WO2013167518A2

WO2013167518A2 - Silicon nitride ceramic and method for the production thereof

Info

Publication number: WO2013167518A2
Application number: PCT/EP2013/059357
Authority: WO
Inventors: Karl Berroth; Frank STEGNER
Original assignee: Schaeffler Technologies AG & Co. KG
Priority date: 2012-05-10
Filing date: 2013-05-06
Publication date: 2013-11-14
Also published as: WO2013167518A3; DE102012207802A1

Abstract

The present invention relates to a sintered silicon nitride ceramic, which consists of, in particular, a component of a rolling or sliding bearing. The invention also relates to a method for producing said type of silicon nitride ceramic. The method comprises a first step in which silicon nitride Si₃N₄ is prepared. Then, a first additive which has a perovskite structure is prepared. Said first additive has a perovskite-type chemical compound having a structure such as CaTiO₃. Said structure is already present prior to the sintering process such that the first additive can be referred to as presynthesized. The first additive is also a sintering aid. In a further step of the claimed method, the silicon nitride and the first additive are mixed to form a mixture. The mixture is subsequently formed into a green compact which is sintered to form a silicon nitride ceramic.

Description

Silicon nitride ceramics and process for their preparation

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

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.

EP 0 587 1 19 B1 shows a silicon nitride sintered body with a high content

Thermal conductivity and a method for its production. According to this

Methods are 2.0% to 7.5% by weight of one or more

Rare earth elements in the form of the respective oxide as an additive added to the silicon nitride. In preferred embodiments, yttria and

Alumina is used as a single additive. In these embodiments, sintering takes place over a period of six hours at a temperature of 1,900 ° C. From DE 39 38 644 A1 a sintered body based on silicon nitride is known, which is intended for use in a rolling bearing. In the production of the sintered body, it is preferable to add 6% of yttrium oxide as an additive to the silicon nitride. DE 602 18 549 T2 shows a roller bearing element with a silicon nitride

Sintered body, which in addition to silicon nitride also 1 mass% to 10 mass%

Contains rare earth metals in the form of oxides. For the production of this

Rolling element takes place after completion of the sintering preferably a Hot isostatic pressing treatment (HIP) in a non-oxidizing atmosphere of at least 300 atm (30 MPa) and at a temperature of 1,600 ° C to 1,880 ° C.

From DE 23 53 093 B2 a method for producing a sintered ceramic based on silicon nitride is known in which alumina powder and

Magnesium oxide powder can be used as additives for sintering. According to a preferred embodiment, the alumina powder and the

Magnesium oxide powder already reacted before sintering. The

Alumina powder and the magnesium oxide powder are for three to ten

Hours at a temperature of 1 .600 ° C to 1 .800 ° C to produce spinels. Then the spinels are finely pulverized to a

Particle size of less than 50 μιτι to achieve.

DE 37 34 274 A1 shows an electrically insulating, ceramic sintered body, which preferably consists of a silicon nitride ceramic. In the production of the sintered body, the spinel MgAI ₂ O ₄ is added as an additive.

DE 40 13 923 A1 shows a method for producing a silicon nitride powder, in which a spinel rare earth element oxide is preferably used as a sintering aid.

DE 694 27 510 T2 discloses a method for producing a sinter

Silicon nitride. In this method, the spinel structure MgO ^■ Al2O3 is used as a sintering additive.

The object of the present invention, starting from the prior art therein, is a silicon nitride ceramic and a method for the production thereof

to provide, with a comparatively aufwandärmere production leads to a Siliziumnitridkeramik, which has both a high strength and fracture toughness, a largely non-porous structure and a lower sintered 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 Si3N is provided. The silicon nitride is preferably provided as a powder.

Furthermore, a first additive is provided as a sintering additive, which is a

Having perovskite structure. Thus, the first additive is a perovskite-type chemical compound having a structure such as CaTiO 3. This structure is already present before a sintering process, so that the first additive can be described as presynthesized. The first additive also acts as a sintering aid. In a further step of the invention

Process are the silicon nitride, the first additive and possibly further

Additives / additives mixed into a mixture. The mixture is then formed into a green body, which is then sintered to a silicon nitride ceramic. The sintering can be done for example by gas pressure sintering or by a hot pressing process. Here, in addition to the silicon nitride and the first

Additive to a component of the silicon nitride ceramic and forms a glassy binder, filling and grain boundary phase.

An essential advantage of the method according to the invention is that the use of the first additive in the form of a presynthesized

Perovskits a particularly homogeneous distribution of liquid phase sintering necessary elements in the Si3N ceramic is achieved. This results in a particularly advantageous sintering behavior, which it u. a. allows to dispense with a hot isostatic sintering compaction of Si3N ceramic. Instead, it is possible

According to the invention a Si3N ceramic with approximately equivalent structure and

Properties also over a less complex Gasdrucksinter- or

Produce 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 La ₂ O ₃ , Nd ₂ O ₃ , P ^ Os) and themselves therefore

normally do not allow to disperse aqueous. The first additive to be used according to the invention in the form of a presensitized perovskite, on the other hand, does not prove to be hygroscopic, but is also stable in an aqueous environment. The mixing of the silicon nitride powder and the at least first additive and the further treatment 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 before, with which the desired fineness,

Homogeneity and surface quality of the mixture can be achieved and targeted. 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. In preferred embodiments of the method according to the invention, the first additive has the following general chemical formula:

RE _x M _2- xO ₃ In this general chemical formula, RE is at least one

Rare earth metal, while M stands for at least one metal. Furthermore, 0.4 <x <1, 6.

In preferred embodiments, x = 1, so that there is a stoichiometrically balanced chemical compound REMO3.

In alternative preferred embodiments, x <1, more preferably x <0.8. In further alternative preferred embodiments, x> 1, particularly preferably x> 1, 2. The embodiments mentioned lead to an over or understoichiometry of the first additive, by which the

Perovskitstrukur is modified if necessary, so that at least one perovskite-like structure is present, which is within the scope of the invention.

The component M is preferably selected from the group formed by Al, Fe, Cr and Mn, wherein the component M may comprise one or more of these metals. Particularly preferably, the component M comprises at least aluminum Al. The component RE particularly preferably comprises at least neodymium Nd.

In particularly preferred embodiments of the process according to the invention, the first additive has the formula NdxAl _2-x O ₃ , which is a concretization of the abovementioned general chemical formula.

In further particularly preferred embodiments of the method according to the invention, the first additive has the formula NdAIO3, which is a

Specification of the general chemical formula given above.

According to another preferred embodiment, the additive has the following general chemical formula:

In this general chemical formula, RE ¹ stands for a first rare earth metal, while RE "stands for a second one different from the first rare earth metal

Rare earth metal stands. Furthermore, u + v + w = 2 and 0.4 <(u + v) <1, 6 and

0 <w <0.4.

The first additive is preferably provided in the form of particles which have an average primary particle size of less than 1 μm. These are therefore nanoscale primary particles, which are preferably formed as nanoparticles are. The phar particles can also be present in particular as granules or agglomerates.

The average primary particle size of the first additive is preferably from about 10 nm to several 100 nm

Preferably, the primary particle size of the first additive is preferably less than 500 nm. Furthermore, the average primary particle size of the first additive is preferably more than 50 nm. 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. Basically, a share of more than

10 wt .-% of an additive for use as a sintering aid 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 ensures, in addition to an improved sinterability, a higher elasticity, an increased

Toughness and improved damage tolerance. In alternative preferred embodiments, the proportion of the first

Additive between 3 wt .-% and 10 wt .-% of the mixture. This proportion has an advantageous effect on strength, hardness, corrosion resistance and high-temperature properties. The proportion of the silicon nitride is preferably between 60 wt .-% and 95 wt .-%, more preferably between

80 wt .-% and 90 wt .-% of the mixture.

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 included

Mixture, the green compact and the sintered silicon nitride ceramic next to the

Silicon nitride only the first additive. The exclusive use of the The first additive as an additive ensures that the first additive is extremely homogeneously distributed in the mixture.

In alternative preferred embodiments of the invention

Method is provided in addition to the first additive, a second additive 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 μιτι.

The second additive is preferably such chemical

Compounds which are already added according to the prior art, the silicon nitride as a sintering additive, colorants or to form a further phase. In particular, this second additive is preferably selected from the group of oxides and nitrides of the elements 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 as well as when using the first additive alone. Also, the third additive is preferably present in the form of primary particles, which an average

Primary particle size of less than 1 μιτι have.

The third additive is preferably such chemical

Compounds used according to the prior art as an additive.

The third additive is preferably MgO, Al 2 O 3, Y 2 O 3 or AlN. Alternatively, the third additive is preferred by a spinel or by a garnet educated. 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 2000 ° 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 made. For both deployment variants of the first additive, a preferred

Primary particle size and a specific surface can be achieved by the subsequent grinding of coarser particles.

The sintering of the green body is preferably carried out by sintering

Gas pressure atmosphere or via a hot press. Since the silicon nitride ceramic produced according to the invention is already densely sintered to a large extent without pores and has high-quality microstructures and properties, preference is given to none

Hot isostatic pressing (HIP) performed. 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 while sintering

resulting sintered skin 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, preferably no measure for subsequent

Reduction of the sinter skin, such as a 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, in addition to silicon nitride, comprises a grain boundary phase which is formed by a chemical compound of silicon nitride and a first additive having a perovskite structure. The silicon nitride is preferably crystallized in the form of β-Si3N-stalk crystals. The grain boundary phase is preferably formed in a glass-like manner between the β-Si 3 N stalk crystals. The grain boundary phase preferably has the general chemical formula RE _a Si _b McO _d N _e , in which RE is a rare earth metal and M is a metal, where a, b, c, d and e are greater than zero.

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

The silicon nitride ceramic according to the invention has in terms of their

Composition, in particular with regard to the chemical composition of the bound in the grain boundary phase first additive and optionally further

Additives and their quantitative composition also favor those

Characteristics, which are preferred for the inventive method

are indicated. Thus, the average particle size of the first additive preferably less than 1 μιτι, more preferably less than 500 nm. Further For example, the average particle size of the first additive is more than 50 nm. The chemical composition of the grain boundary phase

bonded first additive is preferably similar to the chemical composition of the first additive, which is preferably used according to the inventive method. The chemical composition of any other additives present in the sintered silicon nitride ceramic is preferably the same

chemical composition of the other additives, which according to the

Preferred methods of the invention are to be used. 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. So is the

3-point bending strength of the silicon nitride ceramic according to the invention preferably at least 800 MPa, more preferably more than 900 MPa. The crack toughness according to Niihara is at the same time at least 6 MPa ^"0.5 , preferably more than 7 MPa ^{" 0.5} , particularly preferably about 7.5 to 8.7 MPa ^"0.5 The compressive strength of the silicon nitride ceramic according to the invention is preferably at least 2500 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 crystals, which are present in the glassy or

semi-crystalline grain boundary phase are embedded. The needle-shaped crystals ensure a high fracture toughness and damage tolerance of the

Silicon nitride ceramic. The needle-shaped crystals have a large relative length. Accordingly, the acicular crystals have a length and a diameter whose ratio is on average preferably greater than 4, more preferably greater than 8. Preferably, at least one-tenth of the needle-shaped crystals of the silicon nitride ceramic according to the invention has a length and a

Diameter, the ratio is greater than 10, more preferably greater than 20.

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

Was subjected to the procedure. 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 forming the

Silicon nitride ceramic according to the invention at least comprises, also one

Subject 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:

Fig. 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

Technology;

3 shows a micrograph of the silicon nitride ceramic according to the invention in a higher resolution;

4 shows a micrograph of the silicon nitride ceramic according to the prior art in a higher resolution; a micrograph of a cross-sectional area of the edge region of the preferred embodiment of the invention

silicon nitride; and a microsection of a cross-sectional area from the edge region of

Silicon nitride ceramic according to the prior art.

The following are some examples of embodiments of the

Invention explained in which a gas-pressure sintered material is produced.

For the production of silicon nitride ceramics according to the invention, first of all material mixtures according to Tables 1 to 4 were obtained by dispersing the

Silicon nitride powder prepared in aqueous medium with the addition of appropriate additives, such as a condenser and a defoamer. To set 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 carried out by spraying and drying the suspension by means of spraying 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 by machining processes, such as drilling, turning, milling, grinding, etc. reworked as close to final contour as possible in the green state. 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. The sintering of the moldings was then carried out depending on the type of sintering additive used and sintering additive content at temperatures between 1 .700 ° C and 1 .900 ° 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 ceramic was determined by comparing the samples or the determined by the measurement method according to Archimedes component density and the

Helium pycnometry measured on a very pulverulent material sample determined true density of the silicon nitride material determined. From the produced ceramic samples or components were subsequently

Test specimen for the determination of the 3- or 4-point bending strength according to DIN EN 843-1 worked out and subjected to the measurement.

The determination of the modulus of elasticity was carried out by evaluating the stress-strain ratio from the 3- or 4-point bending test according to the standard DIN EN 843-2,

Procedure A.

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

The fracture toughness test was carried out by measuring the cracks coming from the corners of the hardness impressions and calculating according to the formula for the fracture toughness K | _C to Niihara. The statistical evaluation of all determined mechanical characteristics was carried out according to the DIN EN 843-5 standard issued for monolithic ceramics. In order to produce the silicon nitride ceramics according to the invention, preferably one RExAl _2-x O _{3 was used} as the first additive, where RE is a rare earth metal and 0.4 <x <1, 6. This is a perovskite structure, which in the context of the invention is stoichiometric in the case of x = 1 (see Table 1),

superstoichiometric in the case of x> 1 (see Table 2) or substoichiometric in the case of x <1 (see Table 3) the silicon nitride in different

Concentrations were added.

Table 1 shows various proportions of rare earth oxides REO and

Alumina AI2O3 for the inventive provision of various proportions of the first additive, each having a stoichiometric composition for the

Gas pressure sintering. The data in Table 1 are in% by weight.

Sinter10.0 13.5 15 17.5 additive-

proportion of

Shares Shares Shares Shares

RE REO AI2O3 REO AI2O3 REO AI2O3 REO AI2O3

La 7,62 2,38 10,28 3,22 1 1, 42 3,58 13,33 4,17

Ce 7.64 2.36 10.32 3.18 1 1, 46 2.04 13.37 4.13

Pr 7.64 2.36 10.31 3.19 1 1, 46 2.04 13.37 4.13

Nd 7.67 2.33 10.36 3.14 1 1, 51 1, 99 13.43 4.07

Dy 7,85 2,15 10,60 2,90 1 1, 78 1, 72 13,74 3,76

Yb 7.94 2.06 10.73 2.77 1 1, 92 1, 58 13.90 3.60

Table 1

Table 2 shows various proportions of rare earth oxides REO and

Alumina AI2O3 for the inventive provision of various proportions of the first additive, each with a stoichiometric composition (x = 1, 2) for gas pressure sintering. The data in Table 2 are in% by weight. Total salary

sintering additives

10% by weight 13.5% by weight 15% by weight 17.5% by weight

equivalent to

based

the oxides

REO AI ₂ O ₃ REO AI2O3 REO AI2O3 REO AI2O3

element

La 8,27 1,73 11,17 2,33 12,41 2,59 14,48 3,02

Ce 8,29 1,71 11,20 2,30 12,44 2,56 14,51 2,99

Pr 8,29 1.71 11.19 2.31 12.44 2.56 14.51 2.99

Nd 8.32 1.68 11.23 2.27 12.48 2.52 14.56 2.94

Dy 8.46 1.54 11.42 2.08 12.69 2.31 14.80 2.70

Yb 8.53 1.47 11.51 1.99 12.79 2.21 14.93 2.57

Table 2

Table 3 shows various proportions of rare earth oxides REO and

Alumina AI2O3 for the provision according to the invention of various proportions of the first additive, each having a substoichiometric composition (x = 0.8) for gas pressure sintering. The data in Table 3 are in% by weight.

Total salary

Sintering additives 10% by weight 13.5% by weight 15% by weight 17.5% by weight

equivalent to

based

the oxides REO Al ₂ O ₃ REO Al ₂ O ₃ REO Al ₂ O ₃ REO Al ₂ O ₃

element

La 6,81 3,19 9,19 4,31 10,21 4,79 11,91 5,59

Ce 10.92 5.41 11.15 5.65 10.25 4.75 11.96 5.54

Pr 10,91 5,42 11,15 5,65 10,25 4,75 11,96 5,54

Nd 10,98 5,35 11,21 5,59 10,31 4,69 12,03 5,47

Dy 11,30 5,03 11,54 5,26 10,64 4,36 12,41 5,09

Yb 11.47 4.86 11.71 5.09 10.81 4.19 12.61 4.89

Table 3

Table 4 shows various concentrations of the elements Nd, Al and O as well as concentrations of neodymium oxide and alumina for the invention particularly preferred provision of NdAIO3 as the first additive for gas pressure sintering. The data in Table 4 are in% by weight.

Guideline Element calculation of the concentration of transition metal oxides

proportion of

in% Nd Al O Nd ₂ O ₃ Al ₂ O ₃

Share% 10.0 6.58 1, 23 2.19 7.67 2.33 First 13.50 8.88 1, 67 2.95 10.36 3.14

Additive in 15.0 9.87 1, 85 3.28 1 1, 51 3.49

Si ₃ N ₄ 17.5 1 1, 52 2.16 3.82 13.43 4.07

Table 4

Representative Examples Nos. 1 to 7 for the property values of gas pressure sintered ceramics according to the present invention are shown in Table 5.

Table 5

The evaluation of the structure, the microstructure and the sinter skin at the edge of the material was carried out by the production of finely polished material and

Partial grinding and analysis by means of incident light or scanning electron microscopy.

Fig. 1 shows a micrograph of a preferred embodiment of a

silicon nitride ceramic of the present invention (Example No. 6 in Table 5) prepared by the addition of a first additive having a perovskite structure and can be described by the formula NdxAl _2-x O3 with 0.4 <x <1.6. This chemical compound NdxAl _2-x O3 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.

In the micrograph a scale of 50 μιτι is shown. The silicon nitride ceramic shown is approximately free of pores and homogeneously sintered. The very few, black dots, however, indicate small residual pores 01 in the material. The white microstructure constituents in the polished section are due to the introduction of a third additive as coloring agent, which forms crystallized grains 02 having a size of 1 μm to 2 μm.

FIG. 2 shows a prior art silicon nitride ceramic which has been subjected to hot isostatic pressing (HIP) with about 12% by weight of sintering additive content, so that it has a comparatively high compressive strength of more than

3,000 MPa, a 3-point bending strength greater than 900 MPa and a fracture toughness of 6.5 MPam ^"0.5 to 7.0 MPam ^{" 0.5} . By contrast, the silicon nitride ceramic according to the invention shown in FIG. 1 was not subjected to a hot isostatic pressing process, so that it was produced much less laboriously and yet has a comparable structure to the silicon nitride ceramic produced in accordance with the prior art shown in FIG. 2. The inventive

Silicon nitride ceramics also have a compressive strength of about 3,000 MPa, and also the fracture toughness reaches approximately the same at 6.9 MPa ^"0.5

Property values. The slightly lower flexural strength of 835 MPa

Ceramic according to Example 6 of the invention is on the much higher

Sinteradditivzusatz traceable.

The prior art silicon nitride ceramic shown in Fig. 2 also has a dyeing additive included in the silicon nitride ceramic in the form of crystallized grains 03, which are recognizable as white dots.

The grains 03 have a size of about 1 μιτι to 2 μιτι on. Despite sintering and densification by hot isostatic pressing (HIP) are also in the Microstructure of this silicon nitride ceramic fine residual pores 04 with approximately similar frequency and size as in Fig. 1 recognizable.

Fig. 3 shows a further microsection of the preferred embodiment of

Silicon nitride ceramic according to the invention in a higher resolution. The micrograph shows a scale of 10 μιτι. In this micrograph it becomes clear that the silicon nitride ceramic has a morphology with acicular β-Si 3 N crystals 06 which are present in the structure in a three-dimensional and statistically oriented manner, resulting in a quasi-isotropic microstructure and the quasi-isotropic properties of the ceramic. The needle-shaped crystals are surrounded by the glassy or semi-crystalline grain boundary and filling phase formed from the first and optionally third additive and the SiO 2 fraction of the starting Si 3 N powder. The grain boundary and

Filling phase is formed by a chemical compound of silicon nitride and the first additive having a perovskite structure. The needle-shaped

Crystals 06 have a length and a diameter whose ratio is in many cases more than 10 and occasionally up to 24. The needle-shaped crystals 06 cause a high fracture toughness and thus damage tolerance of

Silicon nitride ceramic, so that they represent an in-situ generated fiber reinforcement. FIG. 4 shows a further micrograph of the prior art silicon nitride ceramic shown in FIG. 2 in a higher resolution. It is the same resolution as in Fig. 3, to the invention shown in Fig. 3

Silicon nitride ceramic to compare with the prior art. The silicon nitride ceramic according to the prior art shown in FIG. 4 predominantly has only short acicular β-Si 3 N crystals 07, so that in contrast to

Silicon nitride ceramic according to the invention only a significantly reduced

Fiber reinforcement effect is given.

5 shows a micrograph of a cross-sectional area of the invention

Silicon nitride ceramic. The silicon nitride ceramic shown has no or only a very thin sinter skin on its surface 08, without having to remove or reduce a sinter skin for this purpose by means of a hard-machining step. FIG. 6 shows, in comparison to FIG. 5, a cross-sectional area of a silicon nitride ceramic according to the prior art. This silicon nitride ceramic has on its surface on a sintered skin 09, which must be removed for many applications by a subsequent hard machining, but 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 blends for the hot-pressed material variants were prepared analogously to the inventive process route 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 2,000 ° C with application of an axial or uniaxial pressing pressure of 5 MPa to 40 MPa.

Table 6 shows various proportions of rare earth oxides REO and

Alumina AI2O3 for the inventive provision of various proportions of the first additive, each having a stoichiometric composition (x = 1, 0) for hot pressing. The data in Table 6 are in% by weight.

Total salary

Sintering additives 3% by weight 5% by weight 8% by weight 10% by weight

equivalent to

based

the oxides REO Al ₂ O ₃ REO Al ₂ O ₃ REO Al ₂ O ₃ REO Al ₂ O ₃

element

La 2.28 0.72 3.81 1, 19 6.09 1, 91 7.62 2.38

Ce 2.29 0.71 3.82 1, 18 6.1 1 1, 89 7.64 2.36

Pr 2.29 0.71 3.82 1, 18 6.1 1 1, 89 7.64 2.36

Nd 2.30 0.70 3.84 1, 16 6.14 1, 86 7.67 2.33

Dy 2.36 0.64 3.93 1, 07 6.28 1, 72 7.85 2.15

Yb 2.38 0.62 3.97 1, 03 6.36 1, 64 7.94 2.06

Table 6

Table 7 shows various proportions of rare earth oxides REO and

Alumina AI2O3 for the provision according to the invention of various proportions of the first additive, each having a superstoichiometric composition (x = 1, 2) for hot pressing. The data in Table 7 are in wt .-%.

Total content of sintering additives 3% by weight 5% by weight 8% by weight 10% by weight

equivalent to

based

the oxides REO Al ₂ O ₃ REO Al ₂ O ₃ REO Al ₂ O ₃ REO Al ₂ O ₃

element

La 2.48 0.52 4.14 0.86 6.62 1, 38 8.27 1, 73

Ce 2.49 0.51 4.15 0.85 6.63 1, 37 8.29 1, 71

Pr 2.49 0.51 4.15 0.85 6.63 1, 37 8.29 1.71

Nd 2.50 0.50 4.16 0.84 6.66 1, 34 8.32 1, 68

Dy 2.54 0.46 4.23 0.77 6.77 1, 23 8.46 1, 54

Yb 2.56 0.44 4.26 0.74 6.82 1, 18 8.53 1, 47

Table 7

Table 8 shows various proportions of rare earth oxides REO and

Alumina AI2O3 for the provision according to the invention of various proportions of the first additive, each having a substoichiometric composition (x = 0.8) for hot pressing. The data in Table 8 are in% by weight.

Total salary

Sintering additives 3% by weight 5% by weight 8% by weight 10% by weight

equivalent to

based

the oxides REO Al ₂ O ₃ REO Al ₂ O ₃ REO Al ₂ O ₃ REO Al ₂ O ₃

element

La 2.04 0.96 3.40 1, 60 5.44 2.56 6.81 3.19

Ce 3.28 1, 62 5.46 2.71 10.79 5.28 10.92 5.41

Pr 3,27 1, 63 5,46 2,71 10,78 5,29 10,91 5,42

Nd 3,29 1, 61 5,49 2,68 10,85 5,22 10,98 5,35

Dy 3,39 1, 51 5,65 2,51 1 1, 17 4,90 1 1, 30 5,03

Yb 3.44 1, 46 5.74 2.43 1 1, 34 4.73 1 1, 47 4.86

Table 8

Table 9 shows various concentrations of the elements Nd, Al and O as well as concentrations of neodymium oxide and alumina to the invention particularly preferred provision of NdAIO3 as the first additive for hot pressing. The data in Table 9 are in wt .-%.

Table 9 Representative examples Nos. 8 to 11 for the property values of the ceramic materials hot-pressed according to the present invention are shown in Table 10.

Table 10

LIST OF REFERENCE NUMBERS

01 - residual pores

02 - crystallized grains of a colorant

03 - crystallized grains of a colorant additive

04 - residual pores

05 - -

0 6 - acicular β-Si3N crystals

07 - short acicular β-Si3N crystals

08 - surface

0 9 - sinter skin

Claims

claims

Method for producing a silicon nitride ceramic, comprising the following steps:

- Providing silicon nitride;

Providing a first additive having a perovskite structure;

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 has the following general chemical formula:

RE _x M _2- xO ₃ ,

in which

RE comprises at least one rare earth metal,

M comprises at least one metal, and wherein 0.4 <x <1, 6.

A method according to claim 2, characterized in that x is equal to 1.

A method according to claim 2 or 3, characterized in that M is selected from the group formed by Al, Fe, Cr and Mn.

A method according to claim 4, characterized in that the first additive has the formula NdxAI _2-x O3.

Method according to one of claims 1 to 5, characterized in that the first additive is provided in the form of primary particles having an average primary particle size of less than 1 μιτι.

7. silicon nitride ceramic, obtainable by a method according to one of claims 1 to 6.

8. Silicon nitride ceramic which is sintered and has at least the following constituents:

- silicon nitride; and

a grain boundary phase formed by a chemical compound

Silicon nitride and a perovskite having a first additive is formed.

9. silicon nitride ceramic according to claim 8, characterized in that the

Silicon nitride is crystallized predominantly in the form of Stangelkristallen, wherein at least one tenth of the Stangelkristalle have a length and a diameter whose ratio is greater than 10.

10. Component of a rolling or sliding bearing in the form of a bearing ring or a rolling element, which by a silicon nitride ceramic according to one of

Claims 7 to 9 is formed.