2 Erosion Pulsed

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524

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 24, NO. 2, APRIL 1996

LENS

MIRROR

CO2 L A s q

TO PUMP

;P

i

APS

Fig.

1.

Experimental setup, C

=

cathode,

A =

anode, 14

=

magnet, T H

=

thyristor switching circuitry, APS

=

arc power sup ply, and CP

=

capacitor.

maximum power of 1MW. Vacuum

lop4

torr) was produced

using mechanical and oil diffusion pumps, the pressure was

kept constant throughout the study. The square arc current and

voltage pulses were measured as a function of time using an

oscilloscope. The mean velocity of the moving arc is measured

by dividing the arc trace length by the arc duration time. The

thyristor based electrical circuit used in this system allows

to adjust the arc duration time between 100 ps and 14 ms

over which a constant current

was

maintained (square current

pulse). The arc spot was moved over the cathode surface in the

retrograde direction by the influence of a permanent magnet

positioned at the back of the cathode. The different magnetic

field intensities oriented parallel to the cathode surface were

adjusted by varying the distance between the magnet and

the cathode. The erosion rate values in grams per coulombs

were determined by weighting the cathode before and after

the arcing using a microbalance, then dividing the weight

difference by the total electric charge Q

=

[ I d t passing

through the cathode. Experiments were performed at a constant

total charge of 50 C for each cathode.

Four different types of graphite materials were tested. Table

I gives some important material properties of these cathodes.

Three of these graphite are polycrystalline (ZXF-SQ, AXF-5Q

and PS) with a given pore size, while PYROID graphite has

no

porosity. Velocity measurements were repeated three times

for each type of graphite material to ensure reproducibility.

Erosion rate measurements were made using a series of 2.5-5

ms individual arc pulses with a current of 70 A. The number of

pulses was adjusted to yield the total electric charge of 50 C.

18

16

4

2

0.035 0.040 0.045

0 050

0.055

0 060 0 065

MAGNETIC

FIELD

INTENSITY, Tesla

Fig. 2. Mean arc spot velocity as

a

function of magnetic field intensity

parallel

to the cathode surface for the four types of graphite described in

Table

I

(lines correspond to linear fitting

of

the data).

14

12

2

AXF-SQ

0 2 4 6

8 10

12 14

16

18

ARC SPOT VELOCITY, mls

Erosion rate of the four graphite materials of Fig. 2

as a

function

of

ig. 3.

the

mean

arc spot velocity.

spot velocity increasing with an increase in the magnetic field

intensity for all cathodes used in this study. Very different arc

spot velocities at the same magnetic field intensity can also

be observed for the various types of graphite cathodes. The

movement of the arc spot on graphite cathodes was shown

to be very difficult compared to other metallic electrodes

[9],

requiring much stronger magnetic field values. The results of

Fig. 2 indicate the choice of the type of graphite being used

has strong effect on the arc spot behavior.

111.

RESULTS

A. Effect of Magnetic Field Intensity on Arc

Spot

Velocity

The mean arc spot velocity is given in Fig. 2 using the

four different graphite cathode materials. One can see the arc

B. Effect

of

Arc

Spot

Velocity on Er osion Rate

Fig. 3 shows that for all the above-mentioned graphite

cathodes, increasing the arc spot velocity has the effect of

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KANDAH AND MEUNIER: EROSION STUDY ON GRAPHITE CATHODES

525

lo

r

-

8

w

7

z

v

z

3 5

6

8 -

4

D l

B4.04

Tesla

. XF-5Q

AXF-JQ

v

3

4 5 6 7 8 9

10

A R C SPO T V ELOC ITY ,

m/s

Fig. 4. Erosion rate of the four graphite types as a function of arc spot

velocity at a constant magnetic

field

intensity of 0.04

T.

decreasing the net cathode erosion rate. This was to be

expected, an increase in the arc spot velocity results in a

decrease in the residence time of the spot on a given site of

the cathode, hence a decrease of the localized heating of the

surface. Such a decrease in the local heat load in the cathode

spot zone was shown to reduce the amount and size of the

macroparticles flux emitted by the spot

[8].

One can see again

in Fig.

3

the various graphite cathodes leading to different

erosion rate values whatever the spot velocity. Fig.

4

shows

for a constant magnetic field intensity of 0.04 T the very

surprising result that cathodes having higher spot velocities

due to their surface characteristics give rise to the highest

erosion rate. The materials in Fig.

4

follow the order of the list

in Table

I.

Materials showing the highest erosion rate and spot

velocity have the lowest electrical resistivity and pore sizes,

and also the largest density. The behavior indicated by Fig.

4

may lead to very interesting and important consequences in

view of deposition applications. It was shown previously

[8]

that a reduction of the local heat load delivered to a given

site of the graphite cathode through a decrease of the arc

residence time leads to a reduction in the number, the size,

and the width of the size distributions of the emitted graphite

macroparticles. Increasing the arc velocity through a judicious

choice of the graphite material surface characteristics should

possibly result in the same trend of reducing the macroparticle

emissions. Fig. 4, however, indicates that the total erosion rate

is increasing with increasing arc spot velocity for cathodes of

specific types.

C. Effecto Pore Size on Erosion Rate

Increasing the pore size

of

the graphite material as shown

in Fig. 5 results in a decrease of the erosion rate. This

unexpected result can again be compared to the data reported

by Kandah and Meunier

[8],

showing that an increase in the

cathode pore size increases the number and size of the emitted

macroparticles. This may indicate that an increase in the pore

size of the cathode could lead to

a

decrease in the ion emission.

3

0.0 0 5

1.0

1.5

PORE SIZE,

pm

Fig. 5.

a

constant magnetic

field

intensity

of 0.04 T.

Erosion rate as a function

of

pore size for the four graphite types at

D.

Cathode Morphology

Figs.

6

and

7

show that under similar operating conditions,

different graphite material properties lead to various spot

movement behavior. Graphite electrodes are characterizedby a

very dlifferent arc spot motion even at moderate magnetic field

valuer; when compared to metallic electrodes. This is shown

in the photograph of Fig. 6 giving the erosion traces over

three graphite materials. One can see, however, that materials

with low pore size tend to generate a more important random

movement of the spots, in a way similar to metallic electrodes.

The dlifferent behaviors on Figs. 6 andl7 are typical of the

given materials.

Fig.

7

shows arc traces when a magnetic field of

0.04

T is

applied. These traces are the result of only one arc discharge

on each cathode sample given. One can see the arc splitting

into

a

multiple spot arc root at a current value somewhere

between 110

A

and 160

A

for the PYRClID graphite. A higher

current of 200 A, however, still results

i m

a single spot arc on

the

ZXF-5Q

cathode. Again, this stresses the importance of

the graphite cathode material properties on the cathode spot

characteristics for this material.

IV. DISCUSSION

The

erosion rate for carbon cathode measured by Kimblin

[lo]

is

1.7

x

g/C. This value, however, is shown here to

depend heavily on the cathode material characteristics. In this

work attention was focused on the effect of different graphite

material properties on the spot behavior and the erosion rate.

One can see from Fig. 2 that arc spot velocity depends strongly

on the surface properties of the different graphite cathodes.

The order of the various graphite for decreasing spot velocity

follows that of Table

I

for their structural and electrical

properties. In the absence of any external disturbance on any

continuous homogeneous metallic surface, an arc spot typically

moveis randomly over the surface of the cathode

[ll]

[12]. If



4/5

526

F-

PS

ZXP SQ

1

0 2

la

t = 2 d I= OA

1.8 1650

Fig. 6.

graphite types at zero magnetic field intensity.

Photographs showing the behavior

of

the arc spot over three different

m w

s

the graphite vacuum arc encounters a discontinuity, such as

a crack or a large pore, one might expect a relative low arc

spot

velocity will be observed due to the repetitive formation

of craters in a limited area. As a consequence the local energy

density on the surface would be increased and a deep crater

formed, reducing the motion of the spot and increasing the

erosion rate [SI.

Two different parameters are used here to increase the

graphite arc spot velocity, the external magnetic field, and

the surface characteristics of the cathode. Each was shown

to

have different and opposite effect on the erosion rate. This

work indicates that an increase in the arc spot velocity resulting

from a change in the graphite cathode surface properties leads

to an increase in the total erosion rate. A decrease of the

arc residence time on a given site of the cathode was shown

previously to result in a decrease of the emitted macroparticles

[SI. Combining these two effects could eventually lead to

an increase in the fraction of carbon ions emitted. Larger

deposition rates in arc ion-plating systems together with lower

macroparticles content in the films can thus

be

expected. An

increase of the arc spot velocity induced

by

a change in the

5 0.8 1.8

1400

5 1.2 1.33

2900

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 24, NO. 2, APRIL 1996

PY

I =

I C

Fig. 7. Photographs showing the behavior of the

arc

spot over three different

graphite types at a magnetic field intensity

of

0.04T and different arc currents.

Arc movement is downwards, with laser ignition at the top.

TABLE

I

PROPERTIESF

GRAPHITEATHODES

SED N THIS

WOR K

()Lohm.cm)

PYROID eoom SpeddtyMinerals Inc. ZXF-SQ, AXI-5Q and PS fmm Po w Graphite

Inc

applied external magnetic field was shown here

to

reduce the

total erosion rate. The shorter residence time of the arc spot

on a given site should, however, also correspond to a decrease

in the emitted macroparticles

[SI.



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KANDAH AND MEUNIER: EROSION STUDY ON GRAPHITE CATHODES 521

v.

CONCLUSION [9] M Kandah, Droplets generation mec ha.nisms by grap hite cathodes

in the vacuum arc deposition technique, M.S. thesis, McGill Univ.,

Montreal, Quebec, Canada, 1993.

[ lo ] C .

W

Kimblin, Erosion and ionization in, the cathode spot regions of

vacuumarcs,

J.

Appl. Phys. vol. 44, no. 7, pp. 3074-3081, 1973.

Four graphite materials having various physical, mechan-

ical, and electrical properties -reveal in this study different

cathode spot behavior and erosion rate values. Erosion rate

y Fang,

spot

velocity

of

vacuum arcs,3,

J Phys D: Appl

for these graphite materials are ranging from 1.02 x

g/C

.Phys. vol. 15, pp. 833-844, 1982.

to

1.24

10-4

g/c and strongly depend

on

the type

of

graphite

[12]

J. E.

Daalder, Random walk

of

cathode arc spots in vacuum,

J. Phys.

,D: ppl .

Phys.

vol. 16, pp. 17-27, 1983.

and the arc spot velocity. Cathodes showing less surface dis-

continuities, e.g., higher density and smaller pore sizes, show

higher spot velocities and highier erosion rates. This should

possibly lead also to

a

lower macroparticles emission from

the cathode. In addition, these cathodes are expected to show

a larger fraction of the total erosion rate transported by the

ion flux. The trend toward higher spot velocities and erosion

rates also correlates with a lower electrical resistivity of the

graphite material at room temperature. For any given graphite

structure, however, an increase in spot velocity induced by

higher magnetic field intensity results in

a

reduction of the

erosion rate.

REFEREINCES

[I] D. J. Page,

Industriul Graphite Engineering Handbook

UCAR Carbon

Co.. Inc.. 1991.

A.

W

Koch, A.

W.

Nurnberg, and R. Behrisch, Investigation of

vacuum arcs on graohite cathodes. J.

Nucl.

Mat.. vols. 122 and 123 ,

pp. 1437-1439, Ibs4.

W. D. Davis and H. C. Miller, Analysis of the electrode products

emitted by dc arcs in

a

vacuum ambient, J. Appl . Phys. vol. 40,

no.

5,

pp. 2212-2221, Apr. 1969.

A. A. Plyntto,

V.

N. R yzhkov,

ancl

A. T. Kapin, High speed streams in

vacuum arcs,

Sov. Phys.-JETP

vol. 20,

no.

2, pp. 328-337, Feb. 1965.

T.

Utsumi, Measurements

of

cathode spot temperature in vacuum arcs,

Appl .

Phys. Lett.

vol. 18, no. 6, pp. 218-220, Mar. 1971.

Y. Y. Udris, Disintegration of materials by an arc cathode spot, Radio

Eng. Electron Phys. vol. 8, no. 6, pp. 1050-1056, 1963.

B.

N Klyarfeld, N.

A.

Neretina, and N N. Druzhinina, Metal

sputtering by the cathode spot of a vacuum arc, Sov. Phys.-Tech. Phys.

vol. 14, no.

6, pp . 796799, Dec. 1969.

M. K andah and

J.-L.

Meunier, Study of microdroplets generation from

vacuum arcs

on

graphite cathodes, J.

Vac.

Sci.

Technol.

A , vol. 13, no.

5, pp. 2444-2450, Sept./Oct. 1995.

Munther

Kandah

was born in Jordan on Jan-

uary 21, 1964. He received the B.Eng. and M.Eng.

degrees in chemical engineering from Yarm ouk Uni-

versity, Jordan, in 1987, and McGill University,

Montrkal, QuCbec, Canada, in 1993, respectively

He is currently with the Chemical Engineering De-

partment at McGill University pursuing the Ph D

degree

He worked in the Jordanian Army as

a

Chemical

Engineer from 1988 lo 1990 and at the Jordan

University of Science and Technology (JUST)

as

a Teaching and Research Assistant from 1990 to 1991 His research interests

include diamond-like coatings using vacuum arc technology

Jean-Luc

Meunier

(S83-M85) was

bom

in Acton

Vale, Qutbec, Canada,

on

July

2

1956.

He received

the engine enng degree in physics at Ecole Polytech-

nique FCdCrale de Lau smn e EPFL), Switzerland,

in

1981, and the M.Sc. and Ph.D. degrees

in

plasma

physics from the Institut National de la Recherche

Scientifique (INRS-Enrrgie) of Quebec, Canada, in

1982 and 1986, respectively.

He worked

as a

Resrarch Engineer at the Hydro-

Quebec Research Institute (IREQ) from 1985 to

1986, where his research and development areas

included vacuum arcs, and plasma torch development and control. He then

joined McGill University, Montreal, Qukbec, as a Research Associate from

1986 to 1990 He is presently an Assistant Professor in the Chemical

Engineering Department of McGill University, where he is engaged in

teaching and research in the areas of thermal plasma and vacuum arc

deposition

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