Carbon-boron surfaced carbon filament

Abstract

A high strength composite filament suitable for use as a reinforcement in metal matrices comprises a filamentary carbon substrate having a continuous essentially amorphous carbon alloy coating adhered thereto, the coating consisting essentially of approximately 43-60 atomic percent carbon, remainder boron.

Description

BACKGROUND OF THE INVENTION

In the evolution of filamentary composites, the development of high modulus, high strength carbon yarns has evoked considerable interest. The major utility for these fibers for reinforcement, however, has been limited to the nonmetal, i.e., resin, matrix materials. Commercially available carbon and graphite fibers which exhibit high tensile strength (over 150,000 psi) and Young's Modulus (over 20 .times. 10.sup.6 psi) are typically less than 10 microns in diameter and are well suited for use as reinforcement for the resin matrices. It is recognized, however, that the incorporation of metals into oriented carbon fibers imposes a more stringent set of requirements than does resin impregnation.

In order to effectively utilize carbon filament as reinforcement in metal matrix material, a relatively large diameter carbon fiber, coated with a diffusion barrier, is necessary. One of the paramount problems in obtaining carbon filament-reinforced metal matrix composites of acceptable properties can be attributed to the small size of the reinforcing carbon fibers. To achieve the maximum strengthening effect in a fiber-reinforced article, it is necessary to utilize filaments having a diameter substantially greater than the presently available 10 micron carbon fibers and more particularly having a final diameter greater than approximately 2 mils. It will be appreciated that filaments smaller than 2 mils, such as the 10 .mu. carbon fibers, for example, present problems in proper and complete matrix infiltration, susceptibility to chemical attack and lack of resistance to compressive forces normal to their axes. To offset the effects of reactivity between the fiber and the metal matrix material, it is necessary to provide a barrier coating without, however, degrading the structural properties of the substrate filament.

SUMMARY OF THE INVENTION

The present invention relates to carbon-base filaments and, more particularly, to carbon-base monofilaments at least two mils in diameter suitable for use as reinforcement for metal matrix composites.

The present invention contemplates the production of a high strength, high modulus composite filament comprising a carbon filamentary core at least one mil in diameter, preferably 1 to 2 mils in diameter, having an adherent continuous essentially amorphous coating consisting essentially of a carbon-boron alloy consisting of approximately 43-60 atomic percent carbon, remainder boron. It has been found that such a carbon-boron alloy coating composition, to a thickness of approximately 1/2 - 2 mils, will not only provide a diffusion barrier to the carbon and metal matrix materials but, in addition, will impart high strength and modulus thereto.

The present invention also contemplates the method for making such a filament and, in general, comprises continuously passing a carbon filament through a reactor, heating the filament to 1050.degree. - 1250.degree.C and exposing the heated filament to a reactant gas mixture consisting essentially of methane, boron trichloride and hydrogen, the mass flow ratios of methane/boron trichloride and hydrogen/boron trichloride being 1.16 to 8.08 and 0.5 to 2.0 respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

An understanding of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings, wherein:

FIG. 1 is a simple sketch, taken in elevation, of apparatus used in the production of the carbon-boron alloy coating of the carbon filaments of the present invention; and

FIG. 2 is an enlarged cross-sectional view through one of the filaments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As seen in FIG. 1, the carbon-boron alloy coating is produced on a resistively heated carbon monofilament 4 which is drawn downwardly through a reactor 6 comprising a tubular containment vessel 8, having a gas inlet 10 at the upper end of the reactor and a single exhaust port 12 at the lower end thereof. A reactant gas mixture consisting essentially of methane, boron trichloride and hydrogen is fed to the reactor through inlet 10. The containment vessel may be formed of Pyrex, although a number of other materials including Vycor and quartz will be found satisfacotry. The gas inlet 10 and the exhaust 12 penetrate and are electrically connected to the metallic end plugs 14 and 16 which provide the end closures for the containment vessel and also provide convenient means by which power may be supplied to the wire for resistance heating purposes.

The end plugs are each respectively provided with a well 20 and 22, for containing a suitable conductive sealant 24, such as mercury, which serves the dual purpose of providing a gas seal around the wire where it penetrates the end plugs, and further providing electrical contact between the moving wire and the respective end plugs which are in turn electrically connected through the tubes 10 and 12 and the leads 26 and 28 to a suitable DC power source 30. The upper plug 14 is provided with a peripheral groove 34, which communicates with the mercury well 20 through the passageway 36, to provide peripheral sealing around the plug. Sealing between the end plug 16 and the lower end of the containment vessel 6 is provided by mercury contained in an annular well 38.

The respective plugs are each formed with a centrally oriented orifice 40 and 42 which is large enough to accommodate the free passage of the wire 2 therethrough but which, in combination with the wire, is small enough to retain the mercury, through surface tension forces, in their respective wells.

The hydrogen is considered necessary in the reactant mixture since it acts not only as a coolant for the end plugs 14 and 16 but also acts to even temperature distribution and to catalyze the methane reaction. In practice, the relative proportions, expressed in flow rate ratios of the constituent methane and boron trichloride gases are critical to the formation of useful filaments. It is considered, for example, that with the hydrogen/boron trichloride ratio in the range of 0.5 to 2.0, preferably at 1.0, the methane/boron trichloride ratio must be greater than 1.0 but less than approximately 10, preferably 1.16 to 8.08, and optimally 2.34. At a mass flow ratio of methane/boron trichloride of 1.0 carbon-boron deposition resulted in embrittled filaments which could not be handled without breaking.

By utilizing various gas flow ratios, relative amounts of carbon/boron in the coating layer may be varied. At the optimum ratio, monofilaments with a coating of C(52 percent B) had an average tensile strength of 265,000 psi and Young's Modulus of 41 .times. 10.sup.6 psi. As shown in FIG. 2, passage through the reactor results in a composite filament comprising a filamentary carbon core 46 having a relatively thick adherent coating of essentially amorphous carbon-boron 48. The term "essentially amorphous" for purposes of the present invention is intended to mean that degree of amorphousness which is represented by the appearance of one or two carbon reflections in the X-ray pattern of an X-ray diffraction study. Subsequent to the formation of the carbon-boron layer, like filaments are consolidated and bonded to the desired matrix material by such standard techniques as plasma spray, liquid infiltration or powder metallurgy and braze or diffusion bonding.

Various process techniques and parameters may be utilized in producing filaments of the present invention, as indicated by the following examples. The filamentary carbon substrates utilized were carbon monofilaments commercially available from Great Lakes Carbon and, although of varying diameters as indicated in the examples, all exhibited an average UTS of 150,000 psi Young's Modulus of 4.8 .times. 10.sup.6 psi.

EXAMPLE 1

In a reactor of the type illustrated utilizing a 24 inch long reactor formed from 12 mm Pyrex tubing and a reactant gas mixture of methane, boron trichloride and hydrogen, a carbon-boron alloy coating was produced on a 1.68 mil carbon monofilament. The gas flows were: methane 597 cc/min, BCl.sub.3 74 cc/min and H.sub.2 74 cc/min to yield a CH.sub.4 /BCl.sub.3 ratio of 8.08 and an H.sub.2 /BCl.sub.3 ratio of 1.0. The monofilament was resistively heated utilizing 1900 volts and 140 milliamps at a power setting of 252 watts to a temperature of 1105.degree.C while moving through the reactor at a rate of 40 ft/hr and achieved a final diameter of 3.3 mils. Several runs were made and the average UTS of this filament was 154,000 psi. The coating was subjected to electron microprobe analysis as well as X-ray diffraction studies and found to consist essentially of an adherent essentially amorphous carbon-boron alloy layer consisting essentially of 59.5 atomic percent C, 40.5 percent B.

EXAMPLE 2

The same apparatus and conditions were utilized as in Example 1 except that substrate filament speed was 30 ft/hr and a substrate temperature of 1100.degree.C was obtained by 2100 volts, 120 milliamps and 252 watts. The coated filament had a diameter of 3.2 mils and the average UTS was 153,000 psi. The quality and composition of the coating was the same as in Example 1.

EXAMPLES 3-11

In the reactor apparatus of Example 1, a CH.sub.4 /BCl.sub.3 ratio of 4.97 and an H.sub.2 /BCl.sub.3 ratio of 1.0 was maintained by a methane gas flow of 538 cc/min, a BCl.sub.3 gas flow of 108 cc/min and an H.sub.2 gas flow of 108 cc/min. All of the coatings consisted of adherent essentially amorphous carbon-boron alloy having a composition at 55 atomic percent C, 45 percent B. The conditions and results are given in the following table.

TABLE I __________________________________________________________________________ CH.sub.4 /BCl.sub.3 Ratio at 4.97 __________________________________________________________________________ Final Avg. Current Voltage Power Temp. Substrate Diam Speed UTS Ex. (ma) (volts) (watts) (.degree.C) Diam(mil) (mil) (ft/hr) (psi) __________________________________________________________________________ 3 120 2050 246 1130 1.68 3.3 40 185,000 4 140 1800 252 1105 do. 3.6 do. 166,000 5 160 1650 264 1130 do. 4.0 do. 181,000 6 160 1850 296 1130 do. 3.4 30 188,000 7 160 1825 292 1105 do. 4.3 do. 182,000 8 140 1900 266 1110 do. 4.0 do. 225,000 9 140 1900 266 do. 4.0 do. 188,000 10 90 1600 144 1095 1.36 3.0 40 186,000 11 100 1550 155 1140 do. 3.1 do. 156,000 __________________________________________________________________________

EXAMPLES 12-27

In the reactor apparatus of Example 1, a CH.sub.4 /BCl.sub.3 ratio of 2.34 and an H.sub.2 /BCl.sub.3 ratio of 1.0 was maintained by a methane gas flow of 407 cc/min, a BCl.sub.3 gas flow of 174 cc/min and an H.sub.2 gas flow of 174 cc/min. All of the coatings consisted of adherent essentially amorphous single phase carbon-boron alloy having a composition at 48 atomic percent C, 52 percent B. Conditions and results are given in the following table.

TABLE II __________________________________________________________________________ CH.sub.4 /BCl.sub.3 Ratio at 2.34 __________________________________________________________________________ Final Avg. Current Voltage Power Temp. Substrate Diam Speed UTS Ex. (ma) (volts) (watts) (.degree.C) Diam(mil) (mil) (ft/hr) (psi) __________________________________________________________________________ 12 140 2750 385 1100 2.2 3.1 50 195,000 13 150 2750 413 1135 do. 3.6 do. 240,000 14 160 1700 272 1190 do. 4.2 do. 186,000 15 150 1750 262 1155 do. 4.0 30 270,000 16 140 1950 273 1185 1.60 4.8 do. 187,000 17 160 1700 272 1160 do. 5.2 do. 209,000 18 130 2010 262 1130 1.68 4.4 do. 258,000 19 105 1450 152 1140 do. 3.2 40 283,000* 20 140 1250 175 1135 do. 4.4 do. 266,000 21 100 2500 250 1155 1.36 3.6 do. 296,000 22 100 2325 233 1145 1.33 3.4 do. 297,000 23 100 2275 228 1140 do. 3.5 do. 308,000 24** 160 2400 384 1230/1185 do. 5.2 do. 165,000 25*** 100 2675/2700 268 1100/1150 do. 3.2 do. 297,000 26 100 1440-1610 150 1175 do. 3.4 do. 296,000 27 100 1500 150 1170 do. 3.2 do. 327,000 __________________________________________________________________________ *Individual values as high as 400,000 psi. **Gas flow rates doubled (i.e., CH.sub.4 at 814 cc/min, BCl.sub.3 and H.sub.2 each at 348 cc/min) ***Gas flow rates halved (i.e., CH.sub.4 at 253.5 cc/min, BCl.sub.3 and H.sub.2 each at 87 cc/min)

EXAMPLE 28

In the reactor apparatus of Example 1, a CH.sub.4 /BCl.sub.3 ratio of 1.16 and an H.sub.2 /BCl.sub.3 ratio of 1.0 was maintained by a methane gas flow of 202 cc/min, a BCl.sub.3 gas flow of 174 cc/min and an H.sub.2 gas flow of 174 cc/min. A carbon-boron alloy coating was produced on a 1.36 mil Great Lakes Carbon monofilamentary substrate. The substrate was resistively heated utilizing 1610 volts, 105 ma and 169 watts to 1185.degree.C while moving through the reactor at a rate of 40 ft/hr and achieved a final diameter of 4.2 mils. Several runs were made and the average UTS was 247,000 psi. The coating appeared to be an adherent single phase carbon-boron layer having a composition calculated to consist of 43 atomic percent C, 57 percent B.

Further experimentation was conducted at CH.sub.4 /BCl.sub.3 flow ratios both above and below those given for Examples 1-28. At ratios higher than the 8.08 of Examples 1 and 2, the results were generally less satisfactory. In one series of experiments, for example, with a CH.sub.4 /BCl.sub.3 ratio of 10.15 and an H.sub.2 /BCl.sub.3 ratio of 1.0 maintained by a methane gas flow of 632 cc/min, a BCl.sub.3 gas flow of 62.3 cc/min and an H.sub.2 gas flow of 62.3 cc/min, seventeen separate runs of 1.68 mil carbon monofilament were made. Using parameter ranges of 120-210 ma, 1160-2350 volts, 237-315 watts, 1080.degree. - 1135.degree.C at speeds of 30-40 ft/hr, the coated filaments had a final diameter of 2.6- 4.3 mils and a total average UTS of 99,000 psi. The coating was adherent single phase carbon-boron alloy with a composition consisting essentially of 64 atomic percent C, 36 percent B.

At a CH.sub.4 /BCl.sub.3 ratio lower than 1.16, the results were unsatisfactory. In one investigation, for example, with a CH.sub.4 /BCl.sub.3 ratio of 1.0 and an H.sub.2 /BCl.sub.3 ratio of 1.0 maintained by a methane gas flow of 252 cc/min, a BCl.sub.3 gas flow of 252 cc/min and a hydrogen gas flow also at 252 cc/min, continuous runs could not be sustained because, with decomposition, there was severe embrittlement and consequent fiber breakage.

The carbon-boron coated carbon filaments are suitable for use in the reinforcement of not only resin, such as epoxy, but also metal matrices such as, for example, aluminum, magnesium and when properly coated, titanium and nickel. The filaments of Example 19 were subjected to testing in aluminum matrices. Composites were prepared by winding the coated fialment around 1 mil 2024 Al foil wrapped around a 12 inch diameter mandrel, spraying with polystyrene, removing from the mandrel and cutting into 1 .times. 5 inch pieces. A plurality of pieces (6 to 8) were laid up and diffusion bonded in a hot press at a temperature of 480.degree.C and pressure of 10,000 psi for 20 minutes. The resulting composite had a fiber loading of 45 vol. percent, a UTS of 98,000 psi and a Young's Modulus of 23 .times. 10.sub.6 psi.

By means of the present invention, it will be appreciated that a composite filament has been provided which significantly improves the potential utility of carbon in filament-reinforced structures, particularly insofar as the selection of matrix materials usable therewith is concerned.

What has been set forth above is intended primarily as exemplary to enable those skilled in the art in the practice of the invention and it should therefore be understood that, within the scope of the appended claims, the invention may be practiced in ways other than as specifically described.

Claims

1. A composite filament at least two mils in diameter having a tensile strength of at least 150,000 psi for use as reinforcement in metal matrix composite articles comprising:

a substrate consisting essentially of carbon filament;

a continuous coating adhered to said substrate consisting essentially of carbon-boron alloy, said coating consisting essentially of approximately

2. The composite filament of claim 1 wherein said carbon filamentary substrate is at least one mil in diameter and said carbon-boron alloy

3. The composite filament of claim 1 wherein said alloy coating consists essentially of approximately 43-55 atomic percent carbon, remainder boron.

4. The composite filament of claim 1 wherein said alloy coating consists essentially of approximately 48 atomic percent carbon, remainder boron.

5. The composite filament of claim 3 wherein said carbon filamentary substrate is at least one mil in diameter and said carbon-boron alloy

6. The composite filament of claim 4 wherein said carbon filamentary substrate is at least one mil in diameter and said carbon-boron alloy coating is at least approximately 1/2 mil thick.