Ti-al-x-n coatings for enhanced fatigue properties

Abstract

The invention relates to coatings applied on components comprising functional substrates and methods for producing the inventive coatings and components, wherein the coatings allows producing components having functional substrates whose coating provides them improved fatigue behavior. The inventive coatings being Ti-Al-X-N coatings with controlled level of compressive residual stress, enabling higher lifetime or higher applied stress. The method for producing the inventive Ti-Al-X-N coatings including targeted control of residual stress. The present invention being especially suitable for application areas including airfoils, turbine blades, and turbine components.

Description

[0001] Ti-AI-X-N coatings for enhanced fatigue properties

[0002] The present invention relates to coatings used to improve wear resistance and fatigue properties of functional substrates and a method for producing them.

[0003] The inventive coatings and methods are in particular suitable for improving wear resistance and fatigue properties of airfoils or gas turbine components to be used in aerospace or other industries. Such kind of components are often manufactured of functional substrates made of materials like superalloys or titanium alloys, for example alloys of the type of Ti-6AI-4V or super-alloys of the type Inconel 718. The choice of such functional substrate materials is usually made under the consideration that these substrate materials have to withstand high operating temperature by retaining high strength at the same time, hence functional substrates in the context of the present invention are preferably substrate materials having properties that allow to withstand high operating temperatures (preferably including operating temperatures in a range from 500 °C to 900 °C or higher) in a way that involves retention of strength properties (also called mechanical properties or physical properties) at least in a degree that permits continuation of the use of the components during operation. Functional substrates in this context are for example alloys (in particular, but not limited to Ti- alloys, TiAI-alloys or AITi-alloys) and super alloys (in particular, but not limited to Ni- based super alloys, such as Inconel 718).

[0004] Prior art

[0005] Attempting to improve wear resistance of functional substrates by applying coatings to the surface of such functional substrates is well known. However, coatings of ceramic materials are often brittle, which may have an adverse effect on the fatigue behaviour of the functional substrates during use in the corresponding applications. Furthermore, research on the cyclic degradation mechanisms of different coatings of ceramic materials applied to functional substrates have reached contradictory conclusions, leaving it unclear whether thin film synthesis enhances or compromises fatigue resistance of functional substrates. Due to the strong directionality of their covalent bonds, the dislocation mobility of ceramic materials is typically severely restricted. In contrast to metals, ceramics are unable to relieve the localized stress peaks at the crack front by plastic deformation. Consequently, ceramic bulk and coating materials are characterized by a low fracture toughness and a tendency to exhibit unstable crack growth from the initial fracture event. In other words, the deposition of a brittle ceramic coating appears generally unfavorable to the fatigue properties of a ductile metallic component.

[0006] Objective of the present invention

[0007] It is therefore an objective of the present invention to provide a coating that can be applied on a functional substrate surface to form a coated component, resulting in improved fatigue strength and operational reliability of a such coated component during use in an application - the coated component comprising a functional substrate and a coating.

[0008] A further objective of the invention is to provide a method for producing the inventive coatings and the inventive coated components (correspondingly comprising functional substrates and inventive coatings).

[0009] Description of the present invention

[0010] The above-mentioned objectives are attained by providing physical vapor deposited (PVD) inventive coatings of type Ti-AI-X-N having adjusted fatigue properties.

[0011] The inventors observed that if a brittle coating cannot resist cracking, notch effects lead to increased stress concentrations at the substrate and promotes the initiation of subsurface fatigue cracks but as soon as coating rupture can be prevented, the local stress concentration at the interface is relieved by the distribution of dislocations and the buildup of secondary slip systems. Surprisingly the inventors found that by producing an adjusted residual stress behaviour along the interface between coating and substrate according to the present invention adjusted fatigue properties are attained.

[0012] Hence, the inventors suppose that the adjusted residual stress behavior according to the present invention allows suppressing coating fracture even at elevated subsurface strains and to push crack initiation far into the bulk material and in this manner the coating rupture can be prevented as the local stress concentration at the interface is relieved by the distribution of dislocations and the buildup of secondary slip systems. In other words, the inventive adjusted residual stress behavior results in that the crack nucleation shifts deeply into the interior of the substrate and a higher fatigue resistance is attained.

[0013] According to the present invention, the inventive coatings of type Ti-AI-X-N contain apart from the elements Ti, Al, and N, additional elements, denoted as X, wherein X is one or more chemical elements, preferably X being tantalum (Ta), or being tantalum (Ta) and silicon (Si).

[0014] Preferably the inventive coating of type Ti-AI-X-N comprises one layer containing Ti, Al, X and N, hereafter also called Ti-AI-X-N layer.

[0015] The Ti-AI-X-N layer in the present description, is also called TiAIN-based layer orTiAIN- based coating layer or TiAIN-based thin film or simply TiAIN-based film.

[0016] According to the present invention X was selected to be one chemical element, preferably Ta, or at least two chemical elements, preferably Ta and Si, wherein the elements constituting X were selected with the intention to produce a higher compressive residual stress in the Ti-AI-X-N layer when compared with a similar layer does not containing X. In other words, the similar layer is a Ti-AI-N layer produced in almost same manner as the Ti-AI-X-N but without providing X during formation of the Ti-AI-N layer being considered the similar layer. Hence, the chemical element composition of the similar Ti-AI-N layer in comparison to the chemical element composition of the Ti-AI-X-N layer differing only in the presence of X, thus differing only in the content of X. TiAITaN coatings and TiAITaSiN coatings were also found to be very beneficial for attaining better mechanical properties as well as oxidation resistance than simple TiAIN coatings.

[0017] The inventors surprisingly found that by adjusting the design of the coating is possible to influence the residual stress behavior of the coating in such a manner that a very low compressive residual stress or > -0.5 GPa or a tensile residual stress or > 0 GPa is produced at the interface between coating and the substrate surface in such a manner that no cracks are produced in the coating and also not at the interface between the coating and the substrate, and this resulting in a higher fatigue resistance.

[0018] Different stress-modifying approaches were implemented for attaining the desired residual stresses in the coating: a) Producing the coating having a TiAIN-based layer, including alloying chemical elements X for forming a TiAIXN layer exhibiting higher compressive residual stresses than a TiAIN layer but retaining the very good mechanical properties of the TiAIN layer. b) Applying a negative bias voltage, preferably having a value higher than 20 V for attaining appropriate high compressive residual stresses in the TiAIXN layer. c) Applying a specific metallic interlayer between TiAIXN layer and the substrate surface.

[0019] The combination of these approaches in inventive manner surprisingly results in a very special residual stress behavior of the coating-substrate system and in a high improvement in the fatigue properties in comparison with using known TiAIN or TiAIN- based known coatings.

[0020] The inventors consider that a possible explanation for this impressive improvement can be that once the residual compressive stress field is able to shift fatigue crack nucleation into the interior of the alloy (e.g. titanium alloy or nickel alloy), the HCF properties enhance. Furthermore, the inventors suppose that in this manner a residual tensile stress peak could be shifted into the bulk material, and the deeper the residual tensile stress peak is shifted into the bulk material - achieved through the inventive residual stress design of the coating-substrate system -, the greater the improvement in HCF strength.

[0021] Concretely, for adjusting the residual stress behavior to obtain a higher fatigue resistance according to the present invention, the inventors implemented different stress-modifying approaches, including:

[0022] - Depositing the Ti-AI-X-N layer by applying a negative bias voltage at the substrate to be coated, wherein the value of the negative bias voltage in volts being selected to attain producing a desired high compressive residual stress in the coating and thus a specific residual stress behavior resulting in higher fatigue resistance.

[0023] - Selecting the chemical elements used as X, as well as the process to introduce X in the Ti-AI-N coating for producing the inventive Ti-AI-X-N coating having a desired chemical element concentration to attain producing a desired high compressive residual stress in the coating and thus a specific residual stress behavior resulting in higher fatigue resistance.

[0024] - Designing a specific interlayer to be deposited between the substrate and the Ti-AI- X-N layer to attain producing a desired residual stress behavior in the coating.

[0025] Example of the present invention by using a Ti-alloy of Ti-6AI-4V as substrate:

[0026] In the case of using the Ti-alloy of Ti-6AI-4V as substrate, it was observed that overall, this approach resulted in an HCF enhancement of over 50% compared to the uncoated Ti-6AI-4V, highlighting the importance of a proper residual stress design according to the present invention.

[0027] By combining analysis of high-cycle fatigue tests, synchrotron-based experiments, and the formulation of a linear-elastic stress-failure model, the inventors identified following interrelations: (i) A sufficiently pronounced residual compressive stress state must be present in the TiAIN-based coating layer to avoid that its deposition leads to diminished HCF performance.

[0028] The measurements were conducted assuming isotropic material behavior and a biaxial stress state in the coating under uniaxial bending stress, and the following condition was hold: where oresiduai (also referred to as or) correspond to the value of residual stresses measured in the coating-substrate system by using known transmission X-ray diffraction techniques, preferably X-ray diffraction techniques, and oappiied correspond to the value of the applied uniaxial bending stress. This condition ensures that the residual stresses in the coating, which typically arise from the deposition process or thermal treatments, are sufficiently high to counteract the tensile stresses induced by the applied bending load, thereby enhancing the material’s resistance to crack initiation and propagation under cyclic loading conditions. Thus, for a person skilled in the art, Oappiied can be understood as the stress resulting from the external forces or moments that are applied to a material, typically calculated based on the geometry of the specimen and the magnitude of the applied force. In this specific case, it is described in the context of uniaxial bending, which means the stress distribution across the specimen is primarily one-dimensional, with the maximum stress occurring at the surface of the specimen in the direction of the applied force.

[0029] The results let suppose that once the residual compressive stress field is able to shift fatigue crack nucleation into the interior of the titanium alloy, the HCF properties enhance, and the deeper the residual tensile stress peak is shifted into the bulk material.

[0030] More concretely the present invention relates to:

[0031] A component for use as turbine engine, comprising a substrate made of an alloy or super-alloy, said substrate having a substrate surface coated with a coating comprising a coating layer that is a TiAIN-based layer comprising titanium, aluminum and nitrogen, wherein: • the TiAIN-based layer comprising alloying chemical elements X included in the TiAIN-based layer, so that its chemical element composition in atomic concentration is given by the formula (TiaAlbXc)N with a+b+c = 1 , 0.05 < c < 0.25, 2 / 3 < a / b < 3 / 2, and X being tantalum, or X being tantalum and silicon, so that X = Ta or X = Ta and Si, wherein the chemical concentration can be measured by using known techniques, for example EDS (energy-dispersive x-ray spectrometry), and

[0032] • the TiAIN-based layer exhibiting residual stresses OTT IXN < 0 GPa that can be measured by using known transmission X-ray diffraction techniques, and

[0033] • the interface region between the TiAIN-based layer and the substrate surface comprising a point A and a point B (see inventive examples in Figure 5, without metallic interlayer in (b), and with metallic interlayer in (c) and (d)), where the point A is closer to the TiAIN-based layer than the point B, wherein residual stresses GrA can be measured at the point A and residual stresses GrB can be measured at the point B, by using known transmission X-ray diffraction techniques, e.g. X-ray nanodiffraction techniques, and wherein: o GrA < 0 GPa, and o GrB > -0.5 GPa, and

[0034] O GrA < GrB.

[0035] Preferably, if X = Ta and Si the content of Si may be lower or equal than the content of Ta, wherein the chemical concentration can be measured by using known techniques, for example EDS (energy-dispersive x-ray spectrometry).

[0036] According to an example of the present invention, the TiAIN-based layer having a layer thickness in a range from 3 pm to 50 pm, wherein the layer thickness can be measured by using known techniques, for example Calotest or scanning electron microscopy.

[0037] According to a further preferred example of the present invention, the TiAIN-based layer may exhibit the following residual stresses orA < -0.5 GPa and / or GrB 0 GPa, preferably GrB > 0 GPa, According to a very preferred embodiment of the present invention the coating may comprise a metallic interlayer deposited as interlayer between the TiAIN-based layer and the substrate surface.

[0038] According to a further preferred example of the present invention, the metallic interlayer may have a layer thickness between 100 nm and 1000 nm

[0039] Preferably, the point B is comprised in the metallic interlayer, so that residual stresses orB can be measured at the point B in the metallic interlayer, wherein in particular CrB > 0 GPa

[0040] The metallic interlayer may be made of one or more metallic elements, wherein according to one of the preferred embodiments, one of the metallic elements may be a metallic element comprised in the alloy or superalloy forming the substrate surface.

[0041] Preferably, the metallic interlayer is made of one or more metallic elements, wherein one of the metallic elements is titanium or chromium.

[0042] It should be mentioned at this point that a metallic interlayer can provide significant benefits, such as improved fatigue strength and operational reliability. However, it may also introduce disadvantages, including increased material cost, potential for galvanic corrosion, and the complexity of processing, which could lead to performance issues or reduced long-term reliability in certain environments.

[0043] Preferably, the alloy or super-alloy may be a titanium-based alloy or a titaniumaluminum-based alloy or an aluminum-titanium-based alloy or a nickel-based alloy, for example an alloy of the type of Ti-6AI-4V or a super alloy of type of INCONEL 718.

[0044] According to a further preferred embodiment of the present invention, the metallic interlayer may be a titanium interlayer or a chromium interlayer or a nickel interlayer.

[0045] Particularly good results were observed when the titanium interlayer may exhibit a tensile residual stress in a range from 0.1 GPa to 1.5 GPa. Also, particularly good results were observed when, the metallic interlayer may be a titanium interlayer, and the substrate and the metallic interlayer comprising a thermal expansion coefficient, wherein the the thermal expansion coefficient of the titanium interlayer TECn-interiayer is higher than the thermal expansion coefficient of the substrate Surface TECsubstrate_surface, SO that TECTi-interlayer / TECsubstrate_surface > 0.

[0046] Preferably, the thermal expansion coefficient of the titanium interlayer TEC -interiayer may be maximal 15 % higher than the thermal expansion coefficient of the substrate Surface TECsubstrate_surface, SO that 1 5 =5 TECfi-interlayer * 1 00 / TECsubstrate_surface > 0.

[0047] According to a further very preferred embodiment of the present invention the component may be a turbine engine component, preferably an airfoil or rotor blade for a gas turbine.

[0048] A further aspect of the present invention is a method for producing a component for use as turbine engine, preferably a component according to one of the preceding claims, wherein the component comprises a substrate made of an alloy or super-alloy, said substrate having a substrate surface coated with a coating, comprising a coating layer that is a TiAIN-based layer comprising titanium, aluminum and nitrogen and further comprising alloying chemical elements X included in the TiAIN-based layer, so that its chemical element composition in atomic concentration is given by the formula (TiaAlbXc)N with a+b+c = 1 , wherein X being tantalum, or X being tantalum and silicon, so that X = Ta or X = Ta and Si, wherein the method comprises the steps:

[0049] • Providing a target material, preferably at least one target comprising Ti, Al and X, for deposition on the substrate surface of the substrate,

[0050] • Evaporating the target material with the addition of nitrogen as reactive gas,

[0051] • Depositing the target material on the substrate surface of the substrate, wherein the method is carried out by a targeted adjustment of a bias substrate voltage to adapt the residual stress state of the Ti-AI-X-N layer, preferably within the interface region between the TiAIN-based layer and the substrate surface.

[0052] Preferably, the method may be a PVD method, in particular a cathodic arc evaporation method.

[0053] Preferably, the bias substrate voltage may be varied in a range between -20V to -80V, preferably in a range between -20V to -40V.

[0054] According to an example of the further aspect of the present invention, nitrogen may be added in a pressure range between 2 and 5 Pa, preferably between 3 and 4 Pa

[0055] Moreover, the coating temperature may be varied in a range between 300 and 600 °C, preferably in a range between 400 and 500 °C.

[0056] Preferably, the method comprising measuring the residual stress of the Ti-AI-X-N layer, in particular, within the interface region between the TiAIN-based layer and the substrate surface.

[0057] According to a preferred example of the further aspect of the present invention, the targeted adjustment of a bias substrate voltage may be carried out to adapt the residual stress state of the Ti-AI-X-N layer to provide the following values:

[0058] • the TiAIN-based layer exhibiting residual stresses GI-TIAIXN < 0 GPa,

[0059] • GrA < 0 GPa and orB -0.5 GPa, wherein the interface region between the TiAIN-based layer and the substrate surface comprising a point A and a point B, where the point A is closer to the TiAIN-based layer than the point B, wherein residual stresses orA can be measured at the point A and residual stresses orB can be measured at the point B.

[0060] Preferably, the targeted adjustment of a bias substrate voltage may be carried out to adapt the residual stress state of the Ti-AI-X-N layer to provide the following values: GrA < -0.5 GPa and / or orB > 0 GPa, preferably GrB > 0 GPa, in particular GrA < orB. According to a preferred example of the further aspect of the present invention, a metallic interlayer may be deposited between the TiAIN-based layer and the substrate surface, wherein preferably the metallic interlayer is made of one or more metallic elements, wherein in particular one of the metallic elements is titanium or chromium.

[0061] Specific Description

[0062] Functional substrates in the context of the present invention include, but are not limited to, superalloys like Inconel (e.g. Inconel 718), Waspaloy, C263, and titaniumaluminides like Ti-6AI-4V.

[0063] In order to evaluate fatigue properties of coated functional substrates, the inventors used single cantilever high-cycle fatigue tests with a system of type Discovery DMA 850. Specimens of Ti-6AI-4V of length 17.5 mm were used, where inventive coatings were applied. Reference tests were also performed with specimens in uncoated condition. The specimen were foreseen with a notch on the one edge prior to the start of the fatigue testing, to create a stress concentration point. As depicted in Fig. 1 , one edge of the cantilever was fixed with a clamp immediately next to the notch. The other edge of the cantilever was oscillated with a frequency of 10 Hz between a neutral position of no bending of the cantilever, and a strained position where the cantilever was moved out of the neutral position to create a strained state. The maximum strain position is determined by the maximum displacement out from the neutral position, together with the mechanical properties of the cantilever material. For each combination of functional substrate and coating, as well as for reference tests with specimens in uncoated condition, several tests were performed using for each test a predetermined maximum strain position, corresponding to a predetermined stress state at the position of the notch. The number of cycles that could be performed until breakage of the specimen were recorded.

[0064] In Fig. 2, results of the single cantilever high-cycle fatigue test are presented graphically in a so-called S-N plot. On the horizontal axis, the number of cycles, N, until failure is plotted on a logarithmic scale. On the vertical axis, the applied stress OK,a the maximum strain position is plotted on a logarithmic scale. One datapoint corresponds to one single cantilever high-cycle fatigue test. The relationship between number of cycles and applied stress is indicated with lines for each specimen type (functional substrate and coating combination, respectively uncoated specimens).

[0065] Residual stress and mechanical properties of the Ti-AI-X-N coatings were also characterized and is graphically depicted for exemplary Ti-AI-X-N coatings in Fig. 3. Residual stress can preferably be determined by well-known methods based on X-ray diffraction, such as the sin2psi method. Mechanical properties can preferably be determined by well-known nanoindentation methods.

[0066] The inventors observed that the residual stress of the Ti-AI-X-N coating has a critical influence on the fatigue properties of the coated functional substrates. A marked improvement of fatigue properties was concluded when the residual stress is compressive and exceeded a critical range of 1 -3 GPa. The inventors further concluded that compressive residual stress of lower magnitude than the critical range was detrimental for fatigue properties.

[0067] In an exemplary series of experiments, the residual stress of Ti-AI-X-N coatings was obtained by selecting different substrate bias voltage during the deposition. Three different Ti-AI-X-N coatings were so synthesized on Ti-6AI-4V substrates:

[0068] • -20 V bias resulted in compressive residual stress of 0.61 ± 0.15 GPa, samples denoted as Tio.57Alo.43N'20V.

[0069] • -40 V bias resulted in compressive residual stress of 3.04 ± 0.29 GPa, samples denoted as Tio.56Alo.44N'4OV.

[0070] • -80 V bias resulted in compressive residual stress of 3.60 ± 0.33 GPa, samples denoted as Tio.56Alo.44N’8OV.

[0071] The result of single cantilever high-cycle fatigue test of the Ti-6AI-4V coated substrates and uncoated Ti-6AI-4V specimens are depicted in Fig. 2. As can be observed, the Tio.57Alo.43N'20Vcoated samples showed performance worse than uncoated Ti-6AI-4V. The worse performance can be read out by considering the applied stress OK,a that can be applied at a specific number of fatigue cycles (for example 1x105). The Tio.57Alo.43N'20Vcoated samples show here lower tolerance for applied stress compared to uncoated Ti-6AI-4V. On the contrary, the Tio.56Alo.44N'4OVcoated samples and the Tio.56Alo 44N'8OVcoated samples could both tolerate higher applied stress OK, a at a specific number of fatigue cycles (for example 1x105). The higher tolerance for applied stress is evidence of enhanced fatigue properties of the coated component.

[0072] Disclosed is also an inventive method for producing coatings according to the invention. The method comprises the use of a PVD technique for deposition Ti-AI-X-N coatings onto functional substrates. The method comprises means for controlling the residual stress state of the Ti-AI-X-N. In a preferred embodiment, the PVD technique is cathodic arc deposition, which has the advantage of providing a high degree of ionized species. In a further preferred embodiment, a substrate bias voltage is applied to the functional substrates and the magnitude of the bias voltage is used to control the residual stress of the coating.

[0073] As an example of the method, coating system of type INNOVA was used to realize a PVD coating process of type cathodic arc deposition. The coating system was equipped with 4 targets of composition Ti 50 at% Al 50 at%. The coating system was pumped to vacuum and heated to operating temperature of 450 °C, whereafter an in- situ ion etch was performed. An arc discharge was ignited on each of the four targets, whereafter they were operating with an arc current of 200 A per target. Nitrogen was used as process gas, controlled to a pressure of 3.2 Pa. The substrate bias voltage was used to control the residual stress of the coating. It was found that a negative bias voltage of - 40V can be used to synthesize coatings with compressive residual stress of magnitude 3 GPa.

[0074] In a further aspect of the invention, different atomic ratios of Ti to Al are disclosed for the Ti-AI-X-N coating. The Ti / AI ratio can preferably be in the range of 35 / 65 [at% Ti] / [at% Al] to 45 / 55 [at% Ti] / [at% Al], A high Al-content may be preferrable for oxidation resistance. In a further aspect of the invention, different alloying elements and concentrations are disclosed for the Ti-AI-X-N coating.

[0075] - X may be Si, preferably in a concentration of 1 - 15 at%, when only the constituents Ti, Al, and X are considered for the calculation of composition.

[0076] - X may be Ta, preferably in a concentration of 1 - 10 at%, when only the constituents Ti, Al, and X are considered for the calculation of composition.

[0077] - X may be a combination of Si and Ta, preferably in a concentration of 1 - 25 at%, when only the constituents Ti, Al, and X are considered for the calculation of composition.

[0078] In a further aspect of the invention, different thicknesses of the Ti-AI-X-N coating is disclosed. Preferably, the thickness of the Ti-AI-X-N coating is preferably at least 3 pm and not thicker than 50 pm.

[0079] In a further aspect of the invention, an interlayer is disclosed between the functional substrate and the Ti-AI-X-N coating. The interlayer is characterized as being of different material compared to the functional substrate and the Ti-AI-X-N coating. The interlayer is preferably thinner than the Ti-AI-X-N coating. In one embodiment, the interlayer comprises or consist of Ti.

[0080] Disclosed is also different uses of the Ti-AI-X-N coated functional substrates according to the invention. In one embodiment, the Ti-AI-X-N coated functional substrate is used as an airfoil or part of an airfoil. In another embodiment, the Ti-AI-X-N coated functional substrate is a turbine blade or turbine component of a gas turbine.

[0081] A coated component according to the present invention preferably comprises a functional substrate and a Ti-AI-X-N coating, wherein the Ti-AI-X-N coating has residual compressive stress exceeding 3 GPa.

[0082] A method according to the present invention to produce inventive coatings and inventive coated components (comprising a functional substrate and a Ti-AI-X-N coating) preferably comprises a step in which means for controlling the residual compressive stress of the Ti-AI-X-N coating are used, wherein the magnitude of the substrate bias is preferably used to control residual stress.

[0083] The method preferably includes a step in which a negative bias is applied to the substrate being coated, wherein the magnitude of the applied bias voltage is in a range from -40 V to -80 V.

[0084] In order to facilitate understanding of the invention, some explanations, as well as some particular examples and experiments will be provided below.

[0085] The invention relates to coatings applied on components comprising functional substrates and methods for producing the inventive coatings and components, wherein the coatings allow producing components having functional substrates whose coating provides them improved fatigue behavior. The inventive coatings being Ti-AI-X-N coatings with controlled level of compressive residual stress, enabling higher lifetime or higher applied stress. The method for producing the inventive Ti-AI-X-N coatings including targeted control of residual stress. The present invention being especially suitable for application areas including airfoils, turbine blades, and turbine components.

[0086] Experimental details in Examples using the Ti-alloy known as Ti-6AI-4V and the Ni- superalloy known as Inconel 718:

[0087] Coating deposition:

[0088] TiAIN-based thin films were deposited on substrates of Ti-6AI-4V and Inconel 718 for conducting investigations of the fatigue behavior. The TiAIN-based films were deposited on the substrates in an industrial physical vapor deposition (PVD) machine (Oerlikon Balzers, INNOVA 1.0) using cathodic arc evaporation. For each deposition process, six-inch powder-metallurgically manufactured targets from Plansee Composite Materials GmbH were operated in a pure N2-atmosphere: Either four TiAl targets with equal atomic percentages or four TiAITa targets with a composition of 45 / 45 / 10 at. % or four TiAITaSi targets with a composition of 42.5 / 42.5 / 10 / 5 at. %, respectively. The metallic interlayer was deposited in a pure Ar-atmosphere by two Ti targets with a purity of 99.6 % (Plansee Composite Materials GmbH). For all fatigue tests, pre-notched custom-made Ti-6AI-4V and Inconel 718 fatigue samples have been used. Subsequently, all substrates were pre-cleaned in an ultrasonic bath using acetone and ethanol for five minutes each. The cleaned fatigue specimens were clamped at the father away end from the notch, onto sample holders and placed on a rotating support structure inside of the vacuum chamber. This ensured a continuous three-fold rotation during deposition, promoting the formation of a uniform coating on all sides of the fatigue specimen. As soon as an adequate vacuum was reached, the chamber was heated to 450 °C, followed by an argon etching step at a gas pressure of 0.22 Pa. Then, the 300 nm thin titanium interlayer was coated at a bias voltage of -150 V. Therefore, the targets were subjected to an arc current of 160 A. Subsequently, each titanium aluminum nitride based coating (also called top-coating ot TiAIN-based layer) was deposited at a total N2 pressure of 3.2 Pa, an arc current of 200 A. For the residual stress variation, the bias voltage was either set to -20, -40 or -80 V, respectively. Moreover, the deposition time was adjusted individually to achieve a total film thickness of about 4500 nm for all coating states.

[0089] Coating characterization

[0090] To quantify the top-coating thickness and thus the deposition time, calotesting was conducted with the BAQ kaloMAX II and the resulting indentations were evaluated with the Keyence VHX6000 digital microscope. However, a more precise method was necessary for the determination of the interlayer thickness. Therefore, coated Ti-6AI- 4V substrates were cut in half with the Struers Accutom-10, heat embedded using Struers CitoPress-30 and polished with the Struers TegraPol-31 system. Finally, scanning electron microscopy (SEM) was employed to analyze the cross sections with a backscattered electron detector (BSD) on the ZEISS Sigma 500 VO. For each coating, the determination of the chemical composition, energy dispersive X-ray spectroscopy (EDS) was carried out in top-view configuration with an EDAX EDS detector (15 kV acceleration voltage). Furthermore, the structural constitution was analyzed by X-ray diffraction (XRD) using a PANalytical XPert Pro MPD system with a Cu-Ka radiation source (2=1 .5418 A) in the Bragg-Brentano geometry. High resolution transmission electron microscopy (HR-TEM) on a FEI TECNAI F20 system (200 kV acceleration voltage) provided information on the growth characteristics and grain size distribution. In addition, transmission electron backscatter diffraction (t-EBSD) was utilized to gain deeper insights into the characteristics of the interface, grain size, and crystal orientation. Accordingly, transmission Kikuchi diffraction patterns were acquired using EDAX Apex 3.0 software and subsequently post-processed with EDAX OIM 8.6.

[0091] Indentation hardness (H) and Young’s modulus (£) of the TiAIN-based thin films were determined using a CSIRO UMIS nanoindentation system equipped with a Berkovich diamond tip.

[0092] Fatigue testing

[0093] To evaluate the fatigue properties of coated functional substrates, single cantilever high-cycle fatigue tests were conducted using a Discovery DMA 850 system. Specimens made of Ti-6AI-4V with a length of 17.5 mm were coated with the inventive coatings, and reference tests were performed on uncoated specimens. Prior to testing, a notch was introduced on one edge of each specimen to create a localized stress concentration. The fixed edge of the cantilever was clamped immediately next to the notch, while the opposite edge was oscillated at a frequency of 10 Hz. The oscillation involved cycling the cantilever between a neutral position, where no bending occurred, and a strained position, where the cantilever was deflected to induce a strain.

[0094] The maximum strain position was determined by the displacement from the neutral position, taking into account the mechanical properties of the cantilever material. For each combination of substrate and coating, several tests were performed, each with a predetermined maximum strain, corresponding to a specific stress state at the notch. The number of cycles to failure was recorded for each specimen, providing insight into the fatigue resistance of the different coated and uncoated specimens.

[0095] Residual Stress

[0096] To investigate the residual stress gradients of coated specimens, transmission X-ray nanodiffraction experiments can be carried out. Therefore, the polished cross-section of a coated substrate is first precisely aligned with the monochromatic high-energy X-ray beam, so that its focal point is located in the center of the coating and the substrate-coating interface is parallel to the beam direction. Subsequently, the sample is scanned vertically with a step size not largen than 100 nm, starting in the air above the coating and ending a few micrometers deep in the substrate. For further details regarding the sample-beam alignment and the referenced coordinate system, please refer to Figure 4.

[0097] Subsequently, the recorded diffraction patterns have to be processed using an X-ray diffraction data analysis software, as the distortion of the Debye-Scherrer rings and application of single crystal elastic constants allows the determination of the residual in-plane stress of the metallic interlayer:

[0098] By integrating the diffraction rings in segments of 10 ° from S = -5 to 95 ° , the interplanar spacing (dhki 8 ) for a specific orientation 8 must be retrieved. Under the assumption of isotropic material behavior and biaxial stress state, the orientationdependent lattice strain (Ehki(8y) is expressed as

[0099] Accordingly, for a given orientation 8, it solely depends on the interplanar spacing of the unstressed material which itself can be obtained by performing a linear regression of the dhki(8~) - sm2(dj. By considering small Bragg angles (0) and the fundamental equation of X-ray stress analysis the residual in-plane stress (crr|| ) can be evaluated from the distortion of Debye- Scherrer rings expressed by the term

[0100] Figure captions

[0101] Fig. 1 Single cantilever high-cycle fatigue test setup

[0102] Fig. 2 S-N plot of exemplary results from single cantilever high-cycle fatigue test

[0103] Fig. 3 Coating properties of exemplary Ti-AI-X-N coatings deposited with -20V, -40V, and -80V bias : a) residual stress, b) hardness (H) and elastic modulus (E), c) calculated H / E and H3 / E2ratios.

[0104] Fig. 4 Schematic representation of the synchrotron transmission nanodiffraction experiments. The crystallinity of the material leads to diffraction of the incident X-ray beam, and the Debye-Scherrer rings are recorded by a 2D detector. For in-situ bending of the microcantilever (Z x b x h = 92 x 29 x 24 pm3), the diamond wedge tip of the nanoindentation system was placed at its free end.

[0105] Fig. 5 Residual stress gradients at the coating-substrate interface determined by transmission X-ray nanodiffraction experiments. For comparison purposes, all synchrotron-tested coating systems were synthesized at the same bias voltage: (a) TZ0.564Z0.44ZV-1.51 , (b) Ti & Ti0.564Z0.44ZV-0.95, (c) Ti0.484Z0.38Ta0.14ZV-2.23, and (d) Ti & Ti0.484Z0.38Ta0.14ZV-2.20.

[0106] Fig. 6 Fatigue behavior of uncoated substrate of Inconel 718, as well as substrate of Inconel 718 coated with coating labelled TP29 of TiAIXN layer with X = Ta and Si, having chemical element composition Ti0.46AI035Ta012Si0.07N, and substrate of Inconel 718 coated with coating labelled TP29 of TiAIXN layer with X = Ta and Si, having chemical element composition Tio.46Alo.5Tao.12Sio.o7N and comprising a Ti-interlayer between the substrate and the TiAIXN layer.

Claims

Claims:1 . A component for use as turbine engine, comprising a substrate made of an alloy or super-alloy, said substrate having a substrate surface coated with a coating, comprising a coating layer that is a TiAIN-based layer comprising titanium, aluminum and nitrogen, characterized in that:• the TiAIN-based layer comprising alloying chemical elements X included in the TiAIN-based layer, so that its chemical element composition in atomic concentration is given by the formula (TiaAlbXc)N with a+b+c = 1 , 0.05 < c < 0.25, 2 / 3 < a / b < 3 / 2, and X being tantalum, or X being tantalum and silicon, so that X = Ta or X = Ta and Si, wherein• the TiAIN-based layer exhibiting residual stresses GrTiAixN < 0 GPa, and• the interface region between the TiAIN-based layer and the substrate surface comprising a point A and a point B, where the point A is closer to the TiAIN- based layer than the point B, wherein residual stresses GrA can be measured at the point A and residual stresses GrB can be measured at the point B, and wherein: o GrA < 0 GPa, and o GrB > -0.5 GPa, andO GrA < GrB.

2. The component according to claim 1 , characterized in that if X = Ta and Si the content of Si is lower or equal than the content of Ta.

3. The component according to claim 1 or 2, characterized in that the TiAIN-based layer having a layer thickness in a range from 3 pm to 50 pm.

4. The component according to one of the preceding claims, characterized in that GrA < -0.5 GPa and / or GrB — 0 GPa, preferably GrB > 0 GPa.

5. The component according to one of the preceding claims, characterized in that the coating comprises a metallic interlayer deposited between the TiAIN-based layer and the substrate surface.

6. The component according to claim 5, characterized in that the metallic interlayer has a layer thickness between 100 nm and 1000 nm.

7. The component according to claim 5 or 6, characterized in that the point B is comprised in the metallic interlayer, so that residual stresses GrB can be measured at the point B in the metallic interlayer, wherein preferably GrB > 0 GPa.

8. The component according to one of claims 5 to 7, characterized in that:• the metallic interlayer is made of one or more metallic elements, wherein preferably one of the metallic elements is a metallic element comprised in the alloy or superalloy forming the substrate surface.

9. The component according to one of claims 5 to 8, characterized in that:• the metallic interlayer is made of one or more metallic elements, wherein one of the metallic elements is titanium or chromium.

10. The component according to one of the preceding claims, characterized in that:• The alloy or super-alloy is a titanium-based alloy or a titanium-aluminum-based alloy or an aluminum-titanium-based alloy or a nickel-based alloy.11 . The component according to claim 10, characterized in that:• The alloy is of the type of Ti-6AI-4V or the super alloy is of type of INCONEL 718.

12. The component according to claim 11 , characterized in that the metallic interlayer is a titanium interlayer or a chromium interlayer or a nickel interlayer.

13. The component according to claim 12, characterized in that:The metallic interlayer exhibits a tensile residual stress in a range from 0.1 GPa to 1 .5 GPa.

14. The component according to claim 12 or 13, characterized in that:• the metallic interlayer is a titanium interlayer, comprising a thermal expansion coefficient, wherein the thermal expansion coefficient of the titanium interlayer TECTi-interiayer is higher than a thermal expansion coefficient of the substrate Surface TECsubstrate_surface, SO that TECTi-interiayer / TECsubstrate_surface>0.

15. The component according to claim 14, characterized in that:• the thermal expansion coefficient of the titanium interlayer TECTi-interiayer is maximal 15 % higher than the thermal expansion coefficient of the substrate Surface TECsubstrate_surface, SO that 15 ^ TECTi-interiayer * 100 I TECsubstrate_surface>0.

16. The component according to one of the preceding claims, characterized in that:• the component is a turbine engine component, preferably an airfoil or rotor blade for a gas turbine.

17. Method for producing a component for use as turbine engine, preferably a component according to one of the preceding claims, wherein the component comprises a substrate made of an alloy or super-alloy, said substrate having a substrate surface coated with a coating, comprising a coating layer that is a TiAIN- based layer comprising titanium, aluminum and nitrogen and further comprising alloying chemical elements X included in the TiAIN-based layer, so that its chemical element composition in atomic concentration is given by the formula (TiaAlbXc)N with a+b+c = 1 , wherein X being tantalum, or X being tantalum and silicon, so that X = Ta or X = Ta and Si, wherein the method comprises the steps:• Providing a target material, preferably at least one target comprising Ti, Al and X, for deposition on the substrate surface of the substrate,• Evaporating the target material with the addition of nitrogen as reactive gas,• Depositing the target material on the substrate surface of the substrate,• wherein the method is carried out by a targeted adjustment of a bias substrate voltage to adapt the residual stress state of the Ti-AI-X-N layer, preferably within the interface region between the TiAIN-based layer and the substrate surface.

18. Method according to claim 17, characterized in that the method is a PVD method, preferably a cathodic arc evaporation method.

19. Method according to claim 17 or 18, characterized in that the bias substrate voltage is varied in a range between -20V to -80V, preferably in a range between -20V to -40V.

20. Method according to one of claims 17 to 19, characterized in that nitrogen is added in a pressure range between 2 and 5 Pa, preferably between 3 and 4 Pa.

21. Method according to one of claims 17 to 20, characterized in that the coating temperature is varied in a range between 300 and 600 °C, preferably in a range between 400 and 500 °C.

22. Method according to one of claims 17 to 21 , characterized in that the method comprising measuring the residual stress of the Ti-AI-X-N layer, preferably within the interface region between the TiAIN-based layer and the substrate surface.

23. Method according to one of claims 17 to 22, characterized in that the targeted adjustment of a bias substrate voltage is carried out to adapt the residual stress state of the Ti-AI-X-N layer to provide the following values:• the TiAIN-based layer exhibiting residual stresses GrTiAixN < 0 GPa,• GrA < 0 GPa and GrB > -0.5 GPa, wherein the interface region between the TiAIN-based layer and the substrate surface comprising a point A and a point B, where the point A is closer to the TiAIN-based layer than the point B, wherein residual stresses GrA can be measured at the point A and residual stresses GrB can be measured at the point B.

24. Method according to one of claims 17 to 23, characterized in that the targeted adjustment of a bias substrate voltage is carried out to adapt the residual stress state of the Ti-AI-X-N layer to provide the following values:GrA < -0.5 GPa and / or GrB > 0 GPa, preferably GrB > 0 GPa, in particular GrA < GrB.

25. Method according to one of claims 17 to 24, characterized in that a metallic interlayer is deposited between the TiAIN-based layer and the substrate surface, wherein preferably the metallic interlayer is made of one or more metallic elements, wherein in particular one of the metallic elements is titanium or chromium.

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