DE2454147A1 - METHOD OF MANUFACTURING A SILICON NITRIDE ARTICLE - Google Patents

Description

According to the invention there is a method of manufacture a slip cas £ article made of Si-· liciutn nitride wherein the finished article has a selected final density in the range of about 2.1 g / cm "up to about 3.18 g / cm., The control of the. Final density is achieved by regulating three parameters. The first parameter is that the Silicon metal used to form the article Has particle size distribution, 'which in a spe- area falls. The second parameter consists in regulating the pH value of the

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applied slip or the casting compound (slip). The third parameter is the amount of silicon metal contained in • Is suspended in the slip used in the tumble casting process. A slip with the selected particle size range, • pH value and the desired amount of suspended material, ' is tumble cast to shape the finished item to build. The article is then made from virtually pure silicon nitride to produce the finished article nitrided with the selected final density.

British Patent 779,474 discloses a method of making silicon nitride articles by drop casting described. In this patent it is stated that the silicon metal used for a lintel casting Particle sizes of less than 5 / u, preferably in the size range up to only 3 / u, is milled. This patent specification there is also a method for forming a casting slip from the silicon particles by mixing the silicon metal particles, a carrier and a deflocculant at. The patent also describes that the pH of the slip is adjusted to a range between 7.5 and 10.0 will. The examples of this patent indicate that the density by weight of the silicon metal in the Casting slip or casting material is about 54 to about 56 wt .- #. Although not in the British patent explicitly stated, calculations show that the density using the in the examples of the patent finished articles produced a final density for a finished silicon nitride article in the range of about 2.2 to about 2.45 g / cm.

Those described and claimed in this application Procedures go beyond the teachings of the UK patent

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writing addition, in that the present process technology enables the production of a silicon nitride article with a

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A chosen final density in the range of about 2.1 g / cm up to a maximum theoretical density of 3.18 g / cm. The ability to produce a silicon nitride article Achieving a specific final density is achieved by controlling three parameters in the manufacture of the silicon nitride article. .

The first and most important parameter is the particle size distribution of the silicon metal used to produce the casting slip. It has been found that when the Particle size distribution with a particle size of less starts as 50 / u and has a desired particle size distribution curve has, a larger amount of the material can be suspended in each casting slip. A special one Distribution curve within the permissible distribution curve range produces greater or lesser packing densities during the tumble casting process and thus creates greater or lesser densities in the end product. So there is the first Control parameters of the invention in the selection of a particular size distribution of the silicon metal to achieve a desired To achieve final density.

The second control parameter of the invention is the amount of for the production of a casting slip suspended in the carrier Silicon metal particles. Using materials whose particle size is only within the desired falls within the range specified here, the silicon metal slip can have a minimum of about 70% by weight up to a maximum in the range of about 80% by weight of silicon metal particles contained in the slip. The higher the percentage by weight of silicon metal in the casting slip, the

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the higher the final density of the article produced. The second control parameter is thus the amount of the in the casting slip suspended material.

The third control parameter of the invention is adjustment the pH of the casting slip. The viscosity of the casting slip varies with its pH. By regulation The pH value of the casting slip can reduce the casting slip to a minimum viscosity or a higher viscosity can be set as the minimum viscosity. When the viscosity of the slip is of the minimum or minimum viscosity is corrected away, the density of that generated from the casting slip finished item reduced. The object produced from a special slip with maximum Density is created when the slip has a minimum viscosity. The third parameter thus exists in the regulation of the pH value of the casting slip.

The invention relates to a method for producing a Silicon nitride article, and in particular a method for making a silicon nitride article, wherein the article produced a selected final density in the range of about 2.1 g / cm up to a maximum theoretical density of about 3.18 g / cm.

The process is initiated by using practically pure silicon metal is obtained in a finely divided form. The particle size distribution of the silicon metal is measured to determine if the distribution is within the range indicated by the curve in the graph of FIG limited area falls. If the particle size distribution does not fit within the limited range, will corrects the particle size distribution of the silicon metal,

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so that it falls within the range bounded by the curve in the graph of FIG.

To produce a casting slip, 70 to 80 wt .- # of the silicon metal particles in the correct particle size distribution are suspended in 30 to 20 wt. # of a carrier by adding a small amount of a chemical suspending agent of the particle is used in the carrier. The larger the amount of silicon metal particles suspended in the carrier is, the greater the final density achieved for the object from the special slip.

The pH of the slip can also be adjusted to match the density of the finished product obtained from the particular slip To regulate the subject. The viscosity of the pouring slip is a function of the pH of the slip. The pH value of a slip to achieve a minimum slip viscosity is a function of the particle size distribution of the silicon suspended in the slip, the one used Carrier and the agent used to suspend the silicon. Objects with a maximum final density, will made from a slip with a minimal viscosity. However, if so desired »an item from lower than the full density, you can adjust the viscosity of the casting slip to a value greater than that Increase the minimum viscosity and the resulting article will be less dense. .

After the pH of the containing the suspended particles Carrier has been adjusted to the desired value, the slip becomes practical to form an article pure silicon metal particles subjected to tumble casting. The object thus formed becomes practical for transforming the ' pure silicon metal particles nitrided in silicon nitride

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and thereby producing a silicon nitride article of the selected final density. '.

Figure-1 is a semi-log graph the cumulative percentage of the finer masses against the ' equivalent ball diameter in /u.* The graphic representation represents an enclosed area, which is delimited between two curves, within which a special particle size distribution -, must fall from silicon metal, μιη one Making silicon nitride articles from the silicon metal in accordance with the teachings of the invention.

The invention is described below on the basis of preferred embodiments. .:

In general, the invention is concerned with a method of making a silicon nitride article using of tumble casting techniques with a chemically suspended Slip. The tumble casting process consists in pouring a stable powder suspension, the so-called slip, into an absorbent Mold is poured, which is the negative of the object to be molded. The shape absorbs by capillary action the liquid. Fraction of the slip and leaves a coherent casting that has sufficient strength for handling having. The casting is dried and sintered to achieve the final properties of the component. There is two basic types of tumble casting techniques: solid particle casting and drainage casting of hollow parts. Festivals Castings are made in that the absorbed liquid Fraction is continuously replaced by additional slip until a solid object is formed. Hollow castings are made in a similar manner; however, excess slip is poured out of the mold after the article has reached the desired wall thickness.

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A chemical suspension slip is a slip whose flow properties are largely due to an interaction be controlled in an ionic atmosphere between the carrier of the slip and the suspended material. To the generation of a chemical suspension, the particles to be suspended should fall within a size range in which the surface forces, such as' London-van the Waals forces., instead of gravity, determine their behavior in a suspension. For most metals a chemical suspension can be obtained when the mass of the particles is in the size range 1 to 10 / u falls. In addition to the size, the surface of the particles must be such that electrical double layers are formed can. Oxide layers that can be found on most metals form, provide the means for forming the electrical double layers. ,

Reference is now made to Figure 1 which is a semi-log graph shows where an area is enclosed between a pair of curves. The enclosed Area in the graph defines a large number of specific particle size distributions that is determined in each case by a single continuous curve through the area. It has been found that when the particle size distribution the particle size used to form a slip for the tumble casting technique If silicon metal particles fall within the bounded area in Figure 1, a finished article of high density is achieved can be. Material with a distribution curve close to the right or upper boundary of the area results in silt, from which objects with the highest possible final density can be produced. Material with a distribution, those approaching the lower or left limit of figure T have the lowest end densities. So the

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first scheme the scheme that uses a particle size distribution of silicon metal particles is reached, which are within different ranges of the in the curves of Figure 1 defined full range fall.

The process of the invention can be initiated by obtaining practically pure silicon metal powder in a finely divided form. The material is analyzed in order to develop the particle size distribution curve for it in accordance with the parameters defined for the coordinates in the graph of FIG. 1, ie the cumulative percentage of the mass fines is plotted against the equivalent sphere diameter in Ai. This graphing can be performed by sedimentation techniques, Coulter techniques, or other techniques which give a faithful analysis of the particle size distribution.

After measuring the particle size distribution curve, the particle size distribution is corrected if necessary so that it falls within the range delimited by the curve in the graph of FIG. The correction is made only if the material does not originally fall within this range. Correction can be done by certain Sections of the material can be removed from the sample by regrinding the material to reduce it to a finer state or by mixing in special sections of material.

70 to 80 wt. # Of the silicon metal particles having the selected one Size distribution will be ^ with 30 to 20 wt .- ^ of a suitable Carrier and a small amount of an agent to chemically suspend the particles in the carrier. This mixture creates a gießsohlicke. The carrier can be water or

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be any other suitable vehicle that supports the susr suspended particles practically not influenced. A suitable chemical suspending agent is monoethanolamine.

The casting slip has a viscosity that has a function ' its pH. It has been found that a minimum viscosity can be achieved in every casting slip. For a special size distribution becomes the maximum final strength achievable in a silicon nitride article produced therefrom generated from the slip with the minimum viscosity. Therefore you can regulate the pH of the slip, to get the desired final density. If less than the highest final density is desired for a particular casting slip is, the pH of the casting slip is adjusted so that its viscosity increases and thereby the final achievable density in the object ^ decreases.

Another regulation is the maximum attainable strength of the final article for an article achieved from a slip containing the maximum amount of silicon metal particles suspended therein. So if you can a finished article with a density less than the maximum achievable for the particular one being used If you want to make size distribution in the slip, you can use a smaller amount of the slip of material contained therein, for example about 70 wt .- #.

In general it can be said that the object with maximum density can be made up by the techniques given here: A slip can be made by adding material with a partial size distribution close to the right or top curve in Figure 1 is suspended. The casting slip would also have a minimum viscosity

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and would contain the maximum amount of suspended materials. If, on the other hand, you have an object with minimal • If you want to produce density, this would be produced from material with a particle size distribution closely adjacent to the lower or left curve of FIG. 1, it would be made of to pour a casting slip containing a minimum percentage by weight of silicon material and off a casting slip with a higher viscosity than the minimum viscosity.

The following examples serve to further illustrate the invention without limiting it.

example 1

A silicon metal powder of technical quality with a purity of 98.5% and a nominal size accordingly a sieve passage through a sieve with openings of 44 / u (325 mesh) is made by dry grinding in a porcelain mill with high density balls for a period of time ground for 17 hours. The powder gives the following size distribution due to known X-ray sedimentation techniques, In terms of weight percent (% by weight), the following range was obtained: 0% by weight - 50 / u and above, 10% by weight - 19 to 50 yu, 20% by weight - 14 to 50 / u, 30% by weight - 10 to 50 / u, 40% by weight - 7.2 to 50 / u, 50% by weight 5.5 to 50 / u, 60% by weight - 3.8 to 50 / u, 70% by weight - 2.6 to 50 / u, 80% by weight -1.7 to 50 / u, 90% by weight - 1.2 to 50 / u and 100% by weight - 0 to 50 / u. This material is called a silicon powder of 5.5 / u.

i A chemically suspended slip is made using 6000 g of 5.5 / u silicon metal powder, 2000 g of distilled water, 120 g of Fe ₃ O ₃ (a nitriding aid that

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can be replaced by others, such as iron, calcium fluoride and barium fluoride) and 2 g of mono-tianolamine are used as the suspending agent. The solids are gradually added to the liquid medium. Homogenization and chemical equilibrium of the casting slip are established by rolling the suspension in a polyethylene bottle for 4 days. The properties of the silicon metal slip obtained are as follows: solids content 75.4 %, slip density 1.67 g / cm, pH 7.0 (which results in a minimum viscosity of 64 cP). '

A silicon article is tumble cast using the techniques discussed above. Hollow tubes 6.3 mm (1/4 inch) thick are made by the drain casting techniques. The green density of the silicon metal is 1.71 g / cm. The cast article is removed from the mold and dried in an environment with air circulation at a temperature of 82-93 ⁰ C (180 to 200 ⁰ F) for 48 hrs.. The object is nitrided in a nitrogen-containing gas oven in which the article is at a temperature of 1093 ⁰ C (2000 ⁰ F) for 24 hrs., 126o ³ C (2300 ⁰ F) for 24 hrs., And then 1460 ³ C (2660 ⁰ F) is held for 24 hours. A flowing nitrogen atmosphere at 0.85 nr / min (30 cfm) is used in the nitriding. The article produced has a density of 2.74 g / cm ⁵ and a flexural strength of 3160 kg / cm (45,000 psi). The elastic modulus is 2.1 10 ⁶ kg / cm ² (30 10 ⁶ psi) and the article has excellent hot strength and creep resistance.

An article with a lower final density can consist of the same Casting slip can be produced by either reducing the solids content of the casting slip from about 80% by weight

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is reduced to about 70% by weight 'or by increasing the pH of the casting slip is changed so that the viscosity of the slip increases. Each of these techniques produces an article with a final density less than 2.74 g / ' cm-.

For example, the 5.5 / u slip described above is adjusted to a pH value of 5.5 by adding a small amount of nitric acid. The slip viscosity increases to a value of 256 cP, the body obtained has a green density of 1.50 g / cm and has a final density of 2.47 g / cm after nitriding. A pH value of 6.0 for the slip produces a casting with a green density of 1.84 and a density of the nitrided product of 2.64 g / cm ⁵ .

Example 2

The silicon metal powder is prepared by ball milling a silicon powder for 48 hours in accordance with a sieve passage through a sieve having openings of 44 / u (325 mesh). The silicon powder produced is referred to as 4.5 / u silicon metal powder and has the following particle size distribution: O wt.% - 50 / u and above, 10 wt.% 18 to 50 / u, 20 wt.% - 13 to 50 / u, 30% by weight 9.2 to 50 / u, 40% by weight - 6.5 to 50 / u, 50% by weight - 4.5 to 50 / u, 60% by weight 3.3 to 50 Ai ₉ 70 wt # -. 2.2 to 50 / u 80% by weight -. 1.4 to 50 i, 90 wt% -.. 9 to 50 / u and 100 wt% 0 to 50 / u. A slip which contains 77.1% by weight of solids with a specific weight of 1.73 g / cnr and a pH of 6.3 and a viscosity of 96 cP is made by mixing 7500 g of the 4.5 / u Powder made with 2000 g of distilled water and 1 g of monoethanolamine. From this slip, a mixture is made according to the techniques described in Example 1

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article with a green density of 1.87 g / cm. and a density of the nitrided product of 2.99 g / cnr.

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

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Claims (3)

2454H7 Claims ! 1). A method of making a silicon nitride article having a selected final density in the range of about 2.1 g / cm- ⁵ to a maximum theoretical density of about 3.18 g / cm, thereby gekening. indicates that practically pure silicon metal is obtained in finely divided form, The particle size distribution of the silicon metal is measured to determine if the distribution is within of the by the curves in the semi-logarithmic graph of Fig.. 1 limited area fall, the particle size distribution of the silicon metal is corrected so that it is within the range indicated by the curves in the semi-logarithmic graph of FIG. 1 falls within the bounded range, 70 to 80 weight be.% Of mixed Siliciummetallteilchen with 30 to 20 wt.% Of a carrier and a small amount of an agent for the chemical suspension Siliciummetallteilchen in the carrier, the pH of the carrier containing the suspended particles is adjusted to bring the viscosity to the To achieve the desired final density desired, tumble cast the mixture to form an article of substantially pure silicon metal particles will and the article of substantially pure silicon metal particles to produce a silicon nitride article ι is nitrided with the desired final density. 50 9 8 "2 0 / OB 7 4 2454Κ7 2. A method of making a silicon nitride article having a selected final density in the range of about 2.5 g / cm up to a maximum of about 3.18 g / cm, characterized in that practically pure silicon metal is obtained in finely divided form, The particle size distribution of the silicon metal is measured to determine if the distribution is within the graphical through the curves in the semi-logarithmic Representation of Fig. 1 limited area fall, the particle size distribution of the silicon metal is corrected so that it is within the range indicated by the curves in FIG the semi-logarithmic graph of FIG. 1 falls within the bounded range, 75% by weight up to the maximum suspendable percentage by weight of silicon metal particles at 25% by weight to the minimum% by weight of carrier and a small amount an agent for chemically suspending the silicon metal particles are mixed in the carrier, the pH of the one containing the suspended particles Carrier is adjusted to such a value that the viscosity of the carrier containing the suspended particles is at a minimum, the mixture to form an article of practical pure silicon metal particles are tumble cast and the article of substantially pure silicon metal particles to produce a silicon nitride article with the selected final density is nitrided. 3. A method of making a silicon nitride article having a selected final density ranging from about 2.1 g / cm x ⁵ to a maximum theoretical density of about. 3.18 g / cm, characterized in that 509820/0874 245AH7 a finely divided form of pure silicon metal with a particle size distribution represented by the curves in the semi-logarithmic graphical representation of FIG. 1 limited area is produced, 70 to 80% by weight of the silicon metal particles, 30 to 20% by weight of a carrier and a small amount of an agent for chemically suspending the silicon metal particles are mixed in the carrier, the pH of the carrier containing the suspended particles is adjusted to bring the viscosity to that required set the desired viscosity to the selected final density, tumble cast the mixture to form an article of substantially pure silicon metal particles will and the article of practically pure silicon metal particles to produce a silicon nitride article with the selected final density is nitrided. 509820/0874