FABRICATION OF DENSE ALUMINA THROUGH SLIP

CASTING PROCESS

SITI NORFAZNIE BT AHMAD NAJIB

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

FABRICATION OF DENSE ALUMINA THROUGH SLIP

CASTING PROCESS

This report submitted in accordance with the requirement of the Universiti Teknikal

Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Engineering Materials) with Honours.

by

SITI NORFAZNIE BT AHMAD NAJIB

FACULTY OF MANUFACTURING ENGINEERING

2008

i

ABSTRACT

Dense alumina products were prepared through slip casting method. Slips are prepared

at different alumina and ball clay content. The slips were prepared by mixing the

powders with water as solvent and dispersant at different composition of alumina and

ball clay content in each batch. Three different body formulated were prepared named as

batch A, B and C. The process sequence for ceramic forming by using slip casting are

started by setting the composition, followed by mixing the starting raw material as well

as slip preparation through roller milling to obtain the homogenous slip and casting by

using the plaster of Paris mold, drying of slip cast, into bisque formation and lastly the

specimen was sintered. Mechanical properties and microstructures of dense alumina

product were evaluated, analysis methods used in this project research are measure

degree of shrinkage, Scanning Electron Microscopy (SEM) for microstructural

observation, X-Ray Diffraction (XRD) for phase observation, three point bending test

for flexural strength measurement of the specimen. All the specimen from batch A with

the 0.1% ball clay addition have fracture during the drying stages. Batch C have higher

shrinkage and higher flexural strength compared other two batches. From the

microstructural image less pores for batch C specimen are reveal.

ii

ABSTRAK

Spesimen alumina terpadat dihasilkan menggunakan teknik pengacuanan tuangan..

Bancuhan disediakan dengan menngunakan tiga campuran alumina dan tanah liat yang

berbeza. Bancuhan juga telah dicampur dengan air dan bahan serak. Terdapat tiga

bancuhan iaitu bancuhan A, B, dan C. Urutan proses menggunakan teknik pengacuan

tuangan adalah seperti berikut; komposisi bahan dicampur, bancuhan disediakn

menggunakan „roller milling‟untuk mendapatkan bancuhan yang sebati, bancuhan

dituangan ke dalam acuan, spesimen yang terbentuk dikeringkan dan akhir sekali

specimen dibakar pada suhu tinggi ditetapkan. Sifat-sifat mekanikal dan mikrostruktur

alumina terpadat dinilai. Sifat mekanikal dan mikrostruktur dinilai, kaedah yang

digunakan untuk menganalisis adalah dengan mengukur darjah pengecutan, Mikroskop

Imbasan Elektron iaitu untuk pemerhatian pada mikrostruktur, Pembelauan X-Ray iaitu

untuk pemerhatian pada fasa, ujian pelenturan tiga titik untuk mengukur kekuatan

pelenturan. Didapati semua spesimen daripada bancuhan A telah patah pada bahagian

tengah pada peringkat pengeringan dimana bancuhan ini mengandungi 99.9% alumina

dan 0.1% tanah liat. Bancuhan C mempunyai pegecutan dan kekuatan pelenturan yang

paling tinggi berbanding dengan dua bancuhan yang lain. Daripada gambaran

mikrostruktur spesimen C, ia menunjukkan kurang lubang.

iii

DEDICATION

For my beloved family and friends.

iv

ACKNOWLEDGEMENT

Bismillahirrahmanirrahim,

First of all, I am very grateful to Allah the Almighty for giving me the opportunity to

finish my Projek Sarjana Muda (PSM) technical report from the first word until end of

point. I would like to thank to my supervisor, Dr Azizah binti Shaaban for her support,

willingness to help and give lot of guidance and full commitment for my research

project. I would also thank to Mr Sivarao a/l Subramonian, the PSM Coordinator of

Faculty of Manufacturing Engineering of University Technical Malaysia Melaka that

had manage and guide me during my final year project . Finally, my sincere appreciation

is dedicated to my family and my fellow friends in UTeM who are willing to help me

even though they are also busy with their own PSM project.

v

TABLE OF CONTENTS

ABSTRACT i

ABSTRAK ii

DEDICATION iii

ACKNOWLEDGEMENT iv

TABLE OF CONTENTS v

LIST OF TABLES viii

LIST OF FIGURES x

LIST OF ABBREVATIONS, SYMBOLS & NOMENCLATURE xiii

1. INTRODUCTION 1

1.1 Introduction to Ceramic 1

1.2 Type of Ceramic 2

1.2.1 Traditional Ceramic 3

1.2.2 Advanced Ceramic 3

1.2.3 Ceramic Based Composite 3

1.3 Application of Ceramic

1.4 Objectives 5

1.5 Scope 6

1.6 Problem Statement 6

2. LITERATURE REVIEW 8

2.1. Advanced Ceramic Material 8

2.2 Alumina Ceramic 8

2.3 Properties of Alumina 9

2.4 Ceramic Forming 12

2.5 Slip Casting 13

2.5.1 Slip Preparations 14

vi

2.6 Ceramic Sintering 20

3. METHODOLOGY 23

3.1. Process Sequence 23

3.2. Material 24

3.3. Slip Preparation 25

3.3.1 Mixing 25

3.3.2 Slip Preparation through Ball Mill 25

3.3.3 Casting using Plaster of Paris Mold 26

3.3.4 Drying and Released Slip Casting Part 28

3.4 Bisque Process 28

3.5 Sintering 28

3.6 Finishing Operation 29

3.7 Analysis of Specimens 29

3.7.1 Shrinkages Test 29

3.7.2 Flexural Test 30

3.7.3 Scanning Electron Microscopy (SEM) 31

3.7.4 X-ray Distribution (XRD) 32

4. RESULT AND DISCUSSION 33

4.1. Introduction 33

4.2. Raw Material Analysis 33

4.2.1 Powder Morphology 33

4.2.2 Particle Size Analysis 34

4.2.3 Phase Analysis of Raw Materials 35

4.3 Analysis of Dense Alumina 37

4.3.1 Shrinkages Analysis 37

4.3.1.1 Shrinkage during Drying Process 37

4.3.1.2 Shrinkage during Bisque Process 42

4.3.1.3 Shrinkage during Sintering Process 44

4.3.2 Density of the Sintered Sample 47

vii

4.3.3 Flexural analysis 48

4.3.4 Microstructure Analysis 49

4.3.5 Phase Analysis of Sintered Alumina 52

5. CONCLUSION AND RECOMMENDATIONS 54

5.1. Conclusion 54

5.2 Recommendations 54

6. REFERENCES 56

7. APPENDICES

A Gantt Chart for PSM 1

B Gantt Chart for PSM 2

viii

LIST OF TABLES

1.1 The example application of the traditional ceramic 5

1.2 The example application of the advanced ceramic 5

2.1 High alumina engineering eeramics (grades Al-A4, at least 99%

Al2O3) and their characteristic (Morell 1987).

10

2.2 Engineering alumina grades A6-A9 (80% ≤ Al2O3 ≤ 99% as

requirement) and their characteristic (Morrell 1987)

10

2.3 Mechanical properties of engineering alumina ceramics at ambient

temperature. Data originated from specimens with a cross-sectional

effective diameter of 3-15mm (Auerkari 1996)

11

2.4 Apparent viscosity of the Al2O3 slurries with various amounts of

polycarboxylate ammonium dispersant inclusions

16

2.5 Mechanical Properties of the Alumina Samples Sintered at Different

Temperatures (Kim 2008)

22

3.1 Example of Slip casting composition. 24

4.1 Data shrinkage of reference line for drying 38

4.2 Data shrinkage of reference line for biscuits process 42

4.3 Data shrinkage of reference line for sintering 44

4.4 Density of the sample 47

ix

LIST OF FIGURES

2.1 Step in slip casting. In (1) a porous mold is filled with slurry, (2)

casting, (3) after a period of casting, excess is drained from the mold

leaving a hollow cast and (4) separation from mold.

14

2.2 Particle size distributions for the Al2O3 slurries with 0, 0.2 and 2 mass%

of dispersants

16

2.3 Scanning electron micrographs of polished and thermally etched

surfaces of Al2O3 sintered bodies prepared from the dispersed

(dispersant 0.2 mass %) and the flocculated (dispersant 2 mass %)

slurries

17

2.4 Weibull's plots of the Al2O3 sintered bodies prepared from the

dispersed (dispersant 0.2 mass%) and the flocculated (dispersant 2

mass%) slurries; “P” indicates the sample including the fracture

surface which a spherical ball-like pore is present at the fracture

origin in specimens.

18

2.5 Optical photomicrographs taken in the transmission mode of the

Al2O3 sintered bodies prepared from the dispersed (dispersant 0.2

mass %) and the flocculated (dispersant 2 mass %) slurries

19

2.6 Scanning electron micrographs of a fractured surface of the Al2O3

sintered body prepared from the flocculated (dispersant 2 mass %)

slurry

19

2.7 Microstructure of the alumina sintered at (a) 1550°C (b) 1600°C (c)

1650°C.

21

3.1 Flow chart of the fabrication of dense alumina through slip casting 23

x

Process

3.2 Roller milling machine with alumina pot 26

3.3 Plaster mold of Paris for Solid Casting 26

3.4 Plaster of Paris for Drain Casting 27

3.5 Sintering profile 29

3.6 Sample position for 3 point bending test 30

3.7 Scanning Electron Microscope (SEM) 32

3.8 XRD machine 32

4.1 The Morphology of Alumina Powder using SEM 34

4.2 Distribution Particle Size of Alumina 35

4.3 Alumina powder. 36

4.4 Ball clay powder 36

4.5 Graph average length of reference line during drying process 39

4.6 Percentage of linear drying shrinkage for four day 39

4.7 Specimen bar fracture at the middle 40

4.8 Voids produced inside the specimen bar 41

4.9 Percentage of linear biscuit shrinkage. 43

4.10 Average percentage of linear shrinkage after biscuit 43

4.11 Crucibles after bisque process using drain casting technique with casting

time; 5 minutes, 10 minutes and 15 minutes (a) batch A (b) batch B (c)

batch C

44

4.12 Percentage of Linear Shrinkage after Sintering. 45

4.13 Average Percentage of Linear Shrinkage after Sintering 45

4.14 Comparison Average Linear Shrinkage 46

4.15 Cracking crucible after sintering process 46

4.16 Sample density vs. batch 47

4.17 Flexural strength vs. number of specimen 48

4.18 Comparison of flexural strength 48

xi

4.19 Image microstructure of the sintered sample (a) batch A, (b) batch B and

(c) batch C. At the left and right the image microstructure obtain from

BSE and SEM test respectively.

51

4.20 XRD pattern for each composition (a) Composition 99.9% alumina and

0.1% ball clay. (b) Composition 99.5% alumina and 0.5% ball clay. (c)

Composition 99% alumina and 1% ball clay.

53

xii

LIST OF ABBREVATIONS, SYMBOLS & NOMENCLATURE

Al2O3 - Alumina

MgO - Magnesium Oxide

NH4+ -

Ammonia

NH4+-PMA - Salt of poly (methacrylic acid)

SiC - Silicon carbides

Si3N4 - Silicon nitride

CaSO4.2H2O - Gypsum

˚C - Degree Celsius

% - Percent

Wt% - Weight percent

h - Hour

m - Meter

MPa - Mega Pascal

µm - Micrometer

SEM - Scanning electron microscopy

XRD - X-ray diffraction

ASTM - American Standard Testing Material

1

CHAPTER 1

INTRODUCTION

1.1 Introduction to Ceramic

William (2003) state that the term „ceramic‟ comes from the Greek word keramikos

which means „burnt stuff‟ indicating that desirable properties of these materials are

normally achieve through a high temperature heat treatment process called firing.

Chemically ceramics are nonmetallic, inorganic compounds which are formed by the

action of heat.

Ceramic generally are characterized by high hardness, high compressive strength,

corrosion-resistant, low thermal expansion, high temperature resistant, good chemical

inertness, low density, and low thermal and electrical conductivity. On the other hand,

they are brittle and have low toughness and ductility. The properties possessed by most

ceramics due to differences of bonding. Most of ceramic materials have a mixed

bonding structure with various ratios between ionic and covalent character. The

percentage of ionic bonds can be estimated by using electronegativity determinations.

1.2 Type of Ceramic

Ceramics are classified in many ways. It is due to different in composition, properties

and its applications. Based on their composition, ceramics can be classified into Oxides,

Carbides, Nitrides, Sulfides, Fluorides and etc. Based on their specific applications,

ceramics are classified as glasses, clay products, refractories, abrasives, cements, and

2

advanced ceramics for special applications. According to Kalpakjian (2006) ceramic

classification is based on their engineering applications, ceramics are classified into two

groups as: traditional and advanced ceramics.

1.2.1 Traditional Ceramic

Traditional ceramics are most made-up of clay, silica and feldspar. Traditional ceramics

include objects made of clay and cements that have been hardened by heating at high

temperatures. Traditional ceramic products usually made in large quantities by efficient

and inexpensive manufacturing methods. The examples of traditional ceramic are such

as dinnerware, sanitary ware, pottery, tiles and refractory. Traditional ceramics are most

made up of clay, feldspar and silica.

Clay has been used in a wide range of ceramics product for thousands of years and

continues to be a major component in most ceramic bodies today. Clay is essential and

predominant component in most ceramic product.

Feldspar is the most common igneous rock mineral and the most plentiful mineral in the

earth's crust. Feldspar is a common raw material in the production of ceramics product.

In the fabrication of ceramic material, feldspar also serves as a flux to form a glassy

phase at low temperatures. It improves the strength, toughness, and durability of the

ceramic body and cements the crystalline phase of other ingredients.

Silica is a major ingredient in refractories and whitewares. Silica occurs commonly in

nature as sandstone, silica sand or quartzite. Silica is one of the most abundant oxide

materials in the earth's crust. Silica is a precursor to the fabrication of other ceramic

products and as a material on its own.

3

1.2.2 Advanced Ceramic

Advanced ceramic include ceramics for electrical, magnetic, electronic, and optical

application (sometimes referred to as functional ceramics) and ceramics for structural

applications at ambient as well as at elevated temperatures (structural ceramics).

Advance ceramics include carbides, such as silicon carbide (SiC); oxides, such as

aluminum oxide, (Al2O3) and zirconia (ZrO2); nitrides, such as silicon nitride (Si3N4);

silicates such as kaolinite (Al2Si2O5 (OH) 4) and mullite (Al6Si2O13) and many other

materials, including the mixed oxide ceramics that can act as superconductors.

Advanced ceramic requires modern processing techniques, and the development of these

techniques has led to advances in industry.

Advanced ceramics materials possess unique properties in chemical resistance, corrosion

resistance; wear resistance and dimensional stability at room temperature as well as at

elevated temperatures. Ceramic matrix composites are at the forefront of advanced

materials technology because of their light weight, high strength and toughness, high

temperature capabilities, and graceful failure under loading. Ceramic composites are

considered as enabling technology for advanced aero propulsion, space propulsion,

space power, aerospace vehicles, and space structures.

1.2.3 Ceramic Based Composites

Ceramic composites are being actively developed in many research establishments

primarily to improve either the toughness or hardness to be more suited to a particular

application. This group can be composed of a combination of oxide ceramics and non-

oxide ceramics (granular, platy, whiskers, etc.), oxide and oxide ceramics, non-oxide

and non-oxide ceramics, ceramics and polymers, etc. an almost infinite number of

combinations are possible. Fiber, whisker or particulate reinforced composites can be

made tougher and stronger than traditional un-reinforced ceramics. Ceramic composites

4

offer the mechanical and physical properties unavailable in the past and provide the

opportunities to solve past material problems.

1.3 Application of Ceramic

The applications for ceramics have found for a long time ago. Every day new and

different applications are being discovered. This truly makes ceramics have broad ranges

of functions and applications. Ceramic materials display a wide range of properties

which purpose in many different product areas to facilitate user.

Most people, when they hear the word of ceramics, they will think of art, dinnerware,

pottery, tiles, and brick. The above mentioned products are commonly referred to as

traditional ceramic that have been developed since the time of the earliest civilizations.

These traditional products have been and continue to be important and still represent a

major part of the ceramics industry although in recent year advanced material have been

successfully developed . These advanced ceramics are being used for applications such

as space shuttle tile, engine components, artificial bones and teeth, computers and other

electronic components, and cutting tools (Callister, 2003).

Examples of the wide range application of ceramics can be seeing as mention in the

following tables. The applications of ceramics can be simplifies into two major

categories which are traditional and advanced material as in the Table 1.1 and 1.2

respectively. For the traditional ceramic application it can be divided into five major

industries segment together with several examples. For the advanced ceramic it can be

classified into four categories which consists structural, electrical, coatings and lastly

chemical and environmental together with a few examples.

5

Table 1.1: The example application of the traditional ceramic

Industry Segment Common Examples

Structural clay

products

Brick, sewer pipe, roofing tile, clay floor and wall tile (i.e.,

quarry tile), flue linings

Whitewares Dinnerware, floor and wall tile, sanitary ware, electrical

porcelain, decorative ceramics

Refractories Brick and monolithic products are used in iron and steel, non-

ferrous metals, glass, cements, ceramics, energy conversion,

petroleum, and chemicals industries

Glasses Flat glass (windows), container glass (bottles), pressed and

blown glass (dinnerware), glass fibers (home insulation), and

advanced/specialty glass (optical fibers)

Abrasives Natural (garnet, diamond, etc.) and synthetic (silicon carbide,

diamond, fused alumina, etc.) abrasives are used for grinding,

cutting, polishing, lapping, or pressure blasting of materials

Cements Used to produce concrete roads, bridges, buildings, dams, and the

like

Table 1.2: The example application of the advanced ceramic

Industry Segment Common Examples

Structural Wear parts, bioceramics, cutting tools, and engine

components

Electrical Capacitors, insulators, substrates, integrated circuit

packages, piezoelectrics, magnets and superconductors

Coatings Engine components, cutting tools, and industrial wear parts

Chemical and

environmental

Filters, membranes, catalysts, and catalyst supports

1.4 Objective

The objectives of this project are listed as below:

i. To fabricate the bar specimens and crucibles of dense alumina by using slip casting

technique.

ii. To analyze the effect of different raw materials composition on the final properties

of dense alumina.

6

1.5 Scope

The project involved the theoretical study on the ceramic slip casting process and

fabricates the dense alumina through the slip casting process. The emphases are on the

preparation of slip with correct percentage of composition between alumina and ball

clay. Water are used until slip casting obtain the desired specific gravity ranges between

1.4-1.6. For this experiment, the raw materials used were alumina and ball clay with

different ratios of raw materials constituent. There are three different weight ratios of

powder mixture, which are 1:1000; 5:1000 and 10:1000. The ratio refers to ball clay:

alumina. The slip casting was prepared and was mixed using roll mill for three hour to

make sure all the chemical composition agglomerate homogenously. The ratio of

grinding media is ¼ raw materials. The slip is then casted into the plaster mold of Paris

by using solid and drain casting technique for bar specimens and crucibles respectively.

In this study the bar specimens will be done through the process drying at room

temperature and bisque at 900°C by using the heating rate 10°C/min. Then the green

body will be sintered at 1400°C. The samples dimensions were measured before and

after sinter to get the percent of shrinkage of sintered specimen. The finished specimens

were needed to prepare for testing and evaluations. The sintered specimens were tested

by using X-ray diffraction (XRD) to identify phase composition in every sample.

Scanning Electron Microstructure (SEM) is used to analyze the microstructures in each

sample. Other testing for specimens includes flexural test and shrinkage.

1.6 Problem Statement

Alumina is widely known as advanced ceramic materials which are used in a variety of

applications. High purity alumina (Al2O3 content > 99%) have high thermal

conductivity, low thermal expansion, corrosion resistance and high compressive strength

leads to good thermal shock resistance is ideal for high temperature applications such as

crucibles. Advanced ceramic typically used non-traditional techniques such as hot

7

pressing, vapor deposition, gel casting and injection molding to form ceramic materials

into a recognizable shape or product.

However considering the importance homogeneous distribution of the particles in the

production of ceramic crucibles is essential to obtain a high-density impervious product;

therefore the slip casting technique was chosen to produce the crucibles. Homogeneous

distribution of the particles can be obtained by using the slip casting technique although

it quit conventional to the industries.

Rak (2000) stated the problem in the slip casting contains about 30 wt% water and about

half of it must be absorbed by the plaster of Paris mould in order to reduce the liquid slip

to an acceptably firm body at the mold surface. This forms a dense cast with long

production time and the molds are easily damaged.

High purity alumina forming with slip casting technique produce parts that are fragile

and cannot easily be machined in the green body state. The composition of the purity

alumina ceramic body can be changed to enhance particular desirable material

characteristics by adding little additive. In this project, properties of the final body

ceramic alumina with the different composition of additive is then analyzed.