WO2000032537A2

WO2000032537A2 - Reaction-bonded silicon nitride-based materials and method for producing the same

Info

Publication number: WO2000032537A2
Application number: PCT/EP1999/009341
Authority: WO
Inventors: Lothar SCHÖNFELDER; Gerhard WÖTTING
Original assignee: Cfi Ceramics For Industry Gmbh & Co. Kg
Priority date: 1998-12-03
Filing date: 1999-12-01
Publication date: 2000-06-08
Also published as: DE19855811A1; EP1144337A2; AU3148600A; WO2000032537A3

Abstract

The invention relates to reaction-bonded silicon nitride-based materials which contain silicon nitride (Si3N4), silicon carbide (SiC) and silicon oxynitride (Si2N2O) as crystalline phases, which have a phase consistency of silicon of ≤ 1 % and very good mechanical properties and are very stable in oxidizing conditions at high temperatures. The invention also relates to a method for producing these materials from a mixture of silicon, silicon nitride and organic silicon compounds (preferably polysiloxanes and/or polycarbosilanes) by thermally treating said mixture in a nitrogen-containing atmosphere, so that the organic silicon compounds are pyrolized and nitrided in the presence of silicon. The invention also relates to the use of the materials for producing ceramic components.

Description

Reaction-bound materials based on silicon nitride and process for their production

The invention relates to reaction-bound silicon-containing materials and a method for their production.

In the group of silicon-containing materials, reaction-bonded silicon nitride (RBSN) is of technical importance. RBSN is made from silicon powder in the reaction bond process by reaction with nitrogen-containing reaction gases. This reaction produces a porous silicon nitride (Si ₃ N ₄ ) material which, depending on the shaping process used for the silicon powder, still has a porosity of 15 to 30%. Because of this relatively high porosity, the use of this material in the area of high temperatures and especially under simultaneous oxidizing conditions is severely restricted. The oxidation of the reaction-bound silicon nitride leads to severe losses in the mechanical properties of the material. The reaction-bound silicon nitride material is, however, of importance for that due to the inexpensive silicon raw materials that can be used and easy processing, which can advantageously be carried out in the so-called green state or after pre-nitriding

Manufacture of industrial ceramic products.

From the point of view of improving the mechanical properties and reducing the defects in ceramic or metallic materials and thus improving their reliability, DE 43 18 974 A1 proposes the use of organosilicon compounds as binders in combination with plasticizers. Compared to the commonly used temporary binders, however, organosilicon compounds are also precursors for ceramic materials. This use of organosilicon compounds for the production of ceramic powders, fibers or ceramic materials Pyrolysis processes have been described (DE 28 03 658 AI, J. Amer. Ceram. Soc. 78 (4) 835-848 (1995)).

DE 26 50 083 AI discloses the use of silicone resin for solidifying silicon powder moldings by curing, which enables the joining of individual moldings and machining. A lesson on improving the oxidation resistance of RBSN cannot be derived from this.

The processes described do not provide any information as to how the oxidation stability of reaction-based materials based on silicon nitride can be improved and thus the reduction in the mechanical properties of these materials due to oxidation damage.

The object of the present invention is therefore to develop materials based on silicon nitride with improved properties. The focus is on the mechanical properties, the density or porosity and the behavior under oxidizing conditions at high temperatures. This is intended to significantly improve the application behavior and the area of application for corresponding materials.

DE 30 45 010 C2 describes a SiC-Si ₃ N ₄ composite system which is characterized by high mechanical strength and high resistance to thermal shock. To produce them, special organic silicone polymers with silicon powder with a grain size <44 μm are wet milled, shaped and subjected to a thermal treatment between 1200 ° C and 1800 ° C, which results in a

Microstructure composed of β-SiC and α- and β-Si ₃ N _{4 in} addition to an open porosity of> 7.9%. This structure is described as an interwoven texture with no chemical bond or solid solution with micro vacancies between SiC and Si ₃ N ₄ , which differs fundamentally from conventional SiC-Si ₃ N ₄ composite materials. A disadvantage of the composite system described is that only very special organic silicone polymers can be used for its production. The use of simple polysilanes or polysiloxanes is not possible.

The object of the present invention is achieved by reaction-bound materials which contain silicon nitride (Si ₃ N ₄ ), silicon carbide (SiC) and silicon oxynitride (Si ₂ N ₂ O) as well as any remaining unreacted silicon as the X-ray-detectable crystalline phases, and which contain a unreacted silicon can be produced in which mixtures of silicon, organic silicon compounds and silicon nitride or moldings from these mixtures are thermally treated in a nitrogen-containing gas atmosphere. In the process according to the invention, the organosilicon compounds are pyrolyzed in the presence of silicon and silicon nitride and nitrided in the subsequent step.

The starting substances are preferably in powder form. Powder qualities with average grain sizes <10 μm are advantageously used as silicon powder. Powder qualities with average grain sizes <3 μm are advantageously suitable as silicon nitride powder.

Polysiloxanes, polysilazanes,

Polysilanes or polycarbosilanes or copolymers of these compounds are used. In addition to silicon, these compounds can include carbon, nitrogen, hydrogen and / or oxygen and other heteroatoms such as Contain boron, titanium, zirconium, phosphorus or aluminum.

On the one hand, the organic silicon compounds used have the function of a binder. As a result, molded articles can be produced from the raw material mixture using the known methods, for example cold pressing, hot pressing, isostatic pressing, injection molding or extrusion. On the other hand, they are broken down into ceramic phases and reactants by pyrolysis in an inert atmosphere and are thus included in the structure of the material. Thus, in the process according to the invention, the organic silicon compounds are pyrolyzed, and in the subsequent nitridation step a complex course of the reaction results, on the one hand the nitridation of the silicon powder and on the other hand the reaction of the pyrolysis products of the organosilicon

Compounds comprises a multi-phase material, which contains silicon nitride (Si ₃ N ₄ ), silicon carbide (SiC) and silicon oxynitride (Si ₂ N ₂ O), as well as any remaining unreacted Si as the crystalline phases which can be detected by X-ray.

The material according to the invention thus clearly differs from the material disclosed in DE 30 45 010 C2 with regard to the phase inventory which can be detected by X-ray analysis.

It has proven to be extremely advantageous in the production of the material according to the invention or of components made therefrom that pyrolysis and nitridation generally only result in a small linear shrinkage of <5% and at the same time the open porosity is <13% by volume. The low shrinkage reduces the risk of warping or other damage to complex-shaped parts during the thermal treatment. The significantly reduced open porosity of the material according to the invention compared to conventional RBSN leads to a markedly improved oxidation resistance.

The process according to the invention is advantageously carried out in such a way that the free silicon is converted almost completely, i.e. practically no free silicon can be detected in the material according to the invention by X-ray diffraction, which is the case at contents of <1% by weight.

According to the process according to the invention, silicon nitride powder is added to the starting mixture of silicon powder and organosilicon compounds. This addition above all improves the oxidation resistance of the material at high temperatures. As shown in the examples below goes, these materials have significantly improved mechanical properties after an oxidation treatment compared to those which are made from silicon nitride-free mixtures.

To test the behavior of the materials under oxidizing conditions at high temperatures, appropriate tests were carried out at 1400 ° C to 1500 ° C in air and the mechanical properties and the change in weight as a result of the oxidation were measured. Cyclic oxidations were carried out as particularly stressful operating conditions, i.e. Material samples were repeatedly cooled between 1400 ° C and approx. 100 ° C. Compared to an isothermal

Oxidation treatment means this temperature change places a greater strain on the material. These cyclical loads also simulate real operating conditions, such as B. when using ceramic components for combustion chamber linings and crucibles.

Compared to the pure RBSN material, produced by nitridation of silicon powder, the nitridation products according to the invention, starting from mixtures of silicon powder, silicon nitride powder and organosilicon compounds, have significantly improved properties under oxidizing conditions. This is especially true for the room temperature flexural strength after an oxidizing treatment. In addition, these materials have significantly improved mechanical properties at high temperatures both before and after an oxidation treatment. Corresponding measurements are listed in Table 2. Table 2 shows that the reaction-bound material based on Si ₃ N ₄ according to the invention after an oxidation treatment in air at 1400 ° C., 100 h, statically or cyclically at intervals of 10 h, for example a flexural strength at room temperature of more than 40% of the flexural strength before oxidation. It preferably even has> 80% of the bending strength before the oxidation.

Materials with very good high temperature resistance were previously out

DE 30 45 010 C2 known. However, these materials are very complex as pha- pure SiC-Si ₃ N ₄ composite. It is therefore all the more surprising that materials that contain SiC and Si ₃ N ₄ as well as Si ₂ N ₂ O as crystalline phases and that can be produced using cheaper raw materials and simple process technology achieve such a high level of high-temperature resistance.

The open porosity of the material is of crucial importance for the oxidation of the RBSN. The open porosity is associated with a high inner surface, the reaction of which with oxygen leads to damage to the material structure as a result of SiO ₂ formation. A key figure for the degree of oxidation is the weight increase normalized to the sample surface, given in mg / cm ² . RBSN has a relatively high oxidation-related weight gain and is associated with a strong oxidation-related decrease in strength.

In contrast, the materials according to the invention are characterized by significantly lower open porosities, which are typically below 13% by volume. Compared to the prior art for reaction-bonded silicon-containing materials, the change in weight due to oxidation for the materials according to the invention is significantly reduced and thus the drop in strength is also significantly less. Because of these outstanding properties, the materials according to the invention are suitable for high-temperature applications, in particular under oxidizing conditions such as in turbines and combustion chambers.

This increased oxidation stability can be seen in the certain lower weight increases after the oxidation treatment and above all in the measured flexural strengths after the oxidation (Examples 1b, 1c in comparison to, for example, la). The materials according to the invention, starting from raw material mixtures containing silicon nitride, have only a very slight or no drop in strength as a result of the oxidation.

The materials according to the invention are produced from mixtures of silicon, silicon nitride and an organic silicon compound, the proportion of individual mixture components can vary within a comparatively wide range. Thus the starting mixture can contain 15-90% by weight of silicon, 5 to 60% by weight of silicon nitride and 5 to 60% by weight of the organic silicon compound, preferably polysiloxane and / or polycarbosilane and / or copolymers of these compounds , contain. Si contents below 15% by weight and Si ₃ N ₄ contents above 60% by weight lead to the fact that the porosity reduction resulting from the nitriding of Si to Si ₃ N _{4 is} no longer sufficient to produce a material with an open To obtain porosity <13 vol%. This has a negative impact on the mechanical properties and the oxidation resistance of the material. Si contents> 90% by weight allow only low concentrations of Si ₃ N ₄ powder and the organic silicon compound, which means that the desired phase inventory, which is responsible for the positive properties of the material, no longer occurs. This also justifies the specified lower limits of> 5% by weight for the Si ₃ N ₄ powder and the organic silicon compound. The upper limit for the organic silicon compound is set at 60% by weight. Even higher levels would lead to high shrinkage values and an undesirably high open porosity during pyrolysis.

The organic silicon compounds used can additionally contain heteroatoms, such as B, Ti, P, Zr and / or Al, which after pyrolysis of the organic silicon compound react with matrix constituents or the gas atmosphere to give the corresponding oxides, carbides, nitrides and / or carbonitrides. The increases in volume usually associated with these reactions promote the achievement of the low open porosity of the material according to the invention of <13% by volume. On the other hand, these new formations enable the setting of very specific properties, e.g. regarding the electrical and / or tribological

Properties and / or wetting behavior towards liquids, e.g. Lubricants or (metallic) melts.

To set specific properties, it is also possible to add appropriate inert substances, such as BN, TiN, TiB ₂ or MoSi ₂ , to the starting mixture. A large number of substances are conceivable for this, provided that they are - ö -

tion conditions in the presence of the base materials are thermodynamically stable and neither impair the achievement of the desired phase inventory nor lose their specific properties through reaction.

Furthermore, metals or metallic compounds with a catalytic effect on the nitridation reaction can advantageously be added to the raw material mixtures. In particular, e.g. Molybdenum, manganese or iron in powder form and concentrations <5% by weight have proven to be favorable for the catalysis of the nitridation reaction.

Components which bring about a reinforcement of the materials in the form of fibers, as short or long fibers, whiskers, platelets or particles can furthermore advantageously be introduced into the raw material mixtures according to the invention. In order to achieve the advantageous open porosity of <13% by volume according to the invention, post-infiltrations with the organic silicon compound and additional ones can be carried out

Pyrolysis steps become necessary.

Examples 1b, 1c and 3 show a typical process for producing the materials according to the invention. The material is advantageously produced by intensively mixing Si powder with an average particle size <10 μm and Si ₃ N ₄ powder with an average particle size <3 μm, the organic silicon compound and optionally other of the additives described by wet and / or dry grinding become. Si powder with average particle sizes> 10 μm lead to long nitriding times and the risk that unreacted silicon remains in the material in a concentration greater than 1% by weight. Si ₃ N ₄ powders with average particle sizes> 3 μm can already lead to material inhomogeneities and reduce the mechanical properties and oxidation resistance. For both components, purities of> 98% are advantageous in order to avoid the formation of foreign inclusions of critical size in the material. After the mixed grinding, the shaping is carried out by the customary methods or also by hot pressing, the thermal crosslinking of the polymer, the pyrolysis of the organic silicon compound under inert gas and the nitriding. This takes place for the raw material mixtures according to the invention or shaped bodies produced therefrom in a nitrogen-containing gas atmosphere. In addition to nitrogen, the reaction gas can additionally contain hydrogen and / or ammonia gas. The nitridation reaction can take place either under normal pressure or under elevated gas pressure, preferably from 1 to 100 bar. The maximum temperatures for this nitridation reaction are advantageously from 1300 to 1600 ° C. The temperature-time curve for the nitriding reaction has to be adapted to the respective specific conditions, such as furnace size and component volume.

If the material according to the invention is to be used as the matrix material of long fiber-reinforced ceramic composites, the basic composition is preferably already prepared in an organic medium in which the organic

Silicon compound is soluble. For example, isopropanol, butanol, ethoxylethanol or xylene can be suitable as solvents. The ratio of the solids to the organic medium is to be adjusted so that a viscosity suitable for the further processing of the suspension is present. Further processing can be carried out according to the known fiber coating and winding process, polygons for flat laminate panels, pipes or more complex parts being wound according to the winding cores used. Laminates are then stacked and meshed, and finished parts are meshed directly with regard to the target geometry. The crosslinking follows, possibly after an intermediate processing, the pyrolysis and nitridation, as already described.

Due to their good mechanical properties and in particular their resistance to oxidation degradation at high temperatures, the materials and components according to the invention are suitable for corresponding operating conditions, such as turbines and combustion chambers, and for processing metallic melts. In the following the invention will be described with reference to examples, without that here _¬ derived from a restriction of Wekstoffe and methods of the invention for the preparation thereof.

Examples

Example la (comparative example)

1500 g silicon powder (average grain size 3.0 μm, specific surface after

BET = 3.9 m ² / g) and 500 g of a methylpolysiloxane were mixed in isopropanol and then the solvent was distilled off. After the mixture had dried in a heating cabinet, it was pulverized with steel balls in steel containers on a roller bench and sieved through a 100 μm sieve. The powder was pressed into disks in an axial press and isostatically compressed again at 1600 bar. The

The polymer was crosslinked at 250 ° C., the pyrolysis at 900 ° C. in an inert gas atmosphere. The molded articles are nitrided in a nitrogen atmosphere. The samples were heated up to 1450 ° C, with a total process time of 74 hours.

The properties of the nitrided samples before and after an oxidation treatment are listed in Table 1.

Example lb

Silicon powder (900 g), methylpolysiloxane (500 g), qualities as in Example la, and 600 g of silicon nitride powder with an average grain size of 0.5 μm, BET specific surface area = 13.8 m ² / g were as in Example la mixed and processed.

The properties of the nitrided and oxidized samples are listed in Table 1.

Example lc

The mixture was prepared as in Example 1b, the nitriding of the test specimens was carried out in a nitrogen atmosphere up to 1450 ° C., with an entire sample duration of 130 hours. In addition to the basic characterization properties listed in Table 1, material samples according to Example 1c were further investigated with regard to their cyclic and isothermal oxidation behavior between 1400 ° C. and 1500 ° C. up to 1000 hours of aging.

The properties of the nitrided or oxidized samples determined in this way are summarized in Table 2. According to the X-ray phase analysis, the materials according to Examples 1b and 1c have Si ₃ N ₄ , SiC and Si ₂ N ₂ O as crystalline phases. The Si content is below the detection limit of <1%, the shrinkage during production is well below 5% and the open porosity <13% by volume.

Example 2 (comparative example)

In comparison with the materials according to the invention according to Examples 1b and 1c, silicon powder was used without further additives and, as in Example 1a, nitrided to form reaction-linked silicon nitride (RBS R). The results are shown in Table 1; according to the X-ray phase analysis, the material shows only - and ß-Si ₃ Ν ₄ as crystalline phases.

In order to test the oxidation resistance, the samples were produced according to the

Examples la-c and 2 annealed at 1400 ° C in ambient air. A cyclical temperature-time profile also exposed the materials to a change in temperature. The holding time at 1400 ° C was 10 hours per cycle, a total of 10 cycles were run through. An oxidation cycle was carried out with the following temperature-time control:

Room temperature up to 1400 ° C with 3K / min / 1400 ° C, 10 hours holding time / 1400 ° C to approx. 100 ° C in 10 hours.

To assess the resistance of the materials to oxidation, the

Weight increase normalized to the sample surface A and in mg / cm ² in the Tables 1 and 2 listed. As these results show, compared to pure RBSN (comparative example 2), the materials according to the invention show a significantly smaller weight gain as a result of oxidation.

Accordingly, the residual strengths after the oxidation are significantly higher or there is no decrease in strength as a result of the oxidation (Table 1, Example 1c).

The advantageous effect of the addition of silicon nitride powder to the silicon raw material is illustrated by the examples 1b, 1c according to the invention (Tables 1 and 2). The corresponding materials show no or only a very slight drop in strength as a result of the oxidation, in comparison to the silicon nitride-free mixture (comparative example la).

Table 1

a) Double ring bending strength, sample dimensions: diameter approx. 50 mm, height approx. 4 mm, load ring radius 8 mm, support ring radius 16 mm b) 4-point bending strength, sample dimensions: 3 x 4 x 45 mm, supports 40/20 mm c) cyclic oxidation 1400 ° C, 100 hd) mercury buoyancy method e) mercury pressure porosimetry f) based on room temperature bending strength before the oxidation treatment

Table 2

Oxidation tests for material samples according to example lc

Mechanical properties

4-point bending strength, test specimen dimensions: 3 x 4 x 45 mm supports: 40/20 mm

Room temperature - flexural strength before oxidation 255 MPa

Flexural strength 1400 ° C 380 MPa

Flexural strength 1400 ° C, after 100 h cyclic oxidation 1400 ° C 350 MPa

Room temperature bending strength after cyclic oxidation, 1400 ° C, 100 h 260 MPa after isothermal oxidation, 1450 ° C, 100 h 280 MPa after isothermal oxidation, 1500 ° C, 100 h 230 MPa after isothermal oxidation, 1500 ° C, 1000 h 259 MPa

Specific weight changes after oxidation

Weight change Δm / A [mg / cm ² ] cyclic oxidation, 1400 ° C, 100 h 0.55 isothermal oxidation, 1450 ° C, 100 h 0.32 isothermal oxidation, 1500 ° C, 100 h 0.26 isothermal oxidation, 1500 ° C, 1000 h 0.80 Example 3

Mixtures of silicon, silicon nitride and methylpolysiloxane were produced according to Example 1b with variation of the individual mass fractions and the nitrided samples were characterized (Table 3).

Table 3

a) Double ring bending strength, sample dimensions: diameter 50 mm, height 4 mm, load ring radius 8 mm, support ring radius 16 mm b) mercury pressure porosimetry c) mercury buoyancy method

The materials resulting from the variation of the starting compositions all have an open porosity <13% by volume. Si ₃ N ₄ , SiC and Si ₂ N ₂ O phases were determined by X-ray diffraction in different ratios, the residual silicon content is below the detection limit of <1%, the shrinkage occurring during production is clearly below 5% .

Claims

claims

1. Reaction-bonded materials based on silicon nitride, characterized in that they contain silicon nitride (Si ₃ N ₄ ), silicon carbide (SiC) and silicon oxynitride (Si ₂ N ₂ O) as crystalline phases and the phase inventory of silicon is <1% .

2. Materials according to claim 1, characterized in that the open porosity is <13 vol .-% and the linear shrinkage during pyrolysis and nitridation is <5%.

3. Materials according to at least one of claims 1 and 2, characterized in that after an oxidation treatment in air at 1400 ° C, it has a room temperature bending strength of over 40% of the bending strength before the oxidation.

4. Materials according to claim 3, characterized in that the oxidation treatment in air is carried out cyclically between 100 ° C and 1400 ° C, that 5 cycles are carried out and that the holding time at 1400 ° C is 10 hours each.

5. Materials according to at least one of claims 3 and 4, characterized in that it has a room temperature bending strength after the oxidation of more than 80% of the bending strength before the oxidation.

Materials according to at least one of claims 1 to 5, characterized in that they contain molybdenum, manganese, iron and / or compounds of these elements in a concentration below 5 wt .-%.

7. Materials according to at least one of claims 1 to 6, characterized in that they contain inorganic short or long fibers, whiskers, platelets and / or particles of inert substances.

8. Materials according to claim 7, characterized in that they contain inorganic long fibers.

9. A method for producing a material according to claims 1 to 8, characterized in that 15 - 90 wt .-% silicon with an average particle size <10 microns, 5 - 60 wt .-% silicon nitride with an average

Particle size <3 microns and 5 - 60 wt .-% of an organic silicon compound intensively mixed by wet and / or dry grinding, shaped, crosslinked and pyrolysed under inert gas and then at T <1600 ° C in N ₂ or N ₂ -inert gas atmosphere Gas pressures of <100 bar can be nitrided.

10. The method according to claim 9, characterized in that the mixture of silicon, silicon nitride and an organic silicon compound additionally molybdenum, manganese, iron and / or compounds of these elements in a concentration below 5 wt .-% and / or inorganic short or long fibers , Whiskers, platelets and / or particles of inert substances are added.

11. The method according to at least one of claims 9 to 10, characterized in that polysiloxanes, polysilazanes, polysilanes and / or polycarbosilanes and / or copolymers of these compounds are used as organic silicon compounds.

12. The method according to at least one of claims 9 to 11, characterized in that the organic silicon compounds heteroatoms such as.

Contain boron, titanium, phosphorus, zirconium and / or aluminum.

13. The method according to at least one of claims 9 to 12, characterized in that according to known fiber coating and winding processes laminates are produced from a preferably organic suspension of the starting components, these after crosslinking to larger

Units are stacked and laminated, followed by pyrolysis and nitridation.

14. Use of the materials according to at least one of claims 1 to 8 as a ceramic component, in particular at high temperatures under oxidizing conditions, such as in turbines and combustion chambers, as well as processing metal melts.