JP2004010382A - Surface-coated silicon nitride sintered compact - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface-coated silicon nitride sintered compact having a coating layer with high adhesive strength and excellent in corrosion resistance. <P>SOLUTION: The surface coated silicon nitride sintered compact is composed of a substrate of a silicon nitride-based sintered compact including RE<SB>2</SB>Si<SB>2</SB>O<SB>7</SB>and/or RE<SB>2</SB>SiO<SB>5</SB>(wherein RE is a rare earth element) as a grain boundary crystalline phase and a coating layer formed on the surface of the substrate. The coating layer is comprised of a crystalline phase of RE<SB>2</SB>Si<SB>2</SB>O<SB>7</SB>and/or RE<SB>2</SB>SiO<SB>5</SB>(wherein RE is a rare earth element). The coating layer contains at least two rare earth elements, one of which is the same as the rare earth element composing the grain boundary crystalline phase of the substrate. <P>COPYRIGHT: (C)2004,JPO

Description

【００５１】
【表２】

【００５２】
本発明の試料Ｎｏ．１〜１２及び１７〜２９は、耐エロージョン性が１０％以下で、熱サイクル試験で１００回以上クラックの発生が無く、曝露試験での減肉量が１０μｍ以下であった。
【００５３】
一方、被覆層がＲＥ_２Ｓｉ_２Ｏ_７及び／又はＲＥ_２ＳｉＯ_５以外である本発明の範囲外の試料Ｎｏ．１３〜１６は曝露試験による減肉量が２００μｍ以上であった。
【００５４】
また、窒化珪素質焼結体基板の粒界相に含んでいる希土類元素を被覆層に含まない本発明の範囲外の試料Ｎｏ．３０〜３３は耐エロージョン性が１５％以上で熱サイクル試験９０回以下でクラックが発生した。
【００５５】
【発明の効果】
本発明の表面被覆窒化珪素質焼結体は、ＲＥ_２Ｓｉ_２Ｏ_７及び／又はＲＥ_２ＳｉＯ_５（ＲＥは希土類元素）を粒界結晶相として含む窒化珪素質焼結体からなる基材と、該基材表面に形成された被覆層とから構成された表面被覆窒化珪素質焼結体において、該被覆層がＲＥ_２Ｓｉ_２Ｏ_７及び／又はＲＥ_２ＳｉＯ_５（ＲＥは希土類元素）の結晶相からなり、且つ該被覆層に少なくとも２種類の希土類元素を含むとともに、該２種類の希土類元素のうち一方が、前記基材の粒界結晶相を構成する希土類元素と同一することにより、耐食性が高く、耐エロージョン性も高く長期信頼性の高い表面被覆窒化珪素質焼結体を実現できる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a surface-coated silicon nitride sintered body having excellent strength properties from room temperature to high temperature and excellent fracture toughness and oxidation resistance, and particularly relates to a turbine rotor, a turbine blade, a nozzle, a combustor, a scroll, The present invention relates to a surface-coated silicon nitride sintered body suitably used for a gas turbine engine component such as a nozzle support, a seal ring, a spring ring, a diffuser, a duct, and a shroud.
[0002]
[Prior art]
Conventionally, silicon nitride based sintered bodies have been applied to engineering ceramics, particularly as heat engine structural materials, because of their excellent strength, hardness and thermal chemical stability. Such a silicon nitride-based sintered body is generally manufactured by adding a sintering aid such as Y ₂ O ₃ , Al ₂ O _3, or MgO to silicon nitride powder and firing it. By using such a sintering aid, high density and high strength characteristics are obtained. Such a silicon nitride sintered body is used, for example, as an engine component. However, as the operating conditions of the engine component increase, the silicon nitride sintered body further increases in strength and oxidation resistance at high temperatures. Improvement is required.
[0003]
In response to such demands, improvements have been made with a focus on improvement of sintering aids, grain boundary phases, firing conditions, and the like, and formation of an oxide protective film on the surface of a sintered body.
[0004]
For example, in Japanese Patent Application Laid-Open No. 9-183676, the mechanical strength and oxidation resistance at high temperatures are improved by coating the surface of a sintered body mainly composed of silicon nitride or sialon with a glass layer mainly composed of SiO _2. It is proposed to improve.
[0005]
However, the method of forming a glass layer on the surface as described in JP-A-9-183676 has an effect of improving characteristics under static conditions, but is exposed to a high-temperature, high-pressure, high-speed gas in an actual engine. Then, the glass layer is rapidly consumed due to evaporation of SiO ₂ , the life of the glass layer is short, and there is a problem that the glass layer does not use a protective film.
[0006]
Therefore, the present applicant first forms a coating layer having a crystalline phase such as disilicate or monosilicate having excellent oxidation resistance on the surface of the silicon nitride sintered body, thereby causing peeling or cracking of the coating layer. It is suggested that can be prevented.
[0007]
[Problems to be solved by the invention]
However, due to recent diversification and high performance of engines, the combustion temperature of gas has been increasing year by year, and thermal stress is generated due to the temperature distribution at the time of starting and stopping, and collision of minute contaminants in the combustion gas atmosphere. The peeling of the coating layer due to cracks or the like is a problem. In order to solve such a problem, a coating layer having a strong adhesive force is required.
[0008]
Accordingly, an object of the present invention is to provide a surface-coated silicon nitride-based sintered body having a coating layer having high adhesion and excellent corrosion resistance.
[0009]
[Means for Solving the Problems]
In the present invention, two kinds of rare earth elements are present in the coating layer as a crystal phase of disilicate or monosilicate, and one of the rare earth elements is a disilicate or a grain boundary crystal phase of a silicon nitride sintered body as a base material. It is based on a novel finding that the inclusion of a coating layer on a substrate can be enhanced by including it in a monosilicate, and in particular, a disilicate or monosilicate composed of two rare earth elements is mixed in the coating layer. In addition, the crystal phase of one rare earth element should be present almost continuously from the grain boundary of the silicon nitride based sintered body of the base material to the surface of the coating layer, and in particular, only the coating layer should contain the crystal phase of the specific rare earth element. In addition to the high adhesive force, wear due to fine particles is suppressed.
[0010]
That is, the surface-coated silicon nitride-based sintered body of the present invention is composed of a silicon nitride-based sintered body containing RE ₂ Si ₂ O ₇ and / or RE ₂ SiO ₅ (RE is a rare earth element) as a grain boundary crystal phase. In a surface-coated silicon nitride sintered body composed of a material and a coating layer formed on the surface of the base material, the coating layer is made of RE ₂ Si ₂ O ₇ and / or RE ₂ SiO ₅ (RE is a rare earth element). ) And the coating layer contains at least two rare earth elements, and one of the two rare earth elements is the same as the rare earth element constituting the grain boundary crystal phase of the base material. In particular, the coating layer is preferably made of A ₂ Si ₂ O ₇ and / or A ₂ SiO ₅ and B ₂ Si ₂ O ₇ and / or B ₂ SiO ₅ (A and B are rare earth elements; B).
[0011]
In particular, it is preferable that at least one of the rare earth elements contained in the coating layer is different from the rare earth element contained in the grain boundary crystal phase of the base material. Thereby, the consumption by the fine particles can be effectively suppressed.
[0012]
Preferably, the RE of the coating layer is at least one of Lu, Yb, and Er. Thereby, the chemical bonding force of the coating layer at a high temperature can be sufficiently increased, and the generation of cracks due to minute flying objects in the gas and the generation of cracks due to thermal stress can be suppressed.
[0013]
Further, the amount of excess SiO ₂ contained in the coating layer is preferably 5 mol% or less. Thereby, the corrosion resistance of the coating layer can be further improved.
[0014]
Further, it is preferable that RE in the grain boundary phase of the silicon nitride based sintered body is at least one of Lu, Yb, and Er. As a result, the melting point and softening point of the grain boundary phase at a high temperature can be increased, the high-temperature strength of the substrate can be increased, and the coefficient of thermal expansion of the substrate can be reduced. Strength can be increased.
[0015]
Further, the base material contains 70 to 99 mol% of silicon nitride, 0.5 to 10 mol% of a rare earth element (RE) in terms of oxide, and excess oxygen by the following formula:
SiO ₂ / RE ₂ O ₃
In the formula, SiO ₂ indicates an excess oxygen amount in conversion,
RE ₂ O ₃ indicates a rare earth element content in terms of oxide.
Is preferably 2 or more.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The surface-coated silicon nitride-based sintered body of the present invention has a crystal phase of a rare earth element (RE), that is, RE ₂ Si ₂ O ₇ (disilicate), on a surface of a substrate made of the silicon nitride-based sintered body.
Or RE ₂ SiO ₅ (monosilicate)
It is important that a crystal phase of the compound represented by the following formula is formed.
[0017]
The coating layer formed of RE ₂ Si ₂ O ₇ (disilicate) or RE ₂ SiO ₅ (monosilicate) is made of conventional SiO ₂ , ZrO ₂ , Al ₂ O ₃ , mullite, cordierite, YAG or the like. Compared with the protective film, it is very stable even in a high-temperature oxidizing atmosphere, so that excellent corrosion resistance is exhibited. Further, since the melting point is as high as 1600 to 1800 ° C., the heat resistance is excellent, and the life at a high temperature is long. In particular, it is preferable to use disilicate in terms of excellent high-temperature stability and corrosion resistance.
[0018]
Further, in the present invention, the silicon nitride sintered body of the base material contains RE ₂ Si ₂ O ₇ and / or RE ₂ SiO ₅ (RE is a rare earth element) as a grain boundary crystal phase and the rare earth element contained in the coating layer. It is important that at least two of these are included, and one of them is included as a grain boundary crystal phase of the base material. Thereby, since the crystal grain boundary phase and the coating layer of the base material of the same rare earth element exist continuously with very high consistency, the adhesion between the base material and the coating layer can be increased.
[0019]
The crystal phase of the coating layer may contain a composite oxide containing two kinds of rare earth elements, but the composite oxide is regarded as one kind, and the two kinds of rare earth elements are each made of disilicate or monosilicate, It must be mixed. That, A, and B and rare earth elements, A, when B is different elements from each other, rather than ABSi ₂ O ₇ and / or ABSiO composite oxide such as _5, A 2 _Si 2 _{O 7} and / or _{A 2} It is preferable that SiO ₅ and B ₂ Si ₂ O ₇ and / or B ₂ SiO ₅ are mixed.
[0020]
Alternatively, it is sufficient that ABSi ₂ O ₇ and A ₂ Si ₂ O ₇ , or ABSi ₂ O ₇ and B ₂ SiO ₇ are each present as a crystal phase in the coating layer, and further, AB ₂ Si ₂ O ₇ and / or A composite oxide such as a composite oxide such as AB ₂ SiO ₅ may be present in the coating layer together with A ₂ Si ₂ O ₇ and B ₂ SiO ₅ .
[0021]
Further, by making at least one of the two or more crystals different from the grain boundary crystal phase of the base material, a residual stress is developed on the film surface, and the generation of cracks due to the impact of flying fine particles is suppressed. However, the effect of suppressing wear and extending the life can be enhanced.
[0022]
Further, in the present invention, the amount of excess SiO ₂ contained in the coating layer is 5 mol% or less, particularly preferably 3 mol% or less, further preferably 1 mol% or less, most preferably 0.5 mol% or less. This is very important.
[0023]
That is, according to the present invention, since the crystal phase of RE ₂ Si ₂ O ₇ (disilicate) or RE ₂ SiO ₅ (monosilicate) itself has very excellent corrosion resistance, the coating layer has excellent corrosion resistance. Have. However, even if the corrosion resistance of such a crystal phase is good, this coating layer is made of a polycrystalline material, and its crystal grain boundary exists, and does not contribute to the crystal phase at this crystal grain boundary. When impurity oxygen (excess oxygen) exists as SiO ₂ , the SiO ₂ (that is, excess SiO ₂ ) reacts with water vapor or the like to vaporize, and is flown by the combustion gas, and as a result, the grain boundary phase falls off, The coating layer is easily broken, and the silicon nitride-based sintered body is easily corroded via the grain boundary phase that has become a void.
[0024]
Therefore, according to the present invention, by reducing the amount of excess SiO _{2 in the} grain boundary phase due to the excess oxygen to the above range, the generation of such a grain boundary phase can be suppressed, and as a result, the excess amount of the crystal in the coating layer passes through the grain boundaries. Diffusion of oxygen or water vapor can be prevented.
[0025]
The lower limit of the excess SiO ₂ amount is most desirably 0. However, if the SiO ₂ amount in the entire coating layer is smaller than the stoichiometric composition of disilicate or monosilicate, YAM other than disilicate or monosilicate may be used. In some cases, a nitrogen-containing crystal phase such as iron or apatite is precipitated. Therefore, more preferably, when the coating layer is composed of only disilicate, the SiO ₂ / RE ₂ O ₃ ratio (molar ratio) in the coating layer is preferably 1.9 to 2.3. Is preferably 0.9 to 1.2 when is composed of only monosilicate, and when the coating layer is a mixed crystal of disilicate and monosilicate, the SiO ₂ / RE ₂ O ₃ ratio ( (Molar ratio) is preferably in the range of 0.9 to 2.3.
[0026]
In the present invention, the rare earth element which is a component of the crystal phase is a Group 3a element of the periodic table, and specifically, Y, Lu, Yb, Er, Dy, Ho, Sm, Tb, Sc, Gd and Tm and the like. Among these, Lu, Yb, and Er are preferred.
[0027]
This is because the ionic radius of Lu, Yb, and Er is small and the bonding force between the ions is large, so that the disilicate phase and the monosilicate phase have high melting points. Because of these properties, the disilicate and monosilicate phases of Lu, Yb, and Er can provide excellent high-temperature strength, high hardness and high toughness, and have high chemical stability at high temperatures. realizable.
[0028]
In the present invention, it is preferable that the substrate having the coating layer on the surface is a silicon nitride sintered body having a crystal phase at a grain boundary of silicon nitride crystal grains as a main phase.
[0029]
That is, even if the coating layer is provided on the surface of the silicon nitride sintered body as the base material, oxygen diffuses in the coating layer for a long period of time, reaches the silicon nitride sintered body, and the sintered body is If oxidized, its mechanical properties may deteriorate. For example, a silicon nitride crystal changes to silicon dioxide, which causes mechanical property deterioration. However, the presence of a crystal phase at the grain boundary of the silicon nitride crystal in the silicon nitride sintered body can protect the silicon nitride crystal from oxidation and corrosion. For example, the oxidation rate of the silicon nitride sintered body can be reduced. It slows down and effectively controls the decrease in mechanical properties due to oxidation due to the diffusion of oxygen into the sintered body, thereby maximizing the properties of the silicon nitride sintered body and dramatically increasing its life. Can be extended.
[0030]
Further, the crystal phase present at the grain boundary of the silicon nitride crystal is preferably a crystal composed of, for example, a rare earth element (RE), Si (silicon), and O (oxygen). Similarly, it is desirable to be a disilicate phase or a monosilicate phase represented by the chemical formula: RE ₂ Si ₂ O ₇ or RE ₂ SiO ₅ . That is, by providing such a crystal phase at the grain boundaries of silicon nitride crystal grains, the wettability of the coating layer with respect to the substrate made of the silicon nitride-based sintered body is improved, and the grain boundary crystal phase is removed from the substrate. Since it is continuous with the coating layer, the adhesion between the two is strengthened, the difference in thermal expansion between the substrate and the coating layer can be reduced, and peeling of the coating layer can be more effectively prevented.
[0031]
In the present invention, the silicon nitride sintered body used as the base material preferably contains a rare earth element and excess oxygen in addition to silicon nitride as a main component.
[0032]
Specifically, the content of silicon nitride is desirably in the range of 70 to 99 mol%, particularly 85 to 99 mol%, in order to sufficiently exhibit high-temperature strength. Also, Al or O may be dissolved in the silicon nitride to form SIALON.
[0033]
The rare earth element component is derived from the sintering aid and is a component of the above-described grain boundary crystal phase. Such rare earth elements are preferably at least one of Lu, Yb, and Er in order to obtain high chemical stability at a high temperature, and Yb, Lu, and Er are easily sinterable and have a high sintering property. It is also advantageous in terms of strength.
[0034]
The content of the rare earth element in the sintered body base material is preferably 0.5 to 10 mol% in terms of oxide in order to obtain a dense silicon nitride sintered body excellent in high-temperature strength and high-temperature creep. Particularly, 1 to 7 mol% is desirable. For example, if the content of the rare earth element is less than the above range, the sinterability is reduced, and it is difficult to obtain a substrate made of a dense silicon nitride-based sintered body. When the element is contained, the properties of high-temperature strength and high-temperature creep tend to deteriorate.
[0035]
The excess oxygen is mainly present as SiO ₂ and means the amount of oxygen obtained by subtracting the amount of oxygen used for the oxide of the rare earth element from the total amount of oxygen in the silicon nitride sintered body. In the present invention, this excess oxygen amount is represented by the following formula:
SiO ₂ / RE ₂ O ₃
In the formula, SiO ₂ indicates an excess oxygen amount (mol) in terms of SiO ₂ ,
RE ₂ O ₃ represents the rare earth element content (mol) in terms of oxide.
Is preferably in the range of 2 or more, particularly 2 to 3.5, more preferably 2.1 to 2.7. That is, by including excess oxygen in such an amount, a disilicate phase or a monosilicate phase having high resistance to oxidation and corrosion can be formed at the grain boundaries. For example, if the amount of excess oxygen is less than the above range, it is difficult to precipitate such a crystal phase at the grain boundary.
[0036]
In the present invention, the sintered body substrate may contain metal components such as Mg, Ca, and Fe. However, these metals form oxides having a low melting point and crystallize grain boundaries. Therefore, it is preferable to control the amount to 1 mol% or less, particularly 0.5 mol% or less, more preferably 0.1 mol% or less in terms of oxide in order to inhibit the high temperature strength.
[0037]
Next, a method for producing the surface-coated silicon nitride sintered body of the present invention will be described. According to the present invention, a surface-coated silicon nitride sintered body is manufactured by manufacturing a base material and then forming the above-described coating layer on the surface of the base material.
[0038]
【Example】
The following silicon nitride powder, rare earth element oxide powder and silicon oxide powder were used as raw material powders for the base material.
Silicon nitride powder:
BET specific surface area: 9 m ² / g
Α rate of silicon nitride; 99%
Oxygen content: 1.1% by mass
Amount of cationic metal impurities such as Al, Mg, Ca, Fe; 30 ppm or less Rare earth element oxide (RE ₂ O ₃ ) powder:
RE; Lu, Yb, Er, Y
Purity: 96%
Average particle size: 1.5 μm
Silicon oxide powder:
Purity: 99.9%
Average particle size: 2 μm
A mixed powder composed of the above silicon nitride powder (89.5 mol%), RE ₂ O ₃ powder (3 mol%) and silicon oxide powder (7.5 mol%) was prepared, and a binder and a solvent, methanol, were added. The mixture was pulverized for 50 hours with a rotary mill using a ball to prepare a slurry.
[0039]
After drying the obtained slurry, it was subjected to rubber press molding under a pressure of 298 MPa to produce a molded body having a diameter of 60 mm and a thickness of 20 mm.
[0040]
The compact was fired by gas pressure firing (GPS) at a firing temperature of 1900 ° C. for a firing time of 10 hours to obtain a substrate. A crystal phase of RE ₂ Si ₂ O ₇ (disilicate) was precipitated at the grain boundaries on all the substrates.
[0041]
The amount of SiO ₂ in the base material in Table 1 was obtained by pulverizing the base material, obtaining the total oxygen amount by chemical analysis, and converting the oxygen amount excluding the oxygen amount in the added rare earth oxide into SiO _2. This is a calculated value.
[0042]
Next, under the conditions shown in Table 1, a mixed powder of RE ₂ O ₃ powder and SiO ₂ powder was dispersed in distilled water in which PVA was dissolved to prepare a slurry, and the slurry was sprayed to obtain the base material obtained above. It was applied uniformly on the surface. Further, as shown in Table 2, a plurality of RE ₂ Si ₂ O ₇ powders and / or RE ₂ SiO ₅ powders were dispersed in distilled water in which PVA was dissolved to prepare a slurry, and the slurry was obtained by spraying. It was applied uniformly on the surface of the substrate.
[0043]
The sample No. 14-16, instead of the above mixed powder, SiO ₂ powder, using a ZrO ₂ powder or _Al 2 _{O 3} powder to obtain the same manner as described above to surface-coated silicon nitride sintered body.
[0044]
Then, after drying, heat treatment was performed in a nitrogen atmosphere under the conditions shown in Tables 1 and 2 to obtain a surface-coated silicon nitride sintered body.
[0045]
Various characteristics and the like were measured for the obtained sintered body by the following methods, and the results are shown in Tables 1 and 2.
[0046]
Thinning amount:
The surface-coated sintered body of the sample was exposed to a gas stream at 1200 ° C., a pressure of 0.4 MPa, and a gas flow rate of 50 m / s for 100 hours, and the amount of reduced wall thickness was measured.
[0047]
Crystal identification:
The crystal of the grain boundary phase of the substrate and the crystal of the coating layer were identified by X-ray diffraction measurement. In Table 1, the monosilicate phase is indicated by RS, and the disilicate phase is indicated by R2S. In addition, SEM observation and EPMA analysis were also performed to analyze differences in rare earth elements.
[0048]
Erosion resistance:
Glass powder having an average particle diameter of 100 μm was sprayed on the surface-coated silicon nitride sintered body with air at a pressure of 2 kg / cm ² , and the erosion resistance was evaluated from the weight change before and after the test.
[0049]
Heat cycle test:
A thermal cycle of 1300 ° C. and 300 ° C. was applied, and the adhesion of the coating layer was evaluated by the number of times a crack was confirmed by microscopic observation. The test was evaluated by the average number of three test pieces.
[0050]
[Table 1]

[0051]
[Table 2]

[0052]
Sample No. of the present invention In Nos. 1 to 12 and 17 to 29, the erosion resistance was 10% or less, no cracks occurred 100 times or more in the heat cycle test, and the thickness loss in the exposure test was 10 µm or less.
[0053]
On the other hand, Sample No. having a coating layer other than RE ₂ Si ₂ O ₇ and / or RE ₂ SiO ₅ , which is outside the scope of the present invention. In Nos. 13 to 16, the thinning amount by the exposure test was 200 µm or more.
[0054]
In addition, the sample No. which does not include the rare earth element contained in the grain boundary phase of the silicon nitride based sintered body substrate in the coating layer and is outside the scope of the present invention. In Nos. 30 to 33, erosion resistance was 15% or more, and cracks occurred in 90 or less heat cycle tests.
[0055]
【The invention's effect】
The surface-coated silicon nitride-based sintered body of the present invention includes a base material made of a silicon nitride-based sintered body containing RE ₂ Si ₂ O ₇ and / or RE ₂ SiO ₅ (RE is a rare earth element) as a grain boundary crystal phase. And a surface-coated silicon nitride sintered body composed of a coating layer formed on the surface of the base material, wherein the coating layer is made of RE ₂ Si ₂ O ₇ and / or RE ₂ SiO ₅ (RE is a rare earth element). By comprising at least two types of rare earth elements in the coating layer, and one of the two types of rare earth elements is the same as the rare earth element constituting the grain boundary crystal phase of the base material, A surface-coated silicon nitride sintered body having high corrosion resistance, high erosion resistance and high long-term reliability can be realized.

Claims (7)

A base material made of a silicon nitride sintered body containing RE ₂ Si ₂ O ₇ and / or RE ₂ SiO ₅ (RE is a rare earth element) as a grain boundary crystal phase, and a coating layer formed on the base material surface In the constituted surface-coated silicon nitride sintered body, the coating layer is composed of a crystal phase of RE ₂ Si ₂ O ₇ and / or RE ₂ SiO ₅ (RE is a rare earth element), and the coating layer has at least two types. A surface-coated silicon nitride-based sintered body, characterized in that: The surface-coated silicon nitride based sintered body according to claim 1, wherein at least one of the rare earth elements contained in the coating layer is different from the rare earth element contained in the grain boundary crystal phase of the base material. The coating layer contains A ₂ Si ₂ O ₇ and / or A ₂ SiO ₅ and B ₂ Si ₂ O ₇ and / or B ₂ SiO ₅ (A and B are rare earth elements, but A ≠ B). The surface-coated silicon nitride-based sintered body according to claim 1 or 2, wherein: 4. The surface-coated silicon nitride sintered body according to claim 1, wherein RE of the coating layer is at least one of Lu, Yb, and Er. Surface-coated silicon nitride sintered body according to any one of claims 1 to 4, wherein the excess amount of SiO ₂ contained in the coating layer is not more than 5 mol%. The surface-coated silicon nitride-based sinter according to any one of claims 1 to 5, wherein RE in the grain boundary phase of the silicon nitride-based sintered body is at least one of Lu, Yb, and Er. body. The above-mentioned base material contains 70 to 99 mol% of silicon nitride, 0.5 to 10 mol% of a rare earth element (RE) in terms of oxide, and excess oxygen by the following formula:
SiO ₂ / RE ₂ O ₃
In the formula, SiO ₂ indicates an excess oxygen amount in conversion,
RE ₂ O ₃ represents a rare earth element content in terms of oxide.
7. The surface-coated silicon nitride sintered body according to claim 1, wherein the molar ratio represented by is 2 or more.