Amorphous Boron-Carbon Alloy in Bulk Form and Methods of Making the Same

Application Ser. No. 12,930, assigned to the same assignee and of even filing date herewith, describes cutting tools made of amorphous boron-carbon alloys.

1. Field of the Invention

The field of art to which this invention pertains is gas or vapor deposition of article-forming material onto a mold surface and articles so produced.

2. Description of the Prior Art

Attempts have been made in the past to produce hard, high modulus of elasticity, amorphous or extremely fine grained materials for such uses as reinforcements for composites, and other high strength applications. The problem in this area has been formation of such amorphous mateial in bulk form without crystallization. To insure the desired amorphous form of the boron-carbon alloy, the prior art has been limited to production of the alloy in relatively thin layes, e.g., less than 10 mils thick. Furthermore, permanently attached substrates have been used to provide the requisite strength for such thin samples. Note, e.g., U.S. Pat. No. 3,867,191. However, it would be much preferred to be able to produce boroncarbon alloys in bulk form without the resulting crystallization of such bulk production attempts in the past.

According to the present invention, a method of producing amorphous boron-carbon alloy in bulk form without crystallization is described. The amorphous boron-carbon alloy is vapor deposited on a releasable mold or reaction surface from a mixture of carbon and boron containing gases. By utilizing a releasable mold or reaction surface and deposition according to the present invention, not only can a variety of sized and shaped articles be produced but such articles can be produced in bulk form without crystallization and with such properties as no discernible grain boundaries, no sharp X-ray diffraction pattern, smooth surface, high modulus of elasticity and hardness. Reactive gas flow conditions producing Reynolds numbers of 40 to 120 allow the formation of amorphous structures with thicknesses far in excess of the less than 10 mils of the prior art (note U.S. Pat. No. 3,867,191).

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawing.

The FIGURE demonstrates graphically the realtionship of Reynolds numbers of grain sizes produced in processes of the present invention.

According to the method of the present invention, a reaction surface of the desired shape is subjected to a mixture of a carbon containing gas, a boron containing gas, and a reducing gas, at elevated temperatures to produce amorphous boron-carbon alloy in bulk form without crystallization. By amorphous is meant an extremely fine grained material, i.e., the absence of detectable grain boundaries when viewed with an electron microscope and extremely broad peaks when viewed with X-ray diffraction apparatus.

The carbon containing gas can be a C.sub.1 to C.sub.5 containing hydrocarbon or halogenated derivative thereof, with CH.sub.a X.sub.b wherein a+b=4 and X is Cl, Br, F, I, or combinations thereof, being preferred and CH.sub.4 being most preferred. The boron source can be diborane of preferably BX.sub.3 with X defined as above and BCl.sub.3 being most preferred. And hydrogen is the preferred reducing gas used.

By controlling the Reynolds number flow condition of the deposition, which also depends to some degree on the temperature of deposition and ratios of reactants, the bulk samples (greater than 10 and preferably greater than 40 mils) are produced. Such deposition conditions are also evidenced by such things as the absence of grain boundaries in the boron-carbon alloy produced and the lack of sharp X-ray diffraction patterns in the boron-carbon samples produced by the present invention.

The Reynolds number is a ratio defined in terms of the velocity of reactive gas flow, the diameter of the reaction vessel, the viscosity of the gaseous reactants and their density. Numerically this ratio is defined by the following formula:

where

D=diameter of the reaction chamber,

v=velocity of flow of the gas mixture in the reaction chamber,

.rho.=density of the gas mixture, and

.mu.=viscosity of the gas mixture.

Reynolds numbers of 40 to 120 result in bulk deposition according to the present invention without crystallization, with Reynolds numbers of 60 to 100 giving the smallest detectable grain sizes, which would be the most preferred embodiment of this invention. The grain sizes can be calculated from X-ray diffraction data according to well-known and accepted procedures. Note Elements of X-Ray Diffraction, B. D. Cullity (Addison-Wesley Pub. Co., 1956).

The temperature of reaction of the gases is preferably 950.degree. C. to 1300.degree. C. At temperatures below 950.degree. C., excess carbon is produced tending to result in a sooty reaction product. And above 1300.degree. C., reaction takes place too rapidly tending to result in crystalline boron formation intertwined with the carbon. While temperature ranges between 950.degree. C. and 1300.degree. C. give useful products according to the present invention, optimum results are obtained between 1120.degree. C. and 1170.degree. C. Generally, useful articles above 40 mils in thickness can be obtained over these temperature ranges in one to two hours.

The ratio of reactants may vary somewhat. Note the Table. However, to obtain the preferred minimal grain size, the molar ratios of carbon containing gas to boron containing gas is 0.5/1.0 to 6.0/1.0, carbon containing gas to boron+carbon containing gases 0.3/1 to 0.9/1, and carbon containing gas to total gaseous reactants (C, B and reducing gases) 0.15/1 to 0.5/1.

In an exemplary system, a mixture of methane, boron trichloride, and hydrogen gases having a density of 7.99.times.10.sup.-4 cm/cc and a viscosity of 4.times.10.sup.-4 poise were flowed through a cylindrical glass reaction chamber with a diameter of 1.22 cm at a rate of 26.67 cm/min. The system, with a Reynolds number of 64.99, produced bulk amorphous boron-carbon alloy with no crystallization.

A flat glass plate and a glass rod were placed in a cylindrical glass reaction vessel 1.22 cm in diameter. A mixture of 650 cc of CH.sub.4, 278 cc of BCl.sub.3 and 780 cc of H.sub.2 gases were flowed through the chamber contacting the plate and rod at an internal chamber temperature of 1182.degree. C. The chamber was at atmospheric pressure and the flow rate was 850 cc per minute. A Reynolds number of 90 was calculated for the system. A 50 mil thickness of baron-carbon alloy was measured on the plate and rod after one and a half hours. The structures after being removed from the rod and plate by light tapping of the composites were found to have smooth surfaces, showed no discernible grain boundaries under electron microscope examination, or sharp X-ray diffraction pattern, and were found to contain 43% by weight carbon upon analysis. The grain size of the bulk amorphous boron-carbon alloy produced was calculated to be between 15 and 18 A.

As demonstrated graphically by the FIGURE, the products with the optimum properties are obtained with Reynolds numbers between 60 and 100, designated as points a and b on the graph. Such values produce grain sizes less than 30 A and optimally less than 15 A. Products within this range have the optimum hardness, strength, modulus of elasticity and other properties associated with the highly amorphous, very fine grain size structures produced in these examples.

A series of four runs were also made varying reactant flow rates and reactant ratios to demonstrate the relationship of various Reynolds numbers to grain sizes and percent carbon in the final products. As can be seen from the Table, by varying the carbon containing gas flow rate from 300 to 800 cc/min., the boron containing gas flow rate from 150 to 500 cc/min., and the reducing gas flow rate from 780 to 800 cc/min., (and gaseous reactant ratios

Most common uses of the bulk boron-carbon of the present invention are as reinforcements for composites, such as in an epoxy or other polymer matrix, as a backing for mirrors, as amorphous semiconducting devices, as continuous filaments, and in cutting tools (note Ser. No. 12,930). For example, as a mirror backing, the amorphous material having no grain boundaries would have isotropic expansion. Also, in the bulk amorphous structure each grain would not polish differently, almost like a cubic material. This would be ideally suited for the subsequent deposition of a reflecting layer. Such material would also be useful in the whole area of electronics, such as thermistors and semiconductors where there is a change of properties with a change in temperature. The stability of a boron-carbon substrate in such utilities would be particularly advantageous.

Although this invention has been shown and described with respect to a preferred embodiment, it will be understood by those skilled in this art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.