JPH06122557A - Production of sintered compact of silicon nitride - Google Patents

Abstract

PURPOSE:To obtain a sintered compact of silicon nitride keeping flexural strength a fixed level or higher, having improved fracture toughness value and a high Weibull coefficient. CONSTITUTION:In production of a sintered compact of silicon nitride by blending silicon nitride powder with yttrium oxide and spinel as a sintering auxiliary to give mixed powder, molding the mixed powder and sintering the molded article in a non-oxidizing atmosphere, the production consists of a molding process for molding the mixed powder comprising 4-12wt.% sintering auxiliary based on the whole mixed powder, having the ratio of yttrium oxide/spinel of 3-10, into a molded article of a given shape, a sintering process for sintering the molded article at 1,600-1,700 deg.C in a non-oxidizing atmosphere to 98% or higher theoretical density and a heat-treating process for heating the prepared sintered material at 1,750-1,900 deg.C to grow beta-Si3N4 pillar-shaped crystal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon nitride sintered body having excellent fracture toughness.

[0002]

2. Description of the Related Art Sintered bodies containing silicon nitride as a main component have excellent properties such as heat resistance, thermal shock resistance, touch resistance, and wear resistance, so that they are used at high temperatures, such as bearings. ,
It is attracting attention as a material suitable for mechanical seals, high temperature bearings, nozzles, gas turbine parts and the like.

However, since it is difficult to sinter silicon nitride by itself, it is known to add a sintering aid to sinter. For example, JP-A-59-131
No. 574, silicon nitride powder 70 to 94% by weight, yttrium oxide powder 2% by weight or more and spinel powder 2
Disclosed is a method of molding a mixed powder having a weight ratio of not less than yttrium oxide powder and a spinel powder of 6 to 30% by weight and sintering the mixture powder in a non-oxidizing atmosphere. According to this method, since liquid phase sintering is performed and spinel is absorbed as a solid solution in silicon nitride, grain boundaries are strengthened and a high-strength sintered body is obtained.

[0004]

In the above-mentioned conventional manufacturing method, the spinel of the sintering aid has an effect of lowering the liquid phase sintering start temperature at the time of sintering, which facilitates the sintering of silicon nitride. There is. However, in order to obtain such an advantage, yttrium oxide and spinel must be used in a weight ratio of about 1: 1. In this case, more spinel than is necessary for sintering is added. Therefore, the bending strength of the sintered body is improved, but the fracture toughness value is not always sufficient. Therefore, a silicon nitride sintered body having a high bending strength and an improved fracture toughness value has been desired.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silicon nitride sintered body having a bending strength maintained at a certain level or more and further having a fracture toughness value improved.

[0006]

The present inventor diligently studied the mixing ratio of yttrium oxide as a sintering aid and spinel and the sintering temperature. Then, in the case where the ratio of yttrium oxide and spinel is in a fixed range, the compounding amount in silicon nitride is specified, sintering is performed to densify, and then heat treatment is performed at a temperature higher than the sintering temperature to maintain high strength. The present invention has been completed by discovering that the fracture toughness value becomes high.

That is, in the method for producing a silicon nitride sintered body of the present invention, a mixed powder obtained by adding yttrium oxide and spinel as a sintering aid to silicon nitride powder is molded into a predetermined shape and then in a non-oxidizing atmosphere. A method for producing a sintered silicon nitride sintered body, wherein the amount of the sintering aid is 4 to 10% of the total mixed powder.
A forming step of forming a mixed powder having a yttrium oxide / spinel ratio of 3 to 10 in an amount of 12% by weight into a molded body having a predetermined shape;
Sintering step of sintering to 98% or more of the theoretical density in a non-oxidizing atmosphere in the temperature range of ℃, and the obtained sintered body from 1750 to
Β-Si ₃ N ₄ by heating in the range of 1900 ° C
And a heat treatment step of growing columnar crystals.

The sintering aid compounded with silicon nitride in the present invention is
It consists of yttrium oxide and spinel. Atmospheric pressure sintering is difficult only with yttrium oxide. Therefore, by adding spinel, a liquid phase is formed at a low temperature, and at a normal pressure and 1
Densification becomes possible by sintering in the temperature range of 600 ° C to 1700 ° C. In the molding step, the mixed powder in which the silicon nitride powder and the above-mentioned sintering aid powder are mixed is molded into a predetermined shape by a normal molding method. As the molding method, conventionally used molding methods such as hydrostatic pressure molding, injection molding, and slurry casting can be applied.

In the sintering step, the molded body is heated and sintered in a non-oxidizing atmosphere to be densified to obtain a sintered body having a theoretical density (hereinafter referred to as relative density) of 98% or more. The sintering step can be carried out at normal pressure in a non-oxidizing atmosphere according to a conventional method. The heating temperature is 1600 to 1700 ° C. and is maintained until the relative density reaches 98% or more. If the held sintering temperature is less than 1600 ° C., the relative density is not sintered to 98% or more. Further, when the sintering temperature exceeds 1700 ° C., the sintering and the remarkable growth of β-Si ₃ N ₄ columnar crystals proceed at the same time, so that the β-Si ₃ N ₄ columnar crystals cannot grow uniformly and the sintered body is It is not preferable because the strength varies.

In the sintering step, the usual method is to hold at a temperature of 1600 to 1700 ° C. for 1 to 4 hours to sinter, but the temperature rising rate is sufficiently slowed down to a temperature of 1700 ° C. or less during the temperature rising. If the relative density can be sintered to 98% or more, the same effect can be obtained without holding the sintering temperature for a certain time.
The relative density in the sintering step is 98% or more, preferably 99%
% Or more.

The sintered body having a relative density of 98% or more is then heat treated. Heat treatment is 1750 to 1900
It is made by heating in the temperature range of ℃. During this period, β-Si ₃ N ₄ columnar crystals grow in the sintered body. If the treatment temperature in the heat treatment step is less than 1750 ° C., β-Si ₃ N ₄ columnar crystals do not grow sufficiently, and the fracture toughness value of the obtained sintered body is small,
The Weibull coefficient is also small. On the other hand, when the treatment temperature exceeds 1900 ° C., β-Si ₃ N ₄ columnar crystals grow too much, resulting in loss of homogeneity, which becomes a starting point of fracture, resulting in variations in strength and a reduced Weibull coefficient.

By this heat treatment, β-Si ₃ N in the sintered body
_In the growth of _four columnar crystals, the sintered body is densified and there are no pores or voids, so abnormal growth is suppressed and it grows uniformly. As a result, the obtained sintered body is dense and has β-Si.
₃ N ₄ columnar crystals are uniformly present. This makes it possible to improve the fracture toughness value without lowering the strength of the sintered body obtained, and the variation in strength is small.

The amount of this sintering aid is in the range of 4 to 12% by weight based on the total amount of powder forming the sintered body. It is more preferably 5 to 10% by weight. When the total amount of the sintering aid powder exceeds 12% by weight, the sintered body is densely sintered by the sintering aid, but since the existence of grain boundaries increases, the fracture toughness is not improved, which is not preferable. On the other hand, if the total amount of the powder of the sintering aid is less than 4% by weight, the relative density of the sintered body will be lowered and the sintered body will not be densified.
It is not preferable because the fracture toughness decreases.

The ratio of yttrium oxide to spinel is in the range of yttrium oxide / spinel = 3-10. More preferably, it is 4-9. If the ratio of yttrium oxide is large, β-Si ₃ N ₄ columnar crystals are likely to be formed in the sintered body. However, if the ratio of yttrium oxide exceeds 10, the amount of spinel becomes too small and a dense sintered body cannot be formed, which is not preferable. If the ratio of yttrium oxide to spinel is less than 3, β-Si ₃ N
_Since the columnar crystals of ₄ are not sufficiently formed, the toughness is not improved, which is not preferable.

The particle size of the mixed powder for molding the sintered body is 1
In order to enhance the sinterability at 700 ° C. or lower, the thickness is preferably 1 μm or less, more preferably 0.1 to 0.4 μm. If the particle size is large, the sinterability is deteriorated, and it becomes difficult to perform dense sintering, and the fracture toughness cannot be improved, which is not preferable.

[0016]

In the manufacturing method of the present invention, the sintering aids are specified to yttrium oxide and spinel, the total amount thereof is specified, and the ratio of yttrium oxide to spinel is set to a predetermined range, and the mixed powder thereof is used. After heat-sintering the formed compact in the range of 1600 to 1700 ° C. to make the relative density of the sintered compact 98% or more, 1750 to 1900 ° C.
Is heat-treated in the temperature range of. As a result, the obtained silicon nitride sintered body is one in which β-Si ₃ N ₄ columnar crystals are uniformly grown by densification.

That is, β-Si ₃ N _{4 is produced} while suppressing abnormal grain growth which becomes a fracture source and causes variations in strength by heating a sintered body which has been densely sintered to a temperature higher than the sintering temperature. The columnar crystals can be grown uniformly. As a result, the fracture toughness increases, and external defects such as processing scratches can be eliminated to produce a high-strength silicon nitride sintered body.
This is because the presence of spinel, which is a sintering aid, enables the sintering of silicon nitride at the above temperature, densification and sintering at normal pressure. Then, the degree of densification of the sintered body becomes β-Si in the densified sintered body after the relative density becomes 98% or more and is then heat-treated at a temperature higher than the sintering temperature.
Columnar crystals of ₃ N ₄ grow uniformly, and a homogeneous sintered body is obtained. As a result, the sintered body can be reinforced by the β-Si ₃ N ₄ columnar crystals and maintained at high room temperature and high temperature strength, and the fracture toughness value can be greatly improved. Further improvement in fracture toughness makes it less susceptible to defects such as pores in the sintered body and external surface defects such as processing scratches, reducing the variation in bending strength and significantly increasing the Weibull coefficient. .

[0018]

EXAMPLES The present invention will be specifically described below with reference to examples. (Example 1) 8% by weight of silicon nitride powder (average particle size 0.2 μm, α-conversion rate approximately 100%) with yttrium oxide powder (average particle size 0.3 μm, purity 99.9%) as a sintering aid. And spinel powder (average particle size 0.3 μm, purity 99.9%)
Of 2% by weight (Y ₂ O ₃ : MgAl ₂ O ₄ ratio 4), ethanol was added thereto and mixed in a silicon nitride ball mill.

[0019]

[Table 1] . Next, after removing ethanol and drying, 200 k of mixed powder
It was pressure-molded at a pressure of gf / cm ² into a prism shape of 5 × 6 × 40 mm. The molded body was packed in a thin rubber, sealed in a vacuum, pressurized by a CIP method at a pressure of 3000 kgf / cm ² , and then sintered in a nitrogen atmosphere under the heating conditions shown in Table 1 (holding at 1650 ° C. for 4 hours). A sintered body having a relative density of 99.8% was obtained, and this sintered body was heat-treated at 1800 ° C. for 4 hours.

The relative density corresponding to the theoretical density is n
-A value obtained by dividing the bulk density obtained by the butanol substitution method by the theoretical density, and the relative density after the completion of sintering is the sample obtained by cooling the sample sintered under the same conditions without furnace heat treatment. Sought by. The fracture toughness value, room temperature 4-point bending strength, and Weibull coefficient of the obtained sintered body were measured. The results are shown in Table 1.

As for the bending strength, room temperature 4-point bending strength based on JIS R 1601 was measured for 50 samples, and the average strength and Weibull coefficient were obtained. Fracture toughness value is JIS
R 1607, measured based on SEPB method. Since the relative density is 99.8% before the heat treatment, the β-Si in the dense sintered body does not progress further during the heat treatment.
A columnar crystal of ₃ N ₄ grows uniformly and becomes a sintered body having high strength and no variation, and the Weibull coefficient shows a large value of 51.3. (Example 2) As shown in Table 1, the powder sample of Example 1 was used for sintering, the temperature rising rate was slowed down to 2 ° C / min for sintering, and the sintering holding time was eliminated to obtain a relative density of 99. Heat treatment was performed under the same conditions as in Example 1 except that the sintered body was 0.99%.

As shown in Table 1, the obtained sintered body exhibited the same strength and Weibull coefficient as those of the sintered body obtained in Example 1. (Comparative Example 1) As shown in Table 1, the sample of Example 1 was sintered at the same temperature rising rate as in Example 1 to give 17 as in Example 2.
If the sintering holding time at 00 degrees was not given, the sintering was not completed at 1700 ° C, and the relative density was 93.0%. When this is heat-treated at 1800 ° C. for 4 hours, the sintering further progresses and the β-Si ₃ N ₄ columnar crystals also noticeably grow. Therefore, in the sintered body, β-Si
_{If the 3} N ₄ columnar crystals are non-uniformly present and the strength and the fracture toughness values vary, the Weibull coefficient becomes as low as 24.7.

Further, the samples of the sintered bodies of Example 1 and Comparative Example 1 were examined for bending strength at 1000 ° C. in air. The average value of the bending strength of the examples was 1060 MPa and the Weibull coefficient was 48.7. On the other hand, in the comparative example, the bending strength is 980.
The Weibull coefficient at MPa was as small as 16.2. (Examples 3, 4, 5) The addition amount of the sintering aid in Example 1 was changed to 9% by weight of yttrium oxide powder and 1% by weight of spinel powder (Y ₂ O ₃ : MgAl ₂ O ₄ ratio 9). Instead, a sample of a sintered body was prepared under the sintering conditions and heat treatment conditions shown in Table 2, and the room temperature was 4
The average strength of the point bending strength, the Weibull coefficient, and the fracture toughness value were measured. The results are shown in Table 2.

In Example 3, the sintering conditions were a temperature rising rate of 2 ° C./min, and the temperature was maintained at 1700 ° C. for 4 hours to obtain a sintered body having a relative density of 98.6%. The heat treatment was performed at 1800 ° C. for 4 hours.
In Example 4, the temperature rising rate was 2 ° C./min and the temperature was maintained at 1700 ° C. for 8 hours to obtain a sintered body having a relative density of 99.2%.
It was carried out at 800 ° C. for 4 hours. In Example 5, the temperature rising rate was 2
C./min. And held at 1700 ° C. for 4 hours to give a relative density of 98.
A 6% sintered body was used, and heat treatment was performed at 1900 ° C. for 4 hours.

As shown in Table 2, the fracture toughness value is 9 or more, the bending strength is 1100 MPa or more, and the Weibull coefficient is 40.
The above was a homogeneous sintered body. (Comparative Examples 2 and 3) The sintering conditions and heat treatments shown in Table 2 were performed with the same addition amount of the sintering aid as in Example 3. The strength of the obtained sample was measured in the same manner as in Example 1. The results are shown in Table 2.

Comparative Example 2 was 1700 at a temperature rising rate of 2 ° C./min.
There was no holding time at ° C. At this time, the relative density was 94%, which was not sufficiently sintered. This was heat-treated at 1800 ° C. for 4 hours. In this case, since the sintering and the remarkable growth of the β-Si ₃ N ₄ columnar crystals simultaneously proceed during the heat treatment, the β-Si ₃ N ₄ columnar crystals do not grow uniformly and the strength varies and the Weibull coefficient is 13.2. And low.

Comparative Example 3 has a temperature rising rate of 2 ° C./min and a temperature of 1700 ° C.
After holding for 4 hours, a sintered body having a relative density of 98.6% was obtained. This sintered body was heat-treated at 1950 ° C. for 4 hours. In this case, since the heat treatment temperature is high, the columnar crystals grow too much and the uniformity is lost to serve as a fracture starting point, and the strength varies and the Weibull coefficient is as low as 23.1.

[0028]

[Table 2] (Example 6) The amount of the sintering aid added in Example 1 was adjusted to 4.5 wt% of yttrium oxide powder and 0.5 wt% of spinel powder.
When the temperature rising rate was changed to 2 ° C./minute and held at 1700 ° C. for 8 hours instead of (Y ₂ O ₃ : MgAl ₂ O ₄ ratio 9), the relative density was 98.8%. . This was heat-treated at 1800 ° C. for 4 hours to prepare a sample of a sintered body, and the average strength of room temperature 4-point bending strength, the Weibull coefficient, and the fracture toughness value were measured. The results are shown in Table 2. Fracture toughness value is 9.2 MPa√m
With a bending strength of 1190 MPa and a Weibull coefficient of 42.2, a homogeneous and high-strength sintered body was obtained. (Comparative Example 4) Using the sample of Example 6, the same conditions as in Example 6 were measured except that the sintering conditions were changed to a temperature rising rate of 2 ° C / min and no holding time of 1700 ° C was used, and the strength and the like were measured. did.
The results are shown in Table 3. The strength is almost the same as in Example 6, but the Weibull coefficient is small at 12.1 and the variation is large.

[0029]

[Table 3] (Example 7) 4 wt% yttrium oxide powder added amount of sintering aid and spinel powder 1 wt% of Example 1 (Y ₂ O _3: MgA
A sample of a sintered body was prepared under the sintering conditions and heat treatment conditions shown in Table 4 instead of the ratio of l ₂ O ₄ 4), and the average strength of room temperature 4-point bending strength, the Weibull coefficient, and the fracture toughness value were measured. The results are shown in Table 4. Since the relative density at the end of sintering is 98.8% and 98% or more, the sample obtained by heat treatment is β-S.
i ₃ N ₄ columnar crystals are uniformly grown, exhibiting high strength, and a homogeneous sintered body having a Weibull coefficient of more than 40 can be obtained. (Comparative Example 5) Using the sample of Example 7, the same treatment as in Example 7 was carried out except that the sintering conditions were changed, and the strength and the like were measured. The results are shown in Table 4.

In this case, the amount of the sintering aid is small, and the sintering is not sufficiently performed unless it is held at a predetermined sintering temperature, and the relative density after the completion of sintering becomes 92.9% and 98% or less. In the heat treatment stage, the progress of sintering and the growth of columnar crystals proceed at the same time, and the distribution of columnar crystals becomes non-uniform. As a result, the average value of the bending strength and the fracture toughness value shows a predetermined value, but the variation is large and the Weibull coefficient is small. (Comparative Example 6) The amount of the sintering aid added in Example 1 was 6% by weight of yttrium oxide powder and 4% by weight of spinel powder (Y ₂ O ₃ : MgA).
A sample of the sintered body was prepared under the sintering conditions and heat treatment conditions shown in Table 4 in place of the ratio of l ₂ O ₄ of 2.5), and the average strength of room temperature 4-point bending strength, the Weibull coefficient and the fracture toughness value were measured. did. The results are shown in Table 4.

If the ratio of Y ₂ O ₃ : MgAl ₂ O ₄ is less than 3, the fracture toughness value of the obtained sample is 8 MPa√, even if the sintering conditions and heat treatment conditions are within the range of the present invention. It is 7.6, which is less than or equal to m, and has a low strength to be destroyed by processing scratches / damages and a small Weibull coefficient.

[0032]

[Table 4]

[0033]

According to the method for manufacturing a silicon nitride sintered body of the present invention, since the sintered body has almost no pores when it is densely sintered by 1700 ° C. to 98% or more of the theoretical density,
After that, the sintered body is heat-treated at a temperature equal to or higher than the sintering temperature and β-Si ₃ N
₄ When growing columnar crystals, they can be grown uniformly without abnormal growth toward the pores, and a homogeneous sintered body can be obtained. As a result, while maintaining high bending strength,
A sintered body having a fracture toughness value is obtained. Further, since the obtained sintered body has high fracture toughness, there is little variation in strength and the Weibull coefficient can be increased.

Claims (1)

[Claims] 1. A method of manufacturing a silicon nitride sintered body, which comprises molding a mixed powder of silicon nitride powder to which yttrium oxide and spinel are added as a sintering aid into a predetermined shape and then sintering the powder in a non-oxidizing atmosphere. And a molding step of molding the mixed powder in which the amount of the sintering aid is 4 to 12% by weight of the total mixed powder and the yttrium oxide / spinel ratio is in the range of 3 to 10 into a molded body having a predetermined shape. A sintering step of sintering the molded body to a temperature of 1600 to 1700 ° C in a non-oxidizing atmosphere to 98% or more of the theoretical density, and heating the obtained sintered body in the range of 1750 to 1900 ° C. And a heat treatment step of growing β-Si ₃ N ₄ columnar crystals.