DE2356921A1 - METHOD FOR MANUFACTURING SILICON NITRIDE CERAMICS - Google Patents

Description

"concerning

Process for the production of silicon nitride ceramics

It is "known to manufacture silicon nitride ceramic bodies by First silicon nitride was produced, this was then broken and ground, and powdered silicon nitride was sintered under pressure will. All of these processes are very complex, and the ceramic bodies obtained are not very dense.

The invention now relates to a method for reducing the costs of producing silicon nitride molded bodies, wherein these come as close as possible to the theoretical density and in the process according to the invention the intermediate stages of the Breaking and grinding of silicon nitride, which is known characterized by extraordinary hardness, should be avoided.

According to the process according to the invention, the moldings are thereby manufactured by first shaping silicon powder accordingly with compression, then the green compact in a nitrogen atmosphere burns until the silicon is completely nitrated (Reaction sintering), whereupon the reaction sintered body is pressure sintered into the final shape. It has now been found that the manufacture high "dense body according to the invention to pro-

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products that are stable to high temperatures and in addition, even at elevated temperatures, low Creep and good impact resistance can be identified.

Since the starting material is finely divided silicon and it can be produced by reaction sintering, see above silicon and nitrogen are converted to silicon nitride, with no change in dimension. You have noticeable leeway in the shaping of the end products during the initial powder compaction or - in which roughly the end shape is formed.

According to a preferred embodiment of the method according to the invention the body is infiltrated with a flux before pressure sintering. The flux is preferably magnesium oxide in an amount ranging from $ 1 to $ 5

Additional material such as silicon dioxide, silicon carbide, graphite, boron nitride, aluminum, titanium or iron can be added Silicon powder can be added before the first compaction in order to improve special properties of the finished product.

The pressure sintering takes place at elevated temperatures, a maximum of 1600 ° to 1850 ⁰ C.

Complete conversion of the silicon powder pellet to an interlocking crystal aggregate of silicon nitride is achieved in a nitrogen atmosphere at a temperature just below the melting point of silicon until complete conversion to silicon nitride has taken place. This generally takes about 12 hours, depending on the cross-sectional area of the compacts. The reaction starts at 1200 ° C, but is faster at temperatures in the vicinity of 1400 ° C. Although the reaction is exothermic, it is preferable, especially in the case of large cross-sectional areas, not ^{to go above 135O 0} C in order to prevent local melting of silicon powder into lumps that would be inaccessible to reaction sintering.

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prevent. The nitration can preferably also be carried out in two stages v / earth (GB-PS 887 942). Although there are no dimensional changes or noticeable distortions during reaction sintering, it is necessary to provide sufficient excess material to prevent shrinkage during the subsequent pressure sintering to compensate. It may also be necessary to provide excess material on certain areas that will eventually be worked off v / ground, so that the molded body obtained has a high surface quality.

The silicon nitride body is then compacted under slight pressure in a graphite mold, coated with a boron nitride paint, while the temperature is increased to a maximum of 1600 ° to 185O ⁰ O and at the same time the pressure is increased to 2.35 kg / mm (1.5 t / sq_.in ) is increased. These conditions are then maintained in the mold for a short time, depending on the material thickness, and then slowly removed. This is then followed by tempering to remove the residual stress. By compacting the silicon powder into a body in an approximately final shape and by using graphite molds and possibly a simple cutting operation, it is possible to produce shaped bodies largely in the final shape, so that the costly reworking costs are kept to a minimum.

The process can be simplified by infiltrating a Flux in the porous sintered body prior to the step of pressure sintering erns. Magnesium oxide, magnesium nitride, calcium oxide, aluminum oxide, iron oxide, beryllium oxide, Beryllium nitride and calcium nitride.

A modification of the inventive method for "production various materials based on silicon nitride technology for unusual combinations of physical and mechanical properties, especially the components for tribological and high temperature systems, is that additional material is incorporated into the silicon powder prior to the reaction sintering. - 4 -

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Examples of these materials are dispersions from 1 to 50 vol.? ' such substances in silicon nitride as silicon dioxide, Silicon carbide, graphite, boron nitride, aluminum, titanium or iron in finely divided or coarse-grained form in a random arrangement or as oriented wires or fibers or in sandwiches structure.

Through selective placement of reinforcement materials during In the initial stages of production it is possible to produce parts with locally better properties, such as surface hardness, electrical Conductivity, thermal insulation capacity, or with special properties with regard to the connection of Ceramic and metal. It is also possible to modify the structure of these products and the properties through further ones To influence hot processing, which can also aim to achieve special shapes from the material.

At the special request of a customer, bodies with a practically theoretical density were made of silicon nitride containing 1 to 5 YoI. -90 Magnesium oxide, produced, with additional dispersions of) 10 to 30 Vol .- ^ o silicon carbide for tribological and high-temperature properties | with 10 to 30 Vol .- $ graphite for storage properties _| with 10 to 30 fo boron nitride for tribological and wear properties and finally 10 to 30 fo iron for impact resistance and properties. Room temperature.

It is obvious that by making slight modifications to the above procedure, special alloys and composites for can produce special requirements.

Claims 81 / XIX

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Claims (6)

Claims 1. Process for the production of bodies from silicon nitride starting from silicon powder by reaction sintering in nitrogen atmosphere, characterized in that one Silxcium powder compacted into the desired shape, then in Reaction sintered in a nitrogen atmosphere until complete nitration and then sintering the sintered body under pressure ,. approximately , ., . until the final shape. · 2. The method according to claim 1, characterized geken'.n draw t that the reaction-sintered shaped body is infiltrated with a flux and then pressure-sintered. 3. The method according to claim 2, characterized in that the flux is 1 "to 5 vol .- $ magnesium oxide applies. 4. The method according to claim 1 to 3, characterized in that the silicon powder is additionally silicon dioxide, Silicon carbide, graphite, boron nitride, aluminum, titanium and / or iron, especially in an amount of 10 to 30 clogs. ■ '· ■ 5. The method according to claim 1 to 4? characterized in that, for pressure sintering, the reaction-sintered molding is heated under pressure to a temperature of a maximum of 1600 to 185O ⁰ C. 6. The method according to claim 5, characterized in that that one pre-presses the reaction sintered compact in a graphite mold under leiöxtem pressure and with increasing - 2 - 409822/1034 - t - 1A-44 028 ² Press pressure to 2.35 kg / mm the temperature to a maximum of 1600 ° - 1850 ⁰ G increases. 81 / XIX 409822/1034