JPH0848565A - Silicon nitride sintered compact and its production - Google Patents

Abstract

PURPOSE:To crystallize a high m.p. oxynitride at grain boundaries and to obtain a silicon nitride sintered compact excellent in strength at room temp. and high temp. by limiting a sintering aid and conditions in sintering. CONSTITUTION:Silicon nitride powder is mixed with 1-16wt.% lanthanide oxide, preferably Tm2O3, Yb2O,3 Lu2O3 or mixture of them and 0-5wt.% silica, and the resultant mixture is compacted and fired at 1,700-2,200 deg.C in nitrogen under 1-100atm pressure to obtain the objective silicon nitride sintered compact consisting of 98-80wt.% silicon nitride grains and 1-20wt.% crystalline oxynitride represented by Ln4Si2O7N2 (Ln is a lanthanide or Y) or 98-80wt.% silicon nitride grains, 1-15wt.% crystalline oxynitride represented by Ln4Si2O7N2 and 1-15wt.% Ln2SiO5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride sintered body excellent in room temperature and high temperature strength which is used as a structural material in the fields of automobiles, precision machinery, chemical plants, cutting tools and the like, and a method for producing the same. It is a thing.

[0002]

BACKGROUND OF THE INVENTION Sintered silicon nitride is excellent in mechanical properties such as room temperature and high temperature strength, fracture toughness, friction and wear resistance, and is used for manufacturing automobile engine parts, ball bearings and non-ferrous metals. Used for machine parts, cutting tools, etc.

Conventionally, various oxides were mixed as raw materials of silicon nitride as a sintering aid, and after molding, they were heated to a high temperature in nitrogen to produce a high density sintered body. Oxides effective as sintering aids are magnesia, alumina, and rare earth metal oxides. The sintering aid reacts with silica, which is an oxide layer on the surface of the raw material at a high temperature, to form a liquid phase. Sintering proceeds by the diffusion of silicon nitride in the liquid phase. When cooled after sintering, a part of the liquid phase is crystallized, but most remains as a glass phase at the grain boundaries.

The most frequently used auxiliaries are magnesia and yttria alumina. After sintering, the sintered body is composed of silicon nitride particles and a small amount of glass phase in which the liquid phase is solidified. Some may crystallize in the glass phase as oxides or oxynitrides.

However, when a stress such as bending or tensile is applied to such a sintered body from the outside, there is a drawback that a crack progresses through a grain boundary and breaks. The temperature is 800 to 10
At 00 ° C., the glass phase softens and the strength sharply decreases. The degree of this decrease largely depends on the chemical composition of the grain boundary phase. This is because the softening temperature of glass is proportional to the melting temperature of the metal-Si-O system in the grain boundary phase. Therefore, the high-temperature strength is higher when the yttria-alumina system is used than when the sintered body produced by using magnesia is used. Even if the yttria-alumina system is used, a glass phase having a low melting point exists in the grain boundaries of the sintered body, and the strength is 20 to 40% lower than the value at room temperature at 1200 ° C or higher.

Recently, studies have been conducted to deposit Y ₂ Si ₂ O ₇ having a high melting point at grain boundaries using an yttria-silica type auxiliary agent (J. Am. Ceram. Soc. No. 75, 2050 pages
(1992)). In this system, the nitrogen-containing apatite (N phase, Y ₁₀ Si ₇ O ₂₃ N ₄ ) or W phase (YSiO ₂ N) shown in FIG.
Alternatively, the glass phase having a composition close to that becomes the second grain boundary phase next to Y ₂ Si ₂ O ₇ . Softening temperature of N phase and W phase is 1500 ℃
Low as below. In order to prevent the formation of these phases, the raw material of the chemical composition in the _{Si 3 N 4 -Y 2 Si 2} O 7 -Si 2 ON 2 S
The raw material powders are mixed so as to be around i ₃ N ₄ . Even so, since the raw material is powder, the reaction is non-uniform and a small amount of the above-mentioned low melting point crystal phase or glass phase which cannot be predicted from the phase diagram is formed. Further, there is a large difference in thermal and mechanical properties between the silicon nitride particles and Y ₂ Si ₂ O ₇ which is a grain boundary phase, and microcracks are generated at the grain boundaries during cooling to lower the absolute value of strength.

Such conventional materials cannot have a high room temperature and / or high temperature strength due to the presence of a low melting point crystal phase or glass phase at the grain boundary or the oxide grain boundary phase. .

In order to solve the above-mentioned problems of the prior art, the present invention examines the crystallization aid system and the sintering conditions to crystallize a high melting point oxynitride at the grain boundaries, It is an object of the present invention to provide a silicon nitride sintered body having excellent room temperature and high temperature strength.

[0009]

[Solving Means] A sintering aid forms a phase that melts at a temperature equal to or lower than the sintering temperature in order to promote liquid phase sintering, and remains as a high melting point crystalline phase at a grain boundary after sintering. There is a need to. These two points are the conditions of the sintering aid for obtaining a sintered body having high heat resistance. This is often the reason why lanthanide metal oxides are used as auxiliaries. However, as already mentioned, the phase diagram of the lanthanide metal oxide-SiO ₂ -Si ₃ N ₄ system is complicated (Fig. 1 (a)), and only the desired high melting point oxynitride is regarded as the grain boundary phase. It was difficult to manufacture a sintered body that does.

Therefore, the present inventor has studied a system containing a lanthanide metal oxide, which has not been studied so far, and found that the lanthanide metal was thulium (Tm), ytteribium (Yb), lutetium (Lu) alone or in a mixture thereof. It has been found that, for example, the phase diagram becomes simple and the coexistence of low melting point compounds or glass phases can be prevented.

A system containing Yb ₂ O ₃ is described in J. Mater. Sci. 28.
No., page 3529 (1993), and completed as shown in FIG. 1 (b). In this phase diagram, Yb ₄ Si ₂ O ₇ N ₂ (J phase) and Yb ₂ SiO ₅
(S phase) has high melting points of 1850 ° C. and 1980 ° C., respectively. As shown in FIG. 1 (a), these two phases cannot exist in equilibrium with the silicon nitride particles in the yttria system. Thulium (Tm) and lutetium (Lu) alone or in a mixture thereof have similar phase diagrams.

Further, even if the above three kinds of lanthanide metals are mixed with other lanthanide metals, if the former is the main component,
Its action is basically unchanged.

As described above, the chemical composition of the whole sintered body is Si _{3
N 4 -Ln 4 Si 2 O 7} N 2 -Ln 2 SiO 5: controls the mixing ratio of raw material powders as in silicon nitride side in (Ln the lanthanide metals), at room temperature when sintered under appropriate conditions Also, a high-strength sintered body can be manufactured at high temperature. The present invention has been completed based on these findings.

That is, according to the present invention, 98 to 80% by weight of silicon nitride particles and 1 to 20% by weight of crystalline oxynitride L are used.
The gist is a silicon nitride sintered body characterized by being made of n ₄ Si ₂ O ₇ N ₂ (where Ln is a lanthanide metal or yttrium).

In another aspect of the present invention, 98 to 80% by weight of silicon nitride particles and 1 to 15% by weight of crystalline oxynitride L are used.
n ₄ Si ₂ O ₇ N ₂ (where Ln is lanthanide metal or yttrium) and 1 to 15% by weight of Ln ₂ SiO ₅ (where Ln is lanthanide metal or yttrium). The main point is the union.

The manufacturing method is as follows:
-16% by weight of lanthanide metal oxide and 0-5% by weight of silica are mixed and, after molding, 1 to 100 atm of nitrogen is used.
It is characterized by firing at 700 to 2200 ° C.

Further, another manufacturing method is as follows:
~ 16% by weight lanthanide metal oxide and 0-5% by weight
Of silica and 0 to 1% by weight of alumina are mixed, and after molding, at 1850 to 2200 ° C. in nitrogen of 10 to 100 atm.
It is characterized in that it is heat-treated at 1300 to 1700 ° C. after firing.

[0018]

The present invention will be described in more detail below.

In the production method of the present invention, an ordinary sintering raw material is used as a starting raw material, but it is desirable to use a powder having a high purity and a fine average particle size of 0.6 micron or less and a narrow particle size distribution.
As the sintering aid to be added, it is preferable to use Tm ₂ O ₃ , Yb ₂ O ₃ , Lu ₂ O ₃ alone or in a mixture thereof.

In addition, the chemical composition of the whole sintered body is Si ₃ N _{4
-Ln 4 Si 2 O 7 N 2} -Ln 2 SiO 5: as in (Ln the lanthanide metals) inside, if necessary, the addition of silica. Since 1 to 3% by weight of silica exists as an oxide layer on the surface of the silicon nitride powder, it is not necessary to additionally add silica as long as the amount of lanthanide metal oxide added is small. The addition amount is in the range of 1 to 16% by weight of lanthanide metal oxide and 0 to 5% by weight of silica. If the amount added is less than this range, sintering will be difficult. The object can be achieved even if the addition amount is larger than this, but there is a drawback that the price of the lanthanide metal oxide is high and the product becomes expensive. The addition amount is better if both high density and crystallization of grain boundaries can be achieved. Desirably, the lanthanide metal oxide is 6 to
The range is 12% by weight and the amount of silica is 1 to 3% by weight. The lanthanide metal oxide / silica molar ratio in the grain boundary phase must be in the range of 1 to 0.5. If the amount of silica exceeds this range, Ln ₄ Si ₂ O ₇ N ₂ will not be produced. If it is less than this range, it should be noted that the unmelted Ln ₂ Si ₃ O ₃ N ₄ phase is generated and the sintering stops in the middle.

In order to improve the sinterability, it is also effective to add a small amount of lanthanide metal oxide other than the above-mentioned three kinds or alumina. In addition, as a grain boundary phase, Ln ₄ Si ₂ O
_{If 7} N ₂ alone or a mixture with Ln ₂ SiO ₅ is the main component,
Even if a small amount of other crystal phase or glass phase coexists, the sinterability and the mechanical properties of the sintered body do not change.

The raw material mixture is molded into a predetermined shape by die molding, hydrostatic molding, injection molding, cast molding or the like. This molded body is sintered by various methods, and the sintering conditions are, depending on the sintering method, 1700 to 22 in 1 to 100 atm of nitrogen.
The temperature is 00 ° C.

[0023] In the simplest way, the mixture is put in a graphite mold coated with boron nitride powder and heated at 1700 to 1800 ° C in nitrogen under a pressure of 100 to 500 atm for 10 minutes to 4 hours by a hot pressing method. is there. An inexpensive product can be manufactured by the pressureless sintering method in which the molded body is heated at 1700 to 1800 ° C. in nitrogen at 1 atm. Relative density 95 obtained by pressureless sintering
˜98% of the sintered body is heated to 1650 to 2000 ° C. in nitrogen of 1000 to 2000 atm for high temperature hydrostatic sintering (H
When the treatment is carried out by the IP method, residual pores are removed and a sintered body having a higher density can be obtained. In gas pressure sintering in which a compact is sintered at 1850 to 2200 ° C. in nitrogen at 1 to 100 atm,
Since silicon nitride can be heated under more stable conditions than atmospheric pressure sintering, it can be sintered at a higher temperature. The higher the sintering temperature, the higher the pressure of nitrogen needs to be. Minimum required nitrogen pressure is 1900
The pressure is 5 atm at 10 ° C, 10 atm at 2000 ° C, 20 atm at 2100 ° C, and 30 atm at 2200 ° C.

After sintering, 1300 to 17 lower than the sintering temperature
When kept at 00 ° C for 1 to 20 hours, most of the glass phase remaining at the grain boundaries is crystallized.

Next, examples of the present invention will be described.

[0026]

[Example 1] Silicon nitride powder (Ube Industries, E-10 type) 86.4% by weight, ytterbium oxide (Shin-Etsu Chemical, purity 99.9%) 12.0% by weight, and precipitated silica (Kanto) 1.0% by weight, which is obtained by calcining (Chemical) to remove adsorbed water
And were wet-mixed with a ball mill made of silicon nitride using n-hexane as a dispersion medium. After the mixture was dried, it was placed in a mold with a diameter of 15 mm coated with boron nitride powder and placed in a nitrogen atmosphere for 2
The mixture was heated to 1750 ° C. for 2 hours under a pressure of 00 atm. The relative density of the sintered body was 96.8%. The crystalline substance measured by powder X-ray diffraction had β-Si ₃ N ₄ as a main component, and Yb ₄ Si ₂ O ₇ N ₂ was observed as the second phase. A small amount of Yb ₂ Si ₃ O ₃
N ₄ , Yb ₂ SiO ₅ and traces of α-Si ₃ N ₄ were present. According to a transmission electron microscope, a small amount of glass phase was observed between β-Si ₃ N ₄ and the crystal phase of grain boundaries. As a result, a silicon nitride sintered body containing Yb ₄ Si ₂ O ₇ N ₂ which is a high melting point oxynitride as a main component was obtained as a grain boundary phase.

[0027]

Comparative Example 1 Yttrium oxide (manufactured by Shin-Etsu Chemical Co., Ltd., purity 9) was added to silicon nitride powder so that the same molar ratio as in Example 1 was obtained.
9.9%) 7.3% by weight and silica 0.6% by weight. A sintered body was produced from the mixture by the same procedure as in Example 1. The relative density of the sintered body was 75.6%, and a high-density sintered body could not be obtained. The crystalline substance measured by powder X-ray diffraction contains β-Si ₃ N ₄ as a main component, and Y ₂ Si ₃ O as another phase.
_{There was 3} N ₄ , YSiO ₂ N and traces of α-Si ₃ N ₄ .
Y ₂ Si ₃ O ₃ N ₄ does not contribute to sintering because it is stable and does not melt at high temperatures. It is YSiO ₂ and the glass phase present in a small amount that contribute to the sintering, and about half of the added auxiliaries. For this reason, the amount of liquid phase at high temperature was small, and high density could not be achieved.

[0028]

Examples 2 to 5 The sintering aid having the composition shown in Table 1 was added to the silicon nitride powder of Example 1 and mixed and dried in the same manner as in Example 1. After molding the mixture at a pressure of 200 atm,
It was hydrostatically pressed at 2000 atmospheric pressure to obtain a molded body of about 4 × 5 × 45 mm. The compact was sintered at a predetermined temperature in a nitrogen atmosphere under the conditions shown in Table 2. After surface-grinding the sintered body, the relative density, JI
Table 2 shows the results of room temperature strength measurement by 4-point bending (outer span 30 mm, inner span 10 mm) according to S-R1601 and the crystalline composition of the grain boundary phase by powder X-ray diffraction. Also, 120
The high temperature four-point bending strength (high temperature strength) at 0 ° C. measured on the sample of Example 3 was 690 MPa. In any of Examples 2 to 5, Ln ₄ Si ₂ O ₇ N ₂ alone or Ln ₂ Si
A silicon nitride sintered body having a grain boundary phase coexisting with O ₅ was obtained, which was a high-density and high-strength sintered body.

[0029]

Comparative Example 2 A sintering aid having the composition shown in Comparative Example 2 in Table 1 was added to the silicon nitride powder of Example 1, and mixed and dried in the same manner as in Example 1. After molding the mixture at a pressure of 200 atm, isostatically press at 2000 atm, about 4 x 5 x 45
The molded body had a size of mm. The compact was sintered at a predetermined temperature in a nitrogen atmosphere under the conditions shown in Table 2. Although a high-density sintered body was obtained,
According to powder X-ray diffraction, the grain boundary phase was in a glass state.

[0030]

Example 6 The same powder mixture as in Example 1 was used with an inner diameter of 18 mm.
Primary molding was carried out at 200 atm by using the above mold, and then hydrostatic molding was carried out at 2000 atm. The pellets were sintered in nitrogen at 10 atmospheres at 1950 ° C. for 2 hours and subsequently at 1500 ° C. for 5 hours.
Heat treated for hours. The relative density of the sintered body was 99.%. The crystal phase of the grain boundary detected by powder X-ray diffraction is Yb ₄ Si ₂ O
Only ₇ N ₂ . The four-point bending strength of the sintered body measured in the same manner as in Examples 2 to 5 was as high as 813 MPa.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

As described in detail above, according to the present invention,
98 to 80% by weight of silicon nitride as a main component and L at grain boundaries
A silicon nitride sintered body is obtained which has n ₄ Si ₂ O ₇ N ₂ alone or coexists with Ln ₂ SiO ₅ . As described above, a sintered body having a compound with a high melting point at the grain boundary cannot be manufactured conventionally, and it is possible only when the rare earth metal (Ln) contains thulium (Tm), ytterbium (Yb), and lutetium (Lu) as main components. became.

[Brief description of drawings]

FIG. 1 (a) is a phase diagram of Si ₃ N ₄ —SiO ₂ —Y ₂ O ₃ system, (b).
Is a Si ₃ N ₄ —SiO ₂ —Yb ₂ O ₃ system phase diagram, in which J is Ln ₄ Si ₂ O ₇ N ₂ , K is Ln ₂ Si ₃ O ₃ N ₄ , and S is Ln ₂ Si.
O ₅ and W represent LnSiO ₂ N, and N represents Y ₁₀ Si ₇ O ₂₃ N ₄ .

Claims (8)

[Claims] 1. 98-80% by weight of silicon nitride particles,
A silicon nitride sintered body characterized by comprising 1 to 20% by weight of a crystalline oxynitride Ln ₄ Si ₂ O ₇ N ₂ (where Ln is a lanthanide metal or yttrium). 2. 98-80% by weight of silicon nitride particles,
1 to 15% by weight of crystalline oxynitride Ln ₄ Si ₂ O ₇ N ₂ (where Ln is lanthanide metal or yttrium), and 1 to
A silicon nitride sintered body comprising 15% by weight of Ln ₂ SiO ₅ (where Ln is a lanthanide metal or yttrium). 3. The silicon nitride sintered body according to claim 1, wherein the lanthanide metal is thulium (Tm), ytteribium (Yb), lutetium (Lu), or a mixture thereof. 4. The lanthanide metal is thulium (Tm), ytteribium (Yb), lutetium (Lu) alone or in a mixture thereof with another lanthanide metal or yttrium, and the former is 70% of the lanthanide metal. atom%
The above is the silicon nitride sintered body according to claim 1 or 2. 5. A silicon nitride powder is mixed with 1 to 16% by weight of a lanthanide metal oxide and 0 to 5% by weight of silica, and after molding, 1700 to 2200 ° C. in nitrogen of 1 to 100 atm.
The method for producing a silicon nitride sintered body according to claim 1, 2, 3, or 4, wherein the firing is performed at a temperature of 10 to 20. 6. Silicon nitride powder in 1-16 wt% lanthanide metal oxide, 0-5 wt% silica and 0-1.
After mixing alumina by weight% and after molding, baking was performed at 1850 to 2200 ° C. in nitrogen at 10 to 100 atm, and then 130
The method for producing a silicon nitride sintered body according to claim 1, wherein heat treatment is performed at 0 to 1700 ° C. 7. The amount of lanthanide metal oxide added is 6 to 1.
The method according to claim 5 or 6, which is 2% by weight. 8. The method according to claim 5, wherein the addition amount of silica is 1 to 3% by weight.