Ceramics with corrosion protection layer, methods for their manufacture and their use

Description

[0001] The invention relates to a ceramic, in particular a silicon-containing non-oxide ceramic (SiNOK) with a corrosion protection layer applied at least partially to the surface of the ceramic, consisting of a multi-layer system containing a corrosion-stable cover layer which represents a diffusion barrier against hot gas, oxygen or other substances that have a corrosive effect on the ceramic, such as corrosive melts and slags, etc., and at least one adhesion-promoting base layer arranged between the ceramic and the cover layer, wherein the base layer consists of a mixture containing ZrO2 and ZrTiO4 and / or Al2TiO5, in each case stabilized or unstabilized. The invention also relates to the production of this corrosion-protected ceramic and its use.

[0002] Ceramic materials, especially non-oxide ceramic materials such as SiC and Si3N4, are materials that can withstand high tribological, mechanical, and thermal stresses. They exhibit comparatively low densities while simultaneously exhibiting high strength and hardness. Fiber-reinforced composite materials are also used, in which reinforcing fibers are embedded in the non-oxide ceramic materials. The latter can also consist of the corresponding non-oxide, silicon-containing ceramic material, such as silicon carbide. In addition to the aforementioned properties, fiber-reinforced ceramic materials possess more damage-tolerant, quasi-ductile fracture behavior, which broadens the range of applications for ceramic materials as engineering materials.

[0003] Fiber-reinforced non-oxide ceramics, in particular, are suitable for replacing high-temperature superalloys, such as those commonly used in gas turbines, due to their high thermal stability, damage tolerance, and creep resistance. They potentially enable an increase in interior temperatures in the hot gas area of ​​gas turbines by several hundred K, which in turn leads to a corresponding increase in efficiency. This results in a reduction in fuel consumption and, accordingly, in CO2 emissions.

[0004] During the combustion of hydrocarbon compounds in gas turbine combustion chambers, corrosive hot steam is formed. This leads to accelerated oxidation of the surface of silicon-containing oxide and non-oxide ceramics, which in turn forms volatile Si-O x -H y compounds upon contact with the hot steam. Their volatilization and the progressive oxidation of these ceramic materials can cause material removal at rates in the range of 0.2 µm / h to 0.5 µm / h.

[0005] The use of silicon-containing oxide and non-oxide ceramics for these applications therefore requires effective protection through corrosion protection coatings, also known as environmental barrier coatings (EBCs). Typically, coating systems with multiple superimposed layers are used. These include a basic adhesion promoter layer on the substrate, the so-called bond coat, intermediate layers to reduce thermal stresses resulting from different thermal expansion coefficients, and top coats. Intermediate layers can also serve as adhesion promoters between top coats and another adhesion layer.

[0006] At least one cover layer serves as the actual corrosion protection layer. It should therefore be impermeable, provide a diffusion barrier at least against hot gas and oxygen, and be hydrothermally resistant. It should also be resistant to other corrosive media. Examples include alkaline mineral particles (e.g., calcium magnesium aluminosilicates - CMAS), which, in the case of aircraft gas turbines, are drawn in with the ambient air and can then form corrosive melts and slags in the hot gas path of a gas turbine.

[0007] Patent EP 2 344 590 B1 describes a layer system in which an adhesion layer is applied to the surface of a superalloy substrate or a ceramic substrate, which adhesion layer consists predominantly of a rare earth silicate and in which further oxides or titanates or silicates can be contained in a proportion of 0.1 - 30 wt.%.

[0008] This layer system can also include additional overlying layers, which can be implemented between the substrate surface and a suitable cover layer to reduce thermal stresses and increase the barrier effect. However, multiple layers increase the manufacturing effort and cost.

[0009] A disadvantage of the adhesion layer described therein is the increased cost due to the predominantly high proportion of rare earth compounds in direct contact with the substrate surface, even though these compounds are primarily required as a diffusion barrier in the corrosion-resistant top layers. Furthermore, the direct chemical bonding of rare earth silicates to the surface of silicon-containing non-oxide ceramics is insufficient, which impairs their function as an adhesion layer.

[0010] It is therefore an object of the invention to provide a ceramic with a layer system for corrosion protection, wherein the layer system should be as simple as possible, have good adhesion and have a long service life under high temperatures and corrosive conditions.

[0011] According to the invention, this object is achieved with the ceramic, in particular with a silicon-containing non-oxide ceramic, having the features of claim 1. This can be coated, for example, using a wet-chemical application method according to claim 8, although other common coating methods such as plasma spraying, chemical or physical vapor deposition, sputtering, powder deposition by build-up welding, powder coating, or the like are also possible. Advantageous embodiments of the invention can be realized with features specified in the subordinate claims.

[0012] According to the invention, a ceramic, in particular a silicon-containing non-oxide ceramic, is provided, in which at least in some regions on the surface of the ceramic there is applied a corrosion protection layer consisting of a multi-layer system containing a hydrothermally stable cover layer containing a rare earth silicate, a rare earth titanate, a rare earth oxide or mixtures thereof, which represents a diffusion barrier or barrier layer against hot gas, oxygen or other substances that have a corrosive effect on the ceramic, such as corrosive melts and slags, etc., and at least one adhesion-promoting base layer with a thermal expansion coefficient α ZS arranged between the non-oxide ceramic with a thermal expansion coefficient (CTE) α NOK and the cover layer. The base layer is according to the invention made of a mixture containing ZrO 2 and Al 2 TiO 5 and optionallyZrTiO4, whereby the proportions of these components are chosen so that the ratio of the thermal expansion coefficients α ZS / α NOK is in the range of 0.75 to 1.25.

[0013] The mixture preferably contains ZrOz and Al 2 TiO 5 with a molar ratio of 1 : 10 to 10 : 1 or ZrOz and ZrTiO 4 and Al 2 TiO 5 with a molar ratio of 1 : 0.05 : 0.05 to 1 : 5 : 5. It is preferred that a molar ratio of ZrTiO 4 and Al 2 TiO 5 between 1 : 1 is maintained.

[0014] The non-oxide ceramic is preferably selected from the group consisting of or containing silicon nitride (Si 3 N 4 ), silicon carbide (SiC), silicon carbide fiber reinforced silicon carbide (SiC / SiC) and combinations or compounds thereof.

[0015] The adhesion between the ceramic surface and the base layer is preferably achieved by zirconium silicate reactively formed from SiO2 and ZrO2. This ZrSiO4, formed by sintering, enables adhesion between the ceramic surface and the base layer, whereby additional phases are also formed, e.g., Zr5Ti7O24, mixed phases of ZrTiO4 and ZrSiO4, and yttrium titanate.

[0016] Stabilized zirconium oxide has a CTE in the range of approximately 10 10 6 K -1 . The CTE for common SiNOKs, in contrast, is in the range of 2 10 -6 K -1 to 5.5 10 -6 K -1 . However, the CTE of the corrosion protection coating systems must be adapted to the CTE of the respective SiNOK to avoid thermal stresses, especially during thermal cycling and thermal shock. The latter can lead to cracks or even spalling of the coating system, resulting in a loss of corrosion protection.

[0017] By adding the appropriate proportions of the titanate(s) (ZrTiO4 and / or Al2TiO5) to the ZrO2 used in stabilized or unstabilized form, the CTE can be adapted to the substrate material. If Al2TiO5 is used, it can preferably be stabilized with MgO. The preferred proportion of MgO, based on the Al2TiO5, is in the range of 0.1 mol% to 5 mol%. The ZrO2 used can also be partially or fully stabilized, e.g., through yttrium, calcium, magnesium, cerium, scandium, or ytterbium doping. In this case, the ZrO2 possesses additionally increased fracture toughness.

[0018] Al 2 TiO 5 exhibits high thermal shock and thermal cycling resistance, which is due, among other things, to its low CTE of < 1.6 * 10 -6 K -1 . This is due to the anisotropic thermal expansion of the Al 2 TiO 5 crystallites. Upon thermal expansion or contraction, the crystallites form a network of microcracks, whose free-standing volume is available for the thermal expansion of neighboring crystallites. The lower adhesion of Al 2 TiO 5 to the substrate surface of SiNOK can be compensated for by the ZrO 2 content in the base layer material.

[0019] ZrTiO 4 also forms a microcrack network during thermal expansion and contraction. However, the CTE of ZrTiO 4 alone ranges between 8 * 10 -6 < K -1 < to 8.5 * 10 -6 < K -1 < . In combination with other phases, such as ZrO 2 , lower CTE values ​​can be achieved, comparable to those of SiNOK. ZrTiO 4 also binds insufficiently to the surface of a SiNOK, which can be compensated for with an appropriate amount of ZrO 2 , similar to Al 2 TiO 5 .

[0020] The material used according to the invention for the base layer is resistant to the formation of cracks and spalling due to the combination of thermal shock-resistant and crack-resistant materials with a good chemical bond to the substrate surface. It therefore has a property profile suitable for EBC, in particular when the non-oxide ceramic according to the invention is used in turbines, in particular gas turbines or their components.

[0021] A hydrothermally stable top layer can be applied to the surface of the base layer according to the invention. This is, for example, a layer of diyttrium monosilicate, diyttrium disilicate, or mixtures thereof doped with 0.1–5 mol% Al 2 O 3 . Instead of yttrium silicates, however, other rare earth silicates, such as ytterbium silicate, can also be applied. Furthermore, rare earth titanates such as diyttrium mono- and dititanate, as well as analogues, can also be applied. Rare earth oxides such as yttrium oxide or ytterbium oxide can also be applied, although excessive discrepancies in the CTE must be avoided. A mixture of the above-mentioned components is also possible as a top layer.

[0022] According to the invention, a method is also provided for producing a ceramic, in particular a silicon-containing non-oxide ceramic, with a corrosion protection layer applied at least in regions to the surface of the non-oxide ceramic, comprising a multi-layer system containing a corrosion-resistant cover layer which represents a diffusion barrier or barrier layer against hot gas, oxygen or other substances having a corrosive effect on the ceramic, in which a) a mixture containing particles of ZiO2 and ZrTiO4 and / or Al2TiO5 and at least one organic binder is provided as an aqueous suspension, b) the mixture is applied to the surface of the substrate to be coated, c) a thermal treatment is then carried out up to a maximum temperature of 1500°C, wherein the organic components are expelled and the base layer is densified by sintering, d) a corrosion-stable top layer is then applied to the base layer, by means of a mixture containing particles of a rare earth silicate and / or a rare earth titanate and / or a rare earth oxide or mixtures thereof and an organic binder, which is then subjected to a thermal treatment up to a maximum temperature of 1600°C, wherein the organic components are expelled and the top layer is densified by sintering.

[0023] The suspension may preferably contain dispersing additives and / or rheological additives and / or defoamers. These can also be removed before or during the thermal treatment.

[0024] The thermal treatment used to form the base layer can be carried out in several temperature stages with predefined heating rates and holding times. The heating rate can be increased from one stage to the next, and then, in the final stage, a continuous cooling at a constant rate is carried out until the normal ambient temperature of approximately 25 °C is reached.

[0025] By maintaining a specific proportion of ZrO2, stabilized or unstabilized, with one or both titanates, the thermal expansion coefficient of the interlayer material can be closely matched to the thermal expansion coefficient of the substrate material and the cover layer material, so that thermal stresses during operation can be largely avoided. Furthermore, the materials used can achieve high fracture toughness and thermal shock resistance. Especially when used in gas turbines, they can therefore withstand the frequent temperature changes and mechanical stresses.

[0026] The base layer can be easily baked onto substrate surfaces under oxidizing conditions. This is especially true for fiber-reinforced SiNOK and, in particular, SiC-based materials.

[0027] The ZrSiO4 formed by the chemical reaction of SiO2 and ZrO2 prevents or inhibits further oxidation of the substrate material during thermal treatment. This offers advantages in terms of cost, time, and process control, as an inert environment, rinsing cycles, and precise monitoring of the process atmosphere are eliminated. The chemical bonding of the base layer to the substrate surface is very good. The application of a top layer and its firing are easily possible and enable good adhesion to the base layer.

[0028] The thermal expansion coefficient of the base layer material can also be well matched to that of the rare-earth-based top layer material. When using suitable corrosion-resistant top layer materials, no adverse chemical reactions occur between the base layer and the top layer, thus maintaining the corrosion protection effect of the top layer. Furthermore, the titanates used and the ZrO2 exhibit a very low thermal conductivity of < 2 W / mK, so the applied corrosion protection layer also functions as a thermal barrier coating (TBC) to protect the substrate material from high temperatures.

[0029] Another important advantage is the potential elimination of additional intermediate layers, which might be necessary to adjust the CTE or improve barrier performance. Cost-intensive rare earth compounds are used only in the top layer, which conserves the use of scarce resources and makes the described layer system economically attractive.

[0030] The ceramics coated with the described corrosion protection coating system, in particular the SiNOK, are suitable for use in turbines, especially in gas turbines, as well as for other applications with hot gas loads or corrosive loads in heat process engineering, energy technology and process engineering.

[0031] The layer system provided according to the invention enables a simple and cost-effective construction of a ceramic according to the invention. This ceramic is both mechanically and thermally stable, and especially chemically stable. Multiple additional layers that can fulfill specific functions and produce specific effects are unnecessary.

[0032] The invention will now be explained in more detail using a wet-chemical application process as an example. Example

[0033] First, an aqueous suspension of ZrTiO 4 particles (synthesized in-house, calcination temperature 1600 °C for 2 h, bimodal particle size 0.5 µm and 2 µm) and ZrO 2 particles (CS10, Saint Gobain, d 50 value 1.3 µm) is prepared in a molar ratio of 1:2. For this purpose, 0.6 g of deionized water and 0.24 g of BYK Anti-Terra 250 (wetting and dispersing agent) are placed in a 25 mL plastic beaker and mixed. Then, 2.4 g of the powder mixture of ZrTiO 4 and ZrO 2 in a molar ratio of 1:2, as well as 1.16 g of ZrO 2 grinding beads (diameter 3 mm) are added. Furthermore, one drop of the defoamer BYK LP-C 22787 is added, the beaker is closed and sealed with a lid, and treated for 1.5 minutes at 1000 rpm in a Speedmixer VLC 150.1 (Hauschild).

[0034] After this, another drop (approx. 0.03 mL) of the binder BYK LP-C 22346 is added, and the closed and sealed beaker is treated again for 1.5 min at 1000 rpm.

[0035] Before the coating process, the substrate surface to be coated (in this case SSiC, Hexoloy SA, Saint Gobain Industriekeramik GmbH) is thoroughly cleaned with ethanol and dried at 60 °C for 15 minutes in a circulating air drying cabinet.

[0036] The aqueous suspension is applied to the substrate by brushing, spraying or dipping and dried in air for approximately 30 minutes at room temperature and then at 60 °C in a circulating air drying cabinet (at least 1.5 hours).

[0037] If necessary, the surface of the dried suspension can be smoothed mechanically by sanding. For this purpose, sandpaper with a grain size of 400, for example, can be used.

[0038] Following drying, a thermal treatment for debinding and sintering is carried out in a furnace in which an air or a non-oxidizing atmosphere (e.g. argon, nitrogen) can be used.

[0039] In the first stage of the thermal treatment, heating takes place from 25 °C to 1200 °C at a heating rate of 2.5 K / min for debinding. In the second stage, heating continues to 1300 °C at 1 K / min, and this temperature is held for 1 h. In the third stage, heating takes place from 1300 °C to 1400 °C at 1 K / min, and 1400 °C is held for 1 h. Subsequently, in a fifth stage, the temperature is increased from 1400 °C to 1475 °C at a heating rate of 1 K / min, and this temperature is held for 2 h.

[0040] After this holding time, cooling to 25°C takes place at a rate of 2.5 K / min.

[0041] The intermediate layer thus obtained has a thermal expansion coefficient of approximately 5.0 ·10 -6< K -1< .

[0042] Subsequently, a top layer consisting of yttrium silicate is applied to the outward-facing surface of the intermediate layer. The layer preparation described above is carried out analogously, but instead of the ZrTiO4-ZrO2 mixture, a powder mixture of 2 / 3 diyttrium disilicate and 1 / 3 diyttrium monosilicate (synthesized in-house, calcined at 1600 °C for 2 hours, average particle size 850 nm) is added to the deionized water. 1 mol% aluminum oxide (TM-DAR, Taimei Chemicals Co. Ltd.) is added.

[0043] The suspension for the top coat is applied in the same way as for the base coat, by brushing, spraying, or dipping. The top coat is then dried in the same way.

[0044] To complete the coating system, a further thermal treatment is performed for debinding and sintering of the top layer. For this purpose, the sample is again placed in a furnace with an air or inert gas atmosphere. It is heated to a temperature of 1450 °C at a heating rate of 2.5 K / min, held at this temperature for 5 hours, and cooled to room temperature at a heating rate of 2.5 K / min.

Claims

1. Ceramic with a corrosion protection layer applied at least in certain areas to the surface of the ceramic and comprising a multilayer system, comprising a corrosion-resistant top layer comprising a rare earth silicate, a rare earth titanate, a rare earth oxide or mixtures thereof, which forms a diffusion barrier or barrier layer against hot gas, oxygen or other substances having a corrosive effect on the ceramic, and at least one adhesion-promoting base layer with a coefficient of thermal expansion αZS arranged between ceramic with a coefficient of thermal expansion αNOK and top layer, wherein the base layer consists of a mixture containing ZrO2 and Al2TiO5 and optionally ZrTiO4, each stabilized or unstabilized, and the proportions of these components are selected so that the ratio of the coefficients of thermal expansion αZS / αNOK is in the range of 0.75 to 1.25.

2. Ceramic according to claim 1, characterized in that the mixture comprises ZrO2 and Al2TiO5 in a molar ratio of 1 : 10 to 10 : 1 or ZrO2 and ZrTiO4 and Al2TiO5 in a molar ratio of 1 : 0.05 : 0.05 to 1 : 5 : 5, preferably a molar ratio of ZrTiO4 and (stabilized) Al2TiO5 of between 1:1 is maintained.

3. Ceramic according to any one of the preceding claims, characterized in that the mixture is free of rare earth silicate.

4. Ceramic according to any one of the preceding claims, characterized in that the ceramic is a non-oxide ceramic, wherein the non-oxide ceramic consists of or contains silicon nitride (Si3N4), silicon carbide (SiC), silicon carbide fiber reinforced silicon carbide (SiC / SiC), combinations or composites thereof.

5. Ceramic according to any one of the preceding claims, characterized in that the adhesion promotion between the surface of the ceramic and the base layer is realized by ZrSiO4 reactively formed from SiO2 and ZrO2.

6. Ceramic according to any one of the preceding claims, characterized in that the Al2TiO5 contained in the base layer is stabilized with MgO.

7. Ceramic according to any one of the preceding claims, characterized in that the ceramic is a silicon-containing non-oxide ceramic.

8. Process for the production of a ceramic with a corrosion protection layer applied at least in some areas to the surface of the ceramic and comprising a multilayer system containing a hydrothermally stable top layer which forms a diffusion barrier against hot gas and oxygen, in which a) a mixture containing particles of ZrOz and Al2TiO5 and optionally ZrTiO4, each stabilized or unstabilized, and at least one organic binder is provided as an aqueous suspension, b) the mixture is applied to the surface of the substrate to be coated, c) followed by thermal treatment up to a maximum temperature of 1500 °C, whereby the organic components are expelled and the base layer is compacted by sintering, d) then a top layer is applied to the base layer by applying a mixture containing particles of a rare earth silicate and / or a rare earth titanate and / or a rare earth oxide or mixtures thereof and an organic binder is subjected to a thermal treatment up to a maximum temperature of 1600 °C, whereby the organic components are expelled and the top layer is compacted by sintering.

9. Process according to the preceding claim, characterized in that a passivation layer or a structural component containing SiO2 is present on the surface of the ceramic to be coated with the base layer, to which the suspension is applied, and which, during the thermal treatment at a temperature > 1000 °C, enables adhesion of the base layer to the surface of the non-oxide ceramic by chemical reaction of the SiO2 with the ZrOz to form zirconium silicate.

10. Process according to any one of the two preceding claims, characterized in that the thermal treatment is carried out in several temperature stages with predeterminable heating rates and holding times, whereby the heating rate is first increased from one stage to a subsequent stage and at the end, in the last stage, continuous cooling is carried out at a constant rate until the normal ambient temperature is reached.

11. Process according to the preceding claim, characterized in that the organic binder is selected from the group consisting of polyvinyl alcohol or acetate (PVA), polyethylene glycol (PEG), polyvinyl butyral (PVB), acrylates, latex binders, cellulose derivatives, polysaccharides, and mixtures thereof.

12. Process according to any one of claims 8 to 11, characterized in that the ceramic is a silicon-containing non-oxide ceramic.

13. Use of the ceramic according to any one of claims 1 to 7 in thermal process engineering, process engineering, power generation, and in turbines, in particular in gas turbines.