DE2829167A1 - METHOD OF MANUFACTURING OBJECTS FROM DENSE BETA SILICON NITRIDE - Google Patents

Description

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Patent application

Applicant: NORTON COMPANY

Worcester, Massachusetts OI6O6, USA

Title: Process for the manufacture of objects from dense β-silicon nitride

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Applicant: Norton Comp,

description

The invention relates to a method for producing Objects made of fully compressed silicon nitride, in which silicon nitride is placed in a mold and up to Conversion into ß-silicon nitride is heated under a low but sufficient pressure for densification, whereupon temperature and pressure for complete densification of the formed β-silicon nitride can be increased.

US Patents 3,468,992, 3,54g,402, 3,819,786, 3,830,652, 3,835,211 and 3,839,540 are assumed as prior art.

While according to the invention a new hot pressing process of particular Is important, other common methods such as sintering and reaction bonding are also possible, based on their applicability on the fact that they can be used to preform a product which is subsequently to the is subjected to the method according to the invention, especially when the configuration of the target body is relatively simple.

A sintering process for the production of silicon nitride is known from the prior art, in which fine silicon nitride is mixed together with up to 25 % of a binder in the form of boric acid, boron oxide or boron phosphate, the mixture in a form at room temperature and at a pressure of 500 to 5000 kg / cm

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compressed and this compact is then sintered in a nitrogen atmosphere at a temperature between 850 and 1250 ⁰ C to essentially Oc silicon nitride.

One known method of reaction bonding or reaction sintering uses a mixture of silicon powder, 37 µm, and up to 35 % temporary organic binder, e.g. B. butyl methacrylate and Trichloräthylenj mixed, a portion of the solvent removed, so that a stiff, sticky mass is obtained at room temperature, which is flexible at 50 ⁰ C and tough. This preformed object is molded at room temperature and then ^{heated to 130 °} C. in order to remove residual solvent. The molded body is finally brought to an ^A ufheizgeschwindigkeit of 50 ° / h to 1000 ⁰ C in a nitrogen stream, thereby decomposed and the temporary binder is removed. The preform, made essentially only of silicon, is brought to a maximum temperature of 1450 ° C. under a static nitrogen pressure of 0.07 kg / cm. In this way, reaction-bonded <X silicon nitride with a density of 2.24 g / cm is obtained.

A method for producing silicon nitride objects by hot pressing silicon nitride is also known from the above prior art. The difference from the above method is the compound added to the silicon nitride powder before hot pressing. One method employs 1 to 3.5 weight percent of an yttrium compound and the other employs a lithium compound. The hot pressing takes place in a graphite mold at 1550 to 1800 ⁰ C under a pressure of 420 to 490 kg / cm or at a temperature between 1395 and 1900 ⁰ C at a pressure of only 140 kg / cm.

Another known method is first by reaction sintering a preform is formed and then hot-pressed. Such a two-stage process can also be carried out according to the invention come into use. According to this prior art, silicon powder, 3 μm, with magnesium oxide as a flux and mixed with a temporary organic binder, the mixture is heated to remove the binder, then is applied to the desired shape compacted, but with small oversize. That

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Silicon is then nitrided at a maximum of 130o C ⁰ in a 90% nitrogen atmosphere to a body of porous silicon nitride having a density of-1.7 to 2.7 g / cnr. This body is then placed in a graphite mold and hot-pressed under a pressure of 280 kg / cm and at a temperature of 1750 ° C. onto the body of the final dimension and density. In another embodiment, the nitration is carried out in a modified form, first at 1250 ° C, whereupon the temperature is then increased to 14OO ° C and finally beta-phase silicon nitride is carried out at 1550 ^o C, the reaction of the silicon to be in the main. The porous body of low density is then coated with a release agent, placed in the graphite mold and hot-pressed at 280 kg / cm and 175O ° C to the desired object with a density of 2.2 to 2.7 g / cm.

Finally, a method for hot pressing of silicon nitride with simultaneous conversion of the modification into the β modification is known. For this purpose, iX silicon nitride powder containing 2 % MgO is brought into a graphite mold of the appropriate shape and then hot-pressed to the desired density. For this you need Ί500 to 1850 ⁰ C and a pressure of 70 to 350 kg /

cm. As soon as the temperature and pressure are reached, these process conditions are at least another 10 minutes for the modification change maintained. But you can also first hot-press silicon nitride powder to the appropriate density and then still heat for the modification change, which then no longer requires pressure.

The invention now relates to a hot-pressing process for the production of objects from high-density silicon nitride, silicon nitride changing from the Λ-modification to the ß-modification during hot pressing. In the main-silicon nitride powder ix, optionally in admixture with conventional sintering aids or fluxes is introduced into a hot press mold made of graphite and slowly to 1500 ⁰ C under a pressure below the full densification of the silicon nitride Instead

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finds, ζ. B. between atmospheric pressure and 100 kg / cm ^, but preferably at about 35 kg / cm. This temperature and the low pressure are maintained for a sufficiently long time that practically a complete conversion of the Oi -modification into the β-modification can take place. The temperature is then increased to 1850 ^° C. and the pressure to at least 140 kg / cm, as a result of which the β-silicon nitride is completely densified.

It is essential to the invention that "silicon nitride first in the ß-modification is converted without complete compression, which is only followed by compression. Achieved this one by varying the temperature and pressure. The modification change is achieved without any significant compression up to a pressure of a maximum of 100 kg / cm at 14OO ° C at atmospheric pressure at a temperature of up to 1850 ° C or corresponding temperature-pressure combinations. The densification of the ß-silicon nitride then takes place at 1500 to 1800 ° C at one pressure

of at least 140 kg / cm.

In contrast to the prior art discussed above, where first the compression of the starting material and only then the Modification change takes place under pressure, the procedure according to the invention is reversed, namely without any significant compression made the modification change and only then compressed in the desired manner. The main problem with this The prior art is that during the modification change under pressure, the expensive graphite molds often break go, possibly due to the change in volume of the silicon nitride due to the phase change.

Not necessarily, but preferably, should be in the inventive Process, the grain size of the powder used should be a maximum of about 3 μm. It is well known that products become more maximal Strength is achieved if you start from a powder with a maximum grain size of 3 / µm. The source material should be

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consist of at least 50 % d-silicon nitride.

For an optimal procedure according to the invention, the phase change should take place between 1400 and 1750 ⁰ C / a pressure of 0 to 70 kg / cm ² in 30 to 180 minutes. For compaction, the temperature is then increased to 1600 to 1850 G and the pressure to at least 140 kg / cm, and the objects are kept under these conditions for 10 to 60 minutes. These process conditions can be varied considerably in both directions depending on the presence of other substances such as sintering aids, the thickness and shape of the articles to be produced, the proportion of tf-modification versus β-modification in the starting material and the like.

The invention is further illustrated by the following example. example

4 kg ^ '- silicon nitride and 120 g magnesium carbonate were mixed and the mixture was placed in a ball mill lined with tungsten carbide using tungsten carbide balls, 8.5 to 12.7 mm in diameter, and ground with 9 1 isopropanol for 16 hours, then over a 500 mesh screen sieved and dried at 90 ⁰ C. The obtained cake was crushed in a drum with rubber balls in 2 hours.

2.254 kg of this mixture were now placed in a graphite mold, whose mold surface was lined with boron nitride. The mold allowed a 158.75 "x to be made 158.75 mm. The mold was fully assembled and the Contents then hot-pressed according to the following program:

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a) temperature rise from room temperature to 1500 ^° C. within 100 min under a pressure of 35 kg / cm;

b) pressure and temperature were maintained for 90 minutes;

c) the pressure increased every 10 minutes by 17.5 kg / cm
210 kg / cm increased at a temperature of 1500 ⁰ C;

d) the pressure of 210 kg / cm is maintained and the temperature is increased from 1500 to 1740 ^° C. within 40 minutes;

e) these conditions were maintained for about 25 minutes;

f) cooling and relieving the mold.

The molded body of the above dimensions obtained has a density of 3.22 g / cm3 and a transverse flexural strength of 9,103.5 kg / cm under 4-point loading and passed essentially completely made of ß-silicon nitride.

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Claims (4)

DR. INO. V. AVinsSTirOKK _{M. () 0} MÜH V. HKX J) O I) H-IC. ν. l'KCHJIAKN ^si; "WKKi nnsTHASSE 2 I) H. IN". I). ItBUKKIfS tki.kion (OKO) UOSOSl Dill·. INO.lt.CJOKTK τβι, βχ JS 24 070 ^ Q, * jj »Q 1 £ v *? I'ATHNTANWAI.TE "'"' "! '!" "" "." V 1A-50 956 Applicant: Norton Comp. Claims ii Process for the manufacture of articles from high density β-silicon nitride by hot pressing of Silicon nitride, characterized in that silicon nitride is essentially <# - modification at high temperature and under a pressure that does not lead to complete densification of the silicon nitride, and converted into the β-modification / subsequently by increasing the compression of the ß-silicon nitride performs the compression of temperature and / or pressure. 2. The method according to claim 1, characterized in that the modification change between 14OO ° C under a maximum pressure of 70 kg / cm and 175O ⁰ C under atmospheric pressure and the compression of the ß-silicon nitride between 1500 and 1850 ⁰ C under a Pressure of at least 140 kg / cm. 3. The method according to claim 1 or 2, characterized in that one silicon nitride with a mean Grain size of a maximum of about 3 / µm and a content of ^ -silicon nitride used by at least about 50%. 4. The method according to claim 1 to 3, characterized in that the modification change in up to 180 minutes and compaction in 10 to 60 minutes. 8192 009883/0933