DE2451581A1 - METHOD FOR MANUFACTURING SILICON NITRIDE OBJECTS - Google Patents

Description

DR. A. KOHLEfR M. SCHRCEDER PATENT LAWYERS

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TELEPHONE: 37 WT UZ ' θ MUNICH UO

TELESRAMMEi CARBOPAT MÖNCHEN PRANZ-UOSEI ³ H-STRAeS

US-563 - S / Le, * H0 I ΟίΠ

FORD WERKE AG, COLOGNE

Process for making silicon nitride articles '--- "" ■

The invention relates to an improved method of making silicon nitride articles. In general Such an article is made by adding silicon metal particles to the general shape of the article shaped v / ground. The object thus formed is placed in a gaseous environment containing sufficient nitrogen contains in order to convert the silicon into silicon nitride, heated. The improvement discussed here consists in the fact that the gaseous environment consists of a mixture of etv / s 0.5 to about 4-, 0 volume ^ »hydrogen ^ the remaining volume content of the gaseous environment consists of nitrogen.

Silicon nitride has a wide variety of uses due to its physical and chemical properties.

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BATH ORIGINAL

properties. To name a few, these uses include thermocouple protection tubes, foundry crucibles, substrates for electronic applications, and structural components for gas turbine engines. Silicon nitride can be produced by a number of different processing techniques, each technique yielding a different final density. Also, each technique has a certain limitation on the final shape that can be created. \.

Simple shapes with $ 95 to $ 100 theoretical density can by hot pressing silicon nitride powder under Formation of the finished item can be produced. On the other hand, complex shapes can be created by slip casting techniques which produce a finished article with a theoretical density in the range of $ 80 to $ 85. In the slide casting technique, silicon metal particles become first poured into the desired shape. The silicon metal must then be converted into the silicon nitride in a nitriding process be convicted.

As an alternative to slide casting, silicon nitride articles are used complex shape with theoretical densities from $ 70 to $ 75 manufactured by injection molding techniques. In this technique, a mixture of silicon metal particles and a thermoplastic material is formed. This mixture is forming Injection molded to the shape of the item. To then ©: !! Measures include heating the object to get the thersiö = burn out plastic material, and a nitriding of the Object for producing the finished silicon nitride object. When the injection molding technology to form more complex Forming is used, it has generally been found that the typical strength for the finished silicon nitride counterstaiu'l

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is approximately "at 1200 kg / cm ² (17,000 psi) (modulus of rupture at four point bend).

U.S. Patent 3,591,337 discloses a method of manufacture of a silicon nitride article in which both Silicon as well as silicon nitride particles can be used to make the article. This patent specification also describes that the object is in a 90 volume ^; Nitrogen and 10 volumes ^ hydrogen containing atmosphere is nitrided. British patent 717 555 describes methods of making silicon nitride articles. In this patent it is stated that the nitriding stage either in practically pure nitrogen can take place, or in an atmosphere of technical nitrogen tempering hydrogen consisting of 93 volumes ^ nitrogen and 7 volumes ^ hydrogen.

The present invention provides an improvement over this the method according to the prior art and compared to the method of nitriding in an atmosphere consisting of 100 volumes ^ of nitrogen. Two basic advantages are achieved using the improved nitriding process. One advantage is that the finished Object has a higher strength. The second advantage that can be achieved with the improved method is, "that the finished article has improved creep resistance or has fatigue strength, which is particularly favorable when the material is used for high temperature applications will.

The invention relates to an improvement in a method for the production of articles from silicon nitride, wherein Silicon metal particles are molded into the shape of a finished article. The shaping of the finished article can

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be brought about by that Silieiurametallteilchen χ be mixed with a thermoplastic material, and '4, the mixture thus formed "limiting form is injected into a configuration of the finished article term f If _-.'^-'-: M of the object on dieee Way is trained, "he will be on. ⁴ ^ j heated to a suitable temperature to produce the thermoplastic Γ ίίΙ \

Burn out material, leaving silicon metal behind. ; ^: ^ I'f The silicon metal object thus formed is then ner in Egg ".W gaseous Umgefoung ₉ enough nitrogen to _excess:; | leadership of the silicon in silicon nitride contains heated, ¹ - '* ^$!..

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The special procedure described here is used both for>'| Improvement of the strength as well as the long-term strength of the finished object compared to the values that can be achieved for these properties when the gaseous; ^; v Environment consists practically of pure nitrogen "The ver \: | improvement within the process consists in the fact that the gaseous environment is made up of a mixture of about 0.5 to about fii 4.0 volumes of hydrogen, with the remainder; '|. Volume of the mixtures of nitrogen is "The most OF PREFERRED ^l -3 area stems substance in the gaseous ambient · j% is a mixture of about 1 volume ^ is hydrogen, wobei- _; i ^ is the remaining volume of nitrogen. '.- --- f

Even better values are given for the degree of strength of the ': _; finished object if the gaseous environment>, ί is kept in a static state around the object during the Nltri-, ■; · ■: dierungvSstufe. In particular, were excellent - ^; Low results obtained when the static gaseous environment was maintained at a pressure of about 0.2 a tu (3 psig): '. will. ■ 'V ;.

Preferred embodiments of the invention are described below. · '" ^:

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ORlGlWAL INSPECTED

In this description, the method for producing. · -Ν, · the silicon object to be nitrided is based on a _; /: ■:: J ' ^: ' f injection molding process described. It is of course clear that ';' the improved method of the invention can be applied to any object to be nitrided, regardless of how it was initially made. Examples "^ game as the improved method can be applied to Siliciumge-> ι genstände be applied, by Gleitguß (slip casting) * | or have been made by other forming methods;. \ 'A

■ ^; v | In the production of an object by injection molding : A process, silicon metal powder is generally a ■; ■? selected particle size distribution with a thermoplastic:; | elastic binder with the formation of a molding compound or compound | mixed. In a typical molding compound, the SiIi- ^,? Calcium metal particles about 60 to 66 volumes ^ with the remainder, J, made of thermoplastic material. "--1!

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The particle size of the silicon metal is an injection molding process is of importance for the implementation%. Two ; Parameters for the size distribution are generally comparable to ^r Remember, that the maximum particle size and the average | Particle Size “If the silicon metal becomes too coarse (for example, greater than about 200 microns in diameter) it will not be fully nitrided. On the other hand, when the silicon metal powder is too fine, the material Range from 10 to 13 microns.; ·

When the molding compound is formed, the compound is introduced into the cylinder of an injection molding machine. The machine ■■ '' -. heats the thermoplastic material above its melting point. '.

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ORIGINAL INSPECTED

Pressure is applied to the cylinder and the molding compound, "',' ff ;, is shot into a cold mold with the configuration of the item to be manufactured. The thermoplastic. ■ Material solidifies to the desired shape, i.e. the silicon metal particles carried along in the general desired shape can be determined.

The object formed in this way from silicon metal and thermoplastic material is then slowly heated in an oven. heated to a temperature of about 29O ³ C (55O ⁰ F). The heating schedule can be three days so that no stresses are created in the object during the heating process. During this heating process, the thermoplastic binder is burned off. This process leaves silicon metal in the desired shape. The silicon metal is then made according to the invention nitrided.

The nitriding process of the invention is carried out in a resistance heating furnace fitted with a mullite muffle tube to control the atmosphere. The ends of the muffle pipe are closed using metal end caps *. Two types of gaseous environments can be switched on '} together be used with the furnace, a flowing,' / ^ atmosphere or a static atmosphere. In the case that} is used a flowing ^ atmosphere can complex atmospheres containing nitrogen and hydrogen mixtures contain "/ are produced from the individual pure gases by these gases are mixed in a Gasproportionierer, wherein the volume of the gases using regulated by flow meters. The mixture so produced is passed through a drying tower to remove any moisture in the gas. . '

A static atmosphere can also be created in the heating furnace.

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CiRIGSNAL INSPECTED

be turned. A static atmosphere is an Atmos- '-ν ..; phere, in which a constant pressure in the pipe by _Schliemann: ν ^ SEN of the exhaust valve is held. Generally lies. ■: the desired pressure at about 0.21 a.tü (3 psig). If the. If the pressure drops below the desired pressure, more ^w ; "": | of the gas mixture is allowed to enter the furnace tube. When using * ..- ·, complex atmospheres in a static system, the / '. ■;> atmosphere in general premixed ... * .- '"■'", r: 4 ?: '

When using either the flowing or the static atmosphere, once the atmosphere is inside the socket pipe, the gas first migrates over a getter or trap material. In the special cases to be described below, the getter material is silicon. The purpose of the ^getter: i is any excess oxygen from the gas' - to remove ■ system. The gas then treats the silicon objects to be nitrided. I ..; ". ■ · ■; ■" ■ ,. -. * ■

Different time-temperature cycles can be used in the NXtridation stage. Typical examples of i Nitridierungszyklen include the suspension of the object f at a nitrogen-containing atmosphere during 36 hours at 126CPC J (2300 ⁰ F) and then for 4 hours at 146O ⁵ C 1 (266O ⁰ F). An alternative to this is treatment for Λ.36 hours at 126tfC (230O ⁰ F) and then for 24 hours at 1460PC (266O ⁰ F). In general, the two-step nitridation process comprises a temperature value below ^{: one} half the melting point of the silicon metal and the second temperature above the melting point of silicon in order to achieve complete conversion of silicon to silicon nitride in reasonable times .

Various attempts have been made using different Atmospheres, either static or flowing, and '

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different gas compositions carried out. The results of the tests are described below. Using a 100 $ nitrogen atmosphere along with a 36/24 hour nitriding cycle, the finished article had a weight gain of $ 61, a final strength of 1390 kg / cm ² (19,700 psi) and a density of 2.35 g / cm ⁵ . When the same material was treated in an atmosphere of 1/2 $ hydrogen and 99.1 / 2 $ nitrogen, the article after nitriding had a weight gain of $ 63 and a strength of 1670 kg / cm (23,700 psi). The ^re: HOLUNG the muddled with an atmosphere of hydrogen 1 $; i. and 99 $ nitrogen gave weight gains "^, a strength of 1930 kg / cm (27,415 psi) and a density of 2.38 g / cm. An atmosphere of 1.8 $ hydrogen and 98.2 $; · nitrogen gave a weight increase of $ 62, a Pestig- ^l ness of 1730 kg / cm ² (24,650 psi) and a density of 2.31 g / cm using an atmosphere 4 »0 $ hydrogen and?. Containing $ 96 nitrogen resulted in a weight gain of $ 63, strength of 1680 kg / cm (23,800 psi) and density of 2.40 g / cm All of these tests were conducted using a static atmosphere . ·: ·

Using a flowing atmosphere, using : the same nitriding cycle, yielded 100 liters of stick- \; fabric atmosphere has a strength of 1220 kg / cm (17,350 psi) at a density of 2.35 g / cm. When the atmosphere used in the nitriding was 1 $ hydrogen and 99 $ nitrogen, a strength of 1790 kg / cm (25,550 psi) was recorded at a density of 2.32 g / cm.

In view of the above results, it is evident that two things occur. The use of 1/2 to 4 $ hydrogen in combination with nitrogen for a nitride leaching atmosphere creates strength values in the nitrided parts.

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objects above the strength value that is achieved, when only nitrogen is used. The strength that can be achieved is also greater when the atmosphere is static.

The creep properties of the silicon nitride materials prepared as described above were also measured using a flexural test method. The stresses were calculated from the known deflection of the test piece. * The results show a decrease in steady state strain rate and plastic deformation when about 1/2 to about 4 volumes of hydrogen are mixed with the nitrogen for use in the nitriding process. In a flowing atmosphere of $ 100 nitrogen, the steady state strain rate was 8.0 χ 1-0 "^ in / in / hr and the plastic deformation after 100 Hours was 1.296 In the case where 1/2 $ hydrogen and 99 1/2 volume $ nitrogen were used in the static state, the rate of expansion in the steady state was 8.9 x cm / 2.5 cm / h (3.5 χ 10 "" - * in / in / hr), and the plastic deformation after 100 hours was $ 0.55. In the case where 1 volume of hydrogen and 99 volumes of nitrogen were applied under static conditions, the strain rate was 4, 6 χ 10 ~ ⁵ , cm / 2.5cm / h (1.8 10 ~ ⁵ in / in / hr), and the plastic deformation after 100 hours was $ 0.33. In the case where 1.8 volume ^ Hydrogen and 98.2 volumes ^ nitrogen were used as the atmosphere, the steady state strain rate was 3.8 χ ^ 0 ~ ^ cm / 2.5 cm / h (1.5 x 10 "- ^ in / in / hr) , and the plastic deformation after 100 hours was $ 0.27. In the case where 4.0 $ volume hydrogen and 96 volume $ nitrogen were used in a static state, the strain rate was at steady state 3 * 3 × 10 ^-5 cm / 2.5 cm / h (1.3 x 10 "^ in / in / hr),

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and the plastic deformation after 100 hours "was 0.. It is important to note / that the values of the stretch rate and plastic deformation a minimum in the range of 0.5 "to 4 volumes ^ of hydrogen. This is the same area where the strength reaches a maximum.

The invention has been described above on the basis of "preferred embodiments" without being limited thereto.

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Claims (8)

Patent claims: 1. A method of making silicon nitride ^articles in which silicon metal particles are molded into the general shape of the article, the silicon thus formed:! is object in a gaseous environment containing sufficient nitrogen to convert the silicon into silicon nitride, heated, characterized gekennzeich-% net that in the method for improving the strength. | and the fatigue strength of the finished article in relation to the values available for these properties when the gaseous environment consists practically of pure nitrogen, "; the gaseous environment consists of a mixture of about 0.5 to about 4.0 volumes of hydrogen, the remaining volume consists of nitrogen. 2. The method according to claim 1, characterized in that the mixture of the gaseous environment Contains about 1 volume ^ hydrogen and 99 volumes ^ nitrogen. " 3. The method according to claim 1 or 2, characterized in that the gaseous environment around the; Object is maintained in a virtually static state during the conversion of the silicon to silicon nitride. 4. The method according to claim 3. draws that the static gaseous environment at a pressure of about 0.2 atu (3 psig) is maintained. 5. A method of making silicon nitride articles in which silicon metal particles are coated with a thermoplastic Material are mixed to form a deformable mass by injection molding, the deformable mass by injection molding ORIGINAL INSPECTED "5 09819/1020 into a shape to form the general shape of the desired finished article is injected, the molded article to a temperature sufficient to burn out the thermoplastic material is heated to leave an article of substantially pure silicon and the silicon object in a gaseous environment that contains enough nitrogen to convert the silicon into silicon nitride contains, is heated, characterized in -that in the process of improving the Strength and creep strength of the finished object compared to the values available for these properties if the gaseous environment consists of practically pure nitrogen A gaseous environment consists of a mixture of about 0.5 "to about 4.0 volumes ^ of hydrogen, with the remainder Volume consists of nitrogen, is formed. 6. The method according to claim 5 »characterized in that the gaseous ambient mixture is about Contains 1 volume ^ hydrogen and 99 volumes ^ nitrogen. 7. The method according to claim 5 'or 6, characterized that the gaseous ambient mixture around the object is in a practically static state is held while the silicon is being converted to silicon nitride. 8. The method according to claim 7, characterized in that the static gaseous ambient mixture is maintained at a pressure of about 0.2 a-tü (3 psig). '5 09819/1020