DE102004054193A1 - Hard coating on any, preferably flexible substrate, comprising at least two morphologically different layers useful in applications involving friction pairs has outer layer comprising hard layer of amorphous, diamond-like carbon (DLC) - Google Patents

Abstract

Hard coating on any, preferably flexible, substrate comprising at least two morphologically different layers where the first layer directly on the substrate surface is a metal, ceramic, oxide or carbide layer and the outer layer is a hard layer of amorphous, diamond-like carbon (DLC). An independent claim is included for a substrate coated with the hard layer.

Description

The The present invention relates to abrasion and high surface pressures stable Hard material coating on flexible substrates. Especially The invention relates to a coating with an abrasion-resistant surface layer based on amorphous carbon and an underlying, against high surface pressure stable thin carbon-free hard layer, which optionally has an adhesion promoter layer with the surface layer may be connected from amorphous carbon.

Such a surface layer based on amorphous carbon (DLC = diamond like carbon) is an amorphous, partially hydrogen-free, partly hydrogen-containing carbon layer with high hardness and modulus of elasticity and a very low coefficient of friction, which allows coated substrates to reduce the coefficient of friction in friction pairings under reduced or to use without lubricant use and thereby to guarantee a high abrasion resistance in itself and the friction partners used. This is true even in cases where the friction partner is uncoated and thus does not itself have Abrasionsverschleißschutz. Such DLC shafts are used in the EP 0 724 023 the FRAUNHOFER SOCIETY described in more detail.

in the In general, a gas-phase DLC layer is formed by PVD (Physical Vapor deposition; physical vapor deposition) or CVD (Chemical Vapor Deposition) on the substrate surface deposited. As with all thin ones Hard coatings, however, made by CVD or PVD there is a risk that the layer will be exposed to high surface pressures in pressed in the base material becomes (egg shell effect), since the layer with a thickness of approx. 1 μm to 5 μm without one corresponding support effect of the substrate material can not absorb mechanical loads. To thin Hard material layers nevertheless for Applications where high surface pressures to be able to use They are therefore usually on a very hard substrate material (Ceramics, hardened Steel) or the substrates are pretreated (Plasma nitriding of steels or titanium alloys) with which the substrate surface is cured.

Of the Invention was therefore based on the object, a coating for any to provide compliant substrates, which the On the one hand, the coated substrate has the high surface hardness typical of DLC high modulus of elasticity, low coefficient of friction and high resistance to abrasive wear and on the other hand is stable against high surface pressures and thus can absorb the mechanical loads occurring.

The Task is solved by that the coating for the compliant substrate is made up of two or three morphologically distinct ones Layers composed, with the first, resting directly on the substrate surface Location a self-supporting hard layer with a thickness> 50 microns and the outermost layer one Layer of DLC is. Optionally, lies between the hard layer and the DLC layer still a primer layer.

When First, the hard coat is applied, e.g. a self-supporting one metallic layer with and without alloying elements or a ceramic one Spray layer can be, which after different, in the state The technique known methods deposited on the substrate surface can be, e.g. by plasma spraying, flame spraying, high-speed flame spraying (HVOF) and laser deposition welding. Metallic and ceramic spray coatings containing plasma spraying, flame spraying, or HVOF are cheap, porous, very thick and rough and require one Post processing.

Applied by laser deposition welding Hard coatings are thick and rough and must also be reworked become. Other possible Hard coatings consist of metal nitrides, in particular titanium nitride and Chromium nitride.

Preferred hard coatings consist of hard chrome or chemical nickel with different phosphorus contents. Most preferred is a hard chromium layer which may be electrodeposited onto the substrate surface by methods known in the art. The substrate to be coated can also be non-conductive in its core, for example a plastic, but it must have a conductive surface for electrodeposition. For this purpose, the substrate surface only after in the Galvano Technique usually degreased and activated (dekapiert). Further pretreatment is usually not required. As the deposition electrolytes, chromium and chromium alloy electrolytes of both the sulfuric acid and the mixed acid type (ie, the mixture of sulfuric acid and hexafluorosilicic acid) can be used.

The Layer thickness of the deposited hard layer should be above 50 μm, there, as the expert knows, Chromium layers from a layer thickness of about 50 microns to even corrosion resistant and become self-supporting, i. no influence on the substrate hardness anymore the surface properties is present. The parameters that govern the surface texture while the galvanic deposition can be set, are preferably the Composition of the electrolyte, the viscosity of the electrolyte, the exposure time the substrate to be coated in the electrolyte and thus the deposition time, the deposition temperature, the cathodic current density in the electrolyte, the pH of the electrolyte, the electrode material and the electrode spacings, the Hydrodynamics of the electrolyte and, if necessary, the degree of rotation of the substrate to be coated and the shape of the current and / or voltage pulses.

A suitable chromium electrolyte may have, for example, the following composition: chromic acid 260 g / l sulfuric acid about 1% of the chromic acid concentration 3-valent chrome 4 g / l

The galvanic deposition can then be carried out at 50 ° C at a current density of 25 A / dm ² over a period of 4 to 5 hours.

When next becomes the surface subjected to the applied first hard layer of a plasma cleaning. The plasma cleaning can be organic layers or contaminants on a nanometer or micrometer scale gentle on substrates Completely remove. At the same time, the workpieces for subsequent process steps, such as. another plasma coating, optimally prepared. In many cases the user can start the plasma cleaning and subsequent activation and compensation steps (e.g., coatings with PVD, CVD)) in one and the same equipment carry out, ventilate without interruption to have to.

at the surface cleaning Low pressure plasmas are used for the gentle treatment of workpieces which allow a treatment temperature of below 100 ° C. The surface cleaning in low-pressure plasma systems takes place at a negative pressure of 0.1 to 2 mbar. Therefor are mostly vacuum containers used in which the workpieces to be cleaned treated batchwise become. The vacuum chamber of appropriate plants is equipped with a process gas filled. Typical gas mixtures contain noble gases, preferably argon or Helium, and in particular oxygen or air, in rare cases also hydrogen and fluorine-containing gases, e.g. Tetrafluoromethane. The mixture depends on the specific requirements of the cleaning process, which depending on the type of contamination, the substrate material and the Subsequent step result. Leave temperature-sensitive plastics Treat yourself as well as metal, glass and ceramics.

One electric field in the form of high-frequency voltages in the kHz, MHz (radio frequency) or GHz range (Micro-bore) transmits energy into the system and accelerates free charge carriers, the gas particles through Ionize shock. Dependent of the composition of the gas mixture become, for example, organic Impurities (oils, Fats) are oxidized and removed as a result from the surface. reducing Plasmas, on the other hand, are suitable for inorganic deposits, such as metal oxides into metals. This turns the cleaning process as the sum of a physical and a chemical process represents.

Physically: If you switch the workpiece as a cathode, positive ions from the plasma are accelerated onto the workpiece. Release on impact the ions by momentum transfer molecules or fragments from the surface from. This process is often referred to as "sand blasting with ions".

chemical: In case of organic contamination the plasma gas exists in the Usually made of oxygen and possibly argon. Arise in plasma Oxygen atoms, excited states of the oxygen molecule and ions. These high-energy oxygen particles form in the Reaction with hydrocarbons the harmless chemical compounds Carbon dioxide and water.

To the plasma cleaning of the surface of the first hard layer An adhesion promoter layer can be applied. Because in dependence the nature of this hard layer and the process parameters used for the Plasma cleaning the hard layer in total more or less is activated (creation of free valences, formation of defects etc.) may also apply to the application of a primer layer on a case-by-case basis be waived.

When Adhesion promoters come into question those metals that have a high affinity for oxygen and form carbides, e.g. Titanium. The preferred primer layer It is made of chrome. Such adhesion promoters can be on Deposits all metallic substrates that endure 200 ° C.

The Adhesive layer is deposited in vacuo. Suitable for this are the CVD process (CVD = Chemical Vapor Deposition) and the PVD method (PVD = Physical Vapor Deposition), which are well in the prior art are known. In the CVD method, the chamber as the anode and the to be coated parts connected as a cathode. In between is ignited a plasma, wherein the layer material is introduced as a vaporous compound is going through heat and the plasma is cracked. For this one then needs evaporable Metal donor media (precursors) and a chemical reaction, at which then creates the free metal.

exemplary Procedures for the PVD method is the thermal evaporation from tungsten boats, electron beam evaporation, magnetron sputtering and arc evaporation. The latter method works with an arc discharge in a vacuum. The structure and mode of action of evaporators according to the principle The vacuum arc discharge based on that within a vacuum vessel between a cathode made of the conductive material to be evaporated exists and an isolated arranged anode usually the grounded chamber wall, by known methods an arc ignited becomes. Usually becomes an anodic ignition electrode briefly typed on the cathode. Alternatively, special HF plasmas can be used for ignition or focused laser radiation can be used.

Of the Arc can generate currents between a few 10 A and 100 A at voltages between 20 V and Conduct 50V. He steps with the cathode in the so-called focal spot in contact and leads there to an erosive material removal.

In order to can be a particularly high energy efficiency in conjunction with a high degree of ionization of the removed material and a high coating rate in any installation position of the evaporator be achieved.

By the very high energy density in the focal spot, which has a diameter of only a few μm However, it comes to an extremely fast heating, which too an explosive evaporation and ionization of the cathode material leads, which results in the emission of macroscopic particles (droplets) which has the homogeneity possibly affect the applied layer negatively.

Therefore, in order to meet particularly high demands on the homogeneity and reproducibility of the layer to be applied, a method is preferably used in which for the evaporation of the cathode material, a locally variable focused laser beam pulse is conducted with such an energy density on the surface of the cathode that on Place a localized dense plasma is formed. At the same time, a high-current pulse is switched on by suitable means, which in combination leads to a vacuum arc discharge between the cathodic evaporation material at the point of incidence of the laser beam pulse and the anode. Although at the point of impact of the laser beam, explosive evaporation of the cathode material in the manner typical for cathodes occurs, but without the formation of interfering macroparticles (droplets). The procedure is more detailed in the DD 275 883 described.

A has after such a vacuum arc discharge applied chromium primer layer a thickness of about 5 to about 50 nm and is different from the underlying, possibly cracked, first hard layer, always crack-free. Depending on the process variant used they contain, as described above, droplets or free from Be droplets.

The final layer of the coated material consists of an amorphous carbon layer (DLC = diamond like carbon). This is applied either directly to the first hard-coated hard layer (if no primer layer is needed) or to the primer layer. In the latter case, plasma surface purification of the surface of the adhesion promoter layer is no longer necessary, since it was deposited in a vacuum.

The Carbon layers are mainly composed of carbon atoms together and have an amorphous structure. The DLC layers are divided into hydrogen-containing and hydrogen-free. For the DLC layers can also be incorporated metallic and non-metallic elements. Of the Incorporation of metals increased the adhesion to the substrate and increases the toughness. Non-metallic additives reduce the surface energy and thus reduce the adhesion or wetting tendency. An overview into the different (amorphous) DLC layers is in the following Given table.

Table 1: Amorphous carbon films (DLC)

When PVD process is called the physical vacuum deposition. It serves for the separation of in particular hydrogen-containing DLC layers (such as a-C: H and a-C: H: W). In the PVD method, the to be deposited Solid sublimated by a physical mechanism and subsequently deposited as a layer on the substrate. The mechanism is done by thermal or kinetic energy supply. The PVD method are therefore distinguished in vapor deposition, sputtering, ion plating and molecular beam epitaxy.

During sputtering, ions generated by a plasma are accelerated in an electric field, for example by a magnetron, and bombarded with targets of, for example, chromium (Cr) or tungsten (W) with high energy. The ions are provided either by an ion source or by an ionized working gas in the form of a plasma. The working gas used is usually inert argon at a pressure of typically 0.3 Pa. The pulse of the accelerated gas ions initiates a collision cascade on target impact. The collisions release atoms and secondary electrons from the target material. The dissolved, positive parts or ions are deposited on the surface of the approximately 200 ° C heated substrate. The deposition can also be optimized by applying a plasma to the negative bias voltage of about 500V on the substrate. The stress leads to better adhesion, a higher density and a higher hardness of the layer. The sputtering process can be carried out with additional reactive gas (reactive sputtering). In reactive sputtering, reactive gases, such as nitrogen and acetylene, are added to the working gas (argon), which contribute to the film formation process. Nitride layers are formed by means of nitrogen and carbon layers by means of acetylene. These are the gases by collisions with ionized the argon atoms. The layer rate is about 2 μm / h in the reactive PVD sputtering process.

The PACVD process (plasma-activated chemical vapor deposition) also serves for the deposition of hydrogen-containing DLC layers (such as a-C: H and a-C: H: Si: O). In the PACVD procedure are by a non-thermal Plasma triggered chemical reactions that lead to layer deposition. The Substrate temperatures for depositing the PACVD layers are included about 500-750 ° C. Thereby can On the one hand, layers are deposited, which only at low temperatures arise, and on the other hand coated heat-sensitive substrates become. In the PACVD process, one or more gaseous starting materials, so-called precursor gases, such as acetylene or hexamethyldisiloxane with a pressure of 0,5-500 Pa undiluted or by means of an inert carrier gas (e.g., argon) is passed through the reaction chamber. The gases become by applying a high-frequency AC voltage of, for example, 13.56 MHz ionized to a plasma. That with a bias voltage of 100-200 V negative poled substrate attracts the positive ions, so that the film formation process starts. The layer rate is in the PACVD sputtering between 1 and 6 μm / h.

According to the invention as the outermost layer a tetrahedral hydrogen-free amorphous carbon film (ta-C) prefers. Hydrogen-free amorphous carbon layers can by Ion beam method, by pulsed laser ablation (PLD) or pulsed Arc discharge in vacuum from a graphite target generated Ions or plasma jet are separated, to which no hydrogen or hydrocarbon-containing components. For a ta-C layer Particularly suitable are pulsed arc methods, since only with these technically relevant deposition rates (> 10 μm / hour) achieve. Preferably, a laser-controlled pulsed Vacuum arc used in which ignited with a pulse laser arc and deliberately over a made of high-purity graphite, designed as a roller, rotating Carbon cathode (target) out becomes. By using current pulses> 1 kA and variation of the arc burn time between 10 and 100 μs can be both the amount of material per pulse, as well as the structure and thus the properties of the deposited amorphous layer be specifically influenced.

Of crucial importance is the temperature during the deposition process. Extremely high modulus or hardness values are achieved if the temperature at the substrate is well below 100 ° C., and preferably not above room temperature. Thus, such an amorphous carbon layers can be (in particular tetrahedral, hydrogen-free amorphous carbon (ta-C, Diamor ^®) are particularly favorable to temperature sensitive materials, such as. For example, high speed steels, Al, Al alloys, brass, bronze and, in particular to plastics In order to guarantee the desired abrasion resistance, the ta-C layer should have a thickness of> 300 nm DE 195 02 568 and the DE 198 50 217 described by the FRAUNHOFER SOCIETY.

Amorphous diamond-like carbon layers (DLC layers) consist of a highly cross-linked hydrocarbon matrix (hydrogen content 0-40 at%). They may have a proportion of sp ³ -bonding structures (in particular ta-C). The sp ³ content determines the properties of the DLC layer. For example, the density increases linearly to the sp ³ component. In addition, a high sp ³ content at the same time high deposition temperature and high deposition pressure reduces the compressive residual stresses. The reduction leads to a better adhesion of the DLC layer to the substrate, so that larger layer thicknesses are possible.

typical Properties of such DLC layers are layer thicknesses of approx. 0.1 to 10 μm, Hardening between 1 and 90 GPa, a low elastic modulus and low surface roughness, low coefficients of friction of about 0.05 to 0.25 and a low thermal conductivity and a low adhesion tendency compared to the most metals.

A such from two or three morphologically different layers Existing coating makes a substrate against abrasion and high surface pressures resistant, especially if they are on compliant substrates, such as e.g. Plastics, is applied.

Claims (10)

Abrasion and high surface pressure resistant hard material coating on any, especially resilient substrates, characterized in that the coating consists of at least two morphologically different layers, wherein the first, directly on the substrate surface resting layer is a metallic or ceramic, oxide or carbide hard layer and the outermost Location is a hard layer of amorphous Kohnstoff (DLC). Hard material coating according to claim 1, characterized that is between the metallic or ceramic, oxidic or carbide hard layer and the amorphous carbon layer a bonding agent layer is made of a metal that has a high affinity to oxygen and forms carbides. Hard material coating according to claim 1 or 2, characterized characterized in that the layer of the first layer is a hard chrome layer is. Hard material coating according to claim 3, characterized the hard chrome layer has a thickness of> 50 μm having. Hard material coating according to claim 2, characterized in that the primer layer is chromium. Hard material coating according to claim 5, characterized in that that the chromium adhesion promoter layer is crack-free. Hard material coating according to claims 5 and 6, characterized in that the chromium adhesion promoter layer has a thickness of about 5 to about 50 nm. Hard material coating according to one of claims 1 to 7, characterized in that the DLC layer is a tetrahedral, hydrogen-free amorphous carbon layer (ta-C) is. Hard material coating according to claim 8, characterized in that the DLC layer has a thickness of> 300 nm. With a hard material coating according to one of claims 1 to 9 coated substrate.