Abstract:
An improved sintered material and product. A nanometer size reinforcement powder is mixed with a micron size titanium or titanium alloy powder. After the reinforcement powder is generally uniformly dispersed, the powder mixture is compacted and sintered, causing the nano reinforcement to react with the titanium or titanium alloy, producing a composite material containing nano and micron size precipitates that are uniformly distributed throughout the material.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with United States Government support under SBIR Grant No. DE-FG02-03ER-83679. The United States Government has certain rights in this invention. 
     The claimed invention resulted from joint work by IAP Research, Inc. and The Timken Company under the SBIR program. 
    
    
     RELATED APPLICATION 
     This patent application is related to that entitled “METHOD OF PRODUCING UNIFORM BLENDS OF NANO AND MICRON POWDERS,” Ser. No. 11/531,768, filed Sep. 14, 2006, and which is hereby incorporated by reference and made a part hereof. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to powders for compaction and sintering and, more particularly, to micron size powders having nano size reinforcements. 
     2. Description of the Related Art 
     Composite materials containing two different components are well known. Reinforced concrete provides a simple example.  FIG. 1A  illustrates a non-reinforced concrete beam  3 . A load  6  will tend to deform the beam  3 , inducing cracks  9  on the tension side  12 , which can cause the beam  3  to fail. 
       FIG. 1B  illustrates a reinforced concrete beam  18 , which contains a steel reinforcing bar  21  on the tension side  12 . The reinforcing bar  21  absorbs the tensile load on the tension side  12 . The concrete itself, being high in compressive strength, absorbs the compressive load on the compression side  24 . 
     The reinforced beam of  FIG. 1B  can carry a larger load than the non-reinforced beam of  FIG. 1A  by properly combining the desirable properties of two different materials, steel and concrete. 
     Similar principles of reinforcement have been applied to sintered materials. Studies exist where reinforcements of small whiskers or fibers of high tensile strength, having diameters in the range of several microns, have been incorporated into the sintered materials. Similarly, particulate reinforcements of micron size (1-100 microns) have been incorporated into matrix powder and sintered to obtain sintered composite body. 
     For example, titanium particulates, larger than 10 microns, have been mixed with carbon or boron fibers, also larger than 10 microns. The mixture is then compacted and subjected to a sintering process, wherein the carbon or boron reacts in situ with the titanium, to form precipitates of titanium carbide or titanium boride reinforcement. 
     It is known that nano grain size materials show unique properties such as mechanical, optical, electrical, and catalytical properties. 
     However, difficulty has been encountered in attempts to fabricate very small reinforcements, in the nanometer range, within a powder matrix in the micron range. Specifically, if the titanium micron powder of the example immediately above is mixed with titanium carbide particulates in the nanometer range, using ordinary agitation techniques, it is found that the carbide powder agglomerates together, forming islands, and does not become uniformly distributed throughout the titanium powder matrix. 
       FIG. 2  illustrates a specific example of non-uniformity. A micron size powder was mixed with a nano size powder, compacted and sintered. The dark islands indicate the titanium carbide produced by the nano powder, and the surrounding white regions indicate the titanium metallic regions. 
     What is needed is a system and method that overcomes one or more of the problems of the prior art, and that attains a more uniform distribution of nano size powders within a micron size matrix. 
     SUMMARY OF THE INVENTION 
     In one form of the invention, an approach has been developed which effectively disperses nano size discontinuous reinforcements generally uniformly throughout a micron size titanium or titanium alloy powder. 
     An object of the invention is to provide an improved process for blending nano size discontinuous reinforcements with micron-size titanium or titanium alloy powders. 
     A further object of the invention is to provide a process for blending nano-powders with micron powders of titanium or titanium alloy, which produces a generally highly uniform distribution of both entities throughout the mixture. 
     A further object of the invention is to provide blends of titanium or titanium alloy powders of micron and nano powders as precursors in a sintering process, wherein the nano powders form reinforcement. 
     In one embodiment of this invention, superior mechanical properties (that were not possible with micron reinforcements) is made possible by combining nano powder reinforcements with micron size powders. 
     In one aspect, one embodiment comprises a material, comprising: a collection of relatively soft particles, each smaller than 200 microns, and ranging in size from S1 to S2; and a coating of relatively hard particles embedded into each soft particle, the hard particles ranging in size from &lt;S1/1000 to &lt;S2/1000. 
     In another aspect, one embodiment comprises a material, comprising relatively soft particles, each smaller than 200 microns; and relatively hard particles, each smaller than 100 nanometers, which are uniformly dispersed among the relatively soft particles. 
     In still another aspect, one embodiment comprises a method comprising: combining a nano-sized powder of one material with a micron-sized titanium or titanium alloy powder; and ball-milling the particles to produce a mixture in which the number of nano-sized particles in any volume is substantially proportional to the surface area of micron-sized particles in the volume. 
     In yet another aspect, one embodiment comprises a method, comprising: combining micron size particles of a titanium or titanium alloy with nano size particles of a reinforcement; agitating the combined particles in a ball mill; and compacting and sintering the agitated particles, to thereby produce enhanced hardness in a sintered product. The hardness of sintered composite body was proportional to an amount of reinforcement particles used. The composite hardness is much higher than that of pure metal. The composite hardness is also higher than that made with micron size reinforcement. 
     In another aspect, one embodiment comprises a method, comprising: combining micron size particles of a titanium or titanium alloy with nano size particles of a reinforcement; agitating the combined particles in a ball mill; and compacting and sintering the agitated particles, to thereby produce a sintered product with modulus of elasticity that is higher than that of pure metal and composite made with micron size reinforcement. The sintered composite body modulus of elasticity increased linearly with amount of nano reinforcements. 
     In still another aspect, one embodiment comprises a composite, comprising: sintered titanium or titanium alloy and filaments or plates of reinforcement, said reinforcement being harder than the matrix, uniformly distributed throughout the matrix, and of diameter less than 500 nanometers. 
     In yet another aspect, one embodiment comprises a method, comprising: uniformly dispersing nanometer size reinforcements throughout a micron size titanium or titanium alloy powder; compacting and sintering the titanium or titanium alloy powder with reinforcement, to produce a sintered composite material having nanometer sized ceramic bodies uniformly distributed throughout the material. 
     These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate a non-reinforced concrete beam; and a reinforced concrete beam, respectively; 
         FIG. 2  displays a very heterogeneous microstructure that was achieved after compaction and processing of powder blends mixed using conventional blending methods—the bright areas are metallic phase from micron-size powder, and the dark areas are agglomerated ceramic nano powder regions; 
         FIG. 3  illustrates an optical micrograph of a sintered substance in which micron sized titanium powder was mixed with nano sized TiC reinforcement using the invention&#39;s processing techniques. The light gray areas are titanium and dark areas are TiC. 
         FIG. 4  shows nanometer sized titanium carbides surrounded by dislocation tangles via transmission electron microscopy image; 
         FIG. 5  shows micron size titanium carbides in the needle and platelet form in a composite matrix via transmission electron microscopy image; 
         FIG. 6  is a plot of hardness of the titanium composite reinforced with nano particulate TiC of three different weight concentrations (12, 18, and 25 percent); 
         FIG. 7  is a plot of modulus of elasticity increase as a function of TiC concentration for different concentrations of titanium carbide within titanium; and 
         FIG. 8  is a plot of higher hardness enhancement with nano size TiC reinforcements as compared with micron size TiC reinforcements for 6 and 25 weight % titanium carbide particulates. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A micron size titanium powder, ranging in size from 1 microns to 200 microns, is mixed with a nano size reinforcing powder, ranging in size from 1 nanometers to 100 nanometers, or even greater than 100 nanometers to less than 1 micrometer. The uniform blends are prepared by ball milling micron size titanium powders with nano size reinforcements. 
     The amount of nano size powder lies in the range of 1 to 50 percent, by weight, of the micron size powder. For example, if the powder blend weighs 1,000 grams, then 50 percent by weight of nano size powder would weigh 500 grams. 
     The reinforcing powder may also comprise nano and fine size particulates, or sub micron to nano size whiskers, nanotubes of carbon. For example, the reinforcing powder may contain one or more of the following: 
     titanium carbide (TiC), 
     titanium boride (TiB), 
     titanium nitride (TiN), 
     titanium diboride (TiB2), 
     titanium carbonitride (TiCN), and 
     alumina (Al2O3). 
     As mentioned earlier, the mixture is ball-milled, which can be performed using a Unitized Jar Mill (Model No. 784 AVM), manufactured by U.S. Stoneware located at 700 E. Clark Street, East Palestine, Ohio 44413. In one example, the balls were ¼-inch and 3/16-inch alumina, and the mill was run at 110 rpm speed, in a dry condition for two hours in air at room temperature. 
     The mixed powder was then compacted using dynamic magnetic compaction, known in the art, and then sintered at 1260 degrees centigrade for 4 hours under vacuum using known processes. Testing of the sintered composite product using optical microscopy indicated that the titanium matrix and titanium carbides were uniformly dispersed throughout the sintered body, as shown in  FIG. 3 . 
     The addition of the nano reinforcement enhanced one or more of the following properties: strength, wear, corrosion resistance, optical, electrical, thermal, and/or catalytic properties of the sintered composite, compared with the sintered body from micron powder alone. 
     Further, in many cases, the degree of enhancement changes as the concentration of the nano reinforcement changes, sometimes in direct proportion. 
     It was discovered that a degree of enhancement in hardness is proportional to a weight percent of reinforcement. Also, the degree of enhancement in elastic modulus increased linearly with weight percent of reinforcement. 
     Additional Considerations 
     1. The micron powder discussed above, which is also termed the matrix material, was titanium metal. Other matrix materials may include the following: 
     Ti-6Al-4V, 
     Ti-6Al-2Sn-4Zr-2Mo, 
     Ti-6Al-2Sn-4Zr-6Mo, 
     Ti-6Al-2Zr-2Sn-2Mo-2Cr-0.25Si, 
     Ti-5Al-2Sn-2Zr-4Mo-4Cr, 
     Ti-10V-2Fe-3Al, 
     Ti-15V-3Cr-3Sn-3Al, 
     Ti-15Mo-3Al-3Nb-0.2Si, 
     Ti-4.5Al-3V-2Mo-2Fe, 
     Ti-1Al-8V-5Fe, and 
     Ti-35V-15Cr, 
     wherein 
     Ti refers to titanium, 
     Al refers to aluminum, 
     V refers to vanadium, 
     Sn refers to tin, 
     Zr refers to zirconium, 
     Mo refers to molybdenum, 
     Cr refers to chromium, 
     Si refers to silicon, 
     Nb refers to niobium, and 
     Fe refers to iron. 
     The numerals refer to relative molar concentrations, not relative weights. 
     2. In addition to the reinforcement materials discussed above, whiskers or tubes of carbon can be used as mentioned earlier, of diameter 10 nanometers to 100 nanometers. 
     3. Numerous approaches, known in the art, can be undertaken to compaction of the powder mixture, prior to sintering. These approaches include dynamic magnetic compaction, conventional pressing, isostatic pressing, and other types of high speed powder compaction and hot isostatic pressing. Thus, it should be understood that both static and dynamic compaction can be used. 
     4. In one embodiment, sintering is done at temperatures between 1150 and 1300 centigrade, under high vacuum of 10-6 to 10-7 Torr. 
     5. In the sintered state, the composite body can include needles, plates, acicular bodies, spheroids, and irregularly shaped precipitates. These structures can lie in the nano- or micron size range, and are, in whole or in part, responsible for the enhancement in performance which is achieved. 
     6. A specific example of increase in hardness is the following. A sample of micron size titanium metal powder of weight 0.75 W was prepared. Nano size titanium carbide weighing 0.25 W was added, and the mixture—with a total weight W—was processed as described herein. An increase in hardness was measured, from 60 HV, for pure sintered titanium, to 697 HV, for the mixture. HV refers to hardness measured on the Vickers scale. 
     7. The material of point  6 , immediately above, was found to display an increase in modulus of elasticity from 100 GPa to 157 GPa. GPa refers to giga-Pascals. 
     8. One mechanism which is believed to be involved will be explained. The micron size particles (titanium metal in the example above) are softer than the nano size particles. The low energy ball milling causes the harder nano size particles to become embedded within the softer micron size particles. That is, the nano size particles act as a coating. 
     This coating behavior can be viewed as introducing the following features. One, the effective total number of particles present in the mixture is not equal to the sum of the small and large particles. Instead, many small particles are bound to each large particle to provide a hybrid particle, the total number of hybrid particles is less than the total sum of the small and large particles. This reduced number of particles, each being larger and more massive than the original large particles, may be responsible for the good “flowability” observed in the mixture after ball milling. Good “flowability” is desirable for compaction to high density body prior to the sintering process. 
     Specific Embodiments 
     1. Titanium powder of size 50-60 microns was coated with 12 percent by weight titanium carbide powder of size 20 to 40 nanometers and blended in the ball milling process as described above. The mixture was subjected to dynamic magnetic compaction and sintered at 1260 degrees centigrade for 2 hours under high vacuum. The microstructure via optical microscopy is shown in  FIG. 3 . 
     It was found that the titanium carbide was well distributed throughout the final product. Further, the titanium carbide exhibited one of two morphologies. The first was a long needle or plate structure, several microns in length, with many or all being longer than 5 microns. The second was clustered acicular particles of dimension in the tens of nanometers. 
     2. Micron size titanium powders were mixed with nano size titanium carbide powders and processed as described herein. The sintered product of titanium matrix containing 25 percent by weight of titanium carbide reached a hardness of 60 HRC as shown in  FIG. 6 . The hardness for different weight percentages of titanium carbide is shown in  FIG. 6 . The hardness increases as the titanium carbide content increases. 
     3.  FIG. 8  compares the hardness between titanium (Ti) composites with nano and 1-5 micron reinforcements. The results for TiC composites with 6 and 25 weight % are shown in  FIG. 8 . Higher hardness was observed with nano reinforcement. 
     4.  FIG. 7  indicates how modulus of elasticity for the sintered product increases as the weight percentage of titanium carbide within titanium metal increases. For example, the modulus increases from about 110 GPa (arrow D) to 157 GPa (arrow E) by addition of 25 percent by weight of nano size titanium carbide. 
     5. A second type of nano size or fine particles can be added. For example, the second nano particles may be harder than the micron particles, and also harder than the first nano particles. In this example, the second nano particles will become embedded into the larger micron particles, and also into the first nano particles. The second nano particles will thereby form a blended layer. 
     Conversely, the second nano particle can be softer than both the first nano particles and the micron particles. The hard particles embed into the softer particles. 
     Notice that Transmission Electron Microscopy (TEM) of the sintered composite samples showed carbide dispersion throughout the microstructure.  FIGS. 4 and 5  show carbide structures in 12 weight %, TiC composite. These carbides exhibited one of two morphologies. The first is that of a long needle or plate microstructure (microns in length) while the second is that of clustered acicular particles with dimensions of the order of tens of nanometers.  FIG. 4  shows nanometer size carbides surrounding dislocations. Again,  FIG. 5  shows the micron size carbides in needle and platelet form. 
     DEFINITIONS 
     1. One definition of a micron size powder is a powder of particle size ranging from 1 to 200 microns. 
     2. One definition of a nano size powder is a powder of particle size ranging from 1 to 100 nanometers. 
     3. The dimension of a filament, whisker, or tube is determined by its smallest outer dimension, such as its outer diameter. Thus, a carbon tube which is 10 microns long, with an inner diameter of 100 nanometers and an outer diameter of 200 nanometers would be a 200 nanometer tube. 
     4. Titanium carbo-nitride, TiCN, is considered to be both a carbide and a nitride of titanium. 
     5. The phrase “S1/1000” means the quantity S1 divided by one thousand. 
     6. A definition or illustration of “uniform” can be derived from the following perspective. The concentration of nano particles in any volume can be proportional to the surface area of the micron particles in that volume. 
     Of course, if sufficient nano particles are added so that they do not all bond to the larger particles, this situation may not occur. The unbonded nano particles will occupy spaces between the larger particles, and may agglomerate into small islands. For example, if a given volume contains a single large micron size particle, and if nano particles coat the large particle in a single layer, then the number of nano particles depends on the surface area of the large particle. 
     Similarly, if the nano particles coat the micron particle in two or more layers, then again the number of nano particles depends on the surface area of the micron particle. If two different micron particles are present, and are coated with nano particles, then the number of nano particles again depends on the total surface area of the micron particles. Therefore, the concentration of the nano particles, in terms of number of particles in a selected volume, will be proportional to the surface area of the micron particles within that volume. 
     7. “Diameter” of an irregular particle refers to the largest cross-sectional dimension, as viewed through a microscope. “Diameter” of a filament is the diameter of its cross section. “Diameter” of a plate is the smallest dimension of its cross section, which is probably the thickness in most cases. 
     8. Although not shown, some of the parts or products made from the above process and powder composite include: bearings and their components, disks, cylinders, rods and tube like shapes, power train components, drive shaft and friction components, and filters. 
     Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.