US2677627A - Process of coating carbonaceous material with silicon carbide - Google Patents

Description

May 4, 1954 H. R. MONTGOMERY E-r AL 2,677,627

PRocEss oF coATING cARBoNAcEous MATERIAL WITH sILIcoN CARBIDE med oct. 26. 1951 Inventors HAROLDY E Mo/vnsomf/ey Jw WALTER SzYMAszL-K ceorney Patented May 4, 1954 UNITED STATES PATENT OFFICE PROCESS OF COATING CARBONACEOUS MATERIAL WITH SILICON CARBIDE sachusetts Application'October 26, 1951, Serial No. 253,302

Claims. (Cl. 117-406) The invention'relates to a process for coating carbonaceous material such as graphite with silicon carbide.

One object of the invention isto produce superior exhaust'nozzles for guided missiles. Another object is to produce superior rocket nozzles.

'Another object is to produce superior combustion chambers. Another object isto produce superior "crucibles Anotherobjectof vtheinvention is to provide a process for forming silicon carbide in situ on graphite or other carbonaceous pieces with. simple equipment. Another object is to provide a process for coating articles with silicon carbide which vproducesrheavy and strongly Aadherent coatings. .Anotherobject-of the invention is to provide a .thoroughly practical process for the manufacture of Venturi tubes and the like.

Other objects will be in part obvious or in part pointed out hereinafter. In the drawing the single figure is a vertical axial sectional view of Iwith Venturi bores that'will withstand 'intense heat for a long enough time to be quite successful for use in such guided missiles and rockets.

We start with pieces of graphite because graphite can readily be machined. For example the shapes I are made from slid'cylinders of graphite by boring them and then shaping the bores in a turret lathe using shaped tools. 'These A`shaped tools are made 'of soft steel and this "greatlysimplies the problem of making the tools to Vthe 'exact shape desired; a problem which would be difficult using hard tool material because the Venturi shape is not simple. Two tools are usedfor shaping the bores, one for each end, and each tool cuts on each side thereof thus to balance the cutting forces and then a third tool is used .to nish the smallest diameter oi the boreat the `limit of machining by the other tools to make a bore smooth from end to end without vany tool marks and of perfect Venturi shape.

The shapes `I Awhich are to be nozzles are set on graphite bars 2 which are preferably triangular in cross section and are supported by a :circulargraphite plate 3 resting on graphite bars 4, which can be square or rectangular in cross section, the bars 4 being placed across the open top of an inner graphite crucible 5 lled almost full with silica sand t. The graphite crucible 5 rests on graphite blocks 'i on the bottom of an outer graphite crucible 8 having a graphite cover 9.

Through the cover 9, conveniently in the center thereof, is a bore l0. In a counterbore of the bore Iii is a graphite tube I! which acts as a chimney. Extending into a dead end bore into the cover at an angle to the vertical is a graphite tube i2 with two bores I3 and I4 and a cut-out i5 at the inner end; this is a pyrometer tube and the reason for having two bores and a cut-out is to blow gas, suchas nitrogen, through it from the outer end to get rid of fumes which would obscure the dead end of the bore in the cover the color of which is matched to the hot wire of the pyrometer in order to determine the temperature of the apparatus.

Outside of the cruclble 8 is coiled copper tubing i5 through which vwater is owing and through the metalvof which an alternating current of electricity at high frequency is flowing. The coiled copper tubing it is the primary of high frequency induction apparatus; the secondary is the Crucible 8 and the cover ii. Inside of the coil Iii-is a sheet of asbestos Il coiled into a'cylinder and inside of the asbestos sheet Il and under the Crucible and over the cover 9 is comminuted zirconia i8 which is very heavy and is a good insulator of heat and, where-cool as on the outside, a non-conductor of electricity. The bottom of the coil It and the lower layer or zirconia rest upon fire clay bricks I9.

The process may now best be described by giving an illustrative example.

sample I For coating certain nozzles and cylinders with silicon carbide the apparatus described was used the crucible having been thirty inches in diameter and the other dimensions of the crucible 8 and of the other parts of the apparatus having been in proportion about as shown in the drawing. The nozzles in this case were 31/4 inches long and 2% inches in diameter with Venturi bores about the same shape as in the shapes l; but these nozzles were smaller in proportion to the apparatus and there were thirteen of them. The apparatus having been assembled with the nozzles on bars 2, the induction machine (not shown) was started and delivered electric current at a frequency of 1000 cycles a second with a power or" 240 kilowatts to the coil iii. The electromotive force was 365 volts. This input was maintained for two hours and twenty minutes whereupon the E. M F. was dropped to 285 volts and the power to 170 kw. and this input was maintained for one hour more and then the current was turned off completely and the apparatus was allowed to cool. The nozzles were found to be coated with silicon carbide inside and outside to a depth of about .020" to .030. The temperature inside of the Crucible 3 for the last hour was approximately 2350 C.

rllhe process is believed to work as follows: The silica sand 6 melts at about 1700 C. and boils at 2230 C. Therefore when the Crucible 5 reached about 2240" C. the liquid silica boiled violently. At first the gases passed off through the chimney II but later that became plugged with condensed silica and then an appreciable pressure built up within the Crucible 8; sometimes the cover 9 with the heavy zirconia I 3 thereon has been lifted and when that happened the power was cut down for a while. In the particular run described in Example I, however, the Cover 9 did not rise.

The silica vapor in the Crucible 8 on striking the pieces I reacts with the graphite (carbon) to form silicon carbide which is thus integral with the pieces i. Of course the inside of the Crucible 8 and the under side of the cover 9 are also coated with silicon carbide but that is not a detriment. The irst time the apparatus is used a large amount of silica sand is used up coating the inside of the Crucible 8; thereafter less silica sand is used up because the coating process proceeds more slowly after the initial stages as the graphite does not have such ready access to the silica vapor. But the silicon carbide layer is somewhat porous so the reaction and the coating and lining of the pieces I takes v,

less time than in the irst operation. The operation described in Example I was done with apparatus which had been used before. As an incident of the process the plate 3 and also the crucible 5, the bars 2 and 4 and the blocks l are likewise coated with silicon carbide.

Our process is much more effective than the process of coating graphite and the like by heating it in the presence of silicon. Silicon carbide dissociates at 2500" C. so therefore in any process for coating anything with silicon carbide the temperature has to be kept below that ligure, but silicon does not boil until it reaches 2600 C. so the process of coa-ting graphite with silicon carbide by heating it in the presence of silicon is dependent upon the presence of a minor proportion of silicon vapor in the atmosphere and the process is slow and it is diliicult to form a thick coating using the silicon process. But in our process the atmosphere in the Crucible 8 is heavily charged with silica vapor SiO2, the coating of silicon carbide is relatively quickly formed and thick strong coatings can be made.

The reason why we use a plate 3 to cover the Crucible 5 instead of laying the bars 2 across an open top Crucible 5 with the pieces i on the bars 2 is that when the silica boils a blast of vapor is formed which will corrode the pieces I, that is to say eat them away whenever the vapor impinges thereon. In other words the plate 3 is a baille to break up the flow and to cause turbulence and to shield the pieces I from a fast moving stream of gaseous silica. At all events the setup illustrated is effective While removal of the plate 3 frequently spoils the pieces I. The baille plate 3 thus cuts down the velocity of the silica gas and any other arrangement to cut down the velocity of the gas at the locus of the pieces I Could be used. This feature may be referred to as protecting the pieces from direct impact of the silica gas. We will now give another example of the process of the invention.

Example II Using a graphite Crucible 8 of the same size as already described and other parts in proportion about as shown in the drawing we coated twelve graphite pieces to form nozzles. The pieces were 3% inches long and 21/8 inches in diameter and the bores had about the same Venturi shape as the bores in the pieces I in the drawing. For one hour and a half the coil I 6 was energized with electric current at 315 volts, 230 kilowatts and a frequency of 1000 cycles per second, then for one hour at 315 volts, 240 kilowatts and the same frequency, then for one hour at 285 volts, 170 kilowatts and the same frequency. The apparatus had been used before so the inside of the Crucible B and the under side of the cover 9 and the Crucible 5 and plate 3 were coated with silicon Carbide. The temperature of 2350 C. was reached and held for the last hour. The pieces were well Coated, the coating was about .02 to .030" thick and absolutely integral with the underlying graphite. The outside diameter grew only .004" to .006 (.002" to .003" on the radius) showing that a good deal of the graphite was Converted to silicon carbide. (The Venturi bore was coated with silicon Carbide .020" to .030 thick and was everywhere reduced in diameter by .004 to .006".) Thus it is not a case of merely forming a coating on a piece of graphite, an outer layer and also an inner layer of graphite is converted to silicon carbide while more silicon Carbide forms thereover. The same result was also achieved in Example I.

Example III Using similar apparatus as in Examples I and II with an old crucible 8 well coated with silicon Carbide on the inside and an old cover 9 well coated with silicon carbide on the under side but with a new plate 3 and Crucible 5 (the blocks 1 were omitted in this run in order t-o leave enough space for the pieces) the coil I 6 was energized at 320 volts, 220 kilowatts, 1000 Cycles to coat four graphite pieces to make nozzles, 8 inches in diameter, 12 inches long with Venturi bores, and to coat four hollow Cylinders to make chambers, 8 inches outside diameter, 71/2 inches inside diameter and 10 inches long. The above input was maintained for one hour then the load increased to 240 kilowatts, voltage same. This input was maintained for another hour whereupon, with the same voltage the load increased to 250 kilowatts. At this time, namely at the end of two hours, the chimney II was found to be plugged. After another half hour the voltage was dropped to 250 and the load became 170 kilowatts. The temperature had now reached about 2350 C. The latter input, 260 volts, 170 kilowatts was maintained for another hour holding the temperature inside the Crucible 8 at 2350o C. or thereabouts. Then the current was cut 01T and `the apparatus was al1owedto'cool.

The` pieces were found to be coated every-whereby a coating 3.020" to .030 deep but ha'd grown (outside and inside) only .002 -to '.003 on the radius. Of course this growth makes the bore smaller by the amount stated.

The chimney Il (which was 24 inches high) took on a coating of silicon car-bide in its bore adjacent the lower endl but frequently became plugged with silica about half way up. There was always some plugging of the chimney with silica .but this plugging didv not in every run completely block the chimney although in many runs it did. This plugging with silica rather conclusively shows that the atmosphere in the crucible was silica and not silicon. However we believe that the silica in vapor phase may have included some silicon monoxide, SiO, as well as the vapor phase silicon dioxide, SiOz. The oxygen lost would combine with carbon to form carbon monoxide. All air originally in the crucible 8 had been driven out of the chimney Il long before the plugging occurred. The same chimney tube ll was, however, used over and over again since the silica plugging it was removed with no great diic'ulty with iron rods.

The Venturi bore may, in some cases, be a bore comprising a pair of frusto-cones on a single axis with the small ends towards each other but more often the bore is a non-symmetrical negative hyperboloid. An hyperboloid is a volume clened by rotating an hyperbole. around an axis. The hyperboloid is negative when its surface is concavo-convex. The bore is non-symmetrical when one mouth is larger than the other one. If the pieces are made of amorphous carbon hard cutting tools are required. Such tools are more difcult to shape to curves such as hyperbolas and therefore it is an advantage to cut the pieces out of graphite which can be cut with soft steel since the soft steel tools can be more easily shaped. The cutting tools are usually made to approximate shape in a lathe and then iinished by hand to accurate hyperbole. shape using files and abrasive cloth.

A great advantage of our process is that there L is so little growth (and what growth there is is evenly distributed) so nozzles can be made within tolerances of plus or minus one thousandth of an inch everywhere. Allowance is, of course, made for such growth as is obtained.

For the manufacture of very large nozzles the process of the invention can also be used for coating sectors of cylindrical pieces with Venturi bores so that when the sectors are assembled a complete nozzle will be formed. In one case the shape consisted of twenty sectors which were coated with silicon carbide and eventually assembled in a steel shell with refractory cement.

Within the limits of this invention `any carbonaceous material can be coated which will remain in the solid phase at 2230 C. This includes amorphous carbon and graphite and pieces of other material mixed therewith. The range of top temperatures for carrying out this invention is from the boiling point of silica (which we believe is close to 2230o C.) to the incipient decomposition o1 silicon carbide (we believe it starts to decompose at about 2450 C.

although decomposition is not rapid until about 2500 C.). Furthermore at above 2450 C. the boiling of the silica appears to be too violent for the best results. The temperature should be maintained -between these limits for at least one half hour to produce an adequate coating.

We have found it is really-immaterial whether the chimney H becomes plugged or not and in any event the pressure inthe crucible 8 never rises much above one pound per square inch gauge pressure and frequently is at atmospheric pressure.

Graphitehas great thermal shock resistance and can be easily machined into desired shapes. However it oxidizes readily at high temperatures. Silicon carbide is very refractory and not easily oxidized even at the temperatures of the jets' in rockets and other missiles. It is resistant to the erosive effect of the jets, the gases of which sometimes reach a velocity of 4000 feet per second, while graphite is not resistant to such erosive effect. And silicon carbide is diicult to shape and almost impossible to machine. Hence by coating graphite with silicon carbide, we utilize the good properties of each material without detriment from the lbad properties of either.

Features of the invention are that the pieces to be coated are spaced from the comminuted silica (silica sand) and are not buried in anything, i. e. they are surrounded by air in the first place and during the process they are surrounded by gaseous silica with some carbon monoxide mixed therewith instead of being buried in the sand. Another feature is that 4the heating is done by electric induction which makes possible excellent control without which control the results are sporadic and uncertain. Furthermore, induction heating makes possible uniform heating which is highly desirable.

It will thus be seen that there has been provided by this invention a process for coating carbonaceous material (especially graphite) with silicon carbide in accordance with which the various objects hereinbefore set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments might be made of the mechanical features of the above invention and as many changes might be made in the embodiments above set forth it is to be understood that all matter hereinbefore set forth or shown in the drawing is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. Process of coating a carbon body with silicon carbide comprising placing said carbon body in a closed carbon container containing comminuted silica spaced from said body, heating the container to the boiling point of the silica while protecting the body from direct impact of the gaseous silica formed and keeping the temperature below the temperature of incipient decomposition of silicon carbide and maintaining the temperature of the container between the boiling point of silica and the temperature of incipient decomposition oi silicon carbide for at least one half hour, the coating being formed by the reaction of silica gas with the carbon material.

2. Process according to claim 1 in which the body is a body of graphite.

3. Process according to claim 2 in which the container is made of graphite.

4. Process according to claim l in which the container is made of graphite.

5. Process for the manufacture of articles coated with silicon carbide comprising machining a piece of graphite to the required shape, placing it in a closed carbon container with a quantity of comminuted silica therein but spaced from the comminuted silica, heating the container to the boiling point of the silica while protecting the `piece from direct impact of the gaseous silica formed and keeping the temperature below the temperature of incipient decomposition of silicon carbide and maintaining the temperature oi the container between the boiling point of silica and the temperature of incipient decomposition of silicon carbide for at least one half hour, the coating being formed by the reaction of silica gas with the graphite.

6. Process according to claim 5 container is made of graphite.

7. Process according to claim 1 in which the heating is done by electric induction whereby to secure substantially uniform heating of the silica and of the carbon bcdy.

8. Process according to claim 7 in which the carbon body is a body of graphite.

in which the 9. Process according to claim 8 in which the container is made of graphite.

1G. Process according to claim 1 in which the heating is done by electric induction whereby to secure substantially uniform heating of the silica and of the carbon body and in which the container is made of graphite.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,658,647 Miller Feb. 7, 1928 2,431,326 Heyroth Nov. 25, 1947 2,439,983 Morgan et al Apr. 20, 1948 2,463,791 Morgan Mar. 8, 1949

Claims (1)

1. PROCESS OF COATING A CARBON BODY WITH SILICON CARBIED COMPRISING PLACING SAID CARBON BODY IN A CLOSED CARBON CONTAINER CONTAINING COMMINUTED SILICA SPACED FROM SAID BODY, HEATING THE CONTAINER TO THE BOILING POINT OF THE SILICA WHILE PROTECTING THE BODY FROM DIRECT IMPACT OF THE GASEOUS SILICA FORMED AND KEEPING THE TEMPERATURE BELOW THE TEMPERATURE OF INCIPIENT DECOMPOSITION OF SILICON CARBIDE AND MAINTAINING THE TEMPERATURE OF THE CONTAINER BETWEEN THE BOILING POINT OF SILICA AND THE TEMPERAATURE OF INCIPIENT DECOMPOSITION OF SILICON CARBIDE FOR AT LEAST ONE HALF HOUR, THE COATING BEING FORMED BY THE REACTION OF SILICA GAS WITH THE CARBON MATERIAL.

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