JPH075389B2 - Silicon nitride sintered body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silicon nitride sintered body having excellent mechanical strength, thermal shock resistance, wear resistance, and oxidation resistance.

(Prior Art) Since silicon nitride is excellent in mechanical strength, thermal shock resistance, wear resistance, oxidation resistance and the like, it has recently attracted attention as a high temperature structural material such as a gas turbine component. However,
Since silicon nitride has poor sinterability and is difficult to sinter by itself, it is generally difficult to obtain a high-density and high-strength silicon nitride sintered body.

As a conventional method for producing a silicon nitride sintered body, a so-called reactive sintering method in which silicon is nitrided and sintered, and a method in which a sintering aid such as MgO is added to silicon nitride powder and hot pressing are known. ing. However, in the former reaction sintering method, since there is almost no firing shrinkage, a high density product cannot be obtained,
Therefore, there is a drawback that a high-strength silicon nitride sintered body cannot be obtained. In addition, the latter method by hot pressing can obtain high density and high strength, but has the disadvantage that it can only be manufactured in a relatively simple shape due to the restrictions associated with the manufacturing method, and it is economically expensive. there were.

In addition to these manufacturing methods, the applicant has begun to use silicon nitride powder in Be
Manufacturing in which at least two or more of O, MgO, and SrO, and one or more sintering aids of rare earth element oxides are added, mixed sufficiently, and then molded, and then fired in a nitriding atmosphere or an inert gas atmosphere As a method, the Japanese Patent Publication No. 55-46997 "Sintered Silicon Nitride and its Manufacturing Method" was invented.

(Problems to be Solved by the Invention) Japanese Patent Publication No. 55-46997 previously invented by the present applicant is useful for providing a high-density silicon nitride sintered body having high mechanical strength and thermal shock resistance. the case, it may MgO-SiO ₂ based crystal phase are produced in the glass phase consisting of a sintering aid and SiO _2, it is good abrasion resistance by easily occur variations in strength due No oxidation resistance was obtained.

(Means for Solving Problems) An object of the present invention is to eliminate the above-mentioned problems and to have high strength and excellent wear resistance and oxidation resistance that can be used, for example, in sliding materials and automobile parts. The object of the present invention is to provide a good silicon nitride sintered body having no heterogeneous phase.

In the silicon nitride sintered body of the present invention, the higher the oxygen content, the better the oxidation resistance, and the less the cause of the generation of the heterogeneous phase. However, if the oxygen content is too high, it occurs in the grain boundary phase. The amount of glass phase increases, and the softening thereof reduces the high temperature strength. Also, if the oxygen content is too low, the grain boundary phase
The MgO-SiO ₂ system crystal phase is unevenly generated, resulting in wear resistance,
This may cause a problem in oxidation resistance.

Therefore, the amount of oxygen contained in silicon nitride, that is, SiO ₂
Amounts too high or too low will affect the properties.

The present invention comprises 80 to 97% by weight of silicon nitride and 1 to 10% by weight of CeO ₂
If, MgO, and each 0.5 to 5 wt% of SrO, consists composition containing a SiO ₂ .55-10.5 wt%, the weight ratio of SiO ₂ / MgO is 1.
A silicon nitride sintered body of 1 to 2.1, which does not generate a crystal phase in the grain boundary phase, so the amount of oxygen is controlled so that the weight ratio of SiO ₂ / MgO is 1.1 to 2.1. The purpose of the present invention is to specify the composition of a silicon nitride sintered body and to provide a silicon nitride sintered body having high strength and excellent thermal shock resistance, high wear resistance, and oxidation resistance.

As a method of increasing oxygen in the silicon nitride sintered body which is the object of the present invention, a method of adding a compound such as SiO ₂ or MgSiO ₃ or a method of calcination and oxidizing the raw material itself can be considered. It may be manufactured using the method.

It is generally known that silicon nitride has two crystal structures of α phase and β phase, but when it is heated to a temperature of 1400 ° C or higher, the transition from α phase to β phase begins, and it becomes 1600 ° C or higher. Then, the transfer rate becomes faster. Especially when CeO ₂ is added, this α
The transition from the β phase to the β phase is significantly promoted. On the other hand, when SrO and MgO are heated to 1600 ° C or higher, they react with the trace amount of SiO ₂ present in Si ₃ N ₄ or the oxygen of the oxide added as a sintering aid to form MgO-SiO ₂ crystals or liquids. While making a phase,
Some of them form a solid solution by forming a solid solution in Si ₃ N ₄ , and the actions of both occur at the same time as the transition from α phase to β phase, which promotes densification, and Si ₃ N ₄ particles are firmly bonded. It is a thing.

Moreover, when CeO ₂ which is a rare earth element oxide coexists with SrO, MgO, etc., it is uniformly distributed at the interface of Si ₃ N ₄ particles and is bonded to _two or more components of SiO, MgO and SiO ₂ and has a high melting point. It is extremely effective in densifying silicon nitride by forming a phase or crystal. The obtained sintered body is composed of very fine and uniform particles, which greatly contributes to the densification and high strength of the silicon nitride sintered body according to the present invention.

The silicon nitride powder raw material used in the present invention preferably contains at least 30% or more of α phase. The reason is that, as described above, the transition from the α phase to the β phase of silicon nitride is involved in the sintering, so that the transition is promoted and the densification is sufficiently performed. Impurities in the silicon nitride powder cause evaporation of impurity components during high-temperature firing, causing pores and reacting with additional components to form a liquid phase with a low melting point, significantly deteriorating the high temperature characteristics of the sintered body. Therefore, it is desirable that the purity of silicon nitride is at least 97% or more. Furthermore, since silicon nitride has poor sinterability, it is necessary to make the raw material grain size fine and increase the surface tension of the raw material powder, and it is necessary to use fine grains having a grain size of 2 microns or less.

Next, when the amount of the additive component increases, the silicon nitride becomes densified, but the excessive additive component forms a large amount of glass phase between the silicon nitride particles and remarkably deteriorates the characteristics as a high temperature material, which is not preferable. Also, MgO and SrO are less than 0.5% each, total 1%
Less than 1% and the total amount of CeO ₂ is less than 1%, the densification is not sufficient and the mechanical strength is small. Therefore, it is suitable that CeO _{2 is} 1 to 10% by weight, preferably 1 to 5% by weight, and MgO and SrO are 0.5 to 5% by weight, preferably 1 to 4% by weight respectively, and 2 to 8% by weight in total.

Further, the firing temperature needs to be 1600 ° C. or higher in order to obtain a high-density sintered body, but if it exceeds 1900 ° C., the decomposition of silicon nitride becomes severe, which is not preferable. Further, the firing atmosphere needs to be fired in a nitrogen atmosphere or an inert gas atmosphere in order to prevent decomposition and oxidation of silicon nitride.

Hereinafter, a specific example will be described.

Example 1 Silicon nitride containing 90% by weight or more of α phase having an average particle size of 0.5 to 0.7 μm and CeO 2 having an average particle size of 1.8 μm as a sintering aid.
₂ , SrCO _{3 of} 3.2 μm and MgO of 0.5 μm were used.

As a method of increasing the amount of SiO _2, that is, the amount of oxygen, the amount of SiO ₂ was changed and added, and wet grinding was performed with water.

After the completion of the wet pulverization, the slurry was dried and molded into a rectangular parallelepiped having a size of 60 × 60 × 8 mm at a pressure of 200 kg / cm ² , and then hydrostatic pressing was performed at a pressure of 2500 kg / cm ² . The obtained molded body was fired under N ₂ atmosphere at 1750 ° C. for 1.5 hours. The obtained sintered body was measured at room temperature and 1000 ° C. for four-point bending strength in accordance with JIS R-1601 “Test method for bending strength of fine ceramics”.

Regarding the SiO ₂ / MgO weight ratio, the silicon nitride sintered body was crushed and subjected to chemical analysis, the oxygen content contained in the oxide sintering aid was subtracted from the total oxygen content, and the subtracted oxygen content was calculated.
It was converted to SiO ₂ and used as the SiO ₂ / MgO weight ratio.

The above test results are shown in Table 1.

As is clear from Table 1, the SiO ₂ / MgO weight ratio is 0.8,1.0.
Since the sample had a small amount of SiO ₂ , a MgO—SiO ₂ system crystal phase was generated, and no crystal phase was observed in the weight ratio range of 1.1 to 2.2. This is considered to be because the glass phase component increased due to an increase in the SiO ₂ content, and the glass phase was likely to become uniform, and the crystal phase did not precipitate. Regarding the strength, the samples with a crystalline phase did not show a large difference in the high temperature strength, but there was a variation in the room temperature strength, showing a slightly low value of about 50 kg / mm ² . At high temperature strength, when the SiO ₂ / MgO ratio was 2.2, which was the highest, the value was 29 kg / mm ² , and it was considered that the grain boundary phase tended to soften due to the increase in the amount of the glass phase, indicating a low value.

From the above, it was found that the optimal SiO ₂ / MgO weight ratio in the range of 1.1 to 2.1 is that the crystal phase does not occur and the high temperature strength does not decrease.

In addition, the samples with and without the crystalline phase were examined for wear resistance and oxidation resistance. As a result, in the abrasion test, the sample without the crystal phase did not cause chipping and had a good surface state, but as for the sample with the non-uniform phase, there was also a sample in which chipping occurred, and the surface state was the crystalline phase. The surface was rough compared to the sample without. Regarding the oxidation resistance, it was held at 1100 ° C for 1000 hours. The results are shown in FIG. In FIG. 1, the vertical axis shows the relationship between the weight increase and the room temperature strength after the oxidation treatment with respect to the change in the weight ratio of SiO ₂ / MgO. When the SiO ₂ / MgO ratio is within the range of this claim, 1.1 to 2.1, there is almost no weight increase (marked by ●), which is good, but when it is around 1.1 or less, the surface layer of silicon nitride is oxidized and suddenly increases. There is. This is also clearly shown in the room temperature strength (marked with ◯) after the oxidation treatment, and the surface layer of the sintered body with a large amount of increased oxidation is oxidized and becomes whitish, and when the SiO ₂ / MgO weight ratio is 0.8. , About 36 kg / mm ^{2, which} is a considerable decrease.

Due to the above-mentioned characteristics, it is important for the silicon nitride sintered body of the present invention to prevent the formation of a crystal phase, prevent the deterioration of high temperature strength, and secure the characteristics of wear resistance and oxidation resistance. The silicon nitride sintered body having a SiO ₂ / MgO weight ratio of 1.1 to 2.1, which is the scope of the present invention, is sufficiently characterized in all of the characteristics and is characterized by being a stable region.

(Effects of the Invention) According to the silicon nitride sintered body of the present invention described above, a good silicon nitride sintered body in which a crystal phase is not generated in the sintered body and which has high strength and is excellent in wear resistance and oxidation resistance It has a great industrial effect to provide the body.

Thus, the silicon nitride sintered body of the present invention has a mechanical strength,
Because of its excellent properties such as heat resistance, wear resistance, and oxidation resistance, for example, mechanical parts such as bearings and valves,
There are many uses as high-temperature structural materials such as automobile parts such as turbochargers and high-temperature roll materials for steelmaking.
It is extremely useful industrially.

[Brief description of drawings]

FIG. 1 shows the influence of the change in the SiO ₂ / MgO weight ratio on the weight increase and the room temperature strength after the oxidation treatment in the oxidation resistance test of the silicon nitride sintered body of the present invention held at 1100 ° C. for 1000 hours. It is a characteristic curve figure which shows.

Claims (1)

[Claims] 1. Silicon nitride 80 to 97% by weight, CeO ₂ 1 to 10% by weight, MgO and SrO 0.5 to 5% by weight, and SiO ₂ 0.55 to 10.5% by weight.
And a SiO ₂ / MgO weight ratio in the range of 1.1 to 2.1.