WO2022174973A2

WO2022174973A2 - Coated article combining high corrosion and erosion resistance

Info

Publication number: WO2022174973A2
Application number: PCT/EP2022/000013
Authority: WO
Inventors: Lin SHANG; Sebastien Guimond; Juergen Ramm
Original assignee: Oerlikon Surface Solutions Ag, Pfäffikon
Priority date: 2021-02-17
Filing date: 2022-02-03
Publication date: 2022-08-25
Also published as: WO2022174973A3

Abstract

This invention relates to a gas or steam turbine engine component formed by a substrate with a coating system deposited on at least part of the substrate for enhanced corrosion and erosion resistance of the turbine engine component, whereas the component is preferably made of stainless steel or titanium alloy or superalloys and the coating system comprises at least one layer of aluminum nitride and at least one layer of titanium aluminum nitride and wherein the at least one titanium aluminum nitride layer most distant to the substrate is closer to the substrate as the aluminum nitride layer most distant to the substrate.

Description

Coated article combining high corrosion and erosion resistance

Technical Field

The present invention relates to a low alloy steel, stainless steel, titanium alloy 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 nickel-based superalloys, 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 erosive, oxidative and corrosive environment at moderated to elevated service temperature, having an inventive erosion, oxidation and corrosion 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 erosion resistance requirements and at the same time fulfill the corrosion resistance requirements.

For steam turbines, water droplet erosion is a well-known problem. At the back end of the turbine, water droplets condense and cause erosion of the blade leading edge.

For industrial gas turbines, as gas turbine operators look to enhance operating plant capability and flexibility, power augmentation systems are used to provide a power increase at peak loading, taking full advantage of high peak load electricity prices and therefore offering significant performance advantages and attractive financial payback

CONFIRMATION COPY options. Wet Compression is thus designed to increase the power output of the gas turbine by reducing compressor inlet temperatures, intercooling the air mass flow within the compressor and hence an increased mass flow throughout the turbine. A consequence of injecting liquid droplets into the compressor inlet is damage to the leading edge of the compressor blades, where erosion and cavitation of both the protective coating and blade substrate material is observed.

Water droplet erosion of the blade leading edge can cause (i) a reduction in blade chord length and ultimately a reduction in output energy and efficiency and (ii) vibration that can result in fatigue damage of the blade and consequent damage of the turbine system.

Moreover, 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. Among the various ionic species, which reach the surface of turbine components by water droplets, are H⁺, Cl , Br, F , S² among others. Electrochemically induced corrosion besides erosion at the blade 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. Moreover, 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 wt% 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 superalloys 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 low alloy steel, stainless steel, chromium- based superalloys, nickel-based superalloys and titanium 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 oxidize 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, particularly corrosion process of the steel turbine blades under the attack of H⁺, Cl and S² , which is affected by application environments, such as temperature, humidity and pH value. If a coating is to be applied in order to increase the corrosion resistance, all of these factors have to be taken into account. Hence although applying an erosion resistant coating to a metal substrate could enhance erosion resistance, however, if the coating property was not properly adjusted to the corrosive environment, it could be otherwise disadvantageous for the corrosion resistance of the so coated substrate material.

Problem to be solved

The present invention aims to provide a coating system for a low alloy steel, 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 erosion and corrosion 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. steel or superalloys. The inventive coating system comprises an optional metallic or nitride substrate layer, a layer deposited either directly on the surface, or on the metallic or nitride substrate layer, consisting of titanium aluminum nitride (TiAIN), and a top layer, which can consist of either a monolayer aluminum nitride (AIN) or a monolayer aluminum oxide (AI2O3) or a multilayer system of oxides or nitrides or any combinations thereof. 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 much enhanced 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 Schematic illustration of one possibility to form the inventive coating system.

Figure 2 X-ray diffractogram of a TiAIN Monolayer on a 1.4938 stainless steel substrate.

Figure 3 X-ray diffractogram of an AIN Monolayer on a 1.4313 stainless steel substrate.

Figure 4 Cross sectional morphology of AIN monolayer on cemented carbide substrate by scanning electron microscopy.

Figure 5 Pictures of a 1.4313 stainless steel substrate coated with an aluminum nitride (AIN) monolayer. Pictures show the coating before and after it was tested in a neutral salt spray test (NSST) according to DIN EN ISO 9227 for 2500 h.

Figure 6 Pictures of a low alloy steel substrate coated with an aluminum nitride (AIN) monolayer. Pictures show the coating before and after it was tested in a neutral salt spray test (NSST) according to DIN EN ISO 9227 for 120 h.

Figure 7 Pictures of a 17-4PH 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 for 2500 h.

Figure 8 Schematic illustration of an inventive multilayer coating, comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers. Figure 9 Pictures of a 1.4313 stainless steel substrate coated with a multilayer coating comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers. Pictures show the coating before and after it was tested in a neutral salt spray test (NSST) according to DIN EN ISO 9227 for 2500 h.

Figure 10 Pictures of a low alloy steel substrate coated with a multilayer coating comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers. Pictures show the coating before and after it was tested in a neutral salt spray test (NSST) according to DIN EN ISO 9227 for 500 h.

Figure 11 Pictures of a 17-4PH stainless steel substrate coated with a multilayer coating comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers, for which the multilayer coating 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 for 2500 h.

Figure 12 Mass loss of a monolayer aluminum nitride (AIN) coating on a 1.4313 stainless steel substrate, a monolayer titanium aluminum nitride (TiAIN) coating on a 1.4313 stainless steel substrate and a multilayer coating comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers on a 1.4313 stainless steel substrate in a water droplet erosion test.

Figure 13 Mass loss of an uncoated 1.4313 stainless steel substrate, a standard galvanic coating on a 1.4313 stainless steel substrate and a multilayer coating comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers on a 1.4313 stainless steel substrate in a cavitation test. Figure 14 Normalized erosion time of an uncoated 1.4313 stainless steel substrate, a standard galvanic coating on a 1 .4313 stainless steel substrate and a multilayer coating comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers on a 1.4313 stainless steel substrate in a solid particle erosion test at 30° impact angle according to ASTM G76.

Figure 15 Normalized erosion time of an uncoated 1.4313 stainless steel substrate, a standard galvanic coating on a 1 .4313 stainless steel substrate and a multilayer coating comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers on a 1.4313 stainless steel substrate in a solid particle erosion test at 90° impact angle according to ASTM G76.

Figure 16 Schematic illustration of an inventive multilayer coating, comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers.

Figure 17 Schematic illustration of an titanium aluminum nitride (AIN) monolayer, deposited on a metallic or nitride substrate layer, which is deposited on the substrate.

Figure 18 Schematic illustration of an inventive coating system, with a titanium aluminum nitride (TiAIN) monolayer deposited on a substrate, a metallic or nitride substrate layer deposited between the substrate and the titanium aluminum nitride (TiAIN), and an aluminum nitride (AIN) monolayer on top of the titanium aluminum nitride (TiAIN) layer.

Figure 19 Schematic illustration of an inventive coating system, consisting of a multilayer system on top, and a metallic or nitride interface between the substrate and the top layer.

Figure 20 Schematic representation of one embodiment of the inventive coating system. Figure 21 Schematic illustration of a cathodic arc evaporation set-up to deposit the inventive coating on a substrate sample.

Implementation of the present invention

The objective of the present invention is to provide a coating system on a substrate, comprising an optional metallic or nitride substrate layer containing but is not limited to e.g. chromium (Cr), aluminum (Al), chromium aluminum (CrAI), chromium nitride (CrN), aluminum chromium nitride (AICrN) or titanium nitride (TiN), or any combinations thereof, which is deposited directly on the substrate material, and a top layer which is either a monolayer or a multilayer system. The top layer includes at least one layer of titanium aluminum nitride (TiAIN), which is deposited either directly on the substrate, or on the metallic or nitride substrate layer. Furthermore the top layer includes one or more oxide or nitride layers, which are deposited on the titanium aluminum nitride layer (TiAIN). The coating thickness of the inventive coating system ranges from 0.5 to 50 pm, but is preferably chosen to be between 1 and 30 pm.

Titanium aluminum nitride (TiAIN) has excellent water droplet and solid particle erosion resistance due to its high hardness and fracture toughness. If a titanium aluminum nitride (TiAIN) monolayer with a certain composition is used as a top layer, the coating thickness can range from 0.5 to 50 pm and is preferably chosen to be between 1 and 25 pm. Figure 1 shows a schematic illustration of one possibility to form the inventive coating system: titanium aluminum nitride (TiAIN) monolayer (7) coated on a substrate (5). The titanium aluminum nitride (TiAIN) layer shows a cubic crystal structure, as can be seen in the X-ray diffractogram provided in Figure 2, 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 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, as aluminum nitride (AIN) has excellent corrosion and oxidation resistance.

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 to 50 pm, but is preferably chosen to be between 1 and 30 pm, most preferably between 1 and 15 pm. The aluminum nitride (AIN) layer exhibits a hexagonal Wurtzite (w-AIN) crystal structure with (002) texture, as can be seen in the X-ray diffractogram provided in Figure 3. 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: 52 ± 2 at%, N: 48 ± 5 at%, but is not limited to this composition. The indentation hardness was measured to be 20 ± 2 GPa. The indentation modulus was measured to be 255 ± 15 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. Besides the hexagonal Wurtzite (w-AIN) phase, Al peaks were also detected, which come from the Al microdroplets in the coating. The Al microdroplets are believed to be beneficial for corrosion and oxidation resistance since it acts as sacrificial or healing reservoir when the coating is exposed to corrosive and oxidative environment, Al reacts and oxidizes first thus retards corrosion or oxidation progress. The aluminum nitride (AIN) coating exhibits interrupted columnar microstructure as can be seen in Figure 4. The columns are interrupted by platelet-shaped microdroplets. The microdroplets are marked by arrows in Figure 4. Such microdroplets are formed when the cathodic arc in an almost explosive manner locally heating the aluminium target, thereby not only vaporizing aluminum from the target surface but as well ejecting liquid aluminum microdroplets from the target surface which due to their size do not fully react with the reactive gas such as nitrogen and/or oxygen. Therefore, depending on the coating process parameters, the amount of Al microdroplet present in the coating varies.

Various temperatures and thicknesses were chosen in order to test different variants of aluminum nitride (AIN). All samples coated with an aluminum nitride (AIN) layer at temperatures between 200-600 °C showed excellent corrosion resistance in neutral salt spray test (NSST) according to DIN EN ISO 9227 for 2500 h, i.e. no corrosion. An example of a 1.4313 stainless steel substrate coated with an aluminum nitride (AIN) monolayer, before and after 2500 h in a neutral salt spray test (NSST) is shown in Figure 5. No red rust could be observed at the coated surface. Another example of a low alloy steel substrate coated with an aluminum nitride (AIN) monolayer, before and after 120 h in a neutral salt spray test (NSST) is shown in Figure 6. No red rust could be observed at the coated surface. Also no corrosion could be found after testing in Na2S04/NaCI (pH = 3) salt steam atmosphere for 500 h. 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 17-4PH stainless steel substrate, coated with an aluminum nitride (AIN) monolayer, for which the aluminum nitride (AIN) was scratched, before testing and after 2500 h in a neutral salt spray test (NSST), is shown in Figure 7. No red rust was observed after 2500 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. Furthermore, oxidation of aluminum nitride (AIN) layer coated IN718 samples at 650 and 900 °C for 24 h in air showed no phase change and no interdiffusion between aluminum nitride (AIN) and IN718, this shows that the aluminum nitride (AIN) has excellent oxidation resistance.

For the above described samples, the aluminum nitride (AIN) monolayer was applied by cathodic arc deposition, using aluminum (Al) targets with a purity of at least 99.5 wt%. The arc current for the at least one aluminum (Al) target is chosen to be between 130 and 180 A for a circular target with a diameter of 15 cm. A nitrogen (N2) gas flow with a pressure between 0.9e-3 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. Varying nitrogen (N2) pressure leads to different ratio of aluminum (Al) microdroplets/Wurtzite aluminum nitride (w-AIN) phase, which allows tuning of the corrosion resistance of the aluminum nitride (AIN) coating according to the application environment.

As shown in the above described examples, the corrosion and erosion resistance of titanium aluminum nitride (TiAIN) layer 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 for the application. However changing the composition of the titanium aluminum nitride (TiAIN) layer may not provide sufficient corrosion protection for the coated substrate. Take steel substrate for example, the titanium aluminum nitride (TiAIN) coating is conductive and compared with steel is electrochemical ly nobler, therefore, upon corrosion attack, the steel substrate gets corroded underneath the nobler titanium aluminum nitride (TiAIN) coating. Moreover, the coating contains defects such as microdroplets, which may act as initiation sites for pitting corrosion at the steel substrate. Pitting corrosion is known to be detrimental for turbine blade as it may cause corrosion fatigue damage. The inventors found that the deposition of an electrically insulating layer, preferably an aluminum nitride (AIN) or aluminum oxide (AI2O3) layer (8) on top of the titanium aluminum nitride (TiAIN) layer (7) is especially beneficial for providing excellent corrosion protection for the coated substrate. The electrically insulating layer acts as a physical and electron transport barrier to H⁺, Cl , S² and other ions diffusion, which makes up the disadvantage of the conductive titanium aluminum nitride (TiAIN) layer. In addition, the Al microdroplets in the aluminum nitride (AIN) or aluminum oxide (AI2O3) layer are believed to act as sacrificial or healing reservoir when the coating is exposed to corrosive and oxidative environment, forming surface sealing oxide and sealing internal defects, thus provides excellent corrosion protection. An example of an inventive multilayer system is shown in Figure 8. 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. Combining especially aluminum nitride (AIN) or aluminum oxide (AI2O3) with 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.

The inventive multilayer coating, comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers as shown in Figure 8 shows excellent corrosion resistance in neutral salt spray test (NSST) according to DIN EN ISO 9227 for 2500 h, i.e. no corrosion. An example of a 1.4313 stainless steel substrate coated with the multilayer coating, before and after 2500 h in a neutral salt spray test (NSST) is shown in Figure 9. No red rust could be observed at the coated surface. Another example of a low alloy steel substrate coated with the multilayer coating, before and after 500 h in a neutral salt spray test (NSST) is shown in Figure 10. No red rust could be observed at the coated surface. Also no corrosion could be found after testing in Na2S04/NaCI (pH = 3) salt steam atmosphere for 500 h. Moreover neutral salt spray tests (NSST) according to DIN EN ISO 9227 were performed with aluminum nitride (AIN) coated samples, for which the multilayer coating 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 17-4PH stainless steel substrate, coated with the multilayer coating, for which the multilayer coating was scratched, before testing and after 2500 h in a neutral salt spray test (NSST), is shown in Figure 11 . No red rust was observed after 2500 h, neither for the coated nor for the scratched area. Furthermore, oxidation of the multilayer coated IN718 samples at 650 °C for 24 h in air showed no phase change and no interdiffusion between the multilayer coating and IN718, this shows that the multilayer coating has excellent oxidation resistance.

In addition to the excellent corrosion and oxidation resistance, the inventive multilayer coating, comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers showed excellent resistance against water droplet erosion. 1.4313 stainless steel substrates were coated with the multilayer coating, AIN monolayer coating and TiAIN monolayer coating. Water droplet erosion test was performed using a droplet impact speed of 488 m/s. The droplet exhibits a Sauter mean diameter of 88 pm. The test duration was 10 h with a test internal of 1 h. As shown in Figure 12, the mass loss of the multilayer coating and the TiAIN monolayer coating exhibits very slow progress, after 10 h test, the mass losses for the multilayer coating and TiAIN monolayer coating are 3.7 and 0.7 mg, respectively. TiAIN monolayer exhibits excellent water droplet erosion resistance, applying AIN coating on top further enhances its corrosion resistance and still maintains its excellent water droplet erosion resistance. Moreover, the inventive multilayer coating, comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers showed excellent resistance against cavitation. A 1.4313 stainless steel substrate was coated with the multilayer coating 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 10 h. As shown in Figure 13, the mass loss of the multilayer coating is only 7% of an uncoated 1.4313 stainless steel substrate, and only 3% of a 1.4313 stainless steel substrate coated with a standard galvanic coating known from the state of the art. These results show that the multilayer coating exhibits excellent cavitation resistance and achieved significant improvement compared to the state of the art galvanic coating.

Furthermore, the inventive multilayer coating, comprising titanium aluminum nitride (TiAIN) and aluminum nitride (AIN) layers showed very good resistance against solid particle erosion. 1.4313 stainless steel substrates were coated with the multilayer coating and solid particle erosion tests were performed with the so coated samples at 30° and 90° impact angles according to ASTM G76 standard. White corundum AI2O3 particles with an average size of 50 pm were used for testing. The nozzle to sample distance was 10 mm. The particle was fed with 2 g/min feed rate, the resulted particle speed was 70 m/s. The wear depth was evaluated and normalized to get erosion time per unit wear depth. The longer it is needed to erode a unit wear depth of the sample, the better the erosion resistance of the sample is. As can be seen in Figure 14, at 30° impact angle, the erosion time of the multilayer coating is 30 times longer than that of the uncoated 1.4313 stainless steel sample, and 450 times longer than that of the 1.4313 stainless steel substrate coated with a standard galvanic coating known from the state of the art. As shown in Figure 15, at 90° impact angle, the erosion time of the multilayer coating is 10 times longer than that of the uncoated 1.4313 stainless steel sample, and 210 times longer than that of the 1.4313 stainless steel substrate coated with a standard galvanic coating known from the state of the art. These results show that the multilayer coating exhibits excellent solid particle erosion resistance and achieved significant improvement compared to the state of the art galvanic coating.

As already shown, the titanium aluminum nitride (TiAIN) coating exhibits high solid particle erosion, water droplet erosion and cavitation resistance, however, it is not as corrosion resistance as the aluminum nitride (AIN) coating. Combining titanium aluminum nitride (TiAIN) coating with a top corrosion resistant layer can further enhance its corrosion resistance while maintaining its excellent erosion resistance. The top corrosion resistant layer can be the aluminum nitride (AIN) layer or oxide, as schematically shown in Figure 8, or it can be multilayer of nitrides or oxides or any combinations thereof, as schematically shown in Figure 16.

In order to further improve the performance of the coating, an optional metallic or nitride substrate layer can be deposited directly on the substrate material. A metallic or nitride interface can be designed to improve the adhesion of the coating system to the substrate material. Especially for applying the coating on parts with sharp edges, thick nitride coating tends to delaminate due to stress, therefore depositing a metallic or nitride substrate layer which can adapt to the substrate material chemistry and act as a transition for the coating stress to the substrate is beneficial. Also applying nitriding process, especially plasma nitriding using a combination of nitrogen (N2) and hydrogen (H2) to the substrate material before coating is believed to be beneficial for achieving high hardness at the interface, thus providing good adhesion to the hard coating as well as enhancing corrosion, solid particle and water droplet erosion resistance of the coating and base material system. Furthermore, reducing surface roughness of the coating by polishing reduces the amount of surface irregularities which act as erosion initiation sites, thus elongating the incubation period, enhancing water droplet erosion resistance.

A schematic illustration of an titanium aluminum nitride (TiAIN) monolayer (7), which is deposited on a substrate (5), using a metallic or nitride interface (6) between the titanium aluminum nitride (TiAIN) layer (7) and the substrate material (5), is shown in Figure 17. The deposition of a metallic or nitride interface (6) containing chromium (Cr), aluminum (Al), chromium aluminum (CrAI), chromium nitride (CrN), aluminum chromium nitride (AICrN) or titanium nitride (TiN), or any combinations thereof, leads to improved adhesion between the substrate and the coating. Another possibility to apply an inventive coating system, is to deposit a titanium aluminum nitride (TiAIN) layer (7) on a substrate, deposit a metallic or nitride substrate layer (6) between the substrate (5) and the titanium aluminum nitride (TiAIN) layer (7), and apply an aluminum nitride (AIN) layer (8) on top of the titanium aluminum nitride (TiAIN) layer (7), such as is shown in Figure 18. A metallic or nitride substrate layer (6) 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 19. The substrate materials to be coated include but are not limited to steel, superalloys and titanium alloys. The inventive coating is especially suitable to be applied on substrate materials such as low alloy steel, high-chromium (9-18 wt%) containing steel, e.g. 1.4313 stainless steel, 1.4938 stainless steel, 17-4PH 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 titanium aluminum nitride (TiAIN) (7) and alternating layers of titanium aluminum nitride (TiAIN) (7) and aluminum nitride (AIN) (8) is deposited on the substrate material. The coating system shown in Figure 20, as well as the arc deposition method described below, and shown in Figure 21 , are to be seen as only one example but are not limited to this variant. As shown in Figure 20, a titanium aluminum nitride (TiAIN) layer is deposited directly on the substrate. An aluminum nitride (AIN) layer is deposited on the titanium aluminum nitride (TiAIN) layer, followed by another titanium aluminum nitride (TiAIN) layer.

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 21 ) contains four targets, all of them set up in a way as to work as cathodes. 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 1.0e-2 mbar. As shown in Figure 21 , 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 200-600 °C. Magnets, which are not shown in this figure, are located behind the targets, and the magnetic field can be adjusted in order to achieve variation of the coating properties. Shutters (3) can be installed in front of the targets (A, B, C, D), to allow coating different layers, but are not compulsory.

According to another embodiment of the present invention, which is shown in Figure 18, a nitride substrate layer, for example consisting of aluminum chromium nitride (AICrN), is deposited directly on the substrate. A layer containing or consisting of titanium aluminum nitride (TiAIN), aluminum nitride (AIN) is then deposited on the nitride substrate layer. A top layer containing either oxides or nitrides or both, e.g. consisting of aluminum nitride (AIN) is then deposited on top of the titanium aluminum nitride (TiAIN) layer.

According to another embodiment of the present invention, which is shown in Figure 6, an titanium aluminum nitride (TiAIN) layer, preferably with the composition Ti: 25 ± 2 at%, Al: 25 ± 2 at%, N: 50 ± 5 at%, is deposited directly on a nitrided substrate material, such as for example 1 .4313 stainless steel. The monolayer is preferably deposited on the nitrided 1 .4313 stainless steel at 200-600 °C with a coating thickness of 15 pm and exhibits an extraordinary water droplet erosion 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 Gas or steam turbine engine component formed by a substrate with a coating system deposited on at least part of the substrate for enhanced corrosion and erosion resistance of the turbine engine component is disclosed in this description. The component is preferably made of stainless steel or titanium alloy or superalloys and the coating system comprises at least one layer of aluminum nitride (AIN) and at least one layer of titanium aluminum nitride (TiAIN) and wherein the at least one titanium aluminum nitride layer most distant to the substrate is closer to the substrate as the aluminum nitride layer most distant to the substrate.

In the turbine engine component the aluminum nitride layer may comprise microdroplets of aluminum which act as sacrificial and/or healing reservoir when the coating is exposed to corrosive and oxidative environment, whereas the microdroplets preferably have an outer skin of nitride.

In the turbine engine component the at least one aluminum nitride layer may be part of a multilayer which forms a top layer and/or the at least one titanium aluminum nitride layer is part of a multilayer which forms an interlayer.

In the turbine engine component may be directly deposited on the substrate a substrate layer which is a layer formed from a metal or metals or a layer formed from a nitride however which is not aluminum nitride.

In the turbine engine component the coating may have a thickness of between 0.5 pm and 50 pm and preferably between 1 pm and 30 pm.

In the first turbine engine component the at least one AIN layer may comprise oxygen.

A second gas or steam turbine engine component is disclosed with a coating system for enhanced corrosion and erosion resistance of the turbine engine component, whereas the component is preferably made of stainless steel or titanium alloy or superalloys and the coating system comprises at least one layer of alumina (AI203) and at least one layer of titanium aluminum nitride (TiAIN) and wherein the at least one titanium aluminum nitride layer most distant to the substrate is closer to the substrate as the alumina most distant to the substrate.

In the second turbine engine component the alumina layer may comprise microdroplets of aluminum which act as sacrificial and/or healing reservoir when the coating is exposed to corrosive and oxidative environment, whereas the microdroplets preferably have an outer skin of oxide.

In the second turbine engine component the at least one alumina layer may be part of a multilayer which forms a top layer and/or the at least one titanium aluminum nitride layer is part of a multilayer which forms an interlayer.

In the second turbine engine component may be directly deposited on the substrate a substrate layer which is a layer formed from a metal or metals or a layer formed from a nitride.

In the second turbine engine component the coating may have a thickness of between 0.5 pm and 50 pm and preferably between 1 pm and 30 pm.

In the second turbine engine component the at least one alumina layer may comprise nitrogen.

In the first and/or second turbine engine component the coating may comprise at least one aluminum nitride coating and at least one alumina coating. The at least one titanium aluminum nitride layer most distant to the substrate may be closer to the substrate as the at least one aluminum nitride layer most distant to the substrate and may be closer to the substrate as the at least one alumina layer most distant to the substrate.

A method for manufacturing a gas or steam turbine engine component was disclosed to produce a first and/or a second turbine engine component with the layers as described above, where the method comprises the steps of: - placing the component into a cathodic arc physical vapor deposition system with a coating chamber

- evacuating the coating chamber of cathodic arc physical vapor deposition system in order to create a vacuum - depositing the coating according to one of the previous claims by means of physical vapor deposition, preferably in such a way that Al microdroplets are included into the at least one aluminum nitride and/or the at least one alumina layer.

In the method the layers may depsosited and the percentage of oxygen and/or nitrogen may be used to control the density of the aluminum microdroplets as described above.

References

1 Coating Chamber 2 Carousel

3 Shutter

4 Sample

5 Substrate

6 Metallic/nitride Interlayer 7 Titanium Aluminum Nitride (TiAIN) Layer

8 Aluminum Nitride (AIN) or aluminum Oxide (AI203) Layer A, B Al Cathodes C, D TiAI Cathodes N2 Reactive Gas Ar Working Gas

Claims

1. Gas or steam turbine engine component formed by a substrate with a coating system deposited on at least part of the substrate for enhanced corrosion and erosion resistance of the turbine engine component, whereas the component is preferably made of stainless steel or titanium alloy or superalloys and the coating system comprises at least one layer of aluminum nitride (AIN) and at least one layer of titanium aluminum nitride (TiAIN) and wherein the at least one titanium aluminum nitride layer most distant to the substrate is closer to the substrate as the aluminum nitride layer most distant to the substrate.

2. Turbine engine component according to claim 1, characterized in that the aluminum nitride layer comprises microdroplets of aluminum which act as sacrificial and/or healing reservoir when the coating is exposed to corrosive and oxidative environment, whereas the microdroplets preferably have an outer skin of nitride.

3. Turbine engine component according to one of the claims 1 or 2, characterized in that the at least one aluminum nitride layer is part of a multilayer which forms a top layer and/or the at least one titanium aluminum nitride layer is part of a multilayer which forms an interlayer.

4. Turbine engine component according to one of the previous claims characterized in that directly on the substrate deposited is a substrate layer which is a layer formed from a metal or metals or a layer formed from a nitride however which is not aluminum nitride.

5. Turbine engine component according to one of the previous claims, characterized in that the coating has a thickness of between 0.5 pm and 50 pm and preferably between 1 pm and 30 pm.

6. Turbine engine component according to one of the previous claims, characterized in that the at least one AIN layer comprises oxygen.

7. Gas or steam turbine engine component with a coating system for enhanced corrosion and erosion resistance of the turbine engine component, whereas the component is preferably made of stainless steel or titanium alloy or superalloys and the coating system comprises at least one layer of alumina (AI2O3) and at least one layer of titanium aluminum nitride (TiAIN) and wherein the at least one titanium aluminum nitride layer most distant to the substrate is closer to the substrate as the alumina most distant to the substrate.

8. Turbine engine component according to claim 7, characterized in that the alumina layer comprises microdroplets of aluminum which act as sacrificial and/or healing reservoir when the coating is exposed to corrosive and oxidative environment, whereas the microdroplets preferably have an outer skin of oxide.

9. Turbine engine component according to one of the claims 7 or 8, characterized in that the at least one alumina layer is part of a multilayer which forms a top layer and/or the at least one titanium aluminum nitride layer is part of a multilayer which forms an interlayer.

10. Turbine engine component according to one of the previous claims 7 to 9 characterized in that directly on the substrate deposited is a substrate layer which is a layer formed from a metal or metals or a layer formed from a nitride.

11. Turbine engine component according to one of the previous claims 7 to 10, characterized in that the coating has a thickness of between 0.5 pm and 50 pm and preferably between 1 pm and 30 pm.

12. Turbine engine component according to one of the previous claims 7 to 11, characterized in that the at least one alumina layer comprises nitrogen.

13. Turbine engine component according to one of the previous claims, characterized in that the coating comprises at least one aluminum nitride coating and at least one alumina coating.

14. Turbine engine component according to claim 13, characterized in that the at least one titanium aluminum nitride layer most distant to the substrate is closer to the substrate as the at least one aluminum nitride layer most distant to the substrate and is closer to the substrate as the at least one alumina layer most distant to the substrate.

15. Method for manufacturing a gas or steam turbine engine component according to one of the previous claims, characterized in that the method comprises the steps of:

- placing the component into a cathodic arc physical vapor deposition system with a coating chamber - evacuating the coating chamber of cathodic arc physical vapor deposition system in order to create a vacuum

- depositing the coating according to one of the previous claim by means of physical vapor deposition, preferably in such a way that Al microdroplets are included into the at least one aluminum nitride and/or the at least one alumina layer.

16. Method according to claim 15 characterized in that a coating according to claims 6 and 12 is deposited and the percentage of oxygen and/or nitrogen is used to control the density of aluminum microdroplets as described in claim 2.