Patent Application
20050003104
The invention relates to a method for producing by vacuum coating a UV-absorbing wear protection layer that is transparent in the visible range. Preferred applications of such layers are transparent wear protection layers on plastics for exterior applications, e.g., for windows on vehicles and buildings and for other transparent or decorative plastic parts.
It is known to produce transparent wear protection layers by varnishing methods with organic hard varnishes. However, such layers have only a limited wear resistance and in general have an unsatisfactory weather resistance and UV resistance.
By applying transparent oxide layers in vacuum much higher wear resistances are achieved with much lower material expenditure. The coating is carried out by means of vaporization, sputter or plasma CVD methods (G. Kienel: Vakuumbeschichtung, Vol. 5, VDI Verlag, Düsseldorf, 1993). However, the inorganic oxide layers produced in this way have a much lower flexibility than the organic coatings produced by varnishing. The good properties of the oxide layers that can be achieved through vacuum coating are thus impaired during use and further processing of the coated substrates.
Attempts have therefore been made to combine the high flexibility of the organic coatings with the high wear resistance of the inorganic oxide layers. One example is the so-called “organically modified ceramic layers” (ORMOCER layers) that are produced according to the sol-gel method and applied like varnish layers (R. Kasemann, H. Schmidt: New Journal of Chemistry, Vol. 18, 1994, Part 10, page 1117). However, they require similarly large layer thicknesses as conventional varnish layers. Moreover, although the wear resistance is better than with varnish layers, it is by no means as good as with thin oxide layers applied in vacuum.
It is also known to produce organic layers with inorganic oxide content such that the organic layers are deposited in vacuum with the aid of plasma polymerization, whereby metallo-organic or silicon-organic vapors are used as monomer for the plasma polymerization and through simultaneous oxygen inlet metal oxide or silicon oxide molecules are also formed and stored in the growing organic polymer layer (JP 2/99933). The oxide content in the organic polymer layer can be varied depending on the monomer used and depending on the oxygen content. In this manner harder or less hard layers can be deposited which feature good wear resistances as well as a relatively high flexibility. However, this method has the disadvantage that in order to achieve layers of the right quality, only deposition rates of a few nanometers per second are possible. This method is therefore unsuitable for the economic coating of large areas.
It is known to avoid this disadvantage by combining the method of plasma polymerization with the method of plasma-aided high-rate deposition (DE 195 48 160 C1). The plasma-aided high-rate deposition permits the depositing of hard, wear resistant oxide layers with coating rates of up to 1000 nanometers per second or more, while the simultaneous plasma polymerization of the monomers released into the deposition zone causes an increased flexibility of the oxide layers deposited at a high rate. However, the layer-substrate composites produced in this manner have an inadequate UV resistance for many applications. Although the layers themselves are UV-resistant in many cases, in particular with a high inorganic oxide content, the UV radiation passing through the layer damages the plastic substrate lying below it and thus impairs the adhesive strength of the wear protection layers on the substrate.
When depositing transparent wear protection layers through plasma CVD it is known to arrange a UV-absorbing intermediate layer under the actual wear protection layer in order to prevent the UV rays from reaching the plastic substrate, thus avoiding damage to the substrate (U.S. Pat. No. 5,156,882). Both the UV-absorbing intermediate layer (ZnO, TiO2, CeO2, V2O5) and the wear protection layer are deposited by means of plasma CVD with coating rates of less than 1 nm/s, so that this method is not suitable for the coating of large amounts of substrate at an acceptable cost.
It is further known when depositing wear protection layers by means of plasma CVD to incorporate the UV-absorbing material (oxides, silicides, carbides, borides, nitrides, sulfides, fluorides, selenides, or tellurides of the elements La, Ce, Zn, In, Sn, Al, Si, Ge, Sb and/or Bi) in the wear protection layer by means of a PVD method preferably by sputtering (DE 198 24 364 A1). Here the depositing rate is also limited by the plasma CVD process or the PVD process (sputtering) to values in the order of magnitude of 1 nm/s, so the process is not suitable either for coating large amounts of substrate at low cost.
Finally, it is known when depositing wear protection layers by means of plasma CVD to incorporate the UV-absorbing material (oxides, oxynitrides or nitrides of Ce, Zn, Ti, Va, Pb, Ni or Sn) in the wear protection layer or a bottom layer by the evaporation of organic compounds with or without plasma action (DE 199 01 834 A1). Here, too, only depositing rates of a few nanometers per second are achieved, so a process duration of approx. 10 min. results for depositing a layer system, which does not permit a cost-effective coating of large amounts of substrate.
The object of the invention is to create a method for depositing UV-absorbing wear protection layers that are transparent in the visible range by means of vacuum coating with which higher deposition rates and thus lower coating costs are possible than with the previously known methods. The method should be suitable in particular for depositing strongly adhesive, weather-resistant and UV-resistant layers with high wear resistance on plastic surfaces.
The object is attained through a method with the features of claim 1. Advantageous embodiments of the method are described in claims 2 through 22.
An essential feature of the invention is the use of reactive or partially reactive plasma-aided high-rate deposition to deposit an inorganic compound for UV absorption in combination with the plasma-aided high-rate deposition for depositing an inorganic compound for wear protection. Previously inorganic or also organic compounds for UV protection were deposited only by methods with a low coating rate, such as, e.g., through plasma CVD or sputtering, because the UV-absorbing properties were not otherwise achieved. Due to the high evaporation temperatures, vacuum coating methods with a high coating rate, such as, e.g., electron beam high-rate evaporation, lead in part to a decomposition of the UV-absorbing compounds and thus to the loss of the UV-absorbing properties. It has now been found that with the deposition of an inorganic compound for wear protection and an inorganic compound for UV-absorption through reactive or partially reactive plasma-aided high-rate deposition at the same time or immediately one after the other, layers can be deposited that feature an excellent UV-absorption in addition to a high wear resistance, despite a very high coating rate.
Above all SiOx and AlxOy are suitable as inorganic compounds for wear protection, especially as these compounds can be deposited at particularly low cost. SiOx can be deposition by partially reactive plasma-aided electron beam high-rate evaporation of quartz with coating rates of over 50 nm/s, preferably several 100 nm/s to over 1000 nm/s. The use of quartz granules is particularly cost-effective and suitable. AlxOy can be deposited through reactive plasma-aided high-rate evaporation of aluminum by means of electron beam evaporation or boat evaporation with coating rates of several 100 nm/s. The reactive plasma-aided high-rate boat evaporation of aluminum wire is particularly cost-effective.
The depositing of the UV-absorbing layer advantageously takes place by the partially reactive or reactive plasma-aided high-rate evaporation of a UV-absorbing compound or a nonvolatile component of such a compound with coating rates of at least 10 nm/s, preferably 20 to 200 nm/s. The oxides and oxynitrides of Ce, Zn, Ti, Va, Pb, Ni and Sn, i.a., are possible as inorganic compounds for the UV-absorption.
According to the invention a particularly good UV-absorption is achieved if the inorganic compound for the UV-absorption is evaporated by partially reactive plasma-aided high-rate evaporation of CeO2, TiO2 or ZnO or by fully reactive plasma-aided high-rate evaporation of Zn, respectively using oxygen as the reactive gas, immediately before, during or after the deposition of the inorganic compound for wear protection. The coating rates that can be used are preferably 20 to 200 nm/s, so that, due to the low layer thicknesses or the smaller contents in the layer, approximately the same coating times are possible as when depositing the inorganic compounds for wear protection.
The use of plasma sources with high plasma density on the basis of vacuum arc discharges, hollow cathode glow discharges or ECR microwave discharges are recommended in order to successfully carry out the method. The use of hollow cathode arc sources that can be arranged next to one another in appropriate numbers for coating large areas has proven to be particularly suitable.
The inorganic compound for wear protection and the inorganic compound for UV-absorption can be deposited on the substrate to be protected in the form of separate layers one after the other in a different order. It is also possible to deposit the inorganic compound for wear protection and the inorganic compound for UV-absorption on the substrate at the same time in the form of one or more consecutive mixed layers. The starting materials for the two different compounds are thereby preferably evaporated from two crucibles arranged next to one another or concentrically to one another. If both starting materials have approximately the same evaporation temperature, they can also be mixed together before evaporation and evaporated from the same crucible.
In order to achieve high productivity it is advantageous to deposit each one of the consecutive layers in a separate deposition zone and to guide the substrate to be coated at uniform speed over the deposition zones arranged one after the other. If the productivity requirements are not so high, and an identical or similar layer is to be deposited several times in a row, the substrate can also be guided over the same deposition zone several times.
To further improve the layer properties it is advantageous in addition to let in vaporous organic monomers in one or more of the deposition zones. Through the highly dense plasma, during the plasma-activated high-rate deposition the let in organic molecules or fractions thereof are incorporated into the deposited layer and partially cross-linked with one another. This leads to an increased elasticity and to a lower brittleness of the layers. Moreover, an improved adhesive strength, improved antifriction properties, a change in the wetting properties or also a further improvement in the UV-absorption can be achieved, depending on the choice of monomer.
Depending on whether the organic molecules are to be incorporated over the layer thickness uniformly or in the form of a gradient, it is expedient to let in the organic monomer uniformly over the entire deposition zone or preferably at the beginning, in the middle or at the end of the deposition zone. It can also be advantageous to ensure through an appropriate arrangement of monomer inlet, high-rate evaporation and plasma sources that first only the molecules of the organic monomer are deposited on the substrate and only then the molecules of the inorganic compound.
A particularly advantageous embodiment of the invention lies in guiding the substrate to be coated over three deposition zones successively and depositing
It is also possible to first deposit the ZnOx layer for the UV-absorption and subsequently one or more SiOx layers for wear protection. Depositing several SiOx layers in several consecutive separate deposition zones is particularly advantageous in the coating of temperature-sensitive plastic substrates, because a heat transfer can take place between the SiOx coatings into the interior of the substrate.
A further advantageous embodiment of the invention lies in guiding the substrate uniformly over two deposition zones and depositing
When SiOx is used as the inorganic compound for wear protection, the UV-absorbing wear protection layers produced with the method according to the invention should feature a minimum thickness of approx. 5 μm in order to achieve a wear resistance comparable to that of glass.
With a multiple layer system of SiOx layers for wear protection and a ZnOx layer for UV-absorption, it is advantageous
The recommended total layer thickness is approximately 5 to 10 μm depending on the performance requirements and the associated degree of organic modification. The thickness of the UV-absorbing ZnOx layer should be approx. 0.2 to 0.5 μm depending on the UV-sensitivity of the substrate and on the application.
With a UV-absorbing wear protection layer using SiOx/CeOx mixed layers, the total layer thickness should likewise be approx. 5 to 10 μm depending on the performance requirements, whereby the CeOx content converted to the layer thickness should be approx. 0.5 to 2 μm depending on the requirements for UV-absorption, which corresponds to 5 to 50% of the SiOx content, depending on the SiOx thickness. It has proven to be advantageous when two SiOx/CeOx mixed layers are arranged one above the other, to select the CeOx content and the content of organic molecules in the mixed layer facing the substrate to be higher than in the mixed layer facing away from the substrate.
The method according to the invention will be explained in more detail on the basis of an exemplary embodiment and a relevant drawing.
SiOx, as the inorganic compound for wear protection, is deposited in the first deposition zone 2 through the electron beam high-rate evaporation of quartz granules 5 from water-cooled crucibles 6 rotating around the vertical axis. Depending on the width of the plastic substrate 1 to be coated, several crucibles 6 are arranged next to one another over the coating width. The electron beam 7 necessary for the evaporation is generated in an axial electron beam gun 8 and deflected over several crucibles 6 lying next to one another. The plasma activation in the deposition zone 2 takes place by means of one or more plasma sources 9 that act over the entire coating width. In the current example a row of hollow cathode arc sources arranged next to one another is provided in order to achieve a particularly high plasma density, above all at the beginning of the deposition zone 2. Nozzles 10 are provided in the direct vicinity of the plasma sources 9 to let in a vaporous organic monomer. In the present example hexamethyldisiloxane (HMDSO) is used because it leads to particularly favorable layer properties when incorporated into the SiOx layer. It is expedient to let in oxygen as reactive gas via the nozzles 10 in addition to the organic monomer in order to deposit highly transparent colorless SiOx layers. It has proven advantageous to carry out the plasma activation and the organic modification preferably at the beginning of coating, i.e., at the start of deposition zone 2. It is particularly advantageous for the adhesion of the organically modified SiOx layer to the plastic substrate 1 if at first only the molecules of the organic monomer and afterwards in addition the molecules and molecule fragments of the evaporated quartz granules 5 strike the plastic substrate 1, which can be achieved by a shielding 11 between the nozzles 10 for letting in the monomer, and the rest of the deposition zone 2. The average coating rate within the deposition zone 2 is approx. 400 nm/s, so that with a length of the deposition zone of 0.5 m and a substrate transport speed of 3 m/min, a thickness of the organically modified SiOx layer of approx. 4 μm is achieved. In this manner a layer is deposited in which the content of organic molecules on the side of the layer facing the substrate is almost 100% and on the side facing away from the substrate is approx. 20%.
In the second deposition zone 3, the deposition of ZnOx as the inorganic compound for UV-absorption takes place through plasma-aided, reactive evaporation of zinc 12. The evaporation takes place out of a thermally heated oven evaporator 13 with an emission slit for the zinc vapor which extends over the entire coating width. Oxygen is let in as the reactive gas via special nozzles 14 that likewise extend across the entire coating width. The special nozzles 14 are laid on a positive potential so that in this manner electrons are drawn from the plasma of the adjacent SiOx deposition zones 2 and 4 into the deposition zone 3 and there likewise produce a plasma. Through this plasma the zinc vapor emitted from the oven evaporator 13 and the oxygen emitted from the special nozzles 14 are excited and ionized, which leads to an improved chemical reaction between zinc and oxygen and to a densification of the ZnOx layer deposited in the deposition zone 3. The average ZnOx coating rate in the deposition zone 3 is approx. 40 nm/s so that with a length of the deposition zone of 0.5 m and a substrate transport speed of 3 m/min, a ZnOx layer of approx. 400 nm thickness is deposited.
Analogous to the first deposition zone 2, in the third deposition zone 4 another SiOx wear protection layer is deposited through electron beam high-rate evaporation of quartz granules 5. In contrast to the first deposition zone 2 a largely homogenous organic modification of the SiOx layer occurs here by letting in the organic monomer at a greater distance from the substrate and without special shielding between the nozzles 10 for letting in the monomer, and the rest of the deposition zone 4. Here too the organically modified SiOx layer is deposited with a coating rate of approx. 400 nm/s so that with a substrate transport speed of 3 m/min and a length of the deposition zone 4 of 0.5 m, a wear protection layer approx. 4 μm thick is deposited. The organic molecule content of the layer of is approximately constant across the layer thickness and is about 20%.
In the first deposition zone 15 quartz granules 5 and CeO2 granules 17 are evaporated simultaneously from several separate evaporation crucibles 6 arranged over the entire deposition width with the aid of one or more electron beams 7 that are generated in one electron beam gun 8 or several electron beam guns 8 arranged next to one another. One crucible 6 for the evaporation of quartz granules 5 and one crucible 6 for the evaporation of CeO2 granules 17 can thereby be arranged alternately across the deposition width. As shown in
A second SiOx/CeOx mixed layer with a thickness of approx. 4 μm is deposited in the second deposition zone 16 analogous to the first deposition zone 15. However, it is expedient to reduce the CeOx content in the layer compared with the deposition zone 15 to approx. 10% in order to achieve a greater hardness and wear resistance of the second SiOx/CeOx mixed layer. The thickness content of the CeOx in the second SiOx/CeOx mixed layer deposited in the deposition zone 16 is thereby approx. 0.4 μm. Moreover, in the present example the organic modification of the second SiOx/CeOx mixed layer has been omitted, since an increase in the layer flexibility can already be registered through the CeOx addition analogous to the addition of the organic molecules.
| Number | Date | Country | Kind |
|---|---|---|---|
| 101 53 760.3 | Oct 2001 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP02/11728 | 10/19/2002 | WO |
