Thermal spray powders, their production and their use

Abstract

 A method of forming a binder-free agglomerated powder, which comprises:
   placing first and second materials in a drum (28) of a mechanical agglomerator, the drum having a continuous curved inner wall (34) and the mechanical agglomerator having impact means (50) disposed in the drum adjacent the drum inner wall and means (32) for providing relative movement between the impact means (50) and the drum inner wall (34);
   processing the first and second materials in the mechanical agglomerator by centrifugally forcing the first and second materials between the impact means (50) and the drum inner wall (34) such that forces of shear and compression cause the first and second materials to agglomerate to form agglomerated particles which are composites of the first and second materials; and
   classifying the agglomerated particles to form a thermal spray powder fraction.
A thermal spray powder, which comprises mechanically agglomerated particles having a first component and a second component, substantially all of the particles ranging in size from about 0.5 µm to about 177 µm and the powder having an average particle size of from about 44 µm to about 150 µm.
A method of forming a thermal spray coating, which comprises providing a thermal spray powder fabricated by mechanical agglomeration in a drum having an impact member; and thermal spraying said powder onto a target to form a coating.

Description

[0001] The present invention relates generally to thermal spray powders and thermal spray processes. More specifically, the present invention provides binder-free agglomerated powders, methods for manufacturing these powders and a method of forming thermal spray coatings using these powders.

[0002] It is known that composite coatings can be made by a number of methods which are referred to generally as thermal spray processes. Thermal spray processes are used in numerous industries to form coatings on metallic and non-metallic substrates. The relative sophistication of these processes and of the coatings so formed has increased rapidly in recent years resulting in the fabrication of high-tech composite materials. In essence, discrete particles are heated (often melted or softened) and accelerated in a high energy stream. In this state, the particles impact a target. Under proper conditions, high quality coatings are formed. It will be appreciated by those skilled in the art that while a number of parameters dictate the composition and microstructure of the final coating, the nature of the particles which are sprayed determines in large part the characteristics of the coating.

[0003] Thermal spray powders are used in both plasma spraying and combustion flame spray processes. Plasma spraying employs a high velocity gas plasma to spray a material. The plasma is formed by flowing a plasma forming gas through an electric arc which partially ionizes the gas into a plasma stream. The recombination of ions and electrons then creates an extremely hot, high velocity gas jet exiting the plasma gun nozzle. Particles are injected into the gas either inside or outside the gun. The particles which are sprayed typically range in particle size from about 5 to 150 µm. The temperature of the jet may reach 10,000°C and the sprayed particles may attain supersonic velocity. In combustion flame spraying, a fuel gas and an oxidant gas are flowed through a nozzle and then ignited to produce a diffusion flame. The material to be sprayed is flowed into the flame where it is heated and propelled toward a substrate. The powder may be injected axially into the flame in a carrier gas. Some flame spray guns utilize a gravity feed mechanism to introduce the powder into the flame front.

[0004] A number of prior art thermal spray powders and methods of forming thermal spray powders are known in the art. As stated, the characteristics of the powder are critical in determining the properties of the final coating. Moreover, powder properties also dictate whether a selected powder can be successfully sprayed in a particular thermal spray application. Although it is known to form composite materials by simultaneously spraying two or more materials, at times using two distinct thermal spray guns, the use of composite powders is preferred. Thus, in a number of applications, composite coatings are formed by thermal spraying a powder which consists of individual composite particles. These discrete particles are formed of two or more materials by agglomerating the materials under controlled conditions.

[0005] Various methods exist for producing agglomerated particles. In once such method, known as spray drying, a slurry of two discrete materials suspended in a binder solution is sprayed into a heated chamber. The resultant dried agglomerated particles which contain binder are then classified by size. The agglomerated powder is then sprayed utilizing one of the aforementioned thermal spray methods to form a composite coating. Other such methods which involve mixing particles with binder and then drying likewise leave binder solids in the dried agglomerates. Although in a number of applications the presence of the binder does not interfere with the spray process or with the desired characteristics of the final coating, in many instances the presence of a binder in the thermal spray powder is undesirable.

[0006] More specifically, both organic and inorganic binder materials may degrade coating performance. For example, it is known that thermal-induced changes may occur during thermal spraying at the interface of two different materials of a composite particle. As the materials chemically react or form an alloy layer, the capacity of the sprayed powder to form high performance coatings having excellent adhesive properties may be enhanced. The ability of the materials to interact in this manner, however, is inhibited by the presence of a layer of binder which physically separates the discrete materials. In other words, a binder may form a barrier to material interaction thus interfering with the fabrication of coatings having desired characteristics. Although organic binders may be employed which are vaporized or oxidized during the thermal spray process, vaporization or oxidation may not be rapid enough or complete. This is particularly true where plasma spraying is conducted under vacuum conditions or in an inert atmosphere, since conventional composite powders are formed with organic binders which generally do not fully vaporize or oxidize under these conditions.

[0007] It is also known that spray coatings for prosthetic devices must meet stringent requirements with respect to the nature and purity of the materials which are utilized. In many instances, the presence of a binder material or a residue arising from it is not acceptable for bio-implant coatings.

[0008] Conventional methods do exist for forming thermal spray composite powders in which the binder content is at least partially removed prior to spraying the powder. In these methods a vaporisable or oxidizable binder is used to create an agglomerated particle. The dried particles are then sintered at elevated temperatures to remove the binder. Sintering alters the characteristics of the particles and this may result in an unwanted effect in the final coating. In US-A- 4 773 928, for example, a process is disclosed for producing plasma spray powders in which a homogeneous powder of a base metal and chromium, aluminium and yttrium is formed which is then used to make a slurry in an aqueous solution of a binder. The slurry is spray dried to produce agglomerates. Particles ranging in size from 20 µm to 53 µm are removed from the agglomerates and are then sintered in a reducing atmosphere to remove the binder. It is claimed that this process results in a homogeneous plasma spray powder.

[0009] Another approach to the binder problem is disclosed in US-A- 4 028 095. Therein, free flowing powders for thermal spray applications are produced by spray drying a slurry of finely divided particles of a metal in a solvent-binder system to produce agglomerates. Here, however, the binder is a soluble compound of the metal. It is stated that as these powders are heated in a reducing atmosphere above the combustion temperature of the binder, the binder is converted to a base metal and harmless byproducts such as nitrogen and water. It is also stated that use of these powders to form a flame spray coating avoids contamination of the coating as well as contamination of the spray equipment and the work area, problems which often occur with the use of conventional binder systems.

[0010] A number of processes are also known for producing powders of homogeneous particles. These processes include conventional ball milling techniques by which a raw material is reduced to particle size and the use of attritors to form powders. Ultra fine particles having an average size of less than 5 µm may be produced using an attritor or a hammer mill. It is also known to utilize high energy mechanical alloying using attritor type mills. For example, in US-A- 4 300 947, a process for the preparation of oxidation and corrosion resistant cobalt, iron, or nickel based alloy powders from a plurality of constituents in powder form is disclosed. It is stated that the constituents, one of which includes an active metal such as aluminium, are alloyed in an attritor-type mill. This process has a number of steps, including wet mixing and stripping the alloy powder in the presence of an alloy enhancing agent in a quantity sufficient to completely engulf the alloy powder.

[0011] Conventional mills such as high energy ball mills and attritors tend to rip apart, recombine and, possibly, alloy the original particles. These conventional mills also often require the use of processing fluids which may contaminate the resulting powders.

[0012] In contrast to the use of ball milling-type apparatus and material attritors for forming powders, US-A- 4 529 135 discloses an apparatus which includes a treating cylinder or drum which is rotated at high speeds to produce a centrifugal force. This force presses a material in the chamber against the inside chamber wall. Treating members disposed in the rotating chamber impact and compress the material in cooperation with the inside surface of the rotating cylinder. In one embodiment, the treating members include pulverizing elements having inclined surfaces adjacent the cylinder wall. The pulverizing or grinder pieces may be disposed in a stationary position or they may rotate in the direction of the rotating cylinder. Although these types of apparatus are generally designed specifically for comminution of materials, under certain conditions they can produce agglomerated particles.

[0013] Similarly, in US-A- 4 733 826, another machine for pulverizing materials is described which includes impact plates disposed on one side of a rotary plate and classifying blades disposed on the other side of the rotary plate. The plate is rotated at high speeds in a chamber wherein a material is reduced to particle size.

[0014] Still further, in US-A- 4 915 987 a method for improving the surface quality of solid particles by impacting the surface of large particles with fine particles is disclosed. In this method the large particles and the small particles are introduced into a collision chamber wherein a rotating plate is rotated to create a high speed gas flow within the chamber. The smaller particles are caused to impact upon and be fixed to the surface of the solid particles, thereby improving the surface quality of the solid particles.

[0015] It is clear from the foregoing that there exists a need for binder-free agglomerates having unique properties which can be produced without an intermediate step of removing a binder. It is also clear that there is a need for a method of forming a thermal spray coating having superior properties by utilizing composite powders which are binder-free and which have superior mechanical and chemical characteristics. The present invention addresses these needs and others.

[0016] According to the present invention there is provided a method of forming a binder-free agglomerated powder, which comprises:
   placing first and second materials in a drum of a mechanical agglomerator, the drum having a continuous curved inner wall and the mechanical agglomerator having impact means disposed in the drum adjacent the drum inner wall and means for providing relative movement between the impact means and the drum inner wall;
   processing the first and second materials in the mechanical agglomerator by centrifugally forcing the first and second materials between the impact means and the drum inner wall such that forces of shear and compression cause the first and second materials to agglomerate to form agglomerated particles which are composites of the first and second materials; and
   classifying the agglomerated particles to form a thermal spray powder fraction.

[0017] According to the present invention there is also provided a thermal spray powder, which comprises mechanically agglomerated particles having a first component and a second component, substantially all of the particles ranging in size from about 0.5 µm to about 177 µm and the powder having an average particle size of from about 44 µm to about 150 µm.

[0018] According to the present invention there is further provided a method of forming a thermal spray coating, which comprises providing a thermal spray powder fabricated by mechanical agglomeration in a drum having an impact member; and thermal spraying said powder onto a target to form a coating.

[0019] In accordance with the present invention there is provided in one aspect a method of forming a binder-free agglomerated powder for thermal spray applications by mechanical agglomeration which comprises in one embodiment the steps of placing a first material and a second material in a rotatable drum in which at least one treatment member is suspended. The drum is generally cylindrical, having a continuous curved inner wall. The treatment member has an impact surface which is positioned adjacent the continuous curved portion of the drum. The materials are processed in the chamber by being centrifugally forced against the continuous curved surface of the chamber, whereupon the materials move between the impact surfaces of the treating members and the continuous wall surface. Forces of shear and compression are thereby exerted on the materials, causing the materials to agglomerate. This effect can be enhanced by external heating (e.g. by a hot air gun). The resultant binder-free agglomerated particles are a composite of the two materials. In one embodiment, the treating member or members are rotated along the same direction as the rotation of the rotating chamber. In still another aspect, the drum is stationary and the treatment members rotate in the chamber to produce a similar result. Another member, generally stationary, scrapes treated material off the chamber wall.

[0020] In another embodiment, the present invention provides binder-free agglomerated particles for use in thermal spray processes which are formed by the methods of the present invention. In one aspect, the first material is a first metal or metal alloy and the second material is a second metal or metal alloy. In another embodiment, the first material is a metal or an alloy of metals and the second material is a ceramic. In still another embodiment, a plastic is present as one of the materials or as a third material. In still another aspect, the first material is a titanium alloy and the second material is hydroxylapatite.

[0021] In still another embodiment, the present invention provides a method of forming a coating by thermal spraying binder-free agglomerated powders made in accordance with the present invention on a substrate. The coating is most preferably applied by plasma spray or high velocity oxyfuel combustion spray. Coatings containing metals or alloys whose oxidation would impair performance in service are sprayed in a non-oxidizing atmosphere.

[0022] In a particular embodiment the present invention provides a method for forming thermal spray powders by mechanical agglomeration or mechanical fusion. Particularly preferred material treatment apparatuses for use in the present invention are those described in US-A- 4 529 135. Other types of preferred particulate material treating apparatus for use in the process of the present invention are those disclosed in US-A- 4 789 105. In a less preferred embodiment, particulate material treating apparatus used in the present invention are those disclosed in US-A- 4 733 826.

[0023] A wide variety of materials may be utilized in forming the novel thermal spray powders of the present invention. For example, the first material may comprise one or more metals selected from Fe, Ni, Co, Cu, Cr, and their alloys. A preferred second material useful in the present invention when the preferred first material is one or more of the aforementioned metals is a metal selected from Al, Ti, Ta, Mo, Si, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Hf, Co, Ni, Fe, and their alloys. It has been found that a combination of these first and second materials generate a product which, when thermally sprayed, exhibits exceptional adhesion to metal substrates. The resulting composite particles are from about 70 to about 98 percent by weight first material and from about 2 to about 30 percent by weight second material.

[0024] It is to be understood that the present invention is also suitable to form the particles set forth in our European Patent Application 913 (our File PO83625EP) (which claims priority from United States Patent Application Serial No. 07/615557.

[0025] Another preferred combination of first and second materials in the present invention is the use of a metal or alloy as the first material selected from Fe, Ni, Co, Cu, Cr, Al, Ti and their alloys, and a second material which is a ceramic material. Preferred ceramics for use in the present invention are selected from oxides, carbides, borides, silicides, silicates, phosphates, spinels, titanates, perovskites, forms of carbon and combinations thereof. The resulting composite particles are from about 50 to about 98 percent by weight first material and from about 2 to about 50 percent by weight second material.

[0026] In another embodiment, the preferred materials for use in the present invention are a first material, comprising one or more metals, most preferably Fe, Ni, Co, Cu, Al, Ti, V and their alloys, a second material comprising one or more relatively soft ceramics, such as fully or partially stabilized zirconia, phosphates of calcium, machinable ceramics, oxides such as aluminum oxides, spinels, titanates, perovskites and a third material comprising one or more high temperature plastics, most preferably polyimide, aromatic polyester, PEEK, PEK, polysulfone and liquid crystal polymers. Also preferred are polyetherimide, polyethersulphone, polyarylsulfone, polyamide-imide, polyphenylene sulfide and polybenzimidazole. The most preferred plastics for use herein are those described in US-A- 3 238 181, US-A- 3 426 098 and US-A- 3 382 203, and those plastics disclosed in the aforementioned European Patent Application 913 (our File PO83625 EP). The resulting composite particles are from about 5 to about 70 percent by weight first material, from about 5 to about 70 percent by weight second material, and from about 20 to about 90 percent by weight third material. The process of the present invention produces binder free agglomerated particles in which the metal and/or ceramic component fully clads the plastic such that the plastic is protected from oxidation or decomposition during spraying. In some cases the metal component smears to form a continuous cladding layer.

[0027] In some instances it may be suitable to utilize a metal as a first material and a plastic as the second material, where the resulting composite particles are from about 5 to about 70 percent by weight first material and from about 20 to about 95 percent by weight second material. A ceramic as the first material and a plastic as the second material may be suitable where the resulting composite particles are from about 5 to about 70 percent by weight first material and from about 20 to about 90 percent by weight second material. The aforementioned preferred metals, ceramics and plastics are preferred for this use.

[0028] As will be explained more fully below with reference to Figs. 3 to 5 of the drawings, the respective particle sizes of the first and second materials which are placed in hopper 22 may vary depending upon the desired final product. Where the first material is provided as generally spherical particles of metal or metal alloy, and the second material is selected from Al, Ti, Si, and other reactive metals and their alloys, the average particle size of the first material is from about 44 to about 100 µm, where the particles range in size from about 10 to about 177 µm, and the second material preferably has an average particle size of from about 1 to about 20 µm, where the particles range in size from about 0.5 to about 44 µm.

[0029] Where the first material is a metal and the second material is a hard or soft ceramic material, the first material preferably has an average particle size of from about 44 to about 100 µm, where the particles range in size from about 10 to about 177 µm, and the second material preferably has an average particle size of from about 1 to about 20 µm, where the particles range in size from about 0.5 to about 44 µm.

[0030] Where the first material is a plastic and the second material is a metal, the first material preferably has an average particle size from about 44 to about 100 µm, where the particles range in size from about 10 to about 177 µm; and the second material preferably has an average particle size from about 1 to about 44 µm, where the particles range in size from about 0.5 to about 62 µm.

[0031] Where the first material is a plastic and the second material is a ceramic, the first material preferably has an average particle size from about 44 to about 100 µm, where the particles range in size from about 10 to about 177 µm; and the second material preferably has an average particle size from about 1 to about 44 µm, where the particles range in size from about 0.5 to about 62 µm.

[0032] Where the first material is a metal, the second material is a ceramic and the third material is a plastic, the first material preferably has an average particle size from about 1 to about 44 µm, where the particles range in size from about 0.5 to about 60 µm; the second material preferably has an average particle size from about 1 to about 44 µm, where the particles range in size from about 0.5 to about 62 µm; and the third material preferably has an average particle size from about 44 to about 100 µm, where the particles range in size from about 10 to about 177 µm.

[0033] The present invention will now be more particularly described with reference to, and as illustrated in, the accompanying drawings, in which:-

Fig. 1 illustrates in front elevational cross section a mechanical fusion agglomerator useful to produce the binder-free agglomerated powders of the present invention;

Fig. 2 is a plan view of the apparatus depicted in Fig. 1;

Fig. 3 illustrates surface fusion to produce binder-free agglomerated particles in accordance with the present invention;

Fig. 4 illustrates surface fusion and spheroidization to produce binder-free agglomerated particles in the present invention;

Fig. 5 illustrates the formation of binder free agglomerated thermal spray powders in accordance with the present invention wherein one of the starting materials comprises agglomerated particles; and

Fig. 6 illustrates the process of forming a thermal spray coating on a bio-implant in accordance with the present invention.

[0034] Referring to Fig. 1, a first material and a second material are agglomerated in particulate material treating apparatus 20 to form the novel binder-free thermal spray powders of the present invention. It is to be understood that while in this particular embodiment the process of the present invention is described with reference to two materials, it may be possible to incorporate a plurality of different materials, for example, three, four, five or more different materials which are used to form composite particles in accordance with the present invention. The first and second materials are placed in material feeder or hopper 22 which is shown diagrammatically in Fig. 1.

[0035] The first and second materials are fed through inlet 24 into chamber 26 of drum or cylinder 28. Passage or inlet 24 is defined by shaft 30 which is rotated in one embodiment of the present invention by motor 32 (shown integral with hopper 22). (Referring to Fig. 2 of the drawings, drum or cylinder 28 includes a continuous curved inner wall 34 which is of course geometrically consistent with the shape of a cylinder). Drum 28 is mounted on support 36 which is in turn connected to shaft 38. Shaft 38 is attached to base 40. Mounted within base 40 is motor 41 which rotates shaft 38, support 36 and drum or cylinder 28 in the direction of arrow A shown in Fig. 2 in one embodiment of the invention. Hopper 22, motor 32 and shaft 30 are mounted to any convenient support structure (not shown). Referring to Figs. 1 and 2 treatment members 42 and 44, which comprise posts 46 and 48 and conditioning members 50 and 52, respectively, are attached to one end of shaft 30. Conditioning member 50 is in the nature of an anvil and conditioning member 52 comprises a blade or scraper.

[0036] As the first and second materials enter chamber 26, chamber 26 is rotated in the direction of arrow A by virtue of motor 41. The floor of drum 28 is raised at the center portion to assist in the movement of the materials toward inner wall 34. This movement occurs primarily due to the centrifugal force exerted on the first and second materials as drum 28 rotates. It will be appreciated that the dimensions and thus capacity of apparatus 20 may vary depending upon the amount of material to be processed. In general, it is preferred to use the highest rotational speed available for a particular apparatus which would not degrade the apparatus or materials. The rotational speed should be sufficient to soften the surface of at least one of the powders.

[0037] As the first and second materials move by centrifugal force to inner wall 34, they are carried first to conditioning or impact member 50 which preferably has an arcuate geometry which mates with the curvature of inner wall 34. Impact member 50 does not, however, contact inner wall 34. This arcuate nature of impact member 50 is shown best in Figure 2 of the drawings. In another embodiment, conditioning members 42 and 44 are in motion. That is, motor 32 is activated and thus shaft 30 and treatment members 42 and 44 rotate relative to drum 28. In the stationary embodiment, material continuously moves between impact members 50 and inner wall 34 as drum 28 spins. The spacing between impact member 50 and inner wall 34 is such that the first and second materials collect and bind between impact member 50 and inner wall 34. Materials which becomes layered or the walls of the drum by virtue of its interaction with impact member 50 are preferably scraped off using blade 52 whereupon it is continually mixed and "kneaded" to form agglomerated particles. This accumulation, binding and scraping of the materials causes a gentle mechanical and frictional processing of the materials such that they combine to form composite particles having many superior characteristics for thermal spraying. In order to further soften the materials during the process, heat gun 53 may be provided which supplies heat directly to a portion of drum 28. Where temperature must be limited to prevent damage to the powder or the apparatus, cooling air or fluid may be provided instead. The preferred spacing between each impact member 50 and inner wall 34 is approximately 2 to about 10 µm. An inert atmosphere may be provided to prevent oxidation of materials during processing.

[0038] The amount of material processed in a batch system will vary widely. It is believed, however, that up to about 30 kilograms of material can be processed in a single batch depending on the preferred drum dimensions. In the preferred ranges, processing times will generally be between 5 to 30 minutes. In the preferred embodiments, drum 28 revolves at between 100 and 2600 rpm, wherein velocities near the upper range (e.g. greater than 1500 rpm for the smallest drum sizes) are preferred. Processing temperature is normally close to the maximum tolerated by the powders and the materials and seals of the apparatus, but no higher than 250°C for the standard apparatus. Specially constructed apparatus may be able to tolerate higher temperature. The finished particles are then classified to provide a powder in which the average particle size is from about 44 to about 150 µm, where the particles range from 0.5 to about 177 µm in size.

[0039] In still another embodiment of the present invention, the novel binder-free agglomerated powders of the present invention are formed in treatment apparatus 20 by simultaneously rotating drum 28 and conditioning or treatment members 42 and 44. Accordingly, motor 32 is activated to rotate shaft 24 which in turn rotates members 42 and 44 in the direction of arrow B. In other words, both chamber 28 and members 42 and 44 are moving in the same direction, but at different speeds. It is preferred that the rotation of members 42 and 44 lag behind that of drum 28. For example, where drum 28 is rotating at a speed of 100 to about 2600 rpm, it may be suitable to rotate members 42 and 44 at a speed of from about 100 to about 2600 rpm. Therefore, it will be understood that the present invention provides relative movement between the drum and at least one conditioning member.

[0040] Referring now to Figs. 3 to 5 of the drawings, the formation of binder-free composite thermal spray powders in accordance with the present invention is shown diagramatically. Accordingly, in figure 3, metal particle 100 is shown in the presence of ultrafine particles of a metal 102. After agglomeration in accordance with the present invention, a thermal spray composite particle 104 is formed having a cladding layer of metal. With reference to figure 4, an irregularly shaped metal particle 106 is shown, again surrounded by ultrafine particles of another metal 108. After agglomeration in accordance with the present invention, a thermal spray composite particle 110 is formed having a clad metal coating and having an altered geometry due to the force exerted on irregular metal particle 106 during the agglomeration process.

[0041] In Figure 5, a spherical metal particle 118 is shown along with a pre-formed agglomerated particle 120. Following agglomeration in accordance with the present invention composite particle 122 is formed wherein the metal particle is coated by the pre-formed agglomerate material. Depending on the nature of the materials used, the starting particle size, and the parameters of the mechanical agglomerator, these and other composite particles may be formed where the principles of the present invention are faithfully observed.

[0042] In one particularly preferred embodiment of the present invention, a titanium alloy, most preferably Ti-6A1-4V is agglomerated in accordance with the present invention with hydroxylapatite [Ca₁₀(PO₄)₆(OH)₂]. Accordingly, a hydroxylapatite powder having an average particle size of about 10 to about 44 µm, where the particles range in size fromabout 3 to about 62 µm, and titanium alloy powder having an average particle size of about 62 to about 125 µm, where the particles range in size from about 44 to about 165 µm, are agglomerated in treatment apparatus 20. Approximately 10 to about 90 percent by weight titanium alloy and about 10 to about 90 percent by weight hydroxylapatite may be suitable to make a binder-free composite thermal spray powder. The first and second materials are agglomerated for about 5 to about 30 minutes under an inert atmosphere such as, for example, argon until substantially all of the hydroxylapatite particles adhere to the Ti alloy particles. Treatment members 42 and 44 are stationary and drum 48 is moving at approximately 100 to 2600 rpm. The materials reach a temperature no higher than 250°C. The resultant composite particles comprise approximately 10 to about 90 percent by weight titanium alloy and about 10 to about 90 percent by weight hydroxylapatite. Depending on their relative size, either hydroxylapatite or Ti alloy particles form the core.

[0043] In still another embodiment of the present invention, a method of forming a coating is provided in which the composite particles formed in accordance with the present invention are thermally sprayed. More specifically, and referring now to Fig. 6 of the drawings, composite particles manufactured in accordance with the present invention, for example the aforementioned titanium alloy/hydroxylapatite powder are thermally sprayed utilizing thermal spray gun 200 (shown schematically). One preferred thermal spray apparatus for use in the present invention is that disclosed in our European Patent Applications 89309077.9 (EP-A-0361709) and 89309078.7 (EP-A-0361710).

[0044] From approximately 10 to about 90 g/min. of titanium alloy/hydroxylapatite powder is sprayed using thermal spray gun 200 to form a coating on target 202, shown here as an artificial hip joint. A coating of from about 150 to about 1500 µm thick is formed on the surfaces of target 202. Thermal spraying may also be carried out using other suitable oxyfuel or plasma spray guns. Thermal spraying may be carried out in vacuum, or under an inert atmosphere of, for example, nitrogen or under atmospheric conditions. The feed rate and other parameters of the process may vary depending upon the spray equipment and the material being sprayed. Other preferred powders made in accordance with the present invention for coating bio-implants are composites containing hydroxylapatite and plastic or a cobalt alloy. Composites of titanium alloys and plastics are also particularly preferred for use in forming coatings on bio-implants.

[0045] It is to be understood that the present invention encompasses not only the aforementioned method of forming binder-free composite particles, but also the particles and powders formed by this process, as well as the methods of spraying these particles to form coatings and coatings formed by the process of the present invention. The above powders may also be suitable for application by non-thermal spray methods (e.g. compaction and sinking, hot isostatic pressing, etc.).

Claims

1. A method of forming a binder-free agglomerated powder, which comprises:
   placing first and second materials in a drum of a mechanical agglomerator, the drum having a continuous curved inner wall and the mechanical agglomerator having impact means disposed in the drum adjacent the drum inner wall and means for providing relative movement between the impact means and the drum inner wall;
   processing the first and second materials in the mechanical agglomerator by centrifugally forcing the first and second materials between the impact means and the drum inner wall such that forces of shear and compression cause the first and second materials to agglomerate to form agglomerated particles which are composites of the first and second materials; and
   classifying the agglomerated particles to form a thermal spray powder fraction.

2. A method according to claim 1, wherein the first material is a first metal selected from elemental metals and alloys thereof.

3. A method according to claim 2, wherein the first material is a metal selected from Fe, Ni, Co, Cu, Cr, Al, Ti and alloys thereof.

4. A method according to any of claims 1 to 3, wherein the second material is a second metal selected from elemental metal and alloys thereof.

5. A method according to claim 4, wherein the second material is a metal selected from Al, Ti, Ta, Mo, Si, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Hf, Co, Ni, Fe, and alloys thereof.

6. A method according to any of claims 1 to 3, wherein the second material is a ceramic.

7. A method according to claim 6, wherein the ceramic is selected from oxides, carbides, borides, silicides, silicates, phosphates, spinels, titanates, perovskites, forms of carbon and combinations thereof.

8. A method according to claim 7, wherein the ceramic is selected from phosphates of calcium, oxides, spinels, titanates, perovskites and combinations thereof.

9. A method according to claim 8, wherein the oxides are selected from fully or partially stabilized zirconia and aluminium oxides.

10. A method according to claim 3, wherein the ceramic is a machineable ceramic.

11. A method according to claim 3, wherein the ceramic is a hydroxylapatite.

12. A method according to any of claims 1 to 11, wherein there is also added to the drum a third material.

13. A method according to claim 12, wherein the third material is a plastic.

14. A method according to claim 13, wherein the plastic is selected from polyimides, polyesters, PEEK, PEK, and liquid crystal polymers.

15. A method according to any of claims 1 to 14, wherein

(a) the first material is a powder having an average particle size of from about 44 µm to about 100 µm; and/or (b) the second material is a powder having an average particle size of from about 1 µm to about 20 µm; and/or (c) the third material, if present, is a plastic which is a powder having an average particle size of from about 44 µm to about 100 µm.

16. A method according to claim 1, wherein

(a) the first material is a plastic and the second material is a ceramic; or

(b) the first material is a metal and the second material is a plastic; or

(c) the first material is a ceramic and the second material is a plastic; or

(d) the first material is a plastic and the second material is a metal.

17. A thermal spray powder, which comprises mechanically agglomerated particles having a first component and a second component, substantially all of the particles ranging in size from about 0.5 µm to about 177 µm and the powder having an average particle size of from about 44 µm to about 150 µm.

18. A powder according to claim 17, wherein

(a) the first component is a first material as defined in claim 2 or 3; and/or

(b) the second component is a second material as defined in any of claims 4 to 11.

19. A powder according to claim 17 or 18, wherein there is also present a third component.

20. A powder according to claim 19, wherein the third component is a third material as defined in claim 13 or 14.

21. A powder according to claim 17, wherein the first component and/or second component and/or (if present) third component is/are the first, second and third materials, respectively, as defined in claim 16.

22. A method of forming a thermal spray coating, which comprises providing a thermal spray powder fabricated by mechanical agglomeration in a drum having an impact member; and thermal spraying said powder onto a target to form a coating.

23. A method according to claim 22, wherein the thermal spray powder is fabricated by a method as defined in any of claims 1 to 16.

24. A method of forming a coating on a bio-implant, which comprises:
   providing a bio-implant;
   placing first and second materials in a drum of a mechanical agglomerator, the drum having a continuous curved inner wall and the mechanical agglomerator having impact means disposed in the drum adjacent the drum inner wall and means for providing relative movement between the impact means and the drum inner wall;
   processing the first and second materials in the mechanical agglomerator by centrifugally forcing the first and second materials between the impact means and the drum inner wall such that forces of shear and compression cause the first and second materials to agglomerate to form agglomerated particles which are composites of the first and second materials;
   classifying the agglomerated particles to form a thermal spray powder fraction; and
   thermal spraying the powder with a thermal spray gun to form a coating on the bio-implant.

25. A method according to claim 24, wherein the thermal spray powder is obtained by a method as defined in any of claims 1 to 16.

26. A method according to claim 24, wherein the first material is a titanium alloy and the second material is hydroxylapatite.

Drawing