EP0621079A1

EP0621079A1 - Dense oxide coatings by thermal spraying

Info

Publication number: EP0621079A1
Application number: EP94106071A
Authority: EP
Inventors: Joseph P. Mercurio; Edward P. Gianella
Original assignee: Perkin Elmer Corp
Current assignee: Applied Biosystems Inc
Priority date: 1993-04-20
Filing date: 1994-04-19
Publication date: 1994-10-26
Also published as: CA2119430A1; BR9401543A; JPH06312149A

Abstract

A thermal spray gun (10) includes a nozzle member with an axial conduit (34) that conveys a powder stream of heat fusible oxide ceramic in a carrier gas. The conduit (34) terminates at the nozzle face in a plurality of radially divergent orifices (65). A gas cap (14) extends from the nozzle member and defines a combustion chamber (82). An annular flow of a combustible mixture is injected from the nozzle member coaxially into the combustion chamber (82). Air is injected adjacently to the gas cap wall so that a spray stream containing the ceramic is propelled through the open end, preferably supersonic. An aluminum oxide coating sprayed thereby should comprise substantial alpha phase and have good dielectric breakdown strength.

Description

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).
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.
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 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

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 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.
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.
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

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 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.
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.
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.
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.
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.
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.
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.
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.)
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.
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 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 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.
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.

Claims

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.
The thermal spray gun of claim 1 wherein the divergent orifices diverge at an angle with the axis between 10° and 30°.
The thermal spray gun of claim 1 wherein the plurality of radial divergent orifices is in a range of 2 to 8 in number.
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.
The thermal spray gun of claim 1 wherein the combustion chamber converges forwardly.
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.
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.
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.
The method of claim 8 wherein the divergent orifices diverge at an angle with the axis between 10° and 30°.
The method of claim 8 wherein the plurality of radial divergent orifices is in a range of 2 to 8 in number.
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.
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.
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.
The method of claim 13 wherein the oxide ceramic comprises predominently aluminum oxide.
The method of claim 14 wherein the oxide ceramic consists of high purity aluminum oxide.
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.
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.
The article of claim 17 wherein the coating is characterized by a cross sectional structure of laminated lenticular grains.
The article of claim 18 wherein the oxide ceramic comprises predominently aluminum oxide.
The article of claim 19 wherein the oxide ceramic consists of high purity aluminum oxide.
The article of claim 20 wherein the aluminum oxide comprises at least 30% alpha phase.
The article of claim 21 wherein the coating is characterized by a dielectric breakdown strength of at least 25 volts/micron.
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.
The article of claim 23 wherein the coating is further characterized by a dielectric breakdown strength of at least 25 volts/micron.