[0001] 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
[0002] 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.
[0003] 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.
[0004] Rocket types of powder spray guns recently became practical, one example being described
in U.S. Patent No. 4,416,421 (Browning).
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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
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. 80-85% 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
[0010] 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.
[0011] 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 combusting
of the combustible mixture, the powder stream mixes into the combusting mixture. Pressurized
gas is injected adjacently to the gas cap wall radially outward of the annular flow
of the combustible mixture, whereby a spray stream containing the ceramic is propelled
through the open end.
[0012] 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 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.
[0013] 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 least 25% alpha phase, and be
characterized by a dielectric breakdown strength of at least 25 volts/micron.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a longitudinal section of a thermal spray gun of the present invention.
[0015] FIG. 2 is an enlargement of the forward end of the section of FIG. 1.
[0016] FIG. 3 is a view taken at 3--3 of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
[0017] 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.
[0018] 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 cylinder valve
26 in the open position as shown in FIG. 1, oxygen is passed by means of a hose
40 through its connection
19 and valve
26 into a passage
42 from whence it flows into groove
35 and through passage
38. A similar arrangement is provided to pass fuel gas from source
21 and a hose
46 through connection
18, valve
26 and a passage
48 into groove
36, 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.
[0019] 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
56 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
59.
[0020] The combustible mixture flowing from the aligned grooves
52 thus passes through the orifice (or orifices or an annulus)
59 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
61 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 of a threaded retainer ring
64 and extends forwardly from the nozzle.
[0021] Nozzle member
54 is also provided with an axial conduit
62, for 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
65, 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.
[0022] A diagonal passage (not shown) extends rearwardly from tube
33 to a powder connection
65. A carrier hose
66 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
10.
[0023] 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.
[0024] 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
86 at slot
84 also converges forwardly at an angle with the axis, most preferably between about
12° and 16°, e.g. 14.5°. Slot
84 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.
[0025] Also, with valve
26 in a lighting position aligning bleeder holes, an air hole
90 in valve
26 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.)
[0026] 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.
[0027] 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.
[0028] The combustion gas may be propane or hydrogen or the like, but it is preferable that
the combustion gas be propylene gas, or 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.
[0029] 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.
[0030] 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
[0031] A flat copper substrate was prepared by light grit blasting with 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 assignee's U.S. Patent No. 4,900,199 and
sold by Perkin-Elmer as a Metco
(TM) Type DJP
(TM) powder feeder was used to feed the powder at 23 gm/min (3 lbs/hr) in a nitrogen carrier
at 8.8 kg/cm² (125 psig) and 12 l/min (25 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.
[0032] 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.
[0033] 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.
[0034] 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.
1. A thermal spray gun for useful spraying oxide ceramic powder to produce a dense and
tenacious ceramic coating, comprising:
a nozzle member with a nozzle face and an axial conduit adapted to convey a powder
stream of heat fusible powder in a carrier gas, the axial conduit terminating at the
nozzle face in a plurality of radially divergent orifices;
a gas cap extending from the nozzle member and having an inwardly facing cylindrical
wall defining 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;
combustible gas means for injecting an annular flow of a combustible mixture of a
combustion gas and oxygen 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 combusting of the combustible mixture, the powder stream mixes directly
into the combusting mixture; and
outer gas means for injecting an annular outer flow of pressurized non-combustible
gas adjacent to the cylindrical wall radially outward of the annular flow of the combustible
mixture, whereby a spray stream containing the ceramic is propelled through the open
end.
2. The thermal spray gun of claim 1 wherein the divergent orifices diverge at an angle
with the axis between 10° and 30°.
3. The thermal spray gun of claim 1 wherein the plurality of radial divergent orifices
is in a range of 2 to 8 in number.
4. The thermal spray gun of claim 1 wherein the combustible gas means is adapted to inject
the annular flow of combustible mixture at a pressure in the combustion chamber of
at least two bar above atmospheric pressure such that the spray stream is sonic or
supersonic.
5. The thermal spray gun of claim 1 wherein the combustion chamber converges forwardly.
6. The thermal spray gun of claim 5 wherein the combustion gas means is disposed to inject
the annular flow into the combustion chamber from a circular location on the nozzle
face, the circular location having a diameter approximately equal to that of the open
end.
7. The thermal spray gun of claim 6 wherein the open end is spaced axially from the nozzle
face by a shortest distance of between approximately one and two times the diameter
of the circular location.
8. A method for producing a dense and tenacious ceramic coating with a thermal spray
gun including a nozzle member with a nozzle face and 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 having an inwardly facing cylindrical wall defining a combustion
chamber with an open end and an opposite end bounded by the nozzle face, the method
comprising firstly injecting an annular flow of a combustible mixture of a combustion
gas and oxygen from the nozzle member coaxially into the combustion chamber, combusting
the combustible mixture in the combustion chamber, conveying a powder stream of heat
fusible oxide ceramic in a carrier gas through the axial conduit and the divergent
orifices into the combustion chamber proximate and radially inward of the annular
flow of combustible mixture so as to mix the powder stream directly into the combusting
mixture, secondly injecting an annular outer flow of pressurized non-combustible gas
adjacent 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, and directing the spray stream toward a substrate so at to produce thereon
a dense and tenacious coating of the oxide ceramic.
9. The method of claim 8 wherein the divergent orifices diverge at an angle with the
axis between 10° and 30°.
10. The method of claim 8 wherein the plurality of radial divergent orifices is in a range
of 2 to 8 in number.
11. The method of claim 8 wherein the step of firstly injecting comprises injecting the
annular flow of combustible mixture into the combustion chamber from a circular location
on the nozzle face, the circular location having a diameter approximately equal to
that of the open end.
12. The method of claim 11 wherein the open end is spaced axially from the nozzle face
by a shortest distance of between approximately one and two times the diameter of
the circular location.
13. The method of claim 8 wherein the step of firstly injecting comprises injecting the
annular flow of combustible mixture at a pressure in the combustion chamber of at
least two bar above atmospheric pressure such that the spray stream is sonic or supersonic.
14. The method of claim 13 wherein the oxide ceramic comprises predominently aluminum
oxide.
15. The method of claim 14 wherein the oxide ceramic consists of high purity aluminum
oxide.
16. The method of claim 15 wherein the aluminum oxide is in the form of powder particles
having a size between about one and 30 microns.
17. An article comprising a metal substrate with an dense and tenacious oxide ceramic
coating thereon, the coating being produced by the method of claim 8.
18. The article of claim 17 wherein the coating is characterized by a cross sectional
structure of laminated lenticular grains.
19. The article of claim 18 wherein the oxide ceramic comprises predominently aluminum
oxide.
20. The article of claim 19 wherein the oxide ceramic consists of high purity aluminum
oxide.
21. The article of claim 20 wherein the aluminum oxide comprises at least 30% alpha phase.
22. The article of claim 21 wherein the coating is characterized by a dielectric breakdown
strength of at least 25 volts/micron.
23. An article comprising a metal substrate with an dense and tenacious aluminum oxide
coating thereon, the aluminum oxide comprising at least 25% alpha phase, and the coating
being characterized by a cross sectional structure of laminated lenticular grains.
24. The article of claim 23 wherein the coating is further characterized by a dielectric
breakdown strength of at least 25 volts/micron.