DE102012012227A1 - Producing silicon carbide based ceramic sintered body that is useful e.g. for fabricating semiconductors, comprises sintering powder body in inert gas atmosphere, and performing gas pressure sintering in nitrogen-containing atmosphere - Google Patents

Abstract

The process comprises sintering a powder body in an inert gas atmosphere at a pressure of 1-100 mbar and at a temperature of 2000-2200[deg] C, and performing a gas pressure sintering or a sintering-hot-isostatic press in a nitrogen-containing atmosphere. The powder body: comprises silicon carbide (88 wt.%), a boron-containing additive such as boron carbide in which boron is present in an amount of 0.1-1.5 wt.%, carbon (0.5-3 wt.%) and/or a carbon source material that supplies free carbon in the sintering process, and a nitrogen-containing additive. The process comprises sintering a powder body in an inert gas atmosphere at a pressure of 1-100 mbar and at a temperature of 2000-2200[deg] C, and performing a gas pressure sintering or a sintering-hot-isostatic press in a nitrogen-containing atmosphere. The powder body: comprises silicon carbide (88 wt.%), a boron-containing additive such as boron carbide in which boron is present in an amount of 0.1-1.5 wt.%, carbon (0.5-3 wt.%) and/or a carbon source material that supplies free carbon in the sintering process, a nitrogen-containing additive in which nitrogen is present in an amount of 0.01-2.3 wt.%, an aluminum-containing additive in which aluminum is present in an amount of 0.01-2 wt.%, and a temporary process auxiliary material; and is prepared by mixing starting materials together in an aqueous solution and drying to form a shaped body. The aluminum-containing additive is included in the powder body only when the powder body contains the nitrogen-containing additive. An independent claim is included for a silicon carbide based ceramic sintered body.

Description

The present invention is in the field of ceramics, in particular dense silicon carbide (SiC) based materials with defined electrical resistance. It relates in particular to a process for producing a ceramic sintered body based on silicon carbide, starting from readily available raw materials of industrial quality, and thus obtainable dense ceramic sintered bodies, which in particular by a specifically adjustable electrical DC resistance in the range of 10 ² to 10 ⁸ Ω · cm , as well as good mechanical properties.

Dense, SiC-based ceramic materials and structural or structural components made from them have been established in wide areas of technology since the basic developments of Prochazka and Greskovich in the 1970's due to their outstanding combination of technically-interesting properties. These technically relevant properties include a high strength up to high temperatures of 1500 ° C and more, a high hardness and wear resistance, a comprehensive corrosion resistance from the strongly acidic to the strongly basic range, a low thermal expansion, a very good thermal conductivity, good tribological properties and usually electrical semiconductor properties (= about 10 ² -10 ⁵ Ω · cm). Therefore, such components are widely used as seals of mechanical seals, as sliding bearings, as wear-resistant components in processing technology, as structural parts of high-temperature and chemical plants, in semiconductor production, etc. However, there are always restrictions on certain applications, resulting from that Although the SiC material would be well suited in terms of its mechanical, chemical and / or thermal properties, but the electrical conductivity or electrical resistance are inappropriate. For these cases, a procedure is needed to set the electrical resistance targeted, without affecting the other important properties in an unacceptable extent or change.

Silicon carbide components with certain electrical properties are known per se and are widely used as electronic components. So describes DE 19842078 B4 (Winnacker) high-resistance substrate materials with electrical resistances of> 10 ¹² Ω · cm, based on high-purity SiC, which is doped with Al, B and / or Ga, and V, Ti, Cr and / or Mo and N, around the electronic conduction bands to block. These materials and components made from them have in common that their specific electrical resistance is achieved by targeted doping of high-purity SiC starting material and is explained by an influence on the electronic conductivity bands. Such an approach is not acceptable for the large scale production of SiC materials and components due to the limited availability and high cost of the high purity SiC required. Rather, they must be made from engineering raw materials with a much higher impurity content.

In EP 0219933 B1 (Boecker) also discloses a sintered SiC material with high electrical resistance. The material has boron nitride precipitates (BN precipitates) in the SiC grains and unbound carbon between the SiC grains of the sintered material, giving it a DC electrical resistance of 10 ⁸ Ω · cm and more. The materials are produced by sintering in a nitrogen-containing atmosphere, sintering temperatures of at least 2250 ° C. being required to achieve the desired high electrical resistances. This is only a guide to the realization of high resistance values, but no teaching specifically for influencing the electrical resistance in the range "low resistance" given. In addition, depending on the size of the BN precipitates, it can be assumed that they have a negative effect on the strength of the SiC materials.

JP 2001130971 A (Odaka) also describes a sintered SiC material of high density and high volume resistivity, which is obtained by sintering a mixture of SiC, a non-metallic sintering aid and Si ₃ N ₄ . As another requirement, a total impurity level of 10 ppm is specified which requires the use of high purity SiC powder. As mentioned above, this is not acceptable for the large scale production of SiC materials and components due to the limited availability and high cost of the high purity SiC required.

JP 2006240909 A (Yoshioka) discloses SiC materials of high density, whose volume resistivity in the range 10 ⁶ -10 ¹³ Ω · cm can be targeted. As sintering additives, free carbon, B or B compounds and Si ₃ N ₄ are used. The sintering takes place under atmospheric pressure without special control of the gas atmosphere.

In EP 0626358 B1 (Chia), however, a sintered SiC material and its production is disclosed, which should have a low electrical resistance coupled with good mechanical properties. As sintering additives B, Al and C are used, whereby both the B and the Al can be introduced in the form of suitable compounds. The sintering is carried out in vacuum or in an inert atmosphere at a gas pressure below, just at or above atmospheric pressure, the samples were each dusted with additional sintering aid. The resulting material is claimed to have a DC conductivity of at least 0.05 (Ω · cm) ^-1 and higher, which corresponds to a maximum DC electrical resistance of 20 Ω · cm. The presence of nitrogen, whether as a constituent of the mixture to be sintered or of the sintering gas, should be avoided. The proposed dusting with sintering aids is uneconomical because it requires intensive cleaning or even grinding of the surfaces of the sintered components.

US 5,011,639 (Urasato) discloses a method of producing a sintered SiC material having a very high volume electrical resistivity of 10 ¹⁰ -10 ¹³ Ω · cm. The sintering takes place in a sintering atmosphere containing 0.01 to 2% by volume of nitrogen. According to the claim, the SiC powder in the form of β-SiC is specially prepared by pyrolysis of a methylhydrogen silane, which is unacceptable because of the associated high expense for the large-scale production of SiC components and components.

US 2010/0130344 A1 (Mikijelj) describes a dense SiC material with very high electrical resistance, which contains specified amounts of B as a sintering aid, as well as N and O, but no free carbon. As a result, pressureless sintering is made difficult, so that the compaction is carried out by hot pressing, which is considered to be very economically disadvantageous and limits the range of manufacturable parts. While it is well known that this densification process results in substantially dense materials having desirable properties, it allows only the production of simple geometric shapes such as disks, cylinders or short tubular bodies. However, if a more complex component is required, this must be worked out very elaborately using diamond tools by grinding, drilling or other special processing methods from these geometrically simple forms, which is complex and causes high costs. Large, complex-shaped parts are therefore not produced at all by the hot pressing process, which hampers its technical and economic usability enormously. It can be according to US 2010/0130344 A1 In addition, only SiC materials with high electrical resistance, but not obtained with low electrical resistance.

EP 0771769 A2 (Easler) discloses the preparation of high density ceramic bodies by pyrolysis of a mixture containing a carbon source, α-SiC powder, a boron source and an aluminum source. The crucial factor here is the simultaneous use of boron and aluminum. For mixing these components, a synergistic effect is induced which leads to the desired high density of the α-SiC body. The preparation of the mixture of the starting materials is carried out by customary processes, wherein the treatment in organic solvents is preferred. In the examples, the treatment is carried out in a mixture of toluene and ethanol. The sintering conditions are of minor importance. It can be sintered under pressure as well as without pressure. The sintering can be carried out in an inert atmosphere and / or in a vacuum, wherein an inert atmosphere in the sense of EP 0771769 A2 also includes a nitrogen atmosphere. Under vacuum understands EP 0771769 A2 the wide range from 13.3 Pa to 26.6 kPa, corresponding to 0.13 to 266 mbar. The disclosed combined process of sintering in an inert atmosphere and in a vacuum is performed so that sintering under argon and sintering alternate in vacuum. Sintering under argon of low pressure is not described. There is also no information as to whether and how SiC materials with specifically adjusted electrical resistance can be obtained by the method described.

Other methods of presenting SiC-based materials with low electrical resistance use additions of conductive second or more (inorganic) materials that have electrical conductivity. The best known route to this is the so-called silicization, in which a precompressed, but still porous SiC-based body is brought into contact with liquid or gaseous Si or with Si compounds or alloys. These substances penetrate into the existing porosity and fill it, possibly in reaction with other phases, such as free carbon, completely, creating a dense body. About the remaining free silicon of usually at least 8 vol .-%, the dense body receives a low electrical resistance (= about 10 ⁰ -10 ^-3 Ω · cm). However, the free silicon is disadvantageous to the extent that this type of material for various applications, eg. B. for temperatures above 1380 ° C or in strongly basic media is no longer suitable.

Other such processes use high levels of graphitized carbon (particles, short fibers, nanotubes, etc.) or other thermally stable conductive inorganic compounds such as carbides, nitrides, carbonitrides, borides, silicides, etc. of Ti, Zr, Hf, W, Mo, Nb. Ta etc. These phases have in common that they must be added at least in such a concentration that sets a so-called percolation, ie a continuous contact or skeleton structure, which generally corresponds to at least a concentration of 32 vol .-% , It is therefore clear that these phases clearly influence the properties of the SiC-based material and that it is no longer suitable for a large number of the desired applications.

Other approaches use oxidic sintering additives such as rare earth oxides (SEO), z. As Y ₂ O ₃ , Nd ₂ O ₃ , Yb ₂ O ₃ and others, usually in combination with Al ₂ O ₃ or AlN to compress SiC and adjust certain properties. These additives are capable of forming a liquid phase at high temperatures, with the participation of the oxygen always contained to some extent in SiC, which enables sintering and densification by so-called liquid-phase sintering. In this way, high-strength, tough, SiC-based materials can be realized. So revealed EP 1070686 B1 (Schwetz) such a procedure, with the SiC-based materials can be obtained with quite high electrical resistivities. In this case, a powder molded body is produced and gas pressure sintered at an argon pressure of 2 to 10 bar. Disadvantages here are, on the one hand, the need to admit rare earth oxides, whose availability on the world market has declined sharply in recent years, and, on the other hand, the comparatively complicated sintering under pressure. In addition, these materials lose by the fate of an amorphous or semi-crystalline grain boundary phase between the SiC grains above 1000 ° C rapidly strength and the comprehensive corrosion resistance of B and C as sintering additives "dry", ie sintered by diffusion SiC materials is lost.

Thus, there is no indication in the prior art of SiC-based sintered materials using the typical sintering aids B (boron) and C (carbon) which also have the typical "dry" sintered SiC materials mechanical, chemical and thermal properties , By small additions, which hardly affect the mentioned typical properties of these materials to set the specific electrical resistance at room temperature targeted between "high" and "low".

The object of the present invention was to provide a process for the production of SiC-based materials, which makes it possible to produce dense ceramic sintered bodies from readily available raw materials of technical quality, and by simple measures the DC electrical resistance of the materials in the range of 10 ² to 10 ⁸ Ω · cm (25 ° C) targeted. A further object of the invention is to provide SiC-based materials which have an electrical direct current resistance in the range from "strongly insulating" (= approx. 10 ⁸ Ω.cm) to "highly conductive" (= approx. 10 ² Ω. cm) and at the same time have good mechanical properties which allow their use under high mechanical and / or thermal loads. In particular, these materials or components or components made therefrom from raw materials of technical purity and by a simple aqueous treatment process of the starting mixture and a technically and economically favorable sintering process should be inexpensive and competitive to produce. Such sintered structural or design components are for many areas of technology, such as the high-temperature furnace construction, the machinery, equipment and apparatus construction with the special field of sealing technology, the semiconductor and sputtering, but also for electronics and chemical and sensor technology of great importance.

It has now been found that ceramic materials based on silicon carbide with specifically adjusted electrical DC resistance at 25 ° C. in the range of 10 ² to 10 ⁸ Ω · cm and advantageous mechanical properties can be obtained if a defined mixture of starting materials under low pressure in sintered in an inert gas atmosphere.

• a. mindestens 88 Gew.-% Siliziumcarbid,
• b. mindestens ein borhaltiges Additiv, in einer Menge von 0,1 bis 1,5 Gew.-%, berechnet als Bor,
• c. Kohlenstoff und/oder mindestens ein Kohlenstoffquellenmaterial, das beim Sinterprozess freien Kohlenstoff liefert, in einer Menge, die 0,5 bis 3 Gew.-% Kohlenstoff entspricht,
• d. mindestens ein stickstoffhaltiges Additiv in einer Menge von 0 bis 2,3 Gew.-%, berechnet als Stickstoff,
• e. und mindestens ein aluminiumhaltiges Additiv in einer Menge von 0 bis 2,0 Gew.-%, berechnet als Aluminium, mit der Maßgabe, dass ein aluminiumhaltiges Additiv nur dann enthalten sein darf, wenn der Pulverformkörper auch mindestens ein stickstoffhaltiges Additiv enthält,

wobei sämtliche Mengenangaben auf die Summe der Bestandteile des Pulverformkörpers bezogen sind, und wobei das Sintern in einer Inertgasatmosphäre bei einem Druck von 1 bis 100 mbar durchgeführt wird.The subject of the present invention is therefore a process for producing a ceramic sintered body based on silicon carbide, comprising sintering a powder shaped body containing

• a. at least 88% by weight of silicon carbide,
• b. at least one boron-containing additive, in an amount of 0.1 to 1.5 wt .-%, calculated as boron,
• c. Carbon and / or at least one carbon source material that provides free carbon in the sintering process in an amount corresponding to 0.5 to 3 wt.% Carbon,
• d. at least one nitrogen-containing additive in an amount of 0 to 2.3% by weight, calculated as nitrogen,
• e. and at least one aluminum-containing additive in an amount of 0 to 2.0 wt .-%, calculated as aluminum, with the proviso that an aluminum-containing additive may be included only if the powder molded body also contains at least one nitrogen-containing additive,

wherein all quantities are based on the sum of the constituents of the powder molding, and wherein the sintering is carried out in an inert gas atmosphere at a pressure of 1 to 100 mbar.

The quantities are the total quantities of the respective components. For example, if the powder molded article contains a plurality of boron-containing additives, the total amount of all boron-containing additives must be in the range from 0.1 to 1.5% by weight, calculated as boron, not the amount of each individual boron-containing additive. In other words, the total amount of boron in the powder molded article may not exceed 1.5% by weight.

A ceramic sintered body is understood to mean any three-dimensional body which is obtained by sintering a ceramic powder molded body and, if appropriate, mechanical and / or chemical aftertreatment. It may be a simple geometric shape, such as a disk, a cylinder or a short, tubular body. However, the sintered body may also have a complex geometry adapted to the respective application. In the literature is often spoken of a ceramic material instead of a ceramic sintered body, so that both terms are used synonymously in the context of the present application.

The method according to the invention is a simple sintering method in which the desired shape of the ceramic sintered body and thus of the desired component or the desired component is made of a preform produced by various known methods (synonym: "green body", "green body", " Pulverformkörper ") already in the unsintered state (" green state ") is generated. In the simplest case, the powder molded body, z. B. by molding, already produced in the desired shape. In order to realize more complex shapes, the powder molding can be finished with simple tools by turning, drilling, milling, sawing, etc. Conventional methods for producing the powder molded articles are axial or isostatic pressing, extrusion, slip casting, injection molding, etc. The powder molded articles are made larger by the shrinkage occurring during sintering, so that after sintering a body close to the final contours is present Function surfaces, as well as to set very precise dimensions or shape and position tolerances must be edited. This significantly lower processing cost is a great technical and economic advantage over the hot pressing process and also the hot isostatic pressing (HIP). The electrical DC resistance at 25 ° C is according to the invention very easy to adjust by selecting the starting materials in a range of 10 ² to 10 ⁸ Ω · cm. The ceramic sintered bodies obtainable in this way are distinguished not only by the desired electrical properties but also by a density of at least 94% of the theoretical density, determined by means of the Archimedes water displacement method, and advantageous mechanical properties. SiC sintered bodies from a degree of compaction of at least 94% of the theoretical density have no open porosity according to theory and are therefore also referred to as "dense".

According to the invention, suitable shaped powder articles are sintered under comparatively mild conditions. The sintering takes place in an inert gas atmosphere at a pressure of 1 to 100 mbar. Under an inert gas atmosphere is an atmosphere to understand that contains no ingredient that reacts under sintering conditions with one of the constituents of the powder molding. In particular, the sintering atmosphere is free of reactive gases, in particular free of N ₂ , H ₂ and O ₂ . Preferably, helium, neon and / or argon are used in an atmosphere of noble gases. Particularly preferably, the sintering takes place in an argon atmosphere.

The inventively provided low gas pressure during sintering has been proven and promotes the temperature homogeneity in the sintering furnace and the convective heat transfer. A significantly higher pressure leads to even more convection and thus to heat loss by promoting the heat transfer to the usually cooled metallic furnace wall. Furthermore, there is a sintering obstruction due to the inclusion of inert gas in closing pores. Lower. Prints as 1 mbar preclude convective heat transport and promote temperature gradients, since the main heat transfer occurs during sintering by radiation, which can very easily lead to shading in the area of the sintered material.

The sintering pressure according to the invention is preferably 2 to 80 mbar, particularly preferably 3 to 60 mbar, very particularly preferably 4 to 40 mbar and particularly preferably 5 to 25 mbar.

It has been found that it is possible to obtain SiC sintered bodies under the conditions according to the invention which have a density of at least 94% of the theoretical density and can therefore be referred to as "dense".

If sintered in a nitrogen-containing atmosphere instead of in an inert gas atmosphere, no density ≥ 94% of the theoretical density could be achieved under atmospheric N ₂ pressure even at 2200 ° C. Under reduced gas pressure is always obtained at the same sintering temperatures in N ₂ atmosphere, a lower sintering density than in an inert gas atmosphere. This shows the technological advantages of sintering under reduced inert gas pressure, also under the aspect of energy efficiency, since at atmospheric gas pressures the heat losses are significantly higher, recognizable by the much higher electrical power required to reach the target temperature. This shows that the sintering of SiC powder molded articles in N ₂ -containing atmosphere is not an effective means to adjust certain N ₂ contents in the sintered bodies, since it comes to a sintering obstruction, combined with problems to achieve the desired range of properties and it is also still energetically unfavorable.

Preferably, the sintering is performed at a comparatively low temperature for a sintering process. Preferably, the sintering temperature is from 2000 ° C to 2200 ° C, more preferably from 2025 ° C to 2150 ° C, and most preferably from 2050 ° C to 2125 ° C.

Sintering temperatures <2000 ° C usually lead to densities <3.0 g / cm ³ , ie to SiC sintered bodies having a density of less than 94% of the theoretical density in the case of SiC sintered mixtures mixed with customary B and C concentrations. and thus can not be described as dense. In contrast, sintering temperatures> 2200 ° C. can already lead to excessive grain growth, which deteriorates the mechanical properties of the SiC sintered bodies.

Usual hold times at the specified sintering temperatures are, depending on the size, complexity and in particular wall thickness of the powder shaped body to be sintered, for 30 minutes to 5 hours, preferably 1 to 3 hours.

Preferably, the sintering takes place in furnace systems, which allow all thermal processes in a cycle, without having to convert the material to be sintered from one unit to another. Thermal processes should be understood to mean the "debinding" / annealing of the temporary process auxiliaries, if appropriate the pyrolysis of carbon source material ("C suppliers") and finally the sintering process itself. This approach helps to operate cost-effectively and to largely avoid rejects and damage. According to the invention, the sintering is therefore preferably carried out in a single thermal process, d. H. the sintering of the powder shaped body corresponds to a combined Binderausbrand-, pyrolysis and sintering cycle.

The powder molded article to be sintered according to the invention contains at least 88% by weight of silicon carbide. Preferably, it contains at least 90% by weight, more preferably at least 91.5% by weight of silicon carbide.

Preferably, the silicon carbide is α-SiC, more preferably a technical grade α-SiC containing impurities selected from Fe, Mg, Ti and Ca in a total concentration of 250 ppm to 750 ppm. In this case, the silicon carbide is preferably already pre-processed to at least a fineness of 10 m ² / g (according to BET) and / or a d-95 characteristic of the particle size distribution of ≦ 5 μm (determined by means of laser diffraction).

The powder molded article to be sintered according to the invention also contains at least one boron-containing additive in an amount of from 0.1 to 1.5% by weight, calculated as boron. It preferably contains at least one boron-containing additive in an amount of from 0.2 to 1.2 Wt .-%, particularly preferably 0.3 to 1.0 wt .-%, each calculated as boron.

A boron carbide is preferably used as the boron-containing additive, particularly preferably a finely divided, technical B ₄ C powder with a d-95 characteristic of the particle size distribution of ≦ 10 μm, determined by means of laser diffraction.

Finally, the powder molded article to be sintered according to the invention contains free carbon and / or at least one carbon source material which gives free carbon during the sintering process, in an amount which amounts to 0.5 to 3% by weight, preferably 0.75 to 2.75% by weight. %, particularly preferably 1.0 to 2.5 wt .-% corresponds to carbon.

Carbon is referred to as "free" or "unbound", which is not chemically bonded to another element, as is the case in silicon carbide, for example.

The free carbon can be added, for example, in the form of elemental carbon, for example in the form of graphite. Preferably, however, a carbon source material is used, ie a carbon-containing compound which pyrolyzes under sintering conditions and thereby releases free carbon. For example, phenolic resins or commercially available aqueous C-pastes can be used.

In addition to the mandatory constituents, the powder molding to be sintered according to the invention may optionally contain at least one temporary processing aid. This facilitates the production of the powder molded article from the mixture of the starting materials. Temporary process auxiliaries are the customary substances known to the person skilled in the art, for example organic polymers, such as polyvinyl acetates or polyethylene glycols. It should be noted that the temporary process auxiliaries may contribute to the proportion of free carbon in the powder molding and, in this case, should be taken into account when determining the amount of free carbon. Apart from that, the processing aids are not considered in the determination of the amounts of the components of the powder molding. In other words, the quantities of the constituents of the powder molded article relate to the sum of the constituents of the powder molded article without the optionally present temporary process auxiliaries.

To set the desired DC electrical resistance in the ceramic sintered body, the powder molded article preferably further contains at least one nitrogen-containing additive in an amount of up to 2.3% by weight, calculated as nitrogen. Preferably, the content of the nitrogen-containing additive is 0.01 to 2.3 wt .-%, particularly preferably 0.1 to 2.2 wt .-%, each calculated as nitrogen. In this case, the nitrogen-containing additive used is preferably Si ₃ N ₄ and / or AlN, particularly preferably Si ₃ N ₄ . According to the invention, the amount of nitrogen-containing additive must not exceed the limit of 2.3 wt .-%, calculated as nitrogen, in the powder molded article, since it has been found that when using higher amounts, the resulting sintered bodies are no longer tight.

Even a content of 0.5 wt .-% Si ₃ N ₄ powder in the powder molded body increases the electrical resistivity of the sintered body produced therefrom by about 2 orders of magnitude compared to the use of a corresponding powder molded article without the addition of a nitrogen-containing additive. Addition of 1 and 2 wt .-% Si ₃ N ₄ lead to an increase in electrical resistance by about 3 orders of magnitude without affecting the good mechanical properties of the sintered body negative. However, increasing the concentration of the Si ₃ N ₄ additive to 4 or 6% by weight surprisingly reduces the specific electrical resistance again. An explanation for the causes of this unusual behavior could not provide the investigations carried out. This may be related to exceeding the solubility limit of N in SiC. Even higher addition amounts of Si ₃ N ₄ powder cause that no dense sintered bodies are obtained. Therefore, the maximum amount of nitrogen-containing additive in the powder molded body is 2.3 wt .-%, calculated as nitrogen.

If the powder molded article contains at least one nitrogen-containing additive, it may further contain at least one aluminum-containing additive in an amount of up to 2.0% by weight, preferably from 0.01 to 2.0% by weight, more preferably from 0.1 to 1.9 wt .-%, each calculated as aluminum. The aluminum-containing additive used is preferably Al and / or AlN. Naturally, AlN acts both as a nitrogen-containing additive and as an aluminum-containing additive. Contains the powder molded body only an aluminum-containing additive, but no nitrogen source, can be achieved with the addition of small amounts of the aluminum-containing additive no effects on the mechanical and electrical properties of the sintered body and on the addition of high amounts of the aluminum-containing additive no dense SiC sintered body more.

Particularly preferred aluminum-containing additive is a passivated aluminum powder, very particularly preferably a passivated Al powder with a fineness of ≦ 10 μm (d-95 characteristic of the particle size distribution of ≦ 10 μm, determined by means of laser diffraction). The use of passivated Al powder allows aqueous treatment of a mixture of the starting materials in the preparation of the powder molded body.

In a particular embodiment, the powder molded body used consists exclusively of the aforementioned abovementioned constituents in the stated amounts. In other words, in this case, the amounts of silicon carbide, boron-containing additives, carbon and / or carbon source material, and optionally nitrogen-containing additives and aluminum-containing additives add up to 100%, with only temporary process auxiliaries being permitted as further constituents of the powder molding.

The preparation of the powder molded article can be carried out by the known methods. For this purpose, the starting materials are generally prepared and mixed together in a liquid medium, wherein a so-called slip is formed, the slip then optionally dried and formed the slurry or the resulting granules during drying to form a shaped body.

The homogenized and dispersed slip obtained in the combined treatment can either be used directly for the production slip-molded molded body or in a simple and cost-effective manner, by z. As spray drying, fluidized bed drying or other suitable technical process can be converted into a well-processed granules, which is then brought into the desired shape.

In principle, in addition to the already mentioned Schlickergießen also the film casting, extrusion or injection molding can be applied for the shaping. Larger, flat components are preferably formed by axial (die) pressing, but larger complex shaped components are preferably preformed by isostatic pressing (CIP) of the granulated raw material mixture and by a so-called green machining with conventional tool steel or carbide tools by turning, drilling, Milling, etc. designed as near net shape, but increased by the factor of shrinkage occurring during sintering. These are also technically and economically favorable methods and can be used in the economical production of the SiC-based sintered bodies.

Preferably, the treatment of the starting materials is carried out aqueous.

After sintering under the conditions according to the invention, various post-treatment steps may follow for the production of the end products from the sintered shaped bodies. Thus, for example, sintering at atmospheric pressure, gas pressure sintering or sintering-hot isostatic pressing in N ₂ or an N ₂ -containing atmosphere can be performed. As a rule, however, this is not necessary for achieving the desired properties of the sintered shaped bodies.

As a rule, only closely toleranced dimensions, required shape and position tolerances, and surface conditions, in particular by grinding, are generated, which requires the use of diamond tools due to the hardness of the sintered SiC parts. Due to the possibility of realizing complex parts by green machining with only a small measure compared to the final dimensions, this results in significantly less hardworking effort and very significant economic advantages over a hot-pressing or HIP process.

The resulting ceramic sintered bodies based on silicon carbide are distinguished not only by their high density and the specifically set electrical DC resistance but also by advantageous mechanical properties and thus by a unique property profile.

Another subject of the invention are therefore ceramic sintered bodies based on silicon carbide, which are obtainable by the method according to the invention.

Preferably, they have a density of at least 94% of the theoretical density, as determined by the Archimedes water displacement method, and a DC electrical resistance at 25 ° C. of 10 ² to 10 ⁸ Ω · cm, and more preferably a 4-point bending strength at room temperature of ≥ 300 MPa, a static modulus of ≥ 375 GPa, a fracture toughness K _IC of ≥ 2.3 MPa · m ^0.5 , and a hardness HK0.5 of ≥ 22 GPa.

Another object of the invention are ceramic sintered bodies based on silicon carbide, which has a density of at least 94% of the theoretical density, determined by the water displacement method according to Archimedes, and a DC electrical resistance at 25 ° C of 10 ² to 10 ⁸ Ω · cm, as well furthermore, a 4-point bending strength at room temperature of ≥ 300 MPa, a static modulus of ≥ 375 GPa, a fracture toughness K _IC of ≥ 2.3 MPa · m ^0.5 , and a hardness HK0.5 of ≥ 22 GPa exhibit.

The determination of the 4-point bending strength at room temperature (RT-BF) is carried out after DIN EN 843-1 on rod-shaped test specimens measuring 4 × 3 × 45 mm in 4-point bending with a support distance of 40/20 mm. The minimum number of flexural strength samples to be tested is set at 8.

The determination of the modulus of elasticity (modulus of elasticity) is carried out simultaneously with the determination of the 4-point bending strength over the bending of the sample in the bending test and in standard works of the Strength Teaching Calculation. This so-called static modulus of elasticity is easy to determine and well suited for comparative measurements of similar materials.

The fracture toughness K _IC is determined by the most reliable SEVNB method in a global comparison experiment, described in "J. Kübler: Fracture Toughness of Ceramics using SEVNB Method: Round Robin "; VAMAS Report No. 37 (ISSN 1016-2186) ,

Hardness is softened on polished surfaces DIN EN 843-4 using a low load hardness tester and a Knoop indenting diamond at a load of 5 kp (HK0.5) as this method results in much better formed hardness impressions with fewer bursts than the commonly used Vickers indentation diamonds, and thus better and more reliably evaluable is.

• – Messgeometrie: mit Schutzringelektrode
• – Elektrodendurchmesser der „guarded electrode”: 10 mm
• – Innendurchmesser der „guard electrode”: 12 mm.
• – Spaltbreite zwischen den Elektroden: 1 mm.

The determination of the specific DC electrical resistance at room temperature (25 ° C) is carried out on special test specimens of dimensions 20 × 20 × 3 mm according to ASTM D 257 with the following arrangement:

• - Measuring geometry: with guard ring electrode
• - Electrode diameter of the "guarded electrode": 10 mm
• - Inner diameter of the guard electrode: 12 mm.
• - gap width between the electrodes: 1 mm.

The electrodes are applied with conductive silver. The area of the guarded electrode must be corrected. The measurement uses an "Electrometer high resistance meter, Keithley 6517A" voltage measuring device and the "Resistivity Test fixture Keithley 8009" sample holder, which are designed for high resistance measurements by means of an elaborate shielding (triax cable). The measuring voltage is 10 V, in individual cases also measurements are made at 1 V, as well as at 100, 200 and 250 V. 2 samples of the same material are measured in order to rule out incorrect measurements due to poor contact etc. The determined individual values are, if the deviation is not> 5%, averaged.

The effective electrode area is calculated as follows: A = π × (D ₁ + B × g) / 4

From this results for the specific volume conductivity: σ _v = 1 / R · t / A

The conversion of the specific volume conductivity into the specific electrical resistance occurs according to: R = 1 / σ _v [Ω · cm]

A:

effektive Elektrodenfläche (vgl. hierzu ASTM D 257 Table 1 )

D1:

Durchmesser der Messelektrode

g:

Spaltbreite

B:

Korrektur abhängig von g/t (vgl. hierzu ASTM D 257 Appendix X2.2.3. ).

With: t: thickness of the sample

A:

effective electrode area (cf ASTM D 257 Table 1 )

D1:

Diameter of the measuring electrode

G:

gap width

B:

Correction dependent on g / t (cf ASTM D 257 Appendix X2.2.3. ).

The 4-point flexural strength at room temperature in the case of the sintered bodies obtainable by the process according to the invention is preferably in each case from 300 to 650 MPa.

The static modulus is preferably 375 to 425 GPa each.

The fracture toughness K _IC is preferably 2.3 to 5 MPa · m ^0.5 each

The hardness HK0.5 is preferably 22 to 27 GPa each.

The pressureless sintered SiC materials according to the invention are generally largely homogeneous without any detectable crystalline or glassy secondary phases by X-ray analysis.

Preferably, they have as the mineral phase mainly α-SiC with the polytypes 4H, 6H and 15R. Preferably, their microstructure is fine-grained with a maximum grain size of about 25 microns, resulting from the relatively "gentle" sintering conditions. In particular, this texture, together with the high sintering density ≥ 94% of the theoretical density lead to the overall good mechanical properties.

The ceramic sintered bodies according to the invention based on silicon carbide can therefore be used in a wide variety of applications.

Sintered bodies with average specific resistance (about 10 ³ -10 ⁶ Ω · cm) can be used for example as sliding bearings and mechanical seals, as heat exchanger components, kiln furniture, etc. In particular, for tribologically stressed parts such as slide bearings and mechanical seals and sintered body with lower electrical resistance are suitable (about 10 ³ -10 ² Ω · cm) to reduce static charges and Elektrokorrosionseffekte. Also for electrolyte cells, sputtering targets, etc., such sintered bodies are desired.

The sintered bodies with high electrical resistance (about 10 ⁷ -10 ⁸ Ω · cm) can be used for example in the field of high-temperature furnace construction to replace low-strength graphite or expensive, low-solid boron nitride. Due to the significantly higher strength of the sintered body according to the invention over the said established materials, structural parts can be made smaller and lighter and thereby increase the useful volume and the process efficiency.

Sintered bodies with a high electrical resistance also have potential as substrates (substrate) and heat sinks for electronic components due to their good thermal conductivity and the coefficient of thermal expansion which matches silicon.

Another object of the invention is therefore the use of ceramic sintered body according to the invention with low electrical resistance at room temperature (about 10 ² -10 ³ Ω · cm) and components made therefrom as plain bearings and mechanical seals, especially in the so-called mixed friction region and under dry running , as well as in other tribological applications to avoid electrostatic charges, for electrolysis cells, electrodes and similar devices and apparatus, as well as for sputtering materials, and for the production of complex shaped components by electrical discharge machining (erosion / EDM).

A further subject of the invention is the use of the ceramic sintered bodies according to the invention with high electrical resistance at room temperature (about 10 ⁷ -10 ⁸ Ω · cm) and components produced therefrom as structural components for high-temperature furnace construction, semiconductor production, and heat sinks in electrical and electronic applications.

The following examples serve to further explain the subject matter of the present invention. However, they do not represent any limitation of the basic concept underlying the invention.

Examples

The amounts indicated below are in percentages by weight unless stated otherwise.

Material # 0:

The starting material for the production of this material was a press and sinterable SiC batch available from HC Starck GmbH, Goslar, which already contains sintering auxiliaries and process auxiliaries, having the following specification: Base powder: α-SiC, spec. surface area (BET) = 14 m ² / g sintering additive B (as 64C) 0.5% by weight C - total 32.5% by weight C - free (pyrolyzed) 2.6% by weight Fe 0.02% by weight al 0.02% by weight Ca <0.01% by weight

Hereinafter, this raw material is also referred to as Premix, Pmx or S-SiC Pmx-1.

For the preparation of specimens for determining the mechanical properties and the electrical resistivity at room temperature (25 ° C), the premix was uniaxially pressed into rectangular plates of 70 × 60 × 7 mm, these plates are sealed in PVC bags under application of a vacuum and isostatic densified at 2000 bar. The resulting powder molded articles were sintered in an inert gas furnace in a combined Binderausbrand-, pyrolysis and sintering cycle at 2100 ° C for 1.5 h under 10 mbar Ar atmosphere using graphite as a kiln. Material # 0 is also referred to below as a reference material.

After sintering, the sintering density was determined by means of the Archimedes water displacement method and the percent theoretical density calculated in relation to the assumed theoretical density of 3.19 g / cm ³ . The result is a sintered density of 3.15 g / cm ³ , which corresponds to 98.7% of the theoretical density.

The test specimens required for material characterization were produced by cutting and grinding using diamond tools. On these test specimens, the 4-point bending strength at room temperature (RT-BF), the modulus of elasticity (modulus of elasticity), the fracture toughness K _IC , the hardness HK0.5 and the specific electrical DC resistance at 25 ° C (Spec. El. R.) as indicated above.

The values determined for the sintered body # 0 are summarized in Table 1. These show a typical spectrum of properties for a high-quality sintered SiC material.

Material # 0: 1- # 0.6 (# 0.5: according to the invention; # 0.1- # 0.4 and # 0.6 not according to the invention)

In order to demonstrate the influence of the sintering conditions on the properties of the resulting sintered body, the procedure described in the preparation of material # 0 was repeated, each time varying the sintering conditions. Test sintering was performed in a laboratory oven under atmospheric and reduced N ₂ and Ar gas pressures. The respective sintering conditions and the result of these tests are summarized in the following Table 2: Table 2: Sintered density as a function of the sintering conditions Material # raw material sintering conditions sintered density Th. D. T [° C], t [h], gas pressure [bar], gas type g / cm ³ % 0.1 S-SiC Pmx-1 2200 ° C, 1 h, 1 bar, Ar 3.04 95.3 0.2 S-SiC Pmx-1 2200 ° C, 1 h, 1 bar, N ₂ 2.94 92.2 0.3 S-SiC Pmx-1 2100 ° C, 1 h, 1 bar, Ar 3.02 94.7 0.4 S-SiC Pmx-1 2100 ° C, 1 h, 1 bar, N ₂ 2.90 90.9 0.5 S-SiC Pmx-1 2100 ° C, 1 h, 0.1 bar, Ar 3.12 97.8 0.6 S-SiC Pmx-1 2100 ° C, 1 h, 0.1 bar, N ₂ 2.98 93.4 Th. D .: Theoretical density

The results show that under sintering under a nitrogen atmosphere no dense sintered bodies could be obtained in any case. In no case was a density ≥ 94% th. D. achieved. On sintering under an argon atmosphere, on the other hand, dense sintered bodies result.

Both during sintering under atmospheric N ₂ pressure and under atmospheric Ar pressure, the resulting materials after sintering at 2,200 ° C strong grain growth, which can not be expected that the inventive mechanical properties desired could be achieved.

On the other hand, if gas pressure below 2,1 bar was sintered at 2,100 ° C., the sample sintered under 0.1 bar Ar achieved a surprising increase in density compared with the sample sintered at 2100 ° C. under 1 bar Ar and even in N ₂ atmosphere Sintered sample reaches a higher density than below 1 bar N ₂ . Thus, from this experiment, favorable sintering conditions for the S-SiC base composition result in a sintering temperature between 2025 and 2150 ° C. at an Ar gas pressure of ≦ 100 mbar, preferably ≦ 80 mbar.

The results of these experiments also demonstrate the technological advantages of sintering under reduced Ar gas pressure, such as the aspect of energy efficiency, since at atmospheric gas pressures the heat losses are much higher and require significantly higher electrical powers to reach the target temperature. Further, these tests show that sintering SiC powder articles in an N ₂ -containing atmosphere is not an effective means of adjusting certain N ₂ contents in the sintered bodies because of a sintering obstruction, coupled with problems, of the desired spectrum of properties to reach.

Materials # 1-6:

These materials were prepared by redispersing in each case 500 g of the abovementioned premix S-SiC Pmx-1 with addition of H ₂ O-deionized and defoamer in a ₂ l polyethylene bottle using ZrO ₂ milling beads of 3 mm on a tumble mixer. Increasing amounts of Si ₃ N ₄ powder were added to the batches (SN-HQ, HC Starck GmbH, Goslar):
# 1 = 0.5 parts by weight, # 2 = 1 part by weight, # 3 = 2 parts by weight, # 4 = 4 parts by weight, # 5 = 6 parts by weight, # 6 = parts by weight (each to 100 parts by weight of SiC premix).

The adjusted solids content of the resulting slurry was 55 wt .-%. 0.25% by weight of the substance KX 1030 (Zschimmer & Schwarz, Lahnstein) was added as defoamer. The redispersion and mixing of the Si ₃ N ₄ powder was carried out for 10 h. The resulting redispergate was sieved to remove the ZrO ₂ grinding beads over 250 microns and for separating any remaining agglomerates over 63 microns and the resulting pure slip freeze-dried. After drying in a drying oven in air at 70 ° C overnight, the respective material was granulated by sieving <150 microns and this sieved material is pressed into samples and sintered, as described above in the preparation of material # 0.

After determination of the sintering density, specimens were prepared at room temperature (25 ° C.) for determining the mechanical properties and the specific electrical resistance, and the corresponding measurements were carried out. The results obtained here for the development materials # 1- # 6 are again listed in Table 1.

These results show that Si ₃ N ₄ addition amounts up to 4 wt .-% have only a small effect on the sintering behavior. Additions from 2 wt .-% Si ₃ N ₄ show a negative, but still uncritical influence on the mechanical properties achieved.

Material # 6 with 10 wt .-% Si ₃ N ₄ addition no longer reached the inventive target density of ≥ 94% theoretical density under the selected sintering conditions. Furthermore, the results show that the room temperature electrical resistivity has a strong dependence on the amount of added Si ₃ N ₄ powder. Even an addition of only 0.5 wt .-% Si ₃ N ₄ powder (material # 1) surprisingly increases the electrical resistance to the material # 0 already by about 2 orders of magnitude, the addition of 1 wt .-% Si ₃ N ₄ powder (material # 2) even by almost 3 orders of magnitude, without the well-valued mechanical properties of the reference material are significantly below. Still further surprising is that a further increase of the Si ₃ N ₄ addition to 4 and to 6 wt .-% causes no further increase in the electrical resistivity at room temperature, but the values go down to near the level of the reference material. This was not to be expected and enables the person skilled in the art to set a specific electrical resistivity of a SiC-based material at room temperature by means of a specific addition of Si ₃ N ₄ .

Material # 7:

To test whether the results with runs # 1- # 6 are related to the use of the SiC premix, approach # 7 was used to prepare a complete, separate sintered batch. For this purpose, the α-SiC powder "UF-15" of HC Starck GmbH, Goslar, was used, which has the following characteristics: Base powder: α-SiC, spec. surface area (BET) = 15 m ² / g Total carbon 29.5% by weight Fe ≤ 0.05% by weight al ≤ 0.02 wt% Ca ≤ 0.01% by weight

This powder (200 g) was mixed with the addition of 0.5% by weight of B in the form of B ₄ C (HC Starck GmbH, Goslar, Grade HS), 2.0% by weight of carbon in the form of the C-paste. " Derusol 325 "(Degussa) with known C-yield after pyrolysis and 2.0 wt.% Si ₃ N ₄ powder (SN-HQ, HC Starck GmbH, Goslar, Si ₃ N ₄ calculated on the total weight SiC + B + C) treated aqueous by planetary mill. The grinding crucible used was Si ₃ N ₄ , and the grinding beads were 400 g of ZiO ₂ with a diameter of 3 mm. The solids content was adjusted to 55% by weight with addition of 0.5% by weight of liquefier KV5088 and 0.5% by weight of defoamer and wetting agent KX1030 (both from Zschimmer & Schwarz, Lahnstein). As binder and plasticizer, 0.5% by weight of PVA (Mowiol 4-88) and 1.5% by weight of PEG (PEG 400) were added. The grinding time was 5 h.

After this grinding, the slip was removed from the grinding vessels, sieved to remove the ZrO ₂ milling beads over 250 microns and for separating any remaining agglomerates over 63 microns, and the resulting pure slip freeze-dried. The further processing of the batch and the characterization of the material obtained after sintering were carried out as described in the preceding examples.

The results are shown in Table 1. These show that with this method results in a material with good mechanical properties and the determined electrical resistivity at room temperature with about 10 ⁸ Ω · cm in the same order of magnitude as that of the material # 3. Thus, for the resulting specific electrical resistance at room temperature, the type of treatment, if no uncontrolled contamination takes place, plays a minor role compared to the addition of Si ₃ N ₄ , which increases the resistance to the reference material by about 3 orders of magnitude. This thus allows the skilled person to set by the addition of Si ₃ N ₄ targeted a certain electrical resistivity of a SiC-based material at room temperature.

Material # 8:

The process for producing material # 7 was modified as follows: The starting powder used again was the α-SiC powder "UF-15" from HC Starck GmbH, Goslar, containing 0.5% by weight B in the form of B ₄ C (HC Starck GmbH, Goslar, Grade HS), 2% by weight of carbon in the form of the C paste "Derusol 325" (Degussa) with known C yield after pyrolysis and 1.5% by weight of a passivated Al powder "Aquapor 9009" (Schlenk, Roth) were added. In preliminary tests it was verified that the passivation is sufficient for an aqueous preparation of the batch without dangerous gas evolution by reaction of the Al with H ₂ O. The solids content was adjusted to 55% by weight with H ₂ O-deionized, in addition, 0.5% by weight of liquefier KV5088 and 0.5% by weight of defoamer KX1030 (both from Zschimmer & Schwarz, Lahnstein) were added. The dispersion and homogenization took place in a PE bottle with ZrO ₂ milling beads of about 3 mm on a tumble mixer in 10 h. The resulting slurry was sieved and freeze-dried as in the previously described examples. The further processing of the batch, the sintering and the characterization of the material obtained after sintering were also carried out as described in the preceding examples, the results are listed in Table 1. These show that this approach under the applied sintering conditions (and also after variation of these) the target density of the invention ≥ 94% th. D. did not reach. A more detailed characterization of the mechanical properties was therefore not.

Material # 9:

Since the procedure for producing material # 8, ie the sole addition of an aluminum-containing additive, had not led to the goal of reduced specific electrical resistance of a SiC-based material at room temperature, various other simultaneous additives such as Si ₃ N ₄ , AlN and also VC tested. It surprisingly turned out that an addition of 0.7% by weight of Si ₃ N ₄ to batch # 8 (to 100% by weight) leads both to an improvement in the sintering behavior and to good mechanical properties which are desired according to the invention aimed at reduced specific electrical resistance at room temperature. The raw materials used were identical to those of Example # 8, as Si ₃ N ₄ powder was in turn SN-HQ (HC Starck GmbH, Goslar) used. The preparation of the batch, the sintering and the characterization were carried out as described for example # 8 (= production of material # 8), the results are tabulated. 1 listed.

The investigations and analyzes carried out give no clear explanation for the mode of action of the Si ₃ N ₄ addition. It is conceivable, however, that between the Si ₃ N ₄ and the Al to a reaction reaction to form AlN, which is known to be incorporated in SiC as a solid solution (mixed crystal) to form a defect structure. This defect structure could be responsible for reducing the specific electrical resistance at room temperature of material # 9 compared to the reference material # 0 by about 3 orders of magnitude. Regardless of whether this explanation can be confirmed, the described measure of the simultaneous addition of Al and Si ₃ N ₄ to a sinterable SiC base approach is a possibility to purposefully reduce the specific electrical resistance of a SiC-based material at room temperature.

Material # 10:

The procedure for the preparation of material # 9 was repeated, but an identical starting composition was prepared by planetary mill, as was described for the preparation of material # 7. The sample preparation, sintering and characterization were carried out as described in the preceding examples, the results obtained are listed in Table 1.

These results show that this method resulted in a material with good mechanical properties which are desired according to the invention and the determined electrical resistivity at room temperature of approximately 7 · 10 ² Ω · cm is virtually identical to that of the material # 9.

Thus, the type of treatment is irrelevant to the resulting room temperature electrical resistivity, provided that sufficient deagglomeration and homogenization of the various components is achieved and that no uncontrolled contamination occurs.

Material # 11:

To analyze the effect of Fe contamination in the treatment, a 1.5 kg batch of a composition as in the preparation of # 9 and # 10 materials was described using a Vibratom mill in a steel container and steel balls (4.5 kg , 10-15 mm) water over a period of 5 h. The resulting slurry was processed identically further as described in the examples mentioned. From the freeze-dried powder, an Fe content of 0.55% by weight was analyzed, resulting from attrition of the mill and grinding balls during processing. Further sample preparation, sintering and characterization were carried out as described in the preceding examples, the results obtained are listed in Table 1.

These results show that this method in turn results in a material with good mechanical properties, but the room temperature electrical resistivity is about half an order of magnitude higher than that of the # 9 and # 10 approaches to Fe contamination-free preparation. Thus, a targeted contamination by the treatment or an addition of Fe can be regarded as a measure to increase the electrical resistivity at room temperature and thus the resistance range of the materials # 9 and # 10 of about 5 · 10 ² Ω · cm to the of the reference material # 0 of about 2 × 10 ⁵ Ω · cm.

Material # 12:

In order to check whether, instead of the additive combination Al and Si ₃ N ₄ , addition of AlN in a batch according to # 9 leads to a similar reduction in the electrical resistivity at room temperature, such an approach was established. The 1.5% by weight of Al added in the production of material # 9 were replaced in an equivalent amount, based on the resulting aluminum content, by AlN without further addition of Si ₃ N ₄ . Thus, the formulation had the following composition:
α-SiC powder "UF-15", HC Starck GmbH, Goslar; with 0.5 wt .-% B in the form of B ₄ C (HC Starck GmbH, Goslar, Grade HS), 2 wt .-% carbon in the form of the C paste "Derusol 325" (Degussa) with known C-yield after pyrolysis, and 1.5 wt .-% Al in the form of AlN (2.3 wt .-% AlN, Grade C, HC Starck GmbH, Goslar). The resulting base composition was thus: α-SiC + 0.5B + 2C + 1.5Al = 100% by weight.

The attempt to prepare this approach as aqueous, as described in the preparation of material # 8, failed as expected. The reaction of AlN with H ₂ O led to (H ₂ -) gas evolution, which led to a dangerous bloating of the PE bottle. Thus, this route is not economically feasible on an industrial scale.

Nevertheless, in order to analyze the effect of the AlN additive, it was reprocessed using isopropanol. For this purpose, 500 g of said base composition were dispersed with the addition of 1 kg of isopropanol in a 2 liter polyethylene bottle by means of 2 kg ZrO ₂ grinding beads of 3 mm diameter on a tumble mixer for 10 hours and homogenized. The suspension thus obtained was sieved to remove the ZrO ₂ grinding beads over 250 microns and for separating any remaining agglomerates over 63 microns and dried pure slurry obtained by means of an explosion-proof rotary evaporator. After drying in a drying oven in air at 70 ° C overnight, the material was granulated by sieving <150 microns and this sieved material is pressed into samples, sintered, and characterized, as described in the preceding examples.

The results obtained are shown in Table 1, which shows that the material sinters very well and has an electrical resistance slightly above the resistances of materials # 9 and # 10. This demonstrates that instead of the combination of Al and Si ₃ N ₄ AlN can also be used for the specific adjustment of the electrical resistivity at room temperature. However, since these AlN-containing approaches can not be treated aqueous, their preparation is associated with a much higher technical and costly effort.

Material # 13:

To check whether an intrinsically high specific electrical resistance at room temperature can be increased by an afterglow below N ₂ , a sample of material # 3 in a gas pressure sintering plant was annealed for 2 h at 1850 ° C. and a pressure of 10 exposed to bar N ₂ and then again determined the electrical resistance, as described above. In order to avoid surface effects, the test plate was sanded on both sides after the thermal treatment and thereby removed about 0.1 mm of material. The result of the determination of the electrical resistance in Table 1 shows that the thermal after-treatment under 10 bar N ₂ again increased the resistance by almost one order of magnitude. Thus, thermal aftertreatments of SiC-based materials produced according to the invention at temperatures above 1800 ° C. in a gas atmosphere of at least 1 bar N _{2 are} suitable for increasing the specific electrical resistance of the SiC material. The term "thermal aftertreatment" is intended to encompass all thermal treatments in N ₂ -containing atmosphere between 1 bar (sintering), 10-100 bar (gas pressure sintering) and 2000 bar (hot isostatic pressing or HIPing) downstream of the actual sintering step in an inert gas atmosphere.

Material # 14:

The aim of this approach was to demonstrate that the advantageous mechanical and electrical properties are also achieved with an industrial-scale approach. For this purpose, exactly the same raw materials and additives as described for the production of material # 7 were used. The total amount of the batch was 100 kg of solid. The starting materials were dispersed in a Löstesirl in water and then deagglomerated in a continuous process through a stirred ball mill and homogenized. The mill used had a PU lining, SiC break granules <3 mm were used as grinding media. The circulation pump was adjusted so that the batch (calculated) passed the mill 10 times in 1 h, after 1 h the grinding was stopped; the rotor speed was 1,500 rpm. To This homogenization grinding, the resulting slurry was screened 63 microns and granulated by means of a spray dryer with centrifugal. The tower discharge temperature was controlled via the slurry feed amount of the spray dryer so that it did not exceed 120 ° C. About the speed of the spinner, the dryer was operated so that an average granule size resulted by 80 microns. The resulting granules were screened <180 microns and from the passage through the plates were isostatically pressed for the production of samples for material characterization, as well as larger pellets for test parts of kiln installations.

As the results of the material characterization in Table 1 show, the good mechanical properties desired according to the invention and the desired high electrical resistivity at room temperature (25 ° C.) of approximately 2 × 10 ⁸ Ω · cm were also achieved in this procedure.

From the isostatically pressed moldings tubular supports of about 5 cm outside and 2.6 cm inside diameter and 40 cm in length, as well as some cross-holes were prepared by the green compacts (powder molding) processed and in the described, in a Furnace cycle combined heat treatment, pyrolysis and sintering cycle were thermally treated. The parts remained crack-free and defect-free and reached a sintering density of 3.12 g / cm ³ . The finished after a minor hard machining parts were tested in their intended application, where they are fully proven.

This example shows that according to the invention SiC-based materials and components produced therefrom can be produced cost-effectively on a larger scale with established technical processes, which is of great economic importance.

Thus, the invention gives guidance on the properties of SiC-based materials and components made therefrom, such as a sintering density ≥ 94% th. D., a strength of at least 300 MPa, a static modulus of at least 375, a fracture toughness K _IC of at least 2.3 MPa · m ^0.5 , a hardness HK0.5 of at least 22.0 GPa, and a specific To set electrical resistance at room temperature between 10 ² to 10 ⁸ Ω · cm targeted, and to produce these materials and components produced therefrom in an economically favorable way.

An overview of the materials produced and their determined material properties is shown in Table 1: Tab.1: Properties of the produced materials material sintered density Th. D. strength Modulus K _IC Hardness HK0,5 Spec. El. R. Erf.gemäß g / cm ³ % MPa GPa MPa · m ^1/2 GPa Ω · cm Yes No # 0 3.15 98.7 465 385 2.8 22.5 2.4 · 10 ⁵ Yes #1 3.15 98.7 472 388 2.8 22.6 1.6 · 10 ⁷ Yes # 2 3.16 99.1 428 383 2.8 22.5 1.6 · 10 ⁸ Yes # 3 3.15 98.7 360 382 2.7 22.3 8.3 · 10 ⁷ Yes # 4 _{3 , 15} 98.7 365 384 2.5 22.1 8.6 x 10 ⁶ Yes # 5 3.08 96.6 355 375 2.3 21.4 4.2 · 10 ⁵ Yes # 6 2.90 90.9 - - - - - No # 7 3.14 98.4 362 384 2.7 22.4 2.4 · 10 ⁸ Yes #8th 2.80 87.8 - - - - - No # 9 3.13 98.1 307 378 2.8 22.0 3.2 · 10 ² Yes # 10 3.15 98.7 308 380 2.7 22.2 7.1 · 10 ² Yes # 11 3.15 98.7 325 382 2.6 22.3 3.6 · 10 ³ Yes # 12 3.18 99.7 335 388 3.1 22.6 1.8 · 10 ³ Yes # 13 3.14 98.4 375 385 2.9 22.5 6.7 · 10 ⁸ Yes # 14 3.15 98.7 410 390 3.2 22.7 2,0 · 10 ⁸ Yes Th. D .: Theoretical density
Spec. El. R .: Electrical DC resistance at 25 ° C
According to: according to the invention

In ( 1 ), the determined DC electrical resistances at 25 ° C for various of the materials described above are shown graphically. From this figure, the bandwidth of the adjustable by the inventive method specific electrical resistance of SiC-based sintered materials at room temperature (25 ° C) can be seen. As can be seen, sintered bodies with DC electrical resistance at 25 ° C could be obtained in the entire desired range of 10 ² to 10 ⁸ Ω · cm.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19842078 B4 [0003]
• EP 0219933 B1 [0004]
• JP 2001130971 A [0005]
• JP 2006240909 A [0006]
• EP 0626358 B1 [0007]
• US 5011639 [0008]
• US 2010/0130344 A1 [0009, 0009]
• EP 0771769 A2 [0010, 0010, 0010]
• EP 1070686 B1 [0013]

Cited non-patent literature

• DIN EN 843-1 [0053]
• "J. Kübler: Fracture Toughness of Ceramics using SEVNB Method: Round Robin "; VAMAS Report No. 37 (ISSN 1016-2186) [0055]
• DIN EN 843-4 [0056]
• ASTM D 257 [0057]
• ASTM D 257 Table 1 [0062]
• ASTM D 257 Appendix X2.2.3. [0062]

Claims (10)

A method for producing a ceramic sintered body based on silicon carbide, comprising sintering a powder shaped body containing a. at least 88% by weight of silicon carbide, b. at least one boron-containing additive, in an amount of 0.1 to 1.5 wt .-%, calculated as boron, c. Carbon and / or at least one carbon source material that provides free carbon in the sintering process in an amount corresponding to 0.5 to 3 wt% carbon, d. at least one nitrogen-containing additive in an amount of 0 to 2.3% by weight, calculated as nitrogen, e. and at least one aluminum-containing additive in an amount of 0 to 2.0 wt .-%, calculated as aluminum, with the proviso that an aluminum-containing additive may be included only if the powder molded body also contains at least one nitrogen-containing additive, wherein all quantities are based on the sum of the constituents of the powder molding, and wherein the sintering is carried out in an inert gas atmosphere at a pressure of 1 to 100 mbar. A method according to claim 1, characterized in that the powder molded body further contains at least one temporary processing aid. A method according to any one of claims 1 or 2, characterized in that the sintering is carried out at a temperature of 2000 ° C to 2200 ° C. Method according to at least one of claims 1 to 3, characterized in that the powder molded article contains at least one nitrogen-containing additive in an amount of 0.01 to 2.3 wt .-%, calculated as nitrogen. A method according to claim 4, characterized in that the powder molded article contains at least one aluminum-containing additive in an amount of 0.01 to 2.0 wt .-%, calculated as aluminum. Method according to at least one of claims 1 to 5, characterized in that it is the silicon carbide is α-SiC, preferably of technical grade α-SiC, the impurities selected from Fe, Mg, Ti and Ca in a total concentration of 250 ppm to 750 ppm, and / or the boron-containing additive is boron carbide. Method according to at least one of claims 1 to 6, characterized in that for the preparation of the powder molded article whose starting materials are treated together in aqueous and mixed, dried and formed into a shaped body. Method according to at least one of claims 1 to 7, characterized in that after sintering in an inert gas atmosphere sintering at atmospheric pressure, gas pressure sintering or sintering-hot isostatic pressing in N ₂ or an N ₂ -containing atmosphere is performed. Ceramic sintered body based on silicon carbide, obtainable by a process according to at least one of claims 1 to 8. Ceramic sintered body according to claim 9, characterized in that it has a density of at least 94% of the theoretical density, determined by the Archimedes water displacement method, and a DC electrical resistance at 25 ° C of 10 ² to 10 ⁸ Ω · cm, and preferably further 4-point bending strength at room temperature of ≥ 300 MPa, a static modulus of ≥ 375 GPa, a fracture toughness K _IC of ≥ 2.3 MPa · m0.5, and a hardness HK0.5 of ≥ 22 GPa.

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