JP2747635B2 - Method for producing silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a silicon nitride sintered body, and more particularly, to a method for manufacturing a thick molded body, which can surely and densely sinter up to the center of the molded body, and has excellent mechanical properties. The present invention relates to a method for producing a silicon nitride sintered body having strength and high toughness.

[0002]

2. Description of the Related Art A sintered body mainly composed of silicon nitride has excellent properties such as high strength, high heat resistance, high thermal shock resistance, high wear resistance, and acid resistance. The use of is expected.

[0003] Since silicon nitride itself is difficult to sinter, a sintered body is prepared by adding various sintering aids to silicon nitride powder as a raw material during sintering. A method of firing the body under an inert gas atmosphere is generally employed. Particularly in recent years, in order to promote a sintering reaction, to obtain a sintered body having high density and excellent mechanical strength, and to prevent thermal decomposition of silicon nitride at a high temperature, it is necessary to use nitrogen gas or the like. Firing is performed under the condition that the active gas pressure is high.

In the above-described method of firing a ceramic compact under a condition in which the pressure of the inert gas atmosphere is increased, as shown schematically in FIG. 5, the pressure of the inert gas and the firing temperature are simultaneously increased. When the holding temperature T for sintering is reached,
The firing pattern in which the inert gas pressure is also substantially equal to the holding pressure P is most generally used.

[0005]

However, when a thick silicon nitride sintered body is manufactured by a conventional firing pattern as shown in FIG. 5, for example, the center of the sintered body often has a low density. . This may be for the following reasons.

That is, when the compact formed of the silicon nitride powder and the sintering aid powder is heated and starts sintering, first, as shown in FIG. Progress. In the initial stage of such firing, the inside 21 of the molded body 2 has not been sintered yet, and due to the densified surface portion 20, the inside 21 has an external pressure (inert gas pressure) of the molded body 2. The gas having the same pressure as that of the gas is confined. When the sintering of the compact further proceeds, this densified layer grows gradually from the surface to the inside (FIG. 6).
(B)), the densified layer 20a gradually becomes thicker. Therefore, the gas trapped in the inside 21 of the molded body 2 is more difficult to escape to the outside of the molded body 2, and the gas pressure is increased. Therefore, the closer to the center of the molded body, the more the densification is delayed by the residual gas, and the resulting sintered body 2
The density in the vicinity of the inside 22 of a (FIG. 6 (c)) decreases. In particular, when the sintering reaction actually starts (see FIG.
When the external pressure (inert gas pressure) in
Since the gas pressure remaining inside the molded body increases (the amount of remaining gas increases), it becomes more difficult to densify the inside.

For the above reasons, in the conventional method, especially when a thick sintered body is manufactured, the inside tends to have a low density and the mechanical strength is inferior.

In order to avoid such inconveniences, various sintering methods have been proposed. For example, JP-A-60-186474
When firing a molded body composed of a mixed powder of a silicon nitride powder and an oxide auxiliary, (a) the liquid phase formation temperature of 1200 to
In the temperature range of 1600 ° C., the temperature is 0.5 to 0.5 in the vicinity of the temperature at which the liquid phase is completely formed, and (b) in the temperature range of 1400 to 1850 ° C., which is the gas generation temperature, in which the gas generation is particularly remarkable. It discloses a method of firing a molded body while holding it for 3 hours.

According to the above method, defects such as so-called "swelling" due to generation of gas during firing can be reduced, but the mechanical strength of the obtained sintered body is not yet sufficient, and the toughness is large. Absent. When ceramics such as silicon nitride sintered bodies are used for structural ceramics and various mechanical parts, not only mechanical strength but also toughness is important. If the toughness is low, it is hard to say that the ceramic member is reliable.

[0010] Accordingly, an object of the present invention is to provide a silicon nitride sintered body which is capable of achieving internal densification even when a relatively thick molded body is sintered, and thus has excellent mechanical strength and high toughness. It is to provide a method by which can be manufactured.

[0011]

Means for Solving the Problems As a result of intensive studies in view of the above-mentioned objects, the present inventor has found that when firing a molded body composed of silicon nitride powder and a sintering aid in a nitrogen-containing atmosphere, first of all, The molded body is heated under a low first nitrogen-containing atmosphere pressure,
At a point in time after reaching the first holding temperature near 800 ° C. for a while, the nitrogen-containing atmosphere pressure is rapidly increased to a certain level or more, and 30 to 18 after reaching the first holding temperature.
Hold for 0 minutes, and after reaching the second nitrogen-containing atmospheric pressure, 19
A firing program in which the temperature is raised to a second holding temperature of 00 ° C. or more and held at this second holding temperature for 30 to 240 minutes enables the compact to be sintered densely inside the compact, The present inventors have discovered that a needle-like crystal structure can be developed therein, and a sintered body having high strength and high toughness can be obtained, and the present invention has been reached.

That is, the method of the present invention for producing a silicon nitride sintered body by sintering a molded body composed of silicon nitride powder and a sintering aid in a nitrogen-containing atmosphere has a first step of 200 kg / cm ² or less. The temperature was raised under a nitrogen-containing atmosphere pressure P ₁ ,
10 to 100 after reaching the first holding temperature T ₁ of 1800 ° C.
Minutes, the nitrogen-containing atmosphere pressure is increased to 300 kg /
with rapidly boosted to reach cm ² or more second nitrogen-containing atmosphere pressure P _2, the first holding temperature T ₁ of
To hold 30 to 180 minutes, the second nitrogen-containing atmosphere pressure _{P 2} a second 1900 ° C. or more after reaching the holding temperature _{T 2}
Until the temperature was raised, and wherein the holding the second 30 to 240 minutes holding temperature _{T 2} of the.

Hereinafter, the present invention will be described in detail. First, raw materials used for manufacturing a molded body will be described.

(A) Silicon Nitride Powder The silicon nitride powder preferably has a low oxygen content for the purpose of improving the high-temperature strength of the sintered body. However, if the oxygen content is too small, the sinterability will decrease. In the method of the present invention, it is preferable to use a silicon nitride powder having an oxygen content of about 0.5 to 2.0% by weight. If the oxygen content is less than 0.5% by weight, the sinterability decreases, and if it exceeds 2.0% by weight, the high-temperature strength decreases. The average particle size of the silicon nitride powder is 0.3 to 1.
It is preferably about 0 μm.

The specific surface area of the silicon nitride powder is 8 to 12
It is desirably about m ² / g. Further, the amount of metal impurities is desirably 200 ppm or less. When the specific surface area and the amount of metal impurities are in the above ranges, it is possible to suppress the growth of abnormal grains in the structure of the sintered body and to obtain a fine structure, thereby improving the mechanical strength of the sintered body. Can be.

(B) Sintering Aid As the sintering aid, a mixture of an Al compound and a Group IIIa element compound or a Group IIIa element compound can be used.
Here, group IIIa elements are scandium, yttrium,
And lanthanum series elements. Examples of compounds of the Al and IIIa group elements include oxides and organic acid salts. These are mainly added to the silicon nitride powder in a powder state. In order to improve the strength (green strength) of the molded body,
A whisker-like sintering aid composed of the above components can also be used.

Al compounds include Al ₂ O ₃ and Al ₂ TiO ₅
Etc. are preferred. Further, as the IIIa group element compound, Y ₂
O ₃ , yttrium oxalate, Nd ₂ O ₃ , Yb ₂ O _{3 and the} like can be mentioned. Further, a solid solution such as 3Y ₂ O ₃ .5Al ₂ O ₃ can also be used. More preferably, Al ₂ O ₃ and Y ₂ O ₃ are used as sintering aids.

When powdery Al ₂ O ₃ and Y ₂ O ₃ are used as sintering aids, the average particle size of the Al ₂ O ₃ powder is 0.4.
About 0.5 μm, the average particle size of Y ₂ O ₃ powder is 0.4 to 2
It is preferably about μm.

The compounding ratio of the silicon nitride powder and the sintering aid as described above, when used as a slightly different but, Al ₂ O ₃ and Y ₂ O ₃ and the sintering aid by sintering agent used, Al ₂ It is preferable that O ₃ is 0.5 to 2% by weight, Y ₂ O ₃ is 2.5 to 5% by weight, and the balance is substantially silicon nitride. Al ₂ O ₃ content is 2
If the content exceeds 10% by weight, oxidation resistance and strength at high temperatures decrease,
On the other hand, if the content is less than 0.5% by weight, the densification of the sintered body does not proceed, and oxidation resistance and high strength cannot be obtained. On the other hand, Y ₂ O ₃
If the amount exceeds 5% by weight, the oxidation resistance at high temperatures decreases,
If it is less than 2.5% by weight, the densification of the sintered body does not progress,
Oxidation resistance and high strength cannot be obtained.

Next, the method of the present invention will be described. First, using the above-mentioned silicon nitride powder and a sintering aid, a compact is manufactured by the following method.

Silicon nitride powder, Al ₂ O ₃ powder, Y ₂ O ₃
Mix with a sintering aid such as powder. This mixing can be performed by a known method, for example, a ball mill, a disperser, or the like. In addition, it is preferable to perform mixing by a ball mill by adding ethanol or the like to the mixed powder. Further, it is preferable to use a ball made of silicon nitride as the ball used for mixing. Thereby, it is possible to minimize mixing of impurities during mixing.

The obtained mixed powder is formed into a desired shape by a known method using a die press or a cold isostatic press (CIP). At the time of molding, a molding aid such as a polyvinyl alcohol solution can be added as needed.

Next, the compact obtained above is fired. In the present invention, the pressure of the nitrogen-containing atmosphere is controlled as shown below in accordance with the change in the firing temperature.

The firing of the molded body is typically performed according to a firing pattern schematically shown in FIG. Here, the firing temperature pattern is shown in the upper part of FIG. 1, and the pressure change pattern of the nitrogen-containing atmosphere is shown in the lower part. Hereinafter, FIG.
The method of firing the compact will be described below.

First Step (I) In the first step (I) of firing, the compact is held at a first holding temperature T _1.
, But the pressure of the nitrogen-containing atmosphere is kept relatively low. First, it is preferable that the heating rate be about 6 to 10 ° C./min. On the other hand, the nitrogen-containing atmosphere pressure may be gradually increased as shown in FIG. 1, or may be kept at a certain relatively low pressure. However,
It is necessary to apply pressure such that silicon nitride does not decompose, and the nitrogen-containing atmosphere pressure in the first stage (I) is 5 to 200 kg.
/ Cm ² is preferable. More preferably, 10-5
0 kg / cm ² .

The first will enter into the second stage (II) the temperature reaches the first holding temperature T ₁ of rising second stage (II), was the second step (II) but, a nitrogen-containing atmosphere pressure above It is kept low to the level of the atmospheric pressure in step (I). In the example shown in FIG. 1, the second stage (I
While setting the first nitrogen-containing atmosphere pressure at I) to a constant pressure P _1, it is not necessary to the during the second stage (II) and constant pressure, first in the first stage (I) some may vary as long as it is within the range of nitrogen-containing atmosphere pressure P _1. More preferably the first nitrogen-containing atmosphere pressure P ₁ 10
５０50 kg / cm ² .

The first holding temperature T ₁ is 1600 to 1800
° C. If T ₁ is less than 1600 ° C., it is difficult to promote the sintering reaction satisfactorily. If T ₁ exceeds 1800 ° C., silicon nitride may be thermally decomposed. Preferred first holding temperature T ₁ of is 1,750 to 1,800 ° C..

In the present invention, the first holding temperature T ₁ does not need to be a constant temperature as shown in FIG. 1, and may slightly change within the temperature range of T _1. If there is T
_It may change to about ₁ ± 30 ° C.

In the second step (II), a densified layer starts to be formed on the surface of the molded body. At this stage, the nitrogen-containing atmosphere pressure P ₁ is still low, so the densified surface layer is formed. , The gas pressure confined inside the compact is low (the gas amount is small). In addition, the densified surface layer has not been completely cured yet, and the gas generated inside the molded body during firing is relatively easily released to the outside.

The required time Δt (= t ₂ −t) in the second stage (II)
₁ ) is 10 to 100 minutes. If the time Δt is less than 10 minutes, a relatively soft densified layer is not formed on the surface of the molded body, and therefore, the same densification as that obtained by the conventional method in the third step (III) described later is performed. A cured surface layer will result. Then, the gas is confined inside the compact during sintering, and it is not possible to achieve densification inside the sintered compact. On the other hand, when Δt exceeds 100 minutes, the sintering of the densified layer excessively proceeds to cause abnormal grain growth, which causes a decrease in strength. Preferably, the time Δt is 30 to 60 minutes.

Third stage (III) A predetermined time Δ from time t ₁ when the first holding temperature T ₁ is reached.
In time t ₂ that t has elapsed, turn sharply raise the nitrogen-containing atmosphere pressure. At this time, the atmospheric pressure (holding pressure) after pressure increase
P _{2 is set} to 300 kg / cm ² or more. Holding pressure P ₂ is 30
When it is less than 0 kg / cm ² , even if almost the entire inside of the sintered body has a high density, it is not possible to make the minute pores remaining in the sintered body slightly smaller, so that The voids become defects and the mechanical strength decreases. According to the study of the present inventors, when the holding pressure is set to 300 kg / cm ² or more, the maximum diameter of the hole can be extremely small, and the mechanical strength can be improved. Preferably holding pressure P
_{2 is set} to 500 to 2000 kg / cm ² .

[0032] In the present invention, preferably the holding pressure P ₂ and at least twice the boosting before the pressure P _1. Holding pressure P
₂ densification inside the sintered body (density) is difficult to be less than twice the pressure P _1.

Time required for the third step (III) (t ₃ -t ₂ )
Is preferably 60 to 120 minutes, which is set in consideration of the length of the required time (t ₂ −t ₁ ) of the second step (II) described above. Specifically, the second stage (II) and the third stage (II
The total time (t ₃ −t ₁ ) of _I ) is set to be 30 to 180 minutes.

Fourth stage (IV) 30 to 180 from the beginning of the second stage (II) (time t ₁ )
After a lapse of minutes (time t ₃ ), the temperature is raised to the second holding temperature T ₂ while maintaining the nitrogen-containing atmosphere pressure at the second nitrogen-containing atmosphere pressure P ₂ . The heating rate at this time is 6 to 10 ° C /
Minutes is preferred. When the second holding temperature _{T 2} is reached, it holds 30 to 240 minutes at that temperature. That is, in FIG. 1, t ₅ -t ₄ is set to 30 to 240 minutes.

The second holding temperature T ₂ is 1900 ° C. or higher, preferably 1900 to 1950 ° C. If the second holding temperature T ₂ is less than 1900 ° C., no needle-like crystal structure is formed in the silicon nitride sintered body, and a sintered body having high toughness cannot be obtained. Incidentally, the second holding temperature T ₂ need not necessarily be fixed to a constant, it may vary slightly as long as it is within the temperature ranges described above.

Fifth Step (V) After the high-temperature and high-pressure firing in the fourth step (IV) is completed, cooling is performed. The cooling of the fifth stage (V) is furnace cooling (cooling)
It may be. Also, the high nitrogen-containing atmosphere pressure may be released immediately.

According to the above-mentioned method, for example, the molded body is 1
Even in the case of a thick material having a thickness of about 00 mm, it is possible to densely sinter the vicinity of the center portion, and does not leave defects such as voids and the like that lower the mechanical strength. Further, a needle-like crystal structure can be developed in the sintered body.

The size of the needle-like particles in the needle-like crystal structure of the silicon nitride sintered body produced by the above-described method of the present invention is 50 to 70 μm on average of the major axis. The aspect ratio is 10 to 15. Further, the density at the center is 3.2 g / cm ³ or more.

[0039]

According to the method of the present invention, when the nitrogen-containing atmosphere pressure in the initial stage of sintering is still low (by the second stage described above), a relatively soft surface dense layer is formed, and The gas existing inside the compact can easily escape to the outside of the compact. Further, the pressure of the gas confined inside the molded body is also reduced (the amount of the gas confined is reduced), and the gas component can be easily diffused inside the molded body. Next, as a third step, the pressure of the nitrogen-containing atmosphere is rapidly increased, so that densification is ensured without forming voids inside.

In the present invention, since the sintered body is kept at the high second holding temperature of 1900 ° C. or more for a specific time, a dense needle-like crystal structure is formed in the sintered body, and the sintered body has high toughness.

[0041]

The present invention will be described in more detail with reference to the following specific examples. Example 1 Silicon Nitride Powder (SN-E10, manufactured by Ube Industries, Ltd.) 9
6.5% by weight, 2.5% by weight of Al ₂ O ₃ powder (AKP30, manufactured by Sumitomo Chemical Co., Ltd.) and 1.0% by weight of Y ₂ O ₃ (fine powder, manufactured by Japan Yttrium Co., Ltd.) 500 g of ethanol to 500 g of a powder mixture
For 16 hours using a silicon nitride ball.

The obtained mixture was dried by a rotary evaporator, and dried by CIP (pressure of 3000 kg / cm ² ).
It was formed into a size of 40 mm × 50 mm in height.

FIG. 1 shows the above-mentioned molded body in a nitrogen gas atmosphere.
Was fired in the firing pattern shown in FIG. Here, the temperature rising rate in the first stage (I) is set to 10 ° C./min, and the first holding temperature T ₁
Was set to 1800 ° C., and the second holding temperature T ₂ was set to 1950 ° C. Note that the second holding temperature T _{2 at} the beginning of the fourth stage is
The heating rate up to 6 ° C./min.

[0044] For addition pressure was first nitrogen gas atmosphere pressure _{P 1} in the second stage 30kg / cm ^2, a second nitrogen-containing atmosphere pressure _{P 2} and 1000 kg / cm ^2.
Further, regarding the time of each step, the required time of the second step (Δt = t ₂ −t ₁ ) and the required time of the third step (t ₃ −
t ₂ ) was 60 minutes each. The 1 time _(t 5 -t ₄₎ to hold the second holding temperature _{T 2} in a fourth stage
20 minutes. The cooling in the fifth stage was furnace cooling.

When the strength of the obtained sintered body was measured in accordance with JIS R1601, an average of 20 sintered bodies was 95 kg / mm.
^{Was 2} . In addition, when the fracture toughness was measured in accordance with JIS R 1607, it was 7.0 MPa (m) ^1/2 . Further, the Weibull coefficient was also obtained. FIG. 2 shows the values of the fracture toughness and the Weibull coefficient. Furthermore, the density of the central portion of the sintered body was measured and found to be 3.2 g / cm ³ .

When the structure at the center of the sintered body was observed with a scanning electron microscope, a dense needle-like crystal structure was observed. An electron micrograph is shown in FIG.

COMPARATIVE EXAMPLE 1 In the firing program shown in FIG. 1, the temperature was not raised to the second holding temperature (T ₂ = 1950 ° C.) in the fourth step, and the first to fourth steps were performed through the second to fourth steps. A sintered body was manufactured in the same manner as in Example 1 except that the holding temperature was maintained at 1800 ° C.

The strength of the obtained sintered body was measured in the same manner as in Example 1. The average strength was 93 kg / mm ² . Similarly, the fracture toughness and Weibull coefficient were determined. FIG. 2 shows the values of the fracture toughness and the Weibull coefficient. Furthermore, the density at the center of the sintered body was measured in the same manner as in Example 1, and it was 3.2 g / cm ³ .

When the structure at the center of the sintered body was observed with a scanning electron microscope, no sufficient needle-like crystal structure was found. An electron micrograph is shown in FIG.

[0050]

As described in detail above, according to the method of the present invention, even a relatively thick sintered body having a thickness of about 100 mm can be reliably densified at the center thereof, and the mechanical strength can be reduced. A high sintered body can be manufactured. Further, according to the method of the present invention, a silicon nitride sintered body having a dense needle-like crystal structure can be obtained, so that a sintered body having not only mechanical strength but also high toughness can be obtained.

The method of the present invention is not only for the purpose of densifying the inside of a thick sintered body, but also for a system in which the sintering aid is reduced or a system in which the sintering aid has a high melting point (for example, Y _2). O ₃ and
The present invention can also be applied to sintering of a system that is difficult to sinter, such as a system using a mixture with Yb ₂ O ₃ as a sintering aid.

As described above, the silicon nitride sintered body according to the method of the present invention has a high density, a large mechanical strength, and a high toughness. It is suitable for lumber and the like.

[Brief description of the drawings]

FIG. 1 is a graph showing a typical example of a change pattern of a firing temperature and an inert gas pressure in a method of the present invention.

FIG. 2 is a graph showing a fracture toughness value and a Weibull coefficient in the sintered bodies of Example 1 and Comparative Example 1.

FIG. 3 is an electron micrograph showing a particle structure in a sintered body of Example 1.

FIG. 4 is an electron micrograph showing a particle structure in a sintered body of Comparative Example 1.

FIG. 5 is a graph schematically showing a typical example of a firing pattern of a conventional ceramic molded body.

FIG. 6 is a schematic cross-sectional view showing a sintering behavior during firing of a ceramic molded body.

[Explanation of symbols]

 Reference Signs List 2 Molded body 2a Sintered body 20 Densified surface layer 21 Inside molded body 20a Densified layer

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Yoshikatsu Higuchi 1-4-1 Chuo, Wako-shi, Saitama Pref. Honda Technical Research Institute Co., Ltd. (56) References

Claims (2)

(57) [Claims] 1. A method for producing a silicon nitride sintered body by sintering a compact comprising silicon nitride powder and a sintering aid in a nitrogen-containing atmosphere, wherein the first nitride of 200 kg / cm ² or less is used . The temperature was raised under a nitrogen-containing atmosphere pressure, and when 10 to 100 minutes had elapsed after reaching the first holding temperature of 1600 to 1800 ° C., the nitrogen-containing atmosphere pressure was increased to 300 kg / cm ² or more . > together rapidly boosted to reach a nitrogen-containing atmosphere pressure, and held for 180 minutes 30 to the first holding temperature, before
After reaching the second nitrogen-containing atmosphere pressure, the temperature is raised to a second holding temperature of 1900 ° C. or more, and
A method for producing a silicon nitride sintered body, which is held for 0 to 240 minutes. 2. The method for producing a silicon nitride sintered body according to claim 1, wherein the second holding temperature is 1900 to 2000 ° C.