FR2643088A1 - Process for coating based on an element of metallic type of a substrate made of ceramic oxide and ceramic oxides thus coated - Google Patents

Abstract

Process for coating based on an element of metallic type which has a number of possible valencies of a substrate made of ceramic oxide in mono- or multifilament form or in bulk form using reactive CVD in a leakproof enclosure, characterised in that the said substrate is brought into contact with a reaction gas mixture comprising hydrogen and at least one halide of the said element of metallic type at a temperature of at least 500 DEG C, in order to give rise to the formation of a surface layer consisting of compounds whose elements originate partly from the substrate itself. Application: composite materials.

Description

COATING METHOD BASED ON A NETAL-LEAGUE-TYPE ELENENT OF A SUBSTRATE OF CERANIC OXIDE AND CERANIC OXIDES
WHAT ARE THUS COATED.

 The present invention relates to an improved method for coating a ceramic oxide substrate in mono- or multifilament form or in massive form with a compound based on a metal type element having several possible valences. It also relates to the products obtained by this process.

 By "ceramic oxide" is meant any material for high temperature use which contains oxygen in combined form. We can quote for example the mullite 2Al203,2Si02 or simply the alumina A12O3.

 It is known to use alumina fibers as reinforcement of aluminum matrix. It has been shown that the incorporation of this type of fiber leads to a composite material whose performance is better than that of unreinforced aluminum. There has also been noted a significant increase in the breaking strength, in the modulus of elasticity, in the abrasion resistance, and finally in the resistance at high temperature.

 However, the reinforcing role is limited, since it is established that the alumina is poorly wetted by aluminum, which results in insufficient charge transfer between the alumina fibers and the aluminum matrix. , therefore a weak physico-chemical adhesion.

 It was then proposed to overcome this drawback, either by covering the alumina fiber with a thin film of silica, or by introducing a small amount of silica to the gel used to manufacture the fiber, and by subjecting the fiber to a treatment. adequate thermal causing in particular, the diffusion of silica towards the surface of the fiber. The effects of this silica coating remain limited, however, for two reasons.

On the one hand, aluminum ends up reducing the silica used to ensure better adhesion between the fiber and the matrix, and on the other hand, it is not possible to use as matrix the most aluminum alloys widespread on the industrial level, that is to say those containing magnesium, because in this case, this metal, even in small quantities in the alloy, reacts on silica and alumina, resulting in significant degradation of the fiber. In addition, more significant degradations are foreseeable in the case of titanium matrices or ceramic oxides, taking into account the high reactivity of alumina vis-à-vis these compounds.

 The invention overcomes these drawbacks. It relates to a coating process based on metallic type elements, and in particular on titanium, not only of alumina fibers but of all other current ceramic oxide fibers. Mention may be made, for example, of silicoalumilates, in particular mullite, zirconia, magnesia, and Yttrium oxide. As already said, this process can be extended to two- or three-dimensional structures of various shapes.

 It should be noted that the invention is doubly different from the titanium metallization processes described in the prior art. On the one hand, these methods consist in depositing titanium from a gas phase which does not react with the substrate surface and on the other hand, these methods relate only to metallic substrates: steels, iron and copper. In addition, the reaction and the products resulting from it are poorly controlled.

 Finally, the yield of this reaction remains low, even at 1600 K and under an atmosphere (TEYSSANDIER F.

State thesis in science, Grenoble, 1986; YEAN DH and
RITTER DR Metall. Trans. 1974, 5, 2473-2474).

We sought to improve this process by subjecting a pre-reduction of titanium tetrachloride by passing the TiCl4 / H2 mixture over titanium shavings (WAKEFIELD
GF Report AFML-TR, 66-397, 1966). Finally, the Japanese document P 106 2906 shows that copper and steel substrates can be covered with titanium at 850 "C by passing a current of TiC14 entrained by a carrier gas previously heated to 950" C in contact with titanium. metallic. However, this type of process is limited to a simple metallization of a metal substrate.

 A method of depositing a layer of titanium dioxide (TiO2) on glass containers has also been described. This process consists in entraining titanium tetrachloride by a current of air or oxygen and in reacting it on a glass wall worn between 550 "and 6000C (AS SANYAL J. MAKERJI: Glass Technology 26, 1985, 152154) However, this process, which consists of simply depositing a lubricating layer of TiO2, in no way involves the glass substrate in the reaction.

The present invention relates to a coating method based on a metallic type element having several valences of a ceramic oxide substrate in mono or multifilament form or in massive form, by
CVD reactive in a sealed enclosure.

 This process is characterized in that said substrate is brought into contact with a reaction gas mixture comprising hydrogen and at least one halide of said metallic type element at a temperature at least equal to 500 "C, in order to cause the formation of a surface layer consisting of compounds, the elements of which partly come from the substrate itself.

 In other words, the present invention consists in reacting a gas phase under certain temperature conditions with a ceramic oxide substrate, the result of which tends to form a coating based on one part of the element. of metallic type constituting the metallic type compound included in the gas phase and on the other hand of the substrate.

Advantageously, in practice
- the metallic type element is chosen from the group consisting of transition metals and semiconductor elements
- the metallic type element consists of titanium
- the coated ceramic oxide is chosen from the group consisting of silica, alumina, mullite, magnesia, zirconia and Yttrium oxides
the coating process is carried out continuously, the substrate in mono- or multifilamentary, mono-, bi- or three-dimensional form running at constant speed in the sealed enclosure, in which a gas stream consisting of a mixture of hydrogen 2 flows, and halide of the metal element at a temperature at least equal to 500 "C
- The sealed enclosure also comprises in dispersed solid form said metallic type element, the latter never being in contact with the ceramic oxide substrate;
the coating is carried out discontinuously, the substrate being in massive form, the sealed enclosure further comprising in dispersed solid form said element of metallic type, and the reaction gaseous mixture consisting of hydrogen chloride
- Hydrogen chloride is introduced into the sealed enclosure under reduced pressure
- The gas mixture is introduced into the airtight enclosure, and consists of Hydrogen and Argon chloride.

 If the temperature of the enclosure is below 500C, it has been possible to show that the kinetics of the reaction were very slow, or even zero. On the other hand, if this temperature reaches or exceeds 900 "C, the mechanical qualities of the coated substrate are then affected.

 Finally, the invention relates to the wicks of filaments and the mono, bi or three-dimensional structures of wicks of filaments and massive objects of ceramic oxides coated in this way.

 The manner in which the invention can be implemented and the advantages which ensue therefrom will emerge more clearly from the following exemplary embodiments, given by way of indication but not limiting in support of the appended figures.

 Figure 1 is a schematic longitudinal view of an installation of a reactor suitable for the treatment of wicks in ceramic oxide continuously, according to the invention.

 Figure 2 is a section of the fibers obtained according to the present invention.

 The reactor used to continuously coat wicks of ceramic oxide in accordance with the present invention is of a type similar to that described in patent application No. 86 17157 of the Applicant.

 On a support (1) made of stainless steel, two blocks (2) and (3) for housing the pulleys (4) and (5) are placed, detailed in FIG. 2. These blocks (2) and (3) also stainless steel have a flange (6) provided with a Viton seal (7) to allow the operations of loading and unloading the wicks (10).

 The two blocks (2) and (3) are interconnected by a cylindrical tube (8) made of silica, connected by sleeves (11) and (12) provided with sealing rings.

 Each block (2,3) contains a pulley (4,5) mounted on a horizontal axis (9) and driven by a motor (13).

 Between the two drive pulleys (4,5), a wick (10) of parallel filaments of a ceramic oxide is unwound.

 It goes without saying that the treatment enclosure (2,3,8) is waterproof.

 As already said, the operating mode is carried out differently depending on whether one operates continuously or discontinuously. The various operating modes will now be described in more detail.

EXAMPLE 1
The example described below relates to the coating of metallic compounds based on titanium on a mullite substrate, ie a silico-aluminate corresponding to the general formula 3 A1203, 2 SiO2.

 In a closed and sealed enclosure, a preform is introduced, consisting of a three-dimensional assembly of wicks of silico-aluminous filaments containing 70% by weight of alumina A1203, 28% by weight of silica SiO2 and approximately 2% by weight of B2O3 . The enclosure is filled with Hydrogen chloride HCl at a pressure of 270 hectopascals.

 This enclosure contains metallic titanium in the form of chips, in order to increase the reactive surface of the latter. These chips are introduced in sufficient quantity to reach an atomic ratio Ti / Cl close to 100.

 The enclosure is heated uniformly in volume and is brought to the temperature of 700 ° C. by any suitable means, and in particular, by an electric resistance furnace or an induction furnace. This temperature is maintained for seven minutes.

 It should be noted that the preform is not in contact with the titanium shavings, but rests on a bearing structure made of silica, which also ends up passivating.

 The action of HC1 on the Titanium leads to the formation of a complex gas phase containing, in addition to the residual hydrogen H2, various titanium chlorides, in particular Tical4, TiC13 and Tical2, the proportions of which vary according to the temperature .

 At the end of the treatment, the preform is uniformly coated with a very adherent black color deposit.

 The same kind of coating can be obtained using a transition metal chosen in particular from Zirconium, Hafnium, yttrium, Niobium, Vanadium and Tantalum, or a semiconductor element. It is the same if one chooses this element among any of the transition metals.

EXAMPLE 2
In a closed and sealed enclosure, flat substrates of optical silica are introduced. The enclosure is filled with hydrogen chloride HC1 at a pressure of 270 hectopascals. This enclosure contains titanium in chips in sufficient quantity to reach an atomic ratio Ti / Cl equal to 100.

 The enclosure is brought to 600 "C for one hour.

 At the end of this treatment, the substrates are coated with a very adherent brown deposit which is difficult to scratch, having the appearance of a mirror.

EXAMPLE 3
A strand of parallel silico-aluminous filaments (70% alumina, 28% silica, and 2% by weight of B203) consisting of 780 individual filaments from 10 to 12 zm in diameter and around 200 Dtex. This wick is marketed by 3M under the registered trademark "NEXTEL 440".

 This wick is maintained under a slight positive mechanical tension in order to ensure good spreading and to avoid any loose floating or any vibration effect. This scrolling takes place between the two pulleys (4,5) of the reactor described. The wick (10) is maintained at a temperature of 700 "C over an area close to twenty centimeters.

 The enclosure also has titanium shavings distributed in the cylindrical tube (8); but never come into contact with the streaks that pass by.

 A gas mixture consisting of titanium tetrachloride entrained in the opposite direction of the movement of the locks by a stream of hydrogen is then introduced by means of the introduction orifice (15) into the reactor, at atmospheric pressure. The TiC14 is maintained at 25 "C. and the hydrogen flow rate is of the order of nine liters per hour. The hydrogen / chloride chloride volume ratio is then equal to 59.6. This gaseous mixture is slightly diluted by introduction of argon, due to the saturated vapor pressure of Tical 4. The reactor also has an outlet (16) for the reacted gas mixture.

 A wick is obtained in which all the filaments are coated in a homogeneous manner, identifiable by the blackish color taken by the wick. This coating perfectly matches the initial shape of the filaments which retain all their flexibility.

 The thickness of the coating is constant and close to 0.1 micrometer.

The mechanical characteristics of the filaments thus treated are determined by means of 20 cm test pieces subjected to traction. We determine
- the breaking stress: o = 1,750 MPa
- Young's modulus: E = 190 GPa
For comparison, the same characteristics of the wick before treatment were determined - breaking stress: 2 = -2000 MPa
- Young's modulus: E = 190 GPa
There is therefore only a slight decrease in the mechanical qualities of the fibers thus treated.

 It should be noted that the titanium tetrachloride / hydrogen ratio, as well as the temperature of the titanium shavings and the hydrogen flow rate, and finally the running speed, are chosen according to the thickness of the coating desired.

EXAMPLE 4
Example 3 is repeated above, without introducing titanium shavings in metallic form into the cylindrical tube (8).

 Results are obtained comparable to those obtained in Example 3, on the condition of heating the enclosure to 900 ° C., which shows any interest in introducing titanium in the form of shavings, making it possible in particular to limit energy expenditure , and on the other hand to avoid a risk of deterioration of the wicks at high temperature.

 The coating is formed by chemical reaction between the gas phase and the ceramic oxide. The coating is easily identifiable due to its blackish color. Whatever the experimental conditions and whatever the chemical nature of the ceramic oxide constituting the substrate, the coating has a multilayer structure, as can be seen in FIG. 2, representing an elementary filament (20) coated in continuous by the process according to the invention. The outermost layer (21) consists of a solid solution of Oxygen in Titanium. The composition of this phase called TiOX depends on both the experimental conditions and the chemical nature of the ceramic oxides.

In all cases, x is between 0.35 and 0.5. The internal layer (22) in contact with the substrate (23) corresponds to one or as the case may be several intermediate compounds based on Titanium and of the element or elements previously combined with oxygen in the ceramic oxide, in particular silicon, Aluminum, Magnesium, Zirconium,
Vanadium, Yttrium. This has many advantages. Indeed, the outer layer (21), due to its composition Tir, has interesting bonding properties, in particular with Aluminum and Titanium.

 In the example shown, the elementary filament (20) consists of mullite, of general formula 3 A1203, 2 SiO2. The outer layer (21) is composed as already said of Tir ,, x being between 0.35 and 0.5, the inner layer consists of a complex phase containing Titanium, Aluminum, Silicon and a low amount of Oxygen.

 It is shown that the Titanium provided by the gas phase and the Oxygen coming from the substrate diffuse in the opposite direction, the first towards the ceramic oxide constituting the substrate and the second towards the surface of the coating. It is therefore a cross-diffusion through the internal layer of the coating in contact with the substrate. Indeed, the multilayer structure of the coating, and the fact that its thickness increases linearly as a function of the square root of the treatment time, shows that the formation of the coating is governed by a diffusional process. In fact, the kinetics of the reaction being slow and strongly slowing down by itself at the locations of the substrate where the deposit has already formed, the gas phase can continue to react on the bare parts of the substrate. This therefore results in a continuous and regular deposition, and especially in in the case of treatment of a two- or three-dimensional structure based on multifilament wicks, penetration of the reaction gas mixture into the core, that is to say into all the filaments located in the core of the wick. As also, the reaction requires the participation of ceramic oxide, this reaction is limited to the surface of the substrate. In addition, as the coating is formed from the ceramic oxide itself, the adhesion between this coating and the surface ceramic oxide is excellent. In other words, there is self-regulation of the deposit. of metallic compound on the ceramic oxide by passivation of the latter as the reaction progresses.

 In the case of wicks, the individualized filaments of ceramic oxides comprising a multilayer coating, they are characterized in that said coating of each individual filament is regular, homogeneous and has a constant thickness of between 0.1 and 1 micrometer , which constitutes a favorable thickness, taking into account the fact that the average diameter of the ceramic oxide fibers is between 10 and 20 micrometers, and having regard to the adhesion and chemical protection properties of the ceramic oxide fibers. On the other hand, it has been possible to show that if this coating has a thickness exceeding one micrometer, a rapid degradation of the mechanical qualities and a tendency of the filaments to become brittle is then observed.

 The method according to the invention can be used for any form of substrate based on ceramic oxide, in particular based on alumina, silica, mullite, zirconia, magnesia and yttrium oxide.

Thus, the method according to the invention allows it to obtain advantages that the methods known to date did not allow to obtain.
- the formation of a deposit of metallic character, adherent, continuous and homogeneous, conductor of electricity and heat and retaining the flexibility of the fiber or basic wick, without affecting its metallic properties
- the possibility of obtaining entities chemically compatible with many metals, and which can therefore serve as an interface between reinforcement and matrix
The products thus obtained can therefore be favorably used for example in the context of electrolytic plating.

 The fibers obtained also retain the same flexibility as the initial fibers. In this way, they can be used in all applications of composite materials, by introduction into an appropriate matrix. Thus their field of application covers that of ceramic oxides proper.

Claims (10)

 1 / Coating method based on a metallic type element having several possible valences of a ceramic oxide substrate in mono or multifilament form or in massive form by reactive CVD in a sealed enclosure characterized in that one puts in contacting said substrate with a reaction gas mixture comprising hydrogen and at least one halide of said metallic type element at a temperature at least equal to 500 ° C., in order to cause the formation of a surface layer consisting of compounds whose elements partly come from the substrate itself.  2 / coating method according to claim 1, characterized in that the metallic type element is selected from the group consisting of transition metals and semiconductor elements  3 / A coating method according to one of claims 1 and 2, characterized in that the metallic type element consists of titanium, and in that the coated ceramic oxide is chosen from the group consisting of silica, alumina, mullite, magnesia, zirconia and Yttrium oxides  4 / coating method according to one of claims 1 to 3, characterized in that it is carried out continuously, the substrate being in mono or multifilament, mono-, bi or three-dimensional scrolling at constant speed in the sealed enclosure , in which a gas stream circulates consisting of a mixture of hydrogen H 2 and halide of the metallic type element & a temperature at least equal to 500 "C  5 / A coating method according to claim 4, characterized in that the sealed enclosure also comprises in dispersed solid form said metallic type element, the latter never being in contact with the ceramic oxide substrate.  6 / coating process according to one of claims 1 to 3, characterized in that it is carried out discontinuously, the substrate being in massive form, the sealed enclosure further comprising said metallic element in the form dispersed solid, and in that the reaction gas mixture introduced into the enclosure consists of hydrogen chloride  7 / A coating method according to claim 6, characterized in that the hydrogen chloride is introduced into the enclosure under reduced pressure.  8 / A coating method according to claim 6, characterized in that the gas mixture is introduced into the enclosure at atmospheric pressure and consists of Hydrogen chloride and Argon.  9 / ceramic oxide material coated with a compound based on a metallic type element having several possible valences obtained by the implementation of the method according to one of claims 1 to 9, characterized in that it has at least two superimposed layers, of constant thickness, each of them consisting of a compound based on said metallic type element and at least one of the elements used in the composition of the ceramic oxide, forming a passivation barrier.  10 / Material according to claim 9, characterized in that it is in filamentary form, the coating based on a metal type compound is regular, homogeneous, adherent and has a constant thickness between 0.1 and 1 micrometer .