Protective layers against environmental influences (environmental barrier layers) for Ti-Al materials

Abstract

The present invention relates to a surface coating for protecting a substrate having a Ti-Al material, which preferably comprises one or more of the materials set out in Table 1, which coating comprises a layer sequence with at least one layer forming a diffusion barrier for Ti, which layer sequence preferably conforms to one or more of the layer sequences set out in the rows of Table 1, and which coating comprises an oxidation barrier, in particular tailored to the diffusion barrier, which oxidation barrier is preferably tailored according to Table 2, and in particular the surface coating comprises a heat shield, which heat shield is tailored to the oxidation barrier, preferably according to Table 3.

Description

[Technical Field]

[0001] The present invention relates to a surface coating for protecting Ti-Al based materials with high mechanical strength against corrosive wear, in particular oxidative wear, which can be achieved by introducing intermetallic Ti-Al phases into these materials. The invention also relates to a layer system which can be used as a thermal barrier layer. The invention also relates to a method for producing this surface coating. Within the framework of the following description, various barriers are mentioned. In each case, the following is meant:

[0002] Environmental Barrier Layer A protective layer comprising one or more individual layers intended to protect the substrate surface from harmful environmental influences such as oxidation, corrosion, evaporation, volatilization, and erosion.

[0003] Diffusion Barrier The task of a diffusion barrier is to prevent or allow only limited diffusion of elements between the substrate and further layers, such as an oxidation barrier layer. Typically, a diffusion barrier is realized by a diffusion barrier layer.

[0004] Oxidation Barrier An oxidation barrier according to this specification prevents or dramatically reduces the diffusion of oxygen into the diffusion barrier layer or into the interface between the diffusion barrier layer and the substrate surface. Typically, the oxidation barrier is realized by an oxidation barrier layer.

[0005] Heat shielding section The task of the heat shield is to protect the substrate material from excessively high temperatures and thus make it possible to use the substrate material in a temperature range above its operating temperature from the viewpoint of mechanical strength. The layer (heat shield layer) is a layer that is sufficiently thick and / or has a sufficiently low thermal conductivity so that the desired temperature reduction is achieved at that thickness. [Background technology]

[0006] Ti-Al-based materials are preferred and desirable for aircraft turbine components due to their low density and high strength. Therefore, Ti-Al-based materials are currently being investigated as a replacement for Ni-based superalloys, particularly in the aircraft industry (B.P. Bewlay et al., Materials at High Temperatures 33 (2016) p. 549; N.P. Bedture, Nature Materials 15 (2016) p. 804). Furthermore, these materials are being applied in other fields, such as high-performance automotive technology (T. Tetsui and Y. Miura, Mitsubishi Heavy Industries, Technical Review 39 (2002) p. 1) and the nuclear industry (H. Zhu et al., The Journal of The Minerals, Metals & Materials Society 64 (2012) p. 1418). However, these materials have the disadvantage of being poorly resistant to oxidation at high temperatures and subject to diffusion processes, which can lead to deterioration of their mechanical properties. Summary of the Invention [Problem to be solved by the invention]

[0007] The underlying problem of the present invention is to create a surface coating for Ti-Al substrates that is oxidation resistant at high temperatures and, in particular, does not exhibit a deterioration in mechanical properties.

[0008] This problem is solved by a surface coating according to claim 1.

[0009] The underlying problem of the present invention is to create a method for producing a surface coating for Ti-Al substrates, which is oxidation resistant at high temperatures and, in particular, does not suffer from degradation of its mechanical properties.

[0010] This problem is solved by a surface coating according to claim 14.

[0011] High temperatures may be understood in particular to mean temperatures above about 900°C.

[0012] In the context of turbine applications, Ti-Al-based materials have been particularly studied, which may contain the intermetallic phases γ-TiAl and α2-Ti3Al and additional dopants with other elements, as described, for example, in Table 13 on page 308 of "Materials for Gas Turbines - An Overview, Advances in Gas Turbine Technology" by NR Muktinutalapati, edited by Dr. Ernesto Benini, (2011) (ISBN: 978-953-307-611-9). As already mentioned, the surfaces of these materials must be protected against oxidation, and diffusion processes occurring at high temperatures both in the substrate material and in the region between the substrate material and the oxidation protective layer must be prevented or, to a desired extent, controlled. Therefore, efforts are generally made to provide protective coatings (Figure 1) consisting of diffusion and oxidation barrier layers. Furthermore, the possibility of extending the application of these materials to higher service temperatures by providing an additional thermal barrier layer applied on top of the oxidation barrier layer is being investigated. For Ni-based superalloys, such a thermal barrier layer is realized as follows: as a first layer on the superalloy, a so-called MCrAlY layer is applied as an interface (intermediate layer). This MCrAlY layer has a similar fcc (face-centered cubic) structure to the superalloy, but has a slightly higher Al content than the superalloy substrate. When heated in an ambient atmosphere above approximately 1000 °C, it forms a dense thin aluminum oxide layer, the so-called oxide scale, on its surface. This MCrAlY layer can be combined with a thick (200 μm to 1000 μm) and additionally porous YSZ (yttria-stabilized zirconia) layer, through which a temperature reduction of up to 150 °C can be achieved, thereby enabling the use of the superalloy substrate at higher temperatures. This approach is of course also advantageous for Ti-Al based substrates, but is difficult to implement because the requisite heating up to 1000 °C weakens the substrate material already before the protective layer can be formed.

[0013] To improve the oxidation resistance of Ti-Al materials by surface coating, a stable interface between the coating and the Ti-Al substrate material is an important prerequisite. Some of the reasons for this are explained below. Even during coating, the increased energy input at the substrate surface can lead to diffusion processes occurring both near the surface of the substrate to be coated and within the initial layer. These diffusion processes depend on the type of metal vapor used for the coating and the process gases added during the coating process. For example, when a layer consisting of TiN or TiAlN is to be applied, i.e., when the layer is synthesized using a nitrogen plasma, nitrogen diffusion into the substrate occurs. This nitrogen diffusion weakens the mechanical properties in the region near the surface of the substrate to be coated. This weakening effect is even more dramatic if the Ti-Al substrate surface combines with oxygen during coating. This results in the formation of Ti-O compounds, which have poor mechanical properties. Simultaneously with these oxidation processes, diffusion processes that promote interfacial instability, which is usually manifested as void formation, also accelerate.

[0014] As with superalloys, it is desirable to extend the application range of Ti-Al materials to higher temperatures. This can be achieved by applying a thermal barrier layer, which can provide a temperature reduction. The thermal barrier layer can be an additional layer system applied on top of the layer system of FIG. 1 (as shown in FIG. 2). However, a simpler approach can be taken, namely, by simply extending the layers described for the environmental barrier coating (EBC) in FIG. 1 with a thermal barrier layer, for example, by providing a substantially thicker oxidation barrier (as shown in FIG. 3). Particularly suitable for the thermal barrier layer are layer materials that can provide a particularly large temperature reduction, i.e., materials with low thermal conductivity but good mechanical stability at high temperatures. For the Ti-Al materials to be protected here, temperatures above 900°C can already be considered high, since this allows a substantial expansion of the application range of these Ti-Al materials (see "Materials for Gas Turbines - An Overview, Advances in Gas Turbine Technology" by NR Muktinutalapati, edited by Dr. Ernesto Benini, 2011 (ISBN: 978-953-307-611-9), p. 308, table 13). In other words, the thermal barrier layer does not necessarily have to be based on a YSZ material, which is thermally stable up to about 2000°C, but can also be a material with a thermal stability up to about 1500°C or even lower. It is desirable for such a material to have a low thermal conductivity, and it is particularly important that the coating process allows for the production of a layer morphology characterized by porosity, but which is nevertheless mechanically stable.

[0015] Solution A first solution to this problem is the layer system according to FIG. 1, in which the interface between the layer and the substrate is either diffusion-proof or stabilized by (limited) diffusion processes, i.e. no defects (voids) are formed (or only a negligible number are formed).

[0016] Furthermore, the inventive layer system according to FIG. 1 has a stable oxidation barrier on its surface or forms such a stable oxidation barrier during operation, preventing oxygen from reaching the interface between the substrate and the layer.

[0017] These two requirements are essential prerequisites for a layer system that guarantees the stability of the Ti-Al substrate material surface in a given temperature range, which is on the one hand predetermined for the specific application and on the other hand dependent on the mechanical strength of the Ti-Al substrate, which depends inter alia on the chemical composition, crystalline structure and crystallite size.

[0018] A further aspect of the layer system according to the invention is that in a variant according to the invention the environmental barrier layer of FIG. 1 is extended with a thermal barrier layer, as shown in FIG. [Brief explanation of the drawings]

[0019] FIG. 1 is a schematic diagram of a layer system according to the present invention for an environmental barrier layer for a Ti-Al material. It comprises a Ti-Al substrate material 101, which may be, for example, a material listed in Table 13 on page 308 of "Materials for Gas Turbines—An Overview, Advances in Gas Turbine Technology" by NR Muktinutalapati, edited by Dr. Ernesto Benini (2011) (ISBN: 978-953-307-611-9). However, its chemical composition can also be modified. Within the scope of the present invention, a first layer 102 is deposited on this substrate material, which, in terms of its function, is a diffusion barrier for the temperature range for the specific use of the material. This diffusion barrier simultaneously ensures adhesion of the entire layer system to the substrate material. The task of this layer is to prevent or allow only limited diffusion between the substrate and subsequent layers, which leads to improved adhesion on the substrate but is limited by the amount of material available (thin layer). A thin metal layer 121 can be advantageously used to improve adhesion on various substrate materials and to improve the adhesion of layer systems for different application areas. Furthermore, it may be useful to create a gradient 122 in the chemical composition of the metal-silicon layer before the desired metal-silicon layer composition is coated as an intrinsic diffusion barrier (102).

[0020] The second layer 103 of the layer system prevents the diffusion of oxygen into the diffusion barrier or into the interface between the diffusion barrier and the substrate surface (oxidation barrier). Depending on the layer material, it may be advantageous to generate a protective oxide scale 123 on the Me-Si diffusion barrier before depositing the oxidation barrier in an oxygen environment in order to prevent the diffusion of active oxygen from the oxygen plasma required for the synthesis of the oxide oxidation barrier.

[0021] Essential characteristics of an effective environmental barrier layer are the prevention of Ti diffusion to the surface of the layer system after atmospheric baking and the detection of good adhesion between the layer system and the substrate after annealing in ambient atmosphere.

[0022] drawing Figure 1 1 is a diagram of a layer system according to the invention for an EBC on a Ti—Al substrate, consisting of a substrate 101, a diffusion barrier 102 and an oxidation barrier 103. FIG.

[0023] In Figure 1, the following meanings apply: 101 Ti-Al based substrate to be protected against oxidation 102 Diffusion barrier layer provided as an intermediate layer (interface) between the Ti-Al base substrate and the oxidation barrier layer 121 Metal adhesion layer 122 Gradient Layers in Metal-Silicon Layer Materials 123 Oxide scale 103 Oxidation barrier layer that seals the layer system against the environment at its surface against oxidation processes

[0024] Figure 2 Layer system according to the invention for an EBC on a Ti-Al substrate, consisting of a substrate 201, a diffusion barrier 202, an oxidation barrier 203 and a further thermal barrier layer 204.

[0025] 2 shows the expansion of the oxidation barrier layer 203 with a further layer 204 having the function of a thermal barrier layer. Generally, such a thermal barrier layer is thermally stable within a given temperature range and preferably has a low thermal conductivity, which is provided by the material of the layer (e.g., oxide) or achieved by increasing the porosity of the layer.

[0026] In Figure 2, the following meanings apply: 201 Ti-Al based substrate to be protected against oxidation 202 Diffusion barrier layer provided as an intermediate layer (interface) between the Ti-Al base substrate and the oxidation barrier layer 203 Oxidation barrier layer that seals the layer system against the environment at its surface against oxidation processes 204 Heat shield layer

[0027] Figure 3 Layer system according to the invention for EBC on a Ti-Al substrate, consisting of a substrate 301, a diffusion barrier 302 and an oxidation barrier 305 (which consists of 331 and 332), which is extended to a greater layer thickness and has a layer morphology at 332 that becomes more porous with increasing layer thickness.

[0028] 3 shows a further layer system with a simplified layer structure compared to that of FIG. 2. In this layer system, the oxidation barrier layer 203 and the thermal barrier layer 204 in FIG. 2 are combined into one layer 305, which fulfills both functions. This can be achieved by a gradient in the layer morphology, i.e., for example, by making the layer 305 denser near the interface 331 to achieve good adhesion, but then transitioning stepwise and / or continuously to a columnar structure or other type of porous structure 332 as the layer thickness increases, as suggested in FIG. 3. Denseness can be understood in particular to mean that the layer 305 is not porous near the interface 331, or is less porous, especially compared to the porous structure 332.

[0029] In Figure 3, the following meanings apply: 301 Ti-Al based substrate to be protected against oxidation 302 Diffusion barrier layer as an intermediate layer (interface) between the Ti-Al base substrate and the oxidation barrier layer 305 The combination of oxidation barrier layer and thermal barrier layer is based on substantially the same material system (as the oxidation barrier), but starts with a dense morphology at the interface with the diffusion barrier, as in layer 231 of FIG. 2, and then changes to an increasingly porous morphology as in layer 332 from FIG. 3 with increasing layer thickness.

[0030] Figure 4 FIG. 1 shows an XRD spectrum of a Mo—Si diffusion barrier layer according to the present invention prepared with a silane flow of 90 sccm, detecting the coexistence of Mo and MoSi 2 phases in the layer.

[0031] Figure 5 FIG. 1 shows the XRD spectrum of a Mo—Si diffusion barrier layer according to the present invention made with a silane flow of 180 sccm and having primarily the MoSi 2 phase.

[0032] Figure 6 FIG. 1 is a cross-sectional REM image of an environmental barrier layer consisting of a 4.9 μm thick Mo—Si layer (diffusion barrier) and a 2.7 μm thick Al—Cr—O layer (oxidation barrier).

[0033] Figure 7 REM cross-section of the environmental barrier layer in Figure 6 after annealing at 800°C for 20 hours in air. The Al-Cr-O layer (oxidation barrier) shows no change in thickness or morphology after annealing. At the interface with the substrate, limited diffusion occurs over a total area of ​​approximately 8 μm, including the diffusion barrier, but does not result in the formation of pores.

[0034] Figure 8 REM cross section of a Mo-Si diffusion barrier omitting the oxidation barrier. In this simplified version, immediately after coating (also performed at 450°C), the REM cross section shows no signs of diffusion processes at the interface. This layer was made with a silane flow of 180 sccm and has the XRD spectrum characteristic shown in Figure 5.

[0035] Figure 9 REM cross-section of the Mo-Si diffusion barrier shown in Figure 8 after annealing at 800 °C for 20 hours in air. In the interface region, a diffusion process occurs within a range of approximately 2 μm. Furthermore, Al diffuses toward the layer surface, forming an Al-O scale.

[0036] Figure 10 Figure 9 shows a cross-sectional REM image of the Mo-Si diffusion barrier (although at 10x magnification), which more clearly shows the approximately 200nm thick Al-O scale. DETAILED DESCRIPTION OF THE INVENTION

[0037] Layer-based manufacturing process The coating process can be carried out as a combination of physical vapor deposition (PVD) and plasma-enhanced chemical vapor deposition (PECVD), i.e., both methods are used to achieve layer synthesis, sometimes especially simultaneously. PVD methods can be, for example, electron beam evaporation, sputtering, and / or cathodic spark evaporation. CVD methods are essentially based on additional gas inflows, which can be used to inject various gaseous precursors into the coating system used, which are then decomposed and excited in plasma. Advantageously, the same coating system is used for PVD and CVD. The plasma required for CVD can be generated using a plasma source present in PVD, such as a cathodic spark source. However, it can also be generated by other methods, such as a separate low-voltage arc discharge. These methods are known to those skilled in the art.

[0038] Below, examples are given that explain and illustrate the production of a layer system according to the invention, but the description of this exemplary layer system should not limit the more general idea of ​​the invention.

[0039] First, the manufacturing process of the layer system according to the invention will be explained with reference to FIG. The TiAl substrates are placed in the coating system and fixed in appropriate holders. These holders are mounted on a substrate holder system, which can be stationary during coating and / or rotated once, twice, and / or three times. The coating system is rotated for approximately 10 -5The pressure is reduced by a pump until the pressure reaches a pressure of less than 100 mbar. The substrates are then pretreated. In this example, they are heated to the desired temperature (200°C to 600°C) using a radiant heater. The substrate surface is then pretreated in the equipment, for example, by sputtering with Ar gas ions. For this cleaning process, a negative voltage (substrate bias) is typically applied to the substrate. After these pretreatment steps, in this example, the negative substrate bias is set to a value, for example, -40 V, and this value is maintained during coating. As an example, we will describe the synthesis of a Mo-Si layer for a diffusion barrier layer using a combination of PVD and CVD processes. This coating begins with the ignition of a cathodic spark discharge on a Mo target, which is connected as the cathode of the spark discharge and is therefore evaporated by the cathodic spark. A reactive gas is introduced simultaneously, or, as described later in this example, with a short delay. A rare gas or a rare gas mixture can also be introduced. In this example, the spark discharge is operated with a source current of 220 A. A 90 sccm silane flow is added with a time delay of 2 minutes. A bias voltage of -40 V is applied to the substrate. In this way, the substrate is coated with a Mo-Si layer, the chemical composition of which can be controlled over a wide range via the Mo target evaporation rate (spark current) and the silane flow, thereby enabling the layer's chemical composition to be set. As an example, an X-ray diffraction pattern (XRD spectrum) of such a Mo-Si layer is shown in Figure 4. The characteristic Bragg peaks in this spectrum prove that, with the above coating parameters, a layer essentially consisting of Mo and MoSi2 is synthesized. By adjusting the Mo coating rate with the silane flow, the existing Mo-Si compounds can be almost freely selected, as depicted in the Mo-Si phase diagram. Figure 5 shows the XRD spectrum of a Mo-Si layer prepared with a silane flow of 180 sccm (instead of 90 sccm in Figure 4).In this spectrum, essentially only the Bragg peaks for MoSi2 synthesized in a mixture of the two crystalline structures can be recognized, i.e., the higher silane flow shifts the chemical composition of the synthesized layer toward chemical compounds with a higher Si content. In certain applications, this layer can be used only as an environmental barrier layer, i.e., when an oxide scale forms on its surface. This point will be explained in more detail below based on Figures 8-10.

[0040] The subsequent process step, i.e., the deposition of the oxidation barrier, is performed without interrupting the vacuum following the deposition of the Mo-Si layer. The transition between the deposition of the diffusion barrier and the deposition of the oxidation barrier can be abrupt; for example, the spark discharge on the Mo target is stopped, followed by the silane flow, before the oxidation barrier coating process begins. However, a process transition can also be selected during the coating of the oxidation barrier (in this example, an Al-Cr-O layer), which is performed smoothly, as described here. For this purpose, a cathodic spark discharge on an Al-Cr target (preferably with a target composition of Al (70 atomic percent) - Cr (30 atomic percent)) is ignited (spark current 180 A) during the last 3 minutes of the diffusion barrier coating process, i.e., while still in this process step. After a few minutes, an additional flow meter is set to gas inflow, which supplies a 400 sccm oxygen flow to the coating system. A few minutes can be understood to mean, in particular, about 0.5 minutes to about 15 minutes, preferably about 2 minutes to about 9 minutes, preferably about 2 minutes, preferably about 9 minutes, or particularly preferably about 4 minutes to about 6 minutes. In further embodiments of the present invention, a few minutes can also be understood to mean a different time period. After the oxygen flow has stabilized (about 1 minute), the Mo target is switched off and the silane flow meter is set to close the silane gas inflow, i.e., to switch off the silane flow. As a result, an Al-Cr-O layer is formed on and slightly overlapping the Mo-Si diffusion barrier.

[0041] If the layer system according to the invention is to be produced according to FIG. 2 or FIG. 3, the next step in the process is either to make the oxidation barrier layer thicker (as shown in FIG. 3) and correspondingly more porous (for example by increasing the oxygen gas flow), or (according to FIG. 2) to deposit the layer using a corresponding target, for example a target made of Zr—Y, to form a YSZ layer as described in Table 3.

[0042] Examples of layers made according to the present invention A layer system including and / or consisting of a Mo-Si diffusion barrier and an oxidation barrier is shown by way of example in FIG. 6, which is a cross-section of the layers taken with a scanning electron microscope (REM) and corresponds to the layer system according to the invention shown in FIG. 1. This layer system consists of a 4.9 μm thick Mo-Si layer (diffusion barrier) and a 2.7 μm thick Al-Cr-O layer (oxidation barrier). FIG. 6 illustrates a layer configuration that satisfies the above-set criteria for an environmental barrier layer for a Ti-Al substrate material. The diffusion barrier prevents the diffusion of Ti into the applied layer and the diffusion of the elements of the applied layer into the substrate material. According to the process description, this method also allows for the creation of a compositionally gradient Me-Si layer by adapting the coating speed at the interface toward the substrate, which favors layer adhesion. In this context, it should also be mentioned that, prior to the Me-Si layer, another metal layer can be applied to the substrate as an adhesion layer to further improve the adhesion of the layer system. For example, this can be a metal consisting of a Me-Si layer, or an adhesion layer metal can be used and graded to the Me and / or Me-Si layers.

[0043] Such a layer system also fulfills a second requirement for an environmental barrier layer: the Al-Cr-O layer prevents oxygen penetration and therefore diffusion to the interface between the substrate and the layer system.

[0044] The requirements established for realizing an environmental barrier layer on a Ti-Al substrate are met in the above examples. Si, when coated, prevents diffusion at the interface with the substrate material. This applies not only to pure Si layers, but also to Me-Si layers, which ensure better adhesion to the Ti-Al substrate and are substantially more mechanically stable than pure ceramic-like Si layers. Figure 7 shows a REM cross-section of the layer system shown in Figure 6, deposited at a substrate temperature of 450 °C and after a 20-hour bake test at an ambient temperature of 800 °C. Comparison of both layer systems reaffirms the excellent stability of the Al-Cr-O oxidation barrier, which remains unchanged. Conversely, in the Mo-Si diffusion barrier, diffusion processes occur at the interface with the Ti-Al substrate, but do not lead to pore formation or interface destabilization.

[0045] We also investigated a simplified variation of the environmental barrier layer depicted in Figure 1. In this example, the coating of the Ti-Al substrate was terminated after the application of the Mo-Si diffusion barrier; thus, the deposition of the oxidation barrier was omitted. In this simplified variation, again immediately after coating at 450 °C, no signs of diffusion processes were observed at the interface in either the REM cross section or linear scans (Figure 8), confirming a very abrupt transition in the layer morphology between the substrate and the Mo-Si diffusion barrier. The coated Ti-Al substrate was then baked at 800 °C in ambient atmosphere for 20 h. This revealed (detected using EDX measurements across the layer cross section) that diffusion occurred within a limited region at the interface, resulting in the enrichment of Al directly toward the substrate, followed by the formation of a Ti-Si region before the transition to Mo-Si. Again, as shown in the REM layer cross section in Figure 9, diffusion or rearrangement processes at the interface with the substrate did not result in the formation of pores, and the layer remained stable after baking. However, what is particularly important is that rearrangements at the interface lead to additional diffusion of Al at the layer surface, and as a result of this diffusion and the baking process, an oxide scale (Figure 10) forms on the surface of the Mo-Si layer. This oxide scale is sufficient as a protective layer for many applications. Furthermore, in an effort to adapt the layer system to the structure shown in Figures 2 or 3, it is advantageous to coat the environmental barrier layer in Figure 1 with a thermal barrier layer. This thermal barrier layer, such as Al-Cr-O, does not have very good oxidation barrier properties, but it can reduce oxygen diffusion (as is the case with YSZ). Thus, during the above-mentioned process of Al diffusion toward the silicide surface, an Al-O barrier layer is formed, which stops any possible oxygen diffusion through the thermal barrier and acts as an oxidation barrier.

[0046] This oxide scaling process is important in other contexts, but also when the transition from the Me-Si diffusion barrier to the oxide barrier layer is problematic in the sense that the use of oxygen plasma to deposit the oxide oxide barrier layer would attack the diffusion barrier. In such cases, the Al-O scale can be achieved without breaking the vacuum by exposing the surface of the Me-Si diffusion barrier to non-plasma activated oxygen at high temperatures.

[0047] The inventors have tested a series of layer materials for their diffusion barrier properties for Ti-Al materials and found that Me-Si layer materials are suitable for such diffusion barriers. The selection of a specific Me-Si compound must be based on the specific substrate material and application conditions, such as the operating temperature and the choice of oxidation barrier, which in turn depends on the corrosion conditions of the application. Another important property of Me-Si is its corrosion resistance. Me-Si layers were deposited on low-alloy steel and subjected to salt spray tests according to ASTM B117. These layers were found to be stable and corrosion-free even after 1000 hours.

[0048] Table 1 lists many of these Me-Si compounds. According to the present invention, they form a good diffusion barrier against Ti-Al materials and can be further coated with an oxidation barrier layer, thereby achieving an environmental barrier layer according to FIG. 1. Table 2 lists some PVD oxide layers that are particularly suitable as oxidation barrier layers, especially in combination with the above-mentioned Me-Si compounds. All of these contain Al as a substantial element in addition to O. In addition, Table 3 lists preferred layer materials for thermal barrier layers. These include oxide layer materials with thermal conductivities below 5 W / (m×K) and consist of Al-, Y-, and Zr-based oxides. By varying the coating parameters, they can be produced as dense layers or as porous layers with a columnar structure.

[0049] Figure 3 shows the combination of the environmental barrier layer of Figure 1 with a substantially thicker oxidation barrier layer. In this layer system, the oxidation barrier layer, in addition to its original function, also assumes the function of a thermal barrier due to its greater thickness. This thicker oxidation layer achieves a temperature reduction, thereby reducing the thermal load on the Ti-Al material and making it suitable for higher operating temperatures. In principle, for this thermal barrier, the oxidation barrier layer can be used as a starting material and further coated in this material system, whereby greater porosity can be introduced into the layer by modifying the coating parameters (e.g., increasing the oxygen flow rate) to thereby reduce the thermal conductivity. This is a preferred approach because it is simpler and more economical than the possibility shown in Figure 2 (in this case, coating the oxidation barrier layer with a material different from the oxidation barrier, i.e., a material based on yttria-stabilized Zr oxide).

[0050] Exam Description Here, exemplary tests carried out on the coatings according to the invention were carried out at 800°C in ambient atmosphere for 20 and 100 hours, respectively. As a demonstration substrate for the results, Ti50Al50 cast material was used, although this is not considered a limitation of the invention. These materials contain no doping, allowing for more subtle differences in the diffusion process to be detected.

[0051] An essential feature of an effective environmental barrier layer system is that it prevents the diffusion of Ti to the surface of the layer system after the baking process in air, while at the same time providing good adhesion between the layer system and the substrate material.

[0052] [Table 1]

[0053] [Table 2]

[0054] [Table 3]

[0055] A surface coating for protecting a substrate having a Ti-Al material is disclosed, the coating comprising a layer sequence with at least one layer, preferably a layer sequence corresponding to one or more of the layer sequences shown in the rows of Table 2, the coating comprising an oxidation barrier tailored to a diffusion barrier, preferably an oxidation barrier tailored according to Table 3, and the surface coating comprising a heat shield, preferably a heat shield tailored to an oxidation barrier according to Table 4.

[0056] A method for producing a surface coating is disclosed, wherein the coating is applied using a PVD method and using a CVD method, and wherein the coating is preferably performed in only one coating system.

[0057] Regardless of the claims, a surface coating for the protection of a substrate having a Ti-Al material, preferably a surface coating having one or more of the materials listed in Table 1, is also required, which coating is a layer sequence with at least one layer forming a diffusion barrier for Ti, preferably corresponding to one or more of the layer sequences listed in the rows of Table 1, which coating comprises an oxidation barrier adjusted to the diffusion barrier, preferably adjusted according to Table 2, and the surface coating comprises a heat shield, which is preferably adjusted to an oxidation barrier according to Table 3.

[0058] Regardless of the claims, protection is also sought for a method for producing a surface coating as described in the previous paragraph, which coating is applied by means of a PVD method and by means of a CVD method, and which coating is preferably carried out in only one coating system.

[0059] Within the framework of the present disclosure, layer system and surface coating may (but need not) be used as synonyms and are therefore, inter alia, the same thing.

Claims

1. A surface-coated Ti-Al substrate, the surface coating comprises a layer sequence with at least one layer forming a diffusion barrier preventing diffusion of Ti from the Ti-Al substrate; the diffusion barrier comprises a Me—Si layer made of Mo—Si, Ti—Si, Cr—Si, Ni—Si, Al—Si, Zr—Si, Nb—Si, Hf—Si, Y—Si, Ta—Si or W—Si; The surface coating comprises an oxidation barrier adjusted to the diffusion barrier as follows: a), b), c), d), e), or f). a) when the diffusion barrier is a layer made of Mo—Si, Ti—Si, Cr—Si or Ni—Si, the oxidation barrier is made of at least one of Si—O, Al—O, Al—Cr—O and / or Cr—O; b) when the diffusion barrier is a layer made of Al—Si, the oxidation barrier is made of at least one of Al—O and / or Al—Cr—O; c) when the diffusion barrier is a layer made of Zr—Si, Nb—Si or W—Si, the oxidation barrier is made of at least one of Si—O, Al—O and / or Al—Cr—O; d) when the diffusion barrier is a layer made of Hf—Si, the oxidation barrier is made of at least one of Si—O, Al—O, Al—Cr—O, Hf—O and / or Al—Hf—O; e) when the diffusion barrier is a layer made of Y—Si, the oxidation barrier is made of at least one of Al—O, Al—Cr—O, Al—Y—O and / or Y—O; f) when the diffusion barrier is a layer of Ta—Si, the oxidation barrier is at least one of Al—O, Al—Cr—O, Al—Ta—O and / or Ta—O, and The surface coating comprises a heat shield adjusted to the oxidation barrier as follows: i) when the oxidation barrier is a layer made of Si—O, Al—O, Al—Cr—O and / or Cr—O, the heat shield is made of at least one of Al—Cr—O and / or YSZ; ii) when the oxidation barrier is a layer made of Hf—O and / or Al—Hf—O, the heat shield is made of at least one of Al—Cr—O, YSZ, Hf—O and / or Al—Hf—O; Surface-coated Ti-Al substrate.

2. 2. The surface-coated Ti-Al substrate of claim 1, wherein the diffusion barrier is disposed between the oxidation barrier and the substrate.

3. 3. The surface-coated Ti-Al substrate according to claim 1, wherein the heat shield is disposed directly on the oxidation barrier.

4. A surface-coated Ti-Al substrate as described in claim 1 or 2, characterized in that one layer fulfills the functions of both the oxidation barrier and the heat shield.

5. 5. The surface-coated Ti-Al substrate of claim 4, wherein the one layer has a gradient layer morphology, in which the density of the one layer is highest in the vicinity of the substrate and the layer transitions stepwise and / or continuously to a porous structure as the distance from the substrate increases.

6. Surface-coated Ti-Al substrate according to any one of claims 1 to 5, characterized in that a metal layer is deposited on the substrate directly between the substrate and the diffusion barrier.

7. 7. The surface coated Ti-Al substrate of claim 6, wherein the diffusion barrier is deposited on a gradient layer, and the gradient layer is deposited on the metal layer.

8. Surface-coated Ti—Al substrate according to any one of claims 1 to 7, characterized in that the oxidation barrier comprises Si—O and / or Al—O and / or Al—Cr—O.

9. The surface-coated Ti—Al substrate according to any one of claims 1 to 8, characterized in that the heat shield comprises Al—Cr—O and / or YSZ.

10. The surface-coated Ti—Al substrate according to claim 6, characterized in that the metal layer contains Cr and / or Al.

11. The surface coated Ti-Al substrate of claim 7, wherein said gradient layer is adapted to said diffusion barrier.

12. A surface-coated Ti—Al substrate according to any one of claims 1 to 11, characterized in that a transition layer is arranged between the oxidation barrier and the diffusion barrier.

13. The surface-coated Ti—Al substrate of claim 12, wherein the transition layer comprises Si—O.

14. The method for producing a surface-coated Ti—Al substrate according to any one of claims 1 to 13, wherein the method comprises the following steps A), B) and C): A) introducing a Ti—Al substrate into a coating system; B) pretreating the Ti—Al substrate in the coating system; C) applying a surface coating by PVD and CVD methods, in which the target is evaporated by spark discharge and reactive gases are introduced simultaneously or at short intervals, and the application of the surface coating is carried out in only one coating system; A method comprising:

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