JPH07187865A - Surface-coated silicon nitride-based heat resistant member - Google Patents

Abstract

PURPOSE:To improve oxidation resistance by coating the surface of an Si3N4- based sintered compact with ZrSiO4. CONSTITUTION:A mixture of Si3N4 powder with SiO2 and one or more kinds of oxides (RE2O3) of group IIIa elements of the Periodic Table such as Y2O3 and La2O3 is compacted. In the mixture, the molar ratio of SiO2 to RE2O3 is <=2. The resulting compact is fired in an N2 atmosphere to produce an Si3N4- RE2O3-SiO2 sintered compact consisting of about 0.1-10mol% RE2O3, about 0.2-40mol% (expressed in terms of SiO2) excess oxygen and the balance Si3N4. Prescribed amts. of ZrO2 powder and SiO2 powder are mixed and a binder is added to prepare the slurry. This slurry is applied to the surface of the Si3N4- based sintered compact, dried, allowed to remove the binder and fired in an Ar atmosphere to obtain the objective surface-coated Si3N4-based heat resistant member with a ZrSiO4 coating layer having about 10-1,000mum thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride heat resistant member having a surface coating layer which is excellent in heat resistance and corrosion resistance at high temperatures and which is excellent in thermal shock.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been applied to engineering ceramics, in particular, as members for gas turbines because of their excellent heat resistance, thermal shock resistance and oxidation resistance.

A silicon nitride based sintered body has a Y content with respect to silicon nitride.
Sintered materials such as ₂ O ₃ and Al ₂ O ₃ are added and fired. The sintered body is required to have room temperature strength and high temperature strength, as well as oxidation resistance at high temperature. To be done. In order to meet such demands, various methods have been proposed, such as studying the type of sintering aid to be added and crystallization of the grain boundary phase in the sintered body.

However, although improvements in the sintered body itself have made it possible to obtain a certain level of mechanical strength, the oxidation resistance is more severe under high-oxidation conditions. Since it is becoming a sexual atmosphere, it has not yet obtained sufficient durability.

Therefore, as one method for imparting such oxidation resistance, the surface of the silicon nitride sintered material is coated with a metal oxide having excellent heat resistance and oxidation resistance, such as zirconia. It is proposed in Japanese Examined Patent Publication No. 5-8152.

[0006]

However, zirconia is a silicon nitride sintered body (coefficient of thermal expansion of 3 to 4 × 10 ^-6 /
The coefficient of thermal expansion is higher than that of (° C) (about 10 × 10 ^-6 /
There is a problem that peeling or cracking occurs in the coating layer due to repetition of temperature rise and decrease, and heat resistance is significantly deteriorated. For such a problem, it is possible to form an intermediate member having an intermediate thermal expansion property between the silicon nitride sintered body and zirconia, but it has not been practically solved yet.

Further, when a member coated with zirconia using a silicon nitride sintered body having a low oxidation resistance as a base is exposed to a high temperature oxidizing atmosphere for a long time, diffusion of oxygen occurs through the coating layer and silicon nitride is produced. There is also a problem that the sintered body is oxidized and a low-melting glass is formed at the interface between the coating layer and the sintered body, and peeling occurs due to repeated thermal fatigue.

The present invention solves the above problems,
An object of the present invention is to provide a surface-coated silicon nitride heat-resistant member which is excellent in thermal fatigue even when kept in a high temperature oxidizing atmosphere of 1000 ° C. for a long time, and in which peeling of a coating layer does not occur and which has excellent corrosion resistance and heat resistance. is there.

[0009]

To suppress cracks and peeling of the surface coating layer, it is important for the present inventor to reduce the difference in coefficient of thermal expansion between the silicon nitride sintered body and the surface coating layer. As a result of repeated studies from the viewpoint that the above, it was found that the use of zircon (ZrSiO ₄ ) in the surface coating layer can significantly suppress the occurrence of peeling and cracking due to repeated temperature rising and lowering, leading to the present invention. It was

That is, the surface-coated silicon nitride heat-resistant member of the present invention is characterized in that the surface of a substrate made of a silicon nitride sintered body is coated with zircon (ZrSiO ₄ ).

The present invention will be described in detail below. The silicon nitride heat-resistant member of the present invention is composed of a base body made of a silicon nitride sintered body and a coating layer formed on the surface of the base body.

The coating layer in the present invention is composed of zircon. Zircon is a 1: 1 compound of ZrO ₂ and SiO ₂ . This coating layer is formed on the surface of the substrate of the silicon nitride sintered material in an amount of 10 to 1000 μm, particularly 50 to 500 μm.
It is formed with a thickness of m.

On the other hand, the silicon nitride-based sintered body constituting the base body contains silicon nitride as a main component, but the silicon nitride-based sintered body according to the present invention has heat resistance and oxidation resistance as the base body. Therefore, the composition of the sintered body is Si ₃ N ₄ —SiO ₂
It is desirable to be composed of a ternary system represented by —RE ₂ O ₃ (RE: Group 3a element of the periodic table). Among them, from the viewpoint of oxidation resistance, it is preferable that the RE ₂ Si ₂ O ₇ crystal phase (RE: Group 3a element of the periodic table) be precipitated at the grain boundaries of the silicon nitride crystal. In the composition, SiO ₂ is so-called excess oxygen existing in the sintered body, and specifically, from the total oxygen amount in the sintered body, an oxide of Group 3a of the periodic table in the sintered body is used. Is the amount of oxygen remaining excluding oxygen bound to the elements when forming an oxide stoichiometrically, most of which is oxygen contained in the silicon nitride raw material or a component mixed as addition of SiO ₂ or the like. Is.

In the sintering process, these RE ₂ O ₃ and SiO ₂ exist as a liquid phase to increase the sinterability by the reaction with the silicon nitride particles, but if they remain as a glass phase in the grain boundary phase after cooling, they are burned. since deteriorates the oxidation resistance and at the same time will reduce the high temperature strength of the sintered body, RE ₂ Si ₂ these components with excellent oxidation resistance at high melting point at the grain boundary by a predetermined cooling process or a heat treatment By precipitating as an O ₇ crystal phase, the high temperature characteristics of the sintered body can be improved.

The composition of the silicon nitride-based sintered body is 0.1 to 10 when the oxide (RE ₂ O ₃ ) of Group 3a element of the periodic table is used.
Mol%, particularly 0.3 to 5 mol%, excess oxygen (converted to SiO ₂ ) is 0.2 to 40 mol%, particularly 0.6 to 20 mol%,
The balance is preferably made of silicon nitride. Examples of the Group 3a element of the periodic table used in the present invention include Y and lanthanoid elements, but in particular Er, Yb and Lu.
Is preferred.

In some cases, Al, Ca, Mg, Fe, etc. are contained as unavoidable impurities in the sintered body, but these elements easily form a low melting point substance as an oxide,
Since these components tend to deteriorate the high temperature characteristics of the sintered body, it is desirable to control these components to 0.5 wt% or less in terms of oxides.

In addition, the above Si ₃ N ₄ --RE ₂ O ₃ --S
In addition to iO ₂ , TiC, TiN, WC, W within the range that does not adversely affect the characteristics of the sintered body, especially the crystallization of the grain boundaries.
It is also possible to add carbides, nitrides, oxides, carbonitrides and the like of elements of Groups 4a, 5a and 6a of the periodic table such as O ₃ , Nb ₂ O ₅ , Cr ₂ O ₃ and Cr ₂ C.

As a method for producing the above-mentioned silicon nitride sintered body, silicon nitride powder, Group 3a element oxide powder of the periodic table, and silicon oxide powder in some cases are used as raw material powder, and these are used in the above-mentioned composition. Weigh and mix to within range. At this time, as another form, metal silicon powder can be used instead of silicon nitride. The form of addition of the Group 3a element oxide (RE ₂ O ₃ ) of the Periodic Table is a compound powder consisting of one or more of RE ₂ O ₃ and silicon oxide, or an oxide of one or more of silicon nitride and RE ₂ O ₃ and oxidation. A compound powder composed of silicon can also be used.

The mixed powder thus obtained is molded into a desired shape by a known molding method, for example, press molding, cast molding, extrusion molding, injection molding, cold isostatic molding, etc., and then a known firing method. For example, a hot press method, normal pressure firing, nitrogen gas pressure firing, and further, HIP treatment after these firings, and glass seal HIP firing are performed to obtain a dense sintered body.

When silicon powder is used in the molded body, the molded body is in the nitrogen-containing atmosphere at 800 ° C to 1500 ° C.
The heat treatment is performed at a temperature of 1 to nitrid the silicon contained in the molded body to generate silicon nitride, and then the firing is performed.

At least RE ₂ S is contained in the grain boundaries of the sintered body.
In order to precipitate i ₂ O ₇ (RE is a Group 3a element of the periodic table) crystals, the composition in the compact should be SiO ₂ / RE.
_{The 2} O ₃ molar ratio may be controlled to be 2 or less, and may be maintained during cooling after firing, or by primary holding in the cooling stage, or by heat treatment after firing.

On the other hand, the coating layer made of zircon is prepared by mixing a predetermined amount of ZrO ₂ powder and SiO ₂ powder, adding a binder, and applying a slurry to the surface of the silicon nitride sintered body or spraying the slurry. Or bake at a high temperature once, or the mixed powder is once treated at a high temperature to synthesize zircon and then pulverized to form a coating layer by the same method. In addition, the coating layer can be formed by spraying zircon powder on the surface of the substrate by a well-known plasma spraying method.

The zircon coating layer is formed by coating or spraying zircon slurry on the surface of the silicon nitride sintered material and baking it at a high temperature. Alternatively, instead of zircon powder, a mixed powder of equimolar% of zirconia powder and silica powder may be used. Alternatively, the coating layer may be formed by plasma spraying.

[0024]

The thermal stability of the surface-coated heat-resistant member depends on the difference in thermal expansion between the silicon nitride sintered body and the surface coating layer. When the difference in thermal expansion is large, a large thermal stress is generated when the temperature is raised or lowered, and the surface coating layer is peeled or cracked. Therefore, it is important to reduce the difference in thermal expansion in order to increase the thermal stability. According to the present invention, as a result of studying the material of the surface coating layer based on the above viewpoint, the coating layer has a small difference in thermal expansion from the silicon nitride sintered body, has high heat resistance, and is excellent in heat insulating property. By using, it is possible to prevent the coating layer from cracking or peeling even when the temperature is raised and lowered repeatedly. This makes it possible to provide a silicon nitride heat-resistant member having a surface coating layer having excellent corrosion resistance and heat resistance.

[0025]

EXAMPLE Silicon nitride powder (BET specific surface area 8 m ² / g, α ratio 98%, oxygen content 1.2% by weight, metallic impurities 0.03% by weight) as raw material powder, Yb ₂ O ₃ 3 mol%, Si
9 mol% of O ₂ was weighed and mixed, and this was press-molded and then fired at 1850 ° C. for 4 hours under a nitrogen gas pressure of 10 atm.
A disc-shaped sintered body having a diameter of 50 mm and a thickness of 5 mm was obtained.

When X-ray diffraction measurement was performed on the obtained sintered body, Yb ₂ Si ₂ O ₇ and Si ₂ N ₂ O were detected. The bending strength at 1400 ° C of this sintered body was measured by JI
When measured by the four-point bending test of SR1601, 720
It was MPa. In addition, when the increase in oxidized weight was measured after holding at 1400 ° C in the atmosphere for 24 hours, it was 0.1 mg.
Was / cm ² .

Next, the mixed powder having the composition shown in Table 1 was used to form a coating layer by a slurry coating method and a plasma spraying method. The coating layer was formed by the slurry coating method by coating the surface of the sintered body with the slurry made of the powders shown in Table 1, drying and binder removal, and then heat-treating at 1400 ° C. in an argon atmosphere for 1 hour to prepare a sample. It was made. The coating layer was formed by plasma spraying by spraying the powder shown in Table 1 on the surface of the sintered body as a spraying agent to prepare a sample.

A repeated thermal fatigue test was conducted on each of the obtained samples. The test was carried out by placing the sample in an electric furnace maintained at 1000 ° C. and holding it for 15 minutes, then taking it out of the furnace and allowing it to cool.
It was put in the furnace again, and this was performed as one cycle for a maximum of 30 cycles, and the number of times until cracking occurred in the coating layer was confirmed. The results are shown in Table 1.

[0029]

[Table 1]

* Indicates a sample outside the scope of the present invention.

In Table 1, the powder composition is ZrO ₂ : S.
Samples No. ₂ and 5 using the mixed powder of 1: 1 of io 2 are Z
As a result of the X-ray diffraction measurement as in the case of Sample Nos. 1 and ₄ using the rSiO ₄ powder, it was confirmed that the crystal was composed of zircon (ZrSiO ₄ ) crystals.

According to the results shown in Table 1, the samples Nos. 1, 2, 4, and 5 having zircon as the surface coating layer were 50 times in the repeated thermal fatigue test between room temperature and 1000 ° C.
Although cracks did not occur even after repeated use, samples No. 3 and 6 having zirconia as the surface coating layer, on the other hand, suffered peeling or cracking a small number of times and had low thermal fatigue properties.

[0033]

As described in detail above, according to the present invention,
It is possible to provide a heat-resistant member having a surface coating layer that is excellent in high-temperature strength and oxidation resistance and is also excellent in repeated thermal fatigue properties between room temperature and high temperature. As a result, it is possible to expand the application to various heat resistant members such as heating members for boilers as well as heat engines such as gas turbines.

Claims (1)

[Claims] 1. Zircon (ZrS) is formed on the surface of a silicon nitride sintered body.
A surface-coated silicon nitride heat-resistant member characterized by being coated with iO ₄ ).