (about 2-3

am"') so it is an ideal material to reinforce . Much research is being carried

out on using Sic or ZrOz reinforcements in a mullite matrix [12,13].

Mullite has remarkable high temperature microstructural stability so that

an appropriately processed sample can retain its room temperature strength upto

1300°C. Its low thermal expansion results in good thermal shock resistance,

which coupled with excellent chemical inertness and low thermal conductivity

makes mullite a very attractive ceramic. However its toughness is low, limiting

many of its potential applications and therefore making it a perfect choice of

matrix to wh~ch, a toughening second phase can be introduced [14].

Mullile based composites with dispersed zirconia particles have been

widel) studied. A variety of processing techniques have been reported to make

zirconia mullite composite including mechanical mixing of mullite and zirconia,

mechanical mixing of alumina and zircon and sol-gel mixing of silica, alumina

and zirconia [15]. Mixed powder processing results in a more heterogeneous

microstructure than reaction sintering whereas sol-gel technique can produce

extremely fine evenly dispersed microstructures. The sol-gel route provides

greatly improved strength compared to other techniques but with inferior

toughness. This may arise because of the critical particle size for transformation

of the zirconia is around 113 that of reaction sintered ceramic.

Even though zirconia additions improve strength and toughness values,

the benefits are largely restricted to low temperature because the chemical

driving force for transformation of the high temperature t-phase to the room

temperature polymorph decreases with increasing temperature.

Most of the ceramic nanocomposites studied in the last few years involve

S i c as a reinforcing phase and several Sic containing systems including the

A1203-Sic: mullite-Sic, Zr02-Sic, MgO-Sic and Si3N4-Sic systems, have been

investigated mainly for mechanical performance [16-181. Ceramics that

incorporate silicon carbide whiskers andfor partially stabilized zirconia (PSZ)

are higher than conventional monolithic ceramics. However, at high

temperatures. the instability and deterioration of S i c containing CMCs via

oxidation are of particular concern. Niihara observed that the incorporation of

small amounts of (5 to 10 vol.%) of nanosized (0.3 pm dia.) S i c particles into

an alumina matrix could significantly enhance the strength compared to pure

alumina. The unindented strengths were reported to increase from 350 MPa for

alumina to over 1 GPa for the 5 vol.% Sic composites [15]. The Sic particles

were found to strongly inhibit grain gowth of the alumina matrix. In addition to

this, enhanced densification is reported in presence of ultrafine S i c particles

were found to strongly inhibit grain growth of the alumina matrix. In addition to

this, enhanced densification is reported in presence of ultrafine Sic particles

(-1.5 pm) [ I 51. The improved strength of alumina-Sic nanocomposites has

been observed and a variety of strengthening mechanisms have been proposed to

explain this phenomenon. A method for processing Sic-mullite-A1203

nanocomposites by reaction sintering of green compacts prepared by colloidal

consolidation of a mixture of Sic and alumina powder was reported by Sakka

et al. [19]. Here the surface of the Sic was first oxidised to produce SiOz and

thus reducing the S i c particle size to nano size. The surface Si02 reacted with

alumina to produce mullite, resulting in Sic-M-A120j nano composite (Fig.5.2).

-

Consolidation Oxidation Reaction sintering

Fig.5.2 Schematic illustration of the process steps used to produce

nanocomposites by reaction sintering.

A nev development in the processing of engineering composite ceramics

is throileh precoated powders. [20,21]. This can be accomplished by sol-gel and

insitu solution precipitation process. The benefit of using coated powder is by

achieving enhanced homogeneous distribution, fast densification and

improvement of homogeneity of microstructure of the sintered material, and the

reported high green compact strength. The strength is found to increase with

increasing coating content and reaches a saturation value, which approximately

corresponds to that of the pure coating material. The probable reasons for high

green strength are finer particle size of coating layer, and the relatively high

compaction density. Heterogeneities in a green compact typically densify at

rates different from that at which the matrix phase densifies. Such

heterogeneous packing systems can be densified by coating the hard second

phase particles with a polymer, preventing contact between the non-sinterable

particles and the surrounding matrix. The polymer coating can be burned off

before sintering, yielding a free volume for the matrix phase to 'shrink fit'

around the nonsinterable particle during sintering, pore size distribution is found

to be different for the coated and uncoated composites (221. Poor distribution

due to second phase agglomeration can cause matrix rich areas free of

reinforcing particles, lowering strength and toughness. As a result, a progressing

crack \\auld seek weak matrix regions and effectively avoid energy-dissipative

contact with the reinforcing particles. An ideal microstructure can also be

approached by the coating technique. Coated powders greatly reduce or

eliminate second phase contacts, enabling a more homogeneous phase

distribution 1231. I he effect of coated particles on densification for two phase

composite is represented in Fig.5.3.

(a) (b)

Fig.5.3 Schematic diagram of the effect of coated particles on densification for

two phase composite, (a) discrete particlelwhisker mix before and after

densification: (b) coated whisker before and after densification

In ceramic processing, powder coating is investigated mostly for

improving the structural homogeneity of the products [24]. The process of

coating silicon nitride from oxide sols as sintering aids, via methods such as

solution precipitation [25] and colloidal coating [26] has been studied by various

groups to enhance sintering [27]. A general flow chart for the preparation of

composite from coated powder is shown in Fig. 5.4 The coating process for

AI2O3-Sic nanocomposite involves the deposition of an alumina precursor by

precipitation into the surface of Sic particles dispersed in an aqueous slurry

[20]. X-ray diffraction experiments performed on pre and post calcined coated

platelet powders showed the coating to be an aluminium sulphate hydrate

compound before calcining and crystalline y-A1203 after calcining.

Particle phase

Milling (1) Milling (2) Attrition (3) Surface charge control

Particle phase suspension

I

Composite

Shapindsintering

b

I composites I

Mixindgelation

Fig. 5.4 A general flow chart for the preparation of composite from coated

powder.

Recently plasma sprayed mullite and mullitelyttria stabilized zirconia

(YSZ) dual-layer coating have been developed to protect silicon-based ceramics

Coated particulates

Characterisation

Calcination

from environmental attack. The effect of interfacial contamination by alkali or

alkaline earth metal oxides on the durability of the mullite coated S ic system

indicates that the porosity increased as the mullite purity decreased or the

temperature increased 1281. The coating of a-alumina particles with amorphous

Si02 layers by precipitation from a solution of tetraethylorthosilicate has been

described by Sacks [29] to produce micro composites. The ability to coat

particles successfully by this process depends on the exploitation of

heterogeneous nucleation and growth of the precipitated material on the surface

of the particles. All these reports indicates that the use of coated precursors of

the matrix phase is being made to produce better nanocomposites. Thin, uniform

coating of the matrix phase over the dispersoid provide the same surface

characteristics and hence results in better homogeneity of the sintered ceramic.

Such coatings sometimes function as an interface between the dispersoid and the

matrix phase in order to control properties of thermal expansion as in the case of

S i c particles coated with mullite before introduction to an alumina matrix.

Further, coated precursors can be consolidated with improved green densities

and hence can be sintered to higher densities. By suitably adjusting the condition

of coatings, it is possible to obtain either the mullite phase at as low as 980°C or

a reaction formed dense coating at less than 1300°C. This present work

describes a novel process for the preparation of mullite coated S ic powders

using sol-gel method involving diphasic systems. The results of the investigation

are presented and discussed.

5.2 EXPERIMENTAL PROCEDURE

5.2.1 Activation of S i c Powder

As received Sic powder (-180 nm size), Ibiden, Ogaki (Japan) was

washed with dilute 1% HF solution to remove possible traces of silica associated

with the powder . The HF treated powder was then washed thoroughly with

double distilled water, finally with acetone and dried. Further this powder was

thermally act~vated at 250°C for three hours in air. This activated S ic powder

was used for the synthesis of mullite-Sic nanocomposite precursors.

5.2.2 Preparation of Boehmite Sol

500gm of AI(NO& 9Hz0 (Qualigen Chemicals, India) was dissolved in

4L of distilled water. The solution was heated to 90°C and dilute (-10%)

ammonia was added dropwise till the precipitation was complete at pH 7.5.

During the addition of dilute ammonia solution the temperature of the solution

was maintained -90°C. The precipitate was filtered at the hot condition and

washed with distilled water several times to remove the free nitrates. The

precipitate was aged for 24 hours and peptised into a stable sol at pH 3.5 by the

addition of dilute nitric acid.

Estimation of alumina in the boehmite Sol

To estimate the amount of alumina in the boehmite sol, 10ml of the stable

sol was pipetted out into a previously weighed platinum crucible and dried at

110°C in an air oven. This was heated to 1000°C for 2 hours in an electric

fiimace. By knowing the weight of the empty crucible and the weight of the

sample after heating to 1000°C, the amount of alumina in the boehrnite sol can

be estimated. 11 duplicate experiment was also conducted. In the present case the

concentration of alumina in the boehmite sol was 1 .895~10.~ glml .

5.2.3 Preparation of Mullite Coated SIC Nanocom~osite Precursor

Boehm~te sol prepared by the hydrolysis of AI(N03)3 9H20 and

tetraethylorthosilicate (TEOS) were used as alumina and silica source

respectively. Four different mullite-Sic nanocomposite precursors were

prepared by ~arying the amount of Sic loading in the mullite matrix i.e.

precursor containing 5 vol.% Sic, 10 vol.% Sic, 15 vol?h S ic and 20 vol.%. In

a typical expenment for the preparation of 30gm mullite 5 vol% S ic composite,

1.5 gm of Sic was dispersed in 200ml of ethanol-water mixture (1:l). This was

then ball milled for -5 hrs in a PVC container with alumina as the grinding

media to get a good dispersion of S i c particles in the medium. Mullite precursor

sol is prepared by mixing boehrnite sol and E O S . 1024 ml of boehmite sol is

taken and to this 27.6 ml of TEOS is added dropwise during vigorous stirring.

The mixture was stirred for about 5 hours to result a homogeneous distribution

of alumina and silica species (to get 23.5 gm mullite phase) . To this mixture

S i c suspension was added, stirred for 1 hr and the whole suspension with the

dispersed Sic particles were flocculated to pH 7.5 by the addition of dilute

ammonia solution. The coated powders were filtered, washed and dried in an air

oven at 80°C. Similar experimental procedure was repeated for the preparation

of 10 vol.%, 15 vol.% and 20 vol.% Sic- mullite composites.

The characterisation of the mullite coating on the green samples as well

as calcined at 600' & 800°C were carried out using a Transmission Electron

Microscope (TEM, Model: JEOL 100 CX, USA). The TEM samples were

prepared as follows. A suspension containing coated S ic particles (0.5 wt %)

was made in 50 volume percentage ethanol-isopropanol medium by controlled

ultrasonic stirr~ng. A drop of the suspension was deposited on to a carbon coated

TEM grid and dried under vacuum. The grid was observed under TEM.

The gel coated composite precursors with Sic volume particles 5,10,15 . and 20% were calcined at 1000°C in N2 atmosphere for 2 hours with a heating

rate of 5"CImin. The calcined powders were then ball milled in ethanol for 12

hours and the hard agglomerates were removed by sedimentation and dried in an

air oven at 50°C. The dried powders were then compacted into bars of

dimensions 40mm x 5mm x 4mm by uniaxial pressing at 25 MPa followed by

cold isostatic pressing (CIP) at 500 Mpa. The bars were then subjected to gas

pressure sintering at 1550°, 1650' and 1700°C for a period of one hour under N2

atmosphere in graphite crucibles over S i c powder bed. A gas pressure of 60 bar

was applied at the sintering temperature. The sintered densities of the fired

samples were measured by Archimedes method with water as the reference

liquid. The thermomechanical analysis of the samples were done (after calcining

the coated powder at 1000°C in nitrogen atmosphere) on Netsch Dilatometer at

a heating rate of 10 "Clmin. upto 1000°C and up to 1700°C at a heating rate of

30°C/min. in U2 atmosphere. . XRD of samples were recorded in Philips PW

1710 Diffractometer.

5.3 RESULTS AND DISCUSSION

5.3.1 Characterisation of Coating

The coating on S ic particles with the mullite precursor was first

identified through analysing the intensities of the peaks in the XRD pattems of

the uncoated SIC and mullite gel coated Sic . The XRD patterns of the uncoated

S ic and mullite gel coated Sic, and the same calcined at 600°C are presented in

Fig. 5.5. The intensities of the S i c peaks are found to decrease on coating with

the mullite precursor. However the reduction in intensity decreases with increase

in concentration of Sic particles from 5-20 vol.%, which may be due to the

increase in SIC' particle per unit area. The coating was further analysed through

x - Sic' - Boehmite

0 - y - A1203 x

0

f k

- e

___1

e-

x X

Fig 5.5 XRD patterns of (a) uncoated Sic (b) Mullite gel coated 5 vol % Sic

(c) MuUite gel coated 15 ~ 0 1 % Sic (d) Mullite gel coated 20 vol % Sic

(e) Mullite precursor at 600°C (0 Mullite precursor coated 5 vol %

Sic (600°C) (g) Mullite precursor coated 15 vol % Sic (600°C)

(h) Mullite precursor coated 20 vol % Sic (600°C)

Fig. 5.6 (a) TEM photograph of mullite - 15 vol. % SIC nanocomposite gel precursor.

Fig. 5.6 (b) TEM photograph of mullite - 15 vol. % S i c nanocomposite precursor calcined at 600°C .

Fig. 5.6 (c) TEM photograph of mullite - 15 vol. % Sic nanocomposite precursor calcined at 600°C ( at a higher magnification).

TEM technique (Fig.5.6). The general observations in this regard are, the

starting gel particle size of mullite precursor is about 100nm and is microporous,

with an average pore size of <5nm. They are elongated with a maximum length

of -250nm and thickness of 100-600nm. On calcination at 600°C, there is a

uniform shrinkage of these particles to an average size of 70nm, both in length

and thickness. The porosity decreases during calcination at this temperature and

the particle appear denser. On further calcination at 800°C, a partial fusion of

the glassy phase appear to have taken place, thereby giving a glassy coating over

the S i c particle and the individual gel particle morphology disappears. In any

case the method of coating adopted in this procedure has been successful in

obtaining fully coated S ic particles.

5.3.2 Densification Features

The sintering behaviour of sol gel mullite precursor coated S ic

composites were analysed using a dilatometer and the curves are presented in

Fig.5.7. The coated S ic was calcined at 1000°C prior to analysis and resulted in

a monolayer of transitional alumina and amorphous silica mixture over the

surface of nanosized S ic particles. The typical heating cycle followed for

dilatometric analysis was 10°C/min up to 1000°C and 30°C/min up to 1700°C in

N2 atmosphere and soaking for 1 hr at that environment.

Time (ruin)

Fig. 5.7 Dilatometric curve of mrlllite-Sic nar~ocomposite precursors

157

- C

-

--5

l , I I I , , , 1 , I I

Fig. 5.7 Dilatometric curve of mullite-Sic nanocomposite precursors

5

0 h s V

& 9 .:

--lo $j

--I5

5 0 100 150 200 2 50

Time (min)

l 5

0 h

s V

--5 & 9 a .- i - a

10 v,

---15

Mullite -20 Vol% SIC d

-

~ ~ ~ ~ ~ ~ t 7 n 9 1 ~ l 0 5 0 100 I Time (min) 150 I I 200 I , , 250 i u ̂e 2 1000 G

500

-

0

In all the cases, the sintering of mullite precursor coated S ic occurs in

one step indicating the increased reactivity of composites due to coating.

However, the extend of densification decreases with increasing vol.% Sic . The

composite containing 5 vol.% S i c shows no change upto 1000°C and no

shrinkage is observed. The shrinkage starts approximately around 1200°C which

is associated with mullitization and the rate of densification rapidly occurs in the

range 1270-1300°C. The maximum shrinkage occurred in this range indicates

the rate of pore elimination is fast and so the sintering of the green compact. In

the case of 10 vol.% Sic, the densification starts at 1220°C and the rate of

densification is maximum between 1300-1350°C. The mullite-Sic

nanocomposite precursor with high vol.% Sic requires additional driving force

for complete densification. This is clear from the dilatometric curve of mullite -

10 vol.% S ic in which the maximum shrinkage is only 17.5 % in the range 1000

to 1350°C. The maximum shrinkage in samples with IS vol.% and 20 vol.% Sic

is less than 15%, indicating the incompletion of pore removal. The coating is

more porous with increase in Sic content. In all the cases, the advantage of

prevention of S i c oxidation at high temperatures was observed. Since the

thermal expansion coefficient of mullite and the S i c is nearly identical, the

interface mismatch between mullite-Sic is avoided and reactive alumina present

in coating reacts only with silica which forms liquid phase at lower temperatures

and highly favours the densification process. The high temperature reactions of

AI2O3-Sic reported by Ahmed ef al., shows the sinterability of A1203 , which

has high ionic character and low surface to grain boundary energy ratio [17]. So

it is the thickness and nature of coating, decides the mullite-Sic sintering. Since

the coating is not as uniform as in the case of 5 vol.% S ic precursor, the

sintering of samples containing higher vol.% S ic becomes difficult at this

particular sintering schedule.

The sintered densities of the coated nanocomposite samples at different

temperatures are presented in Table 5.2. The results indicate that the densities of

the composite are strongly dependent on the volume fraction of particulate

dispersions. In all the cases sintering has been carried out under N2 atmosphere

mainly to avoid the oxidation of Sic. The original property of the composite

may be lost due to oxidation of Sic. In order to avoid such situations, the

present work has been carried out in Nz atmoSphere and also the coating present

on the Sic prevents oxidation. For optimal sintering results, it is necessary that

the green compact should be as dense as possible prior to firing. When the

powder is uniaxially pressed, compact cracking and end capping can result

which may, during sintering will combine with the damage created by

inclusions to create a defective sintered part. However in the present work, the

damage in the initial compact was minimized by the application of a just enough

uniaxial load for structural integrity (25hPa) followed by a relatively high

pressure cold isostatic pressing (500 MPa). The green compact densities were

found to be approximately 50% of the theoretical.

Considering the sintered densities of the samples, there is a decrease in

density with the increase in vol.% of Sic. This may be due to the clustering of

the reinforcing particles to form networks, which may hinder the densification

of the con~posite. Two mechanisms can be suggested for this hindrance to

densification 1301. The excluded volume inside network clusters make extra

deformation of the matrix and continuous reinforcement network support the

stress associated with the densification. Each of these mechanisms can be

related to the inability to sinter composites with higher volume fractions of

reinforcing phase to closed porosity. In the first mechanism, the matrix is

required to flow into areas kept devoid of matrix particles by the reinforcing

clusters. A large external load as used in hot pressing, may break the clusters

and force in the matrix material. While in free sintering, which is thexase here

no external load exists to break the clusters and hence the excluded volume

remains. In the second mechanism part of the matrix are shielded from the

densification stress by load-bearing non-deformable networks. Again a large

external load can overcome the stress-bearing capability of the networks, but in

free sintering the sintering stress alone is not usually adequate to accomplish this

and hence the void remains and density decreases. Even in such eases the

densities of the coated powder composites are superior to those of the

mechanically mixed composites [18].

Sample- Sintered Density Sintered Density Sintered Density Mullite (% TD at (% TD at (% TD at with 1550°C/lh) 1650°C/lh) 1 700°C/l h)

5 vol.% S ic 92.72 94.66 96.93

10 vol.% Sic 91.49 93.31 93.98

------------------.-------------------------------------------------------------------------------

Table 5.2 Relative densities of Mullite-Sic samples sintered at various temperatures.

5.3.3 Microstructural Features

Fractographs of 5 and 15 vol. % Sic incorporated mullite composite are

presented in Fig. 5.8. The coated precursor route has shown excellent dispersidn

characteristics with 5 vol.% Sic. Silicon carbide particles of size around 200 nm

appear to have dispersed in mullite having grain size about 1-2 pm and has high

density, random closed pores of submicron size are also seen. Under the present

conditions of preparation the silicon carbide particle appear to get preferentially

distributed at the grain boundaries and this could be a feature of the coated

162

Fig. 5.8 (a) Fractograph of mull.ite - 5 voi. % Sic composite indicating well distributed Sic particles in the mullite matrix

Fig. 5.8 (b) Fractograph of mullite - 5 vol. % Sic composite indicating Sic partides of average size 200 nm distributed at the grain boundary of mullite grain of average size 1-3 pm.

Fig. 5.8 (c) Fractograph of mul1ite - 5 vol. % SiC composite indicating possible inhibition of mullite grain growth interspaced between tcvo SIC particles.

Fig. 5.8 (d) Fractograph of muIlite - 15 vol. % Sic composite indicating occurrence of agglomerates of mullite coated SiC particles acting as a week point in the composite.

precursors. The dispersoid also exhibits very good interface properties. Fracture

is often seen around the S ic particles, thus deciding the fracture mode. There

appears a strong influence for Sic particles in controlling the grain growth in

mullite, especially when placed at two opposite ends of a mullite grain.

However as the vol.% of SiC dispersion increases, an increasing tendency for

segregation of the coated S ic particles is observed. The size of such aggregates

vary in the range 1-2 pm. This may have occurred due to the localised

flocculation of a gel at the early stage of blending the precursor,

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