GB2141442A - Apparatus and method for the production of metallic coatings by ion-plating - Google Patents

Abstract

The use of atriode electrode arrangement to maintain the glow discharge at argon gas pressures below 1 mu m of Hg, enables coatings to be deposited by ion plating under low argon gas pressures. The resulting coatings exhibit non-porous, non-columnar grain structures and therefore improved corrosion and oxidation resistance.

Description

SPECIFICATION Improvements in or relating to coatings and apparatus therefor The present invention relates to the production of coatings on metal substrates by an ion vapour deposition process.

For many years cadmium coatings produced by electroplating have been the preferred method of corrosion protection for steel and titanium alloy components subjectto harsh environments. However, in recent years concern has increased over pollution caused by cadmium bearing effluents.

Whilst other problems related to corrosion of alumi nium alloys components secured with cadmium plafed fasteners have also fostered the search for alternatives.

An apparatus described by Steube in United States patent No 3926147 relates to the coating of small components, particularlyfasteners for aircraft use, by ion vapourdeposition of aluminium. The apparatus essentially comprises a rotatable perforated barrel type vessel in which the articles to be coated are tumbled, simultaneously maintained within a diode glow discharge and are subjected to a source of ionised aluminium atoms. The glow discharge of the apparatus described is produced in an argon atmos phere at a pressure of between 10 and 20 lim Hg. A shortcoming ofthis operational requirement is that the aluminium coating thus produced possesses a porous and columnar microstructure.The major disadvantages of such a microstructure are that it reduces corrosion and/or oxidation resistance and may also impairadhesion of the coating to the substrate. To overcome these disadvantages coated components produced by the method described are generally furthersubjected to surface peening and heattreatment It is an object of the present invention to produce corrosion and oxidation resistant coatings on compo nentsthecoatings being non-porous and not requiring post-coating process operations prior to use.

Coatings having a non-porous microstructure may be produced by generating the coating at lower argon gas pressures. However, depending upon the geometry of the apparatus in question a diode glow discharge may be extinguished at pressures below 5 pm Hg.

According tone aspect of the present invention apparatus for producing ionvapourdepositedcoat- ings on articles comprises; a low pressure chamber; a rotatable carrier to hold articles to be coated, the carrier allowing substantiallyfree access of coating material to the articles; at least one source of ionisable coating material; a source ofionisablegasfora glow discharge; an electron source and an anode and means to control the relative potentials thereof; means to control the electrical potential of the articlesto be coated and means to control the electrical potential of the low pressure chamber rerative to the potentials of the articles to becoated, the electron source and the anode; and whereby the apparatus may be so set that during operation ofthe apparatus coatings may be deposited on the articles having a dense, non-porous and non-columnar microstructure.

It has been found with apparatus ofthe present invention that a glow discharge may be sustained at argon gas pressures below 1 um Hg. Thus the deleterious effects of higher gas pressure are reduced resulting in dense, pore-free structures.

Preferablythe low pressure chamber is at earth potential and all other potentials are controlled relative to the earthed chamber.

The electron source is preferably in the form of a hot wire filament. Use of an anode and filamentcombina- tion raises the ionisation efficiency of the system and it is possible to achieve improvements of a factor of up to about 100 in comparison with a diode arrangement.

The result of improved ionisation efficiency is increased current density to the articles being coated again leading to coating densification.

According to a second aspect of the present invention a processforthe deposition of ion vapour coatings onto an article comprises the steps of; placing the article to be coated in a rotatable carrier, the carrier allowing substantially free access of coating material to the article to be coated; subjecting the article two an ion bombardment cleaning stage; subjecting the article to a source of coating material atoms wherein at least some of those atoms are ionised by passage through a glow discharge; and wherebythe conditions of producing the glow discharge are such that the coating deposited on the articles has a non-porous and non-columnar grain structure.

In a preferred embodiment ofthe present invention the rotatable carrier is maintainedata relatively high negative potential of about 1 to 2 KV: the low pressure vessel and electron source are maintained at ground potential and the anode within the low pressurevessel is maintained at about 100 to 200 V.

Coating materials may be either elemental or alloys.

Pure aluminium has been found to give dense, non-columnar grained microstructures. Other coat ingsbasedon aluminium-zincand aluminium-magne- sium alloys have also been deposited. Preferably for elemental coatings such as aluminium the source of coating material may be a melt withinthe low pressure chamber, the ions being formed from the metal vapour within the chamber. If only a relatively small quantity of coating material is requiredthe molten source may also be used for alloy coatings.

Sputter ion plating from a solid target may be used but is much slower, typically only onetenth the rate of coating achieved byevaporation. This technique does, however, have the considerable advantage of ease ofsource preparation for alloy coatings.

In orderthatthe invention may be more fully understood one example will now be described with reference to the accompanying drawing, which shows: a schematic view of an apparatus for coating loose articles.

Referring nowto the drawing. The apparatus includes a stainless steel vacuum chamber 10 having a removable base 11 and top 12. Within the chamber 10 is cylindrical vessel indicated generally as 13, having a permeable outer way 1 fabricated from a stainless steel mesh having an aperture size of 25 sq mm.

Within the cylindrical vessel 13 is a hollow cylinder 15 electrically isolated from the vessel 13 and permeable outerwall 14byPTFEinsulators 16and 17. Insulators 16 and 17 also constitute bearings and insulator 17 incorporates a drive connection such that the vessel 13 and outer permeable wall 14 are rotatable whilst leaving the hollow cylinder 15 stationary. Drive to the vessel 13is provided by a shaft 18 via the bearing insulator 17. Drive to the shaft 18 is provided via an electrical isolator l9from a gearbox 20. Drive to the gearbox 20 is provided by a shaft 21 driven by a variable speed motor 22 mounted outside the chamber 10 on the removable top 12.The vessel assembly 13 is supported on a cradle 23 itself supported by a water cooled mounting 24 secured in the removable top 12. A negative potential is applied to the vessel 13 from a variable DC voltage source 25 via a brush electrode 26 and connector wire 27. Also included in the removable top 12 is an inlettube and valve 28to admit argon gas from a pressurised cylinder 29 (not shown). The chamber base 11 has a conduit 30 through which the chamber 10 may be evacuated by a vacuum pump 31. Atungsten wire filament 32 of 1 mm diameter and 40 mm in length is installed in the base 11. The filament 32 has a separate power supply 33 for heating thereof, the filament 32 being a thermionic emitter.An anode 34 is installed on the opposite side of the base 11,the anode being in the form of a semi-cylindrical copper plate 60 mm high and 160 mm diameter. The anode 34 has its own variable DC voltage potential supply 35. Installed between the filament 32 and anode 34 is the coating material source indicated generally as 36. The coating source comprises a resistance heated radiantheater37, a crucible 38 containing the coating material, aluminium 39. The heater37 has its own lowvoltage, high current power supply 40. Temperature of the crucible 38 and coating material 39 is governed by a thermocouple 41 controlling the power supply 40. The components 42 to be coated are contained loosley withinthe-vessel 13. Aviewing port 43 isprovided in the side of the chamber 10.

In operationthevacuum chamber 10 is pumped down bythe pump 31 to a pressure of 2 x 105torrand then backfilled with argon gas via the supply tube and valve28toa pressure of 10pm Hg. The chamber 10 is earthed and the vessel 13 made to rotate at 20 to 5 rpm and therebytumblethe components42. The hollow cylinder 15, shaft 18, gearbox 20, shaft 21, motor 22 and cradle 23 areall maintained at earth potential. The permeable wall 14 has a negative voltage of 2 KV applied to it via the brush electrode 26 and wire27 from the variable DC voltage source 25.By biasing the vessel wall 14 with KV potential a glow discharge is initiated within the chamber 10 and ions fro the plasma generated bombard the tumbling components 42thereby exerting a cleaning action in remov ing the various surface oxides etc.Asthe components are cleaned the discharge current density attains an equilibrium value which is taken to be that for a clean surface Whenthis stage is reached the argon gas pressure within the chamber 10 is reduced to 1 pom Hg and the filament 32 heated by the power source 33 to give thermionic emission. The anode 34voltage is adjusted to +200 V by the DC potential source 35 thus initiating a self-sustaining glow discharge between the filament 32 and anode 34.The temperature of the aluminium coating material 39 is raised bythe heater 37 to a temperature ofabout 1 2000C. Some of the aluminium atoms evaporatedfrom the molten metal 39 will become ionised inthe plasma discharge and be accelerated towards the components 42 being tumbled in the rotating vessel 1t Dense, non-porous coatings are thereby produced on the components.

Thickness of the coating will bedependentuponthe length of time the components are subjected to the process.

Itwill be apparent to the person skilled in the artthat alternative electrode arrangements are possible in order to achieve the object of the invention. For example, in the embodiment described above hot filament 32 is maintained at earth potential. Itwould be feasible to dispense entirely with the separate anode 34 and applythe positive potential of 200 V directly to the hot filament 32.

The relative potentials between the electrodes used in the system are important,forexample, a negative voltage of 200 V could be applied to the hotfilament 32 and chamber 10 and zero (or earth) potential to the anode 34 to achieve a similar effect.

The high negative potential applied to the vessel 13 may be in the range of several hundred volts to several kilovolts. However, once the voltage applied to the vessel 13 and consequently the components 42 rises above about 5KV sputtering of material from the vessel and components will begin to occur. Thus a potential will be reachedwherethere is an equilibrium between material being sputteredaway and material being ion-plated onto the components.

The initial ion bombardmentstageto clean the articles can varyfrom a few minutes to about one hour but is normally about 10 to 20 minutes.

When aluminium is the coating material the evaporator source is usually maintained at a temperature in the range 1000 C to 1300 C.

The rate of coating deposition will vary depending upon the potentials employed and thetemperatiiè and number of evaporator sourcesuse::lRates may vary from very low to 5pm per minute(i;rnuItipIe sources are employed) and atypical ratewould be about 0.5 m per minute. The thickness of coating will be dependent upon the application in which the articles are to be used. In the case offasteners for aircraft use the coating thickness required is normally in the range5to25 Ctm.

It will be appreciatedthattheooating material source may also be substantially more sophisticated than that described above. Suitable feed systems to replenish the evaporator may be employed. Systems using continuous-feed wire coils orslug feeds whereby small ingots are periodically added to the evaporator may be used.Where a molten metal source is employed the heating means are not restricted to resistance or radiant heating.Othertechniques such as induction or electron beam melting may alternatively be used Further refinementstothe apparatus may also be made such as, for example, the provision of a removable shutter between the coating material source and the articles to be coated. A shutterwould allowthecoating material source to be brought to temperature concurrently with the sputter cleaning of the articles and prevent any deposition of coating material onto an insufficiently clean surface.

The principle of the invention may also be utilised not onlyfor ion-plating relatively small irregularly shaped components such as those in the above example but could also be used for coating relatively large components. Such as, for example, forgings in high strength alloys for aircraft undercarriage applications. In this case the apparatus may be modified by substituting for the cylindrical vessel 13 other fixtures able to receive individually mountable components.

Such fixtures may comprise rotatable end plates analogous to those shown in thefigure forming the end plates of the rotatable vessel 13. In place of the mesh 14 would be attachment means on the end plates allowing individual components to be mounted between the end plates in the axial direction. In practice such components may be mounted in, for example, batches offourspaced at 900 intervals on the end plates. The individual components may also be themselves rotatable in the carrier by, for example, suitable sun and planet wheel gear meansthus ensuring that all surfaces of the components may be uniformly coated.

Claims (16)

1. Apparatus for the production of ion vapour deposited coatings on articles comprising; a low pressure chamber; a rotatable carrier to hold articles to be coated, the carrier allowing substantially free access of coating material to the articles; at least one source of ionisable coating material; a source of ionisable gasfora glow discharge; an electron source and an anode and means to control the relative potentials thereof; means to control the electrical potential of the articlesto be coated and means to control the electrical potential of the low pressure chamber relative to the potentia Is of the articles to be coated, the electron source and the anode; and whereby the apparatus may be so set that during operation of the apparatus coatings may be deposited on the articles having a dense, non-porous and non-columnar microstructure.

2. Apparatus according to claim 1 and wherein the at least one source of ionisable coating material is molten metal.

3. Apparatus according to claim 1 and wherein the at least one source of ionisable coating material is a solid sputtering target.

4 Apparatus according to any one preceding claim and wherein the electron source is a hot wire filament

5. Apparatus according to claim 4 and wherein the electron source and anode are combined into a single electrode.

6. Apparatus according to any one preceding claim and wherein there is a removable shutter interposed between the at least one source of ionisable coating material and the articles to be coated.

7. Apparatus according to any one preceding claim and wherein the rotatable carrier comprises rotatable end plates having attachment meansforthe mounting of individual components in an axial direction between the end plates.

8. Apparatus according to claim 7 and wherein the rotatable carrier its further provided with means to rotate the components mounted between the carrier end plates.

9. A process for the deposition production of ion vapour deposited coatings on articles comprising the steps of; placing the articles to be coated in a rotatable carrier, the carrier allowing substantially free access of coating material to the articles to be coated; subjecting the articles to an ion bombardment cleaning stage; subjecting the articles to a source of coating material atoms wherein at least some of those atoms are ionised bypassagethrough a glow discharge; and whereby the conditions of producing the glow discharge are such that the coating deposited on the articles has a non-porous and non-columnar grain structure.

10. A process according to claim 9 and wherein the coating material is aluminium.

11. Aprocess according to claim 9 and wherein the coating material is either an aluminium-zinc alloy or an aluminium-magnesium alloy.

12. A process according to claim 10 and wherein the source of aluminium is a bath of the molten metal.

13. A process according to claim 11 and wherein the coating material source is a sputtering target of the solid alloy or a combination of the elemental constituents.

14. A process according to any one of claims 9 to 13 and wherein the glow dischargeforthe ion plating part of the process is sustained at an inert gas pressure of about 1pm of Hg.

15. Apparatus substantially as hereinbefore described with reference to the accompanying specification and drawing.

16. A process substantially as hereinbefore described with reference to the accompanying specification and drawing.