WO2020098967A1

WO2020098967A1 - Coated article exhibiting high corrosion and erosion resistance including ain-layer

Info

Publication number: WO2020098967A1
Application number: PCT/EP2019/000311
Authority: WO
Inventors: Lin SHANG; Juergen Ramm; Beno Widrig; Mirjam Arndt
Original assignee: Oerlikon Surface Solutions AG Pfaeffikon
Current assignee: Oerlikon Surface Solutions AG Pfaeffikon
Priority date: 2018-11-13
Filing date: 2019-11-12
Publication date: 2020-05-22
Anticipated expiration: 2021-05-13

Abstract

The present invention relates to a coating system for enhanced corrosion and erosion resistance of a gas turbine engine component is disclosed, whereas the component is preferably made of stainless steel and/or superalloys and the coating system comprises a layer, the layer is consisting of aluminum nitride (AIN).

Description

Coated article exhibiting high corrosion and erosion resistance including AIN- layer

Technical Field

The present invention relates to a stainless steel or superalloy article having an oxidation, corrosion and erosion resistant coating thereon. More particularly, the invention relates to a high chromium containing steel article and to titanium alloys, such as the ones employed in the gas turbine engine for land-based and aero gas turbines, and to steam turbine engines, and exposed to oxidising, corrosive and erosive environment at moderated to elevated service temperature, having an inventive oxidation, corrosion and erosion resistant coating thereon.

Furthermore the present invention relates to a PVD method, particularly a cathodic arc deposition method, to apply the inventive coating to the article.

State of the art

There have been strong efforts to develop coatings for gas turbine components, in order to improve the corrosion and erosion resistance of the base material. Although several coating solutions for this application do exist, the current need for a further increase in performance and lifetime of turbine compressor components calls for improvements even for already well-established and widely used coating materials. Considering the development of coating systems for this application, one specific difficulty is to fulfill the requirements for the corrosion resistance of the coating and at the same time fulfill the requirements for the erosion resistance of the coating.

Industrial gas turbines are frequently operated in regions which require different protection with respect to corrosion, such as those near chemical or petrochemical plants, where various chemical species may be found in the intake air, or those at or near ocean coastlines or other saltwater environments where various sea salts may be present in the intake air, or combinations of the above, or in other applications where the inlet air contains corrosive chemical species.

CONFir!MATiOM COPY Water droplet exposure can result from use of on-line water washing, fogging and evaporative cooling, or various combinations of these processes, to enhance compressor efficiency. Among the various ionic species, which reach the surface of turbine components by water droplets, are Cl-, Br, F^‘, S2 among others. Electrochemically induced corrosion and erosion phenomena occurring at the leading edge can in turn result in cracking and even breaking of the airfoils thus bringing huge damage and economic loss to the whole engine. And finally, oxidation may occur in hot steam or in ambient at higher temperatures.

For example, stainless steel turbine compressor components, such as e.g. airfoils, of industrial gas turbines have shown susceptibility to water droplet erosion and corrosion fatigue of the airfoil surfaces. Using titanium alloys, nickel-based or cobalt-based superalloys instead of stainless steel for the components can improve the corrosion resistance, however this may not solve the water droplet erosion problem, since the metallic materials are ductile and susceptible to erosion. Furthermore a redesigning process of the turbine components would be needed due to their different metallurgical and mechanical properties. Furthermore the mentioned substrate materials may not be able to withstand the elevated temperatures occurring at later stages of an industrial gas turbine. For example, the application temperature for titanium alloys is limited to around 540°C. Compressor blades of land based gas turbines are often made of 12% chromium containing martensitic stainless steel. Chromium is the key ingredient for the corrosion resistance of stainless steels, this kind of martensitic stainless steel is designed for service in high temperature applications up to 650° C, e.g. for turbine blades. However higher temperatures occur in some parts of an industrial gas turbine. Special austenitic stainless steels and nickel-based alloys are capable of a better performance, but at much higher cost.

Another approach is to deposit a coating, preferably a thin-film coating, on a well- established substrate material, e.g. on a stainless steel substrate, which is widely used for components of industrial gas turbines, such as e.g. airfoils, and design the said coating system in such a way, as to enhance the corrosion and erosion resistance of the said component. Standard substrate materials of components of industrial gas turbine compressors, e.g. blades, include stainless steel, chromium-based alloys, nickel-based alloys and titanium-based alloys. In contrast to using other materials instead of stainless steel for the component, this method needs no redesigning of the components, since a thin-film coating is deposited in a way, such that the dimensions of the components are changed only on the level of micrometers. Gas turbine components are often protected by environmental or overlay coatings, which inhibit environmental damage. Different types of coatings providing protection on various components may be employed depending upon factors, such as whether the application involves exposure to air or combustion gas, and temperature exposure.

One type of coating, an anti-wear coating, is described by Uihlein et al in US9427937B2, especially for components which are subject to erosion under mechanical stress, in particular for gas turbine components. The coating consists of at least two different individual layers, which have been applied in a multiply alternating manner to a surface of a component, which is to be coated. The described coating system comprises a ceramic main layer, which is deposited directly onto the substrate, and a quasi-ductile, non-metallic intermediate layer. Thereby the quasi-ductile, non- metallic intermediate layer is configured in such a way, that the energy is withdrawn from cracks, which grow in the direction of the substrate material, by crack branching. This leads to a slow down or even stop of the formation of cracks, providing an increased life time for the so coated component. This patent focuses on the mechanical stress applied to a component of a gas turbine. However turbines can be operated in highly corrosive environments, such as close to chemical or petrochemical plants, or in saltwater containing environments, such as at the coastline. It would therefore be desirable to have an erosion resistant as well as corrosion resistant coating applied to industrial gas turbine components.

Other authors relate to erosion and corrosion resistant coatings for airfoils. A sacrificial and erosion-resistant turbine compressor airfoil coating is described by Lipkin et al in US20100226783A1. The airfoils which are to be coated, can be made of various types of stainless steel, such as 300 series, 400 series and type 450 stainless steel, and superalloys. The coating system described in this document consists of at least two different kinds of layers, one of which is erosion resistant, the other one is corrosion resistant, whereas the sacrificial coating is more anodic with reference to the airfoil surface than the erosion resistant coating. Among the materials, noted as especially useful for the sacrificial coating, are Al, Cr, Zn, Al-based alloys, Cr-based alloys, and many more. The erosion resistant coating may comprise metal nitrides such as AIN, TiN, TiAIN, TiAICrN, and many more. According to this patent, either the sacrificial coating or the erosion-resistant coating can be applied directly to the surface of the stainless steel component. If the sacrificial coating is deposited directly on the surface of the stainless steel substrate, the erosion resistant coating is deposited on the sacrificial coating, and vice versa. The sacrificial layer may be disposed as a thin film or thick film layer by any suitable application or deposition method, including chemical vapour deposition (CVD) and physical vapour deposition (PVD), for example filtered arc deposition and more typically by sputtering. The coating system provides enhanced water droplet erosion protection, enhanced galvanic and crevice corrosion resistance, and improved surface finish and antifouling capability for turbine compressor airfoil applications. However some materials mentioned in the description of said text, such as the group of metal nitrides including AIN, TiN, TiAIN, TiAICrN, and many more, exhibit different properties for erosion and corrosion resistance.

Besides the desire to increase the corrosion and erosion resistance, higher operating temperatures for gas turbine engines are sought in order to increase the efficiency. However, as operating temperatures increase, the durability of the components within the engine must increase accordingly.

Hazel et al disclose in EP1595977B1 a superalloy article having oxidation and corrosion resistant coating thereon. The invention particularly relates to a superalloy article, such as one employed in the turbine and compressor sections of a gas turbine engine, and exposed to oxidising and corrosive environments at moderate to elevated service temperatures, having an oxidation and corrosion resistant coating thereon. Significant advances in high temperature capabilities have been achieved through the formulation of nickel- and cobalt-based superalloys. However, the components of a gas turbine engine are often simultaneously exposed to an oxidative/corrosive environment and elevated temperatures. In order to avoid damage of the turbine engine components, some of the components are protected by environmental or overlay coatings, which inhibit environmental damage. The type of coating that is chosen for a specific application or component depends on various factors such as if the application involves exposure to air or combustion gas, and temperature exposure. Turbine and compressor disks and seal elements for use at the highest operating temperatures are made of nickel-based superalloys selected for good elevated temperature toughness and fatigue resistance. These superalloys have adequate resistance to oxidation and corrosion damage, but that resistance may not be sufficient to protect these components at the operating temperatures now being reached. It has been shown that application of an aluminum nitride overlay coating to turbine disks, rotors and other components exposed to similar temperature and environment provides an effective environmentally protective coating towards ingested salts and sulfates. The overlay coating typically has good adhesion, minimal diffusion into the base substrate and limited or no debit on low fatigue properties. During engine operation and/or high temperature exposure, the overlay coating may oxidise to form a stable metal oxide on the surface of the coating providing further improved oxidation and corrosion resistance. The protective coating can also be readily reconditioned and repaired if necessary. However the use of a superalloy article can have some disadvantages, such as the ones previously mentioned.

Corrosion processes on metallic surfaces can be very complex. Considering turbines, particularly industrial gas turbines, reactions with Cl and S are the ionic species, which predominantly influence the corrosion resistance of metallic surfaces. Reactions of said kind are also depending on and varying with the humidity. Besides that, the kinetics of said chemical reactions is also highly dependent on the temperature. If a coating is to be applied in order to increase the corrosion resistance, all of these processes have to be taken into account. Applying an erosion resistant coating to a metal substrate could otherwise lead to increased erosion resistance, but could be disadvantageous for the corrosion resistance of the so coated substrate material.

In many cases the formation of a sacrificial layer, as known from the state of the art, is a good approach. A sacrificial layer can be formed at steel substrates, if the interface substrate/coating provides a sufficiently high aluminum (Al) content. The formation of said sacrificial layer on the basis of aluminum (Al) is sufficient to protect e.g. a component in a certain application, but only if applied to substrates, which need protection against iron corrosion. This is the case for e.g. high chromium containing steels, such as the ones commonly used for compressors of industrial gas turbines. However the formation of a sacrificial layer made of aluminum (Al) might not be sufficient to ensure an appropriate corrosion resistance of turbine blades. Factors such as sulfur containing air or elevated service temperatures can have a negative impact on said aluminum (Al) sacrificial layers. Improved control mechanisms of interface reactions in combination with an enhanced corrosion resistance is therefore the main goal of the present invention.

Diffusion processes are boosted by chemical processes and elevated temperatures, and often driven by specific elements comprised in the substrate material, which is the case for e.g. chromium (Cr) and nickel (Ni) comprising substrates. However, providing a dedicated interface can reduce corrosion as well as diffusion processes. Particularly advantageous is the use of PVD for elements which additionally have a high melting point. This is due to the fact that high melting point materials usually form amorphous phases when the metallic vapour is condensing at the substrate surface during the deposition. Another advantage of using PVD, especially cathodic arc deposition to produce said coating, is that the amount of droplets in the coating is decreased in comparison to other low melting point materials. The coating generated accordingly, is therefore denser and shows less defects than other state of the art coatings, which leads to a suppression of corrosion processes.

Problem to be solved

The present invention aims to provide a coating system for a stainless steel, titanium or titanium aluminide article, and for cobalt-based, nickel-based or iron based superalloys, particularly for gas turbine or steam turbine compressor components, which shows enhanced corrosion and erosion resistance compared to state of the art coatings, and which is deposited preferably on a steel substrate by a physical vapour deposition (PVD) method, particularly by cathodic arc deposition.

Another aim of the present invention is to disclose a physical vapour deposition (PVD) method, particularly a cathodic arc deposition method, to deposit the inventive coating system on a substrate.

Solution of the problem according to the present invention - Description of the present invention The present invention discloses a coating system for enhanced corrosion and erosion resistance of gas turbine engine components at moderate to elevated service temperatures, whereas these components are made of e.g. stainless steel or superalloys. The inventive coating system comprises an optional metallic interlayer, an intermediate layer deposited either directly on the surface, or on the metallic interlayer, consisting of aluminum nitride (AIN), and a top layer, which can consist of either a monolayer or a multilayer system of oxides or nitrides. It could be shown in various standardized corrosion tests, that the inventive coating exhibits an enhanced corrosion resistance compared to previous coating systems which are known from the state of the art. Furthermore the inventive coating system also shows improved erosion resistance in various standardized tests.

The present invention furthermore relates to a physical vapour deposition (PVD) method, particularly to a cathodic arc deposition method, for depositing an inventive coating system.

Description of figures

Figure 1 XRD diffractogram of an AIN Monolayer on a 1.4313 stainless steel substrate

Figure 2 Schematic illustration of one possibility to form the inventive

coating system.

Figure 3 Pictures of a 1.4313 stainless steel substrate coated with an

aluminum nitride (AIN) monolayer, before and after it was tested in a neutral salt spray test (NSST) according to DIN EN ISO 9227

Figure 4 Pictures of a 1.4313 stainless steel substrate coated with an

aluminum nitride (AIN) monolayer, for which the aluminum nitride was scratched. Pictures show the coating before and after it was tested in a neutral salt spray test (NSST) according to DIN EN ISO 9227. Figure 5 Mass loss of an uncoated 1.4313 stainless steel substrate, a standard slurry coating on a 1.4313 stainless steel substrate and an aluminum nitride (AIN) coating on a 1.4313 stainless steel substrate in a single particle erosion (SPE) test with an impact angle of 20°.

Figure 6 Mass loss of an uncoated 1.4313 stainless steel substrate, a standard slurry coating on a 1.4313 stainless steel substrate and an aluminum nitride (AIN) coating on a 1.4313 stainless steel substrate in a single particle erosion (SPE) test with an impact angle of 90°.

Figure 7 Mass loss of an uncoated 1.4313 stainless steel substrate, a standard slurry coating on a 1.4313 stainless steel substrate and an aluminum nitride (AIN) coating on a 1.4313 stainless steel substrate in a cavitation test.

Figure 8 Schematic illustration of one possibility to form the inventive coating system.

Figure 9 XRD diffractogram of a TiAIN Monolayer on a 1.4313 stainless steel substrate.

Figure 10 Schematic illustration of an inventive multilayer coating,

comprising aluminum nitride (AIN) and titanium aluminum nitride (TiAIN) layers.

Figure 11 Schematic illustration of an aluminum nitride (AIN) monolayer, deposited on a metallic interlayer, which is deposited on the substrate.

Figure 12 Schematic illustration of an inventive coating system, with an aluminum nitride (AIN) monolayer deposited on a substrate, a metallic interlayer deposited between the substrate and the aluminum nitride (AIN), and a titanium aluminum nitride (TiAIN) monolayer on top of the aluminum nitride (AIN) layer. Figure 13 Schematic illustration of an inventive coating system, consisting of a multilayer system on top, and a metallic interface between the substrate and the top layer.

Figure 14 Schematic representation of one embodiment of the inventive coating system

Figure 15 Schematic illustration of a cathodic arc evaporation set-up to

deposit the inventive coating on a substrate sample.

The invention will now be discussed in detail and on the basis of examples and with the help of the figures.

According to the present invention a coating system on a substrate is provided, consisting of an optional metallic interlayer containing but is not limited to e.g. niobium (Nb), chromium (Cr), zirconium (Zr), hafnium (Hf) or molybdenum (Mo), or any combinations thereof, which is deposited directly on the substrate material, and a layer system which is either a monolayer or a multilayer system. The layer system includes at least one intermediate layer of aluminum nitride (AIN), which is deposited either directly on the substrate, or on the metallic interlayer. Furthermore the layer system optionally includes one or more nitride layers, which are deposited on the aluminum nitride layer (AIN).

Aluminum nitride (AIN) has a very low oxidation rate and forms a protective oxide layer. The coating thickness of the aluminum nitride (AIN) comprising coating of the inventive coating system ranges from 0.5 pm to 50 pm, but is preferably chosen to be between 1 pm and 30 pm, most preferably between 1 pm and 15 pm . If the said layer consists of solely aluminum nitride (AIN), this layer exhibits a hexagonal Wurtzite (w-AIN) crystal structure with (002) texture, as can be seen in the XRD diffractogram provided in Figure 1. The lattice constants of this structure are a = 3.113 A and c = 4.984 A. The composition of this stoichiometric compound is preferably chosen to be Al: 50 ± 2 at%, N: 50 ± 5 at%, but is not limited to this composition. The indentation hardness was measured to be 24 ± 1 GPa. The indentation modulus was measured to be 281 ± 7 GPa. The coating hardness of aluminum nitride (AIN) 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 minimize substrate interference.

In order to determine the properties of aluminum nitride (AIN), a 1.4313 stainless steel substrate was coated with an aluminum nitride (AIN) layer, which was deposited directly on the substrate, as shown in Figure 2. Various temperatures and thicknesses were chosen in order to test different variants. All samples coated with an aluminum nitride (AIN) layer at temperatures between 300-500 °C showed excellent corrosion resistance in neutral salt spray test (NSST) according to DIN EN ISO 9227 for 2000 h, i.e. no corrosion.

An example of a 1.4313 stainless steel substrate coated with an aluminum nitride (AIN) monolayer, before and after 1500h in a neutral salt spray test (NSST) is shown in Figure 3. No corrosion could be found at the samples. Also outstanding corrosion resistance was achieved in a Na2S04/NaCI solution.

Moreover neutral salt spray tests (NSST) according to DIN EN ISO 9227 were performed with aluminum nitride (AIN) coated samples, for which the aluminum nitride (AIN) was scratched, so that the substrate was not covered anymore by aluminum nitride (AIN) and therefore exposed to the salt fog.

An example of a 1.4313 stainless steel substrate, coated with an aluminum nitride (AIN) monolayer, for which the aluminum nitride (AIN) was scratched, before testing and after 500 h in a neutral salt spray test (NSST), is shown in Figure 4. No corrosion was observed for 500 h, neither for the coated nor for the scratched area. This finding proves that the aluminum nitride (AIN) forms a sacrificial layer at the steel substrate. Aluminum nitride (AIN) is also a good insulator. This is another reason for the reduction or prevention of corrosive processes in the coating-substrate interface.

In addition to the excellent corrosion resistance, the aluminum nitride (AIN) coatings showed very good resistance against solid particle erosion. A 1.4313 stainless steel substrate was coated with an aluminum nitride (AIN) monolayer and a solid particle erosion test was performed with the so coated sample at 20° and 90° impact angles.

As can be seen in Figure 5, at an impact angle of 20°, the mass loss of the aluminum nitride (AIN) coating is 25 times smaller than that of the uncoated 1.4313 stainless steel sample, and 170 times smaller than that of the 1.4313 stainless steel substrate coated with a standard slurry coating known from the state of the art. As shown in Figure 6, at an impact angle of 90°, the mass loss of the aluminum nitride (AIN) coating is 15 times smaller than that of the uncoated 1.4313 stainless steel sample, and 30 times smaller than that of the 1.4313 stainless steel substrate coated with a standard slurry coating known from the state of the art. The anisotropic behavior of the erosion protection is a result of the configuration of the cathodic arc source and can be modified by the source parameters, e.g. by the modification of the source magnetic field. Moreover, the resistance of aluminum nitride (AIN) coatings against cavitation shows excellent results. A 1.4313 stainless steel substrate was coated with an aluminum nitride (AIN) monolayer and a cavitation test was performed by immersing the sample in 25° C water. Shockwaves at the immersed sample surface were generated using a sonotrode with a frequency of 20 kHz and a peak to peak amplitude of 50 pm. The test duration was 20 h. As shown in Figure 7, the mass loss of the aluminum nitride (AIN) coating is 45 times smaller than the one of a uncoated 1.4313 stainless steel substrate, and 80 times smaller than that of a 1.4313 stainless steel substrate coated with a standard slurry coating known from the state of the art. For the above described samples, the aluminum nitride (AIN) monolayer was applied by cathodic arc deposition, using aluminum (Al) targets with a purity of 99.5%. The arc current for the at least one aluminum (Al) target is chosen to be between 130 A and 180 A. A nitrogen (N2) gas flow with a pressure between 0.9e-3 mbar and 3.2e-2 mbar is inserted into the vacuum chamber, in order to form a hexagonal Wurtzite (w-AIN) crystal structure with (002) texture. Using a lower nitrogen (N2) pressure leads to the formation of both, aluminum (Al) and Wurtzite aluminum nitride (w-AIN) phases.

As already shown, the aluminum nitride coating exhibits highly desirable properties, especially regarding corrosion resistance, single particle erosion and cavitation, which can be even further improved, by depositing at least one oxide or nitride layer, e.g. titanium aluminum nitride (TiAIN) on top of the aluminum nitride (AIN) layer. A schematic illustration of this embodiment of the inventive coating system is shown in Figure 8. It is commonly known that nitrides and oxides generally exhibit a high hardness and good erosion resistance. However, depositing one or even more nitride layers on or in combination with at least one aluminum nitride (AIN) layer, leads to an exceptional enhancement of the corrosion and erosion resistance of the so coated sample. The inventors found that titanium aluminum nitride (TiAIN) is preferably deposited on top of an aluminum nitride layer, as shown in Figure 8.

If a titanium aluminum nitride (TiAIN) monolayer with a certain composition is used as a top layer, the coating thickness can range from 1 pm to 50 pm and is preferably chosen to be between 1 pm and 25 pm. The titanium aluminum nitride (TiAIN) layer shows a cubic crystal structure, as can be seen in the XRD diffractogram provided in Figure 9, and a lattice constant of a = 4.171 A. 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.

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 cubic titanium aluminum nitride (TiAIN) and hexagonal Wurtzite aluminum nitride (w-AIN) 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-AIN) 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. As shown in the above described examples, the corrosion and erosion resistance of nitride layers can be varied according to the ratio of e.g. aluminum (Al) to titanium (Ti) and nitrogen (N). This offers a possibility to adjust the coating to a specific

environment, depending on if the erosion resistance or the corrosion resistance is of higher importance. However changing the composition of the nitride layers may not be sufficient to provide the corrosion resistance needed in an extreme environment. The inventors found that the deposition of a multilayer system including at least one aluminum nitride (AIN) layer, and at least one nitride layer, preferably a titanium aluminum nitride (TiAIN) layer, is especially beneficial. An example of an inventive multilayer system is shown in Figure 10. Since a change of the ratio of the metals in the at least one nitride layer, e.g. a change of the ratio of aluminum (Al) to titanium (Ti) in a titanium aluminum nitride (TiAIN) layer can lead to different phases, a high flexibility is offered by this coating system. Moreover aluminum nitride (AIN) also forms different phases depending on the nitrogen (N2) gas flow. Combining especially aluminum nitride (AIN) and titanium aluminum nitride (TiAIN) thus offers a broad range of parameters to adjust the properties of the inventive coating system in order to fulfill the requirements of a specified environment.

In order to further improve the performance of the coating, an optional metallic interlayer can be deposited directly on the base material. The metallic interface can be sacrificial or non-sacrificial. A schematic illustration of an aluminum nitride (AIN) monolayer, which is deposited on a substrate, using a metallic interface between the aluminum nitride (AIN) layer and the substrate material, is shown in Figure 11. The deposition of a metallic interface containing niobium (Nb), chromium (Cr), zirconium (Zr), hafnium (Hf) or molybdenum (Mo), preferably consisting of said metals, or any combinations thereof, leads to improved adhesion between the substrate and the coating, as well to an improved corrosion resistance of the so coated substrate.

However one possibility to apply an inventive coating system, is to deposit an aluminum nitride (AIN) layer on a substrate, deposit a metallic interlayer between the substrate and the aluminum nitride (AIN), and apply a titanium aluminum nitride (TiAIN) coating on top of the aluminum nitride (AIN) coating, such as is shown in Figure 12. A metallic interlayer can also be applied, if a multilayer system according to the present invention is to be deposited on a substrate, an example of which is shown in Figure 13. The substrate materials to be coated include but are not limited to stainless steel, superalloys and titanium alloys. The inventive 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 aluminide (TiAI) and iron-based, cobalt-based and nickel-based superalloys.

An embodiment of the invention will be described by way of example, which is meant to be merely illustrative and therefore non limiting.

According to one embodiment a multilayer system consisting of alternating layers of TiAIN/AIN, AIN, and TiAIN is deposited on the substrate material. The coating system shown in Figure 14, as well as the arc deposition method described below, and shown in Figure 15, are to be seen as only one example but are not limited to this variant. As shown in Figure 14, a TiAIN/AIN layer is deposited directly on the substrate. An aluminum nitride (AIN) layer is deposited on the TiAIN/AIN layer, followed by another TiAIN/AIN. A titanium aluminum nitride (TiAIN) layer is then deposited on the TiAIN/AIN followed by another TiAIN/AIN which represents the top layer of this coating system.

The said coating system is deposited on a sample (4) using an arc deposition method. In order to apply the inventive coating system to a sample (4), using the inventive coating method, a sample (4) is placed in a vacuum coating chamber (1 ). The substrate (4) is placed rotatable in the center of said vacuum chamber on a carousel (2). The inventive 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 order and number of the targets can be of any desired kind. The set-up shown in this particular example (Figure 15) contains four targets, all of them set up in a way as to work as cathodes. As can be seen in Figure 15 the targets are mounted at the walls of the vacuum coating chamber. In order to produce the inventive coating system described in this specific embodiment, cathodes A and B are targets comprising aluminum (Al) as main component, and cathodes C and D are targets comprising titanium aluminum (TiAI) as main component. The target positions are to be seen as only one example of the present invention and are not limiting. In order to generate the nitrogen (N2) containing layers, a non-zero amount of N2 is inserted into the vacuum chamber through the gas inlet. In this example the N2 pressure was set to 3.2e-2 mbar. As shown in Figure 15, 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 300-500°C. Magnets, which are not shown in this figure, are located behind the targets, and the magnetic field can be adjusted in order to influence the coating. Shutters (3) can be installed in front of the targets (A, B, C, D), to allow different coating layers, but are not compulsory. The coating thickness of the individual layers of the multilayer system described herein can be chosen to be typically between 0.01 pm and 5 pm. One example of said coating system produced by said method was deposited on a substrate sample, exhibiting a coating thickness of 11 pm.

According to another embodiment of the present invention, which is shown in Figure 3, a metallic interlayer (10), for example consisting of niobium (Nb) or chromium (Cr), is deposited directly on the substrate (9). An intermediate layer (11 ) containing or consisting of aluminum nitride (AIN) is then deposited on the metallic interlayer. A top layer (12) containing either oxides or nitrides or both, e.g. consisting of titanium aluminum nitride (TiAIN) is then deposited on top of the aluminum nitride (AIN) layer.

According to another embodiment of the present invention an aluminum nitride (AIN) monolayer, preferably with the composition Al: 50 ± 2 at%, N: 50 ± 2 at%, is deposited directly on a substrate material, such as for example 1.4313 stainless steel. The monolayer is preferably deposited on the 1.4313 stainless steel at 300° C with a coating thickness of 8 pm and exhibits an extraordinary corrosion resistance.

The aluminum nitride (AIN), if exposed to oxidising ambient, forms a native oxide at the surface. In general, this oxidation can also be performed in the vacuum system by controlled plasma oxidation. This again improves the oxidation and corrosion resistance of aluminum nitride (AIN). A Coating system for enhanced corrosion and erosion resistance of a gas turbine engine component is disclosed, whereas the component is preferably made of stainless steel and/or superalloys and the coating system comprises a layer, the layer is consisting of aluminum nitride (AIN).

The AIN layer can be an intermediate layer and the coating system comprises a top layer, where the intermediate layer is positioned between a surface of the component and the top layer consists of either a monolayer or a multilayer system.

The intermediate coating can be either directly coated on the surface of the

component or coated on a metallic interlayer positioned between the intermediate coating and the surface of the component.

The top layer can comprise a monolayer or a multilayer system of oxides or nitrides and preferably consists of a monolayer or a multilayer system of oxides or nitrides. The top layer can comprises for example a TiAIN layer. The top layer can comprise for example at least one TiAIN/AIN layer system.

The layer consisting of aluminum nitride can comprise hexagonal structure as can be measured by XRD.

The layer consisting of AIN can comprise metallic Al droplets. Droplets are particles created by the Arc deposition PVD process. In the case of reactive arc deposition such particles are in most cases not fully reacted through, therefore having an outer shell of reacted material and an inner core of metal.

Labeling in the Figures

1 Coating Chamber

2 Carousel

3 Shutter

4 Sample 5 Substrate

6 Metallic Interlayer

7 Aluminum Nitride (AIN) comprising layer

8 Nitride Layer

A , B Al Cathodes

C , D TiAI Cathodes

N₂ Reactive Gas

Ar Working Gas

Claims

1 . Coating system for enhanced corrosion and erosion resistance of a gas

turbine engine component, whereas the component is preferably made of stainless steel and/or superalloys and the coating system comprises a layer characterized in that the layer is consisting of aluminum nitride (AIN).

2. Coating system according to claim 1 , characterized in that the AIN layer is an intermediate layer and the coating system comprises a top layer, where the intermediate layer is positioned between a surface of the component and the top layer consists of either a monolayer or a multilayer system.

3. Coating system according to claim 2, characterized in that the intermediate coating is either directly coated on the surface of the component or coated on a metallic interlayer positioned between the intermediate coating and the surface of the component.

4. Coating system according to one of claims 2 or 3, characterized in that the top layer comprises a monolayer or a multilayer system of oxides or nitrides and preferably consists of a monolayer or a multilayer system of oxides or nitrides.

5. Coating system according to one of the claims 2 to 4, characterized in that the top layer comprises a TiAIN layer.

6. Coating system according to claim 5, characterized in that the top layer

comprises at least one TiAIN/AIN layer system.

7. Coating system according to one of the claims 1 to 6, characterized in that the layer consisting of aluminum nitride comprise hexagonal structure as can be measured by XRD.

8. Coating according to one of the claims 1 to 7, characterized in that AIN

comprises metallic Al droplets