JPH0741367A - Silicon nitride sintered compact and production thereof - Google Patents

Abstract

PURPOSE:To improve the strength by incorporating SiN as a main component and a group IIIa element in The periodic Table and holding a specific composition in a burnt molded body containing SiO2 or the like to keep fracture toughness on the polished mirror surface. CONSTITUTION:The molded body is obtained by incorporating the oxide (RE2O3) of group IIIa element in The Periodic Table or RE2O3 and SiO2 into SiN. Next, by sintering the molded body under a N2 containing atmosphere at 1600-2000 deg.C, a sintered body, (a) mainly containing SiN, (b) containing group IIIa element (RE) in The Periodic Table and oxygen and (c) satisfying I1<2.3, I2<1.9, -0.3<=I1-I2<=0.4 when the mol ratio SiO2/RE2O3 of the excess oxygen expressed in terms of SiO2 to Re expressed in terms of oxide (RE2O3) in the surface part of the sintered compact is I1 and the mol ratio in the center parts of the sintered compact is I2, is obtained. And a silicon nitride sintered compact >=5.5 MPa.m<1/2> in fracture toughness (K1c) of the mirror surface is produced by 0.06 mm-polishing the surface of the sintered compact.

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 useful as a structural material for a heat engine such as a gas turbine and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has attracted attention as a material excellent in strength, hardness, and thermal and chemical stability at high temperatures, and its application as engineering ceramics, particularly as a structural material for heat engines has advanced. Has been.

Generally, since silicon nitride itself is difficult to sinter, it is necessary to add a Group 3a element oxide of the periodic table such as Y ₂ O ₃ or Al ₂ O ₃ as a sintering aid. After molding it, 1500-20 in a non-oxidizing atmosphere containing nitrogen
A high-density sintered body is obtained by firing at a temperature of 00 ° C.

[0004]

Problems to be Solved by the Invention When a normal sintered body is commercialized, the surface thereof is subjected to polishing. However, for example, when a complex shaped product such as a turbine rotor is manufactured,
The entire surface of the sintered body cannot be polished, and an unpolished portion, a so-called burnt surface, remains.

In the case of manufacturing by the conventional method as described above, decomposition is apt to occur due to reaction between silicon nitride and various sintering aids in a non-oxidizing atmosphere during firing, and therefore, near the burnt surface. However, there is a problem in that the toughness and strength are lowered, and the stabilization of the mechanical properties of the complex shaped product is hindered.

As a method for suppressing the decomposition due to the reaction between the silicon nitride and the sintering aid component at the time of firing, for example, JP-A-63 / 1988
No. 190759 proposes to use a nitrogen atmosphere containing SiO as the atmosphere during firing, but it is the current situation that the strength and toughness in the vicinity of the burnt surface have not been increased to a practical level.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The inventors of the present invention have made detailed investigations on the cause of the deterioration of strength and toughness in the vicinity of the burnt surface. Due to the composition of the surface of the sintered body, specifically SiO ₂ / RE ₂ O ₃ (RE: Periodic Table 3a
It has been found that this is because the ratio of group elements) is lower than the internal ratio.

Therefore, according to the present invention, firing is performed by controlling the firing atmosphere so that the fluctuation range of the composition of the surface of the sintered body after sintering with respect to the composition of the surface of the molded body is within a specific range. The inventors have found that the toughness and strength in the vicinity of the burnt surface of a bonded body can be increased to a practical level, and the present invention has been completed.

That is, the silicon nitride-based sintered body of the present invention contains silicon nitride as a main component, contains a Group 3a element (RE) of the periodic table and excess oxygen, and has the above-mentioned periodic table on the surface of the sintered body. Third
Oxide equivalent of group a element (RE ₂ O ₃ ) and S of excess oxygen
When I ₁ is the molar ratio represented by SiO ₂ / RE ₂ O ₃ in terms of iO ₂ and I ₂ is the molar ratio in the center of the sintered body, I
₁ <2.3, I ₂ <1.9, -0.3 ≦ I ₁ −I ₂ ≦
0.4 is satisfied and the surface of the sintered body is 0.05 mm
Fracture toughness (K ₁ c) on polished mirror surface is 5.5 MPa
^-It is characterized in that it is at least m ^1/2 . Further, the method for producing a silicon nitride sintered body of the present invention is based on periodic table 3a element oxide (RE
₂ O ₃ ) or oxide of Group 3a element of the periodic table (RE
₂ O ₃ ) and SiO ₂ are fired at a temperature of 1600 to 2000 ° C. in a nitrogen-containing atmosphere, the molar ratio of SiO ₂ / RE ₂ O ₃ in the above-mentioned compact is I ₃ , I ₃ <1.9, I ₃ −I ₁ ≦ 0.6, where I ₁ is the molar ratio in the surface portion of the final sintered body.
The firing is performed in an atmosphere containing SiO so as to satisfy the above condition.

The present invention will be described in detail below. The silicon nitride-based sintered body in the present invention has silicon nitride as a main component in terms of composition,
Desirably, it is contained in a proportion of 70 to 99 mol%, and contains the Group 3a element (RE) of the Periodic Table and excess oxygen as other components. Here, the excess oxygen is the total amount of oxygen contained in the sintered body, and is the amount of oxygen remaining after removing oxygen mixed in stoichiometrically as a Group 3a element oxide of the periodic table. It is composed of impurity oxygen in the silicon nitride raw material or oxygen added as SiO ₂ .

According to the present invention, the oxide (RE ₂ O ₃ ) conversion amount of the Group 3a element of the periodic table contained in the sintered body and the surface of the sintered body when the excess oxygen is converted into SiO ₂ Part of S
Assuming that the molar ratio represented by iO ₂ / RE ₂ O ₃ is I ₁ and the molar ratio at the center of the sintered body is I ₂ , I ₁ <2.3, I ₂
It is important to satisfy <1.9, -0.3 ≦ I ₁ −I ₂ ≦ 0.4.

In the present invention, I ₁ and I ₂ are limited to the above range because I ₁ is larger than 2.3,
If I ₂ is larger than 1.9, the high temperature strength at 1400 ° C. is not sufficient, and if I ₁ -I ₂ is smaller than -0.3, the toughness and strength of the surface of the sintered body, especially the burnt surface, are not improved. ,
When it exceeds 0.4, the high temperature strength inside the sintered body is lowered, and when the value of I ₁ -I ₂ deviates from the above range, the internal difference becomes large and the internal difference causes a large composition difference. In some cases, stress may occur and cracks may occur.

Further, the sintered body of the present invention has a fracture toughness (K ₁ c) of 5.5 MPa · m ^{1/2 on the} surface of the sintered body, for example, a mirror surface obtained by polishing the burnt surface after firing by 0.05 mm. The above is also important. This is an important factor at least for increasing the mechanical strength of the burnt surface, and when the fracture toughness value is lower than 5.5 MPa · m ^1/2, voids and the like which are unavoidably present on the burnt surface. This is because the defects easily lead to destruction and the strength of the burnt surface cannot be increased.

[0014] the desired range of the I _1, I ₂ in the present _{invention, I 1 ≦ 1.8, I 2} ≦ 1.8,0 ≦ I 1 -I 2
≦ 0.3.

Next, the method for producing a silicon nitride sintered body according to the present invention will be described. First, silicon nitride powder is the main component as a starting material, and the third component is a periodic table as an additional component.
An a-group element oxide powder, and optionally a silicon oxide powder is used. In addition to the above powders, it is added in the form of a complex compound powder of a Group 3a element oxide of the periodic table and silicon oxide, or a complex compound powder of silicon nitride-group 3a element oxide of the periodic table-silicon oxide. You can also The silicon nitride powder used is α
Type and β type may be used, and their average particle diameter is 0.
4 to 1.2 μm is suitable.

After that, these powders are mixed at a predetermined ratio. The mixing ratio is 70% silicon nitride component.
˜99 mol%, and an additive component such as an oxide of a Group 3a element of the periodic table and silicon oxide is adjusted to a ratio of 1 to 30 mol%. Then, this mixed powder is formed into a desired molding means,
For example, a die press, a cold isostatic press, an extrusion molding, or the like is used to form an arbitrary shape.

After that, the molded body is fired in a non-oxidizing atmosphere containing nitrogen. The firing method is normal firing,
In addition to the hot pressing method, a method of firing in a nitrogen gas pressurized atmosphere of more than 1 atm, preferably 5 atm or more, or the sintered body obtained by the above-mentioned firing method is an inert gas 500 to 20.
For example, a hot isostatic firing method of firing under 00 atm can be adopted.

According to the present invention, in the above-mentioned sintering process,
SiO in the formed body in terms of oxide (RE ₂ O ₃ ) of Group 3a element of the periodic table and SiO _{2 of} excess oxygen.
Assuming that the ratio represented by ₂ / RE ₂ O ₃ is I ₃ , and the molar ratio in the surface portion of the final sintered body is I ₁ , I ₃ <1.
9, it is important to fire so as to satisfy I ₃ −I ₁ ≦ 0.6. Particularly preferably, I ₃ ≦ 1.8, I ₃ −
I ₁ ≦ 0.

SiO ₂ / R on the surface of the molded body and the sintered body
The reason why the E ₂ O ₃ ratios I ₁ and I ₃ are limited to the above range is that I
_{When 3} is 1.9 or more, the final sintered body is made of SiO during the sintering process.
_{This is because the 2} / RE ₂ O ₃ ratio I ₁ may exceed 2.3, and a high-strength sintered body cannot be produced stably, and I ₃ -I ₁ is 0.6. When it exceeds, it means that decomposition and volatilization during the sintering process is severe, and SiO ₂ / RE ₂
This is because the fluctuation range of the O ₃ ratio becomes large, the toughness in the vicinity of the surface of the sintered body decreases, and the high temperature strength of the sintered body is not improved.

Further, as described above, in order to suppress the variation of the SiO ₂ / RE ₂ O ₃ ratio within the above range in the process of changing the compact into the sintered body, it is necessary to finely control the atmosphere. . Specifically, in the atmosphere, Si ₃ N ₄
Nitrogen gas at a decomposition equilibrium pressure of Si ₃ N ₄ or higher is introduced to suppress the decomposition of Si ₃ N ₄ , and silicon nitride powder and periodic table 3a
Group element oxide powder, SiO ₂ powder, Si powder and SiO ₂
It is also effective to arrange these impurity components in the firing furnace because they are mixed powders of powders or Al ₂ O ₃ and are unavoidably contained in the compact and easily volatilized as a low melting point substance.

[0021]

[Function] Usually, the surface of the sintered body after firing is roughened due to decomposition and volatilization of the components of the sintered body.
There is a phenomenon that mechanical properties are deteriorated as compared with a polished surface. In the case of simple shaped products, polishing the surface can prevent deterioration of properties due to roughness during firing, but in the case of complicated shaped products, there are inevitably places that cannot be polished, so the burnt surface The mechanical properties of the surface were unavoidable. When the cause of the deterioration of the mechanical properties of the burnt surface was examined in this way, the SiO ₂ component was mainly decomposed during the sintering process, and specifically the composition of the surface of the sintered body, specifically SiO ₂ / RE ₂ O ₃ (RE : Periodic Table 3a
It was found that the ratio of group elements) was lower than the internal composition.

Further, according to the present invention, the factors that can enhance the strength of the burnt skin surface were examined. When the toughness in the vicinity of the burnt skin surface was high, cracks generated through the burnt skin surface were directly under the burnt skin surface. It was found that in the high toughness region of, the progress was suppressed and the fracture of the sintered body was suppressed. This is clear from the relationship between the toughness value near the burnt surface and the high temperature strength in FIG. As is clear from FIG. 1, in order to achieve a strength of 600 MPa or more on the burnt surface, it is necessary to set the toughness in the vicinity of the burnt surface to 5.5 MPa · m ^1/2 or more.

According to the present invention, in the firing process, when the SiO ₂ / RE ₂ O ₃ ratio of the molded body is reduced so that the variation is small, the SiO in the sintered body and the burnt surface are reduced.
The difference in the ₂ / RE ₂ O ₃ ratio is also small, and at the same time, the toughness in the vicinity of the burnt surface is not reduced, the toughness is improved, and as a result, the mechanical properties of the sintered body having the burnt surface can be enhanced. .

Further, in order to obtain practical strength, Si
The O ₂ / RE ₂ O ₃ ratio needs to be controlled within a specific range. The ratio of the surface of the sintered body is I ₁ <2.3 and the ratio of the inside of the sintered body is I _1.
By setting ₂ <1.9, -0.3 ≤ I ₁ -I ₂ ≤ 0.4, and the ratio I ₃ <1.9, I ₃ -I ₁ ≤ 0.6 in the molded body, bending at 1400 ° C Strength 600 kg / mm ²
The above is achieved. Further, according to the above configuration, 140
Oxidation resistance at 0 ° C. is also improved, and 0.15 mg / cm ² or less can be achieved as will be apparent from Examples described later.

The elements of Group 3a of the periodic table in the present invention are preferably Y, Yb, Er, Lu, Ho, Dy and the like.

According to the silicon nitride sintered body of the present invention, Al, Mg, Ca, Fe, etc. may be contained as impurities. However, since these components easily form a low melting point substance, In order to increase the high temperature strength, it is desirable to control the amount of these components to 0.5% by weight or less, particularly 0.1% by weight or less.

Further, in addition to the above composition, the silicon nitride sintered body of the present invention contains 10% by weight or less of carbides, nitrides, carbonitrides, borides and the like of metals such as W, Ti, Nb, V and Mo. May be included in the ratio.

[0028]

EXAMPLE As a starting material, silicon nitride powder (average particle size 0.5 μm, impurity oxygen amount 1% by weight, purity 99%),
An oxide powder of a Group 3a element of the periodic table having a purity of 95% or more and a silicon oxide powder were prepared, and these were weighed and mixed so as to have the composition shown in Table 1. The amount of SiO ₂ in the formulation in Table 1 is the total amount of added SiO _{2 and the} amount of impurity oxygen in the silicon nitride powder converted to SiO ₂ . Then, this mixed powder was press-molded. When the composition of the obtained molded body was analyzed by ICP analysis, it was almost the same as the blended composition.

Next, this compact was placed in a silicon carbide jar in a firing furnace equipped with a carbon heater, and Si powder and SiO ₂ powder together with the compact were mixed at a weight ratio of 1: 1.5. The mixed powder and the silicon nitride powder mixed in 1. were blended in the amounts shown in Table 1 and fired in an atmosphere of 1800 ° C. and a nitrogen gas pressure of 10 atm for 5 hours.

A flat plate having a thickness of 10 mm was cut out from the obtained sintered body from the inside of the sintered body and the surface including the burnt surface, and the test was conducted on SiO ₂ / RE ₂ O ₃ ratio, flexural strength and oxidation. Weight increase and fracture toughness were measured.

The SiO ₂ / RE ₂ O ₃ ratios I ₁ and I ₂ were subjected to ICP analysis and nitrogen-oxygen simultaneous analysis using the above test pieces, and the amount of SiO ₂ was quantified by ICP analysis from the total oxygen amount. The remaining oxygen amount obtained by subtracting the amount of oxygen bonded to the oxide from the amount of Group 3a element of the periodic table is Si.
It is converted to O ₂ .

The bending strength was measured based on JISR1601 by cutting a JIS test piece from the above-mentioned test piece (the surface test piece includes the burnt surface) and measuring the bending strength at room temperature.

The bending strength of the internal test piece at a high temperature of 1400 ° C. was also measured.

After measuring the strength of the test piece including the burnt surface, the test piece is used to measure the burnt surface 0.01 to 0.0.
The surface was polished for 5 mm and mirror-finished, and the fracture toughness K ₁ c was measured by the IM method. Furthermore, the increase in oxidized weight is 1 in the atmosphere.
It was calculated from the weight change after oxidation treatment at 400 ° C. for 24 hours. The results are shown in Table 1.

[0035]

[Table 1]

As is clear from Table 1, the sample N in which the SiO ₂ / RE ₂ O ₃ ratio I ₁ of the surface portion of the final sintered body is 2.3 or more, or the internal ratio I ₂ is 1.9 or more.
Nos. 7, 8, 9, and 17 have low strength at 1400 ° C., and I ₁ -I ₂ is less than -0.3.
In No. 1, the toughness and strength in the vicinity of the burnt surface are low, and I ₁ -I
_In Sample Nos. 8 and 10 in which ₂ exceeded 0.4, the high temperature strength inside the sintered body decreased. Sample Nos. 1 to 9 and 12
FIG. 1 shows the toughness in the vicinity of the burnt surface and the burnt surface strength.
Plotted on. As is clear from FIG. 1, the higher the toughness near the burnt surface, the higher the burnt surface strength.

In addition, the SiO ₂ / RE ₂ O ₃ ratio I of the molded body
_{When 3} is 1.9 or more, I ₁ of the final sintered body exceeds 2.3 in the sintering process as in sample No. 17, and the high temperature strength is low. The characteristics were not exhibited. Sample No. 1, in which I ₃ -I ₁ exceeds 0.6
In Nos. 2, 11 and 18, the toughness in the vicinity of the surface of the sintered body was lowered and no improvement in the high temperature strength of the sintered body was observed.

In contrast to these comparative examples, the samples of the present invention all have a burnt surface toughness of 5.5 MPa · m ^1/2 or more, a room temperature strength of the burned surface of the sintered body of 600 MPa or more, and an internal temperature of 650 MPa. Above, 1400 ℃ internal strength 600MP
A or more was achieved.

[0039]

As described above in detail, according to the present invention, the sintering process of the molded body and the S in the obtained sintered body are performed.
By controlling the iO ₂ / RE ₂ O ₃ ratio to satisfy a specific condition and increasing the toughness in the vicinity of the surface of the sintered body, the strength on the burnt surface can be increased. As a result, high strength can be imparted even to a product having a complicated shape that cannot be surface-polished, and a highly reliable product can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between toughness near a burnt surface and burnt surface strength.

Claims (2)

[Claims] 1. An oxide conversion amount (RE) of a group 3a element of the periodic table on a surface portion of a sintered body containing silicon nitride as a main component, containing a group 3a element (RE) of the periodic table and excess oxygen.
₂ O ₃₎ and the excess oxygen in SiO ₂ equivalent amount SiO ₂ / RE
_When I ₁ is the molar ratio represented by ₂ O ₃ and I ₂ is the molar ratio of the center of the sintered body, I ₁ <2.3, I ₂ <1.9, −
0.3 ≦ I ₁ −I ₂ ≦ 0.4 is satisfied, and the fracture toughness (K ₁ c) on the mirror surface obtained by polishing the surface of the sintered body by 0.05 mm is 5.5 MPa · m ^1/2 or more. A silicon nitride-based sintered body characterized by: _2. A molding containing silicon nitride containing a group 3a element oxide of the periodic table (RE ₂ O ₃ ) or a group 3a element oxide of the periodic table (RE ₂ O ₃ ) and SiO _2. When the body is fired at a temperature of 1600 to 2000 ° C. in a nitrogen-containing atmosphere, SiO ₂ / RE ₂ in the formed body is used.
When I ₃ is the molar ratio represented by O ₃ and I ₁ is the molar ratio in the surface portion of the final sintered body, I ₃ <1.9, I ₃ −
A method for producing a silicon nitride sintered body, which comprises firing so as to satisfy I ₁ ≦ 0.6.