GB2051875A - Preparing metal coatings - Google Patents

Abstract

A process for coating a solid substrate material, e.g. a carbon fibre, with a uniform layer of a valve metal or compounds thereof including nitrides, carbides or carbonitrides. The substrate surface is exposed to a gaseous nitrogen compound, or to nitrogen gas together with hydrogen and a volatile halide of the valve metal. It is thought that nitrogen provides nucleation of the surface for subsequent metal or metal compound deposition. The coatings may be used in the construction of superconductor elements.

Description

SPECIFICATION Preparing metal coatings This invention relates to processes for coating surfaces, and in particular to processes for the chemical vapour deposition of a metal or metal alloy surface layer.

In many applications it is necessary to coat a solid surface with a thin metal film. Various techniques have been employed in the production of such coatings, but such techniques in general suffer from the disadvantage that where a very thin coating, e.g. 0.05 microns, is required it is very difficult to achieve a uniform coating. This non-uniformity of thin films is inherent in the nature of these prior art deposition processes. The metal tends to deposit initially at selected sites on the surface, further deposited metal then growing out from these sites.

Our co-pending application No. 7918659 (E. L. Bush - E. J. Workman 22-1) describes and claims a process for depositing by chemical vapour deposition a substantially uniform valve metal coating on a particulate insulating substrate, the process including introducing a nucleating agent during the deposition process, the nucleating agent being such that it provides for deposition of an initial surface coating of the valve metal via an irreversible reaction.

That application was directed towards the production of metal coated insulating powders e.g. for the manufacture of electrolytic capacitors. We have now found that the technique canwbe employed to deposit a uniform metal coating on other solid surfaces, for example sheets or fibres. Iri particular we have found that the process may be employed to provide valve metal alloy coatings on carbon tibres in the manufacture of superconductor elements. The metal film may provide a base for further metal deposition or for deposition of one or more metal compounds.

According to one aspect of the present invention there is provided a process for depositing by chemical vapour deposition a substantially uniform coating on a solid surface, wherein a nucleating agent is introduced to the surface during the deposition process, the nucleating agent being such that it provides for deposition of an initial surface coating via an irreversible reaction.

According to another aspect of the invention there is provided a process for depositing by chemical vapour deposition a substantially uniform layer of tantalum and/or niobium or compounds thereof as hereinafter defined, on a fibrous substrate, the process including exposing the fibres together with tantalum and/or niobium to nitrogen gas a gaseous nitrogen compound together with a hydrogen halide, wherein the hydrogen halide reacts with the tantalum and/or niobium to provide a gaseous species which reacts with nitrogen or the nitrogen compound thereby depositing a uniform surface layer on said fibres via an irreversible reaction.

The term 'valve metal' as employed herein is understood to include not only individual valve metals but also alloys thereof.

In order to provide a tantalum, or other valve metal, coating of e.g. not greater than 0.3 to 0.1 microns thickness it is essential to employ a deposition process which provides uniform growth rather than "fill-in" of island growth. We have found that uniform growth of a metal film on a surface can proceed only after a complete nucleation of the surface has occurred. This necessitates a high degree of supersaturation of the depositing metal species.

In many metal deposition processes the chemical reaction is thermodynamically reversible, the forward and reverse reactions being stimulated by control of temperature and the partial pressures of the various gaseous species. Thus, for example, the deposition of such valve metals as tantalum and niobium may proceed via the following typical reactions::-

1000 C + excess H TaCI4 + 2H2 x - Ta +4HCL 5000C 10000C + excess ThCle+221H2 Ta 4---=------------ Ta + 5HCL 4000C 1 000 C + excess NbCl4 + 2H2 > Nb + 4HCL 5000C 10QOOC + excess H, NbCi5 + 22H2 Nb + Q2 Nb + SHCL 40000 Even at 10080C and a 20-fold excess of hydrogen, the conversion of tantalum chloride to tantalum metal is only 96% efficient.The result is that, in the absence of extensive nucleation, once tantalum growth has been initiated on parts of the surface there is a tendency for these regions to grow at the expense of the remainder of the surface and that the remainder of the surface remains uncovered.

We have found that it is possible to modify the chemical transport reaction to provide an irreversible deposition step to give a metal-compatible deposit such that metal deposition then proceeds uniformly over the nucleated surface. Such a modified reaction process is postulated as follows:-

5000C Ta + 4HCI = TaCl4+2H2 100000 reversible 10000C reversible 8 , + nucleating gas RX Intermediate TaCI4 adduct irreversible 10000C Ta + TaX + HCI We have made the unexpected discovery that this reaction sequence may in fact be achieved with the aid of certain nitrogen containing reactive gases, e.g. ammonia.When ammonia is employed as the nucleating gas, then the tantalum deposits over the complete surface of all the particles within the reaction region. The tantalum film thus deposited may contain up to 10 weight % nitrogen, possibly in the form of one or more nitrides. Further deposition of tantalum by the normal thermodynamically reversible process results in the build up of substantially uniform layers of tantalum. The electrical and physical properties of the final coated product are substantially unaffected by the thin interfacial underlayer of nitrided tantalum.

Ail attempts to achieve uniform surface coverage with tantalum without initial treatment with a nucleating gas have proved unsuccessful unless uneconomically large quantities of tantalum are deposited. Ammonia is relatively cheap and readily obtainable and is thus an attractive choice as the nucleating gas. However, other nitrogen containing gas may of course be employed, and we have achieved successful nucleation with, e.g., ammonium chloride, hydrazine-hydrochloride, hydroxylamine hydrochloride or their other halides and, under certain conditions, even nitrogen gas. We have also found that nucleation can be achieved not only with nitrogen compounds but also other materials which react in an irreversible way to form a surface layer compatible with a subsequent depositing species.

Thus, volatile borides, sulphides, phosphides and silicides may all be employed as nucleating agents.

Nitrogen compounds are to be preferred however as they are relatively cheap and, in general, easier to handle.

The technique may be applied to the coating of carbon fibres with tantalum/niobium alloy compositions or with the metal compounds, e.g. nitrides, carbides or carbonitrides, the coated fibres subsequently forming superconductor elements. It is well known that tantalum/niobium alloys of suitable composition exhibit superconducting properties when cooled to within a few degrees of absolute zero. It is also known for example that niobium carbonitride has superconducting properties.

In a typical deposition process, carbon fibres are exposed, together with a mixture of tantalum and niobium powder, to a flow of hydrogen at a temperature of 9000C to 1 1000C. Nucleation of the fibres is then effected by exposing them to an atmosphere of hydrogen containing e.g. 10 volume % ammonia and 10 volume % hydrogen chloride for a period of 5 minutes. During the nucleation period metallic tantalum and niobium react with the hydrogen chloride to form volatile chlorides whereby tantalum and niobium are deposited on the fibre surface via an irreversible reaction to yield a nitrided alloy surface.

Further deposition of tantalum and/or niobium or the corresponding nitrides, carbides or carbonitrides can then take place via the normal reversible reaction process.

The process is not of course restricted to alloy deposition, and if required a single metal can be deposited.

In an alternative fibre coating process, the fibres may be subjected to a conventional metal coating treatment to provide non-uniform regions of a valve metal. The fibres are then exposed to hydrogen at- temperature of 900--1 1000C. Nucleation of the fibre surface is next effected by exposure to an atmosphere of hydrogen containing 10 vol % ammonia and 10 vol % hydrogen chloride for a period of 5 mins. During the nucleation period the valve metal reacts with the hydrogen chloride to form a volatile metal chloride which then deposits the metal uniformly on the nucleated fibre surface via an irreversible reaction thus providing a nitrided metal surface. Further deposition of metal or metal compounds can then be effected.

The conditions necessary for metal deposition on fibres or on other surfaces correspond to those set out in our co-pending application No. 791 8659 in reiation to powder coating.

Claims (11)

1. A process for depositing by chemical vapour deposition a substantially uniform coating on a solid surface, wherein a nucleating agent is introduced to the surface during the deposition procsss, the nucleating agent being such that it provides for deposition of an initial surface coating via an irreversible reaction.

2. A process for depositing by chemical vapour deposition a substantially uniform layer of tantalum and/or niobium or compounds thereof as hereinbefore defined on a fibrous substrate, the process including exposing the fibres together with tantalum and/or niobium to nitrogen gas or a gaseous nitrogen compound together with a hydrogen halide, wherein the hydrogen halide reacts with the tantalum and/or niobium to provide a gaseous species which reacts with nitrogen or the nitrogen compound thereby depositing a uniform surface layer on said fibres via an irreversible reaction.

3. A process as claimed in claim 2, and wherein said tantalum and/or niobium is provided as a non uniform coating on at least some of said fibres prior to the deposition process.

4. A process as claimed in claim 2 or 3, and wherein said fibres are carbon fibres.

5. A process as claimed in claim 2, 3 or 4, and wherein the reaction temperature is within the range 9000Cto 1 1000C.

6. A process as claimed in any one of the claims 1 to 5, wherein the nucleating species is ammonia, an ammonium halide, a hydrazine hydrohalide or a hydroxylamine hydrohalide.

7. A process as claimed in claim 1, wherein the nucleating gas is a halide or hydride of boron, sulphur, phosphorus or silicon.

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8. A process as claimed in any one of claims 1 to 7, wherein the thickness of the deposited material is less than 1 micron.

9. A metal deposition process substantially as described herein.

10. A coated material prepared by a process as claimed in any one of claims 1 to 9.

11. A coated carbon fibre prepared by a process as claimed in any one of claims 1 to 9.