Making silicon carbide bodies

Details Making silicon carbide bodies Making silicon carbide bodies

1983-01-21

1984-11-13

Cooper, Jack

Plastic and nonmetallic article shaping or treating: processes

Carbonizing to form article

264 60, 264 63, 264 65, 423345, 501 88, C04B 3556, C01B 3136

Patent

active

044825129

DESCRIPTION:

BRIEF SUMMARY
This invention relates to making silicon carbide bodies and provides an improved method of making reaction-bonded, silicon carbide bodies.
It is known, from Published U.K. Patent Application No. 2 043 111 A, to make a reaction-bonded silicon carbide body by processes which include the steps of preparing a porous preform compising compacted particulate silicon carbide and a carbon-yielding binder, heating the preform firstly to convert the binder to carbon and secondly, while the preform in a vacuum or inert atmosphere is in contact with a source of silicon in the form of a compacted silicon cake, made from ingredients including particulate silicon and a carbon-yielding binder, to cause melting and migration into the preform of silicon from the silicon cake.
According to the present invention, in a method of making a reaction-bonded silicon carbide body by a process comprising the above steps, the silicon cake is made essentially only of silicon metal in flake form and the binder, the amount of binder used in making the silicon cake being at least 10% by weight of the silicon, and the binder, when mixed with the silicon, is in such liquid state that it wets the silicon substantially throughout and provides a coating of binder on surfaces of substantially all the silicon flakes.
In all the processes described in the above-mentioned Specification No. 2 043 111 A, the silicon source is a silicon-carbon mixture containing finely divided elemental carbon, preferably including an organic binder which yields carbon on heating, and in the form of a "compact". In the present specification the term "silicon cake" is used to distinguish from the silicon carbide preform which is also called a "compact" in Specification No. 2 043 111 A.
It is a surprising and advantageous feature of the present invention that finely divided elemental carbon is not an essential ingredient of a silicon source to be used in contact with a silicon carbide preform to be reaction bonded by heating.
Since the binder is the only carbon source in the silicon cake and wets the silicon throughout, there is no requirement for effecting uniform dispersal of finely divided elemental carbon through the silicon particles, so that an ingredient and a production step are saved, and furthermore it is found that, on heating the silicon cake, there evolves a silicon carbide cage structure stable enough to withstand, during the reaction, the load of stacked preforms and silicon cakes but not so stable as to impede migration of molten silicon from the cake into the preform or, on completion of the process, to prevent easy removal of the silicon carbide cage as a friable residue in cleaning or finish-machining of the preforms.
The binder used in the silicon cake is preferably a water soluble binder, so that it can be diluted to ensure wetting of the silicon flake, and in particular sodium lignone sulphonate.
The following is an example of the way in which the invention can be carried out.


EXAMPLE

All quantities in percentage parts by weight. Manufacture of a silicon impregnated reaction-bonded silicon carbide body.


1. SILICON CARBIDE PREFORM



Raw Materials
to improve mixing)


Process

(a) 100 lbs. above mix wet milled, using pebbles +10% wt. iron balls with 7 gals. water and 1% SK (Sodium Lignone Sulphonate) as milling aid. Milled for 31/2 to 4 hours.
Resulting slurry is allowed to stand for 48 hours. Supernatant is decanted, wet cake mixed cold, dried overnight at 100.degree.-105.degree. C. in an air drier. The dried cake is hammer milled and sieved to 180 mesh (about 58u).
(b) Yield approximately 80% wt. of passing 180 mesh powder.
(c) Powder pressed into preform blanks--10 ton/sq. in. producing Green density of order 1.94-1.96.
(d) Kilned in induction furnace 50.degree. C./hr. to 700.degree. C.
Kilning drives off pitch volatiles, converts pitch to pitch coke, gives a machinable preform.
(e) Machining Preform machined to size approximately 0.25 to 1 mil oversize on each dimension.


2. MANUFACTURE OF SILICON CAKE



Raw Materials
mesh (approx. 516u

REFERENCES:
patent: 4325930 (1982-04-01), Vallet
patent: 4364974 (1982-12-01), Lask

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