JPH04219374A - Silicon nitride-based sintered compact and its production - Google Patents

Abstract

PURPOSE:To improve transverse rupture strength and fracture toughness extending from ambient to high temperatures by adding a group Va element oxide to an Si3N4-RE2O3 (rare earth element oxide)-Al2O3-SiO2-based mixture. CONSTITUTION:A mixture is prepared by mixing 60-93mol% Si3N4 powder having 0.4-1.2mum grain diameter with RE2O3 (e.g. Er2O3), Al2O3 and SiO2 so as to afford 6-39.9mol% total content of the RE2O3, Al2O3 and SiO2 and 0.1-1.0mol% group Va element oxide (e.g. Nb). The resultant mixture is formed and then burned at 1700-1800 deg.C in a nitrogen-containing nonoxidizing atmosphere to provide a silicon nitride-based sintered compact, having >2 molar ratio of SiO2/RE2O3, (1/15)=(5/4) molar ratio of the Al2O3/(RE2O3+SiO2) and <=1.5mum average crystal grain diameter (minor axis) at 2-15 aspect ratio and constructed from an Si3N4 crystal phase, a crystalline grain boundary phase composed of Si, 0 and N, a grain boundary phase containing RE and Al and an oxynitride phase of the group Va element.

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 that has excellent strength properties from room temperature to high temperatures and is used particularly for automobile parts, gas turbine engine parts, etc., and a method for producing the same.

0002

BACKGROUND OF THE INVENTION Silicon nitride sintered bodies have been used as engineering ceramics, particularly for heat engines such as turbo rotors, because of their excellent heat resistance, thermal shock resistance, and oxidation resistance.

[0003] Recently, in order to produce a high-density and high-strength sintered body, rare earth element oxides and aluminum oxides have been added as sintering aids, as disclosed in Japanese Patent Publication No. 52-3649 and No. 58-5190. is being considered.

[0004] Also, recently, N is added to silicon nitride sintered bodies.
A method of increasing the strength and toughness of a sintered body by adding compounds of metals such as b, Ta, Ti, W, etc. is disclosed in JP-A-64-
It is proposed in No. 87568.

[0005]

[Problems to be Solved by the Invention] However, when yttrium oxide and aluminum oxide are used as sintering aids, the sinterability is improved and the density can be increased, which improves the strength at room temperature and high temperature to some extent. However, it is still insufficient for practical use, and further improvement in strength is required.

[0006] Furthermore, although improvements in room temperature strength have been observed in the method of adding Ta, Nb, Ti, etc. to the sintered body, no study has been made regarding improvement in high temperature strength.

[0007]

OBJECTS OF THE INVENTION The object of the present invention is to develop a material with sufficient strength properties to be used in automotive parts, gas turbine engine parts, etc. from room temperature to high temperatures, and in particular, to have sufficient bending strength from room temperature to high temperatures of 1000°C. An object of the present invention is to provide an excellent silicon nitride sintered body and a method for manufacturing the same.

[0008]

[Means for Solving the Problems] The present inventors have discovered that in order to improve the high-temperature properties of a sintered body, the composition of the sintered body, the shape of the silicon nitride crystal phase in the sintered body, and the Based on the viewpoint that it is important to control the sub-phases present at the grain boundaries of the phase, we have developed a system consisting mainly of silicon nitride with the addition of rare earth element oxides, aluminum oxide, and silicon oxide. Further, an oxide of a Group 5a element of the periodic table is added, and this is shaped and fired to precipitate a crystalline grain boundary phase consisting of silicon, oxygen, and nitrogen in the resulting sintered body. discovered that a sintered body having excellent high-temperature properties can be obtained by precipitating a phase consisting of an oxynitride of an element of group 5a of the periodic table.

The present invention will be explained in detail below. The silicon nitride sintered body of the present invention has silicon nitride as a main component, and contains rare earth elements, aluminum, excess oxygen, and elements of group 5a of the periodic table as additive components.

[0010] Here, excess oxygen refers to the total amount of oxygen in the sintered body, the stoichiometric amount of each element in the sintering aid, rare earth element oxides, aluminum oxide, and oxides of Group 5a elements in the periodic table. This is the remaining amount of oxygen excluding bonded oxygen, and most of it is oxygen contained in the silicon nitride raw material, or
It is mixed as an additive such as SiO2, and in the present invention, it is considered that it exists entirely as SiO2.

The sintered body of the present invention has a silicon nitride crystal phase as its main phase, most of which is β-Si3 N4
It consists of crystals, but in some cases a small amount of aluminum oxide (Al2O3) is dissolved in solid solution, forming Si-Al-O-N (
They may also form sialons. In addition, this main phase generally has a columnar shape, but the average crystal grain size (minor axis) is 1
．． It is preferable that the thickness is 5 μm or less and the aspect ratio is 2 to 15 from the viewpoint of improving strength and toughness.

According to the present invention, a major feature is that a crystalline phase consisting of silicon, oxygen and nitrogen exists as a subphase at the grain boundaries of the main phase. This crystalline phase is, for example, Si2
It consists of a silicon oxynitride crystal phase represented by N2O, and the presence of this crystal phase can increase high-temperature strength.

According to the present invention, it is also important that, in addition to the above-mentioned crystalline phase, an oxynitride phase of an element of group 5a of the periodic table is present. This crystal phase suppresses abnormal grain growth of the silicon nitride crystal phase and improves strength at room temperature and high temperature.

[0014] The above three types of crystal phases are generated in the sintered body of the present invention, and grain boundary phases consisting of silicon, aluminum, oxygen, and rare earth elements are formed at the grain boundaries of these crystal phases. . This grain boundary phase itself may be either an amorphous phase or a crystalline phase.

[0015] Furthermore, the sintered body of the present invention contains 60% silicon nitride.
present in a proportion of ~93 mol%, in particular 68-85 mol%,
The total of the oxide equivalent amount of Group 3a elements of the periodic table, the oxide equivalent amount of aluminum, and the SiO2 equivalent amount of excess oxygen is 6.0 to 39.9 mol%, especially 14.2 to 31.8 mol%.
Furthermore, it is important that the composition consists of 0.1 to 1.0 mol %, particularly 0.2 to 0.8 mol %, of Group 5a elements of the periodic table in terms of oxides.

[0016] This is because if the total amount of rare earth elements, aluminum, and excess oxygen is less than 6.0 mol%, there will be a shortage of liquid phase during the sintering process, so it is necessary to increase the sintering temperature. Grain growth occurs, causing a decrease in strength, and if it is greater than 39.9 mol%, there will be a large number of grain boundary phases in the sintered body, leading to a decrease in high-temperature strength and further deterioration of oxidation resistance at high temperatures. Therefore, if the Group 5a element of the periodic table is less than 0.1 mol%, silicon nitride particles grow, causing a decrease in strength, and if it is more than 1.0 mol%, the acicular formation of silicon nitride particles is suppressed. This is because the fracture toughness decreases.

Furthermore, according to the present invention, when the oxide equivalent amount of Group 3a elements of the periodic table is RE2 O3 and the oxide equivalent amount of aluminum is Al2 O3, the excess oxygen Si
The molar ratio expressed by SiO2 /RE2 O3 with respect to O2 equivalent amount is 2 or more, especially 2 to 25, and Al2 O
It is desirable that the molar ratio expressed by 3/(SiO2 +RE2 O3) is 1/15 to 5/4, particularly 0.1 to 1.0. This is because when the SiO2/RE2O3 molar ratio is 2 or less, a crystalline phase consisting of silicon, oxygen and nitrogen is difficult to form, resulting in a decrease in strength.
If the l2O3/(SiO2 +RE2O3) molar ratio is smaller than 1/15, the sinterability will be reduced and it will be difficult to obtain a high-density sintered body, and if it is larger than 5/4, Al2O
3, the formation of low melting point substances increases, resulting in deterioration of strength and oxidation resistance at high temperatures.

The rare earth element used in the present invention is Y
Among these, Y
b2 O3 , Er2 O3 , Ho2 O3 , Dy
2 O3 is preferred in terms of uniformity and strength of the sintered body.

[0019] Also, as an element of Group 5a of the periodic table, N
Among these, Nb has the greatest effect.

According to the method for producing a silicon nitride sintered body of the present invention, silicon nitride powder, rare earth element oxide powder, aluminum oxide powder, and group 5a element oxide powder of the periodic table are used as raw material powders, and when In some cases, silicon oxide powder is further used.

Silicon nitride powder itself is α-Si3N
4, β-Si3 N4 can be used, and it is desirable that their particle size is 0.4 to 1.2 μm.

According to the present invention, using these powders, silicon nitride (Si3N4) is contained in an amount of 60 to 93 mol %, especially 6
8 to 85 mol%, rare earth element oxide (RE2 O3
), aluminum oxide (Al2O3), excess oxygen (
The total amount of (SiO2 equivalent amount) is 6.0 to 39.9 mol%
, especially 14.2 to 31.8 mol %, and 0.1 to 1.0 mol %, especially 0.2 to 0.8 mol % of the oxide of Group 5a element of the periodic table, and mixed. . The excess oxygen (SiO2) at this time is the total amount of the impurity oxygen contained in the silicon nitride powder converted to SiO2 and the added silicon oxide powder.

The mixed powder thus obtained is molded into a desired shape by known molding methods such as press molding, casting molding, extrusion molding, injection molding, and cold isostatic pressing.

Next, the obtained molded body is subjected to a known firing method,
For example, hot press method, normal pressure firing, nitrogen gas pressure firing, HIP firing after these firing, and
A dense sintered body is obtained by firing using glass seal HIP firing or the like. The firing temperature at this time should be below 1900°C, especially at
A non-oxidizing atmosphere containing nitrogen gas at 700 to 1800°C is desirable.

According to this firing, most of the additive components are present in the grain boundaries of the silicon nitride crystal phase, but among these, the elements of group 5a of the periodic table are oxidized by nitrogen substitution of a part of the oxide. Nitride is produced. On the other hand, other additives form a crystal phase consisting of silicon, nitrogen, and oxygen, and a grain boundary phase consisting of silicon, aluminum, oxygen, and nitrogen.
If a crystal phase consisting of silicon, nitrogen, and oxygen is not generated depending on the conditions, the obtained sintered body is
A crystalline phase can be generated by processing in a non-oxidizing atmosphere at .degree.

[0026]

[Operation] Silicon nitride crystal grains must have an appropriate aspect ratio and size, and in order to obtain high reliability, it is necessary that there are no abnormally grown grains. For this purpose, an appropriate amount of grain boundary phase is required, and in addition to the conventional addition of rare earth element oxides and aluminum oxide, the addition of SiO2 lowers the melting point of the liquid phase generated during the sintering process. , it becomes possible to perform low-temperature firing, suppress the growth of silicon nitride particles, and form a fine structure.

Furthermore, by adding an oxide of an element of Group 5a of the periodic table, the oxide changes to an oxynitride during the sintering process, so that it does not dissolve in the grain boundary phase composed of other components and forms an oxynitride phase. Since the oxynitride phase remains, it is possible to further suppress the growth of silicon nitride particles and form a fine and uniform structure with a uniform aspect ratio. Furthermore, silicon in the sintered body,
By forming a crystalline phase consisting of oxygen and nitrogen, components other than the silicon nitride crystalline phase are stabilized at high temperatures, thereby increasing the strength of the sintered body at room and high temperatures as well as its toughness.

[0028]

[Example] Example 1 Silicon nitride powder (BET specific surface area: 8 m2) was used as the raw material powder.
/g, alpha rate 98%, oxygen amount 1.2% by weight) and various rare earth element oxide powders, silicon oxide powders, aluminum oxide powders, and periodic table group 5a element oxide powders as shown in Table 1. After blending and mixing the composition, 1t/cm2
It was press-molded into a mold.

[0029]

[Table 1]

The obtained molded body was placed in a silicon carbide sagger and fired at 1750°C for 4 hours in a nitrogen gas stream at normal pressure using a carbon heater to obtain a sintered body. .

The obtained sintered body was polished to a shape specified by JIS-R1601 to prepare a sample. For this sample, the specific gravity was measured based on the Archimedes method, the average crystal grain size (minor axis) of the silicon nitride crystal, and its average aspect ratio were measured from electron micrographs. A folding strength test was conducted, and the fracture toughness was also measured. Furthermore, the crystals in the sintered body were identified by X-ray diffraction measurements. The results are shown in Table 2.

[0032]

[Table 2]

According to the results in Tables 1 and 2, Si3
When N4 is less than 60 mol%, RE2 O3, Al2 O
3, No. where the total amount of SiO2 exceeds 39.9 mol%
The sintered bodies of , 1 have decreased high temperature strength, and Si3 N4
exceeds 93 mol%, RE2 O3, Al2 O3
The sintered body No. 10, in which the total amount of SiO2 was less than 6 mol %, was insufficiently densified and its strength was reduced. Also, Section 5a
The room temperature strength of the sintered body No. 6 containing less than 0.1 mol% of group elements decreases, while the sintered body No. 6 containing group 5a elements exceeding 1.0 mol%
, 9 had decreased fracture toughness and room temperature strength.

In contrast to these comparative examples, all of the sintered bodies of the present invention exhibited excellent bending strength and fracture toughness. Further, the SiO2 /RE2 O3 molar ratio is greater than 68 and 85 or less, and Al2 O3 /(SiO2 +RE2
O3) N with a molar ratio greater than 0.1 and less than 1.0
o, 2, No, 5, No, 7, No, 8, No, 11,
The sintered bodies No. 14, No. 15, and No. 16 exhibited even better bending strength and fracture toughness.

Example 2 Among the samples in Table 1 of Example 1, No. 3, No. 10,
Regarding the sintered bodies No. 12, No. 13, further, nitrogen gas containing argon gas 2000 atm, 1850° C., 1
Hot isostatic pressure treatment was carried out under the conditions of: Note that no compositional change was observed during HIP. The obtained sintered body was evaluated in the same manner as in Example 1, and the results are shown in Table 4.

[0036]

[Table 3]

According to the results in Table 3, the density of the sintered bodies was improved by the HIP treatment, but the strength of sample No. 10, which had a small amount of Si3N4, could not be recovered even by the HIP treatment.

[0038]

[Effects of the Invention] As detailed above, according to the present invention, S
i3 N4 -RE2 O3 -Al2 O3 -Si
By adding oxides of Group 5a elements of the periodic table to the O2 system and precipitating Group 5a oxynitrides and crystal phases consisting of silicon, oxygen, and nitrogen in the sintered body, silicon nitride crystal grains are In addition to suppressing grain growth, it is possible to stabilize additives such as sintering aids in high temperature ranges, and as a result, high strength and toughness can be obtained from room temperature to high temperature.

Therefore, it can be used as a structural material for heat engines such as gas turbines and turbo rotors, as well as structural materials used at various room temperatures and high temperatures.

Claims (5)

[Claims] [Claim 1] A silicon nitride crystal phase, a crystalline grain boundary phase consisting of silicon, oxygen and nitrogen, a grain boundary phase containing at least a rare earth element and aluminum, and an oxynitride phase of a Group 5a element of the periodic table. A silicon nitride sintered body comprising: [Claim 2] Silicon nitride is 60 to 93 mol%, the total amount of rare earth elements in terms of oxides, aluminum in terms of oxides, and excess oxygen in terms of SiO2 is 6 to 39.9 mol%, periodic table The silicon nitride sintered body according to claim 1, wherein the Group 5a element is present in an amount of 0.1 to 1.0 mol% in terms of oxide. [Claim 3] Oxide equivalent amount of rare earth element (RE2 O
3), the SiO2 equivalent amount of excess oxygen, and the oxide equivalent amount of aluminum (Al2 O3), SiO
The silicon nitride sinter according to claim 1, wherein the molar ratio expressed by 2 /RE2 O3 exceeds 2, and the molar ratio expressed by Al2 O3 / (RE2 O3 + SiO2) is 1/15 to 5/4. body. [Claim 4] 60 to 93 mol% of silicon nitride, and a total amount of rare earth element oxide, aluminum oxide, and silicon oxide of 6 to 93 mol%.
39.9 mol %, and 0.9 mol % of the oxide of Group 5a element of the periodic table.
After molding a mixture prepared to have a concentration of 1 to 1.0 mol%, it is sintered in a non-oxidizing atmosphere containing nitrogen to form a silicon nitride crystal phase and a crystalline grain boundary phase consisting of silicon, oxygen and nitrogen. , a grain boundary phase containing at least a rare earth element and aluminum, and an oxynitride phase of an element of group 5a of the periodic table. 5. Addition amount of the rare earth element oxide (RE2
O3), the amount of silicon oxide added (SiO2),
In the amount of aluminum oxide added (Al2O3), the molar ratio expressed as SiO2/RE2O3 exceeds 2, and Al2O3/(RE2O3 +Si
5. The method for producing a silicon nitride sintered body according to claim 4, wherein the molar ratio represented by O2) is 1/15 to 5/4.