HIGH TEMPERATURE PROTECTIVE COATING

Abstract

The invention relates to a high temperature protective coating based on MCrAlY coating, with M at least one element out of the group of Ni, Co and Fe, for a component of a turbo machine, especially a gas turbine, the coating containing at least at least 1.75 vol.-% chromium borides and the coating consisting of the following chemical composition (in wt.-%): 10-27 Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 M, with M being a different element out of said group compared to the remainder, and the remainder being M and inevitable impurities. A preferred embodiment is a coating with the following chemical composition: 10-27 Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 Co and the remainder being Ni and inevitable impurities.

Claims

1. High temperature protective coating based on MCrAlY coating, with M at least one element out of the group of Ni, Co and Fe, for a component of a turbo machine, especially a gas turbine, the coating containing at least 1.75 vol.-% chromium borides and consisting of the following chemical composition (in wt.-%): 10-27 Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 M, with M being a different element out of said group compared to the remainder; the remainder being M and inevitable impurities.

2. The coating according to claim 1, wherein the coating consists of the following chemical composition (in wt.-%): 10-27 Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 Co; the remainder being Ni and inevitable impurities.

3. The coating according to claim 1, wherein the coating consists of the following chemical composition (in wt.-%): 10-27 Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 Ni; the remainder being Co and inevitable impurities.

4. The coating according to claim 1, wherein the Cr content is 21-25 wt.-%, preferred 22-25 wt.-%.

5. The coating according to claim 1, wherein the Al content is 4-6 wt.-%.

6. The coating according to claim 1, wherein the Si content is 1.5-2.6 Si wt.-%.

7. The coating according to claim 1, wherein the Ta content is 1.5-3 wt.-%.

8. The coating according to claim 1, wherein the Y content is 0.01-1 wt.-%.

9. The coating according to claim 1, wherein the B content is 0.1-1 wt.-%.

10. The coating according to claim 1, wherein the M content, with M being a different element out of said group compared to the remainder, is 0-1 wt.-%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

[0039] FIG. 1 shows in a schematic overview MCrAlY alloy classes (according to the known prior art) and their oxidation resistance and hot corrosion resistance;

[0040] FIG. 2 shows the calculated phase fracture dependent on temperature in the range between 600 C. and 1400 C. for an advanced NiCrAlSiTaCoBY coating according to an embodiment of the invention;

[0041] FIG. 3 shows the dependence between the boron content and the Cr.sub.2B volume fraction of the standard MCrAlY (0 wt.-% B) and 4 different advanced metallic coating systems according to embodiments of the present invention and

[0042] FIG. 4 shows tensile test results at ambient temperature (left part) and at 600 C. (right part) for common state of the art NiCrAlY coatings and for NiCrAlSiTaCoBY coatings according to several embodiments of the present invention.

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION

[0043] The invention describes an advanced high temperature resistant MCrAlYB coating class containingas the main factorthe element boron, leading to the formation of chromium-borides, in higher amount (at least 1.75 vol.-% chromium-borides) compared to similar state of the art coatings. M is at least one element out of the group of Ni, Co and Fe. In addition Si and Ta are alloying elements in said MCrAlYB coating according to the invention.

[0044] Some examples of preferred embodiments are coatings consisting of the following elements (given in wt.-%), wherein the balance is always Ni and inevitable impurities:

TABLE-US-00001 TABLE 1 Chemical composition of several exemplary embodiments of the coating according to the invention Element Coating Ni Cr Al Si Ta Co B Y AC-I Balance 24.02 5.3 2.34 1.24 1.02 0.23 0.45 AC-II Balance 23.04 5.1 2.08 1.48 2.04 0.46 0.40 AC-III Balance 22.07 4.9 1.82 1.72 3.06 0.69 0.35 AC-IV Balance 21.08 4.7 1.56 1.96 4.08 0.92 0.30

[0045] The coating is applied onto the surface of a metallic component, for example a gas turbine blade made of a Ni-base superalloy.

[0046] The application is done under air, vacuum or inert gas by one of the following thermal spray processes: [0047] Low pressure plasma spray (LPPS) [0048] Vacuum plasma spray (VPS) [0049] Atmospheric plasma spray (APS) [0050] High velocity oxygen fuel (HVOF) [0051] Physical vapour deposition (PVD) [0052] Chemical vapour deposition (CVD) [0053] Electrochemical deposition
or by any other suited application process which is state of the art.

[0054] The coating microstructure (phase distribution), at thermodynamic equilibrium, was calculated using Thermo-Calc method. The results for coating composition AC-III (see Table 1) are shown in FIG. 2. The Cr.sub.2B volume fraction is constant over the complete test temperature range, while the -Cr fraction is decreasing with increasing the temperature and does not more exist at temperatures above about 760 C. In addition, the volume fraction of is significantly decreasing with increasing temperature.

[0055] FIG. 3 shows the dependence between the boron content and the Cr.sub.2B volume fraction of the standard MCrAlY (0 wt.-% Bsee Table 2) and the four different advanced metallic coating systems according to the invention with their chemical composition (given in wt.-%) described in Table 1.

TABLE-US-00002 TABLE 2 Chemical composition of the tested reference standard coating Element Coating Ni Cr Al Si Ta Co B Y MCrAlY Balance 25.0 5.5 2.6 1.0 0.5 0.5

[0056] The nominal content of Ni, Ta, Co, and B in the four samples of the embodiments according to the invention was increased, whereas the Cr, Al, Si and Y content was decreased. The adjustment of the coating microstructure is simple, as the volume fraction of borides is linearly increasing with the boron content.

[0057] The advanced NiCrAlSiTaCoBY coating microstructure is comprised of a -matrix which contains , -Cr and Cr.sub.2B precipitates. Formation of undesirable phases like -Cr or -(NiAl), which have a significant influence on the ductile to brittle temperature (DBTT) and on the coefficient of thermal expansion, is avoided. The risk of stress accumulation in the coating (overlay) leading to surface cracking and stress build-up when used as bond coat eventually causing TBC spallation is significantly reduced.

[0058] Main hardening effect for NiCrAlY alloys is precipitation hardening. With increasing temperature, the volume-fraction of and -Cr precipitates is significantly decreasing (see FIG. 2). In consequence, mechanical properties change and e.g. ultimate tensile strengths is significantly decreased. If compared to common NiCrAlY alloys, the NiCrAlSiTaCoBY coating has increased high temperature strength due to precipitation hardening by thermodynamically stable CrB and/or Cr.sub.2B precipitates.

[0059] Tensile test results for various NiCrAlSiTaCoBY coating compositions (embodiments of the present invention) in comparison to a known state of the art NiCrAlY coating compositions (as reference material) are shown in FIG. 4.

[0060] NiCrAlSiTaCoBY coatings offer higher tensile ductility at lower temperatures and higher tensile strengths at comparable strain (<6%) for higher temperatures.

[0061] As a matter of fact, the disclosed advanced coating class according to the invention does perform much better in cyclic loading. Enhanced tensile strength, respectively creep resistance, at elevated temperature and less crack probability and severity due to increased ductility at low temperature do lead to a significantly extended lifetime of the high temperature protective layer.

[0062] The hot corrosion resistance will be increased, due to a diffusion-controlled dissolution of the CrB and/or Cr.sub.2B phase which is acting as a chromium reservoir during long term service.

[0063] Boron is known to be a fast diffusing element. In case of chromium depletion in surface near regions during operation due to oxide formation, the CrB and/or Cr.sub.2B precipitates will dissolve and progressively release chromium which is needed to form a protective chromium-oxide-scale. Furthermore, the advanced coating promotes formation of highly protective alumina scales which increases the coating service lifetime in base-load operation with respect to oxidation.

TABLE-US-00003 TABLE 3 Chemical composition of several exemplary embodiments of the coating according to the invention (in wt.-%) Element Coating Ni Cr Al Si Ta Co B Y AC-V Balance 10.0 3.0 1.0 0.1 0.1 0.01 AC-VI Balance 27.0 12.0 4.0 3.0 7.0 3.0 3.0

[0064] Coatings with a chemical composition at the lower specified range (see embodiment AC-V in Table 3) show a significant ductility, toughness respectively, increase. These coatings are especially optimized for application in high-cyclic operation with less oxidation and corrosion attack. On the other hand, coatings with a chemical composition at the upper specification range (see embodiment AC-VI, in Table 3) deliver best protection from oxidation and hot corrosion at increased ductility (compared to standard MCrAlY). These coatings are especially optimized for cyclic and base-load mode with extended service lifetime intervals (compared to the current state-of-the-art MCrAlY's).

[0065] The key advantages of the present invention are: [0066] High temperature protective coating with increased lifetime in (high-)cyclic operation mode and at least same lifetime in base-load modus [0067] High temperature protective coating with increased ductility at low operation temperatures (T<500 C.) due to an optimized microstructure with reduced volume fraction of brittle phases like -Cr, or [0068] High temperature protective coating with increased tensile strength, respectively creep resistance, at higher operation temperatures (T500 C.) due to dispersion strengthening effect of CrB and/or Cr.sub.2B precipitates [0069] High temperature protective coating with increased (w.r.t. standard MCrAlY) oxidation/hot corrosion properties due to presence of CrB and/or Cr.sub.2B precipitates acting as chromium reservoirs in depleted areas [0070] Adjustable strengthening and reservoir effect due to linear relation between boron content and volume-fraction of CrB and/or Cr.sub.2B precipitates