Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2 il 9 ~ ~0 ME-4104
DENSE OXIDE COATINGS BY THERMAL SPRAYING
This invention relates to the thermal spraying of oxide ceramics,
particularly to a method and an apparatus for producing dense and
tenacious coatings of oxide ceramics, and more particularly to
coatings of aluminum oxide useful for electrical insulation
BACKGROUND OF THE INVENTION
Thermal spraying, also known as flame spraying, involves the
melting or at least heat softening of a heat fusible material
such as metal or ceramic, and propelling the softened material in
particulate form against a surface which is to be coated. The
heated particles strike the surface where they are quenched and
bonded thereto. A thermal spray gun such as described in U.S.
Patent No. 3,111,267 (Shepard et al) is used for the purpose of -~
heating and propelling the particles. In this type of thermal ~ ~
spray gun, the heat fusible material such as a metal or oxide is ~-
supplied to the gun in powder form. Such powders are comprised
typically of small particles, e.g., between 100 mesh U. S.
Standard screen size (149 microns) and about 2 microns. Heat for
powder spraying is generally from a combustion flame or an arc- -
generated plasma flame. The carrier gas, which entrains and
transports the powder, may be one of the combustion gases or an
inert gas such as nitrogen, or it simply may be compressed air.
Quality coatings of many thermal spray materials have been
produced by spraying at high velocity. Plasma spraying has been
a successful high velocity process in many respects but it can
suffer from non-uniform heating and/or poor particle entrainment
which results from feeding powder laterally into the high
velocity plasma stream.
Rocket types of powder spray guns recently became practical, one
example being described in U.S. Patent No. 4,416,421 (Browning).
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This type of gun has an internal combustion chamber with a high
pressure combustion effluent directed through a long nozzle or
open channel. Powder is fed into the nozzle chamber to be heated
and propelled by the combustion effluent.
s A short-nozzle spray device is disclosed for high velocity
spraying in U.S. Patent No. 4,865,252 (Rotolico et al). Powder
is fed axially into a combustion chamber within an annular flow
of combustion gas. An annular air flow is injected coaxially
outside of the combustion gas flow, along the wall of the
chamber. The spray stream with the heated powder issues from the
open end of the combustion chamber.
; In the device of the Rotolico Patent, an additional annular inner
flow of pressurized air is injected from the nozzle into the
combustion chamber coaxially between the combustible mixture and -
the axial powder/carrier gas. Devices based on this patent, with
axial powder feed and the annular inner flow, have been quite
successful in producing high quality metallic and carbide ~-
coatings with a minimum of material buildup inside the gas cap.
The high velocity oxygen-fuel (HVOF) spraying has been
particularly advantageous for effecting dense coatings of metals
and carbides low in oxide content. Although disclosures have ~ -
mentioned HVOF for ceramic spraying (e.g. in the aforementioned
U.S. Patent No. 4,416,421), in practice the high velocity
combustion process has not allowed sufficient heating for
refractory oxide ceramic powder particles to be well melted or
heat softened. The result has been low deposit efficiency and
little improvement in coating quality over other conventional -~
thermal spray processes.
Aluminum oxide (alumina) is a typical refractory oxide material
useful for the thermal spraying processes, for example to produce
electrically insulating coating layers. Although low velocity
combustion spraying is satisfactory for some applications, plasma
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spraying of this oxide is used for the higher quality coatings of
aluminum oxide. However, because of the rapid cooling of the
spray particles on the substrates, the alpha phase of alumina is
low and the metastable gamma phase is the most prevalent form,
e.g. 80-8s~ gamma. Such coatings have a dielectric strength in
the range of 12 to 20 volts/micron. Coatings with higher levels
of the stable alpha phase and thereby higher dielectric strength
are desired.
SUMMARY OF THE INVENTION
An object is to provide an improved thermal spray gun for
spraying oxide ceramic powder to produce a dense and tenacious
ceramic coating. Another object is to provide an improved high
velocity oxygen-fuel thermal spray gun. A further object is to
provide an improved method for producing a dense and tenacious
ceramic coating, particularly of aluminum oxide. Yet another --
object is to provide a novel article comprising a metal substrate
with dense and tenacious oxide ceramic coating thereon,
particularly of aluminum oxide with incrased alpha phase. An
additional object is to provide a coating of alumium oxide high
in dielectric breakdown strength. Other objects will become
apparent from the following descriptions.
Foregoing and other objects are achieved, at least in part, by a
thermal spray gun that includes a nozzle member with an axial
conduit adapted to convey a powder stream of heat fusible oxide
ceramic in a carrier gas, the axial conduit terminating at the ;~
nozzle face in a plurality of radially divergent orifices. A gas
cap extends from the nozzle member and defines a combustion
chamber with an open end and an opposite end bounded by the
nozzle face, the combustion chamber being receptive of the powder
stream from the divergent orifices. An annular flow of a
combustible mixture is injected from the nozzle member coaxially
into the combustion chamber proximate and radially outward of the
powder stream issuing from the divergent orifices so that, with a
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combusting of the combustible mixture, the powder stream mixes
into the combusting mixture. Pressurized gas is in~ected
adjacently to the gas cap wall radially outward of the annular
flow of the combustible mixture, whereby a spray stream
S containing the ceramic is propelled through the open end.
Objects are also achieved by a method which utilizes a thermal
spray gun including a nozzle member with an axial conduit
terminating at the nozzle face in a plurality of radially
~; divergent orifices, and a gas cap extending from the nozzle
member and defining a combustion chamber with an open end and an
opposite end bounded by the nozzle face. According to the
method, an annular flow of a combustible mixture of a combustion
gas and oxygen is firstly injected from the nozzle member
coaxially into the combustion chamber, and the combustible
mixture is combusted in the combustion chamber. A powder stream
of heat fusible oxide ceramic is conveyed in a carrier gas ~ --
through the axial conduit and divergent orifices into the
combustion chamber proximate and radially inward of the annular ~
flow of combustible mixture so as to mix the powder stream -
~;~20 directly into the combusting mixture. An annular outer flow of
pressurized non-combustible gas is injected adjacently to the ~ ~-
~ cylindrical wall radially outward of the annular flow of the
;~ ~ combustible mixture whereby a spray stream containing the oxide
ceramic is propelled through the open end. The spray stream is
directed toward a substrate so at to produce thereon a dense and
:~:
tenacious coating of the oxide ceramic. Preferably the
combustible mixture is injected at a pressure in the combustion
chamber of at least two bar above atmospheric pressure such that
the spray stream is sonic or supersonic.
In a preferred aspect the oxide ceramic is a refractory oxide,
most preferably aluminum oxide. Thus an article may be produced,
comprising a metal substrate with an dense and tenacious oxide
ceramic coating thereon, the coating being produced by the
foregoing method. The aluminum oxide coating should comprise at
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least 25% alpha phase, and be characterized by a dielectric
breakdown strength of at least 25 volts/micron.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal section of a thermal spray gun of the
present invention.
FIG. 2 is an enlargement of the forward end of the section of
FIG. 1.
FIG. 3 is a view taken at 3--3 of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
, ~ -
An example of apparatus for carrying out the invention is
illustrated in FIG. 1. A thermal spray gun 10 has a gas head 12 ~-
with a tubular member in the form of a gas cap 14 mounted
thereon, a valve portion 16 for supplying fuel, oxygen and air to
the gas head, and a handle 17. The valve portion 16 has a hose
connection 18 for a fuel gas, a hose connection 19 for oxygen and
a hose connection 20 for air. The three connections are connected
respectively by hoses from a fuel source 21, oxygen source 22 and
air source 24. Orifices 25 in a cylindrical valve 26 control the
flow of the respective gases from their connections into the gun.
The valve and associated components include a pair of valve
levers 27, and sealing means for each gas flow section that
include plungers 28, springs 29 and o-rings 30.
A cylindrical siphon plug 31 is fitted in a corresponding bore in
gas head 12, and a plurality of o-rings 32 thereon maintain a
gas-tight seal. The siphon plug is provided with a tube 33
having a central passage 34. The siphon plug further has therein
an annular groove 35 and a further annular groove 36 with a
plurality of inter-connecting passages 38 (two shown). With
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cylinder valve 26 in the open position as shown in FIG. 1, oxygen
is passed by means of a hose ~0 through its connection ~9 and
valve 26 into a passage ~2 from whence it ~lows into groove 35
and through passage 38. A similar arrangement is provided to
s pass fuel gas from source 21 and a hose ~C through connection 18,
valve 2C and a passage 48 into groove 3C, mix with the oxygen,
and pass as a combustible mixture through passages 50 aligned
with passages 38 into an annular groove 52. Annular groove 52
feeds the mixture into a plurality of arcuately arranged passages
53 in the rear section of a nozzle member 54.
~,
Referring to FIG. 2 for details, nozzle member 54 is conveniently
constructed of a tubular inner portion 55 and a tubular outer -
portion 56. (As used herein and in the claims, "inner" denotes
toward the axis and "outer" denotes away from the axis. Also
"forward" or "forwardly" denotes toward the open end of the gun;
"rear", "rearward" or "rearwardly" denotes the opposite.) Outer
portion SC defines an outer annular orifice means for injecting
the annular flow of the combustible mixture into the combustion
chamber. The orifice means preferably includes a forward annular
opening 57 with a radially inward side bounded by an outer wall
58 of the inner portion. The orifice system leading to the
~;` annular opening from passages 53 may be a plurality of arcuately
spaced orifices or an annular orifice S9.
The combustible mixture flowing from the aligned grooves 52 thus
; 25 passes through the orifice (or orifices or an annulus) S9 to
produce an annular flow which is ignited in annular opening 57.
A nozzle nut 60 holds nozzle 54 and siphon plug 28 on gas head
12. Two further O-rings Cl are seated conventionally between
nozzle 54 and siphon plug 31 for gas tight seals. The burner
nozzle 54 extends into gas cap 14 which is held in place by means
I of a threaded retainer ring 64 and extends forwardly from the
nozzle.
Nozzle member 54 is also provided with an axial conduit 62, for
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the powder in a carrier gas, extending forwardly from tube 33.
The axial conduit terminates at the nozzle face in a plurality of
radially divergent orifices 65, also shown in FIG. 3. Four such
divergent orifices (two shown) are in the present example. The
exact number of orifices is not critical; from 2 to 8 is
satisfactory. The orifices preferably are arcuately spaced with
an angle to the axis 63 between 10 and 30, for example 23. The
outer orifice 59 or ring of orifices for the combustible mixture
should be proximate the divergent orifices C5, so that the
combusting mixture is proximate the powder stream issuing from
the divergent orifices, and the diverging powder stream mixes
directly into the combusting mixture.
A diagonal passage (not shown) extends rearwardly from tube 33 to
; a powder connection 65. A carrier hose C6 and, therefore,
central bore 62, is receptive of powder from a powder feeder 67
entrained in a carrier gas from a pressurized gas source 68 such
as compressed air by way of feed hose 66. Powder feeder 67 is of
the conventional or desired type but must be capable of
delivering the carrier gas at high enough pressure to provide
powder into the chamber 82 in gun lo.
~ ,
Air or other non-combustible gas is passed from source 24 and a
hose 69 through its connection 20, cylinder valve 26, and a
;~ passage 70 to a space 71 in the interior of retainer ring 64.
Lateral openings 72 in nozzle nut 60 communicate space 71 with a
cylindrical combustion chamber 82 in gas cap 14 so that the air
may flow as an outer sheath from space 71 through lateral
openings 72, thence through an annular slot 84 between the outer
surface of nozzle 54, and an inwardly facing cylindrical wall 86
defining combustion chamber 82 into which slot 84 exits. The
flow continues through chamber 82 as an annular outer flow mixing
with the inner flows, and out of the open channel at open end 88
in gas cap 14. Chamber 82 is bounded at its opposite, rearward
end by face 89 of nozzle 54.
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Preferably at least the outer end of combustion chamber wall 86
converges forwardly from the nozzle at an angle with the axis,
most preferably between about 2 and 10, e.g. 5. Wall 8C at
slot 8~ also converges forwardly at an angle with the axis, most
preferably between about 12 and 16, e.g. 14.5. Slot 8~ further
should have sufficient length for the annular air flow to
develop, e.g. comparable to chamber length 102, but at least
greater than half of such length 102.
Also, with valve 26 in a lighting position aligning bleeder
holes, an air hole 90 in valve 2C allows air flow for lighting,
and the above-indicated angles and dimensions are important to
allow such lighting without backfire. (Bleeder holes in valve 26
for oxygen and fuel for lighting, similar to air hole 90, are not
shown.)
In the gas head 12, central bore 62 is 2.0 mm diameter, and the
open end 88 of the gas cap is 0.95 cm from the face of the nozzle
(length 102). Thus the combustion chamber 82 that also entrains
the powder is relatively short, and generally should be between
about one and two times the diameter of open end 88.
- 20 A supply of each of the gases to the cylindrical combustion
chamber is provided at a sufficiently high pressure, e.g. at
least 2 bar (30 psi) above altmospheric, and is ignited
conventionally such as with a spark device, such that the mixture
of combusted gases and air will issue from the open end as a
sonic (choked) or supersonic flow entraining the powder. The
heat of the combustion will at least heat soften the powder --
material such as to deposit a coating onto a substrate. Shock
diamonds should be observable. Because the flow is under
expanded, an expansion type of nozzle exit is not necessary to
achieve the supersonic flow.
The combustion gas may be propane or hydrogen or the like, but it
~; is preferable that the combustion gas be propylene gas, or
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21 19 ~ 3 0 ME-4104
methylacetylene-propadiene gas ("MPS"). These latter gases allow
a relatively high velocity spray stream and excellent coatings to
be achieved without backfire. For example with a propylene or
MPS pressure of about 7 kg/cm2 gauge (above atmospheric pressure)
to the gun, oxygen at 10 kg/cm2 and air at 5.6 kg/cm2 at least 8
shock diamonds are readily visible in the spray stream without
powder flow.
The invention is preferably carried out with a heat fusible oxide
ceramic powder having a size distribution generally between 1 and
30 microns, advantageously between 5 and 20 microns. Suitable
thermal spray oxides are aluminum oxide, titanium dioxide, and
composite alumina-titania powder.
Particular benefits are attained with high purity aluminum oxide
(Al2O3). A coating of this material should have no more than -
0.25~ porosity measured using the line intercept method, and
~; should comprise at least 25% alpha phase, compared with less than
15~ for a conventional coating. Also such a coating should have
a dielectric breakdown strength of at least 28 volts/micron.
Otherwise such coatings of this or other oxides will have the
typical cross sectional structure of HVOF coatings, viz.
laminated lenticular grains representing the flattened particles
of powder melted and sprayed at high velocity.
,~ .
Example ~ ~
- .-
A flat copper substrate was prepared by light grit blasting with
; 25 177-590 grit alumina under 2.5-3.2 kg/cm2 (35-45 psig) air
pressure. A 99~ pure aluminum oxide powder having a size
distribution of 20 to 5 microns was thermal sprayed with the
preferred apparatus described above with respect to FIGS. 1-3.
Oxygen was 9.4 kg/cm2 (135 psig) and 300 l/min (633 scfh), -~
propylene fuel gas was 4.5 kg/cm2 (65 psig) and 97 l/min (206
scfh), and air was 5.2 kg/cm2 (75 psig) and 328 l/min (694 scfh).
A high pressure powder feed of the type disclosed in the present
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211.~1~0 ME-4104
assignee's U.S. Patent No. 4,900,199 and sold by Perkin-ElJner as
a Metco~ Type DJP~ powder feeder was used to feed the powder
at 23 gm/min (3 lbs/hr) in a nitrogen carrier at 8.8 kg/cm~ (125
psig) and 12 l/min (2S scfh). Spray distance was 13 cm and
traverse rate was 4.5 m/min. The resulting coating was ground
conventionally to a thickness of 250-300 microns.
An analysis by x-ray diffraction revealed that the coating
contained 35% alpha phase, 55% eta phase and 7.4% beta phase,
compared with a conventional plasma sprayed coating containing
20% alpha and 80% gamma.
Dielectric breakdown strength was measured with the simple
conventional method of touching an electrode to the coating
surface and applying a voltage between the probe and the
substrate. Voltage was increased in increments until breakdown
occurred. This was repeated at 5 locations across the surface.
The breakdown strengths ranged from 30 to 40 volts/micron
thickness of coating. This range compares with typical strengths - -
of 12 to 20 volts/micron.
While the invention has been described above in detail with
reference to specific embodiments, various changes and ~
modifications which fall within the spirit of the invention and ~ -
scope of the appended claims will become apparent to those
skilled in this art. Therefore, the invention is intended only
to be limited by the appended claims or their equivalents.
,10 '~ ~-
