NO874395L - PROCEDURE FOR THE PREPARATION OF ENGINEERING CERAMIC POWDER WITH ADDITIVES. - Google Patents

Description

The invention relates to a method for the production of engineering ceramic powders whose individual particles are coated with an oxidic additive consisting of a different material.

Furthermore, the invention relates to S13N4 powder and the use of these powders, and the engineering ceramic powder which is obtainable by the method according to the invention for the production of ceramic shaped bodies.

Increasing importance in material technology for so-called engineering ceramic powders. Engineering ceramic powders within the scope of the invention are ceramic powders of one or more substances from the group BN, B4C, A1N, A1203, SIC, Si3N4, TIC, TiB2, Zr02, including partially stabilized and fully stabilized ZrC>2, and/or mixed forms thereof.

The production of shaped bodies from engineering ceramic powders takes place by shaping and sintering methods.

However, many engineering ceramic powders are, in their pure form, only slightly or not at all sinter-active. By adding additives, however, the ability to sinter can usually be improved, and thus a strong compaction of the material is achieved by sintering.

As additives, it therefore has, above all, suitable oxidic materials such as MgO, Y2O3, AI2O3 and B2O3.

The engineering ceramic powders are mixed and/or ground thereto mostly according to DE-A-20 472 555 and US-A-3 992 497 with the additives. When sintering this mixture, however, a complete compression of the material is usually only achieved at very high pressures and temperatures.

In the doctoral dissertation of G.WBttin, TU Berlin 1983, page 27 and pages 83-86, it is proposed to apply salts (eg magnesium acetate) in the form of a solution to the surface of 813^- powders. After spray drying and production of a raw body, by isostatic pressing, the salts separated on the surface of the Si3N4 powder are transferred by annealing in the air into the corresponding oxides.

This method has the disadvantage that volatile decomposition components of the salts escape during the annealing, and can lead to displacement of the bodies or to an undefined pore size distribution in the body. Moreover, the powders obtainable according to this method are not usable for shaping by mud casting, as the additives would again be dissolved in the mud.

Another possibility for applying avoxidic additives to the surface of ceramic powders consists in the so-called alkoxide-sol-gel method (compare Horizons of Powd.Metall-urgy, Part. II, Proceedings of the 1986 Intern. Powd. Met. Conf. and Exhib., publisher W.A.Kaysser, W.J.Huppmann, pages 1151-1154, H.Kubo, H.Endo, K.Sugita "Sintering Behavior of ultra-fine Alumina-coated Silicon Carbide".)

The SiC powder is then mixed with a solution of aluminum isopropoxide (I-C3H7O) qA1) dissolved in an organic solvent. After a drying, a hydrolysis of alkoxides to a boehmite (AlOOH) gel respectively -Sol and/or pyrolysis of alkoxides respectively the AlOOH gel, you get the SiC powder with an AlOOH coating. This coating is then transferred at 1200°C to aluminum oxide.

However, this method has the disadvantage that a working inorganic solvent is required. Moreover, this method cannot be easily transferred to other additives, as many alkoxides of elements that are transferable to oxidative additives form easily volatile compounds, which leads to problems in their handling. Not least, this method also has the disadvantage that alkoxides are mostly not readily available cheap. The task of the invention now consisted in providing a method for the production of ceramic powders whose individual particles are coated with an oxidic additive consisting of a different material, which does not have the mentioned disadvantages.

Surprisingly, it was now found that the task can be solved as oxidative additives are applied in the form of salts or salt mixtures soluble in aqueous media and thermally decomposable to oxides, on the individual particles of the ceramic powder, and by thermal decomposition are transferred to the corresponding oxides.

The object of the invention is thus a method for the production of engineering ceramic powders whose individual particles are coated with an oxidative additive consisting of a different material. The method is characterized in that the additive is applied in the form of a salt or salt mixture that is soluble in an aqueous medium and can be thermally split into oxide, and the individual particles, and by thermal decomposition is transferred to the corresponding oxide form.

Preferably, salt or the salt mixture is used in such quantities that the ceramic powders are coated with 0.5 to 25 wt% of an oxide additive after the thermal cleavage.

Different substances can be used as salt or salt mixture. However, preference is given to those which, after the thermal decomposition, form an oxidative additive from the group Alkaline earth oxides B2O3, AI2O3, Ga203, Sc203, Y2O3, La203, TI02, Zr02, HfO2, Cr203, oxides of the rare earth metals, mixtures or mixed oxides of two or more of the aforementioned oxides.

Particularly preferred is an embodiment of the method according to the invention where the salt or salt mixture is dissolved in an aqueous medium, mixed with a ceramic powder and the salt or salt mixture from the mixture is applied as a coating to the individual particles of the ceramic powder by spray drying, freeze drying or a precipitation reaction.

However, it is also possible to apply the salt or the sales mixture to the individual particles of the engineering ceramic powder by another type of drying, for example by introducing the mixture into a water-extracting solvent. In another variant of the method according to the invention, the engineering ceramic powders are sprayed with salt solution and then dried.

Especially in the case of non-oxidizing engineering ceramic powders, it is advantageous to carry out the thermal decomposition under inert gas or in a vacuum, whereby a possible oxidation of the powder can be avoided. Nitrogen or argon is preferably used as inert gas.

Hydroxides, carbonates, nitrates, salts of organic acids from the group of carboxylic acids, hydroxycarboxylic acids or aminocarboxylic acids or mixtures thereof are preferably used as salts soluble in aqueous media.

In order to obtain products with the highest possible degree of purity, it is appropriate to use salts that can be decomposed and/or burned to oxide without residue. It is therefore particularly advantageous to use salts such as formates, hydroxides and nitrates, which can be completely decomposed without the supply of oxygen. A thermal decomposition of the oxides can in these cases be carried out under inert gas or in a vacuum, whereby, as mentioned, oxidation of the non-oxidative engineering ceramic powder is avoided.

However, even when using salts that are not completely cleavable without oxygen supply (for example acetates), oxidation of the non-oxidizing engineering ceramic powders can be prevented when the thermal decomposition is started in the first place at a temperature below 600°C in air, and then continues under protective gas or 1 vacuum at temperatures of 600-1200°C.

The method according to the invention is universally applicable to all engineering ceramic powders. Engineering ceramic powders whose individual particles are coated with oxidic additives are obtained in a simple and economical way.

According to this method, non-oxidic and especially nitrided engineering ceramic powders can be particularly advantageously coated with oxidic additives.

The coated engineering ceramic powders can be completely compressed by sintering without the aforementioned disadvantages occurring.

A further advantage of the method according to the invention lies in the fact that it is possible to apply homogeneous mixed oxides by using corresponding salt solutions on the individual particles of the powder.

The method according to the invention is particularly suitable for laying Si3N4 powders with oxidative additives. The obtainable Sis^ powders are distinguished by particularly favorable sintering properties and a weight loss of less than 1 weight-$ when annealing up to 1000°C. Such Si3N4 powders are not known to date.

The object of the invention and therefore also the Si3N4~ powder, which is characterized by the fact that their individual particles are coated with oxide additive amounts from 0.5-25 wt- 56.

Preferred are such Si3N4 powders whose individual particles are coated with an additive from the group of alkaline earth oxides, B2O3, A1203, Ga203, Sc203, Y203, La203>Ti02, Zr02, Hf02, Cr203, oxides of the rare earth metals, mixtures or mixed oxides of two or several of the aforementioned oxides.

Si3N4 powders having a surface enrichment factor K greater than 10 are particularly preferred.

The surface enrichment factor K is a measure of the surface coating of the Si3N4 powder single particles with additives, can be detected by deep profile investigations with secondary ion mass spectrometry (SIMS; F. Schulz, K. Wittmaack, J. Maul, Radiation Effects 18, 211-215 (1973); S. Hofmann, Appl. Phys. 9, 59-66

(1976); S. Hofmann, Surface and Interface Analysis 2, 148-160

(1980); R. G. Gossink, Glass Technology 21, 125-133 (1980)). Factor K is the ratio of the quantitative composition of the surface referenced to that of the volume (ie the beginning of the depth profile to the end of the depth profile). For the SIMS measurements, the following apparatus parameters have proven suitable: primary ions Ar<+> (argon cations), 12 keV (kiloelectron volts), current IO-<7>A (amperes) at a beam diameter of 1.2 mm , mixture of the usually non-conductive powder to be measured with pure Ag (silver) powder in a ratio of 1:4. If Si3N4~ particles and the additive particles are present next to each other, the SIMS investigations show a time-independent linear intensity progression of the ions characteristic of the individual substances. However, if the Si3N4 particles are coated with additives, at the beginning of the measurements, predominantly the ions of the additives and less the ions of Si3N4 appear in the spectrum. Only when the covering layer has been removed by ion shielding does the intensity of the ions formed from Si3N4 increase, those of the ions from the additives decrease until an equilibrium is reached, i.e. the spectra show a strong time-dependent course. With the above-mentioned apparatus parameters, the equilibrium setting is reached after 60 minutes at the latest, which corresponds to decomposition from 500 to 1000 Å. With SIMS investigations, it is also possible to find quantitative statements. After an adjustment with mixtures of Si3N4 powders and additives with known additive concentration, the ion intensities measured with SIMS can be converted into concentrations of the individual substances. As a measure of the quality of the surface coating, a surface enrichment factor K is defined using the measured concentrations:

If there is no coating of Si3N4~ powders with additives, the measured concentrations at the beginning of the measurement (t=0; sample surface), and after a longer ion protection (t=60 minutes; layer at a depth of approx. 500-1000Å), are equal . No surface enrichment is present, and a factor of K=l appears. If, on the other hand, there is an ideal closed encasement of the Si3N4 powder particles with additives, the concentration of Si3-N4 powders on the sample surface (t=0) is equal to 0, resulting in a surface enrichment factor of K = (infinity). In real samples, the value K=°° is not achieved.

This method can be applied to all ceramic powders with additives. Products with a surface enrichment factor K greater than 10 have a sufficient coating with an oxide additive to improve the sintering properties.

The object of the invention is also the use of the Sis^ powders according to the invention, and the engineering ceramic powders obtained according to the method according to the invention for the production of ceramic moldings.

Due to the very small weight loss when annealing up to 1000°C and the high degree of surface coating with oxidic additives, the aforementioned powders can be particularly advantageously used in the production of ceramic molded bodies. or injection moulding), as also with the subsequent sintering method (for example pressureless sintering, gas pressure sintering, hot pressing, hot isostatic pressing) the disadvantages mentioned can be avoided.

The method according to the invention shall be explained in more detail by means of an example.

As characteristic sizes of the product coated with oxidative additives, the surface enrichment factor K and the growth loss upon annealing at 1000°C were determined. For comparison, the surface enrichment factor K of a corresponding powder that was produced by grinding together with oxidative additives was determined.

Example.

From 100 g of Si3N4~powder, 95 ml of an approx. A 12 weight-* solution of yttrium formate dihydrate in water and 20 ml of a 18 weight-* solution of aluminum formate in water are prepared in a stirring apparatus into a suspension. This is dried in a spray dryer. The dry product is calcined under air for 1 hour at 600°C, and then under nitrogen for 1 hour at 1000°C. After the calcination, the Si3N4~ powder was coated with 5.3 wt-* Y2O3 and 1.1 wt-* AI2O3, referred to the total amount of powder.

On the calcined product, deep profile measurements are carried out with SIMS (primary ions Ar<+>, 12 keV, current 1.10-<7>A at a beam diameter of 1.2 mm).

From the measured ion intensities, the following concentrations c (indicated in weight-*) appear:

on the sample surface (t = 0):

cAl 2 O 3 = 25.1*; cY 2 O 3 = 57.3*; cSi 3 N 4 = 17.6*; in the inner samples (t = 60 minutes): cAl203= 4.4*; cY203= 13.2*; cSi 3 N 4 = 82.4*; and a surface enrichment factor of

After annealing to 1000°C, no weight loss could be observed.

Comparative example 1

A suspension is prepared from 140 g Si3N4 powder, 16 Y2 O3 powder, 3.6 g Al2 O3 powder and 350 ml isopropanol. This is ground for 1 hour in a pipework ball mill with Al203 beads. The grinding beads are then separated and the suspension is dried in a spray dryer.

Deep profile measurements with SIMS are carried out on the dried product. From the measured ion intensities, the following concentrations c (indicated by weight-*) appear:

on the sample surface (t = 0):

cAl 2 O 3 = 1.2*; cY 2 O 3 = 7.9*;

cSi 3 N 4 = 90.9*;

in the inner sample (t = 60 minutes):

cAl 2 O 3 = 1.6*; cY 2 O 3 = 4.7*;

cSi 3 N 4 = 93.7*.

The surface enrichment factor therefore amounts to:

Comparative example 2

A suspension is prepared from 93.6 g Si3N4 powder, 5.3 g Y2 O3 powder, 0.3 g Al2 O3 powder and 350 ml isopropanol. This is ground for 1 hour in a pipework ball mill with A^C^ beads. The grinding beads are then separated and the suspension is dried in a spray dryer.

After the spray drying, the powder mixture was composed of 5.5 wt-* Y 2 O 3 , 1.9 wt-* Al 2 O 3 , and the remainder Si 3 N 4 .

Deep profile measurements with SIMS are carried out on the dried product. From the measured ion intensities, the following concentrations c (indicated by weight-*) appear:

on the sample surface (t = 0):

cAl 2 O 3 = 2.2*; cY 2 O 3 = 1.9*;

cSi 3 N 4 = 95.9*;

in the inner test (t = 60 minutes):

cAl 2 O 3 = 3.8 cY 2 O 3 = 1.9*;

cSi 3 N 4 = 94.3*.

The surface enrichment factor therefore amounts to:

The very small surface enrichment factors K show that when grinding Si3N4 powders with the oxidic additives, only a small surface coating of the individual Si3N4 powder particles is achieved.

Using the method according to the invention, on the other hand, a significantly improved surface coating of the 813^ powder single particles with the oxidic additives is achieved.

Claims (12)

1. Process for the production of engineering ceramic powders whose individual particles are coated with an oxidic additive consisting of another material, characterized in that the additive is applied to the individual particles in the form of a salt or salt mixture that is soluble in an aqueous medium and can be thermally split into oxide, and is transferred by thermal splitting into the corresponding oxide form. 2. Method according to claim 1, characterized in that the salt or salt mixture is used in such quantities that the ceramic powders after the thermal cleavage are coated with 0.5-25 weight-* of oxide additive. 3. Method according to claim 1 or 2, characterized in that the additive is an alkaline earth oxide, B2 O3 , AI2 O3 , SC2 O3 , Ga2 03 , Y2 O3 , 1^2 03 , T102 , ZrC"2 , HfO2 , 0203, ethoxide of the rare earth metals , a mixture or a mixed oxide from two or more of the said oxides. 4. Method according to one or more of claims 1 to 3, characterized in that the salt or salt mixture is dissolved in an aqueous medium, mixed with the ceramic powder and the salt or salt mixture from the mixture is applied by spray drying, freeze drying or a precipitation reaction as a coating on the individual particles of the ceramic powder. 5. Method according to one or more of claims 1 to 4, characterized in that the thermal cleavage is carried out under inert gas or in a vacuum. 6. Method according to one or more of claims 1 to 5, characterized in that the salt soluble in an aqueous medium is a hydroxide, a carbonate, a nitrate, a salt of an organic acid from the group of carboxylic acids, hydroxycarboxylic acids or formic acids, or a mixture thereof. 7. Method according to one or more of claims 1 to 6, characterized in that the engineering ceramic powder is a non-oxidizing powder. 8. Method according to one or more of claims 1 to 7, characterized in that the engineering ceramic powder is a nitride powder. 9. Si2 N4 powders, characterized in that their individual particles are coated with an oxidative additive in amounts from 0.5 to 25 weight-*, and the Si3 N4~ powders when annealing up to 1000°C have a weight loss of less than 1 weight-*. 10. Si3 N4~ powders according to claim 9, characterized in that the additive is an alkaline earth oxide, B2 O3 , AI2 O3 , Ga2 03 , SC2 O3 , Y2 O3 , La2 03 , T102 , ZrC>2 , HFO2 , CV2 O2 , an oxide of the rare earth metals, a mixture or a mixed oxide of two or more of the said oxides. 11. Si3 N4 powders according to claim 9 or 10, characterized in that the Si3 N4 powders have a surface enrichment factor greater than 10. 12. The use of the Si3 N4 powders according to one or more of claims 9-11 and the ceramic powder obtained according to a method according to one or more of claims 1-6 for the production of ceramic shaped bodies.