DE4106228A1 - Prodn. of polycrystalline silicon nitride or carbide sinter prods. - using di:carbonyl cpd. to produce complex cpd(s). with sintering agents thereby preventing presence of water which can lead to oxidn. - Google Patents

Abstract

Method of producing polycrystalline silicon nitride or silicon carbide sintered bodies consists of mixing together Si and/or Si3N4 powder or SiC powder of average grass size below 5 microns with one or more sintering agents plus dispersion and/or reinforcement component, is an organic solvent or solvent mixt. and/or H2O followed by grinding. Prior to, during or after grinding dicarbonyl cpd. is added in an amt. which is stoichiometrically above that required to form rare earth metal oxide dicarbonyl complexes. After sepn. from the liq. phase and drying, the complex compounds are decomposed at upto 600 deg.C to provide the dispersed sintering agent. USE/ADVANTAGE - Producing sintered bodies of Si3N4 and SiC as cutting tools and ceramic parts in armatures and motor construction. By using a dicarbonyl compound, no significant increase in the oxygen content of the powder material is detected which leads to a higher high temp. strength in the sintered body as the prod. of low viscosity glass phase in the grain boundaries is prevented. The sintering agent is very well dispersed in the powder.

Description

The invention relates to the field of construction ceramics and relates to a process for the production of polycrystalline Silicon nitride or silicon carbide sintered bodies which, for. B. as cutting materials and as ceramic wear parts in Valve and engine construction are used.

In the manufacture of polycrystalline silicon nitride or Silicon carbide materials become too strong for sintering Materials usually added sintering aids. These sintering aids are mostly oxides for silicon nitride of the elements yttrium, rare earths, magnesium, aluminum, Strontium, scandium or nitrides, such as e.g. B. aluminum nitride. In addition to conventional sintering aids, silicon carbide is used like boron, aluminum, their carbides and carbon too Oxides of yttrium, aluminum and rare earth elements used (I.-M. Lihrmann et al., Proc. Science of Sintering, 1989, Plenum Press New York, p. 367). To ensure a homogeneous distribution of these sintering aids in the sintering silicon nitride powder and / or mixture or silicon carbide powder and / or mixture to achieve different Procedure applied. It is introduced in the patent specification WP C 04 B / 30 61 616 by means of mixed grinding. The disadvantage of such a mixed grinding in the dry state or in suspension is the need  large meals with limited dispersity and homogeneity the regrind the use of expensive and more wear-resistant Make grinding units necessary. The distribution of the Sintering aid not to the desired extent, which is also on the reliability of the sintered body produced affects. Furthermore, rare earth oxides introduced into the cold pressed moldings react with air humidity and the Moldings are mechanically damaged while increasing the volume.

A higher homogeneity of the distribution of the sintering aids succeeds by precipitating hydroxides or oxalates according to the Patent DE 37 17 831 and subsequent calcination to the Oxides. The advantage of better dispersity is as follows Disadvantage against: rare earth oxides used as sintering aids can only be anhydrous at temperatures around 1000 ° C obtained so that the water accordingly at these high temperatures the equation

Si₃N₄ + 6 H₂O → 3 SiO₂ + 4 NH₃

reacts with the sinter-active silicon nitride to form silicon dioxide. This increases the oxygen content in the powders to be sintered undesirable, which affects the high temperature properties due to the formation of more or less viscous Glass phase between the silicon nitride grains results.

The introduction of sintering aids via metal nitrates according to the published patent application DE-OS 37 11 191 takes place from homogeneous Solution, but the metal nitrate on the silicon nitride do not stand out and fix in a high distribution. It also arises  in the decomposition of active oxygen, the sintered Silicon nitride powder according to the equation

Si₃N₄ + 6 O₂ → 3 SiO₂ + 2 N₂

e.g. T. oxidized, which in turn worsened the high temperature properties has the consequence. The disadvantage of this manufacturing process is that in silicon nitride sintered bodies produced in this way by the inadequate High temperature properties limited the area of application is.

For highly disperse silicon carbide powders, the disadvantages of the above method in an analogous manner.

The problem specified in the invention is the problem based on specifying a method by which reliability and the high temperature properties of polycrystalline sintered bodies be improved.

In the process according to the invention, the dicarbonyl compounds used implemented into complexes and this or that used dicarbonyl metal oxide complexes over the liquid Phase noticed. These conspicuous complexes are stored as very small particles finely distributed on the surface of the others Particles of the powder mixture. After disconnecting the Grinding liquid is raised to temperatures up to 600 ° C. This will make the finely divided small particles thermally decomposed from dicarbonyl metal oxide complexes and it the oxides of these compounds are present. These oxides act as  Sintering aids that are very finely divided in the powder mixture. It can be seen that none of these results from this powder preparation significant increase in the oxygen content of the powder mixture entry. Although the sintering aid content in the molded body remains unchanged remains, becomes a higher high temperature strength this sintered body compared to those produced by mixed grinding reached. It also increases reliability reached, which is measured in the form of the Weibull module.

An additional advantage is that the good Distribution of the sintering aid in the green body a high sintering activity is achieved. This can reduce the proportion of sintering aids be reduced. This also has an advantageous effect on the high temperature properties of the sintered body out.

Furthermore, the invention is intended to be based on several exemplary embodiments to be discribed.

Example 1-3

Silicon nitride powder with a specific surface area of 10.5 m²g ^-1 , a degree of contamination of 0.5% and an oxygen content of 1.55% is mixed with a mixture of oxides of the rare earth elements neodymium, prasodymium, lanthanum, samarium and cerium (hereinafter as didymium oxide , R₂O₃, designated) and aluminum oxide (composition see table 1) and with 20 ml of acetylacetone (pentanedione-2,4) mixed for 300 min in 85 ml of 96% ethanol and then heated to the boiling point of the ethanolic solution and cooled again. The powder mixture is then dried in vacuo, granulated and pressed at 250 MPa to give shaped bodies 5 × 60 × 6 mm. The moldings are then heated in air to a temperature of 500 ° C. in 10 h and cooled again. The moldings are then sintered at 1700 to 1950 ° C. under a nitrogen pressure of up to 80 bar. After appropriate finishing (grinding of the outer surfaces by approx. 0.3 mm), the four-point bending strength σ⁴ and the Weibull modulus m at room temperature as well as the four-point bending strength σ⁴ at 1200 ° C are determined. These strength properties have been compared with those of sintered moldings of the same starting composition, but produced using the mixed grinding process in a planetary ball mill. The results are shown in Table 1.

Table 1

Characterization of the sintered body

Examples 4 and 5

Silicon nitride powder is used according to Example 1 with a mixture made of aluminum oxide and didymium oxide according to Table 2 and with 15 ml acetylacetone (pentanedione-2,4) 250 min in 85 ml 96% Mixed ethanol. When using yttrium oxide this was added as yttrium oxide acetylacetonate. Subsequently the powder mixture is dried, granulated, on a Temperature of 500 ° C warmed and after a holding time of 2 h cooled down. Then the powder mixture in a graphite matrix hot pressed at 1840 ° C for 60 min under a pressure of 30 MPa. For those with diamond tools made of densely sintered discs cut and finished test specimen (3.5 × 60 × 4.5 mm) the following values are obtained after measuring the strength (see table 2).

Table 2

Characterization of the sintered body

Example 6

80 g silicon powder with a specific surface area of 5 m² / g, a degree of contamination of 0.4% and an oxygen content of 1.1% with 9.5 g of R₂O₃ (see Example 1), 2.2 g Al₂O₃ and 25 ml trifluoroacetylacetone 250 min in 75 ml pure Mixed ethanol. The powder mixture is then dried, granulated and with 200 MPa to form with the Dimensions 5 × 60 × 6 mm pressed. Then the moldings gradually (first stage at 500 ° C) to a temperature of 1420 ° C heated and nitrided. Then the sintering takes place of the rods at temperatures of 1700-1950 ° C at one Nitrogen pressure of 80 bar. So prepared Si₃N₄ rods show the mechanical properties given in Table 3.

Table 3

Characterization of the sintered body

Example 7

α-SiC powder with a specific surface area of 16.1 m² / g and an oxygen content of 0.85% as in Examples 1-5  with a mixture of didymium oxide and aluminum oxide accordingly Table 4 and with 15 ml acetylacetone (pentanedione-2,4) in 300 min Mix 85 ml of 96% ethanol mixed. The sum of the solids is 100 g. The powder mixture is then dried, granulated, heated to a temperature of 500 ° C and cooled after a holding time of 2 h. Then the powder mixture in a graphite matrix at 1950 ° C for 120 min under one Pressure of 30 MPa hot pressed. For the test specimens produced analogously to Examples 4 and 5 the following strength values measured and produced with analog Test specimens without using the manufacturing method according to the invention compared.

Table 4

Characterization of the sintered body

Claims (3)

1. A method for producing polycrystalline silicon nitride or silicon carbide sintered body, in which silicon and / or silicon nitride powder or silicon carbide powder with an average grain size of the powder of <5 microns, with one or more sintering aids, in silicon carbide powder with one or more oxidic sintering aids, the Powder mixture storage and / or reinforcing components can be added, mixed in a liquid phase from organic solvents or solvent mixtures and / or water, it being possible to grind at the same time, then the powder mixture is separated from the liquid phase and dried and further processed to polycrystalline sintered bodies takes place, characterized in that the powder mixture before or during or after the mixing or mixed grinding dicarbonyl compounds in an amount which is greater than the stoichiometric amount to form rare earth metal oxide dicarbonyl is complex, and / or dicarbonyl metal oxide complexes are added, these are mixed with the powder mixture and / or ground, and that after separating and drying the powder mixture and before or during further processing into sintered bodies, the dicarbonyl metal oxide complexes are decomposed at temperatures up to 600 ° C. 2. The method according to claim 1, characterized in that as Dicarbonyl compounds acetylacetone, its derivatives and / or the acetylacetonates of the rare earth oxides lanthanum oxide, Yttrium oxide, aluminum oxide or magnesium oxide can be used. 3. The method according to claim 1, characterized in that at Adding dicarbonyl compounds after mixing and / or grinding the suspension to the boiling point of the liquid phase is heated and then cooled again.