JPH10158097A - Oxidation preventing coating film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a coating film inhibiting the formation of cristobalite in an oxidizing atmosphere at a high temp., suppressing the progress of oxidation toward the interior and having enhanced oxidation preventing performance by diminishing <111> oriented grains in a polycrystalline β-silicon carbide coating film preventing the oxidation of a substrate formed by a chemical vapor phase reaction-vapor deposition method. SOLUTION: The fraction of <111> oriented grains in a polycrystalline β-silicon carbide coating film is made lower than that in a random oriented polycrystalline body. When the polycrystalline β-silicon carbide coating film formed on a graphite substrate by a chemical vapor phase reaction-vapor deposition method using gaseous SiCl4 , CH4 and H2 is exposed to an oxidizing atmosphere at >=1,500% deg.C, cristobalite having a high coefft. of thermal expansion as well as amorphous SiO2 is formed. Since the formation of cristobalite depends on the amt. of <111> oriented grains in the β-silicon carbide coating film, the oxidation preventing ability of the coating film is enhanced by diminishing <111> oriented grains.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon carbide oxidation protective film utilizing a chemical vapor deposition method.

[0002]

2. Description of the Related Art Although the strength at high temperatures is very good,
A carbon-based material coated with a silicon carbide thin film on the surface of a carbon-based material that is easily oxidized and volatilized and eroded in a high-temperature oxidizing environment is resistant to severe environments in the aerospace and energy fields and has a high ratio. It is expected as a structural material having strength. As such an oxidation protection film, a polycrystalline silicon carbide silicon carbide film has conventionally been adopted by a chemical vapor deposition method, and a silicon dioxide layer which is hardly permeated by oxygen atoms on the surface of the silicon carbide film in an oxidizing atmosphere. Was formed, thereby suppressing the oxidation inside the silicon carbide film and preventing the base material from being oxidized and volatilized.

[0003]

When a silicon carbide film is placed in an oxidizing atmosphere, a dense amorphous silicon dioxide layer is formed on the surface of the film, and the diffusion coefficient of oxygen atoms in this layer is extremely small. Therefore, it is possible to suppress the progress of acid north into the interior, but when the ambient temperature is 1500 ° C. or higher, a high-temperature phase (β) cristobalite, which is crystalline silicon dioxide, is also formed on the film surface in addition to the amorphous silicon dioxide phase. Become. This high-temperature cristobalite has a larger coefficient of thermal expansion than amorphous silicon dioxide, and when it cools, it shrinks greatly, causing cracks,
Further, at a temperature of 200 to 270 ° C., a displacement-type phase transformation to the low-temperature phase (α) cristobalite occurs, whereby the volume further shrinks and cracks progress. When this is exposed to a high temperature again, there has been a problem that oxygen atoms proceed from the cracks to the inside, the cristobalite phase region is separated, and oxidation proceeds rapidly, and finally the base material is attacked. The present invention
An object of the present invention is to solve such a problem, and an object of the present invention is to provide a silicon carbide oxidation protective film having an effect of suppressing the generation of cristobalite.

[0004]

SUMMARY OF THE INVENTION The present invention is a polycrystalline beta silicon carbide coating which is formed on a carbon-based substrate by chemical vapor deposition to protect the substrate from oxidation.
> The fraction of crystal grains to be oriented is <
111> an oxidation protective film characterized by being less than the fraction of oriented crystal grains, and suppressing the formation of cristobalite from the silicon carbide film in a high-temperature oxidizing atmosphere.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a polycrystalline beta silicon carbide film for protecting a substrate from oxidation, if the number of <111> oriented crystal grains is small, even in a high-temperature oxidizing atmosphere at 1500 ° C. or higher. Utilizing the low generation of cristobalite from the silicon film, it promotes the formation of a dense amorphous silicon dioxide layer with a small diffusion coefficient of oxygen atoms and suppresses the progress of oxidation to the inside of the substrate It protects against oxidation volatilization and wear.

In a chemical vapor deposition method, a polycrystalline beta silicon carbide film is formed by changing film forming conditions such as a film forming reaction temperature, a precursor gas flow rate, a precursor gas flow ratio, and a film forming reaction pressure. The fine structure such as the surface morphology, crystal grain size, crystal grain shape and crystal grain orientation can be changed.

[0007] The term "crystal grain orientation" as used herein refers to the nature of what crystal orientation of each crystal grain is parallel to the surface normal of the silicon carbide film. For example, <111
> Orientation means that the <111> crystal axis of most of the crystal grains is parallel to the surface normal of the silicon carbide film. Further, “random orientation” means a distribution of crystal orientations in which arbitrary <hkl> crystal orientations appear with equal probability without depending on the value of hkl. Here, hkl is a set of three integers having no common divisor other than 1.

When a silicon carbide film formed by chemical vapor deposition is exposed to a high-temperature atmosphere of 1500 ° C. or more, or an oxidizing atmosphere such as methane combustion exhaust gas, a dense amorphous silicon dioxide layer is formed on the surface. In addition, it is known that cristobalite, which is crystalline silicon dioxide, is formed at the same time.

Although this cristobalite impairs the oxidation protection function of silicon carbide as described above, its ease of formation depends on the amount of <111> oriented grains in the polycrystalline beta silicon carbide, and <111>. The present inventors have found that the more the oriented crystal grains, the more the cristobalite is generated.

From this, it can be seen that the more the <111> oriented beta silicon carbide crystal grains, the lower the oxidation protection function of the silicon carbide film. Therefore, the oxidation protection performance can be enhanced by reducing the number of the <111> -oriented crystal grains.

Note that the cristobalite generation amount is <111
The dependence on the amount of oriented beta silicon carbide grains is presumed to be due to cristobalite nucleating <111> oriented grains as preferential nucleation sites in a high-temperature oxidizing atmosphere. It is considered that the cause of the formation is related to the consistency of crystal atoms at the interface between silicon carbide and cristobalite.

[0012]

[Example] 10 mm x 10 mm x 2 mm or 30 mm
Using an isotropic graphite material (IG-11 manufactured by Toyo Carbon Co., Ltd.) having a size of mm × 30 mm × 4 mm, a silicon carbide film was formed by a thermal chemical vapor deposition method. As a precursor gas, silicon tetrachloride gas, methane gas and hydrogen gas are used, and the film formation temperature is set to 1
200-1300 ° C.

The reason why the graphite substrate was selected in the present embodiment is that the coefficient of thermal expansion of the graphite material is close to that of silicon carbide. This is because cracks can be prevented from occurring due to the difference between the two and the stress generated in the silicon carbide film is small. If the thermal expansion coefficients of the silicon carbide film and the base material are significantly different, cracks will occur in the film or large stress will be generated in the film, and when performing a high-temperature oxidation test, the silicon carbide film will exhibit complex oxidation behavior. As a result, there is a possibility that the intrinsic properties of silicon carbide may be lost.

In each chemical vapor deposition process, several substrates are simultaneously placed on sample supporting pins (3 or 4 pins are used for each sample) fixed in the chemical vapor deposition apparatus. To prevent the formation of a film due to the substrate being shielded by the substrate support as much as possible, but since the film is not formed at the pinpoint support point, half of the estimated film formation time has elapsed. At this point, the substrate was turned over and film formation was continued.

The total film forming time per sample was set so that the film thickness was about 90 μm in consideration of the film forming speed under each film forming condition.

The film microstructure was identified by analyzing the film surface and the film cross section using X-ray diffraction analysis, optical microscope observation, and scanning electron microscope observation. In the X-ray diffraction analysis method, a monochromatic CuKα ray was used as an X-ray source, and a θ-2θ scanning method was used. Scanning electron microscope observation was performed at an electron acceleration voltage of 15 to 25 kV.

In the thermal chemical vapor deposition method, when the film forming conditions such as a film forming reaction temperature, a precursor gas flow rate, a precursor gas flow ratio, and a film forming reaction pressure are changed, a polycrystalline base carbon is formed. The fine structure such as the surface morphology, crystal grain size, crystal grain shape, and crystal grain orientation of the silicon film also changed. Table 1 shows the relationship between the fifteen different film forming conditions of the formed silicon carbide and the obtained film microstructure. No crack was observed in the film.

[0018]

[Table 1]

In Table 1, symbols (1) for crystal grain orientation,
(2) and (3) represent (1) almost randomly distributed, (2) strong <111> orientation, and (3) coexistence of preferential <110> oriented grains and randomly distributed grains. Symbols for crystal grain shape (CL), (GL), (C
S) and (GS) are (CL) columnar, large (> 5 μm) crystal grains, (GL) granular, large (> 5 μm) crystal grains, (CS) columnar, small (<5 μm) crystal grains, (GS) Granular, smaller (<5 μm) crystal grains. Facets exhibit a tendency to faceting on a small scale and represent (F) a faceted surface, (R) a relatively round smooth surface. Pebble indicates a tendency of pebble on a wider scale, and indicates (P) strong pebble, (L) less pebble.

The film forming conditions A to O shown in Table 1 are not universal, and greatly depend on the chemical vapor deposition apparatus to be used. However, it is clear that the film microstructure can be controlled by controlling the film forming conditions. In the following oxidation tests, examples are shown only for the samples prepared under the conditions of A to D. However, the results of A to O shown in Table 1 as the film forming conditions are as follows. This is to show that the structure changes.

The oxidation test was performed in a methane combustion exhaust gas atmosphere and an air atmosphere.

In the oxidation test in the atmosphere of methane combustion exhaust gas,
Four different film forming conditions (film forming condition names A, B, C, and
Using a sample of 10 mm × 10 mm × 2 mm coated with a silicon carbide film having a different microstructure obtained in D),
The methane combustion exhaust gas was introduced into a sample placed in an electric furnace at 0 ° C., and after holding for 8 hours, the temperature was returned to room temperature and the surface was observed.
Further, the same oxidation test was continued. Such a heating / cooling cycle was repeated three times. The heating time at 1500 ° C. is 24 hours. For methane combustion, the molar ratio of air to methane gas is 15: 1, the oxygen gas concentration in the methane combustion exhaust gas is 8 mol%, and the dew point corresponding to the concentration of water vapor is about 50 ° C.
Met.

In the oxidation test in the air atmosphere, a silicon carbide film having a different microstructure obtained under three different film forming conditions (film forming condition names A, B, and C in Table 1) was 30 mm in size.
1600 ℃ open to the atmosphere using a sample of × 30mm × 4mm
After holding the sample in the xenon lamp heating apparatus for 1 hour, the temperature was returned to room temperature, the surface was observed, and the same oxidation test was continued. Such a heating / cooling cycle was repeated five times. The integrated heating time at 1600 ° C. is 5 hours.

After the oxidation test, the sample was analyzed by optical microscopy, electron probe characteristic X-ray spectroscopy, and X-ray diffraction analysis, and the change in sample mass before and after the oxidation test was measured. After the oxidation test, the sample was subjected to elemental analysis by electron probe characteristic X-ray spectroscopy. As a result, oxygen was detected on the surface, and it was clearly recognized that the silicon carbide film was oxidized. When the change in the sample mass before and after the oxidation test was measured, the change in both of the two oxidation tests was below the detection limit of 0.025%, and no significant mass change was observed.

It can be said that, under the conditions of the present experiment, the oxidation protection function is sufficiently working, apart from the degree of the difference. However, the surface condition of the sample was different from each other visually and optical microscope observation after the oxidation test, and the amount of cristobalite, which appeared as white dots in the transparent and glossy dense amorphous silicon dioxide film, depends on the film forming conditions. Significantly dependent. The order is ascending order of cristobalite. In oxidation test in methane combustion exhaust gas, conditions A, D, B, C,
In the atmospheric oxidation test, conditions A, B, and C were in the order of.
This indicates that the oxidation protection performance is higher in this order.

When the sample surface after the oxidation test was subjected to X-ray diffraction, a cristobalite peak was detected. The peak intensity depends on the sample, and in the oxidation test in methane combustion exhaust gas, conditions A, D,
B, C, and in the atmospheric oxidation test, conditions A, B, and C were in order. This is consistent with the result of the above visual observation.

FIGS. 1 to 4 show examples of X-ray diffraction profiles (oxidation test in methane combustion exhaust gas). FIG. 1 is a film formation condition A, FIG. 2 is a film formation condition B, FIG. 3 is a film formation condition C, and FIG. X after three oxidation tests
It is a line diffraction profile. The scales of the vertical axes indicating the diffraction intensities are different from each other, and the full scale is 11.06 in FIG.
8 kcps, 14.567 kcps in FIG. 2, 42.740 kcps in FIG. 3, 17.371 kcps in FIG.
It is. The indices of 111, 220 and 311 relate to beta silicon carbide, and the ratios of the respective peak intensities in FIGS. 1 to 4 are different depending on the film forming conditions, and the orientation of crystal grains is different. I understand. Further, two kinds of peaks marked Si0 ₂ were conspicuously observed in each of the figures. This indicates that cristobalite has been generated. A large amount of amorphous silicon dioxide is also produced, but no remarkable peak appears due to the amorphous nature, and these profiles are not clear.

In order to make the X-ray diffraction results easy to understand, Table 2 shows the two types of cristobalite diffraction peak intensities and silicon carbide 1 in the samples involved in the oxidation test in methane combustion exhaust gas.
Summarize the 11 peak intensities. From this table, it can be seen that the condition A is the smallest for the two types of cristobalite peak intensities I ₁ and I ₂ , and increases in the order of D, B, and C in the following order.

[0029]

[Table 2]

Focusing on the peak intensity of silicon carbide 111 in Table 2, the intensity is the smallest under condition A, and D, B, C
It turns out that it becomes large in order of. This order is consistent with the order of cristobalite peak intensity. Therefore, it can be said that the smaller the peak intensity of silicon carbide 111, the lower the peak intensity of cristobalite.

From these results, it was found that <111
> It cannot be said that the smaller the number of oriented crystal grains, the less the generation of cristobalite. Cristobalite has two types of crystal faces (peaks I ₁ , I
₂ respectively). Here, assuming that one of these cristobalite crystal planes is parallel to the (111) plane of silicon carbide when cristobalite is generated, the more the silicon carbide is oriented <111>, the more the cristobalite is oriented in a plane parallel thereto. Therefore, it is natural that the larger the 111 peak of silicon carbide, the larger the peak of cristobalite in the crystal plane parallel to 111 of silicon carbide.

In order to confirm this possibility, the ratio of I ₂ / I ₁ was calculated and shown in Table 2. If Christobalite is X
The value of I ₁ if oriented in the plane corresponding to the peak that gives I ₁ is increased at a linear diffraction, surface corresponding to the peak that gives the I ₂ does not occur is analyzed Kai no longer parallel to the sample surface Therefore, the value of I ₂ becomes very small. Therefore, the value of the ratio I ₂ / I ₁ is close to zero. Further, if cristobalite is oriented on a plane corresponding to a peak giving I ₂ in X-ray diffraction, the value of I ₂ becomes large,
Since the plane corresponding to the peak giving I ₁ is not parallel to the sample surface and diffraction does not occur, the value of I ₁ becomes very small. Therefore, the value of the ratio I ₂ / I ₁ is close to infinity.

However, looking at the condition C in which the <111> orientation of the silicon carbide crystal grains is strong, it can be seen that I ₂ / I ₁ is rather the maximum in this table. This means that cristobalite is not oriented. Therefore,
The above problem was solved, and it was proved that the smaller the number of the <111> -oriented silicon carbide crystal grains, the smaller the generation of cristobalite.

Table 3 shows the same peak intensities as in Table 2 for the oxidized samples in the atmosphere. In Table 3, an oxidation test was separately performed under the same conditions for two samples manufactured under the same film forming conditions, and the results are shown as test pieces 1 and 2. From this table, the same conclusion as the result of the oxidation test in the methane combustion exhaust gas atmosphere can be said for the oxidation sample in the air atmosphere, and it was found that the smaller the number of the <111> oriented silicon carbide crystal grains, the smaller the generation of cristobalite.

[0035]

[Table 3]

Summarizing the above results, in both the methane combustion exhaust gas atmosphere and the air atmosphere, at a high temperature of 1500 ° C. or higher, the smaller the number of <111> -oriented crystal grains in the silicon carbide film, the smaller the generation of cristobalite. It is concluded. Accordingly, it is preferable that the fraction of the crystal grains having the <111> orientation be as small as possible.
It is preferable that at least the fraction of the bulk polycrystal grains is small, but it is desirable that the fraction be smaller than at least the fraction of the <111> oriented crystal grains in the case of random orientation appearing in the bulk polycrystal. The fraction of <111> -oriented crystal grains in the case of random orientation is 8dΩ / 4π
It is. Where dΩ is the allowable solid angle around the <111> axis, 8 is the multiplicity factor of <111>, and 4π is the full solid angle.

[0037]

As described above, according to the present invention, it is possible to control the conditions of the chemical vapor deposition method to form a silicon carbide film having a fine structure with less generation of cristobalite in a high-temperature oxidizing atmosphere. Thereby, an oxidation protection film having a high oxidation protection performance for a carbon-based substrate can be produced.

[Brief description of the drawings]

FIG. 1 is a chart showing an X-ray diffraction profile after performing an oxidation test on an example of the present invention prepared under film forming condition A.

FIG. 2 is a table showing an X-ray diffraction profile after performing an oxidation test on an example of the present invention prepared under film forming conditions B.

FIG. 3 is a table showing an X-ray diffraction profile after performing an oxidation test on an example of the present invention prepared under film forming conditions C;

FIG. 4 is a table showing an X-ray diffraction profile after performing an oxidation test on an example of the present invention prepared under film forming conditions D.

Claims (1)

[Claims] 1. A polycrystalline beta silicon carbide film for forming a laminate by chemical vapor deposition to protect the substrate from oxidation, wherein the fraction of <111> -oriented crystal grains in the randomly-oriented polycrystalline body is <111> An oxidation protective film characterized by having a fraction less than the oriented crystal grains.