Process For Forming Stoichiometric Silicon Carbide Coatings And Filaments

Abstract

A process whereby stoichiometric silicon carbide is chemically deposited on a resistively heated wire from a reactant gas mixture including methyldichlorosilane and hydrogen together with a carbonizing gas such as methane.

Description

BACKGROUND OF THE INVENTION

It is known that filamentary materials may be produced by pyrolytic techniques wherein the desired material is deposited on a resistively heated wire which is drawn through a gaseous reactant mixture containing the material to be deposited.

In a copending application Ser. No. 618,510 entitled Process for Forming Filamentary Silicon Carbide by Malcolm Basche and Urban E. Kuntz filed on Feb. 24, 1967 and now abandoned, which shares a common assignee with the instant application, there has been described a process wherein silicon carbide is deposited on a resistively heated wire from a gaseous reactant mixture including methyldichlorosilane and hydrogen. In that copending application, there is taught a method for producing continuous filaments of silicon carbide as well as methods for forming composite filaments such as boron with a thin coating of silicon carbide.

Silicon carbide, because of its relative inertness and its strength at elevated temperatures, offers excellent potential as the reinforcement material in fiber-reinforced composites utilizing a wide variety of matrix compositions. The strength of these silicon carbide fibers is, however, dependent upon their composition. While it may be desirable in some instances, as is hereinafter discussed in greater detail, to provide a silicon carbide filament or coating which varies from the stoichiometric composition, control of this variation is essential. This is particularly true in those instances wherein the silicon carbide is utilized to form a diffusion barrier on other filaments such as boron. Silicon or silicon-rich silicon carbide does not provide the desired substrate or matrix compatibility to the same degree as does the stoichiometric composition.

Using the process described in the previously mentioned copending application, satisfactory silicon carbide filaments have been produced in continuous lengths of 9,000 feet or more. It has been found, however, that without very careful attention to the process details, the silicon carbide filaments may be formed with a silicon rich outer layer which, although it might be advantageous in some instances, is nevertheless detrimental to the compatibility characteristics of the filaments in usage with the metal matrices.

SUMMARY OF THE INVENTION

The present invention relates in general to a process for depositing stoichiometric silicon carbide on a resistively heated wire and, more particularly, to a process for forming silicon carbide by chemical deposition from a gaseous reactant mixture including methyldichlorosilane to which a carbonizing gas, such as methane has been added. It contemplates not only the production of filaments which are predominantly silicon carbide but also the production of composite filaments wherein the silicon carbide is provided as a thin layer over a variety of substrates, principally boron.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description which follows it will be convenient to make reference from time to time to the drawings in which: FIG. 1 is a simple sketch, taken in elevation, of a reactor used in the pyrolytic deposition of silicon carbide.

FIG. 2 is a schematic view of the gas system used to provide the gaseous feed for the reactor of Fig. 1.

FIG. 3 is a graph of the pressure-temperature relationship as related to the composition of the deposit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to Fig. 1, the preferred reactor configuration for producing the silicon carbide coating on a resistively heated wire 2 which is drawn downward through the reactor 4, may be seen to comprise a tubular containment vessel 6, having dual gas inlets 8 and 10 at the upper end of the reactor and a single-exhaust port 12 at the lower end thereof. Cooling hydrogen is fed through the inlet 8, and the inlet 10 is used for introduction of the reactant gas mixture including methyldichlorosilane (CH.sub.3 SiHC1.sub.2) and methane, and in some instances, hydrogen. The containment vessel is formed of Pyrex, although Vycor, quartz and a number of other dielectrics are suitable. In a different configuration, various metals are also satisfactory. The gas inlets 8 and 10, and the exhaust 12 penetrate and are electrically connected to the metallic end plugs 14 and 16 which provide convenient means by which poser may be supplied to the wire for resistance heating purposes.

Although the end plugs may readily be seen to differ in overall configuration, they both incorporate a number of common features. They are each formed to provide a well 20 and 22, respectively, 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. A variable resistance 32 is provided in the external circuit to permit regulation of the power supplied to the wire and, hence, temperature control thereof. The upper end 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 further each formed with a centrally oriented orifice 40 and 42 which is large enough to accommodate the free passage of the wire therethrough but which, in combination with the wire, is small enough to retain the mercury, through surface tension forces, in the respective wells.

The hydrogen admitted through the inlet 8 enters the reactant chamber immediately adjacent the wire inlet and is used primarily for cooling purposes at the end plug 14. The reactant gases enter the reactant chamber in an enlarged chimney portion 50, reverse flow therein, and enter the tubular member 6 at opening 52. In order to maintain the methyldichlorosilane at a fixed and controlled level, a condenser is utilized to yield a gas mixture with a fixed dew point. Such a system is set forth schematically in Fig. 2. It has been found that at pressures of about 4 p.s.i., a dew point of between 12.degree. C. and 15.degree. C. produces filaments of good quality and consistency. At a dew point of 25.degree. C. the concentration of said silane appeared too high and resulted in filaments of reduced strength. At a dew point of 0.degree. C., said silane concentration appeared too low. Experiments were run utilizing both hydrogen and argon as the carrier gases and, in all cases, the hydrogen carrier produced filaments having the higher strength. In subsequent experimentation, tests were run utilizing methane as the carrier gas, dispensing with the hydrogen entirely, except that, in most instances a limited amount of hydrogen was used for cooling purposes at the upper mercury seal. Results were perfectly satisfactory. In general, a methyldichlorosilane molar percentage of from 10-60 percent, based on total flow, will be preferred, regardless of the carrier gas composition. In general, it was found that the peak temperature of the wire must be maintained within the range of 1,100.degree.-1,500.degree. C. and preferably within the range of 1,200.degree.-1,400.degree. C.

In the gas system depicted in Fig. 2, hydrogen from a suitable source is introduced through conduit 60, through pressure regulator 62, flowmeter 64, and valve 66, to the evaporator 68. Part of the hydrogen is bubbled through the liquid methyldichlorosilane 70 in the evaporator and the hydrogen-methyldichlorosilane mixture is discharged therefrom through conduit 72 to the condenser 74 which is maintained at the appropriate temperature to provide an output having a dew point of the appropriate range, this output from the condenser being introduced into the reactor 4 through inlet 10 as previously described.

A portion of the hydrogen may bypass the evaporator and be introduced through line 76 to a three-way control valve 80 for reactor purging purposes or to further regulate the composition of the reactant gas mixture. Cooling hydrogen is admitted to the reactor through inlet 8 from conduit 82 through valve 84 and flowmeter 86.

A study was made of the pressure-temperature relationship involved in the deposition of silicon carbide. In the formation of filamentary silicon carbide and in the formation of coatings of silicon carbide on reactive filamentary material, such as boron, it is extremely important that at least a substantial portion of the deposited material is approximately stoichiometric silicon carbide rather than silicon or silicon-rich silicon carbide. For this reason, the carbonizing medium such as methane is also introduced into the reactant gas mixture. The methane addition is introduced to the system through conduit 90, valve 92 and flowmeter 94. The reactant gas mixture may, therefore, be seen to preferentially comprise methyldichlorosilane, hydrogen and methane, although as previously indicated, the hydrogen may be omitted if desired.

The curve shown in Fig. 3 shows that to produce silicon carbide without free silicon, one must work above the curve for the desired results because at any point below the curve either silicon or a mixture of silicon carbide and silicon will be produced. The majority of experiments conducted were run at one atmosphere pressure, but the data accumulated was sufficient to demonstrate the problem.

The temperature distribution in the usual reactor operated at one atmosphere pressure is such that only a limited portion of the filament on which deposition is being effected, lies above the curve. Accordingly, there is a tendency to produce a layer on the wire which is rich in silicon. In a 30 inch reactor, a graded fiber has been produced which is stoichiometric silicon carbide at the surface abutting the wire and silicon-rich silicon carbide at the outer surface of the coating. Many of the problems that researchers have associated with silicon carbide fiber, such as reactivity with the metal matrix materials and strength degradation at elevated temperatures, can be explained by this high silicon content. Similarly, many of the problems associated with the operation of the reactors producing these fibers can be traced to this silicon formation. The addition of a carbonaceous gas such as methane to the reactant gas mixture, according to the present teaching, permits the formation of stoichiometric silicon carbide without the undesirable high silicon contents. As an obvious corollary, in those instances where a graded coating is desired the desired properties can be achieved by the judicious control of the methane content and the temperature distribution in the reactor. It has been found that satisfactory results obtain with a methyldichlorosilane/hydrogen ratio maintained in the reactor in a range of about 1/1:1/3 on a molar basis and with methane comprising 10-60 mol percent of the total gas flow.

In one experiment utilizing approximately 16 mol percent methane in the reactant gas mixture consisting of methyldichlorosilane and hydrogen, silicon carbide filaments having tensile strengths as high as 571,000 p.s.i. were produced without difficulty, and microprobe analysis verified the absence of free silicon in any significant amount.

It should be pointed out that, in some applications, a filament can be produced which is basically silicon carbide having a thin outer layer of pure silicon. The resulting fiber, after careful oxidation of the silicon, would then actually comprise a composite, silicon carbide with a silica (glass) coating. This would be particularly excellent for use with the resin matrix materials.

EXAMPLE 1

In a system of the type described utilizing a 61/2-inch long reactor formed from 9 mm. Pyrex tubing, a silicon carbide coating has been produced on boron fiber at a rate of 760 feet/hour. The filamentary boron substrate was of the type produced by chemical deposition on a resistively heated wire. At an evaporator pressure of 2 p.s.i.g., a hydrogen flow rate through the evaporator of 483 cc./min. was maintained with no bypass hydrogen flow around the evaporator. The condenser was maintained at 14.5.degree. C. and a methyldichlorosilane flow rate of 231 cc./min. was effected. Cooling hydrogen was admitted to the reactor at a rate of 114 cc./min. and the methane addition to the reactant gas mixture was made at the rate of 150 cc./min., resulting in a total gas composition in the reactor of 15.3 mol percent methane, 23.4 mol percent methyldichlorosilane and 61.3 mol percent hydrogen. The wire temperature was established at approximately 1,130.degree. C. In this regard, it should be noted that the maximum temperature must be below that temperature at which crystallization of the boron occurs.

EXAMPLE II

In a test somewhat similar to that described, utilizing a 30 inch long reactor formed from Pyrex tubing having a diameter of 9 mm. silicon carbide filaments have been produced on a tungsten wire at a rate of 200 feet/hour. Gas flow rates and compositions approximated those set forth in the previous example, but the wire temperature peak was maintained in the range of 1,200.degree.-1,400.degree. C. Over 9,000 feet of continuous silicon carbide filament was produced having ultimate tensile strengths as high as 571,000 p.s.i.

In general, the silicon carbide filaments were produced in the range of 3-4 mils. As previously indicated, those produced in processes utilizing the methane addition had strengths on the order of 571,000 p.s.i. while those produced without the methane addition were generally limited to ultimate tensile strengths on the order of 345,000 p.s.i.

Boron fiber is generally produced with a diameter of 3-4 mils. Tests on 3.8 mil boron fiber with a 0.2 mil coating of stoichiometric silicon carbide indicated that the composite fiber had essentially the same ultimate tensile strength as the basic boron or about 460,000 p.s.i.

EXAMPLE III

In a similar test utilizing a reactor formed from 25 mm. tubing 8 inches long, a silicon carbide coating of 0.2 mil thickness was produced on 4.4 mil boron fiber. Methane was utilized as the carrier gas and introduced to the evaporator at a rate of 520 cc./min., the evaporator being held at 14.degree. C. This resulted in a methyldichlorosilane flow rate of 193 cc./min. Cooling hydrogen was admitted at a rate of 200 c.c./min. The total composition of the gas in the reactor, on a molar basis, was 57 percent methane, 22 percent hydrogen and 21 percent methyldichlorosilane A subsequent microprobe analysis revealed no excess silicon or carbon in the deposit.

In the course of experimentation, silicon carbide was deposited on various substrate materials including in addition to tungsten and boron, graphite and tantalum. Further, wire temperatures and gas compositions were varied extensively to optimize the process conditions. Still further, the various filaments were subjected to compatibility testing in various matrix materials, including aluminum, magnesium and titanium. No degradation of the silicon carbide fibers was found after heating them in aluminum at 580.degree. C. for 24 hours, and in titanium at 730.degree. C. for 24 hours. The compatibility of the boron-silicon carbide composite filaments was similarly established in other tests for periods of over 500 hours.

It will readily be seen that by this invention there has been provided a process for producing silicon carbide filaments and silicon carbide coatings whereby the character and chemistry of the silicon carbide may be precisely regulated with the resultant improvement in the quality of the filament produced and in the ease by which the filament may be produced.

Claims

We claim:

1. A process for continuously depositing silicon carbide on a heated wired as it is drawn through a reactor comprising the steps of:

maintaining the wire at a temperature sufficient to effect deposition of silicon carbide on the wire, the peak temperature of the wire being within the range of 1,200.degree.-1,400.degree. C.;

exposing the wire to a gaseous stream consisting essentially of methyldichlorosilane admixed with hydrogen; and

adding methane to the gaseous stream to effect the formation of a coating of essentially stoichiometric silicon carbide,

the methyldichlorosilane/hydrogen ratio in the reactor being maintained within the range of about 1/1:1/3 on a molar basis and the methane comprising 10-60 mol percent of the total gas flow.