WO1999014393A1

WO1999014393A1 - METHOD FOR COATING SUBSTRATES WITH ALUMINUM OXIDE (Al2O3) AND COATING A WORK PIECE USING SAID METHOD

Info

Publication number: WO1999014393A1
Application number: PCT/EP1998/005909
Authority: WO
Inventors: Thomas LÖHKEN; Holger Lüthje; Cornelia Steinberg; Thomas Jung
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1997-09-16
Filing date: 1998-09-16
Publication date: 1999-03-25
Also published as: EP1015654A1

Abstract

The invention relates to a method for coating substrates with aluminum oxide (Al2O3) with at least a proportional crystalline phase of the aluminum oxide during which the substrates are coated by means of hollow cathode gas flux sputtering at temperatures greater than 400 °C and less than 1,000 °C.

Description

Process for coating substrates with aluminum oxide (Al ₂ O ₃ ) and workpiece coated with it

The invention relates to a method for coating substrates with aluminum oxide (Al ₂ O ₃ ) with at least a proportion of the crystalline phase of the aluminum oxide. The invention further relates to a workpiece coated with this method.

Layers of aluminum oxide are characterized by great hardness, very good electrical insulation and high wear resistance. For these reasons, aluminum oxide coatings are applied for a wide variety of applications.

The so-called "alpha modification" of the aluminum oxide, ie the crystalline phase of the aluminum oxide, offers particular advantages, in particular when coating tools. However, this phase can only be achieved under special conditions. If coating with low temperatures, only one coating is used achieved with amorphous aluminum oxide or with the gamma modification, as is proposed, for example, in DE 196 12 344 C1 for coating plastic films.

The gas phase deposition method, mostly referred to as CVD (chemical vapor deposition) method, has proven particularly suitable and suitable for coating with at least a proportion of crystalline phase of the aluminum oxide. It is widely used and usually works at temperatures above 1,000 ° C, such as Prengel et al., CVD coatings based on medium temperature CVD Gamma- and α-AI ₂ O ₃ , in: Surface and Coatings Techn. 68 / 69 (1994) 217-220. At these high, usually still higher temperatures, this method enables a very good and mature coating of high quality and also cost-effective procedure.

CONFIRMATION OPIÜ Unfortunately, due to the high temperatures, the method cannot be used in many cases, namely when the substrates to be coated are unstable at such temperatures or are already undergoing considerable changes in their structure. A typical example is tool steel, which loses its good properties above certain temperatures, so that a coating with Al ₂ O _{3 is} no longer helpful. Other examples include insulators or glasses, which therefore cannot be coated with this process either.

There have therefore been attempts to run these CVD processes at lower temperatures. For example, EP 0 513 662 B1 proposes the use of chromium as an auxiliary in the deposition, as a result of which a hard layer with (Al, Cr) ₂ O ₃ is to be formed, which are also coated with temperatures of around 900 ° C. by means of the CVD process can. The use of the auxiliary chromium does not, of course, result in an aluminum oxide layer, as is actually desired, and the deposition rates are also quite low.

Von Zywitzki and Hoetzsch, Influence of coating parameters on the structure and properties of AI ₂ O ₃ layers reactively deposited by means of pulsed magnetron sputtering, in: Surface and Coating Technology 86-87 (1996) 640-647 and also by Fietzke, Goedicke and Hempel, The deposition of hard crystalline AI ₂ O ₃ layers by means of bipolar pulsed magnetron sputtering, in: Surface and Coating Technology 86-87 (1996) 657 - 663, it is suggested that a coating by means of pulse sputtering instead of the CVD process ( Pulsed magnetron sputtering (PMS). While this process apparently provides a coating of good quality and at relatively low temperatures, the deposition rate of 5 μm per hour is quite low and the outlay on equipment is extremely high. This makes coating very expensive.

In contrast, the object of the invention is a high-quality coating with aluminum oxide with at least partial α-crystalline phase To provide that can be done at relatively low temperatures and is still economical.

This object is achieved in that the substrates are coated by means of hollow cathode gas sputtering at temperatures of more than 400 ° C. and less than 1,000 ° C.

Hollow cathode gas flow sputtering is a well-known and proven method for high-rate coating with amorphous layers at low substrate temperatures. However, the resulting layers, which are known from the prior art, are all X-ray amorphous and, with a hardness of 1,400, measured by the Knoop method at a load of 0.025 pond, are significantly softer than the layers of crystalline α- Alumina.

Since these properties of hollow cathode gas flow sputtering are known and the previous approaches, which actually also require the use of very complicated AC voltages with certain pulse peaks and waveforms for pulse sputtering, the use of hollow cathode gas flow sputtering, which works by means of direct voltage, would never have been considered by the person skilled in the art .

Completely surprisingly, however, it turned out that, when using hollow cathode gas flow sputtering in the temperature range between 400 ° and 1000 ° C., it is, contrary to expectations, possible to deposit aluminum oxide layers which are at least partly α-crystalline, this proportion being very high.

Hollow cathode gas flow sputtering is significantly less expensive than pulse sputtering, which creates a much more economical coating option. According to the invention, the hollow cathode gas flow sputtering preferably takes place at temperatures below 900 ° C., particularly preferably below 600 ° C. This means that tool steels, insulators and glasses can be coated economically and effectively with the hollow cathode gas flow sputter. The deposition rates found in tests could be achieved up to over 50 μm / h, sometimes up to 100 μm / h.

The quality of the coating obtained is such that dielectrics, but also wear protection layers and friction-reducing layers can be achieved.

The coating process is preferably carried out at pressures between 0.1 and 10 hPa. It is particularly preferred if just enough oxygen (O ₂ ) is added that the target of the hollow cathode is not yet oxidized. This oxygen number can be set properly, since the beginning oxidation of the target can immediately be measured in a change in the target properties. This setting enables stoichiometric Al ₂ O ₃ layers to be generated.

When coating by means of pulse sputtering, the targets present there are driven reactively, that is by means of alternating current. The oxide formation takes place there on the target, and the oxidized aluminum oxide particles are transferred to the substrate as a whole. In the case of hollow cathode gas flow sputtering, however, according to the invention the oxide formation only takes place on the substrate itself.

The oxygen content is preferably approximately 0.5 to 2%, in particular approximately 1% of the argon content. Instead of argon, another noble gas can also be used, the proportions then being able to be distributed differently.

It is also particularly interesting that graded layer systems according to the invention can be achieved, that is to say those in which the layer properties differ within the aluminum oxide. In this way, a somewhat softer and more elastic layer can be applied, which then changes into a harder aluminum oxide layer during further coating. This can be achieved simply by adjusting and varying the amount of O ₂ supplied. If necessary, aluminum oxide with γ phase would form and form parts of the layer.

Distances between the hollow cathode and the substrate between 1 cm and 10 cm have proven effective.

In particular, it is preferred if small substrates or substrates are coated with tips. Here, the properties of the hollow cathode gas flow sputtering have a particularly favorable effect, since the gas stream flowing onto the substrate reaches the tips or small components unhindered, without backflow forming on a flat area, which could hinder the deposition process.

It is also possible to embed certain sensory layers or one or more sensors during the coating, this also being able to be produced by varying the reactive gas flow.

In order to further increase the economic viability of the method, it can be provided in a preferred embodiment that the substrates are moved in front of the hollow cathode. As a result, even larger substrates or a plurality of substrates can be coated during a single coating cycle.

An alternative possibility, or one that can also be used simultaneously, is to provide two or more parallel gas flow hollow cathode sputtering sources in the same space, which also facilitates the coating of the layer systems with their gradations.

It has also proven to be an advantage if the substrates are connected to a voltage source. Here, in particular, a DC voltage source with a positive bias voltage and a potential between 10 volts and 1,000 volts is preferred. In the following an embodiment of the invention will be described with reference to the drawing. Show it:

Figure 1 is a schematic representation of a hollow cathode arrangement for performing the method;

Figure 2 is a schematic representation of a coated substrate.

The method according to the invention is shown schematically in FIG. A substrate 10 is a flat disk here. However, other bodies can also be involved, in particular tips and / or cutting edges which are to be coated are suitable.

The substrate is brought from the rear to a desired temperature via a substrate heater 12, here for example 600 ° C.

The hollow cathode arrangement 20 can be seen on the left side of FIG. It has two opposing targets 21. These targets are each plate-shaped, the plates being perpendicular to the image plane. A direct voltage of approximately 300 to 1,500 volts is applied between them. The entire arrangement, including the substrate 10, is in a spatial environment, not shown, in which there is a working pressure of approximately 0.05 to 5 hPa.

A noble gas stream, preferably argon stream, 22 is fed in on the left and flows between the two targets 21 to the right at a relatively high speed.

The superimposition of the two negative glow lights of the two targets 21 and the electrons oscillating back and forth between the two targets 21 lead to an efficient energy transfer, which is reflected in the argon Plasma 22 forming current 22 and to a high plasma density (hollow cathode effect).

After leaving the hollow cathode arrangement 20 at the end of the two plates of the targets 21, the plasma density decreases with increasing distance and, at preferred substrate positions at a distance of approximately 1 cm to 10 cm, is approximately 10 ¹⁸ to 10 ¹⁹ m ^"3. This plasma density is approximately two orders of magnitude The resulting intensive ion bombardment of the growing layer is to a large extent responsible for the good layer quality which can be achieved with this process even without applying a substrate bias Production of hard Al ₂ O ₃ layers is that, in contrast to known magnitron sputtering processes, this process enables a high intensity of ionized species on the substrate, but the energy of the particles is very low and is only a few electron volts, which leads to the desired ion-induced Growth with minor growth disturbances in the layer that forms.

The material sputtered in the hollow cathode 20 is transported to the substrate 10 by the flow of the working gas, ie the argon flow 22. Diffusion overlaps this transport process. Due to the relatively high process pressure, neither a high vacuum pump nor the corresponding peripherals are required as a rule. This significantly reduces the system costs compared to other sputtering processes. A combination of rotary vane and root pump can be used as the pumping station (not shown).

Oxygen O _{2 is} supplied as reactive gas 25 in the desired deposition of aluminum oxide. The reactive gas is supplied outside the hollow cathode arrangement 20 in the process gas stream which is already widening there. If appropriate, the reactive gas can also be nitrogen or Contain carbon to separate admixtures of nitrides or carbides.

Diffusion of the reactive gas 25 between the targets 21 is prevented by the high flow velocity of the argon flow 22 as process gas. This avoids target poisoning that is known from other sputtering methods.

Sufficient reactive gas 25 is preferably used that no oxidation of the targets 21 takes place, not even at the edge regions 27. This can easily be determined by measuring methods (not shown) on the targets 21.

The process parameters can moreover be chosen largely independently of the data of the hollow cathode arrangement 20. A displacement effect is also achieved on the substrate 10 by the intensive process gas stream (argon stream 22), which makes it possible to produce very pure layers even at relatively high residual gas pressures.

During the conditioning of the source, that is to say the hollow cathode arrangement 20, the deposition of layer material can be prevented by means of a shutter 26.

The substrate heater 12 can of course also achieve other temperatures of, for example, 400 ° C. to 1,000 ° C. for the substrates 10.

The parameters for the detailed adjustment of the properties of the deposited aluminum oxide layer are the target power, the intensity of the process gas flow, ie the argon flow 22, the amount of the supplied reactive gas 25, ie the reactive gas flow, the temperature of the substrate 10, the exact position of the Substrate 10 relative to the targets 21 of the hollow cathode arrangement, the total pressure and the process control. In addition (not shown), a bias voltage can be applied between the substrate 10 or the holder of the substrate 10 and the hollow cathode arrangement as a source. This would lead to an expansion of the plasma zone from the source to the substrate 10.

In one exemplary embodiment, indexable inserts made of hard metal P 20 of the type SNMA 120408 have been coated with aluminum oxide. The indexable inserts were subjected to tenside cleaning and installed in the hollow cathode gas flow sputtering system. By means of the built-in substrate heater 12, the plates as substrate 10 were heated to 800 ° C. After reaching the substrate temperature, the gas flow sputtering source, which was coated with aluminum targets 21, was switched on and first operated with argon for five minutes. Thereafter, oxygen was introduced as the reactive gas 25 in the vicinity of the holder of the substrate 10 and the shutter 26 was opened after about three minutes. The coating takes place under constant conditions without substrate bias for 20 minutes. After this time, the shutter 26 was closed and the discharge and the supply of gases and the heating were switched off. The substrates 10 were removed at a temperature of 100 ° C. The layers were 5 μm thick and had a crystalline structure.

FIG. 2 shows a corresponding representation of the coated substrates 10 in which the alpha phase was determined by X-ray diffraction. There is a layer 11 on the substrate 10.

The layers which were produced using the method according to the invention were distinguished, inter alia, by the fact that the hardness was more than 1,800, even more than 2,000, measured by the Knoop method at a load of 0.025 pond. In an adhesion test using the scratch method, it was found that the critical load at which layer 11 failed for the first time was greater than 80 N. The structure was crystalline, the alpha phase was present, and layers 11 showed very good cutting behavior machining in the turning process of CK 45 steel. Reference list

Substrate

layer

heater

Hollow cathode arrangement

Targets

Argon flow

plasma

Reactive gas

Shutter

Edge area

Claims

claims

1 . Process for coating substrates with aluminum oxide (Al ₂ O ₃ ) with at least a proportion of the crystalline phase of the aluminum oxide, characterized in that the substrates are coated by means of hollow cathode gas flow sputtering at temperatures of more than 400 ° C and less than 1,000 ° C.

2. The method according to claim 1, characterized in that the temperatures are less than 900 ° C.

3. The method according to claim 2, characterized in that the temperatures are less than 600 ° C.

4. The method according to any one of the preceding claims, characterized in that as much oxygen (0 ₂ ) is supplied during the coating process that just now no oxidation of the target of the hollow cathode occurs.

5. The method according to any one of the preceding claims, characterized in that so much oxygen (0 ₂ ) is added that the proportion is between 0.5 and 2% of the argon proportion.

6. The method according to any one of the preceding claims, characterized in that the oxygen content is varied during the coating in order to achieve graded layer systems.

7. The method according to any one of the preceding claims, characterized in that pressures between 0.1 and 10 hPa are used.

8. The method according to any one of the preceding claims, characterized in that the distance between the hollow cathode and substrate is between 1 cm and 10 cm.

9. The method according to any one of the preceding claims, characterized in that small-sized or tipped substrates are coated.

10. The method according to any one of the preceding claims, characterized in that the substrates are moved relative to the hollow cathodes during the coating process for coating larger or more substrates.

1 1. The method according to any one of the preceding claims, characterized in that two or more hollow cathodes are provided within the same coating space, so that layer systems or several substrates can be applied simultaneously in one cycle.

12. The method according to any one of the preceding claims, characterized in that the substrate is connected to a voltage source.

13. The method according to claim 12, characterized in that the voltage source is a DC voltage source or a bipolar pulse or a medium or high frequency source.

14. The method according to claim 13, characterized in that a negative bias is applied to the substrate.

15. The method according to claim 14, characterized in that the potential of the negative bias is between 10 volts and 1,200 volts.

16. Tool or workpiece, characterized in that it is completely or partially coated with a method according to any one of the preceding claims.