. ' I f I-

UNIVERSITI TEKNIKAL MALAY 'SIA MELAKA

Mechanical Properties of Mullite-Bonded Silicon

Carbide Sintered at Various Heating

Temperature

Thesis submitted in accordance with the requirements of the Universiti Teknikal

Malaysia Melaka for the Degree of Bachelor of Engineering (Honours)

Manufacturing Engineering (Engineering Materials)

Lim Yee Kai

Faculty of Manufacturing Engineering

March 2008

+ ~ b b * t r ~

I UNlVERSlTl TEKNIKAL MALAYSIA MELAKA - - - v, - 'a - .- "lf*" _=

JUDUL:

Mechanical Properties of Mullite bonded Silicon Carbide Sintered At Various

Heating Temperature

SESI PENGAJIAN: Semester 2 200712008

Saya LIM YEE KAI mengaku membenarkan laporan PSM I tesis (SarjanaIDoktor Falsafah) ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut:

I . Laporan PSM I tesis adalah hak milik Universiti Teknikal Malaysia Melaka dan penutis.

2. Perpustakaan Univeniti Teknikal Malaysia Melaka dibenarkan rnembuat salinan untuk tujuan pengajian sahaja dengan izin penulis.

3. Perpustakaan dibenarkan membuat salinan laporan PSM I tesis ini sebagai bahan pertukaran antara institusi pengajian tinggi.

4. *Sila tandakan (4) (Mengandungi rnaklurnat yang berda j a h keselarnatan atau kepentingan Malaysia yang terrnaktub di dalam AKTA RAHSIA

TERHAD (Meqandungi maklumat TERHAD yang telah ditentukan oleh organisasilbadan di rnana penyelidikan dijalankan)

TlDAK TERHAD

m (LIM YEE KAI) (DR. MOHD WARIKH BIN ABD RASHID) Alamat Tetap: No.2, JLN 512,

Taman Daya, Kepong, 52100 Kuala Lumpur

Cop Rasmi:

Tarikh: 44-4 Tarikh:

* Jika laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak orsanisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.

;;"tsr .w 9 UNfVERSIT! TEKN!KP.L MALAYSIA MELAKA

Karung Berk~rnci 7200, Ayer Kerofl, 75450 Melaka Tel . 06-233 2421, Faks Oti 233 2414

Emall : fko@kctkm.edu mv

FAKULTI KEJURUTERAAN PEMBUATAN

Rujukan K a m ~ (Our Ref) :

Rujukan Tuari (Your Ref): 01 Mei 2008

Pustakawan Perpustakaan Universiti Tekni kal Malaysia Mela ka (UTeM) Taman Tasik Utama, Hang Tuah Jaya, Ayer Keroh, 75450, Melaka

Saudara,

PENGKELASAN LAPORAN PSM SEBAGAI SULlTnERHAD - LAPORAN PSM SARJANA MUDA KWURUTERAAN PEMBUATAN (BAHAN KWURUTERAAN): LIM YEE KAI TAJUK: Mechanical Properties of Mullite bonded Silicon Carbide Sintered At

Various Heating Temperature

Sukacita dimaklumkan bahawa tesis yang tenebut di atas bertajuk " Mechanical Properties of Mullite bonded Silicon Carbide Sintered At Various Heating Temperature" 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,

.............................................................. DR. MOHD WARIKH BIN ABD RASHlD P ensyarah, Fakul ti Kejuruteraan Pembuatan

DECLARATION

I hereby declare that this report entitled "Mechanical Properties of Mullite-Bonded

Silicon Carbide Sintered at Various Heating Temperature" is the result of my own

research except as cited in the references.

Signature

Author's Name 46

: LIMYEEKAI

Date : 301412008

APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a

partial fulfillment of the requirements for the degree of Bachelor of Manufacturing

Engineering (Engineering Materials). The members of the supervisory committee are

as follow:

DR. MOHD WARIKH BIN ABD RASHID

(PSM Supervisor)

ABSTRACT

Mullite-bonded porous silicon carbide samples are produced by the reaction

sintering in air from silicon carbide (Sic), alumina (A1203), and using graphite as the

pore former. The sintering process is based on the oxidation of Sic and the

mullitization between A1203 and oxidation derived Si02 (Cristobalite). At

mullitization temperature (1450°c), S ic particles are bonded by the mullite

(3A1203.2Si02) and Si02. Sintered samples will be characterized by density

determination and three-point bend tests. Morphology and microstructure analysis

study will be done by scanning electron microscopy, while the phase composition

will be investigated by X-ray diffraction. Mullite phase transformation start at

1450 '~ which was the optimum sintering temperature for mullite transformation.

The mullite content increase with sintering temperature concurrent with decrease of

A1203. Porous S i c ceramics were bonded by the mullite (needle like) and oxidation-

derived $302 . The densities was increase while the open porosity was decreases with

increasing of sintering temperature. Flexural strength was direct proportion to the

sintering temperature but inverse proportional to the percentage of open porosity.

Highest content of mullite and mechanical properties achieved at 1500°c,

ABSTRAK

Poros Mullite-bonded silikon karbida dihasilkan oleh reaksi pensinteran

dalam udara dari silikon karbida (Sic), alumina (A1203), dan menggunakan grafit

sebagai pembentuk liang. Proses sinter adalah berasaskan pengoksidaan Sic dan

mullitization antara Al2O3 dan pengoksidaan terbitan Si02 (Kristobalit). Di

mullitization suhu, zarah-zarah S ic adalah diikat oleh mulit (3A1203.2Si02) dan

Si02. Sampel yang tersinter akan dijalankan ujian ketumpatannya and kelenturan tiga

mata. Morfologi dan mikrostruktur kajian analisis akan dibuat oleh mikroskop

elektron pengimbasan (SEM), manakala komposisi fasa akan disiasat oleh belauan

sinar X (XRD). Mulit permulaan penjelmaan fasa pada 1450 '~ merupakan optimum

pensinteran suhu untuk mulit tranformasi. Mulit peningkatan dengan pensinteran

suhu serentak dengan pengurangan A1203. S ic yang poros seramik adalah diikat oleh

mulit (bentuk jejarurn) dan pengoksidaan terbitan Si02. Ketumpatan adalah

meningkat manakala keporosan terbuka adalah berkurangan dengan penambahan

pensinteran suhu. Lenturan kekuatan hfidala berkadar terus dengan pensinteran suhu

tetapi berkadar songsangan dengan peratusan keporosan. Kandungan mulit dan ciri-

ciri mekanikal tertinggi dicapai pada 1 500°c,

ACKNOWLEDGEMENTS

I would like to thanks and deepest gratitude from bottom of my heart for all the

support, encouragement and inspiration that I obtain to finish these thesis.

My special thanks to my supervising lecturer, Dr. Mohd Warikh, which we

had a good working relationship and who offered help, guidance and encouragement

during prepare this thesis.

I also thank to my family for their constant encouragement and support. To

all of my lecturers, technician, and friends who help, guide and encourage me.

TABLE OF CONTENTS

Declaration

Approval

Abstract

Acknowledgement

Table of Contents

List of Figures

List of Tables

Sign and Symbols

1 INTRODUCTION

1.1 Overview

1.2 Statements of problem

1.3 Objectives

1.4 Scopes of the project

1

. . 11

. . . 111

v

vi

ix

xii

xiv

2 LITERATURE REVIEW

2.1 Overview of Ceramic Material 5

2.2 Silicon Carbide 6

2.3 Mullite 9

2.4 Strength o f ceramics 12

2.5 Sintering Process 13

2.6 Reaction Sintering 14

2.7 Effect of sintering temperature on the reaction bonded S i c 14

3 METHODOLOGY

3.1 Introduction

3.2 Fabrication of mullite-bonded porous silicon carbide

3.2.1 Raw material aggregate

3.2.2 Dry milling

3.2.3 Powder preparation with binder

3.2.4 Green Compact Preparation

3.2.5 Sintering

3.3 Determination percentage of open porosity in porous

ceramic

3.4 Test Method for Flexural Strength of ceramic

3.5 Sample Preparation (Cold Mounting)

3.5.1 Vacuum Impregnation Process for porous ceramic

3.6 Grinding and Polishing for Mounting Sample

3.6.1 Abrasive grinding procedures

3.6.2 Polishing procedure

3.6.3 Precaution step in polishing technique

3.7 Scanning Electron Microscope (SEMIEDX) Observation

3.8 Phase Composition Analysis by Using XRD

3.8.1 Sample preparation

3.8.2 XRD operation

4 RESULTS AND DISCUSSIONS

4.1 Raw Material Analysis

4.2 Energy Dispersion X-ray (EDX) analysis

4.3 In-situ reaction bonding behavior

4.4 Phase Composition Analysis

4.5 Microstructural evolution

4.6 Density and porosity of porous S ic ceramic

4.7 Flexural strength of porous S ic ceramic

5 CONCLUSION AND FUTURE WORKS

5.1 Conclusion

5.2 FUTURE WORKS 62

5.2.1 Improve imaging methods of optical microscopy 62

5.2.2 Three-point bending test specimen 62

5.2.3 Sintering Profile 63

6 REFERENCES 64

APPENDICES

A Glossary

B Properties of the sintered mullite ceramics

C Property of Sintered Silicon Carbide, Sic

D Percentage of Porosity

E Flexural Strength

F Particle size measurement for raw material: (a) S ic (b) A1203 (c)

Graphite

G Gantt Chart: (a) PSM 1 (b)PSM 2

LIST OF FIGURES

FIGURES TITLES PAGES

2.1 Crystalline Sic grain from the Acheson process

2.2 Nonmetallic hard materials in the system B-C-Si-N

2.3 Crystal structure of the Sic polytype 3C and 6H

2.4 The ceramic phase diagram (A1203.Si02)

2.5 Schematic indication of the distinction between densifying

and nondensifying microstructural changes resulting from

atom transport during the sintering of ceramic powders

2.6 Plot of oxidation degree of Sic vs. sintering temperature

2.7 Changes of oxygen content for the following systems:

(a) Sic containing 3 wt.% of carbon, (b)pure Sic

2.8 Average pore radius vs. sintering temperature and

composition: (a)pure Sic, (b) Sic containing 0.5 wt.% of

boron, (c) Sic containing 3 wt.% of carbon as well as

(d) S ic containing 3 wt.% of carbon and 0.5 wt.% of boron

3.1 Process flow of the steps taken in fabricating and testing on 20

the porous Sic specimens

3.2 Steps taken in ball milling: (a) clean for bowl and steel ball 22

(b) filling powder and steel ball (c) clamping of cap and

tightening with lids (d) milling

3.3 Preparation of binder solution: (a) Hot distilled water 23

(b) dissolving binder (c) mix binder solution with powder

(d) filter the excess water

3.4 Powder preparation: (a) slurry after filtered 1 day (b) dry in 24

oven (c) crush into powder27

3.5 Uniaxial die press process: (a) filling powder (b) clamp and 25

compact (c) eject green compact

Heating profile

Steps taken in identify samples' density: (a) calculate for

bulk volume (b) drying (c) find the dry weight (d) find the

density

Three point fixture configuration

Steps taken in 3-point test method: (a) alignment for spans

distance (b) place sample (b) apply load (c) brittle fracture

sample

Cold mounting step for porous ceramic: (a) stirring

(b) filling resin into mold (c) vacuum impregnation

(d) eject sample

Steps taken in abrasive grinding: (a) edge rounding

(b) surface grinding (c) change grit (d) cleaning

Steps taken in polishing: (a) apply diamond lubricant

(b) apply diamond suspension (c) sweep suspension to the

whole pad (c) polishing

(a) Sample for 1400°c, 1450°c, 1 5 0 0 ~ ~ . (b) Disperse raw

material powder: Silicon carbide, Alumina oxide, Graphite,

Binder (PVA)

Scanning Electron Microscope (SEMIEDX): (a) vacuum

chamber (b) operation unit

Sample preparation using spinning stage: (a) Press and

clamp specimen stage (b) fi l l in powder (c) press and mate

both templates (d) finish sample

Sample preparation using flat stage: (a) filling powder (b)

press and compact (c) even or flatten the powder's surface

X-ray Diffraction scanning: (a) spinning stage (b) flat stage

4.1 Single crystal S ic platelets (plate-like) or flaky shape. SEM 37

1 OOOX

4.2 Alumina Oxide, A1203 particles. SEM 1000X 39

Graphite, C particle. SEM 1 OOOX

XRD pattern of the composition contain in the green

powder

Sample sintered at 1400'~. SEMIEDX 1 OOOX

Sample sintered at 1400'~. SEM/EDX 2000X

Sample sintered at 1400'~. SEM/EDX lOOOX

Sample sintered at 1 5 0 0 ~ ~ . SEMIEDX 1 OOOX

XRD patterns of the specimens sintered in air, where the

lOum graphite was used, the volume ration of Sic,

Alz03and C in green bodies was 2.1 : 1 : 1.2 and the forming

pressure of green bodies was 16 ton (S is Sic, M is Mullite,

C is Cristobalite, and A is Aluminium Oxide

Microstructure of mounting sample sintering at 1400'~.

SEM,2000X

Grain growth structure and phase transformation for

ceramic sintering temperature. (a) 1 400°c, (b) 1 450°c,

(c) 1 5 0 0 ~ ~

Ceramic sintered at 1400'~. Arrow A show the

transgranular fractures on the S ic grain. The cracks appear

inside the grain of the microstructure. Optical, 200X

Ceramic sintered at 1450'~. The dark areas (Arrow B) were

pore. Pore between particles. The particles contain fine,

internal pores and some coarse pores. Optical. 200X

Scanning Electron Microscope (SEMIEDX): (a) vacuum

chamber (b) operation unit

Sample preparation using spinning stage: (a) Press and

clamp specimen stage (b) fill in powder (c) press and mate

both templates (d) finish sample

Ceramic sintered at 1450'~. The surface of S i c having a

high density of interconnecting void, resulting in a porous,

weak material. The arrow show the smaller closed pores

caused by the gaseous SiO and CO during the heat treating

in porous S i c ceramic. SEM 2000X

4.17 Sample sintered at 1450'~. The first stage of mullitization 55

led to the entrapment of many alumina particles within the

growing mullite grains. The pores form by stacking A1203

particle. The arrows indicate needlelike crystalline mullite

embedded at the necks of surface Sic. SEM 1 OOOOX

4.18 Stable structure with obvious connected pores and the well 56

developed necks appear between S ic particles. SEM 2000X

4.19 Sample sintered at 1500'~. Continued growth of mullite 56

during the second stage required interdiffusion of alumina

and silica through the grains. Arrows Y showed the necks

forming between two adjacent particles. These necks (solid

regions) replace particle boundaries as sintering progress.

Arrows X indicate pores opening into the spongy interior.

SEM 6500X.

4.20 The sintered densities of the fabricated S i c specimens 5 7

4.21 Plot of open porosity vs. sintering temperature; the 57

specimens were sintered at indicated temperatures for 4

hours in air, where 10 graphite was used as the pore former

4.22 Relationship of flexural strength vs. sintering temperature; 59

the specimens were sintered at indicated temperatures for 4

hours in air, where lourn graphite was used as the pore

former

5.1 Beveled surface after polishing

5.2 Suggested sintering profile

LIST OF TABLES

TABLES TITLES PAGES

2.1 Dominant S ic types 8

3.1 Calculation for the powder grammage 20

3.2 Powder compositions and characterization of material used in 20

this study

3.3 Sintering Temperature 25

3.4 The abrasive size using in grinding and polishing 33

4.1 Raw material characteristic 39

4.2 Effect of sintering temperature on density, open porosity and 59

flexural strength

LIST OF ABBREVIATIONS, SYMBOLS, SPECIALIZED

NOMENCLATURE

A1203

3Al2O3 .2SiO2

S i c

PVA

EDX

SEM

XRD

4 v v

VT

0 0

0

P

b

Aluminium Oxide

Mullite

Silicon Carbide

Polyvinyl Alcohol

Energy Dispersion X-ray

Scanning Electron Microscope

X-ray diffraction

Percentage of porosity

volume of void-space

total or bulk volume of material

strength of a nonporous structure

strength of the pore characteristic

porosity

constant depending on the pore

characteristics

CHAPTER 1

INTRODUCTION

1.1 Overview

Ceramic are inorganic, nonmetallic materials and can be crystalline or

amorphous. Ceramic are compounded with metallic and non-metallic element, such

as aluminum or calcium, oxygen or silicon and nitrogen. Normally, sintered ceramics

are required to have minimum porosity. However, there are cases where porosity is

desirable for example in humidity and gas sensors and where there thermal shock

resistance is of overriding importance. Porous silicon carbide ceramic is one of the

potential materials to be used as filters, membranes, catalytic substrates, thermal

insulation, gas burner media, and refractory materials due to of their excellent

mechanical and chemical stability, such as low bulk density, high permeability, high

temperature stability, erosion and corrosion resistance, and excellent catalytic

activity.

The high degree of covalence in S ic ceramic makes it difficult to sinter

Sic by heating of powder compacts at moderate temperature. Hence, many

different sintering techniques have been developed. Among those techniques, hot-

pressing with a sintering aid, pressureless sintering with a sintering aid and reaction

sintering are the important one. In this study, in situ reaction bonding technique is

use in low temperature fabrication of S ic ceramic.

In reaction bonding technique, a secondary phase may add and utilize the

oxidation derived silica to bond S ic particles with mullite bonding. Mullite (3Al2o3

.2Si02) is the intermediate phase in the Si02-A1203 system. It has excellent high

temperature properties with improved thermal shock and thermal stress owing to the

low thermal expansion, good strength and interlocking grain structure. It is believed

that mullite bonding will possesses better high temperature stability and oxidation

resistance, matching thermal expansion and good chemical compatibility between

mullite and Sic.

The effect of charge composition, characteristic of the starting material

(silicon carbide, S i c and alumina, A1203) and pore-forming agent (graphite) on the

formation of highly porous Sic-based by in situ reaction bonding will be studied.

Pores in porous S i c ceramics can be made by burning out the graphite during

sintering process. These can keep the skeleton of the green bodies intact before the

bonding phase formed at higher temperature.

The fabrication process of synthetic mullite porous ceramic involves

precision mixing, forming and sintering. The preparation of S i c green body is using

reaction sintering below the melting point. Then sample is test for its

mechanical properties; microstructure and phase composition analysis. The effect

of sintering temperature on properties of porous S i c ceramics such as porosity and

flexural strength will be study.

1.2 Statements of problem

Ceramic materials face a stiff competition from metal alloy and composites in

the extreme environment applications. In order to compete well, new and low cost

processing method with high performance ceramic is needed. In the meantime,

porous silicon carbide ceramic have wide application in filtration, separation,

catalysis and high temperature structural material. As diesel particulate filter, Sic

ceramic has a higher melting temperature (2700°C), better thermal shock resistance

and high chemical stability but with lower mechanical properties compare to metal

alloy. Further, the S i c ceramic also facing major problem in fabrication process due

to the natural high degree of covalency in S ic makes it difficult to sinter to

high densification by normal heating of powder compacts. As a result, various low

temperature sintering techniques have been developed. This reaction bonded silicon

carbide (RBSC) technique required no machining and low cost manufacturing

technique. This technique had solved the problem of high cost techniques (hot press

and hot isostatic press) of sintering and high cost of machining after sintering

because it able to produce very low shrinkage of sintered ceramic. The reaction

sintering in pressureless environment further reduces the fabrication cost of the

material. The low mechanical strength of the porous S ic ceramic can be improved by

mullite bonding during reaction sintering. Therefore, in this study of alumina and

graphite will be added for reaction sintering in order to produce mullite bonded

which exhibit excellent high temperature strength. According to Ding et al., (2007),

the flexure strength of porous ceramic is highly dependent on the sintering

temperature. The strength of porous ceramics is exponential increase with the

decrease of the porosity and the porosity also inverse proportional to raising sintering

temperature. Subsequently, it is important to determine the optimum sintering

temperature in the fabrication of mullite bonded porous silicon carbide with

improved mechanical properties.

1.3 Objectives

The purposes of this project are:

i. To identify the optimum sintering heating temperature for mullite-bonded

porous S i c ceramic with highest mechanical properties.

ii. To examine the morphology and microstructure of porous S i c ceramic at

elevated temperature.

iii. To investigate the phase composition evolution at elevated temperature

1.4 Scopes of the project

This project studies the effect of elevated sintering temperatures on porous

Sic ceramic by in-situ reaction bonding technique. Mullite formed during the

sintering process will act as the bonding phase between silicon carbide. The effect of

the bonding material on the porous microstructure will be examined under SEM.

Phase evolution was investigated usingX-ray diffraction. The mechanical properties

of porous S i c ceramic will be studied in the aspect of percentage of porosity and

flexural strength.

CHAPTER 2

LITERATURE REVIEW

This chapter reviewed the basic theory on properties and mechanical testing

on mullite bonded porous silicon carbide. It also cover various study and analysis

results and finding from previous researches. Most of the review base on fabrication

and sintering of silicon carbide with the effect of temperature, material composition,

porosity and material characteristic.

2.1 Overview of Ceramic material

Ceramic (Munz et al., 2001) can be classified into different group by

considering their chemical composition, microstructure, or application. We can

distinguish the traditional ceramics and advanced based on their application. The

traditional ceramics include tableware, pottery, sanitary ware, tiles, bricks and

clinker. The advance ceramic can be divided into electronic ceramics (insulators,

substrates, capacitors, varistors, actuators, and sensors), optical ceramics (window,

laser), magnetic ceramics and structural ceramics. The structural ceramic was applied

in mechanical engineering, chemical engineering, high temperature technology, and

in biomedical technology. Special ceramic which were not directly related to the

categories mentioned above were reactor ceramics (absorber materials, breeder

materials, nuclear fuels) and refractory product.

Classification of ceramic was to distinguish between

Silicate ceramics

Oxide ceramics

Non-oxide ceramics

The classification was a mixture of composition (oxide, non-oxide) and

atomic structure (glassy-amorphic, crystalline). The main feature of silicate ceramics

was the glass-amorpic phase with a pronounced pore structure. The main content was

SiOa, with additions of Al.2.0.3, MgO, BeO, ZrO.2. and other oxides. The further

subdivision was between clay-ceramic with mullite (3A1.2.0.3..2Si0.2.) as the main

constituent and other silica ceramics, e.g. cordierite (2Mg0.2A1.2.0.3..5Si0.2.). Clay

ceramics were subdivided into those with coarse grain structures. Earthenware,

tableware, porcelain and tiles belong to the first category. Bricks, clay pipes, and

clinker belong to the latter.

Oxide ceramics were different from silica ceramic due to it have major

crystalline phase with only a small content of glassy phase.

Example of oxides:

A1.2.0.3., BeO, MgO, ThO.2., TiO.2, .UOa., ZrO.z

The properties of single oxide can be modified by additives. Al.2.0.3 .ceramic

was the dispersion toughened ceramic with fine dispersion of particle of ZrOa. or

Tic. The system A1.2.0.3.- Zr0.2. was name ZTA (zirconia-toughened aluminium

oxide).

2.2 Silicon Carbide

According to Munro (1997) sintered silicon carbide ceramics was produced

by using submicrometer powders that extracted from an Acheson furnace and ground

to a fine particle size. Acheson furnace (Saddow et al., 2004) was invented by

Eugene and Alfred Cowles and this furnace was adopted by Acheson to produce

suitable materials that could substitute diamond as an abrasive and cutting material.

Acheson mixed coke and silica in the furnace and discover a crystalline product

which have great hardness, refractability, and infusibility, which was the compound

of carbon and silicon. The product was given by name cccarborundurn" with formula

S ic (Figure 2.1). S i c was sinter at temperature 2500 "C with boron and carbon as

sintering aids to achieve improved densification during sintering. However, Evans et

al. had realized that silicon carbide was very difficult to manufacture and the fully

dense sintered S i c raw material was very expensive.

Riedel (2000) had stated that S ic was the most important carbide due to its

extreme properties and potential commercial applications and its belongs to the

group of nonmetallic hard materials (Figure 2.2) which had great hardness and high

melting point result from a high fraction of covalent bonding. Superhard compounds

were obviously formed by combination of the four low atomic number elements:

boron, carbon, silicon, and nitrogen to form a quaternary system; carbon as

diamond, boron nitrogen as cubic boron nitride, boron-carbon as boron carbide, and

silicon-carbon as silicon carbide belong to the hardest materials.

Figure 2.1 : Crystalline S i c grain from the Acheson process

7