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JUDUL: The Relationship Between Grain Sizes and Hardness of Sintered Alumina Product

SESl PENGAJIAN : 2007-2008

Saya LOH YIN KUAN

mengaku membenarkan tesis (PSMISarjanalDoktor Falsafah) ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut:

1 . Tesis adalah hak milik Universiti Teknikal Malaysia Melaka . 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan

untuk tujuan pengajian sahaja. 3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan

pertukaran antara institusi pengajian tinggi. 4. **Sila tandakan ( d )

(Mengandungi maklumat yang berdarjah keselamatan SULlT atau kepentingan Malaysia yang termaktub di dalam

AKTA RAHSIA RASMl 1972)

TERHAD (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasilbadan di mana penyelidikan dijalankan)

TlDAK TERHAD

&/LkJR Kb+L

(Loh Yin Kuan) (&&dbinti Shaaban)

Alamat Tetap: Cop Rasmi: Eyii'A, &/4h 8/PJ,

7 n r ~ n l?W @dm,

I K L

Tarikh: aq/ , a+) , 0% Tarikh: ROD v * Tesis dimaksudkan sebagai tesis bagi ljazah Doktor Falsafah dan Sarjana secara penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan penyelidikan, atau Laporan Projek Sarjana Muda (PSM). ** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasalorganisasi berkenaan densan menvatakan sekali sebab dan tempoh tesis ini ~ e r l u dikelaskan sebasai SULIT atau TERHAD.

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FAKULTl KEJURUTERAAN PEMBUATAN

Rujukan Kami (Our Ref) : Rujukan Tuan (Your Ref):

25 Mac 2008

Pustakawan Perpustakawan Universiti Teknikal Malaysia Melaka UTeM, No1 , Jalan TU 43, Taman Tasik Utama, Hang Tuah Jaya, Ayer Keroh ,75450, MELAKA.

Saudara,

PENGKELASAN TESlS SEBAGAI SULITnERHAD - TESlS SARJANA MUDA KEJURUTERAAN PEMBUATAN (BAHAN KEJURUTERAAN): LOH YIN KUAN TAJUK: THE RELATIONSHIP BETWEEN GRAIN SIZES AND HARDNESS OF SINTERED ALUMINA PRODUCT

Sukacita dimaklumkan bahawa tesis yang tersebut di atas bertajuk "THE RELATIONSHIP BETWEEN GRAIN SIZES AND HARDNESS OF SINTERED ALUMINA PRODUCT" mohon dikelaskan sebagai terhad untuk tempoh lima (5) tahun dari tarikh surat in i memandangkan ia mempunyai nilai dan potensi untuk dikomersialkan di masa hadapan.

Sekian dimaklumkan. Terima kasih.

"BERKHIDMAT UNTUK NEGARA KERANA ALLAH"

Yang benar, /7

~ d ~ z i z a h binti Shaaban Pensyarah, Fakuiti Kejuruteraan Pembuatan

DECLARATION

I hereby, declared this thesis entitled "The Relationship Between Grain Sizes and

Hardness of Sintered Alumina Product" is the results of my own research

except as cited in references.

Signature

Author's Name Loh Yin Kuan

Date 25 MARCH 2008

APPROVAL

This PSM submitted to the senate of UTeM and has been as partial fulfillment of the

requirements for the degree of Bachelor of Manufacturing Engineering. The

members of the supervisory committee are as follow:

J (Dr. Azizah binti Shaaban)

(Official Stamp & Date)

ABSTRACT

Ceramic is a glazed or unglazed body of crystalline or partly crystalline structure, or

of glass, which body is produced from essentially inorganic, non-metallic substances

and either is formed from a molten mass which solidifies on cooling, or is formed

and simultaneously or subsequently matured by the action of the heat. Alumina is

one of the ceramic material formed of bodies with non-naturally occurring material is

called new ceramic or modern ceramic. Alumina was given attention in this study of

particles size and its hardness due to its most widely used as the synthetic raw

materials for ceramics with excellent heat and corrosion resistance property. Thus, it

has a variety of applications are being developed for machine part used in a lower

temperature range, such as cutting tools, pumps and valves. Objectives of this study

are to produce fine and narrow particles size distribution of alumina through

planetary ball milling and, correlate between particle size and hardness of the

sintered alumina, the changes in grain structure of alumina products will also be

discussed. The main factor to study of particles sizes is correlated with the milling

time, and the hardness of sintered alumina is corresponding to the grain size and

microstructure. The seven alumina specimen with different milled time is fragmented

on impart by mill ball. The milled powder is then compacted by the hydraulic pellet

press and followed by sintering process. The changes in particles size and hardness

of sintered alumina can be investigated via scanning electron microscope and

hardness test respectively to identical the effect of grains size on hardness. As

conclusion, increasing milling time resulted in a finer grain sizes and increase in

hardness of sintered alumina. The Planetary ball mill cannot well-perform to

fragment the particles sizes uniformly due to trapped powder between vial and ball

milling. Finally, mechanical properties of milled particle size and hardness of

sintered alumina will be affected the application and quality of the product.

ABSTRAK

Seramik merupakan bahan licau atau bukan licau dengan struktur kristalografi yang

sepenuh dan separuh, dimana bahan ini dihasilkan daripada inorganik, bahan bukan

logam atau membentuk daripada berat leburan dimana mengalami pemejawapan atau

terbentuk pada masa yang sama.

Alumina adalah salah satu bahan seramik dihasilkan daripada bahan bukan

semulajadi dan sebagai teknologi seramik maju atau bahan baru seramik. Alumina

menitikberatkan dalam saiz partikel dan kekerasan disebabkan kegunaan yang

meluas dengan sifat rintangan haba dan kakisan yang kuat daripada seramik.Dengan

itu,kegunaan alumina dalam penghasilkan bahagian mesin yang memerlukan sifat

suhu rendah, misalannya alat pemotong,pump dan valves amat diperlukan. Matlamat

daripada projek bertujuan menghasilkan saiz dan taburan partikel yang halus dan

kecil melalui bola pengisaran mesin, di mana berhubung kait dengan saiz partikel

dan kekerasan sinter alumina.Akhirnya, memerhatikan perubahan alumina dalam

tumbesaran butir. Faktor utama mempelajari projek ini berhubungkait dengan masa

pengisaran dan kekerasan bagi sinter alumina yang kait dengan saiz butir dan

mikrostruktur. Ketujuh-tujuh alumina spesimen ini mempunyai masa pengisaran

yang berbeza dan dihancurkan oleh media pengisar. Selepas it, serbuk kisaran

dimampatkan oleh mesin hidraulik pemampat dan diikut dengan persinteran.

Perubahan dalam saiz partikel dan kekerasan sinter alumina akan dikaji dengan

Mikroskop Elektron Imbasan (SEM) dan ujian kekerasan untuk mengenalpastikan

kesan saiz butir partikel-kekerasan selepas activity pengisaran dan persinteran.

Kesimpulannya, penambahan masa pengisaran akan mendapat saiz butir yang lebih

halus dan penambahan kekerasanterhadap sinter alumina. Pengisar tidak dapat

menyerata saiz butir dengan menyeluruh kerana terdapat partikel yang melekat pada

media pengisar tidak dapat dikisarkan.

DEDICATION

To my beloved family

ACKNOWLEDGEMENTS

This report inevitably involves many helping hands. First and foremost, the author

would like to thank the author's project supervisor, Dr Azizah binti Shaaban for all

the guidance, thoughtful comment and her patience during the final year project..

The author owes a debt of gratitude to technical staff Mr. Sarman bin Basri, Mr.

Azhar Shah bin Abu Hassan and Mr. Mahader bin Muhamad for their invaluable

cooperation and assistance.

Lastly, the author would like to thank to author family and friends for their supports

and encouragement.

TABLE OF CONTENTS

... Declaration ...................................................................................... 111

Approval ......................................................................................... iv

Abstract .......................................................................................... v

Abstrak ........................................................................................... vi . .

Dedication .................................................................................... vii ...

Acknowledgements ........................................................................... vln

Table Of Contents .............................................................................. ix . .

List Of Figures ................................................................................. xli

List Of Tables .................................................................................. xv

List Of Abbreviations. Symbols; Specialized Nomenclature ........................... xvi

1.INTRODUCTION .......................................................................... 1

1.1 Introduction to Alumina Ceramic .................................................. 1

1.2 Problem Statement .................................................................... 2

............................................................................. 1.3 Objectives 3

.................................................................... 1.4 Scope of the Study 3

1.4.1 Selection of Milling Speed .......................................................... 3

........................................................... 1.4.2 Selection of Milling Time 4

....................................................................... 1.4.3 Sintering profile 5

............................................................................ 1.5 Thesis Layout 5

2.LITERATURE REVIEW ................................................................. 6

2.1 Introduction ................................................................................ 6

2.2 Alumina (Al2O3) ........................................................................... 6

2.2.1 The Alumina Manufacturing Process ............................................. 7

2.3 Ceramic Powder Processing ............................................................ 10

2.3.1 Particles Size reduction ............................................................. 10

2.3.2 Sieving ................................................................................. 12

2.3.3 Doping .................................................................................. 13

................................................................ 2.3.4 Compaction Process 13

..................................................................... 2.3.5 Sintering Process 16

..................................... 2.4 Particle Size-Microstructure-Hardness Relation 19

2.4.1 The Effect of Particle Size and Particle Sizes

................................... Distribution on Sintering and Microstructure 19

........................ 2.4.2 Effect Of Grain Size On Hardness Of Sintered Product 22

........................................................................... 2.5 Fracture Mode 25

....................................................................... 3 . METHODOLOGY 26 ................................................................................. 3.1 Introduction 26

............................................................................. 3.2 Starting Material 26

.............................................................. 3.3 Flow of the Methodology 27

........................................................................ 3.3.1 Milling Process 29

.................................................................................... 3.3.2 Sieving 34

3.3.3 Analysis Powder Alumina by SEM ............................................................... 34

.................................................. 3.3.4 Particle Distribution Analysis, PDA 39

.................................................................... 3.3.5 Compaction Process 39

...................................................................... 3.3.6 Sintering Process 43

............................................................................. 3.3.7 Sectioning 44

............................................................................... 3.3.8 Mounting 45

............................................................... 3.3.9 Grinding and Polishing 46

........................ 3.3.10 Sample Characterization of Sintered Alumina by SEM 48

.......................................................................... 3.3.11 Hardness Test 48

................................. 3.3.12 Analysis of the microstructure and hardness data 49

.......................................................... 3.4 Sample Characteristic Analysis 50

....................................................... 3.4.1 Microstructure Analysis 50

.................................................... 3.4.1.1 Scanning Electron Microscopy 50

................................ 3.4.1.2 Energy-Dispersive X-ray Spectroscopy, EDS 51

........................................................ 3.4.1.3 X-Ray Diffraction, XRD 51

.................................................................. 3.4.2 Physical Analysis 52

3.4.2.1 Determination of the particle size by image analysis of

................................................ Electron photo micrographs 52

......................................... 3.4.2.2 Grain size and mean lineal intercept 52

..................................... 3.4.2.3 Relative density of the sintered alumina 54

..................................................... 3.5 Data Collection Experiment Tools 55

.............................................................. 4 . RESULT & DISCUSSION 56

................................... 4.1 Microstructure Study on Starting Alumina Powder 56

............................. 4.2 Microstructure Observation on Milled Alumina Powder 57

..................................... 4.3 Microstructure Observation on Sintered Alumina 58

...................................................................... 4.3.1 Fracture Surface 61

..................................................................... 4.3.2 X-ray Diffraction 62

.................................................................................... 4.4 Hardness 64

................................................................................. 4.4.1 Density 65

4.4.2 The Relationship between Microstructure (Density) and

................................................................ Hardness of Alumina 66

.................................................................... 4.5 Problem Encountered 67

.................................................... . 5 CONCLUSION & SUGGESTION 69 ................................................................................... 5.1 Conclusion 69

......................................................................... 5.2 Recommendation 70

................................................................................ REFERENCES 72

APPENDIX

A Thesis 1 and 2 Planning Schedule

LIST OF FIGURES

Rotating speed of milling plate versus rotating speed of bowl

Sintering profile

The Bayer process for chemically refining bauxite into alumina

The effect of BPR on the mean particle size of powders. RS: 700 rpm

Schematic representation for the deformation mechanism of the

alumina agglomerates

Solid-State Material Transport

Sintering characteristics of colloidally-prepared sub-micron alumina

powder compacts of two mean particles sizes

Grain size effect on the Vickers hardness of ground alumina

microstructures

Grain size effect on the Vickers hardness of polished alumina

microstructures

Schematic representation of the fracture energies for brittle solid

vs grain size.

Methodology sequence flow chart

Planetary ball milling machine

Initiate the value to zero

Weigh 50g alumina powder

Unscrewing the lid of the bowl

Putting the alumina powder in the bowl

Putting the milling balls in the bowl

Getting out the milled powder by spatula

Scanning electron microscope

Small amount of alumina in alcohol

Container wrapped by alumina foil

Put inside the ultrasonic cleaner to disperse milled alumina powder

Ultrasonic machine

Dispersed alumina powder is put on the tray by riffle splitter for drying

After drying, the powder is scratch and placed on carbon tape

laid on the rotary riffle

Rotary riffle is placed in the SEM for characterization

Hydraulic pellet press

Green density of dry-pressed compacts versus the compacting pressure

Schematic representation of the single-acting, uniaxial, dies

compaction method of the basic geometry

Schematic representation of the single-acting, uniaxial, dies

compaction method of the various major manufacturing stages.

PA, PT and PR represent applied, transmitted and ejection stresses,

respectively (Pa2>Pal, and PT2>PTI). Z and D represent the powder

height and the die diameter, respectively.

Furnace machine

Sintered Alumina

Diamond Cutter Machine

Sectioning Part for Sintered Alumina

Mounting Machine

Mounted Specimen for Al-02, Al-04 and A1-06

Grinding Machine

Polishing machine

Hardness testing machine

4.1 SEM micrograph of starting material -from (a) with 200 magnifications and

(b) 1Kx magnifications 56

4.2 SEM micrograph of starting material -Milled Alumina

(a) A1-02 (b) Al- 04 (c) Al-06 5 7

4.3 SEM micrographs of sintered alumina, Al-02 5 9

4.4 SEM micrographs of sintered alumina, 4 - 0 4 5 9

4.5 SEM micrographs of sintered alumina, Al-06 60

4.6 SEM micrographs of fracture surface of sintered alumina for

(a) A1-02, (b) A1-04 and (c) Al-06 6 1

4.7 XRD pattern from N2o3 powder before sintered, A1-06 63

4.8 XRD pattern from A1203 sintered powder, Al-06 63

4.9 Grain size effect on the Vickers hardness of alumina microstructures 65

4.10 The relationship between relative density of sintered alumina and grain

Size 66

4.11 Density effect on the Vickers hardness of alumina microstructures 67

LIST OF TABLES

2.1 Microstructural Changes observed in the initial, intermediate, and

final stage of solid-state sintering (powder compact) 18

2.2 Some factors influencing solid-state reactivity 19

3.1 Sample Code with difference Milling time 30

4.1 Relative density of sintered alumina versus grain size 65

4.2 Measurements of grain size, density, and hardness of sintered alumina for

powder milled at 2 , 4 & 6 hours. 67

LIST OF ABBREVIATIONS, SYMBOLS, SPECIALIZED

NOMENCLATURE

>

<

f

%

OC

A

A1203

Al(OH)3

Al(OH)4

BPR

d

GSD

h

AL - L

LL

Alpha

Beta

fractional porosity

GammafSpecific surface energy

Micro

Density

Green density

Density of sintered alumina

he ore tical alumina density

change

Approximately

More than

Less than

Or

Percent

Degree Celsius

total surface area of the compact

Alumina or Aluminum Oxide

Hydrated alumina

Sodium Aluminate

Ball to powder ratio

Particle size diameter

Geometric Standard deviation

Hour

Change in length

Mean intercept

The length of the line segments falling within the

PDA

re

rPm

RS

R ,

SEM

T i c

u s .

UTEM

Vol

phase of interest per unit length of the test line

Meter

- Magnesium Oxide

- Number of points where the lines intercept the

features of interest (grain) per unit length of test

line

- Particle distribution analysis

- mean equivalent cylindrical pore radius

- Rotational per minutes

- Rotational Speed

- surface-volume mean equivalent spherical

particle radius

- Scanning Electron Microscope

- Titanium carbide

- United State

- Universiti Teknikal Malaysia Melaka

- Volume

CHAPTER 1

INTRODUCTION

1.1 Introduction to Alumina Ceramic

Alumina (A2o3) is the oxide of aluminum (Al) and consider as a fine ceramic. Alumina

occurs in nature as the mineral bauxite, bayerite, corundum, diaspore and gibbsite.

Alumina has an ionic bonding and there are numerous configurations for each

combination of element. Aluminas have few type crystalline forms which are a- alumina,

p- alumina and y- alumina.

The production of alumina for ceramic is based on the Bayer process, which was for the

refining aluminum. The process involved are mining, dissolution of the alumina at

elevated temperature, addition of flocculants, precipitation of pure gibbsite, regeneration

of the solutions for recycling and finally heating the gibbsite to 1100°C (calcinations) to

give alumina.

Many different forms in which alumina manifests itself give the material a great

versatility, with characteristics ranging from the hardness of sapphire to a softness

similar to that of talc, from a high insolubility to ready solubility in acids or alkalies,

from chemical inertness to high activity, from crystal sizes smaller than a micron to

those of the single-crystal ruby gems.

The applications range from mechanical, chemical, and thermal resistance-demanding

ones such as, refractory, cutting tools, abrasives, dental and bone implants, and space

rockets protection to optical such as ruby lasers, or optical windows such as the radomes

used to enclose the radar antenna equipment in high-speed aircrafts. Alumina used as a

structural material processes properties such as having extremely high melting point

(2,0500C), insoluble in water and organic liquid and very slightly soluble in strong acids

and alkali, high hardness, electrical insulation and the ability to take on diverse shapes

and functions.(Ichinose, 1987). The optical properties of ruby, which is a-Al203 with

0.5 to 2% Cr203, have been amply exploited by the jewel industry, but are also

important in lasers, where rods of single-crystal ruby are used to generate the highly

coherent laser light. Another important electronic application is electrical and electronic

application are circuit substrates,.spark plugs, magnetrons, where good insulators with

low dielectric constant are desirable for the good performance of the circuits, As in the

case of all phases of alumina, the number of applications is steadily

growing.(Ruberto,1998).

1.2 Problem Statement

Alumina ceramics have been widely applied because of the high hardness, good wear

resistance and outstanding mechanical properties. Due to its technological importance,

sintering of ceramic powder compacts has been extensively investigated and many

works have been developed for improving their mechanical properties. Smaller particles

size and narrow size distribution have been reported by earlier researcher, Ma and Lim

in their studies of effect of particle size distribution of agglomerate alumina powder

compact that would promote sinterability and thus producing dense microstructure with

higher hardness properties. However, the research resource of the effect of particle size

of alumina on hardness of sintered alumina is scarce. Therefore, the present work

attempts to study the relationship between grain size and hardness of sintered alumina

and to have a comprehensive and overall understanding on the correlation between

particle size, microstructure and hardness of sintered alumina. In other words, this study

is to understand the effect of particle size on microstructure and the resultant

microstructure on hardness of sintered alumina. Furthermore, the relationship between

the hardness as a function of grain size will be determined.

Mechanical milling is known to be used to decrease the particles sizes. The planetary

ball mill will be used in this study to reduce the particles size of alumina powder to some

extent. Thus, the performance of the planetary ball mill in producing fine powder will be

evaluated in term of microstructure and particle size distribution by using scanning

electron microscope and laser particle size distribution respectively.

1.3 Objectives

The objectives of this study are to produce fine and narrow size distribution of alumina

powder through planetary ball milling and to correlate between particle size and

hardness of sintered alumina. The grain structures of alumina products will be compared

as well.

1.4 Scope of Study

1.4.1 Selection of Milling Speed

Rotational speed (RS) of planetary ball mill is set at 300 revolutions per minutes (rpm).

Rotational speed of 300rpm is selected due to some reasons. The operating rotational

speed of the planetary ball mill is in the range of 40 rpm to 600 rpm. A trial run of

planetary ball mill with 100, 200, 300, 400, 500, and 600 rpm was done. From the

observation, the rotational speed for 100 and 200rpm is slow thus the milling process is

inefficient. Then, strong vibration occurred in 400, 500 and 600 rotational speed.

Dangerous environment is created to student during running time of the experiment.

Furthermore, strong vibration ruins the reliability of the machine as well if the machine

is operated in long period. Therefore, 300rpm is the optimum rotational speed for whole

milling process.

The actual performance speed of planetary ball milled is different from the rotational

speed setting because there have rotating speed for plate and rotating speed for bowl.

Thus, setting of the machine controls the rotating speed of plate. The corresponding

speed between plate and bowl is shown in Figure 1.1. From here, the milling speed is

emphasized on the rotating speed of planetary milling plate.

Speed of Plate Vs Speed of Bowl

600 E 2 500 Y

0 400 . m a rn e 300 * 0 200 'CI

a 0) 100 e

0 140 282 424 566 708

Speed of Bowl (rpm)

Figure 1.1: Rotating Speed of milling plate versus rotating speed of bowl

1.4.2 Selection of Milling Time

Alumina milling time is selected as 0.5, 1, 2, 3, 4, 5 and 6 hours in order to investigate

the particles size distribution for each variable milling time. The selected milling time is

not long since the time frame for the project is limited. Moreover, the millings activities

need to coordinate with laboratory technician in order to avoid clashing with conducted

class because operating noise of the machine is disturbing.

1.4.3 Sintering Profile

Temperature, "C

Figure 1.2: Sintering profile

The specimens consist of 2 hours, 4 hours and 6 hours milled alumina are set to 1250°C

sintering temperature. These three specimens are selected for sintering process in order

to investigate the different grain size and hardness after sintering process. Figure 1.2

shows the sintering profile for the sintering process. As room temperature is estimated

around 30°C, then temperature is ramped up 1 O°C/min until 1 250°C, hold for 1 hour and

cooled down naturally in the furnace, finally final product is released and the properties

are investigated.

1.5 Thesis Layout

The thesis is divided into five chapters. In this thesis, a review of literatures is presented

in Chapter 2 which discusses about the alumina material, alumina processing, and the

previous research work and theories development on the relation of grain size and

hardness. In chapter 3, the flow of the methodology used in this experiment is discussed.

Chapter 4 which is result and discussion and Chapter 5 are conclusion and suggestion.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter will be discussing studies from previous researchers to particular method. It

consists of alumina, alumina manufacturing process, ceramic powder processing,

particles size reduction, sieving, doping, compaction process, sintering process, particle

size, microstructure, hardness relation, the effect of particle size and particle size

distribution on sintering and microstructure and effect of grain size on hardness of

sintered product.

2.2 Alumina (&03)

Alumina, A1203 is the most widely used of synthetic raw material for ceramic. Many of

the oxide ceramic have strong ionic bond. Of these, A1203 has the most stable physical

properties, with the excellent heat and corrosion resistance. A loss of strength at high

temperature is unavoidable, but a variety of applications are being developed for it as

machine parts used in a lower temperature ranges, such as cutting tools, pumps, and

valves.

Although alumina can be represented simply by the chemical formula A1203, its nature

varied considerably depending upon, for instances, its crystalline form, the impurities

present, and the particle diameter. Since the required physical properties also vary

according to be intended use, many different type of crystalline form, such as a- alumina,

p-alumina, y-alumina and q-alumina. a-alumina composed of colourless hexagonal

crystal with properties stated below. p-alumina is the compound of natrium oxide,

Na20.AI2O3. The pure form obtained by calcination at high temperature. Pure a -/d2O3

has a very high melting point (2050 OC), extreme hardness (9 on the Mohs scale of

hardness, which is next hardest to diamond, at lo), high electrical resistivity (1011 m at

50VC), low dielectric constant (E", -- 10 at room temperature), and high thermal

conductivity (40 Wm-1K-1 at room temperature).The excellent of a -Al2O3 in all of

these characteristic, together with its capacity of being efficiency joined to a metal,

makes the material unequalled in a large variety of application.(Ruberto,1998) It is

stable to about 1000°C and contains traces of water or hydroxyl ions. y-Alumina

composed of minute colorless hexagonal crystal with the sp.gr about 3.6 that are

transformed to the a-form at the high temperature. (Somiya, 1987). Then, q-alumina is

observed on heat treatment. The degree of crystallization is only 50-55% and the crystal

size is small, around 6nm. Another noticeable feature of the microstructure is the large

number of pores which gives a low effective density and the service temperature is

900°C. Continue heating results in the progression through the y-alumina with a

contaminant increase in strength and toughness. (Matthews et al, 1994). This different

type of alumina form is chosen depend on intended use among all those commercially

available.

2.2.1 The Alumina Manufacturing Process

Alumina, Al2O3, is produced by using Bayer process in ceramics industries as shown in

Figure 2.1. The basic raw material used is bauxite. The principal operations in the Bayer

process are physical beneficiation of the bauxite, digestion (in the presence of caustic

soda, NaOH at an elevated temperature and pressure), clarification, precipitation and

calcination, followed by crushing, milling, and sizing. During the digestion, most of the

hydrated alumina goes into solution as sodium aluminate: