Patent Publication Number: US-2005132843-A1

Title: Chrome composite materials

Description:
TECHNICAL FIELD  
      This invention relates generally to chrome composite powders and methods for producing the chrome composite powders.  
     BACKGROUND  
      Wear and corrosion resistant materials are of great interest to many industries, including, but not limited to, the heavy machinery, automobile, and aerospace industries. Even as demand increases for complex machined components with long life/duty cycles, low maintenance requirements, and improved performance under harsh conditions, a parallel objective exists to achieve these ends at reduced cost to both the industry and the consumer. Achieving satisfactory wear and corrosion resistance in today&#39;s complex materials has typically required the use of relatively expensive starting/raw materials in combination with lengthy and complex processing techniques. While this route can produce wear and corrosion resistant materials, it can be costly.  
      For example, chrome carbide types of powders, such as Cr 3 C 2  or Cr 7 C 3  are widely used in applications for wear and corrosion resistant coatings, as disclosed in U.S. Pat. No. 6,254,704,. These Cr 3 C 2  or Cr 7 C 3  containing powders are typically made by combining pure, high carbon content precursors, which may be high cost materials, with a nickel material. The resulting chromium carbide materials may be used to generate the chrome carbide powders. A disadvantage of this process is that the resulting chrome carbide powders are costly. The high cost of the chrome carbide powders may be due to the use of expensive high carbon content chrome carbide precursor materials, which are generally produced only on small scales for specialty applications.  
      Using known processing techniques, these high cost chrome carbide powders may be used to produce wear and corrosion resistant materials such as, for example, complex chrome composite powders and coatings. While these materials may offer suitable performance, their substantial cost of production may be prohibitive for many applications.  
      The invention is directed to overcoming one or more of the problems or disadvantages existing in the prior art.  
     SUMMARY OF THE INVENTION  
      One aspect of the invention includes a method of making a composite chrome powder. The method includes selecting a ferrochrome material. The ferrochrome material is mixed together with a nickel-based material, and a composite chrome powder is generated from the mixture.  
      A second aspect of the invention includes a chrome composite powder. The composite powder includes a plurality of particles, wherein at least some of the particles include a carbide-metal matrix composite structure, which has a matrix material of at least one of nickel, nickel-chromium, and iron chrome. A plurality of Fe—Cr-carbide particles are dispersed in the matrix material to form the composite structure.  
      A third aspect of the invention includes a composite material. The composite material includes a nickel-based component and a ferrochrome component dispersed within the nickel-based component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a scanning electron microscope (SEM) micrograph illustrating representative morphology of a composite powder consistent with an exemplary embodiment of the invention.  
       FIG. 2  is an SEM micrograph illustrating representative morphology of an as-sintered composite powder consistent with an exemplary embodiment of the invention.  
       FIG. 3  is an SEM micrograph illustrating representative microstructure of a coating made from a composite powder consistent with an exemplary embodiment of the invention.  
       FIG. 4  is an SEM micrograph illustrating representative coating morphology of high carbon ferrochrome composite powder particles clad with nickel consistent with an exemplary embodiment of the invention.  
       FIG. 5  is an SEM micrograph illustrating morphology of a composite powder formed using an atomization method according to an exemplary embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      A method of making a composite carbide powder is provided. The method may include selecting a ferrochrome material and mixing the ferrochrome material with a nickel-based material. The ferrochrome material can be selected from among many materials that include at least some iron, chrome, and/or carbon. For example, in one embodiment, the ferrochrome material may include at least one of (CrFe) 7 C 3 , (CrFe) 23 C 6 , and (CrFe) 3 C 2 . Further, the ferrochrome material may be selected in powder form, ingot form, or any other form suitable for obtaining the ferrochrome precursor material. Similarly, the nickel based material may be provided in powder form, ingot form, or any other suitable form.  
      The ferrochrome material and the nickel-based material may be mixed in a variety of ways. For example, the ferrochrome material and the nickel-based material, especially when in powder form, may be mixed together with a solvent to form a slurry. Alternatively, the ferrochrome material and the nickel-based material may be melted together to form a melt. The disclosed composite chrome powders, ultimately, may be generated from the mixture of the ferrochrome material and the nickel-based material. Various methods for generating the composite chrome powders may be used including, for example, spray drying and sintering, atomization, sintering and crushing, mechanical blending, chemical vapor deposition cladding, electrochemical cladding, mechanical cladding, and mechanical blending.  
      Various additional materials may be added to the mixture of the ferrochrome material and the nickel-based material. For example, activated carbon and/or graphite powder may be added to the mixture. Similarly, in some embodiments, one or more of silicon, titanium, niobium, vanadium, tantalum, molybdenum, tungsten, and manganese may be added. For certain applications, any one of these materials may be limited to no more than 5% by weight of the chrome composite powder, and a combination of these materials may be limited to 10% by weight of the chrome composite powder.  
      The disclosed composite chrome powders may have a variety of particle sizes. In one embodiment, an average particle size of the composite chrome powder may be from about 3 μm to about 500 μm. In still other embodiments, the average particle size may be from about 10 μm to about 60 μm.  
      The weight percentages of the constituents of the ferrochrome material may vary according to the requirements of a particular application. For example, the selected ferrochrome material may include carbon up to about 14 percent by weight. Further, the ferrochrome material may contain iron up to about 65 percent by weight. In certain embodiments, however, the amount of iron may be limited to less than about 35 percent by weight. The amount of chrome in the ferrochrome material may also vary between about 15 percent by weight and about 75 percent by weight. Further, the ferrochrome material may have a iron to chrome weight ratio of from about 0.2 to 0.5 by weight and a carbon to chrome ratio of from about 0 to 0.2 by weight.  
      Similarly, the weight percentages of the constituents of the composite chrome powder may also be varied according to the requirements of a particular application. For example, the composite chrome powder may include carbon up to about 14 percent by weight. Further, the composite chrome powder may contain iron up to about 65 percent by weight. In certain embodiments, however, the amount of iron may be limited to less than about 35 percent by weight. The amount of chrome in the composite chrome powder may also vary between about 15 percent by weight and about 75 percent by weight. The composite chrome powder may also include up to about 35 percent by weight of nickel.  
      A discussion of several exemplary methods for making the disclosed composite chrome powder follows. As noted above, a high carbon ferrochrome-nickel chrome composite powder may be made with a spraying and drying process. Spray drying is a process that transforms a slurry liquid into a powder by spraying the slurry into a heated environment. When the slurry enters the heated environment, the liquid portion of the slurry is vaporized, which leaves behind the solid particles of the powder. Spray drying can be used to produce dense particles with a controlled size distribution.  
      In the spray drying process, the ferrochrome powder may be mixed with a nickel-based powder to form a powder mixture, the powder mixture may be dispersed in a solvent to form a slurry. A composite chrome powder may then be generated from the slurry. The high carbon ferrochrome powder can be obtained from many industrial and steel making powder vendors, including, for example, FW Winter, Chemalloy, among others. The high carbon ferrochrome powder may be, for example, at least one of (CrFe) 7 C 3 , (CrFe)23C6, or (CrFe) 3 C 2 .  
      While the ferrochrome powder and the nickel-based powder may be selected as individual components, it is also possible to obtain the high-carbon ferrochrome powder and nickel-based powder in a pre-mixed form. The powders may be combined, for example, during a milling process, which reduces average individual particle size down to the micrometer range to promote uniform mixing and a fine composite structure in the final product. Milling of the powders can be accomplished, for example, by use of an Attritor mill operating at about 400 revolutions-per-minute (rpm). A tungsten carbide (WC) milling media may be included in the mill in a milling media-to-powder ratio of about 6:1 to about 8:1 (ratio of weight of WC to powder added). A solvent (e.g., acetone or heptane) can also be added for the step of milling the powder mixture to improve powder distribution during milling. The as-milled powder mixture may include a particle size distribution of about 0.1 μm to about 50 μm.  
      The powder mixture may be dried in air under a fume hood or other ventilation system, though the mixture need not be dried in every case. A sieving machine may be used to separate the WC milling media from the powder mixture after milling is complete.  
      To prepare the as-milled powder mixture for spray drying, the powder mixture may be combined with deionized water or an organic solvent (e.g., acetone, heptane, etc.) to form a slurry with a certain solid content.  
      Additional chemicals can be added to the slurry, including a binder (e.g., polyvinyl alcohol, gum Arabic, wax, etc.), a dispensing agent (e.g. sodium metaphosphate), a plasticizer (e.g. glycerine), and a surfactant (e.g. a synthesized detergent) or other anti-foaming agent. The slurry is then spray dried at an elevated inlet temperature (e.g. about 260° C.) to obtain a powder with an average particle size of about 20 μm to about 80 μm.  FIG. 1 , for example, is an SEM micrograph illustrating representative morphology of a composite powder consistent with an exemplary embodiment of the invention. A distribution of spheroid particles  10  and  12  with particle sizes of about 20 μm to about 80 μm, respectively, are visible in  FIG. 1 .  
      Optionally, making the composite carbide powder using a spray drying method may include sintering the as-spray dried powder at an elevated temperature (e.g. about 1100° C. to about 1280° C.) to form a loosely bonded powder body. After sintering, this powder body can be broken and sieved to form the composite carbide powder. Spray drying can produce a powder, as shown in  FIG. 1 , for example, that is relatively porous. Sintering may serve to densify the powder by, for example, forming metallurgical bonds between individual particles inside each powder particle, or agglomerate. Sintering may be carried out in a batch furnace or push furnace in a reducing atmosphere.  FIG. 2 , for example, is an SEM micrograph illustrating representative morphology of as-sintered composite powder particles consistent with an exemplary embodiment of the invention. As-sintered particles  16  and  18  have a rough spheroid surface, indicative of binder material driven out during the sintering process. The sintered powder shown in  FIG. 2  was sintered at 1140° C. for 45 minutes.  
      In an embodiment of the invention, approximately 1% to approximately 2% activated carbon, graphite, or other carbon-containing powder (e.g. a carbonaceous material) may optionally be added to the powder mixture during the milling process. The presence of this activated carbon may promote conversion of a (CrFe) 7 C 3  phase to a higher hardness (CrFe) 3 C 2  phase during sintering. Specifically, the approximately 1% to approximately 2% activated carbon, graphite, or other carbon-containing powder may combine with chrome or other metals during sintering to form a carbide structure in the final composite carbide powder.  
      The disclosed composite chrome powder may have a composite structure. Particularly, in one embodiment, at least some of the particles of the composite chrome powder may include Fe—Cr-carbide particles dispersed within at least one of a nickel, nickel-chromium, or iron chrome matrix. To make the composite structure powder, a metal powder may be mixed with a ferrochrome powder prior to or during the milling process. The composite structure may be formed when the combination of the metal and ferrochrome powders is spray dried. For example, if a metal powder (e.g. nickel (Ni) or chromium (Cr)) is mixed with the high carbon ferrochrome powder before milling the high carbon ferrochrome powder, a composite structure may be produced. This composite structure may include hard Fe—Cr-carbide particles dispersed relatively uniformly in a softer, tougher Ni matrix.  
      Other composite structures may be generated, however. For example, the chrome composite powder may include particles having a ferrochrome core material. A nickel layer and/or a nickel-chromium layer may be clad on the ferrochrome core material to provide a composite structure.  
      In another embodiment for generating a composite chrome powder, a chrome composite powder may be made using a cladding process. Cladding is a process where a material is applied to the surface of another material and at least partially bound to it. Cladding of composite powder particles may be used to coat ferrochrome carbide particles with Ni, Ni—Cr, or Fe—Cr, for example. The cladding technique may be accomplished by decomposition of a precursor, such as nickel-carbonyl, followed by deposition of the Ni, Ni—Cr, or Fe—Cr onto the composite powder particles. This may produce a softer outer layer of Ni, Ni—Cr, or Fe—Cr on a harder carbide particle.  FIG. 4  shows a SEM micrograph illustrating representative morphology of high carbon ferrochrome powder particles clad with nickel consistent with an exemplary embodiment of the invention. The clad particles are irregular rather than spheroid in shape.  
      In even a further embodiment, a ferrochrome-nickel chrome composite powder was made with a gas atomization process.  FIG. 5  shows the exemplary morphology of a composite powder made using gas atomization.  
      The disclosed composite chrome powders may be used to form various composite materials for use in many applications. For example, these composite materials may be used to form stand-alone parts, composite coatings, etc. The composite materials, like the composite powders from which they may be derived, may include a high carbon ferrochrome material combined with a nickel-based material. In the composite materials, the nickel-based material may be distributed between the carbon ferrochrome particles. This may produce a composite material having a composite structure where the ferrochrome material is dispersed in a nickel matrix.  
      Coatings and/or free standing parts using the disclosed chrome composite powders can be made in a variety of ways. Further, coatings made from the disclosed chrome composite powders may be applied to a variety of objects/substrates (e.g. a carbon steel). For example, powders may be used to form coatings on substrates with any of a variety of application methods including thermal spray processes (e.g., plasma spray, flame spray, HVOF, HVAF, detonation gun spray, and cold spray), laser cladding, plasma welding (e.g., PTA), and sintering (e.g., as associated with one or more powder metallurgy processes).  FIG. 3  shows an SEM micrograph illustrating representative microstructure of a coating made from a composite powder consistent with an exemplary embodiment of the invention.  FIG. 3  is a plan view of a cross section of a coating, showing high carbon ferrochrome powder particles  20  dispersed in nickel matrix  22  over a carbon steel substrate (not shown).  
     EXAMPLE 1  
      In one exemplary embodiment of the invention, a powder mixture was formed by placing 80% high carbon ferrochrome powder (˜325 mesh, corresponding to about 45 μm and smaller individual particle diameter) and 20% carbonyl nickel powder (also ˜325 mesh) into an Attritor mill and wet milling the mixture for approximately six hours at 400 revolutions-per-minute (rpm). A tungsten carbide (WC) milling media was included at a milling media-to-powder mixture ratio of about 6:1 to about 8:1 (ratio of weight of WC to powder added). The powder mixture was milled to an average particle size of about 2 μm. A clear acetone solvent was added for the step of milling the powder mixture.  
      After milling, a sieving machine was used to separate the WC milling media from the powder mixture. Then the clear acetone solvent was poured out, and the milled powder mixture was dried by low temperature baking under a fume hood.  
      A 70% solid content slurry was then prepared for spray drying by combining the powder mixture with deionized water, 1% polyvinyl alcohol, sodium metaphosphate, glycerine, and a synthesized detergent. The slurry was then spray dried at about 260° C. to obtain a powder with an average particle size in the range of about 25 μm to about 70 μm.  
      The as-spray dried powder was then sintered in a batch furnace in a reducing atmosphere at about 1100° C. to about 1280° C. to form a loosely bonded powder body. After sintering, the loosely bonded powder body was broken and sieved to form the final composite carbide powder.  
     EXAMPLE 2  
      In a second exemplary embodiment of the invention, a powder mixture was formed by placing 80% high carbon ferrochrome powder (˜325 mesh, corresponding to about 45 μm and smaller individual particle diameter) and 20% carbonyl nickel powder (also ˜325 mesh) into an Attritor mill and wet milling the mixture for approximately six hours at 400 revolutions-per-minute (rpm). A tungsten carbide (WC) milling media was included at a milling media-to-powder mixture ratio of about 6:1 to about 8:1 (ratio of weight of WC to powder added). The powder mixture was milled to an average particle size of about 2 μm. A clear heptane solvent was added for the step of milling the powder mixture.  
      After milling, a sieving machine was used to separate the WC milling media from the powder mixture. Then the clear solvent was poured out, and the milled powder mixture was not dried prior to preparation for spray drying.  
      A 70% solid content slurry was then prepared for spray drying by combining the powder mixture with deionized water, 1% polyvinyl alcohol, sodium metaphosphate, glycerine, and a synthesized detergent. The slurry was then spray dried at about 260° C. to obtain a powder with an average particle size of about 25  82  m to about 70 μm.  
      The as-spray dried powder was then sintered in a batch furnace in a reducing atmosphere at about 1100° C. to about 1280° C., after which the loosely bonded powder body was crushed and sieved to form the final composite carbide powder.  
     EXAMPLE 3  
      In a third exemplary embodiment of the invention, high carbon ferrochrome particles were clad with Ni. To clad the particles, a predetermined amount of high carbon ferrochrome powder (˜325 mesh, corresponding to about 45 μm and smaller individual particle diameter) was injected into a reaction chamber and fluidized. Carbonyl nickel, which is an organic precursor, was added to the reaction chamber after the fluidized bed was heated. The carbonyl nickel decomposed and a thin Ni film was deposited on the individual powder particles. Operating conditions were adjusted so that a predetermined percentage of Ni was deposited on the composite powder particles.  
     EXAMPLE 2  
      In a fourth exemplary embodiment of the invention, high carbon ferrochrome nickel powder was made by a gas atomization method. A high carbon ferrochrome ingot was melted along with nickel. The raw material proportion was controlled such that the final powder contained about 20% by weight of nickel, with the balance of high carbon ferrochrome. The melt was fed through a nozzle and atomized with and inert gas (e.g., nitrogen or argon).  FIG. 5  is a SEM picture of the composite powder generated.  
      Industrial Applicability  
      The disclosed high carbon ferrochrome precursor materials may be used to produce composite powders for applications including coating of engine parts, cylinders, rods, bearings, joints, cam shafts, axles, etc. These ferrochrome materials may be selected from among those materials commonly used for making stainless steel and tool steel. Thus, these materials may be low cost materials. Use of this low cost precursor may translate into significant cost reduction over existing methods. In fact, based on the cost of the precursor materials, powders produced using carbon ferrochrome precursors may cost less than half as much as powders and coatings produced using known materials and methods. Despite the lower cost, coatings made using the disclosed composite powders may offer similar or better wear and corrosion resistant properties as the existing materials. These composite powders may be used in any industry where wear and corrosion resistant properties are desired.  
      Various composite materials were generated using the disclosed composite powders. For example, coatings were made using various systems including a Sulzer MetcoDiamond Jet HVOF system, a Praxiair JP 5000 HVOF system, a Demoton DSP detonation spray system, and Dolore Stellite 300M PTA system on steel substrate. These coatings included Knoop hardness values of between about 950˜1200 HK at 100˜300 gram load. These coating went through wear and friction evaluation and exhibit superior wear resistance.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the described powders, coatings, and methods of making powders and coatings, without departing from the scope of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims and their equivalents.