WO1992014859A1

WO1992014859A1 - Process and device for reducing droplets during coating of surfaces with hard substances by a pvd process

Info

Publication number: WO1992014859A1
Application number: PCT/EP1992/000348
Authority: WO
Inventors: Hans-Jürgen LOHMANN
Original assignee: Eifeler Werkzeuge Gmbh
Priority date: 1991-02-19
Filing date: 1992-02-19
Publication date: 1992-09-03

Abstract

A process is disclosed for coating objects with hard materials according to a PVD (physical vapour deposition) process, in which the object to be coated is pre-heated. According to this process, the object and/or its surface are heated by exposure to radiation before the surface is coated. In a preferred embodiment of the process, a heating element is used as radiation source, at a high partial pressure of the reaction gas, preferably in the 10-3 to 1 mbar range. A device that can be used as a radiation source to implement the process is formed by a heating element (1) provided with an electrically heatable resistance wire axially arranged in the hollow cylinder. Between the heating element (1) and the inner wall (10) of the reaction chamber (11) a reflector surface (12) is preferably located. The objects (16) obtained by this process are distinguished by improved mechanical properties, as their surfaces coated with hard substances present only, if at all, a small quantity of so-called droplets.

Description

Method and device for reducing droplets in the coating of surfaces with hard materials according to the PVD method

The present invention relates to a method according to the preamble of claim 1 and a device according to the preamble of claim 9, as well as objects obtainable by coating with hard materials according to claim 13.

Objects of all kinds, primarily tools and machine components, are provided with hard material coatings to extend their service life and protect them against stress. The so-called CVD (chemical vapor deposition) process has long been known for coating tools with hard and wear-resistant layers, such as titanium nitride and titanium carbide. The disadvantage of this is the relatively high temperatures, as a result of which structural changes can occur in the tools to be treated. Accordingly, complex post-treatment processes are sometimes necessary.

DD-PS 236,950 describes an improved CVD process. A glow discharge is generated between the reactor wall and the object to be coated, the surface of the object to be coated being first provided with a nitride layer within preferably 60 minutes. The titanium coating then takes place in accordance with the CVD method.

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DE-OS 35 10 608 describes a corrosion-resistant and high-temperature-resistant, lubrication-free, low-friction, sliding system particularly suitable for shut-off devices. The surfaces of this sliding system are provided with titanium nitride and / or titanium carbide, which is coated by the CVD process.

DE-OS 36 23 360 relates to devices between which high pressures and / or high frictional loads occur. The surfaces of these systems are coated by high-frequency sputtering.

The PVD (physical vapor deposition) process avoids the use of these high temperatures and operates in the range from temperatures up to 500 ^° C. Various systems are known as coating systems based on the PVD process, which are essentially characterized by distinguish the type of evaporation sources of the material provided for the coating. What the different process variants have in common is that they take place in a high vacuum. Metal nitrides and carbides and mixtures of nitrides and carbides are known as layers. The selection of the corresponding hard materials for coating the materials depends on the application area of these tools. For example, titanium nitride, titanium carbide and titanium nitride / carbide surfaces are characterized by wear resistance and good friction properties. The titanium nitride or titanium carbide or titanium nitride / carbide (CiNC) coatings are carried out by evaporating the metal in the presence of nitrogenous or carbon-containing gases. The use of ultrapure nitrogen, ultrapure methane or ethylene or acetylene or mixtures thereof have been proven in practice.

DD-PS 241 273 describes a process for coating cast iron in accordance with the PVD process. The The object underlying this patent specification is to provide a technology which leads to a reduction in the graphite content on the component surface and thus enables a pore-free coating of cast iron with titanium nitride. This is achieved by smooth rolling the components before coating.

DE-OS 37 14 327 relates to a metal body for ink transfer and metering in printing units, the surface of which is provided with titanium nitride in a layer thickness between 1 and 8 μm. The body is for the purpose of production to a temperature up to 500 "C, preferred wise between 380 C and 450 ^β ^β C preheated, is evaporated after which the titanium nitride.

The evaporation in a high vacuum on the surface to be coated takes place in a vacuum chamber which must have an outlet to the vacuum pump and an inlet for the reaction gas. The objects to be coated are arranged in a holder which is usually rotated during the exposure process in order to facilitate a uniform coating. The metal is converted into the vapor phase by different physical processes. For example, the process of cathode sputtering, the process of so-called ion plating (ionized reactive evaporation) and the evaporation of the metal by means of arc discharge have been introduced in technology. The arc discharge method has proven to be advantageous in practice, since it is possible to work with several arc evaporators which can be arranged in the coating chamber in any spatial position. In contrast to the other methods, this method works by avoiding a liquid metal phase. The coating material is applied as a cathode (target) to the negative pole of a voltage source. By contacting the cathode surface with an earthed trigger, an arc (high currents, low voltage) is generated which then burns between the cathode (target) and the anode (chamber wall). The arc attacks the cathode at the cathode spot, diameter 10 to 100 μm. This results in extremely rapid heating of microscopic tips of the target surface, which then evaporates as a dense metal plasma with a sufficient number of ions and electrons for an independent discharge. Negative substrate bias (bias) accelerates the titanium ions towards the substrate. Highly ionized and high-energy titanium particles from the plasma condense on the substrate surface after the reaction with nitrogen let into the chamber. The disadvantage of this preferred procedure is the formation of so-called droplets and uncharged titanium clusters. The droplets form discrete areas on the surface of the tool to be coated, which do not combine with the remaining homogeneous coating with sufficient strength. These discrete parts are thus imperfections in the coating, which can also protrude beyond the coating. When subjected to mechanical loads, these so-called droplets can be detached from the coating and thus form pores in the homogeneous coating. The pores are now points of attack for secondary processes, which ultimately accelerate the wear of the tools by destroying the hard material layer.

Another disadvantage of this prior art method resides in the fact that the workpiece to be coated is bombarded with the metal ions or metal atoms at high bias voltage, about 1,000 volts, is heated. Depending on the production target, this takes 10 to 60 minutes. As a result of this rather unspecific heating, by bumping the metal parts onto the tool surface, overheating occurs at sharp-edged transitions, which in turn leads to a non-homogeneous coating when the metal vapor is subsequently deposited and deposited.

The technical problem on which the invention is based consists, on the one hand, of providing a coating process which largely avoids the formation of droplets and ensures uniform heating of the tool to be coated, and on the other hand consists in making objects provided with hard material layers whose surface is characterized by an extremely small number of droplets.

According to the invention, this object is achieved in accordance with the characterizing part of patent claim 1. Subclaims 2 to 9 relate to preferred embodiments of the method according to the invention. A preferred device for carrying out the method according to the invention is claimed in claims 10 to 13. Claims 14 and 15 relate to the objects obtainable by the method according to the invention.

The process measure, the surface and the object to be coated with hard material, before the actual coating process with radiation to the temperature required for the coating, ensure that no overheating occurs at sharp-edged transitions of the object . Rather, the radiation uniformly reaches every contour of the surface to be coated. According to the prior art, this is accelerated in the electric field There are no metal ions which transfer their energy to the tool and thus heat up through impact and other heat transfers. Correspondingly high field densities occur at the tips and edges, so that more energy is transferred there accordingly and the tips and edges are heated up correspondingly more strongly.

The following method steps are preferably carried out in the method according to the invention: a) evacuation of the vacuum chamber charged with the surface to be coated, b) heating of the object to be coated by means of thermal radiation, c) treatment of the surface of the object with an ionized noble gas or mixed, d) switching off the radiation source, e) briefly bombarding the surface of the object to be coated with titanium atoms or ions with a relatively high bias voltage, f) reaction gas entry, with a relatively low bias voltage and treatment of the surface to be coated of the item for a longer period of time.

After the reaction chamber has been evacuated to <10 torr in step a), the surface of the object to be treated is preferably treated with radiation for a period of 45 to 120 minutes, depending on the size of the object to be coated. As soon as a temperature of 300 to 500 ° C. has been reached, a mixture of an inert gas, preferably argon, and hydrogen is admitted into the chamber in order to free the surface of oxides Volts applied and the item with titanium Ion bombarded. This process preferably takes 60 to 90 seconds. This also prepares the surface for later coating. The bias voltage is then reduced to 150 to 500 volts, preferably 150 to 400 volts, and the actual coating of the surface to be coated begins in the presence of the corresponding reaction gas. Depending on the desired surface coating, nitrogen, methane, ethylene or acethylene are used as the pure gases or mixtures thereof as the reaction gases.

In order to suppress the so-called "droplet" formation, a procedure has proven to be advantageous, the partial pressure of the reaction gas being in the range from 10 -3 to 1 mbar, particularly preferably around 10 -2 mbar.

The method according to the invention is preferably suitable for the arc vapor ion deposition method.

Radiation heating, which is arranged on the inner wall of the coating chamber, has proven to be suitable as the radiation source for heating the surface and the object in the reaction chamber.

The device for heating an object in a vacuum chamber, the surface of which is coated by means of the PVD method, has a heating element which is arranged on the inner wall of the coating chamber. The device is electrically heated and has a hollow cylinder with an axial resistance heating wire. This is designed as a PTC thermistor in the area of the electrical supply line. The lumen of the hollow cylinder forming the heating element is preferably filled with a heat-resistant filler. Around To ensure uniform heat radiation, the heating element is either plate-shaped or in the form of heating coils. The area that contains the container to be vaporized with the arc is preferably left out.

In order to avoid heating the inner wall of the reaction chamber, suitable reflectors can be arranged between the heating element and the inner wall of the reaction chamber. Metal foils are particularly suitable. In a preferred embodiment, the reflector surfaces have a curvature parallel to the curvature of the inner wall. It is also advantageous to design this curvature in such a way that the tool to be coated can be focused.

FIG. 1 shows a preferred embodiment of the device according to the invention, which is shown in cross section in FIG. 2 along the line A ... A * of FIG.

FIG. 3 schematically shows a longitudinal section through a heating element of the heating device, which appear in cross section in FIG. 1.

FIGS. 4 to 6 are scanning electron microscope images of surfaces coated with hard materials. FIGS. 4 and 5 show surfaces produced by the method according to the invention, while FIG. 6 shows a surface produced according to the prior art.

FIGS. 1 and 2 show a preferred embodiment of a device according to the invention which can be used in the method according to the invention. The heating element 1, for example a heating conductor, which is arranged on the inner wall 10 of the double-walled water-cooled coating chamber 11 (recipient), consists of arcuate heating coils 5. A reflector surface 12 is arranged between the inner wall 10 and the heating element 1 , the radius of curvature of which runs parallel to the radius of curvature of the semi-circular vacuum chamber 11. In the area of the cathode 14, which carries the metal 15 used for coating (evaporator with evaporator power supply 20), the reflector surface 12 is recessed (FIG. 2). The reflector surface 12 is preferably made of metal, which on the one hand reinforces the heating of the substrate 16 by reflection of the thermal radiation incident on it and on the other hand reduces heating of the wall of the vacuum chamber 11, usually the substrate 16 rotates during the coating process about its own axis and additionally on a rotatable substrate holder 17 which is connected to the substrate voltage supply 19 via the rotary feedthrough 18. The substrate holder 17 preferably has a planetary gear. The recipient is evacuated through the passage 21 leading to the vacuum pumping station, while the process gas enters the inlet 22. The cooling water enters the cavity 24 through the inlet 23 in order to leave the cavity again at the outlet 25.

FIG. 3 shows a cross section through the heating element of the heating element 1. It consists of a hollow cylinder 2, the outer jacket 6 of which is preferably made of Inconel. A resistance heating wire (heating conductor 3) is arranged axially therein. The supply area at the cold end is preferably formed by a PTC thermistor 7. PTC thermistor 7 and heating element 3 are preferably connected to one another at position 8 by welding. The wall of the - 10 -

Hollow cylinder 2 is expanded in the area of the weld. The lumen of the hollow cylinder 2 is preferably filled with a heat-resistant filler 4. Compacted aluminum oxide powder is preferably used for the insulation between the outer jacket and the heating conductor and the cold end.

The heating element is preferably supplied with power by a vacuum-tight feedthrough 26 at the respective cold end 27 of the heating element 1.

FIGS. 4 and 5 each show surfaces that can be obtained by using the method according to the invention. The almost completely smooth surface is clearly recognizable. Only in FIG. 5 is a droplet visible, which is located on the edge of the coating. The white bar appearing on the left edge of the image corresponds to a size of 2 μm. FIG. 6, on the other hand, shows a surface as can be obtained by the method according to the prior art. It can be seen that there is in principle no homogeneous coating at all, but rather the surfaces produced in this way are composed of a large number of droplets, which results in an enormous increase in surface area. The droplets are moreover responsible for the poorer properties of the surfaces compared to the surfaces according to the invention, which are obtainable according to the prior art. The white bar appearing at the lower left edge of the image corresponds to a distance of 5 μm.

Due to the relatively rough surface, as shown in Figure 6, the surface of the so appears coated objects matt. In contrast, the surface of the objects claimed according to the invention is shiny.

The method according to the invention, preferably carried out in a device according to the invention, makes it possible to produce objects coated with hard materials which are distinguished by an extremely small number of droplets. The present invention thus also relates to objects with a hard material layer obtainable by the process according to the invention.

Claims

5- 12 -PATENT CLAIMS:

1. Method for coating objects with hard materials according to the PVD (physical vapor deposition) method with preheated object to be coated, characterized in that the heating of the object and / or its surface by means of radiation prior to coating the surface he follows.

2. The method of claim 1, wherein the following procedural steps are carried out: a) evacuation of the vacuum chamber charged with the surface to be coated, b) heating of the object to be coated by means of thermal radiation, c) treatment of the surface of the object with an ionized noble gas or mixture, d) switching off the radiation source, e) briefly bombarding the surface of the object to be coated with titanium atoms or ions at a relatively high bias voltage, f) reaction gas entry, at a relatively low bias voltage and Treatment of the surface of the object to be coated for a longer period. - 13 -

3. The method according to claim 1 and / or 2, characterized gekenn¬ characterized in that the vacuum in step a) is <10 -5 Torr, the heating of the surface of the object to be coated in step b) to a temperature of 300 to 500.degree. C. is carried out and / or argon is used as the noble gas in step c) and / or the bias voltage in the titanium bombardment in step e) is 900 to 1,500 volts and / or the titanium bombardment in step e) for a period of time from 60 to 90 seconds and / or the bias voltage is reduced to 150 to 500 volts in step f) and / or in step f) the treatment of the object to be coated in the presence of the reaction gas over a period of 30 to 90 Minutes.

4. The method according to at least one of claims 1 to 3, wherein the reaction gas is nitrogen, methane, ethylene, acetylene, oxygen or oxygen-releasing gases or a mixture thereof.

5. The method of claim 4, wherein the partial pressure of

—3

Reaction gas is in the range of 10 to 1 mbar.

6. The method of claim 5, wherein the partial pressure

—2 is about 10 mbar.

7. The method according to at least one of claims 1 to 6, wherein the metal provided for coating is evaporated by means of an arc.

8. The method according to at least one of claims 1 to 7, wherein the object to be coated is heated by a radiant heater which is arranged on the inner wall of the coating chamber.

9. The method according to any one of claims 1 to 8, wherein heating elements such as resistance heaters, microwaves and / or induction heaters are used to generate the radiation for heating the object.

10. A device for heating an object located in a vacuum chamber, the surface of which is coated by means of the PVD method, with a heating element, characterized in that the heating element (1) on the inner wall (10) of the coating chamber (11th ) is arranged.

11. The device according to claim 10, characterized in that the heating element (1) is electrically heated, has a hollow cylinder (2) with an axial resistance heating wire (3) which is formed in the region of the supply line as a PTC thermistor, and the rest ¬ lumen of the heating element (1) forming the hollow cylinder (2) is filled with a heat-resistant filler (4).

12. The apparatus of claim 10 and / or 11 with a reflector surface (12) arranged between the inner wall (10) and the heating device (11).

13. The apparatus of claim 12, wherein the reflector surface (12) has a curvature parallel to the curvature of the inner wall (10).

14. Objects with hard material coating made of TiN and / or TiNC and / or TiC obtainable by coating according to a method according to at least one of claims 1 to 9.

15. Articles according to claim 14, which have a glossy surface.