DE68910072T2 - Wear-resistant coating and process for its production. - Google Patents

Description

The present invention relates to materials and coatings, primarily those which wear easily, such as coatings used as self-grinding seals for turbines. In particular, it relates to an improved wear material and its method of manufacture.

Materials that wear easily in a controlled manner are used in a variety of applications, including as self-adjusting seals. Those skilled in the art will understand that contact between a rotating part and a fixed self-adjusting seal causes the wear material to be worn down to a structure that conforms to the moving part in the contact zone. This means that the moving part shaves away part of the self-adjusting seal so that the seal assumes a geometry that closely conforms to the moving part. This effectively forms a seal with extremely close tolerance.

A particular application of self-grinding seals is their use in turbines. Typically, the inner surface of the turbine casing is coated with a wear material to a predetermined thickness using a spray gun. During operation, as the turbine blades rotate, they expand slightly due to the heat generated. The tips of the rotating blades then contact the wear material and dig precisely defined grooves in the coating without touching the casing itself. These grooves create the precise clearance that the blades need to rotate, thus eliminating the need for an essentially custom-made seal.

In order for the turbine blades to cut grooves into the wearable coating, the material from which the coating is formed must be easily wearable without the blade tips wearing away. This requires that coating requires a careful balance of materials. In this particular environment, the wearable coating must also have good resistance to particle erosion and other deterioration at elevated temperatures. However, as is known to those skilled in the art, these properties are difficult to achieve using conventional methods for producing wear coatings.

In particular, some conventional wear coatings are produced by plasma spraying the filler and metal components in powder form, which requires careful monitoring of a number of parameters. These parameters include the composition of the powder fed, the powder size, and the various operating conditions of the spray gun. However, even when these factors were closely monitored, conventional equipment and techniques have not consistently proven successful in producing high quality wear coatings.

In detail, conventional composite wear coatings are produced by thermal spraying a feedstock selected from two different general types. The simplest of these comprises a mixture of a metal powder and a filler, which is usually a non-metallic powder. That is, a mixture of discrete particles of each component is produced which is then sprayed using a plasma spray gun. However, these powder mixtures often segregate, not only during storage but also in the particle spray stream itself. Both of these adversely affect the microstructure of the coating produced. Particle segregation is known to produce localized zones in the coating which consist predominantly of a single powder component. This in turn produces coatings of non-uniform composition and hardness which have lower utility. This lack of uniformity can also be caused by preferential evaporation or other thermal transformations of one of the powder components, particularly when a plastic material is used as one of the components. In addition, the use of mixed or blended powders also makes it difficult to adjust the ratio of the components to produce layered coatings which require different feedstock mixtures for each layer.

In another general class of spray powders, the two components are bonded together to form composite particles. A number of bonding techniques are known, such as coating a first powdered material with a second material, or simply sticking the two powders together with a suitable binder. However, the binder may not be able to prevent the two dissimilar materials from separating. In addition, not only are the coating techniques expensive, but preferential evaporation of the coating may occur, reducing the compositional balance of the coating, and a single powder composition cannot be used to produce a coating having different properties throughout its thickness.

For many materials, the production of satisfactory wear coatings requires the use of extremely high speeds that cannot be achieved with conventional combustion flame spray guns. Although flame spray guns produce high speeds, they operate at such high temperatures that they can produce vaporization and thermal degradation, such as vaporization of plastic components and oxidation of powder components. The latter is accelerated by the turbulence of the spray stream.

Patent Abstracts of Japan, Vol. 12, No. 372 (C-533) refers to the aim of obtaining a uniform and good thermally sprayed layer of a metal-based composite material by melting wire rods, which serve as a matrix, using an electric arc and blowing away the molten metal together with a finely divided reinforcing material using gas.

According to this abstract, a pair of metal wires are fed through nozzles and an arc is created to melt the wires. Finely divided reinforcing material is carried through the wires by a gas to thermally spray a surface.

It would be desirable to provide a process for forming a wear material that can reduce or eliminate the problem of particle segregation. It would also be desirable to provide such a process with the additional feature of producing high quality wear coatings without significant degradation of the feedstock. It would also be desirable to provide such a process that can produce a composition gradient in the coating, allowing independent control of the feedstock components without the use of complex measurement systems and avoiding the steep temperature and velocity gradients of plasma spraying. The present invention provides a process for forming a wear material that meets these objectives. It also provides a novel wear material formed by the process of the invention.

According to the present invention, a method for producing a wear material is provided, which comprises the following steps:

- axially feeding a filler into a stream of high temperature combustion gases in order to entrain the filler in the stream of high temperature combustion gases;

- atomising a molten metal by means of the stream of high temperature combustion gases containing the entrained filler so that the atomised molten metal is entrained in the stream of high temperature combustion gases together with the filler;

- directing the stream of high temperature combustion gases, containing the entrained filler and the entrained atomised molten metal, towards a target object;

- forming a deposit of the filler and atomized metal entrained by the stream of high temperature combustion gases on the target object, the deposit comprising a wear material having a metal matrix in which the filler is embedded.

In one aspect, the present invention provides a method of making a wear material by introducing a filler, preferably in particulate form, and in particular a powdered non-metallic material such as a plastic material, into a stream of high temperature combustion gases, the filler being entrained by the gas stream. The filler is preferably fed axially into the combustion gas stream, avoiding uncontrolled lateral dispersion as the particles enter the high velocity gas stream. The filler is heated by the combustion gases along an axis which intersects the molten metal and is moved at an extremely high velocity. The stream of high temperature combustion gases carrying the filler atomizes the molten metal such that the molten metal is entrained in the stream with the filler. A composite stream is thus formed which contains both the filler and the atomized molten metal. The composite stream or spray is then directed against a target object whereby the heated filler and molten metal impact the surface of the target object at high speed, forming a layer or coating of wear material. Upon impact, the molten metal forms a substantially continuous metal matrix in which the filler is embedded in the interstices. The resulting coating is easily abradable and well suited for use in the manufacture of self-grinding seals.

According to a preferred aspect, the method of the present invention is carried out using the high velocity flame spraying apparatus disclosed in copending European Patent Application 89 309 078.7 (EP-A-0 361 710). Therein disclosed is a flame spraying apparatus having a housing portion with a bore having a feedstock inlet and an outlet communicating therewith. Preferably, the funnel-shaped constriction is coaxially aligned with the feedstock bore. The housing has a fuel channel with a fuel inlet and an outlet surrounding the feedstock bore and communicating with the funnel-shaped constriction. The gun housing portion further has an oxidant channel with an inlet adapted to receive an oxidizing gas and an outlet communicating with the constriction. Therefore, the constriction receives a fuel and an oxidizing gas separately before the fuel is mixed with the filler feed material. The constriction has a conical wall which is sufficiently spaced from the outlets for the fuel and the oxidizing gas to cause the fuel and the oxidizing gas to mix and partially burn in the constriction. When the fuel and the oxidizing agent are ignited, a flame front is generated in the constriction which rapidly heats the incoming fuel. The resulting chemical reactions release energy which provides the driving force to maintain a continuous high velocity diffusion reaction. In this way, the feed material is accelerated through an outlet in the tip of the conical wall. The tip of the conical wall is aligned with the feed bore, thereby directing the accelerated feed material through the gun barrel to the tip opening in a straight bore nozzle. In one embodiment, the heated combustion gases carrying the feed material are at a temperature sufficient to melt the tip of a metal wire, which is then atomized by the high velocity gas stream. In another embodiment, the preferred spray device is provided with a two-wire electric arc device such that the wire tip is melted by electric arc heating. The molten metal is atomized and entrained in the stream exiting the gun throat, forming a composite spray.

In another aspect, the present invention provides wear materials that have improved uniformity and lower metal oxide content than some conventional sprayed materials. The wear materials comprise a matrix of metal in which a filler, preferably a soft, crumbly non-metallic material, is evenly distributed. In one embodiment, the wear materials of the invention comprise composite self-grinding seals for use in applications such as self-grinding turbine seals. The wear materials and seals of the invention are formed using the method of the invention. In a preferred embodiment, the wear materials of the invention comprise a metal matrix in which a plastic is evenly distributed in the matrix interstices.

According to one embodiment of the present invention, a method for producing a wear material is provided, which comprises the following steps:

- injecting a particulate filler feedstock substantially axially into a stream of high temperature combustion gases flowing through a spray gun such that the particulate filler feedstock is entrained by the stream of high temperature combustion gases;

- atomizing one end of at least one molten metal wire by placing one end of the one molten metal wire in the path of the stream of high temperature combustion gases such that the molten metal is entrained by the stream of high temperature combustion gases together with the filler;

- directing the stream of high temperature combustion gases, containing the entrained particulate filler feedstock and the atomized molten metal, towards a surface to be coated; and

- coating the surface with the particulate filler feedstock and the molten metal from the stream to form a wear coating on the surface, the wear coating comprising a substantially continuous metal matrix having interstices filled with the particulate filler feedstock.

According to another embodiment of the present invention, there is provided a method of making a composite metal matrix wear coating for use as a self-grinding seal, comprising the steps of:

- Injecting a powdered filler feed material by injecting the powdered filler feed material substantially axially introduced into a stream of rapidly expanding high-velocity combustion gases in a spray gun;

- atomizing a molten metal by directing the stream of high temperature combustion gases containing the powdered filler feedstock to the tip of at least one metal wire so as to form a composite stream of high velocity high temperature combustion gases entraining the powdered filler feedstock and the molten metal; and

- depositing the filler and the molten metal on a surface by directing the composite stream onto the surface such that the filler and the molten metal impact the surface at high velocity to form a composite wear coating on the surface acting as a self-grinding seal.

The present invention also provides a novel wear material and a method of making the wear material. In a preferred embodiment, the inventive material is formed as a wear coating on the surface of a part. In the most preferred embodiment, the inventive wear material is a self-grinding seal.

In the process of the invention, a stream of high-velocity, high-temperature combustion gases is formed by means of a combustion spray device, the most preferred embodiment of which is set out in the above-mentioned co-pending European patent application (claiming priority to US patent application Ser. No. 247,024). However, it will be clear to those skilled in the art that in order to accelerate the filler particles other spray systems, preferably high-velocity spray systems, can be used.

The present invention will now be described with reference to the drawing, without, however, being in any way limited to these.

In the drawing represent:

Fig. 1 is a cross-section through a preferred flame spraying device for use in carrying out the method according to the invention, the wire and the wire feed not being shown for the sake of simplicity;

Fig. 2 is a plan view of a preferred flame spraying device for use in accordance with the invention, showing a two-wire arc arrangement;

Fig. 3 is a schematic diagram showing the formation of the flame front in the converging throat of the spray gun and the formation of the parallel-directed composite particle stream which forms the wear material according to the invention; and

Fig. 4 is a microphotograph of a wear material according to the invention in cross section.

With reference to the drawing (and in particular Fig. 1), the flame spraying apparatus, generally designated 10, comprises a burner housing 12 which is shown integral with the cylinder 14. The conical wall 16 of the burner housing 12 has a converging throat 18 in which a continuous high-velocity diffusion reaction is carried out. The feed material feed bore 20 is defined by the feed material feed tube 22 which is closely enclosed by the feed material housing 24. The feed material housing 24 is in the disclosed embodiment provided with a threaded end 26 which is received in the burner housing 12 provided with a threaded bore. A collar 28 may be provided to facilitate positioning of the feed housing 24. The feed housing 24 and the feed feed tube 22 are arranged inside the fuel supply nozzle 30 such that an annular fuel feed tube 32 is defined. Preferably, the end 34 of the fuel nozzle 30 is tapered and pressed into the burner housing 12.

The feed housing 24 has a second collar or flange portion 36 which cooperates with the fuel nozzle 30. The collar 36 has longitudinal channels which are aligned with the feed bore 20. The fuel flowing in the direction indicated by the arrows is not substantially impeded by the collar 36 during operation. The collar 36 has a grooved outer surface so that it can act as a spacer with respect to the fuel nozzle 30 and yet allow substantially unrestricted fuel flow through the annular fuel passage 32. Similarly, the end portion 38 of the fuel nozzle 30 is provided with a series of substantially parallel longitudinal channels. This channel arrangement also allows the end portion 38 of the fuel nozzle 30 to cooperate with the conical wall 16 while allowing an oxidant to flow through the annular oxidant passage 40 into the converging throat 18. The annular oxidant passage 40 is an annular space defined by the sections 42 and 44 of the burner housing 12. The section 44 also creates a conical wall 16. To securely connect the section 44 to the section 42, the section 42 is provided with an internal thread which receives the threaded portion of the section 44.

The fuel supply passage 48, which extends through the end portion 50 of the burner housing 12, leads into the annular fuel passage 32 and is connected to This continuous passage serves as a channel through which a fuel is supplied to the flame front in the converging throat 18. Likewise, the annular oxidant passage 40 is in flow communication with the oxidant inlet passage 42. The end portion 50 has a connector 54 which can be threaded for connection to a feed material supply hose, as will be explained in more detail in connection with the method according to the invention. The filler feed material is introduced into the feed material bore 20 via the connector 54.

Preferably, the cross-sectional area of the feed bore 20 is substantially smaller than the cross-sectional area of the annular fuel passage 32 and the annular oxidizer passage 40 such that the powdered feed material is fed into the converging throat 18 at sufficient velocity to move through the converging throat 18. The feed bore 20 is generally smaller than about 15% of the cross-sectional areas of both the annular fuel passage 32 and the annular oxidizer passage 40. In addition, the ratio of the diameter of the feed bore 20 to the inner diameter of the spray passage 56 is generally 1 to 5. The ratio of the cross-sectional areas is thus generally about 1 to 25.

The cylinder 14, which is a straight bore annular nozzle, has a hollow cylindrical portion 46 which defines the spray passage 56. As will be described in more detail, the high velocity particles of a filler feedstock are driven through the passage 56 as a parallel stream. To avoid excessive heating of the cylinder wall 46 and to create an effect referred to herein as "thermal pinch" - a phenomenon which maintains the parallel orientation of the particle stream and reinforced - a heat exchange jacket 58 is provided which defines an annular heat exchange chamber 60. The heat exchange chamber 60 is confined to the cylinder 14 so that heat is not directly removed from the converging throat 18. In operation, a heat exchange medium, such as water, is passed through the heat exchange chamber 60 via the channels 62 and 64. Hoses (not shown) are connected at each end to the connectors 66 and 68 to circulate the heat exchange medium through the heat exchange chamber 60.

The flame spraying apparatus 10 shown in Fig. 2 includes means for supplying molten metal, which is shown here as a two-wire electric arc device (not shown in Fig. 1 for simplicity). The arc device 70 includes a carriage 72 which houses wire guides 74 and 76. The wire guides 74 and 76 are designed to feed the wires 78 and 80 to the arc zone 82 at a predetermined speed. The angle formed by the wires 78 and 80 is preferably less than about 60° in most applications. In a preferred method, an electric arc of predetermined intensity is ignited and continuously maintained between the ends of the wire electrodes. In another embodiment, the heat of the parallel flow of combustion gases melts the tips of the wires 78 and 80. In some applications, it may be convenient to use a single wire 78, with the heat of the combustion gases melting the wire. In the disclosed embodiment, the wires 78 and 80 are continuously advanced toward an intersection point in the arc zone 82 as they are melted and consumed as atomized molten metal. While the distance of the arc zone 82 from the end of the cylinder 14 is not critical and can be adjusted to control various properties of the coating or article to be formed during the spraying process, the ends of the Wires 78 and 80 are preferably positioned about 4 to 10 cm from the end of cylinder 14 for most applications. The arc and the ends of the molten metal wire should be positioned within the parallel particle stream exiting cylinder 14, ie, along the longitudinal axis of cylinder 14.

A variety of fuels and oxidizers may be used in the present invention. As described in the above-mentioned co-pending European patent application, gas, liquid or particulate fuels or oxidizers may be suitable. Most oxygen-containing gases are suitable as oxidizers. Substantially pure oxygen is particularly preferred. Suitable fuel gases for producing a high-velocity thrust on the spray material in the present invention are hydrocarbon gases, preferably high-purity propane or propylene, which produce highly inert oxidation reactions. Hydrogen and other liquid or gaseous fuels may also be suitable for certain applications. In the present invention, the flame temperature and hence the temperature of the filler feed material can be controlled by appropriate fuel selection as well as by controlling the gas pressures and the residence or dwell time of the feed material particles in the converging throat 18 and the bore 56.

By controlling the composition of the fuel and the gas pressure, a wide range of particle velocities can be obtained. The preferred fuel gas pressure is about 137.9 to 689.5 kPa, more preferably about 275.8 to 482.7 kPa. The pressure of the oxidizing gas is typically in the range of about 137.9 to 689.5 kPa for most applications, preferably about 275 to 551.6 kPa. When operating within these ranges, the velocities of the combustion products exiting the cylinder 14 are in the supersonic range and significantly higher than the velocities of other conventional flame spray guns. under similar operating conditions. It is clear that the nature of the fuel gas and its flow properties determine the speed to a large extent.

In Fig. 3, the flame spraying device 10 is shown schematically in which a filler feedstock 116 is injected through the feedstock bore 20. In this embodiment, the filler 110 is in particle or powder form and is entrained by a carrier gas, preferably one that is inert to the sprayed materials. The flame front 112 and the low pressure region 114 are shown in the throat 18. After atomization of the molten metal tips of the wires 78 and 80, a composite stream 115 is formed which impinges on a target object 116 to form a layer of the wear material 118 therein in accordance with the invention.

A number of fillers are suitable for forming the wear material of the invention. The most preferred filler is a plastic material. The term "filler" as used herein is generally defined as follows: A material which is physically and chemically substantially heat stable prior to molding, during molding according to the invention and in the use environment of the final wear material. Further, the preferred filler has hardness values which are lower than those of the material used to abrade the wear material, i.e. is softer than the material from which the moving part which contacts the wear material is formed. Finally, the preferred filler is chemically stable to the matrix material during molding according to the invention and during operation of the wear coating. If the filler is used in powder form, it must also be flowable. Also, the fillers preferably used in the present invention are not thermally degraded to any significant extent during the process for producing the wear material. Although the filler is preferably in particle form, For example, it is used as a powder, but it can also be used in rod form.

Soft, crumbly fillers are therefore generally preferred here. Particularly preferred fillers are synthetic polymers of the type used as plastics, fibers or elastomers. Natural polymers which have the desired properties can also be suitable. Preferred synthetic polymers or copolymers include acrylic resins, such as polymers or copolymers of acrylic acid, methacrylic acid, esters of these acids, and acrylonitrile. Also preferred for use here are bismaleimides obtained by condensing a diamine with maleic anhydride, for example by condensing methylenedianiline with maleic acid; fluoroplastics such as polytetrafluoroethylene and polyvinyl fluoride; fully aromatic copolyesters such as liquid crystalline polymers, for example those sold under the registered trademarks Xydar (by Amoco Chemicals Corp.) and Vectra (by Hoechst Celanese); Polyamideimides, such as those sold under the registered trademark Torlon (by Amoco Chemicals Corp.); both thermoplastic and thermosetting polyimides; and sulfone polymers, including polysulfones, polyarylsulfones and polyethersulfones; plastic polyesters, such as aromatic polyesters, preferably polyarylates made from iso- and terephthalate and aromatic bisphenol homopolyesters, polybutylene terephthalate, polyethylene terephthalate, wholly aromatic copolyesters; silicone resin; epoxy resin; polyetherketone and polyethylene sulfide. In general, most thermoplastics and thermosetting resins that have the properties described are suitable for use as a filler component in the present invention. The thermoplastics and thermosetting resins suitable for the present invention cover a wide range of molecular weights, for example from about 2,000 to 1,500,000. Plastics with values outside this range as well as monomers and prepolymers may also be suitable.

As mentioned, the filler used here for the wear material should be soft and crumbly in order to obtain a wear material with the desired properties. In addition to polymers, other non-metallic materials which are preferably used as a filler component in the present invention are solid lubricants such as boron nitride, calcium fluoride, molybdenum sulfide, fluorinated (non-graphitic) carbon, fluorinated graphite, non-graphitic carbon, graphite, and combinations of these materials.

Certain soft ceramic materials are also suitable as filler material, for example calcium carbonate; clays per se, for example kaolin and bentonite; calcium phosphates; wollastonite; pyrophyllite; perlite; gypsum; barite; hydrated alumina; silica; and diatomite, including calcined diatomite; and combinations thereof. In general, most non-abrasive minerals which are not excessively hardened by the flame spraying process are acceptable. In addition, it may be desirable to use certain soft metals as a filler component in the present invention.

The filler of the preferred embodiment of the present invention is a powder which preferably has a particle size of about 5 to 100 micrometers, although diameters outside this range may be suitable for certain applications. The filler powder should be flowable within the needs of the spraying equipment and should have a fairly narrow size distribution so that excessively fine or large particles are not present. The techniques for producing these powders are well known to those skilled in the art.

As mentioned, the metal for the metal matrix of the wear material according to the invention is preferably used as a wire, one end of which is in the path of the flow of the combustion gases, through which the filler is entrained, as shown in Fig. 3. A single wire may be used, with melting of the tip being accomplished by the heat of the combustion gases. Alternatively, as shown in Fig. 3, two wires may be used, with an arc melting between the two wire tips. This creates a source of molten metal which is then atomized by the gas stream. If two wires are used, they may be of the same or different metals. Accordingly, the wire must be consumable by either of these means.

The metals suitable for forming the metal matrix in the present invention are preferably supplied in wire form. Preferred metals include aluminum and its alloys, such as aluminum 1100, 1350 and other 1XXX series; aluminum/magnesium alloys as such, for example 5356 and other 5XXX series; aluminum/silicon alloys as such, for example 4043, 4047 and other 4XXX series; aluminum/magnesium alloys as such, for example 5356 and other 5XXX series; aluminum/magnesium/silicon alloys of the 6XXX series; and aluminum/titanium alloys. Also suitable are copper and its alloys, including copper UNDS C101000-C15735; copper/aluminum alloys as such, for example UNS C60600C64400 (aluminum bronze); Copper/nickel alloys as such, for example UNS C70100-C72500. Also suitable are nickel and its alloys, including nickel UNS NO2200, UNS NO2201 and UNS NO2205; nickel/copper alloys, including UNS NO4400, UNS NO4404 and UNS NO4405; and nickel/chromium alloys as such, for example UNS NO4405; and nickel/chromium alloys as such, for example UNS NO6003. Other metals which are suitable for use in forming the metal matrix of the wear coatings according to the invention are nickel and/or cobalt-based superalloys and high-temperature or corrosion-resistant alloys. MCrAlX alloys are preferred, wherein M is Fe, Ni, Co and combinations thereof; X is a rare earth metal including La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf and combinations thereof, or Zr, Si and combinations thereof. Also preferred for use in the present invention are intermetallic compounds including the aluminides of Ni, Ti and the like. Also suitable are steels including low carbon steel, alloy steel and stainless steel. Also acceptable are pure metals including nickel, cobalt, iron, copper, aluminum and any other metal that can be formed into wires.

The thickness of the wire is not critical, but generally is in the range of about 0.762 to 6.35 mm in diameter. Values outside this range may also be suitable. As the tips of molten metal melt and become atomized, the wire or wires are advanced in the direction of the stream at a rate that ensures a constant supply of atomized molten metal.

One of the many advantages offered by the flame sprayer 10 is the ability to regulate the rate at which the particulate filler is injected into the flame front. Unlike some devices, the flame sprayer 10 allows the rates of particle injection, fuel gas flow, and oxidizing gas flow to be regulated independently. The feed particles are injected into the flame front by an independent flow of an inert carrier gas. By independently regulating the flow rates, turbulence in the converging throat 18 is substantially reduced by maintaining the pressure of the carrier gas at a higher value than the pressure of the fuel gas. This increases particle velocities. The pressure of the carrier gas is preferably in the range of about 275.8 to 482.7 kPa, more preferably about 344.7 to 413.7 kPa. Preferably, it is always greater than the pressure of the fuel gas. Although, as noted, the relative dimensions of the outlets 33 and 41 shown in Fig. 3 can vary widely, the inside diameter of the feed supply tube 22 is generally substantially smaller than the cross-sectional area of the annular oxidant passage 40. The ratio of the cross-sectional area of the feed bore 20 to that of the spray passage 56 of the barrel 14 is generally about 1 to 25 to reduce the likelihood of the filler particles contacting and sticking to the inner surface of the barrel 14 during spraying. The phenomenon called "spattering" which occurs at lower carrier gas pressures is avoided by maintaining the carrier gas pressure above about 344.7 kPa while maintaining the fuel gas pressure at about 310.2 to 448.2 kPa and the oxidizing gas pressure at about 482.6 to 620.6 kPa. The spattering results from the radial movement of particles which may adhere to the conical wall 16. It is believed to occur at lower carrier gas pressures due to increased turbulence. Therefore, turbulence is reduced when the carrier gas pressure is maintained at high values.

As the filler particles move into the converging throat 18, the thermal and kinetic energy of the particles increases substantially due to an exothermic continuous high-velocity diffusion. The energetic filler particles flow through the converging throat 18 and form a parallel stream of high-energy particles which are driven in a substantially straight line through the passage 56 of the barrel 14. As mentioned, there is also a reduction in the turbulent radial motion of the spray particles. An axial, substantially non-turbulent flow of the combustion gases and the filler particles, which results in a parallel high-velocity particle stream, is obtained by creating a non-turbulent gas flow into the converging throat 18 and a continuous high-velocity diffusion reaction is maintained. In addition, broadening of the particle stream as it passes through the cylinder 14 is reduced by removing heat from the cylinder wall 46 by means of a heat exchange jacket 58. When the cylinder 14 is cooled in this manner, a thermal pinch is created which further reduces any radial movement of the energetic particles against the side walls of the cylinder 14.

As the collimated particle stream exits the cylinder 14, it moves through the arc zone 82. During this passage, in the most preferred embodiment, the wires 78 and 80 are energized to strike a sustained electric arc. A voltage sufficient to maintain an arc between the ends of the wires 78 and 80 is maintained by a suitable power supply 90. A voltage between about 15 and 30 volts is generally sufficient. When molten metal has formed at the wire ends, the particle stream atomizes the molten metal. To maintain the electric arc and, as mentioned, to provide a continuous supply of molten metal, the wires 78 and 80 are advanced at a predetermined speed. As the molten metal is atomized, a combined or composite particle stream 115 is formed which contains both the filler and the molten metal. Although some turbulence is created by the presence of the wires 78 and 80, the particle stream is maintained in good parallel alignment. The composite stream is then directed toward the target object 116 where it forms the inventive wear material 118.

In a typical preferred commercial self-grinding seal, the metal matrix of the resulting coating comprises about 40 to 95 volumes of the wear material and the filler component comprises about 5 to 60 volumes of the wear material. In a specific application, the method of the present invention is used to form a wear coating on the surface of a part. In a most preferred embodiment, the invention is used to form a self-adjusting seal for a moving part, such as a self-adjusting seal for turbines. In this aspect, the method of the present invention is used to form a wear coating on the inner surface of a turbine shroud. After the coating has solidified, the turbine blades are rotated to cut grooves in the wear coating and form a closely fitting self-adjusting seal.

The following examples describe the present invention in more detail.

Example

Using the spray gun substantially as shown in Figures 1-3, a wear material was prepared as follows: Two 1100 aluminum wires of 1.59 mm diameter were fed into the spray stream at a rate of 34.5 g/min. The filler component was a thermoplastic polyimide which was fed axially into the combustion gas stream in the manner described above at a rate of about 15 g/min. The size of the thermoplastic powder was substantially 45 to 106 micrometers. The oxidizing gas was substantially pure oxygen at a flow rate of 225 l/min. The fuel gas used was propylene at a flow rate of 46 l/min. Two carrier gases were tried: nitrogen at 85 l/min and carbon dioxide at 67 l/min. The distance between the target object and the gun, measured from the arc zone, was approximately 29.2 cm. The velocity of the combustion gas was approximately the speed of sound. The resulting consumable material is shown in cross-section as a photomicrograph in Fig. 4.

Claims (17)

1. A method for producing a wear material, which comprises the following steps: - axially feeding a filler into a stream of high temperature combustion gases and entraining the filler in the stream of high temperature combustion gases; - atomizing a molten metal by means of the stream of high temperature combustion gases containing the entrained filler, so that the atomized molten metal is entrained in the stream of high temperature combustion gases together with the filler; - directing the stream of high temperature combustion gases, containing the entrained filler and the entrained, atomized, molten metal, towards a target object; - Formation of a deposit of the filler and atomized metal entrained by the stream of high temperature combustion gases on the target object, the deposit comprising a wear material having a metal matrix in which the filler is embedded. 2. A method according to claim 1, wherein the flow of combustion gas is formed in a combustion spray gun at supersonic speed. 3. A method according to claim 1 or 2, wherein the filler is in particulate form. 4. A method according to any one of claims 1 to 3, wherein the molten metal is supplied by inserting the tip of at least one metal wire into the stream of combustion gases high temperature, which contains the entrained filler, so that the metal wire tip is melted by the combustion gases. 5. A method according to any one of claims 1 to 3, wherein the molten metal is supplied by providing two metal wires and means for applying an electric current to the metal wires and creating an electric arc between the tips of the wires, the electric arc being sufficient to melt the tips of the metal wires. 6. A method according to any one of claims 1 to 5, wherein the filler is a powder of a synthetic plastic, selected from thermosetting plastics, thermoplastics and combinations thereof. 7. A method according to any one of claims 1 to 5, wherein the filler is a powder of a solid lubricant selected from boron nitride, calcium fluoride, molybdenum sulfide, fluorinated non-graphitic carbon, fluorinated graphite, non-graphitic carbon, graphite, and combinations thereof. 8. A method according to any one of claims 1 to 4, wherein the filler is a ceramic powder selected from calcium carbonate, kaolin, bentonite, calcium phosphate, wollastonite, pyrophyllite, perlite, gypsum, barite, alumina hydrate, silicon dioxide, diatomite, calcined diatomite and combinations thereof. 9. A method according to any one of claims 1 to 8, wherein the filler is supplied in rod form. 10. A method according to any one of claims 1 to 9, wherein the molten metal is selected from aluminum, aluminum/silicon alloys, aluminum/magnesium alloys, aluminum/magnesium/silicon alloys, aluminum/titanium alloys, copper, copper/aluminum alloys, copper/nickel alloys, nickel, nickel/copper alloys, nickel/chromium alloys, cobalt-based superalloys and combinations thereof. A method according to any one of claims 1 to 9, wherein the molten metal is an alloy of the formula MCrAlX, wherein X is selected from the rare earth metals, Y, Hf, Zr and Si, and M is selected from Fe, Ni, Co and combinations thereof. 12. A process according to any one of claims 1 to 9, wherein the molten metal is selected from nickel aluminides and titanium aluminides. 13. A method according to any one of claims 1 to 9, wherein the molten metal is selected from low carbon steel, alloy steel and stainless steel. 14. A method according to any one of claims 1 to 9, wherein the molten metal is selected from the pure metals nickel, cobalt, iron, copper, aluminum and combinations thereof. 15. A method according to any one of claims 1 to 5 and 9, in which the filler is a plastic material and the metal is selected from copper and copper alloys. 16. The method according to claim 1, wherein - the filler is a particulate filler feed material; - the stream of high temperature combustion gases into which the particulate filler feedstock is introduced flows through a spray gun; - the metal is in the form of a metal wire and one end of at least one such metal wire is atomised by placing said one end in the path of the stream of high temperature combustion gases; and - the target object towards which the stream of high temperature combustion gases containing the entrained particulate filler feedstock and the atomised molten metal is directed is a surface on which the deposit is formed in which the interstices of the metal matrix are filled with the particulate filler feedstock. 17. The method according to claim 1, wherein - the filler is a powdered filler feed material; - the stream of high temperature combustion gases is a stream of rapidly expanding high temperature and high velocity combustion gases; the powdered filler feed material is introduced by means of a spray gun and the metal in the form of at least one metal wire and the flow of high-temperature combustion gases, which carries the powdered filler feed material, is directed at the tip of the metal wire in order to atomize the metal; and - the target object at which the stream of high temperature combustion gases containing the entrained powdered filler feedstock and the atomized molten metal is directed is a surface on which an abradable composite layer is formed, the surface of which serves as a self-adhering seal.