DE102012207803A1 - Silicon nitride ceramics and process for their preparation - Google Patents

Abstract

The present invention relates to a sintered silicon nitride ceramic, which in particular forms a component of a rolling or sliding bearing. Furthermore, the invention relates to a method for producing such a silicon nitride ceramic. The method first comprises a step in which silicon nitride Si3N4 is provided. Furthermore, a first additive is provided which is in the form of primary particles having an average primary particle size of less than 1 μm. The first additive acts as a sintering aid. In a further step of the method according to the invention, the silicon nitride and the first additive are mixed to form a mixture. The mixture is then formed into a green body, which is then sintered to a silicon nitride ceramic. In addition to the silicon nitride, the first additive also becomes part of the silicon nitride ceramic.

Description

The present invention relates to a sintered silicon nitride ceramic, which in particular forms a component of a rolling or sliding bearing. Furthermore, the invention relates to a method for producing such a silicon nitride ceramic.

Sintered silicon nitride ceramics are a frequently used material in mechanical and plant engineering, in the chemical industry, in foundry technology, in electronics, and because of their high strength, fracture toughness, wear, corrosion and thermal shock resistance as well as their low density and low thermal expansion in aerospace engineering.

The EP 0 587 119 B1 shows a silicon nitride sintered body with high thermal conductivity and a method for its preparation. According to this method, 2.0 wt .-% to 7.5 wt .-% of one or more rare earth elements are added in the form of the respective oxide as an additive to the silicon nitride. In preferred embodiments, yttria and alumina are used as individual additives. In these embodiments, sintering takes place over a period of six hours at a temperature of 1,900 ° C.

From the DE 23 53 093 B2 For example, a method for producing a silicon nitride-based sintered ceramic using aluminum oxide powder and magnesium oxide powder as additives for sintering has been known. According to a preferred embodiment, the alumina powder and the magnesia powder are already reacted before sintering. The alumina powder and the magnesia powder are heated at a temperature of 1,600 ° C to 1,800 ° C for three to ten hours to produce spinels. Subsequently, the spinels are finely pulverized to achieve a particle size of less than 50 microns.

The DE 37 34 274 A1 shows an electrically insulating, ceramic sintered body, which preferably consists of a Siliziumnitridkeramik. In the manufacture of the sintered body, spinel MgAl ₂ O ₄ is added as an additive.

DE 40 13 923 C2 describes a silicon nitride ceramic with a sintering additive of different oxides which are added in the form of their mixtures and which after the dry mixing of the powders give a spinel-like composition (Sections [0082] and [0094]).

The DE 694 27 510 T2 shows a method for producing a silicon nitride based sinter. In this method, the stoichiometric spinel structure MgO.Al ₂ O _{3 is used} as the sintering additive (max 6% by weight).

The DE 602 18 549 T2 shows a rolling bearing element with a silicon nitride sintered body, which also contains 1 mass% to 10 mass% of rare earth metals in the form of oxides in addition to silicon nitride. To produce this rolling bearing element, after completion of the sintering, preferably a hot isostatic pressing treatment (HIP) is carried out in a non-oxidizing atomic sphere of at least 300 atm (30 MPa) and at a temperature of 1,600 ° C to 1,860 ° C.

The object of the present invention, starting from the prior art, is to provide a silicon nitride ceramic and a method for the production thereof, wherein a comparatively low-wall production leads to a silicon nitride ceramic which has both a high strength and fracture toughness, a largely non-porous microstructure and a small amount Having sinter skin.

The above object is achieved by a method according to the appended claim 1 and by silicon nitride ceramics according to the attached independent claims 7 and 8.

The inventive method is used to produce a sintered silicon nitride ceramic, which is designed in particular as a component of a rolling or sliding bearing, for example as a bearing ring or as a rolling element. The method first comprises a step in which silicon nitride Si ₃ N _{4 is} provided. The silicon nitride is preferably provided as a powder. Furthermore, a first additive is provided as a sintering additive, which is in the form of primary particles having an average primary particle size of less than 1 micron. These are therefore nanoscale particles, which are preferably formed as nanoparticles. The first additive also acts as a sintering aid. In a further step of the method according to the invention, the silicon nitride, the first additive and optionally further additives and additives are mixed to form a mixture. The mixture is then formed into a green compact, which then is sintered to a silicon nitride ceramic. The sintering can be done for example by gas pressure sintering or by a hot pressing process. In this case, in addition to the silicon nitride and the first additive to a component of the silicon nitride ceramic and forms a second phase, in particular a glassy binder, filling and grain boundary phase.

A significant advantage of the method according to the invention is that a particularly homogeneous distribution of the elements required for liquid phase sintering in the Si ₃ N ₄ ceramic is achieved by using the first additive. This results in a particularly advantageous sintering behavior and a pore-free or largely pore-poor microstructure, which makes it possible to dispense with a hot isostatic sintering compaction (HIP) of the Si ₃ N ₄ ceramic. Instead, according to the invention, it is also possible to produce a Si ₃ N ₄ ceramic with an approximately equivalent microstructure and properties via a less complicated gas pressure sintering or hot pressing process.

A particular advantage of the method according to the invention is also that oxidic compounds / elements can be used as sintering additives during the production process, which have hygroscopic properties in intrinsic form (eg MgO, CaO) and therefore can not normally be dispersed in water. By contrast, the first additive to be used according to the invention does not prove to be hygroscopic, but is stable even in an aqueous environment. The mixing of the silicon nitride powder and the at least first additive and the further processing can therefore also be carried out on an aqueous basis, so that it is possible to dispense with a much more expensive and dangerous solvent-based preparation.

Preferred embodiments of the method further provide a wet grinding process for the mixture, with which the desired fineness, homogeneity and surface quality of the mixture can be achieved and adjusted in a targeted manner. Subsequently, a drying of the ground mixture is preferably carried out to a fine, free-flowing granules. The subsequent shaping of the granules to the green body is preferably carried out by pressing into a mold.

The first additive is preferably formed by a multi-cation oxide. These are therefore the cations of at least two different chemical elements, in particular the cations of at least two different metals.

The first additive particularly preferably has a spinel structure, wherein the term spinel structure here expressly includes spinel-like compounds, ie. H. Spinel structures are included with non-stoichiometric compositions. These mixed oxide structures are particularly suitable as sintering aids and are also only slightly hygroscopic, so that an aqueous dispersion is possible.

In alternative preferred embodiments, the first additive has a garnet or perovskite structure.

In preferred embodiments of the method according to the invention, the first additive has the following general chemical formula: M ^II _{1 + x} M ^III ₂ O _{4 + x}

In this general chemical formula, M ^{II represents} at least one divalent metal, while M ^{III represents} at least one trivalent metal. Furthermore, 0 ≤ x ≤ 0.7. Insofar as x = 0, there is a stoichiometrically balanced chemical compound.

In preferred embodiments, the variable x in the general chemical formula given above is greater than zero, so that a superstoichiometric composition of the first additive is given. It is thus a spinel structure or a spinel-like structure, ie a spinel-like structure comprising an excessive proportion of the oxide of M ^II , which reduces the sintering additive-based glass phase during sintering, so that the silicon nitride ceramic at a relatively low melting temperature and completely compacted with a comparatively short sintering time. The variable x is particularly preferably greater than 0.2.

The average particle size of the first additive is preferably from about 10 nm to a few 100 nm. Accordingly, the average particle size of the first additive is preferably less than 500 nm. Furthermore, the average particle size of the first additive is preferably more than 50 nm.

The component M ^{II of} the general chemical formula given above is preferably one or more elements selected from the group formed by Mg, Ca, Ba and Sr.

The component M ^{III of} the general chemical formula given above is preferably one or more elements selected from the group formed by Al, Fe, Cr and Mn.

In particularly preferred embodiments of the method according to the invention, the component M ^II comprises the element Mg. The component M ^III particularly preferably comprises the element Al.

The first additive preferably has the chemical formula Mg _{1 + x} (Al, Fe) ₂ O _{4 + x} , which is a concretization of the general chemical formula given above.

The first additive preferably has the chemical formula Mg _{1 + x} Al ₂ O _{4 + x} , which is a concretization of the general chemical formula given above.

The proportion of the first additive is preferably between 10% by weight and 30% by weight of the mixture. Furthermore, the proportion of the first additive is preferably between 10 wt .-% and 15 wt .-%, more preferably between 12 wt .-% and 13.5 wt .-% of the mixture. In principle, a proportion of more than 10% by weight of the additive for use as a sintering aid is comparatively high. This high proportion leads to a comparatively large proportion of a glass phase in the sintered silicon nitride ceramic. The greater proportion of the glass phase, in addition to improved Flüssigphasensinterbarkeit ensures higher elasticity, higher toughness and improved damage tolerance.

In alternative preferred embodiments, the proportion of the first additive is between 3% by weight and 10% by weight of the mixture. In contrast, this proportion has an advantageous effect on the strength, the hardness, the corrosion resistance and the high-temperature properties.

The proportion of silicon nitride is preferably between 60% by weight and 97% by weight of the mixture, more preferably between 80% by weight and 90% by weight.

In preferred embodiments of the method according to the invention, no further additive of the mixture is added in addition to the first additive. Thus, the mixture, the green compact and the sintered silicon nitride ceramic contain only the first additive in addition to the silicon nitride. The exclusive use of the first additive as an additive ensures that the first additive is extremely homogeneously distributed in the mixture. Preferably, the mixture is free of rare earth elements and their compounds, which are dispensable for the process according to the invention.

In alternative preferred embodiments of the method according to the invention, in addition to the first additive, a second additive is provided and added to the mixture.

Also, the second additive is preferably in the form of primary particles which have an average primary particle size of less than 1 micron.

The second additive is preferably those chemical compounds used according to the prior art as an additive. In particular, the second additive is preferably selected from the group of oxides and nitrides of the elements Fe, Ti, Hf, Zr, Mo, Ta, Nb and Cr and the oxides and nitrides of the rare earth metals. Also, the second additive may comprise several of the compounds mentioned.

The proportion of the second additive is preferably at most 5 wt .-% of the mixture.

In further alternative preferred embodiments of the method according to the invention, a third additive is provided and added to the mixture. The third additive can be added both when using the first and second additive and when using the first additive alone. Also, the third additive is preferably in the form of primary particles which have an average primary particle size of less than 1 micron.

The third additive is preferably those chemical compounds which are already used according to the prior art as an additive for sintering silicon nitride. Preferred is the third additive is MgO, Al ₂ O ₃ , Y ₂ O ₃ or AlN. Also, the third additive may comprise several of said compounds.

The proportion of the third additive is preferably at most 5 wt .-% of the mixture.

In preferred embodiments of the method according to the invention, the sintering is carried out at a temperature between 1,500 ° C and 2,000 ° C, more preferably between 1,700 ° C and 1,900 ° C. In alternative preferred embodiments, the sintering is carried out at a temperature of between 1,700 ° C and 2,000 ° C.

With the method according to the invention, short times for sintering can be realized. Therefore, the duration of sintering is preferably between one minute and 60 minutes, more preferably between 20 minutes and 30 minutes. In alternative preferred embodiments, the duration for sintering is between one hour and four hours, more preferably between two hours and three hours.

The provision of the particles of the first additive preferably takes place in that the substance of the first additive is precipitated from a liquid phase. Alternatively, the provision of the particles of the first additive preferably takes place in that a flame pyrolysis of the substance of the first additive is carried out. For both deployment variants of the first additive, a preferred primary particle size and a specific surface can be achieved by the subsequent milling of coarser particles.

The sintering of the green body is preferably carried out by sintering under a gas pressure atmosphere or alternatively by a uniaxial hot pressing. Since the silicon nitride ceramic produced according to the invention already has a largely pore-free structure and a high strength, preferably no hot isostatic pressing (HIP) is carried out.

An advantage of the method according to the invention is that no or only a thin sintered skin of the silicon nitride ceramic is formed. The sintering skin produced during sintering has a thickness which is preferably less than 0.5 mm, more preferably less than 0.2 mm and more preferably less than 0.1 mm. Thus, there is no need for a measure to subsequently reduce the sinter skin. Therefore, there is preferably no measure for the subsequent reduction of the sintering skin, such as, for example, grinding of the silicon nitride ceramic. Also preferably no measure is taken to reduce the oxygen content in the edge region of the silicon nitride ceramic, such as, for example, a deoxidation treatment before sintering.

Another object of the invention is a silicon nitride ceramic, which is obtainable by the method according to the invention.

A further subject of the invention is a silicon nitride ceramic which is sintered and comprises a second phase in addition to silicon nitride. The second phase is formed by a chemical compound of silicon nitride and a first additive. The second phase has an average size of less than 1 μm

The following description of preferred embodiments relates to both of the above silicon nitride ceramics according to the invention.

The second phase is similar to a binder. Preferably, the two-phase is formed amorphous or partially crystalline.

With regard to its composition, in particular with regard to the chemical composition of the first additive and optionally further additives and their quantitative composition, the silicon nitride ceramic according to the invention preferably also has those features which are specified as preferred for the process according to the invention. This applies to the second phase also with respect to the first additive. Thus, the average size of the second phase is preferably less than 500 nm. Furthermore, the average size of the second phase is preferably more than 50 nm. The chemical composition of the second additive bound in the first additive in the sintered silicon nitride ceramic preferably resembles the chemical composition of the first additive is preferred to use according to the inventive method. The chemical composition of the further additives optionally bound in the sintered silicon nitride ceramic preferably resembles the chemical composition of the further additives which are preferably to be used according to the method according to the invention.

The silicon nitride ceramic according to the invention is characterized in that it has a high strength and at the same time good fracture toughness. Thus, the 3-point flexural strength of the silicon nitride ceramic according to the invention is at least 650 MPa, preferably at least 750 MPa. The Niihara fracture toughness is at least 6 MPam ^-0.5 at the same time. The compressive strength of the silicon nitride ceramic according to the invention is preferably at least 2,500 MPa, more preferably more than 3,000 MPa.

The silicon nitride ceramic according to the invention preferably has a morphology with predominantly acicular β-Si ₃ N ₄ crystals which are embedded in the glassy or semicrystalline grain boundary and filling phase. The acicular crystals ensure a good fracture toughness and damage tolerance of the silicon nitride ceramic. In this case, the needle-shaped crystals have a large relative length. Accordingly, the needle-shaped crystals have a length and a diameter whose ratio on average is preferably greater than 2, more preferably greater than 5.

The silicon nitride ceramic according to the invention is preferably formed without pores, without being subjected to a hot isostatic pressing or a comparable method.

Preferred embodiments of the silicon nitride ceramic according to the invention have a sintered skin which is less than 0.5 mm, more preferably less than 0.2 mm and more preferably less than 0.1 mm thick, without the sintering skin being reduced by a measure after sintering ,

The silicon nitride ceramic according to the invention is preferably designed as a component of a bearing, for example as a component of a sliding bearing or a roller bearing. Thus, a component of a bearing which at least comprises the silicon nitride ceramic according to the invention also forms an object of the invention. In the simplest case, the component of the bearing is formed by the silicon nitride ceramic according to the invention.

The silicon nitride ceramic according to the invention is preferably designed as a bearing ring or as a rolling element.

Further advantages, details and developments of the invention will become apparent from the following description of preferred embodiments of the invention in comparison with the prior art, with reference to the respective tables and drawings. Show it:

1 a micrograph of a preferred embodiment of a silicon nitride ceramic according to the invention;

2 a micrograph of a silicon nitride ceramic according to the prior art;

3 a cross-sectional image of a cross-sectional area of the edge region of the preferred embodiment of the invention Siliziumnitridkeramik; and

4 FIG. 2: a micrograph of a cross-sectional area from the edge region of a silicon nitride ceramic according to the prior art with a sintering skin.

In the following, some examples of embodiments of the invention are first explained, in which a gas-pressure sintered material is produced.

For the production of silicon nitride ceramics according to the invention, material mixtures according to Tables 1 and 2 were first prepared by dispersing the silicon nitride powder and the additives in an aqueous medium with the addition of further appropriate additives, such as a plasticizer and a defoamer. To adjust the sintering activity, these suspensions were then ground in an attritor mill to the desired particle size and specific surface area. The necessary for a subsequent shaping and green working binders, plasticizers and pressing aids were then added to the suspension and this homogenized again. The further processing of the material mixture to a fine, free-flowing and compressible granules was then carried out by atomizing and drying the suspension by means of spray or fluidized bed granulation.

The granules produced were then processed by the molding processes of a cold isostatic pressing (CIP) or a uniaxial dry pressing with cold isostatic densification into green bodies and if necessary in terms of geometry, dimensional accuracy, tolerance and surface quality Machining processes, such as drilling, turning, milling, grinding, etc. in the green state reworked as close to final contour as possible.

Subsequently, a thermal debinding step was carried out to remove all the organic components which are disadvantageous for the subsequent sintering but which are necessary for shaping.

Depending on the type of sintering additive used and sintering additive content, the sintering of the shaped bodies was carried out at temperatures between 1.700.degree. C. and 1900.degree. C. in a gas pressure sintering furnace under non-oxidizing atmosphere with temporary application of a gas pressure of 0.5 MPa to 10 MPa.

The density of the ceramics was determined by comparing the samples or the component density determined by the Archimedes method of measurement and the true density of the silicon nitride material determined by helium pycnometry measurement on a very finely powdered material sample.

From the produced ceramic samples or components were then test specimens for determining the 3- or 4-point bending strength according to DIN EN 843- 1 worked out and subjected to the measurement.

The E modulus was determined by evaluating the stress-strain ratio from the 3-point or 4-point bending test according to the standard DIN EN 843-2 , Method A.

The hardness test was carried out by Vickers HV20 hardness impressions according to the standard DIN EN 843-4 on finely polished material ground.

The fracture toughness test was carried out by measuring the cracks emerging from the corners of the hardness impressions and calculating according to Niihara crack fracture formula KIc.

The statistical evaluation of all determined mechanical characteristics was carried out according to the standard issued for monolithic ceramics DIN EN 843-5 ,

For the preparation of the silicon nitride ceramics according to the invention, in each case one M ^II _{1 + x} M ^III ₂ O _{4 + x} was preferably used as a first additive, where M ^{II is} at least one divalent metal and M ^{III is} at least one trivalent metal and 0 ≤ x ≤ 0 , 7 applies. This is a spinel or a spinel-like structure, which in the context of the invention stoichiometrically in the case of x = 0 (see Table 1) or superstoichiometric in the case of x> 0 (see Table 2) added to the silicon nitride in different concentrations has been. The first additive was in the form of primary particles having an average primary particle size of less than 1 μm.

Tabelle 1 Table 1 shows various proportions of M ^II and M ^III metal oxides M ^II O, M ^III O according to the invention to provide various proportions of the first additive, each having a stoichiometric composition for gas pressure sintering. The data in Table 1 are in% by weight.

Table 1

Tabelle 2 Table 2 shows various proportions of M ^II and M ^III metal oxides M ^II O, M ^III O according to the invention to provide various proportions of the first additive, each having a stoichiometric composition (x = 0.5) for the gas pressure sintering. The data in Table 2 are in% by weight.

Table 2

Tabelle 3 Representative examples Nos. 1 to 3 for the property values of gas pressure sintered ceramics according to the invention are shown in Table 3.

Table 3

The evaluation of the structure, the microstructure and the sintering skin at the edge of the material was carried out by the production of finely polished material and Bauteilanschliffe and analysis by incidental or scanning electron microscopy.

1 shows a micrograph of a preferred embodiment of the silicon nitride ceramic of the present invention (Example no. 3 in Table 3) which was prepared by adding a first additive having a spinel or spinel-type structure and having the formula Mg _{1 + x} Al ₂ O _{4+ x} can be described with 0 ≤ x ≤ 0.7. This spinel or spinel-like structure did not first form during sintering, but was added in this form as an additive to the silicon nitride before sintering. It is thus a presynthesized additive.

The micrograph shows a scale of 50 μm. The silicon nitride ceramic shown is approximately free of pores and homogeneously sintered. The very few black dots indicate small residual pores 01 in the material. The polished white microstructure ingredients are due to the incorporation of a third additive as a colorant, the crystallized granules 02 with a size of 1 micron to 2 microns forms.

2 shows a silicon nitride ceramic according to the prior art, which was subjected to about 12 wt .-% sintering additive content of a hot isostatic pressing (HIP), so that they have a relatively high compressive strength of more than 3,000 MPa, a 3-point bending strength greater than 900 MPa and has a fracture toughness of 6.5 MPam ^-0.5 to 7.0 ^-0.5 MPam. On the other hand, the in 1 silicon nitride ceramic according to the invention has undergone no hot isostatic pressing process, so that it was produced much less expensive and yet a comparable microstructure as in 2 has shown in accordance with the prior art consuming produced silicon nitride ceramic. The silicon nitride ceramic according to the invention also has a compressive strength of about 3,000 MPa. The slightly lower flexural strength of 775 MPam ^-0.5 and the fracture toughness of 6.3 MPam of the inventive ceramic according to Example 3 can be attributed to the significantly higher sintering additive addition.

In the 2 The prior art silicon nitride ceramic shown also has a dyeing additive which is present in the silicon nitride ceramic in the form of crystallized grains 03 is included, which are recognizable as white dots. The grains 03 have a size of about 1 micron to 2 microns. Despite sintering and densification by hot isostatic pressing (HIP), fine residual pores are also present in the microstructure of this silicon nitride ceramic 04 with about the same frequency and size as in 1 recognizable.

3 shows a micrograph of a cross-sectional area of the silicon nitride ceramic according to the invention. The silicon nitride ceramic shown has on its surface 06 no or only a very thin sinter skin, which would have to be removed or reduced after sintering by a hard machining step.

4 shows in comparison to 3 a cross-sectional area of a silicon nitride ceramic according to the prior art. This silicon nitride ceramic has a sintered skin on its surface 07 on, which must be removed by subsequent hard machining for many applications, however, significantly increases the manufacturing cost.

In the following, some examples of embodiments of the invention in which a hot-pressed material is produced will be explained.

The material mixtures for the hot-pressed material variants were prepared analogously to the process route according to the invention already described for gas pressure sintering, but with the addition of a lower content of binder before granulation. The free-flowing powder material was then filled in a hot press mold and compressed at temperatures between 1.700 ° C and 2000 ° C with application of an axial or uniaxial pressing pressure of 5 MPa to 40 MPa.

Tabelle 4 Table 4 shows various proportions of M ^II and M ^III metal oxides M ^II O, M ^III O for the inventive provision of various proportions of the first additive, each having a stoichiometric composition (x = 0.0) for hot pressing. The first additive was in the form of primary particles having an average primary particle size of less than 1 μm. The data in Table 4 are in% by weight.

Table 4

Tabelle 5 Table 5 shows various proportions of M ^II and M ^III metal oxides M ^II O, M ^III O according to the invention to provide various proportions of the first additive each having a stoichiometric composition (x = 0.5) for hot pressing. The data in Table 5 are in% by weight.

Table 5

A representative example No. 4 of the property values of the ceramic materials hot-pressed according to the present invention is shown in Table 6. No additive stoichiometry salary density 3-point bending strength Weibull hardness Cracking ability x Wt .-% % th. D. MPa HV 20 MPam ^-0.5 4 MgAl 0 3 99.6 1012 9.2 nb nb Table 6

LIST OF REFERENCE NUMBERS

01

residual pores

02

crystallized grains of a colorant

03

crystallized grains of a coloring additive

04

residual pores

05

06

surface

07

sinterskin

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

• EP 0587119 B1 [0003]
• DE 2353093 B2 [0004]
• DE 3734274 A1 [0005]
• DE 4013923 C2 [0006]
• DE 69427510 T2 [0007]
• DE 60218549 T2 [0008]

Cited non-patent literature

• DIN EN 843- [0065]
• DIN EN 843-2 [0066]
• DIN EN 843-4 [0067]
• DIN EN 843-5 [0069]

Claims (10)

Method for producing a silicon nitride ceramic, comprising the following steps: - Providing silicon nitride; - Providing a first additive as a sintering aid, which is in the form of primary particles having an average primary particle size of less than 1 micron; Mixing the silicon nitride and the first additive into a mixture; - forming the mixture into a green compact; and - sintering of the green compact into a silicon nitride ceramic. A method according to claim 1, characterized in that the first additive is formed by an oxide with a plurality of cations, wherein the plurality of cations are formed by at least two different metal ions. A method according to claim 1 or 2, characterized in that the first additive has a spinel structure. A method according to claim 2 or 3, characterized in that the first additive has the following general chemical formula: M ^II _{1 + x} M ^III ₂ O _{4 + x} wherein M ^II comprises a divalent metal, MI ^II comprises a trivalent metal, and wherein 0 ≤ x ≤ 0.7. A method according to claim 4, characterized in that the first additive has the formula Mg _{1 + x} Al ₂ O _{4 + x} . Method according to one of claims 1 to 4, characterized in that the proportion of the first additive in the mixture between 3 wt .-% and 20 wt .-% is. Silicon nitride ceramic obtainable by a process according to any one of claims 1 to 6. Silicon nitride ceramic, which is sintered and has at least the following constituents: - silicon nitride; and A second phase formed by a chemical compound of silicon nitride and a first additive and having an average size of less than 1 μm. Silicon nitride ceramic according to claim 8, characterized in that the second phase is formed amorphous or partially crystalline. Component of a rolling or sliding bearing in the form of a bearing ring or a rolling element, which is formed by a silicon nitride ceramic according to one of claims 7 to 9.

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