The Relationship Between Grain Sizes And Hardness Of Sintered Alumina Product.
UTeM Library (Pind.112007)
*
$
-
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,,,
UNlVERSlTl TEKNIKAL MALAYSIA MELAKA
-
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
atau kepentingan Malaysia yang termaktub di dalam
AKTA RAHSIA RASMl 1972)
SULlT
(Mengandungi maklumat TERHAD yang telah ditentukan
oleh organisasilbadan di mana penyelidikan dijalankan)
TERHAD
TlDAK TERHAD
&/LkJR
Kb+L
(Loh Yin Kuan)
Alamat Tetap:
Eyii'A,
Tarikh:
Shaaban)
Cop Rasmi:
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Tarikh:
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* 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).
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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 ini 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 binti
a h 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
..
...
Acknowledgements ........................................................................... vln
Table Of Contents .............................................................................. ix
..
List Of Figures ................................................................................. xli
List Of Tables .................................................................................. xv
List Of Abbreviations. Symbols; Specialized Nomenclature ........................... xvi
Dedication .................................................................................... vii
1.INTRODUCTION ..........................................................................1
1.1
Introduction to Alumina Ceramic .................................................. 1
1.2
Problem Statement ....................................................................2
............................................................................. 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
1.3
Objectives
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
.............................................................. 27
3.3.1 Milling Process ........................................................................ 29
3.3.2 Sieving .................................................................................... 34
3.3 Flow of the Methodology
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
............................................................................. 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
....................................................... 50
3.4.1 Microstructure Analysis
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
3.3.7 Sectioning
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 ....................................................................
5. CONCLUSION & SUGGESTION
67
....................................................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
4.2
56
SEM micrograph of starting material -Milled Alumina
(a) A1-02 (b) Al- 04 (c) Al-06
57
4.3
SEM micrographs of sintered alumina, Al-02
59
4.4
SEM micrographs of sintered alumina, 4 - 0 4
59
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
61
4.7
XRD pattern from N2o3powder before sintered, A1-06
63
4.8
XRD pattern from A1203sintered 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
4.11
Size
66
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
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
f
Or
%
Percent
OC
Degree Celsius
A
total surface area of the compact
A1203
Alumina or Aluminum Oxide
Al(OH)3
Hydrated alumina
Al(OH)4
Sodium Aluminate
BPR
Ball to powder ratio
d
Particle size diameter
GSD
Geometric Standard deviation
h
Hour
AL
Change in length
-
L
Mean intercept
LL
The length of the line segments falling within the
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
PDA
re
rPm
RS
R,
-
Particle distribution analysis
mean equivalent cylindrical pore radius
Rotational per minutes
Rotational Speed
surface-volume mean equivalent spherical
particle radius
Tic
-
Titanium carbide
us.
-
United State
UTEM
-
Universiti Teknikal Malaysia Melaka
Vol
-
Volume
SEM
Scanning Electron Microscope
CHAPTER 1
INTRODUCTION
1.1
Introduction to Alumina Ceramic
Alumina ( A 2 o 3 ) 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
E
600
2 500
Y
0
.ma
rn
400
e 300
*
0
200
'CI
a) 100
0
e
0
140
282
424
566
Speed of Bowl (rpm)
708
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, A1203is 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
conductivity (40 Wm-1K-1
(E",
--
10 at room temperature), and high thermal
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:
*
$
-
x
,,,
UNlVERSlTl TEKNIKAL MALAYSIA MELAKA
-
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
atau kepentingan Malaysia yang termaktub di dalam
AKTA RAHSIA RASMl 1972)
SULlT
(Mengandungi maklumat TERHAD yang telah ditentukan
oleh organisasilbadan di mana penyelidikan dijalankan)
TERHAD
TlDAK TERHAD
&/LkJR
Kb+L
(Loh Yin Kuan)
Alamat Tetap:
Eyii'A,
Tarikh:
Shaaban)
Cop Rasmi:
8/PJ,
&/4h
7 n r ~ nl?W
I
(&&dbinti
@dm,
K L
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 udikelaskan sebasai SULIT atau TERHAD.
UNlVERSlTl TEKNIKAL MALAYSIA MELAKA
Karung Berkunci 1200, Ayer Keroh, 75450 Melaka
Tel : 06-233 2421,Faks : 06 233 2414
Email : fk~bkutkm.edu.rnv
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 ini 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 binti
a h 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
..
...
Acknowledgements ........................................................................... vln
Table Of Contents .............................................................................. ix
..
List Of Figures ................................................................................. xli
List Of Tables .................................................................................. xv
List Of Abbreviations. Symbols; Specialized Nomenclature ........................... xvi
Dedication .................................................................................... vii
1.INTRODUCTION ..........................................................................1
1.1
Introduction to Alumina Ceramic .................................................. 1
1.2
Problem Statement ....................................................................2
............................................................................. 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
1.3
Objectives
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
.............................................................. 27
3.3.1 Milling Process ........................................................................ 29
3.3.2 Sieving .................................................................................... 34
3.3 Flow of the Methodology
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
............................................................................. 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
....................................................... 50
3.4.1 Microstructure Analysis
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
3.3.7 Sectioning
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 ....................................................................
5. CONCLUSION & SUGGESTION
67
....................................................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
4.2
56
SEM micrograph of starting material -Milled Alumina
(a) A1-02 (b) Al- 04 (c) Al-06
57
4.3
SEM micrographs of sintered alumina, Al-02
59
4.4
SEM micrographs of sintered alumina, 4 - 0 4
59
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
61
4.7
XRD pattern from N2o3powder before sintered, A1-06
63
4.8
XRD pattern from A1203sintered 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
4.11
Size
66
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
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
f
Or
%
Percent
OC
Degree Celsius
A
total surface area of the compact
A1203
Alumina or Aluminum Oxide
Al(OH)3
Hydrated alumina
Al(OH)4
Sodium Aluminate
BPR
Ball to powder ratio
d
Particle size diameter
GSD
Geometric Standard deviation
h
Hour
AL
Change in length
-
L
Mean intercept
LL
The length of the line segments falling within the
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
PDA
re
rPm
RS
R,
-
Particle distribution analysis
mean equivalent cylindrical pore radius
Rotational per minutes
Rotational Speed
surface-volume mean equivalent spherical
particle radius
Tic
-
Titanium carbide
us.
-
United State
UTEM
-
Universiti Teknikal Malaysia Melaka
Vol
-
Volume
SEM
Scanning Electron Microscope
CHAPTER 1
INTRODUCTION
1.1
Introduction to Alumina Ceramic
Alumina ( A 2 o 3 ) 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
E
600
2 500
Y
0
.ma
rn
400
e 300
*
0
200
'CI
a) 100
0
e
0
140
282
424
566
Speed of Bowl (rpm)
708
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, A1203is 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
conductivity (40 Wm-1K-1
(E",
--
10 at room temperature), and high thermal
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: