CA2060520A1

CA2060520A1 - Metal matrix composites

Info

Publication number: CA2060520A1
Application number: CA002060520A
Authority: CA
Inventors: Jonathan G. Storer
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1991-03-11
Filing date: 1992-02-03
Publication date: 1994-12-09
Also published as: FR2709134A1; GB2279667A; GB9204716D0; JPH0770668A; ITRM920159A1; GB2279667B; ITRM920159A0

Abstract

Abstract of the Disclosure A fiber reinforced metal matrix composite is provided comprising consolidated coated fibers, said fibers comprising a ceramic, carbon or metal fiber having a coating thereon of a metal matrix material, the coating being substantially uniform and having a morphology which is fine grained and substantially void free and said composite being substantially void free. A
barrier/interface layer made be provided between the fiber and the metal matrix. A process for forming the metal matrix composite is also provided as well as fibers which are coated with metal matrix material.

Description

~ O 6 ~ ~ ~ D 44761 CAN~R

METAL MATRIX COMPOSITES

~his invention relates to fiber reinforced metal matrix composites.
Reinforced metal matrix composite systems were developed in the 1960's. SUch composites provide high strength, high modulus and toughness over the properties of the metal without reinforcement. Reinforcement materials can be in the form of, for example, particles, whiskers, chopped fibers and continuous fibers. Early composites were boron fibers in an aluminum matrix.
Carbon fibers, silicon carbide fibers and refractory metal oxide fibers were later used in various metal matrices. Generally, continuous fibers ~having a large aspect ratio (l/d>10)) provide reinforced metal matrix composites having the highest strength, modulus and toughness.
The reinforcing fibers can be incorporated with the matrix metal in various ways includinq laying the fibers between metal foils, surrounding the fibers with metal powder, or pre-coating the fibers with the matrix metal. In solid state consolidation, the fiber/matrix metal combination is laid up in the desired form in a vacuum encapsulated ampule and then heat, along with pressure, by means of a compressed inert gas, is applied for a time necessary for the matrix metal to flow around the fibers and bond together by interdiffusion.
The consolidation process just of using a hot isostatic press (HIP), is also referred to as HIPping.
The simplest method of incorporation is alternating layers of fiber and metal sheet. This method requires the metal to flow around the fibers and make a bond with the fibers and the next layer of metal during the consolidation process. In this technique, high temperatures and pressures and time are required to get the metal sheet to flow the required distances and the metal being laid up with the fiber may not completely surround the fiber, thus leaving voids in the matrix : ~ . - : .:: : :
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2 0 ~ ~ ~ 2 0 after the consolidation process. ~n addition, metal sheets of important intermetallic compounds, chosen because of their technologically important creep resistance at high temperatures, possess concomitant brittleness which renders metal sheet fabrication difficult.
Plasma spray, chemical vapor deposition and thermal evaporation of theimatrix metal onto suitably laid up fibers are other alternatives. These methods all have a number of problems associated with them.
Plasma spray, which utilizes the melting of large powder particles in a plasma torch, by its very nature, deposits large droplets (50 to 200 micrometers).
When these large droplets are sprayed on small diameter fibers, i.e~, 5-50 micrometers, very uneven coatings occur with the resultant composite being uneven and containing voids. Because sufficient numbers of droplets must form splats on any given deposition area, and thus build up coating volume, the coating formed by large diameter droplets on small diameter fibers also leads to small volume ~raction values for the fiber to total volume ratio, which is undesirable for a strong metal matrix composite. In addition, the deposited droplets are hot and cause the fibers to be thermally stressed, which often leads to damage and even fracture of the fibers.
U.S. Patent No. 4,782,8~4 (Siemers) discloses a method for continuous fabrication of fiber reinforced titanium-based composites. A number of individual silicon carbide filaments are assembled to form a tape.
The tape is passed across a drum in a low pressure plasma deposition apparatus ~here the surface of the drum is formed o~ a polished and cooled refractory metal. The tape is sprayed with a plasma spray of the titanium base alloy metal to form a sheet of such metal on the tape and on the surface of the drum. The deposited sheet including the silicon carbide reinforcement tape is removed from the drum surface by paeling.

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2 ~ 2 0 With chemical vapor deposition processes, the coatings generally are fragile, and not very adherent.
Contamination by carrier gases and unwanted reactant products can lead to poor metallurgical properties.
Cathode sputtering, a type of physical vapor deposition, in an inert gas atmosphere is a technique used to deposit metal onto small diameter ceramic and carbon fibers. The efficiency o~ this technique is quite low. Even with high rate magnetron DC cathode sputtering, the feedstock utilization can only reach 30 in the best circumstances.
U.S. Patent No. 4,853,294 tEverett et al.) discloses carbon fiber reinforced metal matrix composites. The carbon reinforcing fibers are protected from interaction with the matrix material by an inner ~nd an outer barrier layer. The composites can be prepared using physical vapor deposition, conventional casting, chemical vapor deposition or liguid-metal infiltration.
The preferred method uses physical vapor deposition.
Thermal evaporation processes are also used to deposit matrix metal on fibers. With thermal evaporation processes, there is a problem of stoichiometry maintenance. One cannot be sure of obtaining the correct intermetallic compound on the coated fibersO In addition, upward deposition of molten feedstock requires complicated fixturing and transport of fibers in order to get even coating coverage.
The present invention, in one aspect provides a coated fiber having a coating thereon of a metal matrix 30 material, the coating being substantially uniform and having a morphology which is fine grained and substantially void free. Suitable metal matrix materials include aluminum, titanium, magnesium, nickel, iron, copper, chromium, tantalum, tungsten, niobium and various alloys and intermetallics thereof. The fiber to coating volume ratio is preferably in the range of about 20:80 to 70:30, more preferably 30:70 to 50:50.
The present invention, in another aspect, provides a fiber reinforced metal matrix composite ... . . .. .

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comprising consolidated coated fibers, said flbers comprising a ceramic, carbon or metal fiber having a coating thereon of a refractory metal or metal-based ceramic material, the coating being substantially uniform and having a morphology which is fine grained and substantially void free. Preferably, the fiber volume fraction within the composite is greater than 30 percent, more preferably greater than 50 percent.
The present invention further provides a 10 process for making a metal matrix composite comprising the steps of (1) coating ceramic, metal or carbon fibers by ~a) applying sufficient current to a metal cathode to form a plasma of atoms and ions of the cathode material, (b~ positioning said fibers in the path of said plasma, said plasma causing said fibers to billow, and (c) causing said plasma to condense on said fibers to form a refractory metal or a metal based ceramic coating; (2) vacuum encapsulating the fibers; (3) applying sufficient heat and pressure to cause the metal coating to flow and bond the coated fibers together by interdifffusion; and ~4) allowing the bonded fibers to cool to form a consolidated metal matrix composite.
In the present invention, the fibers are evenly and continuously coated with the matrix metal prior to the composite consolidation. This results in the fibers having fully densa coatings of matrix metal, substantially without any voids. When these so coated fibers are laid up in the desired matrix configuration, and then consolidated, it is possible to obtain a high volume fraction of fibers completely surrounded by the matrix metal, with substantially no voids in the matrix metal. This constructiorl provides a high strength, high modulus, and high toughness metal matrix composite structure.
FIGS. 1 and 2 are cross-sectional side views of apparatus suitable for use in the present invention.
FIG. 3A is a schematic diagram shown in cross-section of an encapsulation process useful in the process of the present invention.

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FIG. 3B is a schematic diagram of encapsulated coated fibers as prepared for consolidation.
FIG. 4 is scanning electron photomicrograph of an alpha~alumina fiber coated with titanium aluminide useful in the present invention.
FIGS. 5 and 6 are scanning electron photomicrographs of a metal matrix composite of the present invention at magnifications of 500X and 5000X, respectively.
Fibrous materials useful in the present invention include carbon, and ceramic fibers such as, for example, alumina, alumina silica, alumina-boria-silica, boron, titanium diboride, tungsten, silicon carbide, and silicon nitride fibers. Particularly preferred fibers are alpha-alumina fibers and NextelT~ alumina-boria-silica continuous fibers, available from 3M Company. The fibrous material can he in the form of tow having a finite length or in continuous form, each tow containing from a few to many filaments. Generally, the fiber diameter is in the range of about 5 to 150 micrometers, more preferably 5 to 50 micrometers, most preferably 5 to 30 micrometers.
Metals useful as matrix materials are dependent on the end use, but common ones include ~5 aluminum, titanium, magnesium, nickel, iron, copper, chromium, tantalum, tungsten, niobium and various alloys and intermetallics of these.
By choosing combinations of fibers and matrix materials, metal matrix composites with varying 30 properties can be tailored to meet specific requirements.
For example, for a high stif~ne~s material one would choose a high modulus fiber such as alpha-alumina; for high specific creep resistance a matrix material of titanium aluminide is good; for high strength, alpha alumina fibers have tensile strength in excess of 300 Kpsi; and for thermal--chemical stability, a barrier interface coating of yttrium oxide provides protection at high temperatures.

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The matrix metal coating can be applied to the fibrous material using cathodic arc deposition. The preferred coating thickness depends on the volume fraction of fiber desired in ~he final composite but for fibers in the range of diameters from 5 to 15 microns, the coating thickness of the matrix metal material is in the range of about 1 to 10 microns, more preferably, in the range of about 2 to 5 microns.
An apparatus useful for such deposition in batch applications is shown in Figure 1. Vacuum chamber 10 contains cathode 13 formed of conductive material with which the fiber is to be coated and surrounded by confinement ring 14. Anode 12 forms a portion of chamber 10. Current is supplied form high current power supply 11 to cause an arc discharge to occur between anode 12 and cathode 13 and plasma 19 to form from cathode 13.
Plasma 19 is directed by magnetic solenoid 15 and deposits on rotating fiber tow 20 held in chuck 21.
Insulating ring 17 separates anode 12 from that portion of vacuum chamber 10 in which the fiber tow is suspended.
Optionally, an inert gas can be fad through tube 16.
Another apparatus useful for such deposition but for continuous operation is shown in Figure 2.
Vacuum chamber 30 contains cathode 33 formed of conductive material with which the ~iber is to be coated and surrounded by confinement ring 34. Anode 32 forms a portion of chamber 30. Current is supplied from high current power supply 31 to cause an arc discharge to occur betwean anode 32 and cathode 33 and plasma 39 to form from cathode 33. Plasma 39 i.s directed by magnetic solenoid 35 and deposits on continuous fibers 40 which are fed from spool 41 over festooning rolls 42 and 43 and onto takeup spool 44. Insulating ring 37 separates anode 32 from that portion of vacuum chamber 30 in which the fiber tow is festooned. Optionally, an inert gas can be fed through tube 36.
The inert gases which can be used in the present invention include argon, krypton, xenon, helium and other gases belonging to Group ~ of the periodic .~....... . .. , ~ .

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2060~2a table which are chemically inert in a plasma environment.
The use of such gases generally aids in the development of a more stable discharge from the cathode. Argon is generally preferred due to cost and availability.
When the conductive material of the cathode is connected to the high current power supply, arcs form on the cathode surface. The arcs formed are small, luminous regions which are very mobile and move rapidly over the cathode surface. Due to the high current density contained in each spotl rapid ebullition of the cathode material occurs forming a plasm~ of atoms and ions of the cathode material and this material is deposited on the fibers. The current density at each spot may reach ten million amperes per square centimeter and this leads to the ionization of much of the outflowing vapor.
The high fraction of ions in the plasma flow of the cathodic arc and the high kinetic energy of the ions permits control of the morphology of the deposition and the ability to form very dense coatings~ The coatings are small grained and columnar in structure, with grain size preferably in the range of about 5 to 75 nanometers, more preferably ~5 to 25 nanometers. The coatings are fully dense, i.e., without substantial voids either within or between the columns. Without substantial voids as used herein means one visible void per 0.5 square micrometer of area, particularly when viewed in a transmission electron photomicrograph at 200,000 magnification. A visible void is generally about 5 nanometers or greater in size.
The high activation energy and fluence of the ions also causes the fiber tow to billow out and the fibers to separate from each other, permitting the deposition of substantially uniform coatings, i.eO, having less than 80%, more preferably less than 50%, most preferably less than 30% deviation from the average both lengthwise and in cross section. Further, the fluctuating nature of the ion wind caused by the random nature of the arc motion on the cathode, results in a ..: . ~

0~0~20 shaking of the fiber tow resulting in further separation of the individual fibers.
Excellent adhesion of the coatings to the fibrous material results from deposition by cathodic arc because the high energy ions in the plasma result in strong atom-to-a~om bonding, with some interdiffusion of the coatings and the fibers. This excellent adhesion is evidenced by a lack of spalling away from the break line when a coated fiber is broken.
The interface between the fiber and the metal of the matrix can be a cause for concern. Fiber surfaces may be coated with suitable materials as barrierSinterface coatings to a~ control the adhesion, b) to prevent any adverse chemical reaction between the fiber and the matrix and c) to provide a thermal expansion coefficient matching layer.
The refractory metal or matal-based ceramic barrier interface coatings are preferably also applied to the fibrous material using cathodic arc coating as is 20 used for applying the metal matrix coating.
Refractory metals or metal-based ceramics usaful as barrier/interface coatings in the present invention include, for example, molybdenum, tantalum, tungsten and niobium, oxides of aluminum, yttrium, zirconium, hafnium, gadolinium, titanium, erbium, and other rare earth metals, and carbides of tantalum, tungsten, niobium, zirconium, hafnium, and titanium.
Preferably, the refractory metal or metal-based ceramic i5 applied to achieve a coating thickness of about 20 to 1000 nanometers, more preferably about 50 to 300 nanometers.
Reactive deposition of compounds for barrier or interface layers is possible using cathodic arc with the simple addition of the reactive gas into the coating chamber. Reactive gases which are useful in the present invention for the production of barrier and interface layers include oxygen, nitrogen ammonia, and hydrocarbons. For example, the use of a yttrium cathode and a small amount of oxygen in the chamber results in a ,.

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deposit of yttrium oxide. Thus, non-conductive materials can ba deposited using cathodic arc. The compound of reactive deposition are stoichiometric and generally crystalline, providing sufficient reactive gas is supplied to the system.
It is preferred in metal matrix composites to have a high volume fraction of fibars, that is, if the composite is sectioned, the ratio of fiber area to total composite area would preferably be larger than about 30 percent, more preferably larger than about 50 percent, and most preferably larger than about 70 percent. It is also desirable for the composite to be non-brit~le, free of voids and non-strength producing third phases.
HIPing, a process well known to those skilled in the art, is used to form the fibers into the metal matrix composite. This process utilizes a hot isostatic press for applying heat and pressure to the fibers. The metal coated fibers are put in the desired configuration, vacuum encapsulated, and then heat, generally at a temperature of about 800 to 1200C, along with pressure of about 35 to 300 Mpa, by means of a compressed inert gas, is applied using a hot isostatic press for the time necessary for the matrix metal to flow completely around the fibers and bond together by interdiffusion, generally about 30 minutes to 4 hours. The encapsulation process is shown in FIG. 3A. Protective felt mat 52, prepared from a material such as alumina is placed in the bottom of glass tube 51. Coated fibers 54 which have been wrapped in protective foil 53 made from a material such as titanium are placed in glass tube 51 and another protective felt 52 is placed over the fibers 54 in glass tube 51. A Yacuum is drawn on glass tube 51 containing the coated fibers via vacuum tube 55. The top potion of glass tube 51 is heated, for example, by acetylene torch 57 and sealed by sealing means 56 to form seal 58 to provide the encapsulated fibers 50. FIG. 3B shows several of the encapsulated fibers 50 in a group 60 ready for the HIPping process. HIPiny is further described, for example, in Toops, J., I'HIP rides a new wave of :.. :. : . -: -... .
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Since the matrix metal completely surrounds the fibers, the metal flow is minimal, and the chances for voids is small. Also, because the matrix metal is coated directly on the small fibers useful in the present invention, the consolidated product has a high volume fraction of fiber, e.g., greater than 50%.
The following nonlimiting examples are presented to further describe and illustrate the invention. In the examples the coatad fibers were characterized using the following methods.
Scanning Electron Microscope (SEM) was used to determine the morphology of the coatings. Coated fiber preparation for alpha-alumina fibers, which are typically 8 to 30 micrometers in diameter, and of circular cross section, was as follows. A 0~25 inch t0.64 cm) long section of the coated fiber tow, typically containing several thousand filaments, was fastened to a standard aluminum SEM stub with double sided adhesive tape. The ends of the filaments were left overhanging the edge of the stub. An additional 0.0625 inch (0.16 cm) was cut off the ends of the overhanging tow using a scissors.
The stub and filaments were overcoated with 90 angstroms of gold-palladium and inserted into the SEM. A model JSM-35C SEM from JEOL of Tokyo, Japan, or a model 240 SEM
from Cambridge Instruments Ltd. of Cambridge, England, was used to image the ends of the fibers and the applied coatings.
SEM was also used to ~ualitatively show the consolidated fiber and matrix metal. Sample preparation involved cutting off the end of the consolidated slug perpendicular to the fibers, polishing with diamond and aluminum oxide grit down to a final grit o~ 0.05 microns, and the observing the so polished end.
Inductively Coupled Plasma (ICP) spectroscopy was used to analyze the coated matrix material to quantify the material deposited and to show the maintenance of stoichiometry. Because of the difficulty ,.. : ~ . :

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of removing matrix material from coated ~ibers, glass slides were placed in the coating chamber to collect the coating matarial for analysis with this test. Material was then dissolved from these slides and compared with material scraped from the originating cathode. Shavings were dissolved in a solution of 5 ml HCl, 2 ml HF, and 93 ml H200 The material on the glass slides was first dissolved in an aqueous solution of HCl, and this solution then was treated to obtain the same proportion of HCl, HF and H20 as above. The so prepared samples were then aspirated into either an Applied Research Laboratory model 3580 or a model 3410 ICP
instrument.
The alpha-alumina fibers of the examples were prepared as follows as described in U.S. Patent No.
4,954,462.
An aluminum formoacetate solution of composition Al(OH)l 2tO~CH)o 6 was prepared by digesting aluminum powder in an acetic acid-formic acid solution under reflux conditions. ~ 1000 ml round bottom flask was charged with 516.8 g of deionized water, 44.6 ml of glacial acetic acid and 3301 ml of concentrated formic acid. The solution was brought to a rolling boil and 34.9 g of aluminum metal powder was added to the boiling carboxylic acid mixture in 3 portions of roughly 12 g each over a 2 hour period. An exothermic reaction ensued after the initial addition, and the rate of the reaction was moderated by the occasional addition of room temperature deionized water. The digestion was continued for 10 hours, and the slightly hazy solution was cooled, filtered through a Whatman #54 and an Whatman #5 filter paper. The percent solids was determined by gravimetric analysis to be 9.25%.
An iron nitrate solution was prepared by dissolving 19.73 g Fe(N03)3 gH20 in 800 ml deionized water. Then 9.40 g NH4HC03 was dissolved in 150 g water and slowly added to the rapidly stirred iron nitrate solution over a period of 5 minutes. The solution was then stirred and heated ~maximum temperature 80C) and , ~ . , : , . , - - ~ .

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2 (~ 5 ~ ~ 2 dialyzed by passing deionized water through a dialysis tube which was immersed in the hydrolyzed iron solution.
The haating was discontinued after 40 minut~s but the dialysis was carried out overnight. Ths final dispersion was 0.39 weight percent equi~alent iron.
A spinning sol was prepared by adding 13.3 g of the hydrous iron polymer solution to 162 g of the aluminum formoacetate solution which was being rapidly stirred. To this 2.5 g of 85% lactic acid solution was added. While maintaining rapid stirring, 0.52 g of NALCOTM 2326 colloidal silica (50 angstrom particle size, ammonium ion stabilized, 14.5~ Sio2, available from Nalco Chemical Company) was added. A final addition of 0.09 g Mg(NO3)2 6H2O dissolved in 20 g water was added dropwise to give a nominal oxide composition Al2O3 + 0.35%
e~uivalent iron + 0.5% sio2 + 0.1% MgO.
Fibers were spun, collected and heated to 650C in air at a heating rate of 150C per hour. The preferred fibers were then rapidly heated to 1400C in air in a CMTM Rapid Temp Furnace (CM, Inc., Bloomfield, NJ) and held at this temperature for 10 minutes to convert them to alpha-alumina.

Example 1 An alpha-alumina fiber tow, 4 inches (10.2 cm) long, containing about 5000 fibers, each fiber being about 16 to 30 micrometers in diameter, was fastened without tension, to a 45 RPM rotating clamp. The clamp was positioned in a Model 100 boron nitride confined type cathodic arc apparatus, from Metco Cat Arc division of Perkin Elmer Corp. as shown in Fig. 1 and similar to that describe in U.S. Patent Mo. 3,836,451 (Snaper) and having a magnetic solenoid as described in Gilmore et al., "Pulsed Metallic-Plasma Generator~," Proceeding of the IEEE, V. 60, No. 8~ pp. 977 991, such that the fiber tow was 25.4 cm (10 inches) from th2 cathode. A 5.7 cm (2.25 inch~ diameter, 2.54 cm (l inch) thick gamma titanium aluminide cathode obtainable from Nuclear Metals, Inc. of Concord, Ma., was mounted on a water cooled cathode ~s~

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!~'.'.` ' ' '.' . ~ . , 2a60520 holder, which was installed in a cryopumped vacuum chamber of the apparatus. After the vacuum chamber was evacuated to 2 X 1o~6 torr, the high vacuum pump was throttled, and argon gas at a flow rate of 14 standard cubic centimeters/minute (scc~) was admitted to the chamber. The chamber pressure was 11~6 mrrorr. An arc was ignited on the cathode surface and was regulated to 140 amperes at 3~ volts. A 15.2 cm (6 inch) long solenoid providing a magnetic field of about 50 Gauss, served to duct the titanium aluminide plasma to the fiber tow. During the plasma deposition, the tow of fibers was observed to billow apart, allowing each individual fiber to be coated. The total deposition time, from arc ignition to arc extinction was 25 minutes.
An SEM photomicrograph of this fiber showed the coating to have a fine columnar structure and substantially uniform thickness. Further examination by SEM revealed that all the fibers in the tow were coated to a thickness of 3.3 microns with a variation of 1 micron.
ICP analysis of material removed from a glass slide placed in the vacuum chamber and coated under identical conditions revealed the composition of the coated material to be essentially the same as the material of the starting cathode.
A 5 cm (2 inch) long bundle of the coated ~ibers was wrapped with 3 wraps of 0.05 mm titanium foil to protect the fiber from the encapsulation tube, the number of foil wraps being the amount necessary to compact the coated fiber bundle and yet be able to fit the wrapped construction inside a 0.5 inch (1.27 cm) O.D.
heavy wall Pyrex glass tube. Each end of the wrapped coated fiber bundle was capped with a .25 inch (.63 cm) thick alumina felt mat of SAFFILTM, available from Imperial Chemical Industries Ltd., previously baked out at 1000C in air. The Pyrex tubes were then evacuated to a pressure of 10-4 torr, the samples were hot degassed at about 300C using an acetylene torch until the pressure stabilized at 10-4 torr for 2 minutes to drive off water :.................. : . .
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20~0-l2a vapor and other adsorbates. After cool down, the Pyrex tubes were then sealed off by heating and melting the neck until collapsed and thus sealed.
These sample ampoules were then HIPped in a model SL 1 hot isostatic press, available from Conaway Pressure Systems, Inc~, at 1010C for 2 hours at an applied pressure of 103.4 MPa (15,000 p5i).
An SEM photomicrograph, shown in FIG. 4, of a cross section of the so produced composite showed the fibers to be evenly embedded within an essentially void free dense metal matrix, and the fiber volume fraction to be 45 percent.

Example 2 Using the apparatus and coating method of Example 1, a tow of alpha-alumina fibers was f irst coated with a 200 nm thick coating of yttrium oxide as follows.
A 5.7 cm (2.25 inch) diameter, 2.54 cm ~1 inch) thick yttrium cathode from Research Chemicals, Inc., Phoenix, AZ. was mounted on a water cooled cathode holder, which was installed in a cryopumped vacuum chamber of the apparatus. After the vacuum chamber was evacuated to 2 X
10-6 torr, the high vacuum pump was throttled, and a mixture of argon at a flow rate of 15 sccm and oxygen at a flow rate of 55 sccm was admitted to the chamber. The chamber pressure was 11.6 mTorr~ An arc was ignited on the cathode surface and was regulated to 50 amperes at 50 volts. A 15.2 cm (6 inch) long solenoid providing a magnetic field of 50 Gauss, served to duct the yttrium plasma to the fiber tow. The total deposition time was 4 minutes.
This yttrium oxide coated tow was then further coated with a coating of gamma-titanium aluminide as describe in Example 1, to a thickness of 3.3 microns with a variation of 1 micron.
The SEM photomicrograph of fibers from this coated tow showed the coating to have two layers, each layer with a fine columnar microstructure, and completely void freP.

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2 ~ 6 0 ~ 2 0 ICP analysis of material removed from a glass slide placed in the vacuum chamber and coated under identical conditions revealed the composition of the coated material to be essentially yttrium oxide.
The coated fibers were prepared and HIPped as in Example 1. An SEM photomicrograph of a cross section of the so produced composite showed the fibers to be evenly embedded within an essentially void free dense metal matrix, and the fiber volume fraction to be 45 percent.

Example 3 A tow of 15 micron diameter alpha-alumina fibers was coated with super-alpha2-titanium aluminide using the method and procedure of Example 1. A
super-alpha2-titanium aluminide cathode from Titanium Metals Corporation was mounted on a water cooled cathode holder, the argon flow rate was 14 sccm, the chamber pressure was 10 mTorr, the arc current was 140 amperes at 28 volts, and the 6 inch ~15.2 cm) long solenoid provided a magnetic field of 50 Gauss for plasma ducting. The total deposition time was ~0 minutPsO
The SEM photomicrographs of this fiber showed the coating to be a fine columnar structure and substantially uniform in thickness. Further examination by SEM revealed that all the fibers in the tow were coated to a thickness of 150 nm with a variation of 50 nm.
ICP analysis of material removed from a glass slide placed in the vacuum chamber and coated under identical conditions revealed the composition of the coated material to be essentially the same as the material of the starting cathode.
The coated fibers were prepared and HIPped as in Example 1. An SEM photomicrograph of a cross section of the so produced composite showed the fibers to be evenly embedded within an essentially void free dense metal matrix, and the fiber volume fraction to be 45 percent.

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A tow of 15 micron diameter alpha-alumina fibers were first coated with a 200 nm thick coating of yttrium oxide as described in Example 2, and then coated with a 3.3 micron thick coating of super-alpha2-titanium aluminide as described in Example 3. These so coated fibers were HIPped as describPd in Example 1.
SEM photomi.crographs o~ a cross section of the so produced composite showed the fibers to be evenly embedded within an essentially void free dense metal matrix, and the fiber volume ~raction to be 45 percent.
SEM photomicrographs of this composite are shown in FIGS
5 and 6.
It will be obvious to those skilled in the art ~5 that various other fibers including Textron-SCS (SiC), Sigma (SiC), Saphikon (Al203), Sumitomo (Al203-SiO2), Nextel and TiB2 bare or with prime coatings of yttrium oxide, gadolinium oxide, and other rare earth oxides, may be used and that various other matrix materials such as alpha+beta-Titanium (e.g., Ti-6-4), beta titanium (e.g., Ti-15-3), aluminum alloy, iron aluminide, or nickel aluminide may be used.
The various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention and this invention should not be restricted to that set forth herein for illustrative purposes.

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Claims

1. A fiber reinforced metal matrix composite comprising consolidated coated fibers, said fibers comprising a ceramic, carbon or metal fiber having a coating thereon of a metal matrix material, the coating being substantially uniform and having a morphology which is fine grained and substantially void free and said composite being substantially void free.

2. The composite of claim 1 wherein said ceramic fibers are alumina, alumina-silica, alumina-boria-silica, boron, titanium diboride, tungsten, silicon carbide, and silicon nitride fibers.

3. The composite of claim 1 wherein said fibers have a diameter in the range of about 5 to 150 micrometers.

4. The composite of claim 1 wherein said metal is aluminum, titanium, magnesium, nickel, iron, copper, chromium, tantalum, tungsten, niobium and various alloys and intermetallics thereof.

5. The composite of claim 1 wherein said composite has a fiber to total volume ratio of at least about 30%.

6. The composite of claim 1 further comprising a barrier/interface coating between said fiber and said metal matrix.

7. The composite of claim 6 wherein said barrier/interface coating is molybdenum, tantalum, tungsten or niobium, oxides of aluminum, yttrium, zirconium, hafnium, gadolinium, titanium, erbium, or other rare earth metals, or carbides of tantalum, tungsten, niobium, zirconium, hafnium, or titanium.

8. The composite of claim 6 wherein said barrier/interface coating is applied to achieve a coating thickness of about 20 to 500 nanometers.

9. A process for making a fiber reinforced metal matrix composite comprising the steps of (1) coating ceramic, metal or carbon fibers by (a) applying sufficient current to a metal cathode to form a plasma of atoms and ions of the cathode material, (b) positioning said fibers in the path of said plasma, said plasma causing said fibers to billow, and (c) causing said plasma to condense on said fibers to form a metal matrix material coating; (2) vacuum encapsulating the fibers;
(3) applying sufficient heat and pressure to cause the metal coating to flow and bond the coated fibers together by interdifffusion; and (4) allowing the bonded fibers to cool to form a consolidated metal matrix composite, said composite being substantially void free.

10. The process of claim 9 wherein said ceramic fibers are alumina, alumina-silica, alumina-boria-silica, boron, titanium diboride, tungsten, silicon carbide, and silicon nitride fibers.

11. The process of claim 9 wherein said fibers have a diameter in the range of about 5 to 150 micrometers.

12. The process of claim 9 wherein said metal is aluminum, titanium, magnesium, nickel, iron, copper, chromium, tantalum, tungsten, niobium and various alloys and intermetallics thereof.

13. The process of claim 9 wherein said composite has a fiber to total volume ratio of at least about 30%.

14. The process of claim 9 wherein said coating is applied in an inert atmosphere.

15. The process of claim 14 wherein said inert atmosphere is argon, krypton, xenon, helium or another gas belonging to Group 8 of the periodic table.

16. The process of claim 9 further comprising applying a barrier/interface coating on said fiber prior to step (1) by applying sufficient current to a metal cathode to form plasma of atoms and ions of the cathode material and positioning said fibers in the path of said plasma, said plasma causing said fibers to billow, and causing said plasma to condense on said fibers to form a refractory metal or a metal-based ceramic coating, said coating being substantially uniform and having a morphology which is fine grained and substantially void free.

17. The process of claim 16 wherein said barrier/interface coating is molybdenum, tantalum, tungsten or niobium.

18. The process of claim 16 wherein said process is carried out in a reactive gas atmosphere.

19. The process of claim 18 wherein said reactive gas is oxygen, nitrogen, ammonia or hydrocarbon.

20. The process of claim 19 wherein said gas is oxygen.

21. The process of claim 20 wherein said coating is an oxide of aluminum, yttrium, zirconium, hafnium, gadolinium, titanium, erbium, or another rare earth metals.

22. The process of claim 19 wherein said gas is hydrocarbon.

23. The process of claim 22 wherein said coating is a carbide of tantalum, tungsten, niobium, zirconium, hafnium, or titanium.

24. The process of claim 16 wherein said barrier/interface coating is applied to achieve a coating thickness of about 20 to 1000 nanometers.

25. The process of claim 9 wherein the temperature in step (3) is in the range of about 800 to 1200°C.

26. The process of claim 9 wherein the temperature in claim (3) is about 35 to 300 Mpa.

27. The process of claim 9 wherein step (3) is carried out for a period of 30 minutes to 4 hours.

28. A coated fiber comprising a ceramic, carbon or metal fiber having a coating thereon of a metal matrix material, the coating being substantially uniform and having a morphology which is fine grained and substantially void free.

29. The coated fiber of claim 28 wherein the metal matrix material is aluminum, titanium, magnesium, nickel, iron, copper, chromium, tantalum, tungsten, niobium and various alloys and intermetallics thereof.

30. The coated fiber of claim 28 wherein the fiber to coating volume ratio is in the range of about 20:80 to 70:30.