GB2334039A - Manufacture of diamond fibres incliding a pretreatment step - Google Patents

Abstract

A method of surface preparation of a metallic or ceramic core suitable for serving as a substrate for the chemical vapour deposition of diamond fibre comprises cleaning and degreasing the surface of the core in an organic solvent; and treating the surface of the core by placing the core in a an ultrasonic bath containing a slurry containing diamond powder and applying an ultrasonic agitation to the slurry, the slurry including a metallic powder such as titanium or molybenum which is capable of catalysing a subsequent deposition reaction to produce enhanced diamond nucleation and which has a diameter of less than 45Ám. The manufacture of a diamond fibre on the prepared substrate by chemical vapour deposition and its incorporation into a MMC are also described.

Description

METHOD OF MANUFACTURE OF DIAMOND FIBRES The invention relates to a method of manufacture of diamond fibres by chemical vapour deposition, and in particular to a method of surface preparation of metallic and ceramic fibre cores to produce improved substrates for the deposition of such fibres.

Diamond has a number of extreme properties, including its stiffness and thermal conductivity, that make it particularly attractive for high performance structural applications, for example within the aerospace industry. However, bulk diamond is unlikely to be a practicable structural material in the foreseeable future.

The incorporation of diamond into another material to produce a composite has been recognised as an appropriate route to exploit the properties of diamond. There has been considerable interest in recent years in metal matrix composites (MMCs) for high performance structures. In the main, the matrices for these composites have been either Al or Ti alloys with ceramic reinforcements in the form of particulates or fibres.

Continuous fibre MMCs offer great potential structural advantages. Practical composite manufacture has been demonstrated with continuous SiC fibre for use in Al and Ti matrices. However, the toughness of these materials is relatively low in comparison with the bulk matrix alloy, a result of the relatively large volume fractions of SiC fibre required to give significant improvements in modulus and strength. It is known that diamond can have a modulus of up to 1200 GPa, between two and three times greater than SiC. Thus if diamond could replace SiC in the titanium composites then a much lower volume fraction of fibre would be required to achieve a given composite modulus, with a potential improvement in toughness.

There is also interest in incorporation of such fibres in to polymer matrix composites. Continuous SiC fibres may be manufactured by the deposition of material onto a metallic wire core, tungsten being frequently used. Examples of such fibre manufacturing techniques are described in European Patent numbers 396332 and 396333.

There are a range of techniques capable of producing diamond or diamond-like coatings. These techniques deposit diamond from carbon containing precursor gases and require high temperatures and carefully controlled atmospheres. The most highly developed of these processes is chemical vapour deposition (CVD) of carbon from a suitable carbon depositing reaction gas atmosphere, methane being a particularly preferred reaction gas, the reactive process then being, under suitable activation, CH4 (g)

C (diamond) + 2H2 (g), but a wide variety of carbon containing gaseous compounds or mixtures are known.

It is well established from diamond deposition technology over a range of substrates that minimal diamond nucleation occurs without prior substrate surface preparation.

Improved nucleation can be obtained by abrading a cleaned and prepared surface by agitating with ultrasound in a slurry or mechanically abrading with a paste loaded with diamond powder. The abrasion is believed to have two main effects: first the abrasion creates surface defects which some authorities consider serve as nucleation sites; second crystallites of diamond and other carbon residues may be deposited onto the abraded surface by the slurry treatment and serve as nucleation sites for epitaxial growth of diamond at the CVD stage.

Attempts have been made to make diamond fibres by deposition onto metallic cores using processes analogous to SiC fibre manufacture. Using the abrasive paste method of surface preparation described above tolerable levels of diamond nucleation have been achieved on a laboratory scale. However, the slurry-containing ultrasound bath treatment is of limited effectiveness in stimulating nucleation of diamond on metallic cores, and nucleation is limited and inclined to be erratic and the resultant fibres prone to structural irregularities and defects which limit their effectiveness in structural applications in MMCs. This is unfortunate, since the slurry method can be seen to offer great potential for mechanisation and scaling up to produce diamond fibres on a practical industrial scale, and is also likely to be most practical treatment for very fine fibre cores eg down towards 1 0cm, both ceramic and metallic.

The present invention is directed at providing a method of surface preparation for metallic and ceramic cores based on the slurry-containing ultrasound bath treatment method but which leads to improved rate and consistency of nucleation during deposition, and consequently improved diamond fibre production.

Thus according to a first aspect of the invention, a method of surface preparation of a metallic core suitable for serving as a substrate for the chemical vapour deposition of diamond fibre from a suitable carbon depositing reaction gas atmosphere comprises cleaning and degreasing the surface of the core in an organic solvent; treating the surface of the core by placing the core in an ultrasonic bath containing a slurry containing diamond powder and applying an ultrasonic agitation to the slurry; characterised in that the diamond slurry also includes a metallic powder having a diameter of less than 451lm which is capable of catalysing a subsequent deposition reaction to produce enhanced diamond nucleation.

The addition of a metallic powder to the slurry produces a prepared surface which, on subsequent deposition of diamond onto the core surface, is found to exhibit much enhanced nucleation of diamond microcrystallites, and produces a coating of enhanced integrity compared with that resulting from applying conventional slurry processes. Whilst the invention is not limited by any particular theory, it is believed that the titanium or molybdenum powder serves to catalyse the chemical vapour deposition process by inhibiting the reverse reaction in which early diamond nuclei are attacked by the reactive CVD atmosphere and thereby enhancing survival rates of the diamond nuclei. Titanium and molybdenum are particularly preferred since it is known that both metals exhibit especially high rates of diamond nucleation when they serve as metallic substrates in their own right, so that in this case the diamond slurry also includes a titanium or molybdenum powder having a diameter of less than 45cm.

By analogy with existing technology in relation to the deposition of silicon carbide, a filament of tungsten wire comprises a particularly suitable core material to which the surface treatment can be applied for subsequent deposition such fibres are typically around 35 to 701lm diameter. This treatment has also now been demonstrated to work effectively on ceramic (10-151lm diameter) fibre cores (SiC - Tyranno). A diamond slurry comprising diamond particles of diameter in the range 10pm to 20,um, most preferably substantially 1 5cm, produces optimum results in either case.

According to a further aspect of the invention, a method of manufacture of diamond fibre comprises providing a deposition core comprising a metal or ceramic filament; preparing the surface of the core using the surface treatment above described; depositing a diamond coating onto the core by chemical vapour deposition.

Diamond fibres prepared in this way from metallic (eg W) cores are suitable for use in direct analogy to W -cored SiC in MMCs. Smaller diameter fibres, preferably deposited on ceramic cores (such as 10-15pm SiC) are especially suited to incorporation into polymer matrix composites.

The atmosphere in the deposition chamber may contain any of the gases known to be suitable for the CVD production of diamond coatings. As discussed, methane is a favoured reaction gas, but a wide variety of carbon containing gaseous compounds or mixtures will readily suggest themselves to those skilled in the art.

To provide the necessary activation energy the chemical vapour deposition process step conveniently comprises heating the filament in a deposition chamber containing a gaseous atmosphere which on contact with the hot filament deposits a diamond coating. Any suitable form of heating may be used, but preferably the filament is heated resistively by passage of an electric current. Alternatively, a microwave generated plasma atmosphere may be used to provide the necessary activation.

According to a further aspect of the invention, a method of manufacture of diamond fibre reinforced metal matrix composites comprises the manufacture of a quantity of diamond fibres using the surface treatment and deposition techniques above described, adding to the fibres so produced a suitable metallic matrix material and consolidation of the whole to produce a metal matrix composite. A preferred method is to coat the fibres with a suitable metallic matrix alloy material, for example by electron beam physical vapour deposition, allowing control of alignment of the coated fibres, and to consolidate these into a reinforced MMC, for example by HIPing. Alternatively, powder metallurgical or metal foil routes will suggest themselves to those skilled in the art.

The invention will now be described by way of example only with reference to figure 1 of the accompanying drawings in which a schematic cross section of a deposition apparatus suitable to lay down diamond on a core prepared in accordance with the invention is shown.

50pm diameter tungsten (W) wire was chosen as the core material to serve as the substrate for diamond deposition. In order to compare the effect of varying process parameters on the nucleation of diamond the surface was treated in accordance with the invention using titanium powder and also by prior art methods of abrading with diamond paste and agitating in a catalyst free diamond slurry. These treatments are hereafter abbreviated to ST, P and S respectively Prior art treatment P involved using a commercially available diamond polishing paste of 0.25, 1, 6 and 15 llm grit size to manually abrade the surface of the W wire for 1 minute. Tungsten wire substrates were also immersed in commercially available diamond polishing slurries (containing 6 and 15 ,um diamond particles) and ultrasound applied for 5 minutes (treatment S). A treatment in accordance with the invention was applied using a 15 Fm diamond slurry, as above, but with 1 gram of sub 45 pm Ti powder added per 100 ml of slurry. These substrates were then used directly for diamond deposition trials without further surface treatment. For comparison, substrates were also used with no surface pre-treatment directly off the reel (N) and after only degreasing in acetone (A).

Following the surface pre-treatments, the tungsten substrates were loaded into a hot filament chemical vapour deposition diamond reactor, the schematic layout of which is shown in Figure 1. Tungsten substrates 2 were mounted in a reactor chamber 1 on substrate holders 3 so as to be offset away from the filament 4 during deposition.

Tungsten wire (0.5 mm diameter) was selected for the filament material, and both substrate and filament spring mounted in the CVI) reactor to ensure they remained straight during deposition. This ensured that the filament/substrate distance was kept constant throughout the diamond deposition trials. The filament/substrate distance is influential since it controls the percentages of active gas species available for diamond deposition, and a preferred offset is in the range 3-9mm.

The filament 4 was resistance heated to 2000-22000C using a d.c. power supply 5, the chamber 1 was evacuated to a pressure of 6-7 x 103 torr, and hydrogen and methane, selected as the reaction gases throughout the work, were fed through the chamber via an inlet 6, outlet 7 and pump 8, with a reactor pressure maintained at 50 torr, a hydrogen flow rate of 200 sccm and a methane flow rate of 2 sccm. For preliminary nucleation studies a one hour deposit time was used, and nondimensionalised data for the number of diamond crystals initiated for the different pre-treatments on the W wire substrates may be found in Table 1.

Pretreatment Particle size Nucleation rate (cm) A n/a 0 N n/a l.ixi011 P 0.2511m 9.0 x 1014 1.0 m 1.0 x 1014 6.0 m 1.0 x 1014 15.011m 14.5 x 1012 S 6.0 m 3.4x109 15.011m 6.0 x 1012 ST 15.OCLm 8.8 x 1015 Table 1 Nucleation densitv of CVI) diamond crystals after various surface treatments and one hour diamond deposition As can be seen from the table negligible nucleation is seen in the absence of a surface pre-treatment. When pre-treatment consisted of abrasion with a diamond loaded paste (P) reasonable nucleation rates were observed, and reasonable deposited coatings achieved. It was found that the smaller the nominal size of the paste, the higher the number of crystals observed after 1 hour's growth. There was an observed reduction in diamond grain size. Of these pre-treatments the 0.25 ,um paste gave the highest rates and smallest grain size after the 1 hour deposition.

When the substrates pre-treated with the diamond slurry were examined it was found that the nucleation density was greatly reduced when compared to the fibres treated with pastes containing similarly sized diamond powder. Levels of nucleation are not really satisfactory for diamond fibre production. The addition of sub 45 m titanium powder to the 15 pm slurry resulted in a marked change to the nucleation behaviour compared to that seen previously where titanium was not present. The number of crystals seen on the substrate had greatly increased, as can be seen from the results in Table 1. In fact, this was the only slurry pre-treatment that resulted in a continuous diamond coating. A smaller average grain size of the crystals present after deposition for one hour with the addition of titanium was also observed (0.29clam) than in the case where a 15 pim slurry only pre-treatment was utilised which was also significantly less than that observed with the 0.251lm P treatment.

Thus, by addition of titanium powder to a standard diamond slurry, the slurry treatment was successfully modified to produce deposition results comparable with the best available using the paste treatment.

Claims (17)

1. Claims 1. A method of surface preparation of a metallic or ceramic core suitable for serving as a substrate for the chemical vapour deposition of diamond fibre comprising the steps of: cleaning and degreasing the surface of the core in an organic solvent; and treating the surface of the core by placing the core in a an ultrasonic bath containing a slurry containing diamond powder and applying an ultrasonic agitation to the slurry; characterised in that the diamond slurry also includes a metallic powder which is capable of catalysing a subsequent deposition reaction to produce enhanced diamond nucleation having a diameter of less than 45pom.
2. 2. The method of surface preparation of claim 1 characterised in that the diamond slurry includes a titanium or molybdenum powder having a diameter of less than 45cm.
3. 3. The method of surface preparation of claim 1 or claim 2 applied to a metallic core which is a tungsten wire filament.
4. 4. The method of surface preparation of claim 1 or claim 2 applied to a ceramic core which is a SiC based fibre.
5. 5. The method of surface preparation of any one of claims 1 to 4 wherein the diamond slurry comprises diamond particles of diameter in the range 101lm to 20cm.
6. 6. The method of surface preparation of claim 5 wherein the diamond slurry comprises diamond particles of diameter substantially 1 5lem.
7. 7. A method of manufacture of diamond fibre comprising the steps of: providing a deposition core comprising a metal or ceramic filament; preparing the surface of the core using the surface treatment method of any preceding claim; depositing a diamond coating onto the core by chemical vapour deposition.
8. 8. The method of manufacture of claim 7 wherein the core comprises a metal filament.
9. 9. The method of manufacture of claim 8 wherein the core comprises tungsten.
10. 10. The method of manufacture of claim 8 or claim 9 wherein the core has a diameter of 35-70CLm.
11. 11. The method of manufacture of claim 7 wherein the core comprises a ceramic filament.
12. 12. The method of manufacture of claim 11 wherein the filament comprises silicon carbide.
13. 13. The method of manufacture of claim 11 and 12 wherein the core has a diameter oflO- 15cm.
14. 14. the method of manufacture of any one of claims 7 and 13 wherein the chemical vapour deposition process step comprises heating the filament in a deposition chamber containing a gaseous atmosphere which on contact with the hot filament deposits a diamond coating.
15. 15. The method of manufacture of claim 14 wherein the filament is heated resistively by passage of an electric current.
16. 16. The method of manufacture of claim 14 or claim 15 wherein the atmosphere in the deposition chamber contains methane.

    A method of manufacture of a diamond fibre reinforced metal matrix composites comprising the steps of: manufacture of a quantity of diamond fibres using the method of any preceding claim; adding to the fibres so produced a suitable metallic matrix material; and consolidation of the whole to produce a metal matrix composite.
17. 17. The method of manufacture of claim 10 wherein the fibres are coated with a suitable metallic matrix material by electron beam physical vapour deposition, and the coated fibres are consolidated into a reinforced MMC by HIPing.