US3706614A

US3706614A - Fabrication of composite material by uniting thin fiber coated polymerizable plastic sheets

Info

Publication number: US3706614A
Application number: US714901A
Authority: US
Inventors: Milton E Kirkpatrick; Jo L Reger; Karl P Staudhammer
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1968-03-21
Filing date: 1968-03-21
Publication date: 1972-12-19
Anticipated expiration: 1989-12-19
Also published as: GB1259625A; DE1914318A1

Abstract

Micron-size elongated particles or fibers of high strength material are dielectrophoretically deposited on a thin substrate or low strength material so that the fibers are aligned parallel to each other in a layer of micron-size thickness. A multiplicity of such fiber coated substrates are laminated bonding to form an integral structure.

Description

United States Patent Kirkpatrick et a1.

[4 1 7' Dec. 19, 1 972 154] FABRICATION OF COMPOSITE MATERIAL BY UNITING THIN FIBER COATED POLYMERIZABLE PLASTIC SHEETS [72] Inventors: Milton E. Kirkpatrick, Palos Verdes Peninsula; J0 L. Reger, Los Angeles; Karl P. Staudhammer, Gardena, all of Calif.

[73] Assignee: TRW Inc., Redondo Beach, Calif. 22] Filed: March 21, 1968 [21] Appl. N0.: 714,901

[52] US. Cl ..156/151, 117/93.4 R, 156/272,

156/276,l56/306, 161/170, 204/181 [51] Int. Cl. ..l ..B32b 31/12 [58] Field of Search ..156/151, 179, 276, 300, 309, 156/31 1 182, 306, 272; 1'17/93.4 R;

3,481,822 12/1969 Wilson et a1 ..161/93 2,343,775 3/1944 Land ...117/93.4 X 2,744,041 5/1956 Balchcn... ....156/179 X 3,082,138 3/1963 Hjelt ....156/151 X 3,244,572 4/1966 Nicol ..156/276 FOREIGN PATENTS OR APPLICATIONS 760,530 10/1956 Great Britain ..156/276 OTHER PUBLICATIONS Pohl, J. Applied Physics, 22 (No. 7), PP. 869-871 (1951).

Kirk-X Othmer Encyclopedia of Chemical Techno1ogy-]o1. 8 (2nd. Edition) pp. 23-36 (1965).

Primary Examiner-Carl D. Quarforth Assistant ExaminerE. A. Miller Attorney-Daniel T. Anderson, Gerald Singer and Alfons Valukonis [5 7 ABSTRACT Micron-size elongated particles or fibers of high strength material are dielectrophoretically deposited on a thin substrate or low strength material so that the fibers are aligned parallel to each other in a layer of micron-size thickness. A multiplicity of such fiber coated substrates are laminated bonding to form an integral structure.

1 Claim, 22 Drawing Figures Fig.1.

W Milton E. Kirkpatrick Jo L. Reger Karl P Stoudhommer INVENTORS.

AGENT.

PATENTED DEC 1 9 I972 SHEET 2 0F 5 Fig. 8.

Milton E. Kirkpatrick Jo L. Reger Kori P. Stoudhommer INVENTORS.

AGENT Fig. 4.

PATENTEBUENQ m2 3.706.614

Milton E. Kirkpatrick Jo L. Reger Karl F? Stoudhommer INVENTORS.

AGENT.

PATENTEDUECIQ m2 3,706,614

SHEET 8 0F 5 Fig. 15.

98 Fig. 21.

92 Milton E. Kirkpatrick J0 L. Reger Karl F? Stoudhommer AGENT.

PATENTED 19 1972 3,706,614

sum 5 OF 5 Milton E. Kirkpatrick do L. Reger Karl E Stoudhommer INVENTORS AGENT.

FABRICATION OF COMPOSITE MATERIAL BY UNITING THIN FIBER COATED POLYMERIZABLE PLASTIC SHEETS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the fabrication of composite materials of the kind in which micron-size fibers or whiskers of a high strength material are incorporated in a matrix of light weight lower strength material. The resultant composite combines the high strength of the fibers or whiskers with the desirable properties of matrix material which may, for example, have high strength-to-density ratio, or high melting point.

2. Description of the Prior Art I The basic concept of composite materials consists of embedding a high strength material in a weaker matrix. The higher strength materials commonly 'used are fibers, whiskers, or continuous polycrystalline filaments. A fiber is defined as an elongated particle of micron-sized diameter, with a length-to-diameter ratio of over to l. The fiber material may be non-crystalline, single crystal, or polycrystalline in nature. A filament differs from a fiber in that its absolute diameter is an order of magnitude greater than that of a fiber.

A whisker by definition is a single crystal fiber with a high degree of crystalline perfection which attributes to its ultra-high strengths (above 10 pounds per square inch). Whiskers by nature of their, growth are short, their cross-sections are in the micron range and their length-to-diameter ratio usually ranges from approximately 200 to 10,000. The matrix is a metal, ceramic or plastic. The basic principle for fiber or whisker reinforcement is as follows: fibers or whiskers, having lengths greater than a critical length value, properly aligned to the applied stresses, possessing an adequate concentration, distribute the load throughout the composite more efficiently. Reinforcement of the matrix is accomplished by transfer of the shear stresses between the matrix and the whiskers. The stress on the composite material is consequently transmitted between adjacent whiskers or fibers by the bond with the matrix. The principal role of the matrix is that of a binder for the fibers and whiskers and as a means to transfer stress from one whisker or fiber to the next under any load conditions imposed.

Normally, optimum physical properties are achieved in the composite if the fibers are aligned in the matrix parallel to each other. Heretofore, alignment of whiskers has been achieved by coating the whiskers with a magnetic material such as nickel and subjecting the coated whiskers to a magnetic field. The aligned whiskers form a mat having an overall thickness many times that of a single whisker. The matrix material, such as molten aluminum, is then caused to flow over the mat to fill the interstices and bind the whiskers together. This method suffers from the disadvantage that it is rather difficult to align the whiskers with a magnetic field. Furthermore, this method does not lend itself very easily to continuous, mass production of composite sheets of large size and varied shapes.

Another method according to the prior art consists of mixing a loose slurry of aluminum powder and silicon carbide whiskers, and then extruding the slurry through a fine orifice having a diameter smaller than the length of the whiskers to constrain the whiskers in the direction of flow. The extruded composite is then sintered to unite the whiskers with the aluminum matrix. Here again, this method limits the finished composite article to one of small dimensions and restricted shape.

SUMMARY OF THE INVENTION The foregoing disadvantages of the prior art are overcome by a process which includes depositing fibers on a thin substrate so that the fibers are aligned parallel to each other. According to one embodiment the substrate serves as a matrix material. A multiplicity of such fiber-coated substrates is then combined by diffusion bonding or other laminating processes so that the substrates diffuse together to produce an integral structure in which the aligned fibers are bound together by the matrix material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3;

FIG. 5 is an enlarged fragmentary, perspective view of a laminated article, with portions of layers removed, in which the fibers in adjacent layers are at 45 degree angles with each other;

FIG. 6 is a top plan view showing one step in the process of coating a wire matrix with fibers;

FIG. 7 is an end view showing a multiplicity of fibercoated wires prior to uniting them into an integral structure;

FIG. 8 is a sectional view showing an integral structure resulting from bonding the fiber-coated wires of FIG. 7 according to the invention;

FIG. 9 is a diagrammatic view, greatly enlarged, illustrating the polarization of fibers and matrix particles in a liquid solution subjected to an electric field;

FIG. 10 is an enlarged sectional view showing fibers commingled with matrix particles on a substrate;

FIG. 1 l is an enlarged fragmentary, sectional view of a layered structure, each layer comprising fibers commingled with matrix particles on a substrate prior to bonding of the layers;

FIG. 12 is an enlarged, fragmentary sectional view taken at right angles to the section of FIG. 11;

FIGS. 13 and 14 are enlarged, fragmentary sectional views of the layered structures of FIGS. 11 and 12, respectively, after bonding of the layers;

FIG. 15 is a top plan view illustrating a process of coating a wire with commingled fibers and matrix particles;

FIG. 16 is an enlarged, fragmentary sectional view showing a bundle of wires coated with commingled fibers and matrix particles, prior to bonding;

FIG. 17 is an enlarged, fragmentary sectional view showing the bundle of wires of FIG. 16 after bonding;

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown an apparatus for coating a thin sheet or foil of matrix material with a micron-size layer of reinforcing fibers or whiskers. The process of coating a single sheet or'wire with aligned fibers is disclosed and claimed in a copending, concurrently filed applicationof J. L. Reger et al, Ser. No.

715,088 entitled Fabrication of Composite Material," now abandoned and is not per se the subject matter of this invention. A liquid storage tank holds a liquid suspension 17 of micron-size fibers.- The fibers are preferably single crystal fibers of silicon carbide, aluminum oxide, or silicon nitride, commonly known as whiskers. In cross-section, the crystals are threefour-,

six-, or more-sided polyhedrons and have a diameterin the range of k to 3 microns. The diameter of the crystals can be considered as the diameter of the circle drawn tangent to the sides of the polyhedrondefining the cross-section of the crystal. The crystals are usually 100 to 200 microns in length. For use as strength reinforcing ele'ments, they should have a length to diameter ratio of at least 10 to l.

The solution in which the fibers are suspended may be polar or non-polar liquids. Amyl acetate is an example of a polar liquid, while benzeneand toluene are examples of non-polar liquids that are suitable as suspending media. The suspending medium is preferably of low viscosity so that the fibers can freely move therein. r

A magnetic stirrer or impeller 18 includes a magnetic stirring bar 19 located in the lower section 12. The magnetic stirring bar 19 isactuated by a U-shaped magnet 21 located beneath the storage tank 10 and driven by a motor 23. Rotation of the magnet 21 by the motor 23 causes the stirring bar 19 to rotate. When the stirring bar 19 is rotated, it imparts an upward lift to the fibers and causes them to traverse a honeycomb 20 of glass or plastic tubes or the like located in the intermediate section 14. The fibers thereby exit from the honey-comb 20 and flow into the upper section 16 substantially alongvertical paths.

In the upper section 16, the fibers are subjected to an electric field for the purpose of aligning them parallel to each other along their long dimension. The electric field is provided between a pair of'spaced, parallel, rectangular electrodes 22 and'24 disposed vertically in the upper section 16. The electric field produces lines of force running parallel to each other in the manner exemplified by the dashed lines 26.

An alternating voltage source 28 has one side connected to one electrode 24. The other side of the source 28 is connected to the hub of metal roller 30, over which is fed a continuous strip or foil of matrix material 32 from a roll 34 thereof wound on a takeoff reel 36. A takeup reel 38 that is driven slowly by'a small motor (not shown) winds the matrix material 32 after it is coated in a manner to be described.

Proceeding from the takeoff reel 36, the matrix material 32 traversesa path over the metal roller 30 from which it descends into the liquid suspension 17,

I then under two rollers 40 and 42 located in the bottom portion of the upper section 16, then vertically upwards in contact with the inner surface of the electrode 22, then out of the .tank 10 over a roller 44, over several smaller rollers 46, and ontothe takeup reel 38. The

reels

36 and 38, metal roller 30 .and

rollers

44 and 46 are mounted on a support structure 48. -A heater 49 may be provided to dry the fiber-coated matrix material 32 as it traverses the region between the roller 44 and the takeup reel 38.

The matrix material 32 may be a metal having a high strength to density ratio, such asaluminum, titanium, or the like, or a light weight metal alloy of similar'propertie's. The invention also has utility in strengthening matrix materials having other desirable properties. For example, columbium and tantalum have. high. melting temperatures, and nickel has good oxidation resistance. Thus the composite of matrix material and fibers or whiskers will have the desirable properties of the matrix material combined with the high strength of the fibers or whiskers.

With oneside of the voltage source 28 connected to the metal roller 30, it can be seen that this side of the voltage source is connected to the matrix material 32 by virtue of the physical contact to the metal roller 30. This same side of the voltage source 28 is also connected to the electrode 22 by virtue of the physical contact between the electrode 22 and the matrix material 32.

In cases where it is desired to use a non-conducting matrix material 32, such as a plastic, or an organic polymer, the voltage source 28 may be connected directly to the electrode 22 rather than the roller 32. In this case, the roller 32 need not be made of metal or other electrically conducting material. The electrode 22 is smoothly curved at the top and bottom portions thereof so as to avoid any damage to the matrix material 32 from sharp edges.

Reference in now made to FIG. 2 for a description of how the fibers are caused to deposit on the matrix material 32 along parallel lines. In this view thefibers are identified by the reference numeral 50. It can be seen that the fibers 50 in the liquid suspension 17 are aligned substantially parallel to each other and normal to the surfaces of the

electrodes

22 and 24. The alignment of the fibers 50 is along the electric field lines of force existing between the

electrodes

22 and 24. To the naked eye the fibers 50 appear as long threads. There is in fact an amount of bunching and overlapping that causes the fibers 50 to link together loosely in the direction of alignment and form thread-like chains.

It is theorized that the application of an alternating voltage across the

electrodes

22 and 24 sets up an alternating electric field in the liquid suspension 17 between the

electrodes

22 and 24. During a half cycle of one polarity, the fibers 50 become polarized in a direction opposite that of the electric field. The greater the difference in dielectric constant between the fibers and the liquid, the stronger the polarization of the fibers.

Being free to move in the liquid suspension, the fibers 50 line up parallel to the electric field. When the electric field reverses polarity during the next half cycle, the fibers 50 also reverse their polarity and accordingly still remain fixed in their same position aligned with the electric field.

If the electric field is unidirectional rather than alternating, similar alignment of the fibers 50 is obtained, one difference being that there is no reversal of polarity of the electric field in the liquid or on the polarized whiskers 50. The principal reason for preferring an alternating field to an unidirectional field is to prevent any permanent ions present in the suspension from interferring with the deposition of the fibers S0 in the matrix material 32. By alternating the potential on the

electrodes

22 and 24, the permanent ions have no preferred direction in which to migrate.

While the fibers are suspended in solution with the electric field applied, they become polarized and aligned as explained above. Being polarized, the fibers in close end-to end adjacency to one another are subjected to electrostatic field forces which tend to attract oppositely charged poles of these fibers. These electrostatic forces are believed to be responsible for the formation of threads of interlinked fibers in the liquid suspension.

In the vicinity of the matrix material 32, some of the fibers, interlinked with others or unlinked, come in contact with the matrix material 32, either through electrical attraction therewith or through the agency of surface tension at the liquid miniscus adjacent to the matrix material 32. It is believed that electrical attraction of surface tension forces alone may be sufficient to cause the fibers 50 to attach themselves to the foil of matrix material 32. However, to facilitate attachment of the fibers 50 to the foil, a slight amount of soluble binder or adhesive such as nitrocellulose, or polybutadiene is incorporated in the liquid suspension 17. The binder coats both the fibers 50 and the foil of matrix material 32. The ends of aligned fibers 50 attach themselves to the foil throughout its immersed length. As the foil moves out of the liquid suspension 17, the fibers 50 breaking through the liquid surface hang onto the foil and by surface forces attach themselves vertically to the foil to assure the desired parallel alignment. The fibers 50 deposited on the foil are shown on the left portion of FIG. 2.

The fiber-coated foil moves through the drying region indicated by the heater 49, where the solvent evaporates and the binder is solidified.

The fibers 50 are deposited on the foil of matrix material 32 as a micron-size layer. Some of the fibers deposit as clumps of three or four, while others deposit singly. Some of the fibers are linked end-to-end, like threads, while others are not connected together. However, if a cross-section is takenat any point across the width of the foil, the section would cut across a great number of the fibers 50.

It is speculated that if a non-uniform electrical field is produced in the liquid suspension 17 and if the dielectric constant of the fibers 50 differs from that of the liquid in which the fibers 50 are suspended, then a stronger electrostatic force will be exerted on the polarized fibers 50 that tends to attract and move them towards one electrode or the other. The greater the dielectric constant of the fibers 50, the greater the force of attraction. If the dielectric constant of the fibers 50 is greater than that of the liquid, it is postulated that the fibers 50 will be attracted to the electrode having the higher concentration of electrostatic lines of field. If the dielectric constant of thefibers 50 is less than that of the liquid, then the fibers will tend towards the electrode where the field concentration is lower. A non-uniform electric field may be produced by making one of the electrodes, say electrode 22, narrower than the other electrode, say electrode 24. In such case, the fibers 50 will be attracted .to the narrower electrode 22 if the dielectric constant of the fibers 50 is greater than that of the liquid.

While the electrode 22 next to the foil need not be as wide as the other electrode 24 and may at first glance appear to be superfluous, the electrode 22, by having portions thereof extending beyondthe edges of the foil of matrix material 32, tends to straighten the electrostatic field lines at the edges of the foil and further ensures that the fibers 50 in the edge regions of the foil will be aligned substantially parallel to the whiskers 50 in the central regions of the foil. The abovedescribed process is known to the art as dielectrophoresis. See for example, the article by Herbert A. Pohl entitled The Motion and Precipitation of Suspensoids in Divergent Electric Fields," published in the Journal of Applied Physics, Volume 22, Number 7, July 1951, pages 869-871.

In accordance with an operative embodiment, a roll of aluminum foil of 0.45 mils thickness and 3 inches width was moved at the rate of 0.35 inches per second through a liquid suspension of silicon carbide whiskers. The solution was amyl acetate containing 0.01 percent by weight of Pyroxylin, which is a DuPont trade name for lacquer grade nitrocellulose binder material. The

electrodes

22 and 24 were six inches wide and 2 inches deepin the solution and were places three inches apart. The voltage applied to the

electrodes

22 and 24 was 4,000 volts alternating current.

If a non-conductive matrix material is used such as plastic, the operation of depositing fibers is similar to that describedabove. In this case, however, the voltage is applied directly to both

electrodes

22 and 24. The electric field passes through the non-conductive matrix and the fibers are deposited in the same manner. In this case, a binder is probably essential. Glass fibers, or the like, may be used in place of the silicon carbide whiskers.

In accordance with the invention, fiber-coated foils of matrix material may be laminated in many layers to form a rigid integral structure of any desired thickness. A portion of such a laminated article is shown in FIGS. 3 and 4, FIG. 3 being a section taken across the width of the laminations, while FIG. 4 is a section taken along the length thereof.

Any of the well-known methods of forming laminated structures may be used. For laminating fibercoated metal foils, such processes as diffusion bonding, including hot rolling, may be used. For laminating fiber-coated plastic films, such bonding techniques may include polymerization or thermoplasticizing.

vA process of diffusion bonding has been used to laminate from to 580 layers of aluminum foil reinforced with silicon carbide whiskers. The layers were l060l l 0226 subjected to pressures of from 1,000 to 10,000 pounds per square inch and temperatures of l,000to 1,200 F for to minutes. Higher pressures should be avoided to prevent rupturing of the fibers. The temperature used should be in excess of one-half, but below, the melting temperature of the matrix material.

While the structures of FIGS. 3 and 4 were formed by diffusion bonding of multiplelayers of fiber-coated sheets, it is pointed out that the laminar boundaries or interfaces between the laminar sheets are not clearly discernible. This is due to the fact that the matrix foils are diffused together to form an integral unit of matrix material with the fibers embedded therein disposed in well-defined strata.

A laminated structure of aluminum reinforced with silicon carbide whiskers has exhibited a modulus of elasticity of 30 million pounds per square inch as contrasted with a modulus of 10 million poundsper square inch for commercial purity aluminum. The tensile strength of laminations of fiber-reinforced aluminum is 30,000 to'50,000 pounds per square inch as contrasted ,with a tensile strength of 6,000 to 8,000 pounds per square inch of commercial purity aluminum.

With a metal matrix, a true metallurgical bond is produced in which the laminations are mechanically and chemically bonded together. Such a bond is characterized as an adhesive bond. That is, the laminations are bonded together by means of adhesion without the agency of an intermediate constituent.

As shown in FIGS. 3 and 4, the layers may be laminated so that the fibers run in the same parallel directions, in which case the laminated article may have been preferentially strengthened in one direction.

Alternatively, the laminations may be in the form of crossplys in which the fibers in adjacent plys are oriented at 9045 or at smaller angles to each other. In these cases, the reinforcement may be distributed over different angles. FIG. 5 illustrates a laminated article in which the fibers in adjacent plys are at 45 degree angles with each other.

FIG. 6 illustrates a process of coating a matrix material in the form of a wire. A wire matrix 52 is located centrally within a cylindrical electrode 54. When the wire matrix 52 and cylindrical electrode 54 are placed in a liquid suspension 17, and an electric field applied therebetween, the fibers 50 line up radially along the electric field lines of force. As the wire matrix 52 is withdrawn from the liquid suspension 17, the fibers 50 deposit on the surface of the wire matrix 52 in a manner similar to that described above in connection with the foil matrix material 32.

FIG. 7 shows a multiplicity of fiber-coated wires 52 prior to bonding. FIG. 8 shows a composite article resulting from bonding the fiber-coated wires 52 of FIG. 7. As illustrated in FIG. 8, the wires 52 diffuse together into a solid, continuous mass of matrix material 56, with the fibers 50 embedded therein and aligned parallel to each other. It is understood that the wires 52 may be metal or plastic matrix material. Also the fibers 50 may be single crystal whiskers, or they may be polycrystalline or non-crystalline as discussed previously.

Thus far, there has been described a process for coating a thin sheet or wire of matrix material with a micron-size layer of aligned fibers and then uniting a multiplicity of such fiber-coated wheets or wires into an integral rigid structure. There will now be described alternative processes in which a thin substrate in the form of a wire or sheet, which may be a matrix material or a non-matrix material, is coated with a micron-size layer of commingled matrix particles and aligned fibers. A multiplicity of such layers of commingled matrix particles and aligned fibers, either attached to or removed from the substrate, are united to form an integral structure in a manner similar to that described, such as by diffusion bonding, polymerization, thermoplasticizing, or the like. I v

The process of coating a substrate with commingled fibers and matrix particles is disclosed and claimed in copending, concurrently filed application of Karl P. Staudhammer et al, Ser. No. 715,057, entitled Fabrication of Composite Materials by Codeposition of Matrix and Reinforcing Particles, now abandoned. In practicing one form of the method disclosed in that application, apparatus similar to that of FIG. 1 is used. The storage tank 10 holds a liquid suspension of micron-size fibers and matrix particles. The particlesof matrix material are preferably of sub-micron diameter, and may be particles or powders such as aluminum, nickel, titanium, columbium, or alloys thereof, for example. Alternatively, the matrix particles may be nonmetallic sub-micron particles, such as organic polymers or plastics.

The substrate is preferably formed of the same material as that of the matrix particles. In this instance, the matrix particles will serve to facilitate in the diffusion of matrix material with the fibers, when sheets of fiber-coated matrix material are laminated to form an integral structure, as will be explained.

Reference is now made to FIG. 9 for a description of how the fibers and matrix particles are caused to deposit on a substrate with the fibers aligned along parallel lines. In this view, the fibers are identified by the same reference numeral 50, the matrix particles by the reference numeral 58, and the substrate by the reference numeral 60. The substrate 60 occupies the same position in the apparatus of FIG. 1 as the matrix material 32. The size of the fibers 50 and matrix particles 58 are exaggerated for ease in illustration. It can be seen that the fibers 50 in the liquid suspension 17a are aligned substantially parallel to each other and normal to the surfaces of the

electrodes

22 and 24. The alignment of the fibers S0 is along the electric field lines of force existing between the

electrodes

22 and 24. Some of the matrix particles 58 bunch together around the ends of the whiskers 50 through the agency of electrostatic forces and cause the fibers 50 to link together loosely in the direction of alingment and form threadlike chains.

It is theorized that the application of an alternating voltage across the

electrodes

22 and 24 sets up an alternating electric field in the liquid suspension 17a between the

electrodes

22 and 24. During a half cycle of one polarity, the fibers 50 and matrix particles 58 become polarized in a direction opposite that of the electric field. The greater the difference in dielectric constant between the fibers 50, the matrix particles 58 and the liquid, the stronger the polarization of the fibers 50 and matrix particles 58. Being free to move in the liquid suspnesion 17a, the fibers 50 line up parallel to the electric field. Being polarized, the matrix particles 58 are attracted to the poles of the fibers 50. The negative poles of the matrix particles 58 are attracted to the positive poles of the fibers 50 and the positive poles of the matrix particles 58 are attracted to the negative poles of the fibers 50. When the electric field reverses polarity during the next half cycle, the fibers 50 and matrix particles 58 also reverse their polarity and accordingly the fibers 50 and matrix particles 58 still remain fixed in their same position with the fibers 50 aligned with the electric field.

If the electric field is unidirectional rather than alternating, similar alignment of the fibers 50 and bunching of the matrix particles 58 is obtained, one difference being that there is no reversal of polarity of the electric field in the liquid or on the polarized fibers 50 and matrix particles 58. The principal reason for preferring an alternating field to a unidirectional field is to prevent any permanent ions present in the suspension from interfering with the deposition of the fibers 50 and the matrix particles 58 on the substrate 60. By altemating the potential on the

electrodes

22 and 24, the permanent ions have no preferred direction in which to migrate.

While the fibers 50 and matrix particles 58 are suspended in solution with the electric field applied, they become polarized and the fibers 50 are aligned as explained above. The fibers 50 in close end-to-end adjacency to one another are linked together with the bunched matrix particles 58 through electrostatic forces, as has been explained above. These electrostatic forces are believed to be responsible for the formation of threads of interlinked fibers in the liquid suspension 17a.

In the vicinity of the substrate 60, some of the fibers 50, interlinked with others by the matrix particles 58 or unlinked, but with matrix particles 58 bunched thereon, come in contact with the substrate 60, either through electrical attraction therewith or through the agency of surface tension at the liquid miniscus adjacent to the substrate 60. It is believed that electrical attraction or surface tension forces alone may be sufficient to cause the fibers 50 and matrix particles 58 to attach themselves to the substrate 60. However, to facilitate attachment of the fibers 50 and matrix particles 58 to the substrate 60, a slight amount of soluble binder or adhesive such as nitrocellulose or polybutadiene is incorporated in the liquid suspension 17a. The binder coats the fibers 50 and matrix particles 58 and the substrate 60. The ends of aligned fibers 50 attach themselves to the substrate 60 throughout its immersed length. As the substrate 60 moves out of the liquid suspension 17a, the fibers 50, and matrix particles 58, breaking through the liquid surface, hang onto the substrate 60 and by surface forces attach themselves vertically to the substrate 60 to assure the desired parallel alignment of the fibers 50.

The fibers 50 and matrix particles 58 are deposited on the substrate 60 as a micron-size layer. Some of the fibers 50 deposit as clumps of three or fouror more, with matrix particles 58 attached thereto, while others deposit singly. Some of the fibers 50 are linked end to end with the matrix particles 58, like threads, while others are not connected together. However, if a cross section is taken at any point across the width of the substrate 60, the section would cut across a great number of the fibers 50 and matrix particles 58. Ideally, the greater bulk of the fibers 50 should deposit as a substantially uniform layer of several whiskers deep, as shown in FIG. 10.

It is speculated that if a non-uniform electrical field is produced in the liquid suspension 17a and if the dielectric constants of the fibers 50 and matrix particles 58 differ from that of the liquid in which the fibers 50 and matrix particles 58 are suspended, then a stronger electrostatic force will be exerted on the polarized fibers 50 and matrix particles 58 that tends to attract and move them towards one electrode or the other. The greater the dielectric constants of the fibers 50 and matrix particles 58, the greater the force of attraction. If the dielectric constants of the fibers 50 and matrix particles 58 are greater than that of the liquid, it is postulated that the fibers 50 and matrix particles 58 will be attracted to the electrode having the higher concentration of electrostatic lines of field. If the dielectric constants of the fibers 50 and matrix particles 58 are less than that of the liquid, then the fibers 50 and matrix particles 58 will be attracted towards the electrode where the field concentration is lower. A non-uniform electric field may be produced by making one of the electrodes, say electrode 22, narrower than the other electrode, say electrode 24. In such case, the fibers 50 and matrix particles 58 will be attracted to the narrower electrode 22 if the dielectric constants of the fibers 50 and matrix particles 58 are greater than that of the liquid.

To illustrate how a process for depositing commingled fibers and matrix particles on a substrate may be carried out, a roll of aluminum foil of 0.45 mils thickness and 3 inches width may be moved at a rate of 0.35 inches per second through a liquid suspension of aluminum carbide whiskers and aluminum matrix particles. The, solution may be amyl acetate containing .01 percent weight of Pyroxylin binder material. The aluminum matrix particles may be equal to or less than 44 microns in diameter but are preferably less than a micron in diameter. The electrodes 22 and may be six inches wide and two inches deep in the solution and placed three inches apart. The voltage applied to the

electrodes

22 and 24 may be 4,000 volts alternating current.

If a non-conductive substrate is used such as plastic, the operation of depositing fibers and matrix particles is similar to that described above. In this case, however, the voltage is applied directly to both

electrodes

22 and 24. The electric field passes through the non-conductive substrate and the fibers or whiskers and matrix particles are deposited in the same manner. In this case, a binder is probably essential.

Substrates made of matrix material and coated with fibers 50 and matrix particles 50 may be laminated in many layers to form a rigid integral structure of any desired thickness. Any of the well-known methods of forming laminated structures may be used, such as those previously described in connection with the deposition of fibers above.

FIGS. 11 and 12 show a portion of a layered structure, prior to bonding, each layer of which is similar to the structure of FIG. 10. Each layer includes a substrate 50 of matrix material coated with fibers 50 com- I060l 1 022B mingled with matrix particles 58. The dimensions are greatly exaggerated for ease in illustration and only a few layers are shown, it being understood that an actual layered structure will include several hundred layers. The substrate 60 of matrix material may be five microns thick, the fibers or whiskers two microns in diameter, and the matrix particles 58 may be less than a micron in diameter. The thickness of the fiber layer may be ten. fibers thick, thereby providing a ratio of four to one between the thickness of the fiber layer and the thickness of the substrate 60. P rior to lamination into an integral structure, it will be seen from FIGS. 11 and 12 that there are clear lines of separation between the substrate 60 and the fibers 50 and matrix particles Reference is now made to FIGS. 13 and 14 which show a laminated structure after the layers are bonded according to techniques previously referred to, such as diffusion bonding. In this structure it will be observed that the application of heat and pressure acts to squeeze the fibers 50 and matrix particles 58 together and to cause the matrix particles 58 to fuse together in the spaces between the fibers 50. Furthermore, the substrate 60 of matrix material fuses with the matrix particles 58, forming a solid continuous body of matrix material 61 in which the fibers 50 are embedded. The fibers 50 are squeezed into layers, identified by a thickness 62, separated by a layer of matrix material, identified by a thickness 64 which represents the position formerly occupied by the substrate 60. As a result of the bonding process, the ratio of the thickness 62 to thickness 64 has been reduced to about three to one. Furthermore, it is apparent that the fibers 50 occupy the major portion of the total volume of the laminated structure, and the matrix particles 58 have filled in the voids previously existing between the fibers 50.

A process of diffusion bonding may be used to laminate from 100 to 600 layers of aluminum foil substrates, reinforced with silicon carbide whiskers commingled with aluminum sub-micron matrix particles. The layers are subjected to pressures of from 1,000 to 10,000 poundsper square inch and temperatures of l,000 to l,200 F for 5 to 10 minutes. Higher pressures should be avoided to prevent rupturing of the fibers or whiskers. The temperature used should be in excess of one-half, but below, the melting temperature of the matrix material.

A laminated structure of aluminum foil substrates reinforced with silicon carbide whiskers commingled with aluminum submicron matrix particles can be expected to exhibit a modulus of elasticity of 30 million pounds per square inch as contrasted with a modulus of 10 million pounds per square inch for commercial purity aluminum. The tensile strength of laminations of fiber reinforced aluminum is expected to be 30,000 to 50,000 pounds per square inch as contrasted with a tensile strength of 6,000 to 8,000 pounds per square inch of commercial purity aluminum.

The layers may be laminated so that the fibers or whiskers run in the same parallel directions, in which case the laminated article may have been preferentially strengthened in one direction. Alternatively, as discussed previously in connection with substrates coated with fibers alone, the laminations may be in the form of cross-plys in which the fibers in adjacent plys are oriented at 45, or at smaller angles to each other. In these cases, the reinforcement may be distributed over different angles. U I

FIG. 15 illustrates a process of coating a matrix material in the form of a wire. A wire matrix 66 is located centrally within a cylindrical electrode 68. When the wire matrix 66 and cylindrical electrode 68 are placed in a liquid suspension 17a, and an electric field applied therebetween, the fibers 50 and matrix particles 58 line up radially along the electric field lines of force. As the wire matrix 66 is withdrawn from the liquid suspension 17a, the fibers 50 and matrix particles 58 deposit on the surface of the wire matrix 66in a manner similar to that described above in connection with the foil matrix material or substrate 60.

FIG. 16 shows a multiplicity or bundle of matrix wires 66 coated with fibers 50 and matrix particles 58 prior to bonding. FIG. 17 shows a composite article resulting from bonding the coated wires 66 of FIG. 16. As illustrated in FIG. 17, the wires 66 diffuse together into a solid, continuous mass of matrix material 70, with the fibers 50 embedded therein and aligned parallel to each other. It is understood that the matrix wires 66 and matrix particles 58 may be metal or plastic matrix material. Also the fibers 50 may be single crystal whiskers, or they may be polycrystalline or non-crystalline as discussed previously.

In order to provide an even high density of fibers relative to matrix material, especially where the matrix material is non-metallic, such as a plastic, an alternative method may be used for depositing fibers and matrix particles on a substrate. In this method, the fibers and matrix particles are deposited on a substrate in the form of an endless belt to produce a film of fibers united with matrix particles and the film is then stripped away from the substrate.

Reference is now made to FIG. 18 for a description of this alternative method. An endless belt 72 is driven by a roller 74, over a second roller 76, then over a third and

fourth roller

78 and 80 in the liquid suspension 17a, following which the belt 72 slides across the electrode 22 and over a fifth and final roller 82. The belt 72 may be made of metal, and preferably is coated with a plastic, such as Teflon, to give it a sliding surface from which the deposited film can easily be removed.

The matrix particles in the liquid suspension are preferably made of polymerizable or thermoplastic materials, such as polyethylene, polyurethanes, epoxies, and polyimides, for example. Alternatively, the binder used in the liquid suspension 17a may be a polymerizable material.

In operation, the aligned fibers commingled with matrix particles are deposited on the moving belt 72 in the same manner as described previously in connection with the apparatus of FIG. 1. The fibers and commingled matrix particles form a loosely bound film 84 loosely attached to the belt 72 by means of the binder material. After moving past the roller 82, the deposited film 84 is subjected to radiation from a lamp 86 to dry the solvent from the liquid suspension material 17a and polymerize the matrix particles and/or the binder material and form a coherent film of aligned fibers embedded in polymerized plastic. The radiation from the lamp 86 may be infrared, ultravoilet, or other polymerizing radiation.

The polymerized film 84 is then removed from the endless belt 72 by means of a doctor blade 88 and rolled on a takeup reel 90.

Referring now to FIG. 19, laminated structures of polymerized film 84 may be formed of any desired thickness, only three layers being shown for ease in description. Each layer of film 84 includes aligned v fibers 92 embedded in polymerized matrix material 94.

The layers or films 84 are joined together by a layer of adhesive 96, which may be the same material as the matrix material or a different material.

In an alternative process utilizing the apparatus of FIG. 18, the matrix particles and/or binder material in the film 84 deposited on the moving belt 72 may be partially polymerized by the radiation from the lamp 86 prior to removal by the doctor blade 88. Partially polymerized films 84 may then be stacked in layers as shown in FIG. 20 without the aid of adhesive material, and the layered structure may be further polymerized to produce a finished fiber-reinforced article of desired thickness.

According to still another process, the film 84 deposited on the belt 72 may be subjected to heat from the lamp 86 solely to remove solvent from the binder material without polymerizing either the binder material or matrix particles. As shown in FIG. 21, the film 84a after removal by the doctor blade 88 may consist of polymerizable matrix particles 98 and aligned fibers 92 held together by binder material 100. The films 84a may be stacked together as shown and subjected to heat and pressure to polymerize the matrix particles 98 and decompose the binder material. As shown in FIG. 22, the finished article will consist of aligned fibers 92 embedded in a mass of polymerized matrix material 102.

According to another embodiment, the apparatus of FIG. 1 may be used to deposit metal matrix particles commingled with fibers on a substrate that is of a different material from the matrix particles. Thereafter, during the laminating process, the substrate material may be removed by vacuum evaporation in the case of a metal substrate, or by thermal decomposition, in the case of a plastic or non-metallic substrate. For example, in the case of a metal substrate, aluminum matrix particles and fibers may be deposited on a foil made of lead 'or other metal that vaporized in vacuum at a lower temperature than the aluminum. The foils coated with fibers and aluminum particles are then stacked together and placed in a vacuum furnace where the stacked layers are subjected to heat and mechanical pressure. The lead substrates vaporize, and the vapor therefrom is exhausted, and the aluminum particles merge together by diffusion bonding into a solid continuous matrix containing the aligned fibers.

In the case of a plastic or other non-metallic substrate, similar processing steps as those above described may be performed, except that the diffusion bondin may be carried out at atmospheric ressure. Under he in uence of heat and pressure, he nonmetallic substrate decomposes and vaporizes, while the aluminum matrix particles again form a solid continuous matrix in which the aligned fibers are embedded.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A method of fabricating composite materials, comprising:

dielectrophoretically depositing a multiplicity of fibers from a non-aqueous liquid suspension onto an elongated preformed partially polymerizable plastic sheet by moving said plastic sheet through said suspension while applying a non-uniform electric field to said suspension in a direction transverse to the direction of movement of said plastic sheet and with said plastic sheet in the region of higher field strength to form a micron-size layer of said fibers carried by said plastic sheet, with said fibers aligned parallel to said plastic sheet and to said direction of movement; said fibers having a dielectric constant greater than that of said suspension liquid; said fibers being of micron size diameter and having a length greater than ten times the diameter thereof; said fibers being made of a first material having given physical properties, and said plastic sheet being made of a second material having different physical properties from said first material; partially polymerizing said plastic sheet; stacking a plurality of said fiber-carrying partially polymerized plastic sheets together in layers; and uniting said layers by further polymerizing said fibercarrying plastic sheets to form a unitary composite mass of plastic matrix material reinforced by parallel strata of parallel-aligned fibers.

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