Patent Publication Number: US-9427806-B2

Title: Method and apparatus for forming a gold metal matrix composite

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 61/833,341 filed Jun. 10, 2013 entitled “Method and Apparatus For Forming A Gold Metal Matrix Composite”, which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The described embodiments relate generally to methods for assembly of multi-part devices. In particular, methods for providing a metal matrix composite that is rugged, scratch resistant and presents an aesthetically pleasing appearance are described. 
     BACKGROUND 
     A metal matrix composite (MMC) is composite material with at least two constituent parts, one being a metal. The other material may be a different metal or a non-metal material, such as a ceramic. MMCs are made by dispersing a reinforcing material into a metal matrix. The matrix is the monolithic material into which the reinforcement is embedded. In structural applications, the matrix is usually a lighter metal such as aluminum, magnesium, or titanium, and provides a compliant support for a reinforcement material. The reinforcement material is embedded into the matrix. The reinforcement material does not always serve a purely structural task (i.e., reinforcing the MMC), but can also change physical properties such as a wear resistance, friction coefficient, or thermal conductivity of the MMC. The reinforcement material can be either continuous, or discontinuous. Discontinuous MMCs can be isotropic, and can be worked with standard metalworking techniques, such as extrusion, forging or rolling. In addition, they may be machined using conventional techniques, but commonly would need the use of polycrystalline diamond tooling (PCD). 
     What is desired is a metal matrix composite that presents a cosmetically appealing appearance that is maintained throughout an operating lifetime and is relatively inexpensive to manufacture in both processing and materials. 
     SUMMARY 
     This paper describes various embodiments that relate to assembly of cosmetically appealing devices. In particular embodiment, a precious metal matrix can be formed that provides an overlay for a device that is cosmetically appealing and is also rugged enough to maintain the cosmetically appealing appearance throughout an operating life of the device. 
     According to one embodiment, a gold metal matrix composite is formed. The gold metal matrix composite includes a porous preform that includes a number of ceramic particles and spaces positioned between the ceramic particles. The gold metal matrix composite also includes a gold matrix including a network of gold formed within the spaces of the porous preform. The gold metal matrix composite is characterized as 18 k gold. 
     According to another embodiment, a housing for an electronic device is described. The housing includes a precious metal matrix composite forming at least a portion of an external surface of the housing. The precious metal matrix includes a continuous metal material having at least one type of precious metal. The precious metal matrix also includes a number of ceramic particles dispersed within the continuous metal material. The ceramic particles increase a hardness of the precious metal matrix composite compared to the continuous metal material without the ceramic materials. The precious metal matrix composite includes about 75% precious metal by mass. 
     According to an additional embodiment, a method of forming a gold metal matrix composite is described. The method includes forming a gold and ceramic mixture by coating a number of ceramic particles with gold. The method also includes placing the gold and ceramic mixture into a die having a near net shape. The method additionally includes compressing and heating the gold and ceramic mixture in the die forming a gold metal matrix composite having a shape corresponding to the near net shape. 
     According to a further embodiment, a method of forming a gold and diamond matrix composite is described. The method includes forming a gold and diamond mixture using gold particles and diamond particles. The method also includes modifying or coating a surface of the diamond particles using a wetting agent. The modified or coated diamond surface is suitable for binding with the gold particles. The method further includes compressing and heating the gold and diamond mixture. The wetting agent forms a carbide at the diamond surface, the carbide suitable for binding with the gold during the compressing and heating. 
     It should be noted that for any of the methods described above, the ceramic can take many forms. For example, the metal matrix composite can include in addition to gold any of the following in any combination: boron carbide, diamond, cubic boron nitride, titanium nitride (TiN), iron aluminum silicate (garnet), silicon carbide, aluminum nitride, aluminum oxide, sapphire powder, yttrium oxide, zirconia and tungsten carbide. The choice of materials used with the gold in the metal matrix composite can be based upon many factors such as color, desired density (perceived as heft), an amount of gold required to meet design/marketing criteria, and so on. 
     Other aspects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the described embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The described embodiments may be better understood by reference to the following description and the accompanying drawings. Additionally, advantages of the described embodiments may be better understood by reference to the following description and accompanying drawings in which: 
         FIGS. 1A-1D  show a powder metallurgy process for forming a gold metal matrix composite in accordance with described embodiments. 
         FIG. 2  shows a flowchart detailing the powder metallurgy process in accordance with  FIGS. 1A-1D . 
         FIGS. 3A-3E  show a squeeze casting process for forming a gold metal matrix composite in accordance with described embodiments. 
         FIG. 4  shows a flowchart detailing the squeeze casting process in accordance with  FIGS. 3A-3E . 
         FIGS. 5A-5D  show a modified powder metallurgy process for forming a gold metal matrix composite in accordance with described embodiments. 
         FIG. 6  shows a flowchart detailing the modified powder metallurgy process in accordance with  FIGS. 5A-5D . 
     
    
    
     DETAILED DESCRIPTION 
     Representative applications of methods and apparatus according to the present application are described in this section. These examples are being provided solely to add context and aid in the understanding of the described embodiments. It will thus be apparent to one skilled in the art that the described embodiments may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the described embodiments. Other applications are possible, such that the following examples should not be taken as limiting. 
     In the following detailed description, references are made to the accompanying drawings, which form a part of the description and in which are shown, by way of illustration, specific embodiments in accordance with the described embodiments. Although these embodiments are described in sufficient detail to enable one skilled in the art to practice the described embodiments, it is understood that these examples are not limiting; such that other embodiments may be used, and changes may be made without departing from the spirit and scope of the described embodiments. 
     This paper provides a description of methods and associated apparatuses for providing a metal matrix composite well suited for use as an external structure for a device. In some embodiments, the device is an electronic device or an accessory for an electronic device. In particular embodiments, the metal matrix composite forms a housing or a portion of a housing of an electronic device. In some embodiments, the metal matrix composite includes as at least one precious metal. The precious metal can include, for example, one or more of gold, silver and platinum. In this way, the metal matrix composite can provide a cosmetically appealing and rugged component that can be used to enhance the experience of a user of the device. 
     For the remainder of this discussion, the metal matrix composite includes gold (or predominantly gold) as the precious metal. However, other precious metals, such as silver and/or platinum, can also be used in accordance with described embodiments. In some embodiments, gold and one or more different metals, such as different precious metal, are used in conjunction within a metal matrix composite. 
     In general, an indication of an amount of gold in the metal matrix composite can be expresses in terms of karats (or carats), which represents the amount of gold in a gold alloy, where 24 k represents almost pure gold and 18 k represents 18/24 or 75% gold by mass. More specifically, karat purity is measured as 24 times the purity by mass as:
 
 k= 24×( M   g   /M   m ) where
         k is karat rating of the material,   M g  is the mass of pure gold in the material, and   M m  is the total mass of the material       

     It should be noted that in general usage, due to the inherent softness of elemental gold, gold is generally alloyed to less than 24 k using a number of metals such as silver, platinum, etc. In the context of the following discussion, however, a gold metal matrix composite (gMMC) can include in addition to gold, alloying metals such as silver, and/or a ceramic material as reinforcement materials. The choice of ceramic can depend on material properties desired for the gMMC. Such material properties can include, for example, hardness, corrosion resistance, machinability and color. Color, in particular, can be selected based upon specific ceramic materials. For example, silicon carbide powder can be black or green whereas yttrium oxide powder can be white. In this way, a gMMC can be rendered to reflect light in specific ranges of the visible light spectrum to provide a desired color appearance. 
     In addition to using as little gold as possible while maintaining a specific karatage, a gMMC can be formed that has selected aesthetic properties well suited for providing a favorable user experience. For example, a unit volume of 18 k of gMMC that uses gold in combination with a ceramic as a reinforcement can be less dense, can require less gold, and can be more scratch resistant than that of a unit volume of gold alloy of the same karatage without ceramic. Scratch resistance is generally related to a hardness of the gMMC, which can be measured using a Vickers hardness test. In embodiments described herein, the hardness of gMMC is generally harder than gold alloy of the same karatage. In some embodiments, the gMMC has a hardness of at least 400 Hv, as measured by Vickers test. 
     Moreover, by selecting specific ceramic materials, a gMMC can be scratch and corrosion resistant, can be polished to a high degree to bring out a natural luster, can possess a high degree of machinability (i.e., can be easily machined into any desired shape), and in some cases, provide good heat transfer characteristics. For example, diamond powder can be used with gold to form a gMMC that has superior heat transfer characteristics due to the superior heat transfer characteristics of the diamond reinforcement. However, it should be noted, that in order for gold and diamond to form a viable gMMC, a wetting agent may be required that facilitates wetting a surface of the diamond by the gold. Boron, silicon, titanium, chromium and tungsten are examples of suitable wetting agents that can react with diamond to form a carbide layer that facilitates wetting the surface of the diamond by a matrix metal, which may be necessary for the formation of a gold and diamond MMC. 
     Other ceramic properties of interest can include a size of the ceramic particles. Particles that are too large may hinder polishing of the gMMC since large particles may be removed during a polishing operation and cause pitting of the gMMC surface. Moreover, a large sized particle also has the potential to hinder a sintering process in that large particles have a tendency to form large gaps between particles. The large gaps between particles can hinder the ability of the large particles to coalesce during the sintering operation. In addition, in some embodiments, the size of the ceramic particles are sufficiently small so as to give the gMMC a continuous appearance. That is, the ceramic particles are not so big as to be visibly distinguishable within the gMMC. 
     It should also be noted, that there can be an optimal range of ceramic volume fraction in accordance with a fixed karatage value. The optimal range of ceramic volume fraction can be based upon a desired hardness range of the gMMC. For example, if the ceramic volume fraction is reduced (relatively more gold), then the hardness of the gMMC can be reduced (approaching that of pure gold). As the volume fraction of ceramic increases (with a concomitant decrease in an amount of gold), the hardness of the gMMC generally increases to the point where the gMMC starts to exhibit brittleness. Therefore, an optimal range of ceramic volume fraction can be determined based on desired gMMC material properties, gMMC karatage, ceramic density and other properties. 
     For the remainder of this discussion, a metal matrix composite having gold as at least one metallic constituent and a ceramic as a reinforcement constituent is discussed. In particular, the gMMC is 75% by mass gold and 25% by mass ceramic reinforcement in accordance with an 18 k material. It should be noted, however, that methods described herein are not limited only gold and ceramic metal matrix composites and that any suitable matrix compositions in any suitable karatage can be used in accordance with described embodiments. 
     Since per unit volume, the density of ceramic particles is less than metals generally used to alloy gold (e.g., copper, silver, nickel), a unit volume of 18 k gMMC is less dense and thus requires less gold than a unit volume of gold alloy. Accordingly, the size (density) of the ceramic particles can be tuned to achieve a desired MMC density that can be expressed by the following: 
     ρ 1  is density of gold, ρ 2  is density of ceramic, V 1  is volume of 1 kg of gMMC, k is karatage
 
 V   1 =(1−( k/ 24)/ρ 2 )+(( k/ 24)/ρ 1 ))
 
for k=18
 
 V   1 =(0.25/ρ 2 )+(0.75/ρ 1 )
 
 VF   ceramic =((0.25/ρ 2 )/ V   1 )
 
 VF   gold =((0.75/ρ 1 )/ V   1 )
 
     Accordingly, as k increases (greater proportion of the gMMC is gold), the corresponding volume fraction of ceramic (VF ceramic ) decreases. However, for a constant k, as the density (ρ 2 ) of the ceramic increases, the corresponding ceramic volume fraction (VF ceramic ) decreases. Therefore, as the density of the reinforcement is decreased for a constant k, the mass of gold used for the same part decreases. Moreover, since the density of 18 k gMMC is less than a 18 k metal-based gold alloy, the amount of gold used in the 18 k gMMC is less than that used in a 18 k metal-based gold alloy. 
       FIGS. 1A-1D  show a powder metallurgy process for forming a gMMC in accordance with described embodiments. At  FIG. 1A , gold particles  102  and ceramic particles  104  are blended together forming mixture  106 . Gold particles  102  can be in any suitable form, including in the form of a powder or flakes of gold. Gold particles  102  can be made of substantially pure gold or a gold alloy. Ceramic particles  104  can be made of any suitable type of ceramic materials, such as suitable metal oxides, carbides, borides, nitrides and silicides. In some embodiments, ceramic particles  104  include one or more of garnet, boron carbide, silicon carbide, aluminum nitride, diamond, boron nitride, aluminum oxide, sapphire, yttrium oxide, titanium oxide and zirconia. As described above, the type of ceramic material can be chosen based on factors such as a desired color, density, hardness, corrosion resistance, machinability and polish-ability of a final gMMC. Gold particles  102  and ceramic particles  104  can be blended using any suitable mixing technique. It should be noted that in order to assure good mixing and provide a good basis for subsequent sintering operation, the size of ceramic particles  104  can be selected to minimize an amount of open space between ceramic particles  104  in mixture  106 . As described above, the relative amount of gold particles  102  within mixture  106  will depend upon a desired karatage of the final gMMC. 
     As described above, in some embodiments, a wetting agent is used to assist binding of ceramic particles  104  with gold particles  102  during a subsequent compressing operation and/or sintering operation. Ceramic particles  104  can be coated with the wetting agent prior to mixing with gold particles or the wetting agent can be added to mixture  106 . In some embodiments, the wetting agent modifies the surfaces of ceramic particles  104 . For example, diamond particles can be coated with a wetting agent that modifies the surfaces of the diamond particles by causing carbide to form on the surfaces of the diamond particles. The carbide assists binding of ceramic particles  104  to gold particles  102  during subsequent sintering. In some embodiments, the wetting agent includes one or more of boron, silicon, titanium, chromium and tungsten. 
     At  FIG. 1B , mixture  106  is placed within die  108  having a near net shape that is similar to a final shape of the gMMC. While within die  108 , pressure  110  is exerted onto mixture  106  such that the porosity of mixture  106  is reduced. That is, the density of mixture  106  is increased. The density of mixture  106  after compression is proportional to the amount of pressure  110  applied. In addition, mixture  106  is pressed against die  108  so as to take on the near net shape of die  108 . In some embodiments, heat is applied to gMMC during the compression. After compression, compressed mixture  106  can be removed from die  108  and retain the near net shape. 
     At  FIG. 1C , compressed mixture  106  is placed into oven  112  and exposed to sintering operation. During sintering compressed mixture  106  is heated such that bonding occurs between gold particles  102  and ceramic particles  104  within compressed mixture  106 . Note that in some embodiments, compressing process ( FIG. 1B ) and heating process ( FIG. 1C ) are combined within a single process, sometimes referred to as a Hot Isostatic Pressing (HIP) process. That is, mixture  106  is exposed to a pressure and to heat at the same time. This can be accomplished using a die that is designed to conduct heat to mixture  106  while compressing mixture  106 . Once cooled, gMMC  114  is formed having the near net shape of die  108 . 
     At  FIG. 1D , gMMC  114  can then be removed from oven  112 . In some embodiments, gMMC  114  is the exposed to one or more shaping processes, such as one or more machining or polishing processes, such that gMMC  114  takes on a final desired shape. In some embodiments, gMMC  114  takes on a final shape suitable for housing or a portion of a housing for an electronic device. In some embodiments, gMMC  114  forms an exterior portion of the housing, such as a layer that covers exterior surfaces of the housing. Since gMMC  114  includes a ceramic portion originating from ceramic particles  104 , gMMC  114  has higher scratch resistance and hardness compared to a gold or gold alloy structure. The gold portions of gMMC  114  originating from gold particles  102  give gMMC  114  a gold color and appearance. As described above, the density of gMMC  114  of ceramic particles is less than metals generally used to alloy gold. Thus, a unit volume of gMMC  114  is generally less dense and thus requires less gold than a unit volume of a gold metal alloy. 
       FIG. 2  is a flow chart detailing a powder metallurgy process  200  in accordance with the described embodiments. Process  200  can be carried out by performing at least the following operations. At  202 , gold particles can be blended with a corresponding amount of ceramic particles forming a of gold and ceramic mixture. In some embodiments, the gold particles and ceramic particles are each in the form of a powder. At  204 , the gold and ceramic mixture is formed into a near net shape, by which it is meant that the gold and ceramic mixture is processed in such a way as to take on a form similar to a desired final shape. In one embodiment, the forming into the near net shape can be carried out by compressing the mixture in a die or other container having a shaped interior. At  206 , the compressed mixture can be heated in a sintering operation that causes the gold and ceramic particles to bond with each other. In some cases operations  204  and  206  can be combined into a single operation  208  using Hot Isostatic Pressing, or HIP. 
       FIGS. 3A-3E  show a squeeze casting process for forming a gMMC in accordance with described embodiments. At  FIG. 3A , ceramic particles  302  are combined with mixture  306 , which includes binder  304  and water, within container  310  forming preform composite  308 . Ceramic particles  302  can be in any suitable form, including in the form of a ceramic powder, and can be made of any suitable type of ceramic materials, such as suitable metal oxides, carbides, borides, nitrides and silicides. The type of ceramic material can be chosen based on factors such as a desired color, density, hardness, corrosion resistance, machinability and polish-ability of a final gMMC. Binder  304  can be made of any material suitable for binding ceramic particles  302  together when in aqueous solution and that is removable during a binder removal process. In some embodiments, binder  304  includes a commercially available ceramic binder. 
     At  FIG. 3B , preform composite  308  is removed from container  310  and placed in oven  312  for a drying and binder removal process. Heat from oven  312  removes binder  304  and water from preform composite  308  forming porous preform  314 . In addition, the heat can fuse or sinter ceramic particles together such that voids form between the ceramic particle when the water and binder  304  are removed. In this way, porous preform  314  is formed, which includes voids where binder  304  and water once were. The void volume within porous preform  314  will depend in part on the relative amount of binder/water mixture  306  within preform composite  308 , as well as the size of ceramic particles  302 . In some embodiments, porous preform  314  undergoes one or more shaping processes, such as one or more machining or polishing processes. 
     At  FIG. 3C , porous preform  314  is placed within container  316  and gold particles  318  are added to porous preform  314 . Gold particles  318  can be in any suitable form, including in a powder or flakes, and can be made of substantially pure gold or a gold alloy. In some embodiments, a wetting agent is added to porous preform  314  in order to assist binding of gold particles  318  to porous preform  314 . At  FIG. 3D , porous preform  314  and gold  318  are placed in oven  320 . In some embodiments, container  316  is substantially non-chemically reactive to heat such that preform  314  and gold particles  318  remain within container  316  when placed in oven  320 . Heat from oven  320  can melt gold particles  318  forming molten gold that infiltrates within the voids of porous preform  314  by capillary action. In some embodiments, gold particles  318  are heated to a temperature just over the melting point of gold particles  318 . Pressure (such as by pressurized gas) can be applied within oven  320  while heating in order to assist the infiltration of molten gold within the voids of porous preform  314 . The relative amount of gold particles  318  infiltrated within porous preform  314  will depend upon the void volume of porous preform and a desired karatage of the final gMMC. When the molten gold becomes sufficiently infiltrated within porous preform, gMMC  322  is formed. 
     At  FIG. 3E , gMMC  322  is removed from oven  320  and allowed to cool. As with gMMC  114  manufactured using powder metallurgy described above, gMMC  322  has higher scratch resistance and hardness compared to a gold or gold alloy structure and is generally requires less gold than a unit volume of a gold metal alloy. In some embodiments, gMMC  322  is shaped using, for example, one or more machining or polishing processes. In some embodiments, gMMC  322  is shaped into a housing or a portion of a housing for an electronic device. 
       FIG. 4  shows a flow chart detailing squeeze casting process  400  in accordance with the described embodiments. Process  400  can be carried out by performing at least the following operations. At  402 , ceramic powder and binder (plus water) are combined forming a preform composite. At  404 , the preform composite is dried and sintered, removing both the binder and water and forming a porous preform. At  406 , an optional machining operation can be performed. In some embodiments, the optional machining operation can be used to shape the preform in accordance with a pre-determined final shape of the gMMC. At  408 , gold is added to the porous preform. In some embodiments, the gold is in the form of gold particles (e.g., gold powder or flakes). At  410 , the gold and ceramic preform is heated under pressure to a temperature just above a melting point of the gold. The heat liquefies the gold into molten gold, and the pressure facilitates the infiltration of the molten gold into the ceramic preform by way of capillary action. The result is a gMMC having a pre-determined shape. In some embodiments, the gMMC is further shaped forming a final shape. 
       FIGS. 5A-5D  show a modified powder metallurgy process for forming a gMMC in accordance with described embodiments. At  5 A, ceramic particles  502  are coated with gold forming gold-coated particles  504 . In some embodiments, the coating is accomplished by heating gold or gold alloy material into molten form and blending in ceramic particles  502 . In some embodiments, a wetting agent is added in order to assist binding of ceramic particles  502  and the molten gold. At  5 B, gold-coated particles  504  are placed within die  508  having a near net shape that is similar to a final shape of the gMMC. Pressure  510  is exerted onto gold-coated particles  504  such that the density of gold-coated particles  504  is increased. After compression, compressed gold-coated particles  504  can be removed from die  508  and retain the near net shape. 
     At  FIG. 5C , compressed gold-coated particles  504  is placed into oven  512  and exposed to a sintering operation such that bonding occurs between gold-coated particles  504 . In some embodiments, compressing process ( FIG. 5B ) and heating process ( FIG. 5C ) are combined within a single process, such as a HIP process. Once cooled, gMMC  514  is formed having the near net shape of die  508 . At  FIG. 5D , gMMC  114  is removed from oven  512 . In some embodiments, gMMC  514  is then shaped using one or more shaping processes, such as one or more machining or polishing processes, such that gMMC  114  takes on a final desired shape. Since gMMC  514  includes a ceramic portion originating from ceramic particles  502 , gMMC  514  has higher scratch resistance and hardness compared to a gold or gold alloy structure. As described above, the density of gMMC  514  of ceramic particles is less than metals generally used to alloy gold. Thus, a unit volume of gMMC  514  is generally less dense and thus requires less gold than a unit volume of a gold metal alloy. In some embodiments, gMMC  514  is shaped to form a housing or a portion of a housing for an electronic device. 
       FIG. 6  is a flow chart detailing a modified powder metallurgy process  600  in accordance with the described embodiments. Process  600  can be carried out by performing at least the following operations. At  602 , ceramic particles can be coated with gold forming gold-coated particles. The gold-coated particles can then be compressed at  604  in a manner that reduces spaces between and increasing the density of the gold-coated particles. At  606 , the compressed gold-coated particles can undergo a heating operation having the effect of forming the gMMC. It should be noted that as with process  200  described above, operations  604  and  606  can be combined into a single operation  608  using HIP. 
     Table 1 below summarizes relative gold volume and mass of various 18 k gold samples A-F, in accordance with described embodiments. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Relative Gold Volume and Mass of 18k Gold Samples 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Matrix 
                 Particle 
                   
                 Mass of 
               
               
                 Sam- 
                   
                 Volume 
                 Volume 
                 Part 
                 Gold 
               
               
                 ple 
                 Composition 
                 Fraction 
                 Fraction 
                 Mass 
                 in Part 
               
               
                   
               
               
                 A 
                 18k gold alloy 
                 100%  
                  0% 
                 34.4 g 
                 25.8 g 
               
               
                   
                 (baseline) 
                   
                   
                   
                   
               
               
                 B 
                 Boron carbide/ 
                 28% 
                 72% 
                 16.1 g 
                 12.1 g 
               
               
                   
                 pure gold MMC 
                   
                   
                 (Δ 53%) 
                 (Δ 43%) 
               
               
                 C 
                 Yellow diamond/ 
                 34% 
                 66% 
                 19.1 g 
                 14.3 g 
               
               
                   
                 pure gold MMC 
                   
                   
                 (Δ 44%) 
                 (Δ 36%) 
               
               
                 D 
                 Cubic boron nitride/ 
                 35% 
                 65% 
                 19.9 g 
                 14.9 g 
               
               
                   
                 pure gold MMC 
                   
                   
                 (Δ 42%) 
                 (Δ 34%) 
               
               
                 E 
                 Titanium nitride/ 
                 46% 
                 54% 
                 26.1 g 
                 19.6 g 
               
               
                   
                 pure gold MMC 
                   
                   
                 (Δ 24%) 
                 (Δ 19%) 
               
               
                 F 
                 Red garnet/ 
                 27% 
                 73% 
                 15.5 g 
                 11.6 g 
               
               
                   
                 pure gold cermet 
                   
                   
                 (Δ 55%) 
                 (Δ 55%) 
               
               
                   
               
            
           
         
       
     
     In Table 1, samples B-F are gMMC materials having different compositions. Sample A is an 18 k gold alloy sample, which is a gold metal alloy without any non-metal material (e.g., ceramic particles), and is used as a baseline for comparison with gMMC samples B-F. Samples A-F each have substantially the same volume. That is, they each represent a volume of a part. Matrix Volume Fraction refers to a volume percentage of non-particle material and Particle Volume Fraction refers to a volume percentage of particle material within the different 18 k gold samples. Part Mass refers to a mass of a part having a pre-defined volume and Mass of Gold in Part refers to the mass of gold within the part. Also included for gMMC samples B-F are the percentage change of the mass of the part and percentage change of the mass of gold in the part compared to gold alloy sample A. 
     Sample A (18 k gold alloy) is not a MMC material and, therefore, does not contain any MMC particle material. GMMC samples B-F are each gMMCs have different compositions. In particular, sample 2 is formed from boron carbide particles that are blended with pure gold, sample 3 is formed from yellow diamond particles that are blended with pure gold, sample 4 is formed from cubic boron nitride particles that are blended with pure gold, sample 5 is formed from titanium nitride particles that are blended with pure gold, and sample 6 is formed from red garnet particles that are blended with pure gold cermet. Pure gold cermet refers to a gold and ceramic material. 
     As described above, the choice of materials used in a gMMC can depend in part on the relative amount of gold used in the part. As indicated by Table 1, gMMC samples B-F each have less volume percentage of non-particle material and less gold mass than gold alloy sample A. Thus, a part manufactured using a composition of one or more of gMMC samples B-F can reduce the amount of gold within the part compared to a part made of gold alloy. The data of Table 1 can be used to choose the composition of a gMMC for manufacturing the part. For example, sample B (boron carbide/pure gold MMC) and sample F (red garnet/pure gold cermet) are characterized as having the lowest volume percentage of non-particle material, lowest part masses and lowest gold mass of the listed gMMC samples B-F. Thus, one may decide to use a gMMC having the composition corresponding to either sample B or sample F if such factors are desired. As described above, other factors, such as hardness, scratch resistance, machinability and color, can also be used to determine the composition of gMMC used in a manufactured part. 
     Table 2 below summarizes some cosmetic and physical properties of various 18 k gold samples 1-13, in accordance with described embodiments. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Cosmetic and Physical Properties of 18 k Gold Samples 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Pure Gold 
                   
                   
               
               
                   
                   
                   
                   
                   
                 Matrix 
                 Ceramic 
               
               
                   
                 Particle 
                 Particle 
                   
                 Melting 
                 Volume 
                 Volume 
                 MMC 
               
               
                 Sample 
                 Type 
                 Color 
                 Density 
                 Point 
                 Fraction 
                 Fraction 
                 Density 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 18 k gold 
                 — 
                 19.3 g/cm 3   
                 1060° C. 
                 75% 
                 — 
                 — 
               
               
                   
                 alloy 
               
               
                   
                 (baseline) 
               
               
                 2 
                 Iron 
                 red, pink 
                 2.4 g/cm 3   
                 1250° C. 
                 27% 
                 73% 
                 7.0 g/cm 3   
               
               
                   
                 aluminum 
               
               
                   
                 silicate 
               
               
                   
                 (garnet) 
               
               
                 3 
                 Boron 
                 brown/grey 
                 2.5 g/cm 3   
                 2763° C. 
                 28% 
                 72% 
                 7.2 g/cm 3   
               
               
                   
                 carbide 
               
               
                 4 
                 Silicon 
                 black, 
                 3.2 g/cm 3   
                 2730° C. 
                 33% 
                 67% 
                 8.6 g/cm 3   
               
               
                   
                 carbide 
                 green 
               
               
                 5 
                 Aluminum 
                 light grey 
                 3.3 g/cm 3   
                 2200° C. 
                 34% 
                 66% 
                 8.7 g/cm 3   
               
               
                   
                 nitride 
               
               
                 6 
                 Diamond 
                 yellow, 
                 3.3 g/cm 3   
                 3550° C. 
                 34% 
                 66% 
                 8.6 g/cm 3   
               
               
                   
                 powder 
                 light grey 
               
               
                 7 
                 Cubic 
                 amber 
                 3.5 g/cm 3   
                 2967° C. 
                 35% 
                 65% 
                 9.0 g/cm 3   
               
               
                   
                 boron 
               
               
                   
                 nitride 
               
               
                 8 
                 Aluminum 
                 white/clear 
                 4.0 g/cm 3   
                 2977° C. 
                 38% 
                 62% 
                 9.8 g/cm 3   
               
               
                   
                 oxide 
               
               
                 9 
                 Sapphire 
                 clear or 
                 4.0 g/cm 3   
                 2040° C. 
                 38% 
                 62% 
                 9.8 g/cm 3   
               
               
                   
                 powder 
                 doped 
               
               
                   
                   
                 colors 
               
               
                 10 
                 Yttrium 
                 white 
                 5.0 g/cm 3   
                 2425° C. 
                 44% 
                 56% 
                 11.3 g/cm 3   
               
               
                   
                 oxide 
               
               
                 11 
                 Titanium 
                 yellow 
                 5.4 g/cm 3   
                 2930° C. 
                 46% 
                 54% 
                 11.8 g/cm 3   
               
               
                   
                 nitride 
               
               
                 12 
                 Zirconia 
                 white, 
                 5.9 g/cm 3   
                 2715° C. 
                 48% 
                 52% 
                 12.3 g/cm 3   
               
               
                   
                   
                 black, 
               
               
                   
                   
                 colors 
               
               
                 13 
                 Tungsten 
                 grey 
                 15.6 g/cm 3   
                 2970° C. 
                 71% 
                 29% 
                 18.2 g/cm 3   
               
               
                   
                 carbide 
               
               
                   
               
            
           
         
       
     
     In Table 2, sample 1 is an 18 k gold alloy sample and is used as a baseline for comparison with gMMC samples 2-13. Particle Type refers to the composition each sample, sample 1 being the only non-MMC sample. Particle Color refers to a perceived color of each of the samples. Density refers to the density of the particles in grams per cubic centimeter. Melting Point refers to the melting point of the sample. Pure Gold Matrix Volume Fraction refers to percentage volume of gold within the sample. Ceramic Volume Fraction refers to percentage volume of ceramic material within the sample. GMMC Density refers to the MMC density of each sample. 
     Table 2 provides information related to the appearance (color), amount of gold and physical properties (e.g., density, melting point) of gMMC samples 2-13, which can be used to design a composition of a manufactured part. For example, a gMMC formed from garnet particles (sample 2) can impart a red/pink color a final gold color of the gMMC. Similarly, a gMMC that includes aluminum oxide (sample 8) or titanium oxide (sample 10) can impart a white aspect to a final gold color of the gMMC. In addition, Table 2 indicates that gMMCs formed from garnet particles (sample 2) and boron carbide particles (sample 3) have the lowest density of the gMMC samples 2-13. Thus, gMMCs formed of these particles may be considered for manufacturing parts in which lighter weight is desirable. In some embodiments, two or more of particle types listed in Table 2 are used together in a single gMMC to give the gMMC a desired color. 
     Table 2 can provide information also provides information related to relative densities of gMMC materials using different ceramic materials. As shown, the gMMC densities using different ceramic particles can vary broadly. For example, an 18 k gMMC formed from garnet particles (sample 2) can have a density of 2.4 g/cm 3  while an 18 k gMMC formed from tungsten carbide particles (sample 13) can have a density of 15.6 g/cm 3 . Thus, a part made of a gMMC material can be designed based in part on a desired final density. In some cases, it is desirable that the gMMC have a relatively low density in order to reduce a perceived heft of a part. According to some embodiments, an 18 k gold gMMC having a density of less than about 10 g/cm 3  is formed. According to some embodiments, an 18 k gold gMMC having a density of less than about 5 g/cm 3  is formed. According to some embodiments, an 18 k gold gMMC having a density ranging between about 2 g/cm 3  and about 5 g/cm 3  is formed. 
     Table 2 can also provide information as to other physical properties that can be helpful in deciding the type of ceramic particle to use, including melting point, volume fraction of ceramic particles and gold matrix density. According to some embodiments, an 18 k gold gMMC having a melting point of greater than about 1200° C. is formed. According to some embodiments, an 18 k gold gMMC having a volume fraction of ceramic particles is greater than about 50% is formed. According to some embodiments, an 18 k gold gMMC having a gold matrix with a density of 7.0 g/cm 3  or greater is formed. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of specific embodiments are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the described embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.