2 erosion pulsed
TRANSCRIPT
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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
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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