JPH09157028A - Silicon nitride sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sintered compact excellent in high-temp. strength at 1,200 deg.C and oxidation resistance at 1,000 deg.C by firing at a low temp. of <=1,850 deg.C. SOLUTION: A compact based on Si3 N4 and contg. 1-20wt.% (expressed in terms of oxide) at least one selected from among Y and rare earth elements, 0.01-1wt.% (expressed in terms of oxides) Mn and/or Cu and 0.1-5wt.% (expressed in terms of oxides) W and/or Mo as auxiliary components is fired at <=1,850 deg.C in a nonoxidizing atmosphere contg. nitrogen to obtain the objective silicon nitride sintered compact having such superior characteristics as >=1,050MPa strength at ordinary temp., >=950MPa strength at 1,200 deg.C and <=0.1mg/cm<2> increase in quantity by oxidation after an oxidation test at 1,000 deg.C for 1,000hr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is excellent in strength characteristics and toughness from room temperature to high temperature, and in particular, silicon nitride sintered material used for automobile parts such as piston pins and engine valves, gas turbine engine parts and the like. The present invention relates to a body and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been known to have heat resistance,
Due to its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbochargers.

To produce this silicon nitride sintered material, rare earth element oxides such as Y ₂ O ₃ and Al ₂ are used as sintering aids.
It is possible to add aluminum compounds such as O ₃ and AlN, SiO _{2 and the} like, and densify by firing under atmospheric pressure or a nitrogen pressure atmosphere.
No. 190 has already been proposed.

[0004] In addition, for the silicon nitride-based sintered body, an additive to be added is selected according to its use. For example, a rare earth element oxide is essential, and Al ₂ O ₃ or Mg
Since the liquid phase is generated at a low temperature when O or the like is added, 18
It can be densified by firing at a relatively low temperature of 00 ° C. or lower and normal pressure. According to this method, a sintered body having high room temperature strength can be obtained, and therefore it is often used for applications used at room temperature. ing.

However, in the above-mentioned sintered body, the glass phase generated between the silicon nitride crystal grains by the generated liquid phase is
Since it softens at temperatures over 1000 ° C, 14
It cannot be applied to parts for gas turbines that are exposed to temperatures above 00 ° C.

Therefore, in order to increase the high temperature strength, Al ₂
It has been proposed to form the grain boundary by a crystal phase having a high melting point by compounding a rare earth element oxide and a SiO ₂ component without adding O ₃ or MgO. It is necessary to perform firing in a nitrogen pressure atmosphere of 1900 ° C. or higher.

[0007]

However, in the conventional system in which the rare earth element oxide and Al ₂ O ₃ or MgO are added, depending on the composition, it can be fired at a low temperature of 1800 ° C. or less and the room temperature strength is high to some extent. The strength gradually decreases in the high temperature region, and at 1200 ° C, the strength is only about 500 MPa at most, and such a sintered body cannot be applied to parts used in the high temperature region of 1200 ° C. Further, such a sintered body was insufficient in practical use in oxidation resistance at 1000 ° C.

Further, by not adding Al ₂ O ₃ or MgO as an auxiliary agent, it is possible to improve the high temperature strength to some extent. In particular, it has been reported that a system in which grain boundaries are crystallized has a high strength of 800 MPa or more at a temperature as high as 1200 ° C. However, such a system has a production method of 1900.
Since it has to be fired at a high temperature of ℃ or more, there is a problem that the manufacturing cost is high, such as the need for special equipment capable of high temperature firing.

In any of the above systems, 100
When exposed to an oxidizing atmosphere at 0 ° C., there is a phenomenon in which oxidation rapidly progresses, and it was difficult to obtain good characteristics with respect to oxidation resistance in the medium temperature range.

Therefore, the object of the present invention is to obtain a temperature from room temperature to 120
It has sufficient mechanical properties to be used in automobile parts, gas turbine engine parts, etc. up to a high temperature of 0 ° C, and has excellent oxidation resistance at 1000 ° C, and 185
It is to provide a silicon nitride-based sintered body that can be fired at 0 ° C. or lower, and a method for manufacturing the same.

[0011]

As a result of intensive studies on the above problems, the present inventor has found that Y, a rare earth oxide, and Mn and / or Cu are added as components to Si ₃ N ₄ .
The inventors have found that the above object can be achieved by a sintered body to which W and / or Mo are added at a specific ratio, and have reached the present invention.

That is, the silicon nitride sintered body of the present invention is made of Si
₃ N ₄ as a main component, Y as an auxiliary component, and at least one selected from the group of rare earth elements in an amount of 1 to 20 in terms of oxide.
% By weight, Mn and / or Cu in terms of oxides of 0.0
It is characterized by containing 1 to 1% by weight, and W and / or Mo at a rate of 0.1 to 5% by weight in terms of oxide. Further, according to the silicon nitride sintered body of the present invention, at least one selected from the group consisting of Si ₃ N ₄ as a main component, Y as an auxiliary component, and a rare earth element is 1 to 20 wt% in terms of oxide. , Mn and / or Cu in an amount of 0.01 to 1% by weight in terms of oxide, and W and / or Mo in an amount of 0.1 to 5% by weight in terms of an oxide. It is characterized by being fired at 1850 ° C. or lower in an oxidizing atmosphere.

[0013]

According to the present invention, in addition to the compound of Y and / or the rare earth element as an auxiliary component, Mn and / or C
By adding u, it becomes possible to perform firing at a lower temperature than in the conventional case, and it is possible to suppress grain growth of silicon nitride particles.

However, when Mn and / or Cu is added, Mn and / or Cu form a liquid phase having a low melting point at a high temperature, so that the strength decreases at a high temperature of 1200 ° C. Therefore, in the present invention, W and / or
Alternatively, the addition of a Mo compound can suppress the decrease in high temperature strength.

This is because by adding W and / or Mo and Mn and / or Cu at the same time, Mn and / or Cu form a liquid phase having a low melting point in the firing process to promote densification and In the sintered body after binding, W and / or Mo, Mn and / or Cu, and Si form a compound, or Mn and / or Cu forms a solid solution in the silicide of W and / or Mo. Therefore, the liquid phase having a low melting point is not generated at high temperature, and the strength deterioration at high temperature is suppressed.

With this structure, according to the present invention, abnormal grain growth does not occur, a fine structure is not generated, and a liquid phase having a low melting point is not generated at a high temperature, so that high-temperature strength is not deteriorated. A quality sintered body can be obtained. Moreover, such a sintered body also has high durability against oxidation resistance at 1000 ° C. This is because the sintered body has a fine structure and is not easily affected by the grain boundary phase.

[0017]

According to the present invention, Y and Sm, Er, Y are used as the first auxiliary component for Si ₃ N ₄ .
at least 1 selected from the group of rare earth elements such as b and Lu
1 to 20% by weight, in particular 3-1
5% by weight, 0.01 to 1% by weight of Mn and / or Cu as a second auxiliary component in terms of oxide, particularly 0.05
˜0.5 wt%, as a third auxiliary component, W and / or Mo is 0.1 to 5 wt%, especially 0.5
It is important to include in a proportion of ˜3% by weight.

The content of these components is limited to the above range. First, if the amount of the first component is less than 1% by weight, the liquid phase becomes insufficient in the sintering process, and in order to obtain a dense body. High temperature calcination is necessary, which causes the growth of silicon nitride particles and causes a decrease in strength. If it exceeds 20% by weight, the grain boundary phase increases in the sintered body, resulting in a decrease in high temperature strength of 1200 ° C and 1000 ° C. Deterioration of the oxidation characteristics in the above.

When the oxide conversion amount of Mn and / or Cu as the second auxiliary component is less than 0.01% by weight, the above-described effect of adding Mn and / or Cu cannot be obtained, and the Mn and / or Cu cannot be added at low temperature. It is difficult to obtain a dense body and the strength at room temperature is lowered by the firing of, and if it exceeds 1% by weight, W and / or
Alternatively, even if Mo is added, the high temperature strength decreases.

Further, when the oxide conversion amount of W and / or Mo as the third auxiliary component is less than 0.1% by weight, the effect of adding W and / or Mo cannot be obtained and Mn is not obtained.
And / or the adverse effect of the addition of Cu lowers the high temperature strength, and if it is more than 5% by weight, the high temperature strength and the oxidation property are deteriorated.

Further, according to the present invention, in addition to the above-mentioned first to third auxiliary components, Al is contained in an amount of 7% by weight or less in terms of oxide, and the same effect can be obtained.

The silicon nitride-based sintered body of the present invention has a structure in which a main crystalline phase of β-silicon nitride, Y or a rare earth element, W and / or Mo, Cu and / or Mn, It is composed of a grain boundary phase containing silicon and oxygen.
This grain boundary phase is amorphous, but in some cases, disilicate, H-phase (apatite), K-ph.
There may be a crystal phase such as ase (wollastonite). When an Al compound is added, Al
May also exist in the grain boundary phase, or a small amount of aluminum may form a solid solution in the β-silicon nitride crystal phase to form β-sialon.

The main crystal phase exists as acicular crystals and is a particle having a minor axis of 0.1 to 3 μm and an average aspect ratio (major axis / minor axis) of 2 to 10.

According to the method for manufacturing a silicon nitride sintered body of the present invention, first, at least one selected from the group consisting of Si ₃ N ₄ as a main component and Y as a first auxiliary component and a rare earth element is used. 1 to 20% by weight, particularly 3 to 15% by weight, in terms of oxide, and 0.01 to 1% by weight, particularly 0.05 to 0. 0, in terms of oxide of Mn and / or Cu as a second auxiliary component.
5% by weight, W and / or Mo as third auxiliary component
A molded body containing 0.1 to 5% by weight, particularly 0.5 to 3% by weight, in terms of oxide is produced.

In producing the above-mentioned molded body, the silicon nitride powder itself may be either α-Si ₃ N ₄ or β-Si ₃ N ₄ , and the particle size thereof is 0.4 to 1.
It is preferably 2 μm and the oxygen content is 0.5 to 1.5% by weight.

It is desirable that all of the auxiliary components added to the silicon nitride powder be added in the form of oxides, but carbonates, nitrates, acetates, metal-element-containing organic compounds capable of forming oxides during the sintering process. A compound or the like can be used. However,
Regarding W and / or Mo, any of carbide, silicide and nitride may be used.

The above-mentioned auxiliary components are added to and mixed with the silicon nitride powder in the above proportions, and the mixture is mixed with a desired molding means such as a die press, cold isostatic pressing, extrusion molding, injection molding and the like. To form an arbitrary shape.

As another method, the silicon nitride powder, the silicon powder, and the auxiliary component are mixed as described above so that 80% by weight of silicon nitride can be obtained by nitriding the silicon powder into the silicon powder. It is also possible to produce the above-mentioned molded body by forming the above-mentioned formed body by nitriding it in nitrogen at 1100-1400 ° C. to convert silicon into silicon nitride. According to this method, Mn and / or Cu act as a nitriding aid for Si, permitting nitriding at a relatively low temperature for a short time, and when nitriding at a low temperature, α-Si ₃ N ₄ is generated. This is because it is easily generated, and the α-fraction of the nitride increases, and the strength of the sintered body also increases.

Next, the obtained molded body is subjected to a known firing method,
For example, hot pressing, normal pressure firing, nitrogen gas pressure firing, and further, after these firings, hot isostatic pressing (HIP) firing, or the above-mentioned molded body can be sealed with glass to perform HIP firing.

According to the present invention, the firing temperature at this time is 18
Even at 50 ° C. or lower, particularly 1650 to 1800 ° C., a dense and excellent sintered body can be obtained. In addition,
Of course, the nitrogen pressure at this time is set to be equal to or higher than the decomposition equilibrium pressure so that the silicon nitride is not decomposed.

A preferable firing method is 160
It is best to densify by firing at a low temperature of 0 to 1800 ° C. at a nitrogen pressure of 1 to 5 atm, then raising the temperature to 1700 to 1850 ° C. and firing in high pressure nitrogen at 5 atm or more.

[0032]

EXAMPLES Silicon nitride powder (BET specific surface area 9 m ² / g, α ratio 95%, oxygen amount 1.0% by weight) as raw material powder, S
i powder, Y or rare earth oxide powder, W and / or M
The oxide powder of o, the oxide powder of Mn and / or Cu, and the SiO ₂ powder were blended so as to have the compositions shown in Tables 1 and 2, and then mixed at 1 t / cm ² for molding.

The molded body thus obtained was placed in a silicon carbide based bowl and baked in nitrogen using a carbon heater. Si
When powder is not included, 1750 ° C (N ₂ : 1.2 at
m) for 5 hours, the temperature was raised to 1800 ° C. (9 atm), and the furnace was cooled for 5 hours to obtain a sintered body. Further, when Si powder is included, it is held at 1200 ° C. for 5 hours to perform nitriding, then held at 1750 ° C. (N ₂ : 1.2 atm) for 5 hours, heated to 1800 ° C. (9 atm), and held for 5 hours. Furnace cooling was performed to obtain a sintered body. Further, Sample Nos. 23 to 25 were fired under the same conditions as above except that the final firing conditions were set to the temperatures shown in Tables 1 and 2.

The obtained sintered body was processed into the shape shown in JIS R1601. A specific gravity measurement based on the Archimedes method and a four-point bending bending strength test at room temperature and 1200 ° C. based on JISR1601 were performed on the obtained sample, and the oxidation weight gain after the oxidation test at 1000 ° C. for 1000 hours was measured. The measurement results are shown in Tables 1 and 2.

[0035]

[Table 1]

[0036]

[Table 2]

According to the results of Tables 1 and 2, samples No. 26, 33 and Y containing no Mn and / or Cu were added.
And / or sample No. 2 with a small amount of rare earth element added
In No. 8, the densification is not sufficient and both the room temperature strength and the 1200 ° C. strength are low. In addition, samples No. 27 and 32 in which W and / or Mo were not added were Mn and / or Cu.
Due to the effect, the normal temperature strength is high, but the high temperature strength is greatly deteriorated. Further, in the sample No. 29 containing a large amount of Y and / or the rare earth element added, the high temperature strength and the oxidation characteristics were significantly deteriorated. The sample No. 30 containing a large amount of W and / or Mo was deteriorated in oxidation characteristics.

In contrast to these comparative examples, all the samples of the present invention have room temperature strength of 1050 MPa or more, 1200 ° C. strength of 950 MPa or more, oxidation increase of 0.1 mg / cm ² or less after oxidation test at 1000 ° C. for 1000 hours, and room temperature. It had excellent strength from high temperature to high temperature, and also had excellent oxidation characteristics.

[0039]

As described in detail above, according to the present invention,
It has high strength from room temperature to 1200 ° C, and moreover, 1
It is possible to provide a silicon nitride sintered body having excellent oxidation characteristics at 000 ° C. Therefore, reliability can be improved by applying such a sintered body to automobile parts such as piston pins and engine valves that require high strength from room temperature to high temperatures, and parts for gas turbine engines.

Claims (2)

[Claims] 1. A main component is Si ₃ N ₄ , at least one selected from the group consisting of Y as an auxiliary component and a rare earth element in an amount of 1 to 20% by weight in terms of oxide, and Mn and / or Cu in terms of an oxide. 0.01 to 1% by weight, W and / or Mo
Is contained at a ratio of 0.1 to 5% by weight in terms of oxide. 2. Si ₃ N ₄ as a main component, Y as an auxiliary component, and at least one selected from the group of rare earth elements in an amount of 1 to 20% by weight in terms of oxide, and Mn and / or Cu in terms of oxide. 0.01 to 1% by weight, W and / or Mo
Of a silicon nitride sintered body, characterized in that a molded body containing 0.1 to 5% by weight in terms of oxide is fired at 1850 ° C. or lower in a non-oxidizing atmosphere containing nitrogen. Method.