GB2226334A

GB2226334A - Multilayer coatings

Info

Publication number: GB2226334A
Application number: GB8926530A
Authority: GB
Inventors: David Stafford Rickerby; Robert Edwin Barrell
Original assignee: J J CASTINGS INVESTMENTS; UK Atomic Energy Authority
Current assignee: J J CASTINGS INVESTMENTS; UK Atomic Energy Authority
Priority date: 1988-11-25
Filing date: 1989-11-23
Publication date: 1990-06-27
Also published as: GB8926530D0; GB8827541D0

Abstract

A substrate is provided with a multilayer coating by applying a first layer of a refractory transition metal (e.g. Ti) of thickness 0.1 micrometre or greater, for example 0.5 micrometre or greater, and a second layer of a refractory compound thereof (e.g. TiN), e.g. of thickness greater than 1 micrometre. If desired, the substrate is first provided with preliminary layers: a first preliminary layer of a refractory transition metal (e.g. Ti) of thickness up to 0.5 micrometre, and a second preliminary layer (e.g. TiN), e.g. of thickness from 0.1 to 5 micrometres. The layers are each applied by a physical vapour deposition method such as sputter ion plating. The resulting coating may endow the substrate, e.g. a metal such as a steel, with corrosion or wear resistance or both. The refractory transition metal comprising the first layer may also be Zr, Hf, V, Nb, Ta, Cr, Mo or W; the refractory compound of a transition element may be a nitride, carbide or oxide of an element of groups 4, 5, 6 of the Periodic Table with the exception of radioactive elements such as also ZrN, TiZrN, TiTaN, HfN and carbides of these elements. The substrate may also be Ti and alloys thereof; Ni and alloys thereof; Al and alloys thereof; Ta; Cu; or Mo.

Description

Multilayer Coatings This invention relates to enhancing the corrosion and/or wear resistance of a substrate by means of a multi-layer coating.

The invention provides a method of forming a multi-layer coating on a substrate comprising the steps of (i) applying a first layer comprising a refractory transition metal or alloy thereof by a physical vapour deposition method, the first layer having a thickness of about 0.1 micrometre or greater, for example 0.2 micrometre or greater such as 0.5 micrometre or greater; and (ii) applying a second layer to the first layer by a physical vapour deposition method, the second layer comprising a refractory compound of a transition element and having a thickness of, for example, greater than about one micrometre.

In many cases, the multilayer coating so-produced has been found to endow substrates with resistance to corrosion or wear or both, particularly when the substrate is soft, for example having a Vickers' hardness of less than 300.

Examples of such soft substrates are mild steel and stainless steel.

It is believed that the thickness of the first layer is significant in giving rise to adequate adhesion of the coating to its substrate; use of a thinner layer has been found to be ineffective in this respect. The role of the second layer is to provide wear and/or corrosion resistance which is not provided by the first layer alone.

However, when the substrate is hard, for example having a Vickers' hardness of 300 or greater, steps (i) and (ii) above, peformed directly onto such a substrate, may give a coating that does not adhere satisfactorily to the substrate or give adequate corrosion resistance. In such cases, it may be necessary to first provide the substrate with a preliminary multilayer coating. This may be done by applying first and second preliminary layers to the substrate, each by a physical vapour deposition method, the first preliminary layer beinq in contact with the substrate and comprising a refractory transition metal or alloy thereof and having a thickness of up to about 0.5 micrometre, and the second preliminary layer comprising a refractory compound of a transition element and having a thickness of, for example, from about 0.1 micrometre to about 5 micrometres.Thus, in such cases, the substrate carries in sequence: the first preliminary layer, the second preliminary layer, the first layer and the second layer. The thickness of the first preliminary layer is important for obtaining satisfactory adhesion to the substrate.

The coatings produced by the present method need not necessarily be restricted to the above-identified layers: additional layers may, in some circumstances and when required, be provided.

The present method is applicable to any substrate that is compatible with the coating and its method of application. In practice, the method will be most usually required where the substrate is one that is susceptible to corrosion and/or wear. Thus, the substrate is preferably metallic, specific examples of which are steels such as mild, carbon, stainless, high speed and tool steels; Ti and alloys thereof; Ni and Ni base alloys; Al and Al base alloys; Ta; Cu; and Mo.

The refractory transition metal or alloy thereof comprising the first layer and the first preliminary layer, if provided, may, for example, be a metal of Groups 4, 5 or 6 of the Periodic Table (with the exception of radioactive elements), namely Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. Ti and Zr are particularly preferred.

The refractory compound of a transition element comprising the second layer and the second preliminary layer, if provided, may, for example, be a nitride, carbide or oxide of an element of Groups 4, 5 or 6 of the Periodic Table with the exception of radioactive elements. Examples are nitrides such as TiN, ZrN, TiZrN, TiTaN and HfN and carbides of the elements. TiN and ZrN are particularly preferred.

The transition element in each of the layers of the coating, including or excluding the first and second preliminary layers, whether as the metal as such or in a compound thereof may be the same or different in any two layers. Also, the compound in the second layer may be the same as or different from the compound in the second preliminary layer when present.

A physical vapour deposition method is a method wherein a material is physically caused to be deposited onto a substrate from the vapour state. Such a method has the advantage that it can replicate the surface finish of the substrate and provide a layer which is sufficiently defect free for the purposes of the invention. Examples are sputter ion plating (e.q. dc, rf or magnetron), electron beam evaporation, arc technoloqy such as arc evaporation, and other evaporation techniques e.g. using a hollow cathode, a hot filament or a laser.A varient of sputter ion plating which is particularly noteworthy in the practice of the present invention and involves the transfer of material from a cathode to a substrate in the presence of a DC qlow discharge in a soft vacuum chamber wherein the material is generated from the cathode by the action of ion bombardment, i.e. sputtering, and ultimately diffuses to the substrate to form a coating thereon. Sputter ion plating is described in detail in a number of references in the art, for example in Proc. conf. on 'Ion plating and Allied Techniques', Edinburgh, 1977, CEP consultants, Edinburgh, p. 177 by R.A. Dugdale; Metals Technology 1982, 9,499 by J.P. Coad and J.E. Restall; and NATO Advanced Workshop on 'Coatings for Heat Engines', Acquafredda di Maratea, Italy, April 1984 by J.P. Coad and D.S. Rickerby.

To apply compounds to a surface, sputter ion plating can be performed in a reactive environment, e.g. in a N2 containing environment where it is required to deposit a nitride.

The invention is, for example, applicable to providing coatings wherever corrosion protection and/or wear resistance is desired, for example in the protection of surfaces of artefacts used in the chemical or food industries. Also, the coatings may have a decorative effect.

The invention will now be particularly described by way of example only with reference to the accompanying drawings, wherein Fig 1 is a schematic diagram of an apparatus for coating by sputter ion plating; and Fig 2 is a graph of weight loss against number of revolutions for different coated substrates tested under abrasive wear conditions.

Referring to Figure 1, an earthed cylindrical coating chamber 1 is provided with an externally mounted resistance heater 2 having a cooling jacket 3. The coatinq chamber 1 has a gas inlet vent 4 with an associated baffle 5 and gas outlet vents 6 with associated baffles 7.

The inlet vent 4 communicates with a getter chamber 8 provided with an inlet conduit 9 and the outlet vents 6 communicate with a pumping chamber 10 provided with a pumping port 11.

A substrate 12 is mounted in the coating chamber 1 and is electrically connected to a bias potential power supply (not shown) by a conductor 13 mounted in insulators 14 and 15 positioned in the walls of the pumping chamber 10. A cathode in the form of a series of target plates of which two 16 and 17 are shown is also mounted within the coating chamber 1. The cathode (e.g. 16 and 17) is electrically connected to a cathode power supply (not shown) by a conductor 18 mounted in insulators 19, 20 and 21 positioned in the walls of the pumping chamber 10.

In operation of the apparatus shown in Figure 1, an operating gas is supplied at the inlet conduit 9 and, by operation of a pump (not shown) at the pumping port 11, is drawn into the getter chamber 8 as shown by arrow a and thence into the coating chamber 1 via inlet vent 4. The coating chamber 1 is heated by means of the heater 2 in order to outgas the substrate 12, cathode (e.g. 16 & 17) and evaporate any organic material. Undesired gas and vapour leave the coating chamber 1 via the outlet vents 6 to enter the pumping chamber 10 and are removed via the pumping port 11 as shown by arrow b. A high negative voltage is applied to the target plates (e.g. 16 and 17) by means of the cathode power supply (not shown) to produce a glow discharge with net transfer of cathode material therefrom by sputtering onto the substrate 12 to provide a coating thereon.External heating is not required at this stage since the process generates sufficient power to maintain the operating temperature.

EXAMPLES General procedure The apparatus shown in Figure l was used. The coating chamber I was pumped down to 10-100 m torr pressure with a flowing high purity argon atmosphere purified by passinq over freshly deposited titanium. The coating chamber 1 was heated to a temperature of around 3000C to effect outgassing of the substrate 12 and the cathode (e.g. 16 and 17) and evaporation of any organic material. A high negative voltage (typically 400V to 1500V) was then applied to the cathode (e.g. 16 and 17) to produce a glow discharge with net transfer of material therefrom to the substrate 12 to effect coating thereof. there reactive sputtering was required, a reactive gas was admitted to the coating chamber 1 at a small partial pressure (1 to 100 m torr) during the coating process.

Specific Examples The above procedure was used to coat a stainless steel substrate with a first layer of Ti and a second layer of TiN. Three samples were coated of thicknesses as follows: Example Ti thickness (tom) TiN thickness and coating time (min) (Am) A 0.1 ; 8.5 2.0 B 0.1 ; 45 2.2 1 0.3 ; 120 2.0 The products of Examples A, B and 1 were tested by measuring their respective weight losses when subjected to the standard Taber test for abrasive wear resistance usinq CS 17 wheels (1000 g load). The results are shown in Figure 2 where the vertical axis, representing weight loss, is a measure of resistance to abrasive areas, and the horizontal axis represents the number of revolutions or sliding distance. The results for Examples A and B are shown by hollow and filled circles respectively and the results for Example 1 by hollow triangles. It can readily be seen that the thicker titanium layer of example 1 considerably enhances wear resistance in comparison with Examples A and B where the titanium layers are thinner.

EXAMPLE 2 The above procedure was used to coat a tool steel substrate with the following layers in sequence: a first preliminary layer of Ti of thickness 0.1 micrometre, a second preliminary layer of TiN of thickness 0.5 micrometre, a first layer of Ti of thickness 1 micrometre, and a second layer of TiN of thickness 2 micrometres.

The resulting coating was found to have satisfactory adhesion to the substrate and to endow it with excellent corrosion and wear resistance properties.

Claims

1. A method of forming a multi-layer coating on a substrate comprising the steps of (i) applying a first layer comprising a refractory transition metal or alloy thereof by a physical vapour deposition method, the first layer having a thickness of greater than 0.1 micrometre; and (ii) applying a second layer to the first layer by a physical vapour deposition method, the second layer comprising a refractory compound of a transition element.

2. A method according to claim 1 wherein the first layer has a thickness of 0.2 micrometre or greater.

3. A method according to claim 1 or claim 2 wherein the second layer has a thickness of greater than one micrometre.

4. A method according to any of the preceding claims wherein the substrate is metallic and has a Vickers' hardness of less than 300.

5. A method according to claim 4 wherein the substrate is a mild steel or a stainless steel.

6. A method according to claim 1 wherein, before step (i) is carried out, a first preliminary layer is applied to the substrate by a physical vapour deposition method, the first preliminary layer comprising a refractory transition metal or alloy thereof and having a thickness of up to about 0.5 micrometre, and a second preliminary layer is applied to the first preliminary layer by a physical vapour deposition method, the second preliminary layer comprising a refractory compound of a transition element.

7. A method according to claim 6 wherein the second preliminary layer has a thickness of from about 0.1 micrometre to about 5 micrometres.

8. A method according to claim 6 or claim 7 wherein the substrate is metallic and has a Vickers' hardness of 300 or greater.

9. A method according to any of the preceding claims wherein the refractory transition metal is a metal of Groups 4, 5 or 6 of the Periodic Table with the exception of radioactive elements.

10. A method according to claim 9 wherein the element is titanium or zirconium.

11. A method according to any of the preceding claims wherein the refractory compound is a nitride, carbide or oxide of an element of Groups 4, 5 or 6 of the Periodic Table with the exception of radioactive elements.

12. A method according to claim 11 wherein the refractory compound is titanium nitride or zirconium nitride.

13. A method according to any of the preceding claims wherein the physical vapour deposition method is sputter ion plating.

14. A method according to any of the preceding claims wherein the substrate is or forms part of an artefact.

15. A method of forming a multi-layer coating on a substrate substantially as described herein with reference to either of the examples.

16. A substrate provided with a multilayer coating by a method according to any of the preceding claims.