JPS5837272B2 - Silicon nitride molded body and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Reactively bonded silicon nitride has been used by the British Parr, Martin, Popper and their associated departments, especially the British Admiralty, for the last ten years or so. It has received considerable attention in the technical literature, widely derived from research at the Atomic Energy Research Institute and other UK government agencies.

The first description of the manufacturing process for silicon nitride moldings was in 1960 in ``Special Ceramics'' published by Heywood & Co, Ltd., London.
amics'').

Pages 102 to 135 specifically discuss articles by Parr, Martin, and May.

Additional details of the technology are provided in the December 5, 1965 US Pat.

Other patents mentioning reaction-sintered Si3N' and its uses include British Patent No. 895,769, No. 1.16.
No. 8,499, No. 1,266,506, U.S. Pat.
, 7 5 0, 2 6 8, French Patent No. 2074.9
There are No. 20 etc.

British Ceramic Society (Briti)
The current state of the art for manufacturing certain shapes from reactively bonded silicon nitride is described by Barr and May in the 1967 Bulletin of the Ceramic Society, Vol. 7, pages 81-90.

Such molded products are currently manufactured by the British company AME (Ady
anced Materials Engineeri
ng9Ltd. ) is provided by.

The technique is thought to be based on the Parr and May technique described in the 1967 article mentioned above.

The maximum bending failure strength (t
raverse ruptures t ren gt
h) is approximately 2 5 3 0k9/crlt (3 6,
0 0 0 psi).

The material provided by AME is 2110kg/Ci (30,0
It is advertised as having a bending failure strength of around 0.00 psi).

This material is, for example, Degussa, Hoffman and Carborundum Campa=
It is believed to be representative of other materials offered by other suppliers such as (Carborundum Company).

In commercial distribution, the material used is 2 1 1 0 kg/ca (
None are advertised as having a bending failure strength greater than 30,000 psi).

In the prior art, represented by Parr, May, and other British researchers, silicon powder was
The compressed material is compressed at a relatively high pressure such as by pressing) to form a compressed body.

Prior to machining into the desired part shape, the prior art partially nitrides the compact to provide a "green" compact sufficiently strong to withstand handling during machine operations.

After machining, the material is fired in a nitrogen-containing atmosphere to convert all silicon to silicon nitride.

In British Patent No. 942,082, Parr claims that a mixture of alpha and beta s13N4 produced by the method described above can be converted to beta s t3-d in large quantities by heating at 1.700° C. in a nitrogen atmosphere. He says it can be changed.

As mentioned above (stated by Parr and May), the bending failure strength is 2530 kg/crlt (36,00
0 p s i ) is the maximum intensity reported so far.

However, with the current technology, 3-point load (3-point load)
oading), the bending strength is approximately 1
400kg/crA (20,000psi
)Kara2400k9/crA(30,00
0 psi).

A. E. R. E's Evans and Davidge%J ``Ja→Ruob Material Science 5'' (Journal of M
material Sciences 5) (1 9 7
(2013), pages 314A-325, describe a study of the strength of reaction-sintered silicon nitride obtained by nitriding unmachined simple compacts under various conditions.

The highest strength was obtained by long nitriding the compact at a temperature below 1400'C, the melting point of silicon.

The Ivans and Davidch product has a particle size somewhat larger than that used in the present invention and a final pore size larger than that of the present invention.

Its breaking strength is approximately 2880 kg/tst (4
1.000 psi) (though only measured for length to thickness ratios of only 4 to 1 or 5 to 1).

This bending strength is somewhat higher than that described by other researchers involved in providing molded parts.

Taylor's British Patent No. 1.168,499 is BSA (Birmingham Small Arm
Describe the improvements made by S Company.・Ru.

This is similar to the present invention in that the starting silicon powder compact is sintered in argon.

After machining, the product is fully nitrided.

However, the Taylor patent does not describe the characteristics of the starting silicon powder, nor does it describe the requisite fine particle size, which is important to achieving the benefits of the present invention.

The bending fracture strength of the reaction bonded silicon nitride marketed by BsA Group is approximately 1760k'
; i/crA (25,000 psi) (according to a document believed to have been published somewhat after the issuance of Taylor's patent), Taylor's silicon powder effectively has such a fine particle size. It's obvious that it doesn't have one.

This is typical of other reactively bonded silicon nitride products that are commercially available from other sources.

These products far fall short of the results achieved by the present invention.

The molten silicon nitride used to make molded parts is approximately 280 kg/cA (40,000 kg/cA).
It has been reported to have a stress rupture strength of 0 psi).

It is very difficult to obtain a suitable cross-sectional thickness for this welded part due to the relatively high density of the starting silicon product and the difficulty of completely nitriding the entire cross-section. be.

In the present invention, some of the difficulties of the prior art have been corrected to obtain a final product with very high flexural breaking strength at room temperature, which furthermore has a strength of 1375
It increases as the temperature increases to °C.

Materials produced using Parr and May's technology exhibit maximum strength at temperatures around 1300°C, within the range that can be ascertained. It starts to fall off pretty quickly.

Evens and Davids's strength changes little with temperature.

The present invention provides an isotropic structure and 2. 4-Oft/cc
with a density exceeding 280' J-kg/cfIL (
To provide a silicon nitride shaped body having a bending strength exceeding 40,000 psi), almost all zero pores being 2 microns or less, and a ratio of alpha silicon nitride to beta silicon nitride of about 2. .

The present invention also provides a starting silicon powder that is at least 1.40 f? fine, ie, essentially free of particles having a diameter greater than approximately 10 microns. sintering of this compacted powder in an atmosphere essentially free of nitrogen, carbon, and oxygen, and subsequent combustion and nitriding of the product in a nitrogen-containing atmosphere. 280 1kg/cvl. c 4
A method of forming a silicon nitride compact having a bending strength to break exceeding 0.000 psi) is provided.

The silicon used as the starting material is preferably in the form of very fine particles, with a maximum particle size of 10 microns and an average particle size of about 2 microns or less.

This fine silicon powder is subjected to an equilibrium pressure of approximately 1.5
It is compressed to a compression density of 5'/cc.

It is then sintered in an inert gas atmosphere such as argon.

It is desirable to evacuate and flush the inert gas sintering furnace with argon several times to remove air contaminants as completely as possible.

The sintering operation is preferably carried out at a temperature of 1100°C, the time depending on the amount of silicon.

This step results in a bonded material with sufficient strength to be easily machined.

This intermediate product is completely different from the product of Parr and May in that the product is essentially free of nitrides at this stage.

This sintering operation bonds the silicon particles together and provides a continuous silicon structure.

It is clear that such silicon bonding occurs because the strength of the compressed body is high.

A micrograph of the sintered product also shows the bonding of silicon particles.

The time and temperature spent in this initial sintering operation are determined by the density and size of the compact, the silicon particle size, and the strength required of the compact during the machining process.

For example, for low-density compacts, it may be necessary to extend the sintering time and/or increase the temperature, which ensures sufficient bonding of the silicon particles and creates a continuous structure and high A final product with density and high strength can be reached.

When using fine silicon, the sintering time is shorter and the sintering temperature is lower.

The sintering step 6 provides an important step in creating a continuous silicon structure while giving the pressed pongee body strength to withstand machining.

A continuous silicon nitride structure is created from a continuous silicon structure.

After machining, the silicon mass is nitrided according to a conventional process, ie in a nitrogen-containing atmosphere.

In order that the invention may be fully understood, the following non-limiting examples are provided.

Example 1 The starting material for this process is a finely divided silicon powder with a maximum particle size of 10 microns and an average particle size of approximately 2 microns.

Union Carbide Commercial Grade 200X
D silicon powder is good.

According to chemical analysis, this powder contained 0.5% iron, 0.4% aluminum and 0.1% calcium as impurities.

7. Pour silicon powder into a fine grinder such as one made by Magic Ink.
By passing at a feed rate of 25 Icg (16 pounds)/hour, silicon powder with even smaller particle size was obtained.

The average particle size distribution of this powder is 2 microns and 0.5
The particles had a particle size in the range of ~10 microns.

9.1 kg (20 lb) of this powder was balanced in a double-sealed Visten bag at 3660 k9/cr/t (5 2,0 0 0 ps).
i) Approximately 1 diameter by applying pressure 5. It was compressed into a cylinder of 5 CwL (6 inches) and approximately 35.6 cR (14 inches) high.

Next, take out this cylinder from the bag and
It was placed in a vacuum furnace that was evacuated twice and flushed with argon each time, and burned at 1100° C. for 15 hours.

The product thus obtained was machined into its final shape.

In this example, it was machined into the turbine shaft of a gas turbine.

During the machining process, a copying machine controlled by a master with a diamond set in the tip was used.

The machined turbine shaft has an airtight double-walled approximately 0.1
After loading and sealing a nitriding furnace consisting of a silicon carbide box having a volume of 4 ft3, the nitriding furnace was placed in an electric furnace heated by a silicon carbide resistance element.

The furnace was filled with nitrogen, 0.0 2 8 per hour.
3 1-2 ft" of ammonia was poured into the furnace.

A control valve was used to adjust the pressure in the entire furnace to a constant pressure, and nitrogen was introduced into the furnace.

The temperature was raised to 1200°C and maintained at that temperature for 24 hours.

Increase the temperature by 100℃ every 24 hours to 1450℃
Finally, the temperature was raised to 1450°C, and the temperature was kept at 1450°C for 15 hours.

As a result of analysis, the turbine shaft-shaped test rod manufactured in this way was found to have the following physical properties.

That is @ degree 2.4P/cc, bending breaking strength 3520k
g/cti (5 0, O OO O IEi)
Also approximately 0.4% iron, 0.3% aluminum, 0
．． <1% calcium, 70% alpha silicon nitride, 25%
It was a component of β silicon nitride.

Chemical analysis revealed the presence of approximately 5% silicon carbide by X-ray diffraction.

The pores of the product are less than 2 microns, and the majority are less than 1 micron.

Example 2 (Comparative Example) In this example, the same operation as in Example 1 was performed except that several types of test rods were used.

In this example, the particle size of the starting powder was between 1 and 40 microns, with an average particle size of approximately 14 microns.

While balancing this powder, 2820kg/ca
(40,000 psi), sintered at 1100° C. in an argon furnace, machined to final shape, and then 141° C. as in Example 1 above.
Nitriding was carried out at 0° C. for 15 hours.

The final product showed a 61% weight increase, 2.51ff/c
Density of c, 2 3 2 0k9/crrt (33,
It had a bending breaking strength of 000 psi).

Several specimens were prepared as in Example 1':j6 and Example 2 using a wide range of densities and particle sizes.

These are shown in Table 1 together with the results of Example 1 and Example 2.

In Examples 3, 4.5, the equilibrium pressure was varied to adjust the raw density as well as the final product density.

The results revealed the effect of density on strength.

Example 7 (Comparative Example) The mixture was Carbowax 4000 (manufactured by Carbide & Carbon Chemical Co., Ltd.) 12, which was a slurry solution made by dissolving polypropylene glycol in 100 parts by weight of 250 mesh (average 10 to 20 microns) silicone and methylene chloride solvent. parts by weight, the exact amount being not critical.

The slurry is mixed until the methylene chloride evaporates and the mixture is spread on paper to dry completely.

1 5.2cIrLX5.0 8crfLX0.6 4
A cIrL (6 inch x 2 inch x 14 inch) test bar was hydraulically compressed and placed in a visten bag.
bag), sealed, balanced and compressed at a pressure of 1400 m/cf/t (20.000 psi).

The bar was raised to 250°C at a rate of 10°C per hour and burned in a nitrogen oven at 250°C for 24 hours to remove carbowaxes.

Raise it to 1160℃ at a rate of 40℃ per hour, then 1200℃ for 24 hours, then 1300-1350℃
The temperature was raised to 1450° C. for 24 hours, and the temperature was maintained at a temperature slightly higher than 1450° C. for 12 hours.

The test bar has a theoretical value of 9, taking into account the incinerated carbowax.
It showed a weight increase of 43.8%, which corresponds to 0%.

The density of the sintered product was 2.4 7 F/'cc. 1
5. 2cWLX O. 3 2CrILX0.3 2(
] (6 x 1/8 x 1/8 inches) test bars were cut from this bar with a diamond wheel.

At room temperature, MOR is 1.9ocrI with 3-point loading.
Measured with a span of L (374 inches).

The average value is 2 4 4 01cg/crA (3 4
, 800 psi) and its range is 2380k
glcry (33,500 psi) to 2
540kg/crA (36,200 ps
It is within the range of i).

This had a grain structure similar to other prior art quality products.

Example 8 (comparative example) A method similar to the previously described method, ie BSA technology.

Diameter 1 5.2crrL (6 inches) X3 0.4
cfIL (L2 inch) dimensions, approximately 9.1 kg (
20 lbs.) sticks in double Bisten bags.
2810kg/crtt (4
0,000 psi).

The bar was presintered at 1080° C. in argon and soaked in argon for 15 hours.

A 0.64 (IX 4 inch) thin piece was cut from the sintered rod and heated at 1200°C for 5 hours and at 1330°C for 28 hours for 14 hours.
Fired in a nitrogen oven at 10° C. for 15 hours.

The flakes showed a weight increase of 62.0% compared to a theoretical increase of 93%.

The density was 2.60 ft/cc.

0,3 2crILX0,3 2CrrL (
A 1/8 x 1/8 inch test bar was cut from the burned flake.

MOR at room temperature was measured with a three-point load at a span of 1.90 cIrL (3/4 inch).

The average value of 5 destructions is 2 1 5 0 kg/CJ
(30,600psi), and the deviation is 1
550 kg/crA (2,200 psi
)Met.

The results of X-ray diffraction of this material showed that the amounts of α nitride and β nitride were approximately equal.

The cross section of the broken pieces was examined using a scanning electron microscope at a magnification of 200 times.
Analyzes were performed at magnifications of o and 5,000 times.

The pore diameter was examined by photography.

In the 1000x photograph, coarse pores were observed, with the largest pores having a size of 6 to 10 microns.

A typical pore size is 3 microns.

Approximately 50 of these typical pores could be observed on the photographic surface of approximately IOOXIOO microns.

The photo was taken at an angle of 45°. The crushed surfaces and polished cross sections of representative specimens of the prior art and the present invention were compared by scanning electron microscopy (SEM) at 200x to 1000x magnification.

For these reasons, in the prior art, there are quite a lot of materials with pores larger than 15 microns, whereas
It has been found that the products of this invention have very few pores larger than 15 microns.

The maximum pore size directly affects the strength.

Large pores induce cracks and are considered defects.

Since the method of this invention uses fine particles as a starting material, which essentially do not contain particles with a diameter larger than 10 microns, the pores of the resulting molded product have a size smaller than 2 microns, which gives a high strength This is one of the factors that makes it possible to obtain a molded product.

Judging from the data in Table 1, it is clear that the products of Examples 1 and 5 substantially outperform the prior art.

Although Example 4 is close to the prior art, Example 4 does not represent the preferred technology of this invention.

Examples 2, 3, and 6 are outside the scope of the invention.

Although the exact reason why this invention achieved such great results in terms of strength has not been proven, there are several logical explanations for this success.

First, a fine powder with a substantially uniform particle size is used.

In the sintering process, this fine powder becomes a cohesive mass of silicon having a relatively uniform pore size spread throughout the sintered body.

Once the product is temperature sintered, silicon-silicon bonds form between the various small silicon particles.

This is in marked contrast to most products of the prior art, in which the product that is machined and ultimately converted to silicon nitride consists primarily of products bound by silicon nitride bonds. Ta.

The products of this invention have a homogeneous structure with very fine and evenly distributed pores, rather microscopic crystals.

When examined with a microscope, reflected light and cross-polarized light (crossPolarized light) are detected.
The differences between the two are shown in the light (arized light).

In the case of anti-separation, the product has a uniform distribution of air and a very small average pore size.

Cross-polarized light also reveals a uniform quality of light and a dark lace pattern of microcellular structures.

However, the products of the prior art exhibit a variety of cellular structures.

Testing of the etched and unetched examples of the product reveals only a few recognizable particles even when the product of this invention is magnified io, ooo times.

The very few particles observed have a particle size of about 3 microns.

In contrast, in the products of the prior art, many grain structures are observed, which are unevenly distributed.

The green bottom material of this invention is clearly distinguished from the products of the prior art by its fine particle size, uniformity of structure, uniformity of density, etc.

In other words, it has no large pores that would cause defects and has high strength.

As shown in Example 1, the method produces a uniform small pore product.

As described above, the particle size of silicon is extremely fine, the silicon particles are not compacted into a mass before pressing, and no organic binder is used that would produce foreign matter or pores during combustion.

The product is compressed at very high pressure to ensure maximum green density, minimum pore size and uniform structure.

The uniform and fine particle size of the substance used in this invention is
It is believed that these particles provide a more uniform conversion of silicon to silicon nitride at relatively low temperatures.

This is thought to provide a product with lower stress inside and more uniform physical properties.

Prior art [Special ceramics (Special)
Ceramics), Heywood
Published, 1960, edited by P. popper 13
On page 2] there is a short note stating that no significant nitridation was measured for samples in which silicon powder was heated first in vacuum and then in nitrogen.

According to the conditions used by Potzper and his fellow researchers, vacuum sintering binds the larger silicon particles he used into very large agglomerates, so that the subsequent nitriding operation removes the product. It does not help in converting most of it into nitride.

This appears to be due to a significant reduction in surface area in the case of Potsper et al.

Although sintering in a vacuum or other inert, non-reactive atmosphere is used in the present invention, due to the fine particle size of the starting silicon powder, sintering in a non-reactive atmosphere results in some effective surface area. Although reduced, the remaining surface area is sufficient to effect complete nitridation.

Therefore, the maximum cross-sectional area in any part of the bonded silicon structure is so small (on the order of a few microns) that nitrogen can completely diffuse into the silicon structure and convert virtually all of the silicon structure into silicon nitride. be.

In this invention, the sintered silicon particles have a more continuous structure so that nitridation occurs within the final silicon nitride structure making it more continuous.

Strength testing of the silicon compacts before and after argon sintering shows a fairly significant increase in strength due to sintering, thus indicating the presence of a strong silicon-to-silicon bond.

Sintering provides a continuous silicon framework that is subsequently converted in situ to silicon nitride.

The average bending breaking strength for the argon sintered product of Example 1 increased by 10. 4 kg/crA (148
psi), while after sintering it is 115kg/cr
A (1633 psi).

Similarly, in Example 6, 6.8 kg/ca (97 ps i
) to 8 0. 5kp/cwt (11.48p
si).

From the above examples, it can be seen that the product of this invention is β in the final product.
It was found that the ratio of α-silicon nitride to silicon nitride was high.

It is believed that the ratio of α to β indirectly affects the strength.

This is due to the reduction of large pores.

This is done by filling the alpha matte with the β phase or by filling the alpha mat with the β phase.
This is due to differences in crystal growth between phases.

That is, large pores occur in larger and more discontinuous particles, β.

This seems to be due to the fact that nitriding occurs at a relatively low temperature and is completed at this low temperature.

This is also due to the short diffusion paths in the solid silicon being nitrided due to the small particle size used in the initial compact.

It can be seen that in the above examples the impurities iron, aluminum and calcium are present in small concentrations.

Although the exact role of these impurities is not fully understood,
It has been found that the use of very pure silicon as raw material reduces the mechanical strength only slightly.

Although glass bonds are not believed to exist in silicon-to-silicon sintering, it is believed that these impurities aid the sintering process.

The metal impurities likely act to provide a eutectic alloy, so that sintering occurs at lower than normal sintering temperatures.

Since all silicon powders have some oxide film on their surface, during the sintering process these impurities may help break down this oxide film and provide a silicon-silicon bond.

In the particular case mentioned above, a mixture of nitriding and ammonia is used in the furnace.

Although this is a common procedure, it is considered preferable.

On the other hand, although it has been stated that argon is preferred for sintering the raw silicon condensate prior to machining, other atmospheres such as helium, other noble gases, hydrogen and
Gases that do not react with silicon at temperatures of 0.degree. C. can also be used.

Gases containing nitrogen, oxygen or carbon are generally not suitable for purposes of this invention.

Below are embodiments of this invention.

1. The manufacturing method according to the claims, wherein the inert atmosphere is argon.

2. The manufacturing method according to the claims, wherein the nitrogen-containing atmosphere contains ammonia.

Claims (1)

[Claims] 1 Compressed fine silicon powder essentially free of particles with diameters greater than 10 microns to 1.45P/cr#L
forming a green body having a green density of 100 to 100 ml, and sintering this body in an inert atmosphere essentially free of nitrogen, carbon, and oxygen to produce a relatively uniform pore size throughout the green body; forming a cohesively bonded silicon mass with a pore structure sufficient to permit the diffusion of nitrogen; and thereafter sintering the silicon mass in a nitrogen-containing atmosphere sufficient to convert it to silicon nitride. 2 8 1 2.4 kg/ca (40,000p
s i) A method for producing a sintered silicon nitride compact having a bending strength of greater than or equal to 2, an alpha to beta silicon nitride ratio of greater than 2, and substantially all pores having a size of less than 2 microns.