DD299119A7 - PROCESS FOR PREPARING POLYCRYSTALLINE SILICON NITRIDE INTERCOLORS - Google Patents

Abstract

The invention relates to the field of structural ceramics and relates to a process for the production of polycrystalline Siliziumnitridsinterkoerpern, which are used as cutting materials and as ceramic wear parts in valve and motor construction. The object of the invention is achieved by a process for producing polycrystalline Siliziumnitridsinterkoerpern, wherein the silicon and / or Siliziumnitridpulver with a mean grain size, the powder particles of 5 mm, with one or more sintering agents of the rare earth group, wherein the powder mixture Einlagerungs- and / or reinforcing components may be added, mixed in a liquid phase of organic solvents or solvent mixtures and / or water, which can be ground simultaneously, then the powder mixture is separated from the liquid phase and dried and further processing to polycrystalline sintered bodies, according to the invention thereby dissolved in the powder mixture before or during or after mixing or mixing dicarbonyl compounds in an amount which is greater than the stoichiometric amount to form rare earth oxide dicarbonyl complexes, these with the Mixed powder mixture and / or are ground, and that after the separation and drying of the powder mixture and before or during the further processing into sintered bodies the Dicarbonylmetalloxidkomplexe be decomposed at temperatures up to 600C. Silicon nitride sinters produced according to the invention have improved high temperature properties and increased reliability. {Construction ceramics; Sintered body, polycrystalline; silicon nitride; Cutting material; Verschleiszteil; Powder; Sintering aids; Mix; Mixed grinding; Decomposition}

Description

ZIoI the invention

The aim of the invention is to broaden the field of application of polycrystalline Siliziumnitridsintorkörpern according to the invention over the prior art produced polycrystalline Siliziumnitridsinterkörpern.

Explanation of the essence of the invention

The invention has for its object to provide a method by which the reliability and high temperature properties of polycrystalline Slntsrkörpern be improved.

The object of the invention is achieved by a process for producing polycrystalline Siliziumnitridsinterkörpern, wherein the silicon and / or Siliziumnitridpulver with a mean particle size dei powder particles of <5μπι, with one or more sintering agents of the rare earth group, wherein the powder mixture storage and / or reinforcing components may be added, are mixed in a liquid phase of organic solvents or solvent mixtures and / or water, wherein simultaneously can be ground, then the powder mixture is separated from the liquid phase and dried and further processing to polycrystalline sintered bodies, according to the invention thereby solved in that dicarbonyl compounds are added to the powder mixture before or during or after mixing or mixing in an amount greater than the stoichiometric amount to form rare earth metal dicarbonyl complexes be mixed and / or ground the powder mixture, and that after separation and drying of the powder mixture and before or during further processing into sintered bodies Dicarbonylmetalloxidkomplexe be decomposed at temperatures up to 60O ⁰ C.

Advantageously, acetyl acetone and / or its derivatives are used as Dicai bonylverbindungen.

It is also advantageous when using Dicai honylverbindungen after mixing and / or grinding, the suspension to the boiling point of the liquid phase to heat and then cool again.

In the process according to the invention, the rare earths present as oxides are precipitated on the surface of the powder particles in a finely divided manner during the mixed grinding by complex formation over the liquid phase.

In the case of addition of the dicarbonyl compounds after grinding, the suspension is then heated to the boiling temperature of the liquid phase and cooled again, in which case the complex formation takes place via thermal activation. After the milling liquid has been separated, the temperature is raised to temperatures of up to 600 ° C. As a result, the finely divided small particles of Dicarbonylmetalloxidkomplexen thermally decomposed and there are the oxides of these compounds. These oxides act as sintering aids, which are very finely divided in the powder mixture.

It can be seen here that no significant increase in the oxygen content of the Fv''vergemisches occurs by these Pulveraufbureitunrj. Although the sintering aid content in the shaped body remains unchanged, a higher high-temperature strength of these sintered bodies is achieved in comparison to those produced by mixed grinding. In addition, a higher reliability is achieved in Fo. m of the Weibulimoduls is measured.

An additional advantage is that a high sintering activity is achieved by the good distribution of Sinterhilfsmittei in the green body. As a result, the proportion of sintering aids can be reduced. This also has an advantageous effect on the high-temperature properties of the sintered body.

embodiments

Example 1-3

Silicon nitride powder having a specific surface area of 10.5 m ² g-1 impurity of 0.5% and an oxygen content of 1.55% is mixed with a mixture of oxides of rare earth elements neodymium, prasodymium, lanthanum, samarium and cerium (hereinafter referred to as Didymoxid, R ₂ Oa) and aluminum oxide (composition see Table 1) and mixed with 20 ml of acetylacetone (Pentandion-2.4) for 300min in 85 ml of 96% ethanol and then heated to the boiling point of the ethanolic solution and cooled again. Subsequently, the powder mixture is dried in vacuo, granulated and pressed with 250 MPa to 5 × 60 × 6 mm beads. Thereafter, the molded bodies are heated in air to a temperature of 500 ° C and cooled again.

Subsequently, the moldings are sintered at 1700 to 195O ⁰ C at a nitrogen pressure to 80bar. After a corresponding finishing treatment (grinding of the lateral surfaces by approx. 0.3 mm), the four-point bending strength σ ⁴ and the Weibuli modulus m at room temperature and the four-point bending strength σ ⁴ at 1200 ° C. are determined.

These strength properties have been compared with those of sintered moldings of the same starting composition, but with the method by means of mixing in the planetary ball mill manufactured. The results are shown in Table 1.

Table 1: Characterization of the sintered bodies

No. Si ₃ N ₄ Basic composition the invention Strength in MPa the by mixed grinding ^m RT in mass proportions produced sintered body produced sintered body 8th R ₂ O ₃ Al ₂ O ₃ Cr ⁴ RT O ⁴ 1200 O ⁴ RT O ⁴ 1200 11 91.5 762 530 745 412 12 89.9 770 549 " ¹ RT 756,380 1 88.6 6.8 1.7 756 543 19 743 406 2 7.8 2.3 21 3 8.8 2.6 24

Example 4

Silicon nitride powder is mixed according to Example 1 with a mixture of alumina and didymium oxide according to Table 2 and mixed with 15 ml of acetylacetone (pentanedione-2,4) 250min in 85 ml of 96% ethanol.

Then, the powder mixture is dried and granulated and in a graphite die at 1840 ⁰ C for 60 min under a pressure of 30MPa hot pressed. For the test specimens (3.5 χ 60 χ 4.5) mm cut and finished with diamond tools of densely sintered disks, values are obtained following the measurement of the strength (see Table 2).

Table 2: Characterization of the sintered bodies

Ausgangsjusam. nensetzg.

in mass proportions Si ₃ N ₄ R ₂ O ₃ Al ₂ O ₃

Strength in MPa

According to the invention produced sintered body

sintered body produced by mixed grinding

93.8

5, OR 1,2

D ⁴ RT 852

0 * 1200 657

O ⁴ RT 843

0 * 1200 521

Example 5

80g of silicon powder with a specific surface area of 5m * / g, an impurity level of 0.4% and an oxygen content of 1.1% are mixed with 9.5 g of R ₂ O ₃ (see Example 1), 2.2 g of Al ₂ O ₃ and Mix 25 ml of trifluoroacetylacetone for 250 min in 75 ml of pure ethanol. Subsequently, the powder mixture is dried, granulated and pressed with 200MPa to form bodies with the dimensions 5 x 60 χ 6mm. Thereafter, the moldings are stufi nweise heated to a temperature of 142O ⁰ C and nitrided. Subsequently, the sintering of the rods takes place at temperatures of 1700-1950 ⁰ C at eii.em nitrogen pressure of 80 bar. Si ₃ N ₄ rods produced in this way exhibit the mechanical properties given in Table 3.

Table 3: Characterization of the sintered bodies

Basic composition

in mass proportions Si ₃ N ₄ R ₂ O ₃ Al ₂ O ₃

Strength in MPa

the sintered body produced according to the invention

the sintered body produced by Mischmahlung

91, b '

6.8

1.7

O ⁴ RT 640

0 * 1200 450

^m RT 15

O ⁴ RT 618

σ * 1 200 400

^m RT 10

Examples

Silicon nitride powder is mixed-milled in accordance with Example 1 with a mixture of aluminum oxide and didymium oxide according to Table 4 for 250 minutes in 200 ml of isopropanol. Subsequently, 15 ml of acetylacetone (pentanedione-2,4) are added and the suspension is refluxed for 15 minutes. Thereafter, the suspension is dried in a fluidized bed dryer and granulated. For the test specimens prepared analogously to Examples 1 to 3, the following strength values are measured (see Table 4).

Table 4: Characterization of the sintered bodies

Basic composition

in mass proportions Si ₃ N ₄ R ₂ O ₃ Al ₂ O ₃

Strength in MPa

the sintered body produced according to the invention

the sintered body produced by mixing

91.5

6.8

1.7

O ⁴ RT 770

0 * 1200 525

O ⁴ RT 755

o * 1200 430

Claims (3)

-1- 299 claims: 1. A process for the preparation of polycrystalline Siliziumnitridsinterkörpern, wherein the silicon and / or Siliziumnitridpulver with an average particle size of the powder of <5μιτι, with one or more sintering agents of the rare earth group, wherein the powder mixture storage and / or reinforcing components may be added be mixed in a liquid phase of organic solvents or solvent mixtures and / or water, which can be ground simultaneously, then the powder mixture is separated from the liquid phase and dried and further processing to polycrystalline sintered bodies, characterized in that the powder mixture before or during or after mixing or mixing, dicarbonyl compounds are added in an amount greater than the stoichiometric amount to form rare earth metal dicarbonyl complexes, these are mixed with the powder mixture and / or be gemleilei ι , And that after the separation and drying of the powder mixture and before or during further processing into sintered bodies Dicarbonylmetalloxidkomplexe be decomposed at temperatures up to 600 ^c C. 2. The method according to claim 1, characterized in that acetylacetone and / or its derivatives are used as dicarbonyl compounds. 3. The method according to claim 1, characterized in that upon addition of dicarbonyl compounds after mixing and / or grinding, the suspension is heated to the boiling temperature of the liquid phase and then cooled again. Field of application of the invention The invention relates to the field of structural ceramics and relates to a process for the production of polycrystalline Siliziumnitridsinterkörpern, which come as cutting materials and as ceramic wear parts in fittings and engine used. Characteristic of the known state of the art In the production of polycrystalline silicon nitride materials, sintering aids are generally added for sintering into high-strength materials. These sintering aids are mostly oxides of the elements yttrium, the earth side, magnesium, aluminum, strontium, scandium or nitrides, such as. B. aluminum nitride. In order to achieve a homogeneous distribution of these sintering aids in the silicon nitride powder and / or mixture to be sintered, various methods are used. In patent specification WP 292588, the introduction takes place by means of mixed grinding. The disadvantage of such mixed grinding in the dry state or in suspension is the need for large meals, which require the use of expensive and wear-resistant grinding aggregates with limited dispersity and homogeneity of the ground material. In this case, the distribution of the sintering aid does not succeed in the desired extent, which also has a negative effect on the reliability of the sintered body produced. Furthermore, introduced rare earth oxides in the cold-pressed moldings can react with atmospheric moisture and the moldings are mechanically damaged while increasing the volume. A higher homogeneity of the distribution of the sintering aids is achieved by precipitating hydroxides or oxalates geir.äü the patent DE 3717831 and then calcining to the oxides. The advantage of better dispersity is counteracted by the following disadvantage: rare earth oxides used as sintering aids can only be obtained anhydrous at temperatures around 1000 ° C., so that at these high temperatures the water corresponds to the equation Si ₃ N ₄ + 6H ₂ O -> 3SiO ₂ + 4NH ₃ reacts with the sintering silicon nitride to silicon dioxide. Thus, the oxygen content in the powders to be sintered undesirably increases, resulting in deterioration of the high temperature properties due to the formation of more or less viscous glass phase between the silicon nitride grains. Although the introduction of sintering aids via metal nitrates according to the published patent application DE-OS 3711191 takes place from homogeneous solution, but the metal nitrate can not be noticed on the silicon nitride and fix in high distribution. In addition, upon decomposition of active oxygen, the sintered silicon nitride powder is formed according to the equation Si ₃ N ₄ + 6O ₂ -> 3SiO ₂ + 2N ₂ z.T. oxidized, which in turn results in a deterioration of the high temperature properties. The disadvantage of this production method is that in the case of silicon nitride sintered bodies produced in this way, the field of application is limited by the insufficient high-temperature properties.