DE2707835A1 - CERAMIC OBJECTS AND PROCESS FOR THEIR PRODUCTION - Google Patents

Description

The invention relates to rotationally symmetrical ceramic objects with an inner part or a core and an outer part or a shell, which are permanently connected to one another and whose coefficients of thermal expansion are different. The core is preferably made of silicon nitride and has a density that comes as close as possible to the theoretical density. The shell preferably has the theoretical density, but at least 60 to 75 % of the theoretical density, and consists of 75 to 95% by weight of silicon nitride and 25 to 6% by weight of another material, the preferred particle size of which is about 3 μm 1st. This other material is preferably silicon carbide.

In recent years the interest in silicon nitride as a material for high-temperature applications has increased more and more primarily because of its extraordinary properties, namely high heat resistance, creep resistance, Resistance to temperature changes, resistance to

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chemical degradation and especially resistance to oxidation. There are a large number of special uses for products made of silicon nitride, such as in engines, in particular Hotoren, stators and nozzles in turbines, electrical insulators and as special refractory materials, such as for thermocouple protection tubes, Furnace linings, crucibles and the like in the metallurgical industry.

There are basically two types of silicon nitride, the differences primarily originating from the manufacture of the silicon nitride. There is also high-density silicon nitride, which is obtained by hot-pressing silicon nitride powder, and such a lower density, which is obtained by sintering silicon nitride powder at normal pressure or by the so-called "reaction sintering process" ¹¹ .

In addition to the refractory materials made from silicon nitride only, there are various products based on silicon nitride, however, they contain other substances. From GB-PS 942 082 sintered bodies made of silicon nitride are known, which contain up to 10% by weight of finely divided silicon carbide. The creep resistance of pure silicon nitride is through Silicon carbide with an average grain size of about 7 μm improved. From US-PS 3394026 silicon carbide are contained Silicon nitride products known. The US-PS 3 911 188 brings Ceramic body in sandwich construction with an inner layer and two outer layers, with the outer layers having a smaller one Have thermal expansion coefficient than the inner layers. Of the Sandwich body is made by hot pressing three layers of a fine mixture of refractory products, whereby a monolithic sandwich structure is obtained. The outer layers are silicon nitride while the inner Layer a mixture of silicon nitride and silicon carbide. is. According to GB-PS 1,264,476 silicon nitride products are used reinforced with carbon fibers. Monolithic silicon nitride products obtained by equal proportions of silicon nitride, silicon carbide and up to 25 wt .-% boric acid, boron oxide or a boron salt in the starting mixture according to US Pat. No. 3,468,992.

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If sintering is achieved by hot pressing, so do so various sintering aids such as modifications above to further variations in the composition. Magnesium oxide is a very effective sintering aid. Awake üB-PS 970 639 are the oxides and nitrides of magnesium, Beryllium and calcium as well as the oxides of aluminum and iron to facilitate sintering and increase the density, if they are used in amounts of 0.1 to 25% by weight, preferably about 5% by weight. Lithium nitride is described as an outstanding sintering aid in GB-PS 1 164 795 described, according to which in silicon nitride powder up to 20 wt .-%, preferably 5 wt% lithium nitride is provided.

Hot pressing is known (GB-PS 1 164 795). Silicon nitride powder is filled into the graphite mold, this is put together and while the powder is heated to 1500 a pressure of 210 kg / cm was applied up to 1850 ° C. Though silicon nitride powder alone hot press itself to high density optimal densities are obtained when a sintering aid or a means for densifying the silicon nitride powder is added. For maximum density and optimal physical properties, hot pressing is unique the method of choice. The only disadvantage here is the restriction to relatively simple shapes. Complex shapes can - like easily visible - not obtained after the Ileißpreßverfahren will.

In the usual sintering process for the production of silicon nitride bodies, there are a wide variety of options for shaping the hollow body or green body. One possibility consists in introducing silicon nitride powder, optionally with an additive, into a steel mold and pressing it at room temperature under a pressure of about 3 »63 t / cm. The green silicon nitride molded body is then fired in an inert atmosphere at 1000 ° G, so that a self-binding

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Sintered body is obtained (US-PS 3,468,992). More options Isostatic pressing, slip casting or the like is used to produce the green molding a sinter firing follows. The sintered bodies have a relatively low density} the density can be reduced by further firing increase something.

A variant of the above sintering process is the production of silicon nitride bodies by reaction sintering. Fine Silicon powder is preformed first and this preform in a nitrogen atmosphere as long as 125 ° to 135O ° C held until partial nitration has taken place. Complete nitration of the unreacted silicon then takes place at a temperature of the order of 1700 ° 0. Is to produce an object of complex shape the partially nitrided body is machined to the desired shape and dimension and then finished nitriding made (GB-PS 942 082). Particularly high quality Sxliciumnitridartikel are obtained according to US-PS 3,778,231 » by sintering a preform made of silicon in an inert atmosphere free of nitrogen, the silicon sintered body processed to dimension and only then nitrided and sintered. The improved properties of these products are likely to be based on the unevenness of the initial nitriding, which ultimately has a favorable effect on the final nitriding.

As already pointed out above, the outstanding fire resistance and mechanical properties make silicon nitride particularly suitable for furnace linings, for crucibles and pans in the metallurgical industry or for special furnace constructions, such as those required in the semiconductor industry as seals for piston machines and with particular interest for components it is suitable for gas turbines. For example, US Pat. No. 3,854,189 discloses an eotor for a turbine engine with high-density silicon nitride as the hub

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or manufactured in the middle area, on which a ring of blades made of reaction-sintered silicon nitride lesser Density is attached. This rotor is made by the outer part, i.e. the ring of the blades, through Reaction sintering of a silicon mold is obtained which the hub or the inner part of the rotor is formed by hot pressing of silicon nitride powder.

According to an older proposal, a composite turbine rotor is made of silicon nitride with a sealed inner Part and a less dense outer part. This older proposal is based on the Prior art in the form of US Pat. No. 3,854,189, where the The hub of the rotor is made up of two parts and is bound in the outer part with the help of a refractory mortar. The less dense outer part preferably comprises a protective layer which is resistant to corrosion and / or erosion is resilient. In composite bodies of this type, a fireproof fiber material is used for reinforcement.

Although the composite ceramic rotor represents a significant advance, problems arise due to the formation of cracks in rotor structures with areas of different density. The cracking occurs in the exterior Part in general on or in the immediate vicinity of the interface of the two Botorteile. The object of the invention is now the solution to this problem associated with cracking.

The invention now relates to a monolithic rotatable composite ceramic body with an inner part or core or an outer part or a shell, which differ in composition in terms of quality and / or quantity and the coefficient of thermal expansion of the shell is about 1.5% lower to 6 % higher than that of the core. Whether the difference is positive or negative depends on the relative dimensions of the two parts.

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In the case of the body according to the invention, the core is very dense and the shell is less dense.

It has been surprisingly found that the cracking at the interface of the two parts of a rotor of different density or a rotor with two parts of high density, but different composition based on a large difference in the thermal expansion of the two parts at elevated temperatures. For example, the coefficient of thermal expansion of hot-pressed silicon nitride is 5.08 10 "/ degree and that of the much less dense reaction-sintered silicon nitride 3» 01 χ 10 "/ degree or less, depending on the density. This size of the difference in the coefficient of thermal expansion, ie 2.3 % y , leads to the formation of bites and / or a partial separation of the two parts. By increasing the coefficient of thermal expansion of the outer part to 1.5% below to 6 % above that of the inner part, the problem of crack formation is solved. In general, the outer part has a much smaller thickness (radial) than the inner part. In this case, the coefficient of thermal expansion of the outer part should preferably be 1 to 6% above that of the inner part.

Since the composite body is produced at an elevated temperature and then cooled to room temperature, the difference in the thermal expansion coefficients means that the outer part is under tension and the inner part is under pressure. This has two consequences, namely avoiding the formation of cracks at the interface between the two parts and improving the strength by compressing the inner part.

The outer part of the composite body can be different

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Composition ". Compared to the inner part, which has a lower density. The inner part can consist of silicon nitride, for example, while the outer part is made up of a mixture of silicon nitride and a substance which

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• St has a much higher coefficient of thermal expansion than silicon nitride, such as silicon carbide, mullite, zircon, Xttererde, Thorerde, alumina, tantalum carbide, titanium carbide or mixtures thereof. If silicon nitride is used here,. so all that is included in this, which is used to support the sintering or to increase the density. Although silicon nitride is currently preferred for the inner part and also in a modified form for the outer part, other refractory materials can in principle also be used, such as silicon oxynitride, silicon aluminum oxynitride or the like, which are not only useful but also sometimes desirable, depending on the working conditions Place of use of the items. If the objects are exposed to such extreme conditions, as are given for the rotor in turbine engines , and the Hotor has different densities, it is further advantageous to provide a coating on the less dense, relatively porous outer part of the rotor, which protects against corrosion and / or erosion resistant. This coating can be applied in a conventional manner such as by painting a slurry or glaze and baking, vapor deposition, flame or plasma spraying, or the like.

The inner part and outer part of the composite body can be manufactured in a known manner, such as by hot pressing, Cold pressing and sintering or reaction sintering. Optimal strength of the inner part or the hub is achieved through Hot pressing. The outer part, which can be much less tight than the inner part, should be after one of the others Process are produced. If the shape is complex, such as the blade ring of a turbine, it is advantageous to to press the blank isostatically or to manufacture it by injection molding or slip casting, followed by sintering or Reaction sintering takes place.

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The composite body according to the invention is built up by first completely the outer part and the inner part is made and then diffusion bonding is done by adding the parts placed next to one another. Exposure to heat and pressure (U.S. Patent 3,854,189).

But Han can also do the outer part by cold forming and Sintering or reaction sintering and making the inner part by hot pressing silicon nitride powder with usual Pour in sintering aids directly. One more way, which is particularly desirable is to have the two finished parts, i.e. the outer part cold formed and sintered or reaction sintered and the inner part hot-pressed, with the help of a refractory mortar into a monolithic Unite structure.

The invention is particularly suitable for the manufacture of ceramic rotors for turbine rotors. Due to the high load on a rotor at the required operating speed and the corrosive environment, silicon nitride is the ideal material due to its high strength and relatively high corrosion resistance. This has maximum properties when the silicon nitride has been hot-pressed. The inner part of the rotor, which is the most stressed and which has a relatively uncomplicated geometry, is obtained by hot pressing silicon nitride to a density of 90 to 100 % of theory; the outer part is preferably obtained by reaction sintering. For this purpose, a powder mixture of silicon and the corresponding amount of silicon carbide, each with an average fineness of 10 μm, is made by slip casting, injection molding or pressing and reworking the green mold

? 6formed and then nitrided

The blade ring obtained made of silicon nitride and silicon carbide should preferably contain 75 to 95% by weight of silicon nitride and 25 to 5% by weight of silicon carbide and a density of about 50 to 85 % of theory. exhibit, though

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higher densities are possible.

The preferred composition of the blade ring is 11.4 % silicon carbide and 80.6% silicon nitride, which then has a coefficient of thermal expansion of about 3.11 10 / degree. The coefficient of thermal expansion of the denser inner part is around 3 »08 χ 10 / degree. The difference in the coefficients of thermal expansion of the two files is therefore 1%, but can be up to about 6 fr.

These two parts (and possibly the two halves of the inner part) are made permanent with a refractory mortar connected) at least on the blade area of the rotor should have a relatively impermeable, corrosion-resistant coating made of preferably silicon carbide or silicon nitride are applied to avoid damage by the extreme to prevent corrosive environment in the working space of a turbine engine.

The invention is further illustrated by the following example. example

A composite pane - thickness 4.6 mm, diameter 139 »6 mn, Bore 25.4 mm - with an inner part that has a diameter of 55 »9 mm was manufactured as follows:

500 g of silicon nitride powder containing 1.5 % by weight of MgO ale sintering aids were placed in a ball mill lined with tungsten carbide and ground with a 1/3 filler of 12.7 mm tungsten carbide balls using 500 cm ^ isopropanol as a painting aid for 16 hours , then sieved and dried at 90 ⁰ O. The cake was crushed in a plastic drum with tungsten carbide balls in about 15 minutes. 20 g of this product were poured into a boron nitride

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coated graphite mold - cavity 57 »9 mm diameter -

introduced and essentially based on theoretical density η, ρ A second just such

pressed at 1750 C and 245 kg / cm. Disc - 57.9 mm diameter, 2.3 mm thick, 25 mm bore - was made in the same way. One groove. of 60 ° was on the circumferential surface both discs ground in. The coefficient of thermal expansion was 3.08 10 / degree.

9.08 kg of a mixture of silicon powder 3 μm and silicon carbide powder 3 µm was prepared by mixing 754 kg silicon and 1.5 kg silicon carbide. The powder mix was isostatically pressed into a bar - 305 x 152.5 mm -

under a pressure of 2110 kg / cm. This pole was sintered according to U.S. Patent 3,778,231. A 4.6 mm thick disk was then cut from the rod and placed on a disk with an outer diameter of 139i5 mm and an inner diameter 55, ^ mm edited.

This preform was then nitrided in a furnace in the usual manner. The nitrided disc showed a density of

2.4 g / cm and a coefficient of thermal expansion of about 3.11 x 10 / degree; the two edges of the hole were machined to a 60 ° angle. The V-shaped inner surface of the outer part fit in the thus-machined inner discs · IM this now becomes a monolithic sheet to connect a mortar made of enstatite was prepared by g in a mortar with a Pistill6 SiO ₂ and 4 g of MgO with methylene chloride to a thin paste and smeared the mating surfaces of the three parts with this paste. These were then hot-pressed in a graphite mold lined with molybdenum sheet for one hour at 1570 ° C. and under a pressure of 35 kg / cm. The monolithic disk obtained showed no cracks or other defects.

By coordinating the coefficient of thermal expansion of the

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Interior and exterior parts and assembly at 156o C ⁰ was the inner part due to the larger thermal expansion coefficient s of the outer part up to 1560 ° C under pressure. This compressive force on the inner part further improves the already high strength.

While the invention has always been based on silicon nitride as the base material for the inner part and silicon nitride has been described together with silicon carbide or the other substances mentioned above for the outer part, one can Provide silicon carbide or the other substances for both parts as long as the outer part considerably more Contains silicon carbide, so that the necessary difference in the coefficient of thermal expansion is ensured. So can for example, for the hot pressing of the inner part as a sintering aid for the silicon nitride up to 15% by weight Use Thorerde and / or Xttererde. In this case must a correspondingly larger amount of gate earth, silicon carbide or the like is used for the outer part.

Instead of introducing the necessary difference in the coefficient of thermal expansion by one or more of the above additives in the outer part and, if necessary, also in the inner part , you can also add a certain amount of one substance with a given coefficient of thermal expansion to the inner part and a certain amount of another substance with one Provide a larger coefficient of thermal expansion in the outer part. According to the invention, both parts can therefore be essentially completely impermeable, but differ in their composition.

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/ PATENT CLAIMS;

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Claims (1)

DR. IMG. F. WUBSTHOKF SOCO MONOHSIfOO DR.X.T.PKOHMANN ICWHOEIHMBU DH. IHG. D. BKHHKK ₈ """"«"»·" · " DIW-IKG. H.GOKT * niHiuin ■ PlTMIAiWlLTl ranmPMm »tmn 1A-49 115 PATENT CLAIMS 2707635 Ceramic composite body made of an inner part and an outer part which are permanently connected to one another, the coefficient of thermal expansion of the outer part being approximately 1 »5% less to 6 % greater than that of the inner part. (2) composite body according to claim 1, characterized in that both parts are made of silicon nitride consist with an additive in i'orm of silicon carbide, mullite, zircon, ytter earth, thore earth, alumina, tantalum carbide and / or titanium carbide. (3) A composite body according to claim 1, characterized in that the inner part high-density silicon nitride and the outer part is less dense silicon nitride with the addition of silicon carbide, mullite, zircon, yttria, thoria, alumina, tantalum carbide and / or titanium carbide. (4) composite body according to claim 1 to 3, characterized in that at least the Outer part a corrosion and / or erosion-resistant Owns coating. (5) Composite body according to claims 1 to 4, characterized in that the outer part consists of 75 to 95% by weight silicon nitride and 25 to 5% by weight silicon carbide and has a density of 50 to 85 % of theory. and the inner part made of silicon nitride with a density 709839/071 3 ORIGINAL INSPECTED from 90 to 1üÜ> · ό d.Th. is constructed. (6) composite body according to claim 5, characterized in that the coefficient of thermal expansion of the inner part about 3.08 10 / degree and that of the outer part about 3.08 to 3.26 χ 10 ~ ⁶ / degree. is. (7) composite body according to claim 1 to 6, characterized in that the silicon carbide in the The outer part has a medium size of a maximum of 10 μm. (8) Composite body according to claim 1 to 7 »characterized in that the inner part is made of silicon nitride with a density of about 98 Ja d.Th. and a coefficient of thermal expansion of about 3.17 χ 10 " / grd and the outer part of 80 to 85% by weight silicon nitride and 20 to 15 % silicon carbide with a density of at least 65 % of theory, the grain size of the Silicon carbide is about 3 / um and the V / thermal expansion coefficient of the outer part is about 3.2 χ 10 " ⁶ / degree. (9) Process for producing the molded body according to claims 1 to 8, characterized in that the silicon nitride inner part is made with a density of 90 to 100 % of theory. by hot pressing and the outer part made of 70 to 90% by weight of silicon nitride and 30 to 10% by weight of silicon carbide with a density of 60 to 75% of theory. by molding and sintering or reaction sintering and joining the two parts under heat and pressure. (10) The method according to claim 9, characterized in that for the hot pressing 0.5 to wt .- / 3 sintering aids are used and the outer part is made from a mixture of 57.5 to 80 % silicon powder with a grain size of 3 / um and 42 to 15 / ^ silicon carbide with 709839/0713 an ivornuno from ^: j / um. (11) The method according to claim 9 or 10, characterized in that the connection of the Carries out parts under heat and pressure with a refractory mortar. 709839/071 3