Abstract:
An improved method for producing a uniform distribution of substances within a powder mixture is provided. By control of pH, a metal containing substance is precipitated on a powder and converted into a coating to form a relatively homogeneous powder mixture. The powder mixture may be pressed and sintered to form a composite powder.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to processes for forming coated powders that in turn may be used to form composite powders. The use of composite powders is well known in many fields and is particularly well known in the field of powder metallurgy. In this field, one often uses composite powders in applications in which strength and durability are concerns. For example, composite powders may be used to form cutting tools, mining tools and wear parts.  
           [0002]    In developing a composite powder for use in powder metallurgy, typically one will start with a powder mixture, which is a combination of two or more substances, and then press and sinter the powder mixture. As used herein, the phrase “composite powder” refers to powder mixtures that have been pressed and sintered. When forming the powder mixture it is important to obtain a mixture that is as uniform and homogeneous as possible. If the components of the mixture are not uniformly dispersed in each other and a heterogeneous mixture is produced, the performance of the final product may be diminished.  
           [0003]    The ability to produce a homogenous mixture is dependent upon a number of factors, such as the mixing method and the components&#39; particle sizes and concentrations. However, even when the components are thoroughly mixed, there can be isolated regions in a powder mixture that are devoid of one or more of the components. Additionally, during handling and processing, there can be segregation. The amount of segregation that can take place is dependent in part upon the density differences and particle sizes of the various components in the powder mixture. Thus, the development of ways to increase the homogeneity of the mixture and to maintain this homogeneity is desirable.  
           [0004]    Powder mixtures have traditionally been made by mixing two or more distinct components together through the use of blenders, ball mills, attrition mills and/or vibratory mills. Although these milling techniques are advantageous, they may take several hours or several days. Thus, they can be both time and labor intensive. Further, they are only as efficient as the particular milling device used and do not address the problem of post-mixing segregation.  
           [0005]    A number of other methods have been suggested in order to maximize the distribution of the components of a powder mixture. These methods may be used in conjunction with or independent of the aforementioned milling processes. Many of these methods involve forming a coated powder by causing one component to coat another. The phrase “coated powder” refers to an insoluble powder that possesses a coating. The terms “coat” or “coating” refer to the existence of some type of interaction between two or more particles such as the particles to be coated and the particles that coat them. In powder metallurgy, it is common to refer to a metal as coating another substance that itself, may or may not be a metal. The substance that is coated is referred to herein as an “insoluble powder” or a “constituent powder.” 
           [0006]    Insoluble powders are typically not soluble in or have low degrees of solubility in aqueous solutions. Examples of the types of interactions that allow for a coating are adsorption, chemiosorption, surface tension and any type of adhesion.  
           [0007]    One method known in the industry for forming a coating involves placing an insoluble powder into a liquid containing a soluble metal salt and evaporating the liquid, forcing the metal salt to crystallize onto the surface of the insoluble powder. Under this method, metallic salts and organometallic precursors have been used to coat the surfaces of hard insoluble powders such as WC, (Ti,W)C, (Ta, Nb)C, (Ti, Ta, Nb)C, TiC, TaC, NbC, VC or Cr 3 C 2  through crystallization. Typically, a metallic salt or organometallic precursor is dissolved into a polar solvent. A hard insoluble powder may then be added to the solution. The hard insoluble powder and the polar solvent are then thoroughly mixed to form a slurry. Once the slurry is formed, a drying method is used to evaporate the solvent from the slurry. By drying the slurry, the metallic salt or the organometallic precursor crystallizes onto the surface of the insoluble powder. The dried powder is then heat treated under a reducing environment in order to convert the crystallized coating into a metallic layer on the powder. During the drying process, which is required for the crystallization process, the characteristics of the slurry change dramatically, which can make control of the crystallization process difficult. This can lead to a final product that consists of a mixture of partially coated powder and free metallic particles.  
           [0008]    Another method that is well known to persons skilled in the art for coating powders is chemical vapor deposition. In this method, gaseous precursors are used to coat insoluble powders. This method is expensive and requires complicated processing equipment in order to ensure that the powder is finely dispersed during the coating process. Thus, it is not always economical.  
           [0009]    It is also known to coat the surface of carbide powders through precipitation while the substance to be used as a coating and the carbide powder are in solution. For example, it is known to coat cobalt or nickel onto carbide powders by using a polyol solution. Under this method one first suspends a carbide powder in a solvent such as ethylene glycol. To this mixture, a metal compound in the form of a salt, oxide or hydroxide is added and heated. During this process an organometallic compound is formed that will be dried and crystallized on the surface of the particles. The ethylene glycol acts as both a solvent and a reducing agent, causing a metal coating to form onto the surface of the powder while the particles are in solution. Because the formation and reduction of the metal coating occur in solution, a large surplus of ethylene glycol is necessary for this process, and control of the precipitation conditions is difficult; consequently, adjustment of the quality of the coating is also difficult. In addition, volatile by-products that must be removed by distillation are produced. These factors tend to make this process expensive and useful in only certain applications. For example, it is not flexible enough to be used in both batch and continuous production systems. Finally, in these processes, organic compounds are used as solvents, which renders these systems less environmentally friendly and usable in practice.  
           [0010]    All of the aforementioned methods have a number of disadvantages that render them not ideal for all situations. For example, the chemical vapor deposition method is costly. Further, the crystallization method requires a dynamic event such as drying to control the coating of the powder. This drying process changes the characteristics of the slurry dramatically, which may make the control of crystallization process difficult. This in turn may lead to a final product that consists of a mixture of partially coated powder and free metallic particles. Thus, there is a need for an efficient and rapid method for forming uniform or homogeneous powder mixtures that may be used to form composite materials.  
           [0011]    The present invention provides a cost effective means to coat powders while minimizing the problems of non-uniform component distribution and segregation by providing for a homogenous distribution of components throughout a powder mixture.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides an improved method for forming powders coated with metals. According to the present invention, a metal oxide or metal oxide precursor is chemically precipitated onto a powder such as a carbide powder and subsequently treated to form a coated powder. The coated powder may be pressed and sintered to form a densified component or densified composite material.  
           [0013]    In one embodiment, the present invention provides a method for producing a coated powder comprising a metal layer coated on an insoluble powder. This method comprises:  
           [0014]    a. preparing a solution comprising a metal ion;  
           [0015]    b. combining an insoluble powder with the solution to form a slurry containing constituents;  
           [0016]    c. mixing the constituents of the slurry;  
           [0017]    d. adjusting the pH of the slurry to convert the metal ion into a metallic intermediate that is capable of being precipitated onto the insoluble powder;  
           [0018]    e. precipitating the metallic intermediate onto the insoluble powder to form a precipitate;  
           [0019]    f. recovering the precipitate, said precipitate comprising said insoluble powder and said metallic intermediate; and  
           [0020]    g. treating said precipitate to form a coated powder.  
           [0021]    In a particularly preferred embodiment, the present invention may be used to produce a cobalt coated tungsten carbide by:  
           [0022]    a. dissolving a cobalt salt in hydrochloric acid to form a solution;  
           [0023]    b. adding tungsten carbide to the solution to form a slurry containing constituents;  
           [0024]    c. mixing the constituents of the slurry;  
           [0025]    d. adding ammonia to the slurry to adjust the pH and to form a cobalt intermediate;  
           [0026]    e. precipitating the cobalt intermediate onto the surface of the tungsten carbide to form a precipitate;  
           [0027]    f. washing, filtering and drying the precipitate to form a dried powder; and  
           [0028]    g. heating the dried powder under a hydrogen atmosphere to reduce the cobalt intermediate to cobalt metal, so as to form a cobalt coated tungsten carbide.  
           [0029]    The coated powders that are produced according to the present invention may be pressed and sintered to form composite powders and used in applications in which composite powders made by other processes may be used, such as in cutting tools and wear parts. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0030]    [0030]FIG. 1 is a representation of a high magnification micrograph of several grains of the cobalt coated WC powder.  
         [0031]    [0031]FIG. 2 is a representation of a Scanning Electron Microscope picture of coated WC powder with cobalt (13%).  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    The present invention provides more efficient means for achieving a uniform distribution of the components of powder mixtures than is possible with traditional methods. The processes of the present invention may be used to form both coated powders and composite powders.  
         [0033]    The present disclosure is not intended to be a treatise on either coated powders or the processes for coating powders. Readers are referred to appropriate, available texts and other materials in the field for additional and detailed information on any aspect of practicing this invention.  
         [0034]    The present invention may be used in connection with coating insoluble powders with metals for use as composite materials. Examples of insoluble powders that may be used according to the present invention include carbides, nitrides, and carbonitrides that contain at least one element selected from the group of W, Ti, Ta, Cr, V, Mo and Nb. Powder metals such as Co, Ni, Cu, W, Fe, V, Cr, Mo, and Nb may also be used as insoluble powders. Further, other substances that would be obvious to use to one skilled in the art from reading this disclosure may also be used as insoluble powders. The metals that may be used to form the metal coating include, for example, cobalt, nickel, copper, chromium, vanadium, tungsten, iron and mixtures thereof, as well as other substances that would be obvious to use to one skilled in the art upon reading this disclosure.  
         [0035]    According to the present invention, one prepares a solution that contains ions of the metal that will form the coating on the surface of the powder. The term “solution” means any combination of substances that contains a solvent and a metal ion and includes, without limitation, suspensions, mixtures and other combinations of substances that contain the solvent and metal ion. Preferably, the solution will contain a sufficient amount of metal in the form of ions to form a coated powder that contains the metal in an amount between about 0.1 wt. % and about 30 wt. % based on the weight of the total coated powder. More preferably, the amount of metal will be sufficient to form a coated powder that contains between about 2 wt. % and about 10 wt. % metal based on the total weight of the coated powder. The phrase “metal ion” refers to the ion form of a metal.  
         [0036]    Although various solvents that are known in the industry are suitable for practicing the present invention, preferably the solvent is water, and the solution is an aqueous solution.  
         [0037]    One may prepare the solution according to methods that are discussed below or would be obvious to use to one skilled in the art from reading this disclosure. Preferably, the solution is prepared either by mixing a soluble salt of the desired metal into water or by dissolving the metal into an acid that contains water. Examples of soluble salts that may be used with the former method for preparing the solution include without limitation, cobalt salts such as cobalt chloride, cobalt sulfate, cobalt nitrate, cobalt acetate, or water soluble cobalt organometallic compounds. Examples of acids that may be used according to the latter method for preparation include HCl, H 2 SO 4 , and HNO 3 . Preferably, the solution will have a concentration of less than 180 grams of metal per liter and have a pH range sutiable for maintaining the metal ion in solution. By way of example, a solution comprising cobalt ion may be created by dissolving cobalt metal in hydrochloric acid and water. In this case, the pH of the solution will be less than pH 7, and preferably the pH will be between about pH 3 and about pH 5. For some metal powders, the metal ion solution can be prepared by dissolving the metal powder in a base. One such example is aluminum metal powder dissolved in a base solution composed of NaOH and water.  
         [0038]    The solution that contains the metal ion is then combined with an insoluble powder to form a slurry. Preferably, the combining occurs through mixing. The term “mixing” includes, pouring and/or stirring, as well as any other means or combination of means that enable the formation of a slurry in which the constituents are dispersed in each other. The term “slurry” refers to any combination, suspension or other mixture that contains the solution, which contains the solvent and the metal ion, and the insoluble powder. These different components of the slurry (i.e., the solution and the insoluble powder) are referred to herein as “constituents.” For example, a slurry may be formed by mixing together a solution containing cobalt ions with tungsten carbide.  
         [0039]    After a slurry is formed, the dissolved metal ion is converted into a metallic intermediate by adjustment of the pH of the slurry through the use of an acid or base. The phrase “metallic intermediate” refers to the substance that forms from a reaction between the metal ion and the acid or base and is capable of being precipitated onto the insoluble powder. The metallic intermediate is subsequently precipitated onto the insoluble powder and forms a precipitate. The metallic intermediate will preferably be a metal oxide or metal oxide precursor. The term “precipitate” refers to the substance that forms by precipitating the metal intermediate onto the insoluble powder.  
         [0040]    According to the preset invention, precipitation may be accomplished through the adjustment and control of the pH and temperature. Preferred methods for adjusting pH include the addition of an acid or base, such as ammonia (NH 3 ), oxalic acid ((COOH) 2 ) or alkali metal hydroxides such as potassium hydroxide (KOH) and sodium hydroxide (NaOH). Typically, the pH of the mixture prior to adjustment is such that the metal ion remains soluble. For acid additions, the pH of the mixture is preferably adjusted to less than pH 5, and more preferably between pH 2 and pH 4. For base additions, preferably the pH is adjusted to between pH 7 and pH 10.  
         [0041]    Preferably, the temperature range of the slurry is between room temperature and 75° C. prior to and during precipitation. Although not wishing to be bound by any one theory, it is postulated that the upper limit of the temperature range is the boiling point of the mixture. More preferably, the temperature range is between about 40° C. and about 60° C. Methods for precipitation without adjustment of the temperature may be developed according to the present invention. However, preferably, both the pH and the temperature are controlled. Temperature control is advantageous because it controls the rate of reaction and the coating&#39;s uniformity and quality. The optimum pH range for precipitation will vary slightly with temperature. These variations will be readily apparent to persons skilled in the art.  
         [0042]    For example, a slurry of tungsten carbide and cobalt chloride may be heated to 50° C., and through the addition of ammonia or NaOH, precipitation of cobalt hydroxide onto the surface of the tungsten carbide may be induced.  
         [0043]    After the precipitate is formed, it is recovered. The terms “recovered” and “recovering” refer to the isolation of the precipitate from the other components of the slurry, preferably by filtering, washing and drying to form a dried precipitate. Filtering may, for example, be accomplished by continuous or batch filtration units. Generally, the washing step occurs in combination with the filtration step. Washing may, for example, be accomplished by rinsing with water. Drying may, for example, be accomplished through the use of ovens, hot air dryers, spray dryers, vacuum dryers or other means of water removal. Suitable methods of isolating the precipitate such as by filtering, washing and drying, are well known to persons skilled in the art.  
         [0044]    The dried precipitate is then treated in order to convert the metallic intermediate into a metal coating on the insoluble powder, thereby forming a coated powder. Treatment, by way of example, may be accomplished by heat treating the dried precipitate under a reducing environment. Examples of reducing environments include atmospheres comprising predominantly hydrogen, nitrogen, argon, ammonia, carbon monoxide and/or carbon dioxide. Preferably, the atmospheric pressure of the reducing environment is 1 atm, and the temperature at which reduction takes place is between about 250° C. and about 800° C. This temperature is preferably sufficient to decompose the metallic intermediate coating into metallic form, typically through decomposition. For example, a dried precipitate comprising cobalt hydroxide that has been precipitated on a tungsten carbide powder may be reduced at 500° C. under a reducing environment comprised substantially of hydrogen to convert the coating to cobalt metal.  
         [0045]    The resulting coated powder may be pressed and sintered to form a densified component via methods known in the industry, and may be used in applications that are now known or that come to be known for the use of composite products. The methods for pressing and sintering coated powders are well known to persons skilled the art. For example, a coated powder may be mixed with a pressing agent such as paraffin in heptane and subsequently dried. This powder may be screened, pressed and sintered.  
         [0046]    Under one preferred embodiment of the present invention, one may produce tungsten carbide coated with cobalt metal. In order to create this product, preferably one begins by dissolving a cobalt salt in hydrochloric acid. The cobalt salt may, for example, be cobalt chloride, cobalt sulfate, cobalt nitrate and cobalt acetate. In the presence of hydrochloric acid, or other suitable acid, the cobalt salt will ionize, thereby yielding free cobalt ions. To this solution, one may add tungsten carbide while stirring in order to form a slurry.  
         [0047]    To the slurry, one may add ammonia to form a cobalt intermediate, thereby raising the pH to a pH between approximately 8 and 9 and then adjust the temperature to 50° C. to cause the cobalt intermediate to precipitate onto the tungsten carbide and to form a precipitate. The precipitate is then washed, filtered and dried, to form a dried powder. The dried powder may then be heated to 500° C. under a hydrogen atmosphere, which would reduce the cobalt intermediate to a cobalt metal and form a tungsten carbide powder coated with cobalt metal.  
         [0048]    Composite powders may, for example, be used in the manufacture of densified components made by PM (powder metallurgy) applications, hard-metals, hard-facing and diamond tools.  
       EXAMPLES  
       [0049]    The following examples set forth preferred embodiments of the invention. These embodiments are merely illustrative and are not intended and should not be construed to limit the claimed invention in any way.  
       Example 1  
       [0050]    A starting solution of cobalt chloride (CoCl 2 ) was prepared by dissolving cobalt metal into a solution of hydrochloric acid and water (˜0.14M). The concentrations of the cobalt metal and acid were sufficient to produce a solution containing 176 grams of dissolved cobalt metal per liter of solution. In a separate container, 400 grams of ultrafine WC powder were mixed in 2 liters of water under nitrogen purge and heated to 50° C. To this mixture, approximately 0.136 liters of the cobalt chloride solution were added to form a slurry. After thorough mixing and adjustment of the temperature to 50° C., ammonia (NH 3 ) was slowly added to the slurry to cause precipitation of cobalt intermediate (i.e. cobalt hydroxide) onto the surface of the powder. Through the use of ammonia, the pH of the solution was maintained between 8-9 during precipitation. After the precipitation step was completed, the powder was filtered from solution, washed and dried. The dried powder was then reduced at 500° C. under a reducing environment of hydrogen to convert the cobalt intermediate coating to cobalt metal. The resulting product was a WC powder coated with 6 wt. % cobalt metal. The oxygen content of the resulting cobalt WC powder was 0.3 wt. %. Low oxygen content is critical for the production of WC/Co materials. If the composite powder had a high oxygen content, the sintered WC/Co material would be deficient in carbon due to a reaction between the oxygen and carbon in the WC, which would produce a product that has inferior properties.  
       Example 2  
       [0051]    A cobalt coated WC powder was prepared following the same procedure as in Example 1. The powder was then qualitatively analyzed with an Scanning Electron Microscope equipped with EDX (Energy Dispersive X-ray) analysis capabilities. FIG. 1 is a representation of a high magnification micrograph of several grains of the cobalt coated WC powder. This grain morphology of the cobalt coated powder is very similar to the grain morphology of the starting WC powder. A random EDX analysis on grains a, b and c confirm the presence of a cobalt coating on the WC grain. Thus, good distribution was obtained.  
       Example 3  
       [0052]    15 grams of the powder produced in Example 1 were mixed with a pressing agent (2 wt. % paraffin) in heptane and dried with a rotary evaporator. This powder was granulated by passing through a 100 mesh screen. The screened powder was then uniaxially pressed at 22,000 psi into ⅞″ diameter greenware. The greenware was then vacuum sintered in a graphite furnace at a temperature of 1410° C. for 1 hour. The sintered component had a density of greater than 14.90 grams/cc. Thus, the component was sintered to full density.  
       Example 4  
       [0053]    500 g of WC were dispersed in 1.5 liters of water and ammonia by having the pH at 8.2. A solution of CoCl 2  was prepared with cobalt content 86 g. The slurry was heated up to 50° C. with good mixing. The cobalt solution was added on the vessel at 30 ml/min and the pH of the slurry was kept as constant by controlled and continues NH 3  addition. After the precipitation, the sample was filtered, washed by water, dried and reduced at 500° C. The cobalt content of the reduced sample was analyzed and found to be 13%. The quality of the coating was investigated by Scanning Electron Microscope in the way shown in FIG. 2. The structure shows that the cobalt has coated the WC grains and is uniformly distributed on the surfaces.  
         [0054]    Having thus described and exemplified the invention with a certain degree of particularity, it should be appreciated that the following claims are not to be so limited but are to be afforded a scope commensurate with the wording of each element of the claim and equivalents thereof.