LU87647A1 - Oxidisation resistant carbon coated body - comprising a carbon body, an intermediate glass forming coating and an outer refractory coating on the intermediate coating - Google Patents

Abstract

A coated carbon body comprising (i) a carbon body having a converted porous layer formed thereon, and (ii) a glass forming coating, at least a portion of which is within the said converted layer. Method for the mfr. of the said carbon coated body comprising: a) forming a converted layer on the carbon body by converting a porous layer formed by etching and reacting the body with B2O3. The converted layer contains interconnecting interstices and B4C which is formed by reaction between the B2O3 and the carbon body, and b) applying a glass forming coating over the converted layer, at least a portion of which is within the said converted layer. An alternative method for the mfr. of the said carbon coated body comprises: a) forming the said converted layer on the carbon body; b) applying an intermediate glass forming coating over the converted layer, where the intermediate coating comprises Si or a Si-contg. cpd.; c) subjecting the body to a nitriding atmos. to convert the Si or Si-contg. cpd. to Si3N4, and d) applying an outer refractory coating over the intermediate coating. The glass forming layer comprises cpds. selected from Si3N4, SiON2 and mixts. thereof. The glass forming coating comprises a primary glass forming species selected from B, B4C, B2O3, Si, Si alloys, SiO2, GeO2 and mixts. thereof, and may also contain borides and oxides of Zr, Al, Mg, Hf, Ti, carbides of Zr, Hf, Ti, nitrides of Zr, Hf, Ti, Si and mixts. thereof. The refractory coating comprises carbides, borides or nitrides of Si, Zr, Ta, Hf, Nb and Ti, SiON2, AlB, AlN or mixts. thereof. The nitriding atmos. is selected from N2, NH3, N2H4 and mixts. thereof, and also contains O2 or an O-contg. cpd. to convert the Si or Si-contg. cpd. to an oxynitride of Si, where the O-contg. cpd. is H2O or CO/CO2. The glass forming coating is applied by CVD or by sol-gel technology. The converted layer is formed by heating to at least 1500 deg.C. The nitriding step is carried out at a temp. of 1350-1500 deg.C.

Description

OXIDATION RESISTANT CARBON AND PROCESS FOR PRODUCING THE SAME

This invention relates generally to carbon bodies which have improved resistance to oxidation, and more particularly to a method of making carbon bodies which have improved resistance to oxidation both at high temperatures and at intermediate temperatures. The invention also relates to carbon bodies resistant to the oxidation produced by said method.

STATE OF THE ART

The desire and the need for carbon bodies having a high resistance to oxidation are well known. Carbonaceous materials such as monolithic carbon, graphite, carbon-carbon compounds or carbon fibers have excellent resistance properties relative to their weight at high temperatures, for example. ex. 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 with increasing temperatures, while for metals with conventional structure, their resistance decreases when 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 results in erosion of the body into carbon by the reaction between carbon and oxygen producing carbon monoxide and dioxide. As a result, many attempts have been made to provide carbon bodies with an oxidation resistant coating to allow their use in oxidizing environments and at elevated temperatures.

Major difficulties have been encountered in trying to provide carbon bodies with oxidation resistant coatings. One of the difficulties lies in the very large variation in the coefficient of expansion of the different types of carbon body and in the differences between the coefficient of expansion of the carbon body and that of the coating material. Depending on the raw materials used, the expansion coefficient of the carbon body can be significantly different from that of the oxidation-resistant coating.

The tensions that result from different expansion coefficients between the coating and the underlying carbon body cause the coating to fracture or rupture, especially when the part is subjected to thermal cycles, which leads to the penetration of oxygen into the coating that attacks the underlying carbon body which results in loss of structural integrity.

The porosity of the surface of the carbon body, which results from objects or parts which are not fully made dense, can cause the formation of pinholes in the coating during the application process, which may also result in the possibility of oxygen entering the carbon surface. It has also been found that mechanical vibrations, shocks or impacts of debris and other fragments can cause fracture of fragile protective coatings.

Oxidation resistance at elevated temperatures can be successfully achieved by the method described in US Patent No. 4,515,860 which is incorporated in the present description by reference. The oxidation-resistant carbon body described in this patent has a coating of a silicon alloy, the deposition of which was made by a thermochemical process, said coating containing one or more alloying elements chosen from the group consisting of by carbon, oxygen, aluminum and nitrogen. The amount of silicon present in the coating is greater than the stoichiometric amount and the coating alloy has a non-column grain distribution, with substantially equiaxial grains with an average diameter of less than 1 micron. Because of this exceptionally fine grain size and their uniform distribution in the coating, any cracks that could occur are extremely thin in width and form a mosaic-like pattern. The amount of silicon exceeding the stoichiometric amount fills these fine slits when the body is heated beyond the melting point of silicon, for example above 1410 ° C and reacts with any oxygen to form a vitreous silicon oxide which acts as a filling agent or filling compound which seals the slots. This patent also envisages, on an optional basis, in particular in the case where resistance to fracture at lower temperatures is desired, the use of an intermediate boron layer. Boron reacts with oxygen to form a tight coating of vitreous boron oxide which flows through all the cracks that have formed. In commercial applications, the carbon body is generally subjected to a preliminary treatment in a mixture of chromium acid and sulfuric acid.

The resistance to oxidation produced by the coatings described in US Pat. No. 4,515,860 gives characteristics far superior to those of the coatings known previously from the prior art. However, in certain circumstances, in particular in those where significant temperature variations take place, this protection system could prove to be insufficient for an adequate sealing of the slots which form in the fragile coating so that the carbon body is subjected to an oxidation attack.

DESCRIPTION OF THE INVENTION

The present invention provides a coated carbon body, said carbon body having resistance to oxygen over a wide range of temperatures including relatively low temperatures from 500 to 1000 ° C and high temperatures above 1400 ° C. This invention also provides a method of manufacturing carbon bodies having improved resistance to oxidation over a wide range of temperatures and in environments which involve high temperature variations or cycles.

The present invention further provides properties of resistance to ablation and erosion of carbon bodies at elevated temperatures in oxidizing and non-oxidizing media.

In general, according to the method of the present invention, a carbon body is heated to a high temperature, generally above 1500 ° C. which is sufficient to cause a reaction between the carbon body and a gaseous oxide reagent. boron. The consequence of this reaction is that the body surface of carbon is corroded and results in the formation of boron carbide which is contained in this corroded and transformed surface. This corroded and transformed surface extends over an area with a depth or thickness of approximately 2 to 250 microns. The carbon body thus transformed is subsequently provided with a vitreous coating, at least part of which penetrates into said corroded and transformed surface. The glass coating contains chemical compounds selected from the group consisting of silicon nitride, silicon oxynitride or a mixture of these compounds.

According to a preferred embodiment, the carbon body is provided with an outer coating layer of a refractory material which may contain silicon exceeding the stoichiometric value. The outer refractory lining comprises chemical compounds chosen from the group consisting of carbides, borides or nitrides of silicon, zirconium, tantalum, hafnium, niobium and titanium, oxynitrides of silicon, and by borides of aluminum or aluminum nitrides, or by mixtures of these compounds.

In this preferred embodiment of the present invention, the coated carbon body has an outer refractory lining and an intermediate layer which forms a coating of a glassy substance which reacts with oxygen and the other elements present to form this glassy material . The carbon body also contains an additional protective layer, essentially in the original dimensions of the uncoated carbon bodies, which has been transformed, at least in part, into boron carbide B4C. B4C reacts with oxygen which still succeeds in penetrating the intermediate coating to form B203 which is also a substance of vitreous nature.

It has been discovered that chemical attack on the carbon body surface with gaseous boron oxide creates a very suitable surface for the deposition of a selected intermediate coating and also provides an additional measure of protection against oxidation responsible for the body's carbon oxidation attacks. The oxygen contained in the boron oxide reacts, under the treatment conditions, with the carbon to form carbon monoxide gas. This results in the formation of interstices or interconnected pores extending into and below the surface of the carbon body. Boron reacts with carbon to form boron carbon according to the following formula: 2B203 + TC- ^ B ^ C + SCO. The carbon body surface is not uniformly corroded. As a result, interstices in the form of interconnected pores are formed. Boron oxide reacts with the carbon body to a depth depending on the duration of the contact time. The interstices contribute to the total void volume which occupies about 50% of the volume of the transformed surface. The carbon body surface, including the inner surfaces of the interstices, contains boron carbide.

As mentioned before, the corrosion action of the carbon body with gaseous boron oxide has two beneficial results. First, the interconnected interstices act as a reservoir for the intermediate coating, while increasing the volume of the intermediate coating material available for reaction with oxygen. Second, the gaseous boron oxide stripper 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 boron oxide of a glassy nature. Thus, any oxygen that penetrates the intermediate coating is consumed or absorbed by the boron carbon before it has the possibility of attacking the carbon body.

In order to achieve the desired porous surface, the boron oxide stripper is preferably in the gaseous state. It has been found that the liquid or solid transformed boron oxide is too reactive, and the carbon body surface becomes completely corroded, compared to the formation of interconnected interstices in case boron oxide is used under a form other than gas.

The carbon body to which this oxidation-resistant coating is applied, may consist of either of the carbon structures suitable for the intended use, and may include monolithic graphite, a mixed compound of dispersed carbon fibers in a carbon matrix which in turn can be partially or fully graphitized, or any other suitable form of carbon. The carbon body can, for example, constitute a part of a turbine, the rotor of a pump, the edge of the wing of a spacecraft, or even a part of the nozzle or of the engine of a rocket. . The particular shape or structure of the carbon body is not part of the present invention.

According to the present invention, the body of carbon not yet treated is placed in a suitable reactor, for example in a reactor for the deposition of chemical vapors well known in the art. The carbon body is heated to a temperature above about 1500 ° C and preferably to a temperature between about 1600 ° C and 175 ° C. Higher temperatures are also adequate but are not necessary. The pressure inside the reaction chamber is maintained between about 0.1 Torr and about atmospheric pressure. Argon at a temperature between about room temperature and 1750 ° C is circulated as a carrier gas through the chamber at a flow rate between about 0 and 100,000 standard cubic centimeters per minute (SCCM) for reactors with a inner diameter of about 36 inches and at a flow rate greater than 100,000 SCCM for larger reactors. Boron oxide in gaseous form can be obtained by vaporization of boron oxide or by reaction in the gaseous state, for example. ex. by the reaction of boron trichloride with an oxygen source such as steam or a mixture of hydrogen and carbon dioxide. Increased concentrations and higher reaction temperatures, as well as longer reaction times, cause increased depths of the corrosion action. The boron oxide circulation speed is set between approximately 1 and 7000 SCCM for small reactors and above 7000 SCCM for larger reactors. The reaction time can be set between about 30 seconds and 120 minutes and the depth of the corrosion action is usually between about 2 and 250 microns. If desired, the reaction can be continued until the carbon body is completely corroded. The corroded layer of the carbon body generally occupies an empty volume which approaches 50% of the original volume occupied by the carbon body.

The corroded carbon body is then provided with a vitreous layer of intermediate coating, the function of which is to react with any oxygen which could penetrate a ruptured slit to form a tight vitreous coating preventing oxygen from reaching the carbon surface . In some cases where resistance to abrasion or erosion is not required, the intermediate coating may constitute the only protective layer applied to the carbon body. However, for most environmental conditions, and for better resistance to oxidation, additional exterior coatings are applied to the intermediate layer.

The intermediate coating which takes a glassy form at low temperature contains primary glass formation products selected from the group consisting of silicon nitrides and oxynitrides and mixtures of these components, which can be deposited on the corroded surface of the carbon body by means of any suitable process such as the deposition of chemical vapors or any other technique such as the impregnation technique as under the designation "sol-gel".

The intermediate coating may also contain boron, boron oxide, boron carbide, silicon, a silicon alloy, silicon dioxide, germanium, borides and oxides of zirconium, aluminum, magnesium, hafnium , titanium, zirconium carbides, hafnium, titanium, zirconium nitrides, hafnium, titanium, silicon, and mixtures of these.

The silicon nitrides and oxynitrides can be incorporated into the intermediate coating using two basic process approaches. The initial approach is to nitride metal containing silicon, or silicon which contains compounds which were incorporated into the intermediate coating during a previous treatment. The second approach consists in incorporating the silicon nitride by the direct addition of the compound to the intermediate coating.

In the initial approach, it is preferable to carry out nitriding or oxynitriding after the metal containing silicon has been incorporated into the intermediate layer during a previous processing step.

A reactive nitriding gas is used to transform the metal containing silicon or the chemical compound containing silicon into nitrides or oxynitrides.

The following sources of reactive nitrogen will result in the formation of nitrides: N2, NH3, N2H4 or mixtures of these compounds. It is preferable to use temperatures above about 1000 ° C, but below 2000 ° C. Temperatures between 1350 and 1500 ° C will result in significant formation of nitrides during 24 to 48 hours of reaction. If N2 is used as the nitriding agent, it is preferable to use an almost pure N2 atmosphere.

In case NH3 is used, concentrations above 10 vol. % of N2 are to be preferred. The addition of small amounts of H2 to the reactive atmosphere also revealed a beneficial effect by increasing the rate of nitriding. Oxygen can be incorporated into the nitride. This results in the formation of an oxynitride, the incorporation of oxygen being done by several suitable means. For example, an oxygen-containing compound can be added to the nitriding atmosphere. The following chemical compounds can be used singly or in combination: 02, H2.0, C0 / C02. Low oxygen concentrations in the nitriding atmosphere are preferred. For example, the addition of about 600 ppm (vol) of molecular oxygen to an N2 nitriding atmosphere resulted in a higher degree of transformation of the solid silicon phase from nitrides to oxynitrides.

In the second approach, the product, silicon nitride, is added directly to the intermediate coating layer. This can be achieved by using a technology of the kind as under the designation CVD (chemical vapor infiltration), that is to say chemical vapor infiltration, to penetrate or infiltrate the porous corroded layer of silicon nitride or oxynitride. The technology as under the designation "sol-gel" can also be used. The precursors of the Si-O-N products can be any suitable compound resulting in the formation of nitrides or oxynitrides in the intermediate coating layer.

Preferably, the intermediate coating partially fills the interstices resulting from the attack action by boron oxide. Thus, the void volume produced by this corrosion action is partially filled and the resulting product is essentially the same in terms of its characteristics as the original carbon body.

The silicon can be deposited on the surface of the body in corroded carbon at a temperature higher than the melting point of the silicon, or else the silicon can be deposited at a temperature below its melting point and the coated part can by the thereafter be heated above said melting point. In all cases, the silicon, at the temperature above its melting point, infiltrates and fills the interstices of the corroded surface, thus creating a completely dense surface.

The silicon can partially react with the boron carbide coating which comes from the action of corrosion by boron oxide, according to the following formula: 2Si + B4C_ ^ SiB4 + Sic. In the case where the chemical vapor deposition method is used to deposit silicon, information and data obtained by X-ray diffraction show that simple SiB4 is in fact not formed but that a similar and more complex compound results therefrom , namely B4 (SiThis probably comes from the fact that during the chemical vapor deposition of silicon, a carrier gas is used which contains hydrogen.

In the case where an intermediate coating of a silicon alloy is desired, the silicon can be alloyed with one or more elements useful for this purpose, including for example chromium, aluminum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, tungsten and molybdenum. These elements can be supplied to the interstices at the same time as the silicon by one of the appropriate deposition techniques described above or else they can be introduced subsequently by means of a substitution reaction. The free or combined silicon can be partially substituted for one of the elements mentioned above according to reactions similar to those given below for titanium.

When a carbon body is subjected to thermal cycles which are such that it is exposed both to high temperatures above the melting point of silicon and to low temperatures in the vicinity of the melting point of boron oxide, it may be desirable to use either of these coatings, i.e., the silicon coating and the boron coating.

The boron coating is applied by the chemical vapor deposition technique by heating the carbon body to a temperature above about 500 ° C, preferably between about 800 ° C and 1600 ° C. The pressure is maintained between approximately 0.1 and 760 torr, preferably between approximately 1 and 200 torr. A gaseous mixture of a decomposable boron gas, for example boron trihalide, preferably boron trichloride, hydrochloric acid, hydrogen, and argon of the following composition can be circulated over the carbon body corroded:

The gas temperature is maintained between approximately room temperature and 1600 ° C and the contact time can vary between approximately 30 seconds and 4 hours. A circulation speed (in total) of between approximately 100 and 100,000 SCCM, preferably between approximately 2,600 and approximately 47,000 SCCM, can be used for a reactor whose internal diameter is less than one foot (foot “0, 3048). This results in an intermediate coating of boron having a thickness of about 0.1 micron to 500 microns.

The outer refractory coating may include carbides, borides or nitrides of silicon, zirconium, tantalum, hafnium, niobium, titanium, borides or aluminum nitride or a mixture of these compounds. The refractory lining may additionally comprise silicon oxynitrides.

External refractory linings of silicon nitride or oxynitride can be formed by several means including the chemical vapor deposition (CVD) technique. The following analyzes were found to be effective for the formation of coatings: Sic14 / NH3; Si (CH3) 4 / NH3, SiH "/ NH3.

These coatings were prepared at high deposition rates (> 1 mm / h) by the following reactions: (a) S1CI4 / NH3 and H * at temperatures of 1100-1500 ° C, and pressures of 5-300 Torr; reference is made to K. Niihara, Ceramics Bulletin, Vol. 63, page 1160 (1984) and K. Niihara and T. Hirai, Journel o £ Material Science, Vol. 11, page 604 (1976) (b) of SiF4 and NH3 at about 1450 ° C and 2 torr; reference is made to F.S. Galasso, R.D. Veltri and W.J. Craft, Ceramics Bulletin, Vol. 57, page 453 (1978) and (c) by the reaction of Si (CH3) 4 with NH3 at temperatures of 1100-1500 ° C and 8 Torr; K.F. Lartique, M. Ducarroir and B. Amos, Proceedings IX International Conference CVD, page 561 (1984) and K.F. Lartique and F. Sibieude, Proceedings IX International Conference CVD, page 583 (1984). Both SiCl4 and SiHC13 react very quickly with NH3 at room temperature; however, a high degree of nucleation of the gas phase can be avoided by working at low pressures, by using concentric gas supply tubes in order to minimize mixing between the two reactive compounds, and by very rapid heating. before the filing action; reference is made to W. Hanni and H.E. Hintermann, Proceedings VXII International Conference CVD, page 597 (1981). By these means, coherent films or layers of Si3N4 can be deposited without the formation of Si (NH) 2 or NH4c1.

Both amorphous Si3N4 and crystalline a-Si3N4 can be deposited according to the processing conditions. At low pressure and at temperatures <1300 ° C the amorphous S13N4 is deposited; reference is made to K.H. Lartique and F. Sibieude, Proceedings IX International Conference CVP, page 583 (1984) and to K. Niihara and T. Hirai, Journel of Material Science, Vol. 11 page 604 (1976). At temperatures> 1400 ° C a-Si3N "is the preferred phase. However, the resistance to oxidation of amorphous Si3N4 deposited by the CVD technique is revealed at 1550 ° C. in air as being at least three times greater than that of the crystalline form; reference is made to A.K. Gaind and E.W .. Hearn, Journel Electrochemical Society, Vol. 125, page 139 (1978).

Small amounts of oxygen in the analysis will very quickly cause the formation of silicon oxynitride of variable stoichiometry, for example, when oxygen at a level of 10-100 ppm is introduced into a diluting stream of nitrogen during the CVD process; reference is made to V.A. Wells and M.V. Hanson, Proceedings VII International Conference CVD, page 190 (1979). Silicon oxynitride has also been deposited in the SiH4 / NH3 / H2 reaction including CO2 or NO at temperatures in the range of 850-1000 ° C; reference is made to M.J. Rand and J. Roberts, Journel Electrochemical Society, Vol. 125, page 139 (1978) and to A.K. Gaind and E.W .. Hearn, Journel Electrochemical Society, Vol. 120, p. 446 (1973).

It is generally desirable to provide an outer coating of silicon carbide on the intermediate coating layer. The arrangement to provide such superimposed coatings is described in the state of the art which includes the aforementioned US Pat. No. 4,515,860, these superimposed coatings being able to be produced by the CVD technique.

The following examples which are given to illustrate some ways in which the method according to the invention can be implemented can in no way limit the scope of the claims. They illustrate several embodiments of the present invention. In several of the proposed applications of coated carbonaceous materials, these coated elements will be exposed to a humid environment and / or to water. It has moreover been reported in the literature that coated structures which contain boron have poorer performance in oxidation tests when these structures are exposed intermittently to a humid environment or to water.

A performance test has been developed to assess resistance to oxidation, including the case of intermittent immersion in water. The oxidation resistance was tested by heating coated samples in air in an oven and changing the temperature cyclically, from a base temperature of 650 ° C to a temperature between 1200 and 1375 ° C. The weight of the samples was measured hourly and a given percentage of weight loss was defined as the limit or point of defect or failure. More specifically, the samples were placed in an oxidation oven and subsequently raised to a temperature of 1375 ° C. The test cycle was then completed by repeatedly heating the sample cyclically between 500 ° C and 1000 ° C. After 20 hours of oxidation test, the coated samples are soaked in water for 30 minutes at room temperature. Subsequently, the samples are air dried for 30 minutes, followed by a return to the oxidation test. This test cycle was continued until the samples removed from the oven had a weight loss of> _ 2wt%. The number of hours of the oxidation test cycle (consumed to reach this weight loss criterion) is defined as the life of the coating in the examples which follow.

The carbonaceous material used as base material for the examples which follow was a carbon-carbon composite substrate or support used bi-dimensionally obtained from "Science application international Corporation" and identified by ICC-S1. Each particular sample was approximately 1/2 "X 3/4" X 1/8 "in size.

Example 1

A batch of samples was subjected to a corrosion and transformation action with boron oxide; a temperature of 1651 ° C and a pressure of 23 torr were used for a total treatment time of 32 minutes. The silicon intermediate coating was then applied by treating the batch of samples at 1180 ° C and 260 torr for 45 minutes for each side. The raw materials used were 924 SCCM silicon tetrachloride, 20,000 SCCM hydrogen and 12,500 SCCM nitrogen. These samples were further processed by nitriding in a reactive atmosphere at a temperature of 1400 ° C and a pressure of 760 Torr. The nitriding gas, composed of 1% hydrogen in molecular nitrogen, was circulated over the samples for a period of 24 hours. XRD analysis of the treated samples indicated the formation of silicon β-nitride as the predominant crystal form of silicon nitride resulting from nitriding. Three samples from the batch were then subjected to the oxidation / immersion in water test described above. The average lifespan of these three samples tested was 27 cycles / hour. By way of comparison, a sample not coated with this same substrate has a lifespan of only a few cycles / hour.

Example 2

An additional batch of samples was initially treated by the deposition of silicon as described in Example 1.

The batch of samples was subsequently heat treated at 1450 ° C., 40 Torr in a stream of argon having a circulation speed of approximately 12000 SCCM for a duration of 13 minutes. These samples were then treated in a nitriding atmosphere, using as nitriding gas 1% hydrogen in molecular nitrogen.

The processing conditions were 1500eC, 760 Torr and a 24 hour period. XRD analysis of the treated samples indicates the formation of silicon anitride. These samples were subsequently provided with a refractory coating of silicon nitride surface. The coating conditions used for the refractory surface coating were 1200 ° C, 4 Torr and 60 minutes reaction time per side. The reactors used to constitute these reactive gases were ammonia (100 SCCM), tetramethylsilane (50 SCCM), hydrogen (3000 SCCM) and nitrogen (4000 SCCM). The applied surface refractory coating had a thickness of about 25 microns and a hardness of about 2100 HV. Three fully processed samples were subjected to the oxidation / water immersion test described above, the average cycles / hour to the point of failure for this batch was 60.

Example 3

An additional batch of "carbon-carbon" samples was subjected to the heat treatment at 1450 ° C. described in example 2. Subsequently, a coating of vitreous boron was formed in a CVD reactor at a temperature of 1400 ° C. and a pressure of 150 Torr. A gas stream composed of boron trichloride (700 SCCM), hydrogen chloride (700SCCM), hydrogen (1000 SCCM) and argon (5800 SCCM) was circulated over the samples for a treatment time of 30 minutes per side. The samples were then provided with a refractory surface coating of amorphous silicon nitride as described in example 2. Subsequently, five samples of the fully treated batch were tested in the oxidate ion / immersion in cycle. water and had a lifespan of 60 hours.

Example 4

An additional batch of "carbon-carbon" samples was tested in an identical manner to that described in Example 3, except for what concerns the conditions for treatment of the formation of the refractory surface coating. The refractory surface coating was carried out under exactly the same conditions as those described in Example 3, but in addition 4 SCCM of molecular oxygen were added to the reactive gases. XRD analysis of the resulting coating indicated that it was amorphous. In addition, the results of an X-ray photoelectron spectroscopy indicate that the surface refractory coating has approximately a Si2.70N2 stoichiometry; the stoichiometry was estimated after cleaning the surface by the spraying of argon to eliminate the surface effects. The average performance of five samples from this batch was 52 cycles / hour to the point of failure.

Example 5

A batch of samples was initially treated up to the intermediate coating of silicon as described in Example 1. The samples were then subjected to a nitriding atmosphere containing N2 with high degree of purity at 1500 ° C. for 24 hours. After this treatment, the XRD analysis did not detest the formation of crystalline Si3N4. The same batch of samples was again placed in a nitriding atmosphere. In this process, the high purity N2 has been replaced by high purity N2 containing 1% (vol.) H2. XRD analysis of the samples thus treated clearly indicated the formation of crystalline a-Si3N4. This example clearly shows the dramatic effect that the addition of a small amount of hydrogen has on the degree of nitriding.

Example 6

A batch of "carbon-carbon" samples was treated up to the silicon deposition phase as described in example 1. These samples were then subjected to a reaction in an atmosphere of 2-53% NH3 in argon at 1380 ° C for a period of 24 hours. The resulting samples were analyzed by XRD. This XRD analysis revealed the formation of Si2N20, of silicon oxynitride during the treatment at 1380 ° C. A small amount of air, which was the source of oxygen for the formation of oxynitride, was added during the reaction at 1380 ^ 0 in the reactive atmosphere.

Claims (30)

1. A coated carbon body having improved resistance to oxidation at high temperature comprising: a carbon body, said body having a porous transformed surface layer formed by subjecting said carbon body to a corrosion and reaction action to boron oxide, said transformed surface containing interconnected interstices and boron carbide formed by the reaction of boron oxide with said carbon body and - a coating forming a vitreous layer of which at least part is contained in the transformed surface layer, said coating comprising chemical compounds selected from the group consisting of silicon nitride, silicon oxynitride and a mixture of these elements. 2. A coated body according to claim 1, wherein the refractory coating is applied to the glassy coating layer. 3. A coated body according to claim 2, in which the refractory coating comprises carbides, borides or nitrides of silicon, zirconium, tantalum, hafnium, niobium and titanium, oxynitride of silicon and boride or nitride of aluminum or mixtures of these elements. 4. A coated body according to claim 3, wherein the transformed layer has a depth of about 3 to 250 microns. 5. A coated body according to claim 4, in which the transformed layer has an empty volume of up to 50% of the original volume occupied by the carbon layer. 6. A coated body according to claim 1, wherein the coating forming the vitreous mass partially fills the interstices of said transformed layer. 7. A coated carbon body having improved resistance to oxidation at high temperature comprising: - a carbon body, - said body having a transformed porous layer which has been formed by subjecting said carbon body to a corrosion action and of reaction with boron oxide, said transformed layer containing interconnected interstices and boron carbide formed by the reaction of boron with said carbon body, - an intermediate vitreous coating containing boron contained in said transformed layer, said coating intermediate comprising chemical compounds selected from the group consisting of silicon nitride, silicon oxynitride and a mixture of these elements, and, - an external refractory coating applied to said intermediate coating. 8. A coated body according to claim 7, wherein the refractory coating is silicon nitride. 9. A coated body according to claim 7, wherein the refractory coating is silicon oxynitride. 10. A coated carbon body having improved resistance to high temperature oxidation comprising: - a carbon body, - Said body having a transformed porous layer which has been formed by subjecting said carbon body to an action of corrosion and of reaction to oxide of a corrosion and reaction action on gaseous boron oxide, said transformed layer containing interconnected interstices and boron carbide formed by the reaction of boron with said carbon body, - an intermediate coating formed by a vitreous layer containing boron and silicon, this layer being contained in said transformed layer, said intermediate coating comprising chemical compounds selected from the group comprising silicon nitride, silicon oxynitride, and a mixture of these elements, and, - an outer refractory coating applied to said intermediate coating. 11. A coated body according to claim 10, according to which the refractory coating is silicon nitride. 12. A coated body according to claim 10, wherein the refractory coating is silicon oxynitride. 13. A method of manufacturing a coated carbon body having improved resistance to oxidation at high temperatures comprising: - providing a carbon body, - bringing said carbon body into contact with oxide of boron at a temperature high enough to cause a reaction between the carbon body and the boron oxide to form a transformed porous layer containing interstices interconnected in said body, said layer containing boron carbide, applying a coating forming a layer vitreous on said transformed layer, said coating forming a vitreous layer comprising chemical compounds selected from the group consisting of silicon nitride, silicon oxynitride and a mixture of these compounds. 14. A method according to claim 13, in which an external refractory coating is applied to said vitreous coating. 15. The method of claim 13, wherein the elevated temperature to cause the reaction is at least about 1500 ° C to produce a transformed layer having a depth between about 2 and 250 microns. 16. A method according to claim 13, wherein said transformed layer has an empty volume of about 50% of the original volume occupied by the carbon layer. 17. A method according to claim 13, in which the coating forming a vitreous layer comprises a primary species element for forming the vitreous layer selected from boron, boron carbide, boron oxide, silicon, silicon alloys, silicon dioxide, germanium and a mixture of these compounds. 18. A method according to claim 13, in which the coating forming a vitreous layer also comprises borides and oxides of zirconium, aluminum, magnesium, hafnium, titanium, zirconium carbides, hafnium, titanium, zirconium nitrides, hafnium , titanium, silicon and mixtures of these compounds. 19. A method according to claim 13, in which the intermediate coating is applied by chemical vapor deposition technology. 20. A method according to claim 13, in which the refractory coating is applied by chemical vapor deposition technology. 21. A method according to claim 13, in which the outer refractory lining comprises carbides, borides, or nitrides of silicon, zirconium, tantalum, hafnium, niobium, titanium, boride or aluminum nitride or mixing these compounds. 22. A method according to claim 13, wherein the refractory coating is silicon nitride. 23. A method according to claim 13, wherein the refractory coating is silicon oxynitride. 24. A method according to claim 13, in which the intermediate coating is applied by a technology called "sol-gel". 25. A method of manufacturing a coated carbon body having improved resistance to oxidation at elevated temperatures comprising: - providing a carbon body, - bringing said carbon body into contact with oxide of gaseous boron at a temperature high enough to cause a reaction between the carbon body and the boron oxide to form a transformed porous layer containing interstices interconnected in said body, said layer containing boron carbide, - applying an intermediate coating forming a vitreous layer on said transformed layer, said intermediate coating comprising silicon, or a chemical compound comprising silicon, subjecting said body to a nitriding atmosphere to transform said silicon or compound containing silicon into silicon nitride and, - applying a outer refractory lining on said intermediate lining. 26. A method according to claim 25, in which the nitriding atmosphere is chosen from the group composed of the elements N2, NH3, N2H4 / and by a mixture of these compounds. 27. A method according to claim 26, wherein said carbon body is subjected to a nitriding atmosphere at a temperature of about 1350 to 1500 ° C on average. 28. A process according to claim 27, wherein the nitriding atmosphere contains small amounts of hydrogen. 29. A method according to claim 25, in which the nitriding atmosphere contains oxygen or an oxygen-containing chemical compound to transform the silicon or the silicon-containing compound into silicon oxynitride. 30. A method according to claim 29, wherein the oxygen-containing compound is H20 or C0 / C02