JPH01100065A - Silicon carbide whisker-reinforced silicon nitride sintered compact and production thereof - Google Patents

Abstract

PURPOSE:To obtain the titled sintered compact having excellent folding endurance (at ordinary and high temperature), fracture toughness and oxidation resistance, etc., by sintering and molding a blend composition of a specific SiC whisker with a specific Si3N4 in the presence of oxide of rare earth elements. CONSTITUTION:5-40wt.% SiC whisker having 5-30mu average length, 0.2-5mu average diameter, 2-150 aspect ratio, <=1.0wt.% content of cation impurity (e.g. Al) and SiO2 and 95-60wt.% Si3N4 powder having >=90% alpha ratio are pulverized and blended in the presence of 5-30wt.% one or more oxide (e.g. Y2O3) of rare earth elements used as a sintering auxiliary until the above- mentioned SiC whisker is uniformly dispersed in the range of 5-30mu average length and then molded and sintered under reduction atmosphere or inert gas non-oxidative atmosphere at 1,650-1,850 deg.C until ratio of Si3N4 becomes <=40wt.% to provide the titled sintered compact.

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a silicon nitride-based composite material strengthened using silicon carbide whiskers, and in particular to a silicon nitride-based sintered body with excellent strength and creep resistance at high temperatures and a method for producing the same. The present invention provides a composite material that is widely useful for automobile engine parts such as rotors and ceramic valves, and high-temperature structural parts such as gas turbine rotors, and a method for manufacturing the same. (Prior art) Silicon carbide sintered bodies and silicon nitride-based sintered bodies whose main component is silicon nitride (SiJ*) mainly have covalent bonds as their atomic bonds, and have excellent mechanical strength, oxidation resistance, Because it has excellent properties such as wear resistance, impact resistance, and corrosion resistance, it has begun to be put to practical use, especially as a material for high-temperature structural members such as automobile engine parts and gas turbine rotors, and cutting tool materials. However, despite having these excellent characteristics, compared to metals, they lack quality stability and homogeneity, and from the perspective of improving reliability and high properties, silicon nitride ceramics are being developed with even higher quality. Toughness is desired. By the way, silicon nitride and silicon carbide are both compounds mainly composed of covalent bonds, and are considered to be difficult-to-sinter materials. Therefore,
Silicon nitride and silicon carbide are not sintered alone, but usually by adding several percent to several tens of percent of a sintering aid.
Sintering is promoted by forming a low melting point compound, and the added sintering aid remains after sintering as a glass phase or crystalline phase at the grain boundaries, or remains as a solid solution in the crystals of silicon nitride. However, the sintered body obtained in this way does not contain A1tO3+YzO++LazOa added as a sintering aid.
. MgO, Ca (L rare earth element oxides, etc.) have the advantage of forming low melting point compounds and accelerating sintering as described above, but on the other hand, these low melting point compounds cause a decrease in the mechanical strength of the sintered body at high temperatures. Since there is a disadvantage of a decrease in mechanical strength, studies have been made to consider the type of sintering aid and to reduce its amount as much as possible, but the disadvantage of a decrease in mechanical strength at high temperatures has not yet been resolved. As a countermeasure to this problem, various attempts have been made to obtain improved ceramic sintered bodies by compounding SiC whiskers with ceramics.
This is a manufacturing method in which plates formed using a paste of a whisker mixture are laminated and sintered under pressure, and does not suggest a method of sintering in a powder state. Japanese Patent Publication No. 60-35316 provides high electrical conductivity by dispersing conductive fibrous SiC in 5isNa, and provides a sintered body of Si3N that can be processed by electrical discharge. Since it is necessary to uniformly disperse relatively long fibers of about ~500 μm, care must be taken to avoid agglomeration of SiC fibers due to mixing technology, which can lead to poor sintering and even reliability. There is a risk of a decrease in properties, and further improvements are required in room temperature strength and high temperature strength. In addition, JP-A No. 59-102862 is the above-mentioned JP-A-60
This invention is an extension of No. 35316, which improves electrical conductivity by dispersing conductive fibrous SiC and inorganic powder with specific conductivity in Si3Na. However, it is thought that there is room for improvement, especially in high-temperature strength. JP-A No. 60-200863 discloses SiC whiskers and 4
・Group 5・Contains crystallized products and nitrides, and the rest is Si3
It is a ceramic with a composition consisting of N, which reduces its affinity with iron and can prevent the progression of wear in applications that cause friction with iron, but the transverse rupture strength at a high temperature of 1200°C is
In order to stay within the range of 0 kg/mm, it is necessary to further improve the layer. In Japanese Patent Publication No. 60-55469, a dispersion of silicon nitride powder and a dispersion of fibrous silicon carbide crystals were each filtered. After passing through the powder, the latter is mixed at 5 to 50% by weight relative to the former, and then molded and sintered, which is more time-consuming and troublesome than molding and sintering the powder. Publication No. 60-246268 and Japanese Patent Publication No. 61-291
No. 463 is a product in which SiC whiskers, etc. are added to a sialon base material, the former being β-sialon and the latter being goose sialon, and does not relate to a sintered body in which SiC whiskers are added to a silicon nitride base material. (Problems to be solved by the invention) As mentioned above, composite materials in which SiC whiskers are dispersed in Si3N4-based ceramics have not yet been fully put into practical use.
It was still insufficient for use at high temperatures of 00°C or higher. That is, although the sialon-based ceramics mentioned above have high toughness, their strength deteriorates and their oxidation resistance is poor at high temperatures of 1000°C or higher, and silicon nitride (excluding sialon)
Although base ceramics have better high-temperature properties than sialon-based ceramics, they have problems such as low toughness when used as high-temperature structural materials. (Means for Solving the Problem) The present inventors conducted extensive studies on this point and found that the mechanical properties of a silicon nitride sintered body at high temperatures largely depend on the morphology of its crystal phase and grain boundary phase. In particular, in order to improve toughness and strength at high temperatures, specific SiC whiskers must have a considerable length in the sintered body and be uniformly dispersed;
This is based on the knowledge that a sintered body with preferable characteristics can be obtained when the proportion of α-Si3Nn in Si3N4, that is, the α ratio, is below a certain value, and the outline thereof is as follows. That is, the first invention includes 5 to 40% by weight of SiC whiskers with an average length of 5 to 30 μm, and 5 to 40% by weight of one or more rare earth element oxides.
A silicon nitride sintered body formed by molding and sintering a blended composition consisting of 30% by weight and the balance being silicon nitride, the silicon nitride sintered body is
A silicon carbide whisker-reinforced silicon nitride sintered body characterized in that the α ratio of Jm is 40% by weight or less, and the second invention provides a silicon carbide whisker-reinforced silicon nitride sintered body having an average length of 30 μm in order to produce the sintered body.
5 to 40% by weight of the following SiC whiskers, 5 to 30% by weight of powder of one or more rare earth element oxides, and the balance is α rate 9
0% by weight or more of 5isNa powder is pulverized and mixed so that the average length of the SiC whiskers is uniformly dispersed in the range of 5 to 30 μm, and then molded.
This is a method for producing a silicon carbide whisker-reinforced silicon nitride sintered body, which is characterized by sintering at a temperature of ~1850°C until the sintering rate of SiJ becomes 40% by weight or less. For molding and sintering, the predetermined composition is molded with a mold, isostatically pressed, and injection molded as necessary, followed by hot pressing, normal pressure sintering, gas pressure sintering, and heating in a reducing or inert gas atmosphere. This can be done by isostatic sintering. The SiC whiskers used in the present invention have high hardness and strength from room temperature to high temperatures (and even after sintering, they remain in the whisker shape and are uniformly dispersed within the structure, improving the high-temperature strength of ceramics. , which increases fracture toughness and hardness.SiC whiskers as a starting material used in the present invention have an average diameter of 0.2 to 5 μm and an average length of 5 to 30 μm.
m and an aspect ratio of 2 to 150 is desirable. Also, this whisker is A1*Ca, Mg+Ni
+ Cation impurities such as Fe, Mn, Coo Cr, etc. and Si0g content are 1.0% by weight or less, and whisker-like crystals with few cracks, branches, and surface defects, etc., are highly tough and dense sintered bodies. preferred in terms of obtaining The reason why the amount of SiC whiskers added is 5 to 40% by weight is that if the amount of SiC whiskers is less than 5% by weight, there is almost no effect of adding whiskers to the ceramic sintered body.
This is because no improvement in strength and toughness is observed, and on the contrary, if the content exceeds 40% by weight, the uniform dispersibility decreases due to the anisotropy of the whiskers, and the sinterability also decreases significantly.
The SiC whisker used in the present invention is preferably added in an amount of 0 to 30% by weight, and remains in the sintered body in the form of a whisker even after sintering. Furthermore, it is necessary for SiC whiskers to have an average length of 5 to 30 μm even after pulverization, mixing, molding, and sintering, and the reason is that if the average length is shorter than 5 μm, the effect of whisker addition cannot be seen. This is because no improvement in strength or toughness is observed, and if the length is longer than 30 μm, the sinterability will deteriorate and the agglomeration of whiskers will become significant, and the agglomerated portions generated in the sintered body will become defects and reduce the strength. It is more preferable to have a particle diameter of 5 to 20 μm in order to maintain high toughness and high strength. The rare earth element oxide functions as a sintering aid, forms a glass phase in the sintered body, and is added in an amount of 5 to 30% by weight to form a dense body. If the rare earth element oxide content is less than 5% by weight, too little glass phase (liquid phase) will be produced, so it will not be effective as a sintering aid and will not be densified, and if it exceeds 30% by weight, excessive nitridation will occur. The above range may cause grain growth of silicon, which actually inhibits sinterability and causes strength deterioration, and increases in the amount of glass phase produced, which deteriorates high-temperature properties such as mechanical strength and oxidation resistance. Preferably, it is more preferably 10 to 25% by weight. The reason why the α ratio of 5isNa in the sintered body is set to 40% by weight or less is that a sintered body with an α ratio of more than 40% by weight has low strength and toughness. In order to obtain a sintered body, the α ratio is preferably 40% by weight or less, and more preferably the α ratio is 20% by weight or less. (Example) Examples of the present invention will be described below. Example 1 Average particle size 0. Bttm ex rate 90% by weight 5ixN
a powder of 64% and an average particle size of 1.2. crm's YzO
Laz03 with 6.4% by weight of z powder and an average particle size of 2 μmm
20% by weight of commercially available SiC whiskers having an average diameter of 0.5 μm and different average lengths were added to 9.6% by weight of the powder, mixed by ultrasonic irradiation for 1 hour while stirring with a stirrer in an ethanol solvent, and then dried. Then, it was granulated to obtain a base powder. Next, this base powder was sintered in a graphite mold at a temperature of 1800°C. A sintered body was obtained by hot pressing at a pressure of 200 kg/aJ for 1 hour. The obtained sintered body is 4 w x 3 vnr x 4 Q
After polishing the test piece with the size of H, the density and JI
The bending strength was measured using S-R1601. Furthermore, by mirror-polishing the sintered bodies and observing them with an optical microscope, it was confirmed that the SiC whiskers added in all the blended compositions were present without changing the shape at the time of blending. On the other hand, X-ray diffraction of the obtained sintered body,
It was also confirmed by chemical analysis and carbon quantitative analysis that the SiC whiskers were present with almost the same composition. The results of the sintered body obtained in this example are shown in Table 1. If the average length of the SiC whiskers added is longer than 30 μm, the agglomeration of the whiskers becomes significant and the uniform dispersion in the sintered body becomes poor. It has been found that this deteriorates the sinterability and causes defects in the agglomerated portions, resulting in a decrease in strength. Table 1 also shows the results of measuring the specific resistance of the obtained sintered bodies. Example 2 A base powder was obtained in the same manner as in Example 1, except that SiC whiskers with an average length of 30 μm were crushed in advance and whiskers with average lengths changed as shown in Table 2 were used. Next, this base powder was sintered in a graphite mold at a temperature of 1800°C. A sufficiently dense sintered body was obtained by hot pressing for 1 hour at a pressure of 200 kg/aJ. The flexural strength of the obtained sintered body was measured in the same manner as in Example 1, and the fracture toughness value was further measured by the indentation microrcture method (1M method) at an indentation load of 30 kg. The results of the sintered body obtained in this example are shown in Table 3. Unless the average length of the whiskers dispersed in the sintered body obtained is 5 μm or more, there is no effect of the addition, and the strength of the sintered body is It was found that the fracture toughness did not improve. From the results of Examples 1 and 2, it can be seen that the addition of whiskers has no effect unless the average length of the whiskers to be present in the sintered body is within the range of 5 to 30 μm. Example 3 SiC whiskers with an average length of 30 μm and an average particle size of 0
．． 6μm Si3N4 powder and average particle size of 2μm or less
YzOx, Lag's, CeO,%dlOx
, one or more rare earth element oxides selected from SmzOz and α-^120 with an average particle size of 1 μm as a comparative experiment.
3 and Al1N with an average particle size of 0.5 μm were blended into the composition shown in Table 4, each of which was ground and mixed in an ethanol solvent using a ball mill for 16 hours, then dried and granulated to obtain a base powder. . Next, this base powder was hot pressed in a graphite mold under the sintering conditions shown in Table 4 to obtain a sintered body. The obtained sintered body was subjected to measurements of bending strength and fracture toughness at room temperature in the same manner as in Example 2. Furthermore, the oxidation resistance was evaluated from the bending strength at 1300°C in the atmosphere and the weight change when oxidized for 100 hours at 1300°C. Further, the α-5i3L content in the sintered body was conveniently determined from the diffraction peak heights (α phase peak height Iα, β phase peak height Iβ) by X-ray diffraction using the following formula. Iα (16ri + Iα (Ten Il +)

[Refer to Master Craftsman Examination: Suzuki, Kanno; Simple Quantification Method of α Fraction in Si3N (Ceramic Industry Journal; 92 (8) 1984)] Also, as a result of mirror polishing the obtained sintered body and observing it with an optical microscope, The average length of SiC whiskers present in all sintered bodies was found to be 5-15 μm. The results of the sintered body obtained in this example are shown in Table 4, and these results show that α-5i3N4 in the sintered body is 40
A silicon carbide whisker-reinforced silicon nitride sintered body containing 5 to 40% by weight of SiC whiskers has high toughness, and even at a high temperature of 1300°C, it has higher strength than a sintered body made of sialon as a base material. It was found that the material has excellent oxidation resistance and has sufficiently satisfactory characteristics as a high-temperature structural material. Table 1 (Effects of the Invention) As is clear from the above examples, the present invention provides a sintered body with excellent properties such as flexural strength (room temperature and high temperature), fracture toughness, oxidation resistance, and hardness, and a method for producing the same. This provides a material that is widely useful for automobile engine parts and high-temperature structural parts.

Claims (1)

[Claims] 1) 5 to 40 SiC whiskers with an average length of 5 to 30 μm
5-30% by weight of one or more rare earth element oxides
A silicon nitride-based sintered body is formed by molding and sintering a blended composition consisting of , and the remainder is silicon nitride, and the Si_3 in the sintered body is
A silicon carbide whisker-reinforced silicon nitride sintered body, characterized in that the alpha ratio of N_4 is 40% by weight or less. 2) 10 to 10 SiC whiskers with an average length of 5 to 20 μm
Claim 1 characterized in that the amount is 30% by weight.
A silicon carbide whisker-reinforced silicon nitride sintered body as described in 2. 3) Claim 1 or 2, characterized in that the content of one or more rare earth element oxides is 10 to 25% by weight.
A silicon carbide whisker-reinforced silicon nitride sintered body as described in 2. 4) The silicon carbide whisker-reinforced silicon nitride sintered body according to claim 1 or 3, wherein the α ratio of Si_3N_4 in the sintered body is 20% by weight or less. 5) SiC whiskers 5 to 4 with an average length of 30 μm or less
0% by weight and one or more powders of rare earth element oxides 5-3
0% by weight and the remainder is Si_3N_ with an α rate of 90% by weight or more
4 powder and the average length of the SiC whiskers is 5 to 30.
It is characterized by pulverizing and mixing until it is uniformly dispersed in the μm range, molding, and then sintering at a temperature of 1650°C to 1850°C in a non-oxidizing atmosphere until the α ratio of Si_3N_4 becomes 40% by weight or less. A method for producing a silicon carbide whisker-reinforced silicon nitride sintered body. 6) 10 to 10 SiC whiskers with an average length of 30 μm or less
30% by weight and one or more powders of rare earth element oxides 10
~25% by weight, and the remainder is Si_3 with an α rate of 90% by weight or more
N_4 powder, the average length of the SiC whiskers is 5~
6. The method for producing a silicon carbide whisker-reinforced silicon nitride sintered body according to claim 5, wherein pulverization and mixing are performed until uniformly dispersed in a range of 20 μm. 7) The silicon carbide whisker-reinforced silicon nitride sintered body according to claim 5 or 6, wherein the sintered body is sintered until the α ratio of Si_3N_4 in the sintered body becomes 20% by weight or less. manufacturing method.