EP1999070A1

EP1999070A1 - Sintered material, sinterable powder mixture, method for producing said material and use thereof

Info

Publication number: EP1999070A1
Application number: EP07723198A
Authority: EP
Inventors: Hubert Thaler; Clemens Dr. Schmalzried; Frank Wallmeier; Georg Victor
Original assignee: ESK Ceramics GmbH and Co KG
Current assignee: ESK Ceramics GmbH and Co KG
Priority date: 2006-03-24
Filing date: 2007-03-12
Publication date: 2008-12-10
Also published as: CA2643211A1; US20090121197A1; WO2007110148A1; RU2008142122A; CN101410329A; DE102006013729A1

Abstract

The invention relates to a sintered material based on transition-metal diborides, containing: a) a main phase with between 90 and 99 wt. % of a fine-grained transition-metal diboride, or transition-metal diboride mixed crystal consisting of at least two transition-metal diborides, or mixtures of diboride mixed crystals of this type, or mixtures of diboride mixed crystals of this type and one or more transition-metal diborides, the transition metals being selected from the sub-groups IV to VI of the periodic table, b) a second phase with between 1 and 5 wt. % of a particulate boron carbide and/or silicon carbide and c) optionally as the third phase up to 5 wt. % of a non-continuous, grain boundary phase. The invention also relates to a pulverulent, sinterable mixture for producing a sintered material of this type, to a method for producing the sintered material, preferably by pressureless sintering and to the use of the sintered material as an anti-corrosion material for molten salt and metal, in particular molten material containing cryolite.

Description

Sintered material, sinterable powder mixture,

Process for the production of the material and its use

Field of the invention

The invention relates to a sintered material based on transition metal diborides, pulverulent sinterable mixtures for producing such a sintered material, to processes for producing such sintered materials and to the use of the sintered material as a corrosion protection material for salt and metal melts, in particular kryolithhal- term Melting, for the production of thermocouple protection tubes for cryolite-containing melts, as electrode protection material, electrode material or material for the cell lining in the Al fused-salt electrolysis, as well as electrode material for sliding contacts, welding electrodes and erosion pins.

Background of the invention

Titanium diboride has a number of advantageous properties, such as a high melting point of 3,225 ° C, a high hardness of 26-32 GPa (HV), excellent room temperature electrical conductivity and good chemical resistance.

A major disadvantage of titanium diboride is its poor sinterability. The poor sinterability is due in part to impurities, especially oxygen impurities in the form of TiÜ ₂ , which are contained in the commonly used titanium diboride powders, either by the carbothermal reduction of titanium oxide and boron oxide or by the known as Borcarbidverfahren reduction of me talloxide with Carbon and / or boron carbide are produced. Such oxygen impurities enhance grain and pore growth in the sintering process by increasing surface diffusion.

State of the art

Sintered titanium diboride materials can be made by the hot pressing process. For example, by axial hot pressing with sintered achieved temperatures above 1.800 ⁰ C and a pressure of> 20 MPa densities of above 95% of the theoretical density, wherein the hot-pressed material typically has a grain size of more than 20 microns. However, the disadvantage of the hot pressing method is that only simple body geometries can be produced thereby, while bodies or components with complex geometries can not be produced by this method.

On the other hand, components with more complex geometries can be produced via the pressure-loss sintering process. In this case it is necessary to add suitable sintering aids in order to obtain sintered bodies of high density. Possible sintering additives are, for example, metals, such as iron and iron alloys. By adding small amounts of iron, dense materials with good mechanical properties and high fracture toughnesses of more than 8 MPa m ^1/2 can be obtained. Such materials are described for example in EP 433 856 B l. However, these materials have the disadvantage that they have a poor corrosion resistance due to the metallic binder phase and in particular are not resistant to cryolite and cryolite-containing melts.

EP 0 073 743 B1 describes corrosion-resistant titanium diboride materials in comparison with aluminum melts, titanium hydride and boron being used as compaction additives for their production via a pressure-sintering process. Since these additives exert no obvious grain growth inhibiting effects, it comes at the applied sintering temperatures of up to 2,200 ⁰ C to giant grain growth and, consequently, in reduced strength and increased micro-cracking due to grain sizes above the critical grain size.

It is known in the art that the grain boundaries of titanium tiboride sintered materials are weak points in view of the corrosion resistance to cryolite due to liquid phase infiltration along the grain boundaries.

US Pat. No. 4,500,643 discloses that a sintered material made of pure, fine-grained titanium diboride is resistant to the operating conditions of aluminum alloys.

Melt-flow electrolysis and thus also resistant to cryolite, but that even small amounts of impurities, especially oxides or metals, lead to a dramatic grain boundary corrosion and thus to the decomposition of the component. The titaniboride material described in this US patent has a porosity of 10 to 45% by volume, the pores being interconnected, so that a continuous porosity is present. Due to the open porosity, this material, despite its resistance to cryolite, is not suitable for the separation of various media, in particular it is not suitable as a corrosion protection material for cryolite. For this reason, for example, the material is also unsuitable for the production of thermocouple protection tubes for the Al melt flow electrolyte, and it can not be used as an anode protection material in the Al fused-salt electrolysis. Due to the high porosity, the material is also not mechanically strong enough.

Object of the invention

The invention is therefore based on the object of providing a sintered material which not only has good mechanical properties, but is also corrosion-resistant to salt and metal melts, in particular cryolite-containing melts. Furthermore, the material should have a closed porosity to be effective as corrosion protection. Furthermore, such a sintered material should be producible by a simple and inexpensive process, which also allows the production of moldings with complex geometries.

Summary of the invention

The above object is achieved by a sintered material based on transition metal diborides according to claim 1, a powdery sinterable mixture for producing such sintered sintered material according to claim 9, a method for producing such a sintered material according to claims 17 and 18, and the use of the sintered material according to claims 24-27. Advantageous or particularly expedient embodiments of the subject of the application are specified in the subclaims.

The invention thus provides a sintered material based on transition metal diborides containing a) as the main phase 90-99 wt .-% of a fine-grained transition metal diboride or transition metal diboride mixed crystal of at least two transition metal diborides or mixtures of such diboride mixed crystals or mixtures of such diboride mixed crystals with one or more transition metal diborides, wherein the transition metals from IV. To VI. B) as second phase 1 -5 wt .-% particulate boron carbide and / or silicon carbide and c) optionally as a third phase up to 5 wt .-% of a non-continuous, oxygen-containing grain boundary phase.

The invention further provides a pulverulent sinterable mixture for producing a sintered material based on transition metal diborides, comprising 1) 0.05-2% by weight of Al and / or Si as metallic Al and / or Si and / or a content thereof corresponding amount of an Al and / or Si compound, 2) optionally at least one component selected from carbides and borides of transition metals of IV. to VI. Subgroup of the Periodic Table, 3) 0.5-12% by weight of boron,

4) 0-5 wt.% Boron carbide and / or silicon carbide,

5) 0-5 wt .-% carbon and / or a carbon compound in each case based on the content of elemental carbon, and

6) as the remainder at least one transition metal diboride of IV. To VI. Subgroup of the periodic table, that of the transition metal boride of the above

Component 2) is different.

The invention further provides a process for producing such a sintered material by hot pressing or hot isostatic pressing or gas pressure sintering or spark plasma sintering of a pulverulent mixture as described above, optionally with the addition of organic binding and pressing aids.

The invention likewise provides a process for producing a sintered material as described above by pressure-sintering, comprising the steps: a) mixing a powdery mixture as described above, optionally with the addition of organic binding and pressing aids in water and / or organic solvents to produce a homogeneous powder suspension, b) preparing a powder granules from the powder suspension, c) pressing the powder granules into green bodies of high density , and d) pressureless sintering of the obtained green bodies in vacuo or under protective gas at a temperature of 1,800-2,200 ⁰ C.

The sintered material according to the invention is suitable as a corrosion protection material for salt and metal melts, in particular cryolite-containing melts.

The invention therefore also relates in particular to the use of the sintered material for the production of thermocouple protection tubes for cryolite-containing melts.

The sintered material according to the invention is likewise suitable as an electrode protection material, electrode material or material for the cell lining in the Al fused-salt electrolysis and as electrode material for sliding contacts, welding electrodes and erosion pins.

According to the invention, it has thus been found that the above-mentioned object is achieved by providing a sintered, dense material based on transition metal diborides whose matrix (main phase) consists of a fine-grained transition metal diboride or transition metal diboride mixed crystal or combinations thereof. As a second phase, the material contains particulate boron carbide and / or silicon carbide, which acts as a grain growth inhibitor. Optionally, the material may contain as a third phase an oxygen-containing, non-continuous grain boundary phase. The mixed crystal formation of the main phase has an additional grain growth inhibiting effect, so that a sintered material having good mechanical properties is obtained. Residual contents of impurities, such as, for example, oxygen-containing impurities, may be present in particulate form between the grain boundaries or at the triple points of the grain boundaries. The sintered material according to the invention has a surprisingly excellent corrosion resistance. resistance to salt and metal melts, including cryolite-containing melts.

Detailed description of the invention

As mentioned above, the microstructure of the material according to the invention consists of a transition metal diboride or transition metal diboride mixed crystal of at least two transition metal diborides or mixtures of such diboride mixed crystals or mixtures of such diboride mixed crystals with one or more transition metal diborides. As a second phase is present in a small proportion particulate Borcabid and / or silicon carbide, which is located mainly at the grain boundaries. The boron carbide and / or silicon carbide additionally acts as a particle-reinforcing agent. Furthermore, an oxygen-containing third phase can be present in a small proportion at the triple points of the material. It is important that the oxygen-containing phase does not form a continuous grain boundary film. Optionally, small amounts of particulate carbon and / or particulate boron present may also be present in the material. Furthermore, when using Al or Si or their compounds as sintering aids, low contents of these elements may be present in the main phase. If the oxygen-containing third phase is present, its proportion is preferably up to 2.5 wt .-%.

The main phase preferably has an average particle size of less than 20 μm, more preferably less than 10 μm. The boron carbide and / or silicon carbide of the second phase preferably has an average particle size of less than 20 microns, more preferably less than 5 microns. The determination of the average grain size of the main phase and the average particle size of the boron carbide and / or silicon carbide is carried out by the linear intercept length method on the etched ground.

The transition metals of IV. To VI. Subgroups are preferably selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W.

The main phase is preferably fine-grained TiB ₂ and / or ZrB ₂ and / or a mixed crystal of (Ti ₁ W) B ₂ and / or (Zr ₁ W) B ₂ and / or (Ti ₁ Zr) B ₂ , more preferably a mixed crystal of (Ti ₁ W) B ₂ and / or (Zr ₁ W) B ₂ , including the ternary diborides (Ti ₁ Zr ₁ W) B ₂ . In particular, it is preferably the mixed crystal (Ti ₁ W) B ₂ or the mixed crystal (Zr ₁ W) B ₂ . The proportion of WB ₂ in the main phase is preferably not more than 7% by weight.

The pulverulent, sinterable mixture according to the invention for producing a sintered material according to the invention contains the following components:

1) 0.05-2% by weight, preferably 0.2-0.6% by weight, of Al and / or Si as metallic Al and / or Si and / or an amount of an Al corresponding to this content. and / or Si compound. Al or oxygen-containing Al compounds, in particular Al ₂ O ₃ or boehmite, are preferably used.

2) optionally, preferably> 0.25% by weight of at least one component selected from carbides and borides of transition metals of IV. To VI. Subgroup of the Periodic Table, preferably tungsten carbide. Optionally, as component 2), transition metals of IV. To VI. Subgroup itself and oxides of such transition metals are used. In the case of using transition metal carbides, their proportion may be up to 15% by weight.

3) 0.5-12% by weight, preferably 1-5% by weight of boron in elemental form,

4) 0-5 wt.% Boron carbide and / or silicon carbide,

5) 0-5 wt .-%, preferably 0, 1- 1 wt .-% carbon and / or a carbon compound as an organic carbon carrier, each based on the content of elemental carbon. The added carbon serves to reduce the oxides contained in the starting materials as impurities or oxides formed during sintering. Examples of suitable carbon compounds are dispersed carbon black, phenolic resins and sugars.

6) As the radical at least one transition metal diboride of IV. To VI. Subgroup of the periodic table, which is different from the transition metal boride of the above component 2). As already mentioned above, the transition metals are selected from Ti, Zr, Hf, V, Nb ₁ Ta ₁ Cr, Mo and W. The transition metals transition metal diboride of component 6) is preferably ΗB ₂ and / or ZrB ₂ , more preferably ΗB ₂ .

The above components of the pulverulent mixture are preferably used in the highest possible purity and with a small particle size. For example, the transition metal diboride of component 6) preferably has an average particle size of not more than 4 μm, more preferably not more than 2 μm.

The sintered material according to the invention can be produced in a manner known per se by hot pressing, hot isostatic pressing, gas pressure sintering or spark plasma sintering of a pulverulent mixture as described above, optionally with the addition of organic binding and pressing aids. In this case, customary organic binders such as polyvinyl alcohol (PVA), water-soluble resins and polyacrylic acids and customary pressing aids such as fatty acids and waxes can be used.

To produce the sintered material according to the invention, at least one transition metal diboride of IV. To VI. Subgroup processed with the other powder-shaped components and optionally organic binding and pressing aids in water and / or organic solvents to form a homogeneous powder suspension. The homogeneous powder suspension is then converted into a powder granules, preferably by spray drying. This powder granulate can then be further processed by hot pressing or hot isostatic pressing or gas pressure sintering to form a sintered material.

According to a preferred embodiment, the production of the sintered material according to the invention by Drucklossintern. In this case, a powder granules obtained as described above are pressed into green bodies of high density. For this purpose, it is possible to use all customary shaping methods, such as axial pressing or calcostatic pressing, but also extrusion, injection molding, slip casting and pressure slip casting. The green bodies obtained are then transferred in a vacuum or under protective gas at a temperature of 1,800-2,200 ⁰ C, preferably 1,900-2,100 ⁰ C, more preferably about 2,000 ⁰ C, by pressureless sintering in a sintered material. Vorzugswelse the green bodies are annealed prior to Drucklossintern in an inert atmosphere at temperatures below the sintering temperature to remove the organic binding or pressing aids.

The materials obtained by pressure-sintering have a density of at least about 94% of the theoretical density, preferably a density of at least 97% of the theoretical density. Such density values ensure that porosity, if present, is present as closed porosity. Optionally, the sintered material may be densified by hot isostatic pressing to increase the density and to reduce the closed porosity.

The carbides of transition metals of the IV. To VI. Subgroup of the periodic table selected component of the powdered Ausgangsmi- shear reacts during the sintering process with the added boron to Übergangsmetallborid and boron carbide. The transition metal boride formed and / or the added transition metal boride of the above-mentioned component 2) may form a mixed crystal with the transition metal diboride of component 6), such as titanium diboride. This boride mixed crystal formation has a grain growth inhibiting effect. The boron carbide, both the added and the example of tungsten carbide and boron formed, also acts grain growth inhibiting. In the production of the sintered materials according to the invention, it is important that the oxygen impurities contained in the powder mixture react as completely as possible in order to prevent the formation of continuous, oxygen-containing grain boundary films. This is done by reduction with boron and the added carbon and / or carbon compounds, but also by evaporation in vacuo. Preferably, volatile oxides in the temperature range between 1600 and 1700 ⁰ C can be removed at higher temperatures.

The amounts of added boron and the carbon used and / or the carbon compounds in the starting mixture are calculated so that the following reduction reactions (1) to (3) occur completely: (1) WC + 6 B -> WB ₂ + B ₄ C

(2) TiO ₂ + 4 B -> TiB ₂ + 2 BO (g)

(3) 2 B ₂ O ₃ + 7C -> B ₄ C + 6 CO

In the above reduction reaction (1), WC was representatively selected as the representative of the above-mentioned component 2).

The Al and / or Si or their compounds act as sintering aids, the microstructure formed indicating a liquid-phase sintering process.

The cryolite-resistant and dense, fine-grained material according to the invention is suitable for wear applications. The sintered material according to the invention is furthermore outstandingly suitable as a corrosion protection material for salt and metal melts, such as Al and Cu melts, in particular cryolite-containing melts. Specific applications of the sintered material according to the invention are thermocouple protection tubes for cryolite-containing melts, electrode protection materials, electrode materials or materials for the cell lining in the Al fused-salt electrolysis, as well as electrode materials for sliding contacts, welding electrodes and erosion pins.

Brief description of the attached drawings

FIG. 1 shows a light micrograph of the microstructure of the material obtained in Example 1;

Figure 2 shows a light micrograph of the microstructure of Figure 1 after the cryolite test;

Figure 3 is a photomicrograph of the microstructure of the sintered material obtained in Example 2;

Figure 4 shows a light micrograph of the microstructure of Figure 3 after the cryolite test;

FIG. 5 shows a light micrograph of the microstructure of the sintered material obtained in Reference Example 1; 1 Figure 6 shows a light micrograph of the microstructure of Figure 5 after the cryolite test;

FIG. 7 shows a light micrograph of the microstructure of the sintered material obtained in Reference Example 2;

Figure 8 shows a light micrograph of the microstructure of Figure 7 after the cryolite test;

Figure 9 shows a light micrograph of the microstructure of the sintered material obtained in Reference Example 3;

Figure 10 shows a light micrograph of the microstructure of Figure 9 after the cryolite test; 15

Figure 11 shows a TEM brightfield image of a representative area of the microstructure of Figure 1; and

Figure 12 shows a TEM bright field image (left) perpendicular to the grain boundary of the microstructure from Figure 1 1 and the associated one-dimensional spectrum image (right) along the white line shown in the left image.

The following examples and reference examples illustrate the invention. To evaluate the cryolite resistance, the following test was carried out. 5

cryolite

The sample, together with an amount of pure cryolite completely covering the material, is heated in a closed carbon crucible and kept at 1000 ^° C. for 24 hours. Subsequently, the interface is assessed microscopically.

Example 1 :

5 450 g of TiB ₂ powder (d 50 = 2 μm, 1.7% by weight of oxygen, 0.15% by weight of carbon, 0.077% by weight of iron), 30 g of tungsten carbide (d 50 <1 μm) , 10 g boron amorphous (purity 96.4%, d50 <1 μm), 8 g B ₄ C (d 50 = 0.7 μm) and 2 g Aluminum oxide (boehmite as starting material) is dispersed in aqueous solution together with 10 g of polyvinyl alcohol with an average molar mass of 1,500 as binder, 20 g of stearic acid as pressing aid and 20 g of commercial sugar in aqueous solution and spray granulated. The spray granules are cold isostatically pressed into green bodies at 1200 bar. The green bodies are heated at a heating rate of 10 K / min under vacuum to 2020 ⁰ C and held at sintering temperature for 45 minutes. Cooling takes place with switched off heating power under Ar.

The density of the obtained sintered bodies is 98% of the theoretical density.

A light micrograph of the microstructure is shown in Figure 1.

The resulting microstructure consists of a (Ti, W) B ₂ mixed crystal matrix, particulate B ₄ C and particulate boron (see TEM images in Figure 1 1).

The TEM studies performed on this sample show that the grain boundaries are free of oxygen and other impurities. The (Ti, W) B ₂ mixed crystal also contains low levels of aluminum.

The EDX spectrum recorded on the entire section of Figure 1 1 has only the elements Ti, W, B and Al. There is no oxygen found.

The grain boundary was also examined in the TEM using the high-resolution "spectrum imaging" method, and the line scan across the grain boundary as a function of the electron loss energy (Figure 12) shows neither an oxygen signal (532 eV) at the grain boundary nor a shift in the Ti signal (456 eV), which would occur in a Ti-containing secondary phase.

Subsequently, a sample of the dimension 10 × 10 × 10 mm ^{3 is} subjected to a cryolite test and exposed to a Cryolithschmelze for 24 hours at 1000 ⁰ C. Subsequent microstructural examination of the sample shows that the grain boundaries are stable compared to the cryolite attack (see Figure 2). Example 2:

450 g of TiB ₂ powder (d 50 = 2 μm, 1.7% by weight of oxygen, 0.15% by weight of carbon, 0.077% by weight of iron), 30 g of tungsten carbide (d 50 <1 μm), 10 g of boron amorphous (purity 96.4%, d50 <1 μm), 8 g of B ₄ C (d50 = 0.7 μm) and 2 g of aluminum oxide (boehmite as starting material) are mixed with 10 g of polyvinyl alcohol with a medium Molar mass of 1,500 as a binder and 20 g of stearic acid as a pressing aid dispersed in aqueous solution and spray granulated. The spray granules are cold isostatically pressed into green bodies at 1200 bar. The green bodies are heated at a heating rate of 10 K / min in vacuo to 1650 ⁰ C, the holding time at 1650 ° C is 45 min, then is heated at 10 K / min to 2020 ⁰ C and 45 min held at sintering temperature. Cooling takes place with switched off heating power under Ar.

The density of the resulting sintered bodies is 97.8% of the theoretical density.

A light micrograph of the microstructure is shown in Figure 3.

The resulting microstructure consists of a (Ti, W) B ₂ mixed crystal matrix, particulate B ₄ C and particulate boron.

Oxide impurities in the grain boundary can be removed by evaporation and the reduction of oxides during the additional annealing at 1650 ⁰ C.

The corrosion test in cryolite (24h 1000 ⁰ C) shows no penetration across the grain boundaries (Figure 4).

Reference Example 1:

450 g of TiB ₂ powder (d 50 = 2 μm, 1.7% by weight of oxygen, 0.15% by weight of carbon, 0.077% by weight of iron), 30 g of tungsten carbide (d 50 <1 μm), 10 g of boron amorphous (purity 96.4%, d50 <1 μm), 8 g of B ₄ C (d50 = 0.7 μm) and 2 g of aluminum oxide (boehmite as starting material) are mixed with 10 g of polyvinyl alcohol with a medium Molar mass of 1,500 as binder and 20 g stearin acid as a pressing aid dispersed in aqueous solution and spray granulated. The spray granules are cold isostatically pressed into green bodies at 1200 bar. The green bodies are heated at a heating rate of 10 K / min under vacuum to 2020 ⁰ C and 45 minutes held at sintering temperature. Cooling takes place with switched off heating power under Ar.

The density of the obtained sintered bodies is 97.9% of the theoretical density.

A light micrograph of the microstructure is shown in Figure 5

The resulting microstructure consists of a (Ti, W) B ₂ mixed crystal matrix, particulate B ₄ C, a particulate Ti-Al-BO phase and a continuous amorphous oxygen-containing grain boundary film. Due to the formation of a continuous approximately 2 nm thick oxygen-containing grain boundary film, the material has a grain boundary penetration of cryolite at 1000 ⁰ C. Due to grain boundary corrosion, massive material disintegration occurs (Figure 6)

Reference Example 2:

450 g of TiB ₂ powder (d 50 = 2 μm, 1.7% by weight of oxygen, 0.15% by weight of carbon, 0.077% by weight of iron), 30 g of tungsten carbide (d 50 <1 μm), 15 g of boron amorphous (purity 96.4%, d50 <1 μm), 10 g of B ₄ C (d50 = 0.7 μm) and 2 g of aluminum oxide (boehmite as starting material) are mixed with 10 g of polyvinyl alcohol with a medium Molar mass of 1,500 as a binder and 20 g of stearic acid as a pressing aid dispersed in aqueous solution and spray granulated. The spray granules are cold isostatically pressed into green bodies at 1200 bar. The green bodies are heated at 10 K / min under vacuum to 2020 ⁰ C and 45 minutes held at sintering temperature. Cooling takes place with switched off heating power under Ar.

The density of the obtained sintered bodies is 96.9% of the theoretical density.

A light micrograph of the microstructure is shown in Figure 7 Compared to Examples 1 and 2, corrosion across the grain boundary upon contact with cryolite melt is evident (Figure 8), resulting in grain boundary precipitates that are not cryolite stable.

Example 3:

Production of a thermocouple protective tube:

The spray granulate from Example 1 (bulk density 1, 12 g / cm ³ , residual moisture 0.4%, d50 = 51 microns) is cold isostatically closed on one side

Hollow tube with the dimensions 764 mm in length and 31, 5 mm diameter pressed. The sintering cycle is the same as in Example 1. The

Longitudinal shrinkage is 16.9%, transverse shrinkage 20.6%. The

Sinter density is 98% of the theoretical density. The sintered tube is post-densified at 2000 ^° C. at 1,950 bar. The density after re-densification is 99.3% of the theoretical density.

Reference Example 3: (starting mixture without Al compound as sintering aid)

450 g of TiB ₂ powder (d50 = 2 μm, 1.7% by weight of O, 0.15% by weight of C, 0.077% by weight of Fe), 30 g of WC (d50 <1 μm), and 2O g B amorphous (purity 96.4%, d50 <1 micron) are dispersed together with 10 g of polyvinyl alcohol having an average molecular weight of 1,500 as a binder and 20 g of stearic acid as a pressing aid in aqueous solution and spray granulated. The spray granules are cold isostatically pressed into green bodies at 1200 bar. The green bodies are heated at 10 K / min in vacuo to 2,170 ⁰ C and 45 minutes held at sintering temperature. Cooling takes place with switched off heating power under Ar. The sintered body is subsequently recompressed at 1,950 bar Ar pressure for one hour at 2,000 ⁰ C. The density is 97.9% of the theoretical density.

A light micrograph of the microstructure is shown in Figure 9.

The resulting microstructure consists of a (Ti, W) B ₂ mixed crystal matrix and particulate boron carbide, partly in the grain boundary and partly in the

Mixed crystal grain lies. The average grain diameter is about 100 microns. For compaction, a higher sintering temperature was needed here. The result is a coarse-grained structure.

This material was also subjected to a cryolite test. Compared to Examples 1 and 2, corrosion across the grain boundary upon contact with cryolite melt is evident (Figure 10). The material is not cryolite resistant.

Claims

claims

1. A sintered material based on transition metal diborides, comprising a) as the main phase 90-99 wt .-% of a feinkörnigen Übergangsmetalldibo- or transition metal diboride mixed crystal of at least two transition metal diborides or mixtures of such diboride mixed crystals or mixtures of such diboride mixed crystals with a or more transition metal diborides, wherein the transition metals of the IV. to VI. B) as secondary phase 1 -5 wt .-% particulate boron carbide and / or silicon carbide and c) optionally as a third phase up to 5 wt .-% of a non-continuous, oxygen-containing grain boundary phase.

2. Material according to claim 1, wherein the main phase a) has an average particle size of less than 20 microns, preferably less than 10 microns.

3. Material according to claim 1 and / or 2, wherein the boron carbide and / or silicon carbide of the second phase b) has an average particle size of less than

20 microns, preferably less than 5 microns.

4. Material according to at least one of claims 1 -3, wherein the proportion of the second phase b) 1 -4 wt .-% is.

5. Material according to at least one of claims 1 -4, wherein the third phase c) is present in a proportion of up to 2.5 wt .-%.

6. Material according to at least one of claims 1 -5, wherein the transition metals of IV. To VI. Subgroup are selected from Ti, Zr, Hf, V,

Nb, Ta, Cr, Mo and W.

7. Material according to at least one of claims 1-6, wherein it is in the main phase a) fine-grained TiB ₂ and / or ZrB ₂ and / or a mixed crystal of (Ti ₁ W) B ₂ and / or (Zr, W) B ₂ and / or (Ti, Zr) B ₂ , preferably a mixed crystal of (Ti ₁ W) B ₂ and / or (Zr ₁ W) B ₂ , more preferably the mixed crystal (Ti, W) B ₂ or the mixed crystal (Zr, W) B ₂ .

8. Material according to at least one of claims 1-7, wherein the proportion of WB ₂ in the main phase a) ≤ 7 wt .-% is.

9. A powdery sinterable mixture for producing a sintered material based on transition metal diborides, containing

1) 0.05-2% by weight of Al and / or Si as metallic Al and / or Si and / or an amount of an Al and / or Si compound corresponding to this content,

2) optionally at least one component selected from carbides and borides of transition metals of IV. To VI. Subgroup of the periodic table,

3) 0.5-12 wt% boron, 4) 0-5 wt% boron carbide and / or silicon carbide,

5) 0-5 wt .-% carbon and / or a carbon compound, each based on the content of elemental carbon, and

6) as the remainder at least one transition metal diboride of IV. To VI. Subgroup of the periodic table, which is different from the transition metal boride of the above component 2).

10. Mixture according to claim 9, wherein the proportion of component 1) 0.2-0.6 wt .-% is.

1 1. A mixture according to claim 9 and / or 10, wherein the proportion of the component 2) ≥ 0.25 wt .-% is.

12. A mixture according to any one of claims 9 to 1 1, wherein the transition metal diboride of component 6) has an average particle size of ≤ 4 microns, preferably ≤ 2 microns.

13. Mixture according to at least one of claims 9 to 12, wherein the transition metals of IV. To VI. Subgroup are selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W.

14. A mixture according to any one of claims 9-13, wherein component 2) is tungsten carbide.

15. Mixture according to at least one of claims 9-14, wherein the transition metal diboride of component 6) TiE $ ₂ and / or ZrB ₂ is.

16. Mixture according to at least one of claims 9- 15, wherein the proportion of component 5) 0, 1 - 1 wt .-% is.

17. A method for producing a sintered material according to at least one of claims 1 -8 by hot pressing or hot isostatic pressing or gas pressure sintering or spark plasma sintering a powdery mixture according to at least one of claims 9-16, optionally with the addition of organic binders and pressing aids.

18. A method for producing a sintered material according to at least one of claims 1 -8 by pressure-sintering, comprising the steps: a) mixing a powdery mixture according to at least one of claims 9-16, optionally with the addition of organic binding and pressing aids in water and b) production of a powder granulate from the powder suspension, c) pressing the powder granules into green bodies of high density, and d) pressureless sintering of the green bodies obtained under reduced pressure or under protective gas at a temperature of 1,800-2,200 ° C. ,

19. The method of claim 18, wherein the preparation of the powder granules in step b) is carried out by spray drying.

20. The method of claim 18 and / or 19, wherein the preparation of the green body in step c) by axial pressing, cold isostatic pressing, extrusion, injection molding, slip casting or pressure slip casting follows.

21. The method according to at least one of claims 18-20, wherein the green bodies obtained in step c) are annealed prior to pressure-sintering in an inert atmosphere at temperatures below the sintering temperature.

22. The method according to at least one of claims 18-21, wherein the Drucklossintern in step d) at a temperature in the range of 1, 900-200 ⁰ C, preferably about 2,000 ⁰ C is performed.

23. The method according to at least one of claims 18-22, wherein the non-pressure sintered material is densified by hot isostatic pressing.

24. Use of the sintered material according to at least one of claims 1 to 8 as a corrosion protection material for salt and metal melts, in particular cryolite-containing melts.

25. Use of the sintered material according to at least one of claims 1 to 8 for the production of thermocouple protective tubes, in particular for cryolite-containing melts.

26. Use of the sintered material according to at least one of claims 1 to 8 as electrode protection material, electrode material or material for the cell lining in the Al fused-salt electrolysis.

27. Use of the sintered material according to at least one of claims 1 to 8 as electrode material for sliding contacts, welding electrodes and erosion pins.