U.S. patent number 3,756,193 [Application Number 05/248,815] was granted by the patent office on 1973-09-04 for coating apparatus.
This patent grant is currently assigned to Battelle Memorial Institute. Invention is credited to Donald C. Carmichael, Douglas L. Chambers.
United States Patent |
3,756,193 |
Carmichael , et al. |
September 4, 1973 |
COATING APPARATUS
Abstract
Apparatus for providing a tightly adherent coating on a
substrate comprising a first chamber, means for providing an
ionizable gas such as argon to the first chamber, a cathode
comprising the substrate in the first chamber, a second chamber
adjacent to the first chamber, a wall between the first chamber and
the second chamber, an anode comprising a supply of the coating
material, an exposed surface portion of the material (approximately
planar and parallel to the wall) being in the first chamber and
spaced from the cathode, a source of electrons in the second
chamber, means for directing electrons from the source in a beam
through an opening in the wall and on to the exposed surface
portion of the anode, means for substantially evacuating the second
chamber, and means for providing the cathode with a negative
electric potential relative to the anode. The electron directing
means comprises an electron beam gun that directs the electron beam
initially in a direction approximately parallel to the wall and
away from the exposed anode surface portion, bends the beam through
approximately one right angle to pass through the opening, and then
bends it through approximately two more right angles to strike the
exposed anode surface portion in a direction approximately
perpendicular to the exposed anode surface portion.
Inventors: |
Carmichael; Donald C.
(Columbus, OH), Chambers; Douglas L. (Columbus, OH) |
Assignee: |
Battelle Memorial Institute
(Columbus, OH)
|
Family
ID: |
22940801 |
Appl.
No.: |
05/248,815 |
Filed: |
May 1, 1972 |
Current U.S.
Class: |
118/726;
204/298.05; 204/192.11 |
Current CPC
Class: |
C23C
14/16 (20130101); C23C 14/30 (20130101); C23C
14/32 (20130101) |
Current International
Class: |
C23C
14/30 (20060101); C23C 14/32 (20060101); C23C
14/28 (20060101); C23C 14/16 (20060101); C23c
013/12 () |
Field of
Search: |
;118/49.1,49.5
;219/121EB |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kaplan; Morris
Claims
We claim:
1. Apparatus for providing a tightly adherent coating on a
substrate comprising
a first chamber,
means for providing an ionizable gas to the first chamber,
a cathode comprising the substrate in the first chamber,
a second chamber adjacent to the first chamber,
a wall between the first chamber and the second chamber,
an anode comprising a supply of the coating material, an exposed
surface portion of the material being in the first chamber and
spaced from the cathode,
a source of electrons in the second chamber,
means for directing electrons from the source in a beam through an
opening in the wall and on to the exposed surface portion of the
anode,
said second chamber being fluid-tight except at the opening in the
wall, said opening closely conforming to the cross section of the
beam of electrons passing through it.
means for substantially evacuating the second chamber, and
means for providing the cathode with a negative electric potential
relative to the anode.
2. Apparatus as in claim 1, wherein the electron directing means
comprises an electron beam gun.
3. Apparatus as in claim 2, wherein the gun directs the electron
beam initially in a direction approximately parallel to the wall
and away from the exposed anode surface portion, bends the beam
through approximately one right angle to pass through the opening,
and then bends it through approximately two more right angles to
strike the exposed anode surface portion in a direction
approximately perpendicular to the wall.
4. Apparatus as in claim 3, wherein the exposed anode surface
portion is approximately planar and generally parallel to the
wall.
5. Apparatus as in claim 4, comprising also deflection means for
causing the electron beam to scan a substantial area of the exposed
anode surface portion in a predetermined manner.
6. Apparatus as in claim 4, wherein the substrate is positioned
substantially opposite the exposed anode surface portion.
7. Apparatus as in claim 2, wherein the pressure in the second
chamber is maintained at less than about 10.sup.-.sup.3 torr during
operation of the electron gun.
8. Apparatus as in claim 4, comprising also means for maintaining
the exposed anode surface portion at a predetermined location as
material is evaporated therefrom during coating of the
substrate.
9. Apparatus as in claim 1, wherein the substrate is supported by a
portion of the housing of the first chamber, the support including
an insulative connecting member and a conductive barrier
substantially surrounding the insulative member to prevent any
substantial deposition of coating material on the insulative
member.
10. Apparatus as in claim 1, wherein the gas provided to the first
chamber is nitrogen or an inert gas.
11. Apparatus as in claim 1, wherein the gas provided to the first
chamber is argon.
12. Apparatus as in claim 1, wherein including means to maintain
the pressure in the first chamber at about 5 to 50 microns of
mercury during coating of the substrate.
13. Apparatus as in claim 1, wherein the negative electric
potential provided at the cathode is about 1 to 5 kilovolts.
Description
BACKGROUND OF THE INVENTION
Ion plating is a technique of vacuum coating, newer than vacuum
evaporation (metallizing) and sputtering. In ion plating, the part
to be coated is made the negative electrode or is placed in a
low-pressure (vacuum) dc glow discharge, usually of argon. The
positive ions from the discharge are accelerated by an electric
field and bombard the surface of the part, continuously cleaning it
before and during deposition. The coating material is then
evaporated into the gaseous discharge, where it is ionized. The
coating-material ions in the glow discharge region, which surrounds
the part, are accelerated to all surfaces of the part across the
cathode (Crookes) dark space (between the glow discharge region and
the part). Because the dark space has across it most of the field
gradient (voltage drop) of the discharge, the ions deposit on the
surfaces of the part with high energy, typically forming a very
adherent coating.
Thus two competing phenomena are simultaneously occurring at the
surface of the part: one, the deposition of the coating-material
ions; the other, the sputtering of the deposit by the argon and
coating-material ions. The effective rate of deposition is
determined by the relative rates of these two phenomena, and the
material deposition rate must exceed the sputtering rate to obtain
a deposit. The cleaning action of the sputtering by the ions is
important in establishing the adhesion of the structure of the
deposit.
The foregoing process characteristics give ion plating the
following key advantages:
A. Exposure of the part to be coated to reactive gases or liquids
is avoided. For example, hydrogen embrittlement is not
encountered.
B. The part to be coated can be maintained at room temperature, or
can be heated or cooled. Temperature-sensitive materials, such as
aged or hardened alloys, salts, rubbers and plastics, can be
coated.
C. Excellent adherence is usually obtained even between
combinations of materials that normally do not form adherent
interfaces. The careful cleaning, pretreatment, and handling steps
often required for other coating methods are usually not necessary
for ion plating.
D. Because the coating-material ions are created throughout the
glow discharge surrounding the part, the method has very good
throwing power, and quite uniform coatings can be deposited by ion
plating without rotating the part. Build-up at corners of parts
also is not encountered in this process.
The first two advantages are typical of all vacuum deposition
processes to some extent. The latter two advantages, unique to ion
plating, point the way to important future applications of this
technique. Disadvantages are that masking to block coating of
certain areas of some parts is difficult in ion plating because of
its great throwing power, and that direct deposit-thickness
monitoring during deposition has not been developed. Major
obstacles to increased applications of the process, however, have
been the lack of development of processing parameters and their
interrelationships and of practical source-evaporation systems for
ion plating a wider variety of materials.
The ion plating process was first reported in Mattox, D. M., Film
Deposition Using Accelerated Ions, Report No. SC-DR281-63, Sandia
Corporation, Nov. 1963. It is the subject of Mattox's U.S. Pat. No.
3,329,601, issued July 4, 1967. Investigations of the process were
concerned with gold, aluminum, and chromium coatings applied to
both metal and ceramic parts as reported in Mattox, D. M., Ion
Plating, Report No. SC-R-68-1865, Sandia Corporation, November,
1968. To obtain coatings on ceramics to which a negative potential
cannot be directly applied, a screen wire cage arrangement having a
negative voltage is used around the parts to accelerate the ions to
the parts to be coated. One of the principal early applications was
the coating of a uranium reactor core with aluminum for corrosion
protection.
An interesting application of ion plating for coating high-strength
steel, titanium, and aluminum alloy fasteners with pure aluminum
alloy fasteners with pure aluminum for corrosion protection in
marine environments is described in McCrary, L. E., Carpenter, J.
F., and Klein, A. A., Specialized Application of Vapor-Deposited
Films, Transactions of the International Vacuum Metallurgy
Conference - 1968, American Vacuum Society, New York, N. Y., 1968.
This work included demonstration of the deposition of uniform and
adherent film on screw threads, without any build-up at the thread
crown. Spalvins, et al, have reported some useful descriptions of
ion-plated coatings on complex shapes and of ion-plated coatings of
several alloys deposited using flash evaporation in Spalvins, T.,
Przybyszewski, J. S., and Buckley, D. W., Deposition of Thin Films
by Ion Plating on Surfaces Having Various Configurations, Report
No. NASA-TN-D-3707, July 26, 1966; and in Spalvins, T., Deposition
of Alloy Films on Complex Surfaces by Ion Plating With Flash
Evaporation, Report No. N70-32006, June, 1970. Gold coatings 1300
to 1500 Angstroms thick were deposited on components of a ball
bearing and several other complex shapes. Strong bonding of the
coatings to the substrates and excellent uniformity were obtained.
The alloy coatings which were ion plated using flash evaporation to
vaporize the materials into the glow discharge were lead-tin and
copper-gold compositions. The original compositions of the alloys
were closely maintained in the deposit using this technique and
very good adherence and uniformity were achieved. In these and
other references on the process, it is noted, however, that the
range of materials that have been ion plated is rather limited and
relatively little information is reported on the relationship of
the processing variables involved in ion plating.
The present invention overcomes most of the disadvantages mentioned
above. It also provides higher coating rates, and coatings of
materials having higher melting points, various alloys, ceramics,
glasses, quartz, alumina, beryllia, and other materials not
satisfactory deposited heretofore by ion plating.
SUMMARY OF THE INVENTION
Typical apparatus according to the present invention for providing
a tightly adherent coating on a substrate comprises a first
chanber, means for providing an ionizable gas to the first chamber,
a cathode comprising the substrate in the first chamber, a second
chamber adjacent to the first chamber, a wall between the first
chamber and the second chamber, an anode comprising a supply of the
coating material, an exposed surface portion of the material being
in the first chamber and spaced from the cathode, a source of
electrons in the second chamber, means for directing electrons from
the source in a beam through an opening in the wall and on to the
exposed surface portion of the anode, means for substantially
evacuating the second chamber, and means for providing the cathode
with a negative electric potential relative to the anode.
The electron directing means typically comprises an electron beam
gun that directs the electron beam initially in a direction
approximately parallel to the wall and away from the exposed anode
surface portion, bends the beam through approximately one right
angle to pass through the opening, and then bends it through
approximately two more right angles to strike the exposed anode
surface portion in a direction approximately perpendicular to the
wall and to the exposed anode surface portion, which typically is
approximately planar and generally parallel to the wall. The
apparatus typically comprises also deflection means for causing the
electron beam to scan a substantial area of the exposed anode
surface portion in a predetermined manner. The substrate preferably
is positioned substantially opposite the exposed anode surface
portion.
The pressure in the second chamber preferably is maintained at less
than about 10.sup.-.sup.3 torr during operation of the electron
gun, and the second chamber is fluid-tight except at the opening in
the wall, the opening being only substantially equal to the cross
section of the beam of electrons passing through it.
The apparatus typically includes means for maintaining the exposed
anode surface portion at a predetermined location as material is
evaporated therefrom during coating of the substrate. The substrate
typically is supported by a portion of the housing of the first
chamber, the support including an insulative connecting member and
a conductive barrier substantially surrounding the insulative
member to prevent any substantial deposition of coating material on
the insulative member.
The gas provided to the first chamber may be nitrogen or an inert
gas. A preferred gas is argon, and the pressure in the first
chamber typically is maintained at about 5 to 50 microns of mercury
during coating of the substrate. The negative electric potential
provided at the cathode typically is about 1 to 5 kilovolts.
The substrate typically comprises copper, steel, a refractory
metal, nickel or an alloy thereof, cobalt or an alloy thereof,
alumina, beryllia, or glass; and the coating material typically
comprises gold, copper, nickel, aluminum, stainless steel; an alloy
of iron, cobalt, or nickel with chromium and aluminum, such an
alloy containing also yttrium; glass, or quartz.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partially schematic vertical sectional view of typical
apparatus according to the present invention.
FIG. 2 is a graph of coating deposition rate against gas discharge
pressure for ion plating of a flat plate using apparatus as in FIG.
1.
FIG. 3 is a similar graph for ion plating of a solid cylinder.
FIG. 4 is a similar graph for ion plating of a hollow cylinder.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIG. 1, typical apparatus 10 according to the present
invention for providing a tightly adherent coating 20 on a
substrate 30 comprises a first chamber 11, a second chamber 12
adjacent thereto, and a wall 13 between the first chamber 11 and
the second chamber 12. Means such as a supply of argon 14 and a
variable opening valve 15 provide an ionizable gas to the first
chamber 11. A cathode 16 comprising the substrate 30 is located in
the first chamber 11, as shown.
An anode 17 comprises a supply such as a rod 18 of the coating
material, an exposed surface portion 19 of the material being in
the first chamber 11 and spaced from the cathode 30.
A source of electrons 21 is provided in the second chamber 12, with
means 22 for directing electrons from the source 21 in a beam 23
through an opening 24 in the wall 13 and on to the exposed surface
portion 19 of the anode 17. The electron directing means typically
comprises an electron beam gun 22 that directs the electron beam 23
initially in a direction approximately parallel to the wall 13 and
away from the exposed anode surface portion 19, bends the beam
smoothly through approximately one right angle to pass through the
opening 24, and then bends it smoothly through approximately two
more right angles to strike the exposed anode surface portion 19 in
a direction approximately perpendicular to the wall 13 and to the
exposed anode surface portion 19, which is approximately planar and
generally parallel to the wall 13. The substrate 30 preferably is
positioned substantially opposite the exposed anode surface portion
19.
Means such as a vacuum pumping system 25 is provided for
substantially evacuating the second chamber 12, the pressure in the
second chamber 12 preferably being maintained at less than about
10.sup.-.sup.3 torr during operation of the electron beam gun 22.
This pressure can be monitored by a vacuum gage 27. The second
chamber 12 is fluid-tight except at the opening 24 in the wall 13,
and the opening 24 is only substantially equal to the cross section
of the beam of electrons 23 passing through it. A high voltage
supply 26 provides the cathode 16 with a negative electric
potential of about 1 to 5 kilovolts relative to the anode 17, which
is grounded, as indicated at 36, through the housing of the
apparatus 10.
A power supply and control 28 for the electron beam gun 22 includes
voltages for deflection coils in the gun 22 for causing the
electron beam 23 to scan a substantial area of the exposed anode
surface portion 19 in a predetermined manner. Feed drive means 29
are provided for maintaining the exposed anode surface portion 19
of the rod 18 at a predetermined location as material is evaporated
therefrom as the coating 20 is deposited on the substrate 30. The
substrate 30 is supported by the roof portion 31 of the housing of
the first chamber 11, as by an adjustable support 32 including an
insulative connecting member 33 and a conductive barrier comprising
a pair of spaced cup-like members 34 substantially surrounding the
insulative member 33 to prevent any substantial deposition of
coating material on the insulative member 33.
The gas provided to the first chamber may be nitrogen; an inert gas
such as helium, neon, argon, or krypton; or other suitable vapor. A
preferred gas is argon, and the pressure in the first chamber 11 is
maintained at about 5 to 50 microns of mercury during coating of
the substrate 30 by the variable opening valve 15 from the argon
supply 14. This pressure can be monitored by a vacuum gage 35.
The substrate 30 typically comprises copper, steel, a refractory
metal, nickel or an alloy thereof, cobalt or an alloy thereof,
alumina (Al.sub.2 O.sub.3), beryllia (BeO), or glass, and the
coating material 20 typically comprises gold, copper, nickel,
aluminum, stainless steel; an alloy of iron, cobalt, or nickel with
chromium and aluminum (FeCrAl, CoCrAl, or NiCrAl), such an alloy
containing also yttrium (FeCrAlY, CoCrAlY, or NiCrAlY); glass, or
quartz. These substrate and coating materials have been used with
excellent results. Many other materials can also be used very
satisfactorily.
EXAMPLES
To characterize and evaluate process conditions and parameters, we
used equipment 10 as shown in FIG. 1.
The electron-beam gun 22 was a rod-fed, 10 kw, single position,
270.degree. beam source (Airco Temescal Model RIH-270). This gun
utilizes X and Y water-cooled deflection coils with flush magnetic
poles, a 270.degree. deflected beam for increased filament life, a
water-cooled copper hearth, and rod feeding (as indicated at 18,
37, 29) to the source. It employs a six-turn, 0.030-in.-diameter,
tungsten filament and produces an arrow head spot (generally
triangular) 3/16 to 1/4 in. long, depending on the
filament-to-beam-former spacing and on the size of the orifice 24
in the wall or conductance baffle 13 where the beam 23 enters the
glow discharge region 38 in the first chamber 11. For process
coating applications, the rod-fed type of mechanism 37 was chosen
to feed the electron beam evaporating source rod 18. The rod feeder
37 is mechanically driven by the feed drive 29 and contains the
source material 18 (for the coating 20) which is nominally 1 in.
diameter and 10 in. long. This method provides a large inventory of
evaporant for continuous operation and provides precise control of
the height of the melting pool 19.
The power supply 28 was a constant voltage, 30-kw unit. The power
output provides dc voltage at a constant 10 kv at a total maximum
electron beam current of 3 amp. Using this supply, one 30 kw or
three 10 kw guns may be operated independently of each other, in
the same or in different vacuum chambers 12. In many process
coating applications, more than one source is desired.
To operate an electron beam source in this type of application, the
electron emitting source 22 must be isolated from the high pressure
discharge region 11 of the system. In the development study, a
conductance baffle 13 was used. After the baffle 13 was assembled
into position the system was evacuated and the electron beam 23
turned on. As the electron beam current was increased, the
electrons "burned" an orifice 24 into the baffle, which resulted in
an orifice the same diameter as the electron beam at the highest
beam current that could be used for this type of gun. Once this
orifice 24 was made, a vacuum-discharge pressure range could be
maintained and controlled in the system from
8.5.times.10.sup.-.sup.5 torr to 1.times.10.sup.-.sup.3 torr on the
electron beam gun side 12, and from 5 to 35 microns of mercury
(5-35.times.10.sup.-.sup.3 torr) on the discharge side 11. An ion
gage 27 was used to measure the vacuum, and a Pirani gage 35 was
used to measure the discharge pressure. In this range of
operational pressures the electron beam gun 22 operated well in
excess of 50 hours without any effect on the tungsten filament.
In the initial experiments, the two regions of the system were
evacuated below 1.times.10.sup.-.sup.6 torr and the discharge
region backfilled with argon to 30 microns. The electron beam
source region of the system was maintained at
8.9.times.10.sup.-.sup.4 torr. The substrates were cleaned at 2000
volts and 0.5 ma for 15 minutes. The discharge pressure was then
decreased to the desired coating pressure. The electron beam source
was initiated and the desired power level obtained. At this time,
the electron beam power and the discharge pressure were held
constant and the shutter 39 was opened to commence ion plating;
these conditions were then held constant during the period of
deposition. (The open position of the shutter is shown shown at 39.
Dashed lines indicate its closed position at 39').
To achieve a coating with the desired characteristics, five
parameters require control: glow discharge pressure; evaporant flux
(electron beam power); substrate voltage and current;
source-to-substrate distance; and substrate geometry. The glow
discharge pressure and the evaporant flux are the foremost
parameters to be considered for the ion plating process. They
affect both the ion deposition efficiency and the uniformity of the
coating. The discharge pressure was invetigated from 1 to 30
microns under various conditions. The evaporant flux was measured
in terms of the electron beam power applied to the vapor source and
was investigated from 1 to 10 kw.
To study the effects of substrate geometry, a flat rectangular
plate 0.045 in. thick .times. 0.7 in. .times. 2 in., a solid
cylinder 0.75 in. in diameter .times. 1 in. long, and a hollow
cylinder 0.69 in. I.D. .times. 0.75 in. O.D. .times. 0.65 in. long,
were chosen because these basic configurations are usually found in
most coating applications. Each has a surface area 3 square
inches.
The substrate-to-source distance was held constant at 6.5 inches
normal to the source. The substrate voltage generally was 2000
volts dc; but was varied from 800 to 2000 volts in some experiments
as shown in the appropriate data. The substrate current density
depended on the glow discharge pressure that was used in each
experiment and was in the range of from 0.4 to 0.6 ma/cm.sup.2.
Gold was used as the reference evaporant material because it is
quite sensitive to process changes. Other materials investigated
were aluminum, quartz, type 304 stainless steel, and FeCrAlY alloy.
##SPC1##
How variations in processing parameters affect uniformity of
coating is shown in the table above and in FIGS. 2, 3, and 4. FIG.
2 shows processing parameters for the flat plate obtained with 7.2
kw electron beam source power, 2000 volts on the substrate 6.5
inches from the source. In FIG. 3 the same type of curves were
obtained for the solid cylinder under the same conditions. But
pressure over 30 microns would be needed if uniform coating were to
be achieved at the particular additions. FIG. 4 shows similar
curves for the hollow cylinder under the same conditions, uniform
coating was achieved at about 30 microns. Where uniformity is not
ciritical, higher deposition rates can be used. In general,
uniformity increases as discharge pressure increases and decreases
as coating rate increases. For example, with a discharge pressure
of 10 microns, coating thickness on the back of the flat plate was
40 percent of that on the front with 7 kw of electron-beam power
providing a coating rate of 5.2 mils per hour. At 25 microns (and 7
kw), the rate was 1.5 mils per hour, and uniformity was 100
percent.
The typical structure of gold deposited on copper shows a clean
interface that gives strong adherence that withstands a typical
tape test. Micrographs show excellent coating uniformity around
corners, with no build-up at corners, in contrast with coatings
from other processes. The microstructure of stainless steel on
copper shows a clean interface and a high-density deposit.
Excellent adhesion was obtained with deposition at 12 mils per hour
at 20 microns and substrate at 2000 volts. Micrographs show also
that a FeCrAlY alloy adheres well on a TD nickel substrate.
Deposition at 4.5 mils per hour at 20 microns produces a fine grain
structure, even finer structures can be obtained at different rates
or by heating substrate. Other examples of ion plated parts include
turbine blades with 3 to 15 mils of FeCrAlY alloy, copper
hemispheres with 1 mil 304 stainless steel, titanium honeycombs
with 1/2 to 1 of gold and aluminum, and rf conduits and pulleys
with 1 mil of stainless steel. Insulators ion plated in a wire cage
include porcelain, Al.sub.2 O.sub.3, and BeO, all coated with
stainless steel. The cage forms an electric field around the
insulators during deposition to accelerate coating ions to the
surface of each part. Glow discharge cleaning before the ion
plating yields excellent coating adherence.
In these and various other examples of coating applications for
coating parts as an industrial type of process we characterized the
ion plating process for stainless steel(Type 304), FeCrAlY alloy,
aluminum, glass, and quartz coatings. The characterization curves
in the figures shown for gold are typical for these materials. The
amplitudes and crossover points vary slightly for each material,
but the coating results are of the same order of magnitude.
X-ray fluorescence analysis of the coatings and the stainless steel
source material gave 17.6 weight percent coating of Cr from 20
weight percent source, 7.5 versus 8.5 for Ni, 0.44 versus 0.75 for
Mn, 0.03 versus 0.62 for Si and 0.005 versus 0.04 for P. For the
FeCrAlY alloy, the comparable figures were 24.5 versus 27.6 for Cr,
4.8 versus 6.8 for Al, and 0.58 versus 1.95 for Y.
The curves in FIGS. 2, 3, and 4 are for specific materials, shapes,
voltages, etc. Gas discharge pressures ranging from about 0.5 to
100 microns have also been shown to be useful in various
applications.
Further details, including pictures and micrographs are included in
the article "Electron Beam Techniques for Ion Plating" by D. L.
Chambers and D. C. Carmichael, Columbus Laboratories, Battelle
Memorial Institute, in Research/Development, May, 1971, Volume 22,
Number 5, pages 32-35.
While the forms of the invention herein disclosed constitute
presently preferred embodiments, many others are possible. It is
not intended herein to mention all of the possible equivalent forms
or ramifications of the invention. It is to be understood that the
terms used herein are merely descriptive rather than limiting, and
that various changes may be made without departing from the spirit
or scope of the invention.
* * * * *