EP2203258B1 - Scratch-resistant and expandable corrosion prevention layer for light metal substrates - Google Patents

Abstract

The invention relates to a method for coating the surface of a light metal substrate, said method comprising the following steps: A. the light metal substrate is prepared and optionally the substrate surface to be coated is cleaned, B. the substrate surface optionally cleaned in step A is coated in a plasma polymerisation reactor by means of plasma polymerisation. In step B, at least one organosilicon compound and (a) no other compound or (b) other compounds is/are used as precursor(s) for the plasma, and the light metal substrate is arranged in the plasma polymerisation reactor such that (i) it is located between the region wherein the plasma is formed and the cathode is arranged, or (ii) it acts as a cathode. The invention is characterised in that the method is carried out in such a way that the coating produced by the method comprises: between 5 and 20 atom %, preferably between 10 and 15 atom %, of a carbon part that can be determined by XPS measurement, in relation to the total amount of carbon, silicon and oxygen atoms contained in the coating, a yellow index, determined according to ASTM D 1925, of ≤ 3, preferably ≤ 2.5, and a hardness, to be measured by means of nanoindentation, of between 2.5 and 6 GPa, preferably between 3.1 and 6 GPa.

Description

The invention relates to a method for coating the surface of a light metal substrate, in particular aluminum and magnesium with a layer which is characterized by an advantageous combination of scratch resistance. Dehnbarkeit and corrosion protection distinguishes, as well as coated Leichtmetallsoberstrate.

Untreated surfaces of objects (for example, semi-finished products) made of light metals, in particular aluminum (including aluminum alloys) or magnesium (including magnesium alloys), in particular in certain media, such as alkalis or acids, a strong corrosion susceptibility. In addition, their mechanical stability is comparatively low. Therefore, in particular aluminum surfaces (if necessary refined by a chemical, electrochemical or mechanical pretreatment) are frequently treated by anodization (anodization, anodization, anodization). As a result, the uppermost layer of the metal is converted into a controlled oxide layer. Corresponding anodized surfaces have a metallic luster due to the transparency of the oxide layer produced and are somewhat protected against corrosion and scratching from untreated surfaces. Anodic coatings typically have an open-pored surface structure. Dyes can optionally be incorporated into the pores. Frequently, the openings of the anodizing pores are closed by compaction. For this purpose, alumina hydrate is usually formed in the pore. which improves corrosion protection and longevity. Eloxal layers form an integral part of the aluminum and therefore do not peel off or burst.

A disadvantage of Eloxalschichten is their low ductility. A crack-onset strain (elongation to microcracking) of about 0.4% is typical. which is thus worse than untreated aluminum. This means in the processing of corresponding components of aluminum or aluminum alloys (aluminum substrates). for example, in the case of automobile trims. a strong limitation. Already a slight bend, for example due to a sticking of at least 1 m long trim strip on only one end. causes hairline cracks in an anodized layer. Such cracks also occur regularly when such a component falls from working height to the ground. Hairline cracks are also observed when heated above 100 ° C. as it regularly occurs during welding or hot bending. With such a temperature increase, the material structure changes, which become visible during anodizing and can lead to an undecorative appearance. At the points where hairline cracks run, especially the corrosion protection effect of the anodizing layer fails.

Furthermore, the anodization improves the corrosion resistance of the surface of an aluminum substrate compared to the untreated aluminum substrate. however, it is not sufficient for all applications. Especially against the attack of strong bases (pH about 13.5 or higher) Eloxalschichten protect only insufficient. This has led to it. that anodizing surfaces are used in some technical applications such as e.g. the automotive industry no longer meet the current requirements. Although the scratch resistance of an anodized surface is increased relative to the surface of an untreated aluminum substrate, it is not sufficient for all applications (e.g., not for moldings, covers, lamps, hardware and machine parts for food processing).

An example of a diverse surface stress. a surface treated by an anodizing process is often unable to cope. is again found in the automotive sector, for example in the case of aluminum rims. Should such rims. to obtain their metallic luster, they are not painted after polishing, so a process for producing coatings which hardly or not change the visual appearance is necessary. offer a good corrosion protection, tend not to crack under mechanical pressure and tensile stress, have a high scratch resistance. are not infiltrated at local destruction, are as insensitive to alkalis and cleaning agents. emulate the surface well and have a high temperature resistance and optionally a dirt-repellent surface behavior, so that e.g. can not fix brake dust.

document EP0752483A1 discloses a method for depositing silicon-containing, scratch-resistant protective layers on substrates. The deposition takes place by means of a plasma CVD method, wherein the conversion of the silicon-containing gas (eg HMDSO) to SiO ₂ does not proceed quantitatively.

Other methods for scratch-resistant coating of surfaces are from the documents WO00 / 47290 and US5846649 known.

Object of the present invention was. to specify a procedure. with which a protective layer can be produced on an aluminum substrate, which does not or at least only in a reduced manner eliminate some or all of the disadvantages of anodized layers described above Extent. In particular, such a layer should have an improved crack elongation. Furthermore, improved chemical stability compared to alkaline media and generally an improved corrosion protection effect compared to anodized coatings is desirable. Furthermore, such a layer should have an improved scratch resistance compared to anodized layers and can be produced in an economically advantageous process, in particular at a relatively high speed.

1. A. Bereitstellen des Leichtmetallsubstrates und gegebenenfalls Säubern der zu beschichtenden Substratoberfläche.
2. B. Beschichten der gegebenenfalls in Schritt A gesäuberten Substratoberfläche in einem Plasmapolymerisationsreaktor mittels Plasmapolymerisation, wobei
   • in Schritt B als Precursor(en) für das Plasma eine oder mehrere siliciumorganische sowie (a) keine weiteren oder (b) weitere Verbindungen eingesetzt werden und
   • in Schritt B das Leichtmetallsubstrat im Plasmapolymerisationsreaktor so angeordnet wird, dass es (i) sich zwischen der Zone, in der das Plasma gebildet wird, und der Kathode befindet oder (ii) als Kathode wirkt,
     dadurch gekennzeichnet, dass das Verfahren so geführt wird, dass die durch das Verfahren hergestellte Beschichtung
   • einen durch Messung mittels XPS bestimmbaren Anteil von Kohlenstoff von 5 bis 20 Atom-%, vorzugsweise 10 bis 15 Atom-%, bezogen auf die Gesamtzahl der in der Beschichtung enthaltenen Kohlenstoff-, Silicium- und Sauerstoffatome,
   • einen nach ASTM D 1925 bestimmten Gelbindex (Yellow Index) von ≤ 3, vorzugsweise 2.5 und
   • eine mittels Nanoindentation zu messende Härte im Bereich von 2.5 bis 6 GPa, vorzugsweise 3.1 bis 6 GPa

aufweist.Surprisingly, it has been found that this object is achieved by a method for coating the surface of a light metal substrate, comprising the following steps:

1. A. Providing the light metal substrate and optionally cleaning the substrate surface to be coated.
2. B. coating the optionally cleaned in step A substrate surface in a plasma polymerization reactor by plasma polymerization, wherein
   • in step B as precursor (s) for the plasma one or more organosilicon and (a) no further or (b) further compounds are used, and
   • in step B, the light metal substrate in the plasma polymerization reactor is arranged so that it (i) is located between the zone in which the plasma is formed and the cathode, or (ii) acts as a cathode,
     characterized in that the process is conducted so that the coating produced by the process
   • a fraction of carbon which can be determined by measurement by means of XPS of 5 to 20 atomic%, preferably 10 to 15 atomic%, based on the total number of carbon, silicon and oxygen atoms contained in the coating,
   • a Yellow Index of ≤ 3, preferably 2.5 and according to ASTM D 1925
   • a hardness to be measured by nanoindentation in the range of 2.5 to 6 GPa, preferably 3.1 to 6 GPa

having.

Light metal in the context of the present invention is a metallic material having a specific density of not more than 4.5 g / cm ³ . These include in particular magnesium, aluminum, beryllium and titanium and their alloys. Preferred light metal substrates are magnesium substrates and in particular aluminum substrates.

A coating produced by the process according to the invention is characterized by a hitherto unknown advantageous combination of properties that hitherto were considered incompatible. namely good scratch and corrosion resistance with high ductility (crack elongation preferably greater than or equal to 1%, in particular greater than or equal to 1.5%). In addition, by the method according to the invention also a good substrate adhesion. good optical transparency in the visible range and / or high layer thickness homogeneity can be achieved.

Coatings with a yellow index of 2.5 or below generally have no yellowing discernible to the human eye. However, for coatings intended to replace anodized coatings, minimum yellowing is tolerable, as with a yellow index in the range of greater than 2.5 to 3. The yellow index is further determined substantially by the proportion of Si-H bonds in the coating produced by the process according to the invention, which, as explained below, is crucial for achieving favorable ranges of hardness and elasticity and thus for achieving the advantageous property combination.

Another important parameter is the carbon content in the coating produced by the process according to the invention, which is influenced by the proportion of organic groups. This in turn is just as important for the hardness and elasticity of the coating, which is also desirable for achieving the advantageous property combination. Due to a high elasticity of the coating, this can e.g. be stretched together with the coated article without causing cracking. With very high elasticity of the coating, the substrate can even be plastically deformed without the coating being damaged. As a result, forming processes of the coated material are possible to some extent.

In the case of the coatings which can be produced by the process according to the invention, the scratch resistance, the elongation at break and the corrosion protection properties are improved in comparison to the anodised aluminum layers typical for aluminum. The scratch resistance of the layers is in many cases comparable to that of glass surfaces. It surprisingly turned out that by using the (dry chemical) method according to the invention it is possible to dispense entirely with (wet-chemical) anodizing. This is advantageous because, because of the high energy input, anodizing is an expensive and, from an environmental point of view, a problematic process. In addition, the inventive method makes lower demands on the quality of the aluminum substrate. because it is not necessary. Use anodising grade material. However, an anodizing surface may also be coated, which may be desirable, in particular in the case of colored anodising layers. In particular, the scratch resistance and the corrosion protection properties are improved. In addition, dirt-repellent (low-energy) surface properties can be achieved on the coating surface be set. The achievable layer property function is not only of interest as anodic replacement. but also for the general coating of light metals.

From the DE 197 48 240 A1 is a method for corrosion-resistant coating of metal substrates, in particular of aluminum or aluminum alloys, known by plasma polymerization. As precursor (s) at least one hydrocarbon or organosilicon compound is used. In the DE 197 48 240 A1 there are no data on the proportion of carbon atoms in the plasma polymer coatings produced or their yellow index. The layers disclosed there protect the surface well against corrosion without changing it visually. One limitation, however, is their low scratch resistance. Due to the low deposition rates, the process disclosed therein is not suitable for producing economically higher layer thicknesses, as are necessary for scratch-resistant coatings. In addition, it places very high demands on the surface roughness of the substrate.

In the WO 03/002269 A2 are articles comprising a substrate and a plasma polymer attached to the substrate. Discloses coating comprising O, C and Si. in which the molar ratios of O to Si and C to Si are each in certain areas and can be easily cleaned. However, the coatings disclosed therein have at least 25 atomic% of a higher carbon content than the coatings produced by the process according to the invention and do not have the above-mentioned combination of favorable properties. Also, nothing is said about the yellow index to be set.

Domingues et al. (2002) Electrochimica Acta 47, 2253-2258 discloses an aluminum alloy with a plasma polymer coating that provides some corrosion protection (as detected by electrochemical impedance spectroscopy). However, the disclosed coating lacks, in particular, the good scratch resistance and the high extensibility that are inherent in the coating according to the invention. Furthermore, it is characterized by a high water absorption, comparable to that of organic coatings. Domingues et al. contains no information regarding yellowing or carbon content. Due to the procedure (ratio of the gas flows of oxygen to the organosilicon precursor about 23: 1, for details on the influence of these parameters see below) has the in Domingues et al. disclosed coating a lower carbon content than the coating produced by the inventive method.

In the EP 0 748 259 B1 coatings for soft substrates are disclosed. It is not disclosed that the coatings could be suitable for protecting aluminum substrates from corrosion. The in the EP 0 748 259 B1 disclosed coatings that have a Yellow index of ≤ 3. However, they have a nanoindentation hardness of less than 2.5 GPa.

Further, there is no teaching to control the amount of non-stoichiometric silicon and thus the formation of Si-H to produce particularly hard (well-crosslinked), stretchable layers with little yellowing. The optimized cross-linking ensures optimized corrosion protection.

Thick-film process. e.g. The application of paints or sol-gel coatings, while the necessary corrosion resistance can achieve, but they change the visual appearance. This effect is exacerbated in the case of irregularities such as mechanical damage or lack of adhesion of the thick films.

The method according to the invention is able to meet the need for a cost-effective method for producing a thin layer which does not change the surface color of the aluminum (no intrinsic color and thus has a sufficiently high transmission in the visible range), which polishes the surface structure (eg. grinded, matted), so that no "lacquer gloss" is produced which, in addition to a high corrosion stability, has a high mechanical resistance (scratch resistance, extensibility) and which has a high layer thickness uniformity even with complex geometries.

By varying process parameters, the layer properties can be set within wide limits as described below.

An increase in the self-bias increases the hardness of the layer, its optical absorption in the visible range (and thus the yellow index) and its corrosion protection effect.

By adjusting the hardness of the coatings which can be produced by the process according to the invention, it is possible for a person skilled in the art to achieve an optimum with regard to the scratch resistance of the coating. If the hardness is too low, the deposited plasma polymer layer is not sufficiently scratch-resistant. However, if the hardness is too high, the scratch resistance will also decrease because the layer will then become too brittle. Generally, the scratch resistance of the layer is determined by the appropriate choice of layer thickness and composition. Preferred is a method according to the invention, which is conducted so that the coating produced by the method has a pencil hardness of 4H or higher, determined according to ASTM D 3362. The measurement of the hardness by means of nanoindentation is explained in Example 2.

When adjusting the self-bias, the composition of the gas mixture must also be considered. from which the plasma is generated. For example, a lower self-bias should generally be chosen for a high molecular weight of a precursor than for a low one. The more easily a precursor can be ionized, the lower the plasma power must be. to achieve a certain self-bias. With a high electrical conductivity of the plasma, a low plasma power is needed to achieve a given self-bias.

Preference is given to a method according to the invention, wherein a regulation takes place during step B. such that the self-bias is in the range from 50 to 1000 V, preferably in the range from 100 to 400 V, preferably in the range from 100 to 300 V.

The self-bias can e.g. be reduced with constant plasma power by the plasma excitation frequency is increased.

An increase in the self-bias also causes an improvement in the layer thickness homogeneity. For example, it could be determined in own experiments that on a round substrate with a diameter of 10 cm, the maximum layer thickness can differ from the minimum layer thickness at 100 V self-bias by a factor of 1.1, while this factor at 200 V Self -Bias can be 1.005.

In the method according to the invention, the light metal substrate in the plasma polymerization reactor is arranged in step B in this way. that it is either (i) between the zone. in which the plasma is formed, and the cathode is or (ii) acts as a cathode. Alternative (ii) is preferred here. As the cathode, the substrate acts when it is in direct electrically conductive contact with the part of the cathode which is distinguishable from the light metal substrate or when it is at a sufficiently small distance therefrom. This will achieve a high rate of deposition of the positively charged ions of the plasma. which are attracted to the negatively charged cathode. facilitated. When the substrate itself acts as a cathode. the force is especially increased. with which the positively charged ions impinge on the surface. As a result, the structure of the layer changes in the direction of a smaller proportion of organic (usually consisting mainly of C and H) groups and a correspondingly higher proportion of Si and O. The same effect also occurs (possibly reduced) when the substrate Although not acting as a cathode, but is arranged in the acceleration path of the cations.

Without wishing to be bound by a particular theory, it is assumed. that an increased force such as by an increased ion bombardment, the knocking off of organic groups in both the layer-forming ions and in the facilitated coating, the cleaved groups have a low probability. to be incorporated into the layer. In addition, the force of the impacting ions reduces the internal stress (residual stresses) of the layer, which increases the elongation to microcracking of the layer. For measuring the elongation to microcrack (crack elongation), compare Example 1.

The same observations (higher deposition rate, lower organic content, lower internal stress, higher crack elongation) are made as self-bias is increased. For this reason, hard coatings may have a higher crack elongation than soft ones. With very low self-bias, there is no sufficient impact to relax the coating, which may lead to soft (for example, by nanoindentation to be measured hardness of 1 GPa) and at the same time crack-sensitive coatings.

If, however, the self-bias is increased beyond a favorable range (see also above), coatings are formed which contain too little organic group and too high a proportion of Si-H bonds (see also below). which makes them too hard and too brittle and reduces their scratch resistance. The aim is therefore to be optimum with regard to the hardness and elasticity of the coating produced by the process according to the invention at an acceptable yellow index (see also below).

An increase in the self-bias causes in the resulting layer, on the one hand, the reduction of residual stresses and thus a crack-increasing effect, on the other hand, an increase in hardness and modulus of elasticity and thus a crack-reducing effect. The fact that two opposing effects partially cancel each other out. There is an optimum of crack elongation as a function of self-bias. so that an excellent combination of relatively high hardness and high cracking can be produced.

In addition, an increase in self-bias leads to an increased deposition rate and improved corrosion resistance in the resulting layer.

The layers which can be produced by the process according to the invention are organically modified SiO ₂ frameworks. The organic components can be detected in the IR spectrum by bands at about 2950 cm ^-1 and at about 1275 cm ^-1 . In addition, they can be detected by measuring the surface energy with test inks. The higher the proportion of organic groups, the lower the surface energy. Therefore, the surface energy is greater. the higher the self-bias is set.

In the prior art, the placement of the substrate is such that it acts as a cathode. generally not preferred because of the risk of forming indefinable organosilicon layers on the substrate and countering the desire for a relatively high level of organic groups in the layer. The organic modification increases the flexibility and elasticity of the layer produced. In addition, it reduces its residual stresses (internal stress), which increases the crack elongation of the layer.

Preferably, in a method according to the invention, during step B the self-bias is set on the substrate. The dependence of the deposition rate and the layer properties of self-bias has already been explained. If the self-bias is set directly on the substrate and thus on the object to be coated. this makes it easier to achieve a layer with the desired well-defined properties.

Preferably, in a method according to the invention during step B, a regulation takes place. so that the self-bias is constant. As a result, the structure of the layer can be precisely controlled. Advantages of a self-bias which is as constant as possible are a homogeneous layer structure and a simple process transfer to a variety of substrates or a plurality of substrates. Preferably, during step B, the self-bias is controlled directly. If the plasma power is regulated, the self-bias will generally not be completely constant, but will fluctuate by a certain amount. In such a case, it is preferred. if the total fluctuation range of the self-bias is a maximum of 5% of the time average, preferably a maximum of 3%.

Particular preference is given to a method according to the invention (in particular in combination with one or more features of another method described as preferred or particularly preferred), wherein a regulation takes place during step B. such that the self-bias on the substrate is in the range of 50 to 1000 V, preferably in the range of 100 to 400 V, more preferably in the range of 100 to 300 V, in particular so. that self-bias is constant.

Preferred is a method according to the invention, wherein in step B the light metal substrate is in spatial contact with (i) the cathode or (ii) a part of the cathode. which is distinguishable from the light metal substrate. Alternative (ii), unlike alternative (i), refers to the case. that the substrate itself acts as a cathode. By establishing a spatial contact, the arrangement of the substrate in the plasma polymerization reactor is facilitated.

An increase in the inflow of the organosilicon precursor (s) for the plasma. (in relation to optionally also flowing O ₂ in particular under Keeping constant the total inflow) generally causes a reduction in hardness. an increase in visible absorption (increase in yellowness index), deterioration of anticorrosive effect, and improvement in crack elongation.

With a high number of Si-H bonds in a layer according to the invention, an increased absorption of light of the ultraviolet and blue spectral range can be detected in the UV / Vis spectrum. This leads to an undesirable yellowing (increase in the yellow index). It is therefore desirable. the proportion of Si-H bonds does not become too large. A reduction in self-bias prevents the formation of Si-H bonds in the coating. Likewise, the formation of Si-H bonds can be inhibited by a suitable selection of the amount and / or type of precursor (s). This is usually achieved by a reduction in the inflow of organosilicon precursors, which also reduces the proportion of organic groups in the coating. The occurrence of Si-H bonds can also be detected in the IR spectrum (2150 to 2250 cm ^-1 ).

The formation of Si-H bonds in the coating is also reduced when a sufficient amount of oxygen is added to the plasma, which also reduces the level of organic groups in the coating. Preference is given to a process according to the invention, wherein in step B the plasma is supplied with oxygen (in the form of O ₂ ) and preferably all the substances supplied to the plasma in step B are gaseous before they enter the plasma polymerization reactor. A procedure in the way that the substances supplied to the plasma not only under the conditions of the plasma. but are already gaseous before entering the reactor, facilitates the precise tuning of the dosage of the substances.

An increase in the oxygen influx leads to an increase in hardness, a reduction in absorption in the visible range (reduction in the yellow index). to an improvement of the corrosion protection effect. to a reduction of the proportions of organic groups in the coating and to a reduction of the crack elongation.

In a preferred method according to the invention, oxygen (O ₂ ) is supplied to the plasma. all substances supplied to the plasma in step B are gaseous before entering the plasma polymerization reactor and the ratio of the gas flows of oxygen and further precursor (s) (in particular organosilicon precursors) supplied to the plasma in step B is in the range from 1: 1 to 6: 1 , preferably 3: 1 to 5: 1. In this area, it is particularly easy according to the invention to adjust the desired proportion of carbon in the coating produced by the process and to achieve the desired properties of the coating.

If you get a yellow coating for a given parameter set. so the O ₂ flow can be increased. Alternatively, the inflow of the organosilicon precursor (s) or the self-bias can be reduced. If the coating is too hard and therefore too brittle, the self-bias can be reduced or the inflow of the organosilicon precursor (s) can be increased.

Preferred is a process according to the invention, in particular in one of the embodiments designated as preferred, wherein in step B as precursor (s) for the plasma one or more siloxanes. optionally oxygen (O ₂ ) and preferably no further compounds are used. Siloxanes. In particular, hexamethyldisiloxane (HMDSO) have proven to be particularly suitable precursors to lead a process according to the invention, can be prepared with the coatings with the advantageous combination of properties. Preferably, in a method according to the invention in step B as a precursor (s) for the plasma HMDSO. optionally oxygen and preferably no further compound used. An increase in the inflow of HMDSO in relation to oxygen causes, inter alia, a reduction in the hardness, an increase in the absorption in the visible range and a deterioration of the corrosion protection effect.

Particularly preferred is a method according to the invention, in which oxygen is supplied to the plasma in step B. all substances supplied to the plasma in step B are gaseous before entering the plasma polymerization reactor, the ratio of the gas flows of oxygen and further precursor (s) fed to the plasma in step B is in the range from 1: 1 to 6: 1 and is used as precursor (s ) for the plasma HMDSO. Oxygen and in particular no further connection is used.

Preferably, in the method according to the invention, the plasma polymerization at a temperature of less than 200 ° C. preferably less than 180 ° C and / or a pressure of less than 1 mbar. preferably carried out in the range of 10 ^-3 to 10 ^-1 mbar. If the pressure during the deposition is too high, unwanted powdering of the deposited material may occur. At a temperature greater than 180 ° C, the aluminum becomes progressively softer as the crystal structure changes. At a pressure of less than 10 ^-3 mbar, the plasma can no longer be ignited.

In a method according to the invention, step B is preferably carried out to a thickness of the deposited coating of greater than or equal to 2 μm, preferably greater than or equal to 4 μm. A higher thickness increases the scratch resistance of a given coating.

Preferred is a method according to the invention. wherein in step B, the deposition rate is set to a value of greater than or equal to 0.2 μm / min, preferably greater than or equal to 0.3 μm / min. For example, a value of 0.5 μm / min can be selected. On the one hand, high deposition rates increase the economic efficiency of the process according to the invention and also facilitate the adjustment of the desired layer properties.

Preferably, in a method according to the invention, the light metal substrate is an aluminum substrate selected from the group of substrates consisting of: aluminum or aluminum alloy with a cleaned, uncoated surface; Aluminum or aluminum alloy with superficial oxide layer; anodised (s) (anodized) aluminum or aluminum alloy with dyed or undyed, compacted or uncompacted oxide layer. The aluminum or the aluminum alloy with a cleaned, uncoated surface is optionally mechanically and / or electrically glazed and / or luster-etched or finished by a chemical, electrochemical or mechanical pretreatment, for example, luster-pickled, electropolished or polished. In addition, other light metals, such as e.g. Magnesium and its alloys are preferred as substrates. Also such substrates can be smoothed by smoothing prior to coating according to the invention. or conversion process have been modified.

It is advantageous to carry out the method according to the invention such that in step A the substrate surface to be coated is cleaned by means of a plasma. Such a plasma cleaning improves the layer adhesion. Preferably, in the method according to the invention in step A, a gas or gas mixture is added to the plasma to carry out the plasma cleaning. wherein the gas or gas mixture is selected from the group consisting of: argon. Argon-hydrogen mixture. Oxygen.

In some cases it is advantageous. to carry out the process of the invention so that, following step B within the plasma polymerization reactor non-fragmented organosilicon compounds are present, which react with reactive sites on the surface of the coating to form a hydrophobic surface. This can be achieved by first leaving the unfragmented organosilicon precursor (s) in the reactor after switching off the plasma source, thus giving it the opportunity. to react with the surface radicals of the plasma polymer layer. This can be used to create layers. which are particularly easy to clean. The formation of a near-surface hydrophobic layer can be detected by XPS. In such a procedure, a layer produced by the process according to the invention preferably contains in the upper (5 nm away from the substrate) a carbon content of 40-55 atomic%, a silicon content of 15-25 atomic% and an oxygen content of 20-35 atom -%. based on the total number of Si contained in the coating. C and O atoms. The expert is clear. that this superficial area is applied only after carrying out the steps essential to the invention.

In the method according to the invention, the plasma is preferably generated by means of high frequency (HF). Plasmas, the z. B. generated by means of medium frequency, often lead to coatings that are too brittle.

• in Schritt B wird dem Plasma Sauerstoff zugeführt;
• alle dem Plasma in Schritt B zugeführten Stoffe sind vor Eintritt in den Plasmapolymerisationsreaktor gasförmig;
• das Verhältnis der dem Plasma in Schritt B zugeführten Gasflüsse von Sauerstoff und weiteren Precursor(en) im Bereich von 1:1 bis 6:1;
• während Schritt B erfolgt eine Regelung. so dass der Self-Bias auf dem Substrat im Bereich von 100 bis 400 V liegt, vorzugsweise so, dass der Self-Bias konstant ist;
• als Precursor(en) für das Plasma wird Hexamethyldisiloxan (HMDSO). Sauerstoff und vorzugsweise keine weitere Verbindung eingesetzt;
• in Schritt B befindet sich das Leichtmetallsubstrat in räumlichem Kontakt mit einem Teil der Kathode, der vom Leichtmetallsubstrat unterscheidbar ist;
• die Plasmapolymerisation wird bei einer Temperatur von weniger als 200°C und einem Druck im Bereich von 10^-3 bis 10^-1 mbar durchgeführt;
• in Schritt B wird die Abscheiderate auf einen Wert von größer oder gleich 0.2 µm/min eingestellt;
• Schritt B wird bis zu einer Dicke der abgeschiedenen Beschichtung von größer oder gleich 2 µm durchgeführt;
• das Leichtmetallsubstrat ist ein Aluminiumsubstrat. ausgewählt aus der Gruppe von Substraten bestehend aus: Aluminium oder Aluminiumlegierung mit gereinigter, unbeschichteter Oberfläche; Aluminium oder Aluminiumlegierung mit oberflächlicher Oxidschicht; anodisierte(s) Aluminium oder Aluminiumlegierung mit gefärbter oder ungefärbter. verdichteter oder unverdichteter Oxidschicht.
• oder das Leichtmetallsubstrat ist ein Magnesiumsubstrat, ausgewählt aus der Gruppe von Substraten bestehend aus: Magnesium oder Magnesiumlegierung mit gereinigter. unbeschichteter Oberfläche; Magnesium oder Magnesiumlegierung mit oberflächlicher Oxidschicht.
• in Schritt A wird die zu beschichtende Substratoberfläche mittels eines Plasmas gereinigt. wobei dem Plasma zur Durchführung der Plasmareinigung ein Gas oder Gasgemisch zugesetzt wird. das ausgewählt ist aus der Gruppe bestehend aus: Argon. Argon-Wasserstoff-Gemisch. Sauerstoff;
• das Plasma wird mittels Hochfrequenz erzeugt.

For a very particularly preferred method according to the invention:

• in step B, oxygen is supplied to the plasma;
• all the substances supplied to the plasma in step B are gaseous before entering the plasma polymerization reactor;
• the ratio of the gas fluxes of oxygen and further precursor (s) supplied to the plasma in step B in the range from 1: 1 to 6: 1;
• during step B, a regulation takes place. such that the self-bias on the substrate is in the range of 100 to 400 V, preferably such that the self-bias is constant;
• as precursor (s) for the plasma is hexamethyldisiloxane (HMDSO). Oxygen and preferably no further compound used;
• in step B, the light metal substrate is in spatial contact with a portion of the cathode which is distinguishable from the light metal substrate;
• the plasma polymerization is carried out at a temperature of less than 200 ° C and a pressure in the range of 10 ^-3 to 10 ^-1 mbar;
• in step B, the deposition rate is set to a value of greater than or equal to 0.2 μm / min;
• Step B is carried out to a thickness of the deposited coating of greater than or equal to 2 μm;
• the light metal substrate is an aluminum substrate. selected from the group of substrates consisting of: aluminum or aluminum alloy with a cleaned, uncoated surface; Aluminum or aluminum alloy with superficial oxide layer; Anodised aluminum or aluminum alloy with colored or uncoloured. compacted or uncompacted oxide layer.
• or the light metal substrate is a magnesium substrate selected from the group of substrates consisting of: magnesium or magnesium alloy with purified. uncoated surface; Magnesium or magnesium alloy with superficial oxide layer.
• In step A, the substrate surface to be coated is cleaned by means of a plasma. wherein a gas or gas mixture is added to the plasma to carry out the plasma cleaning. which is selected from the group consisting of: argon. Argon-hydrogen mixture. Oxygen;
• the plasma is generated by means of high frequency.

In another aspect, the present invention also relates to a coated light metal substrate. preferably a magnesium or an aluminum substrate, preparable by the process according to the invention. preferably in one of the embodiments referred to above as preferred.

In the case of a coated light metal substrate according to the invention, the coating preferably has determinable proportions of 5 to 30 atom% by measurement by means of XPS. preferably 10 to 25 atomic% of silicon and 30 to 70 atomic%, preferably 40 to 60 atomic% of oxygen. based on the total number of carbon contained in the coating. Silicon and oxygen atoms. In these atomic percent ranges, the setting of the desired property combination is particularly well possible. The dependence of the proportions of carbon, silicon and oxygen on the arrangement of the substrate and on the self-bias has already been explained above. In addition, these proportions can be influenced by choosing suitable precursors.

In a preferred coated light metal substrate according to the invention, an IR spectrum recorded by the coating has one or more. preferably all of the following bands (peaks) having a respective maximum in the following ranges: CH valence vibration in the range of 2950 to 2970 cm ^-1 . Si-H vibration in the range of 2150 to 2250 cm ^-1 , Si-CH ₂ -Si vibration in the range of 1350 to 1370 cm ^-1 , Si-CH ₃ deformation vibration in the range of 1250 to 1280 cm ^-1, and Si O oscillation greater than or equal to 1150 cm ^-1 .

The location of the maximum of the Si-O-Si oscillation gives information about the degree of crosslinking of the layer. The higher its wave number, the higher the degree of crosslinking. Layers in which this maximum is greater than or equal to 1200 cm ^-1 , preferably greater than or equal to 1250 cm ^-1 , have a high degree of crosslinking, while about non-stick layers with this maximum, typically about 1100 cm ^{-1 have} a low degree of crosslinking.

A detectable Si-CH ₂ -Si vibrational band indicates that in addition to Si-O-Si linkages. Si-CH ₂ -Si linkages are present in the coating. Such a material regularly has increased flexibility and elasticity.

To characterize the coating, the ratio of the intensity of the Si-H band to the intensity of the Si-CH ₃ band can serve. Coatings. where this ratio is less than or equal to about 0.2, are colorless. At a ratio greater than about 0.3, the coatings are yellowish. A coated light metal substrate according to the invention is preferred. wherein in an IR spectrum recorded by the coating, the ratio of the intensity of the Si-H band to the intensity of the Si-CH ₃ band is less than or equal to 0.3, preferably less than or equal to 0.2.

Preference is given to a coated light metal substrate according to the invention, the coating having an absorption constant k of _{300 nm} of less than or equal to 0.05 and / or an absorption constant of k _{400 nm} of less than or equal to 0.01. The relationship between the number of Si-H bonds that cause light absorption in the ultraviolet and blue regions. with the self-bias, an oxygen supply to the plasma and the amount and type of precursor (s) has already been explained above.

Preferably, the coating of a coated light metal substrate according to the invention has a surface energy in the range of 20 to 40 mN / m, preferably 25 to 35 mN / m. The surface energy is determined by the proportions of organic groups and thus by the amount of self-bias, as stated above.

As already stated, the anti-corrosion properties of the coating by z. B. adjustment of the self-bias, the inflows of the organosilicon precursor (s) and are adjusted to oxygen. Preferably, a coated light metal substrate according to the invention after 15 minutes of corrosive attack of NaOH at pH 14 and 30 ° C has no traces of corrosion visible to the naked eye.

As stated above, the crack elongation u. a. determined by the setting of the self-bias. In a preferred light metal substrate according to the invention, the coating has an elongation to microcracking (crack elongation) of greater than or equal to 1%. preferably greater than or equal to 1.5%.

The self-bias influences the layer thickness homogeneity in the above manner. In the case of larger reactors, however, the gas flows are also of great importance. By this example, those reactors are to be understood, in which the reactor chamber (recipient) 2 m ³ or greater. The layer thickness homogeneity, inter alia, is defined by the electric fields generated on the substrate, ie a high field strength means a high deposition rate. Homogeneity can only be achieved. when the electric field strength on the substrate is largely the same everywhere. In general, the layer thickness homogeneity on the arbitrary three-dimensional substrate obeys the Laplace equation. which indicates the solution for the electric field strength on the substrate. Preferably, in a coated light metal substrate according to the invention, the maximum layer thickness differs from the minimum layer thickness by a factor of 1.1 or less.

According to a further aspect, the present invention also relates to the use of a coating which can be produced by a method according to the invention (in particular in a configuration designated as preferred) as a substitute for an anodized layer.

Further aspects of the present invention will become apparent from the following examples. the drawings and the claims.

Examples: Example 1: XPS

XPS measurements (ESCA measurements) were carried out using the KRATOS AXIS Ultra spectrometer from Kratos Analytical. The calibration of the meter was made so that the aliphatic portion of the C 1s peak is 285.00 eV. Due to charging effects, it will usually be necessary. to shift the energy axis to this fixed value without further modification. The analysis chamber was equipped with an X-ray source for monochromatized Al K _α radiation. an electron source equipped as a neutralizer and a quadrupole mass spectrometer. Furthermore, the system had a magnetic lens, which focused the photoelectrons via an entrance slit in a hemispherical analyzer. During the measurement, the surface normal pointed to the entrance slot of the hemisphere analyzer. The pass energy was 160 eV when determining the molar ratios. In determining the peak parameters, the pass energy was 20 eV each.

The measurement conditions mentioned are preferred in order to enable a high degree of independence from the spectrometer type and to identify plasma-polymer products according to the invention.

The reference material used was the polydimethylsiloxane silicone oil DMS-T23E from Gelest Inc. (Morrisville, USA). This trimethylsiloxy-terminated silicone oil has a kinematic viscosity of 350 mm ² / s (± 10%) and a density of 0.970 g / mL at 25 ° C and an average molecular weight of about 13 650 g / mol. The selected material is characterized by an extremely low level of volatiles: after 24 hours at 125 ° C and 10 ^-5 Torr vacuum, less than 0.01% volatiles were detected (per ASTM-E595-85 and NASA SP-R0022A). It was applied by means of a spin-coating process as a 40 or 50 nm thick layer on a silicon wafer; In this case, hexamethyldisiloxane was used as the solvent.

With the procedure described above, the following atomic composition results for the silicone oil DMS-T23E. The binding energies of the electrons are also listed. Table 1: Chemical composition and binding energy of silicone oil DMS-T23E element Si O C Concentration [atomic%] 24.76 25.40 49.84 Binding energy [eV] 102.39 532.04 285.00

Example 2: Measurement of hardness by means of nanoindentation

The nanoindentation hardness of a sample was determined using a Berkovich indenter (manufacturer: Hysitron Inc. Minneapolis, USA). The calibration and evaluation was done according to the established procedure of Oliver & Pharr (J.Mos.Res. 7, 1564 (1992 )). The machine rigidity and area function of the indenter were calibrated prior to measurement. In indentation, the multiple partial unloading method ( Schiffmann & Küster, Z. Metallkunde 95. 311 (2004 )) to obtain depth-dependent hardness values and thus to exclude a substrate influence.

Example 3: Elongation to microcracking

A 0.5 mm thin and 10 cm long coated aluminum sheet is stretched as long. until optically cracks become visible. The crack extension limit is equal to the quotient of change in length to the total length of the aluminum.

Example 4 IR spectroscopy and determination of the intensity ratio of two bands in the IR spectrum

The measurements were carried out using an IFS 66 / S IR spectrometer from Bruker. As a method, the IRRAS technique was used, with the help of which even the thinnest coatings can be measured. The spectra were recorded in the wavenumber range of 700 to 4000 cm ^-1 . The substrate material used was small platelets of very clean and particularly even aluminum. The incident angle of the IR light was 50 ° in the measurement. While the sample was in the IR spectrometer. The sample chamber was continuously purged with dry air. The spectrum recorded under such conditions. that the water vapor content in the sample chamber was so small that no rotation bands of the water could be detected in the IR spectrum. As a reference, an uncoated aluminum plate was used.

The intensity ratio of two bands (peaks) is determined as follows: The baseline in the region of a peak is defined by the two minima. which include the maximum of the band and corresponds to the distance between them. It is assumed. the absorption bands are Gaussian. The intensity of a band corresponds to the area between the baseline and the measurement curve, limited by the two minima. which include the maximum, and can be easily determined by those skilled in the art by known methods. The determination of the intensity ratio of two bands is done by taking the quotient of their intensities. Basic requirement for the comparison of two samples is hereby. that the coatings have the same thickness and that the angle of incidence is not changed.

Exemplary embodiments 1 to 4

An anodised aluminum trim strip as substrate is additionally provided with a transparent, stretchable scratch and corrosion resistant coating.

The substrate is mounted on or in the immediate vicinity of the HF (13.56 MHz) operated cathode (area about 15 x 15 cm), so that the substrate itself acts as a cathode. After the rectangular low-pressure reactor with a volume of about 360 l and an installed nominal suction capacity of 4500 m ³ / h has been evacuated to a pressure of less than 0.02 mbar. oxygen is introduced into the reactor at a flow of 280 sccm. Using a high-frequency plasma discharge (13.56 MHz), a self-bias voltage of 250 V is set on the substrate. Under these conditions, a Sputterätzprozess takes place, in which in particular organic impurities are efficiently degraded. This step as step A of the method according to the invention has a duration of 5 min. The plasma coating is then deposited on the prepurified substrate surface in step B of the method according to the invention. For this purpose, hexamethyldisiloxane (HMDSO) is introduced into the reactor at a flow of 66 sccm and oxygen at a flow of 280 sccm. The self-bias voltage is controlled to set a value of 100V, 250V, 300V, or 400V. After a coating time of 20 minutes, an approximately 4 μm thick layer has been deposited on the substrate. Corrosive attack by NaOH (pH 14. 5 min, 30 ° C) does not cause visible signs of corrosion to the naked eye. In particular, coatings 1 to 3 (see Table 2) are transparent in the visible and UVA range. This is reflected in the very low absorption constant k at 300 or 400 nm. The absorption constants were calculated from the ellipsometric data according to the manual of the WVASE32 spectrometer from JA Woolam Co. Inc. Table 2: Optical constants of the coating at the variation of the BIAS voltage. Self-bias n _{250 nm} n _{300 nm} n _{400 nm} k _{250 nm} k _{300 nm} k _{400 nm} 1 100V 1:58 1:52 1:50 0015 00:00 0 2 250V 1.62 1:58 1:54 0040 12:01 0 3 300V 1.85 1.76 1:58 0110 12:06 12:01 4 400V 2.20 1.93 1.85 0210 0.1 2 12:03

Embodiment 5

A non-anodized aluminum substrate was treated by plasma polymerization under the process parameters listed in Example 2 (66 sccm HMDSO and 280 sccm O ₂ , Self-bias = 250 V). The coating proved to be more scratch-resistant than a compacted anodized coating with a layer thickness of about 8 μm, but a significantly higher crack elongation (greater than 2%) was observed than with an anodized coating.

To determine the scratch resistance on the aluminum substrate, an eccentric device (Starnberger) was used. This was done with a felt plate. which has been loaded with a 1 kg weight. An approximately 8 μm thick anodization surface (not according to the invention), a plasma-polymer coating (not according to the invention, produced by TW Jelinek, surface treatment of aluminum, approximately 500 nm thick, produced according to a process of the prior art.) Eugen G. Leuze Verlag, Saulgau , 1996) and a 4 μm thick plasma polymer coating according to the invention. The anodization surface was already visibly scratched after about 300 strokes after about 300 double strokes and the non-inventive plasma polymer coating after about 500 strokes. In the case of the plasma-polymer coating according to the invention, no visually noticeable scratches could be detected even after 10,000 strokes.

The good corrosion protection properties of the coating produced by plasma polymerization according to the invention apply in acidic (20% sulfuric acid, 45 min, 65 ° C.) as in basic (NaOH, pH 14, 5 min, 30 ° C.) media. After these corrosion tests, no visible damage or no infiltration of coating edges could be detected.

The infrared spectrum of this coating shows FIG. 1 , The CH valence vibrations at 2966 cm ^-1 , the Si-H oscillation at 2238 cm ^-1 , the SiCH ₂ -Si oscillation at approx. 1360 cm ^-1 , and the Si-CH ₃ deformation at 1273 cm can be clearly seen ^-1 , the Si-O vibrations at 1192 cm ^-1 and 820 cm ^-1, respectively. The band ratio (according to Example 4) between Si-H and Si-CH ₃ is about 1: 5. The IR spectrum shows a small peak at 1360 cm-1. This band is correlated with a Si-CH ₂ -Si vibration. Thus, in addition to the Si-O-Si network, there is an Si-CH ₂ -Si network in the coating.

The hardness of the coating was determined according to Example 2. It is 3 GPa. The pencil hardness is 4H.

The coating is homogeneous in depth. The proportions of carbon are about 10%, of silicon about 10%. about 30% of hydrogen and about 50% of oxygen. A scanning electron microscopic examination of the fracture edge of the plasma polymer layer shows, on the one hand, a scaly break as known for brittle materials (glass) and, on the other hand, small step-like fractures expected for crystalline materials.

Embodiment 6

The experiment is carried out as in Embodiment 5. however, in order to increase the deposition rate and thus shorten the process time, a plasma generator oscillating at a frequency of 27.12 MHz was used. With equal gas flows (HMDSO: 66 sccm, O ₂ : 280 sccm) and self-bias voltage (250 V), the deposition rate is increased by a factor of 1.5. The coating properties change only insignificantly and are very similar to those of the coating of Example 5.

Embodiment 7

The anti-corrosion properties of various on technical. Coated aluminum 99.5 deposited coatings were checked in acidic medium (20% sulfuric acid, 45 min, 65 ° C).
a) Variation of self-bias (HMDSO: 45 sccm O ₂ : 50 sccm) Self-bias coating color Corrosion resistance Band ratio Si-H / Si-CH ₃ 50V Colorless - 00:00 100V Colorless - 12:20 150V pale yellow + 12:32 300V yellow + + 0.61
b) Variation of the O ₂ -Hole (HMDSO: 45 sccm, Self-bias: 300 V) O ₂ flow coating color Corrosion resistance Band ratio Si-H / Si-CH ₃ 0 sccm yellow O 0.61 50 sccm yellow O 0, 50 100 sccm very pale yellow + + 12:32 150 sccm very pale yellow + + 12:25

• -- schneller. flächiger korrosiver Angriff
• - flächiger, korrosiver Angriff
• o punktueller korrosiver Angriff
• + am Ende der Messzeit an wenigen Stellen punktueller korrosiver Angriff
• + + kein korrosiver Angriff

The corrosion resistance was evaluated according to the following qualitative five-step scale:

• -- more quickly. extensive corrosive attack
• - surface, corrosive attack
• o selective corrosive attack
• + at the end of the measuring time in a few places selective corrosive attack
• + + no corrosive attack

It is possible to optimize the trade-off between layer properties by choosing a self-bias voltage of about 250 V and a ratio of O ₂ to HMDSO of about 3: 1 or greater. At a ratio of O ₂ to HMDSO of over 4: 1, such as in Example 5, the coating becomes completely transparent.

For the series of measurements given above under a) and b), two different batches of aluminum 99.5 were used. These may differ in their content of metals other than aluminum (e.g., Mg.Cu), which may result in differences in corrosion resistance.

Claims (15)

1. A method for coating the surface of a light metal substrate, with the following steps:

   A. preparing the light metal substrate and optionally cleaning the substrate surface to be coated,

   B. coating the substrate surface optionally cleaned in step A in a plasma polymerisation reactor by means of plasma polymerisation, wherein

   - in step B, one or more organosilicon compounds are used as precursor(s) for the plasma and
   in step B the light metal substrate is arranged in the plasma polymerisation reactor in such a way that (i) it is located between the region in which the plasma is formed and the cathode or (ii) it acts as a cathode,
   characterised in that the method is carried out in such a way that the coating produced by the method has

   - a proportion of carbon, which can be determined by XPS measurement, of 5 to 20 atom %, preferably 10 to 15 atom %, based on the total number of carbon, silicon and oxygen atoms contained in the coating,

   - a yellow index, determined in accordance with ASTM D 1925, of ≤ 3, preferably 2.5 and

   - a hardness, to be measured by means of nanoindentation, in the range of 2,5 to 6 GPa, preferably 3.1 to 6 GPa,
2. The method according to claim 1, wherein the light metal substrate is an aluminium or magnesium substrate.
3. The method according to one of the preceding claims, wherein in step B oxygen is supplied to the plasma and preferably all of the substances supplied to the plasma in step B are gaseous before entering the plasma polymerisation reactor and the ratio of the gas flows of oxygen and further precursor(s) supplied to the plasma in step B is in the range of 1:1 to 6:1, preferably 3:1 to 5.1.
4. The method according to one of the preceding claims, wherein in step B one or more siloxanes, preferably hexamethyldisiloxane (HMDSO), optionally oxygen and preferably no further compounds are used as precursor(s) for the plasma.
5. The method according to one of the preceding claims, wherein a control is conducted during step B, so that the self-bias is in the range of 50 to 1000 V, preferably in the range of 100 to 400 V, preferably in the range of 100 to 300 V
   and/or
   wherein during step B the self-bias is set on the substrate
   and/or
   wherein during step B a control is conducted so that the self-bias is constant
   and/or
   wherein in step B the light metal substrate is in spatial contact with (i) the cathode or (ii) a part of the cathode that is distinguishable from the aluminium substrate
   and/or
   wherein the plasma polymerisation is carried out at a temperature of less than 200°C and/or a pressure of less than 1 mbar, preferably in the range of 10^-3 to 10^-1 mbar
   and/or
   wherein in step B the deposition rate is set to a value of greater than or equal to 0.2 µm/min, preferably greater than or equal to 03 µm/min
   and/or
   wherein step B is carried out up to a thickness of the deposited layer of greater than or equal to 2 µm, preferably greater than or equal to 4 µm.
6. The method according to one of the preceding claims, wherein the light metal substrate is an aluminium substrate selected from the group of substrates consisting of: aluminium or aluminium alloy having a cleaned, uncoated surface; aluminium or aluminium alloy having a superficial oxide layer; is anodised aluminium or aluminium alloy having a dyed or undyed, compacted or uncompacted oxide layer, or a magnesium substrate selected from the group of substrates consisting of magnesium or magnesium alloys having a cleaned, uncoated surface, magnesium or magnesium alloy having a superficial oxide layer.
7. The method according to one of the preceding claims, wherein in step A the substrate surface to be coated is cleaned by means of a plasma and preferably in step A a gas or gas mixture is added to the plasma for carrying out the plasma cleaning, the gas or gas mixture being selected from the group consisting of: argon, argon-hydrogen mixture, oxygen.
8. The method according to one of the preceding claims, wherein the plasma is generated by means of high frequency.
9. A coated light metal substrate which can be produced by a method according to one of claims 1 to 8.
10. The coated light metal substrate according to claim 9, wherein the coating has proportions, which can be determined by XPS measurement, of 5 to 30 atom %, preferably 10 to 25 atom %, of silicon and 30 to 70 atom %, preferably 40 to 60 atom %, of oxygen, based on the total number of carbon, silicon and oxygen atoms contained in the coating.
11. The coated light metal substrate according to claim 9 or 10, wherein an IR spectrum absorbed by the coating displays one or more, preferably all, of the following bands having a respective maximum in the following ranges: C-H stretching vibration in the range of 2950 to 2970 cm^-1, Si-H vibration in the range of 2150 to 2250 cm^-1, Si-CH₂-Si vibration in the range of 1350 to 1370 cm^-1, Si-CH₃ bending vibration in the range of 1250 to 1280 cm^-1 and Si-O vibration at greater than or equal to 1150 cm^-1.
12. The coated light metal substrate according to one of claims 9 to 11, wherein, in an IR spectrum absorbed by the coating, the ratio of the intensity of the Si-H band to the intensity of the Si-CH₃ band is less than or equal to 0.3, preferably less than or equal to 0.2.
13. The coated light metal substrate according to one of claims 9 to 12, wherein the coating displays an absorption constant k_{300 nm} of less than or equal to 0.05 and/or an absorption constant k_{400 nm} of less than or equal to 0.01
    and/or
    wherein the coating displays a surface energy in the range of 20 to 40 mN/m, preferably 25 to 35 mN/m.
14. The coated light metal substrate according to one of claims 9 to 13 which, after a 15-minute corrosive attack by NaOH at pH 13.5 and 30°C, displays no traces of corrosion that are visible to the naked eye
    and/or
    wherein the coating displays a strain to microcracking of greater than or equal to 1%, preferably greater than or equal to 1.5%
    and/or
    wherein the maximum layer thickness differs from the minimum layer thickness by a factor of 1.1 or less.
15. Use of a coating which can be produced by a method according to one of claims 1 to 8 to improve the protection of a light metal substrate against scratching.

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