CN1198964C - High Temperature Protective Coatings - Google Patents

Abstract

The present invention relates to a structural component used for superheat resisting alloys, particularly to a coating composition for gas turbine vanes and blades, which contains the balancing amount of Ni, 0.3 to 1.3 wt% of Y, 18 to 28 wt% of Co, 0 to 1.5 wt% of Mg, and 11 to 15 wt% of Cr, 0 to 0.5 wt% of La plus La, 11.5 to 14 wt% of Al, 0 to 0.1 wt% of B, 1 to 8 wt% of Re, less than 0.1 wt% of Hf, 1 to 2.5 wt% of Si, less than 0.1 wt% of, 0.2 to 1.5 wt% of Ta, wherein 0.3 to 2.0 wt% of Y and La(plus La system), 0.2 to 1.5 wt% of Nb, and less than 2.5 wt% of Si and Ta.

Description

High Temperature Protective Coatings

Field of Invention

This invention relates to an improved type of protective coating for superalloy structural parts, particularly gas turbine rudders and blades.

Background of the Invention

In the field of gas turbine engines, designers have repeatedly pointed to increasing the operating temperature of the engine to increase its efficiency. The oxidation rate of the material also increases significantly with the increase of temperature. Gas turbine components can also experience thermal erosion when corrosive species are drawn into the engine through air and/or impurities in the fuel. Modern structural superalloys are designed to achieve extreme mechanical properties, thereby sacrificing oxidation resistance for a greater degree of corrosion resistance.

In order to improve the service life of gas turbine components, it is customary to use protective coatings, such as aluminum compounds or MCrAlY coatings, where M can be Ni, Co, Fe or their mixtures. Since coated turbine blades are subjected to complex stress regimes during operation, ie during heating and cooling cycles, better high temperature coatings not only provide environmental protection but also necessarily have particularly suitable physical and mechanical properties.

If the protective coating is used as a bond coat for a thermal barrier coating (TBC), there are some additional requirements. When used as a cover coat, that is, without TBC, as long as the activity of Al in the coating is still high enough, the thermally grown oxide can flake off and then grow again; when used as a TBC bond coat, the oxide growth rate and Oxide adhesion is the controlling parameter, as long as the oxide peels off, the TBC will peel off too. In summary, a better high temperature coating must meet the following requirements:

- High antioxidant

- Slow growth of oxide skin (low kp value)

-Good adhesion to oxide skin

-Hot corrosion resistance, better than SX/DS superalloys

-Slow interdiffusion of Al and Cr into the base layer to prevent the deposition of brittle acicular phase species under the coating

-Creep resistance can be compared with common superalloys

- High ductility at low temperatures and low ductile-brittle transition temperature

-The thermal expansion coefficient is similar to that of the base layer over the entire temperature range.

US Patents 5,273,712 and 5,154,885 disclose coatings with a relatively large amount of Re added, which simultaneously improve creep resistance and oxidation resistance at high temperatures. However, Re combines with the high content of Cr in conventional coatings, leading to an undesirable phase structure of the coating and the interdiffusion layer. At moderate temperatures (below 950-900°C), the α-Cr phase is more stable than the γ-matrix in the coating. This results in low rigidity and ductility. Furthermore, the large excess of Cr in the coating compared to the base layer resulted in the diffusion of Cr into the underlying alloy, which enhanced the deposition of acicular Cr-rich, W-rich, and Re-rich phases.

US Patent 4,758,480 discloses a class of protective coatings whose composition is based on the composition of the underlying substrate. The similarity of the microstructure (γ' phase in the γ matrix) makes the mechanical properties of the coating similar to those of the substrate, thus reducing thermal deformation-induced damage during use. However, the content of Al (7.5-11wt%) and Cr (9-16wt%) in the coating is unlikely to provide sufficient resistance to oxidation and/or corrosion for long-term exposure, which is a problem in stationary gas turbines. often.

                     

The main object of the present invention is to provide a new coating for structural parts of gas turbines, in particular blades and rudders, which exhibits improved mechanical properties and provides sufficient resistance to oxidation/corrosion for the long-term exposure often found in stationary gas turbines .

Invention Summary

The present invention discloses a nickel-based alloy which is especially suitable for coating the blades of modern gas turbines. This alloy was prepared with the elements and alloy composition amounts shown in Table 1.

Table 1 Varieties of coating compositions of the present invention Composition elements (wt%) Ni co Cr al Re Y Si Ta Nb La* Mg B coating margin 18-28 11-15 11.5-14 1-8 0.3-1.3 1-2.3 0.2-1.5 0.2-1.5 0-0.5 0-1.5 0-0.1 La*=La+lanthanide Y+La(+La series)≤0.3-2.0Si+Ta≤2.5wt% Hf, C<0.1wt%

The alloys according to the invention simultaneously provide suitable oxidation and corrosion resistance, phase stability during diffusion heat treatment and during service and mechanical properties, especially high ductility, high creep resistance and similarity to the substrate thermal expansion.

This is achieved by a specific phase structure consisting of β reservoir sediments (45-60 vol%) in a ductile γ-matrix (40-55 vol%).

The alloy is prepared by a vacuum melting method, and powdery particles are formed by an inert gas atomization method. The powder is then deposited on the substrate by, for example, thermal spraying. However, other methods of operation may also be used. It is recommended to heat-treat the coating with appropriate time and temperature to achieve a firm bond with the substrate and increase the sintered density of the coating.

Table 2(a) presents a number of different alloys of the composition of the invention that were tested.

Table 2(a) Preferred Coating Compositions Element wt% in composition Ni co Cr al Re Y Si Ta Nb La Mg PC1 margin 24.1 11.8 12.1 2.8 0.3 1 1 0.3 - - PC2 margin 23.8 13 12 3 0.5 1.7 0.5 0.3 - 0.2 PC3 margin 23.8 13 11.8 3 0.3 1 1 0.3 0.1 -

These preferred alloys exhibit the required coating properties: optimum oxidation and corrosion resistance, phase stability during diffusion heat treatment and service, excellent mechanical properties, especially high ductility, high creep resistance and Similar thermal expansion to CMSX4 based substrate.

To illustrate the advantages of these preferred compositions, other alloys of other compositions given in Table 2(b) were also tested. It has been found that alloys EC1-EC8 have inferior properties compared to the preferred compositions _PC1 , _PC2 and _PC3 .

Table 2(b) Other coating compositions Element wt% in composition coating Ni co Cr al Re Y Si Ta Nb f EC1 margin 12 20.5 11.5 - 0.5 2.5 1 - - EC2 margin 12 16 11.5 - 0.3 2.5 1 - - EC3 margin twenty four 16 11 - 0.3 2 1 - - EC4 margin twenty four 13 11 3 0.3 2 - 0.5 - EC5 margin twenty four 13 11.5 3 0.3 1.2 - - 0.5 EC6 margin twenty four 14 11 - 0.3 2 0.5 - 0.5 EC7 margin - 16 8 - 0.5 2 0.5 - - EC8 margin 12 8.5 7 3 0.5 1 3 0.3 0.7

Table 2(c) Composition of CMSX4 (single crystal base material) Element wt% in composition Ni co Cr al Re W Mo Ta Ti f CMSX4 margin 10 6.5 5.6 2.8 6.4 0.5 6.5 1 0.1

The favorable phase structure (β phase in a ductile γ matrix) of the preferred alloy composition is reflected by the room temperature and 400°C tensile test results (Table 3). While tensile specimens coated with EC1 failed at <0.4% strain, specimens coated with the preferred composition elongated >4% and >9% at room temperature and 400°C, respectively.

Table 3 Strain of selected coatings at failure at room temperature and 400°C coating Strain at room temperature failure (%) Strain at failure at 400°C EC1 <0.4 <0.4 EC2 0.8 1.9 EC3 2 4.5 EC4 2.2 4.8 PC1, PC2, PC3 >4 >9

In addition, the TMF experimental data show (Table 4) that the improved coating of the present invention also has excellent TMF performance. The TMF lifetime of the inventive coating was >3000 cycles compared to the EC1 coating which broke at the first cycle and the conventional topcoat which failed after 2000 cycles. That is, very similar properties to uncoated single crystal base alloys.

Table 4. TMF lifetime of selected coatings coating number of cycles at destruction EC1 1 EC2 <10 conventional paint 2000 PC1, PC2, PC3 >3000

The stable phase structure of the preferred composition (45-60 vol% beta and 55-40 vol% gamma) was found to result in extremely high mechanical properties of coated samples or components. The balance of these two phases results in an excellent combination of high TMF resistance and excellent oxidation resistance. Thermal expansion, ductility, and TMF resistance are all above the level of the best γ-γ′ systems (such as single crystal superalloys), and the presence of β-reservoir phases leads to oxidation lifetimes that cannot be achieved by γ-γ′ systems.

It is important to know that only the required element combinations in Table 1 lead to the desired β+γ phase structure (within the required phase ratio) with excellent oxidation/corrosion resistance and good mechanical properties. Excessive amounts of alloying elements such as Cr, Al, Ta, Si, Nb, Co, Re lead to unwanted σ, Heusler or γ-phase deposition.

Al, Cr, Re, and Si below the specified content will reduce the oxidation and/or corrosion resistance. A reduced Ta and Nb content, or at least a lack of one of these elements, increases the oxide growth rate and should therefore be avoided where the coating is used as a TBC bond coating.

Changing the balance between Al, Cr and Co can result in a similar initial phase structure, but this phase structure is expected to be unstable during use. Phase transitions have been shown to exacerbate thermal expansion mismatches between coating and substrate, thereby reducing service life.

While the present invention has been shown and described in terms of a preferred embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention herein.

Brief description of the attached drawings

A more complete appreciation of the invention and its advantages can readily be obtained after a better understanding of the invention has been obtained by the aid of the following detailed description taken in conjunction with the accompanying drawings. in,

Figure 1 shows that the activity of Al varies with the content of Cr in the alloy (other elements in the alloy are as follows: 12.1% Al, 24.1% Co, 3% Re, 1% Si, 0.5% Ta).

Figure 2 shows that the activity of Al varies with the Re content in the alloy (other elements in the alloy are as follows: 12.1% Al, 11.8% Cr, 24.1% Co, 1% Si, 0.5% Ta);

Figure 3 shows that the activity of Al varies with the Si content in the alloy, (other elements in the alloy are as follows: 12.1% Al, 11.8% Cr, 24.1% Co, 3% Re, 0.5% Ta);

Fig. 4 shows that mass per unit area increases with preferred coating composition PC1, PC2, PC3 and experimental coating EC3, EC4, EC5, EC6 and the variation relation of oxidation time when EC8 is oxidized at 1000 ℃;

Figure 5 shows the relationship between the peeling time of the first oxide scale and the composition of the coating at 1050 ° C, expressed in a columnar graph;

Fig. 6 (a) shows that preferred compositions PC1, PC2, PC3 are oxidized at 1000 DEG C during in-situ X-ray analysis and X-ray intensity changes with oxidation time;

Figure 6(b) shows the X-ray intensity as a function of oxidation time from in situ X-ray analysis during oxidation at 1000 °C in the case of transition state oxide formation;

Figure 7(a) shows the equilibrium phase structure of a preferred coating composition;

Figure 7(b) shows the equilibrium phase structure of test coating composition EC7;

Figure 8 shows the coefficient of thermal expansion as a function of temperature for CMSX4, test paint EC7, and alloy compositions of the invention.

Chart Details

It has been found that the oxidation resistance of the alloy is mainly determined by the Al content in the system, that is, by the Al atomic reserves forming the Al ₂ O ₃ protective skin and the activity of Al in the system. The activity of Al is greatly determined by the presence of other elements in the alloy and the Al Diffusion influences the alloy phase structure. The simulation results of Cr, Re and Si's reactivity to Al and thus their influence on the oxidation resistance of the alloy are shown in Figs. 1-3.

When oxidized, the alloy exhibits a weight gain due to the absorption of oxygen. If the growing oxide scale is a protective layer, the weight growth follows the parabolic rate law with the oxidation time. Apparently, a small weight gain indicates slow oxide scale formation and is thus a desirable property.

The experimental data presented in Figure 4 shows that the preferred alloy composition has the least weight change compared to test alloys EC3, EC4, EC5, EC6 and EC8. The poor oxidation behavior of EC8 suggests that sufficiently high amounts of Al and other elements to support Al activity are required in the alloy.

Clearly, certain elements of the preferred composition act by modifying the oxide layer so that it resists oxygen indiffusion or Al outdiffusion. Oxide growth continues until a limiting oxide thickness is reached where exfoliation occurs. As long as the Al content and Al activity in the alloy remain high enough, the Al ₂ O ₃ skin can grow and peel off repeatedly.

Generally, MCrAlY coatings contain 0.5-1wt% Y, which has a strong influence on the oxidation resistance of the alloy. In some formulations, Y acts to improve the adhesion of oxide scales formed on the coating so that flaking is greatly reduced. Various other so-called oxygen active elements (La, Ce, Zr, Hf, Si) have been proposed to replace or supplement the Y content.

In the present invention, Y is added in an amount of 0.3-1.3 wt%, and La and lanthanoids are added in an amount of 0-0.5 wt%. Surprisingly, it was found here that Hf increases the oxide growth rate. The difference in oxidation rates for the preferred alloy composition (ie Hf-free) and the Hf-containing alloys (EC5, EC6 and EC8) is shown in FIG. 4 . Energy dispersive X-ray analysis revealed the presence of hafnium carbide in Hf-containing alloys, which appeared to reduce oxidation resistance.

On the other hand, Nb and Ta were found to increase oxidation resistance by reducing the oxide growth rate. Their cumulative effect is stronger than any one of them alone. In the presence of Ta, even such a small amount of Nb, about 0.2-0.5 wt%, can greatly affect the oxidation resistance (please compare the preferred composition with EC3 and EC4 in Figure 4).

The corrosion resistance of the alloy is mainly determined by the Cr content in the alloy. When tested in a corrosive environment (Na ₂ SO ₄ /CaSO ₄ slag + air/SO ₂ atmosphere) for 2000 hours, various alloy compositions showed corrosion from a few μm to mm deep. While CMSX4 (6.5wt% Cr is fully corroded, the preferred alloy compositions PC1, PC2, PC3 (11-15wt% Cr) show corrosion marks only in the 5 μm area. Low chromium content (<11%) not only results in low corrosion resistance And the activity of Al is also low, thus the oxidation resistance is low. As can be clearly seen from Figure 1, as long as the Cr content> 11%, the Al activity increases greatly. However, the chromium content is too high, especially when combined with the high Al content , will greatly reduce its low-temperature ductility and fatigue life. When the Cr content exceeds 16wt%, the β and γ phases transform into α-Cr and γ′ phases during the use of the workpiece, resulting in a completely brittle phase structure.

Co increases the solubility of Al in the gamma matrix, and as a result suppresses the amount of brittle phases (especially sigma phase) present in the alloy. Comparing the room temperature ductility of samples coated with EC2 and EC3 (Table 3), clearly demonstrates the beneficial effect of Co.

The presence of Si in the alloy increases the activity of Al (Fig. 3), thus increasing its oxidation resistance. However, in order to prevent the deposition of brittle Ni(Ta,Si) phases, Si contents > 2.5 wt% must be avoided. The favorable effect of Ta on oxidation performance, especially when combined with Si, is clearly known from EP Patent No. 0241807. However, computer simulations of the phase structure demonstrated that the total amount of Si+Ta must not exceed 2.5 wt% in order to avoid embrittlement of the coating.

The strengthening of commercial structural superalloys is not only strengthened by the formation of γ' elements (Al, Ti, Ta), but also by the addition of solid solution strengtheners, such as Re, W, Mo, Cr, Co. Since W and Mo were found to be detrimental to oxidation resistance, they have been replaced by Re and Ta without loss of strength. From Figure 2 it can be clearly seen that Re increases the activity of Al in the alloy and thus has a positive effect on the oxidation performance. It is also known that Re promotes the stability of the microstructure and reduces interdiffusion.

The improved coatings of the present invention are also useful as bond coats for thermal barrier coatings (TBC). A typical TBC system is a two-layer material system consisting of a layer of ceramic insulating material (such as ZrO ₂ partially stabilized by Y ₂ O ₃ ) on an MCrAlY bond coat. Because the TBC lifetime is mainly related to the amount of oxide growth at the bond coat/ceramic interface, the oxide growth rate and oxide skin adhesion are among the parameters controlling the lifetime.

Although for surface coatings (ie, no TBC), thermally grown oxides can be regrown by repeated exfoliation, for TBC systems, oxide exfoliation is strictly avoided during use. Oxidation tests were carried out on different coating compositions and the oxidation time (in hours) required for the first flaking to occur was determined.

These data are plotted in Figure 5, from which it can be seen that the preferred coating compositions PC1, PC2, PC3, which characterize the parameter of oxide scale adhesion - the longest time to the first occurrence of flaking.

Most important for TBC bond coats is also the formation of a protective α-Al ₂ O ₃ skin at the initial stage of oxidation. A transition oxide with a higher growth rate than _Al2O3 increases the amount of oxide without increasing its protective properties _.

Therefore, the presence of transition oxides at the bond coat/ceramic interface must be avoided or kept to a minimum. Different pathways have been _proposed , such as Al or Pt diffusion into the exterior of the bond coat, promoting the formation of α- _Al2O3 . However, diffusion-concentrated layers generally have poor mechanical properties due to the deposition of brittle phases.

In situ X-ray analysis of the different alloys during the oxidation phase at 1000°C revealed the following: a protective α-Al ₂ O ₃ skin forms on the preferred compositions PC1, PC2, PC3 within one hour of oxidation, while the transition oxide Not detectable (not even at glancing angles). Apart from α-Al ₂ O ₃ , only AlYO ₃ , which grows close to the Al ₂ O ₃ /substrate interface and promotes mechanical bonding of the oxide scale, appears in the X-ray spectrum. Figure 6(a) shows the results of in situ X-ray analysis of the preferred composition and Figure 6(b) illustrates the situation when transition oxides are formed.

Figure 7(a) shows computer simulation results: phases present in preferred coating compositions. It can be seen that the phase structure consisting of 45-60 vol% β phase and 55-40 vol% γ phase is stable in a wide temperature range (about 900-1280° C.). Only a small amount of alloy volume (<10 vol%) suffers the detrimental phase transformation β+γ→σ+γ′ upon cooling. Such a large phase stable region makes the coating very insensitive to the diffusion heat treatment temperature. In contrast, computer simulations of test paint EC7 (Fig. 7(b)) indicated a stable phase composition only at temperatures &lt;

Phase transitions in an alloy during the heating/cooling cycle have a significant effect on its physical properties and thus the mechanical behavior of the alloy. This is illustrated in Figure 8, which shows the coefficient of thermal expansion of CMSX4 (the base alloy), the preferred alloy composition and alloy EC7. The preferred composition and CMSX4 show almost linear behavior over the entire temperature range, while EC7 deviates from linearity at T ~ 950 °C, which is consistent with the onset of phase transition. Of course, a large difference in the coefficient of thermal expansion between the coating and the substrate leads to high overall mechanical stresses in the coating.

Claims (7)

1. coating composition that is used for the superalloy structural parts, % comprises in weight:

Ni surplus Y 0.3-1.3

Co 18-28 Mg 0-1.5

Cr 11-15 La+La is 0-0.5

Al 11.5-14 B 0-0.1

Re 1-8 Hf ＜0.1

Si 1-2.3 C ＜0.1

Ta 0.2-1.5

Nb 0.2-1.5 Si+Ta ≤2.5。

2. the coating composition of claim 1 comprises:

Ni surplus Si 1

Co 24.1 Y 0.3

Cr 11.8 Ta 1

Al 12.1 Nb 0.3

Re 2.8 Si+Ta ≤2.5。

3. the coating composition of claim 1 comprises:

Ni surplus Y 0.5

Co 23.8 Ta 0.5

Cr 13 Nb 0.3

Al 12 Mg 0.2

Re 3

Si 1.7 Si+Ta ≤2.5。

4. the coating composition of claim 1 comprises:

Ni surplus Y 0.3

Co 23.8 Ta 1

Cr 13 Nb 0.3

Al 11.8 La 0.1

Re 3

Si 1 Si+Ta ≤2.5。

5. the coating composition of claim 1, it comprise one be of value to oxidation-resistance, erosion resistance and mechanical property, by containing the phase structure that the γ matrix β precipitate, that be ductile is formed.

6. the coating composition of claim 1, it is to be deposited upon on the ground that is selected from Ni base and the basic superalloy of Co as one.

7. according to each the coating composition of claim 1-6, it is to be coated with as one to be deposited upon on the ground, and gives the top layer that one deck thermal barrier coating is provided on this coating composition.

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