GB2063920A - Decorative anodised films on substrates - Google Patents

Abstract

The films are produced by depositing an anodisable coating (e.g. Ta, Ti, Zr, alloys thereof and nitrides) onto a substrate (e.g. stainless and mild steel) by sputter ion plating followed by anodising at a controlled voltage in order to control oxide film thickness and hence colour. Anodising is known to generate a thin film on an anodisable material. A particular film thickness may generate a distinctive interference colour if the oxide has a high refractive index. However, not all materials are anodisable and this technique enables the properties of anodisable films to be made more generally available.

Description

SPECIFICATION Anodised Films on Substrates This invention relates to the provision of anodised coatings on substrates, for example, for decorative purposes.

Anodising is a well-known process in which a thin oxide film (e.g. of a few hundred A thickness or greater) may be formed. A particular film thickness may generate a bright, distinctive interference colour if the oxide is of high refractive index. This, however, has limited applicability since not all materials will anodise satisfactorily. Thus, many materials have low corrosion potentials and will only form very thin oxide films at very low voltages before the films break down and the material corrodes. We have now devised a way of making the properties of anodised films more generally applicable.

Thus, according to the present invention a method of providing a substrate with an anodised oxide film comprises the steps of (i) depositing an anodisable coating on the substrate by means of sputter ion plating; and (ii) anodising the coating to produce an oxide film, wherein the anodising voltage is controlled in order to control the thickness of the film.

We have been able to control the thickness of the oxide film reproducibly and uniformly to give, in cases where the oxide has a high refractive index, a variety of colours. Also, we have been able to apply our method to a variety of substrates, including substrates which cannot readily be anodised and which may be relatively inexpensive and easy to fabricate.

An important feature of our invention is the use of Sputter lon Plating in step (i) since this enables a high quality coating of uniform thickness and good adhesion to be obtained, which is required for the anodising step. Thus, the coating should be dense and free from voids, porosity, and cracks which may lead to corrosion of the substrate during subsequent anodising. Sputter lon Plating is a coating technique where material is transferred from a cathode to a substrate in the presence of a DC glow discharge in a soft vacuum chamber and where material is generated from the cathode by the action of ion bombardment, i.e. sputtering, and ultimately diffuses to the substrate.Sputter lon Plating is described in detail in a number of references in the art, for example, "Wire Industry", 44, December 1977, pages 771 to 777; Welding Institute Reprint, Advances in Surface Coating Technology International Conference, London 13-1 5 February 1978, pages 53-59; Proceedings of 'IPAT' Conference, Edinburgh (June 1977) ps. 177--186.

Step (ii) may be carried out by methods known in the art. When the oxide film is insulating and fully formed, the anodising current should drop to zero. However, further anodising may be carried out on the same sample, but at a higher voltage since the oxide film is only insulating at voltages up to the voltages at which it was formed. A higher voltage breaks down the film until it has grown to a sufficient thickness to be insulating again. Thus, repeated anodising at greater and greater voltages can produce a multi-coloured sample. Also, insulating materials such as paint, silicone grease and "Letraset" coverings etc. may be used to mask off areas whilst anodising to achieve a background colour; the masking material may then be removed and anodising carried out at a different voltage to give a two-tone effect.

A simple extension of this procedure may be carried out to obtain a multi-tone effect.

The present invention may be applied to metallic or to non-metallic (e.g. ceramic) substrates.

Examples of metallic substrates are mild, carbon and stainless steels; Al and Al alloys; Ta; Cu; Mo and Ni. Examples of non-metallic substrates are Al203, porcelain and glass.

The anodisable coating may be of materials which are known to anodise such as Nb, Ti, Ta, Zr and also alloys of these elements. The coating may be a nitride coating such as TiN, ZrN, TiZrN, deposited by carrying out the sputter ion plating in a reactive environment (e.g. containing N2). When anodised, such nitride coatings exhibit distinctive colours in comparison with colours obtained by anodising the corresponding element.

The coatings may have a range of thickness. For example, we have made coatings with thicknesses from 0.1 ssm to 1 80 jum.

Whilst the main purpose of our invention is to provide coatings for decorative purposes, coatings may be produced which have application in other areas such as constituting insulating layers for capacitors and microcircuitry.

The invention will now be particularly described, by way of example only, as follows.

GENERAL PROCEDURE Sputter lon Plating An apparatus as described in the abovementioned prior art references was used. A coating vessel consists of a resistance heated stainless steel inner chamber, at the walls of which are suspended a series of target plates constituting the coating material. The substrate to be coated is suspended at the centre of the chamber.

The chamber was pumped down to 10-100 m.torr pressure with a flowing argon atmosphere of high purity. The argon had been purified by passing over freshly deposited titanium. The chamber was heated to a temperature of around 300/3500C by which stage outgassing of the substrate and target and evaporation of organic material has occurred. Further cleaning in some circumstances was achieved by applying a high negative voltage (1000 V) for about 2 hour onto the substrate causing an argon glow discharge to be set up with ion bombardment causing removal of surface atoms on the substrate.

A high negative voltage (400-1000 V) was then applied to the targets to produce a glow discharge with net transfer of material from the targets onto the substrate, i.e. coating of the substrate by deposition. The coating was carried out at 200-3500C; no external heating was required since the above process generates sufficient power to maintain this temperature.

If desired, a negative bias of 20-1 50 V may be applied to the coated substrate causing densification of the coating by resputtering of deposited material and ion polishing.

The above produces a coating of an element or an alloy. If it is desired to deposit a nitride coating, this can be done by admitting nitrogen at a small partial pressure (1-100 m torr) into the chamber during the deposition process. This causes a distinct change in the glow discharge characteristics and generally reduces the rate of deposition, though it does allow very hard, coloured nitride coatings to be deposited.

The deposition rates may vary from 0.5 ym/hour to 10 ym/hour according to power density, though, as stated above, rates are iower when nitrogen is introduced.

Anodising This was carried out by using an electrolytic bath contained in-a glass vessel, a power supply unit (e.g. of 0.110 V and 0--3 A output or more) with current and voltage readout, and a non-corroding cathode (e.g. of Ta or stainless steel) immersed in the bath. The solution used in the bath may, for example, be 1% by weight KOH in water, or ammonium diborate solution made up to pH9 by addition of ammonia solution.

The sputter ion plated substrate was carefully cleaned and connected up as the anode and immersed, either wholly or partly into the bath. A voltage known to give a particular colour was applied.

When the resulting oxide film was fully formed, the current fell to zero. The power supply unit was switched off, the coated substrate removed and washed in water and solvent. If desired, further anodising may be carried out at successively higher voltages to give different colours.

SPECIFIC PROCEDURES Sputter lon Plating A number of substrates were coated with metals and binary alloys at pressures of between 1 5 and 25 m torr. Typical target power densities ranged from 1.78 kW/m2 for Ti to 5.08 kW/m2 for Zr. Typical substrate power densities were 0.41 kW/m2 to 1.18 kW/m2. These power levels ensured deposition of the coating at chamber temperatures of between 2000C and 3000 C. Higher or lower power levels may be used as required. The coating process time was usually 1 to 1 5 hours.

Nitrides were usually deposited as follows. A simple metal coating was deposited for about T hour at power levels as indicated above and at pressures of 20 to 25 m torr. The getter was switched off and nitrogen admitted at a partial pressure of 1-10 m torr. The power levels typically increased by 10% at the targets, because of the different discharge conditions, but at the substrate the power density generally dropped by 4070%. In the extreme case of ZrN, the target power density increased by 12%, though the substrate power density only decreased by 60%. For all the nitride coatings, the temperature in the chamber was maintained by process heat at 200 to 3000C and the total process time was typically 12 to 3 hours. The initial metal coating may be omitted if desired.

Anodising The above coated substrates were anodised in accordance with the general anodising procedure.

We found that Al coatings anodised best in a borate bath whilst other metals, alloys and nitrides could be anodised in either bath, though nitride coatings were found to anodise more quickly in a borate bath.

The results are summarised in the table below which is in two parts and shows the colours achieved and the specific voltage at which they were achieved with a specified voltage range.

ANODISING VOLTAGE RANGE (V)

Coating 0-10 11-20 21-30 31-40 31-40 Substrates Ti 10V 20V 30V 40V Al alloy, Ta mustard mid-blue pale blue yellow/green C. Steel, Stainless Steel, glass Ta 20V, 22V 30V 40V 50V Mo, Stainless dark blue grey/blue ice blue ice yellow Steel, Mild mauve 35V 4 pale green Steel, Al alloy blue glass, steatite AI,O, Zr 10V 15V 30V 40V 45V Ta, Al mustard purple ice blue yellow yellow alloy 20V blue Nb 10V 15V 30V 40V 50V Stainless brown dark blue colourless yellow mauve Steel, Ni 20V 4 purple grey/blue Al 20-110V dark grey # light grey Ta, Stainless with thicker oxide formation Steel TiZr 10V 15V 30V 35V 50V Ta, Mild Steel mustard purple ice blue yellow/ yellow 20V colourless mid-blue 40V light yellow 50V 10V 20V 30V 34V 50V Al alloy, Ta grey/ mid-blue grey/blue ice blue yellow mustard 40V ice yellow TIN 10V 20V 31V 40V Stainless brown light yellow dull white white y8llow/white ANODISING VOLTAGE RANGE (V)

Coating 0--10 11-20 21-30 31-40 41-50 Substrates ZrN 10V 30V 50V Stainless Steel dull brown mid brown white film AIN 20V 25V 40V Ta, Stainless brown mid-blue grey/blue Steel TiZrN 20V 27V, 30V 35V 45V Cu, Al, Ni, brown e grey/blue grey e yellow Stainless Steel purple ice blue 50V 40V pale green ice blue 4 gold ice yellow TiTaN 10V 15V 25V 35V 51V Ta, C. Steel Al, brown dark brown blue ice blue orange Stainless Steel 20V 30V 40V blue yellow # yellow pale blue ANODISING VOLTAGE RANGE (V)

Coating 51-60 61-70 71-80 81-90 91-100 Substrates Ti 60V Al alloy, Ta red/mative C. Steel, Stainless Steel, glass Ta 60V 70V 80V 90V 100V Mo, Stainless yellow orange mid blue -, green e green Steel, Al alloy, purple purple turquoise 110V glass, steatite, pink Al2O3 Zr 51V 62V 75V 90V 95V Ta, Al alloy purple turquoise # yellow/green purple red/mauve peacock blue 80V 101V 70V orange dark green green purple 110V light green Nb 60V 70V 80V 90V 105V Stainless Steel turquoise lime green lilac mauve green Al 20 - 110 dark grey # light grey Ta, Stainless with thicker oxide formation Steel TiZr 60V 70V 75V 90V 105V Ta, Mild Steel purple sea green green pink # red/purple 80V lime yellow/green TiTa 60V 70V 75V 90V 100V Al alloy, Ta pink/lilac vivid blue torquoise gold pink 80V 110V lime mauve TiN 60, 62V Stainless Steel, v. thick Ta white oxide ZrN 70V Stainless Steel thick white oxide ANODISING VOLTAGE RANGE (V)

Coating 5140 61-70 71 80 11191D 91-110 Substrates AIN 60V 80V 85V Ta, Stainless ice blue ice/white white Steel TiZrN 56V 70V Cu, Al, Ni, yellow crimson Stainless Steel pink TiTaN 6OV 7OV 80V 90V 105V Ta, C. Steel Al, orange/brown green/tan maroon/lilac mauve lilac Stainless Steel

Claims (13)

1. A method of providing a substrate with an anodised oxide film which comprises the steps of (i) depositing an anodisable coating on the substrate by means of sputter ion plating; and (ii) anodising the coating to produce an oxide film, wherein the anodising voltage is controlled in order to control the thickness of the film.

2. A method according to claim 1 wherein the substrate is metallic.

3. A method according to claim 2 wherein the metallic substrate is a steel.

4. A method according to claim 2 wherein the metallic substrate is aluminium or an alloy thereof.

5. A method according to claim 1 the substrate is non-metallic.

6. A method according to claim 5 wherein the non-metallic substrate is a ceramic.

7. A method according to any one of the preceding claims wherein the coating has a thickness in the range from 0.1 m to 180 ym.

8. A method according to any of the preceding claims wherein the coating comprises the elements Nb, Ti, Ta or Zr or an alloy of any of the said elements.

9. A method according to any of claims 1 to 7 wherein step (i) is carried out in a reactive environment for depositing a nitride coating on the substrate.

10. A method according to any of the preceding claims wherein anodising is carried out at a first voltage to produce an insulating oxide film and then at one or more subsequent voltages, the or each voltage to produce an insulating oxide film, wherein each voltage is greater than the voltage immediately preceding, thereby to produce an oxide film of variable thickness.

11. A method according to any of the preceding claims wherein part of the substrate carries a removable insulating material whilst anodising is carried out.

12. A method providing a substrate with an anodised oxide film substantially as described herein with reference to any one of the examples.

13. A substrate provided with an anodised oxide film by a method according to any of the preceding claims.