United States Patent c191
`Terentieva et al.
`
`[54] METHOD FOR PROTECTING PRODUCTS
`MADE OF A REFRACTORY MATERIAL
`AGAINST OXIDATION, AND RESULTING
`PROTECTED PRODUCTS
`
`[75]
`
`Inventors: Valentina Sergeevna Terentieva; Olga
`Petrovna Bogachkova; Elena
`Valentinovna Goriatcheva, all of
`Moscow, Russian Federation
`
`[73] Assignee: Societe Europeenne de Propulsion,
`Suresnes, France
`
`[21] Appl. No.:
`
`545,806
`
`[22] PCT Filed:
`
`Mar. 10, 1995
`
`[86] PCT No.:
`
`PCT/FR95/00279
`
`§ 371 Date:
`
`Nov. 8, 1995
`
`§ 102(e) Date: Nov. 8, 1995
`
`[87] PCT Pub. No.: W095/24364
`
`PCT Pub. Date: Sep. 14, 1995
`
`[30]
`
`Foreign Application Priority Data
`
`[RU] Russian Federation ............ 94/008267
`
`Mar. 10, 1994
`Int. Cl.6
`..................................................... C23C 26/00
`[51]
`[52] U.S. Cl ........................... 428/408; 428/446; 428/457;
`428/469; 428/472; 428/697; 428/698; 428no1;
`428no2; 428n04; 156/89; 4211228; 427/397.7
`
`I llll 11111111111111111111111111111111111 Hll 11111111111111111111111
`5,677,060
`Oct. 14, 1997
`
`US005677060A
`[111 Patent Number:
`[451 Date of Patent:
`
`[58] Field of Search ..................................... 428/408, 698,
`428/446,457, 469,472, 677, 658, 701,
`702, 704; 156/89; 427/228, 397.7
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,854,892 1211974 Burgess et al ......................... 29/196.1
`4,190,493
`2/1980 Patel .......................................... 75/252
`1/1992 Chiang et al ........................... 428/408
`5,079,195
`
`Primary EXamine!'-Archene Turner
`Attorney, Agent, or Finn-Weingarten, Schurgin, Gagnebin
`& Hayes ILP
`
`[57]
`
`ABSTRACT
`
`A coating for providing anti-oxidation protection is formed
`at least on the surface of the material and it comprises a
`refractory phase which is formed mainly of the refractory
`silicide Ti(o.4-0.9sihlO<o.6-0.os)Si2 , and which has a branching
`microstructure forming an armature within which a healing
`phase is distributed that is constituted by a eutectic which is
`formed mainly of unbound silicon, of the silicide Ti(o.4-095)
`Moco.6-0.os)Si2 , and of at least the disilicide TiS2 • The coating
`is obtained from a mixture of powders that is deposited on
`the surface of the material and that is subjected to heat
`treatment under an inert atmosphere.
`
`27 Claims, No Drawings
`
`GE-1005.001
`
`

`
`5,677,060
`
`1
`METHOD FOR PROTECTING PRODUCTS
`MADE OF A REFRACTORY MATERIAL
`AGAINST OXIDATION, AND RESULTING
`PROTECTED PRODUCTS
`
`The present invention relates to protecting products made
`of refractory material against oxidation.
`The term ''refractory materials" is used herein to
`designate, in particular, refractory metals or metal alloys
`such as alloys based on niobium or based on molybdenum,
`tungsten, and/or tantalum, or refractory composite materials
`such as carbon-carbon composite materials or composite
`materials having a ceramic matrix, e.g. carbon-SiC (silicon
`carbide) composite materials. Such refractory materials are
`used, in particular, in the aviation or space industries to make 15
`parts that are subjected in operation to high temperatures,
`such as parts of aero-engines or elements of aerodynamic
`fairings (space vehicles).
`A severe drawback common to the above-mentioned
`refractory materials is their poor resistance to oxidation,
`even when exposed to medium temperatures. This consid(cid:173)
`erably limits the possibility of using them in an oxidizing
`medium at high temperature under static conditions, and
`makes such use practically impossible under aerodynamic
`conditions unless protection is provided against oxidation.
`The state of the art concerning protecting refractory
`materials against oxidation is most abundant, in particular
`for composite materials containing carbon. The term "com(cid:173)
`posite material containing carbon" is used herein to desig(cid:173)
`nate a composite material in which carbon is present in the
`reinforcing elements, e.g. in the form of carbon fibers, or in
`the matrix, or in an intermediate layer or "interphase"
`between the reinforcing elements and the matrix.
`Generally, a protective coating is formed on the surface
`of the refractory material, the coating comprising a continu(cid:173)
`ous layer of ceramic that withstands oxidation and that
`constitutes a barrier against the oxygen of the surrounding
`medium. The ceramic used may be a carbide, a nitride, a
`silicide, or an oxide. However, such a ceramic layer is
`inevitably subject to cracking. Microcracks appear in use
`because of the mechanical stresses imposed and/or the
`difference between the thermal expansion coefficients of the
`refractory material and of the protective coating. Similar
`defects may even appear while the ceramic layer is being
`made. The cracks provide the oxygen in the surrounding
`medium with direct access to the underlying refractory
`material.
`To solve that problem, it is well known to make the
`coating so that it forms an outer surface layer that has
`healing properties, or to add such a layer to the ceramic
`layer. While the material is being used, variations in thermal
`and mechanical stresses give rise to variations in the shape
`of the cracks, particularly to their edges moving away from
`and towards each other. The term "healing layer" is used
`herein to designate a layer that is capable, under the condi(cid:173)
`tions of use of the refractory material, of stopping, filling, or
`sealing the cracks while following the movements of the
`cracks, and capable of doing this without itself cracking.
`That is why the healing layer is usually made of substances
`that constitute a glass, or that are suitable for constituting a
`glass under the effect of oxidation, the glass being selected
`so as to exhibit viscous behavior at the working temperature
`of the material.
`Thus, it is known that using a protective coating based on
`silicides provides protection against oxidation at high tern- 65
`peratures because a surface film is formed that is based on
`silica as a result of oxidizing the silicon contained in the
`
`2
`coating. In use, the silica-based film continuously
`re-constitutes itself, so long as a sufficient quantity of
`oxygen is supplied. The silica base has a healing function
`because it passes to the viscous state at high temperatures.
`5 It has nevertheless appeared that in the presence of very high
`energy heat flows at high speed, e.g. in the combustion
`chamber of a direct air flow hypersonic jet engine, the silica
`film does not always regenerate quickly enough. The pres(cid:173)
`ence of intense heat :flows that are localized, particularly in
`10 zones having surface defects, at sharp edges, and also in the
`zones of incidence of shockwaves, can give rise to rapid
`destruction of the surface oxide film and to combustion of
`the refractory material, which combustion can be self-
`sustaining when the oxidation reaction is highly exotherrnal.
`In addition, a healing surface layer or film generally
`presents lower resistance to erosion than does the ceramic
`coating, and in the viscous state it runs the risk of being
`swept away. Unfortunately, in certain applications, in par(cid:173)
`ticular for parts of aero-engines or for fairing elements of
`20 space aircraft, the surface of the material is subjected to a gas
`flow that produces such a sweeping effect. This happens
`whether the healing surface layer is produced and regener(cid:173)
`ated by oxidizing components of the protective coating, or
`whether it is deposited in the form of an additional layer on
`25 the ceramic coating.
`To overcome that difficulty, document EP-A-0 550 305
`proposes making anti-oxidation protection by means of a
`healing phase and a refractory ceramic phase such as a
`carbide, a nitride, a silicide, or a baride, the phases consti-
`30 tuting two inter-penetrating arrays. The protection is made
`on the surface of the product by depositing a mixture
`comprising: a refractory ceramic in finely divided form; at
`least one refractory oxide likewise in finely divided form
`and providing healing properties by forming a glass; and a
`35 binder constituted by a polymer that is a precursor for a
`refractory ceramic. By being cross-linked prior to transfor(cid:173)
`mation into a ceramic, the polymer makes it possible to
`establish a three-dimensional array that holds in place bath
`the refractory ceramic component and the oxide component
`40 (s) of the healing phase. After the precursor polymer has
`been transformed into a ceramic, heat treatment performed
`at a temperature higher than the melting or softening tem(cid:173)
`perature of the particles of the healing phase enables the
`healing phase fillers to bond together. This forms a continu-
`45 ous healing phase that is interpenetrated with the refractory
`ceramic phase, and that is thus made more suitable for
`withstanding abrasion and being swept away.
`However, it is desirable, and this is the object of the
`invention, to further improve the performance of anti-
`50 oxidation protection to make it possible to use refractory
`materials at very high temperatures, typically with the
`material having a surface temperature of up to at least 1850°
`C., and also to guarantee that the healing function is
`continuous, even in surface zones which, because of their
`55 configuration or their location, are exposed to intense heat
`flows or to gas flows at very high speed.
`According to the invention, this object is achieved by a
`coating for protection against oxidation that comprises a
`refractory phase that presents a branching microstructure
`60 forming an armature within which a healing phase is
`distributed,
`the armature-forming refractory phase is mainly formed
`of the mixed refractory disilicide Ti(o.4-0.9s)Mo(0.6-0.os)Si2 ;
`and
`the healing phase is constituted by a eutectic which is
`formed mainly of unbound silicon, of the mixed disilicide
`Ti(o.4-0.9s)Mo(o.6-0.os)Si2 and of at least the disilicide TiSi2 •
`
`GE-1005.002
`
`

`
`5,677,060
`
`3
`The healing phase may further include at least one
`disilicide MeSi2 where Me is a metal taken from groups 3 to
`8 of the periodic classification of the elements (IlJPAC
`standard).
`The protective coating further includes a surface oxide
`film comprising the silica obtained by oxidizing the silioon
`and the silicides contained in the coating. The protective
`coating may further include an outer refractory layer. This
`may be a layer comprising at least one oxide such as a layer
`of silica, alumina, or zirconia glass, or a layer of a non-oxide
`ceramic such as silicon carbide (SIC) or silicon nitride
`(Si3N4 ) e.g. obtained by chemical vapor deposition.
`The exceptional properties of the coating come from the
`particular compositions of the armature of the protective
`coating and of the eutectic uniformly distributed within the 15
`armature, in association with the generation of a surface
`oxide film. These properties include, in particular, the ability
`of the coating to provide protection against oxidation at
`surface temperatures that may be as high as at least 1850° C.
`and for parts having configurations and conditions of use 20
`that are most unfavorable. Performance is enhanced by the
`self-healing ability of the coating provided:
`firstly, by the self-regenerating surface oxide film which
`is constituted essentially by amorphous silica doped with
`other components of the coating; and
`secondly, by the eutectic which constitutes one of the
`structural elements of the coating and which, because of the
`respective compositions thereof, presents good adhesion to
`the other structural component of the coating that forms the
`armature; incidentally, it may be observed that this adhesion 30
`contributes to increasing the resistance to being swept away,
`since the armature retains the eutectic effectively even when
`the temperature exceeds the melting point of the latter.
`The silicide-based armature of the coating remains
`homogeneous over a large temperature range and also has 35
`the ability to accumulate dopants, in particular niobium,
`tungsten, or tantalum coming from the underlying refractory
`material when the latter comprises an alloy of one of said
`elements.
`In addition to its healing function, the eutectic makes it 40
`possible to accelerate the formation and the regeneration of
`the surface oxide film based on amorphous silica, in par(cid:173)
`ticular by facilitating migration, towards the surface, of
`silicon or other dopants that are included in the composition
`of the oxide fihn. Amongst such dopants, boron and yttrium 45
`contribute to facilitating the formation of a uniform surface
`film having improved protective ability. Boron and yttrium
`may be present in the coating in the form of YSi2 , titanium
`boride, and/or yttrium boride.
`The performance of the anti-oxidation protection 50
`obtained by the invention makes it possible to envisage
`using refractory materials provided with said protection in
`applications such as hypersonic jet engines and aero(cid:173)
`spacecraft where operation conditions can be very severe.
`Thus, without requiring complex and expensive cooling 55
`systems, it is possible to make parts for hypersonic jet
`engines or reaction chamber surface portions having sharp
`edges that are the site of intense localized thermal phenom(cid:173)
`ena. In addition, it is also possible to make aerodynamic
`fairing elements for aero-spacecraft, such as the leading 60
`edges of the wings or the nose, in particular for space
`airplanes which are subjected to intense heat flows.
`Another advantage of the invention is that the protective
`coating against oxidation can also be used for protecting
`refractory metal alloys, in particular alloys of niobium, of 65
`molybdenum. of tungsten, or of tantalum, and intermetallic
`compounds or alloys containing dispersed oxide phases, as
`
`4
`well as for protecting refractory composite materials, in
`particular composite materials containing carbon such as
`carbon-carbon or carbon-SiC composites, or for providing
`anti-ignition protection or oxygen compatibility for metals
`5 such as aluminum, titanium, or nickel, as well as for their
`alloys and for their intermetallic compounds and alloys of
`the type TiAl, Ti3Al, TiA13 , NiAl, and Ni3Al.
`Another object of the invention is to provide a method
`enabling the above-defined anti-oxidation protective coating
`10 to be made.
`According to the invention, such a method comprises the
`following steps:
`preparing a mixture containing powders having the fol-
`lowing composition in percentage by weight:
`Ti: 15% to 40%
`Mo: 5.0% to 30%
`Cr: 0 to 8%
`Y: 0% to 1.5%
`B: 0% to 2.5%
`Me: 0% to 10%, where Me is at least one metal other than
`Ti, and taken from groups 3 to 8 of the periodic classification
`of the elements
`Si: balance needed in order to reach 100%
`depositing the mixture on the surface of the material to
`25 be protected; and
`performing at least one heat treatment at a temperature
`that is not less than the melting point of the eutectic of the
`coating.
`Me is preferably selected from Mn, Fe, Co, and Ni.
`The heat treatment comprises a first step under vacuum
`enabling the desired protective coating to be formed and
`enabling the coating to adhere to the surface of the material
`to be protected, and a second step performed in an oxidizing
`medium to enable an oxide film to be formed on the surface
`of the coating. The first step is performed at a temperature
`that is equal to or greater than the melting point of the
`eutectic, generally in a range of about 1300° C. to about
`1600° C. The second step is performed in an oxidizing
`medium at a temperature lying in the range about 1200° C.
`to 1600° C., and preferably at least 1300° C. The second
`step, whose purpose is to achieve pre-oxidation, is not
`necessarily performed prior to the material being used, since
`it can take place on the first occasion that the material is put
`into operation.
`A refractory outer layer may also be deposited on the
`surface of the material provided with the anti-oxidation
`coating. This outer layer may be formed by at least one
`refractory oxide, such as silica, alumina, or zirconia glass, or
`by a non-oxide ceramic, such as SiC or Si3N4 , e.g. obtained
`by chemical vapor deposition.
`A preferred composition in percentage by weight of the
`powder mixture is as follows:
`Ti: about 30%
`Mo: about 10%
`Cr: about 0.2%
`Y: about 0.5%
`B: about 2%
`Me: about 7%
`Si: balance needed in order to reach 100%, in which
`composition Me is preferably iron.
`Various techniques may be used for depositing the pow(cid:173)
`der mixture on the surface of the refractory material.
`The powder mixture is preferably put into suspension in
`a liquid, e.g. water, possibly also containing a dispersing
`wetting agent and/or a transient organic binder such as a
`cellulose varnish, a polymer of the polyvinyl alcohol type,
`etc., so as to make deposition possible by immersion, by
`
`GE-1005.003
`
`

`
`5,677,060
`
`5
`
`5
`brushing, or by spraying in air, which technique has the
`advantage of being simple, quick, and cheap.
`However other techniques for depositing the powder
`mixture can be used, such as cold supersonic spraying,
`spraying by explosion, or thermal spraying, in particular
`plasma spraying.
`The thickness of the deposited layer is selected as a
`function of the thickness desired for the protective coating.
`In general, this thickness is greater than 10 µm and is
`preferably at least 60 µm to 100 µm depending on the surface 10
`configuration of the material (sharp edge or plane surface).
`The application of this type of protection to porous
`materials also includes the possibility of the protection not
`being limited solely to protecting the geometrical surface
`thereof, but also impregnating and filling all or part of the 15
`accessible pores of the materials. This applies, for example,
`to composite materials such as carbon-carbon composites,
`and ceramic matrix composites which present residual
`porosity, and porous metals in the form of foams or fibers.
`Two implementations of the present invention are 20
`described below by way of non-limiting example.
`
`6
`cooling in air from 1300° C. to 20° C. All of the samples
`were subjected to 50 cycles.
`Thereafter, the samples were kept in an oven at 1300° C.
`for 100 hours in air with natural convection. By measuring
`the weight loss of each sample, it was found that the
`oxidation rate was 4.6x10-2 kg/m2-h.
`The samples were then exposed to a temperature of 1775°
`C. for 2 hours to verify that they were operative at that
`temperature. No sign of oxidation appeared on the surface of
`the samples.
`The samples were also tested by being placed in a gas
`flow containing the combustion products of a fuel. The flow
`speed was Mach 2.5, the surface temperature of the samples
`was 1675° C., and the ratio h/Cp was about 10 kg/m2·s,
`where h is the heat transfer coefficient and Cp is the specific
`heat. When the gas flow was directed perpendicularly to the
`plane surface of the samples, no destructive effect was
`observed after 500 seconds, and when the gas flow was
`directed at the edges of the samples, no destructive effect
`was observed after 60 seconds. By way of comparison,
`samples of the same niobium alloy but not provided with the
`protective coating were destroyed in a few seconds when
`subjected to this test.
`
`EXAMPLE2
`
`EXAMPLE 1
`Samples in the form of disks having a diameter of 20 mm
`and a thickness of 1.5 mm were made of a refractory 25
`material formed by a niobium alloy and having the follow(cid:173)
`ing composition (percentages by weight):
`Mo: 4.6%
`Zr: 1.4%
`C: 0.12%
`02: 0.02%
`H2 : 0.05%
`Nb: balance needed in order to reach 100%.
`A coating composition was prepared in the form of a
`mixture of powders in the finely divided state, the average
`size of the powder particles being about 10 µm. The com(cid:173)
`position of the powder mixture in percentage by weight was
`as follows:
`Ti: 30%
`Mo: 10%
`Cr: 0.2%
`Y:0.5%
`B:2.0%
`Fe: 7%
`Si: balance needed in order to reach 100%,
`The powder mixture was put into suspension in water, the
`ratio by weight between the material in suspension and the
`water being 1/1, and it was brushed onto the surface of the
`samples, After drying in air for 40 minutes, it was subjected
`to heat treatment under a vacuum (about 0.65 Pa) at a
`temperature of 1420° C. for 8 minutes. That heat treatment
`served to obtain the desired coating based on silicides with 55
`a thickness that was substantially uniform and with adhesion
`to the surface of the material to be protected. The thickness
`of the coating was 90 µm to 110 µm on various different
`samples.
`Thereafter, oxidation in atmospheric air was performed at 60
`a temperature of 1300° C. for 30 minutes, resulting in the
`formation of a surface film of oxide.
`The samples protected in this way were subjected to
`thermal cycles including thermal shock, each cycle com(cid:173)
`prising an increase in temperature from 20° C. to 1600° C.
`in 5 seconds, a cooling down to 1300° C. over 30 seconds,
`a holding of temperature at 1300° C. for 20 minutes, and a
`
`45
`
`40
`
`Parallelepiped samples having the dimensions 60 mmxlO
`mmx3 mm were cut from a plate of carbon-carbon (C--C)
`composite material having reinforcement, comprising super-
`30 posed layers of carbon fabric, and a carbon matrix. The
`samples were coated in a thin layer of silicon carbide (SIC)
`obtained by chemical vapor deposition, for example.
`A mixture was prepared of finely divided powders having
`a mean particle size of about 10 µm. The composition of the
`35 powder mixture in percentage by weight was as follows:
`Ti: 30%
`Mo: 10%
`Y: 0.5%
`B: 1.5%
`Fe: 7%
`Mn: 1.5%
`Si: balance needed in order to reach 100%.
`The powder mixture was put into suspension in an eth-
`ylsilicate based binder having a ratio by weight of material(cid:173)
`in-suspension to liquid vehicle of 1/1. The suspension was
`brushed onto all of the faces of the samples.
`After drying in air for 1 hour, heat treatment was per-
`50 formed under a vacuum (about 0.013 Pa) at 1440° C. for 5
`minutes. There was thus obtained a silicide-based coating
`fitting closely to the surface of the composite material and
`adhering thereto, the thickness of the coating varying over
`the range 85 µm to 115 µm.
`Oxidation treatment in air was then performed at a
`temperature of 1300° C. for 30 minutes to form a surface
`oxide film.
`The samples protected in this way were tested by thermal
`cycling under a flow of air heated in a high frequency
`induction plasma torch ("Plasmatron"). The characteristics
`of the torch and the test conditions were as follows: flow
`speed 1670 meters per second (rn/s);flowtemperature2153°
`C.; torch power 35 kW; flow pressure 38 Pa to 90 Pa;
`pressure behind the shockwave 0.2 MPa; Mach number 1.9;
`65 distance between the nozzle and the sample 105 mm; area of
`the heated zone on the sample 78.5 mm2
`; flow perpendicular
`to the main surface of the sample. Thermal cycling was
`
`GE-1005.004
`
`

`
`7
`performed with thermal shock in accordance with the fol(cid:173)
`lowing conditions: 5 seconds of heating from ambient
`temperature to 1700° C.; 10 second pause; and cooling to
`about 125° C. to 225 ° C. After 20 cycles, none of the tested
`samples had been destroyed. The measured mass loss rate of 5
`the coating was 0.0009 kg/m2·s for the entire duration of the
`tests.
`Other samples provided with the protective coating were
`tested under a flow of hot air heated in the same heating
`device. Two successive test cycles were performed under the 10
`following conditions respectively: air flow speed 90 mis and
`95 mis; flow enthalpy 5900 kJ and 6200 kJ; stop point
`temperature 3200° C. and 3350° C.; ratio of h/Cp 0.62
`kg/m2·s and 0.66 kg/m2 ·s; distance between the nozzle and
`the sample under test 60 mm and 75 mm; heat flux on hot 15
`wall 2420 kW/m2 and 2680 kW/m2
`; temperature at the
`center of the hot point on the surface of the sample during
`the test cycle 1780° C. and 1710° C.; and pauses at 1780° C.
`for 53 seconds and at 1710° C. for 36 seconds. No sample
`destruction was observed during the tests. By way of 20
`comparison, sinrllar tests performed on similar samples of
`carbon-carbon composite material with SiC coating but
`without the protective coating of the invention always
`caused the samples to be destroyed.
`Above Examples 1 and 2 show that the protection against 25
`oxidation as provided by the present invention is remarkably
`effective, and that this applies at high temperatures for parts
`of unfavorable configuration (sharp edges) and under con(cid:173)
`ditions that are very severe (corrosive gas flows at high
`speed).
`
`30
`
`5,677,060
`
`8
`One of the special characteristics of this coating when
`placed under such conditions is to have very low catalysity
`Kw equal to 2 mis up to 1200° C. and not exceeding 6 mis
`at 1600° C. In association with high emissivity (0.85 at
`1100° C.) this makes the anti-oxidation protection particu(cid:173)
`larly advantageous for protecting structures for re-entry into
`planetary atmospheres.
`
`EXAMPLE4
`
`A traction test piece having a length of 220 mm and a
`working section of 20 mmx3.6 mm and made of C-SiC
`composite material was made and protected as in Example
`3.
`
`The test piece was placed in the air plasma torch described
`in Example 3 so that the body of the test piece was uniformly
`heated while a traction force was simultaneously exerted on
`the test piece corresponding to a stress of 80 MPa. After five
`20-minute tests (i.e. 100 minutes) under stress at 1350° C.
`and under a pressure P=160 kPa, the test piece had not
`broken and its change in mass was negligible.
`At 1500° C. and under a pressure P=386 kPa, the test
`piece had still not broken after being subjected to stress for
`1 hour.
`This shows the ability of the anti-oxidation protection to
`protect a refractory material subjected to heat and to high
`mechanical stresses.
`
`EXAMPLE 5
`
`40
`
`EXAMPLE3
`Samples of C-SiC composite material in the form of
`disks have a diameter of 25 mm and a thickness of 3 mm
`were made using a reinforcement based on carbon fibers and 35
`a matrix of SiC obtained by chemical vapor infiltration.
`The samples were then coated on all their faces by being
`painted with the suspension described in Example 2 and they
`were heat-treated using the same protocol at 1440° C. for 5
`minutes.
`The thickness of the resulting coating was about 100 µm.
`The samples were placed perpendicularly in the gas flow
`from an air plasma torch having the following operating
`characteristics:
`gas speed: Mach 4.5 to 5
`temperature: 7000K
`degree of dissociation: 0.8%
`degree of ionization: 0.1 %.
`The protected samples were inserted in the gas flow for a
`period of 70 minutes during successive tests in which the 50
`distance between the torch and the test piece was adjusted to
`vary the surface temperature of the sample.
`After each test, the relative change Am/m in the mass of
`the sample was measured (a negative value corresponding to
`carbon being oxidized), thereby measuring the degree of
`protection provided.
`For various sample surface temperatures T and pressures
`P. the following were obtained:
`at T=ll00° C. and P=67 kPa: Am/m=+0.06%
`at T=1440° C. and P=160 kPa: Am/m=+o.27%
`at T=l500° C. and P=240 kPa: Am/m=-1.12%
`These results show the effectiveness of the protection
`when subjected to high temperature in a gas flow at very
`high speed. By way of comparison, the same material 65
`without protection was destroyed before the end of the 70
`minute cycle.
`
`55
`
`60
`
`Samples in the form of disks having a diameter of 20 mm
`and a thickness of 1.5 mm were made of refractory material
`constituted by a niobium alloy having the following com(cid:173)
`position (in percentage by weight):
`Mo: 4.6%
`Zr: 1.4%
`C: 0.12%
`02:0.02%
`H2:0.05%
`Nb: balance needed in order to reach 100%.
`A coating composition was prepared in the form of a
`mixture of powders in the finely divided state, the average
`size of the powder particles being about 10 µm. The com-
`45 position of the powder mixture, in percentage by weight,
`was as follows:
`Ti: 30%
`Mo: 10%
`Yb: 1%
`B:2.0%
`Si: balance needed in order to reach 100%.
`The powder mixture was put into suspension in water to
`enable it to be brushed onto the surface of the samples. After
`drying in air for 40 minutes, heat treatment was performed
`in a vacuum (about 0.65 Pa) at a temperature of 1420° C. for
`8 minutes.
`The thickness of the coating lay in the range 90 µm to 110
`µm on the various samples.
`Thereafter, oxidation in atmospheric air was performed at
`a temperature of 1300° C. for 30 minutes. resulting in a
`surface oxide film being formed.
`Diffusion annealing steps were performed for 1 hour and
`for 25 hours at 1450° C. under argon.
`An examination of the layers making up the deposit both
`before and after the annealing steps showed that the coating
`was very stable.
`
`GE-1005.005
`
`

`
`5,677,060
`
`10
`
`9
`The diffusion annealing step affected the composition of
`the various layers of the coating little, regardless of the
`length of the treatment time.
`On samples of the same type, isothennal oxidation tests
`were performed at 1450° C.
`Thermogravimetric analysis showed that this type of
`coating has better behavior than known coatings.
`Oxidation takes place slowly and conditions rapidly
`become linear. Thus, after 10 hours, mass variation was 10
`slightly less than 1 mg/cm2
`
`5
`
`Mo: 10%
`Cr: 0.2%
`Y: 0.5%
`B:2.0%
`Fe: 7%
`Si: balance needed in order to reach 85%. The mixture
`was therefore lacking 15% silicon.
`A composite material test piece made of C-SiC (carbon
`fiber reinforcement and SiC matrix) having a porosity of
`about 10% was used. It was impregnated by being dipped
`under a vacuum, filling its pores and simultaneously coating
`the surface of the material with the composition, and it was
`then baked at 120° C. for half an hour to eliminate the water.
`In a vacuum furnace, elemental silicon was heated in a
`crucible to a temperature of 1440° C. The silicon was then
`liquid.
`The test piece prepared in the above manner was then
`immersed in the liquid silicon, with the liquid silicon thus
`penetrating into the pores, and finally reacting with the
`powder present so as to make the initial composition up to
`100%.
`A composite material was thus obtained that was pro-
`25 tected both on the surface and in bulk by the general
`composition as described in Example 2.
`We claim:
`1. A product made of refractory material protected against
`oxidation by a coating formed at least on the surface of the
`30 material and comprising a refractory phase interpenetrated
`by a healing phase, characterized in that the refractory phase
`is formed mainly by the refractory silicide Ti(o.4-095)Mo(0 .6 _
`o.os)Si2 , and the refractory phase has a branching rnicro(cid:173)
`structure forming an armature within which the healing
`35 phase is distributed, which healing phase is constituted by a
`eutectic formed mainly of unbound silicon, of the silicide
`Ti(o.4-o.95)Mo(o.6-0.os)Si2, and of at least the disilicide TiSi2•
`2. A product according to claim 1, characterized in that the
`healing phase further includes at least one disilicide MeSi2 ,
`in addition to the TiSi2, where Me is a metal taken from
`groups 3 to 8 of the periodic classification of the elements.
`3. A product according to claim 2, characterized in that:
`the coating further includes a surface oxide film compris(cid:173)
`ing silica obtained by oxidizing silicon contained in the
`coating;
`Me is a metal selected from Mn, Fe, Co, and Ni;
`the refractory material is selected from the group consist(cid:173)
`ing of:
`alloys of niobium, of tantalum, of molybdenum, of
`tungsten, intermetallic compounds and alloys con(cid:173)
`taining dispersed oxide phases; and
`aluminum, titanium, nickel, alloys thereof, and inter(cid:173)
`metallic compounds and alloys of the TiAl, Ti3Al,
`TiA13 , NiAI, Ni3Al type, for which the coating
`performs an anti-ignition function on the other hand;
`the refractory material is a composite material containing
`carbon;
`the refractory material is a composite material selected
`from carbon--carbon and carbon-SiC composite mate(cid:173)
`rials.
`4. A method of obtaining a product of refractory material
`that is protected against oxidation, according to claim 3, the
`method comprising the steps consisting in:
`preparing a mixture containing powders having the fol(cid:173)
`lowing composition in percentage by weight:
`Ti: 15% to 40%
`
`•
`
`EXAMPLE6
`
`1\vo alloys liable to self-ignition because of a low ignition
`temperature and high combustion heat, were tested. To this 15
`end, samples wer