Pierson Hugh O. Handbook of carbon, graphite, diamond and fullerene. New Jersey / carbon

Diamond-Like

Carbon

347

The

resulting

coating

can

be

a-C:H or a soft polymer-like

graphite

depending

on the applied

energy.

Deposition

rate is 0.5 to Ppm/hr.

A clean

deposition

surface

is obtained

by chemically

etching

the substrate

followed

by sputter

cleaning

with

argon

just

prior

to the

actual

deposition.

Since

a solid

carbon target

is not needed

and

the

carbon source

is a

gas,

greater

flexibility

is

possible

in

the

positioning

and

geometry

of

the

substrate(s)

as a coating

is deposited

on every

surface

exposed

to the

gas

(as opposed

to sputtering

which

is

essentially

a line-of-sight

process).

Unlike

the

purely

PVD

process,

large

parts

can

be coated

(as long

as they

can be electrically

contacted) with

present

production

equipment.f17)

Deposition

by Ion-Beam Activation.

A typical

ion-beam

activated

system

has a 30 cm hollow-cathode

ion source

with

its optics

masked to 10

cm. Argon

is introduced

to establish

the

discharge

followed

by methane

in

a 28/l

00 ratio

of methane

molecules

to argon

atoms.

The

energy

level

is

100 eV, the acceleration

voltage

is 600 V, and the

resulting

deposition

rate

is 0.5 pm/h.

A

similar

system

has

a

dual

ion-beam.

A

primary

beam

sputters

carbon

while

the

growing

film

is being

simultaneously

bombarded

with

argon

ions generated

from a second

ion source.t18)

Another

system

is based

on a

microwave

discharge

generated

by

electron

cyclotron

resonance

(ECR).f’9jf201

Th e principle

of ECR

is described

in Ch. 13, Sec.

3.3.

Under

normal

circumstances

during

deposition

by

high-frequency

discharge,

the substrate

remains

at low temperature

(c300°C) and a wide

variety

of materials

can

be coated

including

plastics.

This

characteristic is

a major

advantage

of DLC processing over

CVD diamond.

Graphite and diamond

are materials

with a well-defined

structure

and

properties

which vary

within

a relatively

narrow range of values.

DLC is

different

as its structure

and

composition

may vary considerably

and,

as a

result, so do some of its properties. This

is not necessarily

a disadvantage

At the present,

it is not

possible

to deposit

thick

DLC

coatings

as they

tend

to delaminate

and separate

from the

substrate

when

the thickness

is

greater

than

a few

microns.

This

is the

result of high

internal

compressive

stresses

(1.3 - 1.6 GPa) which

appear

to be related

to the hydrogen

content

of the material.t3)f41

Thus

the a-C:H coatings

with

their

high

hydrogen

content

are

more

highly

stressed

than

a-C

coatings

which

have little

hydrogen.

Generally,

the

adhesion

of DLC

to its substrate

is satisfactory,

provid-

ing that

suitable

cleaning

by ion bombardment

is achieved

prior

to deposi-

tion.

Adhesion

to carbide

formers

such

as Si, Ge, MO, Ti, W, and

iron alloys

is particularly

good.

Adhesion

on silicide

formers is improved

by depositing

an intermediate

layer

of Si.t3]

Adhesion

to various

substrates

is shown

in

Table

14.3.[*)

The

test method

is the Sebastian

adhesion

tester.

Coating

thickness averages

10 nm.

The

adhesion

of a-C

and

a-C:H

is generally

similar.

Substrate

Adhesion

(MPa)

Silicon

(111) and (100)

55

Polished

stainless

steel

34

Most

metals

- 30

Fused

silica *

17

CR-39 optical plastic *

13

*Coating

adhesion

exceeded

cohesive

strength

of substrate

An outstanding

characteristic

of DLC coating is its nanolevel

smooth-

ness and

freedom

of pinholes

and

other similar defects.

From that

standpoint,

DLC is different from

CVD

diamond (see Ch. 13, Sec. 3.10). A

350

Carbon, Graphite, Diamond,

and

Fullerenes

DLC coating

planarizes

the surface,

i.e.,

the coating

surface

is smoother

than

the

substrate,

with

surface

roughness

measured

at 0.28

nm for a 100

nm

coating

thickness.t3)

The

material is

an excellent

diffusion barrier to

sodium,

moisture,

and

most gases.t2)

DLC converts

to graphite

at

lower

temperature than

diamond. An

initial

transformation

has been

observed

as low as 250°C.

The

transforma-

tion

is rapid

at 400°C and

proceeds

by

loss

of hydrogen

and

subsequent

graphitization.

A

maximum

long-term

use

temperature

for

DLC is 250 -

300°C.

DLC

coatings

are

generally

optically

transparent

in the wavelength

range

of

2.5 - 18 pm

but

may

have a dark

or

yellow

appearance.

IR

absorption

in the 1 - 1Oprn

range

is very low.

The

index

of refraction of DLC

varies from 1.8 to 2.4 at a 0.589pm

wavelength,

decreasing

with increasing

amount

of hydrogen

(see

Sec. 5.2).

As opposed

to

diamond,

DLC

has

a variable

electrical

conductivity,

which

is

a function

of

hydrogen

content.

It may not

be a suitable

semiconductor

material

since

it has a relatively

low

bandgap,

generally

low

resistivity,

and

low operating

temperature,

although

semiconducting

prop-

erties

have been

reported.tlg)

An

outstanding

property

of

DLC is

its

hardness.

Vickers hardness

ranges

from

2000

to 9000 kg/mm *.

The

large

spread

is due

in part to the

difficulty

of

testing

thin coatings

by

indentation

such

as the

Vickers

test,

since

it is difficult

to

eliminate

the

substrate

effect. Hardness

also

varies

with

the

structure

and composition.

Diamond-Like Carbon

351

5.0 APPLICATIONS OF DLC

DLC coatings

have recently reached

the production

stage

with

appli-

cations

in wear

and

erosion

protection

and

in optics. The cost

is generally

similar

to that

of

carbide

or nitride

films deposited

by CVD or

PVD

The

high

hardness,

low coefficient

of friction,

and

general

chemical

inertness

of DLC coatings

make them well-suited

for applications

involving

wear

and

friction.

In addition,

these

coatings

have

a very

smooth

surface

and

can

be deposited

with

little

restriction

of geometry

and

size.

These

are

important

advantages

and

DLC will likely

gain a foothold in the

hard coating

industry

which

is at the present

dominated

by the carbides

and

nitrides

(Tic,

TIN,

Ti(CN)

and

WC).t6)t171

The

major

drawback

of DLC

is it lack

of high-temperature

resistance

and

thickness

limitation

which

restricts

its

applications

in tool

coating.

In

this

respect,

DLC

is similar

to

CVD

diamond

(see

Ch. 13, Sec. 5.2).

In tribological

applications,

DLC coatings

have

good

potential,

particu-

larly in non-lubricated

conditions

and

in

vacuum

environment.

These

coatings,

when

rubbed

against

themselves,

have

a remarkably

low

coeffi-

cient

of friction

(0.02

- 0.04)

and,

because

of their

superior

hardness,

are

much more

wear

resistant

than

TeflonTM

or

molybdenum

disulfide.

In

addition,

DLC coatings

are very smooth

in the as-deposited

condition,

which

is another

advantage

in friction

applications.

DLC

coatings

are already

found

in a number

of applications,

either

on

a development

basis

or

in

preliminary

production.

These

applications

include

textile

machinery,

bearing

surfaces,

measuring

instruments,

air

bearings,

precision

tooling,

gears,

fluid

engineering

systems,

engine

com-

ponents,

nozzles,

and

rotating

equipment.n7]

Some

typical wear

applica-

tions

are

as follows:

m DLC coatingsforvideo

tapes and high-density

magnetic-

recording

disks

are

applied

by

sputtering

with

facing

targets.

The

disks are

made

of polyvinyl

acetate.

The

coatings

are

extremely

thin

and

smooth

and

allow

the

smallest-possible

clearance

between

the recording

head

and

the

surface

of the

disk.t3)mt23)

. DLC

films

on

smooth metal

substrates

have

a

shiny

appearance

with a

charcoal-black coloration

and are

hard,

do not scratch,

and are chemically

resistant.

They

are being tested

for decorative

and protective applications

such

as jewelry

and

watch parts.flej

DLC is not as good

an optical

material

as single crystal

diamond

but,

as seen

above,

it has many

advantages

as a thin

coating

and is satisfactory

in many

applications.

The

following

is a

review

of present

and potential

optical

applications

of DLC.

DLC in infrared (IF?)Optics.

In Ch. 11, Sec. 6.2,

it was

shown

that

diamond

is transparent

in the

infrared

beyond

7pm.

DLC,

although

not as

transparent

as diamond,

is suitable

and is used

or considered

for

a number

of IR

applications.t2jt241

DLC (a-C:H) provides

an anti-reflection

(AR)

coating

with

an

adjust-

able

index

of refraction

which

varies

with

the

hydrogen

content

as

men-

tioned

above

and

can

be produced

to

match

any

specific

optical

design.

DLC

coatings

are particularly

well-suited

to germanium

windows

with

90%

average

transmission

in the 8 - 12 pm wavelength

range

and to zinc-sulfide

windows

where

they

provide

a reflectance

value of 0.4%

average

in that

same

range.t25j

Adhesion

to the zinc

sulfide

is improved

by an intermediate

film of germanium.t26j

DLC

coatings

also

have an opto-protective

function.

Infrared

window

materials

such as germanium,

magnesium fluoride,

cadmium

telluride,

zinc

sulfide,

and zinc

selenide

are relatively

soft and easily damaged

and eroded

bywind,

rain,

or particle

impact.

They

have

also

poor

resistance

to corrosive

environments.

DLC

coatings

offer

good

protection

with adequate

optical

properties.

However,

their

narrow

IR

bandpass

may

limit

the

range of

applications.

DLC Coatings for Laser Optics.

Laser

window

materials

must

be

capable

of withstanding

high

levels

of fluence (to 100 kW/cm*

or more)

and

tight

bypass-band

specifications.

Tests

carried

out on

a silicon

substrate

coated with DLC (with measurable

amount

of sp3 bonds)

showed

that

the

material

has a high laser-damage

threshold

and

is appreciably

less

dam-

aged

than

common

optical-coating

materials.

These findings

established

the suitability

of DLC for

high-power

laser

windows.t2jt4]

DLC

coatings are found in the following applications:

.

Biomedical: Coatings for

hip joints,

heart

valves

and

other prostheses.

DLC

is

biocompatible

and

blood

compatible.t2st

.

Biochemical:

Coating

for

tissue

culture flasks,

microcarriers,

cell culture

containers,

etc.

DLC coatings are already in production in several

areas (optical and

IR windows)

and

appear particularly well-suited

for

abrasion

and

wear

applications

due to their high hardness and low coefficient of friction. They

have an extremely

smooth surface and can be deposited with little restric-

tion of geometry

and size (as opposed to CVD

diamond).

These

are

364

Carbon,

Graphite,

Diamond,

and

Fullerenes

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