JPH0769735A - Silicon nitride sintered body - Google Patents

Abstract

(57) [Summary] [Structure] Si ₃ N ₄ as a main component, Y as an auxiliary component,
The sum of Sm, Al and excess oxygen, which is the equivalent amount of Y, Sm and Al oxides, and the excess oxygen, which is equivalent to SiO _2, is 5
In an amount of about 25 mol% and Mg in an amount of 0.1 to 15 mol% in terms of oxide, preferably Y ₂ of Y.
O ₃ equivalent, Sm Sm ₂ O ₃ equivalent, Al Al ₂ O
₃ converted amount and SiO ₂ converted amount of excess oxygen SiO ₂ /
The molar ratio represented by (Y ₂ O ₃ + Sm ₂ O ₃ ) is 1 to 7,
The molar ratio represented by Al ₂ O ₃ / (Y ₂ O ₃ + Sm ₂ O ₃ ) is 0.25 to 4, and the molar ratio represented by Y ₂ O ₃ / Sm ₂ O ₃ is 0.3 to _3. The composition is satisfactory. [Effect] High densification can be achieved at a low temperature of about 1700 ° C, and high strength and toughness can be obtained from room temperature to a high temperature of 1000 ° C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has excellent strength characteristics and toughness from room temperature to high temperature, and is particularly useful for automobile parts and gas turbines.
The present invention relates to a silicon nitride-based sintered body used for a bin engine part or the like.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been applied to engineering ceramics, especially for heat engines because of their excellent heat resistance, thermal shock resistance and oxidation resistance.

Since the silicon nitride itself is difficult to sinter, this silicon nitride sintered material has been densified by blending various sintering aids and firing. Examples of the sintering aid used include Y and oxides of rare earth elements (RE ₂
O ₃ ), aluminum oxide (Al ₂ O ₃ ), magnesium oxide (MgO), silicon oxide (SiO ₂ ), etc. are used as an auxiliary agent for easily firing at a comparatively low temperature.
It is also possible to use Y ₂ O ₃ and Al ₂ O ₃ together.
No. 2-3649, Japanese Patent Publication No. 58-5190.

[0004]

However, when Y ₂ O ₃ and Al ₂ O ₃ are used as the sintering aids, it is very effective in increasing the sinterability, but it is difficult to improve the sinterability from room temperature to 1000.
The strength up to a high temperature of ℃ is only about 500 MPa,
It has a drawback that practical strength cannot be obtained when it is used for an engine valve or the like.

Therefore, the present invention provides nitriding excellent in strength characteristics and toughness sufficient to be used in automobile parts, gas turbine engine parts and the like from room temperature to high temperature, and particularly in bending strength from room temperature to high temperature. It is intended to provide a silicon-based sintered body.

[0006]

[Means for Solving the Problems] As a result of intensive studies on the above problems, the present inventor has used two kinds of rare earth element oxides, yttrium oxide and samarium oxide, and has a specific amount of magnesium oxide. It has been found that when added, a sintered body having excellent strength characteristics is obtained.

That is, the silicon nitride sintered body of the present invention is made of Si
₃ N ₄ as a main component, Y, Sm, Al and excess oxygen as auxiliary components, and the sum of the oxide conversion amount of the Y, Sm and Al and the SiO ₂ conversion amount of the excess oxygen is 5 to ₂
It is contained in an amount of 5 mol%, and Mg is converted into oxide,
It is characterized in that it is contained in a proportion of 0.1 to 15 mol%, preferably Y ₂ O ₃ conversion amount of Y, Sm ₂ O of Sm.
₃ converted amount, Al ₂ O ₃ converted amount and excess oxygen S
The molar ratio represented by SiO ₂ / (Y ₂ O ₃ + Sm ₂ O ₃ ) in terms of iO ₂ is 1 to 7, and Al ₂ O ₃ / (Y ₂ O ₃ + S).
m ₂ O ₃ ) has a molar ratio of 0.25 to 4, Y ₂ O ₃
The molar ratio represented by / Sm ₂ O ₃ satisfies 0.3 to ₃ .

The present invention will be described in detail below. The sintered body of the present invention contains silicon nitride as a main component and Y, S as an auxiliary component.
It contains m, Al and Mg as essential components and further contains excess oxygen. Here, excess oxygen means Y ₂ O ₃ , Sm ₂ O ₃ , Al ₂ O ₃ among all oxygen in the sintered body, and the rest except oxygen which is stoichiometrically bonded to each element as MgO. an oxygen amount, oxygen contained in the silicon nitride raw material or is intended to be mixed as an additive such as SiO _2, in the present invention thought of as all present as SiO _2.

According to the present invention, in the content of these auxiliary components, the sum of the oxide conversion amount of Y, Sm and Al and the SiO ₂ conversion amount of the excess oxygen is 5 to 25 mol%, particularly 10 to 20 mol%, Mg in terms of oxide is 0.
It is important to contain 1 to 15 mol%, particularly 0.5 to 5 mol%. This is because if the total amount of Y, Sm, Al and excess oxygen is less than 5 mol%, the liquid phase will be insufficient in the sintering process, and high temperature firing will be required to obtain a dense body. Growth occurs and causes a decrease in strength. On the other hand, if the total amount is more than 25 mol%, the grain boundary phase will increase in the sintered body, resulting in deterioration of high temperature strength and deterioration of oxidation resistance at high temperature. If the amount of Mg is less than 0.1 mol%, the effect of adding MgO cannot be obtained and the toughness becomes low.
If it is more than 5 mol%, the sinterability is impaired and a dense body cannot be obtained.

Further, according to the present invention, Y ₂ O ₃ in terms of Y, Sm ₂ O ₃ in terms of the amount of Sm, molar ratio of Al ₂ O ₃ in terms of weight and SiO ₂ equivalent amount of excess oxygen of Al SiO _{2 / (Y 2 O 3 +} Sm 2 O 3): 1~7,
Especially 1.3 to 2.3, Al ₂ O ₃ / (Y ₂ O ₃ + Sm ₂ O ₃ ): 0.25 to
4, particularly 0.3 to 1.3 Y ₂ O ₃ / Sm ₂ O ₃ : 0.3 to ₃ , particularly 0.5 to 2 is desirable.

This is SiO ₂ / (Y ₂ O ₃ + Sm ₂ O
₃ ) When the ratio is less than 1, the densification is insufficient, and when it exceeds 7, the densification is similarly insufficient. (If it is out of the above range, the liquid phase will be insufficient in the sintering process, and high temperature firing will be required to obtain a dense body, which causes the growth of silicon nitride particles and causes a decrease in strength). Also, A
When l ₂ O ₃ / (Y ₂ O ₃ + Sm ₂ O ₃ ) is less than 0.25, the liquid phase is insufficient in the sintering process, and high-temperature firing is necessary to obtain a dense body. Growth occurs and causes a decrease in strength. When it is larger than 4, the high temperature strength is deteriorated and the oxidation resistance property at high temperature is deteriorated. Further, if the Y ₂ O ₃ / Sm ₂ O ₃ ratio is less than 0.3, the densification will be insufficient, and if it exceeds 3, the oxidation resistance toughness will be deteriorated and the densification will be insufficient.

Further, the silicon nitride sintered material of the present invention is also characterized in that the silicon nitride crystal particles are constituted by a fine structure based on the combination of the above-mentioned additional components. The silicon nitride crystals form columnar crystals, and according to the sintered body of the present invention, the columnar crystals have an average minor axis of 0.4 to 0.8 μm and an aspect ratio (major axis / minor axis) of 3 to 10 It consists of

As a method for producing the silicon nitride sintered material of the present invention, the raw material powder is silicon nitride powder, Y ₂ O.
₃ powder, Sm ₂ O ₃ powder, Al ₂ O ₃ powder, MgO powder, and optionally SiO ₂ powder are used. Using these powders, the total amount of Y ₂ O ₃ , Sm ₂ O ₃ , Al ₂ O ₃ and excess oxygen in terms of SiO ₂ is 5 to 25 mol%, particularly 10 to 10.
20 mol%, MgO 0.1-5 mol%, especially 0.5-
Prepare and mix so as to be 5 mol%. At this time, the excess oxygen (SiO ₂ ) changes the oxygen contained in the silicon nitride powder to Si.
It is the total amount of the O ₂ converted amount and the added SiO ₂ powder. More preferably, SiO ₂ /
(Y ₂ O ₃ + Sm ₂ O ₃ ) is 1 to 7, Al ₂ O ₃ / (Y
₂ O ₃ + Sm ₂ O ₃ ) is 0.25 to 4, Y ₂ O ₃ / Sm
Weigh and mix so that ₂ O ₃ is 0.3 to ₃ . In addition,
Silicon nitride is 75 to 95 mol%, particularly 80 to 90 mol%.
Blended in the amount of.

The mixed powder thus obtained is molded into a desired shape by a known molding method, for example, press molding, cast molding, extrusion molding, injection molding, cold isostatic molding or the like. Then, the obtained molded body is subjected to a known firing method, for example, a hot pressing method, atmospheric pressure firing, nitrogen gas pressure firing, and further, HIP firing after these firing, and
It is fired by glass seal HIP firing or the like to obtain a dense sintered body.

The silicon nitride sintered material according to the present invention contains 1500 to 1750 by using the above-mentioned specific additive component.
A high density body having a relative density of 97% or more can be obtained by performing the treatment at a relatively low temperature of ℃ for 5 to 20 hours.

[0016]

[Function] In order to produce a silicon nitride-based dense sintered body and obtain a silicon nitride-based sintered body excellent in bending strength from room temperature to high temperature, the shape of the main crystal phase of silicon nitride and It is necessary to control the composition and amount of the grain boundary phase composed of silicon nitride and additive components.

The silicon nitride crystal grains are columnar crystals and have a structure structure in which they are entangled with each other. As one of the fracture mechanisms of the sintered body, the silicon nitride crystal grains with abnormal grain growth are the starting points and fracture occurs. It has been known. Therefore, in order to obtain high mechanical strength, it is necessary to suppress the occurrence of such abnormal grain growth.

According to the present invention, Y ₂ O ₃ and Sm ₂ O ₃ are added as rare earth element oxides as additive components, and Al ₂ O is added.
_{When 3} is used in combination, the melting point of the liquid phase generated in the sintering process is lowered and low temperature firing becomes possible, so that the grain growth of silicon nitride particles can be suppressed.

By firing at a low temperature, the growth of silicon nitride crystals can be suppressed, but the columnar crystals themselves of the silicon nitride crystals become small, so that a structure in which the columnar crystals are entangled with each other is difficult to form. Therefore, the toughness tends to decrease. In the present invention, Y ₂ O ₃ and Sm are added as additive components.
By adding an appropriate amount of MgO in addition to ₂ O ₃ , the aspect ratio (major axis / minor axis) of the columnar crystals is increased, and thereby toughness can be increased.

As described above, according to the structure of the present invention, since a sintered body made of fine silicon nitride crystal grains having a large aspect ratio and free from abnormal grain growth is obtained, it has high strength and high toughness. It is possible to obtain a highly reliable silicon nitride sintered body.

[0021]

EXAMPLES Silicon nitride powder (BET specific surface area 12 m ² / g, α ratio 95%, oxygen amount 1.2% by weight) as raw material powder, Y ₂ O ₃ powder, Sm ₂ O ₃ powder, SiO ₂ powder, Al ₂
O ₃ powder and MgO powder were used to prepare the composition shown in Table 1 and mixed, and then a gold-forming mold was formed at 1 t / cm ² .

The molded body thus obtained was placed in a silicon carbide mortar and held at 1600 ° C. for 10 hours in nitrogen at atmospheric pressure using a carbon heater, after which the temperature was raised to 1700 ° C. and further 1
After holding for 0 hour, the furnace was cooled to obtain a sintered body.

The obtained sintered body was ground to a shape shown in JIS R1601. A specific gravity measurement based on the Archimedes method, a four-point bending transverse strength test at room temperature and 1000 ° C. based on JISR1601, and a fracture toughness were measured. Regarding the silicon nitride crystal structure of the obtained sintered body, the average minor axis and the average aspect ratio of each crystal observed from an electron micrograph (3000 times) were determined. The measurement results are shown in Table 2.

[0024]

[Table 1]

[0025]

[Table 2]

According to the results of Table 1, SiO ₂ , Al ₂ O
₃ , in the system to which MgO was added, Sample Nos. 9 and 10 each consisting of only one rare earth element oxide had a large grain size and an aspect ratio smaller than that of the product of the present invention. It was lower than the product. Further, in the sample No. 1 containing a small amount of additive, the sintering was insufficient,
Both room temperature strength and high temperature strength are low. On the contrary, in samples Nos. 17 and 18 in which the amount of the additive was excessive, the high temperature strength was deteriorated. Furthermore, sample N containing no MgO as an additive
In No. 12, the effect of improving toughness was insufficient, and in Sample No. 2 in which the amount of MgO was excessive, strength and toughness deteriorated.

In contrast to these comparative examples, the samples according to the present invention were all room temperature strengths of 1100 MPa or more and 1000
℃ strength 790MPa or more, fracture toughness 7.0MPa ・ m
^It had excellent mechanical properties of ^1/2 or more.

[0028]

As described in detail above, according to the present invention,
A sintered body suitable for parts used at high temperatures such as turbo rotors, which can achieve high density at a low temperature of about 1700 ° C, has high strength from room temperature to a high temperature of 1000 ° C, and can increase toughness. Can be provided.

Claims (2)

[Claims] 1. A main component of Si ₃ N ₄ , Y, Sm, Al and excess oxygen as auxiliary components, and the amount of Y, Sm and Al converted into oxide and the amount of excess oxygen converted into SiO _2. Is contained in an amount of 5 to 25 mol%, and Mg is contained in a ratio of 0.1 to 15 mol% in terms of oxide. Wherein Y ₂ O ₃ in terms of Y, Sm ₂ O ₃ in terms of the amount of Sm, SiO the terms of Al ₂ O ₃ amount and the excess oxygen in the Al
₂ in terms of SiO ₂ / mole ratio represented by _{(Y 2 O 3 + Sm 2} O 3) is _{1~7, Al 2 O 3 / (} Y 2 O 3 + Sm 2
Molar ratio represented by O ₃₎ is 0.25~4, Y ₂ O ₃ / S
The silicon nitride sintered body according to claim 1, wherein the molar ratio represented by m ₂ O ₃ satisfies 0.3 to ₃ .