WO2001068790A2

WO2001068790A2 - Low-friction protective layers that reduce wear and tear and a method for depositing same

Info

Publication number: WO2001068790A2
Application number: PCT/DE2001/001094
Authority: WO
Inventors: Andreas Mucha; Bernd Rother
Original assignee: Mat Gmbh Dresden
Priority date: 2000-03-16
Filing date: 2001-03-16
Publication date: 2001-09-20
Also published as: AU5611801A; WO2001068790A3

Abstract

The invention relates to the field of materials technology and to protective layers, such as they are used on valve insets or deforming tools. The invention also relates to a method for depositing the same. The aim of the invention is to provide protective layers that are applied to different base materials in a relatively simple, essentially stressfree and adhesive manner. The aim of the invention is also to provide the depositing method therefor. According to the inventive method, the protective layers are deposited by means of solid matter sputtering of a graphite target and/or a target containing a graphite. Carbon monoxide and/or carbon dioxide, acting as a working gas or as a component of the working gas, is/are supplied to the target-related gas discharge and/or the partially identical layer-production-related gas discharge before and/or during the deposit. The aim of the invention is also obtained by protective layers that contain more than 95 atom % of carbon, are provided with a mass density ranging from 1.6 to 2.0 g/cm3 and plastic hardness values of 5 to 30 GPa or E module values of 50 to 200 GPa.

Description

Low-friction and wear-reducing protective layers and processes for their deposition

Technical field

The invention relates to the fields of materials technology, chemistry and mechanical engineering and relates to low-friction and wear-reducing protective layers, such as those e.g. on tribologically and / or corrosively stressed components, such as on valve inserts, on ball bearings or on forming tools and on medical implants, and a method for their deposition.

State of the art

Low-friction and wear-reducing protective layers are known ways of improving the service life and / or the performance of tribological pairings. Layers made of carbon or with a high proportion of carbon are of particular importance here, as a rule low friction coefficients in the range of 0.1 can be combined with low wear. Very early indications of such friction and wear-reducing properties of carbon layers deposited from gas discharges in hydrocarbons were given by Schmellenmeier, H., Experimentelle Technik der Physik 1 (1953) 49.

Known layer systems and suitable deposition processes have been described several times (Reinke, S., Kulisch, W., Surf. Coat. Technol. 97 (1997) 23). The following carbon-based layer systems are then specified as low-friction and wear-reducing protective layers:

1. amorphous carbon with hydrogen-saturated internal bonds (a-C.H),

2. predominantly tetragonally cross-linked amorphous carbon with hydrogen-saturated internal bonds (ta-CH), 3. amorphous carbon with hydrogen-saturated internal bonds, as well as with metal inclusions (Me-C: H),

4. amorphous carbon (a-C),

5. predominantly tetragonal cross-linked amorphous carbon (ta-C),

6. Carbon with polycrystalline diamond structure (PD).

The layers are deposited with the exception of the last system with ion support. In contrast, polycrystalline diamond can also be deposited without ion support, but then at temperatures in the range of 800 to 1000 ° C. In contrast, the ion-supported layer deposition enables the deposition of low-friction and low-wear layers even at temperatures below 200 ° C. The ion support serves to generate a primarily mechanically stable carbon crosslinking with the highest possible spatial degree of crosslinking with the highest possible proportion of the tetragonal sp ³ bonds. Both layer-forming or layer-modifying ions can be used to support the layer growth.

The following are known as coating processes:

1. solid atomization (magnetron sputtering) of a graphite target with simultaneous ion bombardment of the substrates, Ar, hydrocarbons, Ar / H ₂ and mixtures of these gases being known as working gases (Surf. Coat. Technol. 97 (1997) 45),

2. Deposition from the plasma of a hydrocarbon atmosphere (Thin Solid Films 40 (1977) L31),

3. the solid atomization of metal targets, hydrocarbons and mixtures of hydrocarbons and inert gases being used as the working gas (Appl. Phys. Lett. 50 (1987) 1056),

4. the deposition by means of mass-separated carbon ion beams,

5. the deposition from the plasma of the cathodic arc evaporation of a graphite cathode (J. Mater. Res. 6 (1991) 101).

It is common to all the methods mentioned that the layer growth and thus the formation of a friction and wear-reducing protective layer must be modified by means of partially high-energy ions and that measures for Charge carrier compensation on the surface of the electrically insulating layer that is formed are required. The latter is usually achieved with a high-frequency substrate voltage.

A general aim of the methods for the deposition of hard and wear-resistant carbon layers is the generation of the highest possible proportion of tetrahedral carbon bonds (diamond bond) in the layers. A simple indication of these proportions is the mass density of the layers. The mass density of diamond with 3.51 g / cm ³ and that of graphite with 2.25 g / cm ³ apply as limit values. Density values in the range of 2.8 g / cm ³ are given for pure carbon layers with high proportions of tetrahedral bonds (J. Vac. Sei. Technol. A12 (1994) 1457-1462). In the case of hydrogen-containing carbon layers, the experimentally determined densities are shifted according to lower values, values of up to 1.35 g / cm ^{3 being} given (Rother, B and Vetter, J .: plasma coating process and hard material layers, Deutscher Verlag für Grundstoffindustrie, Leipzig 1992, p. 105) It is generally known that the wear resistance of pure carbon layers with low internal stresses should correlate with the highest possible proportion of tetrahedral bonds.

All known protective layers deposited using the above-mentioned processes have in common, according to various authors, that they are subject to considerable internal residual stresses and, at the same time, a permanent connection, particularly to metallic base materials, is problematic. Intermediate layers are therefore usually required to connect the protective layer to the base material (Surf. Coat. Technol. 98 (1998) 1518). In addition, the layer thickness often has to remain limited to values in the range of 1 μm due to the high internal residual stresses.

Presentation of the invention

The object of the invention is to provide a carbon-based, low-friction and wear-reducing protective layer and a method for its deposition, the protective layer being relative to the prior art can be easily and largely stress-free and adhered to various base materials.

The object is achieved by the invention specified in the claims. Further training is the subject of the subclaims.

The method according to the invention works on the basis of the solid atomization of a graphite target or a graphite-containing target using carbon monoxide and / or carbon dioxide as the working gas or as a component of the working gas. This means that both target-based and working gas-based carbon act as layer formers. By contrast, the incorporation of oxygen is largely suppressed due to the gaseous oxidation products of the carbon. Target-based carbon are essentially neutral carbon atoms in the energetic ground state with only minimal kinetic energy. Working gas-based carbon, on the other hand, is energetically activated in a variety of ways in the plasma of the gas discharge, which for molecular working gases includes the generation of ions, neutral and ionized molecular components and energetically excited atomic and molecular species. Significant effects for the layer growth are additionally achieved by dissociated oxygen portions of the working gas, which is related to the known preferential oxidation of graphitically bound carbon compared to tetragonally bound carbon.

Overall, there is an apparent coupling between the chemical and physical effects of the layer-forming and layer-modifying particles during the deposition, which enables a relatively simple process design for the deposition of the protective layers according to the invention in comparison with the prior art, and at the same time improved layer properties, in particular increased wear resistance and less internal residual stresses, generated. The relatively simple design of the process is based primarily on the fact that there is no need for additional post-acceleration of the ions and for charge carrier compensation on the substrate by means of high-frequency voltages. At the same time, no hydrogen-containing gaseous carbon carriers (hydrocarbons) are necessary to reduce the hydrogen content in the layers, which can only be achieved with these starting materials according to the prior art by increased ion energies and / or substrate temperatures. In the method according to the invention, hydrogen can only get into the layers from the residual gas or from a gas loading of the graphite cathode. The mechanism already mentioned above applies to the storage of oxygen.

The hardness, the modulus of elasticity and the high wear resistance of the layers according to the invention indicate a high proportion of tetrahedral carbon. In contrast, mass densities in the range of 1.8 + 0.2 g / cm ^{3 are} measured on the layers, which is significantly lower than the mass density of graphite. Similar mass densities are known from fullerene layers (Applied Surface Science 86 (1995) p. 595). However, these are characterized by low plastic hardness and modulus of elasticity as the layers deposited by means of the method according to the invention.

The atomic structure of the layers according to the invention can therefore not be assigned to any known carbon structure via indirect comparisons. A typical feature of this new structure of pure carbon layers according to the invention is the low density in connection with plastic hardness values in the range from 5 to 30 GPa, advantageously from 10 to 15 GPa, and elastic modulus values in the range from 50 to 200 GPa, advantageously in the range from 90 to 150 GPa.

A high level of biocompatibility can also be demonstrated for the protective layers according to the invention. In colonization experiments with osteoplasts, values as good as those on polystyrene substrates specially prepared for cell colonization experiments are measured on the protective layers of different thicknesses.

Best way to carry out the invention

The invention is further explained in several exemplary embodiments. example 1

Polished samples of hardened high-speed steel are positioned in a "vacuum apparatus in such a way that they are fixed directly under a downward-facing round magnetron of 16 cm in diameter equipped with a graphite target. The target-sample distance is 10 cm. The samples are on one Fixed substrate holder, which has an electrical contact.

After positioning, the vacuum chamber is evacuated to a base pressure of 5 x 10 ^~ Pa. The samples are then glow cleaned in an argon atmosphere at a pressure of 10 Pa. The base pressure for the coating is then reduced again to 5 x 10 ^{~ 3} Pa. The gas inlet for carbon monoxide is now opened so that a pressure of 5 x 10 ^"2 Pa is established in the vacuum chamber when the main valve is throttled. When the power is coupled to the magnetron at 700 W, the target's operating voltage is 540 V. After a coating time A layer thickness of 3.6 μm is reached within 2 hours and the process is ended.

The protective layers are evaluated by means of friction tests in an oscillation tribometer. which works on the pin-washer principle. In particular, a hard metal ball with a diameter of 6 mm fixed in a receptacle is loaded with a load of 2 N and rubbed over the surface of the layer with a frequency of 3 Hz and an amplitude difference of 2 mm. With these loading conditions, a constant coefficient of friction of 0.15 is measured after a short running-in phase. After 20 x 10 ³ rubbing cycles, the friction track depth is only 30 to 40 nm.Carbon-based protective layers according to the prior art, such as aC: H layers, are already under 10 x 10 ³ rubbing cycles to a depth of more than 100 under identical load conditions nm rubbed.

The hardness and the modulus of elasticity of the 3.6 μm thick layer was determined by means of recording hardness measurements, a Vickers diamond being used as the indenter. From the measurements, a final load of 50 mN and a penetration depth of 0.4 μm for the layers a plastic hardness of 13.6 + 0.4 GPa and an elastic modulus of 98 + 2 GPa were determined.

No proportions of oxygen and hydrogen, quantifiable by means of optical glow discharge spectroscopy, were found in the layers. Layers that were deposited on Si wafers under identical conditions could be classified as free of macroscopic internal stresses with a layer thickness of 2 μm with a resolution limit of 0.1 GPa.

A mass density of 1.8 + 0.2 g / cm ^{3 was} determined for layers with a thickness of 3.6 μm by weighing large flat samples before and after the coating, the coating being carried out under the same conditions, and by measuring the layer thickness. This value was confirmed by means of suspension tests with detached layer particles of layers of different thicknesses and identically deposited in a mixture of methanol and bromoform.

Example 2

Needle-shaped samples made of stainless steel with a diameter of 1 mm and a length of 50 mm are positioned in a vacuum chamber which is equipped with two magnetrons. The magnetrons are equipped with a graphite and a chrome target. The holder for the samples is designed in such a way that multiple layers can be used to apply a uniform layer over the circumference. After identical to Example 1, the conditioning of the sample surface by means of glow discharge argon up to a working pressure of 1 x 10 ^"2 Pa introduced into the coating chamber as well as set a bias voltage of -2 kV to the substrates.

In a first coating step, an approximately 50 nm thick Cr layer is now deposited on the circumference of the needle. For a second coating step, the graphite target is activated and the chrome target is switched off. In the second coating step, an approx. 30 nm thick intermediate layer of carbon is deposited, which is sputtered with argon as the working gas. The substrates are still biased at -2 kV. The third and last coating step is the deposition of the actual wear protection layer. The deposition is carried out by atomizing the graphite target in a CO atmosphere with the same parameter sets as in Example 1. After two hours of coating time, a uniform layer thickness of 1 μm is achieved on the circumference of the needle.

With the described procedure, the layers could be deposited firmly and evenly on the samples. The same layer properties as for the layers deposited in Example 1 could be measured on parallel coated specimens.

Claims

claims

1. A method for the deposition of low-friction and wear-reducing protective layers, in which the protective layers are deposited by means of solid atomization of a graphite target and / or a graphite-containing target, the target-related gas discharge and / or the partially identical layer-formation-related gas discharge before and / or during the deposition as Working gas or as part of the working gas carbon monoxide and / or carbon dioxide is supplied.

2. The method according to claim 1, in which a graphite-containing target further consisting of aluminum and / or silicon or one or more metals from the group of transition metals or mixtures of these elements is used.

3. The method according to claim 1, in which a graphite-containing target with at least 1% by mass of graphite is used.

4. The method of claim 1, wherein carbon monoxide is supplied.

5. The method according to claim 1, in which a gas mixture of at least 10% by volume of carbon monoxide is supplied to the deposition space before and / or during the deposition as working gas.

6. The method of claim 1, wherein the solid atomization is carried out by magnetron sputtering.

7. The method according to claim 1, in which the substrates to be coated are electrically insulated in the deposition space.

8. The method according to claim 1, in which the substrates to be coated are subjected to a negative bias in the deposition space.

9. The method according to claim 8, in which a high-frequency bias is used as the bias.

10. The method according to claim 1, in which an intermediate layer is applied before the application of the protective layer.

11. The method according to claim 10, wherein an adhesion-promoting intermediate layer made of one of the elements of the 4th to 6th subgroup of the periodic table or elements made of silicon with a layer thickness of up to 100 nm and / or a carbon layer sputtered with an inert gas with a Thickness of up to 100 nm is deposited.

12. The method according to claim 10, in which a simultaneous ion bombardment with ion energies of up to 3 keV is carried out during the deposition of an intermediate layer.

13. The method of claim 12, wherein the ion bombardment is implemented with a bias voltage of up to -3 kV applied to the substrate.

14.Low-friction and wear-reducing protective layers, which consist of more than 95 atomic% of carbon, have a mass density in the range from 1.6 to 2.0 g / cm ³ and plastic hardness values in the range from 5 to determined by means of recording hardness measurements with Vickers diamonds Have 30 GPa or modulus of elasticity in the range from 50 to 200 GPa.

15. Protective layers according to claim 14, in which hydrogen and oxygen are each chemically bound to less than 5 atomic%.

16. Protective layers according to claim 14, which have plastic hardness values in the range from 10 to 15 GPa or elastic modulus values in the range from 90 to 150 GPa determined by means of recording hardness measurements with Vickers diamonds. _

17. Protective layers according to claim 14, which have macroscopic residual stresses of <0.1 GPa up to layer thicknesses of 3 μm on metallic or semiconducting substrates.

18. Protective layers according to claim 14, in which hydrogen and oxygen are each chemically bound to less than 5 atomic% in only part of the protective layers or in a multilayer structure in only one or only part of the layers.

19. Protective layers according to claim 14, in which hydrogen and oxygen are each chemically bound to less than 1 atomic%.

20. Protective layers according to claim 14, in which, in the case of a multilayer structure between the substrate and the wear protection layer, an adhesion-promoting intermediate layer composed of one of the elements of the 4th to 6th subgroup of the periodic table of elements or of silicon with a layer thickness of up to 100 nm and / or one a noble gas sputtered carbon layer is deposited with a thickness of up to 100 nm.

21. Use of protective layers according to claim 14 as boundary layers for the contact of technical materials with biological systems.