FR2690151A1 - Carbon@ bodies resistant to oxidative attack at high temps. - Google Patents

Abstract

A coated carbon body comprises a carbon body having a converted porous layer formed by etching and by reacting the carbon body with gaseous boron oxide, such that the conversion layer contains mutually interconnected intersitices and boron carbide formed by reaction of boron oxide and the carbon body; a glass-forming intermediate layer within the conversion layer; and a heat-resistant outer covering on the intermediate layer.

Description

Oxidation-resistant carbon and process for its re-variation.

 The present application corresponds to a "continuation" of the US patent application n 654 329 filed on September 24, 1984.

 The present invention relates generally to carbon bodies having improved resistance to oxidation, and more particularly to a process for manufacturing carbon bodies having improved resistance to oxidation, both at high temperatures and 'at intermediate temperatures and to oxidation-resistant carbon bodies thus produced.

 The resistance to oxidation of a carbon body constitutes a well-known desired criterion. Carbonaceous materials such as monolithic carbon, graphite and carbon-carbon composite materials of carbon fibers exhibit excellent resistance / weight properties at high temperature, for example at 1400 ° C. and above, and are generally superior to conventional building materials such as metals and superalloys at these temperatures. In addition, the mechanical resistance of a carbon body increases as the temperature increases, while with conventional building metals, the resistance decreases as the temperature increases.

 The use of carbon bodies in high temperature applications has been limited due to the relatively high reactivity of carbon, mainly with oxygen, at temperatures above about 400-500 ° C which give carbon erosion of the body as a result of the reaction between carbon and oxygen, releasing carbon monoxide and dioxide. Therefore, many attempts have been made to provide oxidation resistant coatings for carbon bodies to allow their use in oxidative and high temperature environments.

 In attempts to provide oxidation resistant coatings on carbon bodies, significant difficulties have been encountered. One of them is the wide variation in the coefficient of expansion of different types of carbon body and the differences in the coefficient of expansion between the carbon body and the coating material.

Depending on the raw materials used, the coefficient of expansion of the carbon body is likely to be extremely different from that of the oxidation-resistant coating. The stresses which result from different coefficients of expansion between the coating and the underlying carbon body cause the coating to crack or break, especially when the part is subjected to thermal cycles, allowing oxygen to penetrate into the coating and become fixed. on the underlying carbon body, resulting in a loss of structural integrity.

 The surface porosity of the carbon body, which occurs in articles that are not fully densified, can cause pitting in the coating during the coating process, which can also facilitate the penetration of oxygen into the surface of the carbon. It has also been found that mechanical vibrations, inclusions of impurities and the like are capable of causing cracking of the fragile protective coatings.

 A satisfactory resistance to oxidation at high temperature can be obtained according to the method described in US Pat. No. 4,515,860. The oxidation-resistant carbon body described in this patent includes a coating of silicon alloy deposited thermochemically on the latter, this coating containing one or more alloying elements chosen from the group consisting of carbon, oxygen, aluminum and nitrogen. The amount of silicon in the coating exceeds the stoichiometric amount and the alloy coating has a non-columnar particle size distribution having essentially equiaxial grains with an average diameter less than 1 micron. Due to its exceptionally fine particle size and its uniform distribution in the coating, any crack likely to occur is extremely small in width and has a mosaic configuration. The amount of silicon that exceeds the stoichiometric amount fills these thin cracks when the carbon body is heated to a temperature above the melting point of silicon, for example above 1410il, and it reacts with any presence of oxygen to form a glassy silicon oxide which acts as a filler filling the cracks. This patent also provides, as an alternative, in particular when a crack resistance at lower temperature is desired, to provide an intermediate boron layer. The boron reacts with oxygen to form an oxide sealing material. vitreous boron which flows in any crack which has formed. In industrial practice, the carbon body is usually subjected to a preliminary treatment in a mixture of chromic acid and sulfuric acid.

 The oxidation resistance conferred by the coatings described in US Pat. No. 4,515,860 offers characteristics significantly superior to those of the coatings of the prior art. In certain circumstances, however, particularly in cases where pronounced thermal cycles occur, the protection system may prove to be ineffective when it comes to adequately sealing cracks in the fragile coating to the point that the carbon body is subjected to an oxidative attack.

 The present invention provides a coated carbon body having improved resistance to oxidation over a wide range of temperatures, including at low temperatures of 500-100 ° C and high temperatures above 1400 ° C. In addition, the present The invention provides a process for the preparation of carbon bodies having improved resistance to oxidation over wide temperature ranges and in environments which involve high temperature thermal cycling.

 Still further, the present invention provides resistance to ablation and erosion of carbon bodies in non-oxidizing and oxidizing atmospheres at elevated temperature.

 Very generally, according to the method of the present invention, a carbon body is heated to an elevated temperature, generally above about 1500 ° C., a temperature sufficient to cause a reaction between the carbon body and a reagent. boron oxide gas. This reaction causes attack or corrosion of the carbon body surface and gives rise to the formation of boron carbide which is contained in the converted and attacked surface. The resulting etched and converted surface area has a depth of about 2 to 250 microns. An intermediate vitreous coating is then applied to the converted carbon body. After which, an external coating of silicon carbide with a silicon content which may exceed the stoichiometry is applied to the carbon body.

The carbon body coated in accordance with the present invention has an outer refractory lining and an intermediate vitrifying coating which reacts with oxygen and with other constituents which may be present to form a glass-like material. The carbon body also has an additional protective layer located substantially within the original dimensions of the uncoated carbon body which has been converted at least in part to boron carbide (B4C). The B4C used reacts with any quantity of oxygen which succeeds in penetrating inside the intermediate coating to form
B203 which is also of a glass-like nature.

 It has been found that attacking the carbon body surface with gaseous boron oxide provides an extremely advantageous surface on which the chosen intermediate coating can be deposited and also provides an additional measure of protection against oxidative attack on the carbon body. The oxygen present in the boron oxide reacts to the treatment conditions with the carbon to form carbon monoxide gas.

This gives rise to the formation of interstices or pores communicating with each other and extending inside and below the surface of the carbon body. Boron reacts with carbon to form boron carbide according to the formula 2B203 + 7C - B4C + 6CO. The carbon body surface is not uniformly eroded, which results in the formation of porous-type interstices communicating with each other. Boron oxide reacts with the carbon body to a depth which is determined by the duration of the contact. The interstices participate in the total void volume which occupies up to about 50% of the volume of the converted layer. The carbon body surface, including the inner surfaces of the interstices, contains boron carbide.

 As indicated, attacking the carbon body with gaseous boron oxide provides two advantageous results. First, the interstices communicating with each other serve as reservoirs for the intermediate coating, which makes it possible to increase the volume of the intermediate coating material available for the reaction with oxygen. Second, the attack of boron oxide gas reacts with the carbon in the carbon body to form boron carbide which is contained in the porous surface. Boron carbide reacts with oxygen to form a glassy boron oxide. Thus, any quantity of oxygen entering the intermediate coating is consumed by the boron carbide before it can attack the carbon body.

 To obtain the desired porous surface, the boron oxide attack reagent must be in the gaseous state. It has been found that the liquid or solid boron oxide proves to be too reactive and that the surface of the carbon body undergoes complete erosion, with respect to the formation of communicating interstices, when using oxide of boron other than in gaseous form.

 The carbon body to which oxidation-resistant coatings are applied may be any of the many suitable structural forms of carbon, depending on the intended use, and may include monolithic graphite, a composite material of carbon fibers dispersed in a carbon matrix, which in turn can be fully or partially graphitized or any other suitable type of carbon. The carbon body may, for example, be a turbine part, a pump rotor, a wing end of a spacecraft, or a component of rocket nozzles and reactors. The particular type of carbon body structure does not constitute a part of the present invention.

According to the present invention, the body of untreated carbon is placed in a suitable reaction vessel, for example in a gas phase deposition reactor by chemical process, as is well known in the prior art. The carbon body is heated to a temperature above about 1500 ° C. and more preferably between about 1600 ° C. and about 1750 ° C. Higher temperatures are satisfactory but are not necessary. The pressure in the reaction vessel is maintained between approximately 13.33 Pa (0.1 Torr) and approximately atmospheric pressure. Argon is passed at a temperature between approximately room temperature and 1750 μl through the vessel as carrier gas at a flow rate between about 0 and 100,000 standard cubic centimeters per minute for reactors with an inside diameter up to about 91.44 cm (36 inches) and at a flow rate greater than 100,000 standard cubic centimeters per minute for larger reactors. Boron oxide gas can be obtained by vaporization of boron oxide or by reaction in the gaseous state, for example by reaction of boron trichloride and an oxygen source such as steam or a mixture of hydrogen or carbon dioxide. Increased concentration and increased reaction temperatures cause an increased depth of attack, as is the case with an increased reaction time. The boron oxide flow rate is regulated between about i and about
3 7,000 standard cm3 / min for small reactors and above 7,000 standard cm3 / min for larger reactors. The reaction time can be regulated between approximately 30 seconds and approximately 120 minutes and the depth of the attack is generally between approximately 2 and approximately 250 microns. If desired, the reaction can be continued until the carbon body has undergone a full attack. The attacked layer of the carbon body generally has a void volume corresponding to approximately 50% of the volume originally occupied by the carbon body.

 The vitrified intermediate coating is then applied to the attacked carbon body, the aim of which is to react with any quantity of oxygen liable to penetrate into a crack or rupture and to form a sealing material similar to glass preventing oxygen from reach the carbon surface. In some cases, where it is not necessary for example to have abrasion or erosion resistance, the intermediate coating may be the only protective layer applied to the carbon body. However, for most environments and in order to obtain the best resistance to oxidation, additional external layers are applied to the intermediate coating.

 The low temperature vitrifying intermediate coating comprises a primary vitrifying species which may be boron, boron oxide, boron carbide, silicon a silicon alloy, silicon dioxide, germanium oxide and mixtures of these which can be deposited on the attacked surface of the carbon body by any suitable process such as deposition in the gas phase by chemical process or by other techniques such as sol-gel impregnation.

 The intermediate coating may also contain borides and oxides of zirconium, aluminum, magnesium, hafnium, titanium, zirconium carbides, hafnium, titanium, zirconium nitrides, hafnium, titanium, silicon and their mixtures.

 Preferably, the intermediate coating partially fills the interstices remaining as a result of the attack of boron oxide. Thus, the void volume produced by the etching step is partially eliminated and the resulting product turns out, by its characteristics, to be substantially the same as the original carbon body.

 The silicon can be deposited on the surface of the carbon body attacked at a temperature above the melting point of the silicon, or else the silicon can be deposited at a temperature below its melting point and the temperature can subsequently be increased. the coated part up to a temperature above the melting point. In either case, silicon, at temperatures above its melting point, infiltrates by "wicking" and fills the interstices of the attacked surface, creating a perfectly dense surface.

 The silicon can partially react with the coating of boron carbide which results from the attack of boron oxide according to the formula 2Si + B4C - SiB4 + SiC.

If chemical vapor deposition is used to deposit silicon, the X-ray diffraction data indicates that in reality a simple SiB4 was not formed but a similar and more complex compound, in this case B4 (Si, B, C) H.

This is probably due to the fact that, during the deposition of silicon in the gas phase by chemical process, a carrier gas containing hydrogen is used.

If an intermediate coating of silicon alloy is desired, the silicon can be alloyed with one or more useful elements such as chromium, aluminum, titanium, zirconium, hafnium, vanadium, niobium , tantalum, tungsten and molybdenum. These elements can be brought into the interstices together with the silicon by suitable deposition techniques, as mentioned above, or can be introduced subsequently by means of a displacement reaction. The free or combined silicon can be partially displaced by any of the above mentioned species according to reactions similar to those of titanium as follows
TiC14 (g) + 3Si (s) - TiSi2 (s) + SiCl4 <g)
or 2TiC14 (g) + SiC (s) + SiB4 (s) + 1 / 2C - 2SiC14 + TiC +
TiB2 + 1 / 2B4C
The use of an intermediate boron coating proves to be particularly desirable when it is desired to obtain an oxidation resistance in the temperature range from 500 ° C. to 700 C. Boron oxide is considered to have a melting point in the range of about 450 to 580 * C under ambient conditions.

Thus, the boron oxide which is formed by the reaction of any quantity of oxygen which migrates through cracks or ruptures in the external coating, enters in fusion and flows in the cracks, ruptures, etc. at much lower temperatures than is the case with silicon. This makes boron a desirable intermediate coating when the environment of the carbon portion is below the melting point of the silicon.

 When a carbon body is subjected to thermal cycles in proportions such that it is exposed to high temperatures above the melting point of silicon and to low temperatures near the melting point of boron oxide, it may be desirable to use both a silicon coating and a boron coating.

The boron coating is applied by deposition in the gaseous phase by chemical process, the carbon body being heated to a temperature above about 500 C, preferably between about 800C and about 1600 C. The pressure is maintained between about 13, 33 Pa (0.1 Torr) and approximately 101,323 Pa (760 Torrs), preferably between approximately 1 Torr and approximately 26,664 Pa (200 Torrs). A gaseous mixture of a decomposable boron gas, for example a boron trihalide, preferably a boron trichloride, hydrochloric acid, hydrogen and argon of the following composition, can be blown onto the body in attacked carbon
Gas Flow, standard cm3 / min% of total gas
BCl3 440 - 1500 2.4 - 14.5 H2 200 - 6000 6.6- - 15.8
HCl O - 7400 0 - 19.5
Ar 2000 - 32,000 60.7 - 76.2
The temperature of the gas is maintained between approximately room temperature and 1600 ° C. and the contact time can be varied between approximately 30 seconds and approximately 4 hours. A total gas flow rate of between about 100 and about 100,000 standard cm3 / min, preferably between about 2,600 and about 47,000 standard cm3 / min can be used for a reactor with an internal diameter of less than 0.305 m (1 foot). provides an intermediate coating of boron with a thickness between about 0.1 micron and 500 microns.

 The outer refractory lining may include carbides, borides or nitrides of silicon, zirconium, tantalum, hafnium, niobium, titanium, an aluminum boride or nitride or their mixtures. Additionally, the refractory lining may include a silicon oxynitride.

 It is generally desirable to provide an outer coating of silicon carbide over the intermediate coating. Arrangements of this type of external coating are described in the prior art, including the aforementioned US Patent No. 4,515,860 and they can be produced by deposition in the gas phase by chemical process.

 The following examples, which are given in order to illustrate more specifically some of the ways in which the method of the invention can be implemented, are not intended to limit the scope of the claims appended hereto.

EXAMPLE 1
The carbon-carbon composite material substrate comprising a T-300 material sold by
Avco Systems was heated to a temperature of 1650 ° C with argon and B203 insufflation flows
3 3 of 2030 standard cm3 / min and of 30 standard cm3 / min, respectively. The attack time was 60 minutes, which gave a depth of about 125 microns (5 thousandths of an inch) for a vacuum volume of 50%. Subsequently, insufflations of SiCl4 were made at 925 standard cm3 / min, nitrogen at 10,000 standard cm3 / min and hydrogen at 20,000 standard cm3 / min. The temperature of the element was reduced to 1280C and the insufflation was continued for 20 minutes. A deposit of silicon with a depth of 112.5 microns (4.2 thousandths of an inch) was obtained on the surface of the substrate. The substrate was then heated to a temperature just above the melting temperature of the silicon, for example 1410 ° C., to allow the silicon to infiltrate by wicking in the void spaces, thus filling those this partially. By heating up to 1375 ° C in air, the substrate exhibited excellent resistance to oxidation.

EXAMPLE 2
A carbon-carbon composite material substrate comprising a T-300 material sold by
Avco Systems, supported in a chemical vapor deposition reactor, was heated to a temperature between about 1700 ° C and 1750-C. Argon insufflation was carried out in 2030
3 cm standards / min and a blowing of B203 gas at a flow rate of 10 cm3 standards / min. The insufflation was continued for a period of 60 minutes, which gave an attack up to about 50% void volume at a depth of about 75 microns (3 thousandths of an inch).

After this operation, a deposit of silicon was formed on the substrate at a substrate temperature of 1175C and a pressure of 33.330 Pa (250 Torrs) using an insufflation of SiCl4 at 924 cm3 standards / min, nitrogen at 10,000 cm3 standards / min and hydrogen at 20,000 cm3 standards / min. The deposit resulting from the chemical gas deposition was heated to above the melting point of the silicon and partially filled the void volume in the etched substrate. A boron coating was then deposited on the silicon coating and an outer silicon carbide coating on the boron layer under the conditions which have been described above. The deposit exhibited a very high oxidation resistance during '' thermal shock stress tests up to a maximum temperature of 1375-C in air, showing a weight loss of less than 1% in 24 hours.

EXAMPLE 3
Boron oxide gas can be produced by placing solid boron oxide in a crucible, preferably above the workpiece, and heating the solid material therein to melt the oxide. boron and subsequently to vaporize it. The vapor then descends on the part together with an argon carrier gas to produce the attack on the surface.

EXAMPLE 4
As an alternative to the boron oxide vaporization operation, the argon or hydrogen gas can be saturated with water vapor by bubbling the heated gas through the water. At a pressure of 5,332.80 Pa (40 Torrs) and at ambient temperature, a carrier gas is obtained by this means in which there are equal molar volumes of water and hydrogen or argon. Boron chloride or another boron halide can then be injected into the reaction vessel in a ratio with the carrier gas of approximately 1: 3.

At a substrate temperature of 1600-C, the surface of the substrate will be attacked and converted into B4C in a period of about several hours to a depth of one hundred microns (a few thousandths of an inch) with about 50% of vacuum volume. It is possible to achieve a higher depth of attack at higher substrate temperatures.

EXAMPLE 5
As an alternative to the previous example, boron chloride (BC13) can be mixed with carbon dioxide and hydrogen in equal parts with a ratio between the mixture of carbon dioxide - hydrogen and boron chloride d 'about 3 to 1. At a substrate temperature of 1600-C r the substrate surface is attacked to about 50% of void volume and converted to boron carbide (B4C). The attack speed is significantly slower than that of the previous example due to the presence of large amounts of carbon monoxide resulting from the reaction.

EXAMPLE 6
When depositing the intermediate silicon coating on the part after the etching step, one or more volatile halides of chromium, aluminum, titanium, zirconium, hafnium or vanadium can be added to the halide mixture of silicon - hydrogen in the gas stream. Since metals tend to deposit less easily than silicon, the deposit resulting from silicon alloy will contain small proportions of metals compared to silicon. The conditions under which such deposits can be obtained are similar to those of the previous examples and can follow conventional gas deposition processes.

EXAMPLE 7
As a variant to the previous example, the silicon coating can be combined by depositing the silicon and subsequently by producing a flow of hydrogen and metal halide or argon and metal halide chosen from the group of Example 6. Argon and a metal halide or niobium, tantalum, tungsten or molybdenum can also be used at or below the melting temperature of silicon. Diffusion coating of the metal will occur in the silicon deposit.

EXAMPLE 8
This example was carried out to illustrate the improved resistance to oxidation of carbon bodies whose surface has been attacked using a boron oxide-carbon reaction. Carbon-carbon composite material samples measuring 1.27 cm x 2.54 cm x 0.317 cm (1/2 "x 1" x 1/8 ") were prepared. Six samples, lot A, were processed in accordance with prior art in a saturated solution of chromic acid and sulfuric acid at 121-C for 5 minutes, with stirring, and air-dried at 41 ° C for one hour. These samples did not have a porous surface having interstices communicating with each other and boron carbide was neither in nor on their surface.

 Twelve additional samples were subjected to boron oxide gas attack in accordance with the present invention. The samples were kept immobile in a chemical gas deposition reaction and heated to 170 ° C at a pressure of 5,332.8 Pa (45 Torrs). Argon was used at a flow rate of 2030 cm 3 standards / min to drive the boron oxide vaporized from an induction of boron oxide charge heated in the same area as the sample. The reaction time was 60 minutes.

 Six samples, Lot B r were subjected to a load of 3 grams of a boron oxide attack reagent and six samples, Lot C, were subjected to a load of 10 grams of a reagent d boron oxide attack.

The resulting samples had an etched porous surface formed by communicating interstices containing boron carbide in and on their surface.

We then applied to the Lots A samples,
B and C a vitrifying intermediate coating of boron in a gas phase deposition reactor by chemical process at a temperature of 1400 * C and at a pressure of 19.998 Pa (150 Torrs). A gas mixture of 700 cm3 standards / min of boron trichloride, 700 cm3 standards / min of hydrogen chloride, 6000 cm3 standards / min of argon and 1000 cm3 standards / min of hydrogen were blown into the reactor for 30 minutes per side.

An outer coating of silicon carbide was then applied to the samples in a gas phase deposition reactor at 1400 C and at a pressure of 19.998 Pa (150 Torrs). The first side of the samples was coated for 26 minutes and the second side was coated for 24 minutes. In each case, the gaseous reactive products had the following composition
Reactive product Flow. cm3 standards / min
Methyltrichlorosilane 1250
2 6500
2,300
An additional batch of samples, Lot D, was prepared to provide an intermediate coating of silicon. The boron oxide etching step was carried out as described above, except that a charge of thirty (30) grams of boron oxide was used. An intermediate coating of silicon on both sides of the sample at a temperature of 1175 ° C and a pressure of 33,330 Pa (250 Torrs). The flow rate of the gaseous reaction mixture was 20,000 cm3 standards / min of hydrogen, 10,000 cm3 standards / min of nitrogen and 924 cm3 standards / min of silicon tetrachloride.

The reaction was continued for 45 minutes. The temperature was then brought to 1525 ° C. for ten (10) minutes with an argon flow rate of 14,500 cm 3 standards / min and at a pressure below 13,332 Pa (100 Torrs) for the melting of the deposited silicon.

An outer coating of silicon carbide was applied to the intermediate silicon coating at a temperature of 1400-C and a pressure of 19.998
Pa (150 Torrs). The flow rate of the gaseous reaction mixture was 3750 cm3 standards / min of hydrogen, 6500 cm3 standards / min of nitrogen and 1250 cm3 standards / min of methyltrichlorosilane. The reaction time was 18 minutes. The intermediate vitrifying silicon coating substantially filled the communicating interstices of the porous sample D with respect to the preferred operation of partially filling the interstices of Example E, like this. is described below
Examination with a scanning electron microscope of these samples subjected to attack by gaseous boron oxide demonstrated that the samples which had reacted with a charge of three (3) grams of boron oxide had a lower attacked layer at 10 microns thick, the attacked samples with a load of 10 grams of boron oxide had an attacked layer thickness less than forty (40) microns thick, and the attacked samples with a charge of thirty (30) grams of boron exhibited an attacked layer approximately 150 microns thick.

 Another batch of samples was prepared, Lot E, comprising both a silicon coating and an intermediate boron coating partially filling the interstices of the body with carbon converted by boron oxide gas. The attack on boron oxide was obtained under the conditions indicated above using a charge of 30 grams of boron oxide, thus making it possible to obtain an attacked layer of approximately 150 microns thick. The coatings of boron, silicon, and silicon carbide were applied under the conditions indicated previously in this Example.

Oxidation resistance of samples from
Lots AE has been tested by heating them in air in an oven and subjecting them to thermal cycles from a starting base of 650 ° C to a temperature between 1200-C and 1375 * C. The samples were weighed every hour and a weight loss of 5% was judged to be the point of failure. The results are shown in Table I.

TABLE I
Lots of samples
A B C D E
Average survival time, hours 22 28 49.8 15 30
Standard deviation, hours 1.4 1.4 12 5 19.7
Coefficient of variation 0.06 0.05 0.24 0.33 0.66
The various features of the invention are listed in the appended claims.

EXAMPLE 9
This example illustrates the excellent characteristic of oxidation resistance of a body to carbon carbon attacked using gaseous boron oxide, which gives a surface area of communicating porosity also containing boron carbide. A carbon-carbon composite material, made of two-dimensional T-300 fibers, was coated in accordance with the present invention. Thirty grams of boron oxide were vaporized and blown over the carbon-carbon material at a temperature of 1700-C and a pressure of 5.999 Pa (45
Torrs) for 60 minutes; as the carrier gas, argon was used. Figure 1 shows the microstructure of the carbon-carbon surface layer converted by boron oxide.The transverse micrograph shows the highly porous area having a porosity of about 50% in addition, also appears the communication of internal pores between them . X-ray diffraction analysis of the surface indicates the presence of boron carbide (B4C). Subsequently, an intermediate silicon coating was applied to the composite material using the following process conditions: temperature = 1125-C, pressure = 33,330 Pa (250 Torrs), duration = 45 minutes. The flow rates of the gaseous reactive product were 924 cm3 standards / min of silicon tetrachloride, 20,000 cm3 standards / min of hydrogen and 12,500 cm3 standards / min of nitrogen. for 10 minutes and at a pressure of 5,598 Pa (117 Torrs) in a flow carrying argon at 14,500 cm3 standards / min. The boron was subsequently deposited using the conditions described in Example 8 above. An outer layer of silicon carbide was applied to the parts at a temperature of 1400 ° C and a pressure of 19.998 Pa (150
Torrs). The gas flow rates were 3000 cm3 standards / min of hydrogen, 11.500 cm3 standards / min of nitrogen and 1250 cm3 standards / min of methyltrichlorosilane. The reaction was run for 12 minutes. Analysis of the microstructure of the coating obtained indicated that the intermediate coating had partially filled the interstices of the layer converted by boron oxide. Using the same oxidation resistance characteristic testing method as described in Example 8, the average survival time of the coated carbon body obtained in accordance with this invention was 72 hours.

EXAMPLE 10
A carbon-carbon insert intended for the assembly of a rocket nozzle which had been subjected to an attack of boron oxide at a temperature of 1700-C and a pressure of 2.133.12-3.066.36 Pa (16-23
Torrs) for 46 minutes was heated in a graphite oven to 13504C at a pressure of 5.999 Pa (45
Torrs). An H2 atmosphere was maintained by passing 7,500 cm3 / min of H2 over the rocket nozzle insert, then the part was subjected to a silicon carbide coating operation in which the pressure methyltrichlorosilane (MTS) room temperature equilibrium vapor was introduced for 5 minutes to produce a coating of approximately 0.025 mm (1 thousandth of an inch) of silicon carbide (SiC), which coating adhered to the inspection visual. After this time, 700 cc / min of chlorine was passed through an approximately 20.32 cm (8 inch) bed of Hf pellets in a quartz crucible above the substrate and heated separately up to 480-540-C.

The insufflation of H2 and MTS was continued. Concurrently with the start of the chlorine flow, 400 cm3 / min of methyl chloride were passed in contact with HfCl4 and HfCl3 vapors leaving the Hf reactor, and from there, they were passed over the substrate for the codeposition of SiC and hafnium carbide.

The insufflation of these fluxes was continued for a period of two hours, which gave a coating having a total thickness of 280 microns (11.1 thousandths of an inch).

Results: the insert was removed from the oven
coated nozzle which had been cooled and
placed it on a stand. We applied a
oxy-acetylene torch producing a
flame at a temperature of 1430-1480iC, di
directly on the part, the coating res
both in place.

EXAMPLE 11
We heated a carbon-carbon insert intended for the assembly of a rocket nozzle which had been subjected to a boron oxide attack at 1700-C and a pressure of 2.133-1.599 pa (16-12 Torrs ) for 46 minutes in a graphite oven up to 1350 ° C at a pressure of 5,999 Pa (45 Torrs). An atmosphere of H2 was maintained by passing 7,500 cc / min of H2 above the rocket nozzle insert. Next, the part was subjected to a silicon carbide coating step, in which the room temperature equilibrium vapor pressure of methyltrichlorosilane (MTS) was introduced for five (5) minutes to produce a carbide coating. silicon (SiC) of about 0.025 mm (1 thousandth of an inch) which was adherent to visual inspection. After this period, the introduction of MTS was stopped while the flow of H2 was maintained. A flow of 700 cc / min of chlorine was passed through an approximately 20.32 cm (eight inch) bed of Hf pellets contained in a quartz crucible above the substrate and heated separately to '' at 500-550 C. Simultaneously with the setting
3 route of the chlorine flow, 400 cm3 / min of methyl chloride were passed in contact with the Hf Cl4 and HfCl3 vapors leaving the Hf reactor, and from there, they were passed over the substrate to deposit a coating of hafnium carbide. These flows were continued for a period of two hours.

Results: the insert was removed from the oven
coated nozzle that had been cooled, and
we put it on a stand. We applied
an oxy-acetylene torch producing a
flame at a maximum temperature of 1430
1480C (2600-2700-F), directly on the
piece, the coating remaining in place.

EXAMPLE 12
A carbon-carbon sample which had been subjected to a boron oxide attack according to Example 1 of the present invention was heated in a graphite oven to 1345-C at a pressure of 5.999 Pa (45
Torrs) and an H2 atmosphere was maintained by passing 7,500 cm3 / min of H2 over the sample. Methyltrichlorosilane (MTS) was then introduced into the
3 a flow rate of 750 cm / min. H2 and MTS were passed over the sample to produce a coating of silicon carbide (SiC). These conditions were maintained for 90 minutes, resulting in an SiC coating of about 0.406 mm (16 thousandths of an inch) serving as both an intermediate coating and an outer refractory coating.

Results: after cooling, the heat exchanger was removed
kiln coated with the oven and placed on a
support. A pro oxy-acetylene torch
making a flame at a maximum temperature
1430-1480-C (2600-2700-F) has been applied
directly on the coated sample, the
coating remaining in place.

Claims (26)

 1. Coated carbon body having improved resistance to oxidation at high temperature, characterized in that it comprises: a carbon body, said body comprising a converted porous layer, formed by the attack and the reaction of the carbon body with gaseous boron oxide, said converted layer containing communicating interstices and boron carbide formed by the reaction of boron oxide and said carbon body; an intermediate vitreous coating inside said converted layer; and an outer refractory lining on said intermediate lining.  2. Coated body according to claim 1, characterized in that the intermediate coating comprises a primary vitrifying species chosen from boron, boron carbide, boron oxide, silicon, silicon alloys, silicon dioxide, germanium oxide and mixtures thereof.  3. coated body according to claim 2, characterized in that said intermediate coating also contains borides and oxides of zirconium, aluminum, magnesium, hafnium, titanium, zirconium carbides, hafnium, titanium, silicon, zirconium nitrides, hafnium, titanium, silicon and their mixtures.  4. coated body according to claim 1, characterized in that the refractory coating comprises carbides, borides or nitrides of silicon, zirconium, tantalum, hafnium, niobium, titanium and nitride or boride d aluminum or their mixtures.  5. Coated body according to claim 2, charac terized in that the converted layer has a depth between about 2 and about 250 microns.  6. Coated body according to claim 5, characterized in that the converted layer has a void volume of up to about 50% of the volume originally occupied by the carbon layer.  7. Coated body according to claim 1, characterized in that the intermediate coating partially fills the interstices of said converted layer.  8. Coated carbon body having improved resistance to oxidation at high temperature, characterized in that it comprises: a carbon body, said body comprising a converted porous layer, formed by the attack and the reaction of said carbon body with gaseous boron oxide, said converted layer containing interstices communicating with each other and boron carbide formed by the reaction of boron and said carbon body; an intermediate vitrifying coating containing boron inside said converted layer; and an outer refractory lining on said intermediate lining.  9. Coated carbon body having improved resistance to oxidation at high temperature, characterized in that it comprises: a carbon body, said body comprising a converted porous layer, formed by the attack and the reaction of said carbon body with gaseous boron oxide, said converted layer containing interstices communicating with each other and boron carbide formed by the reaction of boron and said carbon body; an intermediate vitrifying coating containing boron and silicon inside said converted layer; and an outer refractory lining on said intermediate lining.  10. Coated body according to any one of claims 8 and 9, characterized in that the refractory coating is silicon carbide.  11. Coated body according to any one of claims 8 and 9, characterized in that the refractory coating comprises a mixture of silicon carbide and hafnium carbide.  12. Coated body according to any one of claims 8 and 9, characterized in that the refractory coating is silicon nitride.  13. Coated body according to any one of claims 8 and 9, characterized in that the refractory coating is silicon oxynitride.  14. A method for manufacturing a coated carbon body having improved resistance to oxidation at high temperature, characterized in that it comprises steps consisting in: providing a carbon body; bringing said carbon body into contact with a gaseous boron oxide at a high temperature sufficient to cause the reaction between the carbon body and the boron oxide, thus forming a converted porous layer containing interstices communicating with each other in said body, which layer contains boron carbide; applying an intermediate vitreous coating on said converted layer; and applying an outer refractory coating to said intermediate layer.  15. The method of claim 14, characterized in that the elevated temperature is at least about 1500C so that said converted layer reaches a depth between about 2 and 250 microns.  16. The method of claim 14, characterized in that said converted layer has a void volume up to about 50% of the volume originally occupied by the carbon layer.  17. The method of claim 14, characterized in that the intermediate coating comprises a primary vitrifying species chosen from boron, boron carbide, boron oxide, silicon, silicon alloys, silicon dioxide, l germanium oxide and mixtures thereof.  18. The method of claim 17, characterized in that said intermediate coating also contains borides and oxides of zirconium, aluminum, magnesium, hafnium, titanium, zirconium carbides, hafnium, titanium , nitrides of zirconium, hafnium, titanium, silicon and their mixtures.  19. The method of claim 14, characterized in that the intermediate coating is applied by deposition in the gas phase by chemical process.  20. The method of claim 14, characterized in that the outer refractory coating is applied by deposition in the gas phase by chemical process.  21. The method of claim 14, characterized in that the outer refractory coating comprises carbides, borides or nitrides of silicon, zirconium, tantalum, hafnium, niobium, titanium, nitride or boride d aluminum or their mixtures.  22. The method of claim 14, characterized in that the outer refractory lining is silicon carbide.  23. The method of claim 14, characterized in that the refractory coating comprises a mixture of silicon carbide and hafnium carbide.  24. The method of claim 14, characterized in that the refractory coating is silicon nitride.  25. The method of claim 14, characterized in that the refractory coating is silicon oxynitride.  26. The method of claim 14, characterized in that the refractory coating is hafnium carbide.