Patent Publication Number: US-2018029119-A1

Title: Equipartition of Nano Particles in a Metallic Matrix to Form a Metal Matrix Composite (MMC)

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to the field of metal matrix composites. More particularly, the invention relates to a method for uniformly distributing nanometer size ceramic particles throughout a metal matrix. 
     Background 
     Many prior art methods exist to incorporate nano particles into metallic matrices to form metal matrix composites (MMCs). These include: (1) the heat treatment of supersaturated metallic solid solutions to precipitate hard nano particles; (2) mechanical alloying (or ball milling) of metallic powders with ceramic nano particles followed by sintering/metal working at elevated temperature and (3) the precipitation of nano ceramic particles from liquid metal solutions, either as a result of a chemical reaction or upon chilling the liquid. All three of these well-known methods have their limitations. 
     Method (1) provides a very well distributed array of hard nano precipitates in the solute depleted metal matrix. However, these precipitated nano particles become unstable and dissolve back into the metal matrix as the operating temperature of the MMC approaches the aging thermal treatment temperature at which they were formed. This limits the operating temperature of such structural materials. 
     Method (2) suffers from the limitation that it is difficult to create a uniform three-dimensional array of ceramic particles in a metallic matrix without the use of expensive ball milling. Moreover, the grinding media used in ball milling tends to contaminate the nano powder as it is created. In many cases, organic agents are introduced into the ball mill to suppress flocculation of the newly formed nano particles. The anti-flocculating agents tend to be trapped into the metallic particles, which in turn embrittles the metallic matrix during subsequent high temperature sintering and metal working operations. 
     Method (3) is normally rather inexpensive to carry out, but suffers from the fact that it is difficult to form a uniform array of nano particles, and also from the fact that the microstructure of the cast metal matrix is rather coarse and possesses low mechanical strength. 
     A fourth method of creating a uniform array of nano ceramic particles relies on the fact that metallic powder particles naturally oxidize in air. The oxide layer is normally 1 nanometer (nm) to 10 nm thick. Using powder metallurgy to make a solid version of the oxidized particles, and extensive metal working afterwards, leads to a uniform distribution of nano ceramic particles throughout the solid. This approach to equipartition has two limitations. First, the volume fraction of nano ceramic particles that can be added to the MMC is directly related to the surface area of the initial particle size of the metallic powder. For example, in the case of aluminum powder, using this approach with a 1.3 μm average particle size as disclosed in U.S. Pat. No. 8,323,373, one is limited to 2.4 volume % of distributed nano aluminum oxide to the composite created. The second limitation is that 1.3 μm average particle size powder is very expensive and difficult to find commercially. 
     SUMMARY OF THE INVENTION 
     Ceramic particles are normally electrical insulators and can easily support electrical charges on their surfaces when immersed in a polar liquid. With proper blending of a surfactant and the nano particles in a polar liquid, one can create a colloid, at room temperature, where each individual nano particle is physically kept separated from all the others by electrostatic charges. Blending a metallic powder with surfactant-treated nano particles allows one to create a solid MMC with a uniform distribution of nano ceramic particles employing conventional powder metallurgy followed by extensive metal working. 
     Once the ceramic particles are uniformly distributed in the metal matrix, they will remain in place until the metal matrix is totally brought to a molten state thus removing the limitation encountered with the prior art method (1) above. Creating a nano particle dispersion by mixing surfactant treated nano ceramic particles with metallic particles at room temperature eliminates all the limitations noted for prior art methods (2) and (3) above. The advantage that the method of the present invention has over prior art method (4) above is that now we are able to incorporate volumetric concentrations of nano particles quite inexpensively well above the approximately 2% concentration limit available via method (4). 
     The basic steps of the present invention comprise mixing nano size ceramic particles with a surfactant in a polar liquid to produce a colloidal solution, blending the ceramic particles with micron or sub-micron size metallic particles, and then compacting the blended ceramic and metallic particles into a solid mass. The compacted material may be further worked by extruding or rolling to a desired shape and size. 
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the description of the present invention with unnecessary detail. 
     In one embodiment of the invention, a metal matrix composite with a uniformly distributed ceramic component is made using a process comprising several steps as described below. In this particular example, the composite is an aluminum-alumina (Al 2 O 3 ) matrix; however, it is to be understood that the invention is not limited in this regard and may be applied to any other composite comprising a metal-ceramic matrix. Suitable ceramic powders include, but are not necessarily limited to, oxides, borides, carbides and nitrides. 
     First, mix a population of alpha-phase, nano-sized alumina particles (approximately 5 nanometers to approximately 1000 nano meters in diameter) in a polar liquid, such as isopropyl alcohol (IPA, which is also referred to as 2-propanol or isopropanol), with a surfactant and/or dispersing agent that is soluble in the polar liquid, such as certain phosphonic acids, e.g., hexyl phosphonic acid. Alternatively, the alumina particles may be gamma-phase or a combination of alpha-phase and gamma-phase. The role of the surfactant/dispersant is to facilitate wetting of the alumina nano powder in the solvent of choice. For instance, hexyl phosphonic acid rapidly and covalently binds to the surface of de-hydrated alumina nanoparticles using its phosphonic acid group. The hexyl group facilitates nanoparticle dispersion by providing an interface that interacts with the solvent as well as with other dispersing agents or stabilizers. Another advantage of creating a “hexyl-capped” surface is to mitigate the strong pH-dependent suspendability of charged alumina nanoparticles in organic solvents. This is particularly useful for alumina nanoparticles, which are known to have an isoelectric point close to pH 9.0. A “hexyl-capped” surface is expected to have an isoelectric point close to pH 7.0, facilitating dispersion in organic solvents, such as IPA. Thus, de-hydrated alumina nanoparticles are added to a stirring solution of hexyl phosphonic acid in IPA. A probe sonicator (60 Hz) is also placed in the same solution to facilitate full dispersion of the nanoparticles by ultrasonic agitation. As large agglomerates are dispersed, phosphonic acid molecules rapidly cap the surface of these nanoparticles preventing de-agglomerations. 
     Second, during the dispersion process, the pH is adjusted to the optimal value of isoelectric point, determined previously by measurements of zeta potential. Addition of a dispersing polymer, such as poly vinyl pirrolidone (PVP) and continuous ultrasonic agitation helps keep the individual nanoparticles separated and suspended in the organic solvent. The end result is the creation of a stable colloidal alumina solution. Large agglomerates or certain impurities (such as fused nanoparticles or alien material) may be removed by centrifugation. Colloids can be formed at any temperature that reasonably permits solubility of the surfactant and dispersing polymer in the solvent. When IPA is used as a solvent, the most convenient temperature range is between 15-75 degrees Celsius. 
     Third, once a stable colloidal solution is formed (for example, by allowing the solution to stand for 6 hours or exposing it to 20 g for 5 minutes such that no further sediments form and all the nano alumina particles are in suspension), micron or submicron size aluminum metallic particles are added while under vigorous stirring and ultrasonic agitation. The solvent is then removed by evaporation, forming a matrix of alumina nanoparticles uniformly distributed among grains of micron or submicron size aluminum particles. 
     Note that if the centrifuge step is not required, this method may be used to create nano-alumina and micron-aluminum matrix composites in a single step, in any desired concentration, typically ranging from 1% to 40%. This is a significant advantage over the traditional prior art methods for creating these MMCs discussed above. 
     Fourth, in case other components are required in the final MMC, such as boron carbide microparticles or other ceramic microparticles, these components can be added after the addition of the aluminum powder. These other components can be added in a variety of concentrations, typically ranging from 0.1% to 20%. 
     Fifth, the blended powders from the foregoing step are consolidated into a solid mass. This may be accomplished by cold isostatic pressing, (CIP) and sintering, by vacuum hot pressing or by hot powder forging. 
     In the case of CIP or hot vacuum pressing, the compacted solids from the preceding step may then be either extruded or rolled to the final shapes and sizes required by the end user. In the case of extrusion, an extrusion ratio of at least 49:1 is desirable to force the alumina nanoparticles inside the aluminum grains. 
     It will be recognized that the above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.