JPH05163066A - Sintered material of siliceous nitrite - Google Patents

Abstract

PURPOSE:To obtain a sintered material having excellent high-temperature strength and excellent oxidation resistance characteristics from a low temperature to a high temperature. CONSTITUTION:A mixture comprising silicon nitride, an oxide of light rare earth element, an oxide of heavy rare earth element and silicon oxide is molded, sintered and subjected to given heat treatment to form a sintered material comprising a silicon nitride crystal phase as a main phase and a crystal phase shown by the formula RE2Si2O7 (RE; rare earth element) composed of a heavy rare earth element, a light rare earth element, silicon and oxygen at a grain boundary of the silicon nitride phase.

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 which is excellent in strength characteristics from room temperature to high temperature and is particularly used for automobile parts, gas turbine engine parts and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies are excellent in heat resistance, thermal shock resistance and oxidation resistance, and therefore, their application has been promoted for engineering ceramics, especially for heat engines such as turbochargers. ing. This silicon nitride sintered material is generally used for Y ₂ O ₃ , Al ₂ O _{3 and} silicon nitride.
Alternatively, by adding a sintering aid such as MgO, high density and high strength characteristics are obtained. Further improvement in strength and oxidation resistance at high temperature is required for such a silicon nitride-based sintered body when the operating conditions thereof further increase in temperature. In order to meet such demands, various improvements have been attempted so far, such as examination of sintering aids and improvement of firing conditions.

In view of the fact that low melting point oxides such as Al ₂ O ₃ which have been conventionally used as a sintering aid deteriorate the high temperature characteristics, rare earth elements such as Y ₂ O ₃ are used for silicon nitride. A sintered body having a simple ternary composition of elements and silicon oxide has been proposed. In addition, Y composed of Si ₃ N ₄ —RE ₂ O ₃ —SiO _{2 is} added to the grain boundary of the sintered body.
It has been proposed to increase the melting point and stabilize the grain boundaries by precipitating a crystal phase such as an AM phase, an apatite phase, a wollastonite phase, a silicon oxynitride phase, or a disilicate phase.

[0004]

However, by crystallizing the grain boundary, high temperature characteristics are improved as compared with the case where the grain boundary is amorphous, but a stable crystal phase is generated. In addition, a grain boundary phase having a low melting point is formed by a component that does not contribute to crystallization at the same time when a predetermined crystal phase is precipitated, and thus a sufficient effect due to crystallization is not obtained. Was the current situation.

Therefore, it is still insufficient for practical use of such a sintered body, and further improvement in strength and improvement in oxidation resistance are required.

Therefore, the present invention has strength properties sufficient to be used in automobile parts, gas turbine engine parts and the like from room temperature to high temperature, particularly from the viewpoint of oxidation resistance, particularly from room temperature to high temperature of 1400 ° C. It is an object of the present invention to provide a silicon nitride-based sintered body excellent in bending strength and excellent in oxidation resistance from low temperature to high temperature.

[0007]

In order to improve the strength characteristics and the oxidation resistance characteristics of the sintered body, the present inventor controls the composition of the sintered body and the subphase existing in the grain boundary of the silicon nitride phase. As a result of repeated studies based on the viewpoint that it is important to
Silicon nitride is the main component, to which light rare earth oxides and heavy rare earth oxides are added in combination, silicon oxide is added, and this is molded and fired, and then heat-treated during cooling or after cooling to give silicon oxide. And a crystalline grain boundary phase containing a rare earth element,
In particular, it is represented by RE ₂ Si ₂ O ₇ (RE: rare earth element),
By precipitating a crystalline phase in which a rare earth element is composed of a light rare earth element and a heavy rare earth element, a sintered body having excellent strength characteristics from room temperature to high temperature and further excellent oxidation resistance characteristics from low temperature to 1400 ° C. It was found that it can be obtained.

The present invention will be described in detail below.

The silicon nitride-based sintered body of the present invention is composed of silicon nitride as a main component in terms of composition, and contains a rare earth element oxide and excess oxygen as additional components. According to the present invention, it is a great feature that the rare earth element is composed of a light rare earth element and a heavy rare earth element. The light rare earth element is a lanthanide element having an atomic number of 57 to 63, specifically L
a, Ce, Nd, Sm, etc., on the other hand, the heavy rare earth element is a lanthanide element having an atomic number of 64 to 71, and an element number of 39.
Yttrium is included in heavy rare earth oxides.

The excess oxygen is an element when the rare earth element (RE) in the sintered body forms an oxide (RE ₂ O ₃ ) stoichiometrically from the total amount of oxygen in the sintered body. It is the amount of oxygen remaining excluding bound oxygen, most of which is mixed in as oxygen contained in the silicon nitride raw material or addition of SiO ₂ or the like, and in the present invention, it is assumed that all exist as SiO _2. Consider.

The silicon nitride chamber sintered body of the present invention has a silicon nitride crystal phase as a main phase structurally, and most of it is composed of β-Si ₃ N ₄ , and its grain boundaries are Although there are the above-mentioned oxides of light rare earth elements and heavy rare earth elements and excess oxygen (which is considered to exist as silicon oxide), according to the present invention, this grain boundary phase is mainly represented by RE ₂ Si ₂ O _7. It is also important to be composed of crystals. This crystal phase exists as a composite oxide of a light rare earth element compound and a heavy rare earth element compound, or exists as a solid solution of a light rare earth element in the heavy rare earth element compound. Further, this crystal phase exists as a liquid phase having a low melting point due to the reaction with silicon nitride particles during the sintering process, and enhances the sinterability, but when it remains as a glass phase in the grain boundary phase as it is after cooling, it has high temperature strength. At the same time, the oxidation resistance is deteriorated. Therefore, by precipitating the crystalline phase by a predetermined cooling process or heat treatment, it is possible to increase the high temperature strength and simultaneously improve the oxidation resistance.

Further, in order to precipitate the above crystal phase, the amount of excess oxygen in the sintered body in terms of silicon oxide (SiO ₂ ) in terms of silicon oxide (RE) in terms of oxide of the group 3a element (RE) in the periodic table (RE ₂ O ₃₎ )
The molar ratio _{(SiO 2 / RE 2 O 3} ) of 2.0 or more with respect, it is necessary that especially 2.0 to 5.0. This is because when the above ratio is less than 2.0, RE ₂ Si ₂ O is contained in the grain boundary phase.
_In addition to ₇ , a trace amount of a glass layer made of RE-Si-O-N tends to be present, which reduces high temperature strength and deteriorates oxidation resistance. In addition, even if completely crystallized, RE ₁₀ Si
_The apatite phase described by ₂ O ₂₃ N ₄ or RE ₁₀ (SiO ₄ ) ₆ N ₂ or the like and the YAM phase described by RE ₄ Si ₂ O ₇ N ₂ are precipitated to deteriorate the oxidation resistance. This is because it ends up.

According to the present invention, RE ₂ S is mainly formed at grain boundaries.
i ₂ O ₇ crystals are deposited, but the molar ratio (SiO ₂ / R
When the E ₂ O ₃ ) is 2.0 or more and about 2.5 or less, only the RE ₂ Si ₂ O ₇ crystal precipitates in the crystal phase, but when the molar ratio is more than 2.5, other than the RE ₂ Si ₂ O ₇ crystal. Although a slight amount of Si ₂ N ₂ O may be precipitated, there is no problem from the viewpoint of oxidation resistance. However, when a crystal phase mainly composed of Si ₂ N ₂ O is precipitated, there arises a problem that the fracture toughness value is lowered. Therefore, the molar ratio is preferably 2.0 to 2.5.

According to the silicon nitride sintered body of the present invention, the presence of a low-melting metal oxide such as Al ₂ O ₃ , MgO, or CaO hinders the crystallization of the grain boundaries and increases the high temperature strength. The total amount of these oxides is 0.5 for deterioration.
It is desirable to control the content to be not more than weight%.

Next, a method of manufacturing a silicon nitride sintered body according to the present invention will be described. First, silicon nitride powder is used as a raw material powder as a main component, and light rare earth element oxide powder and heavy rare earth element oxide are added as additive components. In addition to adding the powder of the oxides and the powder of silicon oxide, it is also possible to use the compound powder of the rare earth element oxide and silicon oxide or the compound powder of the silicon nitride, the rare earth element oxide and the silicon oxide as an additive component. it can.

The silicon nitride powder used is itself α-Si.
Either ₃ N ₄ or β-Si ₃ N ₄ may be used, and their particle size is preferably 0.4 to 1.2 μm.

According to the invention, using these powders,
70 to 97 mol% of silicon nitride, 3 to 30 mol% of total of light rare earth element oxide, heavy rare earth element oxide, and silicon oxide
In, SiO ₂ of the total amount of light rare earth element oxides and heavy rare earth element oxide and (RE ₂ O _3), silicon oxide (SiO ₂₎
Prepare and mix so that the molar ratio represented by / RE ₂ O ₃ is 2.0 or more. At this time, the amount of silicon oxide (SiO ₂ ) means the amount of impurity oxygen contained in the silicon nitride powder converted into SiO ₂ and the added silicon oxide powder, or the amount of silicon in the silicon-containing compound converted into silicon oxide. It is the total amount.

The mixed powder thus obtained was molded into a desired shape by a known molding method such as press molding, cast molding, extrusion molding, injection molding, cold isostatic molding, and then obtained. A known firing method for the molded body, for example, a hot pressing method, normal pressure firing, nitrogen gas pressure firing,
Further, HIP calcination after these calcinations, glass seal HIP calcination and the like are carried out to obtain a dense sintered body.
If the firing temperature at this time is too high, the silicon nitride crystal grains grow and the strength decreases, so that the firing temperature is 1900 ° C. or less, particularly 16 ° C.
A nitrogen gas-containing non-oxidizing atmosphere at 50 to 1850 ° C. is desirable.

Next, after firing, heat treatment is performed in the cooling process, or the obtained sintered body is heat treated in a non-oxidizing atmosphere. As the heat treatment method, first, the softening temperature Tg of the glass formed at the grain boundaries of the sintered body, silicon nitride and RE ₂ S
i ₂ O ₇ was once held during the crystallization temperature Tc, and RE ₂
Si ₂ O ₇ crystal nuclei are generated. Thereafter, a crystallization temperature Tc, by performing heat treatment by growing the crystal nuclei of RE ₂ Si ₂ O ₇ held between the eutectic temperature Te of the silicon nitride crystal and RE ₂ Si ₂ O ₇ crystals, RE ₂ Generation of Si ₂ O ₇ crystals can be promoted and generation of an amorphous layer existing at the interface can be suppressed.

The softening temperature Tg, the crystallization temperature Tc, and the eutectic temperature Te are determined by firing in the same manner as described above, followed by rapid cooling to room temperature and sintering in which the grain boundary phase is a glass phase. A body is produced, thin pieces are cut out from this sintered body, and the glass composition of this grain boundary phase is determined by the UTW-EDX method using an analytical electron microscope. Next, after molding and firing the mixed powder adjusted to have the same composition as this glass composition,
The glass is rapidly cooled to prepare a glass, and the softening temperature Tg and crystallization temperature Tc of this glass can be determined by the DTA method. Furthermore, a mixed powder of silicon nitride powder and RE ₂ Si ₂ O ₇ powder can be used to perform both Temperature T on the tie line of
e can be obtained.

According to experiments conducted by the present inventors, the softening temperature Tg of a glass composed of various silicon nitrides and RE ₂ Si ₂ O ₇ is about 950 ° C., and the crystallization temperature Tc is about 1100 ° C. Further, the eutectic temperature Te is a temperature around 1550 ° C. Therefore, as the heat treatment temperature, the temperature of the first step is set to around 1050 ° C and the temperature of the second step is set to 1400 ° C.
It is preferable to set them before and after.

[0022]

The major factor that determines the characteristics of the silicon nitride sintered material is the composition and structure of the grain boundaries in the sintered material. It is important that the grain boundaries are crystallized in order to have high strength at high temperature, but it is generally difficult to completely crystallize the glass layer at the grain boundaries. The presence of a large amount of amorphous layer at the grain boundary leads to deterioration of high temperature strength, and also because the solid solution of nitrogen in the amorphous layer reduces the oxidation resistance of the sintered body.

According to the present invention, RE ₂ Si is incorporated into this grain boundary phase.
By precipitating a crystal phase represented by ₂ O ₇ , strength deterioration at room temperature to high temperature can be reduced, and excellent oxidation resistance from room temperature to high temperature can be imparted.

Further, among the rare earth elements forming the grain boundary crystal phase, the light rare earth element has a larger ionic radius than the heavy rare earth element, so that it exists in the silicon nitride raw material and is mixed with silicon nitride at the crystallization stage. It has the property of easily dissolving the impurity element that remains in the amorphous phase generated at the interface and reduces the high temperature strength. However, there is a problem that the oxidation resistance is low only with light rare earth elements, and there is a problem that the high temperature strength is insufficient only with heavy rare earth elements. Therefore, by combining a heavy rare earth element oxide and a light rare earth element, the above problems can be solved and the high temperature strength can be increased.

From this point of view, the light rare earth element (REl) and the heavy rare earth element (REh) preferably have a molar ratio represented by REh / REl of 1 or more.

[0026]

EXAMPLE A silicon nitride powder (BET specific surface area 9 m ² / g, α ratio 98%, oxygen amount 1.2% by weight) was used as a raw material powder,
A heavy rare earth element oxide (REh) powder, a light rare earth element oxide (REl) powder, and a silicon oxide powder were mixed to have a composition shown in Table 1, and then molded with 1 t / cm ² .

These moldings were placed in a silicon carbide jar and the atmosphere was controlled to reduce the composition fluctuation.
Firing was performed in a nitrogen gas stream at atmospheric pressure at 1850 ° C. for 4 hours. Furthermore, for samples other than sample No. 13, 12 hours at 1050 ° C. in nitrogen at 1 atm, followed by 1400
Heat treatment was performed at 24 ° C. for 24 hours.

The obtained sintered body was ground to a shape specified in JIS-R1601 to prepare a sample. Specific gravity measurement based on Archimedes method, JIS-
A four-point bending transverse strength test was carried out at room temperature and 1000 ° C. based on R1601. Further, the sample was exposed to 900 ° C. air or 1400 ° C. air for 100 hours, and the weight change per unit surface area was determined from the weight increase amount and the surface area of the sample. For the composition of the sintered body, a sample was crushed, the oxygen amount was finally converted into CO ₂ and quantified by an infrared absorption method, and the nitrogen amount was measured by thermal conductivity to determine that silicon and elements of Group 3a of the periodic table emit light. It was determined by spectroscopic analysis. The results are shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

According to the results shown in Tables 1 and 2, No. 1 and No. 1 in which the light rare earth element and the heavy rare earth element are not compounded.
No. 7, No. 19 was slightly insufficient in densification, and the high temperature strength or oxidation resistance was further lowered. In addition, SiO ₂ / R
The samples No. 14 and No. 15 having E ₂ O ₃ of less than 2 had deteriorated oxidation resistance. Even if SiO ₂ / RE ₂ O ₃ is 2 or more, the grain boundary phase is not crystallized into RE ₂ Si ₂ O ₇ .
No. 13 had deteriorated high temperature strength.

In contrast to these comparative examples, the sintered bodies of the present invention all showed excellent bending strength and oxidation resistance.

[0033]

As described in detail above, according to the silicon nitride sintered body of the present invention, by precipitating the disilicate crystal phase having two or more kinds of rare earth elements, the strength and oxidation resistance at high temperature can be improved. Can be increased.

Claims (2)

[Claims] 1. A crystal which has a silicon nitride crystal phase as a main phase and is composed of heavy rare earth elements, light rare earth elements, silicon and oxygen at its grain boundaries and is represented by RE ₂ Si ₂ O ₇ (RE: rare earth element). A silicon nitride sintered body containing a phase. 2. An oxide conversion amount of all rare earth elements (RE
₂ O ₃ ) and silicon oxide (Si) of excess oxygen in the sintered body.
The silicon nitride sintered body according to claim 1, wherein the molar ratio represented by SiO ₂ / RE ₂ O ₃ with respect to the O ₂ ) conversion amount is 2.0 or more.