CORROSION RESISTANT COATINGS

Abstract

The present invention relates to a coated substrate comprising a substrate surface coated with a coating comprising at least one layer, wherein the at least one layer comprises titanium, aluminum and nitrogen, wherein—the content of aluminum in relation to the content of titanium in the at least one layer comprising titanium, aluminum and nitrogen satisfy Al/Ti>1 by considering only the respective concentrations in atomic percentage of aluminum and titanium in the at least one layer comprising titanium, aluminum and nitrogen, and—the at least one layer comprising titanium, aluminum and nitrogen exhibits wurtzite phase of aluminum nitride and rutile phase of titanium oxide.

Claims

1. A coated substrate comprising: a substrate surface coated with a coating comprising at least one layer, wherein the at least one layer comprises titanium, aluminum and nitrogen, and a content of aluminum in relation to a content of titanium in the at least one layer comprising titanium, aluminum and nitrogen satisfies Al/Ti>1 by considering only respective concentrations in atomic percentage of aluminum and titanium in the at least one layer comprising titanium, aluminum and nitrogen, and the at least one layer comprising titanium, aluminum and nitrogen exhibits wurtzite phase of aluminum nitride and rutile phase of titanium oxide.

2. The coated substrate according to claim 1, wherein the substrate material is stainless steel or a Ni-based, or Co-based or NiCo-based superalloy material.

3. The coated substrate according to claim 2, wherein the substrate is a part of a component or is a part of an article or is a component or is an article used in the aerospace or power generation industry.

4. The coated substrate according to claim 3, wherein the coated surface is intended to be exposed to air at temperatures in a range from 500° C. to 950° C.

5. A coated substrate comprising: a substrate surface coated with a coating comprising at least one layer, wherein the at least one layer comprises titanium, aluminum and nitrogen, and a content of aluminum in relation to a content of titanium in the at least one layer comprising titanium, aluminum and nitrogen satisfies Al/Ti>1 by considering only respective concentrations in atomic percentage of aluminum and titanium in the at least one layer comprising titanium, aluminum and nitrogen, and the at least one layer comprising titanium, aluminum and nitrogen exhibits wurtzite phase of aluminum nitride, and the substrate is a part of a component or is a part of an article or is a component or is an article used in the aerospace or power generation industry, wherein the substrate material is stainless steel or a Ni-based, or Co-based or NiCo-based superalloy material.

6. The coated substrate according to claim 1, wherein 54/46≤Al/Ti≤80/20.

7. A method for producing a coated substrate according to claim 1, comprising producing the at least one layer comprising titanium, aluminum and nitrogen by: a) depositing a layer comprising titanium, aluminum and nitrogen on at least one surface of the substrate, wherein said layer comprising titanium aluminum and nitrogen is deposited exhibiting wurzite phase of aluminum nitride and having the content of aluminum in relation to the content of titanium satisfy Al/Ti>1, if considering only respective concentrations of aluminum and titanium in atomic percentage and, b) subjecting the substrate coated as indicated in process step a) to a process in which rutile phase of titanium oxide is formed.

8. The method according to claim 7, wherein step a) is conducted by using physical vapor deposition techniques for the deposition of the layer comprising titanium, aluminum and nitrogen and step b) includes exposing at least a part of the substrate coated as indicated in process step a) to temperatures between 500° C. and 950° C.

9. The method according to claim 8, wherein the physical vapor deposition process is a reactive cathodic arc evaporation process.

10. The method according to claim 9, wherein at least a target composed of aluminum and titanium and having an element composition satisfying Al/Ti>1 in atomic percentage is used as a material source and nitrogen gas is used as a reactive gas during deposition of the layer comprising titanium, aluminum and nitrogen.

11. The method according to claim 10, wherein the at least one target has a composition of Al 60 at. % and Ti 40 at. %.

Description

DESCRIPTION OF FIGURES

[0033] FIG. 1 XRD diffractogram of a state of the art TiAlN coating deposited by cathodic arc from a target with a ratio of Al:Ti of 50:50, substrate temperature T=350° C.

[0034] FIG. 2 XRD diffractogram of a state of the art TiAlN coating deposited by cathodic arc from a target with a ratio of Al:Ti of 50:50, substrate temperature T=480° C.

[0035] FIG. 3 XRD diffractogram of an embodiment of an inventive TiAlN coating deposited by cathodic arc from a target with a ratio of Al:Ti of 60:40, substrate temperature T=350° C.

[0036] FIG. 4 XRD diffractogram of an embodiment of an inventive TiAlN coating deposited by cathodic arc from a target with a ratio of Al:Ti of 60:40, substrate temperature T=480° C.

[0037] FIG. 5 Solid Particle Erosion (SPE) results of state of the art and two embodiments of the inventive coating on Inconel718 at impact angles of 20° and 90°

[0038] FIG. 6 Hot Corrosion Resistance test performed at a temperature between 730 and 735° C. (e.g. 732° C.), of one embodiment of an inventive coating system deposited on an Inconel718 substrate, with a substrate temperature of T=350° C.

[0039] FIG. 7 Rotating beam fatigue test results of an embodiment of the inventive coating, deposited by cathodic arc from a target with Al:Ti ratio of 60:40, substrate temperature 480° C.

[0040] FIG. 8 Schematic illustration of a cathodic arc evaporation set-up to deposit the inventive coating on a substrate sample

[0041] FIG. 9 Schematic representation of one embodiment of the inventive coating system

[0042] FIG. 10 XRD diffractogram for a TiAlN coating on a 1.4938 stainless steel substrate

[0043] Subsequently, some embodiments of the present invention will be described by way of example, which is meant to be merely illustrative and therefore non limiting. In order to show the improvement of the corrosion resistance of the inventive coating, the inventive coating as well as several state of the art coatings were tested, and the results of said test are described in the following.

[0044] A state of the art titanium-aluminum (Ti—Al) target with Aluminum (Al) to Titanium (Ti) ratio of Al/Ti=50/50 was provided in order to deposit a thin-film coating on an Inconel substrate using cathodic arc evaporation. This was done for two different substrate temperatures, T=350° C. and T=480° C. The nitrogen gas pressure was set to 3.2 3.2e-2 mbar. For a deposition temperature of T=350° C., a composition of the deposited coating of titanium (Ti) is 24.2 at %, aluminum (Al) is 20.7 at % and nitrogen (N) is 55.2 at % measured by Energy Dispersive X-Ray (EDX) Analysis. For a deposition temperature of T=480° C., a composition of the deposited coating of titanium (Ti) is 23.2 at %, aluminum (Al) is 20.2 at % and nitrogen (N) is 56.2 at % measured by EDX. It can be seen, that the state of the art coatings have approximately the same compositions, independent of the deposition temperature.

[0045] The properties of the state of the art titanium aluminum nitride (TiAlN) coating evaporated from a titanium-aluminum (Ti—Al) target with an aluminum (Al) to titanium (Ti) ratio of Al/Ti=50/50, and deposited at T=350° C. were further investigated. Said coating exhibits a thickness of 14 μm, a stress on a steel substrate of −1.6 GPa, an Indentation Hardness H.sub.IT of 29±2 GPa, an Youngs modulus by indentation E.sub.IT of 390±13 GPa, a surface roughness of R.sub.a=0.26 μm and R.sub.z=3.14 μm and a Critical Normal Load Lc2 of 48 N.

[0046] The properties of the state of the art titanium aluminum nitride (TiAlN) coating evaporated from a titanium-aluminum (Ti—Al) target with an aluminum (Al) to titanium (Ti) ratio of Al/Ti=50/50, and deposited at T=480° C. were further investigated. Said coating exhibits a thickness of 15 μm, a stress on a steel substrate of −1.6 GPa, an Indentation Hardness H.sub.IT of 32±2 GPa, an Youngs modulus by indentation E.sub.IT of 383±20 GPa, a surface roughness of R.sub.a=0.19 μm and R.sub.z=2.62 μm and a Critical Normal Load Lc2 of 62 N.

[0047] In one embodiment of the inventive coating, a titanium-aluminum (Ti—Al) target with aluminum (Al) to titanium (Ti) ratio of Al/Ti=60/40 was provided. As for the above-mentioned state of the art coatings, the deposition of the inventive coating on an Inconel substrate was performed for two different substrate temperatures, T=350° C. and T=480° C. The nitrogen gas pressure was set to 3.2 Pa. For a deposition temperature of T=350° C., a composition of the deposited coating of titanium (Ti) is 18.7 at %, aluminum (Al) is 25.9 at % and nitrogen (N) is 55.4 at % was measured by EDX. Fora deposition temperature of T=480° C., a composition of the deposited coating of titanium (Ti) is 18.5 at %, aluminum (Al) is 25.9 at % and nitrogen (N) is 55.6 at % measured by EDX. It can be seen, that the inventive coatings have approximately the same compositions, independent of the deposition temperature.

[0048] The properties of one embodiment of the inventive titanium aluminum nitride (TiAlN) coating evaporated from a titanium-aluminum (Ti—Al) target with an aluminum (Al) to titanium (Ti) ratio of Al/Ti=60/40, and deposited at T=350° C. were further investigated. Said coating exhibits a thickness of 13 μm, a stress on a steel substrate of −0.8 GPa, an Indentation Hardness H.sub.IT of 33±1 GPa, an Youngs modulus by indentation Err of 362±5 GPa, a surface roughness of R.sub.a=0.25 μm and R.sub.z=3.13 μm and a Critical Normal Load Lc2 of 37 N.

[0049] The properties of another embodiment of the inventive titanium aluminum nitride (TiAlN) coating evaporated from a titanium-aluminum (Ti—Al) target with an aluminum (Al) to titanium (Ti) ratio of Al/Ti=60/40, and deposited at T=480° C. were further investigated. Said coating exhibits a thickness of 12 μm, a stress on a steel substrate of −1.5 GPa, an Indentation Hardness H.sub.IT of 34±1 GPa, an Youngs modulus by indentation E.sub.IT of 331±8 GPa, a surface roughness of R.sub.a=0.24 μm and R.sub.z=3.68 μm and a Critical Normal Load Lc2 of 43 N.

[0050] A comparison of the above described properties of the embodiments of the inventive coatings and the properties of the tested state of the art coatings, show approximately similar properties.

[0051] An XRD phase analysis was performed on the four previously described coatings. Looking at FIGS. 1, 2, 3 and 4, it can be seen that both state of the art coatings, using a titanium-aluminum (Ti—Al) target with an aluminum (Al) to titanium (Ti) ratio of Al/Ti=50/50, and deposited at substrate temperatures of T=350° C. and T=480° C. show only cubic phase. In contrast both embodiments of the inventive coating, using a titanium-aluminum (Ti—Al) target with an aluminum (Al) to titanium (Ti) ratio of Al/Ti=60/40, and deposited at substrate temperatures of T=350° C. and T=480° C. show cubic and hexagonal aluminum nitride (AlN) phases. The difference can also be seen in the Cross Sectional Analysis of the four tested coatings. Both embodiments of the inventive coating, using a titanium-aluminum (Ti—Al) target with an aluminum (Al) to titanium (Ti) ratio of Al/Ti=60/40, and deposited at substrate temperatures of T=350° C. and T=480° C. show less columnar grains, than the state of the art coatings, using a titanium-aluminum (Ti—Al) target with an aluminum (Al) to titanium (Ti) ratio of Al/Ti=50/50, and deposited at substrate temperatures of T=350° C. and T=480° C.

[0052] After annealing the XRD phase analysis was repeated. In addition to the cubic and wurtzite AlN phase the anatase and rutile TiO2 phases were formed. Furthermore, an aluminium oxide was formed, which could not be detected in XRD, but was detected by EDX in Cross Sectional Analysis.

[0053] Furthermore the oxidation resistance of the two state of the art coatings as well as the oxidation resistance of the two above described embodiments of the inventive coating system was tested at T=650° C., 677° C., 732° C. The oxide scale thickness was measured to be approximately 300 nm after 24 h and no significant increase of the oxide scale could be detected after 200 h.

[0054] The erosion resistance of two embodiments of the inventive coating system was compared with the erosion resistance of a state of the art coating and uncoated substrate materials using a Solid Particle Erosion (SPE) test. Therefore four different samples were tested at impact angles of 20° and 90° using corundum abrasive particles (approximately 50 μm) with a nozzle to sample distance of 90 mm, a speed of 90 m/s and an abrasive feed rate of 350 g/min. The erosion resistance of the SPE tested samples is shown in FIG. 5. It can be seen, that the erosion resistance of the tested inventive aluminum titanium nitride (AlTiN 40/60) coating is higher than the erosion resistance of the tested state of the art aluminum titanium nitride (AlTiN 50/50) coating.

[0055] Furthermore, the corrosion resistance of an embodiment of the inventive coating system, directly after deposition and after annealing, was compared to the corrosion resistance of equivalently treated bare Inconel718 and to an equivalently treated Inconel718 substrate coated with a state of the art coating. The state of the art coating was deposited at a substrate temperature of T=350° C. Two embodiments of the inventive coatings were deposited at substrate temperatures T=350° C. and T=480° C. A Neutral Salt Spray Test (NSST) according to DIN EN ISO 9227 was carried out on the samples. After 432 h of NSST the bare Inconel718 substrate shows no corrosion, the Inconel718 substrate coated with a state of the art titanium aluminum nitride (TiAlN) based coating, deposited at T=350° C. also shows no corrosion after 432 h. Furthermore, the Inconel718 substrate coated with a state of the art titanium aluminum nitride (TiAlN) based coating, and deposited at T=350° C., which was thermally exposed to a temperature of T=650° C. for 24 h, shows no corrosion (Ri0). After 432 h of NSST the bare Inconel718 substrate shows no corrosion, while the Inconel718 substrate coated with a state of the art titanium aluminum nitride (TiAlN) based coating, deposited at T=480° C. shows no corrosion after 432 h. Furthermore, the Inconel718 substrate coated with a state of the art titanium aluminum nitride (TiAlN) based coating, and deposited at T=480° C., which was thermally exposed to a temperature of T=650° C. for 24 h, shows no corrosion (Ri0).

[0056] The samples were also tested in a hot corrosion test which was performed at a temperature between 730° C. and 735° C. using salt mixture of magnesium sulfate (MgSO.sub.4) and sodium sulfate (Na.sub.2SO.sub.4) for 260 h. All the tested coatings were deposited on Inconel718 substrates at a substrate temperature of T=350° C. It can be seen in FIG. 6, that the hot corrosion resistance of one embodiment of an inventive aluminum titanium nitride (60:40) coating is much better than the hot corrosion resistance of a tested state of the art titanium aluminum nitride (50:50) coating.

[0057] The coating was also tested in rotating beam fatigue test, as can be seen in FIG. 7. The test was performed using staircase method with a frequency of 100 Hz and a reverse stress cycle of R=−1 at room temperature. The results were evaluated according to ASTM E739 standard. No fatigue debit could be detected.

[0058] According to one aspect of the present invention, the inventive coating can be deposited using a PVD method, preferably sputtering or cathodic arc. An embodiment of applying the inventive coating system with the use of cathodic arc will be described.

[0059] The said coating system is deposited on a sample using a cathodic arc deposition method. In order to apply the inventive coating system to a sample, using the inventive coating method, a sample is placed in a vacuum coating chamber. The substrate is placed rotatable in the center of said vacuum chamber on a carousel. The inventive coating system can be deposited on the sample by using a different amount of targets functioning as cathodes, such as for example two, four, six or even more targets. The order and number of the targets can be of any desired kind. The targets are preferably mounted at the walls of the vacuum coating chamber. In order to produce the inventive coating system described in this specific embodiment, the cathodes are aluminum titanium (AlTi) targets, whereas the ratio Al/Ti>1. The target positions are to be seen as only one example of the present invention and are not limiting. In order to generate the nitride containing layers, a non-zero amount of N.sub.2 is inserted into the vacuum chamber through the gas inlet. In this example the N.sub.2 pressure was set to 3.2e-2 mbar. Preferably an argon (Ar) gas inlet is installed as well, in order to use argon as a work gas. In order to produce the inventive coating system, the coating temperature is chosen within a range between 200-500° C. Magnets are located behind the targets, and the magnetic field can be adjusted in order to influence the coating. Shutters can be installed in front of the targets to allow different coating layers, but are not compulsory.

[0060] The above described examples of the inventive coating system and the methods to deposit the inventive coating system are however not limiting. The ratio of aluminum (Al) to titanium (Ti) found in the target could as well be chosen differently. A ratio within the range of Al: 60 at %, Ti: 40 at % and Al: 90 at %, Ti: 10 at % leads to a cubic titanium aluminum nitride (TiAlN) and hexagonal wurtzite aluminum nitride (w-AlN) formation, for this two-phase-coating preferably a ratio of Al: 80 at %, Ti: 20 at % is chosen. Choosing the composition in order to form a hexagonal wurtzite aluminum nitride (w-AlN) leads to a slight reduction of the hardness and elastic modulus of the coating, but to increased corrosion and oxidation resistance compared to the cubic phase, which is commonly formed for nitrides.

[0061] The inventors found another surprising fact: A coating system which comprises a metallic interlayer and a top layer, which can consist of either a monolayer or a multilayer system of titanium aluminum nitride (TiAlN) exhibits as well improved corrosion resistance. It could be shown in various standardized corrosion tests, that the coating exhibits an enhanced corrosion resistance compared to previous coating systems which are known from the state of the art. Furthermore the coating system also shows improved erosion resistance in various standardized tests.

[0062] According to this, a coating system on a substrate is provided, consisting of one individual interlayer containing niobium (Nb), chromium (Cr), zirconium (Zr), hafnium (Hf) or molybdenum (Mo), or any combinations thereof, which are deposited directly on the substrate material, and a top layer (T) which is either a monolayer or a multilayer system of titanium aluminum nitride (TiAlN) layers, which can exhibit different ratios of titanium (Ti) to aluminum (Al) to nitrogen (N). One version of the coating system is shown in FIG. 9, where a niobium interlayer (6) is deposited on the substrate (5), followed by titanium aluminum nitride (TiAlN) layers with different compositions, which are arranged in an alternating way. However this is only one example and not limiting.

[0063] Turning now back to the first aspect of the present based on an individual interlayer containing niobium (Nb), chromium (Cr), zirconium (Zr), hafnium (Hf) or molybdenum (Mo), or any combinations thereof, which are deposited directly on the substrate material, and a top layer (T) which is either a monolayer or a multilayer system of titanium aluminum nitride (TiAlN) layers, which can exhibit different ratios of titanium (Ti) to aluminum (Al) to nitrogen (N).

[0064] If for example a titanium aluminum nitride (TiAlN) monolayer with a certain composition is used as a top layer (T), the coating thickness can range from 1 μm to 50 μm and is preferably chosen to be between 1 μm and 25 μm. The titanium aluminum nitride (TiAlN) layer shows a cubic crystal structure, as can be seen in FIG. 3, and a lattice constant of a=4.171 Å. The composition is preferably chosen to be Ti: 23±2 at %, Al: 22±2 at %, N: 55±4 at %, but is not limited to this specific composition. The indentation hardness was measured to be 30±2 GPa. The indentation modulus was measured to be 385±12 GPa. The coating hardness was measured using an instrumented indentation test with a Vickers Indenter and a maximum measuring force of 100 mN inside a calo grind. The listed results are average values of 20 single measurements. The hardness values were evaluated according to the Oliver and Pharr method. The indentation depth is less than 10% of the coating thickness to minimise substrate interference.

[0065] The above described example is however not limiting. The ratio of aluminum (Al) to titanium (Ti) could as well be chosen differently. A ratio within the range of Al: 70 at %, Ti: 30 at % and Al: 90 at %, Ti: 10 at % leads to a two phase structure consisting of cubic NaCl and hexagonal Wurtzite structure, i.e. to the formation of Wurtzite AlN (w-AlN). Preferably a ratio of Al: 80 at %, Ti: 20 at % is chosen. Choosing the composition in order to form the hexagonal Wurtzite phase leads to a slight reduction of the hardness and elastic modulus of the coating, but to increased corrosion and oxidation resistance compared to the cubic phase, which is commonly formed for nitrides. Layers produced accordingly, can be applied either as a monolayer, as one or more layers in a multilayer system, or the multilayer system can comprise only layers with compositions of aluminum (Al) to titanium (Ti) which exhibit w-AlN.

[0066] Titanium aluminum nitride (TiAlN) exhibits excellent solid particle erosion resistance and water droplet erosion resistance. However, when tested in a neutral salt spray test (NSST) according to DIN EN ISO 9227, after 24 h red rust was observed at the 1.4313 stainless steel substrate which was coated with a titanium aluminum nitride (TiAlN) monolayer.

[0067] For some applications a multilayer system (T) can be beneficial, in order to find a balance between good erosion resistance and good corrosion resistance. The coating system can for example be deposited as shown in FIG. 2, where the top layer (T) consists of several titanium aluminum nitride (TiAlN) layers, with layers (7) and (8) having different ratios of titanium (Ti) to aluminum (Al) to nitrogen (N). This is one example of how the coating system can be designed, but is not limiting. The ratios of titanium (Ti) to aluminum (Al) to nitrogen (N) could as well be different for every single layer of the multilayer system of the top layer (T).

[0068] The substrate (5) materials to be coated include but are not limited to stainless steel, superalloys and titanium alloys. The coating is especially suitable to be applied on substrate materials such as high-chromium (9-18 wt %) containing steel, e.g. 1.4313 stainless steel, 1.4938 stainless steel, titanium, titanium alloy, intermetallics such as titanium aluminides, Inconel as well as nickel-based, cobalt-based and iron-based superalloys.

[0069] As mentioned before, corrosion processes on metallic surfaces can be very complex. Reactions can be influenced by the presence of various ionic species, humidity, temperature, and other factors. One possibility to improve the corrosion resistance, is to apply a so called sacrificial coating. Whether a metal is suitable to be used as a sacrificial coating in a specific application, depends on the absolute difference between the standard potentials of the metallic interlayer, which is to be used as a sacrificial coating, and the substrate material. In order to determine the standard potential of a metal, the electrode potential of said metal is compared with the standard hydrogen electrode, and is called the standard electrode potential E.sup.0. This can be done for all the metals. Potentials between metals are determined by taking the absolute difference between their standard potentials. A metal with a more negative potential has a higher tendency to dissolve and thus corrode, than a metal with a less negative potential, although kinetic factors may intervene. If the potential of a metal is less than the hydrogen potential, reduction rather than oxidation takes place. Metals which correspond to relatively lower standard potentials E.sup.0 are called active metals, and metals which correspond to relatively higher standard potential or less negative potentials are called noble metals. For example considering two metals, such as zinc (Zn) and aluminum (Al), aluminum (Al) is more active than zinc (Zn) since E.sup.0.sub.Al=−1.66 V, E.sup.0.sub.Zn=−0.763 V.

[0070] In order to improve the adhesion between the substrate (5) and the titanium aluminum nitride (TiAlN) coating, as well as the corrosion resistance of the so coated substrate, a metallic interface (6) containing niobium (Nb), chromium (Cr), zirconium (Zr), hafnium (Hf) or molybdenum (Mo), preferably consisting of said metals, or any combinations thereof, is deposited directly on the substrate. The said materials are metallic materials, which are sacrificial coatings. For a sacrificial coating only materials which exhibit a higher electronegativity than the substrate materials can be used. Checking the standard electrode potentials E.sup.0 of niobium (Nb), chromium (Cr), zircon (Zr), hafnium (Hf) or molybdenum (Mo) with respect to the standard hydrogen electrode, leads to the conclusion that these materials can be used as a sacrificial layer for the above mentioned substrate materials. The metallic interface (6) is thus adjusted to the substrate material in order to minimise the corrosion potential between the substrate material and the titanium aluminum nitride (TiAlN) coating, thus enhancing the corrosion resistance of the coating system. If for example a nickel based alloy (E.sup.0.sub.Ni=−0.257 V) or cobalt based alloy (E.sup.0.sub.Co=−0.28 V) is to be protected against diffusion and corrosion, a sacrificial layer consisting of or containing aluminum (Al) with E.sup.0.sub.Al=−1.662 V is commonly used. However the materials used in this specific application have to exhibit a high melting point, to ensure a certain hardness, since materials with a high melting point are often denser than materials with a lower melting point. For this application, materials exhibiting amorphous phases, and are microcrystalline and glassy, are preferred. A columnar structure of the material is not desired, since this would lead to a decreased hardness. Unfortunately aluminum has a melting point of only 660° C. One possible solution is the use of a sacrificial layer consisting of or containing hafnium (Hf) with E.sup.0.sub.Hf=−1.55 V. As can be seen the reduction potentials E.sup.0 with respect to the standard hydrogen electrode of hafnium (Hf) is slightly lower than that of aluminum (Al). However hafnium has a high melting point of 2231° C., and thus leads to the desired dense structure, and at the same time provides good diffusion and corrosion resistance for a substrate material, such as e.g. titanium. The same effect is seen when using niobium (Nb), chromium (Cr), zirconium (Zr) or molybdenum (Mo) as an interlayer. Depending on the reduction potentials E.sup.0 with respect to the standard hydrogen electrode of the substrate material, the material used for the interface is adjusted.

[0071] An example will be described by way of example, which is meant to be merely illustrative and therefore non limiting.

[0072] According to one embodiment of the example a 1.4313 stainless steel substrate was coated with a coating system. A niobium (Nb) interlayer was deposited directly on the surface, followed by a titanium aluminum nitride (TiAlN) monolayer with a Ti:Al ratio of 1:2, which was deposited directly on the niobium (Nb) interface. The overall thickness of the coating system was chosen to be 6 μm, whereas the thickness of the niobium (Nb) interlayer was 1 μm and the thickness of the titanium aluminum nitride (TiAlN) top layer was 5 μm. The coating temperature was set to 350° C. A set-up providing 4 sources was used to coat said sample with said coating system, two of them consisting of niobium (Nb), two consisting of titanium aluminum (TiAl). The so coated sample was tested for corrosion resistance in a neutral salt spray test (NSST) according to DIN EN ISO 9227.

[0073] Another example uses a chromium (Cr) interlayer and a titanium aluminum nitride (TiAlN) monolayer with a Ti:Al ratio of 1:1 on top. A 1.4313 stainless steel substrate was coated with the said example of a coating system, whereas the coating was applied at a temperature of 300° C., and exhibited a thickness of 16 μm. The sample was coated using 6 sources. The so coated sample was tested for corrosion resistance in a neutral salt spray test (NSST) according to DIN EN ISO 9227.

[0074] One embodiment of a method to deposit a coating on a substrate, is to use an interface consisting of niobium (Nb). Optionally a flow of one or more inert gases (e.g. argon (Ar)) can be introduced into the vacuum coating chamber as work gas. The metallic interface, in this case niobium (Nb), is preferentially applied using an argon (Ar) work gas. For the deposition of the coating system in a PVD vacuum chamber, depending on the desired layers to be coated, either one or more targets comprising aluminum (Al), titanium (Ti) or niobium (Nb), or titanium aluminum (TiAl) targets with various ratios of aluminum (Al) to titanium (Ti) in the solid phase can be used as a material source for supplying the metallic base materials needed to generate the layers of the coating system. For example, aluminum (Al) and titanium (Ti) targets, or titanium aluminum (TiAl) targets with different compositions or ratios of aluminum (Al) to titanium (Ti) are needed for the formation of an titanium aluminum nitride (TiAlN) multilayer system. However for the formation of some layers, such as titanium aluminum nitride (TiAlN) layers, an additional gas flow is required. A nitrogen (N.sub.2) gas flow is introduced to the vacuum chamber to be used as reactive gas to form titanium aluminum nitride (TiAlN).

[0075] The one or more targets can be operated as cathodes in order to deposit the target material onto the substrate, for example by using arc vaporisation techniques or by using any sputtering technique.

[0076] As mentioned before, nitride coatings can be generated by operating the targets in a reactive atmosphere comprising nitrogen. The amount of nitrogen which is incorporated in the titanium aluminum nitride (TiAlN) layer can be varied by changing the amount of nitrogen in the vacuum coating chamber, more precisely the nitrogen gas flow.

[0077] In the context of the present solution the ‘one or more targets comprising aluminum (Al), titanium (Ti), niobium (Nb), zirconium (Zr), hafnium (Hf), molybdenum (Mo) or aluminum titanium (AITO targets with various ratios of aluminum (Al) to titanium (Ti)’ mentioned above, are targets comprising said metals as main components. Preferably the said targets are targets consisting of the above listed metals or any combinations thereof, which can comprise traces of impurities. To deposit a titanium aluminum nitride (TiAlN) coating with a certain titanium (Ti) to aluminum (Al) ratio of u:v, the solid target is manufactured in such a way as to exhibit a titanium (Ti) to aluminum (Al) ratio of u:v.

[0078] In one embodiment of the present solution, an multilayer coating system as shown in FIG. 9, is deposited on a substrate (4) using an arc deposition method with equipment as shown in FIG. 8. The coating system shown in FIG. 9, as well as the arc deposition method described below, are to be seen as only one example but are not limited to this variant. Generally a titanium aluminum nitride (TiAlN) layer can have different compositions Ti.sub.xAl.sub.1-xN.sub.y with x ∈ [0.05, 0.95] and y=1±0.3. As shown in FIG. 2, a TiAlN (7) layer with composition Ti.sub.x1Al.sub.1-x1N.sub.y1 is deposited on a niobium (Nb) interlayer (6). Another TiAlN (8) layer with composition Ti.sub.x2Al.sub.1-x2N.sub.y2 is deposited on the TiAlN (7) layer with composition Ti.sub.x1Al.sub.1-x1N.sub.y1. The titanium aluminum nitride (TiAlN) layers with different compositions Ti.sub.x1Al.sub.1-x1N.sub.y1 and Ti.sub.x2Al.sub.1-x2N.sub.y2 are arranged in an alternating way.

[0079] As shown in FIG. 8, the said coating system is applied to a sample (4) using an arc deposition method. In order to apply the coating system to a sample (4), using the coating method, a sample (4) is placed in a vacuum coating chamber (1). The sample (4) is placed rotatable in the center of said vacuum chamber on a carousel (2). The coating system can be deposited on the sample (4) by using a different amount of targets functioning as cathodes, such as for example two, four or even more targets. The set-up shown in this particular example (FIG. 1) contains four targets, all of them set up in a way as to work as cathodes. As can be seen in FIG. 1 the targets are mounted at the walls of the vacuum coating chamber. In order to produce the coating system described in this specific embodiment, cathodes A and B are targets comprising niobium (Nb) as main component, and cathodes C and D are targets comprising titanium aluminum (TiAl) as main component. First the samples are pretreated in order to prepare them for the coating. The metallic interlayer, in this case niobium (Nb) is then deposited on the sample by switching on the niobium targets (Nb) and using argon (Ar) as a work gas. Therefore an argon (Ar) pressure of 10e-2 mbar is used. In order to generate the nitrogen (N) containing layers, a non-zero amount of N.sub.2 is inserted into the vacuum chamber through the gas inlet, the argon gas flow is sustained during this step of the process. Once the N.sub.2 pressure is stabilised, the titanium aluminum (TiAl) targets are switched on and the argon (Ar) is removed from the chamber. In this example the N.sub.2 pressure was set to 3.2e-2 mbar. In order to produce the coating system, the coating temperature is chosen within a range of 300-600° C. In this example, shutters (3) are installed in front of the targets (A, B, C, D), to allow coating different layers. However coatings of said kind can also be deposited if no shutters are installed. The coating thickness of the individual layers of the multilayer system described herein can be chosen to be at most 5 μm. If the coating is applied according to the above described method, shown in FIG. 8, a gradient is seen.

[0080] A coated substrate was disclosed comprising a substrate surface coated with a coating comprising at least one layer, wherein the at least one layer comprises titanium, aluminum and nitrogen, and wherein: [0081] the content of aluminum in relation to the content of titanium in the at least one layer comprising titanium, aluminum and nitrogen satisfy Al/Ti>1 by considering only the respective concentrations in atomic percentage of aluminum and titanium in the at least one layer comprising titanium, aluminum and nitrogen, and [0082] the at least one layer comprising titanium, aluminum and nitrogen exhibits wurtzite phase of aluminum nitride and rutile phase of titanium oxide.

[0083] Preferably the substrate material is stainless steel or a Ni-based, or Co-based or NiCo-based superalloy material.

[0084] Preferably the coated substrate is a part of a component or is a part of an article or is a component or is an article used in the aerospace or power generation industry.

[0085] Preferably the coated surface is intended to be exposed to air at temperatures in a range comprising temperature values from 500° C. to 950° C.

[0086] A coated substrate is disclosed comprising a substrate surface coated with a coating comprising at least one layer, wherein the at least one layer comprises titanium, aluminum and nitrogen, and wherein: [0087] the content of aluminum in relation to the content of titanium in the at least one layer comprising titanium, aluminum and nitrogen satisfy Al/Ti>1 by considering only the respective concentrations in atomic percentage of aluminum and titanium in the at least one layer comprising titanium, aluminum and nitrogen, and [0088] the at least one layer comprising titanium, aluminum and nitrogen exhibits wurtzite phase of aluminum nitride, and [0089] the substrate is a part of a component or is a part of an article or is a component or is an article used in the aerospace or power generation industry, wherein the substrate material is preferably stainless steel or a Ni-based, or Co-based or NiCo-based superalloy material.

[0090] For the coated substrate the following ratios may be realized: 54/46≤Al/Ti≤80/20, preferably 54/46≤Al/Ti≤70/30.

[0091] A method is disclosed for producing a coated substrate according to any of the previous claims 1 to 4, characterized in that the at least one layer comprising titanium, aluminum and nitrogen is produced by using a process including at least following process steps a) and b): [0092] a) deposition of a layer comprising titanium aluminum and nitrogen on at least one surface of the substrate, wherein said layer comprising titanium aluminum and nitrogen is deposited exhibiting wurzite phase of aluminum nitride and having content of aluminum in relation to the content of titanium satisfying Al/Ti>1, if considering only the respective concentrations of aluminum and titanium in atomic percentage and, [0093] b) subjecting the substrate coated as indicated in process step a) to a process in which rutile phase of titanium oxide is formed.

[0094] The process step a) can be conducted by using physical vapor deposition techniques for the deposition of the layer comprising titanium aluminum and nitrogen and the process step b) can be conducted including exposition of at least a part of the substrate coated as indicated in process step a) to temperatures between 500 C and 950° C.

[0095] The physical vapor deposition process may be a reactive cathodic arc evaporation process.

[0096] A target composed of aluminum and titanium and having element composition satisfying Al/Ti>1 in atomic percentage is used as material source and nitrogen gas is used as reactive gas during deposition of the layer comprising titanium aluminum and nitrogen.

[0097] At least one target may have the composition: Al 60 at. % and Ti 40 at. %

TABLE-US-00001 References 1 Coating Chamber 2 Carousel 3 Shutter 4 Sample A, B Al Cathodes C, D TiAl Cathodes N.sub.2 Reactive Gas Ar Working Gas 5 Substrate 6 Nb Interlayer 7 Ti.sub.x1Al.sub.1-x1N.sub.y1 8 Ti.sub.x2Al.sub.1-x2N.sub.y2 T Top Layer