WO2013123584A1 - Highly filled particulate composite materials and methods and apparatus for making same - Google Patents

Abstract

A composition comprising a high volumetric loading of near spherical particles in a high temperature matrix is disclosed. The matrix material encapsulates the solid particulate material, wherein each individual particle of the solid particulate material is in close contact with an average of at least (6) adjacent particles. A method comprising providing a solid particulate material, providing a mold, providing a means for packing the particles in the mold to provide the high packing density which may include : evacuating the solid particulate material of gas in the void spaces between the particles of the particulate material, evacuating the mold, introducing the evacuated solid particulate material into the mold, providing a fluid matrix material, and introducing the fluid matrix material into the void spaces while constraining the solid particulate material.

Description

HIGHLY FILLED PARTICULATE COMPOSITE MATERIALS AND METHODS AND

APPARATUS FOR MAKING SAME

Listing of Prior Art

1. RK McGeary "Mechanical Packing of Spherical Particles" Journal of American Ceramic Society. Vol. 44, Issue 10, pp. 513-522, October 1961

2. US Patent No. 3,477,967, Dated Nov. 11, 1969 Titled: "Syntactic Foam"; Inventor:

Israel Resnick

3. MM Islam, HS Kim "An Overview on the Manufacturing and Properties of Syntactic Foams Made of Ceramic Hollow Microspheres and Starch" University Publication at University of Southern Queensland, Australia.

4. Patent No. 5,525,557, Dated: June 11, 1996 Title: High Density Green Bodies;

Inventors: VK Pujari, G Rossi, Assigned: Norton Company

5. Patent Application Publication Pub. No. US-2005/0124708A1 dated: Jun. 9, 2005, Title:

"Syntactic Foam" Inventor HS Kim;

FIELD

The present invention relates to particulate based composite materials and methods and apparatus for making same. More specifically, the invention relates to materials and techniques for a new class of materials that are significant improvements on what are generically labeled syntactic materials and syntactic foams allowing the materials to function at much higher temperatures than conventional organic based foam materials.

BACKGROUND

Composite materials are most often viewed as being composed of fibers or fiber-like materials bonded with some form of matrix to transfer the load from the typically weaker matrix to the stronger fibers. However, composites can also be comprised of particles in a "binder" or "matrix" phase that holds the particles together. There are numerous materials comprised of "bonded particulates" including sintered ceramics and metals, metal bonded ceramics, of which cobalt bonded tungsten carbide tooling materials are a typical example. One of the oldest particulate bonded "composites" is simple earthen clay pottery which is essentially glass bonded ceramic. These materials have been utilized for centuries.

Modern methods have in most cases replaced the "mold, dry, and fire" process with other more efficient methods such as slip casting, dry pressing, compression molding, etc. followed by sintering for ceramic and metals. Other, more expensive methods such as hot pressing and reactive sintering have also found some applications.

All of the methods delineated above suffer from a generic problem of "shrinkage" in that the forming process results in a component that has significant void volume that must be eliminated to produce a part with sufficient properties to be useful - unless one wishes to make a filtration component. In nearly all cases the consolidation technique produces a part with final dimensions significantly smaller than the original formed part. In many cases there is also distortion of the original shape due to differential density in the formed part resulting in differential shrinkage.

Particle packing fractions and the maximum number of monosized spheres that can be touching in a close packed structure was demonstrated and calculated millennia ago. McGeary in 1961 (Ref 1) used metallic spheres (shot) and calculated the size ratios and percentages for each size of the shot needed to produce very high packing density - up to 95% by volume. However the spheres were significantly larger, typically greater than 1 mm, than would normally be used in most engineering applications.

During the mid 1960s a new class of materials was developed for deep water exploration vehicles, these materials were called "syntactic foams" and were typically composed of hollow glass spheres imbedded in epoxy or other organic resins. The new materials tended to have much lower shrinkage, since the resin simply filled in the spaces between the particles. However, the syntactic foam of the time still suffered from non-uniform particle packing, filler density variations, and the resulting distortion. Due to the viscosity of the resins, the particle packing density did not approach the theoretical value.

In many cases the volume fraction was well below the theoretical value of 0.74 as demonstrated by US Patent 3,477,967 (Ref. 2) which disclosed a high buoyancy syntactic foam composed of glass microspheres imbedded in a modified epoxy resin. Resnick also disclosed an apparatus and process description utilizing dry spheres poured into a mold and packed tightly utilizing vibration, followed by evacuation of the packed spheres in the mold and drawing of the epoxy resin into the mold to encapsulate the spheres. The encapsulated spheres in the mold are then heated to 300°F while still under a low vacuum to cure the epoxy resin. The procedure produced a foam with roughly 65 volume% glass spheres. The inventor also disclosed the use of coupling agents such as Gamma-aminopropyltriethoxysilane (now a standard coupling agent for glass materials in an organic matrix). This expired patent does disclose many of the procedures described in the examples of the current application, but does not provide a technique to achieve the high particle packing density of the present invention, an does not disclose the use of fugitive and non-fugitive binders as does the present application.

In fact, most of the work on syntactic foam-like materials since the 1960s has utilized larger diameter spherical particles typically in the range of 50 micrometers up to 500 micrometers, with some large structures employing spheres as large as 3-4 inches - designed specifically for large, high buoyancy marine components. In all cases, unlike the present invention, the matrix material was an organic resin such as epoxy and not capable of operation at above 250°C. It was also found that as the particle sizes decreased below 50 micrometers, other forces besides gravity or buoyancy of the particles came into play. In particular, electrostatic forces - particles charging up themselves by rubbing against each other - caused a myriad of problems.

One of the issues preventing attainment of high packing density is the capillary forces of the infiltrating liquid wet the particles and effectively pry the particles apart such that each particle has a layer of liquid between it and the next adjacent particles. This force is significant and causes a volume expansion of dry particles in a compact as the liquid infuses around the particles. Islam and Kim (Ref. 3) developed a mathematical model showing the volume expansion of a hollow sphere compact upon introduction of the "binder" phase. They determined that these forces were greater on small particles than larger particles and there was a limit to the packing density that can be obtained with the simple introduction of a liquid binder that is solidified. Their models showed the expansion and shrinkage of cast syntactic foams and that the flexural strength of the foams was independent of the particle size down to roughly 30 micrometers.

Modern slip casting surfactants in water based systems have evolved to mitigate these problems down to the submicron level but require the use of high liquid content "slips". The high liquid content requires long drying times and rigorous mold design. Recent slip casting formulations can achieve particle packing volume fractions as high as 88% with low shrinkage, but require complicated and careful processing and mold control. Patent No. 5,525,557 disclosed a non-aqueous binder approach to making high density sintered spherical bodies from slip cast or cold pressed compacts. (Ref. 4) The primary use was to produce spherical silicon nitride bearings, but the non- water based binder was useful for suspending any powder that would be subject to hydrolysis in water. The particles were non-spherical and the resulting compact was subject to significant shrinkage.

A Patent Application Publication Pub. No. US-2005/0124708A1 (Ref. 5) discloses a floatation densification procedure for producing close packed syntactic foams from microspheres and liquid binders. The invention utilizes an epoxy binder diluted in acetone to a very low viscosity into which the hollow spheres are dispersed. The hollow spheres float to the top of the mold and pack into a close packed configuration. Once the spheres are packed into a layer of sufficient thickness, the excess liquid is drained off. The inventor was able to reach near theoretical packing density with the procedure if the viscosity of the diluted binder phase was low enough, i.e. the amount of epoxy dissolved in the acetone was sufficiently small.

This disclosed procedure would not provide the ability to produce complex shapes due to the floatation requirement, and therefore is not as advantageous as the present invention. It would also not provide a low density material capable of high temperature applications.

SUMMARY OF THE INVENTION

The invention discloses a material composition and formation technique that produces particulate filled materials with exceedingly low shrinkage upon consolidation. The invention utilizes spherical particles of a narrow size ranges designed to maximize particle-to-particle contact which is defined as a "high particle volume packing fraction" thus giving each particle in the invention a coordination number, which is the number of nearest neighbors to each particle, of at least 6 and preferably 8- 12 out of a maximum of 12 for close-packed monosized spheres. The high packing density also minimizes bulk porosity. A further aspect of the invention is the use of very dilute fugitive and non-fugitive binders in a volatile solvent such as acetone which when it evaporates, will draw the particles into close contact. The cured resin forms a nanometer scale thick layer of "glue" to hold the particles in close contact. This allows removal of the part from the mold for further processing with negligible distortion. A further aspect of the invention is the use of special surface treatments and/or non-aqueous liquids to function as vehicles to control particle surface charges. A still further aspect of the invention is the use of specially formulated pre-ceramic polymer systems that allow conversion of the "binder" phase into ceramic materials to produce ceramic matrix syntactic foam or other ceramic composite materials. Unlike the previous art, these syntactic foams and materials can operate at temperatures comparable to sintered ceramic or high temperature metallic materials but can be formed at low temperatures in low cost molds. The parts can be molded at near room temperature but result in ceramic or metallic components very close to final dimensions and without distortion. Further, unlike previous art, the particulate volume fraction does not have to exceed 0.74 to produce the ultra low shrinkage.

Certain further embodiments of the present invention include a method for forming a particulate filler reinforced composite comprising providing a mold having a cavity therein and an opening in one surface of said mold and communicating with said cavity; providing a particulate filler material; evacuating the mold cavity; loading a quantity of a particulate filler material in the cavity; and introducing a matrix material into the cavity whereby void spaces in the particulate filler are infused with the matrix material while promoting contact among the particles of the filler.

Certain embodiments of the present invention include a method for forming a particulate filler reinforced composite comprising providing a mold having a cavity therein and an opening in one surface of said mold and communicating with said cavity; providing a particulate filler material; evacuating the mold cavity; loading a quantity of a particulate filler material in the cavity; and introducing a matrix material into the cavity whereby void spaces in the particulate filler are infused with the matrix material while promoting contact among the particles of the filler.

The novel materials of this invention are defined as "microspaceframe" materials or "MSF" materials, due to their "space frame" structure composed of closely packed near spherical particles that are surrounded by a supporting high temperature matrix. Specifically unlike prior art, the microspaceframe materials are designed to function at temperatures above 500°C.

In certain embodiments, MSF materials can be partially fired to produce lightweight, solid "green forms" (for use in the manufacture of net shapes) that are much easier and much less expensive to machine into precision molds or parts than are conventional materials. When these green forms are then fired to full hardness, the micro-scale spaceframe prevents shrinking and distortion, thereby maintaining high precision.

Tool wear is also reduced when using green form MSF materials, because of the reduced friction and reduced tooling pressures applied since the spherical particles break away from the tool more easily than non-spherical or acicular particles. Less tool wear results in longer useful tool life, reducing tooling replacement costs. Less tool wear also results in greater precision and accuracy, due to more stable tooling dimensions during forming processes.

Ceramic Engines:

In certain embodiments, MSF materials according to the present invention can be used in the manufacture of a version of an innovative gerotor (generated rotor) engine design by Torxx Group, Inc. of Ontario, Canada, using ceramic materials. The Torxx engine operates at temperatures that only ceramics can withstand and requires very high precision parts. High temperature materials would be desirable for the rotors, gears, bearings, combustor, ports and other parts of the engine.

However, the precision finishing of conventional ceramics, such as silicon carbide, requires expensive grinding using diamond pastes in a process that does not lend itself to low-cost mass production. MSF materials provide the requisite temperature resistance and ultra-high precision tolerances in a process that can readily be adapted to facilitate inexpensive mass production. Also, regular ceramics currently contain many material flaws that lead to parts failure at stresses far below the theoretical strength of the material in its ideal, defect-free form. MSF materials deliver greater practical strengths because of far fewer and smaller material defects, together with a microstructure that terminates flaw propagation.

Higher power density is very desirable for engines. The power density of a gerotor engine, measured in watts per kilogram and in watts per cubic meter, increases directly with a faster rotational speed of the engine rotors. Centrifugal forces on the rotors increases directly with the density of the rotor material and as the square of the rotational speed. The limitation on rotational speed is determined by the strength of the rotor material under rotational stresses. Therefore it would be desirable to have a ceramic gerotor material that is of low density, with high strength under rotational stress, and that is capable of withstanding high temperatures.

Certain embodiments of the present invention include MSF materials useful for producing high- precision, high-performance ceramic bearings and gears.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are illustrative and are not drawn to scale.

FIG. 1 is a cross sectional view showing an example of a mold with the mold cavity filled with a MSF material produced in accordance with an embodiment of the invention. FIG. 2 is a cross sectional view showing the micrometer-scale and / or nanometer-scale structure of a MicroSpaceFrame material, presenting a magnified area of a MSF material, in accordance with an embodiment of the invention.

FIG. 2a is a micrometer scale view of the "fugitive" binder aspect of the invention

FIG 2b is a micrometer scale view of resin infused fugitive binder aspect of this invention

FIG. 3 is a block diagram showing a method of manufacturing MicroSpaceFrame materials according to an embodiment of the present invention.

FIG. 4 is a cross sectional view showing an example of the ultra-precision bonding of mating surfaces areas of two objects by the use of a MicroSpaceFrame bonding material according to an embodiment of the present invention.

FIG. 5 is an enlarged view of a section of the objects of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The invention is comprised of hollow or solid spheres or nearly spherical particles with an L/D of between 0.75 to 2.0. These particles are bonded into a high strength matrix material to form what is termed a "Micro-Space-Frame" material or "MSF" material. The composition of the particles can range from simple glass such as glass spheres or hollow glass "micro-balloons" to hollow or solid ceramic spheres, ceno-spheres, hollow or solid alumina based ceramic spheres, or rounded ceramic particles. In general any hollow or solid near spherical particle that can be used at above 500°C is applicable to this invention. Hollow or solid phenolic resin spheres or metallic spheres can also be used.

The invention also comprises a matrix that bonds the particles together, holds the shape of the molded article and retains the particle contact and packing density. The matrix cushions the particles against compressive and shear forces as well as provides the inter-particle bonding needed for high tensile strength. The matrix can be any high strength material, but is preferably a high temperature stable polymer, a ceramic forming polymer, or a high temperature metal. The invention utilizes a non-aqueous liquid to moderate and control the charge build-up on small diameter particles - specifically those particles below 50 micrometers, and preferably on particles of diameters 10 micrometers and below.

Referring initially to FIG. 1, a cross sectional view is shown of a mold 1 1 with a mold interior void 12 filled with a MicroSpaceFrame (MSF) material 13 manufactured according to one embodiment of this invention.

The composite elements forming the MSF material are generally of micrometer or nanometer size, and therefore at a scale not easily visible to the naked human eye, the MicroSpaceFrame material 13 appears to be a uniform solid. A small circular area 14 of the MSF material 13 is shown magnified in FIG. 2 as circle 21 in order to show the structure of the MSF material. Figures 2a and 2b show another embodiment of the invention, namely the use of non-fugitive binders to hold the particles essentially "in contact" after the evaporation of the solvent and subsequent curing.

The Solid Particle Phase:

A solid particle phase of the MSF material 13 is shown in circle 21 of FIG. 2, consisting of many microspheres 16 of equal diameter. The microspheres 22 have hollow void spaces 23, but in general can be hollow or solid.

Each microsphere 22 is shown in mechanical contact with its nearest neighbors. Only two dimensions are shown in FIG 1, however in three dimensions, each microsphere is, in general, in supporting physical contact with many of its nearest neighbors in all three dimensions. Microspheres at the surface defined by the mold void 12 are also generally in supporting physical contact with the internal surfaces of the mold 1 1.

During formation of a MSF material, each particle is in contact or within 1-5% of its diameter to an average of 10 other solid particles of a MSF material and, for the outermost particles in the spaceframe, with the walls of the mold during formation of the MSF material.

Suitable solid particle material compositions of the particle phase include ceramic, high temperature metal, glass, silica and carbon, with more rigid, higher temperature materials being preferred in order to achieve greater temperature capability and spaceframe rigidity and thereby obtaining greater precision. The solid particle phase illustrated in this embodiment of the invention consists of near spherical shapes, with a length/diameter of no greater than two (2). The particles can be either smooth or with rougher surfaces, but preferably the roughness is no more than 5% of the diameter in order to maximize packing efficiency.

Non-spherical particles tend to "bridge" when flowed into a mold, forming voids and other flaws in the formed material, such as slip planes, where applied stresses can be concentrated, leading to possible material failure under operational stress. Such flaws can also propagate and become larger flaws under stress, particularly under cyclically applied stress, leading to material failure over time.

Spherical particles also flow more rapidly and with less resistance when filling a mold, filling complex mold void contours or shapes with less applied pressure resulting in shorter molding cycle times and fewer problems such as flow blockages.

Therefore, particles with smoother contours are preferred over particles with sharper contours, and solid particles as close as practical to perfect spherical particles are most preferred for maximum material strength in embodiments of this invention.

The microspheres shown in this embodiment of the invention, shown in FIG. 1, are all of equal diameter, however in general the microspheres can also be of up to two discrete and narrow diameter ranges.

However, the smaller diameter particles must be less than or equal to 1/7* of the larger diameter particles, otherwise the smaller particles will interfere with the close packing of the larger particles. More than two discrete particle sizes will cause bridging in small (<50 micrometer) particle systems due to charging and statistical probability effects.

The microspheres shown in this embodiment of the invention are hollow microspheres, however in general the microspheres can be hollow or solid spheres or a mixture of hollow and solid microspheres.

For microspheres of a given material composition and a given diameter, those with a greater shell thickness have greater crush strength, with solid microspheres having the greatest crush strength.

However an offsetting factor is that greater shell thickness results in a denser material. Also, for microspheres of a given material composition and shell thickness, crush strength increases exponentially with decreasing particle diameter. Also for microspheres of a given material composition and a given shell thickness, the density of a microsphere will decrease directly with increasing diameter, since the ratio of surface area to volume ratio of a sphere varies inversely in proportion to the diameter of the sphere.

Therefore, for embodiments of this invention where maximum compressive strength is desired, such as for bearings, solid microspheres of smaller average diameter and composed of a stronger material are preferred. Alternatively, for embodiments of this invention where low material density is desired, such as for parts subject to inertial or centripetal stress (e.g. rotors), or where low mass of parts is important (e.g. parts for aerospace applications), hollow microspheres of lower shell thickness and greater average diameter are preferred, consistent with the desired material strength under operational stresses.

It will be obvious to those skilled in the art that, for a given set of operational criteria, methods such as finite element analysis can determine an optimal tradeoff of material strength versus density, and the optimal composition of microsphere diameters and shell thicknesses.

The Matrix Phase:

A matrix phase of the MSF material 13 is shown in circle 21 of FIG. 2 consisting of a solid material 24, formed from a liquid precursor material that was previously flowed into the mold void 1 1 so as to fill the interstitial spaces defined by the space within the mold void 11 and external to the microspheres 22, and then solidified to form the solid matrix phase 24, consistent with an embodiment of this invention described in FIG. 3.

The solidified matrix material 24 surrounds the microspheres 22 and structurally reinforces the points of contact between the microspheres 22 of the particle phase of the MSF material 13. The void spaces 25 within the matrix phase 24 represent voids that form when the liquid precursor shrinks as it is solidified. Examples of processes that would result in the fluid matrix materials shrinking include: (a) the cooling of a molten metal; (b) the curing of an epoxy resin; (c) thermal decomposition of a precursor material; and (d) a chemical reaction between precursor materials.

The precursor material for the matrix phase can be any liquid material of low enough viscosity to be capable of being introduced into the mold so that the liquid material infiltrates and surrounds the microspheres 22 of the particle phase, and that can be subsequently solidified to form a solid matrix around the particle phase. Suitable liquid materials include, but are not limited to, a ceramic-forming polymer; low viscosity, high temperature organic polymers, metals and alloys, such as molten aluminum, molten magnesium, or molten copper alloys.

Method of Manufacture:

FIG. 3: In one or more embodiments, the present invention encompasses a unique method for making MSF materials. Referring to FIG. 3, in one preferred Method:

Step 31: Microspheres are provided and are optionally coated with a surface coating to enhance bonding to the matrix phase or to impart other desirable qualities, such as absorptive properties for microwaves in stealth materials. While microspheres form the solid particle phase in this embodiment, other suitable particle material can be used for the solid particle phase as described in this specification.

Step 32: The microspheres are evacuated of gases (step 32), with the evacuation being sufficient to allow for the infiltration of a fluid matrix material in subsequent processing step 36, with a minimum of entrapment of gas bubbles, in order to avoid the formation of flawed regions where the microspheres are not bound together by the matrix material. Optionally, this Step 32 can be avoided and the microspheres evacuated only once they are placed in the mold.

Step 33: A mold is prepared by evacuation of gases from the mold, such evacuation being sufficient to allow for the infusion of a fluid matrix material in subsequent processing Step 36, with a minimum of entrapment of gas bubbles, in order to avoid the formation of flawed regions where the microspheres are not bound together by the matrix material.

Step 34: The evacuated mold is filled with the evacuated microspheres while vibrating the mold in order to encourage maximum compaction of the microspheres and to maximize the filling of all voids within the mold.

Step 35: A fluid matrix material is degassed, with the degassing being sufficient to minimize the formation of gas bubbles in subsequent processing Step 36, in order to avoid the formation of flawed regions where the microspheres are not bound together by the matrix material.

Step 36: The degassed fluid matrix material is infused into the mold, filling the remaining void space around the microspheres within the mold void space. Step 37: Optionally, excess fluid matrix material can be removed by centrifuging the mold, leaving a surface coating of the liquid phase material covering the exposed surface areas of the solid phase. The average thickness of the remaining surface coating can be controlled by the duration and speed of the centrifuging process.

Removing excess fluid matrix material can be desirable in order to reduce the density of the finished MSF material. Removing excess fluid matrix material can result in the formation of a network of

interconnected void spaces, which can allow for the exhausting of any gases generated by the solidification process, or it can allow for gas exchange such as the absorption of water vapor during the curing of some phenolic or silicone type of fluid matrix phases. This can speed up the process of solidification and / or it can improve the strength of the finished MSF material by allowing for gases generated by the solidification process to exit the liquid phase normal to the surface layer, rather than allowing gas bubbles to form, causing flaws that may reduce the strength of the finished MSF material.

Fluid Form MSF Material:

Fluid Form MSF Material can be taken from the process after Step 36 or after optional Step 37. Fluid form MSF Materials can be used for purposes such as a bonding material for bonding applications, as a filler material for repair applications, or as a material for use in subsequent molding operations, amongst others.

Without being bound by theory, it is hypothesized that at the micrometer and nanometer scale, gravitational force is a second order force acting on the fluid matrix phase compared to surface tension forces and surface adhesion forces. Surface tension forces of the fluid matrix phase together with surface adhesion forces between the fluid matrix phase and the solid phase surfaces will determine the shape and location of the void spaces between solid surfaces. Therefore in general, the void shapes will be centered between surrounding solid surfaces, as illustrated by the location of the void spaces 25 in FIG. 2.

Without being bound by theory, it is hypothesized that when a Fluid Form MSF Material is made in accordance with the method of manufacture described in FIG. 3, the surface tension forces of the fluid matrix phase together with surface adhesion forces between the fluid matrix phase and the solid phase surfaces will act to minimize the surface area of the liquid phase material, thereby acting to hold together the solid particles in touching contact and to prevent the separation of the contact points between the solid phase particles. This means that a Fluid Form MSF Material will remain a MSF material with the solid phase particles in touching contact even without a containing mold. In one or more embodiments of the present invention, removing excess fluid matrix material as described in Step 37 is preferred for the formation of a Fluid Form MSF Material. If excess fluid matrix material is present, the solid phase particles will be free to move apart without constraint from surface tension forces. If it is then placed in a mold and solidified, such a material would as a result suffer some degree of shrinkage and twisting, since the essential condition of a rigid solid particle spaceframe would no longer be present.

A Fluid Form MSF Material can be prepared in a bulk batch and can then be flowed or injected under pressure into molds; it can be flowed or injected to fill the space between solid shapes and then partially or fully solidified to bond the solid shapes together. In particular, a Fluid Form MSF Material can be used as a bonding material for forming precision bonds between components, as described below under the heading "Precision Bonding and Assembly Using MSF Materials".

Once they have been shaped (for example by molding or pressing) as required for any given application, Fluid Form MSF Materials are generally then further processed in accordance with Step 38 of FIG.3 to produce Green Form MSF Material and / or Step 39 of FIG.3 to produce a Plastic Form MSF Material or a Hard Form MSF Material.

Step 38: Optionally, the fluid matrix material is then partially solidified by a process appropriate to the matrix material, such as cooling, curing, thermal decomposition or chemical reaction. Controlling the portion of the fluid matrix material that is solidified can vary the degree of hardness. The optimum degree of hardness for a Green Form MSF Material for any given application will be determined by the minimum safe hardness required for the Green Form MSF Material to withstand handling and processing, without significant damage, wear or loss of precision.

Green Form MSF Material: Green Form MSF Material is produced at the completion of Step 38 of FIG.3. The result is a soft yet rigid Green Form MSF Material, similar in softness and rigidity to "green" pottery, which is created by partially firing clay materials. These materials are "soft" in the sense that they are easily cut or abraded with relatively low force and have low mechanical strength compared to a fully hardened material.

Step 39: The fluid matrix material is solidified by a process appropriate to the matrix material, such as cooling, curing, thermal decomposition or chemical reaction. Hard Form MSF Material: Hard Form MSF Material is produced at the completion of Step 39 of FIG.3.

Plastic Form MSF Material: Plastic Form MSF Material is produced at the completion of Step 39 of FIG. 3.

Plastic Form MSF material can be extruded or drawn through a die to produce extruded or drawn shapes; it can be pressed or stamped in a die or form; and it can be otherwise formed and fashioned by injection molding, stamping, rolling or other forming processes known to those familiar with the art. Generally, a MSF material will have plastic properties if the matrix material has plastic properties. Plastic Form MSF material may require heating, for example if the matrix material is a thermoplastic, to exhibit plastic properties.

A discovery of this invention is that, if the particle phase is comprised of microspheres, Plastic Form MSF materials will exhibit better properties for being flowed, injected or drawn than does either: the native plastic matrix material on its own; the native plastic matrix material mixed with a lower volume percentage of the same microspheres (for example conventional syntactic foams), or the native matrix material mixed with other particles or fibers. Such improved properties include: lower viscosity; lower pressures and/or temperatures required for processing; greater precision in forming finely detailed or complex shapes or surface patterns; reduced wear to molds, tooling and processing equipment; and greater ease of manufacturing and lower capital and operating costs of manufacturing resulting from these improved properties. The microspheres act as tiny ball bearings which rotating and easily moving past each other in a fluid fashion. This reduces viscosity and acts as a lubricant for cutting or abrasion tools as the tiny microspheres roll under the blade or tool contact surface.

A key discovery of this invention is that when a material is made in accordance with the method of manufacture described with reference to FIG. 3, a MSF material is formed that has advantages over the existing art with respect to one or more of the following desirable properties: low formation distortion, low bulk density; low bulk thermal expansion; low bulk thermal distortion; low thermal conductivity; high thermal and chemical stability; high impact energy absorption; low sound transmission; high mechanical strength; low defect formation; low defect propagation; low cost of production; improved ease of machinability; improved precision net shape casting; improved precision assembly and bonding of component parts; improved establishment and maintenance of precision tolerances and stack-up precision tolerances in three spatial dimensions in simple or complex assemblies of component parts; reduced part counts for component assemblies; and improved ease of manual or automated assembly of components. Microspheres:

The term "microspheres", as used in this description, refers to micrometer-scale hollow particles of approximately spherical shape. Microspheres are also commonly referred to as microballoons or microbubbles. It is to be understood that the solid particle phase of a MSF material can consist of particles with dimensions from 50 nanometers to 1000 micrometers. Examples of hollow solid phase particles suitable for use in the present invention include, but are not limited to: hollow or cellular glass microspheres; hollow phenolic microspheres; hollow ceramic microspheres; cenospheres; and natural perlites.

Among other advantages, a discovery of the present invention is that by incorporating low-density solid phase particles, such as hollow glass microspheres, hollow polymeric microspheres, hollow ceramic microspheres, or natural perlite materials, the density of MSF materials can be reduced to about 0.4 to 0.7 grams per cubic centimeter compared to conventional solid ceramic materials with densities of about 2 to 3 grams per cubic centimeter, or compared to solid metal like aluminum (2.7 grams per cubic centimeter) or stainless steel (about 8 grams per cc), while still maintaining good to excellent material strength because of reduced flaw formation, as discussed above.

Suitable microspheres can include those commercially available, such as those manufactured by 3M, Expancel, Pierce & Stevens Corp., or Emerson & Cuming, Inc. Perlites are natural multi-cellular hollow micro-spheres. Perlites are hydrated rhyolitic volcanic glass containing between two and five percent of chemically combined water, which permits production of an expanded cellular material of extremely low bulk density when the ore is heated to its softening temperature. Cenospheres are hollow microspheres typically produced as a byproduct of coal combustion at thermal power plants, with a density of about 0.4-0.8 g/cc. They have a melting temperature of about 1300 degrees Celsius, making them suitable for use in high-temperature applications. Cenospheres are generally lower in cost than manufactured microspheres and are available from numerous sources, such as Ceno Technologies Inc. Other spheres such as alumina microspheres can withstand over 1800°C and can be used for very high temperature applications.

The true density of these lightweight microsphere filler materials can be in a range from 0.05 to 0.70 g/cc. In one preferred embodiment, the hollow microspheres are hollow glass microspheres with a density of 0.1 to 0.35 g/cc. The materials used for the hollow microsphere materials for the invention can be made of high temperature stable organic resins such as phenolic resin or preferably high temperature stable inorganic materials such as glass, ceramic, perlite, graphite, cenospheres, and other high temperature ceramics, although the invention is not limited to these materials. The shapes of these materials, in general, are generally geometrically spherical with a length to diameter (L/D) ratio of less than two, and single celled, encapsulated with air or other lightweight gaseous materials. Multi-celled microspheres with irregular shapes are also commercially available (e.g., perlite).

As an example of the foregoing, a preferred hollow glass microsphere is the Kl microsphere, which is manufactured by 3M, St. Paul, Minn. The true density of Kl is about 0.125 g/cc, and the materials are made of soda-lime-borosilicate type of inorganic materials. S22 is another hollow glass micro-sphere offered by that supplier. The difference between Kl and S22 is that Kl has a true density of 0.125 g/cc and S22 has a true density of 0.22 g/cc. The diameter of Kl microspheres is much larger than that of S22. S22 may have better crush strength than Kl spheres.

Surface Treatment:

The surfaces of the particle phase can be optionally coated to enhance the strength of surface bonding between the particle phase and the matrix phase. For example, an epoxy silane coupling agent can be used to enhance bonding of an organic matrix material, such as a phenolic resin to an inorganic particle phase, such as glass or ceramic microspheres.

Precision Bonding and Assembly Using MSF Materials:

Fluid Form MSF materials can generally be used as MSF bonding materials to bond together solid parts made of the same MSF solid phase and matrix phase material composition, or from differing

compositions.

Referring to FIG. 4, a cross sectional view is shown of a part 41 bonded to a mating surface of a second part 42 by a bonding layer of MSF Bonding Material 43 manufactured according to one embodiment of this invention. FIG. 4 not drawn to scale.

Part 41 is shown as having joining elements 44 consisting of convex surface protrusions, which mate with complementary concave intrusions of part 32 for the purpose of mechanically strengthening the joint between part 41 and part 42, and for the purpose of aiding the precision location of part 42 with respect to part 42 during assembly. MSF Bonding Materials can also bond parts without the use of joining elements. In general, someone skilled in the art can determine the number, shape and location of such joining elements 44.

The composite elements forming the MSF material are generally of micrometer or nanometer size, and therefore at a scale visible to the human eye, the MSF Bonding Material 43 appears to be a uniform solid. A small circular area 35 of the MSF Bonding Material 43 is shown magnified in FIG. 5 in order to show the structure of the MSF Bonding Material.

A solid particle phase of a MSF bonding material 43 is shown in FIG. 5, consisting of many microspheres 55 of equal diameter, referred to as "monodisperse". The microspheres shown here are solid and are solid and monodisperse, but in general the microspheres for a MSF bonding material can be hollow or solid, and can be monodisperse or a blend of up to three particle sizes.

Each microsphere 55 is typically in mechanical contact with 10 or more nearest neighbor microspheres and with the mating surfaces of part 52 and part 56, providing compressive strength and distribution of compressive forces.

The matrix phase 53 surrounds the microspheres 55 and preferably is present in sufficient quantity to make contact with all of or nearly all of the area of the mating surfaces.

The matrix phase 53 of a MSF bonding material can be any fluid that will solidify and bond to both the particle phase and to both of the mating surfaces with sufficient strength, and that has other physical properties, such as thermal expansively, that are compatible with the materials of the parts being bonded.

The bonding layer MSF bonding material 54 is compressed to diameter of a single microsphere 55, provided that the mating surfaces are smooth and precisely complementary relative to diameter of the microspheres of the solid phase.

A discovery of the present invention is that, used in accordance with the teachings of this invention, MSF bonding materials have the following advantages over conventional bonding materials for precision bonding: a) Low-Shrinkage, Low-Distortion: Conventional bonding materials tend to shrink and twist as they solidify or harden reducing the strength of bonds and reducing the precision with which component parts can be assembled. MSF bonding materials exhibit negligible shrinkage and negligible distortion during bonding, keeping the relative position and relative orientation of a MSF bonded assembly of parts precise to a degree not possible - or costly to achieve - with conventional bonding materials. b) Precision Thickness: With conventional bonding materials, the thickness of the bonding layer is difficult to keep at precisely the optimum thickness over the entire surface area of the mating surfaces as parts are pressed together, which results in weaker bond formation, and loss of precision with respect to relative position and relative orientation of the parts. A bonding layer of MSF bonding materials using monodisperse microspheres always compresses to a bonding layer thickness of precisely the diameter of the microspheres.

For precision bonding of two parts at mating surfaces, MSF bonding material is metered onto the lower mating surface, generally as a droplet at the geometric center of the lower surface, or as a line of MSF bonding material laid as a line along the major geometric centerline of the lower surface. The mating surfaces of the two parts are pressed together, preferably by a precision, six-axis actuator with the associated precision metrology to achieve micrometer or nanometer positioning and precise application force during bonding.

If the parts to be bonded are both comprised of the same MSF material, then generally the preferred MSF bonding material will be that same MSF material in fluid form, in order to ensure optimum compatibility with respect to physical properties, such as thermal expansion coefficient and thermal conductivity.

Syntactic Foam Composed of MSF Materials:

Syntactic foams are composite materials synthesized by filling a metal, polymer or ceramic matrix with hollow particles called microballoons, with microspheres being one type of microballoon. The presence of hollow particles results in lower density, higher strength, a lower coefficient of thermal expansion, and, in some cases, radar or sonar transparency.

The conventional method of producing syntactic foam is to mechanically mix the microballoons into the matrix material. This conventional method has three essential disadvantages: breakage of microballoons during mixing, poor mixing at higher volume fractions of microspheres, and flaw formation. A discovery of the current invention is that syntactic foam materials made in accordance with the teachings of this invention and the method described in FIG. 3 have the following advantages over conventional bonding materials for precision bonding: a) Less Breakage: The shear forces involved in mechanical mixing results in the breaking or disintegration of many microballoons, particularly with more viscous matrix materials, which includes most epoxy, metal, organic polymer and ceramic matrix materials. This breakage generally reduces all of the advantageous properties of syntactic foams. The percentage of broken microballoons generally increases with higher volume fractions of microballoons. Processes to remove the broken microspheres are expensive. Mixing is generally carried out at low speeds to minimize breakage, however this increases processing time and costs. Syntactic foam material prepared in accordance with the method described in FIG. 3 has significantly fewer broken microspheres, since no mechanical mixing is used. b) Better Bonding: With a viscous matrix material, as the volume fraction of microballoons increases, fewer of the microspheres are fully coated by the matrix material during mixing. This means that the bonding of the matrix phase to the particle phase becomes weaker, reducing the strength and the elastic modulus properties of the syntactic foam. Many desirable properties of syntactic foams, such as lower density and greater compressive strength improve with the volume fraction of microballoons, however poor mixing properties often limits the volume fraction of microballoons to less than the theoretical packing volume maximum.

The use of the method described in FIG. 3 produces syntactic foams that have much more complete coating of microballoons, since the fluid matrix is infiltrated into an evacuated particle phase. With the use of Method 20, the volume fraction microballoons approaches the theoretical packing volume maximum, since the mold volume is completely filled with microballoons while the mold is being vibrated. c) Fewer Flaws: Mechanical mixing of microballoons into a matrix material generally entraps gases within the mix. These entrapped bubbles are flaws at which stresses can concentrate, significantly reducing the compressive strength and elastic moduli of the syntactic foam. Mechanical mixing also generally produces non-uniform mixtures with regions of lower or higher concentrations of microballoons within the matrix material. These non-uniform regions are flaws at which stresses can concentrate, significantly reducing the compressive strength and elastic moduli of the syntactic foam. The use of the method described in FIG. 3 produces syntactic foams that have fewer flaws from entrapped gases, since the fluid matrix is infiltrated into an evacuated particle phase.

The use of the method described in FIG. 3 produces syntactic foams that have fewer flaws from nonuniform concentrations of microballoons within the matrix material.

In certain embodiments of compositions and methods of the present invention, particulates (also referred to in certain embodiments as filler or solid phase material) are in mechanical contact which is defined as having the spacing between the particles be less than 1% of the particle diameter; in certain other embodiments, the contact is close contact, which is defined as a particle separation distance of 1% to less than 5% of the particle diameter. An important aspect of this invention is that the contact is sufficiently close to minimize shrinkage of an assembled structure or component during processing.

While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the described invention.

The following examples can illustrate applications of the art. These are meant to be illustrative of the described art and are not to be considered all inclusive.

Example 1 Using a non-fugitive binder to produce a low density insulation material

Hollow ceramic spheres such as cenospheres from Ceno Technologies 50 micrometers in diameter are suspended in a highly diluted ceramic forming polymer such as CSO-105 from EEMS, LLC which is diluted to proximately 5% resin in acetone. The suspension is poured into a mold cavity with walls containing sub-micron level pores and the acetone/resin solution is allowed to evaporate/be absorbed into the mold. A vacuum can be utilized to accelerate the acetone removal. Once the acetone has been removed the part is heated to 90-150°C for two hours to "set" the binder phase. The part is then carefully removed from the mold and placed into liquid tight container. The container is placed into a vacuum chamber where a 250 millitorr vacuum is pulled on the part to remove the air from the pores. A low viscosity liquid ceramic forming polymer such as CSO-105 is allowed to enter the container to fill in the voids in the part. After the part is completely immersed, the part is brought to atmospheric pressure and the excess resin material is allowed to drain off. The part is placed into an oven to cure the resin by heating to 150°C for 2 hours. Once cured, any excess resin is carefully removed and the part placed into a furnace, heated to at least 850°C in air and pyrolyzed for 1 hour then allowed to cool slowly. The resulting ceramic composite can be used as is for low thermal mass insulation or furnace components or further densified to improve strength and hardness.

Example 2 Using a fugitive binder to form a high temperature, high compressive strength engine or thruster component

Ceramic spheres high alumina spheres from 3M of Minesota are sieved and sized to produce narrow particle distributions centered at 20 and centered at 1-2 micrometers respectively. The spheres are blended at a volume ratio of 73% of the 20 micrometer and 27% of the 1-2 micrometer spheres to make the particulate part of the component. The blend is blended into a diluted polymethylmythacrylate solution in a low boiling solvent such as acetone to form a paint-like slurry. The suspension is poured into a liquid tight mold cavity shaped to match the desired configuration. The acetone/resin solution is allowed to evaporate. The application of some heat or a vacuum can be utilized to accelerate the acetone removal. Once the acetone has been removed the part is heated to 50-90°C for two hours to "set" the binder phase. The mold and part are placed into a vacuum chamber where a 250 millitorr or greater vacuum is pulled on the part to remove the air from the pores. A low viscosity liquid SiC forming polymer such as CS-160 from EEMS, LLC, or SMP-10 from Starfire Systems, Inc. is allowed to enter the container to fill in the voids in the part and completely immerse the part in the polymer. After the part is completely immersed, the part is brought to atmospheric pressure and the still immersed part is placed into an oven to cure the resin by heating slowly to 150°C and holding for 2 hours, during this time the fugitive binder is partially replaced by the SiC forming Polymer. Once cured, the part is removed from the mold any excess resin is carefully removed. The part placed into an inert gas furnace and heated to at least 850°C in nitrogen and held for 1 hour at temperature to pyrolyze (convert the polymer to ceramic) then allowed to cool slowly (during this process, the remainder of the fugitive binder is removed, the voids and pores left open by the decomposition of the fugitive binder are then filled with SiC precursor during the subsequent infiltration). The resulting ceramic composite is then vacuum infiltrated with the SiC precursor and pyrolyzed 6-8 more times at up to 1600°C in argon to produce a low porosity, high strength stable in air up to a 1700°C or above.

Example 3 Improving the density of the compact utilizing pressure to remove excess liquid matrix material

Hollow ceramic spheres of a narrow size range centered at 50 micrometers are poured into a ring or gear shaped mold cavity surrounded by heating coils and containing a porous bottom plate over a stiff perforated metal plate. A porous cover is placed on top of the mold to hold the spheres in place. The cover and porous bottom plates are permeable to low viscosity liquids but will not allow the particles to pass through. The cover is backed by a rigid porous metal plate. The porous metal plate is configured so that mechanical pressure can be applied onto the spheres in the mold. The mold has an attached apparatus to cause the mold and particles in the mold to vibrate at a frequency designed to optimize particle movement. The mold is evacuated through an outlet at the top of the mold while a low viscosity (less than 100 centipoise) liquid ceramic forming polymer resin is drawn through the bottom until the liquid completely fills the mold cavity and completely surrounds the particles. Once the mold is filled with liquid, mechanical pressure is applied to the top part of the mold and the mold is caused to vibrate. The combination of pressure and vibration causes the particles to rearrange themselves into a close packed state and the liquid provides some lubrication to assist in particle movement. Pressure is increased until there is no further movement of the upper pressure plate, indicating that the majority of the particles are in close contact - defined by a separation distance of less than 5% of the particle diameter (approximately 2.5 microns). The excess resin is allowed to go back through the porous bottom layer and through the tube back to the resin source. Once the part is compacted fully, the vibration is stopped, and the heaters are turned on. The mold is slowly heated to 120°C and held for one hour to solidify the resin matrix. The part can then be removed from the mold, excess resin removed, and placed into a high temperature furnace. The furnace is heated at 1°C per minute up to 900°C and held for 1 hour, then allowed to cool. The component will have a bulk density of less than 1.0 g/cm² and be stable at up to 1 100°C in air.

Claims

Claims I claim

1. A composite material comprising:

A material composed of spherical or near spherical particles arranged in a close-packed 3 dimensional array bonded together, and in some cases fully encapsulated, in said array by a high temperature (>500°C) stable non-oxide matrix.

2. The composition of claim 1 wherein "close packed" is defined as the minimum average number of three dimensional close contact points is 6 to 12, preferably 9 to 12, out of a possible 12, Additionally, the average spacing of the particles in the close packed array is less than 5% of the particle diameter.

3. The composition of claim 1 wherein the "near spherical" particles are particles that have a L/D of less than two and preferably have a surface roughness of less than 5% of the particle diameter.

4. The composition of claim 1 where particulate the material comprises microscale or nanoscale particles of diameters from 50 nanometers to 1000 micrometers, preferably 50 nanometers to 500 micrometers and most preferably 100 nanometers to 100 micrometers .

5. The composition of claim 1 wherein the particles are solid microspheres or nanospheres.

6. The composition of claim 1 wherein the particles are hollow microspheres or nanospheres.

7. The composition of claims 1-6 wherein the particles are bonded at their contact points by using a very low viscosity liquid that forms a fugitive binder material that temporarily holds the particles in close contact while the matrix material is being introduced and cured.

8. The fugitive binder material in claim 7 wherein binder material is a very low viscosity liquid such as an acrylic emulsion, an acrylate resin, polyvinyl alcohol, or other fugitive binder known in the art that is dissolved in a volatile solvent or in an aqueous emulsion.

9. The composition of claims 1 -6 wherein the particles are bonded together with a non-fugitive binder material that permanently holds the particles in close contact while the matrix material is being introduced and solidified.

10. The non-fugitive binder of claim 9 where material is taken from a class of materials that are very low viscosity liquid precursors to ceramic materials or carbon forming resins.

1 1. The material compositions in claim 10 where the materials are very low viscosity silicon oxycabide ceramic precursor resins, very low viscosity phenolic resins, or any other very low viscosity pre-ceramic polymer.

12. The material compositions in claim 1 1 where the resins are dissolved in an organic solvent or emulsified in water to provide the very low viscosity required.

13. The composite material composition in any one or more of claims 1-12 where the solid or hollow microspheres and nanospheres are composed or one or more of the following: phenolic resin, carbon or graphite, stoichiometric or non-stoichiometric silicon carbide, silicon nitride, silicon carbonitride, silicon nitrocarbide, silicon oxycarbide, silicon oxide, glass, cenospheres, alumina, aluminosilicate, mullite, zirconia, tungsten carbide, high temperature metal nitrides, carbides, borides, or oxides, or high temperature metals.

14. The composite composition of any one or more of claims 1 to 13 wherein the high temperature matrix is comprised of one or more of the materials selected from the group consisting of a ceramic material formed from a liquid pre-ceramic polymer precursor material; a high temperature metal or alloy; or a high temperature organic resin.

15. The composition of claim 14 wherein the ceramic formed from the pre-ceramic polymer precursor is one or more of the following: stoichiometric or non-stoichiometric silicon carbide(SiC), silicon oxycarbide (SiOC,SiCO), silicon nitride (Si₃N₄), silicon nitrocarbide (SiNC), and silicon carbonitride (SiCN), Hafnium carbide, Hafnium Nitride, or other nitride, carbide, or boride forming ceramic precursor polymers.

16. The composition of claim 14 wherein the polymer is low viscosity organic polymer capable of functioning at a temperature of 500°C or higher.

17. The composition of claim 14 wherein the high temperature metal or alloy is taken from, but not limited to: aluminum alloys, copper alloys, iron alloys, nickel alloys, and titanium alloys.

18. A method comprising:

providing a solid particulate material,

providing a mold, evacuating the solid particulate material of gas in the void spaces between the particles of the particulate material, evacuating the mold,

introducing the evacuated solid particulate material into the mold,

providing a fluid matrix material, and

introducing the fluid matrix material into the void spaces while constraining the solid particulate material.

19. The method according to claim 18 wherein the solid particulate material is comprised of micrometer or nanometer sized particles per Claim 4

20. The method according to claim 18 wherein the solid particulate materials comprises microspheres.

21. The method according to claim 18 wherein solid particulate material is selected from the group consisting of one or more of the following ceramic material formed from a liquid precursor material; a metal or a metal alloy; and a high temperature stable organic polymer per claim 13.

22. A method for forming a particulate filler reinforced composite comprising:

providing a mold having a cavity therein and an opening in one surface of said mold and communicating with said cavity; providing a particulate filler material;

evacuating the mold cavity;

loading a quantity of a particulate filler material in the cavity; and

introducing a matrix material into the cavity whereby void spaces in the particulate filler are infused with the matrix material while promoting contact among the particles of the filler.

23. The method in claim 22 whereby the minimum average number of three dimensional close contact points for each particle is 6 to 12 out of a possible 12 and more preferably 9-12 out of maximum of 12. Additionally, the spacing of the particles in the close packed array is less than 5% of the particle diameter.

24. The method of claim 22 wherein a high temperature stable (defined as stable at a minimum temperature of 500°C) matrix material holds the solid particulates in close contact.

25. The method of claim 22 of compacting the particulate filler material in the mold cavity utilizing a combination of vibration and applied pressure.

26. The method of claim 22 whereby the pressure is applied by mechanical, defined as compression of the particles using an applied mechanical load; or hydraulic, defined as the pressure caused by a gas or liquid.

27. The method of claim 22 wherein the filler material is compacted through vibration.

28. The method of claim 22 further comprising evacuating the void spaces in the particulate filler material prior to loading the filler material into the cavity.

29. The method of claim 22 further comprising the step of degassing the matrix material prior to introducing the matrix material into the cavity.

30. The method of claim 22 further comprising coating the particulate filler material with an agent to promote bonding of the particulate filler material to the matrix material.

31. The method of any one of claims 22 to 33 wherein the particulate filler material micrometer scale or nanometer scale particles per claim 4.

32. The method of any one of claims 22 to 33 wherein the particulate filler material comprises microspheres.

33. The method according to claims 22 to 33 wherein solid particulate material is selected from the group consisting of one or more of the following ceramic material formed from a liquid precursor material; a metal or a metal alloy; and a high temperature stable organic polymer per claim 13.

34. The method of any one of claims 22 to 33 further comprising removing excess matrix material from the cavity via wicking into a porous substrate; excess hydraulic or mechanical pressure; or by the use of a vacuum apparatus.