EP0862662A1

EP0862662A1 - Plasma-assisted thermochemical surface-treatment method, installation for carrying out the method and use of the method and installation

Info

Publication number: EP0862662A1
Application number: EP96928307A
Authority: EP
Inventors: Norbert Marie Dingremont; Erich Bergmann; Pierre Collignon
Original assignee: Balzers AG
Current assignee: OC Oerlikon Balzers AG
Priority date: 1995-11-22
Filing date: 1996-09-06
Publication date: 1998-09-09
Also published as: WO1997019203A1; FR2741361A1; FR2741361B3

Abstract

In order, in the plasma-assisted thermochemical treatment of workpiece surfaces, to increase the surface area treated per unit time while at the same time ensuring that the surface microstructure of the workpiece treated is the same as that of an untreated workpiece, the invention calls for a low-voltage discharge (27) to be generated and the workpieces (33) to be charged up to a positive electrical potential (25) with respect to the discharge cathode (7).

Description

Process for plasma-thermochemical surface treatment, plant therefor, and uses of the process or plant

The present invention relates to a method for plasma-thermochemical surface treatment of workpieces, in which a gas with a connecting element, namely in particular with at least one of the elements C, N, O, B, Si, S, is excited and thermally by at least one low-voltage discharge - The workpiece surface is chemically changed, a plant for this and preferred uses of this method or this plant and a workpiece preferably produced therewith.

Thermochemical, plasma-assisted surface treatment processes for workpieces are used industrially on a large scale. Compared to surface treatments using salt baths or in gas phases, they offer the possibility of realizing a large variety of different surface coatings, while varying the resulting properties as a function of the treatment parameters.

In this regard, reference is made to Th. Lampe, "Plasma Heat Treatment of Iron Materials in Nitrogen and Carbon-Gas Mixtures", Progress Reports VDI Series 5, No. 93, Düsseldorf 1985.

In the treatment that is mostly used industrially, namely surface nitriding, an abnormal glow discharge is used, in which the workpieces to be treated are set to cathodic potential. The ion current density on the cathode surface is uniform and increases with the discharge voltage applied. The electrical generators used for the discharge must supply high voltages can, with adjustable and constantly stable power, to ignite the discharge and then maintain it according to the surface to be treated, the working pressure in the treatment recipient and the desired treatment temperature.

Power densities of 0.1 to 4W / cm ² are used (see T. Lampe and S. Eisenberg, Z. Werkstofftech. 17, 183-193 (1986)), and a working pressure is between 1 and 10 ohmPa. The plasmas used are referred to as so-called "weakly ionized plasmas" due to the relationship between the density of electrically charged particles, electrons or ions, and the amount of neutral particles present. These ratios are of the order of 10 ¹⁰ particles / cm ^{3 in} terms of electrons and ions, at an operating pressure of 4 hPa and a total density of 5.8 10 10 particles / cm ³ (see JL Marchand et al., Proc. Of International Conference on Ion Nitriding, Cleveland, USA, 1986).

The functions of heating the workpieces and generating reactive species were decoupled in a further development of the process mentioned. For this purpose, separate heating devices were provided in the treatment recipient, convection or resistance heating devices.

Furthermore, for the implementation of the plasma treatment, generators were used which are capable of delivering variable, pulsed currents with regard to repetition frequency and duty cycle, and thus pulsed plasmas were used. The technological interest in this regard is based on the reduction of arcing and on the fact that the outputs to which the workpieces are exposed - heating output, discharge loading - can be easily adjusted. In this regard, reference is made to R. - 4th

which is applied to the workpieces to be treated can be set with the cathode heating current, while the potential to which the workpieces are placed can be set independently thereof.

This results in better controllability of the species types and their energies which interact with the workpieces to be treated. It is thus possible to reduce the potential applied to the workpieces, which is negative with respect to cathode or discharge - up to approximately -200V - and to carry out the treatment at lower total pressures - in the order of magnitude of 1 Pa and even much less - than is necessary in order to stabilize pure two-electrode discharges.

When using dependent low-voltage discharges, but also independent low-voltage discharges, the ion density is several orders of magnitude higher than with high-voltage plasma discharges, but at the same time the ion impact energy on the workpieces is lower due to the reduced acceleration field. The workpieces can be heated with the aid of an additional heating device or by the discharge itself. In practice, the treatment is carried out by nitriding in a process atmosphere with N, H and C in the form of CH compounds, additionally with argon or a noble gas in general.

For this purpose, see W.L. Grube et al. , J. Heat Treating 2, 1982, 211-216, and H. Michel, Le vide, Les Couches Minces, No. 266, 1993, pp. 197-206.

Gas atmospheres, usually with hydrocarbons of the methane or propane type and with hydrogen, are used for the carburizing of the workpiece surfaces, for which purpose 5 -

V.N. Blinov, Metal Sei. Heat Treatm. 24, 1982, 45-47, and to P. Collignon, These, Universite de Nancy I, 1978.

For the nitration of the workpiece surfaces as customarily workpiece temperatures between 350 ° C and 570 ° C are set einge¬ as are customary for the Karborierung workpiece temperatures between 80O ^c C and 1000 ° C.

As mentioned at the outset, the present invention relates to a method or a plant, the plasma-thermochemical surface treatment being implemented with the aid of a low-voltage discharge. An independent low-voltage discharge, preferably with a hot cathode, is preferably used.

It is the object of the present invention, starting from such a method, to increase the kinetics of the treatment process, in the sense of increasing the quotient "surface treatment effect / treatment time unit". Another object of the present invention is to achieve that the surface microstructure on the finished workpiece remains essentially identical to the surface microstructure on the workpiece that has not yet been treated.

In the previously known treatment processes described above, in which the workpieces are placed at negative potential with respect to the cathode or discharge, which attracts ions, the resulting relatively high ion impact energies lead to a significant increase in the surface roughness. Polished workpiece surfaces have to be polished after the mentioned treatment, which can be extremely costly when producing hard layers. The object is achieved by carrying out the method of the type mentioned at the outset according to the characterizing part of claim 1, namely by placing the workpieces at positive electrical potential with respect to the cathode, be it by utilizing an unbound floating potential, preferably, or by tying up to such a potential.

Preferred embodiments of the method are specified in claims 2 to 5.

A system according to the invention is specified in claim 6, preferred embodiments in claims 7 to 11, preferred uses of both the system and the method in claims 12 and 13. A workpiece according to the invention with preferred embodiments is in claims 14 to 19 specified.

It is surprising that the surface treatment of workpieces mentioned is possible according to the invention with a higher effect per unit of time and at the same time while protecting the original surface structure, although the workpieces are operated at a positive potential with respect to the discharge cathode and thus in the least substantially no ion bombardment takes place.

The invention is subsequently explained, for example, using figures and examples.

Show it:

1 schematically shows a system according to the invention in a first variant for carrying out the method according to the invention in a first embodiment; - 7th

FIG. 2 shows a representation analogous to that of FIG. 1, a second variant of the system according to the invention for carrying out the method according to the invention in a second, preferred form;

3 in the form of a block diagram, further variants and their operating possibilities of systems according to the invention;

4 shows a representation of the treatment kinetics when nitriding two steels,

5 shows a representation of the treatment kinetics in two-electrode processes with glow discharge of an abnormal type and the process according to the invention,

6a, each a micrograph of workpieces according to the invention 6b.

The present invention is preferably used to treat the surfaces of structural steels or tool steels, hot work steels or cold work steels, martensitic stainless steels, austenitic stainless steels or Ti alloys, in particular under Installation of at least one of the elements C, N, O, B, Si or S, or a combination of these elements, but in particular with the participation of N.

For this reason, reference is made below to nitriding.

According to FIG. 1, a cathode chamber 5 is flanged to a vacuum recipient 1, connected via a diaphragm 3, in which a thermally electron-emitting cathode turn 17 is stored, which, as a directly heated cathode, is preferably connected to a high-current generator 9 for the heating current I _H. A gas feed line 11 for the discharge working gas, preferably for argon, opens into the cathode chamber 5. The vacuum recipient 1 is evacuated via a pump connection 13. A reactive gas line 15, which is connected to at least one gas tank 17, opens into the vacuum recipient 1 via a controllable valve 19.

The gas tank 17 contains reactive gas with at least one compounding agent, namely with at least one of the elements C, N, O, B, Si, S, preferably at least with N. The diaphragm 3 opposite is a workpiece carrier anode 21 provided, which is cooled via a cooling medium circuit 23, preferably a water circuit. The workpiece carrier anode 21 is electrically insulated from the recipient 1, which is preferably held at reference potential, preferably at ground potential. Between the cathode 7 and the workpiece carrier anode 21, the DC voltage source 25 is connected for the operation of the dependent low-voltage discharge 27, whereby, by definition, the workpiece carrier anode 21 is set to a positive potential with respect to the cathode 7.

A heating arrangement 29 is provided in the recipient 1, preferably along the recipient wall, enclosing the workpiece carrier anode 21, with heat shields 31a or 31i preferably being provided in this regard on the outside and / or inside. The workpieces 33 are arranged on the workpiece carrier arrangement.

If necessary, the invention can also be used without the provision of the heating device 29, and then the 9 -

Workpieces 33 are heated by the electron current, which can be adjusted by setting the heating circuit I _H at the heating current source 9.

As mentioned, according to FIG. 1, the workpieces 33 are placed on the potential of the discharge anode.

In a preferred manner, however, the workpieces are operated at floating potential in the discharge. According to the invention, they also remain at a potential which is more positive than the potential of the cathode 7.

2, starting from the illustration according to FIG. 1, shows the preferred embodiment variant of a system according to the invention for carrying out the process according to the invention with several variants. The same reference numerals as in FIG. 1 are used for the same parts in FIG. 2. Only the deviations are also described with reference to FIG. 1.

In the recipient 1, the anode is formed with respect to the cathode 7 by at least a part 21a of the inner heat shield cylinder 31i, as a cylinder anode. With electrical insulation sections 43, any cylinder part that is electrically active for the anode function can be separated from the rest of the cylinder part, which is only thermally active. The entire heat shield inner cylinder 3li according to FIG. 1 can also be used as anode 21a. A tool carrier 39 is provided which, as shown with the electrically insulating holder 41, is held at floating potential in the process space 27. Its potential, in any case higher than that of the cathode 7, is set automatically depending on the electrical conditions in the process space 27. Instead of or in addition to the cylinder anode 21a, in any of the mentioned 10 -

Embodiments, one or more ring anodes 21b can be provided, the position of which, as shown by the double arrow z, is chosen along the discharge axis A as required.

The discharge direct voltage source 25, on the one hand, applies the cathode 7, on the other hand the cylinder anode 21a and / or the at least one ring anode 21b, which may be provided, to the respective potentials. This option is shown schematically with the selection unit 40. As further shown in dashed lines, the workpiece carrier 39 can also be tied to a potential that is independent of the anode potential, positive with respect to the potential of the cathode 7, such as by means of a preferably adjustable voltage source 26.

A cooling medium circuit (not shown) can also be provided for the workpiece carrier 39 according to FIG. 2, as has already been illustrated with reference to FIG. 1.

1 (workpieces on the discharge anode), it is advantageous that the ratio of the density of reactive species to the electrical energy introduced (efficiency) is high in that an extremely dense plasma with a given electrical discharge performance is exploited. The disadvantage of this is that a non-optimally homogeneous temperature distribution occurs along the workpiece carrier anode 21, due to the electron current density in the discharge which drops radially with respect to the discharge axis A.

2, the discharge cross section is spread, by providing the cylinder and / or ring anode 21a, 21b, which is the area provided for receiving the workpieces 33 - left

encloses at a distance, then the homogeneity of the density of reactive species and the homogeneity of the temperature distribution over the workpieces 33 is significantly better. This results in a significant improvement with respect to locally identical metallurgical treatment on the workpieces 33 treated at the same time.

3, however, a higher plasma power must be used in order to obtain a density of reactive species on the workpieces which is the same as in the procedure according to FIG. 1.

The operation of the workpieces at floating potential has the further advantage that the heating takes place indirectly in that no discharge current flows over the workpieces. For a given plasma power, a relatively balanced temperature distribution on the workpieces can therefore be achieved, regardless of the plasma density distribution.

By providing the heating devices 29, the workpieces are heated, the influence of the plasma discharge on the workpieces being reduced with respect to homogeneous temperature distribution. This means that plasma performance can also be optimized with regard to the density of reactive species, regardless of the temperature of the workpieces, i.e. an additional degree of freedom is obtained. The thermal characteristics and the characteristics of the plasma reactivity, necessary to carry out a specific treatment, are decoupled.

The advantages achieved according to the invention compared to previously known methods in which the workpieces are placed at the same or negative potential with respect to the cathode are: 12 -

Greatly reduced influence on the surface structure of the treated workpieces by the treatment. For example, the surface of workpieces made of steel type 35NCD16, polished, Ra = 0.05 μm, changes to Ra = 0.1 μm after nitriding at 520 ° C. for four hours. However, if the same workpiece is placed at a potential of approximately -200 V and otherwise treated with identical parameters, the surface roughness after the treatment of Ra = 0.32 μm results due to the ion bombardment. In both cases, the nitriding mentioned results in a nitriding depth of approx. 170μm.

Furthermore, the preparation phase for the thermochemical surface treatments mentioned is always chemical. This means that an argon / hydrogen plasma is used in order to prepare the surfaces to be treated with respect to diffusion, for example of nitrogen, into the material. During this phase, the workpieces are usually kept at floating potential. This makes it possible in the treatment according to the invention with workpieces kept at floating potential to go directly from the pretreatment phase into the treatment phase. Surprisingly, ion bombardment of the workpieces, as is otherwise customary, is not necessary even for rustproof austenitic steels of the types AISI316L. The system costs are thus reduced by the costs of an additional generator in order to put the workpieces on potential.

Furthermore, in the procedure according to the invention with workpieces kept at floating potential, no additional systems are required in order to prevent sparks from forming on the workpieces.

Compared to previously known procedures in which the work 13

pieces are positioned on the discharge cathode, the following advantages are further achieved according to the invention:

The discharge operating sources are considerably less expensive than those necessary for independent discharges, whether the latter are pulsed or not.

According to the invention, the reactive gas consumption is considerably lower, due to lower working pressures. Furthermore, according to the invention, there are no interference problems on the workpieces. The kinetics, i.e. the treatment effect resulting per unit of treatment time is significantly increased, for example by a factor of 2 to 3 for the nitriding of steel during floating operation of the workpieces.

Furthermore, the procedure according to the invention, as has been mentioned, changes the workpiece surface only insignificantly.

The procedure according to the invention makes it possible to implement novel coatings according to the invention, such as single-phase iron e-nitrate, Fe ₂ -3 N6, essentially free of carbon. This type of layer has an increased corrosion resistance, generally compared to layers which can be produced under substantial ion bombardment. The resistance to salt spray according to the DIN 50021 standard of steel 35CD4, treated in an arrangement with ion bombardment, is only 5 hours, while the same steel, treated according to the invention in a floating manner, resists 24 hours.

With preferred provision of the heating arrangement, as shown at 29, and thus the decoupling of the temperature setting of the workpieces from the generation of reactive species, a further - 14

substantial improvement of the process management enables.

3 summarizes various options for operating the workpiece carrier 39 as well as one or more ring anodes 21b and / or the cylinder anode 21a with respect to the cathode 7, analogous to the explanations for FIG. 2, the options again using selection units 42 or 44 are shown graphically.

Examples of the procedure according to the invention based on nitriding:

In the following examples according to the invention, the treatment was carried out with a system according to FIG. 3, the workpieces were operated at floating potential.

After the workpieces have been introduced into the vacuum recipient, the latter is evacuated to 0.02 Pa and the workpieces are heated to the necessary treatment temperature with the aid of the heating system 29 shown in FIG. 3. After a few minutes, argon is let in at a pressure of 0.3 Pa and the hot cathode 7 is heated. The discharge is then created between the hot cathode 7 and the anode 21a. The discharge current is 200A. Hydrogen is now let into the recipient with 100ccm, in addition to the argon. The chemical pretreatment, mentioned above, is carried out while the workpieces are being heated.

When the workpieces have reached their TARGET temperature, the hydrogen flow is reduced to 10cc and then nitrogen is introduced until a total pressure of 0.8Pa is obtained. The workpiece nitriding temperature is then regulated with the help of the heating devices 29 as an actuator during the nitriding treatment. The total pressure should be between 0.5 and - 15th

0.8 Pa. The reactive gas mass flow, in this case the nitrogen mass flow, is also regulated.

The results obtained are summarized in the table below. Here are:

35NCD16: aeronautical construction steel, tool steel for mold making,

Z38CDV5 hot work tool steel,

Z160CDV12: cold work tool steel,

Z40C13: martensitic stainless steel,

Z2CND17-13: stainless austenitic steel.

Table:

4 shows the kinetics of the nitriding, realized at a workpiece temperature of 500 ° C., on the steels 35NCD16 and Z38CDV5. The treatment has resulted in the formation of an iron e-nitrate layer. The following Vickers HV 0.1 hardness values result: 16

for 35NCD16 steel: 900HV, for Z38CDV5 steel: 1090HV.

Furthermore, the kinetics or nitriding speed of 35NCD16 steel in the procedure according to the invention, as shown in FIG. 3, and in the case of nitriding by conventional methods, in which the workpieces were exposed to an ion bombardment, were compared. For the comparison example, a system configuration was used, as shown in DE-PS-37 02 984 or in US-A-4 762 756.

5 shows the kinetics in the procedure according to the invention with the course A, after the previously known one with the course B. It confirms that the nitriding kinetics in the procedure according to the invention is higher than that which is achieved in the aforementioned known procedure. This result is accompanied by an increase in the nitrogen concentration in the nitriding layer to 25 to 30at%, while it is only 20 to 22at% in the nitriding layer realized by the previously known procedure.

With the described method according to the invention, in which ion bombardment of the surfaces to be treated is largely prevented, and the correspondingly configured systems, not only is a better, i.e. faster workpiece treatment possible, which is surprising precisely because of the ion bombardment prevention mentioned, and not only a gentle surface treatment is made possible, as a result of which the original microstructure is changed only insignificantly; In addition, a surface treatment is achieved, from which the resulting workpieces bring significant advantages over those treated according to the known procedure.

While workpieces made from a basic material of steel - 17th

35CD4 surface nitrided in a known manner, i.e. under ion bombardment, a mechanical wear resistance of approx. 1400N resulted in mechanical wear resistance above 1500N on the same basic bodies when treated according to the invention under floating potential, in this case in particular even of approximately 2000N.

The bodies treated according to the invention subsequently survive the salt spray test according to DIN 50021 for longer than 10 hours, even longer than 20 hours or 24 hours, compared to bodies treated by conventional treatment methods under ion bombardment, which no longer meet the test mentioned resisted than 5.5 hours.

6a and 6b show the micrographs of the base bodies according to the invention on 35NCD16 steel on the one hand (FIG. 6a) and on Z38CDV5 steel on the other, with a magnification of 500 times each. It can be seen that the nitrogen concentration N in the e-zone is substantially higher than this concentration in the conventional manner, i.e. Diffusion zones generated under ion bombardment, namely higher than 24at%, in particular in a range between 25at% to 30at% (both limits included). In the conventionally implemented diffusion zones, the N concentration is at most 20at% to 22at%.

On the bodies according to the invention, the ratio of the diffusion zone thickness to the e-zone thickness is 2 ^· 10 to 3 ′ IO ^“6 , preferably approx. 2.5 ^· IO ^{“ 6} . The diffusion zone on the 35CD4 workpiece treated according to the invention was 0.2 mm, while the e-zone was approximately 8 μm thick. The formation of a zone which at least predominantly has the Fe ₂ _ ₃ Ne compound is to be regarded as an essential advantage and leads in particular to the increase mentioned with regard to mechanical and chemical wear resistance.

Claims

19 patent claims:

1. Process for the plasma-thermochemical surface treatment of workpieces, in which a gas with a connecting element, namely in particular with at least one of the elements C, N, O, B, Si, S, by at least one low-voltage discharge excited and thermochemically the workpiece surface is changed, characterized in that the workpieces are connected to a positive electrical potential with respect to the cathode of the low-voltage discharge.

2. The method according to claim 1, characterized in that the workpieces are operated at the anode potential of the low voltage discharge or at floating potential (floating).

3. The method according to any one of claims 1 or 2, characterized ge indicates that the low-voltage discharge is operated as an independent discharge or as a dependent discharge, preferably as a discharge with an electron-emitting hot cathode.

4. The method according to any one of claims 1 to 3, characterized ge indicates that the workpieces are arranged substantially centrally with respect to a ring anode and / or cylinder anode for the discharge.

5. The method according to any one of claims 1 to 4, characterized ge indicates that the workpieces are heated by means of a heating device independent of the discharge.

6. System for carrying out the method according to one of claims 1 to 5 with at least one low-voltage discharge path (7, 21) and a workpiece carrier arrangement (21, 21a, 39) in a vacuum recipient (1), which with one 20 -

Gas tank arrangement (17) is connected, which on the one hand contains a noble gas (Ar) and on the other hand gas with at least one of the elements C, N, O, B, Si, S, characterized in that the workpiece carrier arrangement (39, 21a) either electrically isolated from other parts of the system or connected to an electrical source (25, 37) by means of which it can be connected to an electrical potential which is more positive than the electrical potential which can be applied to the discharge cathode (7;

7. Plant according to claim 6, characterized in that the discharge path is an electron-emitting hot cathode

(7, 9).

8. Plant according to one of claims 6 or 7, characterized gekenn¬ characterized in that the workpiece carrier arrangement (23) comprises a cooling medium circuit.

9. System according to one of claims 6 to 8, characterized gekenn¬ characterized in that a heating arrangement (29) is provided in the recipient (1), preferably arranged along essential areas of the recipient wall and coaxial to the axis of the discharge path, which, more preferably, against the exterior of the recipient and / or the interior thereof is provided with a heat shield arrangement (31a / 31i), the latter being switchable, at least partially, preferably as an anode of the discharge.

10. Plant according to one of claims 6 to 9, characterized gekenn¬ characterized in that the discharge anode (21b / 31i) is formed by a ring anode and / or a cylinder anode which, in this respect at a distance, substantially coaxially to the discharge ¬ axis (A) is arranged, for which the workpiece carrier arrangement (39) is preferably centered. 21

11. Plant according to claim 10, characterized in that the anode (2lb / 31i) simultaneously forms at least part of a heat shield arrangement of a heating arrangement (29).

12. Use of the method according to one of claims 1 to 5 or the system according to one of claims 6 to 11 for the surface treatment of structural steel, tool steel, stainless martensitic or austenitic steels or of Ti alloy.

13. Use according to claim 12 for the nitriding.

14. Workpiece with a base body made of structural steel or tool steel with a nitrided surface diffusion zone of the base body, characterized in that in the direction out of the base body the diffusion zone predominantly has a zone comprising Fe ₂ _ ₃ Ne compound.

15. Workpiece according to claim 14, characterized in that the ratio diffusion zone thickness and zone thickness 2 "10 to 3 '10 ^" ' is preferably approximately 2.5 '10.

16. Workpiece according to one of claims 14 or 15, characterized in that the N concentration at the Fe ₂ _ ₃ Ne zone is higher than 24at%, preferably 25at% to 30at%.

17. Workpiece according to one of claims 14 to 16, characterized in that it withstands a salt spray test according to DIN 50021 for longer than 10h, preferably longer than 20h, preferably even 24h.

18. Workpiece according to one of claims 14 to 17, characterized in that the mechanical surface load capacity is over 1500N, preferably about 2000N. - 22

19. Workpiece according to one of claims 16 to 18, characterized in that the surface structure of the nitrided base body is substantially the same as that of the same, but not nitrided base body.