H.O. Pierson. Handbook of carbon, graphite, diamond and fullerenes. Properties, processing and applications. 1993

Figure

12.5.

High-pressure

diamond

press.

(Photograph courtesy of GE

Superabrasives,

Worthington,

OH.)

Figure

12.6.

Typical

high-pressure

synthetic

diamond

crystals.

.(Photograph

courtesy

of GE

Superabrasives,

Worthington,

OH.)

The

worldwide production

of natural diamond

has risen steadily

since

World War II, in answer

to increasing

demand

(mostly

for gemstones)

and

as a result

of improved

prospecting

and

mining

techniques

and the opening

of new mines.

Production,

which

was

only two tons

in 1947,

reached

an

estimated

twenty

tons

in 1992.

The

major

diamond-producing

countries

in 1990

are

shown

in Table

12 4 [141[151

. .

The

tonnage

production

figures

of Table

12.4

do

not

reflect

the

monetary

value.

Australia

produces

a third of the

world tonnage

but very

few gemstones

(such as the ‘fancy

pink” for which

it is famous).

Nanibia’s

production

is less than

half the tonnage

but more than twice

the value

of that

of Australia.

Yearly

Country

Production

Comments

Australia

7

tons

<5% gem

quality

W. & Central

6

tons

nearly

all

Africa

industrial

grade

Botswana

3 tons

l/3

gems,

largest

gem producer

Russia

2

tons

gem and

industrial,

export

only,

domestic

use unknown

South Africa

1.5 tons

gem

and

industrial

production

declining

Nanibia

small

alluvial

fields

with

(ex SW Africa)

high

% gemstones

292 Carbon, Graphite, Diamond, and Fullerenes

4.4

The Diamond Gemstone Market

As

mentioned

above,

diamond

gemstones

still remain

the

major

use

of diamond

in monetary

terms

in a market

tightly

controlled

by a worldwide

cartel

dominated

by the

de

Beers

organization

of South

Africa.1’)

Some

maintain

that

the

cartel

keeps

the cost

of gemstones

at an

artificially

high

level

and

that

the

supply

is abundant.

Control

of

the price

of

industrial

diamond

is not

quite

as obvious.

The

competition from high-pressure

synthetic

gemstones

is a possibil-

ity.

Gem-quality

synthetic

crystals

up

to

fifteen

carats

are

regularly

produced

but at this time

they

are at least

as expensive

as the natural ones.

A special

case is blue diamond

which

is obtained

by adding

boron to the

metal

catalyst

and

could

be

sold

profitably

in competition

with

the

rare

and

expensive

natural stones.

But synthetics

face a psychological

barrier from the

veryfactthattheyaresynthetic,

although

indistinguishablefromthenatural.t16j

Also the tight

rein of the cartel

and the possibility

of dumping

is always

present.

4.5

High-Pressure

Synthetic

Diamond

Production

The

major

producers

of high-pressure

synthetic

diamond

include

General

Electric

in the U.S.,

de Beers in South

Africa,

Ireland

and

Sweden,

Sumitomo

and

Tome

in

Japan,

and

plants

in the

former

Soviet

Union,

Czechoslovakia,

Germany,

Korea,

and China.

The

growth

of

high-pressure

synthetic

diamond

is

shown

in

Fig.

12.8.t14j

The

market

is now

of

considerable

size

with

an

estimated

production

of seventy

tons

in 1992, all for industrial

applications.

The largest

consumer

is the

U.S.,

closely

followed

by Japan

and

Western

Europe.

High-pressure

synthetic

diamond,

because

of

its

high

purity

and

uniformity,

has taken

an increasing

share

of the industrial

diamond

market

and has

replaced

natural

diamond

in many areas.

Grinding, grooving,

cutting, sharpening,

etching,

and

polishing

are the

main

applications

of industrial

diamond

and

consume

approximately

90%

of the

production

(natural

and

synthetic)

in the processing

of stone,

metal,

ceramic,

glass,

concrete,

and

other

products.

Other

applications in optics

and other

areas

have a more

limited

but growing market.

160

I

I

I

I

I

I

I

140 .. ............

..................

.................

. ............

. .. ... ... .

.. .... .. .....................................

5.1

Industrial

Diamond Powder, Grit, and Stones

Small

synthetic

crystals (cl

mm)

are

used as

powder

and

grit (in six

different

sizes),

in the

manufacture

of fine

grinding,

lapping

and

polishing

compounds,

matrix-set

drill

bits

and

tools, saw blades, and

polycrystalline

diamond

(PCD)

(see

Sec.

5.2).

Larger

crystals

(~1

mm)

are polished

and shaped

and handled as

individual

stones

in

the

manufacture

of drill

bits,

glass

cutters,

abrasive

wheel truers, single-point

turning,

shaping

and cutting tools, heavy-duty

truing and

large

plunge

cutting

tools,

and

wire-drawing

dies.

The main

advantages

of the synthetic

stones

over

the

natural

are the ability

to control

uniformity,

size, crystal habit, crystal friability,

and generally

higher quality

and

consistency.t6)

Natural industrial

diamond

is now limited

to the low-grade

(and cheaper)

applications

such as nail files, low-cost

saw blades, and polishing compounds.

Cutting and Grinding Tools.

Cutting

tools

have

a sharp

edge

for the

purpose

of shaving

and

generating

a material

chip (curl).

This edge must

remain

sharp

for

the

tool

to

perform

properly.

Grinding

tools

are different

in that

they have an

abrasive-coated

surface

which

generates

a powder

(swarm) as opposed

to the chip

of a cutting

tool.

Diamond

performs

well in

these

two basic

operations

but with

limitations

as shown

below.t17)

The

three

requirements

of

a

cutting

or

grinding

tool

material

are

hardness,

toughness,

and chemical

stability.

Diamond

meets

the first since

it is the hardest

material.

However,

it is inherently

brittle,

has low toughness,

and reacts readily with carbide-forming

metals,

thus

limiting

its

use.

Diamond Toughness

and Bonding Techniques.

Several

tech-

niques

are available

to

bond

diamond

to

a

substrate

such

as

resinous,

vitrified,

and

electroplated-metal

bondings.

These

techniques

impart

different

degrees

of toughness

to the

bond

as

shown

Fig.

12.9, and

the

selection

of the

proper

bond

is

essential

to

achieve

optimum

results

in

machining

or grinding

workpieces

of different

moduli

of

resi/jerxe.t1r)t18]

...)

I

Frihble

Diahond

.

2500

(......

.:.

:. .:.:

(Resrn

&

Vltnfled)

Medium-Tough

.)

4””

Diamond

_.

2 .

1500

!

t

Aluminum

_

&

Modulus

of

Resilience

(k

Pa)

Figure 12.9. Selection of diamond

type

as a function

of

modulus of resilience of

workpiece.t’*t

Natural and High-Pressure

Synthetic Diamond

295

The machining

characteristics

of diamond

are

also

influenced

by the

shape

and size of the crystal, and

manufacturers

offer

a great

variety

of

crystals

specifically

suited to

grinding,

cutting,

sawing,

drilling,

or

polish-

ing.[19]

The selection

of grain

size for various applications

is shown

in Fig.

12.10.[201 The

cutting

material

employed

in the

new

ultraprecision

machin-

ing technique

of single-point

turning

is single-crystal

diamond.[21]

Super

finishing hones

Wheelsfor

4

precision grinding

)

4

Fine-gritgrindingwheels

)

Slurrieqpowders

Saw

blades/rotary

4

)

cutters tools

polishing

compounds

I

I

1

I

I

I

0.1

1

10

100

1000

Chemical Stability

of Diamond Tools. Diamond

reactswith

carbide-

forming

metals

such

as

iron.

In

contact

with

these,

diamond

dissolves

rapidly

and

is

considered

unsuitable

to

machine

steel

and

cast

iron.

Likewise,

it is not recommended

for superalloys.

Poiycrystaiiine

Diamond.

Polycrystalline

diamond

(PCD)

is an

aggregate

produced

under

heat

and pressure

from

single crystals

varying

in size from 5 to 20pm, bonded

together

with cobalt

or nickel.[22]

Unlike the

brittle single

crystals,

PCD

is tough

and

well-suited

for

demanding

applica-

tions.

It is similar to the

natural

carbonado

described

in Sec. 2.3.

PCD can

be produced

as blanks, cut into specific

shapes, and

bonded

to atungsten

carbide

substrate for a fast-growing

range

of applications

such

as machining

and cutting

ceramics

and ceramic

composites,

and oil-drilling

bits (Fig.

12.11).