JP3810236B2 - Silicon nitride sintered body and manufacturing method thereof - Google Patents

Description

【００４３】
まず、各種の希土類元素酸化物を５重量％添加してなる本発明の試料Ｎｏ．１〜７は、Ｌが１ｎｍ以下でＷが３ｎｍ以下であり、１０μｍ以下の窒化珪素粒子が８０〜９０％、４０〜１００μｍの粒子が１０〜２０％であり、その結果、１０００℃での強度が９００ＭＰａ以上、破壊靭性が８ＭＰａ・ｍ^1/2以上、疲労寿命が７×１０⁷回以上という優れた特性を有していた。
【００４４】
また、表１に示すように、希土類元素としてＹ₂Ｏ₃を使用し、過剰酸素の酸化珪素換算量であるＳｉＯ₂量およびＡｌ₂Ｏ₃量を変化させた試料Ｎｏ．８〜２９から明らかなように、Ｙ₂Ｏ₃量、ＳｉＯ₂量、Ａｌ₂Ｏ₃量、ＳｉＯ₂／Ｙ₂Ｏ₃比およびＡｌ₂Ｏ₃／Ｙ₂Ｏ₃比が本発明の範囲内にある試料Ｎｏ．８、９、１２、１３、１６、１７、２０、２１、２４、２５、２８および２９は、いずれも窒化珪素質焼結体中におけるβ型窒化珪素結晶相間の非晶質相の厚みＬを２ｎｍ以下で、かつβ型窒化珪素結晶相と粒界結晶相間の非晶質相の厚みＷを５ｎｍ以下であり、その結果、１０００℃での強度を８００ＭＰａ以上、１０００℃の高温雰囲気下で７００ＭＰａの荷重を加えた時の繰り返し疲労寿命を１０⁷回以上にまで高められることが確認できた。さらに１０μｍ以下のβ型窒化珪素結晶相からなるマトリックス７０〜９０％のなかに４０〜１００μｍのβ型窒化珪素結晶１０〜３０％分散させることにより靭性値を７ＭＰａ・ｍ^1/2以上に高めることができることが確認できた。
【００４５】
さらに、出発原料の窒化珪素粉末に対し、１０〜８０重量％の範囲内で珪素粉末に置き換え、成形体を焼成前に、窒素雰囲気下にて１０００〜１４００℃の温度で熱処理してＳｉ粉末を窒化処理して窒化珪素を生成させ、成形体の密度を高めたうえで焼成した試料Ｎｏ．２８および２９は、Ｌが１ｎｍ以下、Ｗが２ｎｍ以下、１０μｍ以下の窒化珪素粒子が８０〜９０％、４０〜１００μｍの粒子が１５〜２５％であり、その結果、１０００℃での強度が１０００ＭＰａ以上、破壊靭性が８．５ＭＰａ・ｍ^1/2以上、疲労寿命が８×１０⁷回以上という非常に優れた特性を有していた。
【００４６】
これに対して、希土類酸化物量が２重量％未満の試料Ｎｏ．１０および１０重量％を越える試料Ｎｏ．１１、酸化珪素量が０．５重量％未満の試料Ｎｏ．１４および５重量％を越える試料Ｎｏ．１５、アルミナ量が２重量％未満の試料Ｎｏ．１８および５％を越える試料Ｎｏ．１９、ＳｉＯ₂／Ｙ₂Ｏ₃比が０．２より小さい試料Ｎｏ．２２および０．７より大きい試料Ｎｏ．２３、およびＡｌ₂Ｏ₃／Ｙ₂Ｏ₃比が０．２より小さい試料Ｎｏ．２６および０．７より大きい試料Ｎｏ．２７は、Ｌ、Ｗがそれぞれ２ｎｍ、５ｎｍより大きく、また１０μｍ以下の窒化珪素の量が７０〜９０％の範囲外、４０〜１００μｍの量が１０〜３０％の範囲外となり、高温強度、破壊靭性および疲労寿命がそれぞれ６４０ＭＰａ以下、６．２ＭＰａ・ｍ^1/2以下、８×１０⁶回以下と低い値であった。
実施例２
次に、組成がＹ₂Ｏ₃量が５重量％、ＳｉＯ₂量が３重量％およびＡｌ₂Ｏ₃量が３重量％含まれる窒化珪素質成形体を１７５０℃の温度で常圧焼成したあと、１９５０℃の温度域２０気圧で焼成後１０００℃で１００時間熱処理し、表２に示した冷却速度で１０００℃まで冷却し、実施例１と同様の条件にて高温強度、疲労寿命、破壊靭性値を測定した。結果を表２に示した。
【００４７】
【表２】

【００４８】
その結果、１０００℃までの冷却速度が５０℃／分以上である試料Ｎｏ．３０〜３２は、１０μｍ以下のβ型窒化珪素結晶相からなるマトリックス７０〜９０％のなかに４０〜１００μｍのβ型窒化珪素結晶１０〜３０％を分散させることができた。また、窒化珪素質焼結体中におけるβ型窒化珪素結晶相間の非晶質相の厚みを２ｎｍ以下で、かつβ型窒化珪素結晶相と粒界結晶相間の非晶質相の厚みを５ｎｍ以下とすることができた。
【００４９】
そして、その結果、１０００℃での強度を８００ＭＰａ以上、靭性値を７ＭＰａ・ｍ^1/2以上、１０００℃の高温雰囲気下で７００ＭＰａの荷重を加えた時の繰り返し疲労寿命を１０⁷回以上とすることができた。
【００５０】
一方、１０００℃までの冷却速度が５０℃／分より遅い試料Ｎｏ．３３および３４は、β型窒化珪素結晶相間の非晶質相の厚みが２ｎｍを越えるとともに、β型窒化珪素結晶相と粒界結晶相間の非晶質相の厚みが５ｎｍを越えるので本発明の範囲外となり、高温強度、破壊靭性および疲労寿命がそれぞれ８００ＭＰａ未満、７ＭＰａ・ｍ^1/2未満、１０⁷回未満と低い値となった。
【００５１】
以上の結果から、本発明の窒化珪素質焼結体は、いずれも１０００℃の高温強度が８００ＭＰａ以上、破壊靭性が７ＭＰａ・ｍ^1/2以上、疲労寿命が１０⁷回以上が達成された。
【００５２】
【発明の効果】
本発明によれば、粒界相の結晶化を促進させて非晶質相の厚みを薄くするとともに、窒化珪素粒子を微細なマトリックスの中に、柱状の窒化けい素粒子を所定量分散させることによって靭性を改善し、高温強度に優れ、疲労寿命の長く、高靭性な窒化珪素質焼結体を得ることができる。
【図面の簡単な説明】
【図１】本発明の窒化珪素質焼結体の結晶構造を示す模式図である。
【符号の説明】
１・・窒化珪素結晶相
２・・粒界結晶相
３・・窒化珪素結晶相間の非晶質相
４・・窒化珪素結晶相と粒界結晶相間の非晶質相[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a silicon nitride sintered body having a long fatigue life at a high temperature and a method for producing the same, and particularly suitable as a heat engine component such as an automotive component such as a piston pin or an engine valve or a gas turbine engine component. It is a thing.
[0002]
[Prior art]
Conventionally, silicon nitride-based sintered bodies known as engineering ceramics have excellent heat resistance, thermal shock resistance, wear resistance, and oxidation resistance, so that they are particularly suitable for heat engines such as gas turbines and turbo-rotors. Applications as parts are underway.
[0003]
Since silicon nitride is a difficult-to-sinter material, in order to improve the sinterability, a molded body in which rare earth element oxides such as Y ₂ O ₃ or aluminum oxide is added as a sintering aid to silicon nitride powder. It is known to obtain a sintered body that is fired under pressure and is composed of an amorphous grain boundary phase mainly composed of a silicon nitride crystal phase and composed of rare earth elements, silicon, aluminum, oxygen, and nitrogen.
[0004]
However, when rare earth element oxide and aluminum oxide are used as sintering aids, the strength at room temperature is high, but in a high temperature range, for example, around 1000 ° C., rare earth elements, silicon, aluminum in the sintered body Further, since the grain boundary phase composed of oxygen and nitrogen has a low melting point and is easily softened in a high temperature range, there is a problem that high temperature strength and fatigue characteristics are greatly deteriorated.
[0005]
In addition, because it has a fine structure to achieve high high-temperature strength, fracture toughness is reduced, and when impact is applied to the silicon nitride sintered body, the crack progresses quickly and is easily broken. There was a problem.
[0006]
Therefore, in order to improve the high temperature strength, Japanese Patent Application Laid-Open No. 7-330437 discloses that a heat treatment is applied to crystallize an amorphous grain boundary phase composed of rare earth elements, silicon, aluminum, oxygen, and nitrogen, thereby increasing the temperature. A silicon nitride sintered body having excellent strength has been proposed.
[0007]
In order to improve the fracture toughness value, Japanese Patent Laid-Open No. 5-186271 proposes a silicon nitride sintered body that promotes the growth of silicon nitride particles to form columnar particles and is difficult to break. .
[0008]
[Problems to be solved by the invention]
However, the silicon nitride-based sintered body disclosed in Japanese Patent Application Laid-Open No. 7-330437 has a high temperature strength, but has a low fracture toughness value, so that it tends to cause chipping when used in an actual heat engine. was there.
[0009]
In addition, since aluminum oxide contained as a sintering aid has an action of suppressing crystallization of the grain boundary phase, an amorphous material existing between the silicon nitride particles or between the silicon nitride particles and the crystallized grain boundary phase. Due to the temperament, the variation in high-temperature strength is large, and the fatigue life in the high-temperature range is short. Parts for heat engines, etc. should be used stably for a long time under harsh conditions exposed to high temperatures. There was a problem that could not be.
[0010]
On the other hand, as disclosed in JP-A-5-186271, grain growth can be promoted and the fracture toughness value can be improved, but on the other hand, the grown particles themselves serve as a fracture source. There was a problem of deteriorating characteristics.
[0011]
As described above, although a method for improving either the high-temperature strength or the high toughness has been obtained, there is a problem that the property is insufficient or the improvement of one property lowers the other property. there were.
[0012]
Therefore, the present invention provides a silicon nitride-based sintered body having excellent strength in a high temperature range from room temperature to 1000 ° C. and having high toughness with excellent chipping property and a long fatigue life, and a method for producing the same. With the goal.
[0013]
[Means for Solving the Problems]
The silicon nitride-based sintered body of the present invention is mainly composed of silicon nitride, the rare earth element is 2 to 10% by weight in terms of oxide, aluminum is 2 to 5% by weight in terms of oxide, and excess oxygen is in terms of silicon oxide. 0.5 to 5% by weight, and the ratio of the oxide equivalent of aluminum to the oxide equivalent of the rare earth element is 0.2 to 0.7, oxidation of excess oxygen with respect to the oxide equivalent of the rare earth element In a silicon nitride-based sintered body having a silicon equivalent ratio of 0.2 to 0.7, a β-type silicon nitride crystal phase, a grain boundary crystal phase comprising rare earth elements, silicon, aluminum, oxygen and nitrogen, and grains The amorphous silicon phase comprises 70 to 90% of a β-type silicon nitride crystal phase of 10 μm or less, 10 to 30% of β-type silicon nitride crystal of 40 to 100 μm, and an amorphous phase between the silicon nitride crystal phases. The thickness is 2 nm or less and the β type The thickness of the amorphous phase between the grain boundary crystal phase and of the silicon crystal phase, characterized in that it is 5nm or less.
[0014]
That is, the silicon nitride sintered body of the present invention promotes the crystallization of the grain boundary phase, and the amorphous phase existing between the silicon nitride particles or between the silicon nitride particles and the crystallized grain boundary phase. By reducing the thickness, the strength of the grain boundary is increased, so that the high temperature strength is improved, and the high temperature sliding at the grain boundary is suppressed, so that the fatigue life can be improved. Furthermore, by dispersing columnar large silicon nitride particles in a matrix made of fine silicon nitride particles with a predetermined particle size distribution, the crack traveling direction is changed, and the crack traveling distance is increased to absorb the fracture energy. , Can improve toughness. Therefore, it is possible to obtain a silicon nitride-based sintered body having excellent high temperature strength, a long fatigue life, and high toughness.
[0015]
Further, since the contents of rare earth elements, aluminum and excess oxygen, and their abundance ratios affect the aspect ratio and the amount of grain boundary crystals, the above-mentioned structure can be obtained by controlling the addition amount and the abundance ratio thereof.
[0016]
Furthermore, in order to produce the above silicon nitride sintered body, the present invention is mainly composed of silicon nitride, 2 to 10% by weight of rare earth element oxide as an auxiliary component, 2 to 5% by weight of aluminum oxide, Excess oxygen is 0.5 to 5% by weight in terms of silicon oxide, and the ratio of the amount of aluminum oxide to the amount of the rare earth element oxide is 0.2 to 0.7. The amount of excess oxygen to the amount of silicon oxide in terms of the amount of the rare earth element oxide The compacts respectively added so as to have a ratio of 0.2 to 0.7 are fired at a temperature of 1700 to 1800 ° C. in a non-oxidizing atmosphere, and then 1900 to 2000 at a nitrogen pressure of 5 atm or more. It is characterized in that the grain boundary phase of the sintered body is crystallized by performing heat treatment at 1000 to 1500 ° C. after firing in the temperature range of ° C. and then rapidly cooling to 1000 ° C. at a rate of 50 ° C./min or more. To do.
[0017]
This is composed of a β-type silicon nitride crystal phase, a grain boundary crystal phase composed of rare earth elements, silicon, aluminum, oxygen and nitrogen, and a grain boundary amorphous phase, and a matrix 70 of β-type silicon nitride crystal phase of 10 μm or less. 90%, 40-100 μm of β-type silicon nitride crystal 10-30%, the thickness of the amorphous phase between silicon nitride crystal phases is 2 nm or less, and between the β-type silicon nitride crystal phase and the grain boundary crystal phase This is to obtain a structure in which the thickness of the amorphous phase becomes 5 nm or less, after firing at a predetermined temperature, promoting grain growth, quenching at a specific rate, and then using under conditions where heat treatment or heat is applied It is important to crystallize the grain boundary phase by doing so.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
The silicon nitride sintered body of the present invention has a grain boundary crystal phase 2 mainly composed of a β-type silicon nitride crystal phase 1 and comprising a rare earth element, silicon, oxygen and nitrogen, as shown in a schematic diagram of a crystal structure in FIG. According to the present invention, the amorphous phase 3 between the silicon nitride crystal phases 1 has a thickness of 2 nm, preferably 1 nm or less, and a silicon nitride crystal. A major feature is that the thickness of the amorphous phase 4 between the phase 1 and the grain boundary crystal phase 2 is 5 nm, preferably 3 nm or less.
[0019]
Thus, not only the grain boundary phase is crystallized, but also the grain boundary amorphous phase 3 remaining between the silicon nitride crystal phases 1 or between the silicon nitride crystal phase 1 and the grain boundary crystal phase 2. By reducing the thickness of the grain boundary amorphous phase 4 to the above range, other high temperature characteristics such as oxidation characteristics and creep can be improved.
[0020]
That is, when the amorphous phases 3 and 4 at the grain boundaries are thin, the amorphous phases 3 and 4 are difficult to soften at a high temperature of about 1000 ° C., and cracks are difficult to develop against external stress. As a result, the repeated fatigue life when applying a load of 700 MPa in a high temperature atmosphere of 1000 ° C. is significantly increased to 10 ⁷ times or more, and further to 3 × 10 ⁷ times or more, and the high temperature strength of silicon nitride-based sintering is increased. Can be 800 MPa or more.
[0021]
By the way, in order to obtain such high-temperature characteristics, rare earth elements such as Y, Er, Yb, Lu, Sm, and Dy with respect to silicon nitride are 2 to 10% by weight, preferably 4 to 8% in terms of oxides. And 2 to 5% by weight of aluminum in terms of aluminum oxide, preferably 3 to 4% by weight, and excess oxygen in an amount of 0.5 to 5% by weight of silicon oxide, preferably 2 And the ratio of the aluminum oxide equivalent of aluminum to the oxide equivalent of the rare earth element is 0.2 to 0.7, preferably 0.3 to 0.6, and the rare earth It is important that the ratio of excess oxygen to silicon oxide equivalent to the element oxide equivalent is 0.2 to 0.7, preferably 0.3 to 0.6.
[0022]
Here, excess oxygen is obtained by calculating the amount of oxygen necessary for the rare earth oxide and the amount of oxygen necessary for aluminum oxide, and subtracting from the total amount of oxygen. The total oxygen amount can be measured by an oxygen analyzer using a combustion method.
[0023]
That is, if the content and ratio of rare earth elements, aluminum, and excess oxygen are out of the above ranges, the grain boundary phase composed of rare earth elements, silicon, oxygen, and nitrogen cannot be sufficiently crystallized or crystallized. This is because the thickness of the grain boundary amorphous phases 3 and 4 cannot be within the above range.
[0024]
Further, in order to have the characteristics of high toughness, a coarse β-type of 40-100 μm, preferably 70-90 μm in a matrix of 70-90%, preferably 80-90% of β-type silicon nitride crystal particles of 10 μm or less, preferably 8 μm or less. It is important to disperse the silicon nitride crystal particles in a proportion of 10 to 30%, preferably 10 to 20%. By controlling to such a structure, the toughness value can be increased to 7 or more, further 8 MPa · m ^1/2 or more, the thickness of the amorphous film phase at the grain boundary can be reduced, and the fatigue characteristics can be further improved. When the proportion of fine particles of 10 μm or less is less than 70%, or when columnar particles are 40 μm or less, there is little deflection and the cracks progress relatively linearly. , High toughness is not achieved. On the other hand, if the columnar particles are 100 μm or more and the ratio is 20% or more, they themselves become a fracture source, and high strength cannot be achieved.
[0025]
Furthermore, in the silicon nitride sintered body of the present invention, as other components, metals of Group 4a, 5a, 6a elements of the periodic table, TiC, TiN, TaC, TaN, VC, NbC, WC, WSi Improved characteristics by including at least one of carbides, nitrides and silicides of elements 4a, 5a and 6a of periodic table such as ₂ and Mo ₂ C, or SiC in the form of dispersed particles or whiskers. It is also possible to do. However, the total content of these is preferably 5% by weight or less.
[0026]
Next, a method for producing the silicon nitride sintered body of the present invention will be described.
[0027]
First, silicon nitride powder is prepared. The silicon nitride powder may be in any state of α-Si ₃ N ₄ or β-Si ₃ N ₄ , the particle size is 0.4 to 1.2 μm, and oxygen is 0.5 to 1.. What is contained in the range of 5% by weight is preferably used.
[0028]
The silicon nitride powder has a rare earth element oxide of 2 to 10% by weight, preferably 4 to 8% by weight, and aluminum oxide of 2 to 5% by weight, preferably 3 to 4% by weight as a sintering aid. The silicon oxide is added in the range of 0.5 to 5% by weight, preferably 2 to 4% by weight, and the ratio of the addition amount of aluminum oxide to the addition amount of the rare earth element oxide is 0.2 to 0.7, preferably 0.3 to 0.6, and the ratio of the amount of silicon oxide to the addition amount of the rare earth element oxide is 0.2 to 0.7, preferably 0.3 to 0 ..6. However, the amount of silicon oxide is a value obtained by adding an amount of silicon oxide powder to which excess oxygen contained as an impurity in silicon nitride powder is converted into silicon oxide.
[0029]
After adding an organic solvent such as ethanol and isopropyl alcohol and a binder to the raw material powder prepared in these ranges, the raw material powder is uniformly mixed by a known pulverization method such as a ball mill, vibration mill, rotary mill, barrel mill, etc. The pulverized product was formed into a desired shape by a known ceramic molding means such as a uniaxial pressure molding method, an equal pressure molding method, a casting molding method, an extrusion molding method, an injection molding method, or a cold isostatic press. A molded body is produced. At this time, the formed body may be cut as necessary.
[0030]
Next, the obtained molded body was fired at normal pressure at a temperature of 1700 to 1800 ° C., preferably 1750 to 1800 ° C. in a non-oxidizing atmosphere, and then 1900 to 2000 ° C., preferably 1950 to 2000 ° C., Baking is performed at 5 atm or more, particularly 10 to 50 atm. By performing such two-stage firing, uniform and fine particles can be generated by low-temperature treatment and densified to some extent, and some of the particles can be grown by subsequent high-temperature treatment. Further, it is important to rapidly cool to 1000 ° C. at a rate of 50 ° C./min or more, preferably 70 ° C./min or more in the cooling process after the firing.
[0031]
In firing, the molded body is embedded in a mixed powder of silicon oxide and Si filled in a closed firing pot or a mixed powder of silicon oxide and silicon nitride, and fired in an atmosphere containing SiO. In this case, decomposition of silicon nitride during firing can be suppressed.
[0032]
Here, the cooling rate is set to 1000 ° C. at 50 ° C./min or more. At a slower cooling rate, the grain boundary phase composed of rare earth elements, silicon, oxygen and nitrogen is partially crystallized or solidified. Therefore, even when used under conditions where heat treatment or heat is applied, the grain boundary phase is insufficiently crystallized, resulting in silicon nitride crystals. This is because the thickness of the amorphous phase 3 between the phases 1 cannot be 2 nm or less, and the thickness of the amorphous phase 4 between the silicon nitride crystal phase 1 and the grain boundary crystal phase 2 cannot be 5 nm or less.
[0033]
That is, according to the present invention, by rapid cooling after firing, crystallization of a grain boundary phase composed of rare earth elements, silicon, oxygen, and nitrogen is suppressed to an amorphous state, and the composition of the grain boundary is made uniform. It is preferable to keep it.
[0034]
Thereafter, the obtained silicon nitride sintered body is subjected to heat treatment in the range of 1000 to 1500 ° C. This heat treatment can also be accomplished by using it in an atmosphere where heat of 1000 ° C. or higher is applied. By such heat treatment, the grain boundary phase composed of rare earth elements, silicon, oxygen and nitrogen in the sintered body is crystallized, and the thickness of the amorphous phase 3 between the silicon nitride crystal phases 1 is 2 nm or less, and The thickness of the amorphous phase 4 between the silicon nitride crystal phase 1 and the grain boundary crystal phase 2 can be made 5 nm or less, and as a result, a silicon nitride-based sintered body excellent in durability with a long repeated fatigue life can be obtained. Can do.
[0035]
Furthermore, according to the production method of the present invention, 10 to 80% by weight of silicon nitride powder as a starting material can be replaced with silicon powder. In this case, the molded body is 1000 1000 under a nitrogen atmosphere before firing. The Si powder may be heat treated at a temperature of ˜1400 ° C. to form silicon nitride to increase the density of the molded body, and then fired under the firing conditions. According to this manufacturing method, shrinkage during firing can be suppressed, and a dense silicon nitride sintered body with high dimensional accuracy can be obtained.
[0036]
【Example】
Example 1
A silicon nitride powder having a BET specific surface area of 9 m ² / g, silicon nitride α ratio of 98%, oxygen content of 1.2% by weight, a rare earth element oxide powder having an average particle size of 1.5 μm as a sintering aid, An aluminum oxide powder having a purity of 99.9% and an average particle diameter of 2 μm, a silicon oxide powder having a purity of 99.9% and an average particle diameter of 2 μm, and a Si powder having a purity of 99% and an average particle diameter of 5.0 μm as required. After blending as shown in Table 1, adding a binder and a solvent and kneading and drying, a molded body was formed by a cold isostatic pressing method at a pressure of 1 t / cm ² .
[0037]
Next, the obtained molded body was put into a slag bowl made of silicon carbide, held at 1300 ° C. for 5 hours in a nitrogen atmosphere at normal pressure, and further fired at a temperature of 1750 ° C. for 5 hours. Thereafter, the temperature was raised to 1950 ° C. and held for 5 hours. Thereafter, the silicon nitride sintered body was obtained by cooling at a rate of 80 ° C./min. Thereafter, this silicon nitride sintered body was heat-treated at 1200 ° C. for 5 hours in a nitrogen atmosphere at normal pressure.
[0038]
And while confirming a composition about the obtained silicon nitride sintered body by ICP analysis and oxygen analysis, a part of sintered body is cut out and heat-treated at a temperature of 1100 ° C. for about 10 hours in a nitrogen atmosphere. Then, polishing is performed on the surface of the silicon nitride-based sintered body, and the thickness L of the grain boundary amorphous phase 3 between the silicon nitride crystal phase 1 and the silicon nitride crystal phase 1 and the grain boundary crystal are measured by a transmission electron microscope. The thickness W of the amorphous phase 4 between the phases 2 was measured. The measurement conditions used a 500,000-fold lattice image, 10 measurement points, and the average values are shown in Table 1.
[0039]
The particle size was calculated by observing the structure with a scanning electron microscope, and from the micrograph, area ratios of 10 μm or less, 40 to 100 μm, and 100 μm or more were determined by image analysis. Further, toughness (K _1c ) was measured according to JIS 1607, and 4-point bending strength measurement at 1000 ° C. was performed according to JIS 1607.
[0040]
Further, the obtained silicon nitride-based sintered body was subjected to a four-point bending test under a stress ratio of 0.1 and a frequency of 40 Hz under a high temperature atmosphere of 1000 ° C., and a repeated fatigue life was measured. did.
[0041]
The results are as shown in Table 1.
[0042]
[Table 1]

[0043]
First, sample No. 1 of the present invention obtained by adding 5% by weight of various rare earth element oxides. 1-7, L is 1 nm or less, W is 3 nm or less, silicon nitride particles of 10 μm or less are 80-90%, particles of 40-100 μm are 10-20%, and as a result, strength at 1000 ° C. Has excellent characteristics of 900 MPa or more, fracture toughness of 8 MPa · m ^1/2 or more, and fatigue life of 7 × 10 ⁷ times or more.
[0044]
In addition, as shown in Table 1, sample Nos. 1 and _{2 in which} Y ₂ O ₃ was used as the rare earth element and the amount of SiO _{2 and the} amount of Al ₂ O _{3 as the} silicon oxide equivalent amount of excess oxygen were changed. As apparent from 8 to 29, the amount of Y ₂ O _{3, the} amount of SiO _{2, the} amount of Al ₂ O ₃ , the SiO ₂ / Y ₂ O ₃ ratio and the Al ₂ O ₃ / Y ₂ O ₃ ratio are within the scope of the present invention. Sample No. 8, 9, 12, 13, 16, 17, 20, 21, 24, 25, 28 and 29 all have the thickness L of the amorphous phase between the β-type silicon nitride crystal phases in the silicon nitride sintered body. 2 nm or less, and the thickness W of the amorphous phase between the β-type silicon nitride crystal phase and the grain boundary crystal phase is 5 nm or less. As a result, the strength at 1000 ° C. is 800 MPa or more and 700 MPa in a high temperature atmosphere of 1000 ° C. It was confirmed that the repeated fatigue life when the above load was applied could be increased to 10 ⁷ times or more. Further, the toughness value is increased to 7 MPa · m ^1/2 or more by dispersing 10 to 30% of 40 to 100 μm β-type silicon nitride crystal in 70 to 90% of a matrix composed of β-type silicon nitride crystal phase of 10 μm or less. I was able to confirm.
[0045]
Further, the silicon nitride powder as a starting material is replaced with silicon powder within a range of 10 to 80% by weight, and the molded body is heat-treated at a temperature of 1000 to 1400 ° C. in a nitrogen atmosphere before firing to obtain a Si powder. Sample No. 1 was formed by nitriding to produce silicon nitride, increasing the density of the compact, and firing. 28 and 29 are 80 to 90% of silicon nitride particles having L of 1 nm or less, W of 2 nm or less and 10 μm or less, and 15 to 25% of particles of 40 to 100 μm, and as a result, the strength at 1000 ° C. is 1000 MPa. As described above, the fracture toughness was 8.5 MPa · m ^1/2 or more and the fatigue life was 8 × 10 ⁷ times or more.
[0046]
On the other hand, Sample No. having a rare earth oxide content of less than 2% by weight. Sample Nos. Over 10 and 10% by weight. 11, Sample No. with a silicon oxide content of less than 0.5% by weight. Sample Nos. Over 14 and 5% by weight. 15, Sample No. having an alumina amount of less than 2% by weight. Sample No. over 18 and 5%. 19, Sample No. with SiO ₂ / Y ₂ O ₃ ratio smaller than 0.2. Sample Nos. Greater than 22 and 0.7. 23, and Sample No. with an Al ₂ O ₃ / Y ₂ O ₃ ratio of less than 0.2. Sample Nos. Greater than 26 and 0.7. In No. 27, L and W are larger than 2 nm and 5 nm, respectively, the amount of silicon nitride of 10 μm or less is outside the range of 70 to 90%, and the amount of 40 to 100 μm is outside the range of 10 to 30%. The toughness and fatigue life were as low as 640 MPa or less, 6.2 MPa · m ^1/2 or less, and 8 × 10 ⁶ times or less, respectively.
Example 2
Next, after firing a silicon nitride-like molded body containing 5% by weight of Y ₂ O _{3, 3} % by weight of SiO ₂ and 3% by weight of Al ₂ O ₃ at normal temperature firing at a temperature of 1750 ° C. , Fired at 20 ° C. in a temperature range of 1950 ° C., heat-treated at 1000 ° C. for 100 hours, cooled to 1000 ° C. at the cooling rate shown in Table 2, and subjected to the same high temperature strength, fatigue life and fracture toughness as in Example 1. The value was measured. The results are shown in Table 2.
[0047]
[Table 2]

[0048]
As a result, Sample No. with a cooling rate of up to 1000 ° C. was 50 ° C./min or more. 30 to 32 were able to disperse 10 to 30% of 40 to 100 μm β-type silicon nitride crystals in 70 to 90% of a matrix composed of a β-type silicon nitride crystal phase of 10 μm or less. The thickness of the amorphous phase between the β-type silicon nitride crystal phases in the silicon nitride-based sintered body is 2 nm or less, and the thickness of the amorphous phase between the β-type silicon nitride crystal phase and the grain boundary crystal phase is 5 nm or less. And was able to.
[0049]
As a result, the strength at 1000 ° C. is 800 MPa or more, the toughness value is 7 MPa · m ^1/2 or more, and the repeated fatigue life when a 700 MPa load is applied in a high temperature atmosphere of 1000 ° C. is 10 ⁷ times or more. I was able to.
[0050]
On the other hand, Sample No. with a cooling rate to 1000 ° C. is slower than 50 ° C./min. 33 and 34, the thickness of the amorphous phase between the β-type silicon nitride crystal phase exceeds 2 nm, and the thickness of the amorphous phase between the β-type silicon nitride crystal phase and the grain boundary crystal phase exceeds 5 nm. Out of the range, the high-temperature strength, fracture toughness, and fatigue life were low values of less than 800 MPa, less than 7 MPa · m ^{1/2, and} less than 10 ⁷ times, respectively.
[0051]
From the above results, all the silicon nitride sintered bodies of the present invention achieved a high temperature strength at 1000 ° C. of 800 MPa or more, a fracture toughness of 7 MPa · m ^1/2 or more, and a fatigue life of 10 ⁷ times or more.
[0052]
【The invention's effect】
According to the present invention, the crystallization of the grain boundary phase is promoted to reduce the thickness of the amorphous phase, and the silicon nitride particles are dispersed in a predetermined amount of columnar silicon nitride particles in a fine matrix. Thus, it is possible to improve the toughness, to obtain a silicon nitride-based sintered body having excellent high temperature strength, a long fatigue life, and a high toughness.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a crystal structure of a silicon nitride sintered body of the present invention.
[Explanation of symbols]
1 .. Silicon nitride crystal phase 2 .. Grain boundary crystal phase 3.. Amorphous phase between silicon nitride crystal phases 4.. Amorphous phase between silicon nitride crystal phase and grain boundary crystal phase

Claims (2)

Mainly silicon nitride, rare earth element 2 to 10% by weight in terms of oxide, aluminum 2 to 5% by weight in terms of oxide, excess oxygen 0.5 to 5% by weight in terms of silicon oxide, and The ratio of the oxide equivalent of aluminum to the oxide equivalent of the rare earth element is 0.2 to 0.7, and the ratio of the silicon oxide equivalent of excess oxygen to the oxide equivalent of the rare earth element is 0.2 to 0. The silicon nitride-based sintered body of .7 is composed of a β-type silicon nitride crystal phase, a grain boundary crystal phase composed of rare earth elements, silicon, aluminum, oxygen and nitrogen, and a grain boundary amorphous phase, and has a major axis of 10 μm. The following β-type silicon nitride crystal particles contain 70-90% by area ratio, 10-30% β-type silicon nitride crystal particles having a major axis of 40-100 μm, and the thickness of the amorphous phase between the silicon nitride crystal phases is 2 nm. And the β-type silicon nitride crystal Phase silicon nitride sintered material, wherein the thickness of the amorphous phase is 5nm or less between the grain boundary crystal phase. Mainly silicon nitride, 2 to 10% by weight of rare earth element oxide, 2 to 5% by weight of aluminum oxide, and 0.5 to 5% by weight of excess oxygen in terms of silicon oxide, and the amount of the rare earth element oxide A molded body obtained by adding a ratio of the amount of aluminum oxide to 0.2 to 0.7 and a ratio of the amount of excess oxygen to the amount of silicon oxide equivalent to the amount of rare earth element oxide to be 0.2 to 0.7, respectively. Baked at a pressure of 1700 to 1800 ° C. under normal pressure in a non-oxidizing atmosphere, and then baked in a temperature range of 1900 to 2000 ° C. at a nitrogen pressure of 5 atm. A method for producing a silicon nitride sintered body, characterized by crystallizing a grain boundary phase of a sintered body by performing heat treatment at 1000 to 1500 ° C. after rapid cooling at a speed.

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