Kernforschungsanlage Julich

Kernforschungsanlage Julich Gesellschaft MIT Beschrankter Haftung

Erno Gyarmati

Aristides Naoumidis

Ashok K Gupta

Rudolf Munzer

Hubertus Nickel

Hartmut Luhleich

Francisco J Dias

Marian Kampel

Hermann Kreutz

Bernd Gottselig

Aristidis Naoumidis

Xiliang Qiu

A layer of titanium carbosilicide Ti.sub.3 SiC.sub.2 on a silicon carbide surface polished for making a joint makes it possible to join silicon carbide bodies together in a hot pressing procedure and obtaining a joint strength comparable to the strength of the silicon carbide material. Such a layer on silicon carbide also makes possible brazed juoints with steel alloy or nickel based alloy parts. The layer may be applied directly by a powder dispersion in a volatile but viscous glycol or by sputtering or else the layer can be made in place from a powder mixture of components, especially TiC.sub.0,8 and Tisi.sub.2 (5:1) or a titanium layer of a thickness in the range of 1 to 3 .mu.m that reacts with the silicon carbide surface. When silicon carbide parts are joined together, the heating up to make the joint also serves to convert a titanium layer into titanium carbosilicide. When silicon carbide is to be joined with metal, a preliminary heating step is necessary to at first convert a powder mixture or a titanium layer on the silicon carbide surface to Ti.sub.3 SiC.sub.2. Alternatively a Ti.sub.3 SiC.sub. 2 surface layer can be formed by a sputtering process. The Ti.sub.3 SiC.sub.2 layer favors brazing of the metal part to the silicon carbide surface as treated. The heating requires reaching a temperature in the region from 1200.degree. to 1600.degree. C. for periods between a half hour to about three hours in the presence of a reducing protective gas.

Method and components for bonding a silicon carbide molded part to

For joining shaped bodies of silicon nitride together, silicon nitride surfaces to be joined are first polished and then put into an apparatus for applying sputtered layers where they are first cleaned by ion bombardment in argon, followed immediately by sputtering with silicon in a nitrogen atmosphere such that a layer is deposited having a nitrogen content exceeding the Si.sub.3 N.sub.4 stoichiometric ratio. This readily provides a layer of the composition Si.sub.3 N.sub.5.5. A complementary nitrogen deficient layer is also provided in the joint before hot pressing, either in the form of a silicon layer that goes between the nitrogen-rich silicon nitride layers or in the form of a nitrogen-deficient silicon nitride layer sputtered onto a polished silicon nitride surface at relatively low nitrogen pressure. The parts are isostatically hot pressed together at 1500.degree. to 1750.degree. C. in a nitrogen atmosphere. The layers which are usually thinner than 1 .mu.m interact, with the disappearance of excess nitrogen and silicon, so that once the joint is produced all traces of jointure tend to disappear. The presence of a small amount of Y.sub.2 O.sub.3 at the joint has a favorable effect.

Process for joining molded silicon nitride parts

Silicon carbon molded parts, whether made of silicon carbide sintered together in the absence of pressure or hot pressed silicon carbide are bonded together at close fitting surfaces by applying a layer not thicker than 1 .mu.m on polished surfaces to be joined, containing at least one carbide and/or silicide forming element from the group Ag, Al, Au, B, Be, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Ni, Pd, Pt, Ta, Ti, V, W and Zr. The surfaces to be joined are than fitted together and heated in an inert or reducing atomosphere at a pressure between 10.sup.-1 and 10.sup.-5 Pa at temperatures in the range 800.degree. to 2200.degree. C. while under a pressure applied which is between 1 and 100 MPa. In particular, the heat treatment range from 1550.degree. C. to 1750.degree. C. under a pressure of 15 to 45 MPa applied pressure in argon at a pressure of from 10.sup.3 to 10.sup.5 Pa argon, for 30 to 60 minutes. Preferably a thin layer is vapor-deposited or sputtered. Cr, Cu, Ni, Pt and/or Pd, and especially Cu and its alloys, have been found particularly effective.

Method of joining molded silicon carbide parts

A method of making shaped bodies of silicon carbide, of graphite coated with silicon carbide or of a graphite-like material with a silicon carbide surface wherein a graphitic body is assembled from preformed parts in the desired shape and is immersed, under a chemically inert atmosphere, in a silicon melt and after penetration of the melt into gaps between abutting surfaces of the body and after reaction of the silicon with the graphite or the graphite-like material to form silicon carbide at the junctions, the body is removed from the melt and is cooled in a chemically inert atmosphere.

Method of making shaped bodies of silicon carbide or of graphite or

Porous silicon carbide bodies are obtained by starting with a powder of particle size falling within a single sieve mesh range fraction and consisting either entirely of carbon particles or a mixture of carbon and silicon carbide particles of the same mesh fraction is mixed or coated with 15 to 30% by weight of a binder, moulded to shape, warmed in the temperature range from 40.degree. to 200.degree. C. to vaporize volatile material and then coked at a temperature rising to 850.degree. C. Then it is siliconized by raising the temperature to the range from 1650.degree. to 1950.degree. C. with gaseous silicon or impregnated with silicon by dipping the body into a silicon melt and convert it to carbide, with the excess silicon thereafter being removed by vaporizing out or by boiling in lye. Sieve mesh fractions in the region of a few hundred .mu.m are preferred, and the density of the porous body after pressing but before siliconizing should lie in the neighborhood of 0.6 to 0.7 g/cm.sup.3.

Method of making porous silicon carbide bodies

For joining molded parts having silicon carbide surfaces, the surfaces are first roughened to a depth of about 100-500 .mu.m by removal of the free silicon either by vaporization or by etching when treating silicon containing silicon carbide layers containing at least 15% weight of excessive silicon. In the alternative, when no excessive silicon is present, the roughening can be done by laser shots pitting the surface. The pores provided by such a step are then loaded with carbon by (repeated) application of a cokable resin followed by coking, said resin can be soaked into the pores or attached as silicon containing resin wafer of cokable material. The surfaces to be joined are united and are heated, preferably at from 1600.degree. to 1800.degree. C. in the presence of silicon that is either made available at the edges of the joint as a liquid or else has been provided in the joint by a synthetic resin foil in which silicon powder is dispersed.

Method of molding bonded parts with silicon carbide surfaces

A significant improvement of the silicon impregnation depth of a carbon precursor body in the siliconizing thereof to make a silicon carbide body is obtained, as well as an accelerated conversion into silicon carbide if activated carbon is used in whole or in part as the carbon powder component of the precursor body. The molding material is preferably 20 to 60% by weight of binder and 20 to 70% activated carbon. This molding material may also usefully contain 10 to 60% by weight of .alpha.-silicon carbide, which may be previously made by siliconizing carbon powder.

Method of producing silicon carbide bodies

In the production of shaped silicon-carbide bodies, especially tubes which onsist predominantly of silicon carbide throughout their thickness for use in nuclear-reactor technology, a carbon-containing preform (body of the desired shape) is contacted with elemental silicon powder at a temperature of 1400.degree. C. to 1500.degree. C. and the silicon-contacted carbon-containing body is thereafter treated at a temperature of 1800.degree. C. to 2000.degree. C. to transform at least the major part of the carbon into silicon carbide.

Method of producing shaped silicon-carbide bodies

A portion of the feed gas is supplied in a pulsating fashion to a fluidized ed reactor containing the articles to be coated mixed with fluidizing particles. The pulse frequency is between 1 and 10 Hz. A continuous flow of a portion of the total gas supply at a rate insufficient to produce turbulence in the bed is provided to facilitate control of the pulsing frequency and reduce the energy necessary for the pulsating gas stream. The gases fed to the reactor are a coating gas and a carrier gas, the flow containing the coating gas including a portion of the carrier gas. Either the pulsed flow or the continuous flow may consist entirely of carrier gas. The fluidizing particles are preferably of the same material as the articles to be coated and are typically of a grain size between 2 and 3 mm.

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Julich, Germany

Erno Gyarmati