Mechanical Properties Of Mullite-Bonded Silicon Carbide Sintered At Various Heating Temperature.

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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

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UNlVERSlTl TEKNIKAL MALAYSIA MELAKA

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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 Itesis (SarjanaIDoktor Falsafah) ini disimpan

di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat
kegunaan seperti berikut:
I.
2.
3.

4.

Laporan PSM Itesis adalah hak milik Universiti Teknikal Malaysia Melaka dan
penutis.
Perpustakaan Univeniti Teknikal Malaysia Melaka dibenarkan rnembuat salinan
untuk tujuan pengajian sahaja dengan izin penulis.
Perpustakaan dibenarkan membuat salinan laporan PSM Itesis ini sebagai bahan
pertukaran antara institusi pengajian tinggi.
*Sila tandakan (4)
(Mengandungi rnaklurnat yang berdaj 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)
Alamat Tetap:
No.2, JLN 512,
Taman Daya, Kepong,
52100 Kuala Lumpur

Tarikh:

44-4

(DR. MOHD WARIKH BIN ABD RASHID)
Cop Rasmi:

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.

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UNfVERSIT! TEKN!KP.L MALAYSIA MELAKA
Karung Berk~rnci7200,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) :

01 Mei 2008

Rujukan Tuari (Your Ref):

Pustakawan
Perpustakaan Universiti Teknikal Malaysia Melaka (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
Pensyarah,
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.

46

Signature
Author's Name

:

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 i c 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
1 4 5 0 ' ~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 oxidationderived $ 3 0 2 . 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 i c 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 1 4 5 0 ' ~merupakan optimum
pensinteran suhu untuk mulit tranformasi. Mulit peningkatan dengan pensinteran
suhu serentak dengan pengurangan A1203.S i c 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 ciriciri mekanikal tertinggi dicapai pada 1500°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

1

Abstract

..
...
111

Acknowledgement

v

Table of Contents

vi

List of Figures

ix

List of Tables

xii

Sign and Symbols

xiv

Approval

1

2

3

11

INTRODUCTION

1.1

Overview

1.2

Statements of problem

1.3

Objectives

1.4

Scopes of the project

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

METHODOLOGY

3.1

Introduction

3.2

3.3

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

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

3.6

Vacuum Impregnation Process for porous ceramic

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

5

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 i c ceramic

4.7

Flexural strength of porous S i c ceramic

CONCLUSION AND FUTURE WORKS
5.1

Conclusion

5.2

6

FUTURE WORKS

62

5.2.1

62

Improve imaging methods of optical microscopy

5.2.2 Three-point bending test specimen

62

5.2.3

63

Sintering Profile

REFERENCES

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 i c (b) A1203(c)
Graphite

G

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

64

LIST OF FIGURES

FIGURES

TITLES

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

PAGES

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 i c 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
compact (c) eject green compact

25

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
~ . Disperse raw
(a) Sample for 1400°c, 1450°c, 1 5 0 0 ~(b)
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) fill 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 i c platelets (plate-like) or flaky shape. SEM

37

1OOOX
4.2

Alumina Oxide, A1203particles. SEM 1000X

39

Graphite, C particle. SEM 1OOOX

XRD pattern of the composition contain in the green
powder
Sample sintered at 1400'~.SEMIEDX 1OOOX
Sample sintered at 1400'~.SEM/EDX 2000X
Sample sintered at 1400'~.SEM/EDX lOOOX
Sample sintered at 1 5 0 0 ~SEMIEDX
~.
1OOOX
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) 1400°c, (b) 1450°c,
(c) 1 5 0 0 ~ ~
Ceramic sintered at

1400'~. Arrow A show the

transgranular fractures on the S i c 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 1OOOOX
4.18

Stable structure with obvious connected pores and the well

56

developed necks appear between S i c 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

57

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;
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

59

LIST OF TABLES

TABLES

TITLES

PAGES

2.1

Dominant S i c 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

Aluminium Oxide

3Al2O3.2SiO2

Mullite

Sic

Silicon Carbide

PVA

Polyvinyl Alcohol

EDX

Energy Dispersion X-ray

SEM

Scanning Electron Microscope

XRD

X-ray diffraction

4

Percentage of porosity

vv

volume of void-space

VT

total or bulk volume of material

00

strength of a nonporous structure

0

strength of the pore characteristic

P

porosity

b

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 i c 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, hotpressing 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 i c ceramic.
In reaction bonding technique, a secondary phase may add and utilize the
oxidation derived silica to bond S i c 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, S i c
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 i c 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 i c 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

S i c 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
as the main
subdivision was between clay-ceramic with mullite (3A1.2.0.3..2Si0.2.)
Clay
constituent and other silica ceramics, e.g. cordierite (2Mg0.2A1.2.0.3..5Si0.2.).
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 i c (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 i c 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