Abstract:
A method for producing a nanostructured, in particular a ceramic-like functional coating on a substrate is described. To that end, using at least one plasma source, a pulsed plasma is produced with which a matrix phase and at least one nano-scale interstitial phase embedded in it are deposited on the substrate via a material input. Preferably a plurality of pulsed plasma sources that are time-correlated or synchronized with each other are used. Also proposed is a nanostructured functional coating, in particular one producible by this method, which is free of chlorine and/or sulfur, and which contains at least one metal and/or at least one element selected from the group oxygen, hydrogen, nitrogen, carbon, helium, argon or neon.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for producing a nanostructured, in particular a ceramic-like functional coating on a substrate, and a nanostructured functional coating producible using the method.  
       BACKGROUND INFORMATION  
       [0002]     Wear-protection coatings, in particular in the form of coatings of hard material based on a carbonitride, are known in single-layer or multiple-layer structure with layer thicknesses in the micron range, both on tools and on parts.  
         [0003]     Disadvantages of such coatings are the often relatively high coefficients of friction, occurrence of intrinsic tensions and resulting reduction of adhesive strength, and the great brittleness in comparison to the hardness. Furthermore, improving the wear-resistance of these coatings by increasing the hardness has a detrimental effect on other desired properties, such as toughness, impact resistance and substrate adhesion.  
         [0004]     In addition to hard material coatings based on a carbonitride, friction-reducing wear-resistant coatings of diamond-like carbon (DLC) and molybdenum disilicide are also known. However, their disadvantages include their relatively low abrasion resistance, inadequate resistance to media, and limited temperature resistance, less than 350° C. in the case of DLC.  
         [0005]     To overcome the named disadvantages, functional coatings that are already nanostructured have also been developed, in which a nanocrystalline phase is present that is embedded in an amorphous or crystalline matrix phase.  
         [0006]     The primary methods used to produce such nanostructured coatings are CVD (chemical vapor deposition) and PVD (physical vapor deposition). In particular, the PVD methods, such as magnetron sputtering or arc vaporization, are characterized by great flexibility in the selection of material, and hence offer the possibility of influencing or adjusting the layer structure in a controlled manner according to the desired application.  
         [0007]     It is proposed for example by R. Hauert et al., Advanced Materials, 11, No. 2 (1999), pp. 175-177, that a PACVD process (plasma activated chemical vapor deposition) be used to embed nano-scale titanium nitride particles having a particulate size of 10 nm to 20 nm in an amorphous silicon nitride matrix or a matrix of amorphous silicon.  
         [0008]     M. Diserens et al., Interface and Coatings Technology, 108 to 109 (1998), pp. 241-246, describe a PACVD method in which a nanostructured (TI, Si)N coating is produced on a substrate in a vacuum chamber at temperatures under 350° C. using a magnetron sputtering method (reactive unbalanced magnetron sputtering). Specifically, nano-scale or nanocrystalline titanium nitride particles having a mean crystallite size of around 20 nm are present there, embedded in an amorphous silicon or silicon nitride matrix. To ensure an additional input of power during deposition of the coating on the substrate, a high-frequency substrate voltage (bias voltage) between 0 V and −140 V is also applied there to the substrate being coated.  
         [0009]     Finally, German Published Patent Application No. 101 04 611 contains a description of a device for coating a substrate with a ceramic-like coating, whereby a material is deposited on a substrate using a source of material and a plasma. There, a power source that is different from the source of material is provided to input power at a defined location into the material which is in front of or on the surface.  
       SUMMARY OF THE INVENTION  
       [0010]     The method according to the present invention for producing a nanostructured functional coating has the advantage over the related art that, because of the pulsed plasma source used, a substantially higher ion density is achieved in the plasma; i.e., it is now possible in a simple way to produce coatings that require a high ion density during deposition. The deposition of such coatings is only possible in conventional PACVD processes with the help of an additional ionization source, which may now be omitted.  
         [0011]     In particular, the particles or atoms deposited or striking the substrate initially exhibit a relatively high kinetic energy, which is sufficient to form nano-scale crystallites on the substrate through diffusion processes, or to promote microcrystallite formation there. In addition, because of the high ion energy, at least a partial shattering of microcrystallites which may be present in the produced plasmas and/or initially deposited on the substrate is achieved, which also favors the desired nanocrystalline growth of the interstitial phase.  
         [0012]     Another advantage of the method according to the present invention is that because of the pulsed plasma source employed and the resulting higher ionization of the materials or atoms utilized for coating, the surface energy in the vicinity of the substrate to be coated is adjustable in a controlled manner to a level necessary for forming the desired nanostructured texture.  
         [0013]     In addition, the method according to the present invention has the advantage of a generally simpler process technology, using a reduced number of system components. For example, it now permits a largely charge-independent and component-independent process management; that is, it is significantly more flexible than known RF techniques, i.e., methods that work with a high frequency substrate electrode voltage.  
         [0014]     Finally, another significant advantage of the method according to the present invention is that the substrate and/or the functional coating deposited on it are heated during production of the coating only to temperatures of a maximum of 250° C., in particular less than 200° C. In this respect it is often possible to dispense entirely with additional cooling of the substrate during deposition of the coating, or only a conventional cooling device having a comparatively low cooling capacity is needed.  
         [0015]     The nanostructured functional coating according to the present invention has the advantage that, in contrast to the coatings known from the related art, which are produced for example by CVD methods, it is producible free of unwanted reaction residues such as residues of chlorine or sulfur from corresponding process gases. In particular, in the functional coatings produced in particular by the method according to the present invention, besides the elements that define the composition of the functional coating produced; such as O, H, N, C and/or other metals, only the noble gas utilized as plasma gas is detectable.  
         [0016]     It is advantageous that with the method according to the present invention both single-layer and multiple-layer coatings, in particular with differing composition or differing properties, may be deposited on parts or cutting tools. In addition, the nanostructured functional coating produced, or at least a partial layer of the functional coating, may have a chemically graduated composition; i.e., the chemical composition of the coating changes as a function of the coating thickness.  
         [0017]     When pulsing the plasma source, it has proven to be advantageous, in the interest of high ion density and efficient deposition, for the plasma source to be pulsed at a frequency of 500 Hz to 100 kHz, in particular 10 kHz to 90 kHz. The shape of the pulses utilized may be varied in a simple way and adapted to the particular application; i.e., depending on the choice made in the individual case it is possible to use at least approximately rectangular, trapezoidal, sawtooth or sine-wave pulse shapes, which also have variably adjustable rising or falling functions. In addition, the pulsing of the utilized plasma sources may be either unipolar or bipolar.  
         [0018]     Finally, it is advantageous for the pulsing of the plasma source to be done with a pulse-to-pause ratio between 1:10 and 2:1, in particular 1:4 to 1:1, with the plasma source being operated with a maximum pulse power of 5 kW to 40 kW. In addition, to carry out the method according to the present invention, aside from the pulsing, advantageous use may be made of established and customary plasma sources such as sputtering sources, preferably magnetron sputtering sources, or else arc vapor sources.  
         [0019]     One particularly advantageous embodiment of the method according to the present invention provides for the use of a plurality of plasma sources that are operated in time correlation or pulsed in synchronization with each other, each of which conveys a material or a compound to the plasma associated with the plasma source for application of the material onto the substrate, possibly after reacting with a reactive gas. Two or three plasma sources are preferably utilized, which are pulsed alternately, in particular in unipolar or bipolar operation.  
         [0020]     Finally, another advantage of the method according to the present invention is that in addition to the pulsed plasma source, an additional power source which is different from the plasma source may also be used if necessary to input power into the material precipitated from the plasma on the substrate, i.e., the functional coating produced, for example to provide a source of activation energy for diffusion processes. This input of power then takes place in the manner proposed in DE 101 04 611.1-45, either in front of and/or on the substrate, by using a microwave unit, an additional ion source unit, a hollow cathode unit, a UV unit to irradiate the substrate with UV light, an electrical field produced between the plasma source and the substrate, and/or by applying a high-frequency substrate electrode voltage (bias voltage). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  shows a schematic sketch of a magnetron sputtering source for carrying out the method according to the present invention.  
         [0022]      FIG. 2  shows a schematic, three-dimensional representation of a section of a produced functional coating. 
     
    
     DETAILED DESCRIPTION  
       [0023]     The present invention is based on a coating system  5  represented in  FIG. 1 , as described in similar form in M. Diserens et al., Interface and Coatings Technology,  
         [0024]      108  to 109 (1998), p. 242.  
         [0025]     In detail, coating system  5  has a grounded vacuum chamber  10 , a first plasma source  11 , a second plasma source  12 , a substrate support  15  with a substrate  16  on it, a gas outlet  18  and a gas inlet  19 . The design also provides for first plasma source  11  to be used in producing a first plasma  13  that is associated with it, and for second plasma source  12  to be used in producing a second plasma  14  that is associated with it, from which plasmas a coating  17  is deposited on the surface of substrate  16  via a material-input.  
         [0026]      FIG. 1  also shows that first plasma source  11  is connected to a first high voltage source  22  and second plasma source  12  to a second high voltage source  20 , with whose aid plasma sources  11 ,  12  are each pulsable initially independently of each other. In addition, an electronic control unit  21  connected to the two high voltage sources  20 ,  22  is provided, which effects a time-correlated pulsing of the two plasma sources  11 ,  12  relative to each other, in particular an alternate pulsing of the two plasma sources  11 ,  12 .  
         [0027]     It should be emphasized at this point that in principle one common high voltage source is sufficient to supply the two plasma sources  11 ,  12 , which then have an appropriate high voltage for producing plasma applied to them through control unit  21 , for example alternately. Furthermore, control unit  21  may also be integrated into one of the high voltage sources  20 ,  22 .  
         [0028]      FIG. 2  shows a nanostructured functional coating  17  produced using coating system  5  in three-dimensional sectional view, a nano-scale interstitial phase  31  being embedded in a matrix phase  30 .  
         [0029]     To produce a nano-scale titanium nitride interstitial phase  31  in a matrix phase  30  of amorphous silicon nitride on a part made of a metal such as tool steel, first plasma source  11  of coating system  5  according to  FIG. 1  is fitted with a cathode having a titanium target and second plasma source  12  with a cathode having a silicon target. At the same time, nitrogen as the reactive gas, into which argon has also been mixed as the plasma gas, is conducted through gas inlet  19  to chamber  10 . Additionally, the two plasma sources  11 ,  12  are alternately pulsed bipolarly with a trapezoidal pulse shape at a pulse frequency between 20 kHz and 70 kHz and a pulse-to-pause ratio of 1:4 to 3:4.  
         [0030]     In particular, one of the two plasma sources  11 ,  12  is active right when a pulse pause is present at the other, i.e., when the intensity of the respective associated plasma  13 ,  14  has reached a minimum, or is about to be extinguished or has already been extinguished. The maximum pulse power of each of the two plasma sources  11 ,  12  is for example 20 kW, corresponding to a mean power of 20 kW for conventional DC magnetron operation.  
         [0031]     The pulsing of plasma  13  or of plasma  14  is thus generally to be understood as a change in the respective plasma intensity over time between a maximum intensity and a minimum intensity, where the minimum intensity is preferably given by extinction of the particular plasma  13 ,  14  in the pulse pauses. Alternately, the minimum intensity may also be given by the particular plasma  13 ,  14  being operated in the pulse pauses with process parameters slightly above those that would lead to extinction, so that it is not extinguished, but the minimum intensity is substantially lower than the maximum intensity.  
         [0032]     Plasma sources  11 ,  12  are preferably sputtering sources, whose efficiency is reinforced by a magnetron; i.e., they atomize a material that is used as the cathode or target. This material is for example in part a metal or metal oxide provided by a plasma source  11 ,  12 , in particular titanium, chromium, copper or zirconium, and in part silicon or carbon provided by the other plasma source  11 ,  12 . In principle, however, just one plasma source  11 ,  12  with an appropriate target, as well as a suitable reactive gas, is sufficient to carry out the method according to the present invention, i.e., to deposit a nanostructured functional coating. Preferably, however, two or more plasma sources  11 ,  12  are employed, with reactive gases added, preferably on a timed basis.  
         [0033]     Finally,  FIG. 1  shows that substrate  16  is preferably positioned on a rotating substrate support  15 , so that it is periodically conveyed past the two plasma sources  11 ,  12  with their respective associated plasmas  13 ,  14 . Thus each time substrate  16  is passed through the zone of influence of plasmas  13 ,  14 , an application of material onto the surface of substrate  16  occurs.  
         [0034]     It should also be mentioned that the production of functional coating  17  in chamber  10  occurs at a working pressure of 10 −4  mbar to 10 −2  mbar, for example 10 −3  mbar. To that end, suitable means—not shown—are provided to ensure an appropriate vacuum in chamber  10 . Substrate  16  is for example a metallic substrate, in particular a metal part or a cutting tool, which is connected to grounded chamber  10  with an electrically conductive connection via substrate support  16 .  
         [0035]     Use of the explained coating method makes it possible, all-in-all, for the substrate and/or the functional coating  17  on the substrate to be heated to a temperature of less than 200° C., in particular merely to approximately 150° C.  
         [0036]     Through analysis of the structure of coating  17  produced as described, it was found that a largely amorphous matrix phase  30  of silicon nitride has been produced, in which a nano-scale or nano-crystalline titanium nitride interstitial phase  31  is embedded.  
         [0037]     Depending on how the process is conducted, the mean grain size of interstitial phase  31  is approximately 1 nm to approximately 100 nm, preferably approximately 3 nm to approximately 30 nm.  
         [0038]     Depending on the material chosen for matrix phase  30 , it is additionally also possible to create a metallic, microcrystalline or crystalline matrix phase  30  instead of an amorphous matrix phase  30 .  
         [0039]     Possible materials for matrix phase  30 , in addition to the described silicon or silicon nitride, also include a metal, an alloy, molybdenum disilicide, amorphous carbon or DLC.  
         [0040]     Suitable materials for the nano-scale interstitial phase  31 , in addition to titanium nitride, also include a metal oxide, a different metal nitride, a metal carbide, a metal carbonitride, or some other hard material phase, in particular zirconium nitride, silicon nitride, titanium carbide, silicon carbide, titanium oxide, chromium oxide, zirconium oxide or tantalum oxide.  
         [0041]     The preferred combinations are silicon nitride as matrix phase  30  and titanium nitride as the nano-scale interstitial phase  31 , or a metal such as copper as matrix phase  30  and zirconium nitride as the nano-scale interstitial phase  31 .  
         [0042]     The composition of the matrix phase  30  to be produced or deposited, as well as of the nano-scale interstitial phase  31 , depends on the materials supplied by plasma sources  11 ,  12  and on the reactive gas supplied to plasmas  13 ,  14 . This is for example the nitrogen already mentioned, supplied through gas inlet  19 , or alternatively oxygen, hydrogen or a gas containing carbon, in particular a hydrocarbon. Also added to the reactive gas is preferably a plasma gas such as argon or helium. It must be emphasized, however, that depending on the materials supplied by plasma sources  11 ,  12  and the desired composition of functional coating  17 , it is also possible in principle to dispense with supplying a reactive gas, for example when a metal oxide is used as the cathode or target in one of plasma sources  11 ,  12 . In this case only the plasma gas is fed to chamber  10 .  
         [0043]     It is also clear that by varying the material fed to first plasma  13  or second plasma  14  as a function of time, for example by changing the nature or quantity of the reactive gas supplied, a functional coating  17  built up of a plurality of partial layers having varying compositions may be deposited. In addition it is also possible to produce a chemically graded composition of functional coating  17  or of the individual partial layers through controlled variation, in particular continuous variation, of the nature and/or quantity of the reactive gas supplied.  
         [0044]     An extension of the exemplary embodiments explained above provides, in addition to plasma sources  11 ,  12 , for use of an additional power source which is different from the latter, to input power into the material deposited on substrate  16  from plasmas  13 ,  14 , this power input taking place in the material located in front of and/or on substrate  16 , which forms coating  17 . Suitable power sources are the devices already described in German Published Patent Application No. 101 04 611, i.e., for example a microwave unit, an ion source unit, a hollow cathode unit, a UV unit, a means of producing an electrical field between plasma source  11 ,  12  and substrate  16 , and/or a high frequency voltage source for applying to substrate  16  a high frequency substrate voltage (bias voltage) with respect to ground.  
         [0045]     Overall, use of the explained exemplary embodiments always produces a high-quality nanostructured, in particular ceramic, coating  17  or a corresponding coating system on substrate  16 , which in addition to the nano-scale interstitial phase  31  also includes, in a further structure, a matrix phase  30 , which is preferably amorphous, or metallic in the form of a refractory network, but may also be crystalline. The proportion of the surface volume of interstitial phase  31  to the total volume is very high, and the interfaces between the nanocrystallites of interstitial phase  31  and the surrounding, in particular amorphous matrix  30  are comparatively sharp. In particular, insertion of a nano-scale hard material phase of this sort produces coatings  17  with hardnesses of over 4000 HV, which exhibit a brittleness reduced due to the nanostructuring produced.  
         [0046]     The thickness of functional coating  17  produced on substrate  16  as explained is usually in the range from 100 nm to 10 μm, in particular 500 nm to 5 μm; along with it, in addition an inherently known adhesion-promoting intermediate layer may also be provided between substrate  16  and functional coating  17 , or a cover coat or flow-in coating, for example of amorphous carbon, may be provided on functional coating  17 .  
         [0047]     For substrate  16 , in addition to planar substrates  16 , three-dimensional parts are also possible, such as drills, shafts, bearing components, pistons, gears or the like.