JPH05178669A - Production of silicon nitride sintered body - Google Patents

Abstract

PURPOSE:To obtain high strength by mixing Si3N4 powder with ZrO2.SiO2 (zircon) as a sintering aid and other oxide sintering aid, compacting and sintering the mixture. CONSTITUTION:Si3N4 powder having 0.1-0.6mum average particle diameter and such high purity as <=100ppm total amt. of metal impurities is mixed with 5-15wt.% sintering aid having 0.05-0.5mum average particle diameter obtd. by mixing zircon with other oxide sintering aid such as Y2O3, or Y2O3+MgAl2O4 in (1:4)-(2:3) weight ratio. The resulting powdery mixture is subjected to isostatic pressing under 1,000-3,000kg/cm<2> pressure to obtain a compact and this compact is sintered at 1,500-1,700 deg.C in an inert atmosphere of Ar to produce a silicon nitride sintered body.

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.

[0002]

2. Description of the Related Art A silicon nitride sintered body is produced by adding a sintering aid to silicon nitride powder, molding and sintering. Various oxides such as Y ₂ O ₃ , MgO and Al ₂ O ₃ are used and proposed as the sintering aid. Although used as a coated base material for a cutting tool, 0.1% to 10% by weight of Y ₂ O ₃ as a sintering aid was added to silicon nitride powder as a sintering aid.
A sintered body to which 1 to 10% by weight of ZrO ₂ is added is disclosed in JP-A-61-101482, and Y ₂ O ₃ is added to a silicon nitride powder as a sintering aid, and further ZrO ₂ and SiO _{2 are} added. It is also conventionally known to add each of these.

[0003]

Conventionally, a silicon nitride sintered body has been pressure-sintered at 1500 ° C. to 170 ° C.
Room temperature to 1000 ° C due to relatively low temperature of 0 ° C
There is no report that the strength exceeds 1000 MPa (in air) in the range of, and further exceeds 600 MPa at 1400 ° C. in air. In order to obtain high strength, a dense and fine sintered body structure was required, and for that purpose it was necessary to sinter at a low temperature, but since it does not sinter under normal pressure, the HIP sintering method is used. Has been.

Therefore, the present invention provides a silicon nitride sintered body which does not require HIP sintering, is pressureless sintered, and is sintered at a relatively low temperature of 1500 to 1700 ° C. to exhibit high strength at high temperature. It is intended to provide a method for manufacturing.

[0005]

According to the present invention, the sintering aid is
ZrO together with various oxide-based auxiliaries as agents₂And SiO ₂
Zircon ZrO₂・ Si
O₂(ZrSiO_Four) Is added in the form of compound
The above-mentioned problems are solved by. ZrO₂And SiO₂
When and are added as individual particles, they are densified, but strength
Will fall.

[0006] Thus, according to the present invention, the silicon nitride powder, molding the homogeneous powder mixture prepared by adding at least ZrO ₂ · SiO ₂ and other oxide-based additive as a sintering aid, 1500 ° C. The molded article Provided is a method for producing a silicon nitride sintered body, which comprises sintering at a temperature of ˜1700 ° C.
The silicon nitride powder as a starting material has an average particle size of 0.1 to 0.1.
It is preferable to use one having a thickness of 0.6 μm, more preferably 0.2 to 0.4 μm. This is because the driving force at the time of sintering becomes large due to the small particle size of the starting material, and when the particle size of the starting material becomes large, the particle size of the sintered particles becomes large, which causes a decrease in strength. Further, the purity is preferably such that the total amount of metal impurities is 100 ppm or less. This is because if the amount of impurities is larger than this, the softening temperature of the grain boundary glass phase decreases, and the strength at high temperature decreases.

The present invention is characterized in that zircon (ZrO ₂ .SiO ₂ ) is added together with the above various oxide type auxiliaries. Densified at a low temperature of 1500 to 1700 ° C. with a silicon nitride powder raw material and various oxide-based auxiliaries,
Moreover, even higher strength can be realized by adding zircon. As an oxide-based auxiliary agent other than zircon, Y ₂ O is typically used.
₃ , Yb ₂ O ₃ , Al ₂ O ₃ , MgAl ₂ O ₄ , TiO
₂ , ZrO ₂ , 3Al ₂ O ₃ .2SiO ₂ (mullite), other rare earth metal oxides, and the like.
These auxiliaries form a liquid phase with the silicon oxide existing on the surface of the silicon nitride powder particles, and become a sufficiently dense sintered body at 1500 to 1700 ° C., so that they are indispensable as accelerators for the liquid phase sintering.

Examples of preferred oxide auxiliaries are Y₂O₃, Y
₂O₃+ MgAl₂O_Four, 3Al₂O₃・ 2SiO
₂(Mullite), Al₂O₃And so on. Y₂O₃And Y
₂O₃+ MgAl₂O_FourIs 1000 ° C. 4-point bending strength 10
Providing a strength of over 00 MPa, mullite is 1300-14
Provides high strength at 00 ° C. Y₂O₃Is acicular β-S
i ₃N_FourIt promotes the formation of crystals and improves the aspect ratio of crystal grains.
By making it larger, the strength and fracture toughness of the sintered body are improved. MgA
l₂O_FourHas the effect of lowering the sintering temperature, and as a result
As a result, a fine structure is developed in the sintered body.

In the present invention, a sintering aid other than the oxide type may be additionally added. For example, AlN, Y ₂ (C
N ₂ ) ₃ and so on. The sintering aid has an average particle size of 0.05 to
0.5 μm, more preferably 0.3 μm, purity 99.0
% Or more is preferable. If the particle size is too large, the sinterability will decrease, and if the amount of impurities is large, the desired strength cannot be obtained.

The amount of the sintering aid depends on the kind of the sintering aid, but generally the total amount of zircon and other oxide-based auxiliary agents is 5 to 15% by weight, more preferably about 6 to 10% by weight. Added. If it is less than 5% by weight, it will not be densified by pressureless sintering and strength will not be obtained. The weight ratio of zircon to other oxide-based auxiliary agent is preferably 1/4 to 2/3, but 1 to 3 is preferable for zircon and mullite. When the amount of zircon is less than these ratios, the effect of improving the high temperature strength is small, and when the amount is large, the densification is hindered.

In addition to the above sintering aid, amorphous Si
When the -NC powder is added, fine SiC particles are generated at the grain boundary triple points and in the grain during the sintering process, thereby
At 00 ° C, softening of the grain boundary phase is prevented, and SiC particles disperse and become resistant to crack growth, resulting in strength (especially 1400 ° C).
Strength) and fracture toughness. This amorphous S
The i-N-C powder is about 30 wt% SiC at the sintering temperature.
And 70% by weight of Si ₃ N ₄ are crystallized, and powder having an average particle diameter of 0.3 μm or less is preferable. Note that SiC powder may be directly added as a powder having a similar particle size.

When the above amorphous Si-N-C powder is added, it is preferably 3 to 20% by weight, more preferably 5 to 15% by weight, and this range is relatively easy to sinter,
The effect of addition becomes remarkable. The starting raw material powder can be molded by a conventional method. That is, typically, the starting powders are uniformly mixed and isostatically pressed at a pressure of about 1000 to 3000 kg / cm ² to form the powder.

Then, it is fired, but in the method of the present invention, it is 1
Baking is performed in a temperature range of 500 to 1700 ° C. 1700 ° C
If it exceeds, the thermal decomposition of zircon becomes remarkable, and the structure does not become fine due to grain growth, so that the strength is lowered. On the other hand, if the temperature is less than 1500 ° C., a sufficiently densified sintered body cannot be obtained and the strength is low. If the firing atmosphere is inert,
For example, nitrogen, argon, helium or a mixed gas thereof is typically nitrogen.

The method for producing a silicon nitride sintered body of the present invention comprises:
In addition to the above, conventional methods can be followed. Further, in the present invention, there is an advantage that a sintered body having a high strength can be obtained by the normal temperature low temperature sintering, but the pressure sintering has an effect of further improving the strength.

[0015]

[Function] The sintering temperature is increased to 1500 to 1 by adding a sintering aid.
When the temperature is set to 700 ° C. and the sintered structure is made finer and zircon is added, the strength at high temperature is improved. Although the detailed function of zircon is unknown, it is considered that zircon increases the viscosity of the grain boundary phase at high temperature and is effective in preventing softening.

[0016]

EXAMPLES Example 1 Si ₃ N ₄ powder (average particle size 0.5 μm or less, α conversion rate 92)
˜97%) as a sintering aid Y ₂ O ₃ powder (average particle size 0.3 μm, purity 99.9%), ZrO ₂ · SiO ₂ powder (average particle size 0.5 μm or less, purity 97% or more) The powder added with was mixed (a ball mill made of silicon nitride) to obtain 200
After pressure molding at a pressure of kg / cm ² , the molded body was packed in thin rubber, vacuum-sealed, and then pressed at a pressure of 3000 kg / cm ^{2 by} CIP. Sintered in a furnace in ₂ atmospheres. The temperature rising rate was 1 ° C./min, and the holding time at the maximum temperature was 4 hours.

Room temperature 4-point bending strength (JI
S R 1601), high temperature 4-point bending strength (JIS A
1604, in the air), and 10 of each were measured, and the average strength thereof is shown in Table 1. The relative density of the sintered body is a value obtained by dividing the bulk density obtained by the n-butanol substitution method by the theoretical density. For comparison, after molding in the same manner as in Example 1, this molded body was sintered in a furnace under N ₂ atmosphere under the conditions shown in Table 2 to obtain the results shown in Table 2.

[0018]

[Table 1]

[0019]

[Table 2]

Example 2 Si ₃ N ₄ powder (average particle size of 0. 0) produced by thermal decomposition method of silicon diimide or direct nitriding method of metallic silicon.
5 μm or less, α conversion rate of 92 to 97%) and Y as a sintering aid
₂ O ₃ powder (average particle size 0.3 μm, purity 99.9%),
MgAl ₂ O ₄ powder (average particle size 0.3 μm or less, purity 9
9.9%), ZrO ₂ · SiO ₂ powder (average particle size 0.5
Powder mixed with μm or less and purity of 97% or more) is mixed (silicon nitride ball mill), pressure-molded at a pressure of 200 kg / cm ² , the molded body is packed in thin rubber, vacuum-enclosed, and then C
After pressurizing with IP at a pressure of 3000 kg / cm ² , the molded body was sintered under the conditions shown in Table 3 in a furnace in an N ₂ atmosphere.
During the densification process, the temperature rising rate was 1 ° C./min, and the holding time at the maximum temperature was 4 hours.

Room temperature 4-point bending strength (JI
S R 1601), high temperature 4-point bending strength (JIS R
1604, in the air), and 10 of each were measured, and the average strength thereof is shown in Table 3. The relative density of the sintered body is a value obtained by dividing the bulk density obtained by the n-butanol substitution method by the theoretical density. After molding in the same manner as in Example 2, this molded body was sintered in a furnace under N ₂ atmosphere under the conditions shown in Table 4 to obtain the results shown in Table 4.

[0022]

[Table 3]

[0023]

[Table 4]

As can be seen in Tables 3 and 4, Y ₂ O ₃ ,
By adding ZrO ₂ and SiO ₂ together with MgAl ₂ O ₄ as ZrSiO ₄ instead of independently and sintering at a low temperature of 1700 ° C. or less, a 4-point bending strength value exceeding 1000 MPa even at 1000 ° C. and in air. Express
Higher strength values were obtained by increasing the atmospheric pressure during sintering (Sample Nos. 29, 30, 31).

The total amount of the sintering aid is preferably in the range of 5 to 15% by weight. If it is less than 5%, it will not sinter densely, and if it is more than 15%, the strength will decrease. When ZrO ₂ and SiO ₂ powder were added separately, Comparative Example No. Although a dense sintered body is obtained as shown in 34, the strength is reduced.

Further, Y ₂ O ₃ --MgAl ₂ O ₄ --ZrO
₂ , Y ₂ O ₃ —MgAl ₂ O ₄ —SiO ₂ , and Y ₂ O
_{In the case of the 3-} MgAl ₂ O ₄ composition, the comparative sample No. 35-3
Although a dense sintered body can be obtained as shown in FIG.
Significant decrease in strength at ℃ Example 3 Si ₃ N ₄ powder: Si produced by pyrolysis of silicon diimide
₃ N ₄ powder (average particle size 0.2 μm, total amount of impurities 50 ppm
, Α-conversion rate 97-100%) Si ₃ produced by the direct nitriding method of metallic silicon
N ₄ powder (average particle size 0.9 μm, total amount of impurities 980 ppm
, Α-ized rate 92 to 97%) Zircon (ZrSiO ₂ ) powder: average particle size 0.2 μm,
Purity 99% Other sintering aids: Auxiliaries in Table 5 and Table 6, both having an average particle size of 0.2 to 0.6 μm and a purity of 99 to 99.9%.

The above powder is blended as shown in Table 5 (Si ₃ N
_{The 4} powders were the balance of the sintering aid, and were weighed with a total of 100% by weight) and mixed with a silicon nitride ball mill in which impurities (inclusions) were prevented as much as possible. 150 kg / cm of the obtained powder
Molding was carried out at a pressure of ² , then packed in a thin rubber bag, sealed in vacuum, and hydrostatically pressurized at a pressure of 3000 kg / cm ² to complete molding.

The compact was sintered under the conditions shown in Table 5 in a nitrogen atmosphere at 1 atm. Temperature rising rate is 1 ° C above 1300 ° C
/ Min was controlled. The holding time at the sintering temperature was 4 hours. The obtained sintered body is machined to JIS-R-1
According to 601 and JIS-R-1604 standard, room temperature and high temperature 4-point bending strength were measured. The number of tests is 15 to 20
The intensity values in Table 5 in this book are arithmetic mean values.

The bulk density of the sintered body was measured by the substitution method using n-butanol, and the total porosity was all 1.5% or less. No pores of 1 μm or more were observed by an optical microscope observation of the polished surface. A molded body was molded in the same manner as in Example 3 except under the conditions shown in Table 6 and was sintered, and the results shown in Table 6 were obtained.

[0030]

[Table 5]

[0031]

[Table 6]

From Tables 5 and 6, Y ₂ O ₃ or Y ₂ O
It is recognized that the high temperature strength can be improved not only by the combination of ₃ + MgAlO ₄ and zircon but also by the combination of various oxide type auxiliaries and zircon. Example 4 Si ₃ N ₄ powder (average particle size: 0.3 μm, purity: 99.
99%), ZrSiO ₄ powder (average particle size: 0.3 μm, purity: 97% or more) as a sintering aid, and 3Al ₂ O.
₃ · 2SiO ₂ powder (average particle size: 0.3 [mu] m, Purity: 9
8% or more) is added in the mixing ratio shown in Table 7, and Si ₃ N ₄ is added.
The resulting mixture was uniformly mixed in ethanol with a ball mill manufactured by K.K. and then dried to obtain a mixed powder.

The above-mentioned mixed powder was pressure-molded at a pressure of 150 kgf / cm ² , the molded body was packed in a thin rubber, and after vacuum filling,
CIP (Cold Isostatic Press)
In the treatment, a pressure of 3000 kgf / cm ² was applied to obtain a molded product having a predetermined shape (6 mm × 5 mm × 45 mm). The molded body was sintered at the maximum heating temperature shown in Table 7 for 4 hours in a furnace in a nitrogen atmosphere to obtain a sintered body. The temperature rising rate at this time was 2 ° C./min.

Regarding these sintered bodies, JIS R 1
According to 604, room temperature in air, 1300 ° C, 1400 ° C
The four-point bending strength was measured for each of 20 pieces at each temperature and the average strength was obtained. The results are shown in Table 7.

[0035]

[Table 7]

As is clear from the results shown in Table 7, in all the sintered bodies according to this example, the high temperature strength at 1300 ° C. was as high as 800 MPa or more, and the high temperature strength at 1400 ° C. could be as high as 700 MPa or more. It was Further, in the present example, the effect of sufficiently improving the strength was obtained even by pressureless sintering. In addition, the sample No. In the sintered body of No. 54, higher strength could be achieved.

The sintered body containing too much mullite decreased in strength at high temperature, and the sintered body containing too little mullite decreased in strength. A sintered body containing too much sintering aid has a high strength at room temperature, but at 1300 ° C.
And the high temperature strength at 1400 ° C decreased. Zircon 2
Addition amounts within the range of 6% by weight and 2-4% by weight of mullite gave favorable results.

The amorphous Si-N-C powder used in Example 5 was obtained by subjecting hexamethyldisilazane and ammonia to a CVD reaction at 1000 ° C.
It is a composite powder produced by thermal decomposition in an N ₂ stream at 1350 ° C., which decomposes into Si ₃ N ₄ and SiC at about 1500 ° C.

Si ₃ N ₄ powder having an average particle size of 0.2 μm and a total amount of metal element impurities of 50 ppm was added to Y having an average particle size of 0.2 μm.
Table 2 shows the composition of ₂ O ₃ powder, ZrSiO ₄ powder, and amorphous Si-N-C powder having an average particle size of 0.1 μm (composite powder of 30 wt% in terms of SiC generated by heating). After weighing and uniformly mixing in ethanol, the mixed powder was molded into a mold, and then a hydrostatic pressure of 3 t / cm ² was applied to the molded body, and the molded body was sintered at a temperature of 1500 to 1700 ° C. After reaching the maximum temperature, the nitrogen gas pressure shown in Table 8 was applied at a pressure increasing rate of 20 atm / min. The strength of the obtained sintered body conforms to JIS standard, and room temperature and high temperature 4-point bending test (J
IS-R-1601, 1604) and the results are shown in Table 8.

[0040]

[Table 8]

Comparing the results of Table 8 with Table 1, it is recognized that the high temperature strength is further improved by further adding the amorphous Si-N-C powder together with the oxide type auxiliary agent and zircon. This is because SiC is generated and dispersed in the grain boundaries and in the grains of the sintered body, so that it has the effect of preventing the softening of the grain boundary phase at high temperatures and of stopping the development of cracks. Is expected to improve.

According to the results of experiments conducted in the same system as described above, the total amount of the oxide type auxiliary agent and zircon is 3 to 10% by weight, and the amorphous Si-N-C powder is 2 to 15% by weight. The addition amount within the range remarkably improves the high temperature strength, which is preferable.

Claims (1)

[Claims] 1. A silicon nitride powder containing at least ZrO ₂ .SiO ₂ (zircon) as a sintering aid, and ZrO ₂ .multidot.
A method for producing a silicon nitride sintered body, which comprises molding a homogeneous mixed powder to which an oxide-based auxiliary agent other than SiO ₂ is added, and sintering the molded body at a temperature of 1500 to 1700 ° C.