DE2650083A1 - non-porous silicon nitride mouldings prodn. - by moulding silicon powder and silicone resin binder, opt. hardening and nitriding - Google Patents

Abstract

Prodn. of Si nitride work pieces involves forming a plastic mixt. of Si powder and an organic binder and opt. hardening in air or an inert protective atmos. at ca. 250 degrees C and then nitriding in a nitrogen atmos. at up to 1400 degrees C. The novelty is that the binder is a silicone resin, which remains in the green moulding on hardening and is nitrided with this. Pref. the green moulding is machined by turning before nitriding. The physical and chemical properties of the binder are changed by heating to 300-1100 degrees C, pref. in air or a protective gas, esp. nitrogen, before nitriding. The work pieces are used esp. as matrix structures for regenerators and recuperators for gas turbine plants. The green moulding has satisfactory strength and very complex moulding can be made without the use of a special adhesive, since individual parts adhere when pressed together and sinter to a monolithic prod. Since the binder remains in the moulding pore formation is avoided.

Description

Process for the production of workpieces from silicon nitride

The invention relates to a method for creating work embroidery made of silicon nitride according to the preamble of claim 1 or according to the preamble of claim 2.

In previous processes of this type, a malleable molding compound was used made of silicon powder with a grain size of up to 50 μm and an organic binder manufactured. Such a molding compound is very malleable and it can by means of the Forming processes known from plastics technology into the desired shape Workpiece shape: cold pressing, hot pressing, isostatic pressing, Extrusion, extrusion, injection molding, rolling, deep drawing, pressing.

So far, hydrocarbon compounds have always been used as binders for the silicon powder used, which gave the mixture the necessary plasticity. The usage however, this binder has some serious disadvantages. The binder must largely before the workpiece is converted from silicon to silicon nitride the mass of the mold are gassed and removed. While removing and gassing of the binder can, due to violent uneven or aii Discharge of vapors and decomposition gases occurring in the wrong places easily changes or damage to the hardening form occurs. Due to thermally induced Erwel When removing the binding agent, it is possible to create a workpiece shape based on its get out of hand with their own gravity and change their dimensions. After removing of the binder always remains a very loosely cohesive light Green part to be damaged made of silicon powder, in which only very little There are binding forces between the individual particles. Such a green part can generally only be handled or transported with great care. at difficult forms must z. B. such a high proportion of binder can be used, in order to obtain the necessary malleability of the mass that after the @removing of the Binder from the molding compound the purticles in the green part in such a loose way and extensive network stand that the corresponding green part is not at all is more transportable. In the case of such workpieces, the binder must remain in place Gas furnace, in which the mold is later nitrided. The minor one Strength of the green part brings high process difficulties and a lot of waste with himself.

For complex workpiece shapes that have a high proportion of binder had to be used to produce the required malleability of the molding compound, Nitridiereu produces porous material with relatively low strength and relatively low density. The volume percentage of the binder in the mixture is in in such cases so high that the poron left behind by the binding agent is by far not through the increase in volume of the individual particles of the mass in the case of the nitrates be filled. Such properties of the material are useful for many purposes, for which in itself a workpiece made of silicon nitride would make sense, unsuitable at high Uinder proportions in the mixture of the plastic hate is the workpiece but also during the nitriding process is endangered by the fact that due to thermal stresses, which can no longer be taken up from the solidifying workpiece, this is destroyed or damaged before reaching its final strength.

The object of the invention is to provide the underlying method in the Modify way, that means: the disadvantages described are avoided, so that one Sufficiently high strength of the green parts or the pre-setting form is guaranteed can be.

To solve this problem, according to the invention in the in the characterizing Proceeding part of claim 1 or of claim 9 mentioned manner.

Avoids the proposed use of silicone resin as a binder the formation of pores when the mold hardens to form a green compact, because the binder remains in the Ferm mass during this process. Even with the nitriding process the binder is not removed from the composite, but it is due to the special chemical composition of the silicone resin chemically converted in such a way that that it can be built into the compound of the basic substance of the mass. The decrease in volume of the binder due to volatilizing constituents is relative small amount. When using silicone resin as a binder, it may also be possible to

the process of curing to a green compact is skipped entirely and after the molding of the workpiece is still soft plastic this immediately are nitrided. In addition, however, has the use of silicone resin the following advantages as a binder. Silicone resin gives the silicon powder an excellent plasticity, so that a good ductility of the molding compound for Complicated shapes are guaranteed Icaml. Because of the whereabouts of the detergent in the wet and its later installation in the composite material is the ratio of binder Z11 basic substance within the mixture of the molding compound to a large extent Limits can be varied, so d! 5 due to a freely selectable proportion of binding agent in the mixture the malleability of the molding compound the complication of the workpiece be optimally adapted to karni. In addition, karnl in a mix of tales the chemical and physical properties of the binder prior to nitriding to be changed. The change can take place in an air atmosphere at temperatures between 300 and at most about 1100 oC, preferably about 500 oO.

If there is a change (oxidation) in an air atmosphere, the for the Take the oxygen required from the air during the oxidation process. It forms more volatile Hydrocarbon, carbon dioxide and silicon dioxide. Silicon dioxide is used during nitriding built into the matrix by converting it into silicon nitride. Instead of in air can also be carried out under a protective gas atmosphere at the specified temperatures inort-behaving protective gas z. B. can be changed under a nitrogen atmosphere. This turns the resin into silicon dioxide, carbon and volatile hydrocarbons decomposed.

During nitriding, silicon dioxide turns into silicon nitride, a hydrocarbon converted into silicon carbide. The evenly distributed silicon carbide, which is only present in a small amount, does not damage the silicon nitride part.

In the cured state, the binder has and with it the corresponding one The green part has a very high strength.

In the green state, the workpieces can therefore be machined very easily edit so that correspondingly good surface finishes and precise dimensions are achievable. The same applies to a large extent to a green part, in which the binder is chemically and physically changed.

lin special field of application of the underlying and of the invention The process is the production of matrix structures for regenerators and recuperators z. B. for waste heat recovery and fresh air heating in gas turbine systems. Such pegenerators and recuperators are monolithic or at the time of their creation Honeycomb structures made of fine ceramics made up of several individual components. While the creation of such workpieces can be the individual components in the desired Relatiylage put together and glued together using an adhesive will. A mixture of silicon powder can preferably also be used as the adhesive and silicone resin as a binder or a silicone resin alone can be used. Thanks to such an adhesive, the state of the individual components when they are joined together and sticking together soft plastic, green, chemically / physically changed or nitrided be; expediently one will choose a condition that has not yet been nitrided. the Like the molding compound itself, the adhesive can initially cure at around 250.degree and then nitrided, but it can also be done immediately after joining are nitrided.

The individual treatment processes for the adhesive connection can be carried out at the same time with the corresponding treatment processes for the molding compound or in separate ones Operations are carried out.

Based on the inventive use of silicone resin as a binder the individual components of complex structures can be used for the plastic molding compound Shapes also joined together in a soft plastic state and at the contact points been pressed together, so that an intimate bond of the molding compounds at the contact points arises. The single components are glued together without any use a separate adhesive mass simply due to the adhesion strength of the still soft plastic ones Molding compound. The individual components sinter during the subsequent further treatment of these parts in the contact area also together to form a monolithic composite.

@s should be mentioned at this point for the sake of completeness that the individual components are not always loose parts before they are assembled must, but that these are also quasi-endless bands of different kinds Cross-sectional shape can be handled in a single or multi-layered layer be wrapped on top of each other.

@both in the basic substance for the common l'ormmasse as well as in silicon nitride powder can be mixed in beforehand with the adhesive. @silicon powder falls in the grinding of rejects to @ Aufgrung @in such a procedure the material from scrap workpieces can be reused in a cost-effective manner will.

The invention is attached to the embodiments shown in the drawings briefly explained below; show: Fig. 1 is a cross section through a regenerator customary for gas turbine installations, FIG. 2 is an axial view the regenerator disc alone, Fig. @ one with a dash-dotted circle III highlighted detail of the regenerator disk according to FIG. 2 in an enlarged view, Fig. 4 shows the construction scheme for a cross flow matrix structure of a recuperator made of flat and corrugated foils, FIG. 5 shows the construction scheme for a matrix structure with exclusively parallel channels and FIG. 6 shows a further embodiment for a matrix structure with channels running in parallel.

The one in rig. 1 is a regenerator disk 1 rotatably mounted in a housing 3 by means of a shaft 2. On the outer circumference of the Disc, a ring gear 4 is arranged over an elastic ring gear holder 5, so that the disc 1 via a drive pinion 6 and a drive shaft 7 in slow Rotation can be offset. The hous se 3 has four connections for the heat exchange stinging modes: namely the exhaust gas feed 8 for the supply of heat-reducing Exhaust gas from a gas turbine system, the exhaust gas discharge 9 for discharging cooled exhaust gas, the fresh air supply 10 for the supply of pre-ventilated fresh air for the Gas turbine system and the fresh air discharge 11 for discharging the warmed up Prisch @ uft. The transition points from the connections to 11 in the pane are via corresponding Seals 12 to 14 sealed.

The regenerator disk has a hub 16 held by the shaft 2 around which a spiral matrix structure 17 is wound. As the shows enlarged individual representation of FIG. 3 is in the embodiment according to 1 to 3, the matrix structure 17 of the regenerator disc 1 by winding a Quasi-endless film strip 1; S emerged from initially still soft plastic molding compound. On one of the foil sides - in the example on the radially inner foil side - are spacer ribs 19 formed at regular intervals, with their largest elevation rest on the outside of the winding and a composite at the contact points 19a with the foil underneath. Ilierbei can be made using a special Adhesive mass made of silicon powder and silicon resin or made of silicon resin alone or also online use of a separate adhesive mass based solely on the remaining ones Adhesive strength of the still malleable molding compound a prebond of the initially still soft plastic Foil 18 on the contact share 19 a with the support ribs 19 are brought about. When using adhesive, this can be applied to the tips of the support ribs shortly before can be applied to the wrap when the film emerges.

To produce the mixture of the malleable molding compound for the tape film 18 Can, for example, a silicon powder with a grain size of up to 50> im and a silicone resin can be used.

Lin suitable silicone resin is e.g. B. Resin No. @ O4 from Dow Corning GmbH, 6000 Frankfurt / Main, or a similar resin from Wacker-Chemie, 8000 Munich. The mixing ratio of powder to resin must be tested and be optimized with regard to the respective desired shape. The form measurement can Silicon nitride powder can be added in the specified grain size range. The one shown Tape 18 with the Cross ribs 19 I can by extruding a first smooth film with a slightly larger wall thickness and then rolling. By rolling, the wall thickness of the film is still somewhat reduced and there will be at the same time the transverse ribs are formed.

After completing the form in this initially still soft plastic State, the regenerator disk is hardened at about 250 ° C in an air atmosphere and a so-called green compact is produced. the binder remains in the molding compound and the binding agent of any adhesive that is used remains the same Adhesive mass back at the adhesive points. The forming agent is rather by the The hardening process itself is hard and, together with the silicon powder, forms a hard one Material.

In this green state of the workpiece, the green part can be machined machined because the strength of the workpiece iii corresponding to this state is high. The regenerator disk can, for example, be turned flat on the two axial front ends or ground so that the required plane parallelism of the opposite Sides and the required surface quality is created.

After the final machining of the green part, it can are now nitrided in a subsequent treatment process. this will be the Workpiece fired in a nitrogen atmosphere at around 1400 OC. Because of this Firing or nitriding takes place a chemical conversion of both the silicon powder as well as the dicing agent instead of the workpiece during the firing process would temporarily lose some of its strength and dimensional stability. the end the Silicon grains are formed from silicon nitride and from the feston decomposition products silicon nitride and possibly some silicon carbide are also produced in the binder. The silicon nitride is mainly produced in the form of so-called whiskers - in fibrous silicon nitride crystals, dio the individual particles of the workpiece compound connect firmly. Due to the peculiarity of the formation of silicon nitride also from the Binder, this happens more firmly than usual.

In cases in which to create a particularly good dildosanikeit the soft plastic molding compound has a particularly high proportion of silicone resin as a binding agent has been entered into the molding compound, some of this binder can before the nitridation chemical and physical properties of the green body are changed, what at temperatures below about 1100 OC in an air odor also in a Nitrogen atmosphere can happen.

For workpieces that are not machined when they are green need to be, nitriding can also be carried out immediately after completion the still soft plastic shape.

In the case of the matrix structure 20 shown in FIG. 4, alternating eilie obeno film 24 and a desired film 25 or 26 are stacked on top of each other and engaged at the contact points 23.

Due to the crosswise arrangement of the longitudinally corrugated film 25 and the transversely corrugated foil 2G arise within the matrix structure due to the respective interposed flat films 24 longitudinal channels 21 and transverse channels 22. One such Matrix structure is particularly suitable for cross-flow heat exchangers, at the global one heat-absorbing and one heat-emitting medium in heat-exchanging To be in contact with each other.

In the similarly constructed matrix structure 3o according to FIG. 5, there are between flat foils 24 corrugated foils 31 arranged with uniform extension of the waves, so that connected between the along the line-shaped contact points 32 Films 24 and 31 result in only uniformly parallel channels 33. This kind the matrix structure is suitable for recuperators in which the heat-absorbing and the heat-emitting medium flows through the matrix structure in countercurrent, alternately in successive levels.

The matrix structure 35 shown in Fig. 6 is made of individually extruded Tubes 36 built up in the extrusion process with a perpendicular to the plane of the drawing lying in the extrusion direction, with the round tubes in the tubes Channels 37 are formed in the extrusion process. The first in a quasi-endlosor form extruded tubes are cross-cut into individual sections of the desired length and at the contact points 3 ;, glued to one another to the desired matrix height. A relatively thin-bodied adhesive with a high proportion of silicone resin can be used here be used so that the adhesive mass by lightly pressing the tube 36 at the contact points 3: J and in the gussets 39, which are expediently complete are to be filled in, easily distributed. It can be useful with this type of matrix structure be to burn the tubes 36 green in initially very long pieces and only in the green state to the required section length to saw up and place these green sections of the tubes at the desired location Assemble matrix structure in the required height. Here the adhesive used during assembly in a further curing process 25o OC can also be hardened to a griilion state, so that the entire Matrix structure is immovable and can be machined before the part is finally nitrided. It is also dexflcable, having a matrix structure the required number of tubes, but this matrix structure in To make the tube very long, for example as long as the curing oven selbor, in this overly long state the initially still soft plastic matrix structure to harden to a green compact and then matrix structures from this quasi-endless green compact to be cut from the desired dimension in the direction of the duct.

Claims (9)

Claims # Method for producing workpieces from silicon nitride, in which a malleable mass of silicon powder and organic binder is plastic is brought into the desired shape, in which the shape is then in air or in an inert gas atmosphere at temperatures in the range is cured to a so-called green compact at 250 OC and then the Green body iii nitrided in a nitrogen atmosphere at temperatures up to 400 OC is, d u r c h e k e n n n z e i c h n e t, that a silicone resin is used as a binding agent is used and that the binder during curing to the green compact in the molding compound remains and is also nitrided during nitriding. 2. Ancestors for the production of workpieces from silicon nitride, at to which a malleable mass of silicon powder and organic binder plastic is brought into the desired shape, within which process as the last process step the mold is nitrided in a nitrogen atmosphere at temperatures up to 1400 OC, d a d u r c h g o -K e n n z e i c h n e t that a silicone resin is used as a binding agent that the nitriding is used immediately after the molding of the mass takes place and that the binder is also nitrided during nitriding. 3. The method according to claim 1, d a d u r c h g o k o n n -z e i c h ii e t that the green compact machined before nitriding begins will. 4. The method of claim 1, 2 or 3, d a d u r c h g c -k e n n notices that the binder before the nitridioren at temperatures between 300 to 1100 ° C changed in its chemical and physical properties will. 5. The method according to claim 4, d a d u r c h g e k e n n -z e i c h Not that the atmosphere is air when it is changed. 6. The method according to claim 4, d a d u r c t1 g c k e n n -z e i c h n e t that the atmosphere is an inert protective gas when changing, in particular is nitrogen. 7. The method according to any one of claims 1 to (;, d a d LL r c h g e k e n n z e i c ii 11 e t that for the production of fully one-piece workpieces, those composed of a plurality of individually shaped, assembled in the desired relative position and single components sintered together, the single components directly assembled in a plastic state and connected to each other Contact points are pressed together to form an intimate bond of the molding compound. @. Method according to one of Claims 1 to @, d a d u r c h g e k e n n z e i c h n c t that for the production of one-piece workpieces, those composed of a plurality of individually shaped, in the desired relative position and sintered together @ single component, the single-pole components preferably in a not yet nitrided state with silicon powder and Silicone resin as a binding agent glued together existing adhesive will and that the adhesive is in a nitrogen atmosphere at temperatures of up to 1400.degree is nitrided, with components of the adhesive and the molding compound in the contact area be sintered together. 9. The method according to any one of claims 1 to 8, d a d u r c h g e k It is indicated that the malleable mass and / or the adhesive mass is silicon nitride powder contains. lo. Method according to one of Claims 1 to 9, in particular according to Claim 7 or, d a d u r c h g e k e n n z e i c h -n e t that the ancestor is applied to the production of matrix structures of ceramic recuperators.