DE3920450A1 - Producing oxidn. and thermo-shock resistant coating on carbon bodies - by forming layers of silicon carbide or nitride followed by glassy layer of silicon oxide opt. with silicon - Google Patents

Abstract

Carbon bodies are protected from oxidation by converting at least part of the surface into SiC, coating with three or more layers of SiC or Si3N4 followed by a layer of glassy SiO2 or mixture of SiO2 and Si. The carbon matrix pref. has a porosity of 2-15 vol.% and contains 30-70 vol.% reinforcement carbon fibre. The carbon matrix surface is converted partially with SiC by surface packing with I or liquid impregnating with Si or by treating with Si vapour at high temp. and correcting to SiC. The layers of SiC or Si3N4 are produced by chemically precipitating from the gas phase and pyrolysing alkylsilanes or alkylhalogensilanes. The final coating of SiO2 or SiO2/Si is tempered to produce the glassy state. ADVANTAGE - The protection layer is dense and adheres well to the substrate. It has good resistance to thermo shock stressing.

Description

The invention relates to the oxidation protection of carbon or Graphite bodies, possibly reinforced with carbon fibers a combination of protecting the substrate and applying Protective layers and a top layer.

From EP-A 2 82 386 it is known carbon bodies or carbon Protect asers against oxidation by using multiple protective layers Silicon carbide and layers of SiO₂ and glazing thereon SiO₂ + B₂O₃ are applied. The layers of glass fill the pores and cracks in the intermediate layer of silicon carbide and create such a dense protective layer.

The healing of cracks in silicon carbide layers with borate glasses has disadvantages, however, because borate glasses are sensitive to them Humidity. By gradual hydrolysis in a humid environment swelling takes place and the glass layers can partly flake off. Furthermore, insulating layers based on B₂O₃ are only for short exposure times suitable, since B₂O₃ at higher temperatures has a relatively high vapor pressure, so that the known protection  layers fail after a short time. Also SiO₂ has not completely proven, because there are reactions with carbon and the resulting CO can detach the layers from the substrate.

EP-B 1 33 315 is directed to a powder mixture based on Al₂O₃, Si and SiC to form a coating on carbon substrates to protect the substrate against oxidation at elevated Temperatures.

The substrate is brought into contact with the powder mixture and then a so-called pack silicification carried out at elevated temperature. The carbon bodies can also be filled into the mixture by brushing with a slurry of the mixture and then Drying and heating to a higher temperature. The on this Wise trained protective layer is more of a change of together settlement in the surface area of the substrate as an additional Layer. The presence of silicon allows for heating in Presence of oxygen the formation of silicon dioxide, which as glassy material becomes liquid at higher temperatures and for Surface sealing contributes. The protective effect can still can be improved by impregnating the base layer with tetraethyl ortho Silicate and heat so that the pores are filled and the substrate be coated.

From US-A 44 25 407 it is known carbon bodies against oxidation to protect it by packing silicon in the surface silicon carbide is formed and then chemically from the To deposit the vapor phase deposited silicon carbide layer.

However, all of these protection procedures have been at long-term highs temperature load proven to be insufficient.

The object of the present invention is a method for protection of carbon or graphite bodies against oxidation and corresponding to create stable bodies, whose protective layers are sufficiently dense and are well adherent and also against thermal shock loads are stable.  

This task is solved by a carbon body or graphite
Body with oxidation protection layers made of SiC, which is characterized in that the carbon matrix in the surface area is at least partially converted to SiC, three or more layers of SiC or Si₃N₄ are deposited thereon and a cover layer made of glazed SiO₂ or a mixture of SiO₂ / Si is present.

This multi-layered structure of the oxidation protection takes place by means of a process in which the carbon matrix in the surface area is at least partially converted to SiC, by pack siliconization or liquid impregnation with silicon or treatment with silicon vapor and conversion of carbon to silicon carbide, application of three or more layers of silicon carbide or Si₃N₄ by chemical deposition from the vapor phase and pyrolysis of alkylsilanes or alkylhalosilanes and subsequent application of a cover layer by chemical deposition from the vapor phase and conversion of the cover layer into glazed SiO₂ and a mixture of glazed SiO₂ / Si by tempering.
Completely surprisingly, it was found that the combination of a conversion of the substrate in the surface area into silicon carbide, application of three or more layers of silicon carbide or silicon nitride and covering of the protective layers and sealing of cracks by glazing with SiO₂ results in long-term oxidation protection even at higher temperatures.

The carbon body or carbon matrix can have an open porosity 2-15% by volume, preferably 3-7% by volume.

The carbon or graphite matrix can be reinforcing carbon fibers contained in an amount of 30-70% by volume, preferably 50-65% by volume, if this is desired for reasons of strength.

The substrate protection in the surface area by siliconizing should Have a depth of 0.01 µm to 0.75 mm and in this area the Carbon at least partially converted to SiC.

Siliconizing or converting carbon to silicon carbide can by so-called packsilicate with a reaction mass  Silicon carbide, the 5-50 wt .-% silicon, preferably 7-25 wt .-% Contains silicon. A reak is very particularly preferred tion mass of 90 wt .-% SiC and 10 wt .-% silicon powder. The too Siliconizing carbon body with a slurry of this Brushed reaction mass or packed into the reaction mass and to temperatures from 1600 ° C to 2000 ° C for 10 minutes to 2 hours warmed up. A siliconization temperature of is particularly preferred 1860 ° C.

With increasing treatment time at elevated temperature, the Penetration depth of the siliconization. Depths of penetration can be up to 750 µm, possibly more, can be achieved. If it is too long Treatment times and high temperatures, however, there is a risk that that the volume increase by SiC to crack formation in the surface area leads. It is therefore preferred to conduct the siliconization in such a way that there is still no appreciable crack formation, which is reflected in an enlargement of the pore volume, occurs.

Instead of pack siliciding, the conversion can be carried out in the surface area also by liquid impregnation with silicon. Thereby in primarily the pores lined with silicon carbide and it comes to no pronounced layer formation of SiC. In the surface area carbon can be impregnated by liquid impregnation with silicon matrix to a depth of 1 µm, but at least up to 0.01 µm deep, at least partially convert to SiC.

The advantage of converting carbon in the surface area into SiC is that the thermal expansion coefficient of the Materials in the surface area to those of silicon carbide or silicon nitride of the protective layers to be subsequently applied largely is approximated. In addition, the liability of those subsequently to be brought up Protective layers on the base body improved.

Three or more layers of silicon carbide or silicon nitride, each about 5-50 µm thick, up to a total layer thickness of 30-300 µm brings.  

It has proven particularly favorable to chemically coat these layers deposit from the vapor phase, possibly also by plasma CVD.

The aim is to make the layers as stoichiometric as possible or Si₃N₄ form, but it is without disadvantages if still small amounts of silicon are present in these protective layers.

It is essential that the individual layers are not too thick and there are at least three layers to make it continuous Avoid cracks as much as possible. The total layer thickness should be more than 30 µm. Layer thicknesses above 300 µm bring none other advantages.

Chemical deposition takes place from the vapor phase of alkylsilanes or alkylhalosilanes at not too high temperatures, so that none Immediate pyrolysis when the steam molecules hit the upper surface takes place, a fine crystalline structure of the Form the layer on the appropriately pretreated substrate adheres well. To ensure that the support points of the to be coated Body are also protected, it is preferred after each Layer to change the position of the body in the coating chamber, so that with the next layer the body has different support points having. In order for the body to be coated to grow and adhere avoiding their contact points or reducing liability the surface of the support device in the coating chamber can be precoated with a thin layer of SiC. It turns out particularly advantageous if support frames made of graphite be used. These are preferably designed so that full flat contact points can be avoided.

When the coated bodies cool, cracks inevitably occur in the silicon carbide or silicon nitride layers.
These cracks can be covered and sealed by a cover layer made of glazed SiO₂ or SiO₂Si with a layer thickness of 0.5-10 µm, preferably 1-3 µm. The cover layer is preferably applied by plasma CVD (sputtering) at temperatures of, for. B. 3000 ° C with combustion of a SiH₄ / N₂O mixture at total pressures of 0.1 to 3.0 mbar and subsequent annealing at 1200 ° C. SiH₄ and N₂O are preferably used in a weight ratio of 1:40. In principle, it is also possible to apply silicone resins and to convert and glaze them at temperatures above 1000 ° C. Another possibility for applying glass layers is to impregnate or coat the carbon bodies provided with SiC layers with metal alkoxides or silica gels or silica sols. After the applied materials have dried on, they are hardened and glazed during tempering. Conventional plasma CVD conditions can be selected for the application of the glass layers. However, it is preferred to subsequently anneal the layers slowly in order to form layers which are as non-porous as possible.

The coals protected according to the invention with a multilayer system fabric bodies also exhibit at temperatures above 1000 ° C Exposure up to 900 hours in an oxygen atmosphere is not essential Oxidation on. Such a result can only be achieved in cooperation of substrate protection in the surface area, multilayer silicon carbide or silicon nitride layers avoiding continuous Reach cracks and final glass top layer.

Starting material for the conversion of the carbon matrix or graphite Matrix in the surface area in SiC are carbon or graphite body, which can optionally be reinforced with carbon fibers. For example, carbon bodies with a fiber content of 55-65% by volume with density (A) of 1.59 g / cm³, (B) of 1.58 g / cm³ and (C) 1.63 g / cm³ can be used. The open one Porosity of this carbon matrix is (A) 7.99% by volume, (B) 5.79% by volume and (C) 9.15% by volume.

With liquid impregnation, the carbon bodies are at one Temperature above 1450 ° C in liquid silicon for a time from 30 seconds to 60 minutes, preferably 1 minute to 10 minutes. There is a reaction between carbon in the surface area and silicon to silicon carbide instead, essentially pores of the substrate are lined with silicon carbide. The porosity the carbon body after the siliconization was, for example, (A)  4.79% by volume, (B) 4.77% by volume and (C) 6.04% by volume. For test specimens of Size 10 × 10 × 10 mm is the weight gain with liquid impregnation for example (A) 12.44%, (B) 6.80% and (C) 11.25%. The liquid impregnation with the predominant lining of the pores Silicon carbide does not lead to pronounced layer formation, however is the carbon matrix in the surface area to a depth of 0.01 µm to 1 µm at least partially converted to silicon carbide. In this way of working, the entire accessible outer and inner Silicified surface of the carbon body.

Instead of liquid impregnation, the carbon bodies can also treated with Si steam at 1600 ° C to 1800 ° C, one Silicification is achieved for 30 minutes to 6 hours.

Instead of liquid impregnation, the conversion to Silicon carbide by so-called pack siliconization, in which the Substrate body in a reaction mass of silicon carbide and silicon packed in powder or slurried in it and enveloped by it will. The reaction mass of silicon carbide and silicon can be from 5-50 wt .-%, preferably 7-25 wt .-% silicon, balance SiC.

Subsequently, the or packed in the reaction mass your encased carbon body in an oven at 1600-2000 ° C, preferably heated to 1860 ° C, with the exception of heating and Cooling times the pure siliconization times are 10 minutes to 2 hours can. The carbon body can directly into a hot Be brought reaction zone or evenly in the course of a or heated for several hours to the reaction temperature.

The starting product for the pack siliciding were carbon bodies Size 10 × 10 × 10 mm with the following density and porosity:

Table 1

Since the reaction is largely determined by diffusion, the reac tion temperature be sufficiently high to achieve a practical reaction to reach speeds, but must not be too high become, because otherwise a strong diffusion-inhibiting thick SiC layer forms. Too long siliconizing times at too high temperatures can cause delamination and cracking in the coals Due to difference, fiber-reinforced carbon bodies result union coefficient of thermal expansion of SiC and the anisotropic Carbon of the body.  

Table 2

Penetration depth in µm of the SiC areas at 1860 ° C treatment temperature and pack siliciding with increasing treatment time

In contrast to liquid impregnation, in which no direct SiC If a layer is formed, training takes place during the pack silicification an SiC layer with weight gain and increase in open Porosity, whereby the siliconizability with longer treatment times strongly depends on the initial porosity. The lower the porosity is, the lower the siliconizability. The reactivity of the Carbon has some influence. The reactivity depends on Manufacturing process and starting composition of carbon or graphite body.

The increase in open porosity after pack siliconization at 1800 ° C and a treatment time of 60 minutes is due to cracking in the coal attributable to fabric. The results of the porosity determination are given in the table below.  

Table 3

Open porosity of the CFC samples after pack siliciding

For samples CFC-III and CFC-IV with higher initial porosity than the other samples prove the chosen siliconization temperature already too high. There is considerable cracking and the samples show delamination already beginning. With low siliciding However, these samples can also be at the desired temperature Silicate in the surface without making it too pronounced Cracking and delamination appear.

For the samples not reinforced with carbon fibers, the average weight gain at 1860 ° C siliconization temperature and 20 minutes treatment time determined. The results are in Table 4 reproduced.  

Table 4

By converting the surface area at least partially into SiC will adjust the expansion coefficient of carbon body and the subsequently applied silicon carbide or Silicon nitride protective layers achieved and the adhesion of these layers improved on the substrate.

On the SiC protective layers to be applied for better adhesion Coals at least partially converted to SiC in the surface area Material or graphite bodies are made with the help of chemical deposition the vapor phase on the heated surface of the body three or more Silicon carbide or silicon nitride layers deposited. The warming the substrate body in the reaction zone of a reactor is carried out by means of Radiant heat. The bodies lie on brackets where the Support area is minimal and were applied after each layer was applied rotated to ensure multiple coating even at the support points put. The coating can in each case under the following conditions be carried out:  

Reaction temperature 1250 ° C, coating time per layer five to six hours, gas pressure in the reactor 60-80 mbar, reaction gas methyl trichlorosilane (H₃CSiCl₃) and hydrogen as a carrier gas. It can but also other alkylsilanes or other alkylhalosilanes are used will. The formation of SiC from methyltrichlorosilane takes place after Reaction equation

H₃CSiCl₃ → SiC + 3HCl

The SiC layers have due to the relatively low temperature the deposition and pyrolysis of the silanes have a fine crystalline structure. Under these conditions, the silane does not immediately open hit the surface decomposed so that also a filling of the Pores can be reached with SiC. The flow rate of the carrier gases proved to be of subordinate importance for stratification.

It is desirable to make the protective layer as stoichiometric as possible Training SiC, however, small amounts of Si are harmless and do not negatively influence the behavior of the protective layer.

In addition to the usual chemical separation from the gas phase Application of the SiC layers from plasma CVD can be used. The The reactivity of the gas mixture is determined by these processes Supply of electrical energy reached. There will be common conditions adhered to.

The layer thicknesses of the test specimens coated with SiC by CVD were determined using a light microscope. There have been made at least 10 measurements and the arithmetic mean educated.  

Table 5

The multiple coating given as an example also enables Coating the support points of the body and avoids continuous Cracks through all protective layers. When cooling the coated Body from the reaction temperature becomes the opposite tensile stress sensitive SiC layer inevitably put under tension, because the Expansion coefficient of the carbon or graphite matrix differs different from that of SiC.

Instead of SiC protective layers, protective layers can also be used Si₃N₄ are formed using CVD techniques, for example using a Si (CH₃) ₄ / NH₃ mixture at temperatures above 1280 ° C and molar ratios of N / Si greater than 2.

A temperature of 1350 ° C. and an N / Si ratio of 3 are preferred and a pressure of 1060 Pa for 5 hours.

It is also possible to work with plasma CVD processes, whereby preferably gas mixtures of halosilanes and ammonia are used be, e.g. B. SiH₄ / NH₃- or SiF₄ / NH₃ mixtures.

The coating takes place at temperatures from 1100 ° C to 1540 ° C, preferably 1400 ° C to 1500 ° C and a pressure of 1.33 x 10² Pa up to 13.33 × 10² Pa. It is preferably deposited alpha-Si₃N₄.  

In order to seal cracks in the protective layers, a top layer is used applied and glazed. Has been particularly preferred and suitable proven to apply plasma CDV for the top layer. At a Total gas pressure of, for example, 0.1 to 3.0 mbar silane (SH₄) and N₂O used in a ratio of 1:40. The temperature was 300 ° C and the deposition rate 75 nm / min. Then the SiO₂ layer annealed under a protective gas atmosphere by slow Heating from 300 ° C to 1200 ° C (heating rate 2 ° Calvin / Min.). The tempering of the glazed layer can also be done among others Conditions take place at temperatures below 1000 ° C. The layer thicknesses the cover layer can be between 0.5 μm and 10 μm, preferably are layer thicknesses from 1 µm to 3 µm. With the help of plasma CDV technology (Sputtering) were, for example, layer thicknesses of 0.5 µm, 1 µm and 3-4 µm applied, glazed and annealed.

To the oxidation resistance of the invention with protection to test the layered carbon or graphite body, were test specimens in a muffle furnace in an oxygen stream, which the Sample flows around evenly, heated to 1260 ° C. The test time was in short-term tests up to 20 minutes, in long-term tests 100 hours. It was 80 1 / h. Oxygen with a purity of 99.95% in the Oven introduced.

After exposure to oxygen at elevated temperature, the samples were cooled under nitrogen and the weight loss by weighing certainly. The short-term exposure was increased by in each case after 5 min. The oxygen load is interrupted at elevated temperature , the samples were cooled and then reheated in the oven were, with the cycles repeated each time, to load to reach times of 20 minutes. In the long-term tests, sub refractions after every 12 hours, 24 hours, 48 hours and 96 hours taken.

Instead of cooling under nitrogen, a second A so-called thermal shock treatment performed by a series of samples rapid cooling, so-called quench cooling at 1260 ° C Samples with water. The quenched samples were then at one hour  Dried 120 ° C and the weight loss determined by weighing. The reproduced values represent mean values of several samples each represents.

The minimum number of SiC layers for a sufficient oxide protection against fire was first determined CVD-SiC-coated samples before applying the top layer checked. The results are shown in Table 6 below given.

Table 6

Burning behavior of the CVD-SiC-coated samples (1-7 coatings at 1260 ° C and 80 liters O₂ / h

The evaluation shows that at least three SiC layers are required are sufficient to achieve adequate protection in the event of prolonged exposure. In the case of short-term loads, further layers do not result in any signifi edge improvement in oxidation resistance. For the long-term ver hold additional layers and the final cover layers required.  

The erosion behavior of test specimens which were provided with glazed SiO₂ layers according to the invention was first determined in a short-term experiment and compared with test specimens which, however, were not siliconized in the surface area. The samples marked with _* are these comparative tests.

In the short-term behavior the differences between the invention occur protection system is not yet clearly visible, only with thermo The system according to the invention has been shown to provide shock treatment with three tiered protection as superior.

Table 7

Cover layer 0.5 µm glazed SiO₂

With long-term exposure of 100 hours, test specimens were also invented protection system according to the invention and a comparative sample following Found skills:

Table 8

The above examples show that the inventive Protective layer system provides long-term protection under high thermal loads is also achieved with thermal shock cooling and intermediate loading, if the glass layer is sufficiently thick.

Table 9

Results after long-term exposure to oxygen for 500 h

This is shown in a long-term test with high thermal loads Superiority of the protection system according to the invention is particularly clear.

Claims (10)

1. Carbon body with oxidation protection layers made of SiC, characterized in that the carbon matrix is at least partially converted into SiC in the surface area, three or more layers of SiC or Si₃N₄ are deposited thereon and a cover layer made of glazed SiO₂ or a mixture of SiO₂ / Si is present. 2. Carbon body according to claim 1, characterized, that the carbon matrix has an open porosity of 2-15 vol .-% points. 3. carbon body according to claim 1 or 2, characterized, that the carbon matrix reinforcing carbon fibers in an amount contains from 30-70 vol .-%.   4. carbon body according to any one of claims 1 to 3, characterized, that the carbon matrix in the surface area at a depth of 0.01 µm to 0.75 mm is at least partially converted to SiC. 5. carbon body according to any one of claims 1 to 4, characterized, that the three or more layers of SiC or Si₃N₄ each 5 to Are 50 µm thick and the total layer thickness is 30 to 300 µm. 6. carbon body according to any one of claims 1 to 5, characterized, that the glazed top layer is 0.5-10 µm thick. 7. A method for forming oxidation protection layers made of SiC on carbon bodies, characterized by

• a) converting the carbon matrix in the surface area at least partially into SiC by pack siliconization or liquid impregnation with Si or treatment with Si steam at elevated temperature and converting carbon to SiC,
• b) applying three or more layers of SiC or Si₃N₄ by chemical deposition from the gas phase and pyrolysis of alkylsilanes, alkylhalosilanes,
• c) subsequent application of a cover layer and converting the same by annealing into glazed SiO₂ or a mixture of glazed SiO₂ / Si.

8. The method according to claim 7, characterized, that the surface area of the carbon matrix is at a depth of 0.01 µm to 0.75 mm is at least partially converted to SiC. 9. The method according to any one of claims 7 or 8, characterized, that the three to seven layers of SiC or Si₃N₄ with one Total layer thickness of 30 to 300 microns are formed.   10. The method according to any one of claims 7 to 9, characterized, that the glazed cover layer is formed 0.5 to 10 microns thick.