GB2261227A

GB2261227A - Surface treatment of metals at low pressure

Info

Publication number: GB2261227A
Application number: GB9123799A
Authority: GB
Inventors: Allan Matthews; Adrian Leyland
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1991-11-08
Filing date: 1991-11-08
Publication date: 1993-05-12
Anticipated expiration: 2011-11-08
Also published as: GB2261227B; GB9123799D0

Abstract

A method of surface hardening metals (in particular stainless steel) comprises heating the component in a chamber to a temperature of less than 550 DEG C, at a pressure of less than 10mTorr (eg 7.5m Torr) containing an element which reacts with the surface of the metal and being in ionized form by use of a plasma in the presence of a gas or gas mixture. This mixture may contain carbon (eg acetylene) or nitrogen containing gases eg nitrogen or a gas containing both carbon and nitrogen. The gas is ionized by a low pressure d.c. negative workpiece bias or r.f. plasma system so that carbon, nitrogen (or both) enter the surface of the metal. An electron emission source, which may be a negatively biased, heated tungsten filament, may provide plasma enhancement independently of workpiece voltage and chamber pressure parameters. In addition, an earthed hot tungsten filament may provide additional substrate heating conventional nitriding may be carried out as a pre-treatment and the method is particularly suitable for treating steels. An apparatus for carrying out the method is also disclosed (see Fig). Hard diffusion layers with superior combinations of wear, fatigue and/or corrosion resistance are produced depending on gas mixture, at a low temperature (as low as 270 DEG C). The range of materials which can be treated by this technique, without the need for post-finishing (either by heat treatment or machining operations), is superior to that achievable by any other single plasma diffusion process. <IMAGE>

Description

SURFACE TREATMENT OF METALS The invention relates primarily to the surface treatment of stainless and low to medium alloy steels. It also has applicability to certain non-ferrous metals, such as titanium and its alloys.

A useful property of stainless steel, in particular, is that it has high resistance to corrosion. However, its use in many engineering applications is limited because the surface hardness is insufficient to resist wear effectively.

Various treatments are known for hardening the surface of ferrous materials. Amongst these are curburizing, nitriding and nitro carburizing.

In gas carburizing, the steel is heated to above 850 C such that it becomes fully Austenitized. It is contacted with a carbon bearing gas, and excess carbon diffuses into the surface of the material. The surface is hardened by quenching into the ferritic phase field to produce a martensitic transformation. Subsequent tempering at low temperature (i.e. generally below 3000C) redistributes the carbides and thus provides the appropriate balance between hardness and toughness. The phase transformations induced by the quenching and tempering process however often cause distortion; there is then a requirement for post treatment machining.

In nitriding, components are heated in the presence of a nitrogen containing gas (such as ammonia) at temperatures typically between 500 and 570 0C and for substantial times (often as great as 60 hours) to diffuse nitrogen into the surface of the material. Since nitriding is a ferritic process, and can provide a good balance between hardening and toughness without quenching, problems with distortion rarely occur. However, the nitriding process traditionally produces both a thick (250-500CLm) diffusion layer and a thin 'white' layer (10am) of y'-Fe4N nitrides at the surface.The white layer properties can to some extent be controlled in terms of thickness and morphology but it is generally friable or porous in nature and (in combination with the underlying diffusion layer/substrate) gives poor corrosion resistance.

In nitro carburizing, the material is typically heated in the presence of a mixture of nitrogen and carbon bearing gases. The treatment temperature is again between 500 and 570 0C and the treatment time is equivalent to nitriding. The nitrocarburizing process also produces a 'white layer' at the surface, however the carbon modifies its structure to produce t-Fe2~3(N,C) nitrides/ carbides. In some contact environments this layer provides improved wear peformance over r and often exhibits superior toughness. Corrosion resistance is also superior to y', especially if a post oxidizing treatment is carried out.

According to the invention, there is provided a method of surface treating metals such as stainless and low to medium alloy steels involving heating the metal at 0 temperatures of between 270 and 550 C in a vacuum chamber at a pressure of less than l0mTorr; the chamber contains gases which include elements for treating the surface of the metal, gas being ionized by a plasma system, and the components being exposed to the ionized gas for a time sufficient to diffuse these elements into the metal and create a hardened surface layer of the required thickness.

By the use of such a method of combination of methods, the treated components require no subsequent finishing operations. Preferably, the method is for treating stainless and low to medium alloy steels.

The ionized gas plasma may be generated by a d.c. negative workpiece bias of > 200V or by a workpiece biased with a radio frequency (r.f.) supply, ideally at a frequency of less than 1MHz. In the former case an additional electron emission source is employed (e.g. a negatively biased, heated tungsten filament) to provide plasma enhancement, independent of workpiece voltage and chamber pressure parameters. In the latter case an earthed hot tungsten filament may also be used to provide additional substrate heating by both radiation and/or electron emission induced plasma enhancement on part of the rf cycle.

Preferably the gas mixture is carbon bearing to produce a carburized surface on the stainless steel. However it may also contain nitrogen to modify the surface layer structure and/or diffusion layer depth/structure. It may also contain hydrogen to act as a cleaning and/or dilution agent or as a catalyst for active treatment species production. It may also contain argon as an inert sputter precleaning or dilution agent.

In the dc. workpiece bias case, for carburizing the temperature may be as low as 4000C, the treatment time more than 10 hours and the pressure 7.5mTorr or less.

Also in this case, the chamber may contain a mixture of argon, hydrogen and acetylene. The weight proportion of acetylene may be small (i.e. < 5%).

In an alternative process, the above mentioned treatment may be preceded by nit riding treatment with the workpiece temperature being 500-570 C the treatment time greater than 5 hours and the pressure around 7.5mTorr or less.

The gas may be nitrogen only, or a mixture of nitrogen and hydrogen. Argon may also be used as a mixture dilutant or as a substitute for hydrogen.

Intermediate to the nit riding treatment and subsequent carburizing treatment, the material may be subjected to an etching stage to deplete the surface of the treated component with respect to nitrogen. In this treatment the 0 workpiece temperature may be 270-550 C, the gas pressure may be 7.5mTorr or less, and the gas mixture may contain argon only or argon and hydrogen but no nitrogen. A carbon bearing gas such as acetylene may however be included in the mixture. The treatment time may vary between 30 minutes and 5 hours.

A third possibility is a single treatment stage, in which the gas mixture contains both nitrogen and carbon to produce a nitro-carburized surface on the material. In this case the treatment temperature may again be as low as 4000C, the pressure 7.5mTorr or less, the treatment time 10 hours or more, and both hydrogen and argon may again be added singly, or together, in the gas mixture for the purposes described above.

With regard to all techniques detailed in the three treatment cycles outlined above, the dc triode plasma system may be replaced by the low frequency rf plasma configuration. In this case all parameters of treatment remain the same except for temperature, which may be as low as 270 C.

The following is a more detailed description of examples of the three embodiments of the invention given with reference to the accompanying drawing.

All examples to be described employ the apparatus shown in the drawing. The apparatus comprises a gas-tight chamber 1 connected to a vacuum pump 2 to exhaust the chamber. A gas supply 3 is connected to the chamber to supply to the chamber a required gas (or gas mixture).

A system to produce a low pressure thermionically enhanced (triode) plasma is enclosed in the chamber 1. The system comprises a filament cathode 4 and anode 5. The sample 6 is connected to either a d.c. voltage source 13 or to an r.f. voltage source 7. The anode 5 is connected to a supply 10. The filament 4 is connected to an electrical source 8 to heat it and to a bias supply 9, if necessary.

Example 1 In the example, a sample 6 of stainless steel was placed in the chamber 1. The chamber pressure was then reduced to less than 0.05mTorr by the pump 2. The pumping speed was then reduced and a gas mixture of 2.5mTorr Argon, 4.8mTorr Hydrogen and 0.2mTorr Acetylene was established in the chamber 1, through the gas supply 3, to create a 7.5mTorr treatment mixture. The sample 6 was then biased to -200V d.c. negative bias by connecting the switch 14 to the power supply 13 to produce a plasma. The filament 4 was biased to -100V negative bias by the power supply 9.

The filament 4 was heated by the a.c. current supply 8 to produce electron emission and raise the current density drawn by the sample 6 to 0.35mAcm The sample 6 was 0 thus heated to 400 C by ion bombardment from the gas mixture and exposed to this mixture for 20 hours to produce a carburized (i.e. carbon rich) diffusion layer 50um deep around the sample.

Example la In this example, a sample 6 of stainless steel was placed in the chamber 1. The chamber pressure was then reduced to less than 0.05mTorr by the pump 2. A gas mixture of 2.5mTorr Argon, 4.8mTorr Hydrogen and 0.2mTorr Acetylene was then introduced to the chamber 1, through the gas supply 3, to create a 7.5mTorr treatment mixture. The sample 6 was connected to earth via switches 11,14. The anode plate 5 was biased to +150V positive bias by the supply 10, initiating a positive plasma and causing ion bombardment of both the chamber 1 and the sample 6. The filament 4 was connected to earth via a switch 12.The filament 4 was heated by the a.c. current supply 8 to produce electron emission and raise the current density drawn by the sample 6 to 0.5mAcm 2. The sample 6 was thus heated to 4000C by ion bombardment from the gas mixture and exposed to this mixture for 20 hours to produce a carburized diffusion layer 50um deep around the sample.

Example 1b In this example, a sample 6 of stainless steel was placed in the chamber 1. The chamber pressure was then reduced to less than 0.05mTorr by the pump 2. A gas mixture of 2.5mTorr Argon, 4.8mTorr Hydrogen and 0.2mTorr Acetylene was then introduced to the chamber 1, through the gas supply 3, to create a 7.5mTorr treatment mixture. The sample 6 was then self-biased to a d.c.-offset of greater than 2kV, but less than 7kV, by connection to the r.f.

supply 7 through the switch 14. The r.f. power was increased to provide a workpiece current density sufficient to raise the temperature of the sample 6 to 270 0C by ion bombardment from the gas mixture. The sample was exposed to this mixture for 40 hours to provide a carburized diffusion layer 30Fm deep around the sample.

Example 2 In this example, a sample 6 of stainless steel was placed in the chamber 1. The chamber pressure was then reduced to less than 0.05mTorr by the pump 2. A gas mixture of SmTorr Hydrogen, 2.SmTorr Nitrogen was introduced to the chamber 1, through the gas supply 3, to create a 7.5mTorr treatment mixture. The sample 6 was then biased to -200V negative bias to produce a plasma, by connecting the switch 14 to the power supply 13. The filament 4 was biased to -100V negative bias by the power supply 9.The filament 4 was heated by the a.c. current supply 8 to produce electron emission and raise the current density drawn by the sample 6 to 0.5mAcm2. The sample 6 was 0 thus heated to 520 C by ion bombardment from the gas mixture and exposed to this mixture for 10 hours to produce a nitrogen-rich diffusion layer of 150um initial depth, around the sample. After 10 hours the gas mixture was changed to replace the 2.5mTorr Nitrogen partial pressure with 2.5mTorr of Argon. The sample was then exposed to the same plasma bias and heating conditions for 2 hours, to deplete the sample surface of nitrogen.

After 2.5 hours, 0.2mTorr of Acetylene was added to the existing Argon/Hydrogen gas mixture, all electrical parameters remaining the same. A further 7.5 hours exposure to the gas mixture at 520 0C produced a surface diffusion layer around lOum depth, rich in carbon, an intermediate layer - 40Fm deep, rich in nitrogen and carbon and a remaining layer - 120-130Rm deep, rich in nitrogen. The total diffusion layer depth was thus 170-180Fm.

Example 2a In this example, identical gas mixtures and exposure times to each gas mixture were employed as those given in Example 2. However, the electrical configuration employed throughout the treatment was identical to that detailed in Example la, with the single exception that the filament 4 was heated by the a.c. current supply 8 at increased power to that in la; providing increased electron emission such that the current density at the sample 6 was increased to 0.7mAcm 2 to provide the treatment temperature of 5200C detailed in example 2. The layer thicknesses produced were similar to those in Example 2.

Example 2b In this example, identical gas mixtures and exposure times to each gas mixture were employed as to those given in Example 2. However, the electrical configuration employed throughout the treatment was identical to that detailed in Example lb, with the single exception that the r.f. power was further increased to provide a workpiece current density sufficient to raise the temperature of the sample 6 to 4000C. The initial nitrogen-rich diffusion layer produced by this technique was 120um in depth. the final layer structure was a surface carbon-rich layer of 5-7Fm in depth and intermediate layer 25-33pm in depth, rich in nitrogen and carbon and a remaining nitrogen rich layer of ~ lOORm in depth. The total case depth was thus l30-140um.

Example 3 In this example, a sample 6 of stainless steel was placed in the chamber 1. The chamber pressure was then reduced to less than 0.05mTorr by the pump 2. A gas mixture of 5mTorr Hydrogen, 2mTorr Nitrogen and 0.5mTorr Acetylene was then introduced to the chamber 1, through the gas supply 3, to create a 7.5mTorr treatment mixture. The sample 6 was subjected to an identical bias configuration to that employed in Example 2, such that a temperature of 5200C was attained. The sample 6 was exposed to ion bombardment from the above mentioned gas mixture for a period of 20 hours, to produce a diffusion layer 150Fm deep round the sample, containing nitrides/carbides, and/or nitrogen and carbon in solid solution.

Example 3a In this example, an identical gas mixture and exposure time to that detailed in Example 3 was employed. The electrical configuration employed was however identical to that detailed in Example la; with the single exception that the filament 4 was heated by the a.c. supply 8 at increased power to that in la, providing increased electron emission such that the current density at the sample 6 was increased to 0.7mAcm 2, to provide the treatment temperature of 5200C detailed in Example 3.

The layer produced was identical in thickness and structure to that detailed in Example 3.

Example 3b In this example, identical gas mixtures and exposure times to those in Examples 3 and 3a were employed. However, the electrical configuration employed was identical to that detailed in lb, with the exception that the r.f. power was further increased to provide a workpiece current density sufficient to raise the temperature of the sample 6 to 400 0C. The layer produced after the 20 hour treatment time was, in this case, between 100 and 120um in depth.

Example 3c In this example, the gas mixture and exposure time were again the same as 3, 3a, 3b above. An r.f. electrical configuration was again employed as in 2b and 3b, however it was supplemented by the filament 4, which was heated by the a.c. supply 8, and earthed through the switch 12. The heated filament 4, provided additional radiant heating/ion bombardment to the sample 6 during certain parts of the r.f. cycle, thus raising its temperature to 520 C. In this case, the layer produced after 20 hours of exposure to the gas mixture was - 200fm in depth.

In the examples above, references to nitrogen or carbon-rich layers are references to layers where the nitrogen or carbon amount in the layer is increased compared to the bulk, and these elements may be present either as separate metal nitrides/carbides, or/and interstitially in the primary lattice(s).

In examples described above, the use of plasma diffusion allows the treatments to be performed at low temperatures. This minimizes the risk of undesirable softening and distortion effects (inherent in treatments which require heating to elevated temperatures) and thus reduces or eliminates the need for subsequent quenching and tempering. It also extends the range of materials (particularly lower - alloy ferrous compositions) which can be treated for wear applications where close tolerances are required but post treatment finishing operations are expensive or impractical.

Claims

1. A method of surface treating metals, involving heating the metal at temperatures of between 270 and 550 0C in a vacuum chamber at a pressure of less than lomTorr; the chamber contains a gas or a mixture of gases including at least one element for treating a surface of the metal, gas being ionized by a plasma system, and the components being exposed to the ionized gas for a time sufficient to diffuse these elements into the metal and create a hardened surface layer of the required thickness.

2. A method as claimed in claim 1, for surface treating stainless and low to medium alloy steels.

3. A method as claimed in claim 1 or claim 2, wherein the ionized gas plasma is generated by a d.c. negative workpiece bias.

4. A method as claimed in claim 3, wherein the d.c.

negative workpiece bias is greater than 200V.

5. A method as claimed in claim 4, including using an electron emission source to provide plasma enhancement independently of workpiece voltage and chamber pressure parameters.

6. A method as claimed in claim 5, wherein the electron emission source is a negatively biased, heated tungsten filament.

7. A method as claimed in claim 1 or claim 2, wherein the ionized gas plasma is generated by a workpiece biased with a radio frequency (r.f.) supply.

8. A method as claimed in claim 7, wherein the frequency of the r.f. supply is less than 1 MHz.

9. A method as claimed in claim 7 or claim 8, including using an earthed hot tungsten filament to provide additional substrate heating by both radiation and/or electron emission induced plasma enhancement on part of the r.f. cycle.

10. A method as claimed in any one of claims 1 to 9, wherein the gas contains a carbon bearing gas to produce a carburized surface on the metal surface.

11. A method as claimed in claim 10, wherein treatment with the carbon-bearing gas is preceded by a nitriding treatment.

12. A method as claimed in any one of claims 1 to 11, wherein the gas contains one or more of nitrogen, hydrogen and an inert gas such as argon.

13. A method as claimed in any one of claims 3 to 6, wherein the gas contains a carbon bearing gas, the temperature is in the range 400 to 550 C, the treatment time exceeds 10 hours and the pressure in the chamber is 4 7. 5mTorr.

14. A method as claimed in claim 13, wherein the gas contains a mixture of argon, hydrogen and a carbon-bearing gas, such as acetylene.

15. A method as claimed in claim 12, wherein the weight proportion of the carbon-bearing gas in the mixture is less than 5%.

16. A method as claimed in claim 14 or claim 15, wherein the gas also contains nitrogen.

17. A method as claimed in any one of claims 7 to 9, wherein the gas contains carbon bearing gas, the temperature is in the range from 2700 to 550 C, the treatment time exceeds 10 hours and the pressure in the chamber is 4 7.5mTorr.

18. A method as claimed in claim 17, wherein the gas contains a mixture of argon, hydrogen and a carbon-bearing gas, such as acetylene.

19. A method as claimed in claim 18, wherein the weight proportion of carbon-bearing gas in the mixture is less than 5%.

20. A method as claimed in claim 18 or claim 19, wherein the gas also contains nitrogen.

21. A method as claimed in any one of claims 13 to 20, wherein said treatment with carbon bearing gas is preceded by a nitriding treatment.

22. A method as claimed in claim 21, wherein during the nitriding treatment the temperature of the workpiece is in the range from 5000 to 570 or, the treatment time exceeds 5 hours and the pressure in the chamber is 7.5mTorr.

23. A method as claimed in claim 21 or claim 22, wherein during the nitriding treatment the gas consists of or contains nitrogen gas, or a mixture of nitrogen and hydrogen, or nitrogen and argon or nitrogen, hydrogen and argon.

24. A method as claimed in any one of claims 21 to 23, wherein between said nitriding treatment and said treatment with carbon bearing gas, the treated metal surface is subjected to an etching stage to deplete the surface with respect to nitrogen.

25. A method as claimed in claim 24, wherein the temperature of the workpiece is in the range 2700 to 5500C during the etching stage.

26. A method as claimed in claim 24 or claim 25, wherein the pressure in the chamber is 7.5mTorr and the gas is devoid of nitrogen.

27. A method as claimed in claim 26, wherein the gas consists of argon only or a mixture of argon and hydrogen.

28. A method substantially as described in the foregoing Examples with reference to the accompanying drawing.

29. An apparatus for the surface treatment of metals for carrying out the method according to any one of claims 1 to 28.