WO1990004044A1

WO1990004044A1 - Surface treatment of metals and alloys

Info

Publication number: WO1990004044A1
Application number: PCT/GB1989/001177
Authority: WO
Inventors: Malcolm Edward Boston; Rhoderick Nigel Turner; Peter Harlow Morton; Thomas Bell; John Lanagan
Original assignee: Tecvac Limited
Priority date: 1988-10-08
Filing date: 1989-10-04
Publication date: 1990-04-19
Also published as: AU4401789A; GB2245601B; GB2245601A; GB9104885D0; ES2017832A6; CA2000320A1; KR900702071A; GB8823668D0

Abstract

The invention provides a method of treating a component made of metal or alloy, in which the component is bombarded with nitrogen ions in a vacuum system at a temperature below that conventionally used for plasma nitriding. The nitrogen ion bombardment is preferably carried out at a temperature below 850°C and for a period of time in the range of 1-2 hours. This method allows surface treatment of a component to be carried out relatively quickly. The invention also provides a method of treating a component made of metal or alloy, in which the surface of the component is cleaned by ion bombardment; the cleaned component is bombarded with nitrogen ions in a vacuum system at a temperature below that conventionally used for plasma nitriding; and a surface coating is then formed on the component. A component is thus produced which has improved properties compared with those obtained hitherto, for example, good adhesion, wear and surface finish properties.

Description

Title; Surface Treatment of Metals and Alloys

Description

This invention concerns the surface treatment of metals and alloys, particularly titanium and titanium alloys.

Background to the invention

It is well known that titanium and its alloys have some very useful properties: they are strong, corrosion resistant, light in weight and easily machinable, weldable and foregeable. However the wear properties of such materials, either between themselves or with other materials, are very poor. In consequence such materials are generally unsuitable for application in circumstances in which rubbing takes place, although they are used in some special situations where no other alternative is practicable, with the material being coated and/or treated in various ways.

None of the coatings and treatments available so far has proved really satisfactory. Coatings generally lack adhesion or hardness. Metallurgical treatments generally damage the fatigue resistance, surface finish, or hardness and/or cause distortion of the bulk material due to the high temperatures at which the treatments are carried out .

In particular plasma nitriding, a process used widely for surface hardening of' some steel alloys, has proved to be disappointing in treatment of titanium. In this process as applied to steel a finished component is introduced into a vacuum system and bombarded with nitrogen ions, by holding the component at a negative DC voltage of some 0.1 to 1 kilovolts. The energy dissipated at the surface of the material by the ion bombardment raises the temperature gradually to 800-900'C (although the heating can be accelerated by additional heating). In this temperature range the nitrogen reacts at the surface with iron, producing iron nitrides. The nitrogen diffuses slowly into the surface during treatment. After a treatment at temperatures up to 1000^βC and lasting up to 60 hours a hard nitride skin some tens of microns thick is established on the component. The nitrogen reacts more rapidly with other alloying elements such as titanium producing titanium nitrides (TiN and Ti_N), which are very hard and stable compounds.

However there are various drawbacks to this process. The effect is inevitably accompanied by changes in dimension, i.e. distortion, generally necessitating re-working or re- finishing of the component by machining, grinding or polishing. In addition, and more importantly, there is a reduction in fatigue resistance of the material probably due to microcracks being opened up in the surface by preferential attack by nitrogen ions at grain boundaries. Moreover the surface finish of the component deteriorates and takes on an "orange peel" appearance.

In another process, a component is coated with titanium nitride by physical vapour deposition (PVD) . Titanium is vaporised from a crucible in a vacuum system by electron beam bombardment. As a first step, the finished machined component, which has 'already been ultrasonically cleaned, is ion bombarded by argon ions for about 30 minutes in the vacuum system, to remove any residual contamination and to raise the temperature of the component to about 300-400*C. Then with the component again at negative voltage titanium evaporation is commenced by bombarding the titanium source with electrons. At the same time nitrogen and argon gas are admitted to the vacuum system so as to create a gas pressure of some 5-15 x 10 millibars (3.3-10 microns): a plasma is created of electrons and ionised nitrogen, argon and titanium. Operating conditions such as plasma pressure and voltage are generally regulated to maintain the temperature of the component in the range 350-450^βC. The component attracts the ions, which give up their charge on arrival at the surface, and the titanium and nitrogen combine to form a stoichiometric compound of TiN. After an hour or so a titanium nitride coating 2 or 3 microns thick is built up on the surface of the component. Under the correct conditions the adhesion is satisfactory. On steel surfaces this treatment greatly improves the cutting and wear performance of many manufacturing tools.

On titanium alloys such treatment provides a wear resistant surface which is adequate for many specialized applications: indeed this process transforms the performance and makes the use of titanium alloys practicable in vehicles (at present mainly racing and rally cars) .

Nevertheless, the coating is relatively thin and brittle. Under high load conditions sub surface deformation of the component can cause breakdown of the coating. In contrast, the comparatively deep diffused layers produced by plasma nitriding do not suffer from high load sub surface deformation. The metallurgical properties of the material are hardly affected by PVD (the treatment temperature is not high enough), there is no distortion, the fatigue resistance is not worsened and can even be improved, and the surface is left with a highly reflecting polish as good as before coating.

If the wear properties of titanium and its alloys could be further improved in a cost-effective way this would significantly open up the range of potential uses for such materials in many industries, particularly the aerospace, automobile and chemical industries for which combinations of the other properties of such materials are of outstanding value.

The Invention

According to one aspect of the present invention there is provided a method of treating a component made of metal or alloy in which the component is bombarded with nitrogen ions in a vacuum system at a temperature below that conventionally used for plasma nitriding.

The nitrogen ion bombardment, or nitriding, is generally carried out at a temperature below 850^βC, typically in the range 600-700^βC.

Nitriding treatment time is not critical, but will typically be in the range 1-2 hours, which is a small fraction of the time of conventional plasma nitriding treatment .

The nitriding may be carried out using nitrogen alone, or nitrogen in combination with other gases such as argon.

After nitriding, a surface coating, particulary of a nitride, may be formed on the component. Such a coating may be formed in conventional manner, e.g. using PVD as described above. Preferred coatings include titanium nitride, although other materials such as aluminium nitride and titanium aluminium nitride may also be used. By way of example, when coating with titanium nitride by PVD, titanium metal in the vacuum system is vaporised by electron beam bombardment in parallel with continued nitrogen ion bombardment. Treatment is generally carried out for between 20 and 40 minutes, say 1/2 hour, and following conventional practice at a temperature in the range 350-450*C, although rather higher temperatures, say up to 600-700^βC, can also be used.

If desired, a further coating, eg of a carbide, oxide or carbonitride, may be formed on top of the first coating.

Prior to the nitriding treatment the component is conveniently cleaned by ion bombardment in conventional manner, e.g. by bombardment with argon ions in a vacuum system as described above. Such treatment is typically carried out for between 1/4 and 3/4 hour, say 1/2 hour, at a temperature in the range 300-400*C, although rather higher temperatures, up to about 500^βC, can also be used. Nitrogen or titanium ions could also be used in similar manner for cleaning purposes. If nitrogen ions are used some plasma nitriding will take place during the cleaning process.

The invention is particularly applicable to the treatment of components of titanium and titanium alloys, but may also be used for other materials such as steel and other ferrous alloys.

In a further aspect the present invention provides a method of treating a component made of metal or alloy, comprising cleaning the surface of the component by ion bombardment? bombarding the cleaned component with nitrogen ions in a vacuum system at a temperature below that conventionally used for plasma nitriding; and then forming a surface coating on the component.

The invention also includes within its scope a component which has been treated by a method in accordance with the invention.

The nitriding treatment of the invention allows surface treatment of a component to be carried out relatively quickly, for example 1/2 hour cleaning, 1-2 hours nitriding, then 1/2 hour coating by PVD.

Further, use of the nitriding treatment of the invention prior to surface coating enables production of a component with improved properties compared with those obtained hitherto, having good adhesion, wear and surface finish properties without any distortion of the component occurring.

In a typical embodiment of the invention a component of titanium or titanium alloy is cleaned by argon ion bombardment in a vacuum system, such as the vacuum systems produced by Tecvac Limited and marketed as their IP35L model. The component is maintained at a negative voltage, and argon gas pumped through the system. Cleaning is carried out for about 30 minutes, with the temperature of the component rising rapidly due to dissipation of energy at the surface of the material by the ion bombardment, and being maintained in the range 300-400'C by suitable regulation of the plasma pressure and the negative voltage of the component.

After cleaning the component is nitrided by introducing nitrogen gas to the vacuum system and bombarding the component intensely with nitrogen ions for 1-2 hours, with the temperature of the component maintained in the range 600-700*C by regulation of the plasma pressure and the voltage of the component.

Immediately following nitriding a titanium nitride coating is laid down on the component by PVD, by vaporising titanium metal introduced to the vacuum system by electron beam bombardment in parallel with continued nitrogen ion bombardment. PVD is carried out for about 30 minutes, with the temperature of the component maintained in the range 350-600*C by regulation of the plasma pressure and the voltage of the component.

The resulting surface appears to have all of the advantages associated with both of the known treatments (plasma nitriding and coating by PVD), without many of the disadvantages. Compared with results obtained previously, adhesion of the titanium nitride coating is improved; the fatigue properties and bulk metallurgical structure of the component are little affected; there is no distortion (as the temperatures employed have not been sufficiently high to produce distortion); and the surface finish is as good as that of the component prior to treatment.

Features of preferred treatment methods incorporating the invention are:

1. To build up a graded structure, with a tough core, an interlayer of increasing hardness and a top surface of greatest possible hardness.

2. To produce the interlayer and top surface by consecutive treatment stages in the same equipment, with one stage immediately following the other without break so as to avoid contamination which can affect adhesion.

3. To produce diffused layers on titanium with benefits of both titanium nitride coating and plasma nitriding at low temperatures and in very fast cycle times, typically one tenth of plasma nitriding times.

4. To produce diffused layers on hardened steel by nitriding at a lower temperature and for a much shorter time than customarily used for plasma nitriding (typically one tenth of that time) followed immediately in the same equipment without a break (ie with one stage immediately following the next, as in 2 above) by coating with titanium nitride or titanium aluminium nitride by a PVD process.

5. To produce diffused layers on metals other than hardened steel by nitriding at lower temperatures and for times shorter than customarily used for plasma nitriding followed immediately (as in 2 above) by coating with titanium nitride or titanium aluminium nitride _by a PVD process.

Claims

1. A method of treating a component made of metal or alloy in which the component is bombarded with nitrogen ions in a vacuum system at a temperature below that conventionally used for plasma nitriding.

2. A method according to Claim 1, wherein the nitrogen ion bombardment is carried out at a temperature below 850°C.

3. A method according to Claim 2, wherein the nitrogen ion bombardment is carried out at a temperature in the range 600-700°C.

4. A method according to Claim l 2 or 3, wherein the nitrogen ion bombardment is carried out for a period of time in the range 1-2 hours.

5. A method according to any one of the preceding claims, wherein the nitrogen ion bombardment is carried out in an atmosphere comprising nitrogen and argon.

6. A method according to any one of the preceding claims, further comprising forming a surface coating on the component after the nitrogen ion bombardment.

7. A method according to Claim 6, wherein the surface coating comprises a nitride.

8. A method according to Claim 6 or ,. wherein the surface coating is formed by physical vapour deposition.

9. A method according to Claim 6, 7 or 8, further comprising forming a further coating on top of the first coating.

10. A method according to any one of the preceding claims, wherein the component is initially cleaned by ion bombardment.

11. A method according to any one of the preceding claims, applied to the treatment of components of titanium and titanium alloys.

12. A method according to any one of Claims 1 to 10, applied to the treatment of steel and other ferrous alloys.

13. A method of treating a component made of metal or alloy, comprising cleaning the surface of the component by ion bombardment; bombarding the cleaned component with nitrogen ions in a vacuum system at a temperature below that conventionally used for plasma nitriding; and then forming a surface coating on the component.

14. A method of treating a component made of metal or alloy, substantially as herein described.

15. A component which has been treated by a method in accordance with any one of the preceding claims.