Method of sintering using polyphenylene oxide-coated powdered metal

Abstract: A polymeric thermoplastic coating material is provided for coating powdered materials and, more particularly, for coating powdered metals which are formed into parts and sintered, so as to form, for example, magnetic cores. The thermoplastic material is polyphenylene oxide which, when properly applied to metal particles to form a magnetic core, is characterised by being sufficiently volatile so as to prevent the formation of contaminants or voids within the sintered article which would be detrimental to the physical properties of the sintered article. Moreover, polyphenylene oxide provides sufficient lubrication and adhesion between adjacent metal particles during an initial compaction process so as to sustain the desired shape of the moulded article and to maximise metal particle density without the use of additional lubricants, thereby preventing the formation of additional contaminants and/or voids within the resultant sintered article from such additional lubricants.

Description: The present invention generally relates to coating metal particles with a polymer for purposes of forming solid metallic articles, such as by sintering. More particularly, this invention concerns an improved coating system, wherein the process of sintering articles from the coated metal particles is facilitated by the use of polyphenylene oxide as the polymer coating. The use of powdered metals, and particularly iron and its alloys, is known for forming permanent magnets, such as soft magnetic cores for transformers, inductors, AC and DC motors, generators, and relays. An advantage to using powdered metals is that forming operations, such as compression or injection-moulding and/or sintering techniques, can be used to form intricate moulded part configurations, such as magnetic cores, without the need to perform additional machining and piercing operations. As a result, the formed part is often substantially ready for securing within its working environment as formed by the moulding process. Before sintering, the powdered metals are moulded by techniques such as compression or injection-moulding. Moulded magnetic cores for AC applications must have low core losses; therefore, the individual metal particles must be electrically insulated from each other. Numerous types of insulating materials, which also act as the binder required for moulding, have been suggested by the prior art, including inorganic materials such as iron phosphate and alkali metal silicates. A list of the different organic polymeric materials suggested by the prior art is extensive and includes amber, phenol-aldehyde condensation products, varnishes formed from China-wood oil and phenol resin, resinous condensation products of urea or thiourea and their derivatives with formaldehyde, polymerised ethylene, butadiene, acrylic acid esters and their derivatives, copolymers of two or more of the above polymers as well as fluorine-type polymers, radical polymerizable monomers such as styrene, vinyl acetate, vinyl chloride, acrylonitrile, divinyl benzene, N-methylol acrylamide, silicones, polyimides, fluorocarbons and acrylics. In addition, it is known to coat a powdered metal with an inorganic undercoating and then provide an organic topcoat. Whilst the above materials generally provide adequate insulation and adhesion between metal particles upon moulding, additional properties are often desirable of a coating material. One such property is the ability to provide lubrication during the moulding operation so as to enhance the flowability and compressibility of the particles, and therefore enable the particles to attain maximum density and strength. This is true not only for moulded articles, where as-moulded strength and density are obviously required, but also for sintered applications, where the moulded articles are further consolidated to attain even greater strength and density. It is often preferable to sinter the magnetic core after the moulding process, such as where the solid magnetic core is intended for DC applications. Sintering fuses the iron particles together to form a solid moulded article and removes the polymer coating through volatilisation. As a result, in addition to the abilities described above, the coating must be capable of being volatilised completely without leaving contaminants and voids within the core. The presence of contaminants or voids within the sintered article reduces the physical strength and properties of the sintered article. In addition, a significant shortcoming associated with the use of the prior-art coatings has been that the coatings will not burn off completely during sintering, thereby leaving a carbonaceous residue within the sintered article. This residue may actually diffuse into the underlying metal particle, causing some degree of deleterious carburization within the sintered article. Polyetherimide, polyethersulphone and polyamideimide have been found to perform well as the coating material for powdered iron, so as to form insulated magnetic cores, particularly with respect to the ability to bind the iron particles together and to resist thermal and chemical attack, and the ability to serve as a lubricant during the compression moulding process. In addition, these polymers adhere well to the underlying metal particle. These polymers are applied to the iron particles using a fluidized bed process which is known in the art. However, there are shortcomings associated with the use of these polymers. Firstly, the above-described polymers may not compression-mould suitably for certain applications due to insufficient lubricity. As a consequence, the magnetic cores may have unsuitably low densities which corresponds to lower magnetic permeability. In addition, the magnet cores tend to stick in the mould cavity, which further results in excessive tool wear and damaged parts. The current solutions to these shortcomings include blending lubricants with the powdered iron before moulding and using mould-release compounds, such as graphite, on the mould cavity prior to the moulding cycle. However, the use of lubricants and mould-release compounds may further reduce the density of the magnetic core and may introduce contaminants, such as carbon, into the material. During sintering, the presence of contaminants can cause voids or stress-risers to be formed within the sintered article, or the contamination may diffuse into the underlying metal particle so as to detrimentally affect the properties of the alloys. In addition, the above-described polymers tend not to volatilize completely upon sintering, therefore adding additional contaminants and/or voids to the sintered article. Making sintered articles from a moulding composition comprising particulate material mixed with a relatively high proportion of preferably 40 - 50 vol% binder, which, inter alia, can be polyphenylene oxide and which is removed by heating the body shaped from the moulding composition, is disclosed in EP-A-329 475. Thus, it would be desirable to provide a coating for powdered metals which has the ability to improve lubrication during the moulding process and to provide adhesion of the metal particles after moulding, so as to attain maximum density and strength of the as-moulded article. In addition, the coating should be readily and cleanly volatilised upon heating during a sintering process, so as not to leave contaminants or voids within the sintered article. Further, it would also be desirable if such a coating could be readily used for sintering of a variety of materials. A method for forming a sintered article according to the present invention is characterised by the features specified in claim 1. Preferred features of the invention are mentioned in the dependent claims. It is an object of this invention to provide a coating material for metal particles, wherein the coating material is sufficiently volatile so as to substantially prevent the formation of contamination or voids within a moulded article at elevated temperatures, such as during sintering. It is a further object of this invention that such a coating material should also serve as a lubricant to facilitate the initial moulding processes of the metal particles prior to sintering, so as to enable maximum density of the moulded article produced thereby. It is yet another object of this invention that such a coating material be capable of sufficiently adhering the metal particles together after the moulding process so as to permit further handling or use of the moulded article prior to sintering. In accordance with a preferred embodiment of this invention, these and other objects and advantages are accomplished as follows. According to the present invention, there is provided a polymeric material for coating powdered metals, wherein the polymeric material is sufficiently volatile at elevated temperatures to prevent the formation of contaminants or voids within a sintered article which was originally formed from the coated metal particles. The preferred coating material also provides sufficient lubrication and adhesion between adjacent metal particles during a compaction-moulding process performed prior to sintering. The above capabilities are particularly advantageous for the manufacture of sintered articles which are initially moulded from the coated particles. The polymeric material found most suitable to provide the above features is polyphenylene oxide. Where an article moulded from the coated particles is to be sintered so as to fuse the metal particles together, such as with DC magnetic cores, polyphenylene oxide is readily volatilised at the elevated sintering temperatures, thereby preventing the formation of contaminants or voids within the sintered article which would reduce the physical properties of the sintered article. In addition, polyphenylene oxide is sufficiently lubricous during the initial moulding step that additional lubricants or mould-release compounds may be eliminated, therefore preventing the formation of additional contaminants or voids in the subsequently sintered article. Polyphenylene oxide can achieve the above advantages whilst being present in relatively low quantities, i.e., less than about one weight percent as compared to the mass of the metal particle. The preferred coating process for purposes of the present invention is a Wurster-type spray-coating fluidized bed of the type known to those skilled in the art, though other coating methods may be used. The fluidized bed serves to recirculate the metal particles within a confined volume numerous times, until each particle has acquired a substantially uniform coating of polyphenylene oxide which is sufficient for purposes of the particular application. The coated metal particles may then be introduced into a suitable moulding apparatus, such as a compression or injection-moulding machine, where the coated metal particles are compressed or injected within a heated mould cavity under a suitably high pressure to compact the coated metal particles to produce a dense, strong and solid article. The article is then appropriately sintered. In addition, the materials and teachings of this invention are readily applicable to a variety of moulding processes used prior to sintering, such as compression or injection-moulding, as well as hot isostatic pressing, cold isostatic pressing, microwave and ultrasonic moulding techniques, as well as others. Other objects and advantages of this invention will be better appreciated from the following detailed description. A polymeric coating material is provided for coating powdered materials and, more particularly, for coating powdered metals which are moulded and sintered under pressure, so as to form, for example, magnets for such applications as AC and DC magnetic cores used in the automotive industry. It is to be noted, though, that the teachings of this invention would extend to the formation of a variety of moulded and sintered articles. According to the present invention, the polymeric material is polyphenylene oxide, which is known in the art by its tradename PPO, an engineering thermoplastics material available from the General Electric Company, U.S.A.. Polyphenylene oxide is characterised by excellent mechanical properties and dielectric characteristics and is useful at a temperature range of greater than about 190°C, as generally determined by a standardised heat-deflection temperature. Polyphenylene oxide is soluble in some aromatic and chlorinated hydrocarbons, thereby permitting its use in the fluidized coating process. All of the above characteristics are advantageous in view of the coating, moulding and sintering processes utilised by the present invention. Furthermore, polyphenylene oxide is insoluble in alcohols, ketones, aliphatic hydrocarbons and water, and is highly resistant to hydrolysis, acids, bases and detergents, thereby making polyphenylene oxide substantially impervious to chemical attack. According to the present invention, when properly applied to metal particles which are compacted to form a moulded article, such as a magnetic core, polyphenylene oxide provides sufficient adhesion between adjacent metal particles to sustain the desired strength and shape of the magnetic core after moulding. Furthermore, it has been determined that the polyphenylene oxide present within the moulded article can be cleanly volatilised therefrom, thereby alleviating the formation of contaminants or voids within the article. Such a capability is advantageous where it is desirable to sinter the moulded article, such as in the case of magnetic cores used in DC motors, so as to fuse the metal particles directly together and thereby improve the physical properties of the sintered magnetic core. Specifically, contaminants and voids within a sintered article would significantly reduce the strength and performance of the sintered article. Because the polyphenylene oxide readily volatilizes at the sintering temperatures, magnetic cores which are compression-moulded from polyphenylene oxide-coated metal particles and then sintered to fuse the metal particles together, produce a physically strong and clean article having high density. Moreover, polyphenylene oxide provides improved lubrication between metal particles during the moulding process prior to sintering. This not only maximises metal particle density, but also, when used in accordance with this invention, polyphenylene oxide can eliminate or reduce the requirement for additional lubricants to be present during moulding. This capability is contrary to the prior art, which must often resort to lubricant additives to enable the metal particles to readily flow into the mould cavity and compact together during the moulding process. Polyphenylene oxide is able to achieve the above advantages whilst being present in low quantities, such as below about one weight percent and most preferably in the range of about 0.40 to about 0.75 weight percent. Though it is foreseeable that greater quantities of polyphenylene oxide could be used, a corresponding reduction in physical properties and/or magnetic properties of the moulded article would result in cases where the moulded article is used in the as-formed condition. The balance of the moulded article, about 99 weight percent, consists of metal particles sized preferably in the range of about 5 to about 400 micrometres, and more preferably in the range of about 125 to about 350 micrometres, to attain magnetic cores of high permeability greater than about 500 GaussOersteds at 300 Hz. The preferred method of coating the metal particles utilises a Wurster-type spray-coating fluidized bed of the type known to those skilled in the art, although other methods which produce a uniform coating on the particles could also be used. The fluidized bed essentially includes a concentric pair of upright cylindrical vessels, one within the other. The outer vessel has its lower axial end closed to form a floor for the outer vessel only, with the inner vessel being suspended above this floor. The floor has perforations of various sizes through which heated air is drawn through both vessels. The perforations are sized such that the majority of the air flow will occur up through the inner vessel and then down between the inner vessel and the outer vessel. Prior to introduction into the fluidized bed, it is preferred, but not necessary, that the metal particles be pre-sorted according to size to promote substantially uniform coating thicknesses on the metal particles during the coating process. The metal particles are first sorted into batches of approximately same-sized particles (e.g., small, medium and large). Each batch is then separately processed and later remixed into any desired particle size distribution. If the above steps are not taken, there is a tendency for the larger and smaller particles to be preferentially coated, leaving the mid-sized particles with a significantly thinner coating. At start-up, a batch of the powdered metal is deposited on the floor and the powder to be coated is circulated at a rate sufficient to coat the particles. According to the batch size and particles sizes, the flow rate of the air will generally be in the range of about 100 to about 200 cubic metres per hour. Also, the air temperature will generally range between about 55°C and 70°C when the coating process begins, but will vary during the coating process with the introduction of the metal particles and as the solvent evaporates. If the air temperature is too low, the solvent will not evaporate upon contact with the metal particle, thereby resulting in a poorly-coated particle, whilst if the air temperature is too high, the solvent evaporates too quickly, thereby also preventing the formation of a uniformly-thick coating on the particles. As the coating process progresses, each of the particles is randomly coated an extraordinarily large number of times, so as to ensure a uniformly-thick coating on the particle. A spray nozzle located on the floor under the inner chamber serves to introduce the polyphenylene oxide, which is dissolved in an appropriate solvent, into the chamber. According to the processing method of this invention, the preferred solvent is chloroform (CHCl 3 ), though other suitable solvents could be used, such as methylene chloride (CH 2 Cl 2 ), monochlorobenzene (C 6 H 5 Cl), and trichloroethylene (CHCl:CCl 2 ). The solution is preferably about 5 to about 15 weight percent polyphenylene oxide, and more preferably about 10 weight percent polyphenylene oxide, so as to maximise the efficiency of the coating procedure, though suitable coating results can be obtained with an extremely large range of polyphenylene oxide solutions. The solution is then sprayed into the fluidized bed at a rate of about 80 grams per minute for a 304.8 mm (12 inch) diameter fluidized bed, with a rate of about 50 to about 100 grams per minute being the preferred range. The preferred spray pressure is about 4.5 x 10 5 Pa (4.5 bar), with about 4 x 10 5 Pa (4 bar) to about 5 x 10 5 Pa (5 bar) being the preferred range. It is to be noted that the deposition parameters may vary considerably, depending on the solvent and deposition chamber used. In addition, as stated previously, other deposition methods may also be employed so long as a substantially uniform coating is obtained. Once coated, the encapsulated metal particles are recirculated by the action of the heated air between the confined volumes defined by the inner and outer vessels. Circulation is continued until each metal particle has acquired a uniform coat of polyphenylene oxide which is sufficient to produce the desired thickness of polyphenylene oxide, preferably between about 0.1 weight percent and about one weight percent, and more preferably between about 0.40 and about 0.75 weight percent, as noted above. Typically, the coating thickness will be in the range of about 0.3 to about 4.5 micrometres for metal particles in the preferred range of about 5 to about 400 micrometres. Thereafter, the coated metal particles may be introduced into a suitable moulding apparatus, such as a compression-moulding or injection-moulding machine. Typical moulding processes used to form, for example, magnetic cores include compression and injection-moulding and are generally performed at mould temperatures ranging from about 221°C (430°F) and 246°C (475°F) with the particles being pre-heated to about 82 ° C (180°F) and 121°C (250°F). At these temperatures, polyphenylene oxide is sufficiently fluid to flow under pressure during the moulding operation whilst also being sufficiently viscous to adhere to the metal particles and provide a lubricating action between adjacent metal particles. As a result, automated handling equipment can be used to process and feed the polyphenylene oxide-coated metal particles throughout the coating and moulding processes, resulting in shorter cycle times, and yet the compaction-moulded article, such as the magnetic core, formed by these processes is characterised by being physically strong and dense, so as to enable immediate handling and use of the as-formed moulded article, if desired. Per the standardised ASTM test entitled "3 Point Modulus Test" for powder metals, the strength of the composite metal having less than about one weight percent polyphenylene oxide and iron particles is about 778.44N (175 pounds) to 889.64N (200 pounds). In that the metal particles are pre-heated to a temperature of about 82 ° C (180°F) to about 121°C (250°F), and the mould cavity is pre-heated to a temperature of about 221 ° C (430°F) to about 246 ° C (475°F) for moulding, metal particles coated with polyphenylene oxide will readily flow into the mould cavity and, when subjected to typical moulding pressures of about 308.89 MPa (20 tons per square inch) to about 772.22 MPa (50 tons per square inch), will flow sufficiently to become compacted and form a moulded article such as a ferromagnetic core whose density is preferably greater than about 7.25 grams per cubic centimetre. In addition, the lubricous nature of polyphenylene oxide persists after the moulding operation to facilitate removal of the moulded article from the mould cavity. As a result, reliance upon the use of release compounds to facilitate the removal of the magnetic core is reduced or entirely eliminated, therefore alleviating the potential for contamination and/or voids from lubricants and/or release compounds during sintering. The labour necessary to apply such release compounds is also eliminated, in addition to a significant reduction in tool wear and part breakage which are associated with the moulded magnetic core not releasing properly. As a result, the use of polyphenylene oxide as a coating material for metal particles is economically advantageous in that it reduces material and processing costs and downtime. The coating and moulding processes described above can be widely varied to alter the physical and magnetic properties of the moulded article, as is known in the art. Following the above processes, the moulded article is then sintered, such as for magnetic cores for DC motor applications or where very high density and strength articles are necessary. However, for purposes of this invention, it is not necessary that an article be formed using the above processes in that an article could be readied for sintering through other methods known in the art. In addition, it is foreseeable that a single sintering step could be used with a loose quantity of particles coated with polyphenylene oxide, eliminating completely the above compaction process. The applications for which sintering is advantageous encompass the use of many types of sinterable particles, such as copper and its alloys, aluminium and its alloys, stainless steel, nickel and its alloys, lead and its alloys, rare-earth-iron-boron alloys and ceramic materials, or any other material which may be sintered. However, because polyphenylene oxide volatilises at generally about 427 ° C (800°F) to about 482 ° C (900°F), the material for the particles must be capable of sintering at or above this temperature. The ability for polyphenylene oxide to be cleanly volatilised is advantageous for use in numerous applications other than the manufacture of magnetic cores. Such applications are entirely within the scope of this invention in that, as an advantageous result of using polyphenylene oxide as the polymeric coating material, the moulded articles formed according to the method of the present invention will be typified by being very dense and strong, thereby permitting the formation of a variety of configurations, including thin-walled, complex configurations. As a specific but not limiting example, iron powder particles commercially available from Quebec Metal Powders, U.S.A. (1001HP iron powder) were coated, moulded and sintered in accordance with the method of this invention. The particle sizes of the iron particles may range from about 44 to about 250 micrometres. However, a very small percentage of the powder may have a particle size as small as 10 micrometres. The powder is about 99.7% Fe, 0.003% C, 0.0005% N, 0.006% S and 0.004% P. The iron powder particles are then coated with the preferred thermoplastic material, polyphenylene oxide, using the above-described fluidized coating method, to a thickness corresponding to an amount of between about 0.1 and about one weight percent as compared to the total mass of the particles. After the iron particles have been coated, a quantity of the coated iron particles is fed into a die mould of a press. The coated iron particles are pre-heated to a temperature in the range of about 82 ° C (180°F) and about 121°C (250°F), and the die mould is heated to a temperature of between about 221 ° C (430°F) and 246°C (475°F). With the coated particles in the die mould, it is compressed at a pressure of about 617.77 MPa (40 tsi) to about 772.22 MPa (50 tsi) for a sufficient duration of time, such as up to about 10 seconds. The thermoplastic polyphenylene oxide material takes on a tacky state during this operation. During the compression-moulding step, the polyphenylene oxide operates as a lubricant which increases the density of the moulded article. The density will exceed about 7.4 grams per cubic centimetre and is substantially uniform throughout the article. After the article has been compression-moulded as described, the compressed article is sintered at a temperature of between about 1093 ° C (2000°F) and 1149 ° C (2100°F), preferably about 1121 ° C (2050°F), for about 15 to about 45 minutes. The polyphenylene oxide is burned off during the high temperature sintering operation since its volatilisation temperature is generally about 427 ° C (800°F) to about 482 ° C (900°F), leaving virtually no contaminants within the sintered article. The iron powder particles no longer have a coating and fuse together to form a dense, strong sintered article. From the above, it will be apparent to one skilled in the art that a significant advantage of the present invention is that there is provided a polymeric coating for powdered metals, wherein the polymeric coating material possesses numerous properties which are beneficial to forming a sintered article, such as a magnetic core from a powdered metal. These properties include the ability to serve as a lubricant and as an adhesive during the initial moulding steps, and being sufficiently volatile at sintering temperatures to prevent the formation of contaminants or voids within the magnetic core formed with sintering of the powdered metal. In particular, where a magnetic core is to be sintered so as to fuse the metal particles thereof together, such as for DC magnetic core applications, the polyphenylene oxide is readily volatilised without leaving contaminants or voids within the sintered article which would otherwise reduce the physical strength of the magnetic core. The polyphenylene oxide coating also provides lubrication between the metal particles to enable higher core densities to be obtained by the moulding process and provides adhesion of the metal particles after moulding to impart sufficient strength so as to permit normal handling and, where appropriate, to permit immediate use of the moulded article. The polyphenylene oxide coating thus can alleviate the requirements for additional lubricants and/or mould-release compounds during moulding, which correspondingly prevents the formation of additional contaminants within the sintered article from these sources. In addition, the method of this invention could be employed to sinter coated particles together to form a solid article using a variety of metals and their alloys or ceramic materials. Foreseeably, almost any type of particulate material could be coated and sintered appropriately. It is also foreseeable that the polyphenylene oxide blend, known by its tradename "Noryl", which is also available from the General Electric Company, U.S.A., could be used successfully as a substitute for polyphenylene oxide. However, Noryl does not exhibit the physical and chemical properties to the level at which polyphenylene oxide is capable, and thus would be expected to provide results somewhat inferior to those obtained with polyphenylene oxide. Therefore, whilst the present invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art; for example, by modifying the processing parameters such as the temperatures or pressures employed, or by substituting appropriate powdered materials, or by utilising the particular materials and methods for use in an alternative application. Accordingly, the scope of the present invention is to be limited only by the scope of the following claims. Reference is made to the disclosures in United States patent application no. 915,587, from which this application claims priority, and in the abstract accompanying this application.

Claims: A method for forming a sintered article comprising the steps of: depositing a substantially uniform encapsulating layer of a fugitive binder on each of a plurality of particles such that said plurality of coated particles comprises less than about one weight percent of said binder; compacting said plurality of particles to form a moulded article; and sintering said moulded article so as to substantially volatilise said binder from said moulded article and to fuse said plurality of particles together, characterised in that said binder consists essentially of polyphenylene oxide. A method according to claim 1, in which each of said particles is a ferromagnetic material. A method according to claim 1 or 2, in which said layer of polyphenylene oxide is deposited to a thickness of about 0.3 to about 4.5 micrometres on each of said particles. A method according to any one of claims 1 to 3, in which each of said particles has a size range of about 5 to about 400 micrometres. A method according to any one of the preceding claims, in which said layer of polyphenylene oxide is deposited on said particles using fluidized bed spray methods. A method according to any one of the preceding claims, in which said step of compacting occurs within a mould cavity at a temperature and pressure which are sufficient to adhere said particles together with said polyphenylene oxide. A method according to any one of the preceding claims, in which said polyphenylene oxide comprises about 0.4 to about 0.75 weight percent of said particles after said depositing step. A method for forming a sintered article according to claim 1, which comprises the steps of: depositing a substantially uniform encapsulating layer of polyphenylene oxide on each of said particles so that said polyphenylene oxide comprises about 0.4 to about 0.75 weight percent of said coated particles, said particles ranging in size from about 5 to about 400 micrometres; compacting said particles within a mould cavity at a temperature and pressure which are sufficient to compact and adhere said particles together with said polyphenylene oxide to form a moulded article; and sintering said moulded article at a temperature so as to substantially volatilise said polyphenylene oxide from said moulded article and to fuse said plurality of particles together. A method according to claim 8, in which each of said particles is a ferromagnetic material. A method according to claim 8 or 9, in which said moulded article is sintered at a temperature of at least 1093°C (2000°F). A method according to claim 8 or 9, in which said layer of polyphenylene oxide is deposited on said particles using fluidized bed spray methods. A method according to claim 8, in which said mould cavity is heated to a temperature of about 221 ° C (430°F) to about 240 ° C (475°F), and wherein said pressure for compacting said particles is about 308.89 MPa to 772.22MPa (20 to 50 tons per square inch).

Inventors: GAY DAVID EARL, WARD ROBERT WALTER, GAY, DAVID EARL, WARD, ROBERT WALTER
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1993
Main classification: B22F1
IPC codes: B22F1, H01F1